Microwave Transmission Networks, Second Edition

Microwave Transmission Networks

ABOUT THE AUTHOR HARVEY LEHPAMER has 30 years of experience in the planning, design, and deployment of wireless and wireline networks, including microwave, fiber-optic and other transmission (transport) systems in Europe, Canada, the U.S., Africa, Mexico, and other parts of the world. Mr. Lehpamer has worked for companies such as Ericsson Wireless Communications Inc., San Diego, U.S.; Qualcomm Inc., San Diego, U.S.; Clearnet Inc., Toronto, Canada; Ontario Hydro, Canada; Lucas Aerospace Inc., Microwave Technologies Division, Canada; Electroproject, Consulting Engineers, Zagreb, Croatia. Mr. Lehpamer is a licensed professional engineer of the province of Ontario, Canada. Today he is an owner and principal engineer of HL Telecom Consulting in San Diego, California (www.HLTelecomConsulting.com). He can be contacted at: [email protected] or [email protected].

Microwave Transmission Networks: Planning, Design, and Deployment Second Edition

Harvey Lehpamer

New York

Chicago San Francisco Lisbon London Madrid Mexico City Milan New Delhi San Juan Seoul Singapore Sydney Toronto

Copyright © 2010 by The McGraw-Hill Companies. All rights reserved. Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without the prior written permission of the publisher. ISBN: 978-0-07-170123-5 MHID: 0-07-170123-0 The material in this eBook also appears in the print version of this title: ISBN: 978-0-07-170122-8, MHID: 0-07-170122-2. All trademarks are trademarks of their respective owners. Rather than put a trademark symbol after every occurrence of a trademarked name, we use names in an editorial fashion only, and to the benefit of the trademark owner, with no intention of infringement of the trademark. Where such designations appear in this book, they have been printed with initial caps. McGraw-Hill eBooks are available at special quantity discounts to use as premiums and sales promotions, or for use in corporate training programs. To contact a representative please e-mail us at [email protected]. Information has been obtained by McGraw-Hill from sources believed to be reliable. However, because of the possibility of human or mechanical error by our sources, McGraw-Hill, or others, McGraw-Hill does not guarantee the accuracy, adequacy, or completeness of any information and is not responsible for any errors or omissions or the results obtained from the use of such information. TERMS OF USE This is a copyrighted work and The McGraw-Hill Companies, Inc. (“McGraw-Hill”) and its licensors reserve all rights in and to the work. Use of this work is subject to these terms. Except as permitted under the Copyright Act of 1976 and the right to store and retrieve one copy of the work, you may not decompile, disassemble, reverse engineer, reproduce, modify, create derivative works based upon, transmit, distribute, disseminate, sell, publish or sublicense the work or any part of it without McGraw-Hill’s prior consent. You may use the work for your own noncommercial and personal use; any other use of the work is strictly prohibited. Your right to use the work may be terminated if you fail to comply with these terms. THE WORK IS PROVIDED “AS IS.” McGRAW-HILL AND ITS LICENSORS MAKE NO GUARANTEES OR WARRANTIES AS TO THE ACCURACY, ADEQUACY OR COMPLETENESS OF OR RESULTS TO BE OBTAINED FROM USING THE WORK, INCLUDING ANY INFORMA TION THAT CAN BE ACCESSED THROUGH THE WORK VIA HYPERLINK OR OTHERWISE, AND EXPRESSLY DISCLAIM ANY WARRANTY, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. McGraw-Hill and its licensors do not warrant or guarantee that the functions contained in the work will meet your requirements or that its operation will be uninterrupted or error free. Neither McGraw-Hill nor its licensors shall be liable to you or anyone else for any inaccuracy, error or omission, regardless of cause, in the work or for any damages resulting therefrom. McGraw-Hill has no responsibility for the content of any information accessed through the work. Under no circumstances shall McGraw-Hill and/or its licensors be liable for any indirect, incidental, special, punitive, consequential or similar damages that result from the use of or inability to use the work, even if any of them has been advised of the possibility of such damages. This limitation of liability shall apply to any claim or cause whatsoever whether such claim or cause arises in contract, tort or otherwise.

I dedicate this book to the memory of my late father, who was a good man and a great engineer.

This page intentionally left blank

Contents at a Glance

Chapter 1 Transmission Network Fundamentals

1

Chapter 2 Basics of Microwave Communications

33

Chapter 3 Microwave Link Design

89

Chapter 4 Planning the Microwave Network

159

Chapter 5 Microwave Network Design

185

Chapter 6 Microwave Network Deployment

217

Chapter 7 Project Management

357

Appendix A American Cable Stranding

417

Appendix B Quick RF Reference Sheet

419

Appendix C Useful Physical Quantities and Units of Measurement

423

Glossary

427

Index

467

vii

This page intentionally left blank

Contents

Preface to Second Edition xi Preface to First Edition xiii

Chapter 1. Transmission Network Fundamentals 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

Transmission Network Media Basic Terminology Transmission Network Topology Transmission Network Performance Network Synchronization Network Delays Security and Encryption References

Chapter 2. Basics of Microwave Communications 2.1 2.2 2.3 2.4 2.5 2.6 2.7

Radio Fundamentals Structure and Characteristics of the Earth’s Atmosphere Radio Propagation Digital Microwave Point-to-Point Systems Other Microwave Systems Basics of Digital Communications Systems References

Chapter 3. Microwave Link Design 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9

Design Process Flowchart The Loss/Attenuation Calculations Fading and Fade Margins Microwave Link Multipath Probability Models Quality and Availability Calculations Rain Attenuation and Outage Models Improving the Microwave System Repeaters References

1 1 7 18 21 24 28 29 31

33 33 39 40 55 73 84 88

89 89 90 99 114 120 127 136 150 158 ix

x

Contents

Chapter 4. Planning the Microwave Network 4.1 4.2 4.3 4.4 4.5

The Microwave Network Planning Process Microwave Systems in Wireless Networks Microwave Systems in Utility Telecom Networks Topology and Capacity Planning References

Chapter 5. Microwave Network Design 5.1 5.2 5.3 5.4 5.5 5.6 5.7

Introduction Spectrum Management Interference Effects and Frequency Sharing Microwave Design Tools Microwave Systems Engineering Tips, Hints, and Suggestions References

Chapter 6. Microwave Network Deployment 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10 6.11 6.12 6.13

Introduction Digital Microwave Radio Digital Multiplexers Cabling and Signal Termination Microwave Antennas, Radomes, and Transmission Lines GIS Data Field Surveys Housing the Equipment Microwave Antenna Mounting Structures Power Supply and Battery Backup Grounding, Lightning, and Surge Protection Microwave Testing and Troubleshooting References

Chapter 7. Project Management 7.1 7.2 7.3 7.4 7.5 7.6

Tracking Microwave Rollout Regulatory Issues Logistical and Organizational Challenges Ethical Issues Frequently Asked Questions References

159 159 161 176 177 183

185 185 185 189 203 204 212 216

217 217 218 256 258 261 288 298 317 320 333 340 342 355

357 357 373 381 392 397 416

Appendix A American Cable Stranding

417

Appendix B Quick RF Reference Sheet

419

Appendix C Useful Physical Quantities and Units of Measurement

423

Glossary

427

Index

467

Preface to Second Edition

It has been six years since the first edition of this book was prepared and published, during which time it was very well received among engineers, project managers, and everyone else interested in learning more about the terrestrial point-to-point microwave systems. During that period of time there have been a number of new developments in the wireless and microwave systems arena that required our attention and, therefore, warranted a new edition of the book. In addition, some information provided in the first edition has been thoroughly updated and in some instances corrected and/or expanded. Many new sections have been added, and practically all the existing chapters and sections of the book have been revised and refreshed with additional new and relevant information, including information on the impact of Ethernet and IP communications on microwave links. A number of useful formulas have been added, as well as their application in solving microwave design-related problems explained in practical examples. There is a new “Frequently Asked Questions” section in Chapter 7 (“Project Management”) that I recommend you consult as often as possible; there is a good chance that your question(s) have already been posted and answered there. The writing of this second edition was helped by many individuals, who were kind enough to comment on the first edition, including the identification of a few errors that inevitably had slipped in. I am convinced that readers will find the resulting 2010 edition of this book a very useful, practical reference in the field of microwave systems engineering. Harvey Lehpamer San Diego, California

xi

This page intentionally left blank

Preface to First Edition

Microwave radio network design is a subset of activities that constitute the overall transmission network design. Transmission networks are sometimes called transport networks, access networks, or connectivity networks. For many wireless carriers, microwave is becoming a popular preference over wireline (leased lines) transport for many reasons, especially as microwave radio equipment costs decrease and installation becomes simpler. Low monthly operating costs can undercut those of typical single (and especially multiple) T1/E1 expenses, proving it to be more economical over the long term—usually two to four years. Network operators also like the fact that they can own and control microwave radio networks instead of relying on other service providers for network components. Most people in the telecommunications field, especially transmission engineers, project managers, and network planners in wireless systems, should have at least a basic understanding of the planning, design, and deployment process of the microwave network. For clarity and technical correctness, we should be very clear and consistent in the terminology used throughout this book. It is important to remember that not all microwave systems are point-to-point, and not all point-to-point systems are microwave. Although many principles are common to other microwave systems, this book predominantly deals with the terrestrial microwave point-to-point systems in 2 to 60 GHz. This book covers all stages of terrestrial microwave point-to-point network build-out from initial planning and feasibility studies to real system deployment. Emphasis is given to practical guidelines and activities involved in putting microwave systems into operation. It describes the process behind planning and creating a business case for a microwave network, including the advantages and disadvantages, and includes discussions that will help executives to make an informed decision about whether to build a microwave network. What is a difference between planning, design, and deployment? Although distinct differences exist in telecommunications projects, xiii

xiv

Preface to First Edition

these three activities in microwave network build-out are somewhat overlapping and mutually dependent. Many times, partial design or redesign has to be performed during the planning stage and/or deployment as well. Planning usually refers to a high-level decision-making process that encompasses budget and schedule definition and identifying team members required for the project. It also includes determination of frequency band, system capacity, network configuration, and performance objectives. System design is an actual detailed link engineering process (which may or may not include site visits) that includes creation of the detailed bill of materials, ordering equipment (MW radio, shelters, towers, and other transmission hardware and software), ordering engineering, installation and other services, and so forth. Deployment (also called implementation) includes all of the field activities such as site and path surveys, tower erection, equipment installation, creation of an asbuilt documentation, and acceptance testing and commissioning. Details and mathematical models of microwave point-to-point link engineering will be discussed only briefly, as they are beyond the scope of this book and its intended audience. In addition, literature and standards are available that provide more details. A comparison of North American and ITU microwave design models and methods is presented, and project management and logistics issues, deployments in different countries, and regulatory and ethical issues are discussed in more detail. This book will be a useful source of information for project managers, sales executives, and nontechnical personnel involved in planning and/or making decisions about microwave network deployment. It will also be helpful to managers and directors of engineering and operations who work for carriers or wireless operators. Additionally, it will serve engineers who are starting their careers in the microwave field, engineers whose main field is not RF or network planning, telecom and microwave engineers who want to expand their knowledge about the business side of microwave network build-out, and instructors and consultants in the telecommunications field—and anyone else who is involved in real-life microwave network build-out. For those who want only an overview of the microwave network buildout, Chapter 7, “Project Management,” will provide easy reading and, at the same time, sufficient high-level information to get them started. More detailed information can be found in other chapters of the book. An extensive glossary provides definitions of many commonly used terms in transmission, RF, and microwave fields. An understanding of these terms is necessary to comprehend the material in this book. Harvey Lehpamer

Chapter

1

Transmission Network Fundamentals

1.1 Transmission Network Media In telecommunications, information can be transmitted between two locations using a signal that can be either analog or digital in nature. In the telecommunications networks today, digital transmission is used almost exclusively, in which analog traffic, such as voice calls, is converted to digital signals (a process referred to as sampling) to facilitate long distance transmission and switching. A high-pitched voice contains mostly high frequencies, while a lowpitched voice contains low frequencies. A loud voice contains a highamplitude signal, while a soft voice contains a low-amplitude signal. Analog signals can be combined (i.e., multiplexed) by combining them with a carrier frequency. When there is more than one channel, this is called frequency-division multiplexing (FDM). FDM was used extensively in the past but now has generally been replaced with the digital equivalent, called time-division multiplexing (TDM). The most popular TDM system is known as the Tier 1 (T1) system, in which an analog voice channel is sampled 8,000 times per second, and each sample is encoded into a 7-bit byte. Twenty-four such channels are mixed on two copper pairs and transmitted at a bit rate of 1.544 megabits per second (Mbps). T1 in North America (E1 in the rest of the world) remains an important method of transmitting voice and data in the public switched telephone network (PSTN).1

1

2

Chapter One

A talking path (i.e., a switched circuit) in the PSTN can be either analog or digital or a combination thereof. In fact, a digital signal can be transmitted over a packet-switched network as easily as a circuit-switched network. Digitized voice is similar to data; therefore, if data can be transmitted over a packet network, then so can digitized voice. One of the most common applications is now known as voice over IP (VoIP). The challenge, of course, is to get the transmitted signal to the destination fast enough (delay-related issues), as in instances in which the conversation may be time sensitive. A second challenge is to get each packet, which is a small piece of a voice conversation, to its destination in the proper order. Three types of media (physical layers) can be used in transmitting information in the telecommunications world: n

n

n

Copper lines (twisted-pair and coaxial cables), for low- and mediumcapacity transmission over a short distance Fiber-optic transmission, for medium- and high-capacity transmission over any distance Wireless transmission, including: n

n

Low (mobile radio) and medium-capacity (microwave point-to-point) over short and medium distances Satellite for low- and medium-capacity transmission over long distances

The terms transmission and transport are used interchangeably in this text, as both are currently in use; the former is preferred in Europe, but the latter is more commonly used in North America. Sometimes transmission refers only to the physical media, while transport can include other OSI Model layers of the data transfer. 1.1.1 Wireline Systems Copper Lines Years ago, copper wire was the only means of transporting information. Technically known as unshielded twisted pair (UTP), it consists of a large number of pairs of copper wire of varying size within a cable. The cable did not have a shield, so the signal (primarily the high-frequency part of the signal) was able to leak out. In addition, the twisting on the copper pair was very casual, designed as much to identify which wires belonged to a pair as to handle transmission problems. Even with these limitations, it was quite satisfactory for use in voice communications.

1.1.1.1

Transmission Network Fundamentals

3

Coaxial cable technologies were primarily developed for the cable TV industry. In the last few years, this technology has been extended to provide Internet services to residences. The high capacity of coaxial cable allows it to support multiple TV channels, and this capacity can also be used for high-speed Internet access. Like fiber optics, the cost of cable installation limits the deployment of new services, and current deployments are not typically in areas that allow this service to be offered to business offices. Fiber optics constitute the third transmission medium, and this is unquestionably the high-bandwidth transmission medium of choice today. Fiber-optic cables can be placed in ducts, buried in the ground, suspended in the air between poles, installed as part of the ground wire on the high-voltage transmission towers optical power ground wire (OPGW), and so forth. Transmission speeds of as high as 10 Gbps have become commonplace in the industry. Of course, laying fiber, on a per-mile basis, still costs somewhat more than laying copper, but on a percircuit basis there is no doubt that fiber is more cost effective. The huge capacity of fiber certainly makes for more efficient communications; however, placing so much traffic on a single strand, for point-to-point communications, makes for greater vulnerability. Most of the disruptions in the long distance network are a result of a physical interruption of a fiber run (called backhoe fade), and the ring configuration is the protection solution used most often in fiber-optic networks. Dense wavelength division multiplexing (DWDM) is a fiber-optic transmission technique that employs light wavelengths to transmit data and can increase the bandwidth of the existing fiber-optic facilities. There is a huge emphasis on scalable DWDM systems enabling service providers to accommodate consumer demand for ever-increasing amounts of bandwidth. In order to squeeze more bandwidth out of their fiber networks, long-haul carriers are deploying DWDM to build backbones that might have dozens of channels riding on a single strand of fiber, with each channel operating at multigigabit speeds. The cost of laying fiber-optic cable can easily reach $70,000 to $150,000 per mile in rural areas and could be much higher in the dense urban areas. This cost does not include any terminal equipment. For that reason, most users opt for either leasing fiber-optic facilities or building their own microwave network.

1.1.1.2 Fiber-optic Systems

1.1.2 Wireless Systems

Wireless communications can take several forms: microwave (point-topoint or point-to-multipoint), synchronous satellites, low Earth orbit satellites (LEOs), cellular, personal communications service (PCS),

4

Chapter One

and so on. For years, microwave radio transmissions have been used in the telecommunications industry for the transport of point-to-point data where information transmissions occur through carrier signals. Microwave carrier signals are typically relatively short in wavelength and can transmit information using various modulation methods. To understand wireless technology, a basic understanding of the radio frequency (RF) spectrum is required. The RF spectrum is a part of the electromagnetic spectrum in which a variety of commonly used devices (including television, AM and FM radios, microwave radios, cell phones, pagers, and many other devices) operate. The electromagnetic spectrum has been used for communications for over 100 years, and it comprises an infinite number of frequencies, from AM radio at 1 MHz to the cellular/PCS band at 2 GHz. Frequencies are measured in cycles per second, or hertz, which are inversely related to wavelength. At low frequencies wavelengths are long, while at higher frequencies wavelengths are very short. Given an equal power level, the longer the wavelength, the greater the distance the signal can travel. Whereas low-frequency signals (such as AM radio) can be transmitted for hundreds of miles, high-frequency signals (such as infrared) can travel only a few feet. The fixed-satellite service basically involves four frequency bands: 4/6 GHz, 7/8 GHz (for military systems), 11/14 GHz, and 20/30 GHz. Although there are numerous bands above 30 GHz allocated to the fixedsatellite service, only one is presently being used. Microwave frequencies and “stationary” satellites allow the use of high-gain, directional antennas, much like the fixed service, reducing the power requirements for the satellite transmitters. The fixed-satellite service includes international, domestic, and military systems and, although they often carry the same type of traffic, each group has its own set of users. International and domestic systems operate at 4/6 GHz and 11/14 GHz, while military systems use 7/8 GHz and frequencies near 20 GHz and 45 GHz. Another of the wireless telecommunications technologies is the low Earth orbit (LEO) satellite system, which are satellites that communicate directly with handheld telephones on Earth. Because these satellites are relatively low (less than 900 miles above), they move across the sky quite rapidly. In a LEO system the communications equipment on a satellite acts in much the same way as a cell site of a cellular system: it “catches” the call transmitted from Earth and usually passes it to an Earth-based switching system. Because of the speed of the satellite, it is frequently necessary to hand off a particular call to a second satellite just rising over the horizon. This is similar to a cellular system, except that in this case it is the cell site that is moving rather than the subscriber.

Transmission Network Fundamentals

5

The RF spectrum in which these carrier transmissions occur is subject to regulation by the Federal Communications Commission (FCC) in the United States, Industry Canada in Canada, Cofetel in Mexico, and globally via the International Telecommunications Union (ITU). Countries that are members of the ITU generally follow the ITU spectrum allocation. Within the RF spectrum, not all frequencies are subject to licensing requirements, and license-exempt bands include the industrial, scientific, and medical (ISM) band (the most widely used license-exempt frequency band) and the Unlicensed National Information Infrastructure (UNII) band. The frequencies that are used for radio communications have slowly moved upward from lower to higher frequencies (shorter wavelengths). Back in the early days of radio, it was easier to generate carrier frequencies of sufficient power at the lower side of the frequency spectrum. With the advancement of new techniques, it became possible to develop new components that use higher and higher frequencies. Microwave and millimeter-wave bands occupy frequencies from around 1 to 300 GHz, but this book discusses the characteristics of the bands for terrestrial microwave (and millimeter-wave) point-to-point systems from around 2 to 90 GHz. 1.1.3

Free-Space Laser Communications

Free-space laser communications systems are wireless, point-to-point connections through the atmosphere that employ the optical part of the frequency spectrum. Therefore, they cannot be categorized as either wireless or wireline systems in a classical sense. They work only under clear line-of-sight conditions between each unit, eliminating the need for securing rights of way, buried cable installations, and government licensing. Free-space laser communications systems can be quickly deployed, since they are small and do not need any radio interference studies. Optical wireless is an attractive option for multigigabit-per-second short range links (typically from a few hundred meters up to 2 km) where laying optical fiber is too expensive or impractical, and where microwave systems do not provide enough bandwidth. This type of optical communication, also known as free-space optical (FSO), has emerged as a commercially viable alternative to RF and millimeter-wave wireless for reliable and rapid deployment of data and voice networks. (See Figure 1.1, which shows wireless transceivers mounted on rooftops.) RF and millimeter-wave technologies allow rapid deployment of wireless networks with data rates from tens of megabits per second (point-to-multipoint) up to several hundred

6

Chapter One TRx A

TRx B Laser beam

Building A

Building B

Figure 1.1 Optical wireless transceivers

megabits per second (point-to-point). However, spectrum-licensing issues at licensed and interference at license-exempt frequency bands can sometimes still limit their market penetration. Although optical losses and space losses can introduce a significant attenuation of the optical signal, the main problem is atmospheric effects. As mentioned earlier, it is desirable to have as much excess margin as possible to mitigate atmospheric effects such as fog. On a sunny day, the atmosphere is clear, and the margin is useful to overcome fades caused by turbulence. On a foggy day, the margin is used to overcome signal attenuation. The dominant atmospheric effect that affects optical communication is attenuation of the signal by scatter and absorption. Molecular scatter and absorption of major atmospheric constituents is relatively insignificant. Although rain and snow can cause attenuation up to approximately 40 dB/km and 100 dB/km, respectively, fog by far is the largest problem. In extremely heavy fog, attenuation as high as 300 dB/km (500 dB/mi) has been reported. Clearly, either link distance or link availability is compromised as part of the network design. Some systems can operate through window glass but with reduced distance. Metallic glass coating prevents any type of radio propagation altogether. Alignment is important both at the transmitter and the receiver. The transmitter has to be pointed accurately to ensure efficient delivery of energy to the receiver. The receiver has to be pointed properly to ensure that the signal entering the receiver aperture makes it to the detector. A great deal depends on how and where the transceivers are located and, in the case when a transceiver is mounted on the roof of a tall building, building sway contributes significantly to pointing error. A high-rise building can sway more than a meter (three feet), and the roof itself often houses air conditioning and ventilation units as well as elevators and other mechanisms that cause vibration in the low

Transmission Network Fundamentals

7

tens of hertz. Moreover, some roofs are not very solid and can deform when someone walks on them. Temperature changes (diurnal and seasonal) and uneven heating by the sun can deform the mount enough to throw the pointing off. Some FSO users have indicated that they need to realign their transceiver units several times a year for this very reason. Optical wireless transceivers and their mounts have enough wind resistance that they can be tilted in heavy winds. To make things worse, building owners, for liability reasons, do not readily grant approval for the installation of penetrating mounts, and nonpenetrating rooftop mounts exhibit even greater pointing fluctuations. Microwave and infrared transmission systems are both good alternatives for short distance network connections (less than two miles). Each system requires line-of-sight and has its advantages and disadvantages. The ease of obtaining frequency spectrum licensing, various weather and atmospheric conditions, topology of the area, and security should be evaluated to determine which system is used. When frequency spectrum licensing is difficult and expensive to obtain, infrared transmission systems have a distinct advantage. Infrared systems are also advantageous if the weather is normally rainy, but not foggy, and there is little smog; when the conditions require spanning a large body of water; and when the area has high levels of electromagnetic interference (EMI). In addition, an infrared transmission system is less inclined to be intercepted. Microwave transmission systems are advantageous in areas that are foggy and have a substantial amount of snow or smog, or if the conditions require spanning longer distances. Microwave systems are the preferred option anytime the distance exceeds two miles. 1.2 1.2.1

Basic Terminology E1, T1, and J1

There are two standards for the first-order digital transmission systems. The T1 system, developed by Bell Laboratories, is used mainly in the U.S.A., Canada, Taiwan, Jamaica, and a few other countries. North American T1 service providers often refer to the signal interfaces between the user and the network as DS1 signals. In the case of userto-user interfaces, the term DSX-1 is used to describe those DS1 signals at the “cross-connect” point; therefore, DSX-1 is a physical interface of the T1 circuit. Most of the countries around the world use the E1 system defined by European Conference of Postal and Telecommunications Administration (CEPT).

8

Chapter One

Details of the T1 systems can be found in literature.2 Here, only a brief description of T1 is given: n

n n

There are 24 DS0 (64-kbps) time slots in a T1 line, providing a total bandwidth of 24 × 64 kbps = 1,536,000 bps (1,536 kbps). Another 8,000 bps are used as framing bits. Adding up the total bandwidth of the 24 DS0 channels and the 8-kbps framing bits yields a 1.544 Mbps T1 data rate.

The North American digital hierarchy (see Table 1.1) starts with a basic digital signal level of 64 kbps (DS0). Thereafter, all facility types are usually referred to as Tx, where x is the digital signal level within the hierarchy (e.g., T1 refers to the DS1 rate of 1.544 Mbps). Up to the DS3 rate, these signals are usually delivered from the provider on twisted-pair or coaxial cables. The basic format for transmission facilities in Japan (see Table 1.2) is similar to the North American ANSI standard and called J1. However, the CMI line coding used in Japan is different from the one used in North America.3 Rather than using 50 percent duty cycle (half-width) “mark” pulses, continuous marks are transmitted, inverting the voltage after each mark. The spaces also have a transition, but in the middle of the pulse period. This ensures that there are always sufficient transitions to maintain synchronization, regardless of whether the signal is all zeros or all ones.

TABLE 1.1

North American Data Rates

Name

Rate

DS0 DS1 DS1C DS2 DS3 DS4

64 kbps 1.544 Mbps 3.152 Mbps 6.312 Mbps 44.736 Mbps 274.176 Mbps

TABLE 1.2

Japanese Digital Hierarchy

Designation DS1 DS2 DS3 DS4 DS5

Bit rate (Mbps) 1.544 6.132 32.064 97.728 400.352

Number of voice channels 24 96 480 1,440 5,760

Transmission Network Fundamentals TABLE 1.3

Name DS0 E1 E2 E3 E4 E5

9

ITU Data Rates Rate 64 kbps 2.048 Mbps 8.448 Mbps 34.368 Mbps 139.264 Mbps 565.148 Mbps

The CCITT Digital hierarchy’s4 basic level is the DS0 rate of 64 kbps (see Table 1.3). These signals are usually delivered from the provider via twisted-pair or coaxial cables. The following is a description of the CEPT digital hierarchy: n

n n

n

There are 30 DS0 (64-kbps) time slots in an E1 line, providing a total bandwidth of 30 × 64 kbps = 1,920,000 bps (1,920 kbps). One 64-kbps time slot (TS0) is used for framing bits. Another 64-kbps time slot (TS16) is used for signaling of voice frequency channels. Adding up the total bandwidth of the 30 DS0 channels, framing, and signaling bits yields the 2.048 Mbps E1 data rate.

1.2.2

PDH, SDH, and SONET

Traditionally, transmission systems have been asynchronous, with each terminal in the network running on its own clock. In digital systems, clocking (timing) is one of the most important considerations. Timing means using a series of repetitive pulses to keep the bit rate of the data stream constant and to indicate where the ones and zeros are located in a data stream. Because these clocks are free running and not synchronized, large variations occur in the clock rate and thus the signal bit rate. Asynchronous multiplexing uses multiple stages; lower-rate signals are multiplexed, and extra bits are added (bit-stuffing) to account for the variations of each individual stream and combined with other bits (framing bits) to form higher-level bit rates. Then bit-stuffing is used again to produce even higher bit rates. At the higher asynchronous rate, it is impossible to access these signals without multiplexing. The Plesiochronous Digital Hierarchy (PDH) signals have the essential characteristics of time scales or signals such that their corresponding significant instants occur at nominally the same rate. The prefix plesio, which is of Greek origin, means “almost equal but not exactly,” meaning that the higher levels in the CCITT (ITU today) hierarchy are not an

10

Chapter One

exact multiple of the lower level. Any variation in rate is constrained within specified limits. The PDH systems belong to the first generation of digital terrestrial telecommunication systems in commercial use. Synchronous network hierarchies were introduced in the late 1980s. The term synchronous means occurring at regular intervals and is usually used to describe communications in which data can be transmitted in a steady stream rather than intermittently. For example, if a telephone conversation were synchronous, each party would be required to wait a specified interval before speaking. Synchronous Digital Hierarchy (SDH) is a newer technology in the field of digital transmission.5 The transmission is carried out in a synchronous mode, hence the name. The most important advantage in adopting this synchronous technology is to enable the mapping of various user bit rates directly onto the main transmission signals, thus bypassing various stages of multiplexing and demultiplexing as was done in the case of earlier PDH technology. North American Synchronous Optical Network (SONET) is a secondgeneration digital optical transport protocol. The fiber-based carrier network uses synchronous operations among such network components as multiplexers, terminals, and switches. The number of new architectures and topologies are made possible as a result of this new technology. For instance, add/drop multiplexers (ADMs) have made it possible to use SONET/SDH terminals in a long chain with bit streams added or dropped along the way in an effective manner.6 By closing the chain at two ends, a ring configuration is possible, which provides enhanced protection features.7,8 SONET-based rings create a robust, high-availability network that can “heal” itself automatically by routing around failures. The optical fiber rings offer security, high bandwidth, low signal distortion, and high reliability. Other advantages include support of network management systems, easy upgrade to high bit rates, and adaptability to the existing PDH. In synchronous networks, all multiplex functions operate using clocks derived from a common source. The North American SONET system is based on multiples of a fundamental rate of 51.840 Mbps, called STS-1 (for synchronous transmission signal, level 1). The facility designators are similar but indicate the facility type, which is usually fiber-optic cable (e.g., OC-1 is an optical carrier supporting an STS-1 signal, whereas OC-3 supports a STS-3 signal, and so forth). Some typical rates are listed in Table 1.4. The international SDH system is based on a fundamental rate of 155.520 Mbps, three times that of the SONET system. This fundamental signal is called STM-1 (synchronous transmission module, level 1). The typical transmission media is defined to be fiber, but the broadband ISDN specification does define a user-network interface (UNI) STM-1

Transmission Network Fundamentals TABLE 1.4

11

STS Data Rates

Name

Rate (Mbps)

STS-1 STS-3 STS-9 STS-12 STS-48

51.840 155.520 466.560 622.080 2,488.320

(155.520 Mbps) operating over coaxial cables. Some typical rates within this hierarchy are shown in Table 1.5. Optical carrier (OC-n) levels describe a range of optical digital signals that can be carried on the SONET network. The general rule for calculating the speed of optical carrier lines is when a specification is given as OC-n, the speed will equal n × 51.840 Mbps. The rate in Table 1.6 is a line rate, referring to the raw bit rate carried over the optical fiber. A portion of the bits transferred over the line is designated as overhead. The overhead carries information that provides OAM&P (Operations, Administration, Maintenance, and Provisioning) capabilities such as framing, multiplexing, status, trace, and performance monitoring. The payload rate (line rate – overhead rate = payload rate) is the bandwidth available for transferring user data such as packets or ATM cells. The SONET/SDH level designations sometimes include a “c” suffix. The “c” suffix indicates a concatenated or clear channel. This implies that the entire payload rate is available as a single channel of communications (i.e., the entire payload rate may be used by a single flow TABLE 1.5

Name STM-1 STM-3 STM-4 STM-16

TABLE 1.6

STM Data Rates Rate (Mbps) 155.520 466.560 622.080 2,488.320

Optical Carrier Data Rates

Name

Rate (Mbps)

OC-1 OC-3 OC-3C OC-12 OC-24 OC-48

51.840 155.520 155.520 622.080 622.080 2,488.320

12

Chapter One

of cells or packets). The opposite of concatenated or clear channel is a channelized link. In a channelized link the payload rate is subdivided into multiple fixed rate channels. For example, the payload of an OC-48 link may be subdivided into four OC-12 channels. In this case the data rate of a single cell or packet flow is limited by the bandwidth of an individual channel. One of the main advantages of the ITU-T SDH system is the fact that it may be the first compatible system used worldwide. A further advantage is the extremely high bit rate transmitted by the system (e.g., nearly 10 Gbps with STM-64). When used in conjunction with DWDM, even much higher rates can be handled. SDH is perfectly suitable to multiplex and transport the traffic of PDH networks, and it also can be used for data transport and leased lines. It follows further from the synchronizing feature that a low-level container, also including its content, can be accessed at any higher hierarchical level. However, a disadvantage of SDH is the necessity to establish a synchronization network. SDH and SONET are both gradually replacing the higher order of PDH systems, but the evolution from PDH to SDH worldwide is a phased process because SDH based networks must have the flexibility to utilize existing PDH transport media. The advantages of higher bandwidth, greater flexibility, and scalability make these standards ideal for asynchronous transfer mode (ATM) networks as well. To maximize the benefits of the SONET/SDH microwave radio, the radio must be capable of complementing a synchronous fiber-optic network. This means that for the microwave radio to integrate with fiberoptic network elements, its design must address a number of parameters, including capacity and growth, network management, maintaining pace with SDH/SONET standards evolution, interface, and performance. Providing microwave radio with the optical interface will allow a microwave network to integrate with the fiber-optic network without the use of multiplexing equipment (unless drop and/or insert of the traffic is required). The same management tools are used for both media. Common expectations for fiber and microwave elements are signal rates and interfaces, overhead processing, service channels, operations systems, and transmission quality. SONET/SDH microwave radios can easily integrate into a new or existing fiber-optic network (see Figure 1.2). Digital microwave systems based on synchronous digital hierarchy (SONET/SDH) can meet the requirements for the high-capacity backbone transmission systems. SDH/SONET radios provide an economical solution when existing infrastructure (towers, shelters, and so on) can be reused, and when rights of way or adverse terrain make fiber deployment very costly or time consuming.

Transmission Network Fundamentals

SONET/SDH MW Radio

FO MUX

FO

MW Radio

13

SONET/SDH MW Radio

MW Radio

FO MUX

FO Terminal

FO

FO Terminal

FO

Figure 1.2 Hybrid microwave/fiber-optic ring

SONET/SDH radio technology is capable of delivering bandwidthefficient bandwidth of 8 bits/s/Hz. For example, the 512-state quadrature amplitude modulation (QAM) technique can pack two STM-1 streams into a single 40-MHz channel using a single carrier. By adding channels in an N+1 configuration, system capacities of up to 14 protected STM-1s can be achieved within one frequency band (for example, in the upper 6-GHz band, 8 bidirectional channels are available). By deploying a dual-band configuration (such as lower 4- and 5-GHz bands) system, capacities of STM-16 and greater are achievable. Although theoretically possible, this can be done only if the spectrum governing bodies allow more than one channel to be accessed by the same user. 1.2.3

ATM

Asynchronous transfer mode (ATM) is the complement of synchronous transfer mode (STM). STM is a circuit-switched networking mechanism whereby a connection is established between two termination points before data transfer commences and torn down when it is completed. In this way, the termination points allocate and reserve the connection bandwidth for the entire duration, even when not actually transmitting data. ATM is a transmission technology that uses fixed-size packets called cells. A cell is a 53-byte packet with 5 bytes of header/descriptor and 48 bytes of payload, or user traffic (voice, data, video, or their combination). Today, telecommunications companies are deploying fiber optics in cross-country and cross-oceanic links with gigabits-per-second speeds. They would like to carry, in an integrated way, both real-time traffic such as voice and high-resolution video, which can tolerate some loss but not delay, as well as non-real-time traffic such as computer data and file transfer, which may tolerate some delay, but not loss.

14

Chapter One

The obvious problem with carrying these different characteristics of traffic on the same medium in an integrated fashion is that the requirements of these traffic sources may be quite different. In other words, the data comes in bursts and must be transmitted at the peak rate of the burst, but the average arrival time between bursts may be quite large and randomly distributed. For these connections, it would be a considerable waste of bandwidth to reserve a bucket for them at their peak bandwidth rate for all times when, on the average, only 1 in 10 buckets may actually carry the data. Thus, using the STM mode of transfer becomes inefficient as the peak bandwidth of the link, peak transfer rate of the traffic, and overall burstyness of the traffic (expressed as a ratio of peak/average) all go up. Terms such as fast packet, cell, and bucket are used interchangeably in ATM literature. ATM networks are connection-oriented packetswitching networks. Future telecommunication networks, including wireless networks, must be able to offer today’s range of services as well as services with new features; e.g., variable bit rates. The requirements of modern networking involve handling multiple types of traffic (voice, video, and data), all with individual characteristics that make very different (and often opposed) demands on the telecommunication channel. The second requirement is reliability and flexibility in the communication links. The greatest problem is that transmissions occur at statistically random intervals with variable data rates. A way of solving this problem is to use a service that takes packets on the transport layer from a higher layer and fragments them in small packets of a fixed size. The delays produced by each packet are going to be short and probably fixed, so, if voice and video traffic can be assured priority handling, they can be mixed with data without diminishing any reception quality. The service that solves this problem is called the ATM adaptation layer (AAL). A new adaptation layer is required to provide the flexibility for network operators to control delay on voice services and to overcome the excessive bandwidth needed by using structured circuit emulation.9 The AAL2 was designed specifically for cost-effective voice transport. AAL2 is used in 3G wireless networks as a backhaul connection between radio base stations (RBSs) and base station controllers (BSCs). Over the last ten years, more and more transmission systems, especially those in wireless networks, have been using ATM over the microwave networks. These radio systems carrying packetized traffic (ATM or frame relay) have to be designed in a way that takes into account the behavior of this kind of traffic. Because ATM is primarily designed for an essentially error-free environment, in the wireless arena, the sources of errors and their consequences on ATM traffic and its Quality of Service (QoS) are being studied today.

Transmission Network Fundamentals

15

ATM traffic requires a very high-quality transmission medium with a good background error rate (also called Residual BER or RBER). Microwave radio and fiber optics are ideal from this perspective, as they both offer in the order of 10–13 background error rates. Microwave radio is subject to fading for a small period of time but, by proper design, this is limited to a specified time period (typically 99.99 percent availability or better). ATM is designed for low BER links, and radio links with a moderate BER can cause unacceptable high cell loss and misinsertion rates.10 By definition, a misinserted cell is a received cell that has no corresponding transmitted cell on the considered connection. Cell misinsertion on a particular connection is caused by defects on the physical layer affecting any cells that were not previously associated with this connection. Although IP-based networks are becoming more and more widely used, ATM still has its applications and will be around for quite some time (the same goes for TDM networks and circuits). 1.2.4

Ethernet Backhaul

1.2.4.1 About the OSI layers The Open System Interconnection Reference Model (OSI Reference Model or OSI Model) is a description of layered communications and computer network protocol design. Basically, it divides network architecture into seven layers which, from top to bottom, are the application, presentation, session, transport, network, data link, and physical layers. It is therefore often referred to as the OSI Seven Layer Model. Internetworking devices such as bridges, routers, and switches have traditionally been categorized by the OSI layer they operate at and the role they play in the topology of a network: n

Layer 2 Bridges and switches operate at Layer 2 (data link layer)

n

Layer 3 Routers operate at Layer 3 (network layer)

Bridges and switches extend network capabilities by forwarding traffic among LANs and LAN segments with high throughput. Layer 2 refers to the layer in the communications protocol that contains the physical address of a client or server station. It is also called the data link layer or MAC layer. Layer 2 contains the address that is inspected by a bridge, switch, or PC NIC. The Layer 2 address of every network device is unique, fixed in hardware by its manufacturer and usually never changed. Traditionally, products that were called switches operated by forwarding all traffic based on its Layer 2 addresses.

16

Chapter One

The spanning tree protocol, implemented in many Layer 2 switches, prevents forwarding loops in switched networks. Unfortunately, this is achieved by shutting down redundant connections and never using them. In contrast, routers are able to keep redundant connections active and make use of this built-in redundancy to increase network reliability and performance. With Layer 2 switching reaching the limits of its potential, the multilayer switch represents the next stage in the evolution of internetworking devices. Multilayer switching is simply the combination of traditional Layer 2 switching with Layer 3 protocol routing in a single product, usually through a fast hardware implementation. Routers perform route calculations based on Layer 3 addresses and provide multiprotocol support and WAN access, but typically at the cost of higher latency and much more complex administration requirements. Layer 3 refers to the layer in the communications protocol that contains the logical address of a client or server station. It is also called the network layer. Layer 3 contains the address (such as IP or IPX) that is inspected by a router that forwards the traffic through the network. The Layer 3 address of a network device is a software setting established by the user network administrator that can and does change from time to time; only devices that need to be addressed by Layer 3 protocols, such as IP, have Layer 3 addresses. Traditionally, routers operated solely on Layer 3 addresses. 1.2.4.2 Carrier Ethernet TDM networks are reliable, well developed tech-

nology with guaranteed and predictable service levels. Unfortunately, they are not suitable for the Ethernet transport and introduction of new, bandwidth-intensive services. Technology—and the cost of course—is driving the migration to IP-based networks, and the new IP services are critical to improving operators’ revenue; therefore, new generations of wireless networks will be IP-based. Network providers are upgrading their IP networks to support not only existing best-effort services but also real-time services and existing Layer 2 services. Best-effort services such as e-mail are able to withstand the significant delay, packet reordering and outages common on most IP networks. On the other hand, real-time services, such as voice-over-IP (VoIP), video, streaming media and interactive gaming, demand a higher level of network performance with low latency and high network availability. Essential services that currently reside on ATM and frame-relay switches cannot be transitioned to a network that is not stable and does not deliver the Quality of Service (QoS) servicelevel agreements require.

Transmission Network Fundamentals

17

As real-time applications continue to increase, network outages will become more visible, and service providers will have to react or lose customers to other carriers. For IP networks to support demanding real-time applications and converged legacy networks, carriers must overcome a number of obstacles; poor router reliability, lack of link protection, disruptive operations, slow convergence time, and multiservice support are just some of the challenges. Even though Ethernet/IP provides significantly cheaper bandwidth, in terms of cost per bit, than legacy ATM and TDM, the technologies so far have rarely been used for transport in mobile access networks, in part due to the lack of sufficient QoS and resilience to guarantee the required service level. Different QoS requirements for voice, data, and video services must be supported by a well-designed IP and Ethernet based network. Driven by the MEF (Metro Ethernet Forum11), a de facto standard “high quality” transport service labeled Carrier Grade Ethernet has been defined to meet these needs. The standard supports high QoS characteristics and a hard SLA (Service Level Agreement). With the increasing importance of Class of Service standards, Carrier Class Ethernet certification, and real-time applications, assuring QoS is a critical element in offering revenue-generating Ethernet services. This assurance comes from properly testing all of the differentiated services using multistream traffic generation and prioritization techniques that did not play a large role in traditional point-to-point services. Carrier Ethernet is a ubiquitous, standardized, carrier-class service defined by five attributes that distinguish it from the regular LANbased Ethernet. Carrier Ethernet attributes are scalability, standardized services, service management, quality of service, and reliability. In wireless networks, mobile backhaul Ethernet can be delivered over a variety of access technologies (see Figure 1.3). MEF has defined the requirements put on network reference points including the user-network interface (UNI) and network-to-network interfaces (NNI). The MEF architecture is based on Ethernet virtual connections (EVC), where an EVC is an association of two or more UNIs over one or more metro Ethernet networks (MEN) that transport Ethernet frames. Each EVC has a set of service attributes (service type, multiplexing support, bandwidth profiles, and performance assurance) that are used to define services in a flexible way and to standardize SLAs. 4G mobile networks require a single, all-IP, packet-based backhaul infrastructure, providing carriers with a significant cost advantage. However, the number of mobile devices and multitude of services, such as traditional voice, voice conferencing, image sharing, video,

18

Chapter One Collocated 2G and 3G Base Stations

Radio Access Network Link Aggregation

Mobile Core Network MSC BSC RNC

MSC TDM/ATM/IP Backbone

Traffic optimization

Carrier Ethernet

BTS DSL

Pico Femto

MSC Microwave Link

WiMAX Pseudowire Node B

Figure 1.3 Carrier Ethernet in mobile backhaul

and high-speed data, strains the infrastructure. Carrier Ethernet will deliver cost, scalability and flexibility of Ethernet networks, but with TDM Carrier Class reliability. New generations of Ethernet microwave radios equipped with adaptive modulation, link aggregation, and XPIC (Cross-polarization Interference Canceller) are already delivering high-speed GigE (Gigabit Ethernet) links. Ethernet microwave radios, adaptive modulation, link aggregation, and XPIC are discussed in more details in other chapters of this book. 1.3 Transmission Network Topology The main objectives of the transmission network are to connect all the points of interest, satisfy the capacity demands, and provide reliable service using different media (microwave, copper, fiber optics, or satellites). During the transmission network build-out, it is an imperative to establish a transmission network plan that will include all present traffic requirements as well as future expansion. As wireless carriers move to 4G mobile technology, huge demands are being placed on carrier backhaul infrastructure. The multiple, highbandwidth, quality-sensitive services that carriers have planned for 4G require an infrastructure that is packet-based, scalable, and resilient, as well as cost-effective to install, operate, and manage.

Transmission Network Fundamentals

19

Network operators worry about two things: how to start deploying the network in phases without spending capital before it is needed, and how to grow these small, initial segments once they see growth coming down the road. That is why scalability is a very important factor. The transmission network topologies can be divided into two groups: n

n

The flat network is a single entity, which means that it can be optimized to have high network utilization. A drawback is that the network topology is sensitive to changes in the traffic distribution. A change in the distribution changes the earlier, well-optimized network to being nonoptimized. The layered network facilitates the design of a network and its subsequent expansion, as the total network is modular. Modular design also makes the network and the traffic routing easy to understand, thus simplifying operation and maintenance (O&M), which will reduce the operator’s O&M costs in the future.

A layered network is divided into various network layers, which are connected via gateways. The layered/modular network is designed subnetwork by subnetwork; i.e., the total demand matrix is divided into demand matrixes for each subnetwork. The new, smaller matrixes are easier to handle and understand than the large ones. In the future, as the services offered to the end user become more and more flexible, the layered approach might be the most suitable topology—assuming that the initial cost is not considered. In wireless networks, the size of the network is assessed based on the number of cell sites and/or required backhaul capacity. Many successful mobile operators protect transmission by using automatic traffic rerouting, assuring additional reliability in normal situations, such as when microwave radio access links suffer cut-off as a result of poor weather conditions, possible fiber-optic cable cuts, or any other human error. With a flexible rerouting transmission system, backup capacity can pass via physically separate routes, given that the problem is not likely to interrupt both routes simultaneously. Rerouting can be arranged for all sites or only critical sites, such as base stations that are labeled as higher priority—for example, a hub site. Hub sites are those sites that collect traffic from more than one other site (typically three to four other sites) and carry that traffic toward the BCS (Base-station Controller) location or fiber-optic ring hub site.*

*

In wireless networks, BSC is the brain of the entire network.

20

Chapter One

For a larger transmission network, using a resilient ring configuration as a high-capacity backbone that carries traffic to the switch is recommended. The ring architecture is considered to be a reliable communication facility, as it provides automatic protection against the following: n

Site hardware (batteries, towers, antenna systems) failures

n

Radio, switch, or multiplexer (MUX) equipment failures

n

Propagation failures in the microwave network

n

Cable cuts in the fiber-optic network

Ring architecture also provides basic user features such as simple operation, fault location, and maintenance. Ring configuration automatically provides alternative routing of E1/T1 traffic and no loss of E1/T1 traffic due to a single failure. Each E1/T1 circuit must be dedicated completely around the ring, and reuse of the same E1/T1 in the opposite direction is not possible. For ultimate reliability, both directions can be 1+1 hardware protected. In legacy PDH networks, additional hardware with built-in intelligence to assess the T1/E1 quality and switch circuits is required. This hardware has to be added at every site, and it is useful for small networks. SONET/SDH have incorporated several protection/switching techniques from their inception. These include linear APS, pathswitched rings, line-switched rings, and virtual rings, providing the ability for a network to detect the problem (under 10 ms) and heal itself automatically in the case of failure, with the restoration time less than 50 ms. Self-healing schemes use fully duplicated transmission systems and capacity for alternate routing of today’s time division multiplexed (TDM) or synchronous transfer mode (STM) circuit facilities. The restoration capacity and the associated transmission systems are essentially unused except in the rare occasions of network failure. Two adjacent rings might be interconnected via shared node. Although expensive and relatively complex to implement, the dualhomed ring architecture is the choice for high-capacity digital service providers. This architecture uses a drop-and-continue feature that ensures that traffic is available to pass between adjacent rings at two separate nodes (i.e., since two rings share two nodes, we say that rings have a shared link). If an entire node is lost, the receiving ring equipment will select traffic from the other node. Although it looks expensive, actually network survivability has a great potential for cost reduction in the future. The new Recommendation ITU-T G.8032/Y.1344 (06/2008) defines the automatic protection switching (APS) protocol and protection switching

Transmission Network Fundamentals

21

mechanisms for Ethernet ring topologies. Ring protection switching occurs based on the detection of events on the transport entity of each ring link. The events are defined within equipment Recommendations ITU-T G.8021. Ethernet ring protection shall support up to 255 ring nodes and in the event of a single ring node or link failure, it will have protection switching time of less than 50 ms. 1.4 Transmission Network Performance In today’s networks, with converged voice and data, performance degradation may be as dangerous and costly as hardware failures. Degraded transmission networks can result in unacceptable signal transmission quality, loss of information, and dropped connections. High availability does not mean just preventing catastrophic failures; it also means preventing quality and performance degradation. High availability (and quality) of the transmission network is an end-to-end network goal. A network management system (NMS) can help identify critical resources, traffic patterns, and performance levels. Transmission network survivability is usually measured in terms of its long-term availability or average network uptime. Most operators expect their network to be continuously available (or at least with as little downtime as possible) to minimize potential loss of revenue. The survivable network has an infrastructure of transmission facilities and reliable network elements that are used to manage them. High network availability at the transport level may be achieved using millisecond restoration schemes provided by self-healing network configurations such as SONET/SDH rings or fast facility protection (FFP). Digital access cross-connects (DACS) in combination with SONET/SDH ring configurations will ensure network availability and survivability. An FFP network comprises two physically diverse routes with identical transmission systems (route diversity). Each route carries half of the working traffic and half of the restoration traffic. The restoration traffic on each route is the duplicate of the working traffic on the other route. If the media on these routes are different (for example, one is fiber optic and the other one is microwave), we call it media diversity. These highly reliable solutions do not come cheap and, in many cases, a compromise between the cost of the network and its deployment time and reliability has to be made. Regardless of the transmission network medium and topology, hardware redundancy is an option when designing the transmission network. Protection types usually employed are 1+1, where one card or module serves as a protection for another one, or N+1, where one card

22

Chapter One

or module protects N other units. The same fiber or frequency is used for working and protection equipment. Linear 1+1 protection switching is different from 1+1 hardware redundancy and means that identical payloads will be transmitted on the working and protect fibers, or working and protect frequencies in case of the microwave system. Linear N+1 protection switching assumes the existence of one protect fiber/frequency for N working fibers/frequencies. A rule of thumb is that, if all the hardware is protected with a 1+1 and/or N+1 configuration, fewer spare parts are needed (discussed in more details later in Chapter 5). In the case of hardware failure, protection will kick in, and the operator will have sufficient time (which may be days or weeks) to order replacement parts from the supplier. In addition, the ring configuration could provide protection against hardware failures as well, so additional hardware protection might not be required. This is something that transmission engineers have to decide, and the decision will be based on both technical and budgetary requirements. It is also important to remember the distinction between terms like performance, availability, quality, and so on. Although the network is operational (available), network performance can still be poor with increased levels of BER, for a number of reasons. In voice networks, that may mean reduced sound quality; in data networks, it can cause constant data retransmissions, incorrect information, and so forth. In other words, the quality of such a network is low and unsatisfactory. For the network operators, one of the key factors to the success is its ability to maintain a high standard of network performance, which can only be achieved by adopting the appropriate QoS metrics and measurement tools. Five nines, the jargon term that means a piece of equipment will function reliably 99.999 percent of the time (statistically, about five minutes of downtime per year), is used widely in the legacy TDM networks but may not be good enough in a world of always-on mobile devices and ever-increasing video consumption. Table 1.7 shows some of the commonly used percentage values for unavailability. The five nines requirement, correctly or incorrectly, has been applied to almost everything—from the telecommunications equipment to microwave paths, fiber-optic links, and sometimes even to the all-inclusive end-to-end link. When talking about availability it is a good idea to define what exactly these five nines include. Unfortunately, to make things even more complicated, five nines will mean different things to different people; so, when we bring up the five nines requirement, it is advisable to have it precisely defined from the get-go.

Transmission Network Fundamentals TABLE 1.7

Unavailable Time per Year

Availability (% of time) 99.9999 99.9995 99.9990 99.9950 99.9900 99.9500 99.9000 99.5000 99.0000

23

Unavailability (% of time) 0.0001 0.0005 0.0010 0.0050 0.0100 0.0500 0.1000 0.5000 1.0000

Unavailable Time per Year (min) 0.5256 2.628 5.256 26.28 52.56 262.8 525.6 2,628 5,256

In North America, it is quite common that, sometimes arbitrarily, network designers assign the same requirement of 99.999 percent of time availability to the microwave path (regardless of its length and number of paths in the system) and a guarantee of bit error rate (BER) better than 10−3 (or 10−6) during that period. In many cases, designers neglect an actual quality of the link due to increased BER and focus only on the availability; the fact that the microwave link (or any other network connection) is available does not mean that the link (and therefore the entire network) is working properly. From that prospective, ITU methods and models, developed and continuously revised over the last 20 years or so, will go a step further and try to incorporate all the possible factors (including equipment) that may affect the network availability, quality, and reliability. Some of the main causes of IP network downtime are router hardware and software failures. In contrast to traditional central-office equipment such as voice and ATM switches, IP routers were not designed to support carrier-grade 99.999 percent availability; typical large IP networks achieve only between 99.95 and 99.99 percent availability. This is much more downtime than the five nines availability benchmark of legacy data networks. New generations of routing platforms have achieved 99.999 percent availability by providing full hardware and software redundancy in a single router; in addition, hitless software upgrades are essential to the delivery of 99.999 percent availability because they eliminate router downtime associated with software upgrades. Studies have shown that link failures accounted for more than 30 percent of the outages in a large IP network. This is not particularly surprising, since IP networks traditionally have been built without locally protected links. Instead, these networks have relied on the ability

24

Chapter One

of routers to route the traffic around failed links, producing unacceptable disruption to real-time and converged services. 1.5 1.5.1

Network Synchronization Synchronization Terminology

Digital systems require all signals within a given transmission level to maintain a frequency relationship. If this relationship is not maintained, information will be lost, or transmission capacity will be underutilized. In addition, varying transmission times, inaccurate timing, and unstable network equipment can also cause bits traveling in a transmission path to arrive at a time that is in variance with their expected arrival. Solving timing-related problems is part of network synchronization. To date, two approaches have been dominant in achieving network synchronization: plesiochronous, used in PDH networks, and synchronous, used in SDH and SONET networks. Digital network connectivity depends on the availability of a reliable synchronization source to provide a timing reference to the network elements. Synchronization networks provide timing signals to all synchronization network elements at each node in a digital network. Buffer elements at important transmission interfaces absorb differences between the average local frequency and the actual short-term frequency of incoming signals, which may be affected by phase wander and jitter accumulated along the transmission paths.12 A slip in T1 is defined as a 1-frame (193 bits) shift in time difference between the two signals in question. This time difference is equal to 125 µs (microseconds), and these slips are not a major impairment to trunks carrying voice circuits. The lost frames and temporary loss of frame synchronization results in occasional pops and clicks being heard during a call in progress. However, with advancements in DS1 connectivity, these impairments tend to spread throughout the network. To minimize them, a hierarchical clock scheme was developed whose function was to produce a primary reference for distribution to switching centers so as to time the toll switches. In carrier telecommunications systems, stratum is used to describe the quality of a clock used for synchronization. The ANSI† Synchronization Interface Standard T1.101 defines profiles for clock accuracy at each

†

ANSI-American National Standards Institute.

Transmission Network Fundamentals

25

stratum level, as does ITU standard G.810, and Telcordia/Bellcore standards GR-253 and GR-1244. n

Stratum 1 Defined as a completely autonomous source of timing that has no other input other than perhaps a yearly calibration. The usual source of Stratum 1 timing is an atomic standard or reference oscillator. The minimum adjustable range and maximum drift is defined as a fractional frequency offset of 1 × 10–11 or less. At this minimum accuracy, a properly calibrated source will provide bit-stream timing that will not slip relative to an absolute or perfect standard more than once every 4 to 5 months. Atomic standards, such as cesium clocks, have far better performance. A Stratum 1 clock is an example of a primary reference source (PRS) as defined in ANSI/T1.101. Alternatively, a PRS source can be a clock system employing direct control from coordinated universal time (UTC) frequency and time services such as global positioning system (GPS) navigational systems. The GPS may be used to provide highaccuracy, low-cost timing of Stratum 1 quality.

n

n

n

Stratum 2 Tracks an input under normal operating conditions and holds to the last best estimate of the input reference frequency during impaired operating conditions. A Stratum 2 clock system provides a frame slip rate of approximately one slip in seven days when in the hold mode. Stratum 3 Defined as a clock system that tracks an input as in Stratum 2 but over a wider range. Sometimes Stratum 3 clock equipment is not adequate to time SONET network elements. Stratum 3E, which is defined in Bellcore documents, is a new standard created as a result of SONET equipment requirements. Stratum 4 Defined as a clock system that tracks an input as in Stratum 2 or 3 but has the wider adjustment and drift range. In addition, a Stratum 4 clock has no holdover capability and, in the absence of a reference, free runs within the adjustment range limits.

Any stratum clock will always control strata of lower-level clocks. Inadequate timing may produce problems in any digital network, so the objectives have to be set very early in the planning process. The master timing sources that are possible to use and recommended are as follows: n

n

PRS DS1 Timing is received from a collocated primary reference source such as a GPS or LORAN-C receiver, which is Stratum 1 level. Dedicated DS1 Timing is obtained by terminating a DS1 dedicated to synchronization.

26

n

n

n

Chapter One

Traffic-carrying DS1 Timing is extracted from a traffic-carrying DS1 (PDH or copper medium and not carried over from SDH/SONET) coming into a site. SDH Timing is extracted from the SDH line signal where there is no influence from the pointer movements phase steps. SONET DS1 Timing is obtained by deriving a nontraffic-carrying DS1 from SONET network elements, an OC-N multiplexer.

1.5.2

Synchronization in Wireless Networks

Telecommunications equipment (for example, cellular base stations) receive their timing signals from the worldwide GPS, an array of satellites that beam timing signals accurate to within 300 ns (nanoseconds) to receiving stations located around the globe. If the link fails, a backup system must be in place to maintain timing accuracy; if there is no backup, the cell goes down and communication is lost, which is an unacceptable condition. The GPS time standard is downlinked from the satellite as long as it is in range of the GPS receiver. However, there are intervals of time when a satellite is not in range (overhead), and no GPS time standard is available. This period without any GPS time update is called the GPS holdover time. One major problem encountered after designing a timing network is evaluating its performance, as any problems related to synchronization can be difficult to detect and even more difficult to troubleshoot. The digital microwave radios and high-level multiplexers are not considered in the synchronization plan, since they internally synchronize on a per-hop basis. Synchronous microwave radios on the market provide a transparent transmission media, which in SDH terms runs in a default regenerator section termination mode (RST). In this mode, the radio obtains synchronization from the aggregate 155 Mbps input into the radio, and it does not offer or require any additional synchronization options. Ethernet and IP are rapidly replacing traditional TDM circuit infrastructure in the communication networks. This creates timing islands in the network, potentially causing service impairments. Mobile services are especially vulnerable as they require precise synchronization at base stations. There are two mobile wireless network synchronization schemes; the first one uses the same technology as a wired network (from T1/E1 circuits). This standard is used to provide frequency-division duplex (FDD) radio-based mobile wireless network synchronization signals for ingress/egress of data and accurate radio frequency (WCDMA, for example).

Transmission Network Fundamentals

27

The second is synchronization of time-division duplex (TDD) radiobased mobile wireless networks, requiring frequency accuracy, phase alignment, and, in certain cases, time alignment between all base stations in the cell network. Examples of TDD radio systems are CDMA, cdma2000, Mobile WiMAX 802.16e, and Long-Term Evolution (LTE). The traditional wired (“circuit” oriented) synchronization signal delivery system cannot be used because there is no phase or time relationship between signal termination points on the clock distribution network.13 The migration toward a packet-based transport network poses the challenge of providing the level of synchronization requirements defined in the 3GPP, 3GPP2 (LTE), and IEEE 802.16e (WiMAX) specifications. The requirements for mobile wireless network synchronization depend on having an inter-base-station aligned timing reference. This is essential to guarantee transport channel alignment for handoff and guard band protection. 3GPP-specified FDD systems require frequency accuracy better than 50 parts per billion (ppb). The 3GPP TDD systems require an inter-basestation time alignment of 2.5 µs to IS-97’s 10 µs, in addition to the 50 ppb frequency accuracy. The IEEE 802.16e mobile WiMAX requirements are 20 ppb of frequency accuracy and 1 µs of phase alignment. Ensuring the fulfillment of these requirements reduces the call drop rate and improves the quality of services by decreasing packet loss. IEEE 1588v2 (2008), also known as Precision Time Protocol (PTP), is the new standard for providing precise timing and synchronization over packet-based, Ethernet networks. Accuracy within the nanosecond range can be achieved with this protocol when using hardware generated timestamps, although the protocol itself has no quality requirement. The transmission of the clock information over a packet network eliminates the need for alternative mechanisms, such as GPS or prohibitively expensive oscillators placed at the receiving nodes and thus providing significant cost savings in network equipment as well as in ongoing installation and maintenance. This synchronization solution transmits dedicated timing packets, which flow along the same paths with the data packets, reducing the cost of synchronization and simplifying implementation. Although IEEE 1588v2 systems add a small amount of additional traffic to the network load, they have several advantages; for example, they work in the data path, the most redundant and resilient part of the network, resulting in reliable operation. In addition, multiple transmission paths reduce redundant clock system costs. They also use a single synchronization session for all base-station traffic. It is important to note that IEEE 1588v2 protocol cannot improve the performance of synchronization or calibration through existing

28

Chapter One

networks unless every network node is replaced with a node that supports an IEEE 1588v2 boundary clock or transparent clock. With IEEE 1588v2 techniques implemented in every node, the network will experience less packet delay variation, which in turn, means that less stable oscillators may be used in network nodes. The IEEE 1588v2 master function will typically be located at switching offices, while the IEEE 1588v2 slave function will be integrated into the base stations and receive the timing packets. The slave function consists of a clock recovery algorithm that works in tandem with the base station’s high-quality oscillator. 1.6

Network Delays

Signal propagation delay or latency describes the delay of a transmission from the time it enters the network until the time it leaves; low latency means short delays, whereas high latency means long delays. Low latency is essential for real-time transmissions. These include live voice conversations (but not voicemail messages, which are time insensitive) and live two-way video (but not entertainment video clips, which also are time insensitive). Latency is a phenomenon not only of mobile networks, but also an outcome of all the networks, terminals, and devices through which transmissions may pass, plus the bottlenecks (and, therefore, delays) they may encounter. Delay can cause protocol time-outs, retransmissions, and disruptions in data circuits and can inhibit voice transmissions. All types of transmission equipment, such as multiplexers, packet assemblers, microwave radios, and digital access cross-connects (DACS), add small amounts of delay as a result of their internal buffering, and satellite links add significant delay to a signal. ATM switches introduce up to a 2 ms delay, and there is a concern about the number of times ATM compression can be used in a tandem link before the overall delay objective is exceeded. Wireless CDMA networks are very sensitive to delays, and vendors recommend that backhaul delay between the cell site and the BSC be below certain limits. For example, the end-to-end delay allowable from the user equipment to a UMTS (WCDMA) radio network controller (RNC) is 7 ms. Values of around 12 ms are used in the cdmaOne and CDMA2000 networks. The emerging set of voice and video services being delivered by 4G networks typically has an end-to-end latency budget of about 10 ms. If 5 ms of this latency budget is allocated to the fiber network, there is 5 ms of delay for the wireless backhaul network. About half of this is allocated to the Ethernet switch layer, leaving about 2.5 ms for the wireless links. In a ten-hop microwave system, this leaves a maximum latency of 0.25 ms per link, requiring a very low latency microwave Ethernet system.

Transmission Network Fundamentals TABLE 1.8

29

Typical Delay Values for the Microwave Hop

Capacity (Mbps)

Delay (µsec)

2×2 4×2 8×2 16 × 2 300 Mbps (Ethernet)

100 75 50 40 100

In addition, utility companies using microwave systems to carry their SCADA (Supervisory Control and Data Acquisition) signals require a very low delay. That is one reason why electrical utility companies still keep their old-fashioned analog microwave systems: they introduce smaller delays than digital microwave systems. The main processing delay in a microwave radio is forward error correction (FEC) buffering, which decreases with an increase in the capacity of the radio link. Total latency of the microwave link is a combination of the radio, free-space, and multiplexer delays. Some typical delays in microwave radio hardware for a point-to-point connection (entire hop) without considering the free-space transit time shown in Table 1.8. Microwave radio manufacturers should be able to supply these numbers for their equipment. 1.7

Security and Encryption

Digital information is often the target of computer hackers, international spies, and criminals. In order to protect the information, in 1977 the National Security Agency (NSA) and the National Bureau of Standards (NBS) adopted the Data Encryption Standard (DES) to protect sensitive, unclassified, nonmilitary digital information from unauthorized access. Encryption is the intentional scrambling or masking of digital data to protect it from compromise. Modern microwave radios often come with some kind of built-in security mechanism, like traffic encryption, for example. It is important to keep in mind to avoid proprietary security and encryption methods. 1.7.1

Data Encryption Standard (DES)

The Data Encryption Standard (DES) algorithm is the most widely used encryption algorithm in the world. For many years, and among many people, “secret code-making” and DES have been synonymous. DES works on bits, or binary numbers—the 0s and 1s common to digital computers. Each group of four bits makes up a hexadecimal, or base 16, number. Binary 0001 is equal to the hexadecimal number 1, binary 1000

30

Chapter One

is equal to the hexadecimal number 8, 1001 is equal to the hexadecimal number 9, 1010 is equal to the hexadecimal number A, and 1111 is equal to the hexadecimal number F. DES works by encrypting groups of 64 message bits, which is the same as 16 hexadecimal numbers. To do the encryption, DES uses “keys,” which are also apparently 16 hexadecimal numbers long or apparently 64 bits long. However, every 8th key bit is ignored in the DES algorithm, so that the effective key size is 56 bits. In any case, 64 bits (16 hexadecimal digits) is the round number upon which DES is organized. In cryptography, Triple-DES is a block cipher formed from the DES cipher by using it three times. Given a plaintext message, the first key is used to DES-encrypt the message. The second key is used to DESdecrypt the encrypted message and, since the second key is not the right key, this decryption just scrambles the data further. The twicescrambled message is then encrypted again with the third key to yield the final ciphertext. In general, Triple-DES with three different keys has a key length of 168 bits: three 56-bit DES keys (with parity bits 3TDES has the total storage length of 192 bits), but due to the meet-inthe-middle attack the effective security, it provides only 112 bits. Triple-DES is slowly disappearing from use, largely replaced by its natural successor, the Advanced Encryption Standard (AES). One largescale exception is within the electronic payments industry, which still uses Triple-DES extensively and continues to develop and promulgate standards based upon it, guaranteeing to keep Triple-DES an active cryptographic standard in the future. By design DES, and therefore Triple-DES, suffer from slow performance in software; on modern processors, AES tends to be around six times faster. Triple-DES is better suited to hardware implementations, and indeed where it is still used it tends to be with a hardware implementation (e.g., VPN appliances and some cellular and data networks), but even there AES outperforms it. Finally, AES offers markedly higher security margins, a larger block size, and potentially longer keys. 1.7.2

Advanced Encryption Standard (AES)

Advanced Encryption Standard (AES) is a symmetric key encryption technique that is slowly replacing the commonly used Data Encryption Standard (DES). It was the result of a worldwide call for submissions of encryption algorithms issued by the U.S. Government's National Institute of Standards and Technology (NIST) in 1997 and completed in 200014. The winning algorithm, Rijndael, was developed by two Belgian cryptologists, Vincent Rijmen and Joan Daemen. AES provides strong encryption and has been selected by NIST as a Federal Information Processing Standard in 2001, and in 2003 the U.S.

Transmission Network Fundamentals

31

Government (NSA) announced that AES is secure enough to protect classified information up to the Top Secret level, which is the highest security level and defined as information that would cause “exceptionally grave damage” to national security if disclosed to the public. The AES algorithm uses one of three cipher key strengths: a 128-, 192-, or 256-bit encryption key (password). Each encryption key size causes the algorithm to behave slightly differently, so the increasing key sizes not only offer a larger number of bits with which you can scramble the data, but also increase the complexity of the cipher algorithm. AES is fast in both software and hardware, is relatively easy to implement, and requires little memory; as a new encryption standard, it is currently being deployed on a large scale. 1.8

References

1. Lehpamer, H., Transmission Systems Design Handbook for Wireless Networks, Norwood, MA: Artech House, 2002. 2. Larus Corporation, Transmission Engineering Tutorials, 1996. 3. Flanagan W. A., The Guide to T1 Networking, 4th ed., Telecom Library Inc., 1990. 4. ITU-T Recommendation G.702, Digital Hierarchy Bit Rates. 5. ITU-T G.803, “Architecture of transport networks based on the synchronous digital hierarchy (SDH),” 03/2000. 6. Goralski, W., SONET, 2nd ed., New York: McGraw-Hill, 2000. 7. Zhou, D., “Survivability in Optical Networks,” IEEE Network, Nov./Dec. 2000. 8. Lewin, B., “SONET Equipment Availability Requirements,” IEEE, 1989. 9. Malis, A. G., “Reconstructing Transmission Networks Using ATM and DWDM,” IEEE Communications, Vol. 37, No. 6, June 1999. 10. Bates, J., Optimizing Voice in ATM/IP Mobile Networks, New York: McGraw-Hill, 2002. 11. http://metroethernetforum.org/, accessed 07/13/2009. 12. Larus Corporation, Digital Network Timing and Synchronization, 1997. 13. Diamond, P., Packet Synchronization in Cellular Backhaul Networks, Semtech White paper, October 2008. 14. Federal Information Processing Standards, Publication 197, Announcing the Advanced Encryption Standard (AES), November 26, 2001.

This page intentionally left blank

Chapter

2

Basics of Microwave Communications

2.1 2.1.1

Radio Fundamentals Brief History of Radio

2.1.1.1 Maxwell’s Equations James Clerk Maxwell (1831–1879) was a Scottish physicist and mathematician whose major discovery of the ether described the vast sea of space that made possible the transmission of light, heat, and radio waves. Maxwell’s discovery of the ether (or the metaphor of it) led to many advances in electronic communications. His extension of the electromagnetic theory of light led directly to Heinrich Hertz’s discovery of radio waves and to the related advances in science and technology of today. Maxwell’s mathematical equations, expressing the behavior of electric and magnetic fields and their interrelated nature, were valid, even though his theory of the ether was not. His calculations were scientific observations resulting in his conclusion that the speed of propagation of an electromagnetic field is approximately that of the speed of light (3 × 108 m/s). Maxwell’s proposal that the phenomenon of light is therefore an electromagnetic phenomenon seemed to fit what he and other scientists could observe about the world around them. Maxwell concluded that visible light forms only a small part of the entire spectrum of possible electromagnetic radiation (EMR). Maxwell’s work of 1873, The Complete Laws of Electrodynamics, defines the relationship between the electric field quantities and the magnetic field quantities, and although a detailed explanation of these 33

34

Chapter Two

laws is beyond the scope of this book, they deserve to be mentioned here at least briefly. James Clerk Maxwell was the first to correctly assemble the complete laws of electrodynamics in this classic text. Modern electromagnetism theory is based on the four fundamental equations known as Maxwell’s equations. Before Maxwell, the laws of electrodynamics, including Gauss’s Law, Ampere’s Law of Magnetostatics, and Faraday’s Law, were laws of electrostatics and did not predict waves. These laws correctly described what is known as the near field (that is, the electrostatic field of an electric charge and the magnetostatic field of a current loop). These laws described the observable impact of electric charges and magnetic fields close to the source but failed to describe the distant impact of these forces. In the static case, when all electric charges are permanently fixed or if they all move at a steady state, the electric field and the magnetic field are not interconnected. This allows us to study electricity and magnetism as two distinct and separate phenomena. Up until Maxwell challenged conventional wisdom, the separation of electricity and magnetism was the accepted state of the world. He corrected Ampere’s Law of Magnetostatics to become Ampere’s Law, so that consistency with the Law of Conservation of Charge occurred. Maxwell added a term indicating that vortices of magnetic fields can be displacement current density (time-varying electric flux density) as well as conduction current density. The resulting corrected equations define the complete laws of electrodynamics and predict electromagnetic waves. Heinrich Rudolf Hertz confirmed experimentally that these waves exist. Maxwell showed that any conductor (e.g., antenna) supplied with an alternating current produces a varying magnetic field (H-field), which in turn produces electric field lines (E-field) in space. This is termed the near field. In the near field both the E- and H-fields are relatively static with no propagation. They only vary in strength as the current varies, with the magnetic flux of the H-field coming out from the antenna and going back in, and the E-field emanating outward. Maxwell also proved that beyond this quasi-static near field, both the E-fields and H-fields at a certain distance detached themselves from the conductor and propagated into free space as a combined wave, moving at the speed of light with a constant ratio of E/H = 120π or 377Ω. The point in which this happens is called the far field. Maxwell’s equations beautifully and correctly describe both the energy storage field and the energy propagation field. It was the physicist Ludwig Boltzmann who said, “There is nothing more practical than a good theory,” and that is absolutely true in the case of Maxwell’s equations; although highly theoretical and mathematical in their nature, they provided an unprecedented contribution to our

Basics of Microwave Communications

35

understanding of the world around us as well as a very practical side in the form of the development of radio communications. The latest developments of quantum physics have shown that Maxwell’s equations are an approximation, albeit a very good one. For most practical applications Maxwell’s equations will suffice. Although widely accepted as an inventor of radio, Guglielmo Marconi, an Italian engineer working in England, based his work on patents and inventions of Nikola Tesla. It is Tesla’s original concept demonstrated in his famous lecture at the Franklin Institute in Philadelphia in 1893 that Marconi used for his radio transmission. Tesla was an inventor of (among other things) alternate current, which is used today in every household. Marconi received, jointly with the German physicist Karl Ferdinand Braun, the 1909 Nobel Prize in physics for their “contributions to the development of wireless telegraphy.” Tesla refused to share a Nobel Prize in Physics for the invention of radio transmission with Marconi. Tesla’s patents from 1900 were reversed in favor of Marconi in 1904 after large private investments (including some by Edison) were made towards Marconi’s company. In 1943, the U.S. Supreme Court upheld Tesla’s radio patent from 1900 and, as it stands today, Nikola Tesla is an official inventor of radio. Although he is often nearly forgotten, Tesla holds over 700 patents, and those are just the ones he remembered to patent.

2.1.1.2 Tesla vs. Marconi

2.1.2

Basic Terminology

Radio waves and microwaves are forms of electromagnetic energy we can collectively describe with the term radio frequency or RF. RF emissions and associated phenomena can be discussed in terms of energy, radiation, or fields. We can define radiation as the propagation of energy through space in the form of waves or particles. Electromagnetic radiation can best be described as waves of electric and magnetic energy moving together (i.e., radiating) through space, as illustrated in Figure 2.1. The magnetic field is perpendicular to the electric field, → and their cross-product→ points towards the direction of propagation (P = E × H.) The vector P in the direction of propagation, and measured in watts per square meter, is the propagation vector or Poynting vector. The existence of propagating electromagnetic waves can be predicted as a direct consequence of Maxwell’s famous equations of 1865. These equations specify the relationships between the variations of the vector electric field E and the vector magnetic field H in time and space within a medium. The E-field strength is measured in volts per meter and is

36

Chapter Two

λ = c/f

Electric field Magnetic field

Direction of propagation

Figure 2.1 Electromagnetic wave

generated by either a time-varying magnetic field or a free charge. The H-field is measured in amperes per meter and is generated by either a time-varying electric field or a current. Maxwell’s first two equations contain constants of proportionality that dictate the strengths of the fields. These are the permeability of the medium m in henrys per meter and the permittivity of the medium e in farads per meter. They are normally expressed relative to the values in free space:

µ = µ0 µ r ε = ε 0ε r

(2.1)

where µ0 = 4π × 10 −7 H/m and ε 0 = 8.854 × 10 −12 F/m are the values in free space, and mr and er are the relative values (i.e., er = mr = 1 in free space and greater elsewhere). Free space strictly indicates a vacuum, but the same values can be used as good approximations for dry air at typical temperatures and pressures. The waves are generated by the movement of electrical charges such as in a conductive metal object or antenna. For example, the alternating movement of charge (i.e., the current) in an antenna used by a radio or television broadcast station or in a cellular base station antenna generates electromagnetic waves. These waves that radiate away from the transmit antenna are then intercepted by a receive antenna, such as a rooftop TV antenna, car radio antenna, or an antenna integrated into a hand-held device such as a cellular phone. The term electromagnetic field is used to indicate the presence of electromagnetic energy at a given location. The RF field can be described in terms of the electric and/or magnetic field strength at that location. Like any wave-related phenomenon, electromagnetic energy can be characterized by a wavelength and a frequency.

Basics of Microwave Communications

37

Frequency (f) is defined as a number of cycles, or periods, per unit of time and is measured in hertz (Hz). The wavelength (l) of a sinusoidal wave is the spatial period of the wave—the distance over which the wave’s shape repeats. In other words, the wavelength l is the distance by which the phase of the sinusoidal wave changes by 2p radians (by definition: kl = 2p, w = kc, and f = w/2p). Electromagnetic waves travel through free space at the speed of light, and the wavelength and frequency of an electromagnetic wave are inversely related by a simple mathematical formula connecting wavelength, speed of light (c), and frequency (f):

λ=

2π 2π c = k ω

c λ= f

(2.2)

The wavelength (l) in centimeters (1 in = 2.54 cm) for a microwave frequency can be simplified as follows:

λ=

30 [cm] f

(2.3)

where f = frequency in gigahertz (GHz). Or, for the wavelength in inches:

λ=

0.9836 [in] f

(2.4)

You can see that the high-frequency electromagnetic waves have short wavelengths, and low-frequency waves have long wavelengths. Variation, or modulation, of the properties of the wave (amplitude, frequency, or phase) then allows information to be carried in the wave between its source (transmitter) and destination (receiver), which is the goal of wireless communications. 2.1.3

Spectrum Considerations

The electromagnetic spectrum includes all the various forms of electromagnetic energy from extremely low-frequency (ELF) energy, with very long wavelengths, to X-rays and gamma rays, which have very high frequencies and correspondingly short wavelengths. In between these extremes are radio waves, microwaves, infrared radiation, visible light, and ultraviolet radiation, in that order.

38

Chapter Two

The RF part of the electromagnetic spectrum is generally defined as that part of the spectrum where electromagnetic waves have frequencies in the range of about 3 kHz to 300 GHz. In the United States, the lowest frequencies currently available for broadband wireless transmissions reside between the 700 MHz and 800 MHz spectrum formerly assigned to television, while point-to-point microwave systems start at about 2 GHz. Further spectrum is available in the United States between 902 MHz and 928 MHz, at 2.3 GHz, at 2.4 GHz, from 2.5–2.7 GHz, and in several bands from 5–6 GHz. Bands located at 2.4 GHz and at 5.8 GHz are widely available (and license-exempt) across the globe. Throughout most of the world, though not in the United States, a band centered at 3.5 GHz is also available for public access data networks and is fairly widely used. Early in 2005, the Federal Communications Commission (FCC) approved a new unlicensed spectrum for broadband data services located between 3.650–3.700 GHz. Transmissions occurring from 3 GHz to approximately 10 GHz are relatively limited in throughput, do not readily conduce to high degrees of frequency reuse, and, perhaps most importantly, share a vulnerability to what is known as multipath distortion. In spite of the vulnerability of lower microwave transmissions to physical obstructions, spectrum in this region, especially below 4 GHz, is in high demand, offering users a combination of high throughput, fairly long distances, and some ability to pass through walls. This spectrum, whether licensed or license-exempt, is the overwhelming choice of broadband network operators around the world. Frequencies between 10–40 GHz are widely used by point-to-point microwave communications systems worldwide, especially after the invention of split-mount microwave radios. Currently in the United States, two millimeter-wave bands are in commercial use for high-speed data transmissions, including millimeter-wave point-to-point systems: a license-exempt band at 59–64 GHz and a licensed band extending discontinuously from 71 GHz to 95 GHz (known as the E band). The FCC is considering the allocation of more bands to be located in the region above 90 GHz. In 2003, the FCC authorized a 50-MHz radio frequency spectrum from 4,940 to 4,990 MHz for noncommercial use by state and local governments and public safety organizations that are dedicated to the protection of life, health, or property. By specifying licensed use of the band only for authorized government and nongovernment public safety groups (police, fire, search and rescue teams, EMS squads, private ambulance companies, municipal utilities, etc.), the FCC ensured a space in the airways free from interference by corporate or general public communications.

Basics of Microwave Communications

39

There are also some quiet zones in which 4.9 GHz operation (as with many other frequency bands) is not allowed—typically border areas and some military sites. For specific locations, consult the FCC Rules Part 2, Section 106, under Footnote US311 for Radio Astronomy Quiet Zones. 2.2 Structure and Characteristics of the Earth’s Atmosphere The Earth’s atmosphere is a collection of many gases along with suspended particles of liquid and solids. Excluding variable components such as water vapor, ozone, sulfur dioxide, and dust, the gases of nitrogen and oxygen occupy about 99 percent of the volume, with argon and carbon dioxide being the next two most abundant gases. From the Earth’s surface to an altitude of approximately 80 km, mechanical mixing of the atmosphere by heat-driven air currents evenly distributes the components of the atmosphere. At about 80 km (50 miles), the mixing decreases to the point at which the gases tend to stratify in accordance with their weights. The lower, well-mixed portion of the atmosphere is called the homosphere, and the higher, stratified portion is called the heterosphere. The bottom portion of the homosphere is called the troposphere. The troposphere extends from the Earth’s surface to an altitude of 8 to 10 km at polar latitudes, 10 to 12 km at middle latitudes, and up to 18 km at the equator. It is characterized by a temperature decrease with height. The point at which the temperature ceases to decrease with height is known as the tropopause. The average vertical temperature gradient of the troposphere varies between 6 and 7°C/km. The concentrations of gas components of the troposphere vary little with height, except for water vapor. The water vapor content of the troposphere comes from evaporation of water from oceans, lakes, rivers, and other water reservoirs. Differential heating of land and ocean surfaces produces vertical and horizontal wind circulation that distributes the water vapor throughout the troposphere. The water vapor content of the troposphere rapidly decreases with height. At an altitude of 1.5 km, the water vapor content is approximately half of the surface content; at the tropopause, the content is only a few thousandths of what it is at the surface. In 1925, the International Commission for Aeronavigation defined the international standard atmosphere (ISA). This is a hypothetical atmosphere having an arbitrarily selected set of pressure and temperature characteristics that reflect an average condition of the real atmosphere. It consists of tables of values at various altitudes, plus some formulas by which those values were derived.

40

Chapter Two

Today, the International Organization for Standardization (ISO) publishes the ISA as an international standard, ISO 2533:1975. Other standards organizations, such as the International Civil Aviation Organization (ICAO) and the United States government, publish extensions or subsets of the same atmospheric model under their own standards-making authorities. 2.3 2.3.1

Radio Propagation Microwave and Millimeter Waves

Microwave transmission is a very attractive transmission alternative for applications ranging from the coverage of the rural, sparsely populated areas of developing countries that have ineffective or minimal infrastructures to the well-developed industrial countries that require rapid expansion of their telecommunications networks. Most of the commercially used terrestrial microwave point-to-point (also called radio-relay) systems use frequencies from approximately 2 to 60 GHz (and lately up to 90 GHz) with maximum hop lengths of around 200 km (125 mi). According to the IEEE, electromagnetic waves between 30 and 300 GHz are called millimeter waves (MMW) instead of microwaves (MW) because the wavelengths for these frequencies are about 1 to 10 mm. Millimeter-wave propagation has its own peculiarities, but the spectrum from 30 to 300 GHz is of increasing interest to service providers and systems designers because of the wide bandwidths available for carrying communications at this frequency range. Such wide bandwidths are valuable in supporting applications such as high-speed data transmission and video distribution. Planning for millimeter-wave spectrum use must take into account the propagation characteristics of radio signals at this frequency range. While signals in lower frequency bands can propagate for many miles and penetrate more easily through buildings, millimeter-wave signals can travel only a few miles or less and do not penetrate solid materials very well. However, these characteristics of millimeter-wave propagation are not necessarily disadvantageous. Millimeter waves can permit more densely packed communications links, thus providing very efficient spectrum utilization, and they can increase security of communication transmissions. The radio frequency propagation mechanisms for microwave and millimeter-wave frequencies include diffraction, refraction, reflection, and scattering (see Figure 2.2). 2.3.2

Line-of-Sight Considerations

Microwave point-to-point communications operate in a propagation mechanism called visibility, so named for its similarity to light propagation.

Basics of Microwave Communications

Terrestrial microwave communications

Satellite

41

Ionosphere

Sky wave (HF only)

Transmitting antenna

Tropopause Troposphere

Refracted wave

Receiving antenna

Direct wave Reflected wave Ground wave (LF/MF only)

Figure 2.2 Radio wave propagation

Microwave radio communications require a clear path between parabolic antennas, commonly known as a line-of-sight (LOS) condition. LOS exists when there is a direct path between two separate points and no obstructions (e.g., buildings, trees, hills, or mountains) between them. Partially obstructed paths can also be examined by including the grazing or diffraction loss from the obstruction in the path calculations. Unobstructed paths are always preferred, and the outage times and percent reliability computed for obstructed paths may not be reliable. There is a critical difference between optical LOS (also known as visual LOS) and radio LOS (or radiovisibility). Visual LOS considers only optical visibility (as seen by the human eye or aided by binoculars) between the ends of the path (see Figure 2.3). Radio LOS takes

on e resnel z First F ht e-of-sig tical lin

Op

Figure 2.3 Optical and radio line of sight

LOS clearance

42

Chapter Two

into account the concept of Fresnel ellipsoids and their clearance criteria. Under the normal atmospheric conditions, the radio horizon is around 30 percent beyond the optical horizon. The early nineteenth century French physicist, Augustin Fresnel, made an important observation about the behavior of light. Fresnel noted that a ray of light passing near a solid object is subject to diffraction, or bending. This diffraction causes the intensity of the original light beam to increase or decrease, depending on how near the object is to the beam. This characteristic of electromagnetic radiation is known as the Fresnel effect, and light and radio waves are subject to the same laws of physics. If an object such as a mountain ridge or building is close to the radio signal path, it can affect the quality and strength of the signal. Diffraction of the radio waves by such objects can affect the strength of the received signal. This happens even though the obstacle does not directly obscure the direct visual path. This area, known as the Fresnel zone, must be clear of all obstructions. It is possible, for example, to have a microwave path with optical LOS but without radio LOS (according to a certain adopted clearance criterion for that particular path). Radio LOS is more stringent than optical LOS, since radiovisibility is always considered using the concept of the first Fresnel zone radius along the path. Sometimes, the extra clearance is used when we want to include possible future growth of buildings or trees near the obstacle. Clearance criteria must be followed along the entire microwave path. In the vicinities of antennas, these far-field criteria are substituted by special near-field clearance criteria. An important objective in planning terrestrial microwave links is to design the radio path in such a way that losses of visibility are extremely rare events. This involves an accurate knowledge of the terrain profile between the terminals and changes in propagation caused by meteorological variations. Sufficient clearance should be guaranteed for the lowest ray to be expected over the path. This can be attained by proper choice of the antenna heights which, however, should not be greater than actually needed—both for economic reasons and because this could increase the risk of fading and signal distortions due to reflections. 2.3.3

Earth Radius and k-Factor

2.3.3.1 Index of Refraction Refraction is a physical phenomenon observed in any medium that has a varying refractive index; it produces the effect of bending a light ray or microwave beam. Refraction in the atmosphere

Basics of Microwave Communications

43

is described by its index of refraction, which is dependent on the humidity, temperature, and pressure of the atmosphere, all of which are a function of height.1 In free space (ideal medium, a vacuum, for example), an electromagnetic wave will travel in a straight line, because the index of refraction is the same everywhere. Within the Earth’s atmosphere, however, the velocity of the wave is less than in free space, and the index of refraction normally decreases with increasing altitude. Therefore, the propagating wave will be bent upward or downward from a straight line. Since the barometric pressure and water vapor content of the atmosphere decrease rapidly with height, while the temperature decreases slowly with height, the index of refraction (and therefore refractivity) normally decreases with increasing altitude. As a tool in examining refractive gradients and their effect on propagation, a modified refractivity, defined as M = N + 0.157h (for h = altitude in meters)

(2.5)

M = N + 0.048h (for h = altitude in feet)

(2.6)

and

is often used in place of the refractivity N. The refractivity distribution within the atmosphere is a nearly exponential function of height (Bean and Dutton, 1968). However, the exponential decrease of N with a height close to the Earth’s surface (within 1 km) is sufficiently regular to allow an approximation of the exponential function by a linear function, a linear function that is assumed by the effective Earth’s radius model. This linear function is known as a standard gradient and is characterized by a decrease of 39 N-units per kilometer or an increase of 118 M-units per kilometer. A standard gradient will cause traveling electromagnetic waves to bend downward from a straight line. Gradients that cause effects similar to a standard gradient but vary between 0 and −79 N-units per km or between 79 and 157 M-units per km are known as normal gradients. N-unit is a dimensionless unit in terms of which refractivity is expressed. Table 2.1 shows normal and anomalous conditions of propagation, discussed in more detail later in this chapter. 2.3.3.2 Effective Earth’s Radius Variations in atmospheric refractive conditions cause changes in the effective Earth’s radius or k-factor from its median value of approximately 4/3 for a standard atmosphere. When the atmosphere is sufficiently subrefractive (large positive values of the gradient of refractive index, low k-factor values), the ray paths will

44

Chapter Two

TABLE 2.1

Refractive Gradients and Conditions

Trapping Superrefractive Normal Subrefractive

N-gradient

M-gradient

< −157 N/km < −48 N/kft −157 to −79 N/km −48 to −24 N/kft −79 to 0 N/km −24 to 0 N/kft > 0 N/km > 0 N/kft

< 0 M/km < 0 M/kft 0 to 79 M/km 0 to 24 M/kft 79 to 157 M/km 24 to 48 N/kft > 157 N/km > 48 N/kft

be bent in such a way that the Earth appears to obstruct the direct path between transmitter and receiver, giving rise to the type of fading called diffraction fading. This fading is the factor that determines the antenna heights. The Earth-radius factor, k, for a certain area can be calculated based on refractivity gradient (G) found from the local charts and using the following formula: k=

157 157 + G

(2.7)

Resulting values of G and k for different propagation conditions are shown in Table 2.2. A standard refractivity gradient of −39 N-units/km corresponds approximately to the median value of the gradient in the first kilometer of altitude in temperate regions (k = 4/3). Unfortunately, the refraction varies considerably from day to day and from one place to another. It is particularly variable over water; due to the high heat capacity of water, the air is nearly always at a different temperature from that of the water, so there is a thermal boundary layer, in which the temperature gradient is far from uniform.

TABLE 2.2

G and k for Different Propagation Conditions

Refractivity Gradient G (N-unit/km)

k

Propagation Conditions

79 0 −39 −79 −157 <−157

0.67 (2/3) 1 1.33 (4/3) 2 ∞ Negative

Subrefraction Normal Normal Normal Superrefraction Trapping (ducting)

Basics of Microwave Communications

45

These temperature contrasts are particularly noticeable near the shore, where the large diurnal temperature swings over the land can produce really large thermal effects over the water, if there is an offshore breeze.2 Ducts exist whenever the vertical refractivity gradient at a given height and location is less than −157 N/km. The existence of ducts is important because they can give rise to anomalous radio wave propagation, particularly on terrestrial or very low angle Earth-space links3. Ducts provide a mechanism for radio signals of sufficiently high frequencies to propagate far beyond their normal line-of-sight range, giving rise to potential interference with other services (for more details, see Recommendation ITU-R P.452). On short paths this ducting phenomenon is not a problem; however, on paths greater than 20 miles, it can cause antenna decoupling and thus becoming a serious factor for consideration. Tropospheric Ducting Forecasts can be found at: http://www.dxinfocentre.com/tropo_eur.html (accessed Oct 13, 2009). Understanding microwave antenna patterns, how to manipulate them, and the effect of Fresnel zones and k-factor issues is a must if one is to successfully engineer microwave links. 3.2.2.3 The Importance of k-Factor in Path Profiles Before computers, to facilitate path profiles, radio transmission engineers introduced the Earth-radius factor k to compensate for the refraction in the atmosphere. Applying appropriate k-values to the true-Earth radius, an equivalent-Earth radius is geometrically obtained and, consequently, straight rays can be drawn (see Figure 2.4). The image shows an equivalent Earth with different ray beam curvatures for different values of the Earth-radius factor. Using the effective Earth’s radius model, ray paths between transmitters and receivers near the Earth’s surface can be approximated by straight lines over a spherical Earth. This model requires the layer of the atmosphere containing both end points of the ray path to have a constant vertical gradient in N and that the elevation angles are small. The model is most useful for terrestrial paths and is not used for paths through the entire atmosphere. The effective Earth radius factor, k, is defined as the factor that is multiplied by the actual Earth radius, a, to give the effective Earth radius ae. The mean Earth radius is in average 6,371 km. Due to the Earth’s curvature and refraction of radio signals by objects, each site must meet a minimum elevation with respect to antenna height. The effects of refraction are significant within an area around the direct path known as the Fresnel zone. The maximum effects caused by the Earth’s curvature and Fresnel zone occur at the midpoint of the link.

46

Chapter Two

ture) ’s curva 1 s Earth k=– w o l l o (F k=∞ k=1 k = 0.5 .33 k=0 Earth surfac

e

True Earth’s curvature

Effective Earth’s radius = k x true Earth’s radius True Earth’s radius = 6,371 km k = 1 used for establishing LOS in most parts of the world k = 4/3 standard atmosphere with normally refracted path k < 1 “Earth’s bulge with path obstruction” is rare but possible in some climates –1 < k < ∞ rare occurrence of entrapment layers in some areas Figure 2.4 Variations of the ray curvature as a function of k

It is usually adequate to use less than the full depth of the Fresnel zone to calculate clearance, and 60 percent of the first Fresnel zone is the generally accepted area that must be kept clear of obstructions. For standard refractivity conditions, k = 1.33 = 4/3, and this value should be used whenever a local value is not provided. It is important to keep in mind that lower k-values will lower the LOS; in other words, they will demand higher antenna heights. Some typical values of k for the U.S. are shown in Table 2.3. 2.3.4

Standard Propagation Mechanisms

Standard propagation mechanisms are those mechanisms and processes that occur in the presence of a standard atmosphere. These propagation mechanisms are free-space propagation, reflection, diffraction, scattering, and tropospheric scatter.

TABLE 2.3

Typical Values of k in the U.S.

Dry mountains (above 1,500 m) Mountains (to 1,500 m) Midwest and Northeast South and West Coast Southern Coast

Summer

Winter

1.20 1.25 1.50 1.55 1.60

1.20 1.25 1.30 1.35 1.50

Basics of Microwave Communications

47

2.3.4.1 Propagation in a Homogeneous Atmosphere The simplest case of electromagnetic wave propagation is the transmission of a wave between a transmitter and a receiver in a homogeneous atmosphere (sometimes also called free space). It is defined as a region whose properties are isotropic, homogeneous, and loss-free; i.e., away from the influences of the Earth’s atmosphere. In free space, the electromagnetic wavefront spreads uniformly in all directions from the transmitter. If a particular point on a wavefront were followed over time, the collection of point positions would define a ray. The ray would coincide with a straight line from the transmitter to the receiver. 2.3.4.2 Reflection Reflection occurs when an electromagnetic wave strikes a nearly smooth, large surface, such as a water surface, and a portion of the energy is reflected from the surface and continues propagating along a path that defines an angle with the surface equal to that of the incident ray. Obstruction dimensions are very large compared to the signal wavelength.4 The strength of the reflected wave is determined by the reflection coefficient, a value that depends on the frequency and polarization of radiation, the angle of incidence, and the roughness of the reflecting surface. For shallow incidence angles and smooth seas, typical values of the reflection coefficient are near unity, i.e., the reflected wave is almost as strong as the incidence wave causing so-called specular reflection. The law of reflection states that for specular reflection the angle at which the wave is incident on the surface equals the angle at which it is reflected. Reflection rays from different surfaces may interfere constructively or destructively at a receiver causing multipath propagation. Diffraction Diffraction occurs when an impenetrable body obstructs the radio path between the transmitter and receiver. Energy tends to follow along the curved surface of an object. Diffraction is one of the important physical phenomena which have been studied using different approaches. Based on Huygens’ principle, secondary waves form behind the obstructing body, and the study is usually mainly based on physical optics. The ability of the electromagnetic wave to propagate beyond the horizon by diffraction is highly dependent on frequency; the lower the frequency, the more the wave is diffracted. Over-the-horizon communication systems, for example, are based on this principle. When the clearance of the radio path over the underlying terrain becomes small, diffraction phenomena take place, and they reduce the

2.3.4.3

48

Chapter Two

received signal strength. To determine how close the radio path can approach an obstacle before diffraction losses begin to occur, we can use the concept of the first Fresnel zone. 2.3.4.4 Scattering Scattering occurs when the radio channel contains

objects whose dimensions are approximately the same as or smaller than the propagating wavelength. Scattering, which follows the same physical principles as diffraction, causes energy to be radiated in many different directions. As frequencies increase, the wavelengths become shorter, and the reflective surface appears rougher, thus resulting in more diffused reflections as opposed to specular reflections. Although not a serious problem for terrestrial microwave point-topoint links, scattering attenuation through clouds can be a serious issue for satellite systems. While cloud-related losses are usually very low below 15 GHz, they become an increasingly serious concern with increasing frequency. The millimeter bands above 40 GHz are virtually unusable for satellite work, and even the Ka band (20–35 GHz) window could be exposed. Of course, the same caveats concerning path length also apply, so for a satellite link, the steeper the elevation angle, the better. At ranges far beyond the horizon, the propagation loss is dominated by troposcatter. Propagation in the troposcatter region is the result of scattering by small imperfections within the atmosphere’s refractive structure. Above the tropopause the temperature is constant; there is little humidity and no movement in the air, thus few irregularities to scatter radio signals. Troposcatter was put into a useful application and used by commercial services and the military in the years 1950–80, before the expansion of satellite services. It is still used today by the military, although mostly just as a backup communications system.

2.3.5

Anomalous Propagation Mechanisms

Anomalous meteorological conditions may occur that considerably change the standard propagation previously described. A deviation from the normal atmospheric refractivity leads to conditions of anomalous propagation such as subrefraction, superrefraction, and trapping (ducting). The effects of anomalous propagation are hard to predict and almost impossible to completely avoid. 2.3.5.1 Subrefraction If the motions of the atmosphere produce a situ-

ation in which the temperature and humidity distribution creates an increasing value of N with height, the wave path would actually bend upward, and the energy would travel away from the Earth. This is

Basics of Microwave Communications

49

termed subrefraction. Although this situation rarely occurs in nature, it still must be considered when assessing electromagnetic systems’ performance. Subrefractive layers may be found at the Earth’s surface or aloft. In areas where the surface temperature is greater than 30°C and relative humidity is less than 40 percent (i.e., large desert and steppe regions), solar heating will produce a very nearly homogeneous surface layer, often several hundreds of meters thick. Since this layer is unstable, the resultant convective processes tend to concentrate any available moisture near the top of the layer, which in turn creates a positive N gradient. This layer may retain its subrefractive nature into the early evening hours, especially if a radiation inversion develops, trapping the water vapor between two stable layers. For areas with surface temperatures between 10 and 30°C and relative humidity above 60 percent (e.g., the western Mediterranean, the Red Sea, the Indonesian Southwest Pacific), surface-based subrefractive layers may develop during the night and early morning hours, caused by advection of warm, moist air over a relatively cooler and drier surface. While the N gradient is generally quite intense, the layer is often not very thick. 2.3.5.2 Superrefraction Superrefractive conditions are largely associ-

ated with temperature and humidity variations near the Earth’s surface. The effect of a superrefractive layer on a microwave system is directly related to its height above the Earth’s surface. If the troposphere’s temperature increases with height (temperature inversion) and/or the water vapor content decreases rapidly with height, the refractivity gradient will decrease from the standard. The propagating wave will be bent downward from a straight line more than normal. As the refractivity gradient continues to decrease, the radius of curvature for the wave path will approach the radius of curvature for the Earth. The refractivity gradient for which the two radii of curvature are equal is referred to as the critical gradient. At the critical gradient, the wave will propagate at a fixed height above the ground and will travel parallel to the Earth’s surface. Refraction between the normal and critical gradients is known as superrefraction.

Ducting Should the refractivity gradient decrease beyond the critical gradient, the radius of curvature for the wave will become smaller than that of the Earth. The wave will either strike the Earth and undergo surface reflection or enter a region of standard refraction and be refracted back upward, only to reenter the area of refractivity gradient that causes downward refraction.5

2.3.5.3

50

Chapter Two

This refractive condition is called trapping (or blackout fading), because the wave is confined to a narrow region of the troposphere. The common term for this confinement region is a tropospheric duct or a tropospheric waveguide. Trapping is an extension of superrefraction, because the meteorological conditions for both are the same. In a discussion of atmospheric ducting conditions on electromagnetic wave propagation, the usual concern is propagation beyond the normal horizon (see Figure 2.5). To propagate energy within a duct, the angle the electromagnetic system’s energy makes with the duct must be small—usually less than 1°. Thicker ducts, in general, can support trapping for lower frequencies. The vertical distribution of refractivity for a given situation must be considered as well as the geometrical relationship of transmitter and receiver to the duct so as to assess the duct’s effect at any particular frequency. Several meteorological conditions could lead to the creation of ducts. If these conditions cause a trapping layer to occur such that the base

Warm air layer Cool air layer

Tropospheric ducting

Rx

Tx Earth

Warm air layer

High-altitude Tropospheric ducting

Cool air layer

Rx Tx Earth

Figure 2.5 Anomalous propagation: ducting

Basics of Microwave Communications

51

of the resultant duct is at the Earth’s surface, a surface duct is formed. Surface-based ducts occur when the air aloft is exceptionally warm and dry compared with the air at the Earth’s surface. Several meteorological conditions may lead to the formation of surface-based ducts. For example, over the ocean and near land masses, warm, dry continental air may be blown over the cooler water surface such as the Santa Ana of southern California, the Sirocco of the southern Mediterranean, and the Shamal of the Persian Gulf. This advection will lead to a temperature inversion at the surface. In addition, moisture is added to the air by evaporation, producing a moisture gradient to strengthen the trapping gradient. This type of meteorological condition routinely leads to a surface duct created by a surface-based trapping condition. Surface-based ducts tend to be on the leeward side of land masses and may occur both during the day or at night. In addition, surface-based ducts may extend over the ocean for several hundred kilometers and may be very persistent (lasting for days). Elevated ducts may vary from a few hundred meters above the surface at the eastern part of the tropical oceans to several thousand meters at the western part. For example, along the southern California coast, elevated ducts occur an average of 40 percent of the time, with an average top elevation of 600 m. Along the coast of Japan, elevated ducts occur an average of 10 percent of the time, with an average top elevation of 1,500 m. It should be noted that the meteorological conditions necessary for a surface-based duct are the same as those for an elevated duct. Evaporation ducts exist over the ocean, to some degree, almost all of the time. The duct height varies from a meter or two in northern latitudes during winter nights to as much as 40 m in equatorial latitudes during summer days. On a world average, the evaporation duct height is approximately 13 m. The duct strength is also a function of wind velocity. For unstable atmospheric conditions, stronger winds generally result in stronger signal strengths (or less propagation loss) than do weaker winds. Since the evaporation duct is much weaker than the surface-based duct, its ability to trap energy is highly dependent on frequency. Generally, the evaporation duct is only strong enough to affect microwave systems above 3 GHz. It is a well known fact that the equatorial regions are most vulnerable to ducts. In temperate climates, the probability of formation of ducts is lower. The ducting probability follows seasonal variations. Conventional techniques used to combat other types of fading, such as increased margins or diversity techniques, have little or no influence on blackout fading. The altitude where ducting conditions occur over land is almost always within the lowest 200 m of the atmosphere. These ducts are caused by

52

Chapter Two

inversions near the surface. Ducting events over land at polar latitudes and over deserts are almost entirely caused by the dry refractivity variability. Midlatitude and tropical events appear at higher altitudes as well, up to 1.9 km and 2.3 km, respectively, while all events at higher altitudes are over the sea. The mean thickness of the layer over which ducting conditions occur is usually very small for near surface events, about 30 to 50 m. Events at higher altitudes show a mean thickness of about 120 m. Events near the surface tend to be highly variable, while events at higher altitude show lower variability. In the tropics they are more variable than at midlatitudes. The mean minimum in the refractivity gradient is lowest for tropical events and increases with altitude. Variability in the minimum refractivity gradient is highest near the surface and decreases with altitude. 2.3.6 Effects of Sand and Dust on Radio Propagation

Propagation through dust and/or sand is less researched and understood than propagation through rain. The atmosphere contains a variety of solid particles in suspension; in soil mechanics, particles with radii less than 60 micron are termed dust, depending on their size 6. Particles with radii greater than 60 micron are typically called sand. The content of dust and sand will be vastly different in different parts of the world. Dust particles are random in shape and cannot be classified as spheres, ellipsoids or otherwise, which complicates their mathematical modeling. The larger the size of the particles, the quicker they fall out of suspension. While aerosols can remain aloft for days and sometimes years, sand will rapidly descend to the ground once the impelling force, such as a violent wind storm, has subsided. In a like manner, aerosols can be carried high up into the stratosphere in even a slight updraft while the larger particles, such as sand, generally remain within a 10 meters of the surface of the Earth. Dust can be found up to 5,000 m above ground. Aerosols and small solid particles do not significantly affect the transmission of electromagnetic waves until the optical frequencies are approached and their particle’s cross-section becomes appreciable. The effect of fine dust and coarse dust (sand) on radio wave transmissions can be noticeable at much lower frequencies, however, although the impact is often difficult to separate from the meteorological phenomena that often accompany the dust storms. In many cases, the strong winds that generate a dust storm may cause antenna misalignment that will lead to a loss of signal strength that is difficult to distinguish from the attenuating effect of dust particles.

Basics of Microwave Communications

53

In other cases, enhanced humidity or even rain that might accompany a severe convective activity in an arid region might cause the dust particles to attenuate more strongly than would be the case in a dry atmosphere due to water uptake into the crystal structure of the particles themselves. A measurement of severity of a dust storm that is used in meteorology is visibility; needless to say that visibility decreases with increasing intensity of dust in a storm. Visibility is related to the mass of dust per cubic meter of air by:

M=

C γ V

(2.8)

where M = mass of dust (kg) V = visibility (km) C, g = constants that depend on the distance from the point of origin of the storm, type of soil, and climatic conditions at the origin The principal criterion applied to defining the occurrence of a dust storm is the visibility that drops below 1 km; with this determining criterion, between 0.1 and 174 days per year of dust storm activity occur for the various regions of the world where such effects are observed. Worldwide research identifies eight distinct dust storm types: planetary winds, cyclogenic, frontal, katabatic winds, haboob, constriction, dust devils, and diurnal winds. There are probably a number of additional local phenomena around the world that may or may not fall into one of the categories listed here. The eight dust storm types were classified into a range of average wind speeds, maximum gusts, average storm widths, average storm lengths, and effective storm heights. In the 1980s, researchers reported that attenuation coefficients from sand particles at microwave frequencies were linearly proportional to frequency and inversely proportional to optical visibility. The attenuation coefficients for distributions of identical particles were linearly proportional to particle radius. Other theoretical analyses have shown that sand and dust particle attenuation at microwave frequencies tends to be significant at very high particle concentrations (visibilities of less than 20 m), or at high moisture contents, or both. Specific attenuation for a visibility of 10 m (30 ft), with high content of water in dust particles, is close to 0.6 dB/km@30 GHz, and 0.07 dB/ km@10 GHz. It was found that for linear polarization the effect of dust storms is almost negligible except for very dense storms; i.e., storms with visibilities of less than a few meters.

54

Chapter Two

Blowing sand and dust storms also occur in some regions of the U.S. The vertical extent of these sand storms is unknown, but it seems unlikely that high concentrations would exceed 1 km. The path length is expected to vary between 0.5 and 3 km, generally resulting in a total additional attenuation due to sand of the order of 1 dB or less. However, data measured in Iraq for an 11 GHz microwave link during a dust storm indicated a median signal reduction of 2 to 4 dB, with signal fades as high as 10 to 15 dB observed7. This discrepancy of calculated and actual results can be explained by anomalies within and surrounding the dust storm, which practically produces its own microclimate and radio propagation conditions not trivial to model mathematically. It is important to recognize that for the terrestrial point-to-point microwave links, primary impact on signal attenuation from a dust storm results not from signal attenuation due to scattering and absorption by the dust particulate but from the atmospheric changes in the index of refractivity (creating reflections and resulting multipath conditions) and the increased loss in signal power due to water vapor attenuation (which is not typically associated with a dust storm in an arid region). 2.3.7 Propagation Impairments Countermeasures

Propagation conditions vary from month to month and from year to year, and the probability of occurrence of these conditions may vary by as much as several orders of magnitude. Some phenomena occur only rarely, requiring many years of observation to make any conclusions. For instance, elevated ducting may occur only several times per year in some locations, and in many locations, rain intense enough to affect propagation paths occurs for less than 1 percent of a year. In parts of India at locations where the monsoon occurs some years but not others, several significant rain events may occur one year but not the following year. From propagation data, it was concluded that, for a well-designed path that is not subject to diffraction fading or surface reflections, multipath propagation is the dominant factor in fading below about 10 GHz. The necessary reduction in path length with increase in frequency reduces the severity of multipath fading. Above 10 GHz, the effects of precipitation tend increasingly to determine the permissible path length through the system availability objectives. The multipath and the rain are two principal causes of fading and they are normally mutually exclusive. Given the split between availability and error performance objectives, precipitation effects contribute mainly to unavailability (link is down) and multipath propagation mainly to error performance (link is not down,

Basics of Microwave Communications

55

but may not be performing well). Another influence of precipitation (e.g., backscatter from rain) may influence the choice of radio frequency channel arrangements. Propagation effects due to various forms of precipitation tend not to be frequency dispersive, while multipath propagation caused by tropospheric layers can be, and this may cause severe distortion of information-bearing signals. The rapid development of digital communication systems has required an improved understanding of these effects and the means to overcome them. Two countermeasures to propagation impairments are commonly used: diversity techniques and adaptive channel equalizers, which attempt to combat attenuation and distortion caused by the propagation medium. The effectiveness of a fading countermeasure is usually expressed in terms of an improvement factor. Adaptive modulation is a new approach to combat rain problems in microwave point-to-point links. More about methods for improving microwave link performance is provided in Chapter 3. 2.4 Digital Microwave Point-to-Point Systems 2.4.1

Applications of Microwave Systems

A significant proportion of business premises lack broadband connectivity, so wireless access provides the perfect medium for connecting new customers. Even if an operator chooses to use license-exempt or pointto-multipoint wireless technologies to connect customers, high-capacity microwave provides the ideal solution for backhaul of customer traffic from access hubs to the nearest fiber point of presence (POP). Wireless operators and wireless networks often face aggressive schedules to provide service for customers and to generate immediate revenue. To turn up their networks, they need to connect their cell sites to switching stations, and they have chosen microwave because of its reliability, speed of deployment, and cost benefits over fiber or leasedline alternatives. Microwave radio is deployed in the existing 2.5 and 3G mobile infrastructures, as well as in emerging 4G networks, to support increased data usage and greater numbers of cell sites needed to support a new generation of mobile services. Companies now have private networks with high-speed LAN/WAN network requirements and need to connect parts of their businesses in the same campus, city, or country. Utility companies use microwave systems to connect remote sites, control gas pipelines, and electric powerline networks and to tie together a corporation’s several operating locations.

56

Chapter Two

Public transport organizations, railroads, and other public utilities are major users of microwave systems. These companies also use microwave systems to carry control and monitoring information (SCADA systems) to and from power substations, pumping stations, and switching stations. Many countries, especially developing countries, need to upgrade their telephone infrastructure systems to the latest digital technology. Microwave has traditionally provided developing nations with the tools of establishing telecommunications quickly over often undeveloped and impractical areas such as desert, jungle, and frozen terrain, where laying cable would be a very difficult and/or expensive undertaking. Governments in many countries use fixed services to provide a number of defense and nondefense functions in locations and circumstances where commercial communication services do not satisfy requirements. They use microwave point-to-point services for control and monitoring of many wide-area systems (e.g., air traffic control) for connecting government mobile radio sites, tactical communications, and communications within test and training ranges8. 2.4.2

Microwave Radio Basics

Microwave point-to-point communication can be achieved by a single connection—for example, a microwave link between two stations located at specified fixed points or multiple cascaded (daisy-chained) links made by a number of intermediate repeaters with or without partial payload drop-insert. The transmitted information can be voice, data, or video as long as it is in a digital format. A typical digital microwave radio consists of three basic components: n n

n

A digital modem for interfacing with digital terminal equipment A radio frequency (RF) unit for converting a carrier signal from the modem to a microwave signal An antenna used to transmit and receive the signal

The combination of these three components is called a radio terminal. Two terminals are required to establish a microwave communications link, in North America commonly referred to as a microwave hop or microwave link (Figure 2.6). Note: In ITU standards, microwave link is the same as end-to-end microwave system in North America, and consists of more than one microwave hop. So, microwave link and microwave hop are not always a synonym.

Basics of Microwave Communications

57

Microwave antenna

Outdoor ODU radio unit

Indoor radio unit IDU

Traffic

IN OUT

ODU

Coaxial cable IDU

Traffic

OUT IN

Figure 2.6 Typical split-mount point-to-point microwave system

We can feed the data and voice traffic into the radio using an electrical or optical interface. In the radio, the digital signals are coded into analog signals and converted to microwaves (with a typical wavelength of a few centimeters). The microwaves are sent using a highly directive parabolic shaped antenna. At the other end, the signals are received and restored to the digital format. The Plesiochronous Digital Hierarchy (PDH) microwave radio provides a transmission medium for digital traffic of standard capacities typically ranging from 1.544 Mbps (1T1) to 45 Mbps (1DS3) in North America, and from 2.048 Mbps (1E1) to 34 Mbps (16E1) based on ITU standards. Sometimes, high-capacity backbones in North America are built for 3DS3 or more and require at least 28 MHz of bandwidth. SDH/SONET microwave radios are typically used for high-capacity backbone systems. The new Ethernet microwave radios, providing in the neighborhood of 1Gbps of throughput, are increasingly being used as well. Radio links may be established between any two points within the line of sight and, depending on the frequency, geographical region, and rain statistics, the typical link distance can be up to 25 mi (40 km). For the longer microwave link hops, additional measures have to be taken to ensure required reliability of the system (e.g., space and/or frequency diversity). In today’s wireless networks, the PDH microwave systems are only used for the low-capacity links. Microwaves, which are only centimeters (or inches) in length, are small relative to the surroundings and hence do not have the bending property. Therefore, to establish a radio link, it is important to have radio LOS between the two radio position sites. One or more radio paths connected in tandem form a microwave system. The radio stations between two terminal stations are called repeater stations (active or passive) (see Figure 2.7).

58

Chapter Two Drop/Insert Repeater station Terminal station

Traffic

Microwave Repeater station link #1

Microwave radio

Microwave radio

Microwave link #2

Passive billboard repeater

Microwave link #3

Terminal station

Microwave radio

Traffic Microwave radio

Figure 2.7 Radio link with repeater

Repeaters can be nonregenerative when the signal is only filtered and amplified, with or without down and up conversions (e.g., in some analog FDM systems), or they can be regenerative when, in digital applications, the signal is demodulated and remodulated before transmission to the next radio hop. Passive repeaters implemented without any active radio components (e.g., two directional antennas connected back to back, a reflector, and so on) are also utilized. Drop-insert is a functionality provided in analog and digital repeaters, where only radio-system specific control and service channels, and possibly part of the payload, are made available for local traffic and system management and maintenance. 2.4.3

Fresnel Zones and Clearance Rules

The most common use of Fresnel zone information on a profile plot is to check for obstructions that penetrate the zone. As emphasized earlier, although line of sight is important, it may not always be adequate. Even though the path has clear line of sight, if obstructions (such as terrain, vegetation, buildings, and others) penetrate the Fresnel zone, there will be signal attenuation. Fresnel zones are specified employing an ordinal number that corresponds to the number of half-wavelength multiples that represents the difference in radio wave propagation path from the direct path. The higher the frequency, the narrower the Fresnel zone and, consequently, the more vulnerability to non-LOS effects (object attenuation). The first Fresnel zone is therefore an ellipsoid whose surface corresponds to one half-wavelength path difference and represents the smallest volume of all the other Fresnel zones. The radius of the first Fresnel zone (Figure 2.8) is the parameter currently employed to establish appropriate clearance.

Basics of Microwave Communications

59

d2

R d1

d = d1 +

d2

Ear th surfac e

d = Distance between antennas (hop length in kilometers) R = First fresnal zone radius in meters f = Frequency in Gigahertz Figure 2.8 Radius of the first Fresnel zone

The general formula (assuming that Rn << d1 and Rn << d2) to calculate the radius of nth Fresnel zone is approximated by: Rn =

nλ d1 d2 f ( d1 + d2 )

(2.9)

From here, for the first Fresnel zone, we have Rfeet = 72.1

Rmeters = 17.3

d1 d2 f ( d1 + d2 ) d1 d2 f ( d1 + d2 )

[ft]

[m]

(2.10)

(2.11)

where d = distance between antennas (hop length in kilometers or miles) R = first Fresnel zone radius in meters or feet f = frequency in gigahertz The Fresnel zone is computed along the path, usually for the distance of each of the terrain points, so the resolution of the computed and plotted Fresnel zone is comparable to the terrain data. Typically, the first Fresnel zone (n = 1) is used to determine obstruction loss, and anytime the path clearance between the terrain and the line-of-sight path is less

60

Chapter Two

than 0.6F1 (60 percent of the first Fresnel zone radius), some knife-edge diffraction loss will occur (see Chapter 3 for more details). The amount of loss depends on the amount of penetration. Profiles are often drawn with the first Fresnel zone and a ratio of 0.6 to provide a quick visual inspection of possible problems caused by obstructions penetrating that zone. Some engineers plot a ratio of 100 percent of the first Fresnel zone to add a bit of headroom for the path design. The refractive properties of the atmosphere are not constant and the variations of the index of refraction in the atmosphere (expressed by the Earth-radius factor k) may force terrain irregularities to totally or partially intercept the Fresnel zone. Clearance can be described as any criterion to ensure sufficient antenna heights so that, in the worst case of refraction (for which k is minimum), the receiver antenna is not placed in the diffraction region. Diffraction theory indicates that the direct path between the transmitter and the receiver needs a clearance above ground of at least 60 percent of the radius of the first Fresnel zone to achieve free-space propagation conditions. Clearance values have to fit the local climate. Clearance can be considered by applying climate-dependent clearance criteria or by properly handling diffraction-refraction fading (k-type fading). Recently, with more information on this mechanism and the statistics of ke that are required to make statistical predictions, some administrations are installing antennas at heights that will produce some small known outage. To summarize all this, we can say that, for normal propagation conditions, the following two clearance criteria have to be satisfied: n

n

The antenna must have clearance of 60 percent or greater at the minimum k suggested for the certain path. The antenna must have clearance of 100 percent or greater at k = 4/3.

In case of space diversity, the antenna can have a 60 percent clearance at k = 4/3 plus allowance for the tree growth, buildings, and so forth—usually at least 3 m (10 ft). Another important use of Fresnel zone information is to check microwave paths for possible reflection points. For even-numbered Fresnel zones (N = 2, 4, …), the difference between the direct path and the indirect path defined by the Fresnel zone radius is a multiple of one halfwavelength. If the geometry of the path is such that an even-numbered Fresnel zone happens to be tangential to a good reflecting surface (e.g., a lake, highway, or smooth desert area, depending on what wavelength is involved), signal cancellation will occur as a result of interference between the direct and indirect (reflected) signal paths.

Basics of Microwave Communications

61

It is possible to set the Fresnel zone to even-numbered values when plotting a profile to see if any potential areas of destructive signal reflection are present on the path. 2.4.4

Near and Far Fields

Let us assume for a moment that electromagnetic waves propagate under ideal conditions, i.e., without dispersion. If high-frequency energy is emitted by an isotropic radiator, then the energy propagates uniformly in all directions. Areas with the same power density therefore form spheres (A = 4p r²) around the radiator (see Figure 2.9). The same amount of energy spreads out on an incremented spherical surface at an incremented spherical radius. That means that the power density on the surface of a sphere is inversely proportional to the radius of the sphere. Since a spherical segment emits equal radiation in all direction (at constant transmit power), if the power radiated is redistributed to provide more radiation in one direction, it results in an increase of the power density in the direction of the radiation. This effect is called antenna gain, and it is obtained by directional radiation of the power. So, the formula to calculate the directional power flux density, S, is

S=

PT GT [W/m 2 ] 4π r 2

where PT = transmitted power (W) GT = gain of the transmitting antenna r = radius of the sphere (meters)

S2 S1 > S2 S1

Figure 2.9 Power flux density

(2.12)

62

Chapter Two

When the range is large, the spherical surface of uniform power density appears flat to a receiving antenna, which is very small compared to the surface of the sphere. This is why the far field wave front is considered planar and the rays approximately parallel. Also, it is apparent that at some shorter range, the spherical surface no longer appears flat, even to a very small receiving antenna. The distance where the planer, parallel ray approximation breaks down is known as the near field. The crossover distance between a near and far field is taken to be where the phase error is 1/16 of a wavelength. The result is a well-known formula for the beginning of the far field of an antenna with the largest dimension D in feet and wavelength l in feet: RFF =

2D2 [ft] λ

(2.13)

If D represents the largest linear dimension of the antenna (the diameter of the parabolic dish antenna), d represents the transmitterreceiver (T-R) separation distance, and l represents the wavelength in free-space, then the following relationships define the far-field region: d >> D d >> λ d >>

2D λ

(2.14) 2

The terms far field and near field describe the fields around an antenna or, more generally, any electromagnetic radiation source. The names imply that two regions, with a boundary between them, exist around an antenna while, in fact, as many as three regions and two boundaries exist. These boundaries are not fixed in space, and they move closer to or farther from an antenna, depending on both the radiation frequency and the amount of phase error an application can tolerate. In the literature, these regions have different names and can be defined in slightly different ways. Usually, two- and three-region models are used. The near field may be thought of as the transition point where the laws of optics must be replaced by Maxwell’s equations of electromagnetism. In the three-region model, near field, far field, and the transition zone are defined as follows. The near field, also called the reactive near field, is the region that is closest to the antenna and for which the reactive field dominates over the radiating fields. In the reactive near-field region, fields vary as 1/r3

Basics of Microwave Communications

63

(power varies as 1/r6). For antennas that are large in terms of wavelength, the near-field region consists of the reactive field extending to the certain distance followed by a radiating near field (see Figure 2.10). The transition zone or radiating near field is the region between the reactive near field and the far-field regions and is the region in which the radiation fields dominate and where the angular field distribution depends on distance from the antenna. In the radiating near-field region, fields vary with 1/r2. The boundary of transition zone is determined by the following equation:

RTRZ = 0.62

D3 [ft] λ

(2.15)

In the radiating near field, the field strength does not necessarily decrease steadily with distance away from the antenna but may exhibit an oscillatory character. Therefore, it is difficult to predict the antenna gain in that region. The far field, or Rayleigh distance, is the region where the radiation pattern is independent of distance. In this area, fields vary with 1/r. Although formulae for the near-field and transition-zone boundary vary in the literature, they all agree on the far-field boundary. Table 2.4 shows some common examples of the distance to the far-field boundary. Equations contain terms in 1/r, 1/r2, and 1/r3. In the near field, the 1/r3 terms dominate the equations. As the distance increases, the 1/r3 and 1/r2 terms attenuate rapidly and, as a result, the 1/r term dominates in the far field. It is also important to notice that far-field expressions are valid if D >> l, which is always the case in microwave systems. Engineers perform link engineering, including Fresnel clearances and path profiles, based on the assumption that microwave antennas

Reactive near field

Radiating far field

Radiating near field

Parabolic dish Feed

0.62

D3 λ

Figure 2.10 Radiating fields of an antenna

2D 2 λ

64

Chapter Two

TABLE 2.4

Far-Field Boundary Distances

Frequency (GHz) 5.8 6 6 6 6 6 7 7 7 7 8 8 11 11 11 18 18 23 23

Antenna Diameter (ft) 2 6 8 10 12 15 6 8 10 12 6 8 2.5 3 4 1 2 1 2

Distance to Far Field (ft) 47 439 780 1,219 1,756 2,743 512 910 1,422 2,048 585 1,040 140 201 358 37 146 47 187

are in the far-field region. For example, a microwave system operating in an 8 GHz band and having 6-ft dishes will have a far field beginning at 178 m (585 ft) from the antenna using the general formula for the far-field boundary. 2.4.5

Link Budget

A microwave engineer starts the microwave link design by doing a link budget analysis. The link budget is a calculation involving the gain and loss factors associated with the antennas, transmitters, receivers, transmission lines, and propagation environment, used to determine the maximum distance at which a transmitter and receiver can successfully operate. The purposes of the transmitter are to generate the carrier frequency that is to be used for the communication, to modulate this carrier frequency with the desired information, and finally, to amplify the signal so that it attains a sufficiently high power level so that it may travel the desired communication distance to the receiver. The receiver amplifies the received signal (which is at this point much weaker than when it was transmitted), filters out any undesirable signals (interfering signals) that the receiver picked up and, finally, detects the existence of information in the carrier frequency.

Basics of Microwave Communications

65

The purpose of transmission lines is to interconnect the antenna with the transmitter/receiver. Transmission lines between the radio equipment and the antenna may consist of coaxial cabling or a (flexible) waveguide. The antenna-coupling unit makes it possible to utilize a common antenna for both the transmitter and receiver. The transmitter and receiver can, for example, be connected to the same antenna either via a duplex filter or a transmitter/receiver switch. Together, feeder cable losses, antenna-coupling losses, and any additional losses (depending on the radio configuration) constitute branching losses. The antenna adapts the generated signal to the surrounding environment (to the propagation medium) and directs the radio waves that are to be transmitted towards the receiving station. When receiving, the antenna receives the signal from the desired direction and sends it to the receiver. Every antenna is typically characterized by its impedance, bandwidth, directivity (radiation pattern), and polarization. Antennas may be more or less an isotropic antenna (it radiates equally in all directions) or an antenna that exhibits extremely high directivity, such as parabolic dish antennas used in microwave point-to-point links. The receiver sensitivity threshold is the signal level at which the radio runs continuous errors at a specified bit rate. Specifications are listed for the 10−3 bit error rate (PDH radios) or 10-6 bit error rate. System gain (in decibels) is defined as the difference between the transmitter output power and the receiver threshold. Lowering the system gain will reduce the fade margin. System gain can be used to reduce antenna sizes or improve the path availability. A given radio system has a system gain that depends on the design of the radio and the modulation used. For example, 99.999 percent system availability (five minutes of outage per year) will degrade to 99.980 percent (two hours of outage per year) if the modulation is changed from 16 QAM to 128 QAM without recovering the system gain reduction and all other conditions remaining unchanged. The gains from the antenna at each end are added to this gain, with larger antennas providing higher gain. The free-space loss of the radio signal as it travels over the air is then subtracted from the system, and the longer the link, the higher the loss. These calculations result in a “fade margin” for the link (see Figure 2.11); fade margin is the difference between the received signal and receiver threshold value (or sensitivity) for given BER, typically 10-6 or 10-3. In most applications, the same duplex radio setup is applied to both stations forming the microwave link. Thus, the calculation of the received signal level is independent of direction. See Chapter 6.2.2 for more details. The radio can handle anything that affects the radio signal within the fade margin. If the margin is exceeded, then the link could go down and

66

Chapter Two

Transmitter 1

Waveguide

Transmitter 2

Splitter

Splitter Receiver 2

Branching losses [dB]

Propagation losses [dB]

Antenna gain [dBi]

Output power (Tx) [dBm]

Antenna gain [dBi]

Receiver 1

Branching losses [dB]

Received power (Rx) [dBm] Fade margin [dB] Receiver threshold value [dBm] Figure 2.11 Radio path link budget

therefore become unavailable. The fade margin is calculated with respect to the receiver threshold level for a given bit-error ratio (BER). The threshold level for BER = 10−6 for older microwave equipment used to be about 3 dB higher than the threshold level for BER = 10−3. Consequently, the fade margin was 3 dB lower for BER = 10−6 than for BER = 10−3. For the new generation of microwave radios with power forward error correction schemes, this difference is more in the 1.0 to 1.5 dB range. After analyzing the link budget, the next step is to analyze rain fading, multipath fading, interference, and other (miscellaneous) losses that could potentially affect the radio signal. 2.4.6 Microwave Systems for Rapid Deployment

“Licensed” RF transmitters communicate using a specific transmit and receive frequency combination that is assigned to the user (licensee). The frequency assignment is coordinated with other users of the same spectrum in the same geographical area. This process

Basics of Microwave Communications

67

provides full disclosure of the frequency assignment and typically avoids interference from any existing licensee in the area. If licensed radios encounter interference, it is typically resolved with the assistance of the regulatory body. The terms license-exempt (also called license-free) and licensed refer to the radio frequency spectrum rules defined by the FCC or an equivalent national government regulatory body. In the United States, the FCC Rules, Part 15 governs the license-exempt frequency spectrum, and the Rules Part 101 govern the licensed frequency spectrum. Licensed products require regulatory approval before deployment, whereas license-exempt products can be deployed without any regulatory approval. A license-exempt system can be installed virtually anywhere within a given country without obtaining an operational license from the regulatory authorities. As such, they are good candidates for the rapid deployment microwave systems. Of course, such a system must already be certified to operate as license-exempt in that country. Manufacturers desiring license-exempt certification must apply to the FCC or equivalent national authority for the type approval to operate the particular product in specific radio frequency bands. It is important to note the following: 1. Not all the license-exempt microwave systems are spread-spectrum. 2. In this text, the term unlicensed is a synonym for illegal, i.e., a microwave link that was installed without obtaining an FCC license. 3. Microwave links that do not require licensing are called licenseexempt microwave links. Spread-spectrum systems were originally developed by the military to counter attempts to detect, decode, or block signal transmissions. The most important nonmilitary characteristics of spread spectrum systems are that they facilitate radio communications in a manner that minimizes the potential to cause harmful interference to other services, and they are able to withstand higher levels of interference than other technologies. Hence, spread-spectrum systems have significant potential to share common spectrum with other services. Spread-spectrum devices operate on a license-exempt basis, if the technical conditions are met, and type approval is mandatory. The two main types of spread-spectrum system are commercially available: direct sequence and frequency hopping. In direct sequence (DS), the incoming information is usually digitized (if not already in binary format) and then Modulo 2 added to

2.4.6.1 Spread-Spectrum Microwave Communications

68

Chapter Two

a higher-speed code sequence. The combined information and the code are then used to modulate a radio frequency carrier. Since the highspeed code sequence dominates the modulating function, it is the direct cause of the wide spreading of the transmitted signal. In frequency hopping (FHSS), the carrier is modulated with coded information in a conventional manner, causing a conventional spreading of the radio frequency energy about the carrier frequency. The frequency of the carrier is not fixed but changes rapidly under the direction of a pseudorandom coded sequence. The wide radio frequency bandwidth needed by such a system is not required to support a spreading of the radio frequency energy about the carrier, but rather to accommodate the range of frequencies to which the carrier frequency can hop. A unique pseudorandom code may be embedded in the signal during the modulation process, enabling a large number of users to occupy the same band, as each receiver will recognize only its own code. This access technique is called code division multiple access (CDMA). Typical bandwidths used by a spread spectrum system range from 500 kHz up to 50 MHz, depending on the data throughput required and the bandwidth available. The bandwidth of the transmitted signal is much greater than the bandwidth of the original message, and the bandwidth of the transmitted signal is determined by the message to be transmitted and by an additional signal known as the spreading code. Users of spread-spectrum devices have been mainly in bands designated for industrial, scientific, and medical (ISM) applications at 900 MHz, 2.4 GHz, and (to a lesser extent) 5.8 GHz. ISM devices generate and internally use radio frequency energy for industrial, scientific, medical, domestic, or similar purposes, but not for communications. Examples of ISM devices are plastic welders, chemical analysis equipment, medical diathermy equipment, wireless microphones, and domestic microwave ovens. These devices radiate incidental emissions that have the potential to cause interference to radio communication equipment. Bands for ISM applications have been specifically designated, via footnotes in the Spectrum Plan (and the ITU Radio Regulations) for the operation of such equipment. In these bands, radio communications services are not protected against any interference caused by ISM equipment. Spread-spectrum communications microwave systems should be used with caution and only as a temporary solution when rapid deployment is required and/or in rural areas only. The reason is that the 2.4-GHz band is also used with a number of other applications within the licenseexempt ISM band, including garage door openers, microwave ovens, Bluetooth systems, Wireless LANs, and so forth.

Basics of Microwave Communications

69

The 2.4-GHz ISM band used to be called the “junk band,” because it was already contaminated by oven emissions. Years ago, 2.43 GHz was allocated to the microwave oven, and it was felt that no one would ever want to co-occupy this band. As pressure to allocate more spectrum to communications was felt, the FCC set up rules for license-exempt ISM operation in this band. Commercial spread-spectrum systems typically use the ISM bands worldwide. These are located as shown below: n

902 to 928 MHz

n

2,400 to 2,483.5 MHz (microwave ovens are located here)

n

5,725 to 5,850 MHz

Commercial spread-spectrum systems in Canada are similar and are based on Industry Canada, Spectrum Management, RSS-210, “Low * Power License-Exempt Radio-communication Devices.” Licensed microwave systems in Canada using the same band are described in RSS139, “Licensed operation from 2,400 to 2,483.5 MHz.” In part for reasons of safety, the transmitter power output level in the ISM band is limited to 1 W (+30 dBm) maximum input antenna power. For similar reasons, and to minimize interference, effective isotropic radiated power (EIRP), or power radiated by the associated antenna system, is limited to 4 W (+36 dBm) maximum in Canada. In the U.S. (2.4 GHz band), for every 3 dB of antenna gain over +6 dBi, the input power to the antenna is reduced by 1 dB from +30 dBm. In the 5.8 GHz band, there are no EIRP limits—only a +30 dBm maximum antenna input power. The 5 GHz band is divided into several sections referred to as the Unlicensed National Information Infrastructure (UNII) bands (see Table 2.5). The UNII-1 band is designated for indoor operations and, therefore, not of interest for point-to-point microwave networks. The UNII-2 and UNII-3 bands are for indoor and outdoor operations, and the 5.725–5.825 GHz UNII/ISM band (called this because of overlap with the ISM band) is intended for outdoor bridge products and may be used for indoor WLANs as well. The expansion of the 802.11a wireless market and the constant push to open up new spectrum for license-exempt equipment use created a requirement for Dynamic Frequency Selection (DFS), a mechanism to allow license-exempt devices to share spectrum with existing military radar and weather radar systems. *

More information about Industry Canada can be found on its Web page: http://www.ic.gc .ca/eic/site/ic1.nsf/eng/home

70

Chapter Two

TABLE 2.5

Unlicensed National Information Infrastructure Bands

Band

Frequency Number of Range (MHz) Channels Max Power Max EIRP

UNII-1 (indoor)

5,150–5,250

4

50 mW

UNII-2 (indoor/outdoor)

5,250–5,350

4

250 mW

UNII-3 (worldwide, indoor/ 5,470–5,725 outdoor) UNII/ISM (indoor/outdoor) 5,725–5,825

11

250 mW

4

1 Watt

200 mW (23 dBm) 1W (30 dBm) 1W (30 dBm) 200 W (52 dBm)

A new FCC Rule addressing Dynamic Frequency Selection (DFS2) in the 5 GHz UNII-2 and UNII-3 bands went into effect for the U.S. and Canada on July 20, 2007. The regulatory requirements for DFS, along with requirements for Transmit Power Control (TPC) and uniform channel loading have been adopted in the U.S., Europe, Australia, and Japan and are being considered by many other regulatory domains looking at adopting the 5 GHz bands for license-exempt and possibly licensed devices. At power-up and throughout operation, each point-to-point terminal scans the band hundreds of times per second and automatically switches to the clearest channel. Some radios will have the 30-day, timestamped database to alert network operators to any interference that does exist and provides statistics that help pinpoint which channels offer the clearest data paths. U-NII is an FCC regulatory domain for 5 GHz wireless devices and power limits are defined by the United States CFR Title 47 (Telecommunication), Part 15—Radio Frequency Devices, Subpart E— Unlicensed National Information Infrastructure Devices, Paragraph 15.407—General technical requirements. IEEE 802.11a and the European HiperLAN standard operate in the U-NII band. Regulations in the license-exempt bands keep changing, so it is recommended you check for the latest update. Regulatory use in individual countries may differ. It is important to remember that some licensed users sometimes operate in the license-exempt bands as well. The license-exempt bands are allocated on a shared basis, and, while there may be no requirement to obtain a license to operate for low-power datacom applications with approved equipment, other licensed users may be allowed to operate with significantly higher power. A particularly important example of this is the operation of U.S. government radar equipment in the U.S. U–NII band at 5.725 to 5.825 GHz.

Basics of Microwave Communications

71

These radars often operate at peak power levels of millions of watts, which can cause significant interference problems to other nearby users in this band. Therefore, it is important to look around the site to determine if there are any airports, military bases, or other installations where such radars may be located. Although the lack of licensing requirements for the ISM band implies ease of installation and minimal capital outlay, it by no means implies a lack of regulatory controls. Most regulations permitting license-exempt spread spectrum penalize the use of directional antennas, and manufacturers using directional antennas are forced to reduce the transmitter output power by the gain of the antenna to comply with an effective radiated power (ERP) limit. From the regulatory perspective, these license-exempt bands come with two major constraints: n

Transmit power limitation of 1 W

n

Minimum processing gain of 10 dB for either FHSS or DS

This implies that the desired data capacity per bandwidth (in other words, bandwidth efficiency) may have to be sacrificed to achieve the processing gain, and the total transmit power is not high enough to support multilevel quadrature-amplitude-modulation (QAM) techniques to increase the data rate. The challenge is to overcome these difficulties and still achieve sufficiently high data capacity. For FHSS systems, IEEE 802.11 defines 79 different hops for the carrier frequency. Using these 79 frequencies, IEEE 802.11 defines 78 hopping sequences (each with 79 hops) grouped in three sets of 26 sequences each. Sequences from the same set encounter minimum collisions, and they may be allocated to collocated systems. Theoretically, 26 FHSS systems may be collocated. However, as synchronization among independent systems is not allowed (synchronization would eliminate collisions), the actual number of systems that can be collocated is around 15. White spaces refer to frequencies allocated to a broadcasting service but not used locally. In the United States, it has gained prominence after the FCC ruled (November 2008) that license-exempt devices that can guarantee that they will not interfere with assigned broadcasts can use the empty white spaces in spectrum. This 300–400 MHz of unused spectrum known as white space is considered a prime spectrum for offering wireless broadband services because it can travel long distances and penetrate through walls. The hope is that the spectrum could be used to augment existing wireless services or eventually be used to create new wireless broadband services.

2.4.6.2 Opportunistic Spectrum Sharing

72

Chapter Two

Depending on the nature of the licensed use, the government (through the FCC) can make under-utilized frequencies available for noninterfering shared access by time, geographic area, direction of signals, or other variables. By establishing guidelines for cognitive radios that can sense and respond to the local spectral environment, the government could in effect create “virtual white space” within spectrum that is already licensed. In the United States, the abandoned television frequencies are primarily in the upper UHF 700-MHz band, covering TV channels 52 to 69 (698 to 806 MHz). U.S. television and its white spaces thus continue to exist in UHF frequencies, as well as VHF frequencies for which mobile users and white-space devices require larger antennas. In the rest of the world, the abandoned television channels are VHF, and the resulting large VHF white spaces are being reallocated for the worldwide (except the U.S.) digital radio standard DAB, DAB+, and DMB. Some U.S. wireless carriers are arguing that it should be licensed and used for backhaul, so it remains to be seen if some type of point-to-point application for the white space will develop. 2.4.6.3 Cell on Wheels (COW) Figure 2.12 shows a portable wireless cellsite trailer. During the wireless network launch, operators bring out these portable towers on a trailer and set them up as a temporary fix until a more permanent site can be found. The cell on wheels (COW) can be equipped with up to a 30-m-long hydraulically expandable mast. They are

Microwave antenna

Up to 30m high

Figure 2.12 Cell on wheels (COW)

Basics of Microwave Communications

73

self-sufficient and capable of operating in the field for extended periods of time, and one person can usually set one up in less than an hour. Natural and man-made disasters can be catastrophic to communications networks. Microwave systems are often used to restore communications when other transmission equipment has been damaged by earthquakes, floods, hurricanes, or other natural disasters; conflicts such as terrorist attack and wars; and other wireless network problems. COW is usually connected via a leased T1/E1 backhaul circuit with the switch or via microwave link to the nearest working cell site. In case of microwave connection, licensed or license-exempt microwave radios can be used, but the preference would be to use a license-exempt spread-spectrum microwave link. The military uses what it calls a tactical transportable antenna system (TTAS) for applications in which rapid deployment is a critical concern. The system consists of a 3-m-diameter reflector mounted on a modified military trailer. The system provides ground-level elevation and azimuth controls and is supplied with guy wires, ground anchors, and waveguide termination behind the reflector. A trained team of two people can erect the antenna in less than 20 minutes. The system is usually used for line-of-sight or over-the-horizon microwave communications in the 5 GHz band. 2.5

Other Microwave Systems

So far, we have talked about point-to-point microwave systems (often simply called microwave systems) that provide wideband communications over the line-of-sight paths. Microwave technology is becoming increasingly useful for accessing individual users and narrow- and medium-bandwidth groups of users, thereby replacing wireline or fitting into the bandwidth gap between wireline and fiber. Expectations are to see more microwave applications for WLL, WLANs, Bluetooth, and other new short-range access technologies. Many of these are nontraditional microwave applications, however, and they are not as easily recognized as the traditional microwave fixed services of the past. The new applications will tend to move the microwave industry closer to the individual user and closer to the mass market. These short-range access markets will be particularly suitable for higher microwave frequency bands where the smaller antenna size, greater frequency reuse, and wider available bandwidths provide major advantages. It is important to note that 3G and 4G technologies (WCDMA, CDMA2000, WiMAX) and WLAN, as well as Bluetooth, are all technologies operating in the 2-GHz microwave band.

74

2.5.1

Chapter Two

Over-the-Horizon Microwave Systems

In addition to terrestrial systems operating within the radio horizon (LOS), there is a different type of point-to-point microwave system. These are called over-the-horizon (OH) systems (see Figure 2.13) and are described in ITU-R P.617. For OH (also called transhorizon) paths, the propagation of the signal over the horizon is possible because of diffraction of the wave over the horizon, scattering of the signal from the troposphere, or both. Diffraction is achieved over Earth’s curvature and over terrain features such as hills. Tropospheric scatter, although very weak, can be the strongest mode of operation at very long distances. For links within the radio horizon, the antennas should be placed to provide direct line-of-sight transmission. Practically, this means that the equipment should be installed on towers or buildings of a certain height that fulfill the line-of-sight requirement and minimize the probability of the fading due to multipath propagation. On the other hand, for over-the-horizon links, a virtual tower height is defined either by the propagation mechanism itself (scattering capability of the troposphere, reflection capability of the meteorite tail) or by the physical placement of the equipment (stratosphere platform, satellite platform). The troposphere is the lowest 10-km part of the atmosphere, and the inhomogenities within the common beam volume of the transmit and receive antennas act as sources that scatter the electromagnetic waves into all directions of space. Thus, components will be generated establishing coupling between transmitter and receiver. The first troposcatter link was put into operation in 1953, and the link allows distances of up to 400 to 500 km to be spanned.

θet

Transmitter

C

θot

a∞

h1 θ∞

h0

A1 L

VE

hts

A

SE

LE

dLt

β∞

θor

d2 d

Figure 2.13 Over-the-horizon microwave systems

Receiver A2

Ds d1

θer

dLr

hrs

Basics of Microwave Communications

75

Troposcatter microwave links have been applied in locations that are difficult to access, such as links connecting oil well islands with the mainland, links within deserts and jungles, and links for military communication (before the large-scale use of inexpensive satellite links). These links are operating in the 350 MHz to 6 GHz range within the bands allocated by the ITU. The parameters of the radio channel require extremely high-gain antennas and transmitting powers, plus the application of multiple diversity systems such as frequency and space diversity, and angle and frequency diversity. The capacity of the transmitted information is limited by propagation time differences over the scattering volume. The siting of transmission links requires some care. The antenna beams must not be obstructed by nearby objects and the antennas should be directed slightly above the horizon. The precise optimum elevation is a function of the path and atmospheric conditions, but it lies within about 0.2 to 0.6 beamwidths above the horizon. For radio paths extending only slightly over the horizon, or for paths extending over an obstacle or over mountainous terrain, diffraction will generally be the propagation mode determining the field strength. In these cases, the methods described in Recommendation ITU-R P.526 should be applied. Generally speaking, over-the-horizon loss (OHLOSS) depends on the diffraction loss and tropospheric scatter loss, which are combined and then adjusted for regional geographic effects. Time variability is added to that value to account for the percentage of time that these losses are exceeded. In addition to the more commonly used over-the-horizon microwave point-to-point systems based on tropospheric scattering, there are other point-to-point systems based on stratosphere platform systems (HAP) and systems utilizing meteorite tails (ITU-R P.843) that cannot be called microwave, since they operate in much lower frequency bands. 2.5.2

Point-to-Multipoint Systems

The legacy telecom networks were designed for the delivery of voice services only. As the demand for Internet and data traffic has increased, the access portion of the network has been unable to achieve the desired speeds because of limitations in the deployed technology. Copper wires, for example, over typical distances to residences are limited to speeds of about a few hundreds kbps using xDSL modems because of the quality of the copper in the ground and cross-connections in the typical local loop. On the other hand, the wireless broadband point-to-multipoint telecommunications platform facilitates the two-way transmission of voice, data, and video. The systems typically operate in different frequency

76

Chapter Two

bands (some licensed and some license-exempt,) which, coupled with a high bandwidth capability in excess of 1 GHz, allows the communication of multimedia services with interactive facilities within a 3 mi (5 km) radius of a central data hub. For the point-to-multipoint (PMP) architecture, the operator installs base stations around the market area that are very similar to traditional cellular systems,9 and each cell contains a hub with multiple radio nodes equipped with sector antennas for PMP and directional antennas for PP connections. Base stations have antennas that transmit and receive on multiple sectors, and typical configuration is four sectors using 90º beamwidth antennas. The subscribers use antennas that are installed on their rooftops, pointing in the direction of the maximum signal strength from the base station. Similar to cellular system, frequencies are reused in neighbor base stations or sectors, as long as the reuse distances are defined so as to avoid interference. These systems provide the bandwidth on demand, which is achieved by use of ATM or IP/Ethernet (and statistical multiplexing) as a transport mechanism. Since the bandwidth is shared with other users, bandwidth available per subscriber is reduced with every new subscriber using the system. For subscribers who demand fixed bandwidth availability, a point-topoint system can be offered that will not share bandwidth with other subscribers. Figure 2.14 shows a typical system architecture based on the dynamic bandwidth allocation and ATM.

PSTN Point-to-multipoint RBS 1

Hub site

AT AT

RBS 2

90 deg Radio node

AT

Point-to-point RBS 3

Circuit emulation shelf

Radio SDH/SONET shelf E3/DS3

ATM Mux or ATM switch

MSC

BSC

Radio node

AT

NMS

AT - access termination To other hub sites Figure 2.14 Microwave point-multipoint system architecture

Basics of Microwave Communications

77

Millimeter-wave (MMW) characteristics dictate short-range lineof-sight propagation (rain attenuation) with minimal refraction and reduced interference. Line-of-sight between the hub site and all the customer sites is required, similar to classic point-to-point microwave systems. Therefore, standard microwave propagation and prediction methods are used for the system design. In addition, frequency coordination with other spectrum owners and PMP service providers must be carefully planned. In urban areas, only low LOS penetration rates are achievable, due to obstructions from buildings, vegetation, and other obstacles. One inexpensive option (at certain frequencies) to increase penetration might be the use of reflected and diffracted waves in the non-line-of-sight (NLOS) areas, if attenuation due to reflection or diffraction is smaller than a certain available system margin.10 The use of reflected waves depends very much on the roughness of the surface under consideration. For very rough building walls with a standard deviation of the roughness close to or more than l/2, only reflected waves of grazing angles can be used. However, since roughness of building walls is often small compared to the millimeter wavelengths, reflection coefficients can be less than the system margin over a wide range of the angle of incidence. PMP can use FDMA, TDMA, or spread-spectrum (DS or FH) overthe-air interface as well as frequency-division duplex (FDD) or timedivision duplex (TDD). There is a debate about TDD versus FDD in point-to-multipoint networks. While TDD requires a single channel for full-duplex communications, FDD systems require a paired channel for communication, one for the downlink (hub to remote), and one for the uplink (remote to hub). In TDD, transmit/receive separation occurs in the time domain, as opposed to FDD, where it occurs in frequency domain. While FDD can handle traffic that has relatively constant bandwidth requirements in both directions, TDD handles better varying uplink/ downlink traffic (bursty traffic—data, Internet) asymmetry by allocating time spent on up- and downlinks. TDD requires a guard time equal to the round-trip propagation delay between hub and remote units and increases with link distance. In FDD, sufficient isolation in frequency between the uplink and downlink is required, so FDD is a simpler but less efficient solution. 2.5.3 Wireless Local Area Networks (WLANs)

Most corporate information systems and databases can be accessed remotely through the Internet Protocol (IP) backbone, but the high bandwidth demand of typical office applications, such as large e-mail

78

Chapter Two

attachment downloading, often exceeds the transmission capacity of cellular/PCS networks. Mobile professionals are looking for a public access solution that can cover the demand for data-intensive applications and enable smooth online access to corporate data services. Wireless LAN access technology provides a perfect broadband complement for the operators’ existing or new 3G/4G services in an indoor environment. Seamless access to modern office tools is one of the most valuable assets for mobile business professionals today. Public wireless LANs can handle large volumes of data at significantly lower costs, offer a migration path to higher speeds, and deliver additional capacity with pinpoint accuracy compared to leading 3G technologies. That is why 3G/4G wireless network operators need public wireless LANs to serve the most demanding users in the most demanding locations. Two standards dominate the WLAN marketplace; IEEE 802.11b has been the industry standard for several years. Operating in the unlicensed portion of the 2.4-GHz radio frequency spectrum, it delivers a maximum data rate of 11 Mbps and boasts numerous strengths. Standard 802.11b enjoys broad user acceptance and vendor support. Many vendors manufacture compatible devices, and this compatibility is assured through the Wi-Fi certification program. Thousands of enterprise organizations that typically find its speed and performance acceptable for their current applications have deployed 802.11b technology. In the U.S., a number of wireless ISPs have emerged that are offering public access services using IEEE 802.11b equipment operating in the 2.4-GHz band. These providers are targeting public areas where business travelers may wish to access corporate intranets or the Internet, e.g., in hotels or coffee shops.11 In some parts of Europe, there are now a number of service providers, both mobile operators and ISPs, offering wireless Internet services based on 802.11b technology in the 2.4-GHz band. Another WLAN standard, IEEE 802.11a, operates in the uncluttered 5-GHz radio frequency spectrum. With a maximum data rate of 54 Mbps, this standard offers a fivefold performance increase over the 802.11b standard. Therefore, it provides greater bandwidth for particularly demanding applications. The IEEE ratified the 802.11a standard in 1999, but the first 802.11a-compliant products did not begin appearing on the market until December 2001. The 802.11a standard delivers a maximum data rate of 54 Mbps and eight nonoverlapping frequency channels—resulting in increased network capacity, improved scalability, and the ability to create microcellular deployments without interference from adjacent cells. Operating in the license-exempt portion of the 5 GHz radio band, 802.11a is also immune to interference from devices that operate in the

Basics of Microwave Communications

79

2.4-GHz band, such as microwave ovens, cordless phones, and Bluetooth (a short-range, low-speed, point-to-point, personal-area-network wireless standard). The 802.11a standard is not compatible with existing 802.11bcompliant wireless devices. The 2.4-GHz and 5-GHz equipment can operate in the same physical environment without interference. IEEE 802.11g is a high-performance standard delivering the same 54 Mbps maximum data rate as 802.11a but operating in the same 2.4 GHz band as 802.11b. The IEEE 802.11 Working Group has been working for the last few years to standardize an upgrade to the 802.11 radio that provides a new set of capabilities dramatically improving the reliability of 802.11 communications, the predictability of 802.11 coverage, and the overall throughput of 802.11 devices. IEEE 802.11n is a proposed amendment to the IEEE 802.11-2007 wireless networking standard to significantly improve network throughput over previous standards, such as 802.11b and 802.11g, with a significant increase in the maximum raw, OSI physical layer (PHY) data rate from 54 Mbps to a maximum of 600 Mbps. IEEE 802.11n builds on previous 802.11 standards by adding multiple-input multiple-output (MIMO) and channel-bonding/40 MHz operation (two adjacent 20-MHz channels, bonded together) to the PHY layer and frame aggregation to the MAC layer. 802.11n is expected to be approved by IEEE-SA RevCom, although many “Draft N” products are already available on the market. Implementers must be able to make an educated choice between deploying 2.4-GHz-only networks, 5-GHz-only networks, or a combination of both. Selecting between these technologies is not a one-for-one trade-off. They are complementary technologies and will coexist in future enterprise environments. Organizations with existing 802.11b networks cannot simply deploy a new 802.11a network on 5-GHz access points (APs) and expect to have similar coverage with 802.11a 54-Mbps data rate compared to an 11-Mbps data rate with 802.11b APs. 802.11n operates in both the 2.4-GHz (802.11b and g) and 5-GHz (802.11a) radio bands. Planning for each of the radio bands should be done independently because of the constraints that are sometimes very different for each band. 2.5.4

Bluetooth

Personal Area Network (PAN) represents the person-centered network concept, allowing the person to communicate with his or her personal devices close to him or her (e.g., personal digital assistants, organizers, handheld computers, cameras, and head-mounted displays, and so on)

80

Chapter Two

and to establish the wireless connections with the outside world. It covers the personal space surrounding the person within the 10 ft (3m) distance. One of the popular solutions is the Bluetooth technology, which allows for the replacement of the many proprietary cables that connect one device to another, with one universal short-range radio link. Bluetooth is a de facto open standard for short-range digital radio and, although Bluetooth is considered to be a point-to-multipoint system, it can also be a point-to-point system, depending on the application. The Bluetooth radio technology (developed originally by Ericsson) built into both the cellular telephone and laptops replaces the cumbersome cable connecting the two devices. Printers, PDAs, desktops, fax machines, keyboards, joysticks, and virtually any other digital device can be part of the Bluetooth system. However, beyond replacing the cables, Bluetooth radio technology provides a universal bridge to existing data networks, a peripheral interface, and a mechanism to form small private ad hoc groupings of connected devices away from fixed network infrastructures. Bluetooth operates in the 2.4-GHz ISM band, a license-exempt portion of the radio spectrum that is already well used and generally available in most parts of the world (Table 2.6). Not only do microwave ovens operate within this range, but so do other RF communications technologies, most notably HomeRF and IEEE 802.11b (WLAN).12 Because this spectrum does not require license, even more uses for it are expected to develop in the future. As the band becomes more widely used, probability of the radio interference will increase. Bluetooth uses FHSS, is a shorter-range and lower-bandwidth technology than 802.11b, and uses frequently changing, narrow bands over all channels. It is important to manage the concurrent operation of 802.11b WLANs and Bluetooth within the enterprise. Task Group 2 of the IEEE 802.15 Working Group is looking at the coexistence issues of IEEE 802.11b WLANs and Bluetooth. To counter this interference, Bluetooth technology incorporates several techniques to provide robust linkages; for example, cyclical redundancy encoding, packet retransmission, and frequency hopping, which can occur up to 1,600 times per second. The hopping pattern

TABLE 2.6

Bluetooth Frequency Bands

Area U.S., Europe, and other countries Spain France

Frequency Band (GHz) Number of Channels 2.4000–2.4835

79

2.4450–2.475 2.4465–2.4835

23 23

Basics of Microwave Communications

81

may be adapted to exclude a portion of the frequencies that are used by interfering devices. The adaptive hopping technique improves Bluetooth technology co-existence with static (nonhopping) ISM systems when these are collocated. The symbol rate is 1 Megasymbol per second (Msps) supporting the bit rate of 1Mbps or, with Enhanced Data Rate, a gross air bit rate of 2 or 3 Mbps. These modes are known as Basic Rate and Enhanced Data Rate respectively. Any time a Bluetooth wireless link is formed, it is within the context of a piconet. A piconet consists of two or more devices that occupy the same physical channel, meaning that they are synchronized to a common clock and hopping sequence). The common (piconet) clock is identical to the Bluetooth clock of one of the devices in the piconet, known as the master of the piconet, and the hopping sequence is derived from the master’s clock and the master’s Bluetooth device address. All other synchronized devices are referred to as slaves in the piconet. The terms master and slave are only used when describing these roles in a piconet. Within a common location a number of independent piconets may exist. Normally the master uses a frequency downstream to the slave and the slave uses the following slot for the upstream communication. The multiplexing technique uses time-division duplex (TDD), i.e., the master and slaves transmit alternatively (a master transmits in the even slots and the slaves in the odd slots). The intervals of time or slots are numbered and each one lasts 625 ms. One major interesting feature of Bluetooth is that it is not dependent on the IP, simplifying the deployment of devices, which do not need to worry about upper layer problems such as address allocation, default router, netmask, and so on. Auto configuration is hence much easier. The technology continues to evolve, building on its inherent strengths: small-form factor radio, low power, low cost, built-in security, robustness, ease-of-use, and ad hoc networking abilities. In April 2009, the Bluetooth SIG (Special Interest Group) formally adopted Bluetooth Core Specification Version 3 High Speed (HS), or Bluetooth 3.13 Bluetooth 3 gets its speed from the 802.11 radio protocol. The inclusion of the 802.11 Protocol Adaptation Layer (PAL) provides increased throughput of data transfers at the approximate rate of 24 Mbps. In addition, mobile devices including Bluetooth 3 will realize increased power savings due to enhanced power control built in. 2.5.5 WiMAX

WiMAX (Worldwide Interoperability for Microwave Access) is a technology based on the IEEE 802.16 air interface standard suite, which provides the wireless technology for nomadic and mobile data access. The first

82

Chapter Two

version of WiMAX was approved as an IEEE Standard 802.16-2001 published in 2002. This standard was created addressing fixed line-ofsight connections by focusing on licensed frequencies in the range of 10–66 GHz. WiMAX offers an alternative to cabled networks such as fiberoptic links, traditional cable or digital subscriber lines, or T1/E1 networks14. The term WiMAX was created by the WiMAX Forum, which was formed in June 2001 to promote conformity and interoperability of the standard. The IEEE 802.16a-2004 standard (a revision of IEEE Std 802.16-2001) is designed for stationary transmission, and the 802.16e-2005 amendment deals with both stationary and mobile transmissions. Although the original 802.16 standard was for the 10 to 66 GHz range, the latest version adds coverage for the 2 to 11 GHz range and supports both licensed and licensed-exempt bands, as well as NLOS (non-line-of-sight) operation, which would be impossible to achieve with the previous version of the standard due to the high frequencies involved and the mandatory need of line-of-sight. The ability to support near-LOS and NLOS scenarios requires additional PHY functionality, such as the support of advanced power management techniques, interference mitigation/coexistence, and multiple antennas. Additional MAC features such as mesh topology and automatic repeat request (ARQ) are also introduced. WiMAX employs orthogonal frequency division multiplexing (OFDM) and supports adaptive modulation and coding, depending on the channel conditions. Wireless systems cover large geographic areas without the need for a costly cable infrastructure to each service access point. The maximum data rate for 802.16a is around 75 Mbps, and the maximum range is 30 miles but, obviously, not at the same time. The maximum data rate for 802.16e is 15 Mbps and the range is 1–3 miles. OFDM technology has been known for decades but its complexity and high cost have prevented widespread use. Recent advancements in integrated circuit technology have made possible the design of costeffective OFDM modems. Because its technical advantages translate directly into benefits for both service providers and customers, OFDM is the key technology for broadband wireless networks and applications. OFDM has been widely used in wire line access applications such as xDSL and cable modems and has also been used in the digital video broadcast industry, especially in Europe. The 802.16a standard supports both time-division duplex (TDD) and frequency-division duplex (FDD), as well as half duplex–FDD. A key feature distinguishing WiMAX from other wireless technologies is per-flow Quality-of-Service (QoS), the ability for a client to

Basics of Microwave Communications

83

have several connections with each having its own QoS characteristics. 802.16 defines four kinds of QoS: n

n

n

n

UGS (Unsolicited Grant Service) Supports constant bit-rate services such as T1 emulation and VoIP without silence suppression. rtPS (Real-Time Polling Service) Provides irregularly timed variable-sized packets for MPEG and VoIP with silence suppression. nrtPS (Non-real-time Polling Service) Supports consistent variable-sized packets for services such as FTP. BE (Best Effort Service)

Supports low-priority applications.

There is no uniform global licensed spectrum for WiMAX, although the WiMAX Forum has published three licensed spectrum profiles, i.e., 2.3 GHz, 2.5 GHz, and 3.5 GHz. Since November 2007, the Radiocommunication Sector of the International Telecommunication Union (ITU-R) has decided to include WiMAX technology in the IMT2000 set of standards, enabling spectrum owners (specifically in the 2.5–2.69 GHz band at this stage) to use Mobile WiMAX equipment in any country that recognizes the IMT-2000 (ITU-R Recommendation M.1457). 2.5.6

LTE

LTE (Long Time Evolution of UTRAN) is a mobile technology upgrade project for 3G networks initiated in 2004 to provide faster data speeds and new services through new radio access technology optimized for IP-based traffic. LTE is part of the GSM evolutionary path beyond 3G, following EDGE, UMTS/W-CDMA, and HSPA. UTRAN stands for Universal Terrestrial Radio Access Network. Performance targets included the average downlink user throughput of 100 Mbps (3X-4X Release 6 HSDPA levels) and 50 Mbps average uplink throughput (2X-3X HSUPA levels). The LTE architecture is called EPS (Evolved Packet System), and includes E-UTRAN (Evolved UTRAN) for access and EPC (Evolved Packet Core) in the core. LTE will be extremely flexible, using a number of defined channel bandwidths between 1.25 and 20 MHz. LTE downlinks use orthogonal frequency division multiple access (OFDMA) to achieve high peak data rates in the high spectrum bandwidth and support data modulation schemes QPSK, 16QAM, and 64QAM. LTE uplinks use SC-FDMA (single carrier-frequency division multiple access) and support BPSK, QPSK, 8PSK, and 16QAM modulation, while E-UTRA defines the radio interface. To suit as many frequency band allocation arrangements as possible, both paired (FDD) and unpaired (TDD) band operation is supported.

84

Chapter Two

LTE can co-exist with earlier 3GPP radio technologies, even in adjacent channels, and calls can be handed over to and from all 3GPP’s previous radio access technologies. The first LTE standards were approved by the 3GPP in January 2008, to be included in 3GPP Release 8. Although some operators are planning to deploy their first LTE networks in 2010, most LTE deployment is not expected to begin until 2011 or even later. LTE systems will coexist with 2G/3G systems including GSM, EDGE, and UMTS. Multimode devices will likely function across LTE/3G or even LTE/3G/2G, depending on market circumstances. There is useful and updated LTE information on the 3GPP website (www.3gpp.org, accessed September 11, 2009). 2.6 Basics of Digital Communications Systems 2.6.1 RF Communications Systems Requirements

Communications systems can be designed as bandwidth efficient, power efficient, or cost efficient, but not all at the same time. Some tradeoffs must be considered in digital RF communications design. Power efficiency describes the ability of the system to reliably send information at the lowest practical power level. For example, designers of hand-held cellular phones put a high priority on power efficiency because these phones need to run on a battery. Cost is also a high priority because cellular phones must be low-cost to encourage more users. Bandwidth efficiency describes the ability of a modulation scheme to accommodate data within a limited (as small as possible) bandwidth. For designers of digital terrestrial microwave radios, their highest priority is good bandwidth efficiency with low BER (bit-error-rate). Microwave radios have plenty of power available and are not typically concerned with power efficiency. They are not especially concerned with receiver cost or complexity because they do not have to build and sell a huge number of units. As you have already seen in this chapter, spectrum is a very expensive and highly controlled commodity and therefore bandwidth efficiency is very important. 2.6.2

Digital Modulation

A sinusoidal electromagnetic wave has three properties: amplitude, frequency, and phase. Any one of these parameters can be modulated to convey information. However, phase and frequency are just different

Basics of Microwave Communications

85

ways to view or measure the same signal change15. In terrestrial radio systems, for example, AM and FM channels represent amplitude and frequency modulation, respectively. In analog signals, the range of values of a modulated parameter is continuous. Over the past 20 years, a major transition has occurred in communication systems, from simple analog amplitude modulation (AM) and frequency/phase modulation (FM/PM) to new, and more complex, digital modulation techniques. Examples of digital modulation include n

QPSK (quadrature phase shift keying)

n

FSK (frequency shift keying)

n

QAM (quadrature amplitude modulation)

In digital signals, the modulated parameter takes on a finite number of discrete values to represent digital symbols. The advantage of digital transmission is that signals can be regenerated without any loss or distortion to the baseband information. To transmit a signal over the air, there are three main steps: 1. A pure carrier is generated at the transmitter. 2. The carrier is modulated with the information to be transmitted. Any reliably detectable change in signal characteristics can carry information. 3. At the receiver the signal modifications or changes are detected and demodulated. In amplitude modulation, the amplitude of a high-frequency carrier signal is changed in proportion to the instantaneous amplitude of the modulating information signal. In frequency modulation, the amplitude of the modulating carrier is kept constant while its frequency (or the phase in phase modulation) is changed by the modulating information signal. A simple way to view amplitude and phase is with the polar diagram, as shown in Figure 2.15a. The carrier becomes a frequency and phase reference and the signal is interpreted relative to the carrier. The signal can be expressed in polar form as a magnitude and a phase. The magnitude can be either an absolute or relative value, and both are used in digital communication systems (Figure 2.15b). Amplitude and phase can be modulated simultaneously and separately, but this is difficult to generate and especially difficult to detect. Instead, in practical systems the signal is separated into another set of independent components: I (in-phase) and Q (quadrature). These components are orthogonal and do not interfere with each other (Figure 2.15c). Polar diagrams are very common in many displays used in digital communications, although it is common to describe the signal vector by

86

Chapter Two Phase change

Am

pl itu de

Amplitude change

Phase

Q

Q value 0°

0°

0°

I

I value

a. Amplitude and phase representation in a polar diagram

b. Simultaneous amplitude and phase change

c. Conversion from polar to rectangular diagram

Figure 2.15 Signal changes in polar form

its rectangular coordinates of I and Q, a rectangular representation of the polar diagram. On a polar diagram, the I axis lies on the zero degree phase reference, and the Q axis is rotated by 90 degrees. The signal vector’s projection onto the I axis is its “I” component and the projection onto the Q axis is its “Q” component. I/Q diagrams are particularly useful because they represent the way most digital communications signals are generated using I/Q modulator. 2.6.3

I/Q Modulator/Demodulator

In the transmitter, I and Q signals are mixed with the same local oscillator (LO) (see Figure 2.16). A 90° phase shifter is placed in one of the LO paths. Signals that are separated by 90° are also known as being orthogonal to each other or in quadrature. Signals that are in quadrature do not interfere with each other. They are two independent components of the same signal. When recombined, they are summed to a composite output signal. There are two independent signals in I and Q that can be sent and received with Radio receiver

Radio transmitter

Q Out

Q 90° Phase shift

Σ

Composite output signal

Local oscillator (carrier frequency)

I Figure 2.16 I/Q modulator/demodulator

Composite input signal

90° Phase shift Local oscillator (carrier frequency)

I Out

Basics of Microwave Communications

87

simple circuits, thus simplifying the design of digital radios. The main advantage of I/Q modulation is the ease of combining independent signal components into a single composite signal and later splitting such a composite signal into its independent component parts. At the receiver, the input signal is mixed with the local oscillator signal at the carrier frequency in two forms. One is at an arbitrary zero phase while the other has a 90° phase shift. The composite input signal (in terms of magnitude and phase) is thus broken into an in-phase (I) and a quadrature (Q) component. These two components of the signal are independent and orthogonal so one of them can be changed without affecting the other. Normally, information cannot be plotted in a polar format and reinterpreted as rectangular values without doing a polar-to-rectangular conversion. This conversion is exactly what is done by the in-phase and quadrature mixing processes in a digital radio. A local oscillator, phase shifter, and two mixers can perform the conversion accurately and efficiently. In order to understand and compare different modulation format efficiencies, it is important to understand the difference between bit rate and symbol rate. The required signal bandwidth of the communications channel depends on the symbol rate, not on the bit rate. Symbol rate =

Bit rate Number of bits per sym mbol

(2.16)

Symbol rate is sometimes called baud rate or modulation rate (note that baud rate is not the same as bit rate). If more bits can be sent with each symbol, then the same amount of data can be sent in a narrower spectrum. This is why modulation formats that are more complex and use a higher number of states can send the same information over a narrower piece of the RF spectrum. Let’s say you have an 8-bit sampler, sampling at 10 kHz for voice. The bit rate—the basic bit stream rate—would be 8 bits multiplied by 10,000 samples per second or 80 kbps. (Note that for the moment we will ignore the extra bits required for synchronization, error correction, etc.). Groups of k bits can then be combined to form new digits, or symbols, from a finite symbol set of M = 2k such symbols. A system using a symbol set size of M is referred to as an M-ary system.16 If one bit is transmitted per symbol, as with binary phase shift keying (BPSK), then the symbol rate would be the same as the bit rate of 80 kbps. If two bits are transmitted per symbol (k = 2), as in QPSK, then the symbol rate would be half of the bit rate or 40 kbps. QPSK is therefore a more bandwidthefficient type of modulation than BPSK, potentially twice as efficient.

88

Chapter Two

QAM is the most widespread digital modulation method in use today for high-capacity terrestrial microwave links, and employs a combination of amplitude and phase modulation. For example, in 16-state quadrature amplitude modulation (16QAM), there are four I values and four Q values. This results in a total of 16 possible states for the signal. It can transition from any state to any other state at every symbol time. Since 16 = 24, four bits per symbol can be sent (k = 4). This consists of two bits for I and two bits for Q. The symbol rate is one-fourth of the bit rate. So, this modulation format produces an even more spectrally efficient transmission; it is more efficient than BPSK, QPSK or 8PSK. Note that QPSK is the same as 4QAM. Due to their spectral efficiency, 32QAM, 128QAM, and 512QAM are commonly used for high-capacity microwave digital radios. Unfortunately, the symbols get closer and closer together and thus more prone to errors due to noise and distortion, as well as interference. More information about efficiency of different modulation schemes and network design considerations is provided in Chapter 6. 2.7

References

1. ITU, Handbook of Radiometeorology, Geneva, 1996. 2. Rec. ITU-R P.453-9, “The radio refractive index: its formula and refractivity data,” 2003. 3. Rec. ITU-R P.834-4, “Effects of tropospheric refraction on radiowave propagation,” 2003. 4. Mojoli, L. F. and Mengali, U., “Propagation In Line of Sight Radio Links” (Part II— Multipath Fading), Telletra Review, 1988. 5. Mojoli, L. F. and Mengali, U., “Propagation In Line of Sight Radio Links” (Part I—Visibility, Reflections, Blackout), Telletra Review, 1988. 6. Ghobrial, S., Sharif, S., “Microwave Attenuation and Cross Polarization in Dust Storms,” IEEE Transactions on Antennas and Propagation, Vol. AP-35, No 4. 7. Comparetto, G., “The Impact of Dust and Foliage on Signal Attenuation in the Millimeter Wave Regime,” Originally published in J. of Space Comm., Vol. 11, no. 1, pp. 13-20, July 1993. 8. Department of Commerce, “Federal Long-Range Spectrum Plan,” prepared by Working Group 7 of the Spectrum Planning Subcommittee, September 2000. 9. Lehpamer, H., Transmission Systems Design Handbook for Wireless Networks, Norwood, MA: Artech House, 2002. 10. Hayn A., Jakoby R., “Radio Propagation Aspects for Digital Microwave Video Distribution System (MVDS) at 42 GHz,” Institut fur Hochfrequenztechnik, 1998. 11. Mason Communications Ltd., “Spectrum Management Strategies for License-Exempt Spectrum: Final Report,” London, England, November 2001. 12. Agilent Technologies, “Investigating Bluetooth Modules: The First Step in Enabling Your Device with a Wireless Link,” Application Note 1333-2. 13. http://www.bluetooth.com/Bluetooth/SIG/ (accessed July 20, 2009). 14. Ahson, S. and Ilyas, M., Editors, WiMAX—Technologies, Performance Analysis, and QoS, CRC Press, Taylor & Francis Group, Boca Raton, FL, 2008. 15. Hewlett Packard, Digital Modulation in Communications Systems—An Introduction, Application Note 1298, 1997. 16. Sklar, B., Digital Communications, Fundamentals and Applications, Prentice Hall, Upper Saddle River, New Jersey, 2nd Edition, 2001.

Chapter

3

Microwave Link Design

3.1

Design Process Flowchart

Microwave link design is a methodical, systematic, and sometimes lengthy process that includes the following main activities: n

Loss/attenuation calculations

n

Fading and fade margins calculations

n

Frequency planning and interference calculations

n

Quality and availability calculations

A preliminary fade margin is a result of the loss/attenuation calculations and is used for preliminary fade predictions in the fading calculation. If interference is present in the frequency-planning calculation, then the threshold degradation is included in the fade margin. The updated fade margin will become the effective fade margin used in the fading predictions. The results of the loss/attenuation and fading calculations will form the necessary input to the quality and availability calculations. The whole process is iterative and may go through many redesign phases before the required quality and availability are achieved (see Figure 3.1). Prediction models for the purpose of performing fading predictions are empirical (empirical comes from the Greek word empeiria meaning experience), i.e., they are not founded on theoretical considerations but are only built upon observation and experience. Empirical models are the result of the application of mathematical regression techniques on

89

90

Chapter Three

Interference analysis

Frequency planning Rain attenuation

Propagation losses Branching losses

Link budget

Fading predictions

Diffraction-refraction losses Multipath propagation

Other losses

Quality and availability calculations

Vigants and Crane propagation models

ITU-R P.530-xx propagation model

Figure 3.1 Microwave link design process

measurement data and therefore result in a relationship that describes a variable’s dependency under certain given conditions. Empirical prediction models often provide a fair description of the fading process for distances and frequencies that lie within the dataranges for which measurements have actually been collected. 3.2 The Loss/Attenuation Calculations The loss/attenuation calculation is composed of three main contributions: propagation, branching, and “miscellaneous” (or other) losses. The propagation losses contribution comes from the losses due to the Earth’s atmosphere and terrain—e.g., free-space as well as gas, precipitation (mainly rain), ground reflection, and obstacles. The branching losses contribution comes from the hardware required to deliver the transmitter/receiver output to the antenna—e.g., waveguides as well as splitters and attenuators. The “miscellaneous” losses contribution has a somewhat unpredictable and sporadic character, e.g., sandstorms and dust storms as well as fog, clouds, smoke, and moving objects crossing the path. In addition, poor equipment installation and less than perfect antenna alignment (field margin) may give rise to unpredictable losses. The miscellaneous contribution normally is not calculated, but it can be considered in the planning process as an additional loss and then as part of the fade margin.

Microwave Link Design

3.2.1

91

Propagation Losses

3.2.1.1 Free-Space Loss Electromagnetic waves are attenuated while propagating between two geometrically separated points. The free-space path loss model is used to predict received signal strength when the transmitter and receiver have a clear, unobstructed line-of-sight path between them. The attenuation is directly proportional to the square of distance and frequency and gives the free-space loss that represents most of the total attenuation caused by wave propagation effects. The frequency and distance dependence of the loss between two isotropic antennas is expressed in absolute numbers by the following equation:

2

 4π df   4π d  = LFSL =   c   λ 

2

[dB]

(3.1)

where d = distance between transmit and receive antennas (m) l = operating wavelength (m) c = speed of light in vacuum (m/s) f = frequency (Hz) It is very important to notice that the Friis free-space path loss model expressed here is valid only for distances that are in the far field of the transmitter antenna. Free-space loss is always present, and it is dependent on distance and frequency. The free-space loss between two isotropic antennas is derived from the relationship between the total output power from a transmitter and the received power at the receiver. After converting to units of frequency and expressing it in the logarithmic (decibel) form, the equation becomes LFSL = 92.45 + 20 log( fGHz ) + 20 log( dkm ) [dB] where f = frequency (GHz) d = line-of-sight (LOS) range between antennas (km)

(3.2)

92

Chapter Three

The constant 92.45 becomes 96.60 if miles are used instead of kilometers: LFSL = 96.60 + 20 log( fGHz ) + 20 log( dmi ) [dB]

(3.3)

Close inspection of the free-space path loss equation yields a relationship that is useful in dealing with link budget issues. Each 6-dB increase in EIRP equates to a doubling of range. Conversely, a 6-dB reduction in system losses (either by way of transmission line loss or on the transceiver end) translates into a doubling of range (the “6 dB rule”). This is not always so straightforward and easy to accomplish, because the total path attenuation is also affected by other negative contributions, e.g., gaseous losses and rain. 3.2.1.2 Vegetation Attenuation LOS between stations is required for point-to-point microwave links. For an unexpected obstacle intercepting the Fresnel zone (e.g., growing vegetation), the additional loss can be calculated. High-resolution path profiles and careful site and path surveys are important to avoid unexpected obstacle attenuation. Vegetation is continuously growing, and the rate of growth is very important. It is important to include a provision for at least ten years of vegetation growth. Foliage losses at millimeter-wave frequencies are significant. An early empirical relationship was developed (CCIR Report 236-2) that can predict the loss. For the case in which the foliage depth is less than 400 m, the loss is given by

L = 0.2 f 0.3 d0.6 [dB]

(3.4)

where f = frequency (MHz) d = depth of foliage (m) This relationship is applicable for frequencies in the range 200 MHz to 95 GHz. For example, the foliage loss at 40 GHz for a penetration of 10 m (which is about equivalent to a large tree or two in tandem) is about 19 dB. This is clearly a very serious attenuation and has to be considered or, even better, completely avoided. Weissberger’s modified exponential decay model, or simply, Weissberger’s model, is a radio wave propagation model that estimates the path loss due to the presence of vegetation on a point-to-point

Microwave Link Design

93

telecommunication link and gives slightly different results.1 If frequency is given in GHz, we have for 0 < d ≤ 14 m 0.45 f 0.284 d L= (dB) 0.284 d 0.588 for 14 < d ≤ 400 m 1.33 f

(3.5)

This model is found to be applicable to cases in which the ray path is blocked by dense, dry, in-leaf trees found in temperate-latitude forests. The ITU Terrestrial Model for One Terminal in Woodland is a radio propagation model and a successor of the Early ITU Model. This model is applicable to the scenario where one terminal of a link is inside foliage and the other end is free. The ITU Single Vegetative Obstruction Model is a radio propagation model that quantitatively approximates the attenuation due to the vegetation in the middle of a telecommunication link. The model is quite complex and does not work for frequencies from 3–5 GHz. These last two new ITU methods are described in ITU-R Recommendation P.833-2 (1999) and could be used for evaluating attenuation through vegetation between 30 MHz and 60 GHz. This model is based on Radiative Transfer Modeling and covers propagation through and the diffraction around vegetation. It should be noted, however, that links passing through vegetation generally vary significantly with time and wind speed. 3.2.1.3 Gas Absorption A major difference in propagation through the real atmosphere versus free space is that there is air present. Nitrogen and oxygen molecules account for approximately 99 percent of the total volume of the atmosphere. Since the absorption bands of nitrogen are located far from the microwave radio communications region of the spectrum, the atmosphere is considered to be composed of a mixture of two “gases”: dry air (oxygen molecules) and water vapor (water molecules). Neither manifests a linear increase with frequency, but instead both exhibit wild fluctuations, with peaks of absorption followed by valleys and then further peaks, but with an obvious overall upward trend. The two absorption peaks present in the frequency range of commercial radio links are located around 23 GHz (water molecules) and 60 GHz (oxygen molecules). Specific attenuation (in dB/km) for water vapor and oxygen are separately calculated and then summed to give the total specific attenuation. The specific attenuation is strongly dependent on frequency, temperature, and the absolute or relative humidity (RH) of the atmosphere (see Figure 3.2).

Total specific gas attenuation (dB/km)

94

Chapter Three

23 GHz T = 40°C RH = 80%

1.0

T = 30°C RH = 80%

0.8 0.6

T = 30°C RH = 50%

0.4 0.2 0

5

10 15

20

25 30

35 40

45

50

Frequency (GHz) Figure 3.2 Gas attenuation versus frequency

Incidentally, the patterns for oxygen and water vapor absorption are quite different, and their peaks and valleys do not coincide. Above 100 GHz, oxygen molecule absorption is quickly reduced to an insignificant level, while the water vapor absorption trend is still upward and manifests a series of high peaks and deeps with the increase in frequency. From 10–30 GHz, absorption of either sort is not a very serious problem, and only one absorption peak of any significance is present, occurring at 23 GHz. Consequently, the entire spectrum category is useful. Above 30 GHz, water vapor absorption rise is very sharp, exceeding 10 dB/km at 60 GHz. Many other atmospheric gases and pollutants have absorption lines in the millimeter bands (e.g., SO2, NO2, O2, H2O, CO2, and N2O); however, the absorption loss is primarily due to water vapor and oxygen only. 3.2.1.4 Attenuation Due to Precipitation Precipitation can take the form of rain, snow, hail, fog, and haze. All of these consist of water particles (haze can also consist of small solid particles). Their distinctions lie in the distribution of the size and form of their water drops. Rain attenuation is, however, the main contributor in the frequency range used by commercial radio links. Rain attenuation increases with frequency and becomes a major contributor in the frequency bands above 10 GHz. The main parameter used in the calculation of rain attenuation is rain intensity (rain rate), which is obtained from cumulative distributions. These distributions are the percentage of time for which a given rain

Microwave Link Design

95

intensity is attained or exceeded and are furnished for 15 different rain zones covering the entire Earth’s surface. The specific attenuation of rain is dependent on many parameters, such as the form and size distribution of the raindrops, polarization, rain intensity, and frequency. The contribution due to rain attenuation is not included in the link budget and is used only in the calculation of rain fading. It is important to notice that rain attenuation increases exponentially with rain intensity (mm/hr) and that horizontal polarization gives more rain attenuation than vertical polarization. Obstacle Loss Diffraction is the mechanism responsible for obstacle loss/attenuation. In fact, obstacle loss is also known in the literature as diffraction loss or diffraction attenuation. Depending on the shape, size, and properties of the obstacle, diffraction calculations can be cumbersome and time consuming. Since microwave paths normally require LOS, relatively simple methods for calculating the obstacle loss are currently employed. One powerful but simple method for calculation of obstacle loss is the single-peak method, which is based on the knife-edge approximation (see Figure 3.3). This method can easily be extended to comprise the three most significant peaks inside the Fresnel zones. There are a number of different methods for estimating diffraction losses, some of them based on the use of serious mathematical calculations. Here, we will show an estimate of the attenuation of the signal (in dB), that results from diffraction over a single obstacle (building or tree), using the knife-edge method.

3.2.1.5

Tx

Rx

h α

β d1

Figure 3.3 Knife-edge diffraction modeling

d2

96

Chapter Three

Parameter v (another definition of the path clearance) will depend on the geometry of the path and can be calculated as follows: d = d1 + d2

v=

2d tan α tan β = λ

2d h h ⋅ ⋅ λ d1 d2

(3.6)

2( d1 + d2 ) v=h λ d1 d2 Notice that v will be positive for obstructed paths and negative for clear LOS paths. After finding v, based on the theory of the diffraction of electric fields over a knife edge, the loss may be approximated by: when − 0.8 ≤ v ≤ 0 6.02 + 9v + 1.65v2  A(v) ≈ 6.02 + 9.11v − 1.27v2 when 0< v ≤ 2.4  when v > 2.4 12.953 + 20 log v

(3.7)

Example: Let’s assume a single tree on an 18-GHz, 5-mi-long path. The distance of the tree to the first site is 1 mi. The obstacle height is 30 ft above center of the Fresnel’s zone and k=4/3. Calculate attenuation due to the diffraction. Note: Keep in mind the consistency of the units for obstacle height, wavelength, and the distance. We will use feet for this example.

1 mi = 5,280 ft d1 = 1 mi = 5, 280 ft d2 = 4 mi = 21, 120 ft

λ=

c = 0.055 ft f

h = 30 ft k=4/3

Microwave Link Design

97

First we calculate parameter v:

v ≈ 30

2 ⋅ 26400 ≈ 2.8 0.055 ⋅ 5280 ⋅ 21120

From here, attenuation is A(v) ≈ 12.953 + 20 log 2.8 ≈ 21.9 dB This is a theoretical analysis of a completely blocked path, something we should never use in real-life microwave path design. For cases where h = 0 (grazing) we have v = 0, and from the formula, we can see that attenuation will be very close to 6 dB. Although sufficient for a quick analysis, an ideal knife-edge rarely occurs in practice, so there is another, modified and more realistic set of formulas that take into consideration the finite radius of the obstacle. If a knife-edge approximation is considered, the values given in Figure 3.4 are reasonable approximations. Having an obstacle-free 60 percent of first Fresnel zone gives 0 dB obstruction loss. In case of more than one obstacle, the well-known approach by Bullington assumes that all the obstacles can be replaced with one equivalent knife-edge obstacle. The Epstain-Peterson method sums all the individual, single-knife obstacle losses. The Deygout method determines the largest obstacle and focuses on the calculations related to it. The Deygout method tends to overestimate the true path losses when there are a large number of obstacles (edges) or when they are very close together (Causebrook offered corrections for those cases). The Giovanelli method is a result of another development of the Deygout method.

100% First Fresnel zone

60% First Fresnel zone

0 dB

0 dB

6 dB

16 dB

Figure 3.4 Obstacle losses and knife-edge approximation

20 dB

98

Chapter Three

The recommended models for diffraction loss are given by the ITU-R Recommendation P.526. The latest version of this document should be used as it is regularly updated and improved. 3.2.2

Ground Reflection

Signal strength [dB]

Reflection on the Earth’s surface may give rise to multipath propagation. Depending on the path geometry, the direct ray at the receiver may be interfered with by the ground-reflected ray, and the reflection loss can be significant. Since the refraction properties of the atmosphere are constantly changing (k-value changes), the reflection loss varies (fades). The loss due to reflection on the ground is dependent on the total reflection coefficient of the ground and the phase shift. Figure 3.5 illustrates the signal strength as a function of the total reflection coefficient. The highest value (AMax) of signal strength is obtained for a phase angle of 0°, and the lowest value (AMin) is for a phase angle of 180°. The reflection coefficient is dependent on the frequency, grazing angle (angle between the ray beam and the horizontal plane), polarization, and other ground properties. The grazing angle of radio-relay paths is very small—usually less than 1°. It is strongly recommended to avoid ground reflection, which can be achieved by “shielding” the path against the indirect ray. For large grazing angles, the difference between vertical and horizontal polarization is substantial. Changing the antenna heights can move the location of the reflection point. This approach, usually known as the hi-lo technique, forces the reflection point to move closer to the lowest antenna by affecting the height of the higher antenna. The grazing angle increases, and the path becomes less sensitive to k-value variations.

+10

Amax

0 Amin

−10 −20 −30 −40

0.2

0.4

0.6

0.8

1.0

Total reflection coefficient Figure 3.5 Signal strength versus reflection coefficient

Microwave Link Design

99

Space diversity also provides good protection against reflection, and it is usually applied for paths over open water surfaces. Obviously, on many paths, particularly at higher frequencies, it is difficult to obtain an accurate estimate of the effective surface reflection coefficient because of various uncertainties such as the surface conductivity, surface roughness, and so on, and the degree of subjectivity currently needed to obtain a calculation. The calculation procedure may only be a rough guide in such situations to help identify problem paths or to help choose one path from another, even if this possibility exists in the first place. The contribution resulting from reflection loss is not automatically included in the link budget. However, when reflection cannot be avoided, the fade margin may be adjusted by including this contribution as “additional loss” in the link budget. 3.3

Fading and Fade Margins

Fading is defined as the variation of the strength of a received radio carrier signal due to atmospheric changes and/or ground and water reflections in the propagation path. Fading types normally considered when planning microwave point-to-point paths are as follows: n

Multipath fading, which is divided into n

Flat fading

n

Frequency-selective fading

n

Rain fading

n

Refraction-diffraction fading (k-type fading)

All fading types are strongly dependent on the path length and are estimated as the probability of exceeding a given (calculated) fade margin. A special type of fading is a fading due to the interference and it will be described in more details as well. 3.3.1

Multipath Fading

Under normal propagation conditions, the receive level is subject to only slight fluctuations of a few decibels peak-to-peak, which can be described by the lognormal distribution. These fluctuations practically have no harmful effect on the system performance as long as the fade margin has been chosen sufficiently high. It is well known that the transmission channel between the antennas of the transmitter and the receiver of a microwave system may diverge

100

Chapter Three

from its normal propagation conditions for short periods of time and experience detrimental propagation effects. Various clear-air fading mechanisms caused by extremely refractive layers in the atmosphere must be taken into account in the planning of links of more than a few miles in length: beam spreading (commonly referred to as defocusing), antenna decoupling, surface multipath, and atmospheric multipath. Most of these mechanisms can occur by themselves or in combination with each other. Multipath fading is the dominant fading mechanism for frequencies lower than approximately 10 GHz. A reflected wave causes a phenomenon known as multipath, meaning that the radio signal can travel multiple paths to reach the receiver. Typically, multipath occurs when a reflected wave reaches the receiver at the same time as the direct wave that travels in a straight line from the transmitter. Multipath propagation gives rise to two kinds of signal degrading effects, i.e., flat fading and frequency selective fading. The flat fading effect is due to thermal noise and interference. Certainly, both flat and selective fading typically occur in combination. Two scenarios of multipath are possible: n

If the two signals reach the receiver in phase, then the signal is amplified. This is known as an upfade. Upfades can also occur when the radio wave is trapped within an atmospheric duct. As can be seen from the following formula, higher upfades are possible for longer paths: Upfademax = 10 log d − 0.03d

(dB)

(3.8)

Path length d is in kilometers and, for the 50 km path, maximum upfade can be up to 16.6 dB. n

If the two waves reach the receiver out of phase, they weaken the overall received signal. If the two waves are 180º apart when they reach the receiver, they can completely cancel each other out so that a radio does not receive a signal at all. A location where a signal is canceled out by multipath is called a null or downfade.

Under fading conditions, the direct signal may be attenuated and/ or distortion increased to the point where frequency selective notches result and dispersive fading occurs. Such distortion results in ISI (intersymbol interference) in the demodulator, an increase in data signal BER, and a possible loss of data signal recovery. Smooth surfaces, such as a body of water, a flat stretch of earth, or a metal roof, reflect radio signals. In Figure 3.6, the body of water

Microwave Link Design

101

Superrefractive layer Refracted wave (multipath) Transmitter

Receiver Direct wave

Reflected wave (multipath)

Water surface

Figure 3.6 Multipath fading

reflects a wave that cancels out the direct signal and could bring down the radio link. Multipath fading was also observed on Earth-to-space links for elevation angles below about 5°; however, the phenomenon is more commonly encountered on terrestrial links.2 All ray components on an Earth-space link usually traverse similar vertical refractive conditions. While there may be some signal-level fading or enhancement due to beam spreading or convergence, the similar impact on all components tends to make the multipath phenomenon less prevalent than on terrestrial links where rays traveling along different heights may encounter distinctly different refractive conditions along their entire lengths. Some important facts about multipath fading are as follows: n

n

Multipath fading is normally more active over bodies of water (lakes, sea, and so forth) than over land. It is common practice on over-water paths to use a low-high antenna pair to move any multipath reflections out of the antenna main beam. It is important to avoid ground reflection. Multipath fading is more likely on paths across flat ground than on paths over rough terrain. Horizontal paths give most flat fading.

102

n

n

n

n

n

n

n

n

Chapter Three

Multipath fading is normally most active during early and later summer (late spring and early autumn). Calm weather favors atmospheric stratification, and that gives multipath fading. The fading season is defined as the so-called worst fading month, usually a summer month, or a fade season, usually 2–4 months per year. In radio links of typical length and in temperate climates, multipath activity lasts approximately three months, so the yearly fading season length is one-fourth of that measured in the worst month. When operating in tropical climates with very long hop lengths, multipath activity may last up to six months with intensity comparable to that of the worst month. A rule of thumb is that multipath fading, for radio links having bandwidths less than 40 MHz and path lengths less than approximately 30 km (20 mi), is described as being flat instead of frequency selective. Increasing path inclination reduces the effects of flat fading. Reducing path clearance (i.e., lowering antennas) will reduce the effect of flat fading, because the risk of multipath propagation is decreased; however, this technique may increase the risk for refraction-diffraction fading. On over-water paths, for example, the path inclination might be adjusted to place the surface reflection on a land surface rather than on water, and even better, on a land surface covered by trees or other vegetation. The reflection point moves towards an antenna that is being lowered and away from an antenna that is being raised. On over-water paths at frequencies above about 3 GHz, it is advantageous to choose vertical polarization over horizontal polarization. At grazing angles greater than about 0.7°, a reduction in the surface reflection of 2 to 17 dB can be expected over that at horizontal polarization. Antenna beam tilting effects have been effectively employed to overcome multipath fading induced by surface multipath or superrefractive/ ducting layers in microwave point-to-point (LOS) links. In both cases, upward tilting of the antenna cuts-off or reduces the radio frequency energy refracted and reduces the multipath fading. As a side note, these approaches and experiments helped to design angle diversity schemes in LOS links.

Generally speaking, links should be sited to take advantage of rough terrain in ways that will increase the path inclination (sometimes referred to as the high-low technique). This approach should be

Microwave Link Design

103

conducted jointly with more specific efforts to use shielding from terrain to reduce the levels of surface reflection. Where towers are already in place, antenna height at one end of the path can be reduced to accomplish this, keeping in mind that some minimum clearance rules still have to be satisfied. Experimental evidence indicates that, in clear-air conditions, fading events exceeding 20 dB on adjacent hops in a multihop link are almost completely uncorrelated. This suggests that, for microwave systems with large fade margins, the outage time for a series of hops in tandem is approximately given by the sum of the outage times for the individual hops. Among the indirect consequences of precipitation, deep fades were experienced in winter months during periods of no precipitation at all. It turned out that these were consequences of reflections due to ice-layers on top of the snow. These can reflect millimeter waves, causing similar multipath effects as over the water propagation. As digital radio continues to operate at higher data rates and with more complex modulation, the need for multipath fade testing during installation and routine maintenance increases. Measurement printout is required for comparison to specified performance documentation during radio line up or for comparison between different radios. A flat fading is a reduction in input signal level where all frequencies in the channel of interest are equally affected. Flat fading implies barely noticeable variation of the amplitude of the signal across the channel bandwidth. Flat fading is dependent on path length, frequency, and path inclination. In addition, it is strongly dependent on the geoclimatic factor (temperature/pressure variations), which is the factor that accounts for the refraction properties in the atmosphere, antenna altitudes, and the type of terrain. Deep flat fading is assumed to follow the Rayleigh distribution. If necessary, the flat fade margin of a link can be improved, including using larger antennas, a higher-power microwave transmitter, lowerloss feed line, and splitting a longer path into two shorter hops in several ways.

3.3.1.1 Flat Fading

Frequencyselective fading implies amplitude and group delay distortions across the channel bandwidth produced by the multipath nature of the transmission media.3 It particularly affects medium- and high-capacity radio links (>32 Mbps). The sensitivity of digital radio equipment to frequencyselective fading can be described by the signature curve of the equipment.

3.3.1.2 Frequency-Selective Fading and Dispersive Fade Margin

104

Chapter Three

Notch depth

The signature parameter definitions and specification of how to obtain the signature are given in Recommendation ITU-R F.1093. The basic principle behind the construction of signature curves of equipment is to split the radio path between a transmitter and a receiver into two signals, one direct and the other reflected/refracted. There is a time delay between both signals before they are finally combined in the receiver. In the laboratory, the split is simulated by a time-delayed signal (the reflected/refracted signal). The equipment signature is a measure of the receiver’s capability to suppress the time-delayed signal. The signature is therefore the level of the signal that is necessary to obtain a certain BER (currently referred to as 10−3 and/or 10−6) in the presence of an interfering signal with a predefined delay and it is measured in the laboratory. Measurements are normally performed at the IF stage (typically 70 MHz). The phase difference between the direct and indirect signals causes a notch (dip) at one of the frequency positions inside the spectrum bandwidth. By changing the phase difference between the direct and indirect signals, the notch frequency will change inside the bandwidth. At every notch frequency, the signal is attenuated until the threshold for a specific BER (10−3 and/or 10−6) is exceeded. The final diagram (Figure 3.7) will illustrate the sensitivity (also known as the notch depth and expressed in dB) of the receiver as a function of the notch frequency. The difference between the highest and lowest notch frequency is the signature bandwidth, expressed in megahertz. It is easier to understand and measure the performance of a microwave radio under multipath receptions when considering just one direct signal and one indirect signal having a different amplitude and

−

∆ƒ

Minimum phase notch

2

20 dB B 40 dB ∆ƒ Frequency

Figure 3.7 Microwave radio signature curve

+

∆ƒ 2

Microwave Link Design

105

relative time delay. Since the amplitude of the reflected signal can be lower or higher than the direct signal, a minimum or nonminimum phase group can be obtained. When the amplitude of the direct signal is higher than that of the indirect signal, the notch is called a minimumphase notch. Conversely, when the delayed signal has higher amplitude than the direct signal, the notch is a nonminimum phase notch. Because a receiver can respond differently to these types of notches, it is important to test the radio under both minimum and nonminimum phase conditions. In general, nonminimum is more severe than minimum phase dispersive fading, but under most conditions, the direct signal typically is usually stronger (minimum phase notch). The M-curve or outage signature is a plot of the minimum phase notch depth versus the notch frequency at which the BER of the link starts to exceed a certain threshold. When the notch depth at a given frequency is greater than the value on the curve, the BER is unacceptable. The W-curve is a plot of nonminimum phase notch versus notch depth. In either case, the curve can be used to calculate a dispersive fade margin that is equal to the area enclosed by the curve and the horizontal frequency axis. The M-curve is the most common test of equalizer performance and is useful for a number of reasons: n

n

It can be used to compare different models of radios; the smaller the M-curve, the better the radio can handle multipath. It can be used for troubleshooting problems on the microwave link.

The dispersive fade margin (DFM), a value expressed in decibels, is the measure of a receiver’s ability to resist dispersive fading. Digital microwave radio manufacturers measure and provide the dispersive fade margin from the typical (measured) fading signatures for their digital radio receivers. To measure the DFM of a digital radio receiver, manufacturers simulate multipath fading conditions either in the field or at the factory. W. D. Rummler of Bell Laboratories has developed a simplified threepath model of multipath propagation and has shown that 6.3 ns is approximately the delay time measured on real microwave links in the U.S. The dispersive fading lab simulation will work at either the carrier (RF) frequency or the intermediate frequency (IF). Most manufacturers of multipath simulation systems incorporate IF in their designs because of relative ease of design and better accuracy at lower frequencies.

106

Chapter Three

The dispersive fade margin is defined as the average depth of multipath fade that causes an outage, independent of thermal and interference fade margins, and can be approximated using the following formula:  2( ∆f ) e− B / 3.8  DFM ≈ 17.6 − 10 log    158.4 

(3.9)

where ∆f = signature width of the equipment B = notch depth of the equipment It is important to remember the following facts about selective fading: n

n

If ∆f and B are not available, the user can define DFM explicitly as well. In Europe, signature width is usually given for the microwave radio; in North America, it is commonly provided in the form of dispersive fade margin (the usual values are 45–65 dB). Increasing the output power so as to reduce the outage time for selective fading does not give any improvement. It only increases the flat fading or reduces the thermal noise power received without having any influence on the effects (amplitude and group delay distortions across the channel) of selective fading.

In addition, it is very important to notice that modern digital microwave radios are very robust and practically immune to dispersive (spectrum-distorting) fade activity. Only a major error in path engineering (wrong antenna size or misalignment) over the high-clearance path can cause dispersive fading problems. 3.3.2

Rain Fading

The principal gaseous absorption is by oxygen and water vapor. Oxygen loss is negligible for frequencies up to about 50 GHz. The first and best known effect of rain is that it attenuates the signal. The attenuation is caused by the scattering and absorption of electromagnetic waves by drops of liquid water. The scattering diffuses the signal, while absorption involves the resonance of the waves with individual molecules of water. Water vapor absorption is highly dependent on the frequency as well as the density of the water vapor (absolute humidity, gm/m3). Water vapor absorption can be significant for long paths (>10 km). Loss has a local maximum at 23 GHz and a local minimum at about 31 GHz. Absorption increases the molecular energy, corresponding to a slight increase in temperature, and this results in an equivalent loss of signal energy.

Microwave Link Design

107

The extent of the attenuation due to rain is primarily a function of the form and the size distribution of the raindrops. Rain fading starts increasing noticeably at about 10 GHz and, for frequencies above 15 GHz, rain fading is normally the dominant fading mechanism. Rain events are statistically predictable with reasonable accuracy if short-integration or instantaneous rain measurements are available. Models that are based on measured cumulative distributions of rain events are currently employed in the prediction of the probability that a certain fade margin will be exceeded. The model estimates the time (normally expressed in percentage of a year) during which a given fade depth (fade margin) is exceeded. Next, the result is converted to worstmonth statistic. The concept of worst month for a certain specific value of the worst month is defined as that month with the highest probability of exceeding that specific value. Other forms of precipitation (snow, hail, fog, and haze) do not affect radio-relay links as much as rain events and are considered negligible. Attenuation is negligible for snow or ice crystals, in which the molecules are tightly bound and do not interact with the waves. For example, at 23 GHz, a 5-mi-long microwave link will have additional attenuation due to a very dense fog of only about 0.7 dB. Snow covering antennas and radomes, the so-called ice coating, can result in different types of problems, such as increased attenuation and deformation of the antenna’s radiation diagram. The rain rate enters into this equation because it is a measure of the average size of the raindrops. When the rain rate increases (i.e., it rains harder), the raindrops are larger, and thus there is more attenuation. Rain models differ principally in the way the effective path length L is calculated. Two authoritative rain models that are widely used are the Crane model and the ITU-R P.530-xx model, but there are a number of other models developed specifically for a certain region and/or application. Heavy rainfall, usually in cells accompanying thunderstorm activity and weather fronts, has a great impact on path availability above 10 GHz. Rain outage increases dramatically with frequency and then with path length. Fading due to rain attenuation is described empirically from link tests and point rainfall data. Location variation is based on selected point rainfall data and radar reflectivity data accumulated around the world. Ten- to fifteen-minute duration fades to over 50 dB have been recorded on an 18-GHz, 5-km (3-mi) path, for example, and increased outage at 23 GHz can require a 2-to-1 reduction in path length compared to 18 GHz for a given availability. Much is known about the qualitative aspects, but the problems faced by the microwave transmission engineer remain formidable. To estimate probability distribution, instantaneous rainfall data is needed.

108

Chapter Three

Unfortunately, the available rainfall data is usually in the form of a statistical description of the amount of rain that falls at a given measurement point over various time periods. Important fact is the total annual rainfall in an area has little relation to the rain attenuation for the area. In some cases, the greatest annual rainfalls are produced by long periods of steady rain of relatively low intensity at any given time. Other areas of the country, with lower annual rates, experience thunderstorms and frontal squalls, which produce short-duration rain rates of extreme intensity. The incidence of rainstorms of this type determines the rain rates for an area, and thus the high-frequency microwave link’s long-term path outage time and “unavailability.” Even the rain statistics for a day or an hour have little relationship to rain attenuation. A day with only a fraction of an inch/centimeter of total rainfall may have a path outage due to a short period of concentrated, extremely high-intensity rain. Another day with several inches/centimeters of total rainfall may experience little or no path attenuation, because the rain is spread over a long time period or large area. The predicted annual outage may not occur for years and then accumulate over a single rainy season for a long-term average. The worst rain outages occur during the heaviest thunderstorms. The gulf coast area from Florida to New Orleans has the most severe thunderstorms in the U.S. As a result, rain outages in microwave systems are most severe in the southeastern U.S. Microwave path lengths must be reduced in these areas to maintain the path availability. Cities such as Seattle, Washington and Vancouver, Canada also receive a large amount of rain. However, there are few thunderstorms, so rain outage is less severe, and longer path lengths are feasible. In the design of any engineering system, it is impossible to guarantee the performance under every conceivable condition. Engineers set reasonable limits based on the conditions that are expected to occur at a given level of probability. For example, a bridge is designed to withstand loads and stresses that are expected to occur in normal operation and to withstand the forces of wind and ground movement that are most likely to be encountered. However, even the best bridge design cannot compensate for a very powerful tornado or an earthquake of unusual strength. Similarly, in the design of a microwave communications link, microwave engineers include a margin to compensate for the effects of rain at a given level of availability. 3.3.3

Refraction-Diffraction Fading

In the real world, the k-factor varies with time and location in accordance with complex physical interactions involving the refractivity gradient

Microwave Link Design

109

(dn/dh) in the lowest part of the atmosphere and other mechanisms as detailed in the propagation “P series” of ITU Recommendations. An important objective in planning terrestrial microwave link systems is to ensure that outages resulting from these variations are extremely rare events; thus, system fade margins, linked to error performance and availability objectives, of the appropriate order are implemented to ensure that this is so. Accordingly, to take account of the statistical nature of radio wave propagation, the application of appropriate propagation prediction models is necessary. Refraction-diffraction fading, also known as k-type fading, is characterized by seasonal and daily variations in the Earth-radius factor k. For low k values, the Earth’s surface becomes more curved, and terrain irregularities, manmade structures, and other objects may intercept the Fresnel zones. The probability of refraction-diffraction fading is therefore indirectly connected to obstruction attenuation for a given value of Earth-radius factor. Since the Earth-radius factor is not constant, the probability of refraction-diffraction fading is calculated based on cumulative distributions of the Earth-radius factor. For high k values, the Earth’s surface gets close to a plane surface, and better LOS (lower antenna heights) is obtained. 3.3.4 Interference Fade Margins and Interference Analysis 3.3.4.1 Interference Fade Margins Interference in microwave systems is caused by the presence of an undesired signal in a receiver. To accurately predict the performance of a digital radio path, the effect of interference must be considered. When this undesired signal exceeds certain limiting values, the quality of the desired received signal is affected. To maintain reliable service, the ratio of the desired received signal to the (undesired) interfering signal should always be larger than the threshold value. Interference into a digital radio will degrade the receiver threshold and result in a lower effective fade margin, thus producing excessive bit errors or frame losses as the radio fades near threshold. In normal, nonfaded conditions, the digital signal can tolerate high levels of interference; however, to protect short-term performance and hop reliability, it is critical to control interference in deep fades. Adjacent-channel interference fade margin (AIFM) (in decibels) accounts for receiver threshold degradation due to interference from adjacent channel transmitters in one’s own system. AIFM is applicable only to frequency diversity and multiline (N+1) protection systems and is not used as long as the minimum allowable frequency separations for

110

Chapter Three

a particular radio are maintained. These minimum frequency separations are included in the manufacturer’s radio specifications. Interference fade margin (IFM) is the depth of fade to the point at which RF interference degrades the BER to 1 × 10−3. It is affected by the frequency congestion, directivity of the interfering and victim system antennas, and so on. The actual IFM value used in a path calculation depends on the method of frequency coordination being used.4 There are two widely used methods: the carrier-to-interference (C/I) and threshold-tointerference (T/I) methods. The C/I method is an older one, developed originally to analyze interference cases into analog radios. Frequencies are selected such that the calculated carrier-to-interference (C/I) ratio for all interfering transmitters is less than some objective value. The objectives are listed in the National Spectrum Manager’s Association (NSMA) interference objective tables. It should be noted that, although often used interchangeably, signalto-noise (S/N) and carrier-over-interference (C/I) are not the same thing. Signal-to-noise refers to the difference between the received signal level (RSL) and the thermal noise floor of the receiver. Noise is random in nature (Gaussian) and does not contain intelligence. On the other hand, because of its frequency relationship and coherent characteristics, an interfering signal is capable of causing destructive results, even at levels below the thermal noise threshold of a receiver. Whenever the receiver’s demodulator acquires and locks on to a signal within its tracking bandwidth, an interfering signal within all or part of the receiver’s bandwidth—which would normally be hidden in the noise—becomes a factor. When the difference in magnitude between the two coherent signals becomes small enough, the resulting inter-symbol interference confuses the demodulator, rendering it unable to track the desired signal. If this condition is short and sporadic the receiver will usually ask the transmitter to retransmit, causing an overall slow-down in data throughput. But, if the condition is more serious, the receiver will become unlocked and the link will go down altogether.

3.3.4.2 Carrier-to-Interference (C/I) Method

In the new T/I method, threshold-to-interference (T/I) curves are used to define a curve of maximum interfering power levels for various frequency separations between interfering transmitter and victim receivers as follows:

3.3.4.3 Threshold-to-Interference (T/I) Method

I = T − (T / I)

(3.10)

Microwave Link Design

111

where I = maximum interfering power level (dBm) T = radio threshold for a 10−6 BER (dBm) T/I = threshold-to-interference value (dB) from the T/I curve for the particular radio Threshold to interference ratio (T/I) is designated as the ratio between static threshold point of carrier level, which can fulfill certain link fidelity, for example, 10−6 BER, and the interference level that would cause 1-dB degradation to the threshold of the protected receiver. This means that for a given BER, the difference between the increased threshold level value due to interference, and the threshold value without interference, is the threshold degradation (TD). TD is assumed to be equivalent to the noise level increase, due to the interfering signal at the input of the receiver. The permissible threshold degradation caused to one fixed-link receiver by one foreign fixed-link transmitter must not exceed 1 dB. The calculation of TD is a two-step process; first, the interfering power level (I) at the input of the victim receiver must be calculated, using link budget calculations and including the antenna discrimination in the direction of the interfering transmitter. Then, the TD due to this interfering signal is calculated and compared to the 1 dB permissible value. The T/I curves of the radio are based on the actual manufacturer’s lab measurements of the radio; T/I can be obtained in the following way: 1. Adjust carrier to a threshold level (T) to achieve required BER, usually 10−6. 2. Increase this carrier level by 1 dB. 3. Inject interference increasingly until the required BER is recovered, record the interference level (I) at this time. 4. Draw the ratio of T/I. The ratio of the initial power level of the desired received signal to the interference power is the T/I ratio. Note that this value will be different for different interferers, especially if the interfering signal is offset in frequency from and/or has a wider spectrum than the victim receiver’s bandwidth. The value of T/I is roughly 6 dB greater than the theoretical threshold value of carrier to noise ratio (C/N) if the interferer produces a thermalnoise-like interference with a bandwidth less than or equal to that of the desired signal. For each interfering transmitter, the receive power level in dBm is compared to the maximum power level to determine whether the interference

112

Chapter Three

is acceptable. This is done instead of calculating C/I values and comparing them to the published objectives. To remain the availability of the affected receiver acceptable despite the interference, for the range of carrier power levels between the clearair (unfaded) value and the fully faded static threshold value, in no case shall interference cause C/I to be less than T/I, unless it can be shown that the availability would still be acceptable under the interference. 3.3.5

Composite Fade Margin

Composite fade margin (CFM) is the fade margin applied to multipath fade outage equations for a digital radio link. The complete expression for describing the CFM for a digital microwave radio is given by CFM = TFM + DFM + IFM + AIFM CFM = −10 log (10 − TFM /10 + 10 − DFM /10 + 10 − IFM /10 + 10 − AIFM /10 )

(3.11)

where TFM = thermal (flat) fade margin, the difference between the normal (unfaded) RSL and the BER = 1 × 10−3 DS1 loss-of-frame point DFM = dispersive fade margin, provided by the radio manufacturer from measurements. It is affected by the complexity of the digital modulation scheme and the types and effectiveness of the adaptive amplitude and/or baseband time domain equalization (if any) used. IFM = interference fade margin. Receiver threshold degradation due to co-channel interference. AIFM = adjacent-channel interference fade margin. This is usually a negligible parameter except in frequency diversity and N + 1 multiline systems. These four fade margins are power added to derive the CFM. The longer the link, the more critical these factors become, since the system gain and composite fade margin determine the range and the reliability performance of a radio under various fading conditions. Often, only the dominant terms (namely, the flat and the dispersive fade margin) are included. 3.3.6

Outages and Availability

A major concern for microwave system users is how often and for how long a system might be out of service. Various statistical models and analysis methods have been developed to predict and measure the outage

Microwave Link Design

113

and availability over a period of time. Performance prediction (related to propagation effects) principally depends on the assessment of two main propagation mechanisms: multipath fading and/or attenuation due to rain. Multipath fading typically gives rise to short-term outages and thus has the most impact on error performance. For modeling multipath fading, propagation prediction methods have been derived that estimate the probability of single-frequency fading. Rain attenuation events typically give rise to long-term outages (and therefore to unavailability) with the durations greater than 10 secs, therefore directly influencing availability of systems operating in the bands above about 10 GHz. The prediction of rain outages is possible through the application of rainfall intensity statistics to modeling methods for rain attenuation. The basis for the dimensioning of the links in a network is usually defined by the operational user requirement, which describes the required availability of a connection and the quality required during the available time. A dimensioning standard developed by ITU is often used in order to obtain an internationally accepted availability and quality for parts of or the entire planned network. ES (errored second) is defined as a second containing one or more bit errors SES (severely errored second) is defined as a 1-sec period during which the BER is worse than 1 × 10−3 An outage in a digital microwave link occurs with a loss of DS1 frame sync (OOF) for more than 10 secs. DS1 frame loss typically occurs when the BER increases beyond 1 × 10−3. Outage (Unavailability) =

SES × 100 [%] t

(3.12)

where t = time period (expressed in seconds) SES = severely errored second, a state defined as any 1-sec period containing a BER of 1 × 10−3 or greater, often accompanied by an out-of-frame DS signal, but with no service disruption Availability is expressed as a percentage as follows: A = 100 − Outage (Unavailability)

(3.13)

Important note: A digital link is unavailable for service or perfor−3 mance prediction/verification after a ten consecutive BER > 1 × 10

114

Chapter Three

SES outage period. This means that outages shorter than 10 secs are not considered unavailability! A distinction should be made between those circuits for which an outage of a few seconds or a few minutes is just an inconvenience, for example, spur cell site in the wireless network. On the other hand, there are circuits for which such an outage might result in a danger to life, great economic loss (important and/or hub sites in wireless network) or other catastrophic consequence. Objectives and/or requirements for different types and applications of transmission networks could be very different. 3.4 Microwave Link Multipath Probability Models 3.4.1 Vigants North American Multipath Probability Model

The Vigants model, used mostly in North America, is also widely used in ITU-R regions and can be found in computer programs as CCIR Rep.338 using KQ geoclimatic factors. This model was created for fade depths of 15 dB or more, i.e., deep fades. The average probability of multipath fading in Vigants North American model (1975) over a long period of time (for example, one year) due to fading is given by P = 2.5 × 10 −6 c f d3 10 − CFM / 10

(3.14)

where P = one-way probability of fading due to all multipath fade activity during the fade season (expressed as a fraction of time, not as a percentage) f = frequency (GHz) d = path length (mi) CFM = composite fade margin (dB) c = climate/terrain factor (4 over water and humid climate, 1 for average terrain and climate, 0.25 for mountains and dry climate) or  50  c = a   w

1.3

(3.15)

Microwave Link Design

115

where a = 2 for humid climate, 1 for average climate, 0.5 for dry climate w = average terrain roughness (over a range of 20 to 140 ft, 50 ft being “normal”) extracted from the path profile The tilting terrain of a high/low path computes to w > 140 ft for c = 0.52 maximum in humid areas using this method. This alternate calculation is tied to the path profile, and therefore is more accurate than using the c factor. Multipath fade probability is highly dependent on the terrain roughness factor, and mountainous paths exhibit least multipath. The probability of a fade of a particular depth increases with the cube of distance. Path length, d, is raised to the power of 3 and hence has a significant effect on the multipath fade probability and, in turn, on the performance objectives. Thus, as the distance is doubled, the probability of a particular fade depth increases by a factor of eight. Or, alternatively, the fade for a given probability increases by 9 dB. So, doubling the distance will increase the free-space loss by 6 dB and increase the probability of fading by 9 dB, thus increasing the overall link-budget loss by 15 dB. Dividing a long path into two identical, shorter paths (and using active repeater), the probability of multipath fading is reduced by factor of four: P ~ d3 , so (d / 2)3 + ( d / 2)3 = d3 / 4. Outage time (or SES) in seconds per year (one-way) can be calculated as follows: Outage (or SES) = T0 ⋅ P [secs /yr]

(3.16)

and fade duration, T0, is expressed as a fraction of a year is given by T0 = 8 × 106 ⋅

t [secs/yr] 50

(3.17)

where t = average annual temperature in degrees Fahrenheit (35°F ≤ t ≤ 75°F) It is important to note that all the path reliability formulas and models, in this text, calculate short-term one-way outage. This reliability method calculates the probability of the receive signal fading below its threshold level due to multipath fading only. The duration of the fade below the threshold level is not considered, and the resultant

116

Chapter Three

outage time is the total time below level. The total time below level can be further broken down into two parts: n n

Fades below threshold that last 10 consecutive secs or longer Fades below threshold that last less than 10 consecutive secs (all other fades)

The ITU terminology for this breakdown is unavailability for fades below threshold lasting 10 consecutive secs or longer and severely errored seconds for all other fades (outages). Multipath fading is a warm-weather phenomenon. This calculation 6 assumes an average three months (8 × 10 sec) fade season, although in reality the fade season is proportional to average annual temperature in the area and can vary between 2.1 and 4.6 months. The most probable multipath times are sunset, around midnight, and after sunrise, during clear days. The question here is whether the result of the link design is acceptable or not, and the answer will probably be different in different situations. The arbitrary requirement (objective) of “five nines” (99.999 percent of the time) may be applied in some situations or some other end-user design objective could be applied as well. Although computed over the fade season (2–4 months), the calculated outage is considered an annual outage and could be compared, for example, to the Bell short-haul (<400 km/250 mi) or Bell long-haul (6,400 km/4,000 mi) design objective. The recommended short-term one-way outage objective for a T1/E1 trunk, regardless of system length, is 1,600 outage secs per year (SES/yr), for a 99.995 percent end-to-end propagation reliability. In North America, using this old Bellcore objective (<250 mi end-to-end) links scales down to 6.4 SES/mi/yr for short-haul and to 0.8 SES/mi/yr for the long-haul links (>250 mi end-to-end). A relaxed 99.999 percent per-hop reliability (320 SES/yr outage) objective (floor) is often assigned to spur links and on short systems of less than about 10 tandem hops. For high reliability links (usually in long-haul systems with many hops in tandem), the per-hop objective may approach or exceed 99.9999 percent, allowing only 20–30 secs of per-hop outage per year. It is important to keep in mind that the propagation outages due to multipath fading are usually very short. A cumulative outage of an hour per year due to multipath might represent thousands of individual outages, each averaging 1 sec or less (1 SES) on a properly engineered path. On the other hand, propagation outages totaling an hour per hop due to rain attenuation might consist of only four or five individual outages averaging ten to fifteen minutes each.

Microwave Link Design

117

The effects of long-term and short-term system outage on E1/T1 circuits are very different. The many short-term outage events do not disconnect circuits nor reduce (in most circuits) data throughout. The few long-term events cause both traffic disconnect and loss of data throughput. Seasonal, terrain, and geoclimatic changes average out on longer systems thus permitting end-to-end guaranteed compliance to a user’s performance requirement. But on a per-hop basis, many links will perform much better (a few somewhat worse) than performance and availability objectives calculated here. Example: Let’s find the probability of multipath fading outage (SES) for a microwave path 30-mi long, at 6 GHz, average terrain roughness of 50 ft, climate-terrain factor c = 1, and t = 40°F, composite fade margin CFM = 36 dB (equal numerically to thermal fade margin).

P = 2.5 × 10 −6 c f d3 10 − CFM / 10 P = 2.5 ⋅ 10 −6 ⋅ 1 ⋅ 6 ⋅ 303 ⋅ 10 −36 /10 P = 1.02 ⋅ 10 −4 (or 0.000102 as a decim mal number) Outage = 1.02 ⋅ 10 −4 ⋅ (8 ⋅ 106 ) ⋅

40 50

Outage ≈ 653 SES/yr (99.998% outage probability) Objective for our link, based on Bellcore short-haul objectives, can be calculated as: Objective = 6.4 SES/mi/yr × 30 mi Objective = 192 SES/yr (99.9994 % outage probability y) Comparing the result of the link design (653 SES/yr) to this objective (192 SES/yr), it becomes obvious that the microwave engineer will have to apply some kind of diversity improvement technique to reduce the probability of outage due to multipath fading. 3.4.2

ITU-R Multipath Probability Model

A method for predicting the single-frequency (or narrowband) fading distribution suitable for large fade depths A in the average worst month

118

Chapter Three

in any part of the world (based on ITU-R p.530-12) 5 and for detailed link design is given as follows: P0 = Kd3.2 (1+|ε p|) −0.97 × 100.032 f −0.00085 hL − A /10 [%]

(3.18)

where f = frequency (GHz) hL = altitude of the lower antenna (i.e., the smaller of he and hr) (m) A = fade depth (dB) and the geoclimatic factor K is obtained from the following equation (if measured data for K are not available): K = 10 −3.9−0.003 dN1 ⋅ sa−0.42

(3.19)

where dN1 = point refractivity gradient in the lowest 65 m of the atmosphere not exceeded for 1 percent of an average year sa = area terrain roughness The term dN1 is provided on a 1.5° grid in latitude and longitude in ITU-R Recommendation P.453. The correct value for the latitude and longitude at path center should be obtained from the values for the four closest grid points by bilinear interpolation. The data are available in a tabular format and are available from the Radiocommunications Bureau (BR). Term sa is defined as the standard deviation of terrain heights (m) within a 110 × 110 km area with a 30s resolution (e.g., the Globe GTopo30 data). The area should be aligned with the longitude such that the two equal halves of the area are on each side of the longitude that goes through the path center. Terrain data are available from the Internet, and the Web address is provided by the BR. If a quick calculation of K is required for planning applications, a fairly accurate estimate can be obtained from: K = 10 −4.2−0.0029 dN1

(3.20)

For example, a typical value for K over much of Northern Europe is 2.4 · 10−4 (dN1 = −200 N-units/km) and as high as 1.4 · 10−3 (dN1 = −464 N-units/km) in areas around the Mediterranean Sea.

Microwave Link Design

119

From the antenna heights he and hr (meters above sea level), calculate the magnitude of the path inclination |ε p| (mrad) using the following expression: |ε p| =

| hr − he | [ mrad] d

(3.21)

where d = the path length (km) he, hr = antenna heights ASL (m) The method just shown is used for small percentages of time, does not make use of the path profile, and can be used for initial planning, licensing, or design purposes. The general rule is that, the smoother the path, the more likely the occurrence of multipath fading. A second method, used for all percentages of time, is suitable for all fade depths and employs the method for large fade depths and an interpolation procedure for small fade depths. The method used for predicting the percentage of time that any fade depth is exceeded combines the deep fading distribution and an empirical interpolation procedure for shallow fading down to 0 dB. A detailed description of a method for all percentages of time is beyond the scope of this book but can be found in the latest revision of the ITU-R Recommendation P.530-xx. Example: Let’s assume that we are designing a 7 GHz link in Northern Europe (or any other region with dN1 = −200 N-units/km, based on ITU maps) and the link length is 40 km. Antenna heights above sea level are 200 and 350 m. We have to find the required fade margin if the reliability requirement of the link is 99.999 percent.

First, with the help of basic trigonometry, we will calculate path inclination: |ε p| =

| hr − he | 150 m = = 3.75 mrad d 40 km

The percentage of time that fade depth A (dB) is exceeded in the average worst month is calculated as: P0 = Kd3.2 (1+|ε p|) −0.97 × 100.032 f −0.00085 hL − A /10 [%] P0 = 2.4 ⋅ 10 −4 ⋅ 403.2 (1 + 3.75) −0.97 × 100.0032⋅7−0.00085⋅200− A /10 [%] P0 = 7.1 ⋅ 100.054 − A /10 [%]

120

Chapter Three

Since reliability is 99.999 percent (the percentage of time during which the fade depth A is not exceeded), P0 = 0.001 percent (the percentage of time during which the fade depth A is exceeded), and solving the exponential equation for A, we get: A = 0.54 − 10 log

P0 0.001 = 0.54 − 10 log 7.1 7.1

A ≈ 39 dB So, the minimum fade margin required to combat multipath on this link would be, in this case, 39 dB. The next step would be to compare this result with the link budget calculation and adjust other (hardware) parameters accordingly. Diversity technique might be required in order to achieve required objectives. 3.5

Quality and Availability Calculations

The main purpose of the quality and availability calculations is to set up reasonable objectives for the microwave path. The entire procedure can be structured in five general steps (see Figure 3.8). Step 1: An appropriate design network model is selected. Step 2: Quality and availability objectives for the corresponding portions and sections of the network model are selected. Step 3: In step 3, the quality and availability parameters are calculated. Step 4: Comparison of the calculations results from step 3 with the objectives from the step 2. Step 5: If the objectives are not met, appropriate network parameters (antenna size, antenna height, output power, channel arrangements, polarization, and so on) are changed, and the quality and availability parameters are recalculated as illustrated in step 3. The procedure is continued in step 4 as an iterative process. For predicting the quality of a microwave radio link, the following fading mechanisms are usually considered: n

Flat fading due to multipath propagation

n

Selective fading due to multipath propagation

n

Fading due to rain

n

Refraction-diffraction fading in the atmosphere, also known as k-type fading

Microwave Link Design

1

Transmission network model selection

2

Allocation of quality and availability objectives

3

Calculation of network quality and availability parameters

Change network parameters

121

5

NO

4

Comparison of calculations results with objectives

Results OK?

YES

To the next step, usually interference analysis.

Figure 3.8 Quality and availability objectives

Multipath fading (flat and selective) is assumed to cause fast-fading events, affecting quality of the link. Rain and refraction-diffraction fading are assumed to cause relatively slow-fading events, contributing to unavailable time. After completion of the quality and availability calculations, the next step is usually interference analysis. 3.5.1

ITU-T Recommendations

3.5.1.1 G-Series Recommendations In digital transmission technology, any bit received in error (a bit error) may deteriorate transmission quality. It is obvious that quality will decrease with an increasing number of erroneous bits. Therefore, the ratio of the number of errored bits referred to the total number of bits transmitted in a given time interval is a quantity that can be used to describe digital transmission performance. The quantity called bit error ratio (BER) is a well-known and commonly used error performance parameter.6 Bit error ratio can be measured only if the bit structure of the evaluated sequence is known. For this reason, BER measurements are mostly

122

Chapter Three

performed out of service using a well-defined pseudorandom bit sequence (PRBS). In practice, the PRBS replaces the information sent in service. Quality parameters defined by ITU-T Recommendation G.821 and applied by ITU-R recommendations are BER based. The quality parameters are errored second ratio (ESR) and severely errored second ratio (SESR). The availability parameter is available time ratio (ATR) or unavailable time ratio (UATR). One of the prime objectives of G.826 was to define all performance parameters in such a way that in-service estimation is possible. Inservice detection of errors in digital transmission is possible, however, using special error detection mechanisms (error detection code, EDC) which are inherent to certain transmission systems. Examples of those inherent EDCs are cyclic redundancy check (CRC), parity check, and observation of bit interleaved parity (BIP). Error detection is capable of detecting whether one or more errors have occurred in a given sequence of bits—the block. It normally is not possible to determine the exact number of errored bits within the block. The basic philosophy of G.826 is based on the measurement of errored blocks, thus making in-service error estimation possible. Block errors are processed in a similar way as bit errors, i.e., the term block error ratio is defined as the ratio of the number of errored blocks referred to the total number of blocks transmitted in a given time interval. The quality parameters are errored second ratio (ESR), severely errored second ratio (SESR), and background block error ratio (BBER). The availability parameter is available time ratio (ATR) or unavailable time ratio (UATR). Some useful error performance definitions are as follows: n

n

n

n

n

n

Block (generic definition) A block is a set of consecutive bits associated with the path; each bit belongs to one and only one block. Errored block (EB) A block in which one or more bits are in error. Errored second (ES) A 1-sec period with one or more errored blocks or at least one defect. Errored second ratio (ESR) The ratio of ES to total seconds in available time during a fixed measurement interval. Severely errored second (SES) A 1-sec period that contains over 30 percent errored blocks or at least one defect. SES is a subset of ES. Severely errored second ratio (SESR) The ratio of SES to total seconds in available time during a fixed measurement interval.

Microwave Link Design

n

n

Background block error (BBE) as part of an SES.

123

An errored block not occurring

Background block error ratio (BBER) The ratio of background block errors (BBE) to total blocks in available time during a fixed measurement interval. The count of total blocks excludes all blocks during SESes and any unavailable time.

Consecutive severely errored seconds (CSES) may be precursors to periods of unavailability, especially when there are no restoration/ protection procedures in use. Periods of CSES persisting for T seconds (2–10) (some network operators refer to these events as failures) can have a severe impact on service, such as the disconnection of switched services. Error performance should be evaluated only during the time the path is in the available state. Measurement of BER and BLER (block error rate) yields comparable results for small BERs, and, for some specific error models, it is possible to calculate BER from a BLER. It is the drawback of this procedure that error models describe the situation found in practice only imperfectly and may be strongly media dependent. Therefore, the result of such a calculation is not very reliable. A hypothetical reference path (HRP) as defined by ITU-T Recommendation G.826 is the whole means of digital transmission of a digital signal of a specified rate, including the path overhead (where it exists) between equipment at which the signal originates and terminates. An end-to-end HRP spans a distance of 27,500 km. The portion of interest is usually the national portion of the HRP subdivided in three classes: access, short-haul, and long-haul (see Figure 3.9). For the purposes of G.826, the boundary between the national and international portions is defined to be at an international gateway (IG), which usually corresponds to a cross-connect, a higher-order multiplexer, or a switch (N-ISDN or B-ISDN). IGs are always terrestrially-based equipment, physically resident in the terminating (or intermediate) country.

Link end

Access

Local exchange

Short haul

Secondary center

National portion Figure 3.9 Hypothetical reference path—national portion

Long haul

International gateway

124

Chapter Three

A large number of devices (test equipment, transmission systems, collecting devices, operating systems, software applications) are currently designed to estimate the G.821 or M.2100 parameters ESR and SESR at bit rates up to the fourth level of the PDH.7 For such devices, the G.826 parameters ESR and SESR may be approximated using the G.821 criteria, but an approximation of BBER is not possible, since the block-based concept and the BBER parameter are not defined in Recommendation G.821. ITU-T Recommendation G.826 applies to both Plesiochronous Digital Hierarchy (PDH) and Synchronous Digital Hierarchy (SDH) systems, while ITU-T R Recommendation. G.828 only applies to Synchronous Digital Hierarchy (SDH) systems. The performance requirements in ITU-T Recommendation G.828 are more stringent than those in ITU-T Recommendation G.826 and compliance with ITU-T Recommendation G.828 will, in most cases, also ensure compliance with ITU-T Recommendation G.821 and ITU-T Recommendation G.826. It should be noted that ITU-T Recommendation G.828 only applies to equipment designed after March 10, 2000 (the date ITU-T Recommendation G.828 was adopted by the ITU). Performance objectives for paths using equipment designed prior to this date are given in ITU-T Recommendation G.826. This is the partial list of some of the most important, relatively new ITU-T recommendations regarding IP-based networks. New versions and addendums are published all the time, so readers should try to acquire the latest revision of the document as well as all the important and most recent additions.

3.5.1.2 Y-Series Recommendations

n

n

ITU-T Recommendation Y.1541 “Network Performance Objectives for IP-Based Services,” (2002) Defines six network Quality-of-Services (QoS) and specifies provisional objectives for Internet Protocol network performance parameters. These classes are intended to be the basis for agreements among network providers and between end users and their network providers. ITU-T Recommendation Y.1540 “IP Data Communication Service – IP Packet Transfer and Availability Performance Parameters,” (2002) Defines parameters that may be used in specifying and assessing the speed, accuracy, dependability, and availability of IP packet transfer of international IP data communication services. Connectionless transport is a distinguishing aspect of the IP service that is considered in this recommendation. The defined parameters apply to end-to-end, point-to-point IP service and to the network portions that provide or contribute to the provision of such service.

Microwave Link Design

n

125

ITU-T Recommendation Y.1561 “Performance and Availability Parameters for MPLS Networks,” (2004) Defines parameters that may be used in specifying and assessing the performance of speed, accuracy dependability, and availability of packet transfer over a label switched path (LSP) on a multiprotocol label switching (MPLS) network. The defined parameters apply to end-to-end, point-to-point LSP and to any MPLS domain that provides or contributes to the provision of packet transfer service.

Today, more attention is being placed not just on availability but also performance of the link during the available time. ITU-T Recommendation M.2301, “Performance Objectives and Procedures for Provisioning and Maintenance of IP-based Networks,” (2002), focuses attention on parameters that significantly affect the quality of service perceived by the customer and the methods of measuring those parameters, including those parameters that affect delay performance at the application layer. The performance of fixed access links, whose routing does not change, is covered, while performance limits for temporary dial-up access links, end-customer owned portions, and MPLS networks are not covered by this recommendation and are for further study. 3.5.2

Quality and Unavailability Objectives

3.5.2.1 ITU Objectives The quality and unavailability objectives for all portions in the hypothetical reference path (HRP) have to be accomplished concurrently. These objectives shall account for effects caused by fading, interference, and other sources of performance degradation. Block allocation is used in the access and short-haul portions, while in the long-haul portion it is a combination of block allocation and lengthrelated allocation. Rain fading and diffraction-refraction fading (k-type fading) give unavailability, whereas multipath (flat and frequency selective) fading gives ESR, SESR, and BBER. Unavailability is the dominating dimensioning factor for frequencies above 15 GHz, whereas quality is the dominating and dimensioning factor for frequencies below 10 GHz (8 GHz in some countries). There is, however, a frequency range between 10 and 15 GHz where both quality and availability might be comparable, and all the mechanisms have to be considered. Unavailability due to hardware failure is, obviously, not path-length related, but unavailability due to radio wave propagation (rain and refraction-diffraction fading) can be strongly length dependent. The ITU-T recommendations G.801, G.821, and G.826 define error performance and availability objectives. The objectives for digital links

126

Chapter Three

are divided into separate grades: high, medium, and local grade. The medium grade has four quality classifications. The following grades are usually used in wireless networks: n

Medium grade Class 3 for the access network

n

High grade for the backbone network

ITU-R recommends objectives for the fixed wireless system availability and performance determination as defined in F.1703 (replaced ITU-R F.1493) and F.1668 (replaced ITU-R F.1491): n

n

ITU-R F.1668-1 (2007) defines “Error performance objectives for real digital fixed wireless links used in 27,500 km hypothetical reference paths and connections.” ITU-R F.1703 (2005) defines “Availability objectives for real digital fixed wireless links used in 27,500 km hypothetical reference paths and connections.”

Availability objectives should be partitioned in order to take into account unavailability events due to propagation issues, equipment failure, human intervention, and other causes. The partitioning of objectives for the different unavailability causes is outside the scope of the ITU recommendations and it is left to the local authorities to define. 3.5.2.2 North American Objectives Transmission network requirements for the reliability (yearly SES) in the microwave networks in North America run from 30 secs (99.9999 percent) for high-reliability links to 26 min (99.995 percent) for single MW hops carrying less-important traffic (or less traffic). Old Bellcore Short-Hop, one-way, quality objectives: n

n

DS1 circuits n

25 mi hop EFS = 99.996% (99.986% requirement)

n

RBER per hop = 2 × 10

−11

(10−10 requirement)

DS3 n

25 mi hop EFS = 99.96% (99.90% requirement)

n

RBER per hop = 8 × 10

−11

Sometimes, these numbers are different and could be arbitrarily proposed by the customer himself; however, it is important to keep in mind that, by definition, higher objective numbers lead to a more expensive MW network. It is important to keep in mind not to over-dimension the network using unnecessarily high design objectives.

Microwave Link Design

3.6

127

Rain Attenuation and Outage Models

The most-used method today for calculating rain attenuation was developed by the International Telecommunications Union Radio Section (ITU-R). However, there are other methods, such as the Crane method (used mainly in the United States) and methods more suitable for specific radio climates, such as wet tropical and equatorial areas. There are differences between the two most popular models, the ITU terrestrial and Crane models, which produce slightly different estimates of the long-term fade probability. Variability is inherent in the estimation of path fading because empirical equations have been derived from measurement points around the world over limited periods of time. Rain attenuation, which is the dominant fading mechanism for millimeter wave paths, is based on nature, which can vary from location to location and from year to year. An underestimate of the required margin to compensate for a given probability of rain outage results in a system that does not meet link availability requirements. It is obvious that the uncertainty of either model or, alternatively, the short-term expectation of fade, is quite large. The uncertainty, as measured by the estimated attenuation standard deviation, is greater than 30 percent and tends to overshadow arguments about the accuracy of the methods. The uncertainty is a result of variations from year to year and location to location. Location to location within a rain zone, we also find a high estimated attenuation standard deviation. In addition, worst-month predictions forecast much higher fade depths than the basic annual predictions. Microwave link design is sometimes a balancing act between contradicting requirements. If availability guarantees are accepted in system contracts, cost incentives may have to be paid for under designing a system and not meeting those guarantees. On the other hand, overestimations of the required margin may result in the overdesign of systems, which results in unnecessary system costs. The Crane models (after Robert K. Crane) are popular for spaceEarth links but also have terrestrial models. There are three versions of the Crane models. The global Crane model was developed in 1980. In 1982, the two-component Crane model was developed, which used a path-integrated technique. A volume cell contribution and a debris contribution for a path were computed separately and added to provide a link calculation. As a refinement of the two-component model, the revised two-component model was introduced in 1989, which includes spatial correlation and statistical variations of rain within a cell. All these models are described in Crane’s book.8 A rain event consists of small “volume cells” of intense rain rate within much larger “debris regions” with a lower rain rate. The dimensions of

128

Chapter Three

these areas are inversely related to rain rate. Volume cells are quite small, generally less than 5.0 km2, with the average volume cell area of about 3.0 km2 over the range of rain rates 20 to 40 mm/hr, which is often of interest as design targets for microwave systems. This could be represented as a rectangle of about 1.6 × 3 km in size (rain cells are usually not circular). Calculation by the ITU model is straightforward by scaling the 0.01 percent rain rate and by using an effective path-length reduction factor to account for the cellular nature of heavy rainfall. Mean cumulative distributions of rainfall zones are defined geographically in ITU-R Recommendation P.837.9 The latest revision is P.837-5 (2007), with the new maps generated using the ECMWF (European Centre for Medium-Range Weather Forecast) ERA-40 reanalysis database. This database is a new product generated by ECMWF using improved assimilation and forecast procedures and covering a longer time period with improved spatial resolution. In addition, the regression coefficients used in the prediction of rainfall rate have been optimized. Rainfall rate statistics with a 1-min integration time are required for the prediction of rain attenuation in terrestrial and satellite links. Data of long-term measurements of rainfall rate may be available from local sources but only with higher integration times. This recommendation provides a method for the conversion of rainfall rate statistics with a higher integration time to rainfall rate statistics with a 1-min integration time. The Crane rainfall zones are defined differently from the ITU zones, with more defined zones in the U.S. than the ITU zones. ∗ Currently, the ITU-R provides Recommendation ITU-R P.530-xx for calculating the average annual, 1-min averaged, rain fade distribution experienced by a terrestrial link. The prediction method refers to Recommendation ITUR P.837 for calculating the average annual, 1-min averaged, rain rate distribution and to Recommendation ITU-R P.838 for the specific attenuation-rain rate relationship. Recommendation ITU-R P.530-xx also provides some guidance for extending these models for the performance of a simple link to predict the performance of more complex links such as multihop links and links utilizing diversity. The ITU has recommended a calculation method for terrestrial systems, ITU-R P.530-xx, and for space-to Earth-links, ITU-R P.618. These models take into account a distance reduction factor to account for the cellular nature of storms and have improved since the original CCIR version.

∗

At the time of writing this book, the latest revision of this model was ITU-R-P.530-12.

Microwave Link Design

129

In most cases, the Crane models predict higher rain attenuation; in other words, they are more conservative than the ITU model. More detailed information on attenuation due to hydrometeors other than rain is given in ITU-R Recommendation P.840. 3.6.1

Crane Rain Outage Model

The method is used to predict the attenuation by rain on terrestrial propagation path. As a first step, the rain climate region of the endpoints of the path has to be determined. Calculating the outage is an iterative process that is based on the calculation of the attenuation. For x ≤ d ≤ 22.5 km,   eµβ d   bβ ecβ x   bβ ecβ d   A = α Rpβ     +  −   µβ   cβ   cβ  

[dB]

(3.22)

For d < x,  eµβ d − 1  A = α Rpβ   µβ 

[dB]

(3.23)

The values of m, x, b, and c can be calculated as follows:

µ=

ln(bx cx ) x

b = 2.3 Rp −0.17 c = 0.026 − 0.03 ln( Rp ) x = 3.8 − 0.6 ln( Rp ) where d = path length (km) Rp = rain rate in millimeters per hour, determined from Crane tables, at the probability p (%) e = base of the natural logarithm (2.71828) a, b = regression coefficients obtained from the table and interpolated if necessary First, it is necessary to determine the rainfall rate required to produce an attenuation equal to the thermal fade margin. After that, determine

130

Chapter Three

the percent of the year this rain rate is exceeded. This value represents the annual two-way rain outage time for the path and is calculated using the rainfall statistics for the required geographic region. If d > 22.5 km, the rain attenuation is calculated using d0 = 22.5 km and a modified probability of occurrence p1.  22.5  p1 = p ⋅   d 

(3.24)

The two-component rain model is an extension of the global model provided by Crane to include statistical information on the movement and size of rain cells and to add several improvements to the original global model for the prediction of rain attenuation statistics. The twocomponent model was first introduced about two years after the global model (Crane, 1982), with later revisions by Crane in 1985 and 1996. The model separately addresses the contributions of rain showers (volume cells) and the larger regions of lighter rain intensity surrounding the showers (debris). The rain distribution for each climate zone is modeled as a two-component function, consisting of a volume cell component and a debris component. The probability associated with each component is calculated, and the two values are summed independently to provide the desired total probability. The rain climate regions of the original global model are used in the two-component model as well. 3.6.2

ITU-R Rain Outage Model

The prediction procedure outlined here is considered valid in all parts of the world, at least for frequencies up to 40 GHz and path lengths up to 60 km. The following simple technique may be used for estimating the longterm statistics of rain attenuation: 1. Obtain the rain rate R0.01 exceeded for 0.01 percent of the time (with an integration time of 1 min). If this information is not available from local sources of long-term measurements, an estimate can be obtained from the information given in ITU-R Recommendation P.837. 2. Compute the specific attenuation, γ R (dB/km) for the frequency, polarization, and rain rate of interest using ITU-R Recommendation P.838. 3. Compute the effective path length, deff, of the link by multiplying the actual path length d (in kilometers) by a distance factor r as follows:

Microwave Link Design

131

deff = d ⋅ r r=

1 1+ d d0

(3.25)

where for R0.01 ≤ 100 mm/hr, d0 = 35e−0.015 R0.01

(3.26)

for R0.01 > 100 mm/hr, use the value 100 mm/hr in place of R0.01: d0 = 35e−1.5

(3.27)

4. An estimate of the path attenuation exceeded for 0.01 percent of the time is given by A0.01 = γ R ⋅ deff = γ R ⋅ d ⋅ r [dB]

(3.28)

5. For radio links located in latitudes ≥ 30° (north or south), the attenuation exceeded for other percentages of time p in the range 0.001 to 1 percent may be deduced from the following power law: Ap A0.01

= 0.12 p− (0.546+0.043 log10 p)

(3.29)

This formula has been determined to give the following factors: 0.12 for p = 1.00 percent, 0.39 for p = 0.1 percent, 1.00 for p = 0.01 percent, and 2.14 for p = 0.001 percent. 6. For radio links located at latitudes ≤ 30° (north or south), the attenuation exceeded for other percentages of time p in the range 0.001 to 1 percent may be deduced from the following power law: Ap A0.01

= 0.07 p− (0.855+0.139 log10 p)

(3.30)

This formula has been determined to give the following factors: 0.07 for p = 1.00 percent, 0.36 for p = 0.1 percent, 1.00 for p = 0.01 percent and 1.44 for p = 0.001 percent.

132

Chapter Three

The attenuation A in the above equations is set to the fade margin and the equation is solved for p. ITU-530-xx clearly states that the equations are valid only in the range from 1 to 0.001 percent. On many practical links, this range will be exceeded, especially on short links with high fade margins. 7. If worst-month statistics are required, calculate the annual time percentages p corresponding to the worst-month time percentages pw using climate information specified in ITU-R Recommendation P.841. The values of A exceeded for percentages of the time p on an annual basis will be exceeded for the corresponding percentages of time pw on a worst-month basis. Both ITU-530-xx and Crane methods calculate the annual probability of rain outage. The annual rain outage probability is translated to worst-month rain outage probability as follows: (3.31)

pw = 2.85 pa0.87 3.6.3 Comparison of ITU-R and Crane Rain Outage Models

The comparison in Table 3.1 of predicted attenuation is provided for places where the ITU and Crane zones overlap.10 ITU zone M does not correspond to a Crane zone very well and is not included in the comparison. Crane D2 and E are irregular through M, with Crane E extending from Florida to Northern Alabama and up to South Carolina. Listed in the table are attenuation values for the same locations using the ITU zone for ITU calculations and the corresponding Crane zone for Crane calculations. All of the Crane models predict a larger attenuation than the ITU model; however, this difference is also about the same as the difference between various Crane models. It is interesting that ITU zone D is Northern California, with less rainfall, and zone E is Southern California, with more rainfall. TABLE 3.1

Rain Attenuation Comparison at 99.99 Percent Availability for a 3 km Path

ITU Zone/Crane Zone

Units

E/F

D/C

K/D2

N/E

Rain rate ITU/Crane ITU-R 530 Crane global Crane two-component Model Crane revised two-component Model

(mm/hr) (dB) (dB) (dB)

22/22 10.8 13.2 13.6

19/29 14.3 17.2 18.4

42/47 22.3 25.7 28.8

95/91 39.2 45.9 52.0

(dB)

12.4

20.0

26.9

51.3

Microwave Link Design

133

The corresponding Crane zones are C for Northern California and F for Southern California, and they predict just the opposite intensity, which is actually correct. For prediction of rain fade attenuation using the ITU 530-xx standard, rain rate at the 0.01 percent exceeded level for the zone of interest is required, plus frequency, path length, and attenuation factors from ITU-R 838. Other percentages are calculated using only the 0.01 percent value. The ITU model consists of simple equations, whereas all of the Crane models, on the other hand, require solution of a number of complex equations to obtain the path-averaged rain rate and a representation of the path profile by exponential functions. Crane’s formulas are used to make predictions about link availability for radio and radar installations. One shortcoming is encountered when trying to extend his result to other than surface to surface and surface to satellite links. Crane’s derivations and formulas are very empirical, sometimes hard to understand, and difficult to extend. Users can apply in their calculations either ITU or Crane models, whichever makes them more comfortable. Their choice of model may depend more on their institutions’ prejudices and traditions or their customers’ “comfort level” with either model than on the requirements for accuracy. It is also important to keep in mind that the more stringent (conservative) method does not necessarily have to be more accurate. On the other hand, a more conservative method of calculation can lead to more expensive network design. 3.6.4

Reducing the Effects of Rain

The most common reason for preferring a lower frequency is the susceptibility of bands above 10 GHz to rainfall attenuation. Although fades caused by rain cells are occasionally observed at lower frequencies (10 to 20 dB fades at 6 GHz have been recorded, even in North America), this type of fade generally causes outages only on paths above 10 GHz. The outages are usually caused by blockage of the path by the passage of rain cells (e.g., thunderstorms) perhaps 4 to 8 km (2.5 to 5 mi) in diameter and 5 to 15 min in duration on the path. Such fading exhibits slow, erratic level changes, with rapid path failure as the rain cell intercepts the path. Things to keep in mind in connection with rain attenuation fades are as follows: n

Multipath fading is at its minimum during periods of heavy rainfall with well-aligned dishes, so the entire path fade margin is available to combat the rain attenuation.

134

n

n

n

n

n

n

n

n

Chapter Three

When permitted, seldom-used crossband diversity is very effective. In this case, the lower frequency path is stable (affected only by multipath fading) during periods when the upper frequency path is obstructed by rain cells. Neither space diversity nor in-band frequency diversity provides improvement against rain-attenuation fade outage. During a rainstorm, both antennas in a space diversity system, and all frequencies in a frequency diversity system, fade together. Intense rain showers that cause extreme attenuation occur in cells of small dimensions, so the probability that two or more links have deep fade events at the same time is very small, and it strongly depends on the geometry of the network. Route diversity with paths separated by more than about 8 km (5 mi) can be used successfully. To develop efficient route diversity methods or site diversity strategies for cellular mobile systems, deep knowledge of the spatial and time correlation statistic is essential. Increased fade margin is of some help in rainfall attenuation fading; margins as high as 45 to 60 dB, some with automatic transmitter power control (ATPC), have been used in some highly vulnerable links for increased availability. Increasing the fade margins, shortening path lengths, using the lower frequency band, and increasing antenna sizes are the most readily available tools for reducing the per-hop annual rain outage in a given area. You can use microwave radios with adaptive modulation. Adaptive modulation is a process of dynamically changing the modulation scheme depending on weather conditions. During good weather conditions an efficient modulation scheme is used providing a high data rate. During heavy rain, adaptive modulation uses a more robust modulation scheme to guarantee the high availability of the link at the cost of a reduced data rate. Because raindrops are oblate rather than spherical, attenuation tends to be greater for horizontally polarized signals than for vertically polarized signals. Vertical polarization is far less susceptible to rainfall attenuation (40 to 60 percent) than are horizontal polarized frequencies. The worst rain outages occur during the heaviest thunderstorms. For example, the Gulf Coast area from Florida to New Orleans has the most severe thunderstorms in the U.S. As a result, rain outages are most severe in the southeastern U.S. Microwave path lengths must be reduced in these areas to maintain the path availability.

When long-term attenuation statistics exist at one polarization, either vertical (V) or horizontal (H), on a given link and are expressed

Microwave Link Design

135

in decibels, the attenuation for the other polarization over the same link may be estimated through the following simple relation: 335 AV + AV AH − 300 AH = 0

(3.32)

This is an important consideration for dual polarization systems that will have to be designed for the worst case situation, i.e., assuming horizontal polarization. 3.6.5

Adaptive Modulation

Received signal

One of the new tools to combat rain fade is called adaptive modulation. Adaptive modulation refers to the automatic modulation (and coding) adjustment that a wireless system can make to prevent weather-related fading from causing communication on the link to be disrupted. When heavy weather conditions, such as a storm, affect the transmission and receipt of data and voice over the wireless network, the radio system automatically changes modulation so that non-real-time data-based applications may be affected by signal degradation, but real-time applications will continue to run uninterrupted (see Figure 3.10). Since communication signals are modulated, varying the modulation also varies the amount of traffic that is transferred per signal, thereby enabling higher throughputs and better spectral efficiencies. For example, 256 QAM modulation can deliver approximately four times the throughput of 4 QAM (QPSK). It should be noted, however, as a higher modulation technique is used, a better signal-to-noise ratio (SNR) is needed to overcome interference and maintain an acceptable BER (bit error rate) level.

256QAM

32QAM 4QAM Time Figure 3.10 Adaptive modulation

136

Chapter Three

The assumption is that while high revenue real-time applications such as video and voice require constant high performance transport, lower revenue non-real-time applications, such as e-mail or Web browsing, require less stringent transport performance. Adaptive modulation is extremely quick, so switchovers can be performed even tens of times per second. When a switchover is performed it will step up or down through the modulation. This enables the system to handle 100 dB/sec fading fluctuations and ensures that the link operates at the highest possible modulation at any given moment. Adaptive modulation cooperates with the equipment’s built-in Layer 2 Quality-of-Service (QoS) mechanism that provides priority support for different classes of service; according to a wide range of criteria it is possible to configure the system to discard only low-priority packets as conditions deteriorate. The modem decreases the modulation when the MSE (mean-squared error) crosses 2 dB above the 10−6 BER threshold in order to change the constellation before any actual errors are detected. When the MSE reaches 4 dB above the 10−6 BER upper threshold, the modem increases the modulation to the next constellation point. The concept of adaptive modulation was introduced into European point-to-point standardization in the ETSI DTR/TM-4147, which first specified the requirements and bit rates for packet data interfaces, effects of flexible system parameters, and the use of mixed interfaces. Recently published, new harmonized standard EN 302 217-2-2 explicitly provides for adaptive modulation within the existing system classes. Systems with lower orders of modulation may be upgradeable to adaptive modulation systems if they continue to comply with the same lower order transmit mask. This means that these systems must adjust the transmit power when using the higher modulation scheme to meet the original mask. In the U.S., FCC Part 101 Regulations do not address adaptive modulation explicitly but provide channeling plans and certain technical parameters within which the transmissions must be limited such as channel bandwidths, minimum capacity requirements, power limitations, transmission masks, and antenna performance. Provided adaptive modulation systems meet these requirements, they are allowed under the FCC provisions. 3.7 3.7.1

Improving the Microwave System Hardware Redundancy

Equipment failure normally results in long interruptions (unavailability), and hardware redundancy often can

3.7.1.1 Hot Standby Protection

Microwave Link Design

137

be the only option of improving the total unavailability figure of a radiolink system. The simplest form of hardware redundancy is that of a standby system operating in parallel with the active system (1 + 1), and it takes over whenever the online (active) system fails. Monitored hot standby is preferred; this term means that the functions of the standby component should work properly and optimally whenever required. A switching device connects the standby and the active system, and the term monitored implies electronic control/supervision. Hardware redundancy improves only the unavailability rate due to hardware failure; it does not affect the unavailability due to propagation effects, and it requires more equipment. Consequently, it is more expensive than a nonredundant system. The most important parameters in the calculation of hardware failure (unavailability) are the mean time between failure (MTBF) and the mean time to repair (MTTR). See Chapter 5.5.2 for more details and examples. MTTR includes the time necessary to detect, report, locate, and repair the failure. Considering that the MTTR of nonredundant systems can be somewhat large in some applications (e.g., microwave systems in remote areas), redundancy is recommended if high availability (low unavailability) is required and short MTTR cannot be accomplished. Unavailability is more suitable to radio-link planning than availability. The main reason for using unavailability is that unavailability contributions can be added together when calculating the total unavailability of a system. The availability of a system should not be mixed with reliability. Availability means the probability of finding a system in operation when it is needed, whereas the reliability is the probability of a system operating as intended during a certain time interval and under certain conditions. The fact is that some parts of the microwave radio are more prone to failure than others; for example, the transmitter is one of those modules that traditionally causes most of the problems. So, some manufacturers have decided to offer, instead of the fully redundant architecture that provides protection against all potential failures in the radio, protection of only the most failure-prone modules. This type of protection offers reliability somewhere between the nonprotected (1 + 0) and fully protected systems (1 + 1), but their price is also significantly lower than that of the fully protected radios. 3.7.1.2 Multichannel and Multiline Protection To achieve high data rates, more radios are sometimes combined together by stacking several radios

138

Chapter Three

on the same antenna. So, if the microwave system has more than one channel, protection can be achieved in two different ways: n

n

Multichannel protection It will have hot standby protection for every channel. In other words, hot standby transmitters and receivers independently protect all working channels in a multichannel system at the same time. The multichannel method combines multiple hot standby radios on the same antenna using rigid waveguide and circulators. Each hot standby radio operates independently and is fully protected. Multiline protection (N+1) Requires an additional RF channel to use as a protection channel and, under normal working conditions, does not carry any traffic. In a multiline protection system, one standby channel will serve as a protection for N working channels. (See Chapter 5, Section 3.3 for more details.)

Multiline protection switches are used to protect against interruptions or degradations to microwave working channels, which may be caused by radio equipment failures or severe propagation impairments. The multiline method combines multiple nonstandby radios on the same antenna. After the working channel returns to normal conditions, the traffic that was previously switched to spare is restored to its normal working path (in case of revertive operation). In nonrevertive operation, the traffic does not return to the working path (channel). Multiline protection can only protect one working channel at a time. As a result, at least theoretically, the equipment reliability is better for a multichannel system but is also more expensive. In practice, simultaneous failure of two channels (path and/or hardware) is an extremely rare event. In addition, frequency planning is more complex when several radios are connected to the same antenna. 3.7.2

Diversity Improvement

Diversity is defined here as a general technique that utilizes two or more copies of a signal with varying degrees of disturbance (i.e., uncorrelated) to achieve, by a selection or a combination scheme, a consistently higher degree of message recovery performance than is achievable from any one of the individual signals separately. The outage probabilities of the single channel can be reduced significantly if the information to be transmitted is simultaneously received over two (or more than two) distinct paths (diversity reception). The paths may be separated by space, angle, or frequency or their combination. After reception, the signals of the two paths are combined and evaluated in an appropriate way. Under normal conditions, only one propagation path exists between the transmitting and receiving antennas of a well-designed LOS

Microwave Link Design

139

microwave link. Multipath fading occurs whenever a low-level reflected signal is radiated out of phase and, at a reduced level, is combined with the desired signal at the victim receiver (ground reflections). Usually, large variations in temperature and humidity (atmospheric refractions) accompany multipath fading, which is characterized by deep, fast, frequency-selective signal attenuation over a certain (usually short) time period. One of the possible methods of limiting signal degradation caused by multipath propagation is to apply one or more of the three different diversity techniques (frequency diversity, space diversity, or angle diversity) or a combination of these. Since, at a given moment in time, each frequency in the bandwidth is affected differently, dispersive fading usually results, and diversity systems can be effective in stabilizing these conditions to a minimum when used with continuous, in-band power combiners in digital microwave applications. The most common forms of diversity in LOS links are frequency and space diversity (Figure 3.11), although angle diversity is also used in

f1 f2

IN

Tx1

Frequency diversity

Rx1 Combiner

OUT

Rx2

Tx2 f

Vertical separation of receive antennas

f

IN

Tx

Rx1

Space diversity

Combiner

Height

Rx2

Optimum antenna separation

Field strength Figure 3.11 Frequency and space diversity

OUT

140

Chapter Three

rare occasions. Diversity does not help to overcome rain-fading events, only multipath and refraction-diffraction fading. Methods are available for predicting outage probability and diversity improvement for space, frequency, and angle diversity systems, and for systems employing a combination of space and frequency diversity. The probability of outage for a diversity system is Ud =

U nd Id

(3.33)

where Ud = one-way probability of outage for a diversity path Und = one-way probability of outage for a nondiversity path Id = diversity improvement factor The terms diversity improvement factor or diversity gain are commonly employed to describe the effectiveness of various diversity configurations. There are a number of different formulas for calculating diversity improvement factor, from internationally accepted empirical and semi-empirical models to more or less proprietary models and formulas developed by equipment manufacturers. The degree of improvement of these techniques depends on the extent to which the signals in the diversity branches of the system are uncorrelated. For narrowband analog systems, it is sufficient to determine the improvement in the statistics of fade depth at a single frequency. For wideband digital systems, the diversity improvement depends also on the statistics of in-band distortion. The diversity improvement factor differs for each type of diversity—space, frequency, hybrid, and so forth. Above 3 GHz, Isd is nearly always higher (better) than Ifd and is therefore selected unless the diversity spacing, Df, exceeds about 5 percent (300 MHz in the 6-GHz band, for example). Below 3 GHz, Ifd is usually larger than Isd and is therefore selected, assuming that spectrumgoverning bodies will allow the use of frequency diversity. Space Diversity Space diversity is the most commonly used diversity option against multipath fading. Space diversity is achieved by using multiple antennas that are separated by a large enough vertical distance to make signals uncorrelated. Space diversity is the simultaneous transmission of the same signal over a radio channel by using two or more antennas for reception. It is typically used on long paths, shorter paths in poor propagation areas, and over-water paths to protect against surface reflections.

3.7.2.1

Microwave Link Design

141

Hitless or errorless switches are used for switching the signal between the main and diversity branches. Combiners are used to combine signals from the main and diversity receivers. When using space diversity, the improvement obtained depends on the extent to which the signals in the two diversity branches of the system are uncorrelated. The antennas are therefore physically separated on a tower or mast, and the vertical distance between the antennas (antenna separation) at the receiver or transmitter is such that the individual signals are assumed uncorrelated (see Figure 3.12). Two important steps are required to establish the optimum antenna separation: n

n

For the k = 4/3, the relative signal should be greater than zero (or very close to zero) on the main and diversity antenna. For any other value of k, simultaneous nulls should not occur on the main and diversity antennas.

Figure 3.12 Space diversity microwave system over the water, Puerto Vallarta,

Mexico

142

Chapter Three

There are two main methods of receiving signals from different diversity paths: baseband switching and IF combining. Baseband switching is a simple method in which radio will choose the signal with maximum S/N ratio or lower BER and is primarily used in PDH microwave radios. IF combiners will, on the other hand, combine signals from different paths and will do that based on certain criteria. Diversity improvement, in the case of IF combiners, is usually a little bit higher than in the case of baseband switching. The appropriate spacing of antennas in space diversity systems is determined by three factors: 1. To keep clearance of the lower antenna as low as possible (within the clearance so as to minimize the occurrence of surface multipath fading) 2. To achieve the specified space diversity improvement factor 3. To minimize the chance that the signal on one diversity antenna will be faded by surface multipath when the signal on the other antenna is faded The space-diversity improvement factor (Isd) for the flat fading component of the fade margin is proportional to the square of the antenna separation. Larger spacing results in higher improvement and a typical space diversity antenna spacing is 30–50 ft. At the same time, Isd for the dispersive component of the fade margin is independent of the vertical antenna separation greater than 10 ft (3.3 m). The improvement increases with dispersive fade margin. Space diversity is highly effective against dispersive fading. For typical DFMs, frequencies and path lengths, the calculated improvement factor for dispersive fading, is very high. In some rare extreme situations (e.g., very long over-water paths), it may be necessary to employ three-antenna diversity configurations. Vigants developed the following formula for calculating space diversity improvement (for baseband switching) factor: f ISD = 7 × 10 −5 ⋅   ⋅ s2 ⋅ v2 ⋅ 10CFM /10  d where f = frequency (GHz) d = hop length (mi) s = vertical antenna separation (ft) CFM = Composite Fade Margin (dB) v = relative voltage gain factor

(3.34)

Microwave Link Design

143

v is a relative voltage gain factor for different gains of the main (GM) and diversity (GD) antennas, both given in decibels, and it can be calculated as: v = 10 − (GM −GD )/ 20

(3.35)

If main and diversity antennas have the same gain (i.e., the same size), v = 1. Example: Using our example in Section 3.4.1, we will now calculate the improvement factor for the space diversity, using the same size 12 ft antennas, with vertical separation of 30 ft. Frequency is 6 GHz.

 6 ISD = 7 × 10 −5 ⋅   ⋅ 302 ⋅ 12 ⋅ 1036 /10  30  ISD ≈ 50 From Section 3.4.1 we calculated that: Non-diversity outage ≈ 653 SES/yr (99.998% reliability) So, after applying space diversity, we get: Space diversity outage =

Non-diversity outage ISD

Space diversity outage =

653 SES ≈ 13 SES/yr (99.99996 % reliability) 50

Now, the reliability of the system meets (and exceeds) the objective of 192 SES/yr, as calculated in Section 3.4.1. It cannot be overemphasized that, although we are using the same model and formulas to design space-diversity systems in different parts of the world, we may not necessarily achieve the same improvement and the exactly the same results. The field results may vary due to the specifics of the local climate and radio propagation conditions. Engineers should be aware of the fact that sometimes certain parameters have to be modified to account for the specifics of the local climate.

144

Chapter Three

Experience has shown that for some of the commonly used formulas for calculating space diversity, the improvement factor may underestimate the improvement for small antenna spacing and overestimate improvement for large antenna spacing on “flat land” microwave links. In space diversity systems, maximum-power combiners have been used most widely so far. Other combiners, employing a more sophisticated approach using both minimum-distortion and maximum-power may give even better performance. Calculation of the outage prediction and diversity improvement factor are described in ITU-R P.530-xx. Space diversity can be combined with angle diversity to further enhance performance if desired, in which case both of the antennas are tilted to provide the additional angle-diversity enhancement. This can be achieved by tilting the main (upper) antenna of a space-diversity pair and the transmitting antenna upward by a small angle. This uptilting of the antennas might result in a loss of flat fade margin in the approximate range 2.5 to 6 dB, the amount depending on whether the tilt is optimized to minimize fading or amplitude distortion. In addition, tilt the diversity (lower) antenna of a space-diversity pair downwards from the local horizontal by a small angle. 3.7.2.2 Frequency Diversity In a frequency diversity system, the radio at each terminal contains redundant transmitter-receiver pairs and simultaneously transmits the same signal over two or more radio frequency channels located at the same frequency band. Each transmitter operates on a different RF channel, and both transmitters are energized and, similarly, each receiver operates on a RF channel that is different but identical to the corresponding transmitter at the far end. When equipment failure or path fading affects an RF channel to the point at which the signal is degraded, a decision is made at the receiver end of the link to switch to a stand-by channel. Using empirical data and mathematical models, Vigants and Pursley developed in 1979 an improvement factor calculation for point-to-point microwave links with frequency diversity:

I FD =

50 ⋅ ∆f ⋅ 10CFM/10 f2 ⋅d

where ∆ f = frequency spacing (GHz); if ∆ f > 0.5 GHz, use ∆f = 0.5 f = frequency (GHz) d = path length (mi) CFM = composite fade margin (dB)

(3.36)

Microwave Link Design

145

This formula is also used by ITU-R P.530-xx, and it applies to FD links (two channels) with the following ranges of parameters: 2 ≤ f ≤ 11 GHz 30 ≤ d ≤ 70 km ∆f ≤ 5% f Like space diversity, frequency diversity is highly effective against dispersive fading. For typical DFMs, frequencies, and path lengths, the calculated improvement factor for dispersive fading is very high. Another form of frequency diversity is crossband diversity, in which the RF carriers are in different frequency bands. One great drawback to frequency diversity is the inefficient use of the available frequency spectrum, and it is prohibited in most countries. In the U.S., the FCC limits frequency diversity to encourage spectrum conservation (FCC Part 101.103(c)). The rules state that frequency diversity transmission will not be authorized in these services if the operator cannot prove that the required communications cannot practically be achieved by other means. In the U.S., where frequency diversity is deemed to be justified on a protection channel basis, it will be limited to one protection channel for the bands 3,700–4,200, 5,925–6,425, and 6,525–6,875 MHz, and a ratio of one protection channel for three working channels for the bands 10,550–10,680 and 10,700–11,700 MHz. In the bands 3,700–4,200, 5,925–6,425, and 6,525–6,875 MHz, no frequency diversity protection channel will be authorized unless there is a minimum of three working channels, except that, if there is a proof that a total of three working channels will be required within three years, a protection channel may be authorized simultaneously with the first working channel. A protection channel authorized under such exception will be subject to termination if applications for the third working channel are not filed within three years of the grant date of the applications for the first working channel. In the 11-GHz band (10,700–11,700 MHz), frequency diversity systems must have a minimum ratio of three working channels to each protection channel. There are 12 frequency pairs in the 40-MHz channel plan and 13 frequency pairs in the 30-MHz channel plan. Therefore, FCC rules will allow multiline systems of 3+1 through 12+1 in the 11-GHz band without a waiver. The rules permit two parallel frequency diversity systems to be installed on the same path, as long as the 3:1 ratio of working to protect channels is maintained.

146

Chapter Three

The federal government bands are administered separately by the National Telecommunications and Information Administration (NTIA) and are not subject to FCC restrictions on frequency diversity or minimum loading. The 8-GHz band (7.125–8.5 GHz) is the preferred Federal Government band for high capacity microwave. This band is not available to common carriers or private users operating under FCC Part 101. NTIA regulations allow 1+1 frequency diversity systems in the federal government bands. If frequency diversity is used in N + 1 operation, and N > 1, we expect that the diversity improvement factor Ifd, will be reduced since there are more than one channel sharing the same diversity channel. Additionally, it is assumed that no more than two of the RF-channels are simultaneously affected by equal fading. Example: Calculate frequency diversity improvement factor on the 6.175 GHz link, 30-mi long, if the frequency spacing is 100 MHz, and the composite fade margin is 36 dB.

I FD =

50 ⋅ ∆f ⋅ 10CFM/10 f2 ⋅d

I FD =

50 ⋅ 0.1 ⋅ 1036 10 6.1752 ⋅ 30

I FD ≈ 17.4 Since the frequency diversity improvement factor is reversely proportional to the link length, its value becomes very large for short path. As mentioned earlier, this formula is only valid for path lengths of more than 30 km (or approximately 19 mi) and should not be used for paths shorter than 19 mi. 3.7.2.3 Hybrid Diversity Hybrid diversity (HD) is an enhancement (SD+FD) of space diversity that uses frequency diversity (when permitted). Hybrid diversity is the most effective of all of the diversity arrangements and is preferred in difficult propagation areas, such as those covering very long distances or transmitting over water. Here, one side of the link has one antenna and the other one has two antennas (SD). It is possible to compute frequency and hybrid diversity improvements for links in regions where regulatory rules or waivers so permit. The hybrid diversity improvement factor, Ihd, is derived either from the space Isd or frequency diversity improvement factor Ifd described previously, or using both values.

Microwave Link Design

147

The higher T/R frequencies must always be assigned to the upper antenna at the space diversity (usually the lower elevation) end of each hybrid diversity link for optimum performance. Quad diversity is a type of hybrid diversity in which both sides of the link have space and frequency diversity, i.e., a four-antenna system. There are a number of different formulas to calculate quad diversity improvement factor. Using microwave radio systems below 10 GHz, hop lengths of about 80 km (50 mi) usually are achieved without problems, utilizing standard diversity techniques.11 When longer distances have to be spanned, however, the problems of propagation increase dramatically. In the early 1990s, Siemens installed one of the longest over-the-water microwave hops for the Mexican telephone company, Telmex. This hop was installed across the Gulf of California in the Baja California region, and was 160 km (100 mi) long. Many “classic” models for estimating the fading behavior of a radio hop assume one reflection point and therefore two superimposed waves at the receiver location. This basic assumption does not apply under extreme conditions such as this one. To meet quality requirements under these very problematic propagation conditions, a customized solution and a number of unusual technical measures were implemented: n n

n

n

Four large 4.6-m parabolic antennas at each side were used. A selection of the best of eight received signals at any time was made (four antennas with two different frequencies). Custom-designed, very low-loss channel branching filters, using waveguide technology without circulators, were implemented. Increased frequency spacing, achieving greater efficiency of frequency diversity reception, was used.

Theory and practice show that a higher number of receiving antennas increase the probability of receiving a strong and less distorted signal. 3.7.2.4 Angle Diversity Angle diversity (sometimes called angle-ofarrival diversity) is a special case of space diversity. Angle diversity (AD) has been used in line-of-sight digital microwave links since the mid-1980s and in troposcatter links since the 1950s. This method used to be popular when digital radios were less robust, but today AD antennas are assigned mostly where installation constrictions (roof mounts problems, space, aesthetics, tower loading, and so on) prohibit SD and thus justify these less-effective, more-costly dishes.

148

Chapter Three

The angle diversity antenna is a single dish with two feeds vertically offset by about 1º (the smaller, the better). Angle diversity is most effective when path outages are dominated by dispersive fade activity (i.e., the dispersive fade outage approaches or exceeds flat fade outage). Research results are divided when it comes to usefulness of angle diversity. For some paths angle diversity can even outperform space diversity, and for some paths angle diversity is almost no better than no diversity at all. Depending on path geometry and climatic conditions, angle diversity improvements of perhaps 20 or even much better can be achieved. Optimum angle-diversity improvements are achieved through an antenna alignment procedure that matches the antenna size and alignment to the path and its climatic characteristics. Angle diversity dishes require a more exacting, long-term alignment procedure than that for space diversity and nondiversity antennas. 3.7.2.5 Route Diversity When a point in a network requires high availability, it is often more spectrum efficient to install multiple links to combat rain than to use higher transmit power on a single connection. The simplest case of diversity is a point connected to a network by two independent links that may operate at different frequencies and polarizations, i.e., route diversity. This scenario is described by the length, frequency, and polarization of each link and the azimuth angle between the two links where they converge at the point being serviced and the integration time of fade measurements. The availability is increased because, when one link has failed due to rain attenuation, the other may still be operational. The increase in availability is quantified using the diversity improvement factor. The higher the correlation of rain attenuation on the two links, the lower the gain from using diversity, as the failure of one link becomes an increasingly better predictor of the failure of the other link. This correlation is determined by the geometry of the link system and the spatial-temporal statistics of rain intensity. 3.7.2.6 Media Diversity Important microwave links can be protected by using completely different media. Most commonly used are fiber-optic systems. In addition, the opposite is true, and sometimes fiber-optic systems are protected by high-capacity microwave links. These solutions are very expensive and should be considered only in exceptional cases. 3.7.3

Antireflective Systems

Antireflection refers to an antenna arrangement technique for reducing the effects of multipath fading. If two identical antennas at either

Microwave Link Design

149

(or both) the receiving site and transmitting site are used, they can be arranged so that their resulting radiation pattern has a null in the direction of the reflection point.12 In addition, this antenna arrangement gives an extra 3-dB gain to the direct ray. If the direct ray arrives horizontally, the two antennas are set in the vertical plane with a spacing of h meters (see Figure 3.13). The output of each antenna is connected into a hybrid through two feeders, which are cut to length so that the direct received signals add in phase at the hybrid output. To trim the phase for optimum performance, a phase shifter is used in one of the feeders. Because of the different path lengths traversed by the reflected signals in reaching each of the two antennas, their phases will not add in phase. The combination of both reflected signals may, under certain conditions, lead to their cancellation. The practical limitations with antireflective systems are n

Changes in k factor will change the angle between the direct and reflected ray.

n

There may be movement of the antenna due to wind.

n

Temperature changes affect the length of the feeders.

n

They exhibit narrow tolerances in carrier frequency.

n

n

A hybrid and delay network, sensitive to temperature variations, should be installed in the equipment room. An antireflective system should not be used for low values of a.

Direct ray α

h

ray ted c e fl Re

Splitter

α Cancellation condition : ϕ = 2π . h . sin α = (2n + 1)π λ Figure 3.13 Antireflective system

Phase shift ϕ

Receiver

150

Chapter Three

If a is fixed by the geometry of the link, the distance h between antennas is as follows:  1  λ h =   (2n + 1)   sin α   2

(3.37)

For example, at a frequency of 6.15 GHz (l = 0.048 m) and a = 1°, h = 1.4 m (for n = 0), and h = 4.19 m (for n = 1). The distance between antennas will depend on their diameter. 3.8

Repeaters

In cases where a direct microwave path cannot be established (i.e., there is no line of sight) between two points, it is possible to establish a path by using a repeater. The function of such a repeater is to redirect the beam so as to pass the microwave beam around or over the obstacle (e.g., a building or hill). The main requirement is that there should be a clear line of sight between the repeater and both sides of the microwave link. This could be an active repeater (two microwave radios connected back to back) if distances are long, or a passive repeater if distances are relatively short. 3.8.1

Active Repeaters

An active MW repeater site contains two complete microwave radio terminals (connected back to back), antennas, waveguides or coax cables, and other components, and it is a much more costly solution than the passive repeater described in the next section. It requires an enclosure for the equipment, power plant, an antenna-mounting structure of some kind, and so on. The best way to avoid use of active microwave repeater sites is to carefully plan and execute the microwave network design and strategically place sites in such a way that they all have a LOS with at least one other site. Active repeaters are used not only in the case of obstructed LOS but also when terminal stations are too far for one microwave hop. Two sites that are 100 mi apart can be typically connected using three, four, five, or even more active microwave repeater stations, depending on the frequency, terrain (LOS), type of equipment (radios and antennas) used, and so forth. 3.8.2

Passive Repeaters

There are two types of passive repeaters in use. One consists of two parabolic antennas connected back to back through a short piece of transmission line. The other, more commonly used, is a plane reflector, flat billboard-type metal reflector that acts as a microwave mirror.

Microwave Link Design

151

Passive repeaters are used to change the direction of the radio-relay signal so as to overcome obstacles in an otherwise direct line of sight between two microwave (radio-relay) stations.13 They are also employed when the cost as compared with an active repeater is too high. A passive repeater is not only less expensive to build than an active repeater, but the operation cost is also substantially reduced. Passive repeaters have a major advantage over active repeaters from the ecology standpoint since it is not necessary to provide access roads and power line rights of way to the repeater site. They require minimal maintenance and can be visited by horseback, foot, or helicopter when necessary. In addition, passive repeaters are almost always less expensive to install and operate, while offering high reliability. 3.8.2.1 Billboard Repeater Plane reflectors reflect the microwave signal

in the same way mirrors reflect light. A single-plane reflector (single billboard) consists of a flat reflexive surface that changes the ray direction to avoid the obstacle (see Figure 3.14). The performance of this setup is given by the reflector surface (height and width) and the angle formed between the incident and reflected ray. Although historically used for low-frequency bands 2-11 GHz, today some manufacturers are actually producing high-performance passive repeaters for frequencies above 11 GHz. The size of the passive plane (billboard) repeaters can vary from 8 × 10 ft (2,300 lb) to 40 × 60 ft (45,000 lb) in size. Passive repeaters are typically designed to withstand wind speed of 125 mph. High-wind models based on 240 mph wind survival can be ordered in areas known to have severe wind conditions. Cold climate models are designed for radial ice in addition to required wind speed.

Single-plane reflector

α

α

Double-plane reflector

Site 2 Obstacle

Site 1

Site 1 Figure 3.14 Single- and double-plane reflectors

Site 2

152

Chapter Three

Passive reflector de-icing kits are available, containing plastic coated nylon fabric and PVC pipe frame to space the fabric out from the reflecting surface. The fabric will not prevent ice buildup but will help shed the ice after a certain accumulation has occurred. If only half of the passive repeater is clear of ice, the received signal will be down only about an acceptable 6 dB. The installation of these covers will reduce the passive repeater gain for about 0.5 dB at 6 GHz and about 1 dB at 11 GHz.14 When a passive repeater is facing south-east, it may be frosted near subzero temperatures very early in the morning. By midmorning, the face skin can be heated by the sun’s radiation, causing extreme temperature difference between the front and back of the panels. This may cause distortion of the panels beyond acceptable tolerances, especially at higher frequencies (above 11 GHz). This will require painting of the face skin with a special flat white paint (instead of the usual black or optional green) to minimize the effects of the sun’s radiation. The effectiveness of a passive repeater is an inverse function of the product of the lengths of the two paths and not the sum of the path lengths, as one might expect. That means that it is highly desirable to keep one of the paths very short. The rule of thumb is that an efficient passive repeater can be designed if the product of the path lengths does not exceed 30 for 6-GHz links and 50 for links at higher frequencies (above 11 GHz). At the same time, the included horizontal angle between the paths (2a) should be less than 130°. Of course, these numbers are just general guidelines and should not be used as firm design criteria. For single billboards, the situation is simple if the billboard is in the far field of both antennas. In that case, the antenna gains and the billboard gains are independent and do not interact with each other. Then, we can simply calculate a total path loss (which is the sum of the two antenna gains and the two-way gain of the reflector) to arrive at the end-to-end path loss through the reflector.15 Net path loss = FSL1 + FSL2 − G1 − G2 − G3

(3.38)

where FSL1 = short-leg free-space losses (dB) FSL2 = long-leg free-space losses (dB) G1, G2 = antenna gains (dB) G3 = free-space, two-way gain of a single passive billboard (dB)

Microwave Link Design

153

There is more than one way to approach the design of the passive repeater. Here, we will first calculate the free-space, two-way gain of a single passive billboard repeater: G3 = 22.2 + 40 log f + 20 log( a cos α )

(3.39)

Passive repeater gain calculations can be complicated when the billboard is in the near field of one of the parabolic antennas. A rule of thumb formula to determine the near-field boundary for the passive repeater is d ≈ 2 ⋅ f ⋅ D2

(3.40)

where d = near-field zone distance (ft) f = frequency (GHz) D = antenna diameter (ft) or the widest projected dimension of passive repeater, whichever is greater In that case, net path loss is Net path loss = FSL1 + FSL2 − G1 − G2 − (G3 − C )

(3.41)

Next, we will check the approximate boundary for the antenna-reflector 1 combination. Near-field parameter K is a function of wavelength l, the effective (or projected) area of the reflector (acosa), and the distance (d) of the closer antenna from the reflector. If its value is less than 2.5, the reflector is considered to be in the near field of the antenna. 1 πλ d 4077d = ≈ K 4 aeff fa cos α

(3.42)

If the antenna and the reflector are in the near field, we will calculate the coupling factor. Coupling factor L is a function of the effective area of the reflector and the parabolic antenna diameter and can be calculated as follows: L=D

0.886 D π ≈ 4 aeff a cos α

(3.43)

154

Chapter Three

With values of 1 and L it is possible to determine a correction factor C K (gain reduction due to near field effect), expressed in decibels. The correction factor C (dB) can be calculated using empirical formulas and graphs, and its value will depend on the parabolic dish diameter, frequency, and the distance between the parabolic antenna and the passive repeater. Its value is usually between 0 and 11 dB (see Figure 3.15). 1  If the passive repeater is in the far field of both terminals  K > 2.5 , 



it is necessary to assume that there are two independent paths. There are no additional losses at the passive end of the circuits since waveguide or other transmission elements are not involved. The transmitted power (EIRP) used for the second path (passive to far end) is equal to the received signal level determined in the calculation for the first path (near end to passive). All other parameters entered are those normally entered for a two-link system. Single passive reflectors described here can be efficiently used when the angle is greater than approximately 50°. For smaller angles, doubleplane repeaters will usually be more efficient. Double-plane reflectors operate by changing the direction of the microwave signal twice. Double-plane reflectors are variations of the one-plane reflector solution, and the most efficient arrangement is the two reflectors with equal

0

6 1. = L

0.8 1.0

L=

L= 1.4 1.2

L=

–5

L=

0.2

–4

L=

–3

L= 0.6 0.4

–2

L=

Gain reduction due to near field effect [dB]

–1

–6 –7 –8 –9 –10 –11

0.15

0.2

0.3 0.4 0.5 0.6 0.8 1.0 1 K

Figure 3.15 Passive repeater correction in near field

1.5

2.0 2.5 3.0

4.0 5.0

Microwave Link Design

155

effective (projected) area located close to each other, i.e., in the near field.16 The rule of thumb is that the additional losses can be kept below 1 dB if 2λ d ≤ 0.13 a cos α

(3.44)

where d = distance between reflectors (ft) acosa = effective (projected) area of reflector with the real area a (sq ft) l = wavelength (ft) This application of double-plane reflectors is limited due to costs and difficulty of the radio link alignment. 3.8.2.2 Back-to-Back Antennas Back-to-back antennas work like ordinary active repeaters, but without generating radio frequency signals. Back-to-back antennas are practical when the reflection angle is large, so they currently are employed in suburban environments, whereas plane reflectors, due to their large size, are used in rural applications. Because of their inefficiency, back-to-back repeaters are used only on very short links, typically less than a few miles. Back-to-back antennas consist of two antennas connected by a waveguide or a short coaxial cable (see Figure 3.16). The total path length, the two additional antenna gains, and the loss introduced by the waveguide are included in the link budget. Therefore, back-to-back antennas introduce considerable losses, and they are less efficient than plane reflectors. The radio path is split into two “legs,” one between Site 1 and the repeater, the second one between the repeater and Site 2, each leg having its own free-space attenuation. The antennas in the repeater

Back-to-back parabolic dish antennas

Path 1

Path 2 Flexible waveguide

Figure 3.16 Back-to-back antennas

156

Chapter Three

contribute with their antenna gain to the compensation of the cumulative free-space attenuation. The total path loss in decibels is calculated as follows: A = ( FSL1 − G1 − G1 R + A1 F ) + ACL + ( FSL2 − G2 − G2 R + A2 F 1 ) [dB] (3.45) where A1F,A2F = feeder and branching losses (dB) G1,G2 = transmitting and receiving antenna gains (dBi) G1R,G2R = repeater antenna gains (dBi) FSL1,FSL2 = free-space loss between the antennas and the repeater (dB) ACL = coupling losses between repeater antennas (dB) Additional gas attenuation, rain attenuation, and obstruction losses can be added as well. The fade margin for a path using back-to-back antennas is calculated as follows: FM = PRx − Pth FM = ( PTx − A) − Pth

[dB]

(3.46)

where FM = fade margin (dB) PTx = transmit power (dBm) A = total path loss (dB) PRx = received signal (dBm) Pth = receiver’s threshold (for given BER) (dBm) It is recommended that antennas at the back-to-back repeater are orthogonally polarized, i.e., one antenna should have vertical and the other antenna horizontal polarization. This means that one leg of the link will have vertical and the other horizontal polarization. 3.8.2.3 Example of Billboard Repeater Calculation Let’s design a passive repeater in the L6 band, 20 × 30 ft size (a = 600 ft2), included angle 80°, 10 ft dishes (G1 = G2 = 43 dB), short leg d1 = 1.5 mi, and long leg d2 = 20 mi. Waveguide losses, as well as connectors, radome, and other fixed losses will be omitted in this simplified example.

4077d1 1 4077 ⋅ 1.5 ≈ 2.15 = = K fa cos α 6.175 ⋅ 20 ⋅ 30 ⋅ cos 40° 2.15 < 2.50

Microwave Link Design

157

The billboard is in the near field of the closer antenna. L=

0.886 D a cos α

=

0.886 ⋅ 10 20 ⋅ 30 ⋅ cos 40°

≈ 0.41

From the curves (refer back to Figure 3.13), we can conclude that the losses due to the near field effect are negligible, i.e., C = 0. The gain of the passive repeater is as follows: G3 = 22.2 + 40 log f + 20 log( a cos α ) = 22.2 + 40 log 6.175 + 20 log(600 cos 40° ) ≈ 107.1 dB FSL1 and FSL2 are calculated using a formula for the free-space loss described in Section 3.2.1.1. For our example, FSL1 = 115.93 dB and FSL2 = 138.43 dB, so: Net path loss = FSL1 + FSL2 − G1 − G2 − (G3 − C ) Net path loss = 115.93 + 138.43 − 43 − 43 − (107.1 − 0) Net path loss = 61.26 dB Example of Back-to-Back Antenna Repeater Calculation Let’s design the back-to-back passive repeater for a short path, where d1 = 0.5 mi, d2 = 1.0 mi, the frequency is 11 GHz, Tx = 27 dBm, Rx = −70 dBm, 4 ft antenna gains are 40 dBi, feeder and branching losses are 2 dB, and coupling losses are 2.5 dB. Additional gas attenuation, rain attenuation, and obstruction losses are neglected for simplicity in this example. From 3.3 we get: 3.8.2.4

FSL1 = 111.4 dB FSL2 = 117.4 dB From 3.45 we get: A = (111.4 − 40 − 40 + 2) + 2.5 + (117.4 − 40 − 40 + 2) A = 75.3 dB

158

Chapter Three

From 3.46 we calculate the receive signal and the fade margin: PRx = 27 − 75.3 = −48.3 dBm FM = −47.8 − ( −70) = 21.7 dB

3.9

References

1. Weissberger, M., ESD-TR-81-101, An Analytical Critical Summary of Models for Predicting the Attenuation of Radio Waves by Trees, Dept. of Defense, Electromagnetic Compatibility Analysis Center, Annapolis, Maryland, 1982. 2. Segal B., “Multipath propagation mechanisms deduced from tower-based meteorological measurements,” Comm. Research Centre, Ottawa, Canada, 2000. 3. Mojoli, L. F. and Mengali, U., “Propagation in Line of Sight Radio Links” (Part II: “Multipath Fading”), Telletra Review, 1988. 4. TIA, Telecommunications Systems Bulletin, TSB-10-F, “Interference Criteria for Microwave Systems,” 1994. 5. ITU-R Recommendation P.530-12, “Propagation Data and Prediction Methods Required for the Design of Terrestrial Line-of-Sight Systems,” 2007. 6. ITU-T Recommendation G.826, “Error Performance Parameters and Objectives for International, Constant Bit Rate Digital Paths at or above the Primary Rate,” 1999. 7. ITU-T Recommendation M.2100, “Performance Limits for Bringing into Service and Maintenance of International PDH Paths, Sections and Transmission Systems,” 1995. 8. Crane, R. K., Electromagnetic Wave Propagation Through Rain, New York: John Wiley & Sons, 1996. 9. ITU-R PN.837, “Characteristics of Precipitation for Propagation Modeling.” 10. Myers, W., “Comparison of Propagation Models,” IEEE P802.16 Broadband Wireless Access Working Group, 1999. 11. Mojoli, L. F. and Mengali, U., Propagation in Line of Sight Radio Links (Part I: “Visibility, Reflections, Blackout”), Telletra Review, 1988. 12. Alcatel Telspace, “Note on Digital Radio Link Calculation (Medium and High Capacity),” SP/LN1/A/04.96. 13. GTE Lencurt Inc., Engineering Considerations for Microwave Communications Systems, 1970. 14. Valmont Microflect, Catalog 41, 1997. 15. Microflect, Passive Repeater Engineering, Manual No. 161A, 1989. 16. Norton, M, “Microwave System Engineering Using Large Passive Reflectors,” IRE Transactions on Communications Systems, Sept 1962.

Chapter

4

Planning the Microwave Network

So far in this book, we have talked about microwave-link design challenges. Even if we were able to design a perfect microwave link, it would never work in isolation from other links in the microwave system we are designing or be far enough away to be immune to the influence of other (external) radio communication systems. Microwave network planning and microwave network design focus on the challenges of making the network work properly in an environment that is full of signals from other users of the RF spectrum. The main difference between microwave network planning and microwave network design is that one cannot build a network based on the outcome and deliverables of the network planning process, whereas network design results are used during the deployment phase to actually build the network. Microwave network planning is a set of preliminary activities and information gathering used only to determine a need and a feasibility (a feasibility study can also be a separate phase) of the microwave network build-out and to consider other options such as building a fiber-optic system or leasing lines from the existing wireline operators. Microwave network planning analyzes the traffic model to arrive at network capacity and dimensions and microwave network architecture, and also to make a preliminary equipment choice. In addition, it will determine budget, time lines, and the work force required for the successful completion of the project and help to get approvals to actually finalize the detailed design and build the network. 4.1 The Microwave Network Planning Process To plan a network effectively, a network planner needs to have a complete understanding of the whole network, understand the business 159

160

Chapter Four

objectives of the operator, and be able to respond to business requirements effectively. At all stages, the emphasis should be on designing a simple network architecture. This will be beneficial in terms of deployment and network management and will provide flexibility to allow for easy network expansion. During the first stage of the transmission network planning process, a few initial questions (customer questionnaire) need to be answered with regard to areas such as economics, the area topology, the existing network, and the services the customer/operator wishes to offer. The following are just a few examples of such questions: n

n

Who is the operator, what economic resources does he have, and what kind of services is the operator going to offer? Are we planning many years ahead or just dealing with today’s demand?

n

Are we expanding an existing network or designing a new one?

n

If there is an existing network, what spare capacity is available?

n

What are the requirements for reliability and performance of the network?

Before transmission network planning can start, some basic activities have to take place to define the operator’s requirements and expectations. These include the following: n

n

n

n

n

n

Identify all the main nodes in the network like switch location, hub sites, collocated sites, and so on. Meet with customers, contractors, vendors, and/or partners) and determine responsibilities for the transmission (leased lines, fiber, MW) network design and deployment. Clearly define and describe in detail the scope of work (SOW). There is no such thing as too detailed a scope of work. Complete the scope and task delineation list (who is doing what). To avoid any future confusion, this document should be as detailed as possible. Sign the nondisclosure agreements (NDAs) with all parties (customer, vendors, partners, and so forth) involved in the project. Identify potential microwave sites, MW link capacity requirements, and MW frequency bands/channels available and/or approved for the project and conforming to relevant ITU-R (or other regional) recommendations.

Planning the Microwave Network

n

n

n

n

n

n

n

161

Identify the available license-exempt microwave spectrum in case rapid deployment microwave systems (spread-spectrum or some other type) are required. Identify existing MW systems in the area and the source of information (microwave frequency coordination). Attain information (drawings, maps, and so forth) of the existing transmission facilities in the area (e.g., MW, fiber optics, copper) as well as PSTN offices and POPs of the local Telco companies. Determine existing tower and other antenna mounting structures’ capabilities, establish whether there is sufficient space for the MW radio equipment and antenna installation (provide site layouts and tower profiles), and verify access to those sites.1 Find out all the customer-specific requirements (preferred equipment and services suppliers, power backup requirements, schedule, internal processes, and so forth). Identify equipment and service resources (for international projects, try to find local companies). Develop a preliminary transmission network build-out schedule.

The purpose of the scope and task delineation list (sometimes also called the responsibility matrix) is to clearly state responsibilities related to all areas in the project. It is of great importance that all aspects are considered, especially when the customer also has to fulfill certain tasks. It is important to notice that, in a large microwave project, like any other project, a compromise must be made between the speed of deployment, reliability of the system, and the price the wireless operator has to pay for the network. Spending more money and time in the beginning and instituting a well-executed planning process will guarantee a reliable network that will continue to work, even under unfavorable conditions. Implementing sound transmission engineering techniques, such as using proven radio propagation models, using the best network topology available (ring topology, for example), hardware redundancy when necessary, and so on, will always prove to be good investment for the future. 4.2 4.2.1

Microwave Systems in Wireless Networks Backhaul in Wireless Networks

Transmission is an important element in any wireless network, affecting both the services and service quality offered, as well as the costs to the wireless operator. Optimization of transmission solutions is thus

162

Chapter Four

certainly worthwhile from the business operator’s point of view. In current wireless networks, transmission has been optimized for the narrowband circuit switched traffic, and this type of traffic will continue to dominate for some time. The rapid growth in the number and diversity of the cell sites of wireless service providers (WSPs) has had a corresponding influence on the size and complexity of their transmission (and backhaul) networks. Since the introduction of cellular radio systems in the mid-1980s, there has been continued growth in microwave communications systems as a key component of the cellular backbone and access networks. In most cases, new wireless operators opted for use of microwave point-to-point systems for economic and deployment timing reasons. In wireless networks, backhaul is defined as the portion of the network that carries the wireless calls from the cell site back to the base station controller (BSC) and mobile switching center (MSC). Calls are then routed to the appropriate service termination points such as PSTN and/or the Internet. Once relegated as a second-tier priority in relation to RF and cell site deployment issues, WSPs today view their backhaul and transmission networks as strategic assets, since this portion of a network can run as high as 20 percent of an annual expense budget. Planning a microwave system in a wireless network is always a dynamic and continuous process. The transmission (backhaul) system has to be able to satisfy present and future capacity demands and provide reliable service. Transmission engineers have to be involved in the planning of the RF/backhaul network from the beginning, and constant cooperation between RF engineers and backhaul network planners is mandatory. 4.2.2 Wireless Network Design for Coverage and Capacity

Successful wireless network operators launch their services by providing adequate geographical coverage in key areas of the network, including major cities’ main roads and selected special tourist/business locations. Fast service rollout for a new network is of vital importance to enable cash inflow and investment payback. Most wireless networks are initially designed for coverage, and they have very low initial capacity requirements due to a small number of customers. Transmission and microwave engineers have to keep in mind that, very soon after the network launch, this strategy will change, and capacity requirements will increase dramatically in a very short period. The transmission network in such a launch phase is not usually optimized for link capacities or cost, but the transmission system and its management system should be selected for their ability to allow future

Planning the Microwave Network

163

network growth. During later phases, when the wireless network is redesigned for capacity, many new base stations will share the main transmission links of the initial launch network. As the subscriber base increases, successful operators add new base stations, base station controllers (BSCs), and switches to serve all cities and improve roadside and indoor coverage. The number of macro-cellular base stations can grow swiftly to hundreds and ultimately to several thousand, and the number of switches could grow to 20, 30, or more, resulting in a rapid increase in the number and capacity of transmission links. Cost optimization plays an increasing role during this phase. New services, mobile and fixed, have to be introduced with minimum rollout, with the same transmission infrastructure used for multiple services. As the competition in a wireless market grows, service differentiation and introduction of new value-added services play an increasing role. Operators who use their own transmission networks (microwave networks for example) tend to begin offering fixed services such as LAN interconnect, voice, IP, and other services in any phase. As a cost-effective strategy, some operators may use the same transmission network for both mobile and fixed services. 4.2.3

3G Wireless Networks

4.2.3.1 About 3G Wireless Networks First generation (1G) mobile systems were analog and focused only on voice traffic, while second generation (2G) represented the transition from analog to digital systems. Third generation (3G) mobile systems evolved to support more bandwidth-demanding services, such as e-mail, text messaging, and image sharing. Typically, 3G mobile networks require two parallel backbone infrastructures, one consisting of circuit-switched nodes and one consisting of packet-based nodes. This type of network infrastructure doubles the capital and operational expenses associated with deploying, maintaining, and operating 3G mobile networks. Universal Mobile Telecommunications System (UMTS), as an example of the 3G mobile system, has been introduced during the first decade of the new century. It is specified by ETSI and the world-wide 3G Partnership Project (3GPP) within the framework defined by the International Telecommunication Union (ITU) and known as International Mobile Telecommunications—2000 (IMT-2000). The 3G systems can support 2 Mbps for indoor environments and at least 144 kbps for vehicular environments. Until recently, the business of telecommunications operators centered around providing basic voice services (i.e., dial tone) to a mass market and voice and data communications connectivity and services

164

Chapter Four

to large and medium-size businesses. The enormous growth experienced by mobile operators and service providers used to be in voice services. Today, more and more of the traffic in mobile wireless networks is data. The arrival of 3G, initially in the form of packet data capabilities for existing networks and subsequently by full 3G capabilities over new broadband wireless systems, has facilitated that change. New technologies and technical solutions enable higher data volumes right now in existing networks. This development will continue with still higher bit rates over the air interface in the 3G UMTS/HSDPA (High-Speed Downlink Packet Access), EDGE (Enhanced Data rates for GSM Evolution), and CDMA EV-DO. These increasing data traffic volumes mean that the share of the packet-based traffic in the total traffic mix in the mobile network is increasing while total traffic volumes are also rising rapidly. Evolution of the circuit switched networks into packet-based and IP networks is taking time, since it has to be done in well-planned and managed steps so that the efficiency of the mobile network is preserved during the changeover phase. In many cases, basic mobile voice services are also growing quickly due to growth in the number of subscribers, which also contributes to the overall traffic increase and continues to require economic solutions for this type of traffic. Therefore, the well-planned steps are vital to manage mobile operators’ cash flows and to make full use of existing investments. It is in the interest of a mobile network operator to direct future transmission network strategy toward this expected increase in the penetration of advanced data services.2 Data traffic is inherently variable, and transporting it over the TDM network is inefficient. In a TDM-based approach (2G wireless networks), time slots are dedicated for connections regardless of whether information is actually being sent. In a multiservice network, the underlying network can be physically subdivided into multiple networks, one for each service (i.e., voice, data, private lines, and so on). Using an ATM-based infrastructure (in 3G wireless networks), much more efficient use of transmission network is possible because it allocates bandwidth on demand based on immediate user needs. Carrier Ethernet and the IP transport will be the only way to build cost-effective backhaul in the next generation (4G) of wireless networks. As previously stated, packet-based information over the mobile network is showing rapid growth, and any reasonable network development plans have to take this into account and plan for a smooth and economic transition and an evolutionary path for the transmission network. Therefore, in broad terms, the transmission network must continue to provide well-engineered and economically optimized solutions for the growing volumes of circuit-based traffic while developing

Planning the Microwave Network

165

the strategy and readiness to cope with the even faster growing data traffic of the future. This type of transmission solution is needed in all parts of the wireless network, both in access networks with many points and low-capacity links and in core networks with high traffic volumes. This means for example that, in a wireless network, a transmission solution is needed that provides for efficient transport of many voice channels and that can evolve to also carry packet-based traffic, whether asynchronous transfer mode (ATM), Internet Protocol (IP), or both. The radio network will be connected to the core network by a backbone network (access and core transmission network), allowing wideband access and interconnection of subscribers. Deterministic and Statistical Multiplexing In 2G wireless networks, deterministic multiplexing is applied, whereby each connection is characterized by a constant bandwidth (e.g., one time slot). The minimum needed bandwidth over the physical link is then simply the sum of the constant bandwidths of the connections. Since the traffic characterization is not probabilistic, statistical gain is not available. The 3G wireless networks use packet-switched (ATM) systems and statistical multiplexing. When several connections from variablebit-rate sources are multiplexed together, a statistical multiplexing gain is obtained because there is a certain probability that traffic bursts on different connections will not appear at the same time. It is possible to maintain the same blocking probability with less bandwidth if statistical multiplexing is used instead of deterministic multiplexing. The price for it is that the quality of service (e.g., packet delay and loss) will not be ensured in a deterministic but in a probabilistic fashion. Statistical multiplexing of data traffic can occur side by side with the transmission of the delay and loss-sensitive traffic such as voice and video.3 Like voice telephony, ATM is fundamentally a connectionoriented telecommunication system. That means that a connection must be established between two points before data can be transferred between them. An ATM connection specifies the transmission path, allowing ATM cells to self-route through an ATM network. Being connection-oriented also allows ATM to specify a guaranteed quality of service (QoS) for each connection.

4.2.3.2

In the process of dimensioning microwave point-to-point systems for ATM traffic, there are a number of issues to be considered. Since bit errors in the microwave system typically appear in multiples and spread less than the ATM header length, the single-bit header correction feature may not improve cell loss rate as much as predicted and intended. The latest

4.2.3.3 The Effect of ATM on Microwave Link Planning

166

Chapter Four

research shows that the bit error rate is degraded approximately one decade from the microwave radio system to the ATM CBR virtual circuit due to the cell loss. A general assumption based on this could be to assume that any BER requirement should be one order of magnitude higher for ATM traffic. In today’s broadband networks (wireless or wireline), the traffic requirements will increase the total transmission capacity needs enormously. Many of today’s 1E1/T1 links will be increased to STM-1/OC-3 and higher capacities and will require high-capacity SDH/ SONET microwave radios. Aside from the capacity, these microwave radios need a very sophisticated error-correction technique to satisfy ATM transport layer requirements. Normally, in a fiber-optic system, BER should be 10−9 measured at the ATM CBR virtual circuit, and the same quality corresponds to BER = 10−10 in the microwave radio system. Existing radio links were planned using parameters for PDH or SDH systems such as severely errored seconds (SES), errored seconds (ES), residual bit error rate (RBER), and background block error ratios (BBER), for which some time percentages of worst-month statistics have been allocated. When planning is based on 64 kbps ISDN paths (ITU-T G.821), several different grades of quality can be applied, such as high grade, medium grade (four subclasses), or local grade. This applies mainly to existing PDH radio links. For mobile systems, one of the medium grade classes typically is applied (Class 3). When planning is based on primary level or above paths (ITU-T G.826), international portion and national portion are specified. National portion has been subdivided into long-haul, short-haul, and access sections. This applies mainly to existing SDH radio links, while new international synchronous paths should be planned according to ITU-T G.828, which also applies to national and private synchronous paths. New ITU-T G.828 specifies recommended block-based error performance parameters for synchronous digital paths that may support circuit switched, packet switched, and leased circuit services. Synchronous digital paths meeting the objectives of G.828 will enable ATM traffic to meet B-ISDN-requirements of I.356. Residual BER should be below 10−11, which also can be measured by a suitable BER test-set. In practice, four-level modulation or FEC easily fulfils this RBER requirement. If old PDH radio links will be utilized (planned according to requirements of G.821) for packetized traffic, recalculations must be done and a fade margin corresponding to threshold level at about BER = 10−5 (2 × 10−5, to be more precise) is needed for the calculations. Multipath outage probability and the rain outage probability during worst-month must be calculated according to ITU-R Recommendation

Planning the Microwave Network

167

P.530-xx and the result compared to values defined in ITU-R Recommendation F.1668-1 (error performance) and ITU-R Recommendation F.1703 (availability). 4.2.4

4G Wireless Networks

4.2.4.1 About 4G Wireless Networks 4G mobile networks require a single,

all-IP, packet-based backhaul infrastructure, providing carriers with a significant cost advantage. However, the number of mobile devices and multitude of services, such as traditional voice, voice conferencing, image sharing, video, and high-speed data, put the high demand on the infrastructure. In addition, 4G mobile technology is placing huge demands on wireless carriers’ backhaul infrastructure. The multiple, high-bandwidth, quality-sensitive services that carriers have planned for 4G technology requires an infrastructure that is packet-based, scalable, and resilient, as well as cost-effective to install, operate, and manage. Network physical links configuration and mediums used to connect nodes are some of the most important problems of mobile communication network planning because they will determine the long-term performance and service quality of networks. Don’t forget the fact that most of the traffic between the users on a wireless network still goes over some kind of wireline transmission network (fiber, copper), and only the last few to a few hundred feet to the end-subscriber are really and truly wireless. Transmission network could also be designed using wireless backhaul, i.e., a microwave system.

4.2.4.2 4G Wireless Network Technologies At the time of writing this text, there are a number of different ideas what 4G should look like. Unlike 3G, no specific standards today spell out what a 4G service, network, or technology (or a combination of different technologies) is going to be. While the specific qualities of 4G networks have not yet been established by any standards body (to be defined as part of the official ITU-R process for IMT Advanced), 4G is expected to offer extremely fast broadband capacity with peak speeds from 100 Mbps and possibly reaching 1 Gbps. In addition, 4G will offer continuous connectivity and extraordinary reliability and quality with significantly less delay to support real-time services. These capabilities will enable the support of multiple streams of high-definition video simultaneously, in combination with a variety of other applications. As importantly, 4G is anticipated to offer wide capabilities for personalization of services, coupled with support for virtually any communications device. Since none of the wireless technologies can cover all the needs,

168

Chapter Four

multiaccess is emerging, and several technologies and/or their combinations may play a role in 4G as it develops: n

n

Orthogonal Frequency Division Multiplexing (OFDM) and OFD Multiple Access (OFDMA) In the OFDM system, data is multiplexed on many narrowband subcarriers, which can be easily generated by using highly efficient digital signal processing based on fast Fourier transform (FFT). OFDMA provides signals that are less prone to interference and can support high data rates and therefore is considered the most important technology for a future public cellular radio access system. Mobile WiMAX An IEEE specification also known as 802.16e and designed to support as high as 12 Mbps data-transmission speeds using OFDMA, the first version of WiMAX was approved as an IEEE Standard 802.16-2001 and published in 2002. This standard was created addressing fixed line-of-sight (LOS) connections by focusing on licensed frequencies in the range of 10–66 GHz. The next important version was 802.16a, published in April 2003. It addresses the use of lower frequencies: 2–11 GHz, with a range of up to 50 km, a bit rate of up to 75 Mbps, and non-line-of-sight (NLOS) support.

n

n

IEEE 802.20, or so-called Mobile Broadband Wireless Access (MBWA) This specification is the first IEEE standard that explicitly addresses the needs of mobile clients in moving vehicles. The design parameters of the specification include support for vehicular mobility up to 150 mph, and 802.20 shares with 3G the ability to support global roaming.3 Multiple-Input Multiple-Output (MIMO) Aimed at the highest bandwidth efficiency and thus the most suitable for ultra-high data rate transmission; one way to get very high bit rates is to use the multiple-antenna technique, which is capable of realizing spectral efficiency that far exceeds that of single-input single-output (SISO) systems. However, MIMO techniques are suitable for flat fading channels, while the broadband wireless channel is the frequency selective channel in which frequency diversity becomes available. Much work on the study of MIMO techniques has been done, and several schemes have been proposed to exploit its potential.4

n

Long Term Evolution (LTE), also known as UMTS Release 8 This project of GSM/UMTS-based technology evolution brings a completely new air interface based on OFDM and MIMO. It’s being developed by the Third Generation Partnership Project (3GPP) and could

Planning the Microwave Network

169

support 45–144 Mbps of traffic with improved spectrum efficiency and low latency. Compared to UMTS, 3GPP LTE is exclusive and solely packet-switched and IP-based, which means that circuit switched core network does not exist. A common trend is emerging regardless of which 3G, or perhaps 4G, evolution path wireless operators are on. All of these new services have two common characteristics. First, they all generate significant amounts of incremental bandwidth, ranging from hundreds of kilobits to several megabits per second and possibly hundreds of megabits per second in the future 4G. Second, the nature of the traffic they generate is IP, or data-oriented, as opposed to TDM voice or circuit-switched data. IP traffic is by its very nature dynamic both in terms of usage and bandwidth requirements, characteristics that are difficult to manage in a TDM-only environment, which is still the most common case today. This growth of IP traffic is significant in that it places a considerable strain on the traditional wireline infrastructure that wireless operators have been using for decades to backhaul traffic from their cell sites into their core networks. For wireless operators, this portion of their network is often referred to as the radio access network (RAN), and getting the traffic back into their network across the RAN is commonly referred to as RAN backhaul. What is sometimes not considered is that wireless networks have a significant dependency on wireline networking. In fact, once the radio traffic generated by mobile devices hits the nearest cell tower, or base station, the traffic is then transported over a mostly TDM-based traditional leased line infrastructure. From a bandwidth perspective, in common 2G networks such as CDMA, 1xRTT, GSM, GPRS, etc., operators generally require only one or two T1/E1 leased lines for every wireless base station (cell site) location. The deployment of the 3G wireless services, however, generally leads to the initial introduction of two to four incremental T1/E1 lines, essentially doubling their access backhaul needs as a starting point, not even considering future growth requirements or the new 4G network requirements. Wireless operators are not overly keen to expose their mobile voice and data backhaul costs. This is because the figure can run from 20–50 percent of operating expenses (OPEX), and often the wireless provider ends up paying a large sum directly to a competitor, such as a local incumbent. Some predictions specify that, even before the 4G deployment, the bandwidth requirements to the cell site will continue to grow and most likely exceed 20 Mbps in 2010.

4.2.4.3 4G Wireless Network Topology

170

Chapter Four

Although fiber optics–based systems can be a feasible solution to the bandwidth problem, the associated cost of fiber-optic solutions, coupled with the deployment challenges, in most cases may prove as strong deterrents in their widespread deployment for future access networks. So, wireless backhaul emerges as a strong candidate for backhaul transmission; microwave links in commonly utilized 6, 18, 23 GHz bands, as well as the new technologies based on free space optical links (also called optical wireless) or 60, 70/80 GHz and above millimeterwave systems have been studied and implemented as alternative access technologies. These technologies, either as standalone or as hybrid systems, will provide high-bandwidth links for future access networks. In cases where every site requires high-capacity links, it makes sense to envision a network where all (or most) sites are connected in the high-reliability network topology. Fully connected and mesh topology provide a high redundancy for all the sites but is definitely not a very cost-effective way of designing the network. A better approach to physical links topology for the 4G RAN has been proposed and analytically argued, taking into consideration a load and routing capabilities.5 It is named the cluster-cellular or, in other words, it is known as a ring topology. Ring topologies have been used for years in high-capacity backbone systems while the rest of the cell sites were connected in some type of star, daisy-chain, or tree topology. An example of the 4G-RAN ring topology is shown in Figure 4.1. In such a topology, base stations are connected to each other and there is a hub (the cluster-main base station) linked to the RNC. Base stations in the ring topology may be connected to each other by optical

Fiber-optic or microwave links

RNC

Fiber hub or “main” base station

Radio base stations (cell-sites)

Fiber-optic ring Figure 4.1 4G RAN topology

Planning the Microwave Network

171

fiber links that are preferred as the dominant links to construct the 4G RAN, mainly from the viewpoint of link capacity. Nevertheless, it should be emphasized that the infrastructure for the fiber-optic networks is very expensive because of high installation costs, so most likely microwave point-to-point links will be used instead. Microwave deployment is also much faster than laying fiber, especially if the fiber ducts are nonexistent. To avoid single point of failure, dual-homed rings may be implemented; it can be seen from Figure 4.2 that failure of any link or site should not cause the failure of traffic from any other site. Although this architecture seems to be more expensive, due to network survivability it offers a high potential for cost reduction in the long run. It is important to emphasize that operators will choose different network topologies based on their own criteria and preference.6 4.2.5 Replacing Leased Lines with the Microwave System About Leased Lines The goal is to build a network that will provide reliable transmission facilities that are capable of delivering enough capacity for the present needs as well as to ensure seamless expansion in the future. Usually, transmission (transport) facilities are either leased or owned (copper, microwave, fiber optic), and sometimes they are a combination of leased and owned (usually microwave) facilities.

4.2.5.1

Fiber-optic or microwave links

RNC

Radio base stations

Fiber-optic ring Figure 4.2 Dual-homed ring network topology

172

Chapter Four

A successful transmission platform includes necessary network functionality for the entire network life cycle, supporting network growth (scalability) and reconfiguration (flexibility) as well as fast multiservice deployment. In many cases, project managers and those involved in the time-tomarket assessment make a conclusion that a faster way to build the network is to lease T1/E1 circuits from the local telephone companies rather than building their own microwave systems. That may not be the case in every situation and may prove, for a number of reasons, to be much more of an expensive and lengthy process than originally anticipated. The projected requirements for high capacity of the backhaul for the new 4G sites will also slow down the use of leased lines. Dedicated service is a circuit between fixed end points, and the circuit is leased (or owned by an end user) from a common carrier. A dedicated line is obtained from the carrier on a monthly basis for unlimited use between the individual points to be connected. The service is available 24 hours a day, 7 days a week, 52 weeks a year, for exclusive use by that user. This service (usually T1/T3, E1/E3) may also be referred to as a leased line, private line, or nailed-up circuit. In selecting a transmission medium for a network, service providers are faced with questions of quality, reliability, time to market, and cost. The temptation during the initial build-out stage is to utilize leased T1/E1 service because it avoids the capital investment required for microwave networks, and it is generally readily available. Once in operation, a maintenance problem is simply addressed by placing a phone call to the carrier to report the problem. However, the decision to use leased facilities may also mean that the service provider sacrifices reliability, control, ownership, and return on investment. It is very important to notice that, even when the transmission network is completely leased, it still requires a great deal of engineering effort from the wireless operator’s side. Engineering cannot be left to the carriers to make decisions on the leased circuits’ routing, installation, project management, and testing without any supervision from the wireless operator. One of the leading causes of delay in getting T1/E1 lines installed is the unprepared equipment room. It is important to coordinate with carriers and cell-site construction team requirements for the equipment room/shelter. This could include ducts or cable ways for the lines, AC and/or DC power, adequate space, heating and air conditioning, solid grounding, and a very common requirement by carriers for a plywood board on the wall for terminal blocks, NIUs, punch-down blocks, and so forth. In addition, for any maintenance problems on the leased line, the time to repair is at the mercy of the carrier. Therefore, it is important to

Planning the Microwave Network

173

establish a good working relationship with the facility (Telco) provider so as to understand the organization of the company and to access key departments when help is needed. Advantages of Microwave Radios The question is whether to build and own a transmission network or to lease facilities (lines) from existing carriers and/or operators. The answer will be different in different situations. It is important to realize that transmission network planning, design, and implementation will directly affect all stages of the wireless network build-out. Introduction of 3G (and lately 4G) wireless networks, with the increased capacity requirements and packet data and/or IP architecture, will also have a great impact on the future transmission network design and deployment. Owned facilities in any network usually involve planning, building, and maintaining the microwave network. Some of the advantages of microwave radios are listed next:

4.2.5.2

n

n

n

n n

n

A MW system meets superior reliability, higher security, and more demanding performance and quality standards. The operator can reuse existing infrastructure such as shelters, towers, and monopoles to set up new microwave radio systems. The operator has total control over the site access and restoration time. Expansion and future relocation are easy. Microwave systems do not require right-of-way permits from local governments as is necessary for buried wireline systems. MW radio has an operational life (>15 yrs) long after the leased line payback time (2 to 4 yrs). Some disadvantages of a microwave network are as follows:

n

n

n

It involves high capital investment by the operator on day one (unless the vendor is financing it). Microwave system design is required, including frequency coordination and spectrum licensing (which may or may not be a big issue or even required). Microwave network operations and maintenance are required.

The last statement requires some additional explanation. In a welldesigned and properly implemented microwave network, there is very little required maintenance. Assuming that high-quality microwave equipment from a reputable vendor is used, long life expectancy and trouble-free operation is almost guaranteed.

174

Chapter Four

4.2.5.3 The Economic Model and the Payback Period Transmission is an important element in any wireless/mobile network, affecting both the services and service quality offered as well as the cost to the mobile operator. Optimization of transmission solutions is thus certainly worthwhile from the operator’s business point of view. Microwave access, based on point-to-point microwave radios, is the dominant technology in base-station access networks, and it offers the fastest means for network rollout and capacity expansion. When using microwave radio transmission, an operator saves on operational expenses as compared to laying his own cable or leasing connections. At least two-thirds of all base station connections worldwide are based on microwave radios; in North America that percentage is much lower. The main problem with leased lines is the high recurring cost that will always exist unless the intention is to replace it with a privately owned system. In addition to the recurring cost, there is a typical service charge, usually several thousand dollars per facility, and a construction charge, from several thousand to hundreds of thousands, if the carrier is required to build facilities to a site. Leased line service costs also include a one-time hookup (service) charge, a monthly lease rate and, in some regions, a monthly inter-LATA or long distance carrier charge. The cost of nonprotected, single T1 service can vary widely by geographic area from a few hundred dollars to over $2,500 for the hookup charge, from under $250 per month to over $700 per month for lease charges, and up to $15 per km per month if distance charges are involved. Another problem with leased facilities is their limited capacity and the fact that leased transmission facility cost is linear with respect to bandwidth. For example, if twice the capacity is desired, then twice the facility must be leased, thereby doubling the cost. Leased facilities are also inflexible in network design, and it is a lengthy and costly process to reconfigure leased lines to address ongoing changes in the network. Protected (i.e., redundant) T1 service is usually not offered in the standard tariff, and it normally results in monthly leasing charges that can approach twice that of single T1 service. Every digital network project is different and requires a detailed “business case” type analysis of “owned versus leased” transmission facilities. While a network is relatively small, it will generally be more cost effective to employ leased transmission lines (assuming, of course, that they are available at the planned site location) due to the ability to closely match supply to demand. However, at some point (which depends on many factors such as cost of leased transmission versus cost of building infrastructure), it will become more cost effective to build and own the infrastructure in some or all parts of the transmission network.

Planning the Microwave Network

175

Most operators these days require an aggressive payback time of 24 months to justify investment in a new network technology. Let us assume that the 4T1 microwave system cost, fully engineered, installed, and tested, is $50,000 per hop (an example is shown in Table 4.1). Not only is the total cost after four years still the same, it will also provide system expansion for an additional three T1s, thus resulting in the cost per T1 actually being only $7,500. On the other hand, leased T1 has recurring costs of $400 per month (typical in the U.S.), and after four years the cost for the leased T1 would grow to $20,200—almost three times that of microwave T1 circuits. In many countries where leased lines are not readily available or cost a lot more, it would be even easier to prove the advantage of building a microwave network. If not one but multiple T1/E1 circuits are required to connect two points, the superiority of the microwave system and its quick break-even point (payback time) are usually very easy to demonstrate. Equipment supplier financing is an attractive means of acquiring microwave systems, particularly for infrastructure build-out requirements. In general, equipment supplier leasing is the financing method preferred by operators. For each of the technologies, it is assumed that there will be ongoing maintenance and operational costs. The maintenance costs are usually assumed to be around 5 percent of the equipment cost on an annual basis, and the operational costs are assumed to be 2 percent, but it is recommended to use some real data (if available) from a similar type of network, in the same country, and using the same type of equipment. The increasing dominance of IP and Ethernet is enabling a convergence of networks and allowing for a wide range of services, business and residential customers and mobile backhaul, to be carried over the same infrastructure. Requirements for huge cell-site capacity, in excess of 20–30 Mbps, in 4G wireless networks, will make quick leasing of T1/E1 backhaul circuits thing of the past.

TABLE 4.1

Comparison of Leased Lines versus Microwave Systems

Expenses

4T1 Microwave System

Up-front charges Monthly charges After 12 months After 12 months per T1 After 48 months After 48 months per T1

$30,000 (turnkey) $ 0 $30,000 $ 7,500 $30,000 $ 7,500

Leased 1T1 $1,000 $400 $5,800 $5,800 $20,200 $20,200

176

Chapter Four

4.3 Microwave Systems in Utility Telecom Networks 4.3.1

SCADA

Microwave radio systems have been used in many utility companies since the early 1950s. Historically, the role of terrestrial microwave (and satellite) communications in the energy industry has been in reaching remote areas; offshore platform communications; pipeline supervisory, control, and data acquisition (SCADA) systems; etc. Electric power and other utility telecommunications networks have some specific requirements that are different from those of other telecom networks, such as (among others) SCADA, powerline fault location, loop and ring protective schemes, and direct transfer trip protective relaying. SCADA systems are used extensively by power, water, gas, and other utility companies to monitor and manage distribution facilities. Direct transfer trip transfers local protective relay tripping signals hundreds of miles to operate a distant circuit breaker. The allocated time to clear a high-voltage fault is typically three cycles, meaning about 50 ms at 60 Hz, and the large part of the sensing and operation times are specific to devices such as protective relays and circuit breakers whose characteristics cannot be changed. The only variable that can be controlled is the communication channel transit time, which includes free-space delays, channel banks, microwave terminals and repeaters, waveguides, and so on. Typical transit (channel delay) time objective for the communication channel is 10 to 15 ms end to end. It is important to notice that transmission and therefore microwave systems for wireless networks, telephone operators, and government or utility companies will have very different sets of requirements to be considered. 4.3.2 Electric Transmission Towers in Telecom Networks

Large electric transmission towers provide a corridor between generation stations and substations, placed in remote, out-of-the-way locations and run through less-expensive territories. Typical voltages on these transmission towers range from 138,000 to 500,000 V. Their location and height (usually over 150 ft) allow for placement of larger antenna arrays and excellent antenna elevation. These towers are ideal as antenna locations for propagating wireless signals over large areas. Precise engineering and extreme care must be used when placing RF and/or microwave antennas near transmission conductors to avoid the danger of electric arcing.

Planning the Microwave Network

177

From a strictly microwave prospective, these towers and power lines are not an obstacle to installing a microwave system, even if they obstruct the LOS of the microwave system. Signal loss due to the obstructed Fresnel’s zone would be only 1 to 2 dB, depending of the type of the tower and its construction. It has to be emphasized here that there shouldn’t be any obstructions in the near field of the antenna. Installation of the microwave systems using electrical utility poles and towers must be carefully examined, since they may not fulfill the twist and sway requirements of the microwave antenna mounting structure—especially for higher frequencies. 4.4 Topology and Capacity Planning 4.4.1 Transmission Network Capacity Requirements

Aside from immediate capacity requirements, the likely future traffic capacity of nodes connected in the transmission network should be considered so as to appropriately dimension the transmission links. Operators usually dictate the transmission network utilization factor. The utilization factor tells us how much of the total capacity of the certain link is used on day one and how much is a growth margin reserved for future expansion. In a microwave network, the utilization factor should not be more than 70 to 80 percent, leaving at least 20 to 30 percent for network capacity upgrades. This rule of thumb may need modifications in the case of long chains with many microwave links connected in tandem and/or other operator-specific requirements. In addition, the total number of links in a chain or ring of nodes should be such that the transmission links between the included nodes can be relatively easily and inexpensively upgraded (e.g., by simply replacing a modem unit or via a simple software change in a microwave link) to accommodate increased node traffic. This expansion should be accommodated within the chosen topology, without the need for major rerouting of transmission paths. For a small, low-capacity network, a PDH solution is usually sufficient whereas, for the higher-capacity systems, an SDH network is a preferred solution.7 The new gigabit Ethernet microwave radios are also being deployed today. The network traffic plan is a document reflecting all these requirements in terms of T1/E1 (or Mbps) plan as well as the channeling plan. A channeling plan describes the routing of individual channels between microwave sites in their respective T1/E1 trunks and is therefore based on the T1/E1 plan. A T1/E1 plan is usually a drawing that shows the traffic flow of the system in terms of T1 or E1 circuits.

178

Chapter Four

4.4.2

Chain/ Tandem Topology

The overall transmission performance of a tandem (also called daisychain) network is largely influenced by the propagation characteristics of the individual hops. It is sometimes possible to achieve the same overall physical connection by using different combinations of hop lengths. Increasing the length of individual hops inevitably results in an increase in the probability of outage for those hops. On the other hand, such an approach could mean that fewer hops might be required, and the overall performance of the tandem network might not be impaired. In the wireless network, this type of configuration consists of linking cell-sites in a chain such that every cell-site in the chain acts as an active repeater for the previous one (see Figure 4.3). This figure illustrates two chains converging to a common switch and, in this particular case, the configuration can also be considered a tree. A common application of chain is the connection of cell-sites along roads (called highway cell sites). Closer to the switch, where the capacity is higher, it is recommended to have some degree of hardware protection (1 + 1 configuration). If some of the microwave sites are in the remote areas where time-to-repair can be long, protected configuration is also recommended. 4.4.3

Simple Star Topology

Figure 4.4 illustrates a common pattern in which all cell-sites are directly connected to the switch to form a star network. The advantage of this configuration is that the cell-sites may be established to expand capacity requirements in a particular area separately from capacity requirements in other parts of the network, and the network may be

2T1 2T1 2T1 4T1

1T1

4T1

1T1

5T1

BSC

3T1

2T1 Figure 4.3 Chain/tandem network configuration

1T1

1T1

Planning the Microwave Network

5T1

2T1

2T1

2T1

2T1

5T1

1T1 BSC

179

1T1

3T1 3T1

Figure 4.4 Simple star topology

gradually taken into service in accordance with the establishment of new sites. This configuration also has the following disadvantages: n

n

It involves a large number of antennas in one place. This may cause space and strength problems for antenna support structures. Large and robust structures are generally more expensive. The high number of incoming routes may lead to problems in finding a sufficient number of available channels.

This configuration is used mainly in leased lines networks and only under special circumstances in microwave networks. 4.4.4 Topology Utilizing Hubs

Figure 4.5 illustrates another option of the star configuration. In this specific case, the connection is made in two stages. The farthest sites are connected first to a common node (hub), which is connected to the switch. The link from the common node (hub) to the switch will generally have higher capacity than the individual cell-site connections. To handle longer distance, it may be necessary to assign a lowerfrequency band to the link between the hub and the switch. Higherfrequency bands are therefore reserved for the connection of the individual cell-sites. The main drawback to the star configurations is generally the vulnerability for hardware failure in the common node: the hub. This type of configuration is suitable for the wireless networks utilizing statistical multiplexing. For example, if the hub site requires 5 T1s for connection to the switch in a 2G network with deterministic

180

Chapter Four

1T1

2T1

2T1

1T1

2T1

2T1 2T1

5T1

6T1

1T1 3T1

BSC

3T1 Figure 4.5 Star network with hubs

multiplexing, we can assume that not all the sites in a 3G network connected to the hub will be fully utilized all the time. Therefore, there is a chance that we may require only 4 T1s (or even fewer) between the hub and the switch and save on the transmission facilities, either leased lines or microwave. 4.4.5

Ring Topology

By using radio links in a ring topology network, each node in the ring (i.e., each base station in wireless network) is provided with two alternative routes (see Figure 4.6). In the event of a failure in one link, Spur sites 1T1

2T1 12T1

12T1

1T1

2T1

1T1 3T1

12T1

12T1

1T1

12T1

BSC

12T1

2T1 Figure 4.6 Ring (loop) topology

1T1

Planning the Microwave Network

181

the traffic can be sent in the other direction of the ring. The main advantage of this configuration is that it improves the availability of the network and can be built using PDH as well as SDH technology. If the ring has sufficient capacity to carry all the traffic from every site in both directions, then complete redundancy has been achieved. The capacity requirement is the total sum of the individual capacity requirements. A PDH ring with maximum capacity of DS3 (28 T1) can then normally handle 14 nodes, with an average capacity of 2 T1s per site. Note that the traffic generated at each node, along with grooming, may result in other capacity requirements. Unavailability time caused by hardware failure is reduced without the necessity of doubling the radio equipment. That means that an unprotected (1 + 0) configuration can be used for all the links forming the ring without sacrificing the availability of the network. Most links in the ring use a higher capacity than would be used in a simple tandem chain. This means that each link works with lower system gain than in a corresponding tandem chain, which is compensated by less fade margin needed due to the ring protection. As a result, the links in a ring-protected network should be able to use smaller antennas. It is important to notice that the physical layout does not necessarily have to form a ring; it is the actual flow of traffic (i.e. logical connection) that determines the ring topology. 4.4.6

Mesh Topology

The mesh topology is a mixture of the previously described configurations and is currently employed to improve the availability to the network (see Figure 4.7). This configuration is not really a cost-effective solution,

1T1

2T1

2T1 2T1

1T1

BSC Figure 4.7 Mesh topology

1T1

182

Chapter Four

so it is somewhat rare. Furthermore, the traffic distribution presents more complexity in the physical layer. Other configurations normally exhibit equivalent reliability at lower cost. 4.4.7 TDMA Transmission Network Optimization

The main purpose for having digital access cross-connects (DACS) in the transmission network is grooming functionality. This way, operators can efficiently “fill” or “groom” DS0s into the T1/E1 circuit and thereby optimize the use of the network links. When the physical capacity on the radio hop is not fully utilized by the existing service, it might be a better solution to install a DACS and add new traffic over the same T1/E1 circuit rather than upgrading the radio. When upgrading the radio, a new frequency plan, new link budget, and new interference calculations have to be made. In addition, for high-priority circuits, the digital access cross-connect can automatically reroute the traffic in a ring/meshed network if a fault in the primary path occurs. A word of caution: Although efficient circuit utilization in a TDMA network is a good thing, and many telecom and wireless operators prefer this solution to network upgrades, it may not leave room for future changes. In other words, if original voice services required 15 DS0s and the other 9 DS0s were left for future expansion, it may not be such a good idea to use them for something else (for example, to overlay data). If you do so, an increase in voice traffic may later create a problem. Overlaying a new network over the existing network without adding any new transmission capacity may prove to be a dangerous thing. Perhaps building a new network now can save lots of time, effort, and money later. For detailed information on planning transmission networks see Reference 6. 4.4.8

Over-Subscription

In TDMA systems traffic is always deterministic, and so 2 T1s/E1s + 3 T1s/E1s is always equal to 5 T1s/E1s; this is not necessarily so in packet-based networks. In the case of, for example, WiMAX networks, operators can decide to over-subscribe the total network capacity in order to improve overall network utilization and cost per line. Over-subscription, sometimes called over-booking, in simplest terms means taking advantage of the fact that, for many systems, absolute peak demand on shared resources rarely occur. You can see examples of over-subscription everywhere around us; airlines aggressively over-subscribe their seat capacity and public telephone networks over-subscribe

Planning the Microwave Network

183

their network switching capacity, relying on the assumption that all the users will never be on the phone at the same time. The point of over-subscription is that system capacity requirements can be significantly reduced if the requirement to handle absolute worst-case scenarios is ignored. Of course, depending on the nature of the traffic, and how aggressively the resource is over-subscribed, there can be exceptional periods where there is more demand than can be served.8 There are two basic scenarios where operators can choose to oversubscribe one or more service flow’s guaranteed bandwidth, or they might choose to over-subscribe their nonguaranteed bandwidth. Generally, over-subscription of guaranteed bandwidth is a practice that operators approach with caution since their customers naturally expect that their service agreements will be honored always. Over-subscription of nonguaranteed bandwidth is less risky, but an operator must still balance their users’ service level expectations against the degree of over-subscription of the network capacity. Operators will have to determine the extent to which they should oversubscribe their networks. If users are told that they can expect “up to” some peak level of service but discover that during busy hours they can only get a small percentage of that service, they will likely be dissatisfied with their service. Marketing departments usually label this as a “typical” level of service. These calculations may affect the required bandwidth for each individual cell site as well as impact the capacity of the entire backhaul network. Typical oversubscription rates can be different for residential service and for business service. 4.5

References

1. Timiri, S., “RF Interference Analysis for Collocated Systems,” Microwave Journal, January 1997. 2. Bates, J., Optimizing Voice in ATM/IP Mobile Networks, New York: McGraw-Hill, 2002. 3. Lehpamer, H., Transmission Systems Design Handbook for Wireless Networks, Norwood, MA: Artech House, 2002. 4. Jun-Seok Hwang et al, “4G Mobile Networks-Technology beyond 2.5 and 3G,” Seoul National University Republic of Korea, PTC 2007 Proceedings, 2007. 5. Jiangzhou Wang, High-Speed Wireless Communications-Ultra-wideband, 3G Long-Term Evolution, and 4G Mobile Systems, Cambridge University Press, 2008. 6. A. Krendzel et al, “Radio Access Network Topology Planning for the 4G Networks,” Tampere University of Technology, Institute of Communication Engineering, Finland, 2004. 7. ITU-T G.803, “Architecture of Transport Networks Based on the Synchronous Digital Hierarchy (SDH),” 03/2000. 8. SR Telecom, “WiMAX Capacity,” White Paper, 2006.

This page intentionally left blank

Chapter

5

Microwave Network Design

5.1

Introduction

After the preliminary microwave network plan has been approved, detailed microwave network design has to be completed. Site acquisition, microwave network design, RF design (in case of wireless network build-out), and interference analysis are done simultaneously. In most cases, the results are mutually dependent. That means that none of these activities can be done without consultations with and input from the other three. It also means that a project manager has to make sure that these groups of experts talk to each other on a daily basis, which can sometimes present a challenge. The results and deliverables of the microwave network design process will be used during the deployment stage for the actual installation and testing of the microwave system. Microwave path (link) calculations are performed as a part of detailed microwave system design, and all the detailed hardware requirements (bill of materials) are defined based on this information. The microwave design software tools are used for detailed path engineering and interference analysis. 5.2 5.2.1

Spectrum Management Availability of Spectrum

Throughout the last 100 years, the perception has always been that there is not enough spectrum available. However, technology continues to resolve this problem by making more spectrum usable. For example, the upper limit of the spectrum managed by the ITU-R has changed throughout the years; 200 MHz (pre-1947), 10.5 GHz (1947), 185

186

Chapter Five

40 GHz (1959), 275 GHz (1971). 300 GHz is the practical limit of radio frequencies and the beginning of the spectrum where electromagnetic radiation starts to become light. Further increases in radio spectrum capacity now require the more efficient use of the existing spectrum. Congestion of the radio frequency spectrum requires sharing many frequency bands among different radio services and among the different operators of similar radio services. National administrations will allocate some or all these bands for fixed microwave radio use in line with local requirements. To ensure the satisfactory coexistence of the systems involved, it is important to be able to predict, with reasonable accuracy, the interference potential among them, using prediction procedures and models that are acceptable to all parties concerned and that demonstrated accuracy and reliability. Radio regulation is managed at the international level by the ITU. Within the U.S., spectrum management is divided among two agencies; the FCC for the private sector and state and local governments, and NTIA (National Telecommunications and Information Administration) for federal government users. The primary agency responsible for interstate and international communications in the U.S. is the Federal Communications Commission (FCC), which is an independent government agency directly responsible to Congress. Each state also has some regulatory authority over intrastate carrier and local service. The Wireless Telecommunications Bureau (WTB) oversees cellular and PCS phones, fixed microwave, pagers and two-way radios. This bureau also regulates the use of radio spectrum to fulfill the communications needs of businesses, local and state governments, public safety service providers, aircraft and ship operators, and individuals. NTIA was created in 1978 as part of the Executive Branch reorganization. It transferred and combined functions of the White House’s Office of Telecommunications Policy (OTP) and the Commerce Department’s Office of Telecommunications. The OTP was created during the Nixon Administration to provide the president a direct hand in the regulation of media. Its advisory function was placed in the NTIA. The NTIA Organization Act of 1992 codified NTIA’s authority and organization. The DoD (Department of Defense) is the largest government user of spectrum in the U.S. It uses the radio spectrum from ELF band through close to 100 GHz in the Extremely High Frequency (EHF) band. Before microwave network planning commences, the operator must determine the available frequency bands and channel plans that are specific to the country (and the local area) in which the network will operate. There has been a recent trend toward spread spectrum

Microwave Network Design

187

microwave links that do not need to be individually licensed. This includes Part 15 transmitters operating in several industrial, scientific, and medical (ISM) bands, the 5 GHz unlicensed (license-exempt) national information infrastructure (UNII) bands, and many new bands that are licensed by geographical area. Users of a common band of radio frequencies must follow a procedure of radio frequency coordination so as to minimize and control potential interference among systems. Frequency coordination is a multilateral process that involves the cooperative sharing of technical operating information among parties utilizing the same spectrum. In the U.S., the procedures are based on the Federal Communications Commission’s (FCC’s) coordination and licensing requirements (found in Rule Part 101) as well as on related industry practices that have evolved over the years. The radio license applicant must determine if the planned radio system will experience any interference from the existing environment, and vice versa. Potential interference can be calculated for three different cases: 1. Interference between microwave stations 2. Interference between microwave stations and Earth stations 3. Interference between microwave stations and a geostationary satellite in orbit In the U.S., the 4 GHz band is shared with the receive portion of an Earth station and is used predominantly by AT&T and some other longhaul carriers although use of this band by terrestrial microwave has declined significantly in the past ten years. The 11 GHz band is also shared with receiving Earth stations using Intelsat or PanAmSat satellites. The number of Earth stations licensed in this band is relatively small at this time, while the terrestrial microwave in this band continues to grow. Regulations for telecommunications are contained in Title 47 of the U.S. Code of Federal Regulations (otherwise known as the FCC Rules), and rules for the use of microwave transmitters in the bands above 3 GHz for common carriers are contained in Part 101. Part 101 consolidates the old Part 21 and Part 94 rules for the bands above 3 GHz into one set of rules for both common carriers and private operational fixed users. All frequency bands under Part 101 are available for both types of user. The FCC does not maintain an online copy of the rules; however, the Government Printing Office (GPO) does have an online search location at the following web-page: www.gpoaccess.gov/cfr/index.html. To find the Part 101 rules, enter 47CFR101 as the search criterion.

188

Chapter Five

5.2.2 Intersystem and Intrasystem Frequency Coordination

Sometimes, an operator may be able to obtain a number of frequency allocations as a “block,” enabling network planning to be performed in advance, without the risk of interference from other users. Most regulatory authorities also operate a local link length policy whereby the length of a particular path will determine what frequency bands are available from which the operator may choose. Typically, the shorter the path, the higher the frequency required. The local requirement for equipment type approval will also vary from country to country, ranging from a simple paperwork exercise to a full product test program to local standards. Type approval is generally the responsibility of the radio supplier, but an operator should ensure that all requirements are satisfied before any links are deployed. In most other cases, the first step is to perform intrasystem frequency coordination (within its own network) and then, if the results are satisfactory, perform intersystem frequency coordination. A radio license applicant must determine if the planned radio system will experience any interference from the existing environment or create interference within it. Potential interference can be calculated for the three different cases described previously. The design of radio links to achieve a particular performance objective is based on equipment and propagation behavior, taking account of intra- and intersystem interference. Many times during the intersystem interference analysis, it may become necessary to change certain parameters of the microwave link and therefore modify the original microwave transmission design. Intersystem frequency coordination includes a detailed frequency search to identify available frequencies for a proposed microwave path based on provided parameters. The study includes a search of all combinations of frequencies and polarizations. Alternative parameters, such as antenna or equipment changes, are studied to maximize frequency availability. Interference analysis (including simulation) of specific interference situations involving space and/or terrestrial systems, including the identification of possible interference mitigation techniques, is done at this stage of the microwave network design. Billboard-type passive repeaters pose additional problems for the interference analysis. A path that includes a passive repeater is not really one path but two, each of which must be analyzed separately. This can be done by treating the passive site as a repeater station looking in two directions so that the azimuths at the active stations are to the passive site, not each other. If necessary, frequency coordination across the border with other countries (transborder coordination) is also performed.

Microwave Network Design

5.2.3

189

Spectrum Sweep

The key aspect of the frequency coordination procedure involves informed radio frequency planning. Radio systems should be designed such that they do not to cause or suffer objectionable interference while operating with other existing or planned systems using the same frequency band. Sharing coordination data among users facilitates this coordination so that accurate and up-to-date information is available with which estimates of potential interference can be made during the system design stage. Radio frequency interference studies and frequency coordination are necessary not only when designing a new system, but also when one is assessing the potential interference effects of other users’ radio construction proposals on existing and planned systems. Thus, coordination is required when one party initiates construction plans as well as when reacting to other parties’ plans. The results of these studies will indicate whether there is potential interference and whether redesign or relocation of the planned MW system is required. In many cases, the most reliable information about the potential interference cannot be generated by calculation, since there may be little or no information about existing terrestrial or satellite systems in the area (this is often a case outside North America and Europe). The best way is to sweep the entire spectrum using test equipment at the future microwave-system antenna location (at the antenna centerline height) and determine the interference potential at that location. Sweeping the frequency spectrum at the ground level, although a much simpler and cheaper solution, will not produce accurate results; it might show the existence of certain potential interference but will not show the correct signal level at the proposed antenna height or the direction of the interferer. The primary tool used for accomplishing the task of interference analysis (spectrum sweep at the microwave site) is the spectrum analyzer, which shows power level as a function of frequency. The result is a spectrum analyzer plot showing all potential interference in the applicable band. 5.3 Interference Effects and Frequency Sharing For the regulations limiting RF emissions, the FCC distinguishes between intentional, unintentional, and incidental radiators. n

Intentional radiators Devices that intentionally emit RF energy, such as transmitters and antennas.

190

n

n

Chapter Five

Unintentional radiators Devices that intentionally generate RF energy for use within the device or a cable system only but not for the purpose of radiation. Examples of unintentional radiators are computer motherboards and receivers with local oscillators. Incidental radiators Devices that are not designed to generate RF energy at all but for which RF radiation may occur as an unwanted side effect. Examples of incidental radiators are dc motors and mechanical switches.

5.3.1

Interference Paths

Interference is the general term for any kind of radiation disturbance on radio-link systems. In this text, however, only interference caused by radiation from other radio systems will be considered. The government requires users of the radio spectrum to frequency coordinate their planned and existing point-to-point microwave radio systems with other users of the radio frequency spectrum.1 Such coordination is a prerequisite for any microwave radio license application submitted by a microwave radio system operator. The license applicant must determine if the planned point-to-point radio system will experience or cause any interference within the existing environment. The results of this calculation will indicate whether there is potential interference and whether a redesign or relocation of the planned MW system is required. In addition, many countries place some very specific requirements on the MW equipment that may be installed. Channel plans, maximum transmit power at the antenna port, and channel separation requirements can differ from country to country. Considering the case of one transmitter and one receiver (which may be collocated), interference may propagate via the following paths (see Figure 5.1). 1. From equipment housing one unit to that of another unit, between components housed in the same cabinet, or among units in the same telecommunications room 2. From the transmitter antenna to the receiver’s equipment housing 3. From the transmitter’s antenna to the receiver’s antenna 4. From the transmitter’s equipment housing to the receiver’s antenna 5. As spurious signals in the power supply system It is assumed that, if one follows local rules and regulations and performs appropriate installation procedures, interference paths 1, 2, 4, and 5 will be eliminated. On that assumption, only interference between antenna (case 3) systems must be considered.

Microwave Network Design

191

3

4

2

Tx

1

Rx

5 Power supply Figure 5.1 Interference paths

5.3.2

Collocation of Radio Stations

Collocation, also known as co-siting, is a general concept that refers to multistation sites consisting of numerous transmitters and receivers installed within a limited geographical area. The site often consists of a number of antennas that are all mounted on the same tower or distributed among a small number of closely positioned towers. Until the 1990s, an operator could install a wireless tower almost anywhere and with minimal objection from local communities. It didn’t take many towers to support an entire region because there were not too many customers in the first place and most of the data was simple voice. Then requirements for towers changed, towers becoming taller and taller, bringing concerns about everything from aesthetics to radiation. Now, with data services proliferating and citizens more astute than ever, there are new trends emerging in tower equipment colocation. Collocation is a logical and creative siting strategy, and it can be approached in two ways. The first approach is for all operators to try to negotiate collocation of their RF and/or microwave equipment on a common tower. The second is to outsource the business of antenna installation and rent tower space from independent companies. Such companies usually offer site engineering, acquisition, and installation services, and they handle routine maintenance.

192

Chapter Five

Based on the latest FCC requirements, wireless service providers in the U.S. will have to prove that they meet safety guidelines for all cell sites constructed, licensed, and activated before October 15, 1997. They must ensure that cell sites comply with safety limits for human exposure to radio frequency emissions. Some of the applicable requirements for existing and/or new operators are as follows: n

n

n

RF emissions of all new cell sites still must be assessed and documented before the facilities are activated. Anytime a licensee renews its operating license, it must document the compliance of all sites. Anytime an operator modifies a site in any way, it must prove that the site remains compliant.

The collocation trend in the industry can actually create compliance challenges that operators otherwise would not have encountered. The reason is they must submit compliance records for their own equipment and for the equipment owned by collocation tenants at the site. This is particularly important for rooftops where multiple operators install transmitting facilities. In certain situations, and depending on the site accessibility to the public, if emissions exceed the maximum allowed exposure levels, any company that contributes 5 percent or more to the RF emissions in that area is responsible for mitigating the problem. If the tower meets certain height criteria or if the site operates at low power levels, the operator could be exempt from such routine procedures, but it must be proven that the site falls within this category. Three different compliance procedures are acceptable. One approach relies on paper studies to calculate exposure levels based on the type of equipment and operating conditions at the site. Another approach uses industry accepted software tools that employ computer modeling and simulation techniques to perform the calculations. A third method is to take actual measurements at the site location. RF interference is a problem not only in collocated systems; it can occur in any RF system that can interact with existing systems. It is simply that collocation additionally complicates interference analysis and control. Microwave equipment has to be included in all calculations aimed at determining interference levels between operators, including safety and RF exposure issues. The main purpose of such a study is to ensure that the installation of the new microwave link will not substantially degrade performance of existing microwave links at and near that location. The applicant company, according to its interference/availability criteria, must determine the allowed degradation that will be caused by the new microwave link.

Microwave Network Design

193

If, for example, the applicant company has an objective of 0.005 percent of unavailability per annum per link (due to propagation only—mainly rain), this objective cannot be exceeded by the sharing addition. From the point of view of the applicant, the degradation caused by the new link must allow all of the individual unavailabilities of all other links to remain below the desired value (0.005 percent per annum per link, in this example). This is a primary condition for the approval of the sharing, but not the only one; infrastructure and legal questions must always be considered as well. No collocations radio equipment can interfere with the existing communications systems. Proponents desiring to obtain approval to collocate their radio equipment or rack space along with antennas mounts, emitters, or any other ancillary equipment usually need to provide, at a minimum, following information in order to start the permitting process: n n

n

n

n

The location of the proposed site. A detailed description of the equipment that is being requested to be located on the site. Radio equipment interior storage space is normally referred to as a full rack, half rack, or floor- or wall-mounted equipment lease space. Identification of the radio frequency ranges, output power levels, commercial electrical power connections, commercial telephone line connections and emergency power requirements. A description of the number, type, and dimensions of all antennas being requested. Certain financial statement(s).

After that, there are a number of additional engineering, financial, and legal documents that will be required to complete the collocation agreement. There are companies that provide maps of tower locations, addresses, and contact phone numbers, in case collocation and/or lease of the tower space are required. They provide accurate, detailed maps and data to assist in determining if collocation of wireless facilities is feasible. 5.3.3

Minimizing Near and Far Interference

The term near interference refers to interference contributions arising from transmitters and receivers situated at the same site (collocation) or in its immediate vicinity. Other interference contributions are termed far interference. Near interference (also known as on-site interference) means that a transmitter in one site interferes with a receiver in the same site.

194

Chapter Five

Intermodulation is a typical form of near interference disturbance. It occurs because of different kinds of nonlinear processes taking place in the equipment that forms the transmitter and receiver. An intermodulated signal is formed by the addition of interference signals and their integer products. Intermodulation disturbances are generally not expected to affect radio links using waveguides and parabolic antennas because of the higher degree of antenna isolation for typical radio-link antennas. In addition, the intermodulation products and frequencies of radio systems operating in other frequency bands usually fall outside the frequency bands used in microwave communications. By allocating the same duplex band (lower/upper) to all the transmitters at the same site, all receivers at the site will automatically operate in the other part of the duplex bands (upper/lower), and near interference, in most cases, can be neglected. Another important characteristic that should be considered when calculating the effect of near interference is the coupling loss between two antennas located at the same site (see Figure 5.2). This is a very important issue when collocating new with existing microwave equipment, and the coupling loss between two antennas should be approximately 80 dB. In reality, this will depend on the distance and angles between the two antennas. Far interference (also known as far-field interference) is present when a received signal is disturbed by signals that are sent on the same channel (co-channel interference) or an adjacent channel (adjacent-channel interference) and are generated by a transmitter located far away from the receiver (Figure 5.3). The influence of far interference is first noticeable during fading conditions as a deterioration of the receiver threshold level; that is, as a decrease of the path’s fade margin. In interference-free reception, the path fade margin is solely dependent on the path parameters.

X1

L = coupling loss X2 = X1 – L

Figure 5.2 Coupling losses between antennas

Microwave Network Design

195

Rx = f1 Tx = f2

MW link 1

Tx = f1 Rx = f2

Interference paths

MW link 2 Tx = f1 Rx = f2

Rx = f1 Tx = f2

Figure 5.3 Far interference

Far interference is often the primary factor that limits the number of paths that can be set up within a given geographical area. Planning an interference-free (in this case, far interference) network will involve the following considerations: n

Knowledge of the geographic locations of the sites, the layout, and dimensioning of the radio-link paths

n

Equipment data

n

Existing network frequency assignments

n

Reasonably accurate radio-wave propagation models

During reception with interference, the fade margin changes, because the receiver’s threshold is degraded—assuming that the bit-error ratio is kept unchanged. The degradation is generally the result of two contributions: the resulting (total) interference level at the receiver and the receiver noise level. The most serious problem caused by interfering transmitters occurs when they transmit at the frequency to which the disturbed receiver is tuned, producing co-channel interference. In some rare cases, serious disturbances may arise even when the interfering signal lies in an adjacent and separate channel rather than the channel containing the

196

Chapter Five

desired signal (adjacent channel interference), but this is normally a minor problem in microwave point-to-point networks. Both horizontal (azimuth) and vertical (elevation) antenna discriminations are generally included in interference calculations. The ability of digital channels to tolerate interference depends on the modulation scheme.2 In particular, a modulation scheme that requires low C/I for a certain bit-error ratio is more tolerant of interference. Robust modulation schemes are, for example, 2PSK and 4PSK, whereas more complex modulation schemes such as 128QAM require much larger C/I-values. 5.3.4

Frequency Planning

5.3.4.1 Frequency Planning Objectives The objective of frequency planning is to assign frequencies to a network using as few frequencies as possible and in a manner such that the quality and availability of the radio-link path is minimally affected by interference. The following aspects are the basic considerations involved in the assignment of radio frequencies: n

n

n

Determining a frequency band that is suitable for the specific link (path length, site location, terrain topography, and atmospheric effects) Prevention of mutual interference such as interference among radio frequency channels in the actual path, interference to and from other radio paths, interference to and from satellite communication systems, and so on Correct selection of a frequency band that allows the required transmission capacity while efficiently utilizing the available radio frequency spectrum

Allocation (of a frequency band) refers to the frequency administration of a frequency band for the purpose of its use by one or more services. This task is normally performed by the ITU. Allotment (of a radio frequency or radio frequency channel) is the frequency administration of required frequency channels of an agreed frequency plan adopted by a competent conference. These frequency channels are to be used by one or more administrations in one or more countries or geographic regions. Assignment (of a radio frequency or radio frequency channel) is the authorization given by an administration for a radio station to use a radio frequency or radio frequency channel under specified conditions.

Microwave Network Design

197

Allotment and assignment are created in accordance with the Series F Recommendations given by the ITU-R. The allotment consists of one or more alternative radio frequency channel arrangements. These arrangements are used in accordance with the rules of the local administration in a country or geographical region. In most applications, however, frequency bands and frequency channels are already selected and provided to operators. Frequency Channel Arrangements Channels are segments (subdivisions) of a frequency range or a portion (frequency band) of the electromagnetic spectrum. Every channel has a specified bandwidth and, depending on the capacity of the link, a certain number of carriers can be accommodated in the band. For instance, a frequency raster may include four adjacent 28-MHz channels (applicable for 34-Mbps links), but each of these channels can be further divided into four 7-MHz channels (applicable for 8 Mbps). To enable four 7-MHz channels to be included within one 28-MHz channel, the center frequencies of the 28- and the 7-MHz channels cannot coincide. Likewise, each 7-MHz channel may be divided in two 3.5-MHz channels (applicable for 2 or 2 × 2 Mbps). The available frequency band is subdivided into two equal halves: a lower (go) and an upper (return) duplex half. The frequency separation between the lowest frequency in the lower half and that of the upper half is known as the duplex spacing (see Figure 5.4). Each RF channel requires two frequencies (transmit and receive). All transmit frequencies are in one half of the band, and all receive frequencies are in the other half. Frequencies are normally assigned so that all frequencies transmitting from a site are either in the high half or the low half of the band. The duplex spacing is always sufficiently large that the intended radio equipment can operate interference-free under duplex operation—

5.3.4.2

Duplex spacing

TX band

RX band Frequency Duplex band separation

Figure 5.4 Frequency band subdivision

198

Chapter Five

meaning that one channel has one frequency for transmitting and one for receiving. The width of each channel depends on the capacity of the radio link and the type of modulation used. The ITU-R recommends frequency channel arrangements according to homogeneous patterns given as follows: n

n

n

Alternated channels An arrangement of radio channels in a radio link in which two adjacent channels are cross-polarized. Co-channel (orthogonal) band reuse An arrangement of radio channels in a radio link in which the same centre frequency is used on two orthogonal polarizations for the transmission of two signals, which may or may not be independent (see also ITU-R Recommendation F.746). Interleaved channels band reuse An arrangement of radio channels in a radio link in which additional channels are inserted between the principal channels. The center frequencies of the additional channels are shifted by a specified value, which is a significant proportion (such as a half) of the channel bandwidth from the center frequencies of the principal channels (see also ITU-R Recommendation F.746).

Channel frequencies may be available on a link-by-link basis or as a channel block and may be freely used by the operator. In the first case, it is common that a local frequency administration coordinates the use of the frequencies among different users. In the second case, it is up to the operator to coordinate the use of the channels within its own network. Local frequency administrations usually keep track of the use of available frequency bands and the corresponding channel distribution. Several operators may be forced to share the same frequency band but different channels, thus making it necessary to control such radio-link parameters as transmitted power, site coordinates, antenna heights, and so on. The most important goal of frequency planning is to allocate available channels to the different links in the network without exceeding the quality and availability objectives of the individual links because of radio interference. Frequency planning of a few paths can be carried out manually but, for larger networks, it is highly recommended that one employ a software transmission design tool. The frequency planning process can be described as follows:

5.3.4.3 The Frequency Planning Process

1. Define the overall structure of the network by determining the location of all the nodes that have to be connected. 2. Allocate the appropriate quality and availability objectives for every portion of the network (no frequencies are involved in this step) and perform quality and availability calculations.

Microwave Network Design

199

3. Estimate the traffic requirements and capacity. It is a good practice to start frequency planning with highest-capacity links in the most concentrated node. This will normally result in the number of frequencies needed in the network, and other links should reuse the same frequencies. 4. In some cases, it may be necessary to use channels from more than one frequency band as a result of the limited number of available channels in the first selected band. 5. Start assigning a duplex half (lower/upper) for the transmitter in the sites of the network. Generally, near interference should be avoided as much as possible by strictly allocating the same upper or lower duplex half to all transmitters (or receivers) on the same site. Generally, two alternatives are possible: n

n

In a chain of sites, there will be alternating lower/upper sites; that is, the transmitter in site 1 is L (lower), site 2 is U (upper), and site 3 is L, and so on. A microwave ring (using only one frequency band) should always have an even number of hops. In a ring with an odd number of sites, the transmitter of the first site will be assigned the same duplex half as the receiver of the last site (which is the first site in a closed ring), causing serious interference.

6. Consider antenna discrimination aspects in the early stages of frequency planning. For instance, in a common site (e.g., a node or hub), the links having sufficient separation angle may use the same channels. In addition to angle separation, distance separation (coupling loss) between two antennas may also provide a certain degree of discrimination. 7. At microwave frequencies, antenna discrimination increases rapidly with angle separation and is an extremely efficient factor in suppressing interference. Thus, if the two links are not closely aligned to a common line and with the (upper or lower) transmitters transmitting in the same direction (in other words, no overshoot), it is normally possible to reuse frequencies between two such links. 8. In congested areas it may be necessary to use antennas that have high front-to-back ratios and large side-lobe suppression. These result in good frequency economy and, in the final analysis, good overall network economy. High-performance antennas may be a suitable alternative. 9. Reuse frequencies and polarization as often as possible. 10. Perform a new quality and availability calculation (after the frequency allocation) and identify links that do not meet the quality

200

Chapter Five

and availability objectives. Far interference calculation is performed, and the receivers having relatively high threshold degradation values are probably a part of the links that do not meet the quality and availability objectives. Make the appropriate changes (polarization, channel, frequency band, antenna size, and so forth) and ensure that a new interference calculation gives lower threshold degradation values. 11. In some situations, higher output power of a transmitter may improve the quality and availability figures without a significant interference contribution to the network. These favorable situations, however, are not very common, so it is not advisable to use a higher output transmit power than necessary. It is a good idea to start frequency planning with the lowest available output power. 12. If the choice is between higher transmitter output power and larger antennas, choose (if possible) a larger antenna. 13. Repeat step 10 until the quality and availability objectives of all portions of the network are accomplished. 14. Based on the fact that transmitting and receiving frequencies change from site to site, there are two types of sites in the microwave system; one is when the receiving frequency is higher than the transmitting frequency, called “high,” and the other one is when the receiving frequency is lower than transmitting frequency, called “low.” High/low site is named based on the receiving frequency. 5.3.4.4 Network Topology and Frequency Planning Interference aspects may severely limit the number of links in a network if appropriate caution is not exercised in the earlier stages of frequency planning (see Figure 5.5). In what follows, some general aspects, based on former sections, are illustrated.

1. Since paths of a chain have very sharp angles, using the same channels by changing polarization (HP/VP) may be a good alternative to using two alternate channels in the chain. 2. Figure 5.5 shows the same channel used alternately with horizontal (HP) and vertical (VP) polarization. Upper (U) and lower (L) duplex halves for the transmitters are illustrated in each site. 3. In the tree configuration, and for sharp angles, polarization discrimination ensures the possibility of using the same channel with different polarization (HP and VP). Both transmitters on the common node have the same duplex half (U).

Microwave Network Design

L Spur site

f1 VP Hub site f1 HP

U

f1 VP

L

f1 HP

L Tree configuration

L U

f1 VP

U

f1 HP

U

Chain/cascade configuration

f1 HP

U

201

U

f2 VP

f1 HP

U U

f1 VP f1 HP

f1 VP

L

L

Ring configuration

L

f1 VP U

f1 HP

f1 HP

U

f2 VP U

Star configuration

Figure 5.5 Frequency planning for different network topologies

4. In the ring configuration, the same channel, with the same polarization, is employed in the perpendicular paths but with different polarization in the parallel paths. The transmitters are alternately labeled upper (U) and lower (L) duplex halves. Although the picture does not represent a physical ring configuration, the logical configuration and traffic flow are indeed ring in nature. If the ring consisted of an odd number of sites, there would be a conflict of duplex halves, and changing the frequency band would be a good alternative. 5. In the star configuration, as noted earlier, all transmitters on the common node should have the same duplex half (L). Keep in mind that this configuration displays a difficult frequency planning scenario and is very sensitive to the geometry (mutual angles). If the node is a concentration point for high-capacity links, wide bandwidth is required, thus making the allocation of smaller channels in other portions of the network quite complicated. It is recommended that the link carrying the traffic out of the hub should use a frequency band other than the one employed inside the cluster.

202

Chapter Five

6. Mesh configuration presents a complicated frequency planning scenario as a result of several conflicts of duplex halves. In addition, it normally requires more channels than do other configurations. Low Frequency Bands and the FCC Loading Rules The low frequency bands available for high capacity radios in North America are shown in Table 5.1. High capacity radios are also available in the 15, 18, and 23 GHz bands. These radios normally use a single RF channel. Maximum path lengths are limited to five miles or less in the high frequency bands due to rain outage. The preferred bands in the U.S. for high-capacity microwave systems are the lower 6 GHz (L6) band (5.925–6.425 GHz) and the 11 GHz band (10.7–11.7 GHz). High-capacity microwave radios typically operate at data rates of 135 to 155 Mbps within a 30 MHz channel bandwidth. FCC Part 101.141(a)(3) defines the minimum loading rules for digital microwave radios. Radios using a 10 MHz channel bandwidth or greater must have 50 percent loading within 2.5 years. This requirement applies to all high capacity 1DS3, 3DS3, and OC3 radios in the lower 6 GHz and 11 GHz bands. A radio is considered 50 percent loaded if at least 50 percent of its DS1 channel capacity is being used. A DS1 channel is considered used if it is connected to a DS1/DS0 multiplexer (e.g., a channel bank). There are no DS0 loading requirements. For non-DS0 services, the next largest DS1 equivalent is considered to determine loading. Systems carrying more than 50 percent digital video are exempt from the loading rules. The lower 6 GHz band is increasingly congested. Furthermore, the L6 GHz band has a large number of licensed satellite Earth stations with each Earth station routinely coordinated for the entire 5,925–6,425 MHz band, and for the entire geosynchronous arc, even if the Earth station actually communicates with only one transponder on one satellite. In U.S., the congestion in the L6 GHz band has led a number of applicants to seek licenses to operate in the upper 6 GHz band (6,525–6,875 MHz)

5.3.4.5

TABLE 5.1

High-Capacity Frequency Bands

Frequency [GHz]

Maximum Bandwidth

Frequency Plan

5.925 – 6.425 6.525 – 6.875 6.425 – 6.930 6.875 – 7.125 7.125 – 8.500 7.125 – 8.275 10.7 – 11.7

30 MHz 10 MHz 30 MHz 25 MHz 30 MHz 40 MHz 40 MHz

FCC Part 101 & Canada FCC Part 101 Industry Canada FCC Part 74 US Federal Government Industry Canada FCC Part 101 & Canada

Microwave Network Design

203

pursuant to waivers that permit them to operate fixed stations in U6 with bandwidths that are greater than the authorized 10 MHz. These waivers issued by FCC were granted upon showing that there were no channels available in the L6 GHz band, that other higher frequency bands were not suitable for the proposed path, and that there were no other alternatives. 5.4

Microwave Design Tools

The International Telecommunications Union (ITU) publishes recommendations for the field of telecommunications. Recommendations for telecommunications are published in ITU-T, and recommendations that have been adapted for radio communication are published in ITU-R. The International Organization for Standardization (ISO) and American National Standards Institute (ANSI) are other organizations that promulgate standards, and they are referenced in this book where applicable. For the proper planning of terrestrial line-of-sight systems, it is necessary to have appropriate and widely accepted propagation prediction models, methods, and data. Methods have been developed that allow the prediction of some of the most important propagation parameters affecting the planning of terrestrial line-of-sight systems. As far as possible, these methods have been tested against available measured data and have been shown to yield an accuracy that is both compatible with the natural variability of propagation phenomena and adequate for most present applications in system planning. Most microwave network design software tools are developed by radio manufacturers and therefore are biased toward the manufacturers’ own equipment. In other cases, the tool may be proprietary and not sold on the open market.3 These tools are sometimes provided to engineering personnel who are working on the customer’s site and performing network design. While some microwave equipment manufacturers insist on using their own software tools, some operators and consultants prefer to use commercially available tools. One such vendor-independent tool is Pathloss 5.0 (and the older 4.0 version). This tool is probably one of the best (and most moderately priced) tools for the complex microwave design, including North American and ITU standards, different diversity schemes, diffraction and reflection (multipath) analysis, rain effects, interference analysis, and others. Radio equipment parameters for equipment from any vendor, channel tables, antenna diagrams, and so on are defined and stored in the default parameters database for easy retrieval. This tool is widely accepted by microwave system design engineers around the world.

204

5.5 5.5.1

Chapter Five

Microwave Systems Engineering System Documentation

In addition to the strictly microwave path engineering part of the project, there is a system engineering portion. System engineers usually provide technical direction and design to guarantee overall system integrity by verifying that all subsystems and contractor-furnished equipment are compatible, and that the desired performance is realized. Systems engineering will also provide transport traffic design and complete system integration as well as the network management system (NMS) and its integration into the other MW and fiber-optic systems. In the next phase, application engineers review and translate the entire system configuration requirements into specific hardware implementations, including standardized interface levels, intra- and inter-rack cabling, and original equipment manufacturer (OEM) integration requirements, and they produce the following system documentation: n

n

n

n

Criteria, methods, standards, and procedures used for MW path engineering A system block diagram that details the main equipment provided by the contractor; all equipment grouped per site with interconnectivity between sites identified A block and level diagram that shows all the equipment provided by the contractor; may also show connections, interfaces, and signal levels to existing equipment at the same site A T1/E1 plan showing the system traffic routing; shows each T1/E1 connection from the originating site to the destination site

n

A channeling plan

n

A synchronization plan

n

Path engineering results

n

n

n n

n

A power consumption document that provides the value of the total power requirements of all equipment provided by the contractor according to the enclosed equipment spreadsheet, on a per-site-basis An equipment list (bill of materials) that includes all the equipment that needs to be provided Site plan A tower profile showing all the radio equipment and transmission lines installed on the tower (see Figure 5.6) Floor plans and equipment layout

N Lightning rod

Path to Z Azimuth = 101°

Top of the light CL = 103' AGL

Top of the steel CL = 100' AGL

Proposed 10' parabolic dish CL = 88' AGL, Azimuth = 101°

Equipment shelter

Path to X Azimuth = 256°

Existing 6' parabolic dish CL = 80' AGL, Azimuth = 256°

G uy

Existing PCS antennas CL = 53' AGL

wi 3'

yw

ad

AG L

Chainlink fence 7'–0"

10'– 0"

AG L

ss ro

Guy tensioners Cable bridge

12'–0"

15'– 0"

10'–0"

Samobor Tower Site Microwave tower site

85'– 0"

Harvey Lehpamer HL Telecom Consulting May 29, 2009 Drawing Number: 0752009

205

Figure 5.6

Acce

Microwave Network Design

Equipment shelter

ire ,5

0'

100' guyed tower

re ,7

L G 'A

Gu

wi

00

uy

,1 re

G

206

n

n

n

Chapter Five

A rack profile that shows the equipment mounting position on the rack Wiring diagrams showing equipment and inter-rack wiring, cabling, waveguides runs, and so forth Geographical system layout

5.5.2

Equipment Availability Calculations

A microwave link can become unavailable for a number of reasons, but this calculation includes only predictable equipment failures. Therefore, it excludes problems caused by misaligned or failed antenna feeder systems, extended loss of primary power, path propagation outages, human error, and other catastrophic events. Short-term (<10 sec) propagation outages are applied to the performance (not availability) objective and will not be used here. It is important to define the terms and parameters used in equipment availability calculations as follows: A = 100 (1 – U) [%]

(5.1)

where A = availability (percentage of time, percent) U = unavailability (percentage of time, percent) For n pieces of equipment connected in series (tandem), U = U1 + U2 + … + Un

(5.2)

For two pieces of equipment connected in parallel, U = U1U2

(5.3)

Unavailability of the microwave radio terminal can be expressed as: U =1−

MTBF MTBF + MTTR

MTTR U= MTBF + MTTR

(5.4)

where 9 MTBF = mean time between failure (MTBF = 10 /FITS) 9 FITS = failures in time (in 10 hrs) MTTR = mean time to repair

MTTR = RT + TT + (1 – P)TR

(5.5)

Microwave Network Design

207

where RT = repair time on site TT = travel time to the site P = probability that a spare module is available when needed TR = time to obtain the spare module (assume 24 hrs) FITS (failures in time) is an internationally used unit for measuring or specifying failure rates. Because individual components or subsystems are generally highly reliable in their own right, the convention has arisen of using a period of 109 hrs (114,155 yrs) as a time unit or time scale on which to quantify failure rates (or conversely MTTFs); a failure rate of one failure in 109 hrs equals 1 FIT. Example: The typical protected MW terminal (1 + 1) has MTBF of 2,200,000 hrs; it takes 0.5 hrs to do the actual repair at the remote site, and the travel time is 3 hrs. With good maintenance practice and spare parts available, we can assume P = 95 % (or 0.95). Let us calculate unavailability of four MW hops connected in tandem (daisy-chain).

MTTR = 0.5 + 3 + (1 – 0.95)24 = 4.5 hr Microwave hop (excluding all other equipment) has two terminals in series, so the unavailability is  2, 200, 000  = 4.091 × 10 −6 U HOP = 2 1 − 2, 200, 000 + 4.5   For the four-hop system, total unavailability is UTOT = 4 × 0.000004091 = 0.000016364 Total availability is ATOT = 100(1 − U TOT ) = 100(1 − 0.000016364) ATOT = 99.9 9984 % It is important to notice that this number includes only microwave terminals, and all the other equipment is excluded. Calculations that are more detailed should include channel banks and multiplexers, power supplies, and other items.

208

Chapter Five

Total unavailability of the microwave link can then be calculated as a sum of the equipment unavailability and the unavailability due to the propagation issues (rain), i.e., path unavailability. 5.5.3 Availability of Different Network Topologies

The question that very often engineers have to answer is related to the selection of the best and most reliable network topology. Each topology has its advantages and disadvantages, but here we will analyze them from the prospective of the individual microwave path availability/reliability. Unavailability could be caused by hardware failures or propagation problems due to rain, or it could be a combination of both. Multipath, under normal propagation conditions, typically does not cause traffic outage and therefore does not contribute to unavailability. So, let us for a moment assume that all the link unavailability values are the same, i.e., U, and calculate and compare average unavailability per cell site for three different network topologies. Shown here is a simplified method of calculation that assumes uncorrelated failures and availability of the switching mechanism (in the ring topology) of 100 percent. When we say a transport network is “100 percent restorable,” we generally mean that it has a sufficient amount and distribution of spare capacity so that any single failure can be withstood without service outage. In principle only dual failures can then cause any service outage. There are increasing numbers of mission critical services calling for as little as 30 seconds (or less) of unavailability per year. This may actually require the ability to withstand certain types of dual failures and motivates analysis of the effects of dual failures on single failure restorable designs. Dual failure analysis is beyond the scope of this book and will not be discussed here. 5.5.3.1 Linear and Star/Hub Topology The following calculation, based on Figure 5.7, shows an average improvement of 40 percent in BTS availability (or reduction in unavailability) for the star/hub network topology over linear (daisy-chaining) topology. Linear (Daisy-chain or tandem) Topology

UTOT = U6 + U7 + U8 + U9 + U10 UTOT = U + 2U + 3U + 4U + 5U = 15U UBTS = 15U/5 = 3U (per BTS)

Microwave Network Design

CELL SITE 2 U2 MW sites in star/hub topology U1

209

CELL SITE 3 U3

CELL SITE 4

U4

CELL SITE 1

U5

CELL SITE 5

MW HUB

CELL SITE 6

U6

MW sites in linear topology

CELL SITE 7

U7

CELL SITE 8

U8

CELL SITE 10

CELL SITE 9

U9

U10

Figure 5.7 Availability in the linear and star/hub topology

Star/Hub Topology

UTOT = U1 + U2 + U3 + U4 + U5 UTOT = U + 2U + 2U + 2U + 2U = 9U UBTS = 9U/5 = 1.8U (per BTS) Ring Topology For a larger transmission network it is recommended to use ring configuration (Figure 5.8) as a high-capacity microwave backbone carrying traffic to the switch location.

5.5.3.2

UTOT = U1 × (U2 + U3 + U4 + U5 + U6) + (U1 + U2) × (U3 + U4 + U5 + U6) + + (U1 + U2 + U3) × (U4 + U5 + U6) + (U1 + U2 + U3 + U4) × (U5 + U6) + + (U1 + U2 + U3 + U4 + U5) × U6

210

Chapter Five

UTOT = U × 5U + 2U × 4U + 3U × 3U + 4U × 2U + 5U × U UTOT = 5U2 + 8U2 + 9U2 + 8U2 + 5U2 UTOT = 35U2 UBTS = 35U2/5 = 7U2 (per BTS) If we assume that the microwave link availability is 99.999 percent (0.001 percent unavailability) we can calculate the average unavailability per BTS for three different topologies. For the linear topology of the five cell-site network, the average unavailability per BTS is 0.003 percent (availability = 99.997 percent); for the star/hub topology, the average unavailability per BTS is 0.0018 percent (availability = 99.9982 percent), and for the ring topology, 0.00000007 percent (availability = 99.99999993 percent). Although this is just an illustration, the advantage of using ring topology from the increased availability prospective is obvious. Let us now consider a secondary ring connected to a node in the primary ring (see Figure 5.9). The secondary ring has links with lower capacity than the primary ring, so the traffic can only flow from the secondary to the primary ring. The total unavailability of a specific node in the secondary ring is calculated by adding the unavailability of the node in the primary ring, cell-site 3, to the unavailability calculated at that specific node in the secondary ring. The unavailability of a specific node in the secondary ring is obtained similarly as in the primary ring. The unavailability of more secondary rings can be calculated in the same way.

MW sites in the ring topology CELL SITE 1 U2

U1

MW HUB

CELL SITE 2 U3

U6 CELL SITE 5

U4

U5

CELL SITE 3

CELL SITE 4

Figure 5.8 Availability in the ring topology

Microwave Network Design

211

MW sites in the ring topology CELL SITE 1 U2

U1

MW HUB

CELL SITE 2

U6

CELL Secondary ring SITE 6

U3

CELL SITE 5

U4

U5

CELL SITE 3

CELL SITE 4

U6

U7

U9

U8

CELL SITE 7

CELL SITE 8 Figure 5.9 Availability in the secondary ring

For example, if we calculate the unavailability of the site furthest away from the MW Hub, cell-site 7, we would get the following: UBTS7 = UBTS3 + (U6 + U7)(U8 + U9) = (U1 + U2 + U3)(U4 + U5 + U6) +(U6 + U7)(U8 + U9) Again, for the sake of simplicity, we will assume that all the unavailabilities are the same (this will never be the case in the real networks, but the calculation principles will be the same,) i.e., U: UBTS3 = 3U × 3U = 9U2 UBTS7 = 3U × 3U + 2U × 2U = 9U2 + 4U2 = 13U2 So, if U = 0.001%, UBTS3 = 0.00000009%, ABTS3 = 99.99999991%, and UBTS7 = 0.00000013%, ABTS7 = 99.99999987%. Important: Note that percentages can be added and/or subtracted directly but cannot (a common mistake) be multiplied or raised to the power without prior conversion into the decimal number. Here is the example of detailed calculation: 2

 0.001  ⋅ 100% = 0.00000009% U BTS3 = 9U 2 = 9 ⋅   100 

212

Chapter Five

Connecting cell-site 6 to cell-site 2 instead of to cell-site 3 is even a better solution, and would create so-called dual-homed secondary ring. N+1 Protection In telecommunications, N+1 is a commonly applied protection method, either for equipment and/or path protection. Here, one channel protects multiple N channels. If the unavailability of the unprotected channels equals U, the unavailability of one protected channel is equal to:

5.5.3.3

U N +1 ≈

( N + 1)U 2 2

(5.6)

So, as a result we get: U1+1 ≈ U 2 , U 2+1 ≈

3 2 U , U 3+1 ≈ 2U 2 ,..., U 7+1 ≈ 4U 2 2

This calculation assumes that the MTBF is the same for all radios, failures occur independently, and the multiline switching is perfect. In practice, the switching is not perfect and will contribute to the equipment outage. Additionally, secondary (low priority) information can be carried by standby channel(s) when they are not required to protect the main information streams. 5.6 Tips, Hints, and Suggestions 5.6.1

Basic Recommendations

In this section, we summarize some of the sound microwave network planning and design techniques that will help reduce the potential of having problems later during the system deployment. Most of the topics discussed here have been either mentioned and/or discussed in more detail elsewhere in this book, but it is a good idea to mention them again, in one place, in the form of a guidance checklist. 1. Use higher frequency bands for shorter hops and lower frequency bands for longer hops. 2. Avoid lower frequency bands in urban areas. 3. Use star and hub configurations for smaller networks and ring configuration for larger networks. 4. Use high-performance antennas in urban areas to minimize interference. 5. In areas with heavy precipitation, if possible, use frequency bands below 10 GHz.

Microwave Network Design

213

6. If bands above 10 GHz have to be used, consider microwave radios with adaptive modulation. 7. Use unprotected systems (1 + 0) only for spur sites, low capacity and/or generally low-importance links. Use protected systems (1 + 1) for all important and/or high-capacity links. 8. Leave enough spare capacity for future expansion of the system. 9. Below 10 GHz, multipath outage increases rapidly with path length. Multipath effects can be reduced with higher fade margins. If the path has excessive multipath outage, the performance may be improved by using one of the diversity methods. 10. The reflection coefficient (and therefore chances of multipath fading) decreases with frequency. 11. Vertical polarization is less susceptible to reflections. 12. Vertical polarization is less susceptible to rain attenuation. 13. Space diversity is a very expensive way of improving the performance of the microwave link, and it should be used carefully and as a last resort. 14. The activities of microwave path planning and frequency planning preferably should be performed in parallel with line-of-sight activities and other network design activities for best efficiency. In addition, start the official process of frequency coordination and licensing process as soon as possible. 15. Use updated maps that are not more than a year old. Magnetic inclination as well as the terrain itself can change drastically in a very short time period. Make sure that everyone on the project is using the same maps, datums, and coordinate systems. 16. The datum or reference ellipsoid selection must be the same for the site data, image, and elevation files. The same map projection must be used for the image and elevation files. 17. Avoid using design software programs with unknown algorithms. This could result in a microwave radio network with poor performance and quality. 18. Consider future network expansions in the frequency planning activity. It must be possible and easy to add new hops as the network expands. 19. Perform detailed path surveys on all microwave hops. Maps are used only for initial planning, as a first approximation (see Chapter 6 for details on performing site/path surveys). 20. Keep in mind during the path surveys that trees and other vegetation grow, and plan for at least 10-yr growth, preferably even 20 yrs.

214

Chapter Five

If trees are already matured, their rate of growth is slower than the young trees. There are very few areas where trees will grow over 100 ft. 21. Make sure that all of the obvious requirements are fulfilled (i.e., enough space for the transmission equipment, available AC and DC power, enough space for antenna installation and panning, access to the site, and so forth). 22. It is very important to use an expert tower company to calculate the loading of the tower and the maximum allowed twist and sway of the structure. These decisions cannot be made on the basis of qualitative perceptions or a gut feeling. Do not try to save money by using the “ballpark” method! 23. Obtain microwave radio, transmission lines, antennas, and so forth only from a reputable and reliable supplier. 24. Hire an experienced project manager, preferably with recent involvement in a similar type of microwave project. Use lead engineer(s) to support and provide help for the less experienced project manager. 25. Waveguide installation is an extremely tricky operation—use only expert waveguide installers and riggers. People installing PCS and cellular antennas are not necessarily qualified for microwave antenna and waveguide installation. 26. Keep a good record of all the design documentation, survey reports, change orders, ATP results, and so on. Many of the long-term test results later will be useful as a benchmark for maintenance and troubleshooting. 27. Finally, do not try to over-dimension the network, as this will make it unnecessarily costly. Every network, regardless of the type, will have brief outages from time to time, and microwave networks are no exception. A network that does not fail is a fiction. 5.6.2

Difficult Areas for Microwave Links

Some areas are more difficult for microwave links than others, and this is usually related to path or atmospheric conditions.4 An example of difficult conditions for microwave links is connecting offshore (gas or oil) platform to the mainland; a very challenging undertaking indeed. First of all, the platform itself is a continuously moving object and any radio solution would have to be able to cope with the continual movement of the sea. Second, some areas where we find these platforms (Middle East or North Africa, for example) have a wide temperature range, i.e., extremely hot during summer days, with much lower

Microwave Network Design

215

temperatures during the night and in winter. This will cause anomalous propagation conditions that alter and can cause significant fading. Third, the distances involved are usually long, and there is a limit on the height of metal structures because of safety regulations. The following is a partial list of recommendations for the design and installation of microwave links in difficult areas: 1. In areas with lots of rain, use the lowest frequency band allowed for the project. Consult a local meteorological station for the “real” rain data and rely on the Crane or ITU rain maps only if no other information is available. 2. Be especially attentive during the design of microwave hops over or in the vicinity of the large water surfaces and flat land areas, as they can cause severe multipath fading. Reflections may be avoided by selecting sites that are shielded from the reflected rays. 3. Swamp and rice fields may cause ground reflections so there is a high probability of multipath fading. The worst time of the year is the rainy season, also called the monsoon season. 4. Hot and humid coastal areas have a high ducting probability. 5. Desert areas may cause ground reflections, but sand does not have a high reflection coefficient. Most critical is the possibility of multipath fading and ducting caused by large temperature variations and/or temperature inversions. 6. Multipath typically occurs at sunrise, sunset, and during the night hours, when the air is calm, and stable refractive layers form in the atmosphere. Multipath is most common during the summer months, when temperature and humidity differences are the most extreme. In North America, multipath outages are most severe near the Gulf of Mexico, the Great Lakes, and the Los Angeles basin, where humid air masses mix with dry continental air masses. Multipath is much less common, for example, in the Rocky Mountain area, where air circulation in the mountains prevents the formation of stable atmosphere. 7. If upfading is a serious problem, smaller antennas, lower transmit power, or receiver attenuators can be used. These changes will improve upfade outage but can be used only on shorter paths, since this approach will reduce fading margins to combat multipath and rain. ATPC will reduce upfade outages without affecting the fade margin. If the angle of the path line to horizontal is more than 0.5°, upfading is not significant and can be ignored, since the microwave signal can penetrate the ducting layers that cause upfade.

216

Chapter Five

5.7

References

1. ITU-R Recommendation P.452-7, “Prediction Procedure for the Evaluation of Microwave Interference between Stations of the Surface of the Earth at Frequencies above About 0.7 GHz,” 1995. 2. TIA, Telecommunications Systems Bulletin, TSB-10-F, Interference Criteria for Microwave Systems, 1994. 3. Lehpamer, H., Transmission Systems Design Handbook for Wireless Networks, Norwood, MA: Artech House, 2002. 4. Henne, I. and Thorvaldsen, P., Planning of Line-of-Sight Radio Relay Systems 2nd ed., Singapore: NERA Telecommunications, 1999.

Chapter

6

Microwave Network Deployment

6.1

Introduction

Microwave deployment (or implementation) is a multidisciplinary activity that involves a number of very specialized experts, regardless of whether it involves a new microwave system or an upgrade or expansion of the existing facilities. The related activities are as follows: n

Program/project management

n

Site/path surveys (also part of the design phase)

n

Site civil work

n

n n

n

n

n

n n

Site preparation, including grounding, lightning protection, and surge suppression Tower and building foundation construction Design, procurement, and erection of the antenna structures and equipment shelters Design, procurement, and installation of power systems (e.g., AC/DC, solar, diesel generators) Procurement, installation, integration, testing, and commissioning of all the equipment required to complete the microwave transmission system Fulfillment of all regulatory requirements (e.g., local authorities, FCC, FAA) Completing as-built documentation Completing training (on-site and off-site), maintenance, technical support, and repair services 217

218

Chapter Six

n

Provisioning for the future upgrades and network expansion

n

Acceptance testing (ATP) and commissioning

Not all the projects will include all of these activities. For example, the upgrade of an existing microwave system will not have tower erection included, but it may take in tower structural analysis and modifications or improvements as required. 6.2 6.2.1

Digital Microwave Radio Microwave Radio Configurations

A standard (all indoor) microwave radio configuration consists of the entire microwave and digital modem part being placed indoors, the microwave antenna mounted outside on the tower, and a waveguide connecting the radio transceiver with the antenna. Today, the most commonly used waveguides for terrestrial microwave point-to-point systems are elliptical waveguides. This solution is acceptable for the lower frequencies, below 10 GHz, and high-capacity (backbone) microwave systems, but it quickly becomes unacceptable as frequency increases. This is a result of the losses in transmission lines (coax or waveguide), which become unacceptably high at higher frequencies. Whenever possible, split configuration microwave radio is replacing the all-indoor configuration. To reduce losses between the transceiver and antenna, the outdoor unit (ODU) containing all the RF modules can be mounted near the antenna. The ODU is connected to the indoor unit (IDU), which contains baseband circuitry, modulator, and demodulator, by means of a single coaxial IF (Intermediate Frequency) cable. The distance between the indoor and outdoor equipment can sometimes be up to 300 m (over 900 ft). The equipment typically operates from a battery supply between −40.5 and −57 V, nominally −48 VDC. The primary DC power is supplied to the indoor unit through a main fuse and a filtering function, which includes an input filter to attenuate the common mode noise. The power to the outdoor unit is supplied from the indoor unit via the IF coaxial cable. 6.2.2

Basic Microwave Radio Parameters

6.2.2.1 Transmit Output Power Transmit (Tx) output power is RF power, usually expressed in decibels referenced to a milliwatt (dBm). It is always necessary to define the interface related to the transmit output power value (e.g., antenna port or radio port). Without this information, it is impossible to calculate the real gain/loss of the radio equipment.

Microwave Network Deployment

219

If the radio includes a booster or a high-power amplifier (usually 2–3 dB higher than standard Tx power) or has an adaptive transmit power control (ATPC) implemented, this factor must be considered in the transmit output value. In addition, if transmit output power can be reduced (e.g., −3 or −6 dB below its nominal value), one must also be aware of it in order to reduce interference levels to acceptable limits. 6.2.2.2 Transmit Frequencies Transmit frequencies, expressed in gigahertz, include the remote site transmit frequency and the local site transmit frequency. In most microwave radio systems, nonintrusive monitor points are available for the measurement of RF output power and transmit and receive local oscillator (LO) frequencies. Therefore, the measurement of transmitter power, along with LO frequencies and power, are recorded on a routine maintenance basis. 6.2.2.3 Equipment Protection The equipment configuration can be either unprotected or protected. Several types of protection schemes for increasing equipment reliability are described in other chapters of this book. Diversity There are a number of diversity methods (space, frequency, angle, as well as some type of their combination) that could be used for the improvement of the microwave link performance. They are discussed in other chapters of this book.

6.2.2.4

6.2.2.5 Link Polarization Polarization is a physical phenomenon of radio signal propagation and refers to the orientation of the electric field vector in the radiated wave. If the vector appears to rotate with time, then the wave is elliptically polarized. The ellipse so described may vary in ellipticity from a circle to a straight line, or from circular to linear polarization. So, in the general sense, all polarizations may be considered to be elliptical. For linear polarization the vector remains in one plane as the wave propagates through space. Linear polarization has two subcategories: vertical or horizontal, and right- or left-handed for circular. The microwave radio link polarization must be of the linear type, either horizontal (H-pol) or vertical (V-pol), or both in some situations. In most cases two antennas that form a link with each other must be set for the same polarization. Antenna Data This includes some basic information about the model and manufacturer, diameter (expressed in meters or feet), and midband gain expressed in dBi or dBd. Terrestrial microwave antenna systems typically use parabolic antennas (solid or grid) in most cases.

6.2.2.6

220

Chapter Six

However, sometimes other types of antennas may be utilized, such as horns and, in the lower microwave frequencies, Yagis and helicals. Bit Rate Bit rate is usually expressed as megabits per second (Mbps). Gross bit rate (GBR) is the final result of the bit rate after inserting the forward error correction (FEC), voice frequency channel, data channel, and control/monitoring channel via the radio equipment.

6.2.2.7

6.2.2.8 Receiver Noise and Signal-to-Noise Ratio Signal to noise ratio (SNR or S/N ratio) is the ratio, usually measured in dB, between the signal level received and the noise floor level for that particular signal. 6.2.2.9 Transmit Spectrum Mask (Spectrum Occupancy) Spectrum analysis is one of the most important measurements in digital radio testing. The spectral occupancy test is a measure of how well unwanted sideband and spurious signals have been suppressed by successive filters in the transmitter. Digital microwave radios operate with well-defined and controlled spectral occupancy; therefore, it is routine practice to measure the occupancy of the radio against predefined limit or masks. All filters and devices (e.g., circulators, isolators, transitions, elbows) must be right in the beginning of the waveguide (elliptical, circular, or rectangular) or coaxial cable in lower frequencies. They must be provided in the form of a decibel-versus-frequency-offset curve. The offset represents the positive or negative frequency offset from the carrier frequency (f0 + ∆ f and f0 − ∆f) and is generally expressed in megahertz. In terms of unwanted emissions (spurious and out of band), the equipment must meet the appropriate specifications (e.g., in the U.S., FCC Part101, Section 101.111, Emission Limitations).

The receive chain (RF + IF + BB) filtering is composed of three curves: the receive RF filter frequency response curve, the receive IF filter frequency response curve, and the receive BB filter frequency response curve. All three curves show the filter response (in decibels) versus the frequency offset (in megahertz).

6.2.2.10 Receive Frequency Response

Attenuator and Additional Losses When a fixed inline attenuator (pad) is used, it is necessary to recognize whether the attenuator belongs to the transmit path only, to the receive path only, or to both paths. Additional losses are divided into two parts, and they both include the portion between the antenna interface and the radio interface. The antenna interface is located between the antenna and the transmitting line (waveguide or coaxial cable). The radio interface is located 6.2.2.11

Microwave Network Deployment

221

between the radio equipment and the transmitting line. The antenna interface consists of the transmit path losses, and the radio interface consists of the receive path losses. Depending on the situation, there could be some differences between these two losses. Overall additional losses are equal to the summation of losses in the transmission line, flex-twists (or jumpers, for coaxial), hybrid devices, switches, circulators, isolators, straight sections, flange adapters, waveguide-to-coaxial adapters, taper transitions, 90° E-plane and H-plane elbows, power dividers, and the radome. 6.2.2.12 Receiver Sensitivity Threshold for TDM Traffic The receiver sensitivity or threshold (Rx) defines the minimum signal strength required for a radio to successfully receive a signal. Receiver sensitivity is a function of the receiver’s noise factor, the noise bandwidth, and the modulation method. A radio cannot receive or interpret a signal that is weaker than the receiver sensitivity threshold. The receiver threshold is the receive power level normally at the antenna interface (equal to antenna port) for a given BER (bit error rate). Receiver thresholds expressed in dBm for a 10−3 BER in PDH radios, and 2 × 10−5 BER in SDH radios (10−6 quite often used as well), are usually provided. ITU-T Recommendation G.826 defines for a data capacity of 155.52 Mbps a data stream with 8,000 blocks (about 20,000 bits/block). The AIS-threshold, where the radio system interrupts the traffic and inserts an AIS-signal (alarm indication signal, also known as all ones) into the following network part, will be reached if 30 percent of the transmitted data blocks are errored. These 30 percent are 2,400 errored blocks of a 155.520 Mbps signal. A data block will be counted as errored if it contains one or more bit errors; the real number of errors is not important. With the assumption that a minimum of 2,400 bit errors are present (one block error per block), the BER will be calculated as BER = 2, 400 bits / 155, 520, 000 bits ≈ 2 × 10 −5 . This means a system that follows the valid SDH/Sonet recommendations has to interrupt the data transmission at a BER∼2 × 10−5. The specified AIS-threshold for PDH systems is at a BER = 10−3 (ITU-T Recommendation G.821), so the requirements for SDH/Sonet systems are therefore higher than for the low-capacity PDH systems. Alarm indication signal (AIS) is a signal transmitted by a telecommunications system to let the receiver know that some remote part of the end-to-end link has failed at a logical or physical level. The AIS replaces the failed data, allowing the higher order system in the concatenation to maintain its transmission framing integrity. The AIS is then forwarded to other components of the transmission systems. There are a number of

222

Chapter Six

types of AIS signals, which signal failure of different logical or physical segments of the system. As the use of Ethernet for long-distance data links has increased, the need for a similar end-to-end OA&M function has led to the development of an Ethernet alarm indication. A new Ethernet alarm indication signal (EthAIS) can provide similar functionality and is an important component of Ethernet OA&M. 6.2.2.13 Receiver Sensitivity Threshold for IP Traffic In wireless networks, the critical performance aspect for the IP-based backhaul is the packet loss for Real Time (RT) traffic. The maximum tolerated packet loss (without service impairment) is 10−4. Since a faulty packet is a lost packet from real-time services perspective, a relation between BER and Packet Error Ratio (PER) needs to be defined in order to enable dimensioning guidelines for packet-based microwave transmission. We can say that

BER = 1 − (1 − PER)

1

PL

(6.1)

where BER = Bit Error Ratio PER = Packet Error Ratio PL = Packet Length in bits In case that packet length is 128 Bytes (or octets)*, we will get as a result a required BER of approximately 10−7. So, for real-time IP-based traffic, 10−7 should be used as the BER criteria for the receiver threshold. Since the receiver threshold value in modern microwave radios, and therefore the calculated fade margin (assuming the general rule of 1 dB/decade), will typically differ for only 0.5 dB or so, the usual methods of designing SDH/Sonet microwave links can be still applied for dimensioning microwave links carrying Ethernet/IP-based traffic. 6.2.2.14 Maximum Receiver Signal The maximum receiver signal is the highest value of the received signal that is safe and would not damage the receiver. A typical value is around −20 dBm. In some radios, a signal approaching the maximum receiver signal value within a few decibels could cause an increase in BER and affect the performance of the link.

*

The size of a byte is typically hardware dependent, but the modern de facto standard is 8 bits, as this is a convenient power of two. The term octet is widely used as a precise synonym of the 8-bit byte where ambiguity is undesirable, such as in communications protocol definitions.

Microwave Network Deployment

223

So, a good rule of thumb is to stay way below the maximum allowed receiver signal for at least 5 dB and preferably even more. 6.2.2.15 Receive Signal Level The receive signal level (RSL) is the expected strength of a signal when it reaches the receiving radio. The following formula defines the RSL:

RSL = Po − Lctx + Gatx − Lcrx + Garx − FSL − Lm

(6.2)

where Po = output power of the transmitter (dBm) Lctx = all the losses (cable, connectors, branching unit, etc.) between the transmitter and its antenna (dB) Gatx = gain of the transmitter’s antenna (dBi) Lcrx = all the losses (cable, connectors, branching unit, etc.) between the receiver and its antenna (dB) Garx = gain of the receiver’s antenna (dBi) Lm = miscellaneous losses (gas attenuation, obstacle losses, misalignment losses, etc.) (dB) FSL = free-space loss (dB) 6.2.2.16 Link Feasibility Formula To determine if a link is feasible, compare the calculated receive signal level with the receiver sensitivity threshold. The link is theoretically feasible if

RSL > Rx

(6.3)

Effective Isotropic Radiated Power (EIRP) Effective isotropic radiated power (EIRP) is defined as the effective power found in the main lobe of a transmitter antenna relative to an isotropic radiator that has 0 dB of gain. It is equal to the sum of the antenna gain (in dBi) plus the power (in dBm) into that antenna. For example, if a 30 dBi gain antenna is fed with 27 dBm of power (including all the Tx/Rx losses), it has an EIRP of:

6.2.2.17

30 dBi + 27 dBm = 57 dBm Example: Assume MW link with the following parameters: n

Frequency = 11 GHz

n

Path length = 7.5 mi

224

Chapter Six

n

Antenna gain = 40 dBi

n

Miscellaneous antenna/Tx/Rx losses = 0.5 dB

n

Transmitter power = 0.13 W

n

Maximum receive power = −15 dBm

n

−3 Rx threshold level = −70 dBm (@10 BER)

Calculate the EIRP, FSL, Receive Signal Level (Rx), and the Fade Margin (see 6.2.2.19 for definition) for this microwave link. Solution:

EIRP = 21 + 40 − 0.5 = 60.50 dBm FSL = 96.6 + 20 log 7.5 + 20 log 11 = 134.93 dB Rx = 21 + 40 − 0.5 − 134.93 + 40 − 0.5 = −34.93 dBm FM = −34.93 − ( −70) = 35.07 dB 6.2.2.18 C/N vs. BER The noise and interference test set is used with a bit-error rate test (BERT) to make one of the most common and important measurements on digital microwave radios, i.e., the C/N curve. This measurement is made at virtually every stage of a radio’s development, production, and use. Later measurements can be made to identify faults or gradual degradation in performance. 6.2.2.19 Fade Margin and Link Availability Fade margin is the difference between the unfaded RSL and the receiver sensitivity threshold (Rx). Each link must have sufficient fade margin to protect against path fading that weakens the radio signals. Fade margin is the link’s insurance against unexpected system outages and is directly related to link availability, which is the percentage of time that the link is functional. The percentage of time that the link is available increases as the fade margin increases, while the link with little or no fade margin may experience periodic (or sometimes quite numerous) outages. 6.2.2.20 Residual BER Residual BER (RBER) is a key radio performance metric; it measures the combined effect of the digital radio’s modulator, transmitter, receiver, and demodulator. Unlike the receiver’s threshold and dispersive fade margin, which are key availability metrics, residual BER characterizes the microwave radio hardware in its normal operating received signal strength range.

Microwave Network Deployment

6.2.3

225

Radio Performance Improvement

Microwave radios take advantage of a number of powerful antifading measures. These include space diversity reception, ATPC, adaptive equalizers, multilevel coded modulation (MLCM)-type forward error correction (FEC), cross-polarization interference cancellers (XPICs), high-speed errorless switching with early warning, and others. 6.2.3.1 Microwave Link Protection The terms protection and diversity are often used interchangeably when applied to microwave links. This is incorrect, since protection commonly improves long-term traffic interruptions (10 CSES or more), whereas diversity arrangements greatly reduce the number and duration of short-term outages (less than 10 CSES). (Note: CSES is the abbreviation for consecutive severely errored seconds.) Nonprotected systems (1 + 0) consist of one indoor unit and one outdoor unit interconnected with a single coaxial cable. In the case of failure of any of the electrical or mechanical components, the entire microwave hop will fail. The monitored hot standby (MHSB) has two transmitters and two receivers that are always online (“hot”). A switch keeps one radio transmitting or receiving until a failure occurs and, at that moment, the signal is switched to the standby radio. The hot standby protection system uses a common frequency channel with only one of the two transmitters at any end of the microwave link active at any particular time. The transmitter in standby mode will be fully operational except that the transmitter will be muted. The MHSB configuration (1 + 1) protects against equipment failures only, not path propagation problems. The hot standby (protected) system configuration provides hitless receiver changeover on each side of the radio relay link in case of receiver equipment failure. If a transmitter fails, there will be a short break in transmission until the standby transmitter is activated. In contrast to receiver changeover, transmitter changeover therefore will not be hitless. In hot standby (protected) configuration (1 + 1), the IDU, the transceiver unit, and the coaxial cable between the IDU and ODU are duplicated. The two transceivers share the same branching unit. A switch at radio frequency level, included in the branching unit, allows for switching between the two transmitters and use of only one antenna. A splitter and switching unit placed between the two IDUs are added. The hot standby branching typically contains an RF switch for transmit direction and a RF power splitter for the receive direction. In addition, there is a version of the protected system with two ODUs and two antennas. The two-antenna configuration has no branching losses and provides better system gain.

226

Chapter Six

Split-configuration radios were designed with ease of installation in mind and to eliminate the waveguide. The equipment can be installed within a few hours. These radios can also be fully indoor mounted if necessary. One coaxial cable between the IDU and ODU is used for 1 + 0 systems, and two cables are used for hot standby systems (1 + 1). A power divider/splitter is (ideally) a lossless reciprocal device that can also perform vector summation of two or more signals and thus is sometimes called a power combiner or summer. Single-antenna protected system configurations require the use of a power combiner. This is true for single-antenna microwave systems with the hot stand-by protection or diversity configurations, when implemented using outdoor mounted RF units. There are typically two versions of power combiners available (Figure 6.1) for use with the outdoor mounted RF unit (ODU), namely the “equal” (or symmetrical) version with 3.5 dB loss per side and the “unequal” (or asymmetrical) version with 1.5 dB/7.5 dB loss. These values can vary and should be obtained directly from the equipment manufacturers. The power combiner uses a stripline directional coupler to split the signal between the two ODUs. The fourth port on the coupler is terminated in a 50 ohm load to dissipate unwanted power from each ODU transmitter. Adaptive Equalizers Adaptive equalizers improve the digital system performance in the presence of multipath fading, linear distortion, or both. The equalizers can only mitigate the dispersive aspects of multipath fading. These adaptive equalizers reshape the pulse so as to minimize the intersymbol interference.

6.2.3.2

SYMMETRICAL Antenna

ASYMMETRICAL Antenna

50 Ω load

50 Ω load

ODU 1 3.5 dB loss

ODU 2 3.5 dB loss

Figure 6.1 Power combiner types

ODU 1 1.5 dB loss

ODU 2 7.5 dB loss

Microwave Network Deployment

227

Adaptive equalizers greatly reduce many of the degradations caused by the radio and the microwave path and therefore allow longer microwave hops. Adaptive equalizers are either spectrum-driven (adaptive slope amplitude equalizers, or ASAEs) operating in the frequency domain or decision-feedback devices (adaptive time domain equalizers, or ATDEs) operating in the time domain. An adaptive frequency domain equalizer is used at intermediate frequency (IF) to control transfer function of the channel. An adaptive time domain equalizer is used in time domain to directly reduce intersymbol interference (ISI) caused by transfer function. Either or both of these equalizers is incorporated into modern highercapacity digital radios as required to meet the long-term performance (outage and quality) objectives over a wide range of atmospheric fade characteristics, path geometries, equipment protection schemes, temperatures, and other characteristics. Forward Error Correction Forward error correction (FEC) is an error correction scheme that adds redundant bits to the payload input to the digital transmitter, thereby increasing transmitted symbol rate and RF bandwidth so as to correct random errors at the receiving terminal. The wider spectrum bandwidth, on the other hand, may cause other problems since it is more vulnerable to dispersive fade outages. The FEC is usually expressed as a fraction, i.e., 1/2, 3/4, etc. In the case of 3/4 FEC, for every 3 bits of data, 4 bits are being sent out, one of which is for error correction. FEC corrects low BERs or so-called “dribbling” or random errors, and it is not effective for BER < 10−4.

6.2.3.3

6.2.3.4 Cross-Polarization Interference Canceller (XPIC) Co-channel operation of microwave systems will double the capacity compared to conventional microwave systems. In co-channel systems, transmission of two separate traffic channels are performed on the same radio frequency using orthogonal polarization. That means that two signals are transmitted—one with horizontal polarity and one with vertical polarity. This works well as long as the discrimination between the two polarizations, called cross-polar discrimination (XPD), is sufficient to ensure interference-free operation. The main challenge with this kind of configuration is cross-polarization interference, whereby energy from one polarization is received in the other. Both multipath and rain fading can result in severe degradation of the XPD level. As the XPD decreases, the interference level in the channel will rise and may cause threshold degradation and errors in the data traffic. Cross-polarization interference could also be the result of equipment imperfections as well.

228

Chapter Six

All this could lower the cross-polarization discrimination (XPD) between the co-channel dual-polarized signals on both channels simultaneously to an unacceptable value, thus degrading the link’s error performance. Reduced XPD will not cause an outage unless cross-polar frequency re-use is deployed. The cross-polarization interference canceller (XPIC) feature is a technology that allows transmission on both the horizontal and vertical polarity of any given frequency pair. XPIC systems filter the crosspolarization interference signal in order to successfully receive or decode the desire signal. This way, the maximum link capacity will effectively double for a given frequency band. Such cancellation is essential to achieve performance that is not limited by cross-polarization interference but rather by the co-polarization attenuation, as with single polarity radios. Improvement achieved by using XPIC is typically around 20–25 dB. Some of the most important XPIC design considerations are as follows: n

n

n

n

n

n

n

n

Ultra-high-performance, dual-polarized antennas have to be used. Dual-polarized antennas have two feed horns that transmit the microwave energy in the vertical and horizontal planes. The ultra-high-performance antenna discrimination between the two polarizations is typically close to 40 dB, which reduces adjacent channel interference. High-XPD antennas are needed for most co-channel dual-polarized (CCDP) applications with the higher efficiency radios. In some cases of low multipath probability or rain attenuation, standard antennas can be used as well. It is preferable to have ATPC operational (both directions) on the XPIC path. For circulator-coupled XPIC equipment, an additional loss for the transmitters and for the receivers must be taken into account in the path calculations. These losses can be sometimes quite high (6 to 8 dB on a transmitter AND receiver side). XPIC should be considered in the initial system design; retrofitting the equipment in the field is almost impossible after it is installed. Some radios with an XPIC option have very high branching losses. High branching losses can have a significant effect on the path design (e.g., larger antenna sizes, higher transmit power, use of space diversity, etc.). Measurements show that the performance of the orthogonal system and its XPD can be significantly improved by adding space diversity.

Microwave Network Deployment

229

The XPD can deteriorate sufficiently to cause co-channel interference and, to a lesser extent, adjacent channel interference. The combined effect of multipath propagation and the cross-polarization patterns of the antennas result in the reductions in XPD occurring for small percentages of time. The following method is used to calculate the multipath outage time in an XPIC system. The method is adopted from ITU-R Recommendation P.530-12. First we calculate XPD0:  XPDg + 5  XPD0 =   40

for XPDg ≤ 35 for XPDg > 35

(6.4)

where XPDg is the manufacturer’s guaranteed minimum XPD at boresight for both the transmitting and receiving antennas. Then we calculate multipath activity factor, h:

η = 1 − e−0.2( R)0.75

(6.5)

For one transmit antenna, Q can be calculated as:  0.7η  Q = −10 log   R 

(6.6)

R is a multipath fade occurrence factor. It can be calculated as P0 /100, corresponding to the percentage of the time P0 (%) of exceeding A = 0 dB in the average worst month, as calculated from Equation 3.18 using the ITU method. Using Vigants, the fade occurrence factor is calculated using the basic outage equation for atmospheric multipath fading: R = 2.5 × 10 −6 c f d3

(6.7)

Calculate the parameter C from: C = XPD0 + Q

(6.8)

Total interference fade margin with and without XPIC is given by: C − C0 / I IFM =  C − C0 / I + XPIF

without XPIC with XPIC

(6.9)

230

Chapter Six

Here, C0/I is the carrier-to-interference ratio for a reference BER, which can be evaluated either from simulations or from measurements. XPIF is a laboratory-measured cross-polarization improvement factor that gives the difference in cross-polar isolation (XPI) at sufficiently large carrier-to-interference ratio (typically 35 dB) and at a specific BER for systems with and without a cross-polar interference canceller (XPIC). A typical value of XPIF is about 20–25 dB. If an XPIC device is not used, set XPIF = 0. The composite fade margin, accounting only for flat fade and interference/XPIC, can be calculated as: CFM = TFM + IFM CFM = −10 log(10 − TFM /10 + 10 − IFM /10 )

(6.10)

Example: So now, for our calculation example from Section 3.4.1, we are going to double the capacity by adding an orthogonal system with XPIC. We are going to neglect changes in the path length due to link budget changes and just analyze the effect of XPIC on the multipath outage time. We will assume the carrier-to-interference ratio for BER=10−6 to be 25 dB. Improvement from the XPIC is 20 dB. Antennas are 8-foot, ultra-high-performance, dual-polarization, with 40 dB cross-polarization antenna discrimination.

R = 2.5 × 10 −6 c f d3 R = 2.5 × 10 −6 ⋅ 1 ⋅ 6 ⋅ 303 R = 0.405

η = 1 − e−0.2( R)0.75 η = 1 − e−0.2(0.405)0.75 η = 0.0965  0.7η  Q = −10 log   R   0.7 ⋅ 0.0965  Q = −10 log   0.405  Q = 7.78 dB

Microwave Network Deployment

231

C = XPD0 + Q C = 40 + 7.78 C = 47.78 dB IFM = C − C0 / I + XPIF IFM = 47.78 − 25 + 20 IFM = 42.78 dB CFM = −10 log(10 − TFM /10 + 10 − IFM /10 ) CFM = −10 log(10 −36 10 + 10 −42.78 10 ) CFM = 35.2 dB Outage = 0.405 × 10 −35.2 /10 ⋅ (8 ⋅ 106 ) ⋅

40 50

Outage ≈ 782 SES/yr (99.9975 % outage probability) Comparing this result with the number of SESes per year in the problem in Section 3.4.1, we can conclude that there is significant difference (increase), even assuming the excellent XPID achieved with high-discrimination antennas and the use of XPIC. ITU-R Recommendation P.530-12 also describes step-by-step procedure for predicting XPD outage caused by precipitation effects. 6.2.3.5 Adaptive Transmit Power Control (ATPC) The radio output power can be controlled in fixed or adaptive mode. In fixed mode, the output power Pout ranges from a minimum level Pfix min to a maximum level Pmax. A value Pset is manually set in 1dB increments, locally or remotely from the management system. In adaptive mode, the automatic transmit power control (ATPC) function is used to automatically control the output power, Pout. The output power is continuously adjusted so as to maintain a minimum input level set from the far-end terminal. Under normal path conditions, the ATPC maintains the output power at a reduced level, resulting in a lower interference level in the radio network. ATPC is a feedback control system that temporarily increases transmitter output power during periods of fading, thus eliminating (or at least reducing) the adverse effects of fade events on digital point-to-point

232

Chapter Six

microwave fixed services. ATPC offers immediate and long-term advantages to the link operator, including reduced average power consumption, extended equipment MTBF, and lower long-term RF interference levels. Propagation statistics indicate that fade events on physically different propagation paths are noncorrelated; thus, the probability of simultaneous sensitivity to interference for two separate systems is small—at least for situations in which multipath fading is the dominant limiting factor. As long as link paths are properly designed with adequate path clearance and are not significantly affected by rain fade events, the ATPC maximum transmit power boost is required only for appropriately short periods of time (<2 sec). Transmit power in excess of coordinated power is maximum 10 dB and is allowed for not more than 0.01 percent of the time (3,250 sec/yr). There are two main advantages to using ATPC: the transmit power less than the maximum power may be used for the calculation of interference into other systems, and calculations of interference into the receiver of a system using ATPC may assume that the wanted signal transmitter is operating at maximum transmit power. 6.2.3.6 Channel Width, Spectral Efficiency, and Modulation Schemes By far the most common form of modulation in digital communication is M-ary phase shift keying (PSK). With this method, a digital symbol is represented by one of M phase states of a sinusoidal carrier. For binary phase shift keying (BPSK), there are two phase states, 0° and 180°, that represent a binary one or zero. With quaternary phase shift keying (QPSK), there are four phase states representing the symbols 11, 10, 01, and 00. Each symbol contains two bits. A QPSK modulator may be regarded as equivalent to two BPSK modulators out of phase by 90°. Some other modulation schemes commonly used in data systems are FSK (including BFSK and QFSK) and QAM (of various levels). A suitable modulation method is selected by taking into account the system requirements. For instance, if spectrum efficiency is not a major issue and/or high interference tolerance is important (congested and/or urban areas), a simple modulation method should be used. Spectral efficiency refers to spectrum utilization as measured in bits/sec/Hz, and some countries and their regulatory organizations do not allow microwave radios to be installed if they do not have a very high spectral efficiency. The features of simple modulation methods are as follows: n

Easy implementation in all frequency bands

n

Robustness against propagation effects

n

High tolerance against all kinds of interferences

n

High system-gain characteristics

Microwave Network Deployment

233

On the other hand, a multistate modulation method improves spectral efficiency and capacity on a route. Sophisticated modulation methods (high levels of QAM and TCM) are required for high-capacity links so that more information can be packed together. Typical applications for these multistate modulation methods are high-capacity trunk, junction, and access networks. The method of modulation and the capacity will directly affect the required bandwidth and the interference tolerance of a radio link. The unit of information is the symbol, and the different schemes use different numbers of bits to define each symbol. For a given symbol rate, the greater the number of bits per symbol, the higher the data rate. For example, 8PSK has eight states in steps of 45°. Shifting the carrier phase by 45° requires 1 Hz of the carrier frequency for 3 bits of the base band (23=8), and the spectral efficiency is 3 bps/Hz. A 2-Mbps baseband modulated with 8PSK requires an RF carrier with a bandwidth of 0.67 MHz. A 140-Mbps baseband therefore requires an RF carrier that has a bandwidth of 47 MHz. On the other hand, 64QAM has 64 states, and phase and amplitude are shifted. One shift requires 1 Hz of the carrier frequency for 6 bits of the baseband (26=64), and the spectral efficiency is 6 bps/Hz. A 2-Mbps baseband modulated with 64QAM requires an RF carrier with a bandwidth of 0.33 MHz. A 140-Mbps baseband requires an RF carrier that has a bandwidth of 23 MHz. As shown in Table 6.1, the STM-1 radio link needs the radio channel 112 MHz with 4QAM, 56 MHz with 16QAM, and 28 MHz with 128QAM. The S/N requirements for receiver threshold using different modulations are also shown in the table. For example, 128QAM needs about 16 dB more S/N than 4QAM. To upgrade the existing radio link from 4QAM 16 × 2 Mbps to 155 Mbps without changing the RF-channel width, 128QAM is needed. A transmit power increase is normally not possible (Ptx max < 30 dBm) beyond more than a few decibels (0 to 3 dB). The required increase in power to attenuate noise and interference for 16QAM, 64QAM, and 256QAM compared to 4QAM is 7, 13, TABLE 6.1

Channel Requirements for Different Modulation Methods

Modulation

Bandwidth (16 ë 2 Mbps)

Bandwidth (155 Mbps)

S/N (10−6)

System Gain (Relative to 4QAM)

4QAM 16QAM 32TCM-2D 64QAM 128QAM 256QAM

28 MHz 14 MHz 14 MHz 14 MHz 7 MHz 7 MHz

112 MHz 56 MHz 56 MHz 56 MHz 28 MHz 28 MHz

13.5 dB 20.5 dB 17.6 dB 26.5 dB 29.5 dB 32.6 dB

0 dB 7 dB 4 dB 13 dB 16 dB 19 dB

234

Chapter Six

and 19 dB, respectively. The result is that these modulation methods, from the microwave link design prospective with a high number of states, are very inefficient in dense city networks containing many randomly oriented links (e.g., a mesh network). If the size of antennas in both ends is doubled, roughly 6 + 6 dB can be gained. Due to practical reasons, antenna gains exceeding about 44 dB cannot be used. With these changes, 12–15 dB is possible and, in the case of short hops, the missing part may be “covered” by the excess system gain margin. If the RF channel can be changed from 28–56 MHz, this kind of capacity upgrade will be less critical, because 32TCM can be used. The system gain increase demand would then be only 4 dB and could be partly covered by transmit power increase (e.g., 3 dB) or with a 50 percent bigger antenna at one end. If antenna changes are planned, the rigidity and available space of the supporting structures must be checked. If antenna changes are not possible, the hop lengths normally must be reduced. The general rule is that, to obtain higher data rates, higher-order modulation schemes must be employed. Sophisticated modulation methods for high SDH capacity links can improve the spectral efficiency, but the price paid for this increased throughput is an increase in operating threshold level (and more susceptibility to interference). In real terms, the trade-off is data rate versus energy per bit, and thus range. Since an increase in bandwidth normally introduces an increase in error rate, error-correcting schemes (overhead) are often included to produce a net gain in data throughput. Detailed information on the subject of spectrum efficiency is provided in ITU-R Recommendation SM.1046. Coded Modulation This is a technique that combines coding and modulation that would have been done independently in the conventional method. Redundant bits are inserted in multistate numbers of transmitted signal constellations. This process is known as coded modulation. Representative examples of coded modulation are block coded modulation (BCM), trellis coded modulation (TCM), and multilevel coded modulation (MLC or MLCM). In BCM, levels are coded by block codes, whereas TCM uses only convolutional codes. TCM uses the same modulation scheme as QAM, but it adds an extra bit to every symbol for error control purposes. On the other hand, different codes can be used for each coded level in MLCM, so MLCM can be seen as a general concept that includes BCM and, to some extent, TCM. These schemes require added receiver complexity in the form of a maximum likelihood decoder with soft decision.

6.2.3.7

Microwave Network Deployment

235

A technique similar to TCM is the partial response, sometimes called a duo-binary or correlative signaling system. A controlled amount of intersymbol interference, or redundancy, is introduced into the channel. Hence, the signal constellation is expanded without increasing the transmitted data bandwidth. There are various methods utilizing this redundancy to detect and then correct errors to improve performance. This process is called ambiguity zone detection or AZD. 6.2.3.8

Receiver Data Switching

Hitless receiver data switching is the most commonly used diversity switching method in microwave radios today. This type of switching between receivers may contribute to additional errored bits, but no hits (service disruptions) occur. Although sufficient in most cases, in a severe fade environment, as well as on long multihop system, it can create enough errors to cause problems. Errorless receiver data switching between receivers is achieved with some type of anticipatory sensing, usually at a BER of 10−8 or before errors occurs. This is the preferred configuration for high-capacity links carrying critical information over a long, multihop microwave system and in a high multipath fade environment with common diversity switching activity. The caution is required in systems in which FEC is utilized, since it can cause problems or completely disable the hitless/errorless switch feature. 6.2.3.9 MIMO MIMO (multiple-input and multiple-output) is the appli-

cation of multiple antennas at both the transmitter and receiver to improve communication performance. It is one of several forms of smart antenna technology. MIMO technology has attracted attention in wireless communications, since it offers significant increases in data throughput and link range without additional bandwidth or transmit power. It achieves this by higher spectral efficiency (more bits/sec/Hz of bandwidth) and link reliability, i.e., diversity (reduced fading). MIMO systems are a natural extension of developments in antenna array communication, i.e. “smart antennas.” While the advantages of multiple receive antennas, such as gain and spatial diversity, have been known and exploited for some time, the use of transmit diversity has only been investigated recently. The advantages of MIMO communication, which exploits the physical channel between many transmit and receive antennas, are currently receiving significant attention.1 In MIMO systems, a transmitter sends multiple streams by multiple transmit antennas. The transmit streams go through a matrix channel, which consists of multiple paths between multiple transmit antennas

236

Chapter Six

at the transmitter and multiple receive antennas at the receiver. Then the receiver gets the received signal vectors by the multiple receive antennas and decodes the received signal vectors into the original information. MIMO systems provide a number of advantages over single-antennato-single-antenna communication. Sensitivity to fading is reduced by the spatial diversity provided by multiple spatial paths. Under certain environmental conditions, the power requirements associated with high spectral-efficiency communication can be significantly reduced by avoiding the compressive region of the information-theoretic capacity bound. Let us assume that we have M transmitting antennas and L receiving antennas, and also that L > M, so that all the transmitted signals can be decoded at the receiver. The main idea in MIMO is that we can send different signals using the same bandwidth and still be able to decode correctly at the receiver. The total capacity of M channels can be approximated by: L   C ≈ M ⋅ B ⋅ log 2  1 + ⋅S/N    M

(6.11)

where C = total channel capacity (bps) B = channel bandwidth (Hz) M = number of transmitting antennas L = number of receiving antennas S/N = signal-to-noise ratio (dB) We can see here that the relationship between the channel capacity and the signal-to-noise ratio (S/N ratio) is logarithmic. This means that trying to increase the data rate by simply transmitting more signal 2 power could be very inefficient. On the other hand, with multiple antennas we get a linear increase in capacity with respect to the number of transmitting antennas. So, the conclusion is that it is more beneficial to transmit data using many different low-powered channels than using one single, high-powered channel. OFDM and COFDM Orthogonal FDM (OFDM) is a special case of FDM. Frequency division multiplexing (FDM) is a technology that transmits multiple signals simultaneously over a single transmission path, such as a cable or wireless system. Each signal travels within its own unique frequency range (carrier), which is modulated

6.2.3.10

Microwave Network Deployment

237

by the data (text, voice, video, etc.). OFDM spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. Figure 6.2 illustrates the difference between the conventional nonoverlapping multicarrier technique and the overlapping multicarrier modulation technique. The “orthogonality” in this technique prevents the demodulators from seeing frequencies other than their own so the guardbands that were necessary to allow individual demodulation of subcarriers in an FDM system are no longer necessary. The use of orthogonal subcarriers allows the subcarriers’ spectra to overlap, reducing the required bandwidth for almost 50 percent and thus increasing the spectral efficiency (measured in bits/sec/Hz). As long as orthogonality is maintained, it is still possible to recover the individual subcarriers’ signals despite their overlapping spectrums. The word orthogonal indicates that there is a precise mathematical relationship between the frequencies of the carriers in the system. If the dot product of two deterministic signals is equal to zero, these signals are said to be orthogonal to each other. Orthogonality can also be viewed from the standpoint of stochastic processes; if two random processes are uncorrelated, then they are orthogonal. The main advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions; for example, narrowband interference and frequency-selective fading due to multipath, without complex equalization filters. In a single carrier system, a single fade or interferer can cause the entire link to fail, but in a multicarrier system, only a small percentage of the subcarriers will be affected; error correction coding can then be used to correct for the few erroneous subcarriers.

FDM

OFDM Number of carriers N=3

Frequency –f

–f/3

f/3

f

BW = 2f Figure 6.2 The OFDM principle

–2f/3 –f/3

f/3 2f/3

BW = 4f/3

238

Chapter Six

Channel equalization is simplified because OFDM may be viewed as using many slowly modulated narrowband signals rather than one rapidly modulated wideband signal. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to handle time-spreading and eliminate intersymbol interference (ISI). This mechanism also facilitates the design of single-frequency networks, where several adjacent transmitters send the same signal simultaneously at the same frequency, as the signals from multiple distant transmitters may be combined constructively, rather than interfering as would typically occur in a traditional single-carrier system. In the telecommunications field, the terms discrete multitone (DMT), multichannel modulation, and multicarrier modulation (MCM) are widely used and are sometimes interchangeable with OFDM. In OFDM, each carrier is orthogonal to all other carriers; however, this condition is not always maintained in MCM. We could say that the OFDM is an optimal version of multicarrier transmission schemes. Coded OFDM, or COFDM, is a term used for a system in which the error control coding and OFDM modulation processes are combined. COFDM systems are able to achieve excellent performance on frequency selective channels because of the combined benefits of multicarrier modulation and coding. From the electronic circuit design perspective, channel equalization becomes simpler than using adaptive equalization techniques with single carrier systems. On the other hand, the peak-to-average power ratio (PAPR) requirement of an OFDM system is a critical parameter in assessing whether the system can be implemented using CMOS technology. The OFDM signal has a noise-like amplitude with a very large dynamic range; therefore, it requires RF power amplifiers with a high peak to average power ratio. 6.2.3.11 Link Aggregation Link aggregation is used to achieve high throughputs by combining the capacity of two or more physical links on a single virtual connection. Link aggregation, or NIC bonding (defined in IEEE 802.1AX-2008), is a computer networking term that describes using multiple network cables/ports in parallel to increase the link speed beyond the limits of any one single cable or port and to increase the redundancy for higher availability. Other terms for link aggregation include Ethernet trunk, port channel, port trunking, link bundling, multilink trunking (MLT), and network fault tolerance (NFT). Link aggregation is designed to overcome two problems with Ethernet connections: bandwidth limitations and lack of redundancy. Even if the microwave link does not include any equipment redundancy, the inherent capabilities provided by the aggregated link means

Microwave Network Deployment

239

that should any one of the paths fail, any high priority traffic (e.g., voiceand video-related services) can be automatically directed over one of the remaining links. Lower priority traffic, such as Internet browsing, can be dropped until the failed path is restored. Some microwave radio solutions offer GigE transport by combining multiple RF links, and then the traffic has to be aggregated in an external switch. Other microwave radios, on the other hand, have link aggregation inside the radio, using an embedded Layer 2 Ethernet switch. This enables the aggregation of two, three, or four physical links to potentially provide more than 1 Gbps of throughput, and optional XPIC can then be employed to maximize spectral efficiency. 6.2.4

Ethernet Microwave Radio

Every year wireless applications demand greater bandwidth than before, so recently mobile operators started deployment of Ethernet as a backhaul technology for 3G and 4G services. Ethernet as a transport technology is appealing because it supports both higher data rates as well as a more attractive cost-per-bit of transported traffic. In years to come, mobile transport networks will be completely based on IP, so wireless operators are already starting to utilize microwave radios with Ethernet capabilities. To be 4G-backhaul-ready, a point-to-point microwave system needs to support high-capacity, low-latency, and legacy solutions. In addition, the system must support network evolution to IP/Ethernet and deliver clock synchronization both for legacy and future services. Last and most important, 4G-ready systems must deliver far lower cost-per-bit than current backhaul solutions based on TDM. One of the base models for Ethernet microwave deployment in the backhaul is native, also referred to as the hybrid model. Native Ethernet transports both TDM and Ethernet natively (i.e., there is no encapsulation of one over another). Using this concept during the migration phase, each cell-site is provided with an option to carry legacy E1/T1 traffic, as well as Ethernet traffic. Usually, the TDM-native E1/T1 ports have priority over the GigE ports and are automatically added or dropped from data stream when connected or disconnected. Synchronization can be handled either over T1/E1s or by using Synchronous Ethernet and IEEE 1588v2. 6.2.5 T/I Curves

The T/I is considered the minimum interference level that will have any significant effect on the radio. The T/I curves of the radio are used for interference analysis and frequency coordination, and they are based on lab measurements.

240

Chapter Six

It is determined by first fading the radio to the 10−6 BER static threshold and then adding an interfering signal until the static threshold is degraded by 1.0 dB. The difference between the interfering signal level and the threshold is defined as T/I. By changing the frequency separation between the interfering signal and the receive frequency, a curve of T/I values is produced. Typically, two different T/I curves are generated for each radio, one using a modulated digital signal as the interferer and the other using a CW tone as the interferer. The modulated curve is used to analyze digital interference cases, and the CW curve is used for FM cases. The digital T/I curve assumes like modulation, and it may not be accurate if the interfering transmitter is not the same radio type as the victim receiver. 6.2.6

Service Telephone Network

A service telephone network can be created using the engineering order wire facilities provided on the SDH equipment. An operator who is seeking to maintain a quality network will require access to all network sites from a central point or from more than one site. There are, in general, two ways to achieve communication to sites. The first is to use the PSTN network and provide a telephone line to each site. The second is to set up a service telephone network for each site. A service telephone network is a system that gives the operator access to sites where there is transmission equipment. Within the design of the SDH equipment is a facility called engineer order wire (EOW), which allows a telephone to be connected to the equipment and enables voice communication via an AUX card. To achieve this, the voice channel is carried by the E1 or E2 bytes in the SDH section overheads. These bytes are dedicated to voice (EOW) by the ITU-T. The EOW is always connected in a party-line network, which means that all NEs configured to have access to E1 or E2 can have simultaneous access to the service telephone. A protocol is used to provide selective or omnibus calls. The EOW interface is a standard analog 2w interface, DC current feeding, hang off status detection, and DTMF signaling. 6.2.7

Duplexers

The duplexer is the main component that can affect the manufacturing time of the microwave radio and therefore the lead times for the microwave network deployment. Any two-way wireless communication requires both a transmitter and a receiver and, in a full-duplex operation, they both operate at the same time. The duplexer is an antennacoupling device that allows a transmitter and a receiver to be connected simultaneously to the same antenna (see Figure 6.3).

Microwave Network Deployment

241

f2 f1

Tx 1

Tx 2

f1

f2

Duplexer

Duplexer

Rx 1

Rx 2

f2

f1

Figure 6.3 Full-duplex traffic with simultaneous transmission

Even if the transmitter and receiver each had its own antenna, fullduplex operation can present a problem, because the power output of the transmitter is greater than the power level of signals the receiver is trying to receive. Therefore, when these two devices are operating at the same time in close proximity, some of the energy from the transmitter will find its way into the receiver, where it will surely be more powerful in comparison to the signals the receiver wants to receive. When the transmitter and receiver are connected to the same antenna, the problem becomes even more acute. For full-duplex to work at all, it is necessary to transmit and receive on different frequencies. This system is called frequency-division duplex. The idea is that the receiver will not be able to “hear” the transmitted signal, because the receiver is selective; it will receive only a frequency (or a small range of frequencies) to which it is tuned and will not receive the transmitted signal if the frequency is outside of the receiver’s tuning range (called the receive passband). Although this fundamental idea sounds simple, there can still be a problem. The receiver obtains its selectivity characteristic by using filters, which pass certain frequencies while rejecting others. The duplexer can be thought of as just a pair of bandpass filters incorporated together in one box. It has three connection ports: the transmit (Tx) port, the receive (Rx) port, and the antenna port. The Tx and Rx ports are usually interchangeable and, in most implementations, the duplexer is a passive device—meaning it neither requires nor consumes any power. Consequently, the duplexer is not user configurable through software control or any other means. Mechanical adjustments that are

242

Chapter Six

made at the time of manufacture should never need readjustment or calibration. The duplexer will have two nonoverlapping passband frequency ranges, and thus one will naturally be higher than the other. It is possible to set up a system to transmit through the higher frequency passband filter and receive through the lower frequency one, or vice versa. These two scenarios are usually described as transmit high or transmit low. The only real requirement is to make sure that the transmit frequency falls within the passband range of one of the duplexer’s filters, and the receive frequency falls within the other. This requires the operator to know the passband frequency ranges of the duplexer and the Tx and Rx operating frequencies when ordering microwave equipment. In practice, one first will determine, to at least a rough degree, what the transmit and receive frequencies will be. Then, a duplexer is chosen with appropriate Tx and Rx passband ranges to accommodate the desired operation frequencies. After the system is installed or ordered, if it is desired to alter either the Tx or Rx frequencies (or both), this can be done as long as any new chosen frequencies fall within the duplexer’s passbands. Otherwise, it will be necessary to obtain a different duplexer (for each end of the link). 6.2.8

Environmental Requirements

Every microwave project is different, and the environmental requirements for the transmission equipment used can vary from country to country and from project to project. Therefore, there is a chance that the equipment will have to be tested and certified to make sure it complies with local standards and regulations as well to make sure it will survive harsh environmental conditions. In case the MW radio is designed in split (indoor-outdoor) configuration, the MW radio’s ODU will be used in an outdoor environment. External temperatures could vary between −40 and +50°C. In that case, the enclosure should be internally insulated with a suitable insulation material to minimize interior temperature variations caused by surface heat absorption, and it should be equipped with a suitable temperature and environmental (cooling and heating) control. The ODU enclosure itself should conform to North American NEMA 4 or equivalent European IEC 529 standards (IP55, Protection from Water, Snow, Dust Entry). If possible, testing should be done by a third party. The equipment supplier should supply test results from an independent test lab, including its detailed environmental testing specification. The ODU will be used for housing electronic equipment and should be shielded from dust, moisture, and any external contaminants that could enter it. The ODU should also be corrosion resistant.

Microwave Network Deployment

243

For indoor applications where HVAC is provided, the transmission/ microwave equipment shall indefinitely satisfy all manufacturers’ published specifications within a temperature range of 0 to +45°C (ideally −10 to +45ºC). For both outdoor and indoor applications, the equipment shall indefinitely satisfy all manufacturers’ published specifications within a humidity range of 0 to 90 percent noncondensing relative humidity. All microwave equipment must satisfy the manufacturers’ published specifications while operating in an altitude pressure density range of −100 to +4,000 m AMSL (above mean sea level). All microwave radio specs have to be guaranteed over the entire temperature range. 6.2.9

Network Management System

The ability to remotely manage equipment from a central management site is an essential requirement of operating any communication network, and microwave networks are no exception. The ability to fully report on and diagnose radio equipment and its performance is very important, since deploying technicians to visit remote tower sites is time consuming and costly. Local management (e.g., configuration and setup, software download, and so forth) at the site is performed from the local craft terminal (LCT), which allows access to all operation and maintenance facilities on the terminal via a web server in the terminal. Remote management can be performed over a DCN from the appropriate NMS. Remote supervision allows monitoring of alarms and performance as well as some configurations. Remote supervision of the SDH microwave network is realized with a connection to one of the terminals in the network. The terminal unit is usually equipped with a 10BASE-T port for this purpose. Each terminal also holds a router that terminates and routes IP messages, which means that the DCN can be extended throughout the transmission network. The router ensures that O&M data on the 10BASE-T port are incorporated in the main traffic over the hop, using either the EOC or DCC channel. For exchange of O&M data between two terminals on the same site, the units can be interconnected by the 10BASE-T ports through an external hub (alternative 1 in Figure 6.4). Another possibility is to let O&M data pass unaffected between the terminals in the EOC/DCC channel (alternative 2). Fault management deals with detection, isolation, and correction of malfunctions. It can be used with performance management as well, to compensate for environmental changes. Also included are maintenance and examination of error logs and action on alarms.

244

Chapter Six

NMS

Microwave hop

DCN

EOC/DCC

EOC/DCC

Alternative 1

STM-1/OC-3 Site A

ADM Alternative 2 2 Mbit/s

Site B

Figure 6.4 Routing O&M traffic in an SDH microwave network

Configuration management addresses the configuration of elements, getting/setting configuration from/to elements and software download functions. Performance management can be described as a set of functions to evaluate and report the behavior of the equipment and to analyze its effectiveness. It also includes subfunctions to gather statistical information. Performance management is based on the ITU-T G.826 (for example, error second ratio (ESR), severely errored second ratio (SESR), background block error ratio (BBER), unavailable state (UAS), etc.) and M.2120 recommendations. The built-in security functions protect the terminal and its services from disturbances caused by illegal activities of nonauthorized personnel. The access control is carried out when the user accesses the terminal from the LCT or when the Java-based user interface is started from the NMS. The user has to log on to the management system with a user name and a password to be able to access the management facilities. Depending on the selected mode, the user will be able to obtain readonly access or read/write access to the system; for example: n

n

Passive users are only able to monitor data. They are not able to collect data or change the network configuration. Active users are able to collect data and change some communication settings but not commands that can make unrecoverable configuration changes.

Microwave Network Deployment

n

n

245

Master users have access to all the commands except those attended with user account administration. Admin users have access to all the commands. The Admin user is the administrator and is responsible for adding, deleting, and managing user accounts and privileges.

6.2.10

Microwave Compatibility and Safety

In addition to the obvious concerns about safety, such as when climbing structures or working with dangerous AC line voltage, there is also the issue of exposure to RF radiation. Much is still unknown, so there is a great deal of debate concerning the safe limits of human exposure to RF radiation. In the U.S., the American National Standards Institute (ANSI) sets safety standards for human exposure to radio frequency (RF) electromagnetic energy. Exposure standards for RF energy are threshold standards. Unlike ionizing radiation, which many people believe to act cumulatively, even at low exposure levels, RF exposure at low levels is not considered to be a cumulative hazard. Threshold standards define the level of RF energy above which there may be health hazards and below which there have been no reported harmful effects. ANSI conservatively set its maximum permissible exposure levels for RF energy at one-tenth (or less) of the threshold for human health effects. The maximum permissible exposure levels for protection against RF energy recommended by ANSI are comparable to those set in other countries. Government agencies recognize and generally accept the ANSI RF safety standard (ANSI/IEEE C95.1).3 Ionization is a process by which electrons are stripped from atoms and molecules. This process can produce molecular changes that can lead to damage in biological tissue, including effects on the genetic material, DNA. This process requires interaction with photons containing high energy levels, such as those of X-rays and gamma rays. A single quantum event (absorption of an X-ray or gamma-ray photon) can cause ionization and subsequent biological damage due to the high energy content of the photon, which would be in excess of 10 eV (electron-Volt) and considered to be the minimum photon energy capable of causing ionization. Therefore, X-rays and gamma rays are examples of ionizing radiation. Ionizing radiation is also associated with the generation of nuclear energy, where it is often simply referred to as radiation. The photon energies of RF electromagnetic waves are not great enough to cause the ionization of atoms and molecules, so the RF energy is characterized as nonionizing radiation, along with visible light, infrared radiation, and other forms of electromagnetic radiation with relatively

246

Chapter Six

low frequencies. It is important that the terms ionizing and nonionizing not be confused when discussing biological effects of electromagnetic radiation or energy, since the mechanisms of interaction with the human body are quite different. The best general rule is to avoid unnecessary exposure to radiated RF energy. This means not standing in front of, or in close proximity to, any antenna that is radiating a transmitted signal. Of course, antennas that are used only for receiving do not pose any danger or problem. For dish-type antennas, it is safe to be near the operating transmit antenna if you are at its back or side, as these antennas are directional, and potentially hazardous emission levels will be present only in front of the antenna. Always assume that any antenna is transmitting RF energy, especially since most antennas are used in duplex systems. Be particularly wary of smaller dishes (1 ft [0.3 m] or less), as these often radiate RF energy in the higher frequency ranges. The general rule is that the higher the frequency, the more potentially hazardous the radiation. It is known that looking into the open (unterminated) end of a waveguide that is carrying RF energy at 10 GHz or more will cause retinal damage if the exposure lasts as little as tens of seconds at a transmit power level of only a few watts. In any case, be careful to ensure that the transmitter is not operating before removing or replacing any antenna connections. On a rooftop and near an installation of microwave antennas, it is important to avoid walking, and especially standing, in front of any of the equipment. If it is necessary to traverse a path in front of any such antennas, there is typically a very low safety concern if you move quickly across an antenna’s path axis. More information can be found in the literature.4–6 6.2.11

Microwave Radio Installation

Installation of the split-terminal configuration, assuming that the entire infrastructure is available (tower, shelter, racks, AC/DC power, pipemounts, and so on), should not take more than one day per hop and two people to do the work. Indoor installation consists of one small module for nonprotected systems and three modules for protected systems (two modems plus the switching module). The usual telecommunications rack in the equipment room consists of the fuse or breaker panel on the top, space for the small rectifier and battery backup at the bottom of the rack, and the microwave radio and DSX panel mounted in the middle of the rack. All-indoor microwave radio installations can take a few days, since they are usually bigger in size and have rigid waveguide components,

Microwave Network Deployment

247

flexible waveguides, flanges, dehydrators, and so forth. They usually need additional bracing and specialized tools to mount and make them operational. The installation team has to have all the necessary tools, installation equipment, and test equipment to complete the project. The basic list of test equipment is as follows: n

n

n

n

n

A bit error test set, covering the data rates, interfaces, and protocols of the equipment to be installed Power meter(s) and associated power sensors covering the frequency bands of the microwave radio equipment Frequency counter(s) covering the frequency bands of the microwave radio equipment Variable attenuators covering the frequency bands and matching the flange/connector type of the radio equipment Multimeter(s)

Radio manufacturers usually provide detailed installation manuals, and in most cases, list of any additional specialized tools required. It is a good idea to attend their training courses and seminars, especially after the new products are being introduced. 6.2.12 Millimeter-Wave Point-to-Point Systems 6.2.12.1 About Millimeter-Wave Radios Every new generation of wireless networks requires more and more cell-sites that are closer and closer together combined with the fast growing demand for the capacity of the transmission links. Millimeter-wave radio has recently attracted a great deal of interest from the scientific world, industry, and global standardization bodies due to a number of attractive features of millimeter-wave to provide multigigabit transmission rate. It is expected that the millimeter-wave radios can find numerous indoor and outdoor applications in residential areas, offices and conference rooms, and libraries. It is suitable for in-home applications, such as audio/video transmission, desktop connection, and supports portable devices. Judging by the interest shown by many leading companies, applications can be divided into the following categories: n

Point-to-Multipoint n

High definition video streaming

n

File transfer

248

n n

n n

Chapter Six

Wireless gigabit Ethernet Wireless docking station and desktop point-to-multipoint connections Wireless ad hoc networks

Point-to-Point n

Wireless backhaul for 3G and 4G wireless networks

n

Campus applications

n

Video relay of uncompressed HDTV

The new millimeter-wave (MMW) radio services (which the Federal Communications Commission (FCC) established in October 2003, together with the allocation, band plan, service rules, and technical standards) will promote the private sector development and use of the millimeter-wave spectrum in the 71–76-GHz, 81–86-GHz, and 92–95-GHz bands (“E” bands). It is expected that millimeter-wave radios will find numerous indoor and outdoor applications in residential areas, offices, conference rooms, corridors, and libraries. Aside from in-home applications such as audio/ video transmission, desktop connection, and support of portable devices, it is suitable for the outdoor point-to-point microwave systems, connecting cell-sites at a one-mile distance or closer and offering a huge backhaul capacity. The 60-GHz band has been allocated worldwide for license-exempt wireless-communications systems. In 2001, the Federal Communications Commission (FCC) set aside a large continuous block of spectrum between 57 and 64 GHz for wireless communications. This allowed all users to be permitted except radar, and the regulatory organizations in United States, Japan, Canada, and Australia have already set frequency bands and regulations for 60-GHz operation, while in Korea and Europe intense efforts are currently underway. A major factor in this allocation with commercial ramifications is that the spectrum is license-exempt—in other words, an operator does not have to buy a license from the FCC before operating equipment in that spectrum. Before this, less than 0.3 GHz of bandwidth was available at lower frequency bands (2.4 GHz, 5.8 GHz, and 24 GHz) for licenseexempt communications. In addition to the high-data rates that can be accomplished in this spectrum, energy propagation in the 60-GHz band has unique characteristics that make possible many other benefits, such as excellent immunity to interference, high security, and frequency reuse. In addition to the very low power levels discussed previously, millimeter-wave systems do not penetrate the human body. High-frequency

Microwave Network Deployment

249

emissions such as 60 GHz are absorbed by the moisture in the human body and are thereby prevented from penetrating beyond the outer layers of the skin. Lower-frequency emissions penetrate and may even pass completely through the human body while the minimal penetration of 60-GHz energy eliminates the debate that currently surrounds the safety of other RF communication systems. At MMW frequencies, RF is generally absorbed at skin but eye damage is a dominant health concern. An FCC limit has been adopted in consultation with four other healthrelated agencies and for 1.5–100 GHz equals 1 mW/cm2 averaged over 2 30 minutes (general public exposure), and 5 mW/cm averaged over 6 minutes for occupational/controlled exposure. 6.2.12.2 70-, 80-, and 90-GHz Bands In 2003, the Federal Communications Commission (FCC) made a ruling on opening up 13 GHz of spectrum at frequencies much higher than had been commercially available before. This spectrum provided for the first time the means to provide economical broadband connectivity at true gigabit data rates and beyond. In 2005, the Commission for European Post and Telecommunications (CEPT) released a European-wide frequency channel plan for fixed service systems in these bands, and the following year, the European Technical Standards Institute (ETSI) released technical specifications covering these bands. Of particular interest is the 10 GHz of bandwidth at 70 and 80 GHz. Designed to coexist together, the 71–76 GHz and 81–86 GHz allocations allow 5 GHz of full-duplex transmission bandwidth; enough to transmit a gigabit of data even with the simplest modulation schemes. With more spectrally efficient modulations, full-duplex data rates of 10 Gbps (OC-192, STM-64 or 10GigE) can be reached. With direct data conversion and low-cost diplexers, relatively simple and thus costefficient and high-reliability radio architectures can be realized. The three spectrum segments of the E-band (71–76, 81–86, and 92–95 GHz) have been allocated as a shared non-federal and federal government service for short-range line-of-sight radios having a transmission capacity comparable to that of fiber-optic communications. Spectrum reuse is based on the pencil beam concept; that is, very high angle discrimination between nearby links. Very stringent requirements are placed on the antenna radiation pattern of at least 50 dBi gain and no more than a 0.6 degree half-power beamwidth. The E-band is the highest frequency spectrum yet allocated to licensed operation, and it contains sufficient space for digital transmission speeds comparable to those of optical communication systems (1.25–5 Gbps). Furthermore, under the licensing rules, a large number of users within a small geographic area will be able to share the E-band allocation. The FCC’s approach to the use of E-band spectrum is nonexclusive, nationwide licensing with site-by-site coordination, but without

250

Chapter Six

extensive FCC action. Thus users will be able to set up short-distance links at locations where the time and cost of installing fiber optic cable are prohibitive. Other than the pencil beam antenna concept to allow very high spatial reuse of frequencies, there are few restrictions imposed on manufacturers of E-band equipment. Thus it is expected that technological developments will occur to make the use of E-band, and perhaps still higher frequency bands, practical and more efficient. Several radios have appeared on the market, using the 71–76 and 81–86 GHz bands as a paired channel. At present they have a fixed transmission speed of 1.25 Gbps full duplex, and their intended applications are high-speed wireless local area networks, broadband access systems for the Internet, and point-to-point communications. Each E-band licensee is assigned the totality of the spectrum in the 71–76, 81–86, and 92–95 GHz bands. The first two bands can be used as a paired channel, i.e., each transceiver transmits only in one of the bands and receives only in the other. Table 6.2 summarizes the technical requirements for licensed E-band millimeter-wave radios. The 92–95 GHz band is intended for indoor applications only, so it is not a part of this discussion. E-Band Licensing Process The FCC ruling permits a novel licensing scheme for millimeter-wave radios, allowing users cheap and fast allocations to prospective users. A 10-year license can be applied for, granted, and purchased in less than 30 minutes for the cost of a few hundred dollars. The FCC will issue an unlimited number of nonexclusive nationwide licenses to non-federal government entities for the 12.9 GHz of spectrum allocated for commercial use. These licenses will serve as a prerequisite for registering individual point-to-point links. The 71–95 GHz bands are allocated on a shared basis with federal government users. Therefore, in order to operate a link under its nonexclusive nationwide license, the licensee will have to do the following:

6.2.12.3

n

n

Coordinate with the National Telecommunications and Information Administration (NTIA) with respect to federal government operations. Register as an approved link with a third-party database manager.

TABLE 6.2

Technical Specs for E-Band in the U.S.

Region

Bandwidth (GHz)

U.S.

1.25 GHz (71–76 and 81–86)

Tx Power

EIRP

Max. Antenna Gain

NS

55 dBW (max)

50 dBi (min) 0.6° (−3-dB points)

Microwave Network Deployment

251

On September 29, 2004, the WTB released an order announcing the appointment of Comsearch, Frequency Finder, Inc., and Micronet Communications, Inc. as independent database managers responsible for the design and management of the third-party 71–95 GHz bands Link Registration System (LRS). Proposed links must be coordinated with the NTIA. The NTIA has developed an automated coordination mechanism that will allow nonfederal government users and the database managers to determine whether a given non-federal government link has any potential conflict with federal government users. A proposed link entered into the NTIA’s automated system will result in either a “green light” or a “yellow light” response based on the proposed parameters. If the proposed link receives a green light, that link will be protected for a period of 60 days in the NTIA’s system; if registration has not been completed through the LRS by the end of that time, the link must be resubmitted through the NTIA’s automated system for coordination with federal government operations. If the proposed link receives a yellow light, an FCC Form 601, Schedule M will need to be filed with the FCC for further coordination with the NTIA through the existing Interdepartment Radio Advisory Committee (IRAC) process. When IRAC clears a proposed link, the licensee will be notified by an FCC letter that the IRAC coordination has been completed. The database managers will also be provided with the status through the ULS nightly batch files for purposes of completing registration of the link. To summarize, a filing with the FCC will be required for links that fit any of the following descriptions: n

Receive a yellow light from NTIA’s automated system

n

Require environmental assessment

n

Require coordination because of a radio quiet zone

n

Are subject to international coordination requirements

Licensees must begin operation of a link within 12 months from the date that the link is registered on the LRS. While licensees need not file a notification of construction completion, it is the responsibility of the licensee to notify a database manager to withdraw unconstructed links from the LRS, and database managers shall remove a link from the LRS if they learn that a link is unconstructed after the required timeframe. In addition, the interference protection date will be rendered invalid for any registered link that does not comply with the 12-month construction requirement. Licensees must meet the loading requirements of 47 C.F.R. § 101.141. If it is determined that a licensee has not met the loading requirements,

252

Chapter Six

then the database will be modified to limit coordination rights to the spectrum that is loaded and the licensee will lose protection rights on spectrum that has not been loaded. See FCC Public Notice DA 05-311, February 3, 2005 (or the latest update for more information on registration process in the 71–76 GHz, 81–86 GHz, and 92–95 GHz bands). There are no current international agreements between and among the United States, Mexico, and Canada with regard to these bands. However, as a general rule, wireless operations must not cause harmful interference across the Canadian and Mexican borders.7 6.2.12.4 60-GHz License-Exempt Band The 60-GHz band has been allocated worldwide for license-exempt wireless-communications systems. In 2001, the Federal Communications Commission (FCC) set aside a continuous block of 7 GHz of spectrum between 57 and 64 GHz for wireless communications. All users are permitted except radar. Regulatory organizations in United States, Japan, Canada, and Australia have already set frequency bands and regulations for 60-GHz operation, while in Korea and Europe intense efforts are currently underway. A summary for the issued and proposed frequency allocations and main specifications for 60-GHz radio regulation in a number of countries is given in Table 6.3. On June 1, 2007, the FCC released ET Docket No. 07-113, which revised the FCC’s rules regarding operation in the 57–64 GHz band. The proposal would amend the requirements in Part 15 of the FCC’s rules applicable to transmitters operating on a license-exempt basis in the 57–64 GHz frequency range. Specifically, the proposal would increase the fundamental radiated emission limit for license-exempt 60-GHz transmitters with very

TABLE 6.3

Region U.S. Canada Japan Australia Korea Europe

Technical Specs for 60-GHz Band License-Exempt Bandwidth (GHz) 7 GHz (57–64) 7 GHz (57–64) 7 GHz (59–66), max 2.5 GHz 3.5 GHz (59.4–62.9) 7 GHz (57–64) 9 GHz (57–66), min 500 MHz

Tx Power 500 mW (max) 500 mW (max)

EIRP 40 dBm (av) 43 dBm (max) 40 dBm (av) 43 dBm (max)

Max. Antenna Gain NS NS

10 mW (max)

NS

47 dBi

10 mW (max)

150 W (max)

NS

10 mW (max)

TBD

TBD

20 mW (max)

57 dBm (max)

37 dBi

Microwave Network Deployment

253

high gain antennas, specify the emission limit as an equivalent isotropically radiated power (EIRP) level, and eliminate the requirement for a transmitter identification for 60-GHz transmitters. The proposal would increase the current Part 15 average power EIRP level from 40 dBm to a new level of 82 dBm minus 2 dB for every decibel that antenna gain is below 51 dBi. The Part 15 peak power EIRP level would increase from 43 dBm to a new level of 85 dBm minus 2 dB for every decibel that the antenna gain is below 51 dBi. These increases would be limited to 60-GHz transmitters located outdoors or those located indoors with emissions directed outdoors, e.g. through a window (although some are concerned that the window installation could seriously impact indoor broadband systems). The changes would allow longer communication ranges for licenseexempt point-to-point 60-GHz broadband digital systems and thereby extend the ability of such systems to supply very high speed broadband service to office buildings and other commercial facilities. At the time of writing this text, these changes are still under consideration. Point-to-point wireless systems operating at 60 GHz have been used for many years by the intelligence community for high security communications and by the military for satellite-to-satellite communications. Their interest in this frequency band stems from a phenomenon of nature; the oxygen molecule (O2) absorbs electromagnetic energy at 60 GHz. Figure 6.5 shows the gaseous attenuation for oxygen absorption and for water vapor absorption as a function of range, over and above the free-space loss.

40 20

15 dB/km @ 60 GHz

10

Attenuation dB/km

4 2 1 0.4 0.2

H2O

0.1 .04 .02

A

.01

B

.004 .002 .001 10

O2 H2O

15

20

25 30

40

O2 A: Sea level T = 20°C P = 760 mm H2O = 7.5 gr/m3

50 60 70 80 90100

Frequency GHz Figure 6.5 Gaseous absorption at 60 GHz

H2O

150

B: 4 km T = 0°C H2O = 1 gr/m3 200 250 300

400

254

Chapter Six

The resonances for frequencies below 100 GHz occur at 24 GHz for water vapor and 60 GHz for oxygen. This absorption occurs to a much higher degree at 60 GHz than at lower frequencies typically used for wireless communications, and it weakens (attenuates) 60-GHz signals over distance, so that signals cannot travel far beyond the intended receiver. This reduction in signal strength enables higher frequency reuse—that is, the ability for more 60-GHz links to operate in the same geographic area than links with longer ranges. 6.2.12.5 Engineering and Installation Precautions

The oxygen absorption affects the range, with the resulting benefits just described; however, link distances of millimeter-wave radios operating in the real world are limited primarily by rain. Link distance increases as level of availability and rainfall rates decrease. Rainfall statistics are well known for locations around the globe so that range and availability can be accurately predicted. In moderate rain regions, the rain attenuation is about twice the oxygen attenuation, and in heavy rain regions, the rain attenuation is more than three times the oxygen attenuation. Therefore, in designing a 60-GHz link to provide robust communication capability in the real world, rain attenuation is actually a larger factor than oxygen absorption, although both have to be taken into consideration. Many high-frequency (10 to 30 GHz) and millimeter-wave (above 30 GHz) links fail to perform properly because rain rates are not properly applied or are not assessed appropriately for a microclimate area. Say, for example, in Town A, just 10 miles north of Town B, we have an average of 48 inches of rain per year compared with Town B’s 23 inches of rain per year (the average rainfall is the mean monthly precipitation, including rain, snow, hail, etc.). The planning for a Town A link may not be the same as that for a Town B link, even though they are in the same rain region. We have to keep in mind that the total amount of rain is not as important as the intensity of the rainfall (rain rate, mm/hr), which also has to be carefully assessed. Many cities have their microclimate areas where temperature and rain rate differ quite significantly from one area to another. Although important in the design of all microwave links in 10–38 GHz bands, exact rain rate data becomes absolutely critical when designing links in even higher millimeter-wave bands (see Figure 6.6). The ITU-R recommendation that can be used for estimating long-term statistics of rain attenuation is considered to be valid in all parts of the world—at least for frequencies of up to 40 GHz and path lengths up to 60 km (37 mi). Most likely certain modifications of the existing models

Microwave Network Deployment

255

100 150 mm/hr 100 mm/hr

50

50 mm/hr

20

18 dB/km @ 70 GHz

25 mm/hr

Specific attenuation Tx (dB/km)

10

4.5 dB/km @ 18 GHz

5

5 mm/hr

2

1.25 mm/hr

1 0.25 mm/hr 0.5

0.2 0.1 0.05 0.02 0.01

1

2

5

10

20

50

100

200

500

1000

Frequency (GHz) Figure 6.6 Rain attenuation curves

may be required to extend their validity to MMW links. Some preliminary long-term measurements on a half-mile link performed in the early 2000s show a significant deviation from the ITU-R recommendations; measured attenuation was typically 1–5 dB higher that the calculated one based on ITU recommendation. It also seems that the lower rain rates caused a larger difference in results than the higher rain rates for the particular percentage of time. Table 6.4 shows approximate results of millimeter-wave link engineering for the typical license-exempt 60-GHz and licensed 70-GHz link using the Crane rain model. The E-band is unaffected by fog, smog, sand, and other small particles in the air. From the installation perspective, if we assume a 2-ft dish at the new, license-exempt millimeter frequency (MMW), 60 GHz for example, the maximum deflection of the antenna and the structure should be 0.5°. For the licensed 70- and 80-GHz bands, this value should be even smaller. This means that these radios should be only mounted on a very sturdy

256

Chapter Six

TABLE 6.4

Millimeter-Wave Link Engineering Maximum Hop Length (Miles) for Rain Regions in the U.S. (Crane 1996)

Frequency 60 GHz

70 GHz

Availability

A

D-2

E

99.999 % 99.995 % 99.990 % 99.999 % 99.995 % 99.990 %

0.48 0.55 0.59 1.4 1.95 2.3

0.32 0.39 0.44 0.68 0.93 1.12

0.26 0.33 0.36 0.53 0.68 0.78

tower or building wall, while all other structures should be completely eliminated. Installation challenges are twofold; first, to ensure that the antenna, together with the mounting structure, has a deflection of 0.5° or better, and second, that people installing it have sufficient experience and/or training with a pencil-wide beam antenna alignment. 6.3

Digital Multiplexers

Today, digital multiplexers are part of every microwave system, whether they are part of the microwave radio equipment or added as an external piece of equipment. Multiplexing is a process in which multiple data channels are combined into a single data or physical channel at the source; conversely, demultiplexing is the process of separating multiplexed data channels at the destination. An example of multiplexing is when data from multiple devices is combined into a single physical channel. Some methods used for multiplexing data are time-division multiplexing (TDM), asynchronous timedivision multiplexing (ATDM), frequency-division multiplexing (FDM), and statistical multiplexing. One method (FDM) involves splitting the frequency band transmitted by the channel into narrower bands. Each of these narrow bands is used to create a distinct channel. In FDM, information from each data channel is allocated bandwidth based on the signal frequency of the traffic, and multiple channels are combined onto a single aggregate signal for transmission. The channels are separated by their frequency. FDM was the first multiplexing scheme to be widely used, and such systems are still in use. However, TDM is the preferred approach today. In TDM, information from each data channel is allocated bandwidth based on preassigned time slots, regardless of whether there is data to transmit. In ATDM, information from data channels is allocated bandwidth as needed, using dynamically assigned time slots.

Microwave Network Deployment

257

In statistical multiplexing, bandwidth is dynamically allocated to any data channels that have information to transmit. Statistical time division multiplexing (STDM) is much improved over TDM because the muxes are intelligent. STDM, or stat mux, offers the advantage of dynamic allocation of available channels and raw bandwidth. In other words, STDM can allocate bandwidth, in the form of time slots, in consideration of the transmission requirements of individual devices serving specific applications. An STDM can also oversubscribe a trunk, supporting aggregate port speeds that may be in the range of 3–10 times the trunk speed by buffering data during periods of high activity. M13 is the most commonly used multiplexer and usually takes the form of a modular, compact unit for multiplexing up to 28 DS1s into a DS3. Compatible with North American standard interfaces, these multiplexers are capable of multiplexing up to seven low-speed DS1 signal groups (each with four DS1s) into one DS3 (44.736 Mbps) signal, with each low-speed signal group consisting of four DS1s. Standard features include continuous performance monitoring and extensive local and remote diagnostics, which provide off-premises restoration, thereby avoiding the cost of on-premises maintenance. Remote monitoring and control are the key features, e.g., remote provisioning, remote inventory, performance monitoring, and remote testing. FMT150 is a fiber-optic multiplexer that multiplexes and transports one, two, or three DS3 signals; up to 84 DS1s; and a maximum of 2,016 voice circuits. Designed to interface with DS1 or DS3 input signals, the typical FMT150 uses 150 Mbps fiber transport plug-ins with up to three DS3 inputs. FMT150 usually features a comprehensive maintenance system and provides complete, instantaneous monitoring and troubleshooting capabilities, including fault analysis of the remote site. It can also monitor alarms and controls at local sites and provide so-called housekeeping alarms. The purpose of subrate multiplexing is to fit more sub-DS0 data circuits into one DS0 (64-kb) channel. The SRDM feeds synchronous or asynchronous data circuits into a single 64-kbps DS0 signal. For example, the SRDM can multiplex 20 sub-DS0 signals into a DS0 channel at 2.4 kbps or 10 at 4.8 kbps or 5 at 9.6 kbps. Subrate multiplexers were very popular during the early TDMA and GSM wireless networks build-out, since they allowed circuit grooming and efficient use of T1/E1 facilities. Drop and insert multiplexers allow a transiting DS3 signal to drop and insert a small number of DS1s without being fully demultiplexed and remultiplexed as would occur with back-to-back terminals. In the case of a regenerative repeater site where mux is not provided, most microwave radios themselves have add/drop capability of one or more DS1 circuits. In this case, additional multiplexers may not be required.

258

Chapter Six

Inverse multiplexers perform the inverse process of traditional muxes. In other words, they accommodate a single, high-bandwidth data stream by transmitting it over multiple, lower-bandwidth channels or circuits. The transmitting mux segments the data stream and spreads it across the circuits on a consistent and coordinated basis, and the receiving mux reconstitutes the composite data stream. Obviously, the two devices must synchronize carefully with each other and with the transmission characteristics of the individual paths and channels in order to minimize errors and delays. An individual communication might spread over multiple switched circuits, dedicated circuits, or channels on multichannel circuits. 6.4

Cabling and Signal Termination

There is very small chance that a microwave engineer working on the design and deployment of a microwave network will get involved in the details of the metallic or fiber-optic cable installation. A microwave engineer may be involved in routing the coax cable connecting outdoor and indoor units of the split-configuration microwave radio.† In addition, interfacing the existing metallic or fiber-optic network at the switch office location and hub sites is very likely. Therefore, the microwave engineer will require some basic knowledge of practical issues involved in this connection, such as bringing cables into the building, terminating them, and interfacing (and cross-connecting) other types of equipment. The National Electrical Code (NEC) identifies three different intrabuilding regions with regard to cable placement: n

n

n

Plenums A compartment or chamber that forms part of the air distribution system and to which one or more air ducts are connected. A room with a primary function of air handling is also considered to be a plenum space. Risers An opening or shaft through which cable may pass vertically from floor to floor in a building. General-purpose areas or risers.

Other indoor areas that are not plenums

Cables are specifically listed for use in each of these areas. The NEC allows the use of a cable with a more stringent listing to be used in an application requiring a lesser listing, but not the other way around.

†

For more details see my other publication, Transmission Systems Design Handbook for Wireless Networks, Artech House, 2002.

Microwave Network Deployment

259

When a cable run will be exposed to both indoor and outdoor environments, there are several options to consider. The first option is to run an outside plant cable for the entire run. The second option is to use an indoor/outdoor cable. The third option is to transition-splice the outside plant cable to an inside plant cable. The consideration and design of proper cable pathways (i.e., conduit, cable trays, riser shafts, and so forth) and termination spaces (i.e., main/ intermediate cross-connects, horizontal cross-connects, and the work area) are as important as the design of the cable network. For more information, see “Commercial Building Standard for Telecommunications Pathway and Spaces,” TIA/EIA-569 or CSA-530 (Canadian Standards Association). The primary focus of this standard is to provide design specifications and guidance for all building facilities relating to telecommunications cabling systems and components. Another useful document is the Telecommunications Distribution Methods Manual, available from the Building Industry Consulting Service International (BICSI). Use the latest revision (presently 12th edition) of the document since new content is constantly added and existing content-modified to reflect the way the industry has evolved. 6.4.1 Metallic Cables and Ground Potential Rise

Wireless operators are increasingly employing electrical utility facilities for the installation of their PCS and microwave antennas. It is important to understand that electric transmission towers, and electric utility environments in general, are dangerous, hazardous locations that are prone to a phenomenon called ground potential rise (GPR). Communications cables (for example, E1/T1 circuits) are susceptible to damage from lightning surges, since they can develop high shieldto-pair voltages, even with low lightning currents on the shield; these voltages could cause lengthy and potentially catastrophic outages. GPR usually occurs during a power fault when the fault current returns to the power neutral source through the earth. Communications cables are considered to be exposed to GPR when the possibility exists that the local ground (at the cell-site) differs from remote ground by 300 V or more. Optical isolators (opto-couplers) are generally used to treat each circuit (including T1/E1 circuits) going into the power station. This protects the circuits and facility, as well as personnel, from electrical hazards associated with GPR. Design of any communication circuits into the power location has to be in accordance with ANSI/ IEEE 367, “Recommended Practice for Determining the Electric Power Station Ground Potential Rise and Induced Voltage From a Power Fault,” and ANSI/IEEE 487,

260

Chapter Six

“Guide for Protection of Wireline Communications Facilities Serving Electric Power Stations.” The only alternative to the GPR study is a letter from the power utility owner, on letterhead and appropriately signed, stating that at no time will the GPR at the site or sites ever exceed 1,000-V peak-asymmetrical, as calculated per ANSI/IEEE 367. The power utility owner must be aware that issuance of the letter constitutes an assumption of liability for injury and/or damage brought about by electrical fault conditions. 6.4.2

Fiber-Optic Cables

Every fiber-optic cable requires proper installation techniques. Building codes and standards, environmental issues, proper design, routing, installation equipment, topologies, applications, and reliability concerns all have to be addressed. Considerations of tensile strength, ruggedness, durability, flexibility, size, resistance to the environment, flammability, temperature range, and appearance are also important in constructing optical fiber cable. From the outside, a fiber-optic cable looks like any other electrical multiconductor cable. However, it is lightweight and flexible as compared to metal conductor cable. Typical fiber cable outside dimensions (ODs) range from less than 1/8 of an inch up to 3/4 of an inch, depending on fiber numbers and cable construction. The most common fiber-optic cable jacket materials are polyethylene (all types), PVC, and polyurethane. Probably the most common mistake made by inexperienced fiber installers is to violate the minimum bending radius by making tight bends in the cable. Tight bends, kinks, knots, and other flaws in fiber cable can result in a loss of performance. The minimum bending radius in traditional fiber cable is usually in the range of 20 times cable OD—considerably higher and more stringent requirement than for electrical cables. However, new fiber technologies are lowering this minimum bend radius. The specific minimum bending radius for a particular cable should be researched in the cable manufacturer’s specifications. 6.4.3

Digital Signal Cross-Connects

Microwave system is usually a part of a bigger transmission network and the digital signal leaving microwave radio and multiplexing equipment has to be terminated at the cross-connect point. Digital signal crossconnect (DSX) devices (also called jackfields) are used to connect one piece of digital telecommunication equipment to another. They simplify

Microwave Network Deployment

261

equipment connections and provide convenient test access and tremendous flexibility for rearranging and restoring circuits. DSX panels are available in numerous configurations and sizes to fit a wide variety of applications, but they all perform the same function. They terminate equipment and provide temporary jack access for centralized testing, cross-connection, reconfiguration, and restoration of various digital circuits. They are used in voice, data, and video networks for patching during equipment installations or breakdowns, network expansion, or traffic-pattern adjustments. At T1 (1.544 Mbps) or E1 (2.048 Mbps) rates, DSX-1 panels connect network equipment such as office repeater bays, channel banks, multiplexers, digital switches, microwave radios, and so on. At the T3 (44.736 Mbps) rate, DSX-3 panels provide terminations for the highspeed (DS3 rate) side of the M13 multiplexers, and the low-speed (DS3 rate) side of the digital microwave radio and fiber-optic systems. The DSX-3 supports networks operating at the DS3 rate and the STS-1 and STS-3 electrical SONET rates. It is usually placed between network elements, such as fiber-optic terminals, multiplexers, broadband digital switches, and digital crossconnect systems, and digital radio components. They are located in a central office, CEV, hut, or cabinet, or on the customer’s premises. A connecting block (also called a terminal block, a punch-down block, a quick-connect block, or a cross-connect block) is a plastic block containing metal wiring terminals used to establish connections from one group of wires to another. Usually, each wire can be connected to several other wires in a bus or common arrangement. There are several types of connecting blocks: M66 clip, BIX, Krone, 110, and others. A connecting block has insulation displacement connections (IDCs), which means that it is not necessary to remove the insulation from around the wire conductor before “punching it down” (i.e., terminating it). Blocks can be reused for at least 500 terminations with less than 1 mΩ of connection resistance. 6.5 Microwave Antennas, Radomes, and Transmission Lines 6.5.1

Basic Antenna Specifications

An antenna is a device that transmits and/or receives electromagnetic waves. Reflector antennas are used extensively where very high gain, high radiation efficiency, and narrow beamwidths are required. Applications include satellite communication systems, navigation systems, terrestrial communications systems, deep space communication systems, and radio astronomy.

262

Chapter Six

The microwave reflector antenna is a highly directional, parabolic, dish-shaped radiator that is connected to the microwave transmitter (through the RF branching network—RF circulators, filters, couplers, and switches) via transmission lines (coaxial cable or waveguide). Initial design and experimentation with microwave antennas began more than 100 years ago, and a number of microwave systems using parabolic antennas grew significantly during the 1930s. During World War II, designs such as pencil-beam and shaped-beam antennas were developed for radar systems used by the Allies. While many advances were made at this time, it was in the 1950s that terrestrial microwave communication systems were deployed, and parabolic reflector designs were utilized on the commercial systems. Microwave antennas are available in many sizes to satisfy the requirements of a particular application. Generally speaking, the larger the antenna diameter, the higher the antenna gain relative to isotropic antenna and the smaller the beamwidth. A feeder is placed with its phase center at the focus of the parabola. Ideally, all the energy radiated by the feed will be intercepted by the parabola and reflected in the desired direction. To achieve maximum gain, this energy should be distributed such that the field distribution over the aperture is uniform. Because the feed is small, however, such control over the feed radiation is unattainable in practice. Some of the energy actually misses the reflecting area and is lost; this is commonly referred to as spillover. In addition, the field is generally not uniform over the aperture but is tapered, with maximum signal at the center of the reflector and less signal at the edges. This taper loss reduces gain, but the field taper provides reduced side lobe levels. During the 1980s, the need became greater for a lower-profile microwave antenna that also exhibited superior radiation pattern performance. Two forces drove this requirement. One was the need to reduce the visual effect of radio communication installations. The other was the need to place more and more microwave “links” in the same geographic area. 6.5.1.1 Antenna Gain The directive gain of an antenna system toward a given direction is the radiation intensity normalized by the corresponding isotropic intensity, and it measures the ability of the antenna to direct its power toward a given direction. The gain of any antenna is essentially a specification that quantifies how well that antenna is able to direct the radiated radio frequency (RF) energy in a particular direction. Thus, high-gain antennas direct their energy more narrowly and precisely, and low-gain ones direct energy more broadly.

Microwave Network Deployment

263

Antennas demonstrate a property known as reciprocity, which means that an antenna will maintain the same characteristics, including the radiation pattern, regardless if it is transmitting or receiving. Most antennas are resonant devices, which operate efficiently over a relatively narrow frequency band. An antenna must be tuned to the same frequency band of the radio system to which it is connected; otherwise the reception and the transmission will be impaired. An antenna’s gain and radiation pattern are fundamentally related; higher-gain antennas always have narrower beamwidth (patterns), and low-gain antennas always have wider beamwidth. For a parabolic reflector microwave antenna (above 1 GHz) consisting of a dish-shaped surface illuminated by a feed horn mounted at the focus of the reflector, antenna gain G is given as G = 20 log10(Dft) + 20 log10(f) + 7.5 (dBi)

(6.12)

G = 20 log10(Dm) + 20 log10(f) + 17.82 (dBi)

(6.13)

or

where dBi = decibels over an isotropic radiator D = antenna dish diameter (feet or meters) f = frequency (GHz) An isotropic radiator is a hypothetical, lossless antenna having equal radiation in all directions. The three-dimensional radiation pattern of an isotropic radiator is a sphere. Antenna gain is related to directivity through the radiation efficiency parameter. Radiation efficiency is a measure of the dissipative power losses internal to the antenna. The preceding formula is based on the efficiency of a parabolic antenna being around 55 percent. Some manufacturers may be able to improve on this value, so the gain given by a manufacturer for a specific antenna should be used when available. In most other cases, the preceding formula will suffice. Engineers commonly refer to half-wave dipole antenna gains. Compared to the gain of an “ideal” isotropic antenna of 1 (0 dB), the gain of a half-wave dipole antenna is 1.64 (2.15 dB). The relationship between the two units, dBi and dBd, is therefore expressed as follows: G (dBi) = G (dBd) + 2.15

(6.14)

Considering both stations of a radio link, the difference between freespace loss comparison using isotropic and half-wave dipole antennas is about 4.3 dB.

264

Chapter Six

Antenna gain and other specifications are defined and measured in the far field of the antenna. In the near field, antenna gain can be up to 10 dB lower than in the far field, and its value is used only for some very specific applications; for example, a back-to-back repeater in which one hop of the system can be only a few hundred feet long. When antenna manufacturers simply specify the gain of an antenna, they are usually referring to the maximum value of gain, G. 6.5.1.2 Half-Power Beamwidth The half-power beamwidth is the angular separation between the half-power points on the main beam antenna radiation pattern, where the gain is one-half the maximum value (−3 dB). Antenna gain and beamwidth are interrelated quantities and are inversely proportional; thus, the higher the gain an antenna has, the smaller the beamwidth. Therefore, increased care must be taken when aligning high-gain antennas to ensure that the antenna is accurately aligned on the center of the main beam—which could be only a few degrees wide. The beamwidth of a parabolic antenna can be approximated by the following formula:

BW ≈

70 [ deg ] f ⋅ D ft

(6.15)

BW ≈

22 [ deg ] f ⋅ Dm

(6.16)

where f = frequency (GHz) D = diameter of parabola (m or ft) Example: A parabolic dish antenna with a diameter of 4 ft is operating at 11 GHz. Determine the approximate gain, beamwidth, and the minimum distance for the far-field region operation. Assume illumination (radiation) efficiency of 55%. D = 4 ft = 48 in

λ=

c f

≈ 0.0273 m = 0.0896 ft

G = 20 log 4 + 20 log 11 + 7.5 ≈ 40.4 dBi

Microwave Network Deployment

BW =

RFF =

70 fD

2D2

λ

=

=

70 11 ⋅ 4

265

≈ 1.6°

2 ⋅ 42 0.0896

= 357 ft

Polarization and Cross-Polarization Discrimination Generally speaking, any two antennas that form a link with each other must be set for the same polarization. This is typically accomplished by the way the antenna (or just the feedhorn) is mounted and as such is almost always adjustable at (or even after) the time of antenna installation. Note that, if the physical waveguide connection at the antenna is vertically oriented, the antenna has horizontal polarization, and vice versa. It is difficult to predict the orientation of the electric field in the nearfield region, as the transmitting antenna cannot be considered a point source in this region. In the far-field region, the antenna becomes a point source, the electric and magnetic components of the field become orthogonal to the direction of propagation, and their polarization characteristics do not vary with distance. For licensed links, the polarization may be specifically dictated by the terms of the license; for license-exempt links, the operator is free to choose, and the choice may be crucial in averting or correcting an interference problem. Note that, for most microwave (dish) antennas, it is not possible to determine the exact type of polarization the antenna is set up for by observation from a distance (such as when viewing a tower-mounted antenna from the ground). If two antennas both had linear polarization, but one had vertical polarization and the other had horizontal polarization, they would be cross-polarized. The term cross-polarization (or cross-pol) is also used to generally describe any two antennas with opposite polarization. Crosspolarization is sometimes beneficial. An example of this would be a situation in which the antennas of link A are cross-polarized to the antennas of link B, where links A and B are two different but nearby links that are not intended to communicate with each other. In this case, the fact that links A and B are cross-polarized is beneficial because it will prevent or reduce any possible interference between the links. Cross-polarization discrimination is the ratio of the signal with the required polarization, PN, and the signal with the orthogonal polarization, P0. 6.5.1.3

XPD = 10 log

PN [dB] PO

(6.17)

266

Chapter Six

On over-water paths at frequencies above about 3 GHz, it is advantageous to choose vertical polarization over horizontal polarization. At grazing angles greater than about 0.7°, a reduction in the surface reflection of 2 to 17 dB can be expected over that at horizontal polarization. 6.5.1.4 Radiation Patterns Radiation patterns describe the distribution

in space of electromagnetic energy generated (or received) by a given antenna. For mobile, portable, and some base station applications, the type of antenna needed has an omnidirectional radiation pattern. The omnidirectional antenna radiates and receives equally well in all horizontal directions, while directional antennas focus energy in a particular direction. For microwave point-to-point links, required antennas are always highly directive. The radiation patterns are presented as polar plots (relative energy level versus angular position) in the E-plane, and H-plane—in other words, in the same plane as the E-field and H-field, respectively. A simplified version, called a radiation pattern envelope (RPE), is often used for design purposes. In this case, the pattern is deliberately “linearized,” and the normal (sometimes wide) fluctuations in the field are removed. Radiation patterns (or the RPE version) are used to design and evaluate system performance as it relates to transmission (EIRP) in any given direction, or reception (RSL) from any given direction, including interference. Four RPEs are always needed when analyzing the behavior of microwave antennas: two parallel envelopes (named HH and VV) and two crossed envelopes (named HV and VH). These acronyms mean n

n

n

n

HH The response of a horizontally polarized antenna port (H-port) to a horizontally polarized microwave radio signal VV The response of a vertically polarized antenna port (V-port) to a vertically polarized microwave radio signal HV The response of a horizontally polarized antenna port to a vertically polarized microwave radio signal VH The response of a vertically polarized antenna port to a horizontally polarized microwave radio signal

Minimizing RF interference is one of the toughest challenges for wireless communications network designers. One of the main causes of interference between point-to-point microwave links is the existence of side lobes (a radiation in any direction other than the direction(s) of intended radiation) and back lobes (the radiation lobe opposite to the main lobe), spurious reflections that radiate (and receive) in directions other than the main beam (see Figure 6.7).

Microwave Network Deployment Side lobes Back lobe

Main lobe

267

–3 dB Half-power beamwidth

–3 dB Figure 6.7 Antenna radiation pattern

Side lobes up to 30° away from the main beam tend to have sufficient energy to interact and interfere with physically adjacent links, causing reduction in signal quality and increases in bit error rate. In an antenna radiation pattern, a null is a zone in which the effective radiated power is at a minimum. A null often has a narrow directivity angle compared to that of the main beam. Thus, the null is useful for several purposes, such as suppression of interfering signals in a given direction. By minimizing side lobes and back lobes, a greater number of point-topoint links may be deployed within a given area without being affected by interference. Consequently, for antennas exhibiting superior side lobe suppression, neighboring links may be located closer together without sacrificing network performance so the greater number of antennas can be practically installed on a single tower. In recent years, significant advancements in the antenna-design areas of polarization discrimination and side lobe reduction have provided the capability of enhanced spectrum efficiency in point-to-point microwave radio communications. 6.5.1.5 Beam Efficiency Beam efficiency is a parameter frequently used

to describe the performance of an antenna. Beam efficiency is the ratio of the power received or transmitted within a cone angle to the power received or transmitted by the whole antenna. In other words, beam efficiency is a measure of the amount of power received or transmitted by minor lobes relative to the main beam. The antenna beam efficiency is absorbed in the definition of gain.

6.5.1.6 Front-to-Back Ratio Apart from the forward gain of an antenna, another important parameter is the front-to-back ratio. This specification is expressed in decibels, and as the name implies, it is the ratio of the maximum signal in the forward direction to the signal in the opposite direction. 6.5.1.7 Bandwidth The bandwidth of an antenna expresses its ability to operate over a wide frequency range. It is often defined as the range

268

Chapter Six

over which the power gain is maintained to within 3 dB of its maximum value, or the range over which the VSWR is no greater than 2:1, whichever is smaller. The radiation pattern of an antenna may change dramatically outside its specified operating bandwidth. The bandwidth can be described in terms of percentage of the center frequency of the band. BW = 100 ×

fH − fL [%] fC

(6.18)

where fH is the highest frequency in the band, fL is the lowest frequency in the band, and fC is the center frequency in the band. In this way, bandwidth is constant relative to frequency. If bandwidth was expressed in absolute units of frequency, it would have different values, depending upon the center frequency. Different types of antennas have different bandwidth limitations. F/D Ratio A common way to define a parabolic dish shape is with the F/D ratio, where F is the focal length, and D is the diameter of the dish; the smaller the ratio, the “deeper” the dish. Most commercial microwave antennas use an F/D ratio of 0.25 to 0.38, with 0.32 to 0.36 the most common. The F/D ratio for a reflector can be determined by measuring the depth of the dish from the plane of the rim to the vertex at the center and using the basic equation for a parabolic curve. Typically, only the measurement from the vertex to the rim is required, since a parabola of revolution consists of the same shape curve for all radial sections.

6.5.1.8

6.5.2

Parabolic Antenna Feed Methods

The horn antenna is a transition between a waveguide and free space. A rectangular waveguide feed is used to connect to a rectangular waveguide horn, and a circular waveguide feed is for the circular waveguide horn. The horn antenna is commonly used as a feed to a parabolic dish antenna, a gain standard for antenna gain measurements, and as compact medium-gain antennas for various systems. The gain of the horn antenna can be calculated to within 0.1dB accuracy from its known dimensions and is therefore used as a gain standard in antenna measurements. It can be seen that the dish antenna provides a very high gain and narrow beam so the alignment of the dish antenna is usually very critical. The parabolic dish is generally fed by a horn antenna (feeder) connected to a coaxial cable.

Microwave Network Deployment

269

Some of the major feed methods are front feed (most commonly used), Cassegrain, and offset feed (see Figure 6.8). The front feed is the simplest method, and the illumination efficiency is typically only 55 to 60 percent. The feed and its supporting structure produce aperture blockage and increase the side lobe and crosspolarization levels. Dual-reflector antennas consisting of a main reflector, along with a secondary subreflector, are used to increase the effective focal length and to provide convenient placement of the feed. The Cassegrain method, consisting of a parabolic reflector and a hyperbolic subreflector, has the advantages that the feed is closer to other front-end hardware and a shorter connection line is needed. Cassegrain antenna aperture efficiencies are typically on the order of 65 to 70 percent. The Gregorian method is similar to the Cassegrain feed, but an elliptical subreflector is used and an illumination efficiency of over 70 percent can be achieved. The subreflectors are held in place by structural leg assemblies that are usually either tripods (three legs) or quadripods (four legs, most common). The offset feed method avoids the aperture blockage by the feed or subreflector and minimizes cross-polarization. The side lobe levels are smaller, and the overall size is smaller for the same gain.8 There are a number of other types of reflectors but they are not used in microwave point-to-point communications. For example, at low microwave frequencies or ultrahigh frequencies (UHFs), a parabolic dish becomes big, so only a portion of the dish is used instead. This is called a truncated parabolic dish (also known as an orange peel paraboloid reflector), commonly used on ships. With the reflector truncated (cut) so that it is shortened vertically, the beam spreads out vertically instead of being focused. This fan shaped beam is used in radar detection applications for the accurate determination of bearing. Since the beam is spread vertically, it will detect aircraft at different altitudes without changing the tilt of the antenna. Front feed Parabolic dish

Cassegrain feed

Feed

Offset feed

Hyperbolic sub-reflector

Offset sub-reflector Figure 6.8 Parabolic antenna feed methods

270

Chapter Six

To make the dish lighter and to withstand strong wind, a dish made of metal mesh instead of solid metal can be used. 6.5.3

Microwave Antenna Selection

Standard, open-grid (for low wind loading), and high-performance dishes, and single or dual polarized antenna models, are available. Many antennas come with galvanized steel mounts based on EIA standards RS195B and RS222. As the antenna beamwidth decreases, antenna alignment (and thus stability) become more critical. Furthermore, weight and wind loading are greater with large antennas. As a consequence, the antenna mounting structure must be several times more rigid (against twist and sway) for each successive increase in antenna size. The selection of antenna size should be based on the results of path analysis and calculations. The antenna size must be determined before a frequency coordination study can be performed and before applying for the license to operate the microwave system.9 The main parameters of interest when choosing the microwave antenna are as follows: 1. Operating frequency band (MHz or GHz) 2. Radiation pattern 3. Gain (dB) 4. Polarization (single or dual polarized) 5. Half-power beamwidth (degrees) 6. Wind load (mph) 7. Front-to-back ratio (dB) 8. Cross-polarization discrimination (dB) (This is the difference between the peak of the co-polarized main beam and the maximum cross-polarized signal over an angle twice the 3-dB beamwidth of the co-polarized main beam.) 9. Isolation (between inputs of single-band, dual polarized antennas) (dB) 10. Additional options for the most microwave antennas that may be ordered from manufacturers are n

Input flanges

n

Antenna color variations

n

Radomes (reflector protectors) of various customized colors

n

High-wind survival options

n

Corrosive environment protection

Microwave Network Deployment

n

Packing type and quantity options

n

Various reflector types

n

Special-purpose antennas: n n

n

271

High-performance or shrouded antennas Ultra-high performance antennas; the improvement over highperformance antennas is a higher front-to-back ratio and better cross-polarization discrimination

Special accessories such as struts, ice shields, and so on

Grid-type dish antennas are usually employed for the low-frequency microwave bands at 1.4 GHz and 2 GHz. At 400 MHz and 800/900 MHz, low-cost Yagi types can be used, but grids are preferred for their better front-to-back ratio performance over Yagi antennas. Most administrations require the use of relatively high-performance antenna for fixed links in these bands to optimize frequency reuse. Grid antennas, on the other hand, are cheaper than the solid types and the use of grids reduces tower wind loads. Antennas operating at the low-frequency bands have wider beam widths than at higher microwave bands, reducing the requirements for tower stiffness providing a further cost saving. 6.5.4

Radomes and Shrouds

High-performance antenna versions include an RF shroud, added to improve side lobe performance. Shrouds appear like “drums” increasing the side profile of the parabolic antenna. The shroud is lined with an RF absorber material, improving side and back lobe radiation. Back lobe radiation can be reduced more than 10 dB by using shrouds. A planar radome is used to protect the antenna against harsh weather conditions and to prevent ice or snow accumulation. Radomes must be electrically transparent at operating frequency; that means that the thickness, L, of the radome must be chosen to be equal to one-half wavelength in the material of the radome. As an example, we can assume that the permittivity er' of the dielectric radome is 2.8 at 6 GHz. The refractive index of radome will be: n = ε r ' = 2.8 ≈ 1.67

(6.19)

The wavelength will be reduced n times in the dielectric material of radome:

λradome =

λ0 n

(6.20)

272

Chapter Six

So, in our example we have:

λ0 = λradome =

c 3 ⋅ 108 m/sec = = 50 mm 6 ⋅ 109 Hz f

λ0 50 mm ≈ 30 mm = 1.67 n

So, in order for this particular radome to be electrically transparent at 6 GHz frequency, it has to be 15 mm thick: L=

λradome 30 mm = = 15 mm 2 2

The amount of loss for a radome may vary from less than 0.5 dB for a typical unheated radome to more than 2.0 dB for a typical heated radome in the high-frequency bands. During a rainstorm, water sheeting on the face of the antenna causes an additional loss called wet radome loss. Preliminary analyses suggest that the effect of water on the antenna radome may be an important mechanism, in addition to rain attenuation along the path.10 This is particularly true in tropical regions where the rainfall rate is very high, and microwave links operating in the higher microwave bands will suffer from high attenuations due to rain. The losses of a wet radome depend not only on the rainfall rate, but also on wind conditions during the rain. In heavy rain situations, values of 15dB or more attenuation have been reported with nylon radomes. The loss for a Teflon-coated radome is 1dB, whereas the loss for a fiberglass or ABS plastic radome depends on the rain rate and frequency band but may be much higher than the Teflon radome. The Hypalon radome material used on some high-performance dishes can be as bad as fiberglass. Because of the lower wet-radome losses, Teflon-coated radomes are recommended for bands that are affected by rain outage. Wet radome losses are lower for Teflon because water tends to form droplets instead of sheets on the slick Teflon surface. Teflon is also used on most highperformance antennas. 6.5.5

Microwave Antenna Installation

The last activity during the microwave radio-hop installation is to align the antennas. The main purpose of path alignment is to physically align the antenna’s azimuth and elevation for maximum signal

Microwave Network Deployment

273

transfer (minimum path loss). Optimal antenna alignment occurs when both transmitting and receiving antennas are precisely aimed at each other in both azimuth and elevation. It is also important to ensure that the two antennas for the link are not cross-polarized. Azimuth is the angle in the horizontal plane with respect to true north, and elevation is the angle (positive or negative) in the vertical plane with respect to the horizontal plane. Notice that the values of the elevation angles on opposite sides of the link are not identical. The difference is a result of the well-known fact that the Earth is not flat. Antenna altitude and antenna height are expressed in meters or feet. These distances are measured to the center (feeder) of the parabolic dish antenna. A coarse alignment can be done by using line of sight, a compass, or some other method that comes with the radio (audible signal, for example). The antenna is finely aligned by adjusting for maximum input power using the outdoor unit’s AGC voltage. At one end of the link at a time, the antenna pointing direction is carefully adjusted to maximize (or peak) the reading on the indicator tool. Once the maximum signal is achieved, the antennas are aligned for elevation optimization. After performing this procedure on both ends, it is very important to obtain the actual received signal level in dBm so as to verify that it is within 0 to 3 dB of the value obtained from the link budget calculation. If the measured and calculated values differ by more than about 8 dB, it is possible that either the antenna alignment is still not correct or that there is another problem in the antenna/transmission line system—or both. It is possible to get a peak reading during the antenna alignment process if one or both of the antennas are aligned on a side lobe, in which case the measured receive level may be 20 dB (or more!) lower than the calculated value indicates that it should be (Figure 6.9).

Dish antenna

Dish antenna

Side lobe maximum

Main lobe maximum

–20 dB

Side lobes

Main lobe maximum

Figure 6.9 Error in antenna alignment

274

Chapter Six

Traditionally, the radios that will be placed at each site are used to complete the task of optimizing the path. However, there are several reasons for not utilizing the radios to complete the process. The radios may not be available at the time the test has been scheduled, or their reliability may be questionable, thus requiring alternative methods. Another possible situation when the radios might not be usable is when the FCC permits have not been granted (in certain frequency bands and/or geographical areas, we are allowed to install radios with FCC permits pending, while in some others we are not allowed until the license has been granted), but the contractor needs to complete the path test on time to meet the customer’s requirements or to stay ahead of expected turbulent weather. In addition, if the anticipated path is questionable, a quick, cost-effective, reliable method is needed to test the link prior to the significant investment in constructing towers, purchasing radios, and obtaining other expensive equipment and hardware. Alternative test instrumentation must be utilized, in lieu of the radios, whenever the previously mentioned circumstances arise. Scheduling of the path alignment test and installation of the associated hardware (e.g., cables, waveguide, antenna) can be facilitated to reduce excess mobilization costs. Some of the most widely used apparatus for this application are signal generators (used as the transmitter) and spectrum analyzers (used as the receiver). The signal generator should be a broadband, synthesized device (phase locked to a reference clock) with accurate output power, equal to or greater than 0 dBm. The spectrum analyzer should be tunable and have at least −100 dBm of sensitivity at the frequency band of interest. One of the most unpredictable and unacceptable unavailability events in the microwave system is power fading due to the ducting, antenna decoupling, and obstruction in coastal and similar humid climates. One of the simplest solutions is to uptilt larger dishes on longer paths (at above 6 GHz) in ducting areas, perhaps 1 dB, to minimize power fading and multipath activity due to the nighttime antenna decoupling. In space-diversity microwave systems, the diversity antenna delay equalization (DADE) has to be set in receivers to ensure errorless or hitless data switching in a severe fading environment. This requirement is a result of different waveguide lengths from the microwave radio to the main and diversity antennas. On most MW radios today, the DADE is automatically adjusted based on the phase difference between main and space diversity received signals. Any system components mounted outdoors will be subject to the effects of wind, and it is important to know the direction and velocity of wind that is common to the site. Antennas and their supporting structures must be able to prevent these forces from affecting the antenna or causing damage to the building or tower on which the components are mounted.

Microwave Network Deployment

275

Antenna designs react differently to wind forces, depending on the area exposed to the wind (wind loading). Most antenna manufacturers will specify wind loading for each type of antenna manufactured. For reasons of safety it is important to know that the near-field regions can constitute radiation hazards at substantial physical distances from an antenna, especially if the antenna is large (D >> l). 6.5.6

Minimum Antenna Height

When planning microwave paths longer than a few miles, the curvature of the Earth becomes a factor in path planning and requires that the antenna be located higher off the ground. The minimum antenna height at each end of the link for paths longer than seven miles (assuming smooth terrain without obstructions) is the height of the first Fresnel zone (60 percent of the FFZ, to be more precise) plus the additional height required to clear the Earth’s bulge, which is at its maximum at the midpoint of the path. The approximate formula is  d  d2 h ≈ 21.65   +  f  6k

(6.21)

where h = height of the antenna (ft) d = distance between antennas (mi) f = frequency (GHz) k = Earth radius factor, k = 4/3 for standard radio propagation conditions For the links at higher frequencies and only a few miles long, the Earth bulge can be neglected. Example: Consider a 30-mile, point-to-point communication link over a flat surface without obstructions. The frequency of operation is 6 GHz and the antennas are at the same height. Find the lowest antenna height that provides the same field strength as in free space (meaning first Fresnel zone has to be cleared). Assume standard atmospheric conditions.

 30  302 ≈ 48.4 + 112.5 ≈ 161 ft h ≈ 21.65   + 4  6 6⋅ 3

276

Chapter Six

We can see that the antenna height needs 48.4 ft for FFZ clearance (60%) plus an additional 112.5 ft to clear the Earth bulge (the Earth is not flat!). The terrain, manmade structures, and vegetation will most likely add additional requirements (height) for the antenna centerline. For k = 2/3, the antenna height would have to be 273 ft above the ground, causing a significant increase in tower requirements (and potentially cost increase as well), so we have to be very careful when selecting k value for our design. We can see that the Earth bulge is the main contributor to the antenna height requirements and therefore microwave hop length limitation. 6.5.7

Antenna Alignment Procedure

As we know, highly focused antennas minimize the possibility of interference between links in the same geographic area, minimize the risk that the transmission will be intercepted, and maximize performance. Directivity is a measure of how well an antenna focuses its energy in an intended direction and point-to-point radios should have highly directional antennas. It is also well known that the narrower the antenna radiation beam, the more difficult it gets to align the antennas on opposite sides of the microwave link. There are several steps involved in the traditional preparation and process of aligning microwave antennas. These steps may include making sure the cable or waveguide transmission line was properly installed, with minimal RF reflection of the microwave signal; that each antenna polarization is properly set up; and that the transmitter output power is calibrated. A voice communication link between the personnel inside the equipment room of each site and the tower technicians, located at each antenna location, needs to be established using two-way mobile radios or cellular phones. The path engineering results are reviewed to determine the expected RSL (receive signal level) for the path under test and any adjustments for output power are applied. Once this setup is complete, the tower technicians are instructed to commence the adjustment of the azimuth alignment (bearing) of the antennas, one at a time. The antennas are panned over their azimuth profile (horizontal alignment) first, and readings of the receiver signal output power are taken. Careful observation of the output power reading is necessary to distinguish the antenna side-lobe to main-lobe response. Once the maximum signal is achieved, the antennas are aligned for elevation optimization (vertical alignment). It is evident that the communication between site to site and tower technician to receiver

Microwave Network Deployment

277

technician needs to be continuous and clear to ensure that the antennas, optimum alignment setting is achieved. For long path and large diameter antennas the alignment has to be done during the propagation quiet time (preferably in mid-day) and redone a few more times over the few days period, in order to avoid multipath activities affecting the alignment. While fine-tuning the alignment, the best way to monitor the signal strength is with an analog voltmeter on the AGC. The importance of an analog meter cannot be overemphasized; digital voltmeters will change numbers and not give a correct indication of the signal change, while the needle swing of an analog meter shows instantly the status of the adjustment. Even the bar graph display on a digital meter will not track the AGC voltage as well as an analog meter. Once the antenna is set at peak level, tighten all adjustments and mounting hardware. Continue to monitor the RSL while doing this to ensure final tightening of the hardware has little to no effect on RSL. A much lower than expected receiver signal level, obtained with good cross-polarization discrimination (>20 dB), is an indication of a problem: faulty antenna feed system, a defective transmission line, or an obstructed path. Verify that calculated insertion loss through the transmission line is within limits and replace antennas or transmission system elements with known good devices if necessary. 6.5.8 Transmission Lines 6.5.8.1 Waveguides and Coaxial Cables There are three basic types of

waveguides: rigid rectangular, rigid circular, and semiflexible elliptical. Short sections of flexible waveguide are also used for connections between antennas and the microwave radio’s outdoor unit. In all cases, it is desirable to keep the number and length of flexible sections as small as possible, since they tend to have higher losses and poorer VSWR than the main waveguide types. Semiflexible elliptical waveguides are the most common type used for terrestrial microwave systems today. They are precision formed from corrugated high-conductivity copper and have an elliptical cross-section. The corrugated wall gives the waveguide increased crush strength, light weight, and relative ease of handling. The coaxial cable is the most widely used transmission line. It consists of two concentric conductors with the space between them filled with a dielectric, such as polyethylene or Teflon. Coaxial cable is used in case of the split configuration (indoor/outdoor units) of the microwave radio. Coax cables are much easier to install, and today they are used more often than waveguides.

278

Chapter Six

There will always be some loss of RF signal strength through the waveguides, cables, and connectors used to connect them to the antenna. This loss is directly proportional to the length of the cable and inversely proportional to the diameter of the cable. Additional loss occurs for each connector used, and this must be considered in the link planning process, i.e. link budget calculation. Cable vendors usually can provide a chart or a table indicating the loss for various types and lengths of cable. 6.5.8.2 Waveguide Installation Elliptical waveguide installation is a very tricky job that must be performed by trained and experienced technicians. Very small deformations can introduce enough impedance mismatches to produce serious problems. Waveguides are typically offered in three versions: standard, premium (low VSWR), and overmoded (low attenuation). Standard waveguides are recommended for low- and medium-capacity microwave systems, while premium elliptical waveguides are recommended for highcapacity microwave systems. Overmoded waveguides are used when long distances or restricted power budgets call for lower than normal attenuation. This is achieved at particular frequency bands by operating the waveguide above the cut-off frequency of higher-order modes. The effect of this is to introduce additional modes (and energy) into the waveguide. Elliptical waveguides may be ordered with the connectors attached and tuned if the precise length is known in advance. If the connectors are to be installed in the field, it may be necessary to sweep and tune the waveguide and connectors to realize optimal return loss. The waveguide must be supported with special hangers at regular intervals to prevent stress, movement, and excessive pulling forces caused by the cumulative weight of the waveguide. Hangers are usually installed at 3-ft (1-m) intervals. A waveguide (as well as coaxial cable) must be grounded using special grounding kits. It is common practice to ground the top of the waveguide run, the bottom of the run, and the radio terminal. For waveguides making the connection from the indoor branching to the outdoor antenna, the important specification is the minimum bending radius (E-plane and H-plane). A few general tips for handling waveguides: n

Waveguides are laid in a single run and connected to the antenna and the transmitter.

n

Do not install a guide that exhibits any evidence of damage.

n

Avoid stepping on or dropping anything on an elliptical waveguide.

Microwave Network Deployment

n

n

n

n

n

n

n

279

Close attention should be paid not to apply any load/force into the H-plane direction, which may crush the elliptical waveguide. Exposed horizontal runs must be protected from the weight of accumulated ice and damage from falling ice or other objects. The radius of curvature at the bending part must not be shorter than bending in the major axis direction. Buried waveguides (not a very common practice) should be below the frost line. A 4-in (10 cm) layer of sand under and over the buried guides is usually adequate to protect the jacket from stones or other sharp objects. Do not cut waveguides without the use of an appropriate flanging tool. Different waveguides require different flanging tools. Check the seal and its groove for dirt or other foreign material and ascertain that it is properly sealed before flange assembly. Keep the ends of the waveguide capped during installation. If installation is, for some reason, interrupted, seal installed line ends and pressurize to the recommended pressure with dry air.

The waveguide (and air dielectric coaxial cables as well) must be pressurized with equipment designed to dry the pressurized air. This action is necessary to displace any water vapor from the waveguide or cable that will significantly increase attenuation. The pressurization equipment may be either a mechanical compressor or a container of compressed gas (which must be replaced regularly) equipped with a pressure regulator. Several types of mechanical compressors are available with various capacities, operating from AC or DC line voltage and in manual or automatic drying cycles. Pressurization equipment is normally attached to the indoor end of the waveguide or coax cable, through a manifold. Since this is a closed system, pressurization equipment operates (turned on) very rarely; increased activity of the compressor could be an indication of the leak in the system and requires troubleshooting. Unfortunately, parabolic antennas and transmission lines in remote areas are quite often a target for practice shooting, and holes in the transmission lines or damaged antenna feeder systems are a common result of such a barbaric act. 6.5.8.3 Coaxial Cable Installation As mentioned earlier, coaxial cable is used in case of a split configuration (indoor and outdoor units) of the microwave radio. Coax cables are much easier to install, and they are presently used more often than waveguides. In addition, by using coax cables, feeder costs are reduced as well.

280

Chapter Six

Some mechanical characteristics of the coaxial cable interconnecting the IDU and ODU or waveguide are also important. These include dimensions over the jacket or outer diameter, distributed weight, and minimum bending radius. For coaxial cables interconnecting IDUs and ODUs operating at intermediate frequency (IF), the maximum acceptable length is vital information for the installation. Usually, the maximum length is about 300 m (984 ft) due to the electrical limitations of the coaxial cable itself. Generally speaking, when the environment is hazardous because of the presence of chemicals, explosive atmospheres, corrosive environments, or shock hazards, or when there are many T1/E1 signals traveling across the line (such as STM-0 or STM-1), it is a good approach to provide physical protection and/or separation. The duct can also act as an obstacle against vandalism, the influence of ultraviolet rays, water penetration, and fire, thus preserving the cable(s) inside it. Most antenna problems are caused by coaxial cable connections that loosen due to vibration, allowing moisture to penetrate the connector interface so it is recommended that all outdoor cable connections be weatherproofed. The IDU-ODU coaxial cable should be grounded, with cable grounding kits, to the tower or station earth to minimize the potential for damage due to lightning or other externally induced interference. It should be grounded as a minimum at the base of the tower and close to the transition into the equipment shelter. Grounding this cable close to the ODU will provide further protection, although it is not essential in areas of low lightning risk. Coordination of lightning protection by the site owner is essential to achieve proper equipment protection. 6.5.9 Installation Safety and Security Issues

The installation team must have enough people skilled in the different aspects of the microwave project (i.e., installers, riggers, and commissioners) to ensure that the project is completed in a professional manner and on schedule. It is obvious that communications tower safety is a life-and-death issue, not only for the men and women who work on the structures, but also for the companies that own them. Despite the common practice of contracting out the construction and maintenance of the towers to other companies, the owners of the structure (company directors, officers, and shareholders) share a legal responsibility with the subcontractor to make sure the work is carried out safely. This situation is not a new one—most jurisdictions require property owners to provide safe job sites regardless of whether they directly employ the workers.

Microwave Network Deployment

281

Communications towers are very high and very narrow and can be extremely dangerous. Therefore, tower construction and microwave antenna installation require special skills (Figure 6.10). Without the cranes to lower pieces into place from above, the towers are constructed with an apparatus known as a gin pole. The gin pole is used to hoist material from the ground and past the worker at the top so that it can be positioned and bolted in place. The work is carried out at the mercy of wind and inclement weather at heights that reach up to a few hundred meters. Typically, communications towers range between 30 and 150 m (100 to 500 ft) in height, while broadcast towers can be much higher. While working on a communications tower, very stringent work procedures and reliable safety equipment are required to protect workers from falls. A subcontractor must carry sufficient liability insurance for services provided within or outside North America, and the copy of the insurance coverage document has to be provided to the project manager prior to the start of any installation. In addition to the specific installation directions supplied by the manufacturer of the antenna, there are some general safety tips antenna installers should adhere to: n

n

n

Keep in mind that the installation and/or dismantling of any antenna near power lines is very dangerous. Never use a utility pole as a support for a guy wire and never climb a utility pole. Do not work on a wet or windy day or if a thunderstorm is approaching or after sundown.

Figure 6.10 Microwave antenna installation

282

n

n

n

n n

n

n n

n

Chapter Six

Height or other restrictions on antennas may apply to the installation depending on the proximity to an airport or local ordinances. Take the time to plan your installation procedure. Each person should have assigned tasks. A team leader or foreman should call out instructions and watch for signs of trouble. Dress properly with rubber soled shoes, rubber gloves, helmets, and long sleeved shirt, and use only an approved safety belt. Do not use a metal ladder. Outdoor antennas should be grounded with an approved lightingarresting device. Local codes may apply. The radio should also be grounded to an earth ground to help protect both the radio and its user. Do not use hot water pipes or gas lines as a ground source. If any part of the antenna, waveguide, or coax cable should come in contact with a power line, DO NOT TOUCH IT OR TRY TO REMOVE IT YOURSELF! Call your local power company immediately and they will remove it. If the assembly starts to drop, get away from it and let it fall. Remember that the antenna mast, cables, and guy wires are all excellent conductors of electrical current.

6.5.10 Impedance Matching and Return Loss Measurements

For an efficient transfer of energy, the impedance of the radio, the antenna, and the transmission cable connecting them must be the same. Transceivers and their transmission lines are typically designed for 50 Ω impedance. If the antenna has impedance different from 50 Ω, then there is a mismatch and an impedance matching circuit is required. Of course, other impedances are also common as well. Voltage standing wave ratio (VSWR) is a measure of impedance mismatch between the transmission line and its load. The higher the VSWR, the greater the mismatch. The minimum VSWR (i.e., that which corresponds to a perfect impedance match) is unity. The combination of the original wave traveling down the coaxial cable (toward the antenna, or the opposite during receive) and the reflecting wave is called a standing wave. The ratio of these two waves is known as the standing wave ratio. Because the standing wave ratio is not always calculated from the voltages, the “V” is sometimes dropped from the acronym and it is referred to as standing wave ratio (SWR). VSWR and SWR are the same thing and can be used interchangeably.

Microwave Network Deployment

283

Return loss is another way of expressing impedance mismatch. It is a logarithmic ratio measured in decibels that compares the power reflected by the antenna to the power that is fed into the antenna from the transmission line. If the antenna absorbs 50% of the signal, and 50% is reflected back, we say that the return loss is −3 dB. An antenna might have a value of −10 dB (90% absorbed and 10% reflected). The relationship between VSWR and return loss is as follows: Return Loss = − 20 log

VSWR − 1 [dB] VSWR + 1

(6.22)

For example, for our earlier example of VSWR = 1.2:1, return loss is calculated to be −20.83 dB. Impedance, in antenna terms, refers to the ratio of the voltage to current (both are present on an antenna) at any particular point of the antenna. This ratio of voltage to current varies on different parts of the antenna, which means that the impedance will be different on different spots on the antenna if you pick any spot and measure it. The impedance for the entire chain from the radio to the antenna must be the same, and almost all radio equipment is built for an impedance of 50 Ω. One common way of visualizing the VSWR is to use a polar plot called a Smith chart. From this plot for the VSWR value, the return loss and the impedance for the different frequencies can be derived. Therefore, it is an important instrument for understanding antennas and transmission lines. VSWR/return loss can be measured for the antenna, transmission line, transmission line and all rigid components, and the entire antenna subsystem (e.g., antenna, transmission line, connectors, and so on). These measurements ensure correct impedance matching between the radio port and antenna port. In addition, the insertion loss of the transmission line and all other components should be measured. Measurement of the antenna subsystem should be performed as follows: 1. Switch off the radio before testing the waveguide. 2. Install a directional coupler between the radio Tx port and the transmission line, and check the polarity as well as the flange connections. 3. Connect the power meter and power sensor to the measurement port on the directional coupler. 4. Switch on the radio power.

284

Chapter Six

5. Measure the reflected power, taking into account the coupling coefficient inside the directional coupler. The reflected power should be around 30 dB below the measured radio output power measured for the system gain calculation. 6. Record the following values: n

Transmit power (dBm)

n

Directional coupler coupling coefficient (dB)

n

Power meter reading (dBm)

n

Measured reflected power (dBm)

n

Return loss (dB)

7. Provide a plot of the sweep across the frequency band, using a reference of 0 dBm. Return loss measurement of just the transmission line may be performed by placing an ideal termination at the opposite end of the transmission line. Insertion loss may be measured by shorting the transmission line. Use a shorting connector for coaxial cable and a shorting plate for the elliptical waveguide. 6.5.11

Antennas for Harsh Environment

6.5.11.1 Corrosive Environment The number of available telecommunica-

tions towers and available space on these towers is getting increasingly scarce, so the network operators are sometimes forced to use sites in unusual locations. For example, from the height perspective, industrial chimneys can be a great option. However, large concentration of emitted gases, which are caused by turbulence and are potentially highly corrosive, are encountered in such locations. This means that the antenna system must be manufactured from corrosion-resistant materials. In addition, any combination of different materials must also be able to resist corrosion. Typically, there are two general types of corrosive environments, marine and industrial. In marine environments, the main contaminant is seawater, while in industrial environments there can be a wide range of corrosive agents, including sulfur and nitrogen.11 The main corrosive agent in the marine environment is sodium chloride, present in seawater in typical concentrations of 3.4 percent, although this varies according to geographical location and climatic factors. Saline atmospheres arise as a result of saltwater becoming suspended in the air due to wind and wave action. The concentration of this salinity and the resulting deposition varies throughout the world.

Microwave Network Deployment

285

In assessing the local environment, many factors need to be considered, including sea and air temperature, prevailing wind direction, local topography, and relative humidity. For increased protection within extreme corrosive and humid environments, most reputable antenna manufacturers offer harsh environment antennas as an option. These antennas come with special corrosion resistant components and finishes and are designed to withstand corrosive weathering environments typical of industrial, shoreline, and offshore environments. The following are features of corrosive-environment antennas: n

Fully epoxy-painted aluminum reflector and shields

n

Epoxy-painted galvanized steel mount

n

Epoxy-painted feed assembly

n

Stainless steel assembly and adjusting hardware

n

Corrosion inhibiting compounds and sealants applied during installation

The following is a list of locations where selection of corrosiveenvironment antennas will most likely be required: n

n

n

n

Antennas sited offshore should always be specified as a corrosive environment. Antennas sited onshore in areas with a sea salt deposition of more than 80 mg/m2/day should be specified as a corrosive environment. Depending on local climatic factors, such conditions can prevail for a considerable distance inland. Antennas situated in close proximity to smelting facilities, chemical works, or fossil fuel power stations (particularly those burning brown coal) should always be specified as corrosive environment. Areas of high general pollution caused by traffic and industry may require corrosive environment antennas, depending on the levels of pollution and the life required from the antenna. In some countries, even usual everyday air pollution is so severe that it warrants the use of these specially protected antennas.

6.5.11.2 High-Wind Environment Typical parabolic antennas are designed for a survival wind speed of 125 mph (200 km/hr). While this is sufficient for the majority of antenna sites, there are some areas that require a more robust antenna. In determining the wind speed experienced by the antenna, not only should the basic maximum ground speed be examined, but also factors unique to the antenna site, like geographic

286

Chapter Six

location, local topography, position of the antenna on the tower, and the probability of wind gusts. These factors work in combination and will generally give rise to a wind speed much higher than that seen at the ground level. The high-wind antenna reinforcement consists of additional struts for mounting between the back ring and the reflector rim, enabling the wind forces to be directed to the most rigid part of the antenna mount. The kit can be installed on site before lifting the antenna onto the tower. Antennas with a wind load kit provide a survival wind speed of 250 km/hr (155 mph) and an operational wind speed of 200 km/hr (125 mph). Some of the high-wind antennas attach via four stainless steel studs to the tower interface steel and not to the standard 4.5 in diameter (115 mm) steel pipe. Some of the high-wind conditions are confined to limited geographical areas, for example, the tornado or twisters of the American Midwest. Others may be known by different names worldwide, for example, hurricanes in the North Atlantic and typhoons in the Pacific. The common feature of these storms is that the mean wind speed is often above the design limit of standard antennas. The winds associated with midlatitude cyclones cause more difficulty. Normally the mean wind speed is well below the antenna design wind speed, but in extreme cases there may be gusts above the standard antenna design limit. Local meteorological data will provide the probability of such extreme events and allow a decision to be made on antenna specification. This data should be used with caution, because extreme winds are usually of a relatively short duration and may occur infrequently. Where a steep-sided valley narrows, causing a funneling effect, extreme wind speeds may result over a localized area. Similar effects occur where there are rapid changes in air temperature, for example, in mountainous areas close to the sea. The main effects of topography are localized, so one should gather as much site-specific information as possible before determining the specification for the antenna. The following are recommendations for the selection of high-wind antennas: 1. Identify the maximum mean hourly wind speed at the antenna site from local meteorological data. 2. Make suitable allowances for local terrain and the height of the antenna on the tower. 3. Determine the wind profile of the area. Is it generally calm with occasional windy periods, or is it a windy area with periods of extreme conditions?

Microwave Network Deployment

287

4. Select the correct antenna for the conditions. Where the antennas may experience severe tropical storms as in, for example, the Caribbean during the hurricane season, even 200 mph (320 km/hr) survival wind speed antennas should be considered. 5. If the antenna will be sited in a marine or corrosive environment, consider specifying a corrosive environment in addition to the highwind antenna. In general, mean wind speed increases with height above ground. This variation corresponds to the relationship:  H VH ≈ V10    33 

p

(6.23)

where VH and V10 are the mean wind speed at H height and 10 m (33 ft) above ground, respectively, computed over 3 secs and with recurrence of 50 yrs. The factor p varies from about 0.1 to 0.4, according to the terrain. A value of 0.17 is frequently used, although it applies only to flat terrain with few trees or obstructions (see Table 6.5). The numbers in Table 6.5 are provided for illustration purposes only and should not be used for actual design; familiarity with local topography and climatic conditions in the specific area are required to determine the antenna wind loading and/or erect the new telecommunications towers. From the antenna loading prospective, it is important to keep in mind that the wind energy is proportional to the cube of its speed, which means that doubling the wind speed increases its energy by a factor of eight. Although they are at the same geographical location, antennas placed at two different heights on the same tall tower do not necessarily have to have the same wind rating. Wind gusts result mainly from the roughness of the Earth’s surface and are accentuated when the air flows over buildings and other obstacles. Gusts may also be caused by temperature convection currents. TABLE 6.5

Wind Speed Above the Ground

Height (Feet) 33 100 200 300 400 500

Wind Speed (mph) 25 30 34 36 38 40

50 60 68 73 76 79

75 91 102 109 115 119

100 121 136 146 153 159

288

Chapter Six

The data for extreme wind speed in gusts is normally measured over a minimum period of three seconds, i.e., peak wind for three seconds. For brief intervals the maximum speed may be higher. Although the mean wind speed may be relatively low, the maximum wind speed seen in a gust will be considerably higher, typically 50 percent higher for a five-second gust in open country. 6.5.11.3 Effect of Ice on Antenna Performance Ice affects the performance of all antennas to some degree, and the problem gets more serious the higher the frequency. The feeder in an antenna transforms electromagnetic signals into electromagnetic waves radiating into free space. The impedance of free space is 377 Ω. If the air immediately surrounding the feeder is replaced by ice, which has lower impedance than air, then the impedance match and radiation patterns of the antenna will change. These changes become progressively worse as the ice loading increases. Parabolic dish antenna elements are usually protected by a radome. This provides an air space between the elements and ice casing so that the lower impedance of the ice layer has only a small effect on the radiators. Detuning is greatly reduced, but radiation pattern distortion may still be encountered (detuning reduces usable antenna bandwidth). For a given ice thickness, deviation from usual performance values become worse as frequency increases. In areas where severe icing and wet snow are common, it is recommended to install a full radome over solid parabolic antennas, and stay away from grid antennas of any type.

6.6 6.6.1

GIS Data General Types of Maps

There are three general types of maps: planimetric, topographic, and orthophoto. Planimetric maps show the positions of features without showing their relationship to the hills and valleys of the land. Examples of features on planimetric maps include rivers, lakes, roads, and boundaries. Planimetric maps include: n n

n

Common road maps, including road atlas and city maps Specific area maps, including replan maps, floor plan maps, storm drain maps, sewer and water system maps Schematic maps, including agency maps and aviation maps

Microwave Network Deployment

289

Topographic maps are different from planimetric maps because they show both the horizontal and vertical (relief) positions of features. Two types of topographic maps include: n

n

Contour maps The most common way to show the shape and elevation of the land. A contour is an imaginary line, where all points on the line are at the same elevation (above or below a specific reference elevation, usually sea level). Contour lines reveal the location of slopes, depressions, ridges, cliffs, and other topographical features. Shaded-relief maps A shadow effect color simulates the terrain. Different color shades are used to accentuate the shape of the physical features. The darker the shading, the steeper the slope.

An orthophoto map is an aerial color-enhanced photograph of the land depicting terrain and other features. Some orthophoto maps are overlain with contour lines and other features commonly associated with topographic maps. These maps are corrected for scale and are the same size as USGS topographic quadrangle maps. 6.6.2

Datums and Geometric Earth Models

When producing maps, the surface of the Earth has to be mapped onto a plane. This is called a projection. A projection from a curved surface like the Earth to a plane cannot be done without distortion; therefore, various projection types with different advantages and disadvantages have been developed and are still in use throughout the world. Every map in the world has been made with one or another of such projections, e.g., UTM, Gauss Kruger, or Lambert Conformal. The same area mapped by two different projections may look slightly or even completely different. A spheroid is a surface obtained by rotating an ellipse around one of its axes. In geography, the shape of the Earth is considered to be a spheroid. However, the Earth is not a perfect spheroid; therefore, several spheroid models have been adopted to approximate the surface in different parts of the Earth. These spheroids are slightly different in shape and size. They are uniquely defined by their equatorial and polar radii, and they are identified by names like Clarke 1866, Hayford, and others. Every map is associated with a projection and a spheroid. This is called a geodetic datum (from Latin, singular for data-given things). Hundreds of different datums have been used to frame position descriptions since the first estimates of the Earth’s size were made by Aristotle. Datums have evolved from those describing a spherical Earth to ellipsoidal models derived from years of satellite measurements. Modern geodetic datums range from flat-Earth models used for plane surveying

290

Chapter Six

to complex models used for international applications that completely describe the size, shape, orientation, gravity field, and angular velocity of the Earth. Referencing geodetic coordinates to the wrong datum can result in position errors of hundreds of meters. The accuracy of a geographic database depends on the source material, data capture technique, and resolution. The source material can be paper maps, aerial photography, or satellite images. Examples of paper maps with different scales and accuracy are topographical, ordnance survey, and city maps. Different nations and agencies use different datums as the basis for coordinate systems used to identify positions in geographic information systems, precise positioning systems, and navigation systems. The diversity of datums in use today and the technological advancements that have made possible global positioning measurements with submeter accuracies require careful datum selection and careful conversion between coordinates in different datums. True geodetic datums were employed only after the late 1700s, when measurements showed that the Earth was ellipsoidal in shape. In North America, there is, for example, NAD83 (an acronym for North American Datum 1983). Sometimes still used is NAD27 (North American Datum 1927). NADCON is the United States Federal standard for NAD27 to NAD83 datum transformations. The GPS is based on the World Geodetic System 1984 (WGS 84). There are numerous other horizontal data, such as Yacare (South America), Tokyo (Japan), Djakarta (Indonesia), and Easter Island (Pacific Ocean). The right choice of the horizontal datum is crucial for good path calculations. Coordinate values resulting from interpreting latitude, longitude, and height values based on one datum as though they were based in another datum can cause position errors in three dimensions of more than a kilometer. 6.6.3

Coordinate Systems

Coordinate systems to specify locations on the surface of the Earth have been used for centuries. In western geodesy the equator, the Tropics of Cancer and Capricorn, and lines of latitude and longitude were used to locate positions on the Earth. Eastern cartographers used other rectangular grid systems as early as 270 CE (common/Christian Era, or A.D. 270, Anno Domini), and various units of length and angular distance have been used over history. The meter is related to both linear and angular distance, having been defined in the late eighteenth century as one-ten-millionth of the distance from the pole to the equator.

Microwave Network Deployment

291

Latitude, Longitude, and Altitude System The most commonly used coordinate system today is the latitude, longitude, and altitude system. The prime meridian and the equator are the reference planes used to define latitude and longitude. The geodetic latitude (there are many other defined latitudes) of a point is the angle from the equatorial plane to the vertical direction of a line normal to the reference ellipsoid. In other words, latitude measures how far north or south of the equator a place is located. The equator is situated at 0°, the North Pole at 90° north (or simply 90°, since a positive latitude number implies north), and the South Pole at 90° south (or −90°). Latitude measurements range from 0° to ±90°. The geodetic longitude of a point is the angle between a reference plane and a plane passing through the point, both planes being perpendicular to the equatorial plane. Longitude measures how far east or west of the prime meridian a place is located. The prime meridian runs through Greenwich, England. Longitude is measured in terms of east, implied by a positive number, or west, implied by a negative number. Longitude measurements range from 0° to ±180°. The geographical coordinates (latitude and longitude) are typically expressed in the sexagesimal form (i.e., degrees, minutes, and seconds). Sexagesimal is a numeral system with 60 as the base; it originated with the ancient Sumerians in the 2000s BCE, was transmitted to the Babylonians, and is still used, in modified form, for measuring time, angles, and geographic coordinates. The geodetic height at a point is the distance from the reference ellipsoid to the point in a direction normal to the ellipsoid. Altitude is measured with reference to mean sea level (MSL), and height is measured with reference to the soil or ground level. In this application, altitudes are known as above mean sea level (AMSL) and heights as above ground level (AGL). It is essential to show the vertical or altimetric reference datum (or VRD) with the altitude, because altitudes change when the vertical datum changes. Sometimes the coordinates can be shown in universal transverse mercator (UTM) format or in values expressing north or east, for example. When the positioning coordinates are obtained from topographical charts, they are normally expressed in UTM format.

6.6.3.1

6.6.3.2 Coordinates Conversion and Distance Calculation

It is important to remember that some GPS receivers are not able to display latitude and longitude readings in decimal degrees. If one of these receivers is being used, it should be set to read in degrees and decimal minutes and then convert the latitude and longitude readings to decimal degrees.

292

Chapter Six

Converting from degrees and decimal minutes to decimal degrees: n n

1 degree = 60 minutes = 3,600 seconds Reading in decimal degrees = degrees + decimal minutes/(60 minutes/ degree)

Example 1: A latitude reading is given as 15° 39.03' N. Latitude in decimal degrees = 15° + 39.03'/(60'/deg) = 15° + 0.6505° = 15.6505° N

Example 2: 1° = 60' = 3,600" 56°26' 45" N = 56° + 26'/60 + 45"/3600 = 56° + 0.43333° + 0.0125° = 56.44583° N

Example 3: 88. 245568° = 88°+ 0.245568(60)' = 88°14.734' 88°14.734' = 88°14' + 0.734(60) " = 88° 14' 44.04"

The following is a lengthy but fairly simple formula for the distance calculation (in miles) between two coordinates on Earth. It assumes that the actual shape of the Earth (called “ellipsoid”) can be neglected and therefore uses mathematical solutions for a sphere. Note that the coordinates are given in decimal degrees and have to be converted into radians before using the formula. The conversion formula looks like this: Coordinates Radians = Coordinates DecimalDegrees ×

π 180°

(6.24)

The Earth’s equatorial radius, or semi-major axis, is the distance from its center to the equator and equals 6,378.1370 km (≈3,963.191 mi). The equatorial radius is often used to compare Earth with other planets. The formula to calculate distance between sites: d = 3, 963.191 ⋅ arccos sin( LAT1 ) ⋅ sin( LAT2 ) + cos( LAT1 ) ⋅ cos( LAT2 ) ⋅ cos( LONG2 − LONG1 ) 

(6.25)

Microwave Network Deployment

293

Example 4: Find the distance between Site A (LAT1 = 38.917900° N, LONG1 = 76.974100° W) and Site B (LAT2 = 38.906200° N, LONG2 = 76.938700° W).

LAT1 = 0.679245 rad

LONG1 = 1.343451 rad

LAT2 = 0.679041 rad

LONG2 = 1.342833 rad

d = 3, 963.191 ⋅ arccos sin(0.679245) ⋅ sin(0.679041) + cos(0.679245) ⋅ cos(0.679041) ⋅ cos(1.341833 − 1.343451)  d ≈ 2.07 mi 6.6.3.3

Universal Transverse Mercator

Universal Transverse Mercator (UTM) is a global coordinate system that is defined in meters rather than degrees-minutes-seconds. UTM is a very precise method of defining geographic locations; therefore, it is commonly used in GPS and GIS mapping. The Universal Transverse Mercator coordinate system was developed by the United States Army Corps of Engineers in the 1940s and named after Gerardus Mercator, a Flemish cartographer who lived 1512–1594. The system was based on an ellipsoidal model of the Earth. For areas within the conterminous United States, the Clarke 1866 ellipsoid was used. For the remaining areas of the Earth, including Hawaii, the international ellipsoid was used. Currently, the WGS 84 ellipsoid is used as the underlying model of the Earth in the UTM coordinate system. When using the UTM coordinate system, a location can be identified within a meter. The UTM grid divides the world into 60 north-south zones; the zones are numbered 1–60, and each zone is 6° wide in longitude. For example, the U.S. mainland goes from Zone 10 to 19 (Figure 6.11). Within each zone is superimposed a square grid, and although the zone lines converge toward the poles, the grid lines do not; therefore, as one travels north from the equator, the grid becomes smaller, although the grid squares remain the same. A tool for converting latitude/longitude coordinates to coordinates in meters on a Transverse Mercator projection can be found on the Internet. UTM uses two coordinates, easting and northing, to determine a location. Locations within a zone are measured in meters east and west from the central meridian, and north and south from the equator. A UTM

294

Chapter Six 126° 120° 114° 108° 102° 96° 90° 84°

78°

72°

66°

10

19 18

11 17

12 13

14

15

16

Figure 6.11 UTM zones in the continental U.S.

coordinate includes the zone, easting coordinate, and northing coordinate; this coordinate describes a specific location using meters.12 UTM coordinates can be abbreviated to the extent of accuracy desired; see Table 6.6 for possible abbreviations for UTM 19 0297475E 4834363N. 6.6.4 The Global Positioning Systems

The Global Positioning System (GPS) is based on information users receive from satellites. The purpose of the GPS is to provide users with the ability to compute their location in three-dimensional space.13 To accomplish that, the receiver must be able to lock onto signals from at least four different satellites. Moreover, the receiver must maintain a lock on each satellite’s signal long enough to receive the information encoded in the transmission. Achieving and maintaining a lock on four or more satellite signals can be impeded because the signal is transmitted at 1.575 GHz, a frequency that is too high to bend around or pass through solid objects in the signal’s path. This is why GPS receivers cannot be used indoors. Outdoors, tall buildings, dense foliage, and terrain that stand between a GPS receiver and GPS satellite will block that satellite’s signal. TABLE 6.6

UTM Coordinates Accuracy

UTM Coordinate

Number of Digits

Area Covered

02974E 48343N

5

(100 × 100) m

029747E 483436N 0297475E 4834363N

6 7

(10 × 10) m (1 × 1) m

Microwave Network Deployment

295

In addition, the signals from the GPS satellites previously were degraded intentionally by the U.S. Department of Defense (DoD) for the purpose of national security. This performance degradation is known as selective availability (SA), and only DoD-approved users have access to satellite signals without SA. However, a policy statement issued by the White House indicates that SA was turned off in 2000, and the accuracy of GPS position fixes has been improved significantly. At this time there is no selective availability in force; however, it can be reactivated at any time without the notice to GPS users. 6.6.5

DEM Data

A digital elevation model (DEM), also sometimes called a digital terrain model (DTM), is a digital file consisting of terrain elevations for ground positions at regularly spaced horizontal intervals. The United States Geological Survey (USGS) produces five different digital elevation products. Although all are identical in the manner the data are structured, each varies in sampling interval, geographic reference system, areas of coverage, and accuracy. The primary differing characteristic is the spacing, or sampling interval, of the data. The five versions of DEM are as follows: n

n

7.5-minute DEM (30 × 30-meter data spacing) is based on 1:24,000 scale topographic maps 1-degree DEM (3 × 3-arc-second data spacing), also referred to as 3-arc second or 1:250,000 scale DEM data

n

2-arc-second DEM (2 × 2-arc-second data spacing)

n

15-minute Alaska DEM (2 × 3-arc-second data spacing)

n

7.5-minute Alaska DEM (1 × 2-arc-second data spacing)

DEMs may be used in the generation of three-dimensional graphics displaying terrain slope, aspect (direction of slope), and terrain profiles between selected points. A DEM file is organized into a series of three records, A, B, and C. The A record contains information defining the general characteristics of the DEM, including its name, boundaries, units of measurement, minimum and maximum elevations, number of B records, and projection parameters. Each B record consists of an elevation profile with associated header information, and the C record contains accuracy data. Each file contains a single A and C record, while there is a separate B record for each elevation profile. Elevation data from cartographic sources are collected from any map series 7.5 minute through 1 degree (1:24,000 scale through 1:250,000 scale). The topographic features (contours, drain lines, ridge lines, lakes, and spot elevations) are first digitized and then processed into

296

Chapter Six

the required matrix form and interval spacing. The digital elevation models distributed within the Department of Defense cover 1 × 1-degree blocks and are called digital terrain elevation data level 1 (DTED-1). The DMA 1-degree DTED-1 data and USGS-distributed 1-degree DEMs are gridded by using the World Geodetic System 1984 (WGS 84). The National Elevation Dataset (NED) is the primary elevation data product of the U.S. Geological Survey (USGS). The NED is a seamless dataset of the latest DEM with the best available raster elevation data of the conterminous United States, Alaska, Hawaii, and territorial islands. The NED is derived from diverse source data that are processed to a common coordinate system and unit of vertical measure and all NED data are public domain. NED data are distributed in geographic coordinates in units of decimal degrees, and in conformance with the North American Datum of 1983 (NAD 83). All elevation values are in meters and, over the conterminous United States, are referenced to the North American Vertical Datum of 1988 (NAVD 88), while the vertical reference will vary in other areas. NED data are available nationally (except for Alaska) at resolutions of 1 arc-second (about 30 meters) and 1/3 arc-second (about 10 meters), and in limited areas at 1/9 arc-second (about 3 meters). In most of Alaska, only lower resolution source data are available. As a result, most NED data for Alaska are at 2-arc-second (about 60 meters) grid spacing. Part of Alaska is available at the 1- and 1/3-arc-second resolution, and plans are in development for a significant improvement in elevation data coverage of the state. These digital cartographic/geographic data files are produced by the USGS as part of the National Mapping Program and are available free of charge for the entire U.S. from the Seamless Data Distribution System (http://seamless.usgs.gov/index.php, accessed Dec 03, 2009). The NED is updated on a nominal two-month cycle to integrate newly available, improved elevation source data. 6.6.6

Magnetic and True North

Some people are surprised to learn that a magnetic compass does not normally point to true north. In fact, over most of the Earth, it points at some angle east or west of true (geographic) north. The direction in which the compass needle points is referred to as magnetic north, and the angle between magnetic north and the true north direction is called magnetic declination. The terms variation, magnetic variation, and compass variation are sometimes used in place of magnetic declination. The magnetic declination does not remain constant in time. Complex fluid motion in the outer core of the Earth (the molten metallic region that lies from 2,800 to 5,000 km below the Earth’s surface) causes the magnetic field to change slowly with time. This change is known as secular variation. Because of secular variation, declination values shown

Microwave Network Deployment

297

on old topographic, marine, and aeronautical charts need to be updated if they are to be used without large errors. Unfortunately, the annual change corrections given on most of these maps cannot be applied reliably if the maps are more than a few years old, since the secular variation also changes with time in an unpredictable manner. The current year’s declination is oftentimes marked in degrees on the map, which is one reason it is important to have a current map. Magnetic field models are used to calculate magnetic declination by means of computer programs. The user inputs the year, latitude, and longitude, and the program calculates the declination. It is possible to use the following web page to calculate declination: http://geomag.nrcan .gc.ca/apps/mdcal-eng.php (accessed January 08, 2010). This program is able to compute values for any location on the Earth since 1960. Navigating by compass requires determining bearings with respect to true or grid north from a map sheet and converting them to magnetic bearings for use with a compass. Important information on the map (see Note 1 on Figure 6.12, which is showing only a part of the much larger map) is the magnetic declination diagram.

Note 1

Note 2 Figure 6.12 Topographical map

298

Chapter Six

Declination diagram will have a star with a straight line representing the North Star (true north) and a line (MN) to the right of it or left of it, depending on the area, to denote the magnetic declination that must be applied for compass correction to use the map for true headings. Another line with GN will also appear, which represents the geographic (grid) north, which is used when reading the map. An additional note for users (Note 2) is stating that “This map is red-light readable,” which is typically found on military maps. It is important to distinguish between grid north, the direction of reference lines shown on the map, and true north. Although declination always refers to the angle between magnetic north and true north, it is often broken into two parts for convenience. In order to change magnetic to true headings, we will add east declination and subtract west declination. In order to convert from true to magnetic headings, we will subtract east and add west declination. In microwave engineering documents, antenna orientation is always given relative to the true north, so it has to be adjusted during the site and path surveys and during equipment installation and alignment. 6.7 6.7.1

Field Surveys Site Surveys

It is recommended that the same company performs site surveys and path surveys because some of the activities overlap and have to be done in a certain sequence. For example, establishing the correct and precise coordinates of the sites is the prerequisite for all other activities (including the path design) and should be done only once. Coordinates measured by different crews and at slightly different location at the site will create confusion. Before visiting a site, the surveyor should perform the following preparations for a site survey: n

n

n

Become as familiar with the site as possible by studying maps and photos of the site and the surrounding area and sites (called a map study). Find out from the network planner the preliminary positions of the neighboring sites and calculate bearings to the neighboring sites from the site to be surveyed. Find out the position of the site, address, directions to the site, and/or coordinates.

Microwave Network Deployment

n

n

299

Program the location (longitude/latitude) in Global Positioning System (GPS) equipment. IMPORTANT: Prepare access with the owner or responsible contact person!

In a first approximation, topographical and digital maps are sufficient to eliminate certain sites if, due to the terrain obstacles, they do not have an LOS with other sites. The second step is a site visit and establishment of LOS availability for shorter microwave systems. For longer microwave hops, a detailed path survey will be required. General information includes location data and site details that the engineer must be aware of when designing a microwave system. On-site activities include the following: n

n

n

n

n

n

n

Compare the photos to the actual location to make sure that you are visiting the right site; determine the site type, which identifies whether this is a rooftop shelter, cabinet or leasehold, or tower. Note site details such as address, site directions, and access. Use the GPS and record the longitude and latitude at an open location on the site in order to receive the best signal. Use the GPS to find out the bearing of the surrounding sites and use the compass to point out the direction. If the site option is located on an empty lot at ground level it will be difficult to take panoramic photos. Identify a high building, tower, etc. in the area and make a note of where the pictures have been taken from, including distance and angle. Use cherry pickers, helicopters, or any other way of establishing and precisely measuring the minimum required antenna height for unobstructed LOS (clear the first Fresnel zone). Make a sketch (with dimensions) of the rooftop or the empty lot and do not forget to point out water tanks, chimneys, parabolic dishes, etc., which can be of great importance at a later stage. Estimate the cable length from antenna to equipment room. Point in the sketch the exact position where the LOS was performed from. If a tower is present, make a sketch of it, showing existing antenna positions, heights, bearing, and the proposed new antenna position and height. Take a picture of the tower and point in it the exact LOS position. Estimate the cable length from tower to equipment cabinet/shelter. Describe in great detail access routes to the roof or vacant lot.

300

n

n

n

n

n

n

n

n

n

n

n

n

n

n

n

Chapter Six

If the site is an existing building, make a note of the height and number of floors (one floor is approximately 9 ft), elevators and their size and capacity, etc. Look out for typical buildings or features that can help you orient yourself. Note the proximity to airports, power lines, military installations, other communications towers, antennas, etc. Note all large off-path objects like water towers, buildings, new constructions, etc. Provide detailed analysis of the types of trees and shrubs in the area (on- and off-path), and estimate rate of growth. For very important switch/hub (and or microwave backbone site locations) sites in the urban areas, visit local city offices and check urban plans for that area. Determine the minimum height to achieve LOS. Describe in the report the terrain profile and possible obstructions along the path. Take pictures of the far-end site using a camera with a zoom lens (300 mm). The surveyor in the far-end site must be using mirrors (preferable) or lamps in order to proof the LOS. A picture without a zoom on bearing to the far-end site is also required. Take pictures at different bearings, every 30°. The accuracy of the bearings is most important. Make sure that the photos are overlapping. Take pictures of the rooftop/front view or vacant lot; these are required. Print bearings on the photos. Note the position of planned or existing cabinet or shelter that will house indoor microwave equipment. Explain the view in both pictures and words. Point out from where panoramic photos were taken on the roof or vacant lot sketch. Ensure that there is available space for the radio equipment and antennas. Confirm that a suitable power (AC and/or DC) source exists for each radio unit. Confirm that each location has a suitable earth ground to support the installation of a lightning arrestor. Cableways to the tower and in the tower should be documented with notes of available space and suggested clamps for the new cables. Take notes that show the dimensions of the cable ladders to determine what clamps to use.

Microwave Network Deployment

301

IMPORTANT NOTE: The surveyor has to be able and allowed (licensed, bonded, and insured) to climb towers if required and have all the required climbing and protection harnesses. Any written observations and comments, such as potential obstacles, buildings, type of surrounding trees and vegetation, power lines and towers, water towers, lakes, rivers, airports, and communication towers, are very important to the microwave designer. Even more beneficial to the engineer are digital photographs that show possible antenna mount locations and the actual LOS paths as seen from the site. Panoramic pictures at every 30º, pictures of obstructions and existing facilities (e.g., shelter, tower), electrical connections, and Telco installations are recommended and can be very useful. For tower sites, all relevant information must be provided, such as the condition of the tower and grounding (if it is an existing site), its capability to accommodate additional microwave antennas (e.g., twist and sway, available space, and so forth). Required loading needs to be calculated if new tower installations are proposed, and these must take into account the antenna wind and ice loading. Engineers will have to address other installation issues, such as building access and permits required by state or local governments. If the equipment is collocated with other users, an additional form with the relevant site and equipment information should be filled out. The ease of service access for maintenance personnel, particularly towermounted equipment, can have a significant impact on costs and repair time. The site access should be described with notes regarding its condition and possible seasonal variations. Photocopied sections of maps with notes identifying the access from nearest highway are recommended. 6.7.2

Path Surveys

Selection of a suitable microwave radio site includes a number of factors. There are economical and engineering benefits to be gained by maximizing the sharing of infrastructure and sites between the various types of elements in the network, particularly regarding expensive civil infrastructure such as towers and equipment housings. Good microwave sites, particularly in relation to hub sites, will be relatively high points to provide the maximum line-of-site availability to other (spur) sites. The surveyor must check all critical points along the path and allow for future obstructions that may impinge the radio path. These can be due to various causes, such as new buildings, tree growth, cranes, etc. It is also important to notice if there are some large bodies of water nearby, since large reflecting surfaces can produce problems due to increased multipath probability at frequencies below 10 GHz and having only optical LOS is not sufficient to provide that information.

302

Chapter Six

Path survey and driving and/or walking the path is the only method that will identify large buildings with huge glass surfaces and water and marshlands areas, potentially dangerous from the multipath propagation perspective, as well as construction sites and new manmade structures that could block the path in the near future. It is important to notice that line-of-sight (LOS) verification is not the same as a path survey, and it does not provide the same detailed information and confidence level on the longer hops. LOS verification can be used on short hops (less than a few miles) and preferably in the dense-urban areas with mostly rooftops and very few towers used for the wireless cell-sites. In checking path clearance by mirror flashing or by any optical or visual methods, it is important to remember that light waves have a slightly different curvature than radio waves and a value of k = 7/6 is usually used for light. Attention should be given to future network growth requirements in all areas, especially if the site is likely to develop into a future hub, as well as to informing landowners about it so as to prevent problems later. Attention should be paid to any local authority planning restrictions and approvals for structures or antenna installations planned. A clear transmission path must exist between the two link nodes of any microwave radio link. Furthermore, as the radio wave disperses as it moves away from the source, there must exist additional clearance over and around any obstructions to prevent attenuation of the transmitted signal. This additional clearance, known as the Fresnel zone, differs for the frequency band of the radio path, where higher frequency translates into a smaller clearance requirement. A path profile is a graphical representation of the path traveled by the radio waves between the two ends of a link. It is a result of path survey. The path profile determines the location and height of the antenna at each end of the link, and it ensures that the link is free of obstructions and propagation losses from radio phenomena such as multipath reflections. A path profile is established from topographical maps, which, by reference to the contours of the map, can be translated into an elevation profile of the land between the two sites in the path. Earth curvature can be added, as well as obstacles. The Fresnel zone calculation can then be applied and an indication of any clearance problems gained. Various software tools are available to assist this process if required, but most rely on the availability of topographical data to the appropriate degree of accuracy being available in digitized format. For longer paths at lower frequencies and higher-capacity systems, it is imperative to actually perform physical path survey and to avoid relying on the maps or aerial photographs. Again, as microwave network planning is an iterative process, if line of sight cannot be achieved, this

Microwave Network Deployment

303

information should be processed back through the network plan and alternative path calculations and site selection performed. Path Survey Activities Completing the path survey requires preparation and planning in radio link design and maps review prior to conducting field survey. Field work includes measurement of the precise coordinates and ground elevation, heights and elevations of towers, vegetation, and buildings; confirming the site location and its address; taking pictures at the site location; recording the FCC ASR‡ number (Antenna Structure Registration) if available, etc. One of the very important planning activities, which can actually prove to be quite lengthy in some cases, is to coordinate and assure access to the site with the owner or responsible contact person. Equipment and tools used by surveyors will be different in different environments and will be based on the methods of surveying and/or establishing LOS, but some typical surveying equipment is listed here:

6.7.2.1

n

Seven-minute (or better) topographic map.

n

Satellite photos.

n

Cell phone or other two-way radio communication equipment.

n

Differential GPS and/or WAAS capabilities.

n

n

n

Compass, used to assist in orientation and provide information for site sketch (walls, fences, buildings, road directions), photographs, antenna alignment, and obstructions. Laser rangefinder; it may replace one or more of the other pieces of equipment listed here. Binoculars; a good quality pair, 7 × 35 or 8 × 50, with fully multicoated lenses. The first number describes magnification, while the second number describes the size of the front lens. For example, 8 × 50 binoculars are often called “night glasses” because they seem so bright in dim light.§

‡

The Antenna Structure Registration Program is the process under which each antenna structure that requires FAA notification, including new and existing structures, must be registered with the FCC by its owner. The owner is the single point of contact for resolving antenna-related problems and is responsible for the maintenance of those structures requiring painting and/or lighting. Note that because the ASR requirements only apply to those antenna structures that may create a hazard to air navigation (either by their height or proximity to an airport), the registration files do not contain a comprehensive record of all antenna structures.

§ Night vision is the ability to see in a dark environment. Whether by biological or technological means, night vision is made possible by a combination of two approaches: sufficient spectral range and sufficient intensity range. Large lenses can gather and concentrate light better, thus intensifying light with purely optical means and enabling the user to see better in the dark than with the naked eye alone.

304

n

n

Chapter Six

Clinometer, for estimating heights of obstacles. The measurements are obtained in percentage or degrees. Altimeter, for measuring altitude because GPS height measurements are not reliable.

n

Digital camera with good resolution (5 megapixel or better).

n

10-ft ladder (may be needed for site survey).

n

n

n

n n

Theodolite, used to measure vertical and horizontal angles to the far end point. These measures are more accurate than any of the other equipments listed. There are analog and digital models; the analog is more accurate, smaller, and lighter, but the readings are easier on the digital model. It is an expensive piece of equipment so it might be advisable to rent it. Measurement tapes for measuring sites, baselines, equipment rooms, etc. Often two tapes are used, a small pocket tape, usually metal, of 3–5 m (10–15 ft) in length for short distances such as details in equipment rooms, height of instrument, etc; and a longer 30–50 m (100–150 ft) tape for site and tower measurements, commonly fiber or nylon. High-intensity flashlight (if flashing method is used) or a 0.5–1 ft precision mirror; special signaling mirrors developed for military use are particularly good. Weather balloons, bright orange-color (if required). Safety equipment; often it is necessary to climb existing towers/masts or the top of buildings. In these cases the surveyors (or riggers) need safety equipment.

Describing and measuring objects that are potential obstacles at the critical points is probably the most important part of the path survey. Here we will demonstrate the basic principal of measuring the height of the tree using trigonometry; most equipment will do these calculations automatically and provide the result on the LCD display. Example: A surveyor measures a height of a tree by using a finder and a built-in three-shot routine (Figure 6.13). If the distance to the tree is 120 ft, the angle to the top is 36.9°, and the angle to the bottom of the tree is 2.9°, find the height of the tree and how tall the surveyor is.

H = H1 + H2 H = d ⋅ tanα + d ⋅ tan β H = 120 ⋅ tan36.9 ° + 120 ⋅ tan2.9 ° H = 90.1 + 6.1 = 96.2 ft

(6.26)

Tree height, H

Microwave Network Deployment

305

H1

α

d β H2

Figure 6.13 Measuring the tree height

The assumption here is that the tree and the surveyor are both on the horizontal surface. The tree is 96.2 ft high and a surveyor is approximately 6'4" ft tall (6'1" + 3"). The microwave engineer will add to the tree height an additional provision for growth, based on the growth rate and the average height of the similar trees in the area. This growth provision could be from a few feet (for already mature trees) to ten or more feet for new growth. 6.7.2.2 Marking a Site If a far end site cannot be identified, it is possible

to mark the site using one of three alternative methods:

n

n

n

Mirror method One of the surveyors is positioned at the far end site holding a mirror. The surveyor at the site being surveyed tries to identify the far end site by looking for sun reflections. Flash method One of the surveyors is positioned at the far end site holding a large electrical flashlight (strobe). The surveyor at the site being surveyed tries to identify the far end site by looking for the light flashes. Balloon method One of the surveyors is positioned at the far end site hoisting a large balloon by a string. The surveyor at the site being surveyed tries to identify the far end site by looking for the balloon. The balloon must not be hoisted higher than necessary, as the length of the string corresponds to the required antenna CL height. The balloon should be big, preferably more then 6 ft (2 m) in diameter. A zeppelin-type balloon is recommended as it is less affected by wind. It should have a bright-orange color as this makes it easier to spot.

306

n

n

Chapter Six

Helicopter method A helicopter can be used in at least two different ways. One is by hovering over the planned site at a level where LOS can be confirmed. In other cases it may also be useful to fly along the planned path to investigate any obstacles. Crane (“cherry picker”) method The surveyor at one end (or both ends) is lifted to the level where line of sight can be anticipated.

The last three methods of marking the site and establishing the LOS are particularly useful for the green-field installations, i.e., when only the empty lot is available and the new tower construction is anticipated. 6.7.2.3 Path Surveys in Dense Urban Area In North America, traditionally, high-density urban areas were reserved for use of leased T1 lines backhaul, while in Europe microwave systems have been used for years. Now, even in big cities in North America, wireless operators are starting to widely deploy high-capacity microwave and/or millimeterwave point-to-point systems. In today’s wireless networks, the density of the cell sites in urban areas is getting so high that most of them are less than a few miles away from each other (with many less than a mile away). In this environment, many high-capacity and high-frequency microwave and millimeter-wave links will be used, requiring different path surveys or LOS verification methods. Towers are rare in high-density urban areas and if they can be found they will probably be monopoles or some type of minimum visual impact structure (wireless cell-sites inside of the advertisement signs, church steeplechase, light towers, flagpoles, artificial chimneys, etc.). Surveyors are the eyes of the microwave design engineer, making sure that every possible piece of information is sent back to the engineering team. This is particularly important in highly urban areas where terrain data is not very useful and even high-accuracy and high-resolution terrain data cannot compensate for the need of good path surveyors. Even in cases when very expensive clutter data is available, many of the paths will still have to be field-verified. Measuring coordinates accurately from the ground level using a GPS is almost impossible when surrounded with very tall high-rise buildings (this is called the urban canyon problem because it occurs primarily in cities). It requires measurements at the rooftop; moreover, on very large buildings coordinates should be measured at different locations on the rooftop. There are a number of LOS verification methods, such as climbing the tower (or from the building rooftop) and using binoculars, flashing the path, balloons, some combination of these methods, etc. Balloons are

Microwave Network Deployment

307

sometimes used just for the verification of the direction of the other side of the microwave link (marking a site), since looking at massive number of rooftops and chimneys surrounding the site, it is difficult to determine which rooftop is the correct one. This will obviously require the cooperation of two surveyors, one on each site’s rooftop. In Figure 6.14, it is important to notice that the microwave link connecting sites A and B is possible (LOS) but only from a limited area on the rooftop of Site A. So, the surveyor should take a coordinate’s measurement and a set of pictures from the exact location(s) where he or

Site B

LoS

Site A

Figure 6.14 Line-of-sight from the rooftops

308

Chapter Six

she recommends the microwave radio and/or the parabolic dish antenna be located. Using a high-intensity flashlight, surveyors can establish an LOS and confirm it with the picture of the flash. A sketch of the rooftop is, in many cases, very useful to the microwave engineer as well. The same challenge is the link planned between Sites A and B shown in Figure 6.15. As shown in Figure 6.15a, measuring coordinates from the building entrance, as commonly done, and performing the survey from those locations, will most likely show that the sites A and B have no line-of-sight. From the rooftops of Sites A and B it can be verified by simple visual inspection (and/or flashing) that a limited window of opportunity exists and the LOS can be established. Using a high-intensity flashlight, surveyors can establish a LOS and confirm that with the picture of the flash (see Figure 6.15b). 6.7.3

Using GPS for Field Measurements

Currently, two global navigation satellite systems (GNSS) are operational: the American Global Positioning System (GPS) and the Russian Global Navigation Satellite System (GLONASS). These so-called Medium Earth Orbiters (MEO) transmit signals in two frequency bands, L1 and L2, and will soon transmit in a third band, L5. The L1

Site A

Site A Flashlight

LoS LoS

Site B Observer

Site B

6.15a Path under survey Figure 6.15 Path survey in urban area

6.15b Flashing the path

Microwave Network Deployment

309

frequency is 1,575.42 MHz; the L2 frequency is 1,227.60 MHz; and L5 is 1,176.45 MHz. Envisioned as an alternative and a complement to the U.S. Global Positioning System (GPS) and the Russian GLONASS, Galileo is a global navigation satellite system currently being built by the European Union (EU) and European Space Agency (ESA). The fully deployed Galileo system will consist of 30 satellites and the associated ground infrastructure and it should be operational by 2013. GPS Measurements A very important fact to note is that the satellites do not go over the North Pole or the South Pole. There are a couple of implications to these orbital patterns when collecting data; when collecting data with a handheld GPS unit, the surveyor should preferably position his/her body to the north in order to block fewer satellites. Similarly, when mapping the position of an object such as a tree, the GPS antenna should be on the south side of the object so that the object blocks fewer satellites. This reasoning is correct in the Northern Hemisphere while it is reversed in Southern Hemisphere. The GPS receiver may require anywhere from several seconds to several minutes to acquire a sufficient number of satellites after it has been activated. This depends on the availability of GPS satellites overhead at the time of the measurement, the presence of obstructions, and the energy level of the receiver’s batteries. The typical GPS measurements procedure is as follows:

6.7.3.1

1. Hold the GPS unit vertically away from body. 2. Wait until you acquire at least four satellites. 3. At one-minute intervals record five measurements: n

Latitude

n

Longitude

n

Elevation

n

Time

n

Number of satellites

4. Choose 2-D or 3-D option. 5. Repeat the measurements five times and calculate the average value of your five measurements. GPS receivers typically present locations to the nearest 0.0001°, but if the receiver records five decimal places for degrees, the surveyor should record all five since the additional information is always useful. Most GPS receivers have internal data and an algorithm to compute the declination after the position is established. It is important to

310

Chapter Six

remember that this data cannot be updated from satellite transmission and therefore becomes outdated after some time. The online calculator computes the estimated values of Earth’s magnetic field, including magnetic declination, based on the current International Geomagnetic Reference Field (IGRF) model. The calculator can be found at the following Web address: www.ngdc.noaa.gov/geomagmodels/IGRFGrid .jsp (accessed January 08, 2010). If the location’s latitude and longitude are unknown, it is possible to use the zip code. While results are typically quite accurate, users should be aware that solar storms can cause intense, short-term disturbances in the magnetic field. In addition, because the Earth’s magnetic field is constantly changing, it is impossible to accurately predict what the field will be at any point in the very distant future. By constantly measuring the magnetic field, we can observe how the field is changing over a period of years. Using this information, it is possible to create a mathematical representation of the Earth’s main magnetic field and how it is changing. Since the field changes the way it is changing, new observations must continually be made and models generated to accurately represent the magnetic field as it is. In this respect, they are accurate to within one degree for five years into the future, after which they need to be updated. In addition, local anomalies distort the World Magnetic Model (WMM, developed by DoD) or IGRF predictions—for example, ferromagnetic ore deposits; geological features, particularly of volcanic origin, such as faults and lava beds; topographical features such as ridges, trenches, seamounts, and mountains; ground that has been hit by lightning; manmade features such as power lines, pipes, rails and buildings; personal items such as ice axes, stoves, steel watches, hematite rings, or even belt buckles, frequently induce an error of 3–4°. The USGS declination chart (GP-1002-D) shows over a hundred anomalies. The following are a few areas (among many more) with welldocumented magnetic anomalies in Canada and U.S.: n

Around the Georgian Bay of Lake Huron

n

Near Timmins, Ontario, west of Porcupine

n

North of Kingston, Ontario, and Kingston Harbor

n

Near the summit of Mt. Hale, New Hampshire

n

n

The Ramapo Mountains, northeastern New Jersey (the compass is rendered useless in some areas) The area 45 mi west of Boulder, Colorado.

6.7.3.2 Offset GPS Measurements In many cases we may need (but not been able to acquire) access to the site to determine the exact location

Microwave Network Deployment

311

of the cell-site or the tower; in such cases, we might have to perform measurements from a distance and correct the results. In many cases it is even desirable to be away from the large metal structures (such as towers and/or parabolic dish antennas) that could affect measurements and perform required measurement from the offset location. The offset GPS measurement procedure is implemented when a valid GPS measurement cannot be collected (for whatever reason) within a 10-ft radius of the site location. The offset procedure requires we determine the true north angle and the distance in feet or meters from where the offset location is to the actual site location and then enter this information into the GPS unit. Incorrectly implementing this process can significantly degrade the accuracy of the GPS measurement. Another method (manual) is described here: if the surveyor can restrict movement to directly north or south from the site, he or she can determine the latitude and longitude of the site using only simple arithmetic. The circumference of the Earth at the equator is 24,901.55 mi (40,075.16 km). But if we measure the Earth through the poles, the circumference is a bit shorter: 24,859.82 mi (40,008 km). By dividing Earth’s circumference of 24,859.82 mi by 360°, we find that there are 69.055 mi (36,4611 ft) in a degree of circumference. By dividing this by 10,000 we find out that there are 36.4611 ft in one tenthousandth of a degree of circumference. GPS receivers typically present locations to the nearest 0.0001°, which is approximately 11.1133 m or 36.4611 ft of latitude on Earth. The distance north or south between the site and an offset location will determine the difference in their latitudes. The following is the typical offset GPS measurements procedure: 1. Determine true north. 2. Move either north or south from the site until GPS reception is good. 3. Measure distance (in feet or meters) from the site. 4. Divide the measured distance by 111,133 m/deg or 364,611 ft/deg to get the latitude correction. 5. In the Northern Hemisphere, add (if you’re reading south of the site) or subtract (if you’re reading to the north of the site) the latitude correction. 6. Record the corrected GPS measurement. Additional points to note: n

Since the surveyor moved only north or south, the longitude will be the same as at the site and does not need any adjustments.

312

n

n

Chapter Six

Remember to reverse the addition or subtraction if you are in the Southern Hemisphere. If the receiver records to five decimal places, you have to round the degrees to the nearest 0.00001.

Example: At the offset location, the surveyor acquired a GPS reading of N 40.3555 and W 95.7958. The surveyor took the reading 200 ft north of the tower. Calculate the coordinates of the site.

200 ft ≈ 0.0005° 364, 611 ft /deg The corrected latitude is obtained by subtracting 0.0005° from the measured value: N 40.3555° − 0.0005° = N 40.3550° The GPS measurement of the tower location should then be recorded as N 40.3550 and W 95.7958. 6.7.3.3. Measuring Elevation and Altitude Scientists have been approximating the shape of the earth for many centuries, and the earliest concept of the earth’s shape was that of a sphere. By the end of the 1600s, it was generally accepted that the Earth was shaped like an ellipsoid, not a sphere, and over time, geodesists have figured out that the Earth’s shape is actually irregular. This representation of the Earth’s shape is referred to as the geoid. There have been many definitions of the “geoid” over 150 yrs or so. This is the one currently adopted at NGS: “Geoid is the equipotential surface of the Earth’s gravity field which best fits, in a least squares sense, global mean sea level.” The notion of an irregular shape is not a reference to topographic influences, such as mountains and valleys, which are considered negligible in the overall calculation of the geoid. The irregular shape is actually a result of variations in the density of the Earth’s crust, which lead to varying gravitational pulls.14 The geoid is a very irregular shape compared to an ellipsoid and, mathematically, it is impractical to define it, so an ellipsoid is used as an approximation for most mapping applications. The size and shape of the ellipsoid used to approximate the Earth’s shape varies, depending on what area of the Earth is of interest; an ellipsoid that approximates one

Microwave Network Deployment

313

area of the Earth may not adequately represent another area. The term spheroid, which is an ellipse that approximates a sphere, is sometimes used when referring to the Earth’s ellipsoid. Now we have to define altitude and elevation. The two terms have slightly different meanings, and their exact definition varies depending on your reference source: n

Altitude is generally used to refer to height above the Earth’s surface.

n

Elevation is generally used to refer to height above a given datum.

As an example, a village on a mountain may have an elevation of 2,500 ft above sea level (topographic elevation or orthometric height), the most common datum used for measuring elevation. An aircraft flying by might be at an altitude of only 1,000 ft, if the pilot was using a local airfield and measuring altitude that way, or maybe 3,500 ft if the pilot was using something close to mean sea level (MSL) as a reference. Orthometric heights for the continental United States (CONUS) are generally referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29) or the updated North American Vertical Datum of 1988 (NAVD 88). The NAVD 88 datum is the product of a vertical adjustment of leveled height difference measurements made across North America and supersedes the NGVD 29. The differences in orthometric elevations between the superseded NGVD 29 and NAVD 88 references are significant, upward of 5 ft (1.5 m) in some places; therefore, it is important that these two reference systems are not interchanged. All measures of elevation are made using the mean sea level as a point of reference. Since the sea level fluctuates daily with the tides, the reference surface that passes through the global mean sea level is called the geoid. It is shaped by the Earth’s gravitational field and thus is not smooth. Since GPS receivers cannot save the complicated geoid shape internally in memory, a smooth, simplified, built-in shape is used, called the reference ellipsoid. The WGS 84 ellipsoid surface was created as a point of reference and as a standard measuring point that, more or less, represents the shape of the Earth, which is not really a perfect but flattened sphere. Because of all this, the elevation of the location measured with a GPS receiver may be significantly different from the elevation determined by other methods (for example, topographic maps). The differences between the geoid and ellipsoid can be hundreds of feet (positive or negative) and it is known as geoid-ellipsoid separation (N) or geoidal undulation. Geoid models are constantly developing; for updated information on developments in the U.S., refer to www.ngs.noaa.gov/GEOID (accessed January 08, 2010).

314

Chapter Six

The relationship between a WGS 84 ellipsoidal height and an orthometric height relative to the geoid can be obtained from the following equation, as depicted graphically in Figure 6.16. h=H+N

(6.27)

where h = ellipsoidal height (WGS 84) H = elevation (orthometric height, normal to geoid) N = geoidal undulation above or below the WGS 84 ellipsoid By convention, the geoid undulation N is a positive height when above the ellipsoid. When measuring over distances of many miles, the difference between the ellipsoid and geoid can cause height errors. In order to reference the GPS measurements to the geoid, a correction is needed. Most geodetictype receivers and software packages use so-called geoid models to correct for this. Since the corrections are not exact, the resulting altitudes are sometimes obviously incorrect (for example, a building at sea level may be corrected to below sea level); surveyors and engineers will double-check elevations when such measurements occur and make appropriate corrections. The other point to consider is that a GPS is inherently less accurate in altitude than lateral measurement and the vertical error is likely to be 1.5–2 times that of the horizontal error, which means the altitude error is within ±23 m (75 ft) 95% of the time. For a vertical fix, the optimum setup is one satellite overhead and three on the horizon, ideally 120° apart in azimuth.

Ellipsoidal height (h) Height of Earth’s surface above or below the ellipsoid i.e., GPS elevation Earth’s surface

Orthometric height (H) Distance between Earth’s surface and geoid (normal to geoid) i.e., topographic elevation Geoid undulation (N) Distance between ellipsoid and geoid

Ellipsoid (WGS 84) Geoid Approximates mean sea level h=H+N Sea Figure 6.16 The geoid and reference ellipsoid

Microwave Network Deployment

315

There are some GPS receivers that have built-in barometric altimeters, which function like those fitted to aircraft. When properly calibrated, these barometric altimeters can measure elevation more accurately than the GPS itself. The altitude/elevation that these units display will vary according to the pressure setting, and the reading may or may not agree with any conventional GPS receiver. Differential GPS The Differential Global Positioning System (DGPS) is an enhancement to GPS that uses a network of fixed, ground-based reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions. These stations broadcast the difference between the measured satellite pseudoranges and actual (internally computed) pseudoranges, and receiver stations may correct their pseudoranges by the same amount. The correction signal is typically broadcast over a UHF radio modem. Depending on the amount of data being sent in the DGPS correction signal, it can reduce the error significantly, the best implementations offering accuracies of better than 4 in (10 cm). In order for DGPS to work properly, both the user’s receiver and the DGPS station receiver must be accessing the same satellite signals at the same time. It is important to note that DGPS cannot correct for GPS receiver noise in the user’s receiver, multipath interference, and user mistakes. The United States Coast Guard runs one such system in the U.S. and Canada on the long-wave radio frequencies between 285 kHz and 325 kHz (radiotelemetry). These frequencies are commonly used for marine radio and are broadcast near major waterways and harbors. There are many operational systems in use throughout the world: according to the U.S. Coast Guard, 47 countries operate systems similar to the U.S. NDGPS (Nationwide Differential Global Positioning System). Unfortunately, broadcasters of the required correction signals generally cluster around larger cities, making such DGPS systems less useful for wide-area navigation. The FAA considered systems that could allow the same correction signals to be broadcast over a much wider area, leading directly to WAAS. A specific form of radio telemetry is the Eurofix system (also known as Loran Comm and Lorsat), which is an application of the concept of integrated navigation. Eurofix combines satellite-based radionavigation systems such as GPS (or GNSS) with the terrestrial radionavigation system Loran-C (or the Russian equivalent Chayka) transmitting the DGPS correction signal. Since Loran-C uses terrestrial transmitters (14 transmitters in Europe, 24 in U.S., 23 in Asia) with high power signals in a different frequency band, it is not vulnerable to the same problems as GPS.

6.7.3.4

316

Chapter Six

Combining Loran and GPS creates a system that is less vulnerable and can achieve a precision of meters (a few feet) even when the GPS signal is interrupted.15 In addition to differential corrections, the Eurofix datalink can also carry other information, such as differential Loran-C information or “short messages” for emergency operations in Europe and the United States. 6.7.3.5 Wide Area Augmentation System Since a GPS unit already consists of a satellite receiver, it made much more sense to send out the correction signals on these frequencies than to use an entirely separate system and thereby double the probability of failure. Wide Area Augmentation System (WAAS) is a system of satellites and ground stations that provide GPS signal corrections, giving even better position accuracy (better than 10 ft) than GPS (20–30 ft) and DGPS (10–20 ft). A WAAS-capable receiver can assure a position accuracy of better than 3 m 95% of the time without having to purchase additional receiving equipment or pay service fees to utilize WAAS. The Federal Aviation Administration (FAA) and the Department of Transportation (DOT) are developing the WAAS program for use in precision flight approaches. Currently, GPS alone does not meet the FAA’s navigation requirements for accuracy, integrity, and availability. WAAS corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each GPS satellite. WAAS consists of a number of ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. The correction information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal. As of 2009, WAAS satellite coverage is only available in North America. There are no ground reference stations in South America, so even though GPS users there can receive WAAS, the signal has not been corrected and thus would not improve the accuracy of their unit. For some users in the U.S., the position of the satellites over the equator makes it difficult to receive the signals when trees or mountains obstruct the view of the horizon. WAAS signal reception is ideal for open land and marine applications. WAAS provides extended coverage both inland and offshore compared to the land-based DGPS (differential GPS) system. The broadcasting satellites are geostationary, which causes them to be less than 10° above the horizon for locations north of 71.4° latitude.

Microwave Network Deployment

317

This means aircraft in areas of Alaska or northern Canada may have difficulty maintaining a lock on the WAAS signal. Other governments are developing similar satellite-based differential systems. For example, in Asia, it is the Japanese Multifunctional Satellite Augmentation System (MSAS), while Europe has the Euro Geostationary Navigation Overlay Service (EGNOS). Eventually, GPS users around the world will have access to precise position data using these and other compatible systems.

6.8 6.8.1

Housing the Equipment Shelters

Telecommunications equipment comes in all shapes and sizes. To properly protect that equipment, shelters are designed and often custom built to house it. Different materials can be used to construct the shelters, including metal frames, concrete, wood frames, and lightweight insulated materials. Each type of material offers benefits for particular environmental conditions and site locations. Wireless operators require shelters that precisely fit their needs, so shelter manufacturers must be flexible enough to design customized shelters. Strict zoning regulations, and the fact that most prime locations are already occupied, require manufacturers to make shelters that can be positioned virtually anywhere or to use small cabinets for the telecom equipment infrastructure. Shelters erected on the top of a 40-story building may need to withstand strong gusts of wind similar to those built for hurricane-prone areas. A bullet- and vandal-proof shelter and cabinet design are also quite often required; all shelters and cabinets must pass certain ballistics requirements as per Bellcore recommendations. To keep the equipment within a telecommunications shelter in proper working order, it is important to control temperature levels in the shelter. Temperature control units often are initially integrated into a shelter, but they can also be added to a structure to create the proper system conditions. In colder climates, the insulation is increased. If the shelter is placed beneath a tower in an area where ice is common, the building’s construction material must be able to withstand ice falling from the tower. Common problems for many shelters are rainwater from leaking roofs and damp walls resulting from water condensation. Grounding with a large buried ground loop, including many outward pointing arms, is probably the best method, but it can be difficult to build in densely urbanized areas. It is important to remember to separate equipment grounding and protection grounding.

318

Chapter Six

Battery backup and/or diesel generators with extended-capacity diesel tanks are required in some areas. It is important to incorporate proper diesel fume exhaust and battery ventilation and also to remember that all batteries produce explosive hydrogen gas. Access roads to the sites can be very costly. The general requirements for access roads is that they are to be permanent, with a minimum width of 3 m (10 ft). The transportation of building material should be feasible without obtaining special permits for long vehicles or traffic police escorts. Prefabricated tower/mast sections consume a great deal of transportation space, but other constructions can be taken apart and packed in an economical manner. 6.8.2

Cabinets

Cabinets have been the time-proven components for housing Telco equipment for many years and have recently become the option of choice for wireless and microwave equipment. They are relatively small, with efficient use of rack space for mounting the payload as well as the power plant. They are exposed to the normal elements such as rain, snow, and temperature extremes that came with the seasons, so they must include several features to protect the equipment and batteries. Two essential features are high- and low-temperature shutdown and temperature compensation. The high/low temperature shutdown feature can work in two ways. First, the system can turn off the microwave and/or Telco equipment while still providing DC power to the cabinet fans or heaters, depending on whether the temperature is low or high. Second, the system can shut down completely if the cabinets use AC fans and heaters. Temperature compensation regulates the amount of battery voltage, depending on the battery temperature, thus prolonging the life of the batteries. Remote monitoring also reduces costs (personnel labor and travel time) by enabling personnel to access the system without being physically present at the site. Remote monitoring via Ethernet connections allows operations to monitor vital data such as temperature, voltage, and alarms as well as capturing engineering data such as peak current draw. 6.8.3

Equipment Room

An equipment room is defined as any space where telecommunications equipment resides. An equipment room may be a room in a building or a shelter that is placed either on the roof of a building or on the ground. In the design and location of the equipment room, one should provide space for expansion and consider water infiltration. Since the

Microwave Network Deployment

319

telecommunications equipment in this room is usually large in size, delivery accessibility should be a consideration as well. The minimum recommended size for this room is 14 m2 (150 ft2). The equipment room for the microwave equipment must fulfill a number of requirements. The building material used for walls, floor, and ceiling must be solid enough that it is possible to drill holes to fasten cable ladders to the wall or the ceiling and to fasten the cabinets to the floor. Floor space must be available for microwave equipment cabinets/racks, additional equipment, and future expansions. The room must have a lockable door that is large enough to allow for equipment transport. If the room has windows, it is recommended to cover them with blinds to minimize the heating effect of direct sunlight. The room must be clean and, preferably, have painted walls and ceiling as well as a painted floor or antistatic flooring to minimize dust. For cabinet leveling purposes, the floor must be leveled to within ±3 mm/2,000 mm, and the floor gradient must be within ±0.1. The floor must be able to carry the extra weight of the equipment or be reinforced. The temperature in the equipment room must be kept within specified equipment limits. Heat generated by the equipment must be removed by ventilation or air conditioning. Lighting protection must be adequate, and a power outlet for the connection of machine tools and test equipment must be available for installation and maintenance. The telecom cabinets and battery racks are mounted on the floor. They may be positioned against a wall, back to back (in case of a small equipment room), or freestanding. Expansion cabinets and racks should be positioned to the right of the main cabinet (facing the cabinet) so as to follow the same standard layout globally. If the installation requirements include earthquake protection, the space between wall and cabinet is to be at least 100 mm and between cabinets at least 150 mm. Certain distances must be considered when planning the room layout to provide a convenient working environment during installation and maintenance work. A distance of 1,000 mm (40 in) in front of the cabinets and racks work is recommended. Space for future expansion must also be considered. Make sure there is free space above the cabinets for exhaust air and below the cable ladder to make it easier for bending antenna cables and waveguides. There must be enough free space above the ladder for maintenance work. The minimum bending radius of cable (i.e., power, switchboard, coaxial, armored, fiber optics, ABAM, waveguides, and so on) shall not be less than the cable manufacturer’s specification. Waveguides are typically run separately from all other cables, preferably in a separate raceway.

320

Chapter Six

The location of the battery backup rack depends on the maximum allowable length of the battery cables. The acceptable power drop and cross-sectional area of the cables will limit the maximum length of the cables. Due to the heavy floor load, the battery rack is preferably placed in the corner of the room and/or on supporting beams. It is important to determine what kind of safety equipment and alarms will be installed at a particular radio site. Fire extinguishers that are easily accessible, an emergency lighting system, and an alarm system are recommended at the radio site. It is recommended that the alarm system at least support an intrusion alarm, a fire alarm, and high/low temperature alarms. 6.9 Microwave Antenna Mounting Structures Many communities have very strict requirements and regulations concerning the siting of towers and antennas. Communities understand that they have to provide a reasonable opportunity for the siting of wireless telecommunications facilities, which enhances the ability of providers of wireless telecommunications services to provide such services to the community quickly, effectively, and efficiently. At the same time, they are sensitive to the effects on aesthetics, environmentally sensitive areas, historically significant locations, flight corridors, health and safety, and so on. Communities permit the construction of new towers only where all other reasonable opportunities have been exhausted, and they encourage the users of towers and antennas to configure them in a way that minimizes their adverse visual impact. Communities also require cooperation and collocation, to the highest extent possible, among competitors so as to reduce cumulatively negative consequences. The construction standard for today’s towers is primarily modeled after American National Standards Institute (ANSI) standards. Communication towers are designed in accordance with “Structural Standards for Steel Antenna Towers and Antenna Support Structures,” ANSI/TIA/EIA-222-G (released in 2006), a nationally recognized standard jointly sponsored by the Electronic Industries Alliance (EIA, formerly the Electronic Industries Association) and the Telecommunications Industry Association (TIA). The ANSI standard is based on equations developed by professional engineers using wind tunnel testing to accurately predict the effect that wind has on telecommunication structures. On January 1, 2006, a new revision (Rev G) of the national standard for tower design (TIA/EIA 222) was issued. The previous revision, which is also still in force, is called Rev F. Revision G made many significant

Microwave Network Deployment

321

changes to Rev F. One of the changes was to introduce a classification system (Reliability Class 1, 2, or 3) for different tower structures that allows engineers to design a greater or lesser safety factor into the tower, depending upon its intended use and the risk of danger to human life. In addition, it provides new wind and ice maps for U.S. counties. Most structures are designed to withstand a forceful wind speed that occurs on the average of once every 50 years. This wind speed is then escalated, with height, to a much higher wind speed at the top of the structure. A gust factor to account for the varying nature of wind is also incorporated into the design of the structure. The ANSI standard lists minimum 50-year return wind speeds for all counties within the United States. Radial ice accumulation is also accounted for in the design. Ice loads are escalated with height since ice accumulation is known to increase with wind speed. Generally speaking, most structures are limited to a 4-degree twist or sway rotation and a horizontal displacement equal to 5 percent of the height of the structure. In addition, more stringent rotation requirements are usually provided for structures supporting microwave antennas. Safety factors specified by the American Institute of Steel Construction and the American Concrete Institute are also used to design tower structures. The same factors used in building design are incorporated into both the structure and foundation design. 6.9.1

Antenna Mounting Structures

6.9.1.1 Monopoles, Self-Support Towers, and Guyed Towers Ranging in height from 25 to 125 ft (7.6 to 38 m), monopoles consist of a single pole, approximately 3 ft (1 m) in diameter at the base, narrowing to roughly 1.5 ft (0.5 m) at the top, and may support any combination of whip, panel, or dish antennas (see Figure 6.17). Monopoles are generally used in rural areas, near freeways, or in areas where buildings are not of sufficient height to meet line-of-sight transmission requirements. In the wireless system, monopoles are used much more commonly than lattice towers. The monopole is a versatile tubular pole developed for the worldwide cellular market, with special attention paid to environmental acceptance. Aesthetic design was considered a top priority during the development stages. The efficiency of the design allows it to be an economical option for a range of heights of up to 40 m (131 ft). The monopole can be used for microwave links and cellular and telecommunications applications. Generally, there are two types of towers: guyed and self-supporting. Each type has different limitations regarding its capacity, stability, and versatility.

322

Chapter Six

Guyed towers are of slender, generally three- or four-sided lattice construction, and are uniform in dimension over their length. They are maintained in their vertical position by guy wires, which are attached at various levels on the tower and anchored to the concrete on the ground. Guy stays stretch over time, and the tensions within these assemblies should be adjusted from time to time. The scope of recommended tower inspection and maintenance is published in both ANSI and CSA publications. Under normal circumstances, a guyed tower requires a radius of 80 percent of the height of the tower. Table 6.7 illustrates the fact that they require a large piece of land to guy a single tower, so they are usually used in rural areas. Self-supporting towers stand on their own and require a base spread of approximately 13 percent of their height. Table 6.8 shows the minimum radius of the circular surface required for the self-support tower erection. These towers are relatively inexpensive up to about 100 ft (35 m) or so but become very costly as their height increases. They are used mostly in urban areas where minimal land is available. Short self-support towers are also sometimes Figure 6.17 Monopole, 15 m placed on the rooftops of the buildings. Steel (galvanized after fabrication), for the most part, is used as the material for the mast construction of towers, and galvanized guy or bridge strand is generally selected for the guy stays that support the guyed tower. TABLE 6.7

Approximate Area Required for Guyed Towers

Guyed Tower Height (Feet) 60 100 200 300 400

Required Area (Feet) 94 × 94 149 × 149 288 × 288 428 × 428 565 × 565

Microwave Network Deployment TABLE 6.8

Approximate Area Required for Self-Supported Towers

Self-Supported Tower Height (Feet) 50 100 150 200 300

323

Required Area for Three-Leg Tower (Radius in Feet) 13.8 17.3 21.9 26.4 34.8

Required Area for Four-Leg Tower (Radius in Feet) 15.4 20.5 25.4 30.6 40.8

The lattice tower can be made using a variety of steel shapes and connection types. A common mast makeup for a multiple microwave dish and line support tower is angle leg, angle bracing, and bolted construction, with a climbing ladder inside the mast. This configuration offers relatively low manufacturing cost, significant strength, high stability, ease of antenna attachment, ease of maintenance, and low shipping costs. On the downside, because of the relatively high, “flat” (angle) projected area of this type of mast, such a tower and foundation system is more expensive than a comparable tower that uses round members (pipe, tube, or solid round) in its makeup. This price difference becomes smaller when the antenna and line load increase. The most economical steel section to use for tower components is hollow tubes, with a circular cross-section (pipe or tube) offering the highest compressive capacity-to-weight ratio as well as lower wind resistance than any other shape. Canadian designers and tower manufacturers have, for the most part, excluded the use of thin-wall and small-diameter hollow sections for primary structural components (legs) in tower construction for a variety of reasons. Even properly vented sections entrap debris and water in the form of ice over time. The entrapped freezing water expands, exerting some pressure on the tube, and aids in the entrapment of debris filtering down the tube, which occasionally blocks the drain holes. Once these holes are blocked, the moisture, along with other chemicals present in it, stay trapped in the tube. The galvanized surface on the inside of the tube section is subject to potential corrosive action, depending on the acidity of the water mixture within it. An inspection of such a tower would not reveal any structural distress until a crack or hole appeared in the wall of the tube. There are many pros and cons in material selection, connection type (welded or bolted), and mast configuration (position of transmission line brackets and ladder, facility to attach antenna mounts) for tower design. Coupled with potential differences in proposed tower accessories (grounding, safety climbing devices, anticlimb devices, waveguide bridges),

324

Chapter Six

towers, even given the same basic specifications, often vary considerably in price. A thorough evaluation of a proposal for the tower and the expertise and reputation of the supplier, along with the price, should be used in the selection of a tower structure. Unlike ground-wired telecommunications (e.g., the land-based telephone system), wireless communications technologies, by their operational nature, require numerous antennas to be mounted at various heights throughout the landscape. To place them at the specific height required by a particular system, these antennas are sometimes mounted on towers, monopoles, tall buildings, or other structures on hilltops. One of the greatest concerns faced by local jurisdictions is the visual impact of wireless communications facilities. Minimum visual impact antenna structures, also called alternative mounting structures, are, for example, a manmade tree, clock tower, church steeple, bell tower, utility pole, light standard, identification pylon, flagpole, or a similar structure. They are designed to support and camouflage or conceal the presence of telecommunications antennas. A number of companies today develop and sell minimum visual impact and multifunction communication structures. Because of their aesthetically pleasing design, these alternative structures can solve unique communication requirements and enhance the permitting process in difficult zoning locations. Each structure naturally blends into the surrounding environment, and its antennas are hidden from view. Examples of these concealed installations include structures that resemble church steeples, with crosses incorporated into the design of the monopole; lighting structures with curved arms and canisters to conceal antennas; signposts that are two- or three-legged structures with optional signage; and the ultimate design—the minimum visual impact tree. The design of the tree pole is site specific to the location, with careful consideration and configuration given to existing trees (see Figure 6.18). Pine (also called mono-pine) and palm tree (also called mono-palm) versions are available, as well as minimum visual impact structures that resemble a saguaro cactus.

6.9.1.2 Minimum Visual Impact Antenna Structures

Other Antenna Mounting Structures The advantage of split-configuration microwave radios is that they can be installed in a very limited space. Poles, wall mounts, and tripods are just some of the examples of antenna mounting structures that could carry the ODU and antenna. New antenna structures on a building can take different forms and shapes. The type of antenna support structure depends on the antenna type, its height, and the existing structure. The antenna

6.9.1.3

Microwave Network Deployment

325

Figure 6.18 A monopole (in the middle) pretending to be a tree

support structure may consist of a self-supported tower on a roof, a guyed tower on a roof, building walls, a tripod, or a pole. A nonpenetrating tripod is often used as a temporary solution for a rapid deployment of the microwave system. For penetrating tripods, there is usually a requirement for architectural analysis and approval, since drilling of the sensitive roof structure has to take place before any rooftop installation is allowed. It is important to ensure that nobody walks in front of the antenna, since that could interrupt the traffic on the microwave link and be hazardous to the person’s health in case of a long-term exposure. 6.9.2

Maximum Allowed Antenna Deflection

It is important to remember that the requirements for the twist/sway of the tower are sometimes much more stringent for microwave than

326

Chapter Six

for RF installations. A typical limitation in twist/sway for the structure (tower and antenna) corresponds to a maximum 10 dB signal attenuation due to antenna misalignment. A simple formula for estimating the maximum allowed deflection (single side) in degrees, for microwave transmission antennas is

α −10 dB =

60λ [ deg ] D

(6.28)

α −10 dB =

18 [ deg ] fD

(6.29)

or simplified even more,

where l = wavelength (m) D = antenna diameter (m) f = frequency (GHz) Table 6.9 shows the maximum allowed antenna deflection for some commonly used frequency bands and antenna sizes. Figure 6.19 illustrates problems due to twist and sway of the antenna structure; although in most cases microwave engineers would like to install microwave antennas as high as possible (position 1 in the figure), from the LOS prospective, it may be more desirable to mount it as low

TABLE 6.9

Frequency (GHz) 2 2 4 4 6 6 7 7 7 8 8 8 11 11

MW Antenna Deflection (–10-dB Points) Antenna Diameter (m) 2.4 3.0 2.4 3.0 2.4 3.0 1.2 2.4 3.0 1.2 2.4 3.0 1.2 2.4

Deflection (−10 dB Points) (°) 3.8 3.0 1.9 1.5 1.3 1.0 2.1 1.1 0.9 1.9 0.9 0.8 1.4 0.7

Frequency (GHz) 13 13 15 15 15 18 18 18 23 23 23 38 38

Antenna Diameter (m) 0.6 1.2 0.3 0.6 1.2 0.3 0.6 1.2 0.3 0.6 1.2 0.3 0.6

Deflection (−10 dB Points) (°) 2.3 1.2 4.0 2.0 1.0 3.3 1.7 0.8 2.6 1.3 0.7 1.6 0.8

Microwave Network Deployment

327

as possible from the twist and sway prospective (position 2) in the case of the less-than-optimal 1 structure type. Due to the shortage of tower space and zoning issues, some operators, in desperation, are even starting to utilize billboards, wooden light poles, and flag poles and mounting microwave equipment on these very unreliable structures. Unless these struc2 tures are specially designed or reinforced to carry microwave equipment, this practice should be avoided. Towers and other antenna mounting structures that do not satisfy these requirements will cause very severe long-term outages (and therefore unavailability of the microwave link) due to misalignment of the antennas. It is very important to use an expert tower company to calculate the Figure 6.19 Twist and sway of the tower loading of the tower and maximum allowed twist and sway of the structure. These decisions cannot and should not be made on the basis of qualitative perceptions or “gut feeling.” 6.9.3

Communication Tower Requirements

6.9.3.1 Tower Locations Three types of areas, based on their potential suitability for wireless facilities, are classified as opportunity areas, sensitive areas, and avoidance areas. It should be noted that collocation of antennas on existing towers or alternative tower structures is encouraged in all areas, including avoidance areas. Opportunity areas are the most likely to provide good sites for the widest range of telecommunications installations, including towers. Opportunity areas include interstate highway corridors, industrial parks, shopping centers, large agricultural tracts, and other locations where properly designed facilities could fit into the landscape reasonably well and are unlikely to become a blighting influence on the surrounding neighborhood.

328

Chapter Six

Sensitive areas, such as high-density housing districts, sites within 500 ft of low-density residential areas, and community facilities such as churches, cemeteries, playing fields, and recreation centers, require more care in site selection, facility design, and screening. Issues such as safety, visibility, property values, and land use compatibility are more likely to arise in these areas than in opportunity areas. Avoidance areas are the least preferred locations for wireless telecommunication towers. Low-density residential districts, ridge tops, historic sites, scenic highways, and most public parks are included in this category. 6.9.3.2 Microwave Requirements Tower site construction involves many

steps: building the access road, bringing in electric and phone lines, erecting the fence and installing other security measures, providing and installing the equipment shelter, erecting the tower and installing transmission lines and antennas, installing microwave equipment, testing the microwave system, and so on. The following factors should be kept in mind during the detailed design and deployment of the MW system:

n

n

n

Ensuring sufficient space on the tower to install and pan the microwave antenna Loading of the antenna-mounting structure (MW antenna, transmission lines, and outdoor MW unit placed on the tower) Maximum allowed twist and sway (antenna deflection) of the antenna mounting structure, expressed in degrees, will depend on the frequency and antenna size

Because of the complexity of existing standards, regulations, and requirements, it is very important that experienced and licensed civil engineers handle the matter of civil construction. Input for civil construction dimensioning must come after thorough transmission site surveys. The survey should produce basic requirements such as tower heights, tower stability, access road existence, and so forth. In addition, it is important to produce documentation on all levels for civil construction and installation to avoid future liability issues. The documentation should include calculations that clearly show how construction requirements are met, including maximum tolerances due to load. Site drawings for electricity, alarms, air condition, and so on are also required. 6.9.3.3 Structural Requirements At the outset, we should consider including a provision for future antennas and transmission lines in the procurement specification. At this time, it is beneficial for the owner

Microwave Network Deployment

329

to consider and plan for potential changes and/or additions to antenna loading that could occur in the future. The costs associated with additional tower strength would be small when compared to the high cost of reinforcing an existing tower and foundations. Generally, if significant unplanned changes in antenna loads are made after the tower is installed, costly reinforcement will likely be required. If the changes or additions are too extensive, then the tower may have to be replaced with a larger one. Keeping that in mind, the best strategy is to create a specification for tower procurement either by using existing staff or by retaining a consultant who specializes in this service and solicit prices from several tower companies. If this approach is used, the specification for tender should include a request for sketches or drawings of the proposed tower and foundations. Towers from several different sources can vary substantially in design and, therefore, price. The design, supply, and installation cost of a self-support tower is approximately twice the cost of an equivalent (between 15 and 76 m in height) guyed tower; beyond 76 m, the difference in cost is even greater. In general, a guyed tower needs considerably larger ground area than a self-support tower, which might be costly in some countries. Large ground area requires considerable fencing, which also can become very costly. Guyed towers are prohibited in some countries (Germany, for example), demanding self-support towers or monopoles. However, self-support towers are generally more expensive than guyed towers, which often can be built very quickly and are much lighter because less required construction material is required. Hence, they have a smaller wind load area. The operational dimensioning is governed by the maximum allowed antenna deflection according to the requirements calculated by transmission engineers. It is important to note that unavailability due to strong wind load does not normally occur simultaneously with flat fading. Unfortunately, exaggerated stability requirements are very common, and stability requirements stating the strongest deflection are prevalent, without any supporting calculations. The result is extremely high construction costs. All external equipment must be properly and thoroughly mounted to avoid vibrations and resonance buildup. Resonance is generally not a problem and is easily avoided by designing a stiff construction having a resonance frequency above 10 Hz (low frequencies give the largest amplitudes). Some important topics to consider in civil construction are quality control, construction of foundations, concrete reinforcement methods and anchor design, and quality of paint and welding points. Steel is extensively used for masts and towers because of its superior strength as compared to aluminum, which requires thicker constructions

330

Chapter Six

that result in a higher wind load. All steel constructions are to be hot-dip (zinc) galvanized for corrosion protection, at least. The effect of twisting can be accounted for by proper tower design or by using torsion protection guiding wires for supported masts. Steel can become brittle at low temperatures. Therefore, a proper choice of metal alloy in the construction is essential to account for temperature-dependent effects. The strain in wires for supported towers creates a need for adjustment, typically after six months of operation and then regularly every few years. Erosion around the foundation can become a problem if the site has been unwisely positioned. Local knowledge is essential for avoiding the problem of erosion. For example, a site in parts of Africa and the Middle East might seem suitable for several years but can suddenly be transformed into a river. Soil testing is also essential when building heavy constructions. It is advisable to take several soil samples for analysis. A soil test is a sample, 6 to 10 m (20–30 ft) deep, that provides information about soil composition, profile, and density. The soil test gives an indication about how to avoid long-term misalignment of the construction due to earth sliding or compression. Earthquake loads rarely govern the design of telecommunications antennas and their supporting structures; however, these structures require special considerations of their response characteristics in regions of high seismicity. The standard provides design criteria to insure sufficient strength and stability to resist the effects of seismic ground motions for self-supporting and guyed antenna supporting structures. Unless otherwise required, earthquake effects are only specified to be considered in very limited areas of high seismic activity. 6.9.3.4 Relevant Standards There are a large variety of country-specific

regulations and standards for civil construction. Country-specific standards for civil construction must be followed, with a suitable choice of additional standards to address customer-specific requirements. Local construction standards govern the requirements on towers and masts. These standards are often locally adapted standards with origins in British or French construction standards. Proper quality control of manufactured material and design are key factors for obtaining a high construction standard and longevity. Minimum recommended tower strength requirements are published in structural standards or are specified by the customer. In Canada, tower strength and design are not regulated, so Canadian Standard S37, “Antenna-Supporting Structures—A National Standard of Canada,” is not a legal requirement.

Microwave Network Deployment

331

In the United States, the tower design and construction standard is ANSI/TIA/EIA-222, “Structural Standards for Steel Antenna Towers and Antenna Supporting Structures—An American National Standard.” The latest Rev G of this standard was published in 2006. Revision G of the standard is the first version of the standard that addresses earthquake loading. The soil structure at a site has a significant effect on the loads resulting from an earthquake. Design parameters are provided for various soil conditions. Some manufacturers produce a commercial, over-the-counter type of tower that is designed to criteria other than these two national standards. Such a product can provide an owner with a low-cost product for a specific and usually limited application. The incidence of failure tends to run higher with structures that do not conform to either the CSA or ANSI standards, and it is advisable to consider the risk and consequences of failure when selecting this kind of product. Usually, these types of towers have been designed to survive a specific and uniform wind velocity with no ice loading, and they have little or no safety factor beyond that loading. The detailed questions and answers for project work are to be produced and evaluated by experienced civil construction engineers in close cooperation with transmission and microwave engineers, whose requirements must be addressed. The typical time for erecting a mast with prefabricated concrete weights or earth anchors is less than five days. Maximum height difference between each leg is ±1 m (±3 ft). The entire mast can be erected with all radio equipment installed at ground level in one or two long sections by using a mobile crane or a helicopter. Alternatively, a hoisting device can be installed at the top of the mast for lifting additional sections and radio equipment manually. The following general timelines can typically apply for towers under 40 m (120 ft):

6.9.3.5 Tower Erection Timelines

Digging the tower foundation Mold and concrete reinforcement Concrete filling and hardening Pit backfilling and earth packing with watering Tower assembly and erection Total time

1 week 2 weeks 1 week 1 week 1 week 6 weeks

Laying a stable foundation is the first concern in erecting a sturdy communication tower. Once concrete is poured, a test should be performed every week to determine the foundation strength of the curing material. At seven days, the concrete should measure 70 percent of its estimated strength. Concrete reaches its normal strength after four to

332

Chapter Six

five weeks, depending on weather conditions. However, tower assembly on ground can start before the foundation is completed, and minor loading of the concrete is possible after one week. Additional Tower Requirements Unauthorized access to towers/ masts can be prevented by fencing with barbed wire, removing ladders, installing flat plates around the lower parts of the construction, and using different types of climbing locks. Many types of fall protection can be incorporated, such as guiding tracks with wire and wheel or simply a climbing cage around the ladder. Service platforms and rest platforms are compulsory accessories in many developed countries. The tower/mast is painted mainly for corrosion protection. A compulsory aviation warning coloring scheme is deployed in some countries. Aesthetic requirements must be considered in other countries. This can become very costly—an additional 50 to 100 percent of invested steel cost if high-quality paint is used. Different types of aviation warning light are required in various countries, including dusk-activated relays. It is widely accepted that lightning protection can be connected to earth through the tower/mast construction. A maximum resistance of 10 Ω to ground is dictated by British standards and is also widely observed elsewhere. However, in some countries (including the U.S.), more stringent criteria are sometimes applied.

6.9.3.6

6.9.4 Tower Procurement

Under a competitive bid situation, it is the designer’s task to make each tower as small as possible given the antenna and transmission line loading, code, deflection, and other customer requirements. The smallest tower, complete with its associated foundations, usually results in the lowest tendered price. The buyer of tower structures should therefore work closely with the radio design and network system planning groups (RF and transmission) in the tower tendering phase. A reputable tower design/manufacturing company is a good source of reliable information regarding pricing, scheduling, and engineering. Such businesses have engineering and technical staffs that interface closely with purchasing, manufacturing, and erection teams. Consulting firms that specialize in towers are another source of unbiased, objective information on creating a specification for towers and tower design loading. These firms are also a source of other professional services such as design, analysis, and inspection, and they can check designs to ensure specification conformance if there exists any doubt about the proposed or installed tower.

Microwave Network Deployment

333

The most vital information that has to be provided to the tower manufacturer is as follows: n

n

Tower load For the microwave application, tower loading of the antenna mounting structures includes antennas (immediate and future requirements), wind, ice, waveguides, coax cables, platforms, the waveguide bridge, outdoor radio units (ODUs), and so on. The detailed loading requirements should be given to the tower designer and, if climatic conditions are unknown, assumptions must be made based on local statistics. Wind speed Sometimes referred to as wind load, this is the force the wind has on the tower and antennas. The governing principles for tower/mast constructions are n

n

n

n

n

Survival Dimension for maximum probable wind load. In Sweden, it is typically 50 to 55 m/s. The U.S. standard is 50 m/s. There is also a high-wind option for up to 70 to 75 m/s. Operation Dimension according to availability objectives, taking normal wind load and gusts into account. Calculation methods are comparable to microwave availability principles. Requirements are, for example, 99.995 percent availability.

Ice load Also known as radial ice, this is the amount of ice in inches formed around each tower member. Soil report This report details the soil conditions present at the site and helps to determine what type of foundation is required. Other design specifications specifications.

These include FCC, FAA, and other

Generally, the minimum recommended requirements for tower strength are either published in structural standards or specified by the customer. These standards are constantly under review by committee and are revised and reissued from time to time to reflect the current knowledge of loading, analysis technique, materials, and workmanship. 6.10 6.10.1

Power Supply and Battery Backup AC Power

Although microwave equipment, as well as most telecommunications equipment these days, requires the usual Telco power (−48 VDC), there is a chance that some additional external equipment may require AC power. There are three distinct AC uninterruptible power supply (UPS) configurations: standby, line-interactive, and online.

334

Chapter Six

A standby UPS (also known as an off-line UPS) consists of a basic battery power-conversion circuit and a switch that senses irregularities in the electric utility. The load is connected directly to the utility power, and power protection is available only when there is an outage. However, some standby UPSes include suppression circuits or power line conditioners to increase the level of protection they offer. A line-interactive UPS offers a higher level of performance by adding voltage regulation features to conventional standby designs. Like standby models, line-interactive UPSes protect against power surges by passing the surge voltage to the load until it hits a predetermined limit. At this point, the unit switches to the battery, but only after passing most surges through to the load. Line-interactive units can provide moderate protection against high-voltage spikes and high-frequency transients, but they do not provide complete isolation for the load. An online AC UPS uses double conversion (AC/DC and DC/AC), providing complete isolation from most types of power problems. 6.10.2

DC Rectifiers

Transmission equipment is usually required to operate off the −48 and/ or +24 V power supply. In wireless networks, battery backup shall provide sufficient reserve capacity to allow the uninterrupted operation of equipment for at least four hours for the cell sites and at least eight hours for the switch office. Customers may specify more or less capacity, as local conditions such as travel time, standby generation of power, and power reliability may vary with location. Unattended telecommunications equipment in cabinets, huts, and CEVs (Controlled Environment Vaults) are often subjected to harsh electrical and environmental conditions. Equipment, particularly batteries, deteriorates very fast under those conditions, so the remote monitoring systems are often used. The equipment shall be capable of successfully operating within published specifications and satisfy all required operational specifications with power variations that do not exceed −56 VDC or fall below −40 VDC at any given time. In newer and more advanced power systems, alarms will notify the user when the voltages get to within 20 percent of these extreme values. In the case of battery drain, it will signal when the equipment is within one hour of losing backup power. Both the constant-voltage and the constant-current charge methods are suitable for charging recombinant lead-acid batteries. Of the two, the most common method used is simple constant voltage, which allows the battery to seek its own current level in overcharge (or float charge). For cyclic applications in which batteries undergo regular charge and discharge, the charge voltage per cell can range from 2.4–2.5 V.

Microwave Network Deployment

335

Although a float charge of 2.25–2.35 V is advisable for batteries in standby use, it is important to consult the battery manufacturer’s literature for proper charge-voltage recommendations. The general design criterion used to calculate necessary capacity for the battery and the rectifiers is  Power Consumption  Rectifier Rating (A) =   System Voltage     Installed Power  × Charging Time × 1.2 + 0.1 ×    System Voltage    (6.30) This formula shows that the rectifier has to be able to recharge empty batteries in x hours (usually 4 hours) while carrying the full load of the system indefinitely. Ideally, the system should be easy to upgrade and expand (modular approach) to accommodate changes and growth. Example: Let us assume that the power consumption of the small microwave terminal is 1,000 W at −48VDC. Dimension the rectifier for the battery system that has to be fully recharged from completely discharged state in 4 hours.

Rectifier Rating =

1000  1000  +  0.1 × × 4 × 1.2   48 48

Rectifier Rating ≈ 21 A + 10 A Rectifier Rating ≈ 31 A A rectifier will have to provide at least 21 A of current (1000/48) for correct operation of the equipment. If the battery was completely discharged, the rectifier will have to provide charging current of 10 A as well (0.1 × 21 × 4 × 1.2). The total current that the rectifier must provide at any given moment is at least 31 A. So the power rating of the rectifier/charger is approximately 50 percent higher than the power consumption of the equipment. Sometimes, in order to increase reliability of the power plant, redundant system design (A + B) is used, where two independent rectifiers and battery systems provide half the repeater load in normal conditions. Should one system fail, the remaining system can power the entire site’s load.

336

6.10.3

Chapter Six

Batteries

Power loss in a communication system can be disastrous for an operator’s business and for an organization relying heavily on such service. The battery is by far the most unreliable part of any system that depends on it. This is true because, if any one of many external variables is incorrect, the natural deterioration process within each cell is accelerated. When the utility power fails and the battery instantly assumes the critical load, the choices and investments that have been made in emergency power equipment will be realized. The accelerating factors are excessive cell temperature, too high or too low cell float voltage, and too many discharge cycles. The first step to minimize the problem is to become aware of and then track the critical variables with an automated continuous monitoring system. The second step is to correct and optimize these critical variables wherever possible. The third step is to immediately notify the user if the critical measured parameters are outside of defined limits. The final step is to automatically identify unfavorable trends and notify the users of a developing problem before it becomes serious. Historically, backup power for telephone networks has been provided using flooded batteries. Over the past several years, there has been a trend toward other types of batteries such as valve-regulated lead-acid (VRLA) batteries (also called gel cell batteries). The term valve-regulated identifies a battery that is equipped with mechanical safety vents that can open under excessive overcharge. Using VRLAs (they require less attention) offers remote monitoring and reduces the number of sites that must be visited for troubleshooting and routine maintenance. This, in turn, reduces the cost of these activities. Absorbent glass mat (AGM) is a newer class of VRLA battery in which the electrolyte is absorbed into a mat of fine glass fibers. The plates in an AGM battery may be flat like a wet cell lead-acid battery in a rectangular case. In cylindrical AGMs, the plates are thin and wound, like most consumer disposable and rechargeable cells, into spirals. Their unique (for lead acid chemistries) construction also allows for the lead in their plates to be purer as they no longer need to support their own weight as in traditional cells. Their internal resistance is lower than traditional cells due to plate proximity, and the pure lead plates have lower resistivity, they handle higher temperatures better, and self discharge more slowly. Their specific power is very good and they can be charged and discharged quite rapidly; however, their specific energy tends to be lower than traditional flooded batteries. AGMs are more expensive than flooded (liquid) batteries but offer very good reliability. Flooded batteries are generally too large to fit into the limited space available for wireless sites applications. Service and maintenance of

Microwave Network Deployment

337

flooded batteries in remote sites would also be very expensive, since the service technicians would have to visit many sites on a regular basis to check electrolyte levels. VRLA batteries do not give off any gases under normal conditions, so special ventilation provisions are not needed. Relative to flooded batteries, VRLA batteries also offer excellent power density, do not require any regular electrolyte maintenance, and can be mounted in any orientation. These characteristics make VRLA batteries ideal for these applications. Equipping power plants with the remote monitoring and control system is a very smart investment, as it reduces both the costs and frequency of personnel visits to the cell sites. They usually generate alarm messages but also can provide vital data about power plant conditions. 6.10.4

Motor Generators

Propane, diesel, or natural gas motors turn electric generators, providing higher power solutions. Motor generators can be used in hybrid power systems, in combination with solar, battery, and/or wind. They require periodic maintenance (as well as regular fuel delivery) and have higher operating and lifetime costs. For remote sites having higher power consumption, this is one of the very few feasible solutions. 6.10.5

Fuel Cells

Battery backup is an efficient solution for telecommunication sites where short power interruptions are expected, from a few minutes up to a few hours. In cases where grid power is unreliable or even nonexistent (remote cell sites and/or microwave repeater sites), alternative solutions to power the site have to be implemented. Presently, diesel generators are widely used for the remote-site power generation. Fuel cell technology is a new option in extended runtime backup power to replace or enhance battery or diesel generator systems. Lower maintenance, lower fuel costs and emissions, as well as longer life, are some of the advantages when compared with traditional diesel generators. Fuel cells are electrochemical devices that convert the chemical energy of a reaction directly into electrical energy. The basic physical structure or building block of a fuel cell consists of an electrolyte layer in contact with a porous anode and cathode on either side. Fuel cells are usually classified by their operating temperature and the type of electrolyte they use. Some types of fuel cells work well for use in stationary power generation plants while others may be useful for small portable applications or for powering cars.

338

Chapter Six

There are several different types of fuel cells, each using a different chemistry. A hydrogen fuel cell uses hydrogen as its fuel and oxygen (usually from air) as its oxidant. Other fuels include hydrocarbons and alcohols. Other oxidants include chlorine and chlorine dioxide. To produce a usable amount of electricity, multiple fuel cells, each producing up to about one volt, are combined into a fuel cell stack. The stack is designed to produce the desired electrical current and voltage for a specific application. The direct current electricity produced can be varied over a wide range by altering the area and number of cells in the stack.16 Hydrogen is the lightest of all elements, causing it to be buoyant and to rapidly disperse when released in air, so a leak is quickly diluted and rendered harmless. Hydrogen is colorless, odorless, and has no taste, it’s nontoxic and nonpoisonous, and there are no significant environmental hazards associated with accidental discharge. A hydrogen fire radiates very little heat compared to a petroleum fire, and for a flammable mixture to exist, a four times higher concentration of hydrogen is required than that of gasoline (4 percent versus 1 percent). An electrostatic spark from the human body is just as likely to ignite gasoline as hydrogen at these minimum concentrations. It is important to keep in mind codes and safety standards for hydrogen storage and siting (refer to International Fire Code, Edition 2009), especially if the site is in the proximity of other manmade structures. 6.10.6

Alternative Energy Sources

6.10.6.1 Solar Energy Solar energy is available almost everywhere in the world and suitable for power supplies everywhere except maybe the polar regions. Solar panels do not require fuel supplies, have no moving parts, and require very low maintenance. Battery plant at remote sites should be designed for 10 to 30 days of battery autonomy, which is recommended for most solar-powered systems (low Arctic locations use 60 to 90 days autonomy). Photovoltaic (PV) power is a semiconductor-based technology (similar to the microchip) that involves converting light energy directly into an electric current that can either be used immediately or stored (e.g., in a battery) for later use. Photovoltaic solar cells produce electricity directly from sunlight, and they are usually made of silicon. The cells are waferthin circles or rectangles, about three to four inches across. Solar cells operate according to what is called the photovoltaic effect, in which sunlight hits the surface of semiconductor material, such as silicon, and liberates electrons from the material’s atoms. Certain chemicals added to the material’s composition help establish a path for the freed electrons, and this creates an electrical current. Through

Microwave Network Deployment

339

the photovoltaic effect, a typical 4-in silicon solar cell produces about 1 W of DC power. Photovoltaic panels/modules are very versatile and can be mounted in a variety of sizes and applications; e.g., on the roof or awning of a building, on roadside emergency phones, and as very large arrays consisting of multiple panels/modules. Many remote sites use photovoltaic cells to generate power for onshore and offshore traffic control systems, microwave radio stations, and so on. They also provide electricity to remote cabins, villages, medical centers, and other isolated sites where the cost of photovoltaic power is less than the expense of extending cables from utility power grids or producing diesel-generated electricity. In remote sites solar energy can be used to replace the electrical utility-based power supply entirely (conventional electricity is unavailable or impractical) or just as an additional power that will help during the peak hours and reduce the electrical power consumption. So, even if grid power is available at these sites, a renewable energy system can provide security as a backup in the event of grid failure. In either case, for every project, a very extensive study must be done to determine the feasibility and cost efficiency of such a system. It is possible to find the solar insolation (insolation is a measure of solar radiation energy received on a given surface area in a given time and comes from a combination of the words “incident solar radiation”) or average wind speeds for any area of interest on Earth by going to the NASA Website at http://eosweb.larc.nasa.gov (accessed January 08, 2010). 6.10.6.2 Wind Turbines In wind turbines wind turns the turbine, gen-

erating electricity from the electrical generator, and DC output charges the battery. Battery reserve for remote sites should be at least 10 days. They are typically low maintenance, requiring only occasional checks for damage to turbine blades, controller and regulator tests, charging tests, and battery tests. A combined wind-photovoltaic hybrid system is quickly becoming the standard in renewable energy installations. Small wind and solar panels are often seen as complementary technologies, as both are used together as a hybrid system to offset the variable resources of the sun and wind. Generally, when it is sunny it is calm and when it is cloudy it is windy. This complementary effect is even greater during seasonal changes. During the winter and spring when the sun is at its least exposure, the wind is at it highest. Siting a wind generator is extremely important for the performance of the system. The ideal location for a small wind turbine is 20 ft above any surrounding object within a 250-ft radius, and having at least 9 mph average wind speed at the site location. A site with an average

340

Chapter Six

wind speed of lower than 9 mph would not benefit year-round from a wind-powered installation. Some of the smallest wind turbines can be mounted on existing telecommunications towers and produce about 8–10 kWh at wind speeds around 9 mph. One of the most crucial pieces of information needed when evaluating the wind energy potential of any given area or site is a reliable definition of the wind resource. Wind maps (like the ones from the NASA Website) are available for most countries, providing information of the wind resources at the planned site location. 6.11 Grounding, Lightning, and Surge Protection 6.11.1

Grounding

Grounding can be described as the science of obtaining a low-resistance path for the dissipation of current into the earth. There are different methods for obtaining a ground but first, a discussion of grounding fundamentals is crucial to understanding and designing a grounding system. Grounding is the physical bonding or connection of equipment by a conductor to earth. Without a proper low-resistance ground, standard protection devices such as breakers and transient voltage surge and lightning protection systems are ineffective. Most communication equipment manufacturers may void their equipment warranties at sites where the ground system performance does not meet their explicit earth-grounding requirements—typically 5 Ω or less. Additionally, good grounding has other benefits, such as enhanced personnel safety; reduction of system noise; and protection from lightning, unwanted voltages and currents, and power surges. Earth is composed of many materials that are variously good and poor conductors of electricity, but Earth as a whole is considered to be a good conductor. For this reason, and as a reference point, the Earth’s potential is assumed to be zero. When an object is “grounded,” it too is thereby forced to assume the same zero potential. If the potential of the grounded object is higher or lower, current will pass through the grounding connection until the potential of the object and earth are the same. The earth electrode is the connection path from the equipment to the earth. The resistance of the electrode, measured in ohms, determines how quickly, and at what potential, energy is equalized. Hence, grounding is necessary to keep an object’s potential the same as that of the Earth’s. Grounding is critical for the human safety, and it is meant to protect equipment from voltage surges and transients. A low-resistance ground

Microwave Network Deployment

341

will keep equipment at or near Earth potential, reducing any voltage difference between equipment and Earth. This can prevent an accident or fatality during human contact. Equipment damage (e.g., to sensitive telecommunications equipment) from surges caused by lightning and other sources can result in the loss of millions of dollars in damage and downtime. For raw land sites, measuring soil resistivity is the first step in the planning process. With this data, a soil resistivity profile is built. This profile is the key to an accurate ground grid design and is the only way to ensure predictable grounding system performance. A soils profile is the collection of soil resistivity levels at 5-, 10-, 20-, and 40-ft depths, and sometimes up to 100 ft. This data is gathered at three or four different locations on the site. This allows the design engineer to determine the length, configuration, and quantity of rods required to achieve the specified ground-system resistance. Shortcutting the planning stage causes many grounding installations to be found lacking on project completion and is responsible for many acceptance test failures. Equipment manufacturers and some regulatory agencies take a serious look at the installed ground-grid resistance measurements before operational approvals are granted. Soil resistivity testing allows a proper design that will save time, money, and effort. At collocated and existing sites, the existing grounding system must be tested and evaluated. This data will be used to determine if a grounding system upgrade is required to protect the new equipment. A lack of planning and poor or nonexistent soil resistivity testing will result in unpredictable ground system performance and ineffective equipment protection. Historically, the site and ground grid testing step has been the most often undervalued one. Some of the most commonly used grounding systems include driven rods, water pipes, chemical wells, ufer grounds,** and electrolytic rods. It is strongly recommended that a lightning rod be fitted at the top of the tower. It should be independently connected to its own dedicated earth at the base of the tower. The microwave radio ODU and cable earth should be connected to a separate earth conductor to the ground. 6.11.2

Surge Suppressors

We use gas-tube surge protectors for microwaves because of their low insertion loss, low voltage standing wave ratio (VSWR), easy installation, and wideband behavior. Split-configuration digital microwave radios are normally housed in weatherproof boxes intended for mounting on towers,

**

An ufer ground utilizes the rebar in a concrete foundation as an earth electrode.

342

Chapter Six

frames, or monopoles, whereas coaxial cables are used for connecting the outdoor radios with indoor equipment. To protect sensitive microwave electronic equipment from the harmful effects of lightning transients (electromagnetic pulses) and induced voltages, inline microwave surge arrestors (or suppressors) are used. They are usually installed in the equipment room in the master ground bar, but they can also be installed outside the equipment room and connected directly to the external ground bar. Coaxes carrying signals between outdoor and indoor radio units operate not only in baseband, including DC for powering the outdoor unit, but also in intermediate frequencies such as 70 or 140 MHz and even higher values. This broadband behavior prohibits the use of quarterwave stub protectors for microwave applications, since they cannot pass DC signals, and their frequency bands are generally incompatible with those bands traveling across the IDU to the ODU coaxial. Two fundamental electrical characteristics of microwave surge arrestors include their ability to pass a DC electric current and a broadband frequency range. The first characteristic allows the arrestor to pass the power used to supply outdoor microwave radio units, such as transceivers, highpower amplifiers, and voltage-operated switches. RF systems using tower-top amplifiers also need DC power. The second characteristic, a bandwidth of hundreds of megahertz, allows the arrestors to pass RF signals between indoor and outdoor radio units. Typical bandwidths include DC to 1 GHz, DC to 2 GHz, and DC to 2.5 GHz. It is important to inspect all surge arrestors periodically to look for visible indications of deterioration that might suggest the need for replacement. It is also possible to develop a specific method for changing gas capsules based on statistics regarding the probability of a lightning strike to towers or monopoles of various heights in various cities. 6.12 Microwave Testing and Troubleshooting 6.12.1

Factory Acceptance Testing

Quality control is the establishment of quality standards for all materials, equipment, and services necessary for the successful execution of the project, followed by systematic measurement of quality actually achieved, comparison with the standards, and corrective action where appropriate. Quality standards should be established jointly and mutually agreed upon by the client and the equipment/service provider. The first article sample is a production component or components submitted as being representative of a specific process using production

Microwave Network Deployment

343

tooling, equipment, methods, technique, standards, personnel, and controls. A first article test (FAT) is conducted to verify that the first article samples meet the performance requirements during and after all specified environmental and durability/endurance conditions. First article inspection is a verification that the first article samples manufactured using the normal production process (i.e., planning, technical/work instructions, material processing systems and controls, tools, fixtures, test equipment, and personnel proficiency) will produce a component that is in compliance with all requirements. Examples of such requirements are dimensional characteristics, material content, process, capability, and performance. Test facilities to be utilized in performance of the first article test, if known at the time of the RFQ response, shall be identified in the quotation, giving the facility name, location, contact, and a phone number. In addition, first article test costs are individual costs for the entire first article test program, including hardware, fixtures, equipment, test procedure development, itemized cost by test parameter, and test report preparation. These costs shall be quoted separately from the cost of production hardware and may be a part of RFQ process as well. Test plans are written by the supplier and/or independent test laboratory and may be amended by the customer. During the entire duration of the microwave project, customers have the right to witness acceptance testing on their equipment during the manufacturing process. This is very useful during lengthy projects where there is a possibility that the quality standards could be changed over the associated period of time. Customers will definitely want to witness acceptance testing in a case in which the first shipment of the equipment possessed some problems that could be directly traced to the manufacturing process and QA. In most cases, when dealing with the reputable equipment supplier, factory acceptance testing will be limited to only the first article inspection. If problems arise in the field during the installation, the customer can request that the supplier include additional tests that may better define the problem and point out a potential solution. 6.12.2

Field Acceptance Testing

The objective of a field acceptance testing document (also called the acceptance test procedure, or ATP) is to outline the requirements and standards that client expects from its subcontractor(s) with regard to field acceptance testing of the provided microwave equipment. Such testing will ensure that all equipment is in proper working order and installed to meet the microwave link engineering specifications.

344

Chapter Six

The record of the standard start-up tests, defined by the client, agreed upon by the microwave supplier, and performed after the MW radio system installation is complete, will be provided by the supplier’s technician and witnessed by operator’s representative (or its designated representative—subcontractor, consultant, or other individual). The system verification document should have authorizations and signatures of people involved in the project and their supervisors. This document typically contains the following information: n

Site-specific information and contact names

n

MW link engineering details

n

n

Visual, mechanical, and physical inspection of the MW radio equipment, cables and waveguides, antenna mounting structures, antennas, labeling, and miscellaneous equipment Electrical measurements on the MW radio, typically including the following information (test results): n n

n n

n

n

Antenna/waveguide/coax return loss measurements (even a perfectly adjusted radio may not operate properly if attached to a damaged or poorly installed antenna system) Input DC voltage Transmitter power verification (Output transmit power shall be measured at the waveguide output/antenna port via a microwave power meter. This value shall be the same as the designed value used in the link engineering sheet, −0.5/+2 dB.) Frequency accuracy measurements that measure and record Tx/Rx local oscillator frequency + 10 ppm Receiver tests including AGC characteristics (The receive signal level shall be measured at the monitor port on the indoor unit and shall equal the calculated value on the link engineering sheet ±2 dB.)

n

Data service channel and VF orderwire testing

n

System alarm and control

n

Loopback capabilities

n

n

Grounding measurements

System gain (Once the output transmit power has been measured, this value, along with the receiver threshold (i.e., sensitivity) of the radio on the opposite end of the link (obtained from the factory acceptance test), should be used to calculate the system gain in decibels.)

Check of pressurization (if installed)

Microwave Network Deployment

n n

345

Short-term BER measurements (1 min each port) Protection switching operation test (Both ends of the microwave link must experience a manual simulated failure to verify switching to standby transmitter.)

n

Power supply redundancy verification

n

Modules and inter-rack cable continuity

n

Coax, ABAM, and FO cables and cross-connects’ continuity

n

User interface functionality

n

Spare module testing

n

Battery backup time for the microwave system

n

Network management system and craft interface verification

n

Long-term (24-hr) BER testing (per hop)

n

Long-term (24-hr) BER testing (for the system, if applicable)

n

Additional Ethernet tests, if required

The document should also contain information that will allow technicians to start troubleshooting the problems and receive immediate technical support, as follows: n

Emergency technical support hotline phone numbers. Examples of emergency technical support include service-affecting problems reported by either side and/or hardware failures that cause an outage or degradation.

n

User manuals and as-built documentation.

n

Warranty, repair, and return procedure description.

A record of the standard turn-up tests has to be provided for a number of reasons. The first and the most obvious one is that it is a part of the overall transmission network acceptance procedure and a proof that the system is functional. The other reason is that it will be the benchmark for all future troubleshooting of the transmission network. During maintenance tests on microwave systems, the results will always be compared against the acceptance testing results. Any deviation from these results should be noted or further investigated. It is also a good practice to include a copy of the manufacturer’s product information that lists the test procedure along with the actual test results and a sketch of how the tests were conducted. In performing tests while bringing a system into service according to ITU-R Recommendation F.1330, it is desirable to avoid the times of year and times of day when multipath propagation is most likely to occur

346

Chapter Six

(especially on long hops and lower frequency bands). Studies carried out in temperate climates indicate that multipath propagation effects are least likely to occur in winter and in the two preceding months. For tests that must be carried out in summer, the period during the day when such effects were observed to be least likely was 10 AM to 2 PM, local time. It is reasonable to assume that the same is true in other seasons. 6.12.3

Bit Error Rate Testing (BERT)

The total time in a digital transmission system is divided into two categories: available and unavailable time.17 A PDH transmission system becomes unavailable if the BER is equal to or worse than 10−3 for more than ten consecutive seconds (or 2 × 10−5 for SDH systems). The following error performance parameters are used in describing transmission quality: n

Unavailable seconds

n

Percent availability

n

Severely errored seconds

n

Percent severely errored seconds

Live data emulation is a pseudorandom pattern that uses a quasirandom signal sequence (QRSS). This pattern provides a good approximation of live traffic with an approximately 50 percent ones density. This pattern generates all possible combinations of a 20-bit binary counter except All Zero and is limited to a maximum of 14 zeros. QRSS tests are very useful to verify connectivity, but the system also has to be tested with a set of tests to make sure that there are no hidden issues (e.g., AMI-B8ZS mismatch, faulty equipment) that could cause intermittent problems later, during operation. This is especially important in mixed microwave and leased line environments, and end-to-end tests should also include all ones (a fixed test pattern of pulses only), one in one (1:1, alternating ones and zeros), all zeros (used to test circuits for clear channel capability), and stress testing. The most commonly used fixed pattern is 3 in 24. The 3 in 24 pattern simultaneously stresses the minimum ones density and the minimum number of consecutive zeros criteria. After installation of a microwave system, out-of-service BER testing is the most useful tool for verifying equipment operation and end-to-end transmission quality. Out-of-service testing allows the user to accurately assess the quality of a T1/E1 and/or T3/E3 circuit by transmitting and analyzing test patterns in place of the live data that is normally present. Because all out-of-service testing requires that live, revenue-generating traffic be interrupted, it is impractical for long-term testing. Thus, this

Microwave Network Deployment

347

type of testing is typically performed when a system is initially installed or when errors are discovered while monitoring data. Two methods of out-of-service testing are typically used to analyze T1/E1 networks: end-to-end testing (two test sets required) and loopback testing (one test set required). End-to-end testing is performed with two test sets, so the analysis can be done simultaneously in both directions, and the direction of errors can be found much faster. The recommendation is to avoid loopback testing and perform end-to-end testing whenever possible. By simultaneously generating a test data pattern and analyzing the received data for errors, the test instruments can analyze the performance of the link in both directions. Most modern microwave radios have built-in a variety of integrated configurable loopbacks for testing and fault finding purpose (see Figure 6.20): n

Baseband loop, near end (1)

n

Baseband loop, far end (2)

n

IF loop IDU, near end (3)

n

IF loop ODU, near end (4)

n

RF loop, near end (5)

In addition, the 64 kbps channel and the E1/T1 wayside channel may be looped but only one loop may be active at the same time. For normal transmission facilities, acceptance tests are commonly run for periods of a few hours at most. However, for microwave systems, it is a good idea to perform long-term tests. Tests should be performed over a period of a few days to cover different atmospheric and weather conditions and ensure that the microwave system will operate well under all these conditions. The in-service method allows live data to be monitored at various access points without disturbing revenue-generating traffic. Because in-service monitoring does not disrupt the transmission of live traffic, it is more suitable for routine maintenance than out-of-service testing.

Traffic IN

Line decoder 2

Traffic OUT

Line encoder

Figure 6.20 Loopbacks

SOH processing

Indoor Unit (IDU) Modulator

Outdoor Unit (ODU) IF IF line interface

1 Demodulator

IF

IF

Coax cable 3

Transmitter

IF line interface 4

IF

RF OUT 5

Receiver

RF IN

348

Chapter Six

6.12.4 Testing Ethernet Transmission Networks 6.12.4.1 Ethernet in Mobile Backhaul The latest industry reports also show that backhaul is probably the most expensive component in deploying a WiMAX (4G) network. A recent industry report estimates that for every dollar spent on building radio access networks, approximately $0.40 will be spent on backhaul, and that this will rise as the costs for WiMAX base stations declines. Ethernet is rapidly displacing traditional leased line technologies and is increasingly utilized in mobile backhaul applications and even for mission-critical data and voice traffic. It is well documented that most network outages for a wireless operator in previous generations of wireless networks originate from the backhaul network, up to 80 percent while all the other causes (BTS, RF, MSC, DCS, etc) account for remaining 20 percent. From this we should conclude that the importance of proper transmission network design and testing cannot be overemphasized. Wireless service providers face new quality of service (QoS) and quality of experience (QoE) challenges beyond just voice quality and service coverage. As wireless operators add more multimedia services, they increase network complexity, bandwidth demands, and performance sensitivity, and significantly raising the user’s expectations. For legacy TDM-based backhaul services (T1/E1), the bit error rate (BER) tests were traditionally used for installation, commissioning and turn-up, as well as round-trip delay and service-disruption measurements. Unfortunately, there are multiple issues related to using BER testing in Ethernet-based networks. New technologies require new methods and tools for qualification in labs and in the field. Ethernet is a Layer 2 switched technology (data link layer in the OSI seven-layer model). Layer 3 is the network layer; this function is most commonly carried out by IP, but it could be another protocol as well. The purpose of Ethernet is to ensure data is transferred over a link in a communications network, while the Layer 3 protocol has the job of ensuring the data is transferred over the whole network, from the original source to the ultimate destination, using any number of separate Ethernet links. The higher layer protocols, Layer 4 and above, have the task of ensuring the integrity of transmitted data and presenting the data to the user or application, and as such of little interest in a transmission environment. Considering all this, it becomes obvious that the hardware loopback, commonly used in TDM networks, might not be the best test approach. The integrity of an Ethernet frame is verified at each switching element, and a single bit error will result in the FCS (frame check sequence) error and resulting in the entire frame being discarded. The errored bit will

Microwave Network Deployment

349

never get to the analyzer and the analyzer will declare a frame loss. For this reason, BER, the most fundamental measure of a SONET/SDH and older PDH service, has no meaning in Ethernet since the ratio of good to errored bits cannot be defined. This does not mean that BER tests should not be used at all; there are still many valuable tests that can be performed. In particular for pure link-to-link Ethernet connections, or even in metro Ethernet, a BER test is useful whenever the traffic does not interface nodes that can drop frames. Many Ethernet and GigE test solutions have adopted the guidelines of the Internet Engineering Task Force recommendation RFC 2544, which was originally designed for verifying the performance of LAN devices at the MAC layer. RFC 2544 describes a set of procedures for measuring the performance of Ethernet equipment but it is also used for the overall network. For Ethernet-based backhaul services, the tests generally used are throughput and latency/frame delay (according to RFC 2544), as well as packet delay variation (according to RFC 3393), but some additional tests are required to validate backhaul services. Measuring frame loss over time will provide an idea of the long-term integrity of the service. Testing the link redundancy with a service-disruption measurement, backhaul providers will ensure that the long-term goal of network availability can be met. The RFC 2544 recommendation provides an outof-service benchmarking methodology to evaluate the performance of network devices using throughput, back-to-back, frame loss and latency tests, etc. The methodology defines the frame size, test duration and number of test iterations. Once completed, these tests provide performance metrics of the Ethernet network under test. In order to ensure that an Ethernet network is capable of supporting a variety of services (such as VoIP, video, etc.), the RFC 2544 test equipment supports seven predefined frame sizes (64, 128, 256, 512, 1,024, 1,280, and 1,518 bytes) to simulate various traffic conditions. Small frame sizes increase the number of frames transmitted, thereby stressing the network device as it must switch a large number of frames. The throughput test defines the maximum number of frames per second that can be transmitted without any error. The test is done to measure the rate-limiting capability of an Ethernet switch as found in carrier Ethernet services18 and it must be performed for each frame size. Although the test time during which frames are transmitted can be short, it must be at least 60 seconds for the final validation. Each throughput test result must then be recorded in a report, using frames per second (f/s or fps) or bits per second (bit/s or bps) as the measurement unit. 6.12.4.2 Testing the Throughput

350

Chapter Six

The test involves starting at a maximum frame rate and then comparing the number of transmitted and received frames. Should frame loss occur, the transmission rate is divided by two and the test is restarted. If during this test there is no frame loss, then the transmission rate is increased by half of the difference from the previous trial. This methodology is known as the half/doubling method and is repeated until the rate at which there is no frame loss is found. 6.12.4.3 Burst Test The back-to-back test (also known as the burstability or burst test) assesses the buffering capability of a switch. It measures the maximum number of frames received at full line rate before a frame is lost. In carrier Ethernet networks, this measurement is quite useful as it validates the excess information rate (EIR). Should a frame be dropped, the burst length is shortened. Should it be received without any errors, the burst length will be increased. The trial length must be at least two seconds long and the measurement should be repeated at least 50 times, with the average of the recorded values being reported for each frame size. The average measurement should be logged in the report. 6.12.4.4 Frame Loss Testing The frame loss test measures the network’s response in overload conditions, a critical indicator of the network’s ability to support real-time applications in which a large amount of frame loss will rapidly degrade service quality. As there is no retransmission in real-time applications, these services might quickly become unusable if frame loss is not controlled. The test instrument sends traffic at maximum line rate and then measures if the network dropped any frames. If so, the values are recorded, and the test will restart at a slower rate (the rate steps can be as coarse as 10 percent, although a finer percentage is recommended). This test is repeated until there is no frame loss for three consecutive iterations, at which time a results graph is created for reporting. The results are presented as a percentage of frames that were dropped; i.e., the percentage indicates the variable between the offered load (transmitted frames) vs. the actual load (received frames). Again, this test must be performed for all frame sizes. 6.12.4.5 Oversubscription Testing The most common reason for frame loss is oversubscription of the available bandwidth. For example, if two 1,000 Mbps Ethernet services are mapped into a single 622 Mbps SONET/SDH pipe (a common scenario), then the bandwidth limit is quickly reached as the two gigabit Ethernet services are loaded. When the limit is reached, frames may be dropped. Under these circumstances

Microwave Network Deployment

351

it might be necessary to know not only how many frames are being dropped but which frames. For example, some network elements claim to be able to prioritize traffic based upon VLAN ID or priority tag. If this function is being used then as the bandwidth limit is reached it should be the low-priority packets which that get dropped and this functionality has to be tested as well. 6.12.4.6 Latency Testing Wireless services are very delay-sensitive (especially real-time traffic), so the ability to provide low latency is critical to the application. For VoIP, this would translate into long delays in the conversation, producing a satellite call feeling. In addition, the service intelligence of the latest OAM standards, such as IEEE 802.1ag and ITU Y.1731, is critical to guaranteeing that service levels essential for wireless backhaul are met. The latency test (also known as end-to-end testing) measures the time required for a frame to travel from the originating device through the network to the destination device. The total delay time is the sum of both the processing delays in the network elements and the propagation delay along the transmission medium. The test can be configured to measure the round-trip time; i.e., the time required for a frame to travel from the originating device to the destination device and then back to the originating device. When the latency time varies from frame to frame, it causes issues with real-time services. For example, latency variation in VoIP applications would degrade the voice quality and create pops or clicks on the line. Long latency can also degrade Ethernet service quality. In client-server applications, the server might time out or poor application performance can occur. The test procedure begins by measuring and benchmarking the throughput for each frame size to ensure the frames are transmitted without being discarded (i.e., the throughout test). This fills all device buffers, therefore measuring latency in the worst conditions. The second step is for the test instrument to send traffic for 120 seconds. At midpoint in the transmission, a frame must be tagged with a time stamp and, when it is received back at the test instrument, the latency is measured. The transmission should continue for the rest of the time period. This measurement must be taken 20 times for each frame size, and the results should be reported as an average.

Another important measurement in Ethernet networking is packet delay variation (PDV). This measurement is critical because excessive packet delay variation can cause major failures in real-time applications such as VoIP or

6.12.4.7 Testing Packet Delay Variation

352

Chapter Six

streaming video. In VoIP applications, excessive PDV will cause dropout effects. In video applications, the image will become choppy and pixelization can occur. At the start of the transmission, the interframe gap between all frames was identical. As the frames navigate through the network, being buffered or routed thought different network devices, the interframe gap begins to vary. The PDV measurement should be done at maximum frame rate as this is where the most variation will occur. The variation in packet delay is sometimes called jitter. This term could be confusing because it is understood differently by different groups of people. “Jitter” commonly has two meanings: the first meaning is the variation of a signal with respect to some clock signal, where the arrival time of the signal is expected to coincide with the arrival of the clock signal. This meaning is used with reference to synchronous signals and might be used to measure the quality of circuit emulation, for example. There is also a metric called “wander” used in this context. The second meaning has to do with the variation of a metric (e.g., delay) with respect to some reference metric (e.g., average delay or minimum delay). This meaning is frequently used by computer scientists and frequently, although not always, refers to variation in delay. 6.12.5 Testing and Troubleshooting Microwave Systems 6.12.5.1 Long-Term BER Measurements As with any other system, prob-

lems on the microwave link can be intermittent or repetitive in nature. They can also be caused by propagation problems, incorrect design, and installation of the equipment, and faulty modules within the radio itself. Short-term measurements may not identify any issues, especially if they are intermittent in nature. Long-term test results from BERT and/or network management system will narrow down potential causes of the microwave link performance degradation. Due to the nature of digital microwave links, error performance changes with time, and because of this, the measurements must be classified statistically for the percentage of time that the error performance was worse than certain thresholds. This requires a long measurement period of a month (or even longer), and the measurements are usually taken during the worst propagation month for a particular route, depending on whether it is dominated by multipath propagation or rain attenuation. Printouts of the long-term dynamic performance test report have to have a precise date and time stamp to correlate the problem with other events, such as radio alarms, weather fronts and rain storms, maintenance

Microwave Network Deployment

353

and manual intervention, software upgrades, switching, fading, local airport flight times, power issues, and so forth. Constellation Diagram In a digital radio, the information contained in the baseband signal is carried by the signal’s amplitude and phase when sampled with reference to a precisely timed recovered clock signal. The phase and amplitude of the baseband signal at the instant of sampling can be plotted on a polar graph to provide a constellation diagram. Constellation diagrams visualize as many similar phenomena as eye patterns do for one-dimensional signals. Constellation analyzer is a special purpose sampling oscilloscope designed for digital radio inservice measurements. For example, a normal constellation diagram will show 16 tightly grouped points for 16QAM as shown in Figure 6.21. Each point contains many samples of the baseband waveform. Data is derived from the part

6.12.5.2

Q

Decision thresholds

Noise margin

I

Figure 6.21 16QAM constellation diagram

354

Chapter Six

(A)

Noisy RF signal reduced noise margin

(B)

Pulse noise or similar interference

(C)

Outer points moved towards the center far-end means that Tx power is too high

(D)

Excessive phase noise in Tx or Rx circuitry

Figure 6.22 Constellation diagram indicating problems

of the graph an individual sample falls into. Where points are tightly grouped, the noise margin is large, reducing the probability of bit errors. If the RF signal is noisy, the points will be more spread and the noise margin reduced, increasing the probability of bit errors.19 Transmitter, receiver, or RF path problems may affect the placement of points or distort their shape, reducing the fade margin for a given signal to noise ratio. This will result in degraded radio BER performance, as listed here and shown in Figure 6.22: A. Gaussian noise shows fuzzy constellation points. B. Tight points with some spread-out samples may be caused by pulse noise or similar interference.

Microwave Network Deployment

355

C. Transmitter overdriving or nonlinearity causes the outer or corner points, which take the most power to transmit, to be moved from the center of their decision boundaries (check and adjust far-end Tx output power). D. Excessive phase noise in the transmitter or receiver (or I/Q modulator) may require replacement of the terminal if the BER performance is unsatisfactory. 6.13

References

1. Daniel W. Bliss, et al.,: “MIMO Wireless Communication,” Lincoln Laboratory Journal, Volume 15, Number 1, 2005. 2. Gregory D. Durgin, Space-Time Wireless Channels. New Jersey: Prentice Hall, 2003. 3. Federal Communications Commission, “Evaluating Compliance with FCC Guidelines for Human Exposure to RF Electromagnetic Fields,” OET Bulletin, Edition 97-01, August 1997. 4. Safety Code 6, Health Canada, “Limits of Human Exposure to Radiofrequency Electromagnetic Fields in the Frequency Range from 3 kHz to 300 GHz” (also available from http://www.hc-sc-gc.ca). 5. Federal Communications Commission/Office of Engineering and Technology, “Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields,” OET Bulletin 65, August 1997. 6. Institute of Electrical and Electronic Engineers, “IEEE Recommended Practice for the Measurement of Potentially Hazardous Electromagnetic Fields—RF and Microwave,” IEEE STD C95.3-1991, 1991. 7. FCC, Public Notice DA 05-311, February 3, 2005. 8. Kai Chang, RF and Microwave Wireless Systems, John Wiley & Sons, Inc., 2000. 9. Andrew Corporation, Catalog 38. 10. Gang Liu et al., The Effects of Wet Radome on a Short Millimetre-Wave Link in Singapore, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore, 2000. 11. Andrew Corporation, Special Environment Antennas—Solutions for Extreme Applications, Bulletin 3522, August 2000. 12. National Wildfire Coordinating Group, National Interagency Incident Management System: “Basic Land Navigation,” NFES 2865, June 2007. 13. E.D. Kaplan, Ed., Understanding GPS: Principles and Applications, Norwood, MA: Artech House, 1996. 14. Xiong Li and Hans-Jurgen Gotze, “Tutorial—Ellipsoid, Geoid, Gravity, Geodesy, and Geophysics,” Geophysics, Vol. 66, No.6 (Nov–Dec 2001), p. 1660–1668. 15. G.W.A. Offermans, A.W.S. Helwig and Dr. D. van Willigen, “The Eurofix Datalink Concept: Reliable Data Transmission Using Loran-C,” Proceedings of the 25th Annual Technical Meeting of the International Loran Association, San Diego, CA, U.S., November 1996. 16. EG&G Services, Parsons, Inc., Science Applications International Corporation, Fuel Cell Handbook, 5th Edition, October 2000. 17. J. Gruber and G. Williams, Transmission Performance of Evolving Telecommunications Networks, Norwood, MA: Artech House, 1992. 18. B. Giguere, “RFC 2544: How it Helps Qualify a Carrier Ethernet Network,” Application Note 183, EXFO, Canada, 2004. 19. Agilent Technologies, “Digital Radio Theory and Measurements,” Application Note 355A, 2005.

This page intentionally left blank

Chapter

7

Project Management

7.1 Tracking Microwave Rollout 7.1.1

Project Management Activities

Project management is the application of knowledge, skills, tools, and techniques to project activities so as to meet (or exceed) customer needs and expectations from a project. Meeting or exceeding customer needs and expectations invariably involves balancing contradicting demands, scope, time, cost, and quality.1 A project is defined as an undertaking of a nonroutine (unique), nonrepetitive nature having prescribed objectives in terms of scope, time, quality, and cost. Within the realm of project management, such projects can be further defined as generally being complex, having a multidisciplinary involvement, and having various phases in their life span. The various phases of a microwave (and any other) network build-out project may be defined differently by different organizations, but they generally fall into the following categories: n

Concept (or feasibility) analysis

n

Network planning and preliminary design

n

Detailed design and engineering

n

Deployment (also called implementation)

n

Testing and commissioning

Each of these phases can be divided into a whole list of subphases and activities. The completion of each of these project phases is usually accompanied by a finished, smaller project of some sort. While project 357

358

Chapter Seven

management skills are quite distinct from engineering design skills, the requirements of good project management are not different from the requirements of good engineering or good management. Effective management of a project calls for the early establishment of policies and procedures for its implementation. During the initial phase of the project, therefore, the project manager, in conjunction with the client and other interested parties, establishes clearly defined and properly documented project policies and procedures and sets design and deployment standards that meet the client’s operational requirements and satisfy the needs of effective management and accountability. Project management services are applicable in all phases of a project, from the initial concept through implementation, to the final commissioning and handover of an operational project.2 It is therefore important that a client be aware of the full scope of services that can be provided. Project management services normally include certain basic activities such as the following: n

Planning and scheduling (time management)

n

Budgeting and estimating

n

Cost control and accounting

n

Quality control

n

Regular reporting

In addition to the basic services, depending on the particular project, project management services could also include a number of other items such as: n

n

n n

n

n

n

Interpreting the client’s requirements, operational needs, and constraints and advising the client as to the suitability of alternative solutions Defining the project requirements, including scope, quality, and overall budget and schedule of work Preparing project policies and procedures Assisting in securing project financing and arranging appropriate financial arrangements Advising the client of required decisions in relation to legal and insurance considerations Advising and assisting the client with respect to the regulatory and approval process with statutory authorities and obtaining the required permits Structuring the project into manageable subentities

Project Management

n

n

n

n

n

n

n

n

359

Prequalifying, recommending, selecting, and negotiating contracts with consultants, vendors, and contractors Managing the design for conformity with the agreed project requirements and budget and administering design changes Suggesting alternatives, evaluating them, and assisting the client to choose among them so as to best meet the needs of the client in terms of scope, time, quality, and cost Identifying to the client the impact (scope, time, quality, cost) of proposed changes so that the client may make well-informed decisions about whether to proceed with the proposed changes, and advising the client of the effect on the project of delayed decisions/approvals Arranging and coordinating the procurement, expediting, and quality control of all required materials, equipment, and services, including those supplied by the client Procuring equipment and services, including prequalification, tendering, contract negotiation, contract administration, and expediting Managing construction/implementation/deployment for conformity with the approved design, including detailed scheduling and coordination, managing inspections, administration of construction changes, approvals of progress claims, completion certificates, management of deficiency and warranty work, commissions, operating manuals, and record documentation Assisting the client in testing and commissioning, startup and/or operating procedures, including staff training

In defining the scope of services to be provided, the client and the project manager should review the preceding list in detail, consider additional items that might be appropriate in the particular case, and establish the items and the related scope to be included in the services contract between them. The project policies and procedures should be specifically developed to suit the size, complexity, and scope of the particular project and normally covers overall project implementation policies, project organization, personnel functions, administrative and control procedures, technical design criteria, documentation standards, quality control procedures, and record keeping.3 7.1.2 Three-Stage Process

Microwave network roll-out is usually a three-stage process consisting of planning, design, and deployment stages (see Figure 7.1). Planning typically refers to a high-level decision-making process that encompasses budget and schedule definition and identifying team

360

Chapter Seven

Customer requirements

MW network planning

High-level MW network plan High-level systems engineering Preliminary BOM

Business case analysis and approvals (internal and external)

PLANNING STAGE

MW network Detailed design MW link and site engineering (BOM) Approvals

Licensing

DESIGN STAGE

DEPLOYMENT STAGE

MW network deployment

Figure 7.1 Three-stage process

members required for the project. It also includes determination of frequency band, system capacity, network configuration, and performance objectives. System design is an actual detailed link engineering process (which may or may not include site visits) that includes creation of the detailed bill of materials, issuing RFQs and RFPs, and ordering equipment (MW radio, shelters, towers, and other transmission hardware and software), ordering engineering, installation and other services, and so forth. Deployment (also called implementation) includes all of the field activities such as site and path surveys, tower erection, equipment installation, creation of an as-built documentation, and acceptance testing and commissioning. What is the difference between planning, design, and deployment? Although distinct differences exist in telecommunications projects, these three activities in microwave network build-out overlap somewhat and are mutually dependent. Many times, partial design or redesign has to be performed during the planning stage and/or deployment as well. 7.1.3

Project Kick-Off Meeting

The project manager is most likely the first person appointed if the new microwave system is even remotely considered. It is all about expectations. The client has expectations concerning delivery times, installation schedules, related costs, and equipment performance.

Project Management

361

The contractors have expectations concerning site access, material delivery, material suitability, and working schedules. Project management establishes nonconflicting expectations by coordinating the client’s desires with the contractor’s abilities. And if/when the unexpected happens, project management adjusts schedules and expectations. The project manager will have to understand the basics of the microwave system build-out process, know which specialists need to be brought into the project, and understand basic time-line and critical path activities. Regardless of whether it is a new microwave system or an upgrade or expansion of the existing facilities, microwave deployment (build-out) is a multidisciplinary activity that involves a number of very specialized experts in their respective fields. It is always a good idea to organize a kick-off meeting and invite all the parties involved in the project. This is a good opportunity for the people to meet and see who else will be working on the project. In addition, this is a good time to identify any “missing links” in the project, i.e., equipment, experts, or anything else that might have been omitted by the project manager or previously thought not to be required. This is a brainstorming session organized by the project manager, but it requires active involvement by all team members. The project system engineer should be present at this meeting, since this person will be responsible for ensuring that the design and installation is performed according to industry, customer, and supplier standards and practices. The project system engineer is the key technical person on the project and has an overall responsibility for the technical integrity of the system design provided. 7.1.4

Network Planning

Network planning (also called preliminary design or nominal design) is usually done in a matter of days or weeks and typically consists of the following activities: n

n

n

n

Discussion of the client’s requirements and project goals. The client defines the system requirements and, often, site locations as well. Preliminary path engineering using topographical maps and visiting only key sites (switch office, big hub sites, and so forth). Identification of material requirements and creation of preliminary BOM. Development of the budget and schedule.

Preliminary path engineering, including routing design and preliminary path analysis, should be done prior to any field trips and

362

Chapter Seven

detailed design. Computer programs are available to plot an initial system map that shows the sites in geographical relationship to each other. Approximate site coordinates and the feasibility of various paths are determined from a topographical map, a digital terrain database (minimum 1:50,000), or old (but not too old) survey information. This map is used, along with traffic requirements, to define the paths between sites and the type of radio for each hop. Results of the preliminary network planning activities and its deliverables (budget, work force, and schedule) form the basis on which the whole project will be assessed and potentially approved or rejected. Of course, due to the short time available as well as very limited input data, the margin of error in these calculations and predictions is quite high. 7.1.5

Project Approval

There is a famous expression, “To be successful, surround yourself with smarter people.” That’s the idea of expert judgment: rely on the experts within your organization, consultants, stakeholders (including the project customers), professional associations, or industry groups for advice. These experts can contribute to the project selection method by offering their opinions, research, and experience. For an engineer, as well as for a project manager, this is probably the most difficult part of any project. It includes convincing nontechnical personnel and executives who may or may not have the budget or any experience with the technology involved and are probably too afraid to commit to spending millions of dollars for something that won’t start bringing any profit for at least two or three years. This is the point at which people with vision stand out from those who are concerned only with the next quarterly results. Usually, transmission (transport) facilities are either leased or owned (copper, microwave, fiber optic) or (most likely) a combination of leased and owned (usually microwave) facilities. In most rural areas and thirdworld countries, there are very few options but to build the microwave system. In North America, however, leased lines are widely available and therefore a good option to consider. The only problem is that this option, although cheap if we consider only the monthly rate, becomes very expensive after a few years, because it is a recurring cost that continues ad infinitum. In many cases, project managers and those involved in the time-tomarket assessment conclude that the faster way to build the network is to lease T1/E1 circuits from the local telephone companies rather than building their own microwave system. That may not be the case in every situation, and it may prove, for a number of reasons, to be a much more expensive and lengthy process than originally anticipated.

Project Management

363

Building the microwave system from the ground up involves many different activities, some of which are very lengthy and very expensive. A business case has to be prepared with due diligence, based on the preliminary microwave network plan, and it cannot be done in a day or two. If hundreds of microwave hops are required, it may take weeks for an experienced engineer to put all numbers and different scenarios together. Vendor financing is one of the most commonly used methods for deferring the initial blow of a huge capital expenditure on a day one. Many vendors are willing to provide financing, assuming they are interested in doing business with a certain customer in a certain part of the world. 7.1.6

Site Acquisition

Everyone wants to use wireless phones, wireless LANs, broadband Internet services, and so on, but few people want the towers and antennas in their neighborhood. With increasingly restrictive zoning requirements and a host of new wireless operators rushing to install thousands of new cell sites in every large city, tools and processes are being developed to increase flexibility in site locations without deteriorating coverage, capacity, and service quality. Site acquisition is a main bottleneck and a critical path in most wireless network build-out projects, and it can take up to six months or longer to acquire the appropriate site. The same goes for microwaveonly sites. The real estate group has to make a decision based on the potential of the site to be leased, the time required to finalize all the required paperwork, zoning restrictions, and other factors. In wireless and/or microwave networks, unfortunately, the optimum site frequently is not available, so compromise is required. Lease negotiations take place after the property for the cell site is selected, and the contractor must negotiate the deal with the property owner, which can be very profitable for the property owner. In accommodating antenna(s) and auxiliary equipment, the property owner could receive a few hundred dollars per month in rural areas and up to a few thousand dollars per month in urban settings. Keep the following in mind during the site acquisition process to help to speed up the process: n

Monopoles are often aesthetic and economical alternatives to self-supporting and guyed towers, and they are more acceptable to planning and zoning committees. They may not be suitable if the loading of the tower is substantial or requirement for twist and sway is very stringent (microwave radios).

364

n

n

n

Chapter Seven

Make all legal documents as simple as possible; long, complicated documents written in language that is not easily understood may cause the property owner to be wary. The document has to satisfy legal requirements and yet allow the nonprofessional to understand it. Address all local community concerns regarding aesthetics, safety, potential health hazards, environmental impact, and so forth as soon and as precisely as possible. In dealing with the public, it is a good idea to avoid using the term microwave, because it makes many people very nervous. Terms like directional RF antenna and RF parabolic antenna, which are also technically correct, can replace the term microwave antenna.

Zoning issues will be different for each jurisdiction, since every city, town, and municipality has its own particular requirements. Negotiating with public officials and civic administrators requires considerable time and experience, so operators quite often hire professionals to do the site acquisition. Many operators optimistically set aside only a few months for acquiring, permitting, and building their initial set of transmission facilities in the effort to launch a new wireless service in a community. They are disappointed when they realize that they did not perform sufficient preliminary investigation, and they end up trying to build cell sites in upscale or historical areas where it is impossible to erect a telecommunications tower. 7.1.7

Detailed Network Design

After the initial planning phase, budget approval, and personnel mobilization, all the prerequisites for the detailed microwave design have been achieved. A microwave radio system requires careful planning and analysis prior to equipment installation. A poorly designed path may result in periods of system outages, increased system latency, decreased throughput, or a complete failure to communicate across the link. Detailed design consists of these main activities: n

Conducting site and path surveys

n

Performing link engineering

n

Performing interference analysis and radio licensing

n

Finalizing the equipment specification (bill of materials, BoM)

Site and path surveys and link engineering are mutually dependent, as not all the sites will be suitable for tower and microwave antenna installation, and any changes in site location can seriously affect the

Project Management

365

network topology and design. A field path survey should determine the exact coordinates of locations where the antennas will be installed, establish the height of each antenna, identify the location and height of current and future path obstructions (for example, a tree may grow to obstruct the path at some later date), and identify the location of possible reflection points. Once the site survey and field path survey have been completed, the final link engineering will be performed. Results of the link engineering process are forwarded to the company that is designing towers, shelters, and other infrastructure, and this information will serve as input data for their design. Interference analysis and frequency coordination play a very important part in proposed route design. Governments usually require users of the radio spectrum to frequency-coordinate their planned and existing microwave radio systems with other users of the radio frequency spectrum. Such coordination is a prerequisite in any microwave radio license application submitted by a microwave radio system operator. The frequency-assignment process varies from country to country. In some countries, the operator will be assigned a frequency band and can then plan the frequency assignment for each hop without asking the authorities for permission. In that case, the operator is also responsible for interference analysis. In other countries, the operator has to apply for frequencies on a per-hop basis. In that case, the authority is responsible for the final interference analysis. Interference typically affects both the “interferee” and the “interferer.” An established system encountering interference may be interrupted, but the new system causing the interference often will not be able to fully or properly function. A new system cannot displace an established system. Once a system is operational, any future systems will have to “steer around” the existing signals so that both systems can live in harmony. When one encounters interference while deploying a new system, it is frequently possible for the established system and the new systems to arrive at some sort of mutual accommodation, enabling both to coexist while avoiding interference. This can be done through tactics such as reducing the transmitting power of one link so it does not interfere with the other link. The objective of the equipment specification is to produce an equipment BoM (Bill of Material) for a proposed network design, including all details of the transmission equipment required to construct the network. The activity will enable correct ordering of equipment, thus ensuring correct equipment availability at the time of installation. It is important to include installation equipment, installation tools, and other equipment necessary so as to complete the implementation.

366

Chapter Seven

The BoM has to be adequate to enable forecasting and ordering (procurement) of the equipment. Any necessary network management system (NMS) equipment and installation services should also be included in the scope of the work. This part is usually handled by a group of people other than those involved in the radio part of the project. 7.1.8

Equipment and Services Procurement

Procurement is the systematic purchasing of all materials, equipment, and services needed for the project, in good time, and in a manner that is cost effective. These generally include (but may not be limited to) those provided by consultants, testing services, suppliers, construction managers, and contractors. Equipment may include all or some of the following items: n

Tower and/or tower upgrades

n

Antennas and transmission lines

n

MW radio equipment

n

n

Other transmission equipment (multiplexers, switches, routers, DACS, etc.) Rectifiers, inverters, batteries, fuel generators, solar power systems, etc. Services may include one or more of the following activities:

n

Transmission/microwave network design n

Network planning

n

LOS verification and/or path surveys

n

Detailed design

n

Installation services

n

Project management

n

Testing and commissioning

Services may also include post-installation operation and maintenance of the transmission network. For a large project, multiple vendors (two or more) are usually selected, and each will share the market and provide backup in case the other has problems with delivering on time or providing the agreed-upon quality.4 A decision is usually made based on the request for quotation (RFQ) response and its compliance with the functional and technical requirements of the client.

Project Management

367

7.1.8.1 Answering Proposals The RFQ (or tender document) response describes, in detail, the equipment and/or services to be supplied. The RFQ is prepared by the customer for the purpose of soliciting hardware, software, and/or services information for evaluation and possible procurement, with a specific project in mind. Answers to questions asked in the RFQ will provide the client with a better understanding, both in financial terms and in view of system integration and capacity aspects, of the equipment and services that the supplier (vendor) can provide. Topics discussed in the RFQ are usually, but are not limited to, commercial conditions of contract, technical conditions, project management, quality assurance and reliability issues, procurement and delivery issues, training and documentation, in-service date, RFQ response due date, and so forth. The request for information (RFI) and request for pricing (RFP) are somewhat less detailed requests that are usually sent to equipment or service providers to solicit information on their products and services. A response to the RFI could be just a collection of data sheets, brochures, user manuals, and similar items. The response to an RFP could consist of a few pages of standard list pricing, usually without any discount or additional considerations. The RFQ can be functional or technical in nature. A functional RFQ is one in which the client describes the system and its functional requirements, and it is the total responsibility of the supplier to make it work (turnkey project). For turnkey contracts, a specific scope-of-work document is also included to define the installation and test services to be covered by the contract. In some cases, different contractors may provide original equipment manufacturer (OEM) and installation services. A technical RFQ is very similar except that the client provides more initial data to the supplier. For example, perhaps a preliminary microwave plan has been done and tentative site locations identified, which will give all the bidders the same starting point. This will eliminate a big discrepancy in the initial approach among bidders and therefore provide more realistic and competitive pricing. If the client takes full responsibility for the microwave network buildout and only needs the equipment, a bid specification is created and sent to suppliers to provide a quote. Installation services may or may not be a part of the bid specification. Sometimes, on large networks, suppliers will install part of the network and train clients’ personnel at the same time. After the initial few hops, the supplier’s staff will leave, and the client’s technicians will complete the rest of the project. In many transmission networks, speed of deployment will be a very critical factor in the process of equipment and/or supplier evaluation, and it must be addressed and discussed in detail. All suppliers are usually provided with the opportunity to individually discuss RFQ proposals;

368

Chapter Seven

after that, final discussions will be conducted with up to three top candidates, after which the financial and legal terms will be determined. It is very important to use the proper terminology while preparing proposals. For example, “shall” and “shall not” identify requirements to be followed strictly and from which no deviation is permitted. “Should” and “should not” indicate that one of several possibilities is recommended as particularly suitable, without mentioning or excluding others; that a certain course of action is preferred but not necessarily required; or that (in the negative form) a certain possibility or course of action is discouraged but not prohibited. “May” and “need not” indicate a course of action that is permissible within the limits of the document. “Can” and “cannot” are used for statements of possibility and capability, whether material, physical, or causal. Proposal Pricing Model A pricing model is usually defined by the client, and it has to be as close to the planned network as possible. The supplier should try to adhere to the requirements and definitions provided in the RFQ as closely as possible. Compliance of the equipment with all of the applicable national and international telecommunications and quality standards, and interoperability with the equipment of other suppliers is usually mandatory. Equipment evaluation is usually based not only on the technical specs and price, but also on the other criteria (e.g., the experience of other customers in the same or other countries, with the same or similar type of equipment, warranty, and customer support). Long-term maintenance costs could exhibit large variations, and it is important to consider this aspect during the system comparison. The RFQ should be structured to simplify the process for the client and supplier alike; responses must specifically address each point set forth in the RFQ and should be clearly answered in the response tables. The amount of information submitted is left to the discretion of the respondent, but it is imperative that pertinent information be submitted and individual topics of interest dealt with completely and concisely. Those suppliers failing to provide complete and accurate responses can be discredited for the quality of the response and appropriately penalized in the response evaluation process. A supplier who chooses a no-bid response to the RFQ should specify, in a cover letter, the reasons for the decision. The usual response time, depending on the complexity of the project and client’s requirements, is two weeks for very small projects and up to eight weeks for large microwave networks. The more details about the future network are provided to suppliers, and the more time they have

7.1.8.2

Project Management

369

to reply, the better are the chances that their estimates will be close to reality, with fewer surprises to follow. The quotes are usually based on uninterrupted/contiguous site field activities. Additional mobilization costs are usually not included in the pricing, and if the installation is delayed due to inclement weather (in U.S., see National Weather Service, www.weather.gov, accessed Oct 13, 2009), an inaccessible site, or incomplete site preparation or construction by others, additional charges may apply. 7.1.9

Site Ready for Installation

In this context, site ready for installation means that the site is ready for the microwave equipment to be delivered, installed, tested, and commissioned. One of the basic assumptions (after establishing LOS, of course) is that there is available space to install the microwave radio, antenna, and all other miscellaneous equipment, and that AC power (and maybe even a DC charger and battery backup) is readily available. Access through site security has to be facilitated in advance so that the installation crew does not waste valuable time waiting to be let into the site. It is important to remember that to install, test, and commission the microwave link, both sites of the link have to be ready for installation. Although it can be done, installing equipment on one site one week and then waiting a few months to finish the installation on the other site that completes the link is not recommended. It is a waste of time and resources and, on a large project with many remote sites, it can amount to a sizeable increase in installation costs. It must first be noted that operators’ capital expenditures consist of civil engineering outlays (network construction and other services) as well as investments in telecommunications equipment. The civil engineering portion, although varying tremendously according to the type of project and location of network deployment, seems to average approximately 60 to 70 percent of the cost for the mobile system gear. While labor costs—the bulk of the civil engineering expenses—are unlikely to fall, there are strong reasons to believe that, as a proportion of total capital expenditures, they will actually decline in coming years. New technologies, after all, do not only offer higher capacity and lower unit cost; increasingly, they offer more efficient operations. 7.1.10

Equipment Installation

Telecommunications equipment comes in all shapes and sizes, so shelters that house such equipment are often custom designed and custom built to properly protect it. For example, split-configuration microwave radios require very little space and can be installed virtually anywhere.

370

Chapter Seven

It is important to note that, after weeks and months of site identification (site acquisition), site construction and equipment installation typically take only a few weeks. These phases include the following: n

Tower erection/upgrade

n

Installing antenna and transmission lines

n

Installing radio equipment

n

Installing other transmission equipment

n

Installing additional power plant

n

Installing other miscellaneous equipment

On split-configuration microwave radios, the same team will install radio, coax cables, antennas, and even the power supply. On large backbone microwave systems, a radio supplier usually provides people to install radios, but large antennas, waveguides, pressurization equipment, and so forth are installed and tested by a different team of people who specialize in this kind of work. Antenna mounting structures could include towers, poles, tripods, walls, and others. During the detailed design and deployment of the MW system, the following are important considerations: n

n

n

n

The existence of sufficient space on the tower to install and pan the microwave antenna The loading of the antenna mounting structure (the MW antenna, transmission lines, and the outdoor MW unit) Maximum allowed twist and sway (antenna deflection) of the antenna mounting structure (in degrees); depends on the frequency and antenna type and size The availability of experienced civil engineers who can handle the complexity of existing standards, regulations, and requirements involved in civil construction It is also very important to keep in mind these safety precautions:

n

All rigging is to be done using safe work practices.

n

All riggers must have an approved and current climbing certificate.

n

No tower climbing is to take place until the rigging company is satisfied that it is safe to do so.

As physical equipment becomes progressively smaller and hence easier to install and commission, huge productivity gains are achieved. Infrastructure vendors are already taking advantage of that trend by preassembling and pretesting equipment at manufacturing sites before

Project Management

371

they are shipped to the installation site. This is designed to minimize costs (and disruptions) in the field. Microwave equipment has also been greatly improved by utilizing split configurations and replacing waveguides with coax cables in many applications. It is a good practice to assign the responsibility of coordinating shipments and deliveries for all equipment and materials on the project. Material coordination is an important key to a successful and efficient implementation plan. 7.1.11

Acceptance Testing

A record of the standard field turn-up tests (defined by the operator, agreed upon by the microwave supplier, and performed after the MW radio system installation is complete) has to be provided, as well as the system verification document. See Chapter 6 for more details on the field tests and deliverables. 7.1.12

As-Built Documentation

An as-built document is usually not completed until each site is integrated; however, it should be stressed that an agreement of its content must be included in the contract at the beginning of the project. It is advisable to prepare a standard installation drawing or a real site and present it to the customer so as to minimize potential misunderstandings later. The as-built document should include at least the following items (note that this refers to microwave sites only): n

n n

n

n

n n

n

n

Link engineering documents Contains all the details of path engineering, including path survey results Site situation plan Floor plan drawing mission equipment

Shows the site location on a map Indicates the location of the installed trans-

Cable way drawing information

Provides indoor cable installation

Antenna placement information Shows the antenna arrangement (tower profile)—height, orientation, polarization, and so on Alarm allocation table Indicates alarm cabling Power distribution equipment

Indicates power distribution for the indoor

Transmission configuration data mation used for software setup

Provides microwave link infor-

Rack layout Shows the layout of the indoor equipment in the transmission rack

372

n

n

n

n

n

n

Chapter Seven

Transmission traffic layout Indicates traffic distribution on T1/E1 level (also called T1/E1 plan); the traffic can also be expressed in Mbps Plant specification Lists equipment used on the site, including installation material (e.g., cables) Product list numbers

Lists main units, including equipment serial

Licenses to operate city, etc.

Includes those by the FCC, FAA, the

Acceptance test document acceptance

Confirms the customer’s site

Factory test results Description of the factory tests and test results performed at the manufacturer’s site during manufacturing and QA (quality assurance) process

The as-built document shows how the equipment is installed in a site. From its contents, it is clear that all installation aspects are considered at this stage. 7.1.13

Commissioning

We can turn the system on when the last two segments are completed; i.e., the as-built documentation is ready and system acceptance tests have been performed. Commissioning is the process of systematically bringing the various components of the project into an operational mode prior to startup and executing a formal handover to the client. Commissioning high-capacity links (backbone systems), hub sites, and ring sites should be performed first. After that, lower-capacity and spur sites should be turned on and commissioned. The client’s technical staff that will operate the completed network normally carry out commissioning and startup. As soon as the equipment becomes operational and tested according to the ATP, the project manager initiates and submits to the client a project final certificate for acceptance. This certificate normally records the formal handover of the completed project and identifies all the documentation (including operational and maintenance manuals, as-built drawings, equipment warranties, and contract completion reports) that the client requires for ongoing operation. 7.1.14

Maintenance Program

Although a properly designed and installed microwave system does not require a great deal of maintenance, a periodic maintenance program has to be implemented. The program has to be established during the

Project Management

373

microwave network-planning phase. Maintenance can be performed by the operator, or it can be contracted out to a third-party company. In preparing a maintenance program, the following must be done: n

n

n

n

Define a maintenance system and arrange for the staffing of maintenance personnel. Establish maintenance centers and determine the types and number of test equipment, tools, and spare parts needed for each center. Prepare a maintenance schedule and forms for maintenance logs and records. Define the target system performance values for providing maintenance and prepare standardized test procedures.

A good NMS, purchased and installed with the microwave radio equipment, will simplify the maintenance tasks and automate the process of periodic maintenance logging and record keeping. The periodic routine maintenance for the equipment is performed in accordance with the relevant equipment manuals and manufacturer’s requirements. Antennas and their supporting tower structure are subjected to dynamic wind loading and, in some cases, seasonal static ice loads. The structural responses are unwanted movements and deformations from the static load, backlash in antenna mounts, slippage in mounting clamps, changes in guy wire tension, foundation settling, and bending of structural members. These changes are not only undesirable structurally, but they could also cause antenna misalignment and therefore degradation of the link performance. As a part of the long-term maintenance program, after a tower and its mounted antennas have been put into service, the mechanical stability of the structures should be monitored. For example, guy tensioners on a guyed tower have to be adjusted from time to time. 7.2 7.2.1

Regulatory Issues FAA, FCC, and NTIA

Under authority granted by the Federal Aviation Act, the Federal Aviation Administration (FAA) has jurisdiction over the following communication facilities: n n

n

Towers that exceed 200 ft in height Towers that are located within 20,000 ft of a major commercial or military airport Towers that are located within 10,000 ft of a general aviation airport

374

Chapter Seven

The FAA reviews the location and height of such towers and may require them to be painted and/or illuminated to prevent possible interference with nearby airport operations. The FAA also reviews possible interference issues with aircraft-to-ground communications that may be caused by transmission facilities located in or near airport flight paths. In the U.S., owners of antenna towers that are taller than 200 ft (60 m) above the ground level, or that may intersect the flight pathways of a nearby airport, must register the structure with the Federal Communications Commission (FCC) and have the structure’s placement studied by the FAA. Previously, antenna registration was the duty of the antenna site licensee, but now (since 1995) the owner of the real property is accountable. FCC Form 854, “Application for Antenna Structure Registration,” is to be used to register structures used for wire or radio communication service in any area where radio services are regulated by the commission. In addition, it is used to make changes to existing registered structures or pending applications, or to notify the commission of the completion of construction or dismantlement of structures. Tower lighting is another of the very important requirements generated by the FAA. Unreported tower light outages can cost a tower owner a lot of money in the form of fines levied by the FCC. However, reducing the potential for a serious aviation accident is enough motivation for reputable tower owners and communications system licensees to maintain tower lighting in good working condition. Each new or altered antenna tower structure registered must conform to the FAA’s painting and lighting recommendations set forth on the structure’s FAA determination of “no hazard,” and it must be cleared with the FAA and filed with the FCC. Although the FAA’s lighting and painting standards are advisory in nature, the FCC’s rules make the standards mandatory. The standards and specifications set forth in these FAA documents are incorporated by reference into the FCC’s rules, making these advisory standards mandatory for antenna towers. The FCC always requires an FAA determination that an antenna tower will not pose an aviation hazard before it will grant permission to build that antenna tower. The FCC is an independent federal regulatory agency that is directly responsible to Congress. Established by the Communications Act of 1934, it is charged with regulating interstate and international communications by radio, television, wire, satellite, and cable, and its jurisdiction covers the 50 states, the District of Columbia, and U.S. possessions. The general objectives of federal telecommunications regulations are to provide efficient use of the electromagnetic spectrum (which is considered to be a public resource) and to develop a domestic telecommunications infrastructure that is able to provide service on the national level and compete on the global level.

Project Management

375

In some circumstances, tower installations must be approved by the FAA, registered with the FCC, or both. To ensure compliance, it is important to review the current FCC regulations regarding antenna structures. These regulations (along with examples) are available on the FCC web site at http://wireless.fcc.gov/siting/(accessed January 06, 2010). In January 2003, the FCC and the Commerce Department’s National Telecommunications and Information Administration (NTIA) executed a new Memorandum of Understanding (MOU) on spectrum coordination. The MOU applies to the coordination of spectrum issues involving both federal and nonfederal users. The FCC is responsible for nonfederal users (e.g., broadcast, commercial, public safety, and state and local government users), and the NTIA is responsible for federal users. The majority of the spectrum is shared between federal and nonfederal users, in which case the FCC and NTIA must coordinate spectrum policy. Other limitations imposed by authorities can also affect microwave radio deployment—e.g., tower height restrictions or limitations on antenna size. These factors can restrict effective radio lengths at the planning stage and should be ascertained in advance of the detailed link design stage. Most local governmental agencies regulate wireless communications facilities via land use regulations contained in respective zoning ordinances and general plans. They are responsible for reviewing and processing applications for discretionary and ministerial permits for such facilities. Local governments also have the broad authority to ensure the public health, safety, and welfare of their citizens. Local jurisdictions regulate wireless communications facilities through the permitting process. Most agencies require a discretionary permit, such as a conditional use permit, to construct a facility. Whether a permit is processed administratively or requires a public hearing varies among local agencies. In general, administrative processing entails lower permit fees and shorter processing times, whereas the public hearing process involves higher permit costs and a longer permit turnaround time. The FCC’s Wireless Telecommunications Bureau (WTB) handles all FCC domestic wireless telecommunications programs and policies except those involving satellite communications. Wireless communications services include cellular telephone, paging, personal communications services, public safety, and other commercial and private radio services. The WTB regulates wireless telecommunications providers and licenses with the intent of preventing interference and conflicts among various operators and services at a given location when such operators use the same portion of the frequency spectrum.

376

Chapter Seven

7.2.2

FCC Frequency Coordination Process

The FCC frequency coordination process involves several distinct but interrelated steps: interference analysis, notification, and response. After the frequency coordination process is completed, the licensee is responsible for maintaining records for the FCC after the license is granted. The licensee must notify the FCC of any address or other administrative changes to the system. The licensee must also track when a license is due for renewal (the current renewal period is ten years) and submit a timely renewal application. The first step in the frequency coordination process is interference analysis. The FCC requires that applicants engineering a new system or making modifications to an existing system must conduct appropriate studies and analyses to avoid interference to other users in excess of permissible levels. This interference analysis is typically performed by the consulting company specialized in this type of work and authorized by the FCC. This analysis is done before issuing a prior coordination notice (PCN) and is performed by recipients of a PCN to verify noninterference. Interference analysis is an iterative process that involves computer simulation of potential interference and an engineering analysis to eliminate interference cases. The process begins with a tentative frequency selection that is consistent with the established frequency plan. Telecommunication Industry Association (TIA) Bulletin 10 criteria and industry-developed guidelines are used for automated calculations. These calculations include co-channel and adjacent channel interference, threshold degradation, adjacent spectrum interference; and potential interference from intermodulation products (see Figure 7.2). In frequency bands shared with satellite Earth stations, an interference analysis is conducted with the applicable ground and space segments. All operational fixed (OF), common carrier (CC), and broadcast auxiliary point-to-point microwave operators are required to complete and submit Form 601 to the FCC. The form must be accompanied by a supplemental document indicating that the frequency coordination process has been completed. Once the appropriate application is filed, the FCC puts the application on notice for 30 days for public comment. If there is no objection, and the application meets all FCC filing requirements, the FCC grants the application. Once an interference analysis has been completed, and prior to system implementation, an operator is required to issue the notification to all “potentially affected parties.” The industry defines an operator as potentially affected if the operator’s facilities (including proposed, applied-for, or operating) fall within a defined coordination distance and operate in the same frequency band. This notice is referred to as a prior coordination notice (PCN) and contains the technical operating parameters and a general description of the proposed system.

Project Management

Operator input

MW transmission design and intra-system frequency coordination

Interference analysis and inter-system frequency coordination Yes Operator approval notice issued Results of the interference analysis are reported and operator approval notice is issued

Yes Modifications required?

Operator approves the microwave link design

No

Prior certification notice (PCN) issued

Notification-PCN sent to all the existing MW link operators within 125 miles radius for comments Comments?

FCC licence application and filing form 601

No

FCC licence application

FCC public notice 30 days for the public response

Comments?

Yes

No

FCC licence issued

Figure 7.2 FCC microwave frequency coordination process

Problem resolution

377

378

Chapter Seven

The recipient of a PCN has 30 days in which to analyze the proposal and provide a response. Every attempt should be made by the receiving party to respond as soon as possible. In most cases, operators utilize an outside agent, commonly referred to as a protection agent, to administer this function. The response to a PCN should include an affirmation of the proposal or, if there are objections, a detailed description of these objections. Typically, a response that raises concerns will contain enough technical data to substantiate the objection. The party issuing the PCN is then required to resolve all potential conflicts to the satisfaction of the objecting party. This may require several rounds of discussion, technical analysis, and negotiation. When both parties have reached an agreeable resolution of the issues, the coordinator of the proposed system issues a document called a supplemental showing. The supplemental showing is akin to a signed affidavit in which the coordinator attests to satisfactorily completing coordination. Form FCC 601 is a multipurpose form used to apply for an authorization to operate radio stations, amend pending applications, modify existing licenses, and perform a variety of other miscellaneous transactions covered in the Wireless Telecommunications Bureau (WTB) radio services. Form FCC 601 is also a multipart form comprising a main form and several optional schedules. Schedule I is a supplementary schedule used to apply for an authorization to operate a radio station in the categories of fixed microwave and microwave broadcast auxiliary services. See Chapter 6 for more specific information on FCC and NTIA licensing process for millimeter-wave radio links in 70- and 80-GHz bands (E-band). 7.2.3

Homologation

Sometimes one of the biggest headaches facing telecommunications equipment suppliers is not technical but rather regulatory. Equipment is very often sold and installed in countries other than the country of manufacture. These countries may have different functionality and safety requirements. To sell their products in other countries, suppliers must go through a certification process (homologation). Quick and efficient approval for the hardware and software in each of the markets usually requires a detailed knowledge of both telecommunications equipment and the local regulatory environment. Such knowledge is typically available from a local company that is intimately familiar with the requirements of the agencies in the countries in which they operate. Quite often, frequency allocations and channeling plans for wireless services (cellular, PCS, microwave, satellite, and so on) are different

Project Management

379

in different countries. To answer questions like, “What’s the dialing/ numbering plan in Nigeria or Croatia?” “What are the specific requirements for a new interface in China?” and “What is the frequency band and channeling plans for the microwave systems in a specific country of interest?” a local presence across the region and broad technical and regulatory expertise are needed. In some countries, the certification process is simple. If equipment is approved by the U.S. FCC and meets certain minimum requirements, it is considered to be certified. In other countries, government regulations are used to exclude foreign manufacturers from their markets. This is done by requiring suppliers to meet very stringent requirements and standards and quite often to customize their hardware designs. While this is not necessarily technically demanding, it is a very time-consuming process. The CE mark is a requirement for all telecommunications terminal equipment (TTE) products sold in the European Union, effective January 1, 1996. The CE marking confirms that a product has been tested and meets the essential requirements of the European Telecom Directive to market it throughout the EU. Obtaining the CE mark allows a product to be sold in EU countries without any further in-country testing. 7.2.4

Other Regulatory Issues

The National Environmental Protection Act (NEPA) requires all federal agencies to implement procedures to make environmental consideration a necessary part of an agency’s decision-making process. As a licensing agency, the FCC complies with NEPA by requiring commission licensees and applicants to review their proposed actions for environmental consequences. FCC rules implementing the NEPA require the licensee to consider the potential environmental effects from its construction of antenna facilities or structures and to disclose those effects in an environmental assessment (EA), which is filed with the FCC for review. The FCC solicits public comment on the EAs and assists its licensees in working with the appropriate local, state, and federal agencies to reach agreement on the mitigation of potential adverse effects. The filing of an EA is required when a proposed facility may have a significant effect on historic properties. The National Historic Preservation Act (NHPA), enacted in 1966, is one of the federal environmental statutes implemented in the FCC’s NEPA rules. Under the NHPA, federal agencies are required to consider the effects of federal undertakings on historic sites. Commission licensees and applicants must comply with NHPA procedures for proposed facilities that may affect sites that are listed or eligible for listing in the

380

Chapter Seven

National Register of Historic Places. This process includes consultation with the relevant State Historic Preservation Officer (SHPO) or Tribal Historic Preservation Officer (THPO) to whether the proposed facility may create an adverse effect on an eligible or listed historic property. Wilderness is a designation made by Congress pursuant to the Wilderness Act (78 Stat. 890; 16 U.S.C. 1131-1136), which established the National Wilderness Preservation System. The act defines wilderness as “an area where the Earth and its community of life are untrammeled by man, where man himself is a visitor who does not remain; an area of underdeveloped federal land retaining its primeval character and influence, without permanent improvements or human habitation and which is protected and managed to preserve its natural conditions.” Permanent structures, including communication towers and antenna facilities, are prohibited in federally designated wilderness areas. A key purpose of the National Wildlife Refuge (NWR) System is to protect and maintain habitat for migratory birds and other wildlife. The National Wildlife Refuge System Improvement Act of 1997 (105 Stat. 57; 16 U.S.C. 668dd-668ee, as amended) provides guidelines and directives for administration and management of all areas in the refuge system. Any proposal to construct a commercial communication tower or antenna facility within a NWR would require a compatibility determination (65 FR 62457-62483; 50 CFR 25, 26 and 29) before a Special Use Permit would be granted from the Service’s Division of Refuges and Wildlife. The commission requires licensees and tower owners (applicants) to consider the impact of proposed facilities under the Endangered Species Act (ESA). Applicants must determine whether any proposed facilities may affect listed, threatened, or endangered species or designated critical habitats or are likely to jeopardize the continued existence of any proposed threatened or endangered species or designated critical habitats. Applicants are also required to notify the FCC and file an environmental assessment if any of these conditions exist. The U.S. Fish and Wildlife Service (FWS) provides information that applicants may find useful regarding compliance with the ESA. In addition, the FWS has formulated and published voluntary guidelines for the siting of towers, and these are designed to address potential effects on migratory birds. These guidelines and an accompanying tower site evaluation form are posted at the U.S. Fish and Wildlife Service, Bird Issues Website (www.fws.gov/habitatconservation/communicationstower.html)[accessed January 06, 2010]. According to the FWS, the guidelines reflect their judgment of “the most prudent and effective measures for avoiding bird strikes at towers.” Migratory birds are a federal trust resource, protected under the Migratory Bird Treaty Act, and communication towers and other

Project Management

381

tall structures represent a potential collision hazard to migrating birds, especially to species of night-migrating birds. The risk of bird collisions is related to tower height, design, lighting, and location relative to migratory bird concentration areas. Most documented bird kills at communication towers involve tall, lighted structures, and birds migrating at night during inclement weather. During these events, birds attracted by the lights congregate and circle around the tower, with mortality resulting from collisions with guy wires, other birds, and the ground, or from exhaustion. However, occurrences of bird collision mortality at communication towers have also been documented during daytime and fair-weather conditions. 7.3 Logistical and Organizational Challenges Among the key people during microwave network deployment is an onsite project manager, who is responsible for the day-to-day management of all project deployment efforts, including overseeing and ensuring the success of all on-site activities and fulfilling customer requirements within the assigned field location. All on-site personnel (employees and/or subcontractors) report to the on-site project manager during the microwave-network deployment process. The on-site project manager is responsible for ensuring that all onsite personnel understand and perform their assignments, establishing on-site working hours and the chain of command, acting as the liaison between the customer and the deployment team, and serving as the main contact point for the on-site customer representative. From a logistical perspective, travel and accommodation, project office space, equipment warehousing, communications infrastructure, and so on are also very important parts of the project manager’s responsibilities. 7.3.1

Project Controls and Reporting

The project control process will be established and maintained by the project manager through a dedicated project organization and the project plan. The project plan is a comprehensive, detailed plan that describes the process that will be implemented so as to meet project objectives. It includes schedules, procedures, guidelines, and process flow illustrations that form the basic foundation of the project. The project schedule contains a milestone timetable for project implementation and illustrates the sequence of events, the responsibilities, and the duration necessary for the microwave network to become ready for commercial launch.

382

Chapter Seven

The project manager will conduct design review meetings and submit monthly reports with regard to project status, problem areas, the following month’s plan, and any unresolved issues. A project manager usually submits monthly project reports to the customer on technical progress, conformance to the project schedule and/or project plan, equipment delivery status, installation, and test results. This report will be used to state any special requests for schedule changes or changes related to reprioritizing site installations. The monthly report shall include, but is not limited to, project status, problem areas, the following month’s plan, and any unresolved issues. 7.3.2 MW Build-Out Process and Documentation 7.3.2.1 Document Control As with any other large-scale project, there are a number of documents produced during the life of the microwave build-out project (Figure 7.3). Document control should provide the administrative support and control of all project documentation; this should include, but is not limited to, contract and contract change proposals, manual updates, drawing control, specification changes, etc. At the end of the project, the documents that should be supplied are technical manuals for the equipment supplied, one set of interim as-built drawings for each site, final as-built drawings following final acceptance, factory test results, relevant ATPs (Acceptance Test Plans), training manuals (if training specified), etc. It is important to recognize that the process described here is an idealized one and applicable to the average project, but is not always followed to the letter, for one reason or another, in every project. Moreover, this brief overview of the engineering process does not deal with the detailed techniques of microwave system design (described in other sections of this book). Experienced engineers know that different projects occasionally require some modification of the normal design process, and they either wisely suggest it when it is appropriate, or otherwise adapt to clients’ particular needs as best they can. For example, preparation of a customer’s questionnaire document (list of questions given to the client/customer in order to better understand and define the project) and identifying communications requirements in this step drive the entire rest of the process. What points need to be connected, with what capacity, and with what level of performance? There are two extreme scenarios here; the customer input could be, in the best-case scenario: “I need X capacity between points A and B (and C, D, etc.),” and the worst-case scenario: “I have no clue, you tell me,” which is okay since engineers can help a client build the plan.

Project Management

Customer questionnaire

CUSTOMER REQUIREMENTS

383

Customer’s input

MW NETWORK PLANNING

Preliminary systems design

RFQ

Equipment and services RFQ

Preliminary MW network plan

Preliminary BOM (factory notification)

Financial Ts & Cs CONTRACT

SOW (equipment and services)

MW NETWORK DESIGN

Site and path reports

MW NETWORK DEPLOYMENT

Change order form

Responsibility matrix

Detailed MW design (including licensing)

Final BOM and equipment order

Final systems design

Site engineering documentation

Punch list ATP As-built documentation Maintenance plan

Figure 7.3 Document control during microwave project

These two extreme cases, as well as anything in between, have to be dealt with quite differently. The process shown here is designed to work smoothly and efficiently although experience shows that occasionally one or more steps in the process result in a “no go,” and portions of the work have to be repeated and/or resolved in a different manner. That is actually a normal (and necessary) part of the process, and if a plan is eventually going to be determined to be infeasible, it’s better to find out as early as possible.

384

Chapter Seven

The cautions for the reader are as follows: first, if input data is not accurate, work (and money) will be wasted; take the time to avoid the “garbage-in, garbage-out” syndrome, and follow the well known and utilized S-curve for the project. Second, if there are unnecessary “rushes to judgment” and the normal sequence is not followed, the overall engineering costs will likely be much higher than they could have been, as portions of the process are necessarily repeated. Remember that the process sequence is designed to deliver critical information to each successive step. When the steps are taken in order and no roadblocks are encountered, the money is well spent; even if legitimate roadblocks are encountered, the money is still well spent as long as little as possible was spent identifying the roadblock. Document control will provide the administrative support and control of all project documentation. This will include, but is not limited to, contract change proposals, manual updates, drawing control, specification changes, and so forth. At the end of the project, the following documents should be supplied: n

Technical manuals for the equipment supplied

n

One set of interim as-built drawings for each site

n

Final as-built drawings following final acceptance

n

Factory test results

n

Relevant acceptance test plans (ATPs)

n

Training manuals (if training is specified)

7.3.2.2 Importance of Record Keeping The document used for tracking day-to-day activities on the microwave project deployment is usually called the master MW site database. It contains all the relevant information about the site and the equipment to be installed. In a wireless network, each cell site may initially have more candidates (i.e., potential site locations) that, over time, will come down to the one final site. It would be a waste of time and money to perform a microwave path survey on all candidates, but they all have to be included in the list while the path survey is performed on the final two or three candidates. The final site location will be chosen based on RF, transmission/microwave, and site acquisition feasibility. Good record keeping is important because, initially, even a small wireless network of 50 cell sites may have 200 to 300 site candidates and the same number of potential microwave hops. In addition, it is very important to make sure that all the suppliers, consultants, and contractors leave behind a detailed written trail in the form of path calculations and path profiles, drawings, test results, as-built documentation, etc.

Project Management

385

Changes and Modifications During the review of the project deliverables, the project manager and project team may discover elements that were not included in the scope statement but should be. On the other hand, the project manager and the team may discover activities in the scope statement that should be removed. Regardless of the reason, when updating the scope statement, the appropriate stakeholders must be notified of the change and the justification of why the change is being made. During the fast-paced build-out phase, things are sometimes done without a written order or without any written trace of who did the work, who ordered it, and why. An important word of caution here is to avoid the trap of performing the work without the written authorizations; a change notification form, also called a change order, must be issued. On a big project, this could amount to a significant amount of money for which, after the fact, no one wants to take responsibility. A change order must contain at least the following information:

7.3.2.3

n

Project name

n

Change order number

n

Description of change

n

Reason for change

n

Change initiator

n

Signed approval

Contractors and consultants must receive written approval for the deliverable prior to starting the new task or making any changes to the previously defined task. 7.3.3

Outsourcing Services

7.3.3.1 About Outsourcing So far, we have discussed the bidding process and hardware procurement. A very similar process is used for choosing the right supplier of services: engineering, installation, testing, project management, and others. It is not unusual to have different providers for microwave equipment, engineering services, and installation services. Due diligence requires that as much attention be paid to the acquisition of services as to the selection of equipment and suppliers. To ensure that the work is completed according to the highest standards, utilize a company that has established processes and procedures for health and safety, quality, customer service, performance measurement, environmental issues, and training and development. Selecting appropriately qualified contractors, consultants, and engineers usually

386

Chapter Seven

results in good engineering designs and can significantly reduce a project’s life-cycle costs. Rather than merely meeting minimum standards, the services of appropriately qualified engineers and/or engineering consultants can enhance a project’s value to clients through rigorous consideration of alternatives, analyses of long-term operating and maintenance costs, and innovative design. It is therefore in the client’s best interest to use a qualification-based selection method that demonstrates the competence of the engineering consultant in the performance of the required engineering services (design, installation, testing, project management, and so forth). The decision about outsourcing has to be very carefully considered.5 In the case of network design and planning, the company is granting strategic responsibility to an external company, so contract management and continuous control over the outsourcer are absolutely necessary. In highly innovative corporate settings, the risk in outsourcing is very high, due to the potential for revealing proprietary and sensitive information to outsiders. Turnkey solutions to large-scale projects can save money. Using a single point of contact to design, build, and commission a telecommunications network involves many obvious benefits, including lower overall costs, reduced cycle time, regulated quality and safety, and simplified administration and project management. By using a single supplier with project-management abilities, it has been proven that overall costs for a project can be reduced by 15 to 20 percent. The theory is that one can reduce cycle time and duplication by using one company to coordinate all aspects of the job, including resource planning (people, equipment, material management), work schedule, and cost management. This will maintain continuity for the project and therefore reduce costs for materials, equipment, and human resources, as it will not involve any downtime or overlap resulting from bad planning. Clients may either retain comprehensive engineering services from a project’s start to finish or develop a work plan for contracting out specific phases of the wireless network project to various parties. They should determine which alternative is appropriate in a particular situation. A preliminary cost estimate will be only as accurate as the defined scope of a project or problem. In cases where very limited preliminary engineering has been undertaken, this estimate will likely reflect the cost of engineering services contemplated in the scope of the project. In cases where more extensive preliminary design has been completed, and the scope of the project has been well defined, this estimate will likely reflect the total project cost.

7.3.3.2 Turnkey Projects

Project Management

387

It is difficult to provide an accurate estimate of the labor cost (services) for the projects in other parts of the world. The engineering team, installation crew, methods, tools, time frame, and test equipment will depend on system configuration, the scope of provision requested of the vendor, the skill level of the labor, and the nature of the technicians and subcontractors available in that country. The availability of construction materials and machines, the weather, site access, transportation, size of system, and the customer’s work schedule will affect planning and costing of the services. Even political situations in some countries can affect network build-out. Sometimes, it is difficult to provide even typical information, since the conditions differ greatly on a country-bycountry and a project-by-project basis. Turnkey contracts are also growing in popularity among operators, in which big wireless equipment suppliers take full responsibility for entire projects, including the construction (and sometimes even the operations) of new telecom networks. 7.3.3.3 Consultants and Contractors Companies quite often face the challenge of implementing a large and/or complex communications system for which, more often than not, they do not have the proper technical and managerial capacity in terms of experience, education, and available work force. Many of these tasks can be given to contracting and/or consulting companies, and there is a significant trend toward outsourcing in the telecommunications industry today. More and more, operators are realizing the benefits of outsourcing services such as network planning, customer care and billing systems, and construction and operations support systems to third parties. It is important to understand the difference between a contractor and consultant when we talk about microwave engineering projects. These two terms are quite often misunderstood and used interchangeably. If we already have a team of engineers and just need an additional engineer to execute the well-known and repetitive processes (e.g., cellular RF of the microwave link design), we hire a contractor. If we need someone to help us with a complex problem, find a new and unique solution, or establish a team of people to undertake a type of project for which we have no previous experience, we hire a consultant. A contractor can be a junior-level engineer with two to three years of engineering experience, but from the consultant we expect many years of relevant engineering experience. Obviously, expectations as well as the pay rates will be significantly different in these two cases.6 7.3.4 Time and Resource Management

It is important to realize that, for the microwave links, both sites must be ready for installation and, for the ring configuration, all the

388

Chapter Seven

sites comprising the ring have to be ready and available to accept equipment installation before the system can be installed, tested, and commissioned. Large microwave networks usually have ring configuration sites and/ or hub sites that have to be completed before the rest of the network can be built. In other words, in most cases, the more important sites (backbone, ring, and hub sites) and the high-capacity microwave links have to be built first so that spur sites and low-capacity systems can be installed afterward. Some typical times per hop/link required for certain activities during the microwave project are shown in Table 7.1. These times can vary widely from project to project. Site and path surveys usually take a few hours on a short hop in an urban area but sometimes can take up to two to three days on long hops (≥30 mi) in rural areas. Note that testing will last at least 24 hours, plus about two hours to set up the equipment and another two hours to remove the equipment and analyze the test results. If a microwave site is in a rural or remote area, the service access should always be considered, especially during times of inclement weather such as heavy snowfall; moreover, it may even be impossible to access the site for a certain number of months. On the other hand, certain areas can only be accessed during the winter when roads are frozen since the rest of the time the entire area is a soft marshland, inaccessible for heavy vehicles. One of the issues many people tend to forget is microwave radio production time. Of course, it is possible to order two or three hops off the shelf within a few days, but large orders are made to order and have to be scheduled. TABLE 7.1

Time and Resource Management

Activity MW link planning MW link design Site/path survey Manufacturing process Site acquisition MW link installation, including antennas MW link testing and commissioning

Time Required Time Required for Backbone People for High-Frequency People Required Link Required Link 2 hr 2 hr 4 hr 14–60 days

1 1 2 n/a

2 hr 4 hr 8 hr 30–180 days

1 1 2 n/a

10–200 days 8 hr

≥2 2*

10–200 days 40 hr

≥2 4†

4 hr

2

4 hr

2

*

Four people may be required if tower climbing is required.

†

Up to ten people may be required for long waveguide pulls.

Project Management

389

For a network of 100 or more microwave hops, it can take a supplier three to six months to produce and deliver—assuming he is not already overwhelmed with another big order. The good thing about this is that such a large number of links can be installed at a rate of only perhaps two to three per day (depending on the number of installation crews deployed), so radio delivery can be staggered with units delivered directly to the sites according to the project installation plan. Because network design, sales, manufacturing, and installation can be happening in different locations (different countries or even continents), other time issues are involved: n

Manufacturing time

n

Manufacturer to port of entry time

n

Customs clearance time

n

Port of entry to warehouse or directly to the site time

Mobilization and resource management are very important issues, and they can be very different and specific during the microwave project. For example, let us say that equipment, engineering, installation, project management, and other costs (turnkey project) were estimated at $100,000 per hop for 20 backbone microwave hops. The total cost would therefore be $2M for the entire project. At the last moment, the client decides to build only 10 hops and use leased T1/E1 lines in the rest of the network. The quick assumption is that the project cost would be half of the initial one, i.e., about $1M. Unfortunately, things are a bit more complicated than that, since the initial quote included certain equipment (including NMS) discounts, which will be reduced for a smaller number of hops. On the other hand, the engineering and installation crew’s mobilization costs, project management costs, and so forth really change very little with the number of hops. It is therefore reasonable to expect an actual cost of $1.1 to 1.2M. For projects in different countries, it is important to be well informed about their local holidays and vacation times. For example, in many European countries, July and August are summer vacations months (and senior stuff members can have up to six weeks of vacation per year), during which very little work progress can be achieved, unless the activities are scheduled in advance and agreed upon by all the parties involved. 7.3.5

Project Management Tools

A number of different project management software tools are on the market (e.g., Microsoft Project), although consulting and project management companies quite often use their own specialized software tools,

390

Chapter Seven

which often are customized for the unique requirements of a communications network. They facilitate tracking and scheduling of the network build-outs and serve as an information storage tool that is used for the network build-out programs. The three main areas that these tools need to address are as follows: n

n

n

Scheduling tracking and reporting at the client, management, and control level Collecting and providing information related to managing the deployment of a large telecommunications network Integrating cost with the schedule to provide cost control and cost forecasting

The scheduling tool must indicate the site development status using the following items: n n

n

Milestones An operating company/client level for reporting progress Activities A project management/project control level for analyzing and forecasting site design, acquisition, and construction activities Tasks A control level checklist used by the disciplines to identify and provide the status of the detailed work tasks required for site completion Site information is collected and recorded by these functional areas:

n

Transmission/microwave engineering

n

Real estate site acquisition and zoning

n

Architectural

n

Building permitting

n

Construction

n

Power/Telco (utilities) coordination

n

Geotechnical

n

Land survey

n

Procurement

Report generation is one of the most important features of every project management tool. Usually, there are a number of different reports to choose from, and a number of customized reports are generally also available. Some of the most commonly used reports in microwave network build-out are the milestones list, activity Gantt chart, activity status report, activity duration report, and activity completion count by week.

Project Management

391

7.3.6 The S-Curve

Closure phase

Execution and controling phase

Planning phase

Initiation phase

The S-curve is a tool for helping with the conceptual understanding of the project and its progress. The S-curve is a display of cumulative costs, labor hours, or other quantities plotted against time/effort. The name derives from the S-like shape of the curve, flatter at the beginning and end and steeper in the middle, because this is how most of the projects look. S-curves allow the progress of a project to be tracked visually over time, and form a historical record of what has happened to date. Analyses of S-curves allow project managers to quickly identify project growth, slippage, and potential problems that could adversely impact the project if no remedial action is taken. As seen in Figure 7.4, the S-curve models progress, as well as other quantities of interest, against time. Early in the life of the project the team is building and learning to work together (Curve A in the figure). Once the team has stabilized it can begin to work more efficiently and the curve begins to accelerate rapidly. Toward the end of the project, work activities slow as the project is brought to successful completion and closure, representing the ideal scenario in the life of the project. Curve B shows a project plan that is too aggressive (a common mistake on large projects) with too much work scheduled too early in

Project Completion

100% 3/4 of the work completed in 2/3 of the time

75% B

A

C

50% 1/4 of the work completed in 1/3 of the time

25%

33% Figure 7.4 S-curves

66% Time

100%

392

Chapter Seven

the project, before the team has even formed and begun to work together. This causes mistakes to occur and requires rework and changes. In addition, the staff could become burnt out at the beginning of the project, which is never a good thing for the project but can be especially detrimental for the project if it happens at the very beginning. Curve C shows that too little work is accomplished in the beginning and the pressure increases as the deliverable dates approach. Both B and C types of the S-curve should be avoided. Typically, most projects fail at the beginning, not the end, especially projects that are running behind schedule. The importance of planning a project is never as evident until the rush to completion. The final actions to complete a project are dependent on the plans and motivations set in the project planning processes. 7.4

Ethical Issues

7.4.1

Code of Ethics

Ethics can be defined as a discipline that deals with what is good and bad, as well as with the moral duty and obligation; it can also be regarded as a set of moral principles or values. Morality is a doctrine or system of moral conduct. Moral conduct refers to principles of right and wrong in behavior. In a sense, then, we can think of ethics and morality as being so similar to one another that we may use them interchangeably to refer to the study of right and wrong behavior in business. Concepts of right and wrong are not simple; today, they increasingly include the more difficult and subtle questions of fairness, justice, and equity. Licensed engineers in North America are required to adhere to certain ethical principles in conducting their day-to-day activities. The author of this book is a firm believer that not only must the ethical aspects of engineering be applied by the licensed engineers, they also should be taught in schools and colleges as a part of any profession and curriculum. One might ask, “What is a code of ethics?” For example, a code of engineering ethics for Canadian licensed engineers7 (a basic guide to professional conduct) states that “…it is the duty of a practitioner to the public, to the practitioner’s employer, to the practitioner’s clients, to other licensed engineers of the practitioner’s profession, and to the practitioner to act at all times with: n

n

Fairness and loyalty to the practitioner’s associates, employers, clients, subordinates and employees Fidelity to public needs

Project Management

n n

n

393

Devotion to high ideals of personal honor and professional integrity Knowledge of developments in the area of professional engineering relevant to any services that are undertaken Competence in the performance of any professional engineering services that are undertaken”

Engineering societies in other countries have similar codes of ethics, while Canada is one of the few countries that actually have an exam in engineering ethics. Through the code of ethics, licensed engineers have a clearly defined duty to society, which is to regard their duty to public welfare as paramount, above their duties to clients or employers. Their duty to employers involves acting as faithful agents or trustees, regarding client information as confidential and avoiding or disclosing conflicts of interest. Their duty to clients means that professional engineers have to disclose immediately any direct or indirect interests that might prejudice (or appear to prejudice) their professional judgment. Licensed engineers are obligated to give proper credit for engineering work, uphold the principle of adequate compensation for engineering work, and extend the effectiveness of the profession through the interchange of engineering information and experience. As co-workers and supervisors, licensed engineers are required to cooperate on project work and must not review the work of other licensed engineers who are employed by the same company without the other’s knowledge. They also must not maliciously injure the reputation or business of other practitioners. 7.4.2 Practical Examples of Ethical Dilemmas

We mentioned some of the basic definitions of a code of ethics, but the question is, “What does that really mean in everyday life and work?” In these electronic times, it is very easy to copy someone else’s work and use it without giving credit (on purpose or by mistake) to the author of the original document. In addition, one person taking credit for the work of a team of people, and never acknowledging others’ contribution to the work and common goal, is very common in today’s workplaces. There are a number of examples from engineering practice that illustrate that a particular practice, although legal, may not necessarily be ethical. Sometimes it is hard to distinguish what falls in what category. For example, what if an engineering company plans to hire consultants for a certain job to be done on their behalf, and they decide that the best person to perform work is the wife of one of the executive directors

394

Chapter Seven

of the engineering company? If we assume that she is perfectly capable and highly qualified to do the work, is there conflict of interest that may prevent the engineering company from hiring her? On the other hand, by not hiring her, they may be losing the best person for the job. This very common dilemma shows up in different shapes and forms in everyday life and work. The following example is derived from the microwave engineering field. It is a common knowledge that in North America (U.S., Canada), only qualified and certified people are allowed to climb communications towers. They have to be trained, licensed, and bonded riggers, equipped with safety devices, harness, helmet, boots, and other appropriate equipment to be allowed to climb a tower. Working in a third-world country on a microwave deployment project, the lead engineer notices that the riggers who are installing antennas and cables have no training, and that they are climbing towers barefoot and without any protective and safety devices. What should be done under these circumstances? The engineer could n

n

n

Ignore the situation and be happy with the reduced costs for training, safety devices, and proper wear. Stop the work immediately, notify superiors (called “duty to report”), and continue only after the situation has been rectified. Quit the job and leave the country immediately.

Readers will not have a problem determining the correct course of action. The engineer has to respect the value of human life and make every effort to ensure its protection. In many cases, companies are concerned not only about human lives but also about potential legal issues and liabilities that come with accidents and the loss of human life. Engineers have to make sure that working conditions are safe and thereby protect not only the interests of the employees but also the company itself. 7.4.3

Ethics, Quo Vadis?

When a decision is made about what is ethical (that is, right, just, and fair) using the conventional approach, three key elements go into such a decision: n

n

We observe the behavior, act, or practice that has been committed or is about to happen. We compare the practice with prevailing norms of acceptability, that is, society’s or some other standard of what is right or wrong (Figure 7.5).

Project Management

n

395

We must recognize that value judgments are being made by someone with his or her personal opinion and bias toward the judgment outcome. This means that two different people could look at the same behavior, compare it with their concepts of what the prevailing norms are, and reach different conclusions as to whether the behavior was ethical or not. This becomes quite complex as perceptions of what is ethical inevitably lead to the difficult task of ranking different values against one another.

Generally speaking, members of society generally agree at a very high level that certain behaviors are wrong. However, the consensus tends to disintegrate as we move from the abstract to specific, real-life situations. If we can put aside the fact that we differ among ourselves (and our view of the events) because of our personal values and philosophies of right and wrong, we are still left with the problematical task of determining society’s prevailing norms of acceptability of the certain behavior, i.e., what metric to use as an acceptable norm. Decision making usually entails the process of stating the problem, analyzing the problem, identifying the possible courses of action that might be taken, evaluating these courses of action, deciding on the best alternative, and then implementing the chosen course of action. Once we leave the realm of relatively ethics-free decisions, decisions quickly become complex, and many carry with them an ethical dimension.

Existing norms of acceptability

Comparison

Behavior or act that has to be evaluated

Observer

Value judgments and perceptions of the observer Figure 7.5 Making ethical judgments

Ethical judgment

396

Chapter Seven

According to LaRue Hosmer,8 there are five important points about the character and nature of ethics and decision making: n

Most ethical decisions have extended consequences.

n

Most ethical decisions have multiple alternatives.

n

Most ethical decisions have mixed outcomes.

n

Most ethical decisions have uncertain consequences.

n

Most ethical decisions have personal implications.

Ethical decision making is not a simple process but rather a multifaceted process that is complicated by the characteristics just described, and there are no simple formulas. The most serious danger is that of falling into an ethical relativism where we pick and choose which source of norms we wish to use based on what will justify our current actions or maximize our freedom. A good ethical decision-making process requires the ability to explore all the aspects of a decision and then to weigh the options surrounding a course of action. As a simple test, ask yourself these questions and try to answer them honestly: n

Is the action under consideration legal?

n

Does the action comply with our society’s values?

n

Is the action right/fair/appropriate?

n

n

If you had to explain your decision on television, would you be comfortable doing so? How will I feel tomorrow about myself?

The sets of questions posed here are intended to produce a process of ethical inquiry that is of immediate use and understanding and help ensure that ethical due process takes place. They cannot tell us whether our decisions are ethical or not, but they can help us be sure that we are raising the appropriate issues and genuinely attempting to be ethical. The generally accepted view of ethics is that ethical behavior resides above behavior required by the law; however, in many respects the law and ethics overlap. To appreciate this, we need to recognize that the law embodies notions of ethics, i.e., the law may be seen as a reflection of what society thinks are minimal standards of conduct and behavior. Both law and ethics have to do with what is deemed right or wrong, but law reflects society’s codified ethics. In spite of this overlap, we continue to talk about desirable ethical behavior as behavior that extends beyond what is required by law.

Project Management

397

Viewed from the standpoint of civilized society, we would certainly say that obedience to the law is a minimum standard of behavior. In other words, what is illegal is definitively also unethical, but what is unethical is not necessarily illegal. Quite often we hear the statement: “Well, legally speaking, this was fine.” However, we should aspire to something higher than just “barely legal” and, most likely, very questionable from the ethical point of view. Instead, the question we should ask ourselves and the society we are living in is whether we are doing the “right” thing. The question is whether our society is becoming less ethical (as it may seem to some people) than, for example, 60 years ago? One of the explanations for why this may be so is that our expectations and awareness due to the modern communications media are much higher today than in early 1950s. So, transparency of what we and companies in our city, state, country, or even globally are doing plays a big role in how we see and judge the world around us. Ethical statements about the company should not be a marketing logo or a selling point in a glossy brochure; instead, they should flow naturally from the company’s behavior, starting with the ethics toward employees, and further extending toward the company’s partners and clients as well as towards other people. 7.5

Frequently Asked Questions

This section will present and attempt to answer a collection of questions I received over a number of years while working in the microwave field.‡ Some of the questions are a reoccurring theme, and they keep repeating in different forms and in different situations. 7.5.1

General Questions

1. Are MW links expensive to deploy? Not necessarily; the cost of point-to-point (PTP) microwave links can vary and will depend on a number of factors (distance, capacity, configuration, etc.). In most cases it will prove to be a cheaper solution than leasing multiple T1/E1 circuits in just over a year. 2. Is leasing T1/E1 circuits much faster than designing, installing, and licensing microwave links?

‡

Some of these questions were discussed in technical papers I wrote for Communications Infrastructure Corporation in 2008.

398

Chapter Seven

Not any longer; the split configuration MW radio (with indoor and outdoor unit) is very easy to install, and the licensing process can be accomplished in a matter of days (much faster than what it used to be). On the other hand, dealing with local carriers to lease T1 circuits can be a very time-consuming process and may require time and expenses due to the additional installation requirements (that is, construction charges). 3. Is leasing T1/E1 circuits an option in the next generation (4G) of wireless networks? No; many predict requirements of over 30 Mbps per cell site in 4G networks. This means that, from a leasing prospective, you would have to lease an equivalent of DS3 to every cell site, which is a very unlikely scenario. Some carriers are constructing fiber facilities all the way to the cell-sites, making that another feasible option in addition to microwave links. 4. What is a “millimeter wave” band? A millimeter-wave band is defined as a 30–300-GHz band. 5. What is a National Radio Quiet Zone? There are some areas in the U.S. where some or all wireless communications are prohibited. The Radio Quiet Zone in Western Virginia is home to the National Radio Astronomy Observatory’s Green Bank Telescope, capable of receiving the quietest radio signals in the universe. The telescope is so sensitive that only diesel-fueled cars are allowed on the property because even spark plugs emit radio frequencies that can wipe out the signals from space. The Zone also provides electromagnetic protection to a secretive Navy research facility at Sugar Grove that monitors communications here on Earth. The FCC has set up a similar radio-coordination zone in Puerto Rico to protect a telescope in Arecibo. Not all radio transmissions are prohibited in the Radio Quiet Zone; exceptions to these restrictions are usually determined on a caseby-case basis, with preference given to public safety concerns. Europe also has a number of radio quiet zones set up in different countries with individual legal and management arrangements. 6. Are free-space laser communications systems a competition to microwave systems? No. Free-space laser communications systems are wireless connections through the atmosphere using the optical part of the frequency spectrum and therefore cannot be categorized as either wireless or a wireline system in a classical sense. They work only under clear line-of-sight conditions between each unit, eliminating the need for

Project Management

399

securing rights of way, buried cable installations, and government licensing. Rain and snow can cause attenuation of up to approximately 40 dB/km and 100 dB/km, respectively, while in an extremely heavy fog attenuation as high as 300 dB/km has been reported. Microwave systems are typically not affected by snow or fog. 7. Should we use path surveys or just try to establish a line-ofsight? That will depend on the situation; for example, in a city where mostly rooftops are used for the wireless sites, LOS using mirrors and/or balloons could be sufficient (and recommended) method for links below a few miles in length. In urban and suburban areas where there are lots of trees and links could be longer, detailed path survey and obstacle height measurements are required. On a 30-mile backbone (high-capacity) links, path survey is the only option. Again, there is an exception to that rule as well, for example, links in the desert or over-the-water links, assuming we can climb high enough to overcome the earth bulge. 8. Is it possible to install microwave equipment on a flag pole or a light pole? It is possible, although that is not a preferred solution; it can only be done if the twist and sway (i.e., how much the structure moves under high wind conditions) is sufficiently low for that antenna size and frequency of operation. Some flag and light poles are specially designed and manufactured for carrying cellular and/or microwave equipment. 9. Are microwave links hazardous to our health? No, unless you are directly exposed to microwave radiation (standing directly in front of the antenna for an extended period of time); microwave antennas are typically located high above the ground, and due to the high directivity of the microwave dish antennas, the intensity of radiation on the ground is negligible. For more information see the safety bulletins and publications at: www.fcc .gov/oet/rfsafety/ (accessed Oct 13, 2009). 10. What is a typical price of the installed microwave link? It is very difficult to define a typical price of the microwave link. Prices can vary anywhere from a few thousand dollars to hundreds of thousands of dollars. It will depend on the radio capacity, path length, tower requirements/modifications, diversity scheme, configuration/protection, special requirements and custom manufacturing (custom painting of the antenna, for example), etc.

400

Chapter Seven

11. Is the cost of a turn-key project for five links in the same geographical area five times more than the cost of one link? No, it is probably significantly less, assuming all five links are designed and installed at the same time; the reason is that engineering costs, project management costs, field crew mobilization costs, etc., do not change that much between one and five links. 12. What is media diversity? Important microwave links can be protected by using completely different media; most commonly used are fiber-optic systems. In addition, the opposite is true and sometimes fiber-optic systems are protected by the high-capacity microwave links. These solutions are very expensive and have to be considered only in exceptionally important cases. 13. What type of transmitter power data is required for the link engineering? Typically, you need an average transmit power. The manufacturers may sometimes specify the maximum peak envelope power emitted from the transmitter or delivered to the antenna. Using the peak output power results in overstating the transmit capabilities of the radio and therefore overestimating the link performance. 14. Can an experienced microwave engineer determine a tower’s allowed loading and twist and sway just by looking at the tower? No. It is very important to use expert tower construction company to calculate the loading of the tower and maximum allowed twist and sway of the structure. In most cases, these conclusions cannot be made just by looking at the tower. 15. Does the microwave link availability calculation include equipment? Not usually. Availability is the percentage of time that the link will be operational. For wireless links, this is generally considered to be exclusively due to rain outages (propagation) and does not usually include equipment failures. An additional analysis may be required to account for availability limitations due to equipment failures. 16. In microwave link design, are “protection” and “diversity” the same thing? No. The terms “protection” and “diversity” are often incorrectly used interchangeably when applied to microwave links. This is not

Project Management

401

correct since protection commonly improves long-term traffic interruptions (10 CSES [consecutive severely errored seconds] or more), while diversity arrangements greatly reduce the number and duration of short-term outages (less than 10 CSES). 17. I am designing a microwave link in a country that does not have very strict control or records of the installed microwave systems; what should I do to make sure there is no interference? In many cases, the most reliable information on the potential interference cannot be received by calculations since there is little or no information on the existing terrestrial or satellite systems in the area (often the case in places outside North America and Europe). The best way is to sweep the entire spectrum using test equipment at the future microwave-system antenna location. Measurements at the proposed antenna centerline will ensure that we will find out not only a frequency but also a direction of the interferer. The result will be a spectrum analyzer plot showing all potential interference in the applicable band. This method, unfortunately, can only ensure interference-free operation at the time of the installation of the link; it cannot ensure that the interference problem won’t appear at some later time due to someone else’s negligent installation of a new link. 18. What is an XPIC and when do I need it? With dual polarity transmission, two signals are transmitted, i.e., one with horizontal polarity and one with vertical polarity. This way maximum link capacity will effectively double for a given frequency channel. The main challenge with this kind of configuration is cross-polarization interference, where energy from one polarization is received in the other. An XPIC (cross-polarization interference canceller) is an adaptive coupling electronic circuit between two orthogonal cofrequency channels or two alternated adjacent channels on the same link, used to reduce cross-polar interference during adverse propagation conditions. 19. What is pseudo-wire? Pseudo-wire (PWE) services enable existing TDM and ATM traffic to be combined with Ethernet traffic and mapped onto a single transport facility. The term circuit emulation, which originally referred to transport of TDM traffic over cell-based ATM networks, is now often used synonymously with TDM pseudo-wire. Circuit emulation is useful when a 4G base station (WiMAX, for example) is sharing a site with an existing GSM/UMTS/CDMA network.

402

Chapter Seven

It enables operators to reduce backhaul costs by combining multiple traffic types onto a single lower-cost facility and to continue to use legacy interfaces on existing 2G/3G base stations. Some microwave equipment manufacturers offer pseudo-wire service in the form of an additional external termination box connected to the microwave radio. As IP, MPLS, HFC, and Ethernet are essentially packet-based rather than circuit-oriented transmission technologies, the pseudowire service must emulate the characteristics of circuit-switched connections while utilizing the packet-based infrastructure. Pseudo-wire must emulate the transparent nature of circuitswitched connections, both for data and timing, in order to provide compatibility with today’s applications, both voice and data, and the customer premise equipment deployed to support them. Doing so requires that a number of challenges be met: we must minimize end-to-end latency and ensure QoS, optimize bandwidth utilization, manage jitter and accommodate packet delay variation, maintain clock synchronization between the two endpoints, and maintain time slot ordering and preserve DS0 alignment. 20. What is “residual BER” and what is its importance for the microwave link? Very weak signals create many bit errors, and as received signal strength increases, the error rate will fall to a very low level, or “error floor.” This error floor is called the “residual” bit error rate or “residual BER.” Residual BER measures the combined effect of the digital radio’s modulator, transmitter, receiver, and demodulator. In essence, it is a single measure of a radio link’s hardware quality. On the other side, as received power increases, the receiver will ultimately reach an overload point where the error rate again increases quickly. 21. What are the consequences of ice buildup on the antenna? Ice buildup on antenna elements will result in an increased SWR (impedance mismatch) that will detune a transmitter system, significantly reducing its output power. Ice can also cause severe transmission line damage, and falling icicles can kill. The easiest way to prevent ice buildup is with special antenna heaters or by covering the antenna system with a fiberglass radome. Unfortunately, radomes will increase the wind load on the tower, while antenna heaters can be expensive and affect the antenna radiation pattern.

Project Management

403

22. What is the frequency coordination? Frequency coordination is a bilateral process that involves the cooperative sharing of technical operating information between parties utilizing the same spectrum. In the U.S., the procedures are based upon the Federal Communications Commission’s (FCC) coordination and licensing requirements found in Rule Part 101 (in 1996, the FCC adopted Rule Part 101, which combined the private and common carrier point-to-point frequency bands under one consolidated Rule Part), as well as related industry practices that have evolved over the years. The frequency coordination process involves several distinct but interrelated elements: interference analysis, notification, and response. 23. What is QRSS and is it sufficient to test the T1 circuit? Whether public or private, T1 circuits and network equipment must be properly tested and maintained to perform to maximum efficiency. For many T1 transmission systems, the bit error rate tests use the QRSS (quasi-random signal sequence, an imitation of the live traffic) pattern, but it is recommended to use some of the stress test patterns as well. Stress patterns will detect B8ZS and AMI mismatch along the T1 circuit, which would not be detected by using just the QRSS test pattern. Mixed T1 circuits containing leased lines, fiber-optic circuits, and microwave links, and tested end-to-end, should also be tested longterm (over 24 hours) and with the multiple test patterns. 24. What is “homologation”? Equipment is very often sold and installed in countries other than country of manufacturing. These countries may have different functionality and safety requirements as well as frequency allocations and channeling plans for wireless services (cellular, PCS, microwave, satellite, etc.). In order to sell their products in other countries, suppliers must go through a certification process (homologation). In many cases, if equipment is approved by the FCC and meets certain minimum requirements, it is considered to be certified. In some countries, government regulations are used to exclude or delay foreign manufacturers from their markets. This is done by requiring suppliers to meet very stringent (and sometimes completely meaningless) requirements and standards and quite often to customize their hardware design. While this is not necessarily technically demanding, certification and homologation could be a time-consuming process, in some countries taking from a few months to more than a year.

404

Chapter Seven

25. Do I need to register my tower? That will depend on a number of factors. Typically, in the U.S., owners of antenna towers that are taller than 200 ft (60m) above the ground level, or that may intersect the flight pathways of a nearby airport, must register the structure with the FCC and have the structure’s placement studied by the Federal Aviation Administration (FAA). Previously, antenna registration was a duty of the antenna site licensee, but now (since 1995) it is the owner of the real property who is accountable. 26. Can a guyed tower be used in the urban/suburban areas of the city? Not usually. Generally, there are two types of towers, guyed and self-supporting. Each type has different limitations regarding its capacity, stability, and versatility. Guyed towers are slender, generally three- or four-sided lattice construction and are uniform in dimension over their length. They are maintained in their vertical position by guy wires, which are attached at various levels on the tower and anchored to the concrete blocks on the ground. Under normal circumstances, a guyed tower requires a radius of 80 percent of the height of the tower. This physical characteristic can require a large piece of land to guy a single tower, and so they are usually used in rural areas only. 27. What is “rusty fence” syndrome? Rusty fence syndrome is an intermodulation on the nonlinear external elements; for example, when two high-powered transmitted signals impinge on some random rusty element such as a steel fence, a rusty metal roof, or even corroded coaxial cable elements or connectors, an electrochemical effect sometimes takes place. The corrosion junction acts like a rectifying diode and mixes all the transmit signals hitting it. This results in a whole list of new signals, called intermodulation products, which are retransmitted. While this effect is typically a random problem (and not very common in microwave point-to-point links), when two signals have an exact frequency spacing equal to the affected receiver’s input, it may receive and accept these retransmitted intermodulation products as its own in-band data. 28. Can I use electric transmission towers to install a microwave system? Yes, partnering with the local utility company could be a good move but could lead to a lengthy approval process for number of different reasons. Some utilities have resisted the idea of adding

Project Management

405

wireless infrastructure development to their workload because of the potential effects on the electric transmission and distribution (T&D) system. Installing antennas on electric transmission and distribution equipment affects wind resistance and torsion stresses on the towers. The design of these structures must account for conditions such as conductor weight, wind loading and ice loading, bracket design, attachment methods, and dead-weight loads. 29. There is a high-powered TV broadcast station close to my planned microwave system, but they use different frequency band. Should I still worry about it? Yes. When a high-powered transmitter, such as a UHF TV broadcast station, is nearby, the affected receiver can be driven into RF overload even though its signal is well out-of-band. The high power signal leaking into the affected receiver will drive the operating point of the front-end amplifier up through its dynamic range characteristic. This destroys the normally required linear amplification process, introducing intermodulation distortion and serious data errors. 30. What is an exclusion zone radius for the wind turbines in the vicinity of my microwave link? There has been very little research done on this topic, which is becoming more interesting as more and more wind farms are being constructed. Obstruction or reflection of radio waves by a wind turbine can degrade the performance of a point-to-point microwave link due to the effect of large blades rotating at approximately 32 rpm (typically there are two or three blades). Thus any significant interfering signal, such as a delayed multipath component, will fluctuate in signal level around 1.0 to 1.5 Hz. The first recommendation is obvious: avoid the near field, as with any other objects. The second recommendation is to keep the first Fresnel zone clear. The third one is that even a clear first Fresnel zone may not be sufficient, so a more stringent requirement of also keeping the second Fresnel zone clear may be implemented. 31. What is the best and most reliable network topology for my microwave network? A ring configuration is by far the most reliable way to connect sites in the network. In large networks it would be impractical, and almost impossible, to connect all the sites in the ring, so typically some kind of combination of ring and star/hub topology is implemented in real-life networks.

406

Chapter Seven

32. What is “antenna concealment”? Antenna concealment is the process of screening, hiding, or “camouflaging” antennas. It can be used to satisfy a municipality or building owner’s need to minimize the visual affect of an antenna installation by disguising it as a flagpole, clock tower, church steeple, rooftop structure, palm tree, pine tree, or any other conceivable design that helps the site visually blend into its environment. 33. How can the capacity of the digital microwave systems be increased? To increase the capacity of digital microwave systems, engineers use a number of advanced techniques; high-level modulation techniques, XPIC, new efficient correction coding techniques (like TCM, MLCM, etc.), antifading techniques like diversity, ATP, adaptive modulation, equalizers, etc. 34. What microwave design tool should we use for the microwave network design? For the proper planning of terrestrial line-of-sight systems, it is necessary to have appropriate and widely accepted propagation prediction models, methods, and data. Methods have been developed that allow the prediction of some of the most important propagation parameters affecting the planning of terrestrial line-of-sight systems. Most microwave network design software tools are developed by radio manufacturers and therefore biased toward their own equipment, or the tool is proprietary and not for the sale on the open market or very expensive to purchase. Choose one of the vendor-independent and widely adopted tools, such as Pathloss 4 or its newest version, Pathloss 5. 35. Where can I get additional practical information about microwave point-to-point systems and transmission networks in general? For additional practical information, refer to another one of my books, Transmission Systems Design Handbook for Wireless Networks, Artech House, 2002, ISBN 1-58053-243-8. 7.5.2

Maps, Datum, and GIS Data Questions

1. What is the difference between true north and magnetic north? True north is used on engineering documents, but unfortunately the magnetic compass does not normally point to true north. In fact, over most of the Earth, it points at some angle east or west of true

Project Management

407

geographic north. The direction in which the compass needle points is referred to as magnetic north, and the angle between magnetic north and the true north direction is called magnetic declination. The terms variation, magnetic variation, or compass variation are often used in place of magnetic declination. It is important to remember that the magnetic declination does not remain constant in time. 2. What is the difference between NAD 27 and NAD 83? Geodetic datum (from Latin, singular for data, given things) defines the reference systems that describe the size and shape of the Earth. Most maps are made based on a datum (horizontal and vertical), which is the origin or reference point from which all points on a map are measured. Several different datums have been used to develop maps; however, commonly used datums include the North American Datum of 1927 (NAD27), the North American Datum of 1983 (NAD83), and the World Geodetic System of 1984 (WGS84). The NAD27 is based on the Clarke ellipsoid 1866 and uses stone in Meades Ranch, Kansas as a reference point, while the NAD83 Geodetic reference system is based on satellites. All North American older topographical maps are based on NAD27. 3. How can I convert geographic coordinates from NAD27 to NAD83? In the U.S., for licensing purposes, geographic coordinates provided to the FCC via the Universal Licensing System (ULS) must be referenced to the North American Datum of 1983 (NAD83). If the source, from which the coordinates were obtained, is referenced to another datum (e.g., NAD27, PRD40), they must be converted to NAD83. Mathematical models and formulas for conversion are between datums are very complex. The online and downloadable tools for coordinate conversion to NAD83 can be found at http://wireless.fcc .gov/uls/index.htm?job=nadcon (accessed August 25, 2009). 4. Where can I find free detailed building and clutter data for my project? The terrain data can range from a minimum terrain data, which is available for the whole world via freely available datasets (Globe, GTOPO30, SRTM), to expensive detailed building and clutter databases. Even these expensive databases will be available only in areas where someone has identified a market opportunity sufficiently strong to justify the dataset creation costs, typically downtown areas of the major metropolitan cities.

408

Chapter Seven

5. What is the consequence of referencing geodetic coordinates to the wrong geodetic datum? Hundreds of different datums have been used to frame position descriptions since Aristotle made the first estimates of the Earth size, and datums have evolved from those describing a spherical Earth to ellipsoidal models derived from years of satellite measurements. Modern geodetic datums range from flat-Earth models used for plane surveying to complex models used for international applications, which completely describe the size, shape, orientation, gravity field, and angular velocity of the Earth. The datum is important for Geographic Information Systems (GIS) and GPS applications to ensure consistency of map data. When using a GPS receiver, the datum must be set to match the horizontal datum on the map. If the datum does not match, there will be errors when plotting data on a map. Referencing geodetic coordinates to the wrong datum can result in position errors of hundreds of meters or worse. 6. Can I use old topographical maps for the path profile and the microwave link design? No. Use updated maps not more than a year old. Magnetic declination as well as the terrain itself can change significantly in a very short time period. Make sure that everyone on the project is using the same maps, datum, and coordinate systems. The datum or reference ellipsoid selection must be the same for the site data, image, and elevation files. The same map projection must be used for the image and elevation files. 7. Why are my GPS positions in the wrong place when I use satellite imagery? In the 3-D (three-dimensional) terrain the image of the terrain is stretched over the terrain. The terrain is most likely not detailed enough to be an exact match, so the photo will be stretched differently. For example, Google Earth uses 30m terrain data for the U.S., but 90m data for most of the rest of the world. So in areas of extreme topography (mountains and canyons, for example), images won’t be “stretched” properly to fit the terrain. In addition, the standard elliptical WGS84 model is too simplistic, and trying to wrap data in the geographic information on such a simple model of the Earth’s shape will introduce additional errors. Users should not have much faith in the precision of satellite imagery for absolute positioning of objects and/or detailed path engineering.

Project Management

409

However, the positioning of objects relative to each other could be precisely determined; for example, the distance from the communications tower to the equipment building. 8. Can we use maps instead of path surveys? No. Perform LOS or path surveys on all microwave links. No matter how good and accurate maps are, they are only used for initial planning as a first approximation. Even if you have a very accurate (and expensive) clutter data, you may need to perform path surveys and/or establish LOS. 9. What is the best resolution for the terrain elevation data that I can find today? USGS (U.S. Geological Survey) is a multi-disciplinary science organization that focuses on biology, geography, geology, geospatial information, and water. The USGS 10 Meter DEM (Digital Elevation Model) data consists of a grid of elevation values posted every 10 meters (actually, every 1/3 arc second). The USGS has released this very accurate terrain data for a large portion of the U.S., especially in the western states. The contours produced from this data follow almost exactly the original contours from the USGS paper 1:24,000 scale Topo maps. Terrain elevation data can be downloaded free of charge from The National Map Seamless Server: http://seamless.usgs.gov/index .php (accessed January 07, 2010). 10. Where can I find the current value for the magnetic declination at a certain location in North America? One of the possible sites providing a magnetic declination calculator is at http://geomag.nrcan.gc.ca/apps/mdcal-eng.php (accessed January 07, 2010). 7.5.3

Radio-Propagation Questions

1. What happens with the microwave link when it rains heavily or during hailstorm? Adverse weather conditions do not affect RF frequencies below 10 GHz (in a temperate climate). Above 10 GHz, the influence of rain can easily be included in the path design. The effects of hail on radio connections are first apparent when hail particle sizes approach the size of radio waves; for example, 150 mm (2 GHz), 9.6 mm (31 GHz), and 6 mm (50 GHz). Fortunately, hail particle sizes greater than 10 mm are not very common and therefore hailstorms cannot be considered an availability limiting factor.

410

Chapter Seven

2. Can rain ever affect microwave links at frequencies below 10 GHz? In most parts of the world rain only starts affecting microwave links above 10 GHz; however, there have been recorded cases in the subtropical regions (and some other areas of limited size) where rain of extreme intensity did affect microwave links at as low as 8 GHz and even 7 GHz. The main factor in calculating the rain attenuation is the raindrop size distribution function. 3. Does fog and snow affect microwave links the same way the rain does? No, effects of snow and fog on microwave propagation are usually negligible. Only on paths at high latitudes or high-altitude paths at lower latitudes can wet snow cause significant attenuation. The attenuation caused by dry snow can be considered as negligible for frequencies below 50 GHz. In addition, heavy snow can affect antenna loading and/or damage microwave antenna feeders, but that is a different type of problem. Attenuation due to fog becomes significant only at frequencies above 100 GHz. More detailed information on attenuation due to hydrometeors other than rain is given in ITU-R Recommendation P.840. 4. Does the presence of excessive smog or pollution have any effect on microwave links? Smog, pollution, and fog are important factors in designing optical links but are negligible in most terrestrial microwave links. See ITU-R P.840 for more information. 5. What happens when a bird flies through my line-of-sight link? Nothing; radio communications rely on propagation through what is known as “Fresnel’s zone.” This can be described as a virtual ellipsoid and, as long as 60 percent of this ellipsoid is visible (free of the obstructions) at either end, the link will not be affected. 6. What is multipath and when do I have to worry about it? Reflection occurs when an electromagnetic wave strikes a nearly smooth large surface, such as the water surface, and a portion of the energy is reflected from the surface and continues propagating along a path that makes an angle with the surface equal to that of the incident ray. Reflection rays from different surfaces may interfere constructively or destructively at a receiver causing multipath propagation.

Project Management

411

In microwave point-to-point systems, multipath can affect microwave links below 15 GHz and can be a serious problem at frequencies below 10 GHz. 7. We work in a rented building and the landlord will not permit us to mount the antenna to the outside structure. Can we still have a microwave link? Yes, although this is not a preferred practice for microwave link installation. Windows will attenuate RF signals very slightly, but a good signal level and/or data rate can still be achieved. However, tinting in the glass or the security wire that is sometimes in windows can mean higher levels of RF attenuation and must be taken into consideration during the path design. 8. Is multipath phenomenon common in a desert? Flat desert areas may cause ground reflections, but sand does not have a high reflection coefficient. In the desert, the most critical factor is the possibility of multipath fading and ducting caused by large temperature variations and/or temperature inversions, not ground reflections. 9. I have a link that seems to perform better in the autumn and winter months than in spring or summer. Why? A possible reason is something that should have been outlined at the time of the site/path survey: it is more than likely there are trees in the link path. During the colder times of year there will not be any leaves on the trees, but in spring and summer performance will deteriorate due to the leaves growing and absorbing some, if not all, of the RF signal. This can be worse on a rainy day when the leaves are wet. If your link is operating at a frequency below 10 GHz (longer paths), problems could also be caused by multipath, which is typically a warm weather phenomenon. 10. What type of modulation should I pick for my microwave link? There is no simple answer to that question. Digital systems employ a variety of modulation techniques. Lower-order modulation techniques, such as frequency shift key (FSK) and phase shift key (PSK) offer a more robust signal and less than optimum RF bandwidth efficiency. Higher order modulation techniques, such as quadrature amplitude modulation (QAM), offer superior RF bandwidth efficiency but are not as robust (i.e., they are more prone to interference). So, it will really depend on application and the location of the microwave link. In areas congested with microwave systems, lower-order modulation may be more desirable.

412

Chapter Seven

11. What is the “system gain” of the microwave link? System gain is the difference between the transmit power and the receiver threshold, expressed in decibels. A higher system gain provides more protection against path fading. Selecting microwave radio equipment based on the system gain value can be used to reduce antenna sizes or improve the path availability. 12. What is the effect of flat fading on analog and digital microwave links? The effect of flat fading for digital and analog microwave links is similar—i.e., signal level decreases and quality degrades. Continued quality degradation will eventually lead to the outage of the link. Digital systems usually exhibit a somewhat higher tolerance to flat fading than do analog systems. 13. What is a “near field” of the antenna? The terms far field and near field describe the fields around an antenna, or more generally, around any electromagnetic-radiation source. The names imply that two regions with a boundary between them exist around an antenna. Actually, as many as three regions and two boundaries exist, and it is important to notice that these boundaries are not fixed in space. Usually, two- and three-region models are used. In the near field, the field strength does not necessarily decrease steadily with distance away from the antenna, but it may exhibit an oscillatory character and therefore it is difficult to predict the antenna gain and radiation pattern in that region. Engineers perform link engineering, including Fresnel’s clearances and path profiles, based on the assumption that microwave antennas are in the far-field region—i.e., that the distance between them is sufficiently large. Any large object in the near field of the antenna may distort radiation pattern of the antenna and, therefore, should be avoided. 14. How can we increase a throughput of the microwave link? Increased spectrum efficiency can be achieved by different methods: n

n

Utilize higher-order modulation schemes, but this will be achieved at the expense of system gain, and therefore resulting in shorter hops. Higher (doubled) capacity in the existing spectrum is achieved by co-channel dual polarization support in the microwave link, i.e., two carriers in the same frequency channel with cross-polarization interference cancellation (XPIC).

Project Management

n

n

413

Even higher capacities can be achieved by using multiple frequency channels. Radios with adaptive modulation/coding can be implemented.

15. What is “adaptive modulation”? Adaptive modulation comprises dynamically changing the modulation scheme depending on weather conditions. During good weather conditions, an efficient modulation scheme is used providing a high data rate; during heavy rain, adaptive modulation uses a more robust modulation scheme to guarantee the high availability of the link at the cost of a reduced data rate. 16. I would like to design my microwave network for 99.999 percent availability per hop. What happens if some of those links cannot achieve better than 99.998 percent availability? Nothing will happen, most likely. We have to remember that the calculations are based on the empirical formulas and probability of experiencing impairments, and the result of the long-term observations on similar links. That means that some installed links may experience better results and some may experience worse results than calculated. We should not reject a link design because of the 1/1000 of the percent difference from our design objective. 17. What is “ducting” and when and where do I have to worry about ducting? Ducts are anomalous radio-propagation conditions and could be a nightmare for an unsuspecting microwave engineer. This refractive condition is called trapping (or “blackout fading”) because the wave is confined to a narrow region of the troposphere. When the refractivity gradient decreases beyond the critical gradient, the radius of curvature for the wave will become smaller than that of the Earth’s. The wave will either strike the Earth and undergo surface reflection, or enter a region of standard refraction and be refracted back upward, only to reenter the area of refractivity gradient that causes downward refraction. There are many areas prone to ducts; it is a well-known fact that the equatorial regions are most vulnerable to ducts. In temperate climates the probability of formation of ducts is lower. The ducting probability follows seasonal variations. Conventional techniques used to combat other types of fading, such as increased margins or diversity techniques, have little or no influence on blackout fading.

414

Chapter Seven

In the U.S., the Gulf States and Florida are most susceptible to ducting, especially during the spring months when the sea temperatures are still relatively low. In the Midwest, the Great Lakes area, and the Northeast, ducting is more common in the fall. Ducting is rare west of the Rockies except along the Pacific Coast and Hawaii. 18. I have been asked to look at a microwave path over 50 miles over the sea between islands. Could you give me a couple of points I need to look out for? The over-the-water paths could be tricky; here are some of the things to look for: n n

n

n

The main concern will be reflections from the water (multipath). For the length of the hop, space diversity will absolutely be required, although it may not be sufficient. Frequency diversity may need to be implemented as well (if allowed). The engineer has to perform very detailed analysis of the antenna placement on the tower for the space diversity improvement to be optimal. In addition, keep in mind that some ducting issues may also appear in certain areas.

19. Should I use vertical or horizontal polarization for overthe-water paths? On over-the-water paths at frequencies above about 3 GHz, it is advantageous to choose vertical polarization over horizontal polarization. At grazing angles greater than about 0.7°, a reduction in the surface reflection of 2–17 dB for vertical polarization can be expected over that at horizontal polarization. 20. How can I avoid interference to/from my microwave link? Keep in mind that entirely interference-free radio networks do not exist. A radio network may, however, be considered as approximating an interference-free network if some general rules and simplifications are applied. Generally, interference may be avoided (or reduced to acceptable level) if the two following conditions are met: n n

Interference signals are sufficiently weak. The frequencies of the receivers are sufficiently separated from interference signals, i.e., there is no frequency overlap.

The first condition may be very difficult to meet, often as the result of frequent occurrence of co-located radio systems (occasionally forced co-location); while the second condition may be attained but requires careful frequency planning.

Project Management

415

21. Which diversity technique has a higher improvement factor—frequency or space diversity? Above 3 GHz, space diversity improvement is nearly always higher (better) than in frequency diversity and is therefore selected unless the diversity spacing exceeds about 5 percent (300 MHz in the 6 GHz band, for example). 22. I understand frequency and space diversity, but what is “angle diversity”? Angle diversity has been used in line-of-sight digital microwave links since the mid-1980s and in troposcatter links since the 1950s. The angle diversity antenna is a single dish with two feeds vertically offset by about 1º. Angle diversity is most effective when path outages are dominated by dispersive fade activity (dispersive fade outage approaches or exceeds flat fade outage). Angle diversity dishes require a more precise, long-term alignment procedure than that for space- and nondiversity antennas. Optimum angle diversity improvements are only obtained through an antenna alignment procedure that matches the antenna size and alignment to the path and its climatic characteristics. Depending on path geometry and climatic conditions, angle diversity improvements of perhaps 20 or even more are achieved. 23. I would like to use a license-exempt microwave point-topoint radio; what should I be concerned about? The environment around us is full of radio signals transmitted by licensed equipment (meaning known and predictable) as well as license-exempt equipment (meaning unknown and unpredictable), and even unlicensed (illegal) radio equipment. Even licenseexempt microwave links require due-diligence in planning, design, and implementation: n

Determine your risk tolerance for interference and ask the tower or structure owner about their interference policy.

n

Determine the distance and pick the frequency.

n

Determine the required bandwidth (link capacity).

n

Perform the path engineering and determine the antenna size required and allowed.

The primary difference between licensed and license-exempt systems is that licensed radio users have a regulatory body that will assist them in overcoming any interference issues that may arise, while license-exempt users must resolve interference issues without governmental assistance. In both cases, proper selection of the frequencies and methodical engineering of the path are the key to

416

Chapter Seven

implementing a microwave path whose potential for interference is greatly reduced. A good deployment practice is to sweep (scan) the antenna locations for potential interference prior to deployment; this can be particularly useful in dense metropolitan areas where it is more likely to encounter interference. 24. We are installing a long over-the-water microwave point-topoint link to the off-shore drilling platform requiring space diversity, but we do not have the capability to install two large antennas with sufficient vertical separation. Can we use horizontal separation of 30 ft instead? Horizontal separation does not improve the microwave point-topoint link’s multipath fading probability. In case you cannot achieve 30 ft of vertical separation, even smaller separation would be beneficial. Some studies indicate that the actual improvement achieved with such a small vertical separation can be much better than the calculated value. Experience has shown that some of the commonly used formulas for calculating the space diversity improvement factor may underestimate the improvement for small antenna spacing and overestimate improvement for large antenna spacing on microwave links. You may have to mount the antennas not only at different heights, but also at different sides of the platform (due to the mounting issues and the limited space). So, horizontal separation is OK, as long as the vertical separation is also present. Additional advice is to make sure that a moored tanker does not block the path. 7.6

References

1. Wysocki, R. K., et al., Effective Project Management, New York: John Wiley and Sons, 1995. 2. Kerzner, H., Project Management—A Systems Approach to Planning, Scheduling, and Controlling, 7th ed., New York: John Wiley and Sons, 2001. 3. Clark, M. P., Networks and Telecommunications—Design and Operations, 2nd ed., New York: John Wiley & Sons, 1997. 4. Situation Management Systems, Inc., Managing Negotiation—Selected Readings on Negotiation Skills, 1996. 5. Mann-Robinson, T. C., Network Design—Management and Technical Perspectives, Boca Raton, FL: CRC Press, 1999. 6. Lehpamer, H., “How to Build a Reliable and Cost-Effective Microwave Network,” Conference Paper, ENTELEC, Houston, Texas, 2006. 7. Andrews, G., Kemper J., Canadian Professional Engineering Practice and Ethics, Toronto, Canada: Saunders College, A Division of Holt, Rinehart and Winston of Canada, Ltd., 1992. 8. LaRue T. Hosmer, The Ethics of Management, Homewood, IL: Richard D. Irwin, 1987.

Appendix

A

American Cable Stranding

When working on international projects, the information shown in Table A.1 will come in handy. Most of the world expresses wire size in terms of its diameter, but in North America, American Wire Gauge (AWG) is the most common unit. AWG is shown in Table A.1 with its exact equivalent value in mm2 and diameter (mm). TABLE A.1

AWG Equivalent in Square Millimeters

AWG Number 1000 MCM* 900 750 600 550 500 450 400 350 300 250 4/0 3/0 2/0 0 1 2

Cross Section (mm2) 507 456 380 304 279 253 228 203 177 152 127 107.2 85.0 67.4 53.4 42.4 33.6

Diameter (mm) 29.3 27.8 25.4 22.7 21.7 20.7 19.6 18.5 17.3 16.0 14.6 11.68 10.40 9.27 8.25 7.35 6.54

Conductor Resistance (Ω/km) 0.036 0.04 0.048 0.061 0.066 0.07 0.08 0.09 0.10 0.12 0.14 0.18 0.23 0.29 0.37 0.47 0.57 (continued) 417

418

Appendix A

TABLE A.1

AWG Equivalent in Square Millimeters (Continued)

AWG Number 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

Cross Section (mm2) 26.7 21.2 16.8 13.3 10.6 8.34 6.62 5.26 4.15 3.31 2.63 2.08 1.65 1.31 1.04 0.8230 0.6530 0.5190 0.4120 0.3240 0.2590 0.2050 0.1630 0.1280 0.1020 0.0804 0.0646 0.0503 0.0400 0.0320 0.0252 0.0200 0.0161 0.0123 0.0100 0.00795 0.00632

Diameter (mm)

Conductor Resistance (Ω/km)

5.83 5.19 4.62 4.11 3.67 3.26 2.91 2.59 2.30 2.05 1.83 1.63 1.45 1.29 1.15 1.0240 0.9120 0.8120 0.7230 0.6440 0.5730 0.5110 0.4550 0.4050 0.3610 0.3210 0.2860 0.2550 0.2270 0.2020 0.1800 0.1600 0.1430 0.1270 0.1130 0.1010 0.0897

* Shown in MCM (circular mills) for larger cross sections; 1 CM = 1 circular mil = 0.0005067 mm2; 1 MCM = 1000 circular mils = 0.5067 mm2. 4/0 is also known as 0000; 1 mil = 1/1000 in = 0.0254 mm.

0.71 0.91 1.12 1.44 1.78 2.36 2.77 3.64 4.44 5.41 7.02 8.79 11.2 14.7 17.8 23.0 28.3 34.5 44.0 54.8 70.1 89.2 111.0 146.0 176.0 232.0 282.0 350.0 446.0 578.0 710.0 899.0 1125.0 1426.0 1800.0 2255.0 2860.0

Appendix

B

Quick RF Reference Sheet

Frequency and Wavelength

Wavelength:

λ= where

c f

f = frequency [Hz] c = speed of light in vacuum ( c ≈ 3 ⋅ 108 m /sec) Basic Rules of Logarithms

log AB = log A + log B log

A = log A − log B B n

log A = n ⋅ log A A = X B ⇒ B = log X A X is the base of a logarithm, and base 10 is most commonly used in radio communications. Base 2 or base of natural logarithms e are used sometimes as well. 419

420

Appendix B

Decibels

The decibel unit is commonly used in radiocommunications because the direct relationship between radio-related power levels covers a wide range of numerical values. The logarithmic nature of the relationship between two power-levels results in values that are easy to handle. The abbreviation for decibel, dB, has a capital B since a bel was derived from Alexander Graham Bell’s last name. Addition or subtraction operations can be easily performed on logarithmic values, simplifying the handling of amplification and attenuation. Humans perceive differences in the sensory impressions of varying intensity in a logarithmic fashion. The decibel is used to compare one power (or voltage level) to another: Gain or loss in decibels = 10log10 where

P2 P1

P1 = input power P2 = output power A +3 dB gain represents a doubling of power, while a –3 dB loss represents half of the power (see the table below). dB GAIN (Power)

Factor

dB LOSS (Power)

Factor

0 dB 1 3 6 10 12 20 30 40

1 (the same) 1.25 2 4 10 16 100 1,000 10,000

0 dB −1 −3 −6 −10 −12 −20 −30 −40

1 (the same) 0.8 0.5 0.25 0.10 0.06 0.01 0.001 0.0001

dBm and dBW

dBm is a power relative to 1 mW. A 100 mW signal is equivalent to a 20 dBm signal. Power = 10log10

Power (mW) 1 mW

[dBm]

or dBW is a power relative to 1 W. A 100mW signal is equivalent to a –10 dBW signal. Power = 10log10 Power (W) [dBW] 1W

Quick RF Reference Sheet

421

dBm units can be converted into dBW units by subtracting 30 dB. dBW = dBm –30 so, for example 0 dBm = 1 mW 30 dBm = 1 W +30 dBm = 0 dBW –30 dBW = 0 dBm A word of caution about dBm: You cannot add dBm to dBm! The two powers must first be converted to milliwatts, then added and the sum reconverted to dBm. For example: 10 dBm + 10 dBm = ? dBm since

10 dBm = 10 mW

10 mW + 10 mW = 20 mW 20 mW = 10 log10 (20/1) = 13 dBm So, doubling the power means a 3 dB increase (from 10 dBm to 13 dBm) in power expressed in dBm. Signal-to-Noise Ratio

The signal-to-noise ratio (S/N) is the amount by which a signal level exceeds the noise level: SNR [dB] = signal level [dBm] – noise level [dBm] EIRP

Effective isotropic radiated power (EIRP) describes the performance of a transmitting system: EIRP [dBW or dBm] = Tx output power [dBW or dBm] + antenna gain [dBi] – line loss [dB] Fade Margin

Fade margin (FM) is an “extra” signal power added to a link to ensure its continued operation if it suffers from signal propagation effects. FM [dB] = system gain + ant. gain (Tx and Rx) – propagation losses – cable/connector/branching loss (both ends added together)

422

Appendix B

System Gain

System gain is the total gain of the radio system without considering antennas and cables. System gain = Tx power – Rx (threshold) sensitivity

[dB]

Free Space Loss

Free-space path loss is the signal energy lost in traversing a path in free space only, with no other obstructions or propagation issues. FSPL = 96.6 + 20log10 (dmiles) + 20log10 (fGHz)

[dB]

FSPL = 92.4 + 20log10 (dkm) + 20log10 (fGHz)

[dB]

Parabolic Antenna Directive Gain

Ga = 10 log η where Ga = antenna directive gain

η = aperture efficiency (usually 55%) Aa = antenna aperture area

λ = wavelength

4π Aa [dBi] λ2

Appendix

C

Useful Physical Quantities and Units of Measurement

About Units of Measurements

Readers of this book will notice the use of different units of measurement throughout the text; the main reason for that is the fact that the world today is a mixture of different units of measurement. So, formulas developed in North America use different units of measurement than those developed by the ITU. All this requires us to pay close attention to what units are used where and to make sure that the units are consistent in our calculations. North Americans probably use a greater variety of units of measurement than anyone else in the world. Caught in a slow-moving transition from customary to metric units, the U.S. uses a fascinating and sometimes frustrating mixture of units to refer to the same things. We measure the length of a race in meters, but the length of the long jump event in feet and inches. We speak of an engine’s power in horsepower and its displacement in liters. In the same dispatch, we describe a hurricane’s wind speed in knots and its central pressure in millibars. Furthermore, English customary units do not form a consistent system, either; reflecting their diverse roots in Celtic, Roman, Saxon, and Norse cultures, they too are often confusing and contradictory. There are two systems for land measurement (one based on the yard and the other on the rod) and a third system for distances at sea. There are two 423

424

Appendix C

systems (avoirdupois and troy) for small weights and two more (based on the long and short tons) for large weights. North Americans use two systems for volumes (one for dry commodities and one for liquids) and the British use a third (the British Imperial Measure). Meanwhile, not too many people know that the legal definitions of the English customary units are actually based on metric units. The U. S. and British governments have agreed that a yard equals exactly 0.9144 meter and an avoirdupois pound equals exactly 0.45359237 kilograms. In this way, all the units of measurement North Americans use everyday are based on the standards of the metric system. Since 1875, in fact, the United States has subscribed to the International System of Weights and Measures, the official version of the metric system. All systems of weights and measures, metric and nonmetric, are linked through a network of international agreements supporting the International System of Units, called the SI, which comes from the first two initials of its French name, Système International d’Unités (ITU-R Recommendation V.430-3: “Use of the International System of Units (SI)”). The key agreement is the Treaty of the Meter (Convention du Mètre), signed in Paris on May 20, 1875. Forty-eight nations have now signed this treaty, including all the major industrialized countries. The SI is maintained by a small agency in Paris, the International Bureau of Weights and Measures (BIPM, for Bureau International des Poids et Mesures). It is updated every few years by an international conference, the General Conference on Weights and Measures (CGPM, for Conférence Générale des Poids et Mesures), attended by representatives of all the industrial countries and international scientific and engineering organizations. At the heart of the SI is a short list of base units defined in an absolute way without referring to any other units. The base units are consistent with the part of the metric system called the MKS system. Physical Constants

c = speed of light in vacuum ( c = 299, 792, 458 m / s ≈ 3 ⋅ 108 m /sec) c = 9.835711 ⋅ 108 ft /sec Frequency

1 GHz = 103 MHz = 106 kHz = 109 Hz

Useful Physical Quantities and Units of Measurement

Distance

1 mi = 1.609 km = 5,280 ft = 63,360 in 1 m = 1.09361 yd = 3.28084 ft = 0.001 km –4 = 6.21371 × 10 mi = 39.3701 in 1 ft = 12 in = 0.3048 m = 0.3333 yd 66 ft = 1 ch (chain) 80 ch = 1 mi Area

1 m2 = 1,550 in2 = 10.7639 ft2 = 1.19599 yd2 1 ac = 208 ft × 208 ft = 43,560 ft2 1 ac = 10 ch2 640 ac = 1 m2 Volume

1 m3 = 61,023.7 in3 = 1.30795 yd3 = 35.3147 ft3 Speed

1 km/hr = 0.277778 m/s = 0.621371 mi/hr = 3.28084 ft/s 1 mi/hr = 1.609344 km/hr Mass

1 lb = 0.453592 kg 1 kg = 2.20462 lb Force

1 N = 0.1019716 kp = 0.224809 lbf Pressure

1 N/m2 = 10–5 bar = 0.0208854 lbf/ft2 = 0.101972 kp/m2 = 9.86923 × 10–6 atm Energy

1 kWh = 3.6 × 106 J = 859.845 kcal = 3412.14 BTU 1 kcal = 4186.8 J = 0.745700 kWhr

425

426

Appendix C

Power

1 hp = 76.0402 kpm/s = 745.700 W 1 W = 0.238846 cal/s = 3.41214 BTU/hr Temperature

°C = 5/9 × (°F – 32) °F = (9/5 × °C) + 32 Throughput

1byte (or octet) = 8bit = 2nibble 1GB = 103MB = 106 kB = 109byte Angle

1 mrad (milliradian) = 0.0572958° 1° = 17.45329 mrad 1° = 60’ = 3,600”

Glossary

3G The third generation of wireless technology. These networks are specified to operate at a minimum of 2 Mbps when stationary and 384 kbps when used at pedestrian user speeds. 3G standards are coordinated through the ITU’s IMT-2000 (International Mobile Telecommunications—2000), the European-based UMTS (Universal Mobile Telecommunications System), and the Third Generation Partnership Project (3GPP), a group formed by GSM-supporting standards bodies. 3GPP Third Generation Partnership Project (see 3G). 4G The next technical strategy in the area of wireless communications. 802.11b The IEEE standard ratified in 1999 that defines wireless LANs

in the 2.4 GHz band.

AAL2 ATM Adaptation Layer 2. AAL2 has been adopted by ITU-T and ATM forum to reduce packing delay. The idea is to multiplex voice packets from several sources into one ATM cell so that the time to fill a cell can be reduced significantly. AALn ATM Adaptation Layer type n.

A designation for 22-gauge, 100 Ω, plastic insulated, twisted pair Western Electric cable, normally used in a central office (CO) for T1 circuits. ABAM cable is no longer available, but it is easy to find a replacement cable that meets the technical requirements.

ABAM

Absorption In radio wave propagation, the irreversible conversion of the energy of an electromagnetic wave into another form of energy as a result of wave interaction with matter.

Radio continually optimizes modulation to transmit the maximum amount of data across the path while maintaining the highest levels of link quality.

Adaptive modulation

ADM Add/drop multiplexer. In an ADM, traffic can be dropped or added

or it can simply go through it.

427

428

Glossary

AFC Automatic frequency control. It is a method (or electronic circuit)

to automatically maintain a tuning of electromagnetic radiation (radio or microwave) signal to desired frequency.

Automatic gain control. A system that holds the gain and, accordingly, the output of a receiver substantially constant in spite of input-signal amplitude fluctuations.

AGC

AIS Alarm indication signal. A signal transmitted to maintain continuity of transmission. The AIS is usually sent to notify the far-end that a transmission fault exists on the line. ALBO Automatic line build-out. Electronic circuits used in the receive equipment of the digital transmission system to establish the proper signal level and pulse shape required by the receiver. AMI Alternate mark inversion. A line code in which the signal carrying the binary value alternates between positive and negative polarities. Angle diversity A technique using multiple antenna beams to receive multipath signals arriving at different angles.

American National Standards Institute. ANSI is a nonprofit, privately funded membership organization that coordinates the development of voluntary national standards in the United States. The ANSI and IEEE standards are often recognized by many government agencies and organizations in both the United States and abroad.

ANSI

Antenna beamwidth The directivity of a directional antenna. Defined as the angle between two half-power (–3 dB) points on either side of the main lobe of radiation.

Automatic protection switch. Provides a network element with the ability to detect a failed unit/line and switch to the spare one; 1 + 1 pairs a protection unit/line with each working unit/line; N + 1 pairs a protection unit/line for every N working units/lines.

APS

ARIB Association of Radio Industries and Business (Japan). ARPU

Average revenue per user.

Adaptive IF (intermediate frequency) slope amplitude equalization.

ASAE

ASL Above sea level.

Glossary

429

Antenna structure registration. The Antenna Structure Registration Program is the process under which each antenna structure that requires FAA notification, including new and existing structures, must be registered with the FCC by its owner.

ASR

ASTM International American Society for Testing and Materials International. ASTM International (ASTM), originally known as the American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, and services. In the United States, ASTM standards have been adopted, by incorporation or by reference, in many federal, state, and municipal government regulations. Asynchronous A type of transmission in which each character is transmitted independently and without reference to a standard clock. ATDE

Adaptive time domain equalization.

ATM Asynchronous transfer mode. ATM is a packet-oriented digital data transmission technology.

An organization originally founded by a group of vendors and telecommunication companies; this formal standards body comprises various committees responsible for making ATM-related recommendations and producing implementation specifications.

ATM Forum

Adaptive transmit power control. ATPC is a feedback control system that increases the transmitting power level during periods of fading. ATPC offers advantages to the link operator that include reduced average power consumption, reduced interference, and extended MTBF.

ATPC

Attenuation Reduction in signal magnitude or signal loss, usually expressed in decibels. AWG American wire gauge. A measurement of wire diameter. The lower the AWG number, the larger the wire diameter. Copper phone wiring usually comes in 24 or 26 AWG. B8ZS Bipolar eight zero substitution; a line coding scheme. Backhaul The portion of the wireless network that carries the wireless calls from cell site radios back to the mobile switching center (MSC) and

430

Glossary

then on to the appropriate service termination points, such as the public switched telephone network (PSTN) and/or the Internet. The information capacity of a communications resource, usually measured in bits per second for digital transmission and hertz for analog transmission.

Bandwidth

Background block error ratio. Background block Error (BBE) is an errored block not occurring as part of a SES. BBER is the ratio of Background Block Errors to total blocks in available time during a fixed measurement interval. The count of total blocks excludes all blocks during SESs.

BBER

Bellcore Bell Communications Research (called Telcordia now). Bellcore provides certain centralized research and standards coordination for the regional Bell operating companies (RBOCs). It also coordinates security and emergency preparedness for the U.S. government. Bellcore was formed in 1984 when AT&T was broken up into the seven RBOCs. BER Bit error rate, also referred to as bit error ratio. The number of

erroneous bits received divided by the total number of bits transmitted.

Bit error rate test. BERT or Bit Error Rate Test is a testing method for digital communication circuits and links that uses predetermined stress patterns comprising of a sequence of logical ones and zeros (QRSS, 3 in 24, All Ones, All Zeros, etc.).

BERT

Busy hour call attempts. The number of call attempts that a telephone system can support during the busy hour of the day. BHCA is a measure of system processor capacity and a factor considered in traffic engineering.

BHCA

BLER Block error rate, applicable to a block of data in which one or more bits are in error:

BLER =

Errored Blocks Received nt Total Blocks Sen

Bluetooth A radio technology developed by Ericsson and other compa-

nies built around a chip that makes it possible to transmit a signal over short distances between phones, computers, and other devices without using wires. Find more information at www.bluetooth.com.

BoM Bill of materials; also called BoQ.

Glossary

431

BoQ Bill of quantity; also called BoM. bps Bits per second. Number of bits in one second. BPV Bipolar violation; the detection of any isolated error. Broadband A classification of the information capacity (bandwidth) of a communication channel. Broadband is generally taken to mean a bandwidth higher than 2 Mbps. BSI British Standards Institute. BSI is a multinational business ser-

vices provider whose principal activity is the production of standards and the supply of standards-related services.

BTS Base transceiver station. Also called RBS, radio base-station. Canadian Electrical Code (CEC) Canadian version of the U.S. National

Electrical Code (NEC).

CAPEX

Capital expenditures.

Carrier A telecommunications provider that owns switch equipment.

A standardized carrier-class service defined by five attributes that distinguish Carrier Ethernet from LAN-based Ethernet: standardized services, scalability, reliability, quality of service, and service management.

Carrier Ethernet

CAS Channel associated signaling. CCC Clear channel capability; usually requires B8ZS line coding on

all elements.

CCIR International Radio Consultative Committee (now ITU-R). CCITT Comité Consultatif International Téléphonique et Télégraphique

(obsolete term).

CCS Common channel signaling. CCS or Common Channel Interoffice Signaling (CCIS) is the transmission of signaling information (control information) on a separate channel to the data, and, more specifically, where that signaling channel controls multiple data channels. Some of the common CCS signaling methods are ISDN and SS7.

432

Glossary

CDMA Code division multiple access. This code division technology was originally developed over 30 years ago for military use. CDMA is a multiple access technique that uses code sequences as traffic channels within common radio channels—it’s used for cdmaOne (IS-95) and CDMA2000 air interface. CDMA2000 A third-generation digital air interface technology. cdmaOne (IS-95) A second-generation digital air interface technology

pioneered by the U.S. firm Qualcomm and further developed in South Korea.

CE Conformité Européenne (the European EMC standard). CEPT Conférence Européenne des Postes et Télécommunications (European

Conference of Postal and Telecommunications Administrations).

CFM Composite fade margin. Typically expressed in decibels.

Carrier Group Alarm. The meaning of CGA is that connectivity on the digital carrier has failed. There are three defined alarm indication signal states, identified by a legacy color scheme: red, yellow and blue.

CGA

C/I Carrier-to-interference ratio. Typically expressed in decibels.

The basic switching process whereby a circuit between two users is opened on demand and maintained for their exclusive use for the duration of the transmission, as opposed to a dedicated circuit that is held open regardless of whether data is being sent.

Circuit switching

Competitive local exchange carrier. Deregulated local telephone companies resulting from the Telecommunications Act of 1996 that are competing for local exchange service, as well as for long distance and Internet service.

CLEC

CO Central office. The building that contains the switches for a local telephone company. CODEC

versa.

Coder-decoder; converts analog voice to digital and vice

COFDM Coded orthogonal frequency division multiplex.

Glossary

433

Comisión Federal de Telecomunicaciones. The primary agency responsible for communications in the Mexico, an autonomous administrative and technical agency, which resides within the Secretaria de Comunicaciones y Transportes (SCT), a cabinet level position whose secretary is appointed by the President. COFETEL was established in 1996 in conjunction with the end of Telmex’s monopoly through the sale of concession for the provision of telecommunications to other companies, including XC Networks. COFETEL is the primary telecommunications regulatory body in Mexico, although the SCT retains certain important responsibilities. On some issues, COFETEL makes decisions requiring little, if any, input from the SCT; while on other issues, COFETEL must obtain the approval of the SCT. COFETEL was formed in part to depoliticize the regulatory process in Mexico.

COFETEL

The linking together of various data structures—for example, two bandwidths joined to form a single bandwidth.

Concatenation

A constellation diagram is a representation of a signal modulated by a digital modulation scheme such as quadrature amplitude modulation or phase-shift keying. Measured constellation diagrams can be used to recognize the type of interference and distortion in a signal and/or for microwave radio troubleshooting purposes.

Constellation diagram

CONUS Continental United States. It refers to the 48 U.S. states located on the North American continent south of the U.S. border with Canada, plus the District of Columbia. The term excludes the states of Alaska and Hawaii, and all off-shore U.S. territories and possessions, such as Puerto Rico. CPE Customer premises equipment. CRC-n Cyclic redundancy check – n bits. A CRC is an error-detecting code, invented in 1961. Cross-connect (DSX Panel, MDF) Distribution system equipment used to terminate and administer communication circuits. In a wire cross-connect, jumper wires or patch cords are used to make circuit connections. In an optical cross-connect, fiber patch cords are used. The cross-connect is located in an equipment room, riser closet, or satellite closet. CSU Channel service unit; the interface from CPE to the public T1 line.

434

Glossary

CW Continuous wave. D4 Fourth-generation digital channel bank. DACS

Digital access and cross-connect system.

DADE Differential absolute delay equalization. DADE is a process typically used in space diversity systems to equalize the signal delay to the main and diversity antenna (result of the difference in the length of the transmission lines). Dark Fiber Dark fiber refers to unused fibers, available for use. dB Decibel. A measure for comparing two quantities. In common usage, when two quantities have dimensions of power, then their dB ratio is 10log(P1/P2). So if P1 is 10 times greater then P2, it is 10 dB greater. If P1 is equal to P2, then it is 0 dB greater. dBdsx Decibels with respect to the standard level at the DSX-1 cross-

connect.

dBm Decibels relative to 1 mW. dBW Decibels relative to 1 W. DC Direct current. DEM Digital elevation model. A digital model describing the elevation of a map area. DFM Dispersive fade margin. Expressed in decibels. DFS Dynamic Frequency Selection. A mechanism to allow unlicensed devices to share a spectrum with existing military radar and weather radar systems. DFS allows automatic detection of radar signal interference and dynamically switches to a clear channel while, for example, operating in the 5.3- and 5.4-GHz frequency bands (U-NII), thus providing greater signal integrity and data rate reliability. DGPS Differential Global Positioning System. DGPS is an enhancement to the global positioning system that uses a network of fixed, groundbased reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions.

Glossary

435

Diffraction A propagation phenomenon that allows radio waves to propagate beyond obstructions via secondary waves created by the obstruction. Classic types of diffractions are smooth Earth and knife-edge. There is no line of sight between the transmitter and receiver. Directional coupler A device that samples the energy traveling in a waveguide for use in another circuit (for example, measurements). Dish antenna A dish-like antenna used to link communication sites by

wireless transmission of voice or data. Also called microwave antenna or microwave dish antenna.

A technique to reduce the effects of fading by using multiple spatially separated antennas (space diversity) to take independent samples of the same signal at the same time. The theory is that the fading in these signals is uncorrelated and that the probability of all samples being below a threshold at a given instant is low. Diversity can also be frequency, angle, hybrid, etc.

Diversity

DoD Department of Defense in U.S. DOT

Department of Telecommunications in U.S.

Drop-insert A functionality provided in analog and digital repeaters, where radio-system specific control and service channels and possibly part of the payload is made available for local traffic and system management and maintenance. DS0 Digital signal, level 0; 64 kbps. DS1

dard. DS3

dard.

Digital signal, level 1; 1.544 Mbps, the North American stanDigital signal, level 3; 44.736 Mbps, the North American stan-

DS/CDMA Direct sequence CDMA (Code Division Multiple Access). Application of direct sequence spread-spectrum (DS/SS) technology in mobile communications. DSL

Digital subscriber line. See also xDSL.

DS/SS Direct-sequence spread spectrum (DS/SS) is a modulation technique. In DS/SS the user data signal is multiplied by a code sequence.

436

Glossary

Mostly, binary sequences are used. The duration of an element in the code is called the “chip time.” The ratio between the user symbol time and the chip time is called the spread factor. The transmit signal occupies a bandwidth that equals the spread factor times the bandwidth of the user data. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. DSU Digital service unit. DSX-1 Digital service cross-connect, level 1; part of the DS1 specification. DTE Data terminal equipment. DTED Digital terrain elevation data. Ducting Ducting is guided propagation of radio waves inside a tropospheric radio-duct. For microwave links this type of anomalous radio propagation mechanism can be a serious problem.

A device used at the end of a transmission line or waveguide to convert transmitted energy into heat so no energy is radiated outward or reflected back.

Dummy load

DWDM

Dense wavelength division multiplexing.

DXC Digital cross-connect. E1 The European equivalent of a North-American T1. Equals 2.048 Mbps. EC European Commission. The EC embodies and upholds the general

interest of the European Union and is the driving force in the Union’s institutional system. Its four main roles are to propose legislation to Parliament and the Council, to administer and implement Community policies, to enforce Community law (jointly with the Court of Justice) and to negotiate international agreements, mainly those relating to trade and cooperation. Enhanced Data GSM Environment. EDGE is a faster version of GSM wireless service. EDGE enables data to be delivered at rates up to 384 kbps on a broadband. The standard is based on the GSM standard and uses TDMA multiplexing technology.

EDGE

Glossary

437

Effective Earth radius The radius of a hypothetical Earth for which the

distance to the radio horizon, assuming rectilinear propagation, is the same as that for the actual Earth with an assumed uniform vertical gradient of atmospheric refractive index. For the standard atmosphere, the effective Earth radius is 4/3 that of the actual Earth radius.

EFS Error-free seconds. Seconds without any errors. EIA Electronic Industries Alliance (formerly Electronic Industries Association). It specifies electrical transmission standards, including those used in networking. EIRP Effective isotropic radiated power in dBW or dBm. The product of the power supplied to the antenna and the antenna gain in a given direction relative to an isotropic antenna. Equal to the transmitted output power minus cable loss plus the transmitting antenna gain:

EIRP = Tx output power (in dBW or dBm) + antenna gain (dBi) – line loss (dB) For example, if a 30 dBi gain antenna is fed with 27 dBm of power (transmit power already includes all the losses), it has an EIRP of 30 dBi + 27 dBm = 57 dBm. EMC Electromagnetic compatibility. EMC is the study of the unintentional generation, propagation and reception of electromagnetic energy and the unwanted effects (electromagnetic interference, or EMI) that such energy may induce. The goal of EMC is the correct operation, without interference, of different equipment in the close proximity of each other. Engset The Engset traffic model is used to explore the relationship between the traffic offered to a trunk group, the blocking that will be experienced by that traffic, and the number of lines provided when there are a finite number of sources from which the traffic is generated. It is used to replace the Erlang B traffic model, which tends to overestimate blocking when the ratio of the number of sources to the number of lines is less than ten.

Ethernet over SONET. A set of protocols that allow Ethernet traffic to be carried over synchronous digital hierarchy networks in an efficient and flexible way.

EOS

Engineering orderwire. A voice or data circuit used by technical control and maintenance personnel for coordination and control actions

EOW

438

Glossary

relative to activation, deactivation, change, rerouting, reporting, and maintenance of communication systems and services. Synonyms are engineering channel, engineering orderwire, service channel. EPL Ethernet private line. Replaces a TDM private line.

Ethernet private LAN. EPL is a data service defined by the Metro Ethernet Forum, providing a point-to-point Ethernet connection between a pair of dedicated User-Network Interfaces (UNIs), with a high degree of transparency.

EPLAN

European Radiocommunications Committee. The ERC was established by CEPT to develop radio communications policy and to coordinate frequency, regulatory and technical matters concerning radio communications.

ERC

Erlang In telecommunications, a nondimensional unit with a value between 0 and 1 that indicates how busy a telephone facility is over a period of time (usually one hour). Agner Krarup Erlang (1878–1929) was a Danish mathematician who invented the formula commonly used to forecast telecommunications traffic. The number 1 applied to a particular telephone circuit would indicate busy 100 percent of the time. Erlang B is a calculation for any one of these three factors if you know or can predict the other two: n

n

n

Busy hour traffic (BHT), or the number of hours of call traffic during the busiest hour of operation Blocking, or the percentage of calls that are blocked because not enough lines are available Lines or the number of lines in a trunk group

An extended version of Erlang B allows you to determine the number of people who, when blocked, retry their calls immediately. Most of the common models of capacity assume that calls are either served immediately or are blocked and overflow. In some applications, blocked calls can be delayed and served later, which leads us to Erlang C distribution, where capacity estimation is a function of delay criteria instead of blocking. It is common for the wireless systems to require GOS (Grade of Service), usually specified as a blocking probability using Erlang B formula, of 2 percent in North America and between 1 and 2 percent in Europe. Engset is replacing Erlang for some specific situations, for example, when the ratio of the number of sources to the number of lines is very low.

Glossary

439

European Radiocommunications Office. The ERO is a permanent resource of the ERC (European Radiocommunications Committee) which provides, among other things, a center of expertise to develop long-term planning proposals and a focal point for consultations.

ERO

ERP Effective radiated power (in a given direction). The product (or

sum, if transmit power and the gain are given in logarithmic units) of the power supplied to the antenna and its gain relative to a half-wave dipole in a given direction.

ESF Extended super frame, a DS1 framing format of 24 frames.

The most widely used wired local area network. Ethernet uses carrier sense multiple access (CSMA) to allow computers to share a network and operates at 10, 100, or 1,000 Mbps, depending on the physical layer used.

Ethernet

ETSI European Telecommunications Standards Institute. A body formed by the European Commission in 1988, which included vendors and operators. ETSI’s purpose is to define standards that will enable the European market for telecommunications to function as a single market. EVC Ethernet virtual circuit (connection). EVC is a logical relationship between Ethernet user-to-network interfaces (UNI) in a provider-based Ethernet service. EVPL Ethernet virtual private line. Replaces frame relay or ATM services. Eye diagram (pattern) An oscilloscope display in which a pseudorandom digital data signal from a receiver is repetitively sampled and applied to the vertical input, while the data rate is used to trigger the horizontal sweep. System performance information can be derived by analyzing the display. An open eye pattern corresponds to minimal signal distortion. Distortion of the signal waveform due to intersymbol interference and noise appears as closure of the eye pattern. FAA

Federal Aviation Administration.

Fade margin A design allowance that provides for sufficient system gain or sensitivity to accommodate expected fading, for the purpose of ensuring that the required quality of service is maintained. In radio communication systems, the difference in decibels between the power

440

Glossary

level at the receiver under nonfading conditions, and the receiver threshold. The variation in signal strength from it normal value. Fading can be either fast or slow. It is normally characterized by the distribution of fades, Gaussian, Rician, or Rayleigh.

Fading

Frame alignment signal. In the transmission of data frames, it is a distinctive sequence of bits used for the frame alignment.

FAS

FB Framing bit. A common practice in data transmission (for example in T-carrier) is to insert, in a dedicated time slot within the frame, a framing bit that is used for synchronization of the incoming data with the receiver. In a bit stream, framing bits indicate the beginning or end of a frame. They occur at specified positions in the frame, do not carry information, and are usually repetitive. FCC Federal Communications Commission. Regulates interstate com-

munications via licenses, rates, tariffs, standards, limitations, and so forth. Commissioners are appointed by the U.S. president. In Canada, the same function is performed by Industry Canada.

FDD Frequency division duplex. FDD means that the transmitter and

receiver operate at different carrier frequencies.

FDL Facility data link; an embedded overhead channel within the ESF

format.

FDM Frequency division multiplexing. FDM means that the total bandwidth available to the system is divided into a series of nonoverlapping frequency sub-bands that are then assigned to each communications channel. FDMA Frequency division multiple access. FEBE Far-end block error. See the new term used, REI. FEC Forward error correction. An encoding technique that allows a limited number of errors in a digital stream to be corrected based on knowledge of the encoding scheme used. FFT Fast Fourier transform.

Frequency hopping. A method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. Type of a spread-spectrum system.

FH

Glossary

441

FHSS Frequency-hopping spread spectrum. See FH.

Also called wireless local loop (WLL). This apparent contradiction in terms signifies a cellular network that is set up to support fixed rather than mobile subscribers. Increasingly used as a fast and economic way to roll out modern telephone services, since it avoids the need for fixed wires.

Fixed wireless or fixed cellular network

Frame relay

Protocol for packet-switched data communications.

Free-space loss The amount of attenuation of RF energy on an unobstructed path between isotropic antennas. Loss of energy as the RF propagates away from a source. Frequency diversity The simultaneous use of multiple frequencies to transmit information. This is a technique used to overcome the effects of multipath fading, since the wavelength for different frequencies result in different and uncorrelated fading characteristics. Frequency response Attenuation versus frequency or, in other words, a plot of how a circuit or device responds to different frequencies.

In radio communications, a number (theoretically infinite) of a concentric ellipsoids of revolution which define volumes in the radiation pattern of a (usually) circular aperture. The cross section of the first Fresnel zone is circular. Most of the radiated energy is contained in the first Fresnel zone. Subsequent Fresnel zones are concentric with the first. Odd-numbered Fresnel zones have relatively intense field strengths, whereas even numbered Fresnel zones are nulls. Fresnel zones result from diffraction by the circular aperture.

Fresnel zones

FSO Free-space optical. FSO is an optical communication technology that

uses light propagating in free space to transmit data between two points. The optical links are usually implemented using infrared laser light.

Gain A measure of directivity. It is defined as the ratio of the radiation intensity in a given direction to the radiation intensity that would be obtained if the power accepted by the antenna were radiated equally in all directions (isotropically). Antenna gain is expressed in dBi. Galileo A global navigation satellite system (GNSS) currently being

built by the European Union (EU) and European Space Agency (ESA).

Geodetic datum A reference that describes the position, orientation, and scale relationships of a reference ellipsoid to the Earth.

442

Glossary

GIS Geographic information system. A common name for software tools to acquire and maintain digital geographic data. GND Ground (0V). In electrical engineering, ground or Earth is the reference point in an electrical circuit from which other voltages are measured. GNSS Global Navigation Satellite Systems. Standard generic term for satellite navigation systems that provide autonomous geo-spatial positioning with global coverage. GoS Grade of service. GPRS General Packet Radio Service, a standard for wireless communications which runs at speeds up to 115 kbps. Grooming

Consolidating or segregating traffic for efficiency.

GSM Global System for Mobile Communications. Originally defined as a

pan-European standard for a digital cellular telephone network, created to support cross-border roaming, GSM is now one of the world’s main digital wireless standards. It uses TDMA air interfaces and is implemented in 900-MHz, 1800-MHz, and 1900-MHz frequency bands.

Guard band A set of frequencies or bandwidth used to prevent adjacent

systems from interfering with each other. Guard bands are typically used between different types of systems, at the edges of the frequency allocations.

HDLC High-level data link control. HDR High data rate system; developed by Qualcomm for CDMA 1.9-GHz

carriers.

HF High frequency. Radio frequencies between 3 and 30 MHz.

WiMAX variation created by the European Telecommunications Standards Institute (ETSI) Broadband Radio Access Networks (BRAN) group. Operates in the 2–11-GHz range.

HiperMAN

HRC Hypothetical reference circuit. Hydrometeors Concentrations of water or ice particles that may exist in the atmosphere or be deposited on the surface of the Earth. Rain, fog, clouds, snow, and hail are the main hydrometeors.

Glossary

443

Hz Hertz; cycles per second. Unit of measurement for frequency. IEC Interexchange carrier. It is defined as any carrier that provides

inter-LATA communication, where a LATA is a local access and transport area. An IXC carries traffic, usually voice traffic, between telephone exchanges.

IEEE Institute of Electrical and Electronics Engineers. Professional organization that defines networking and other standards. IETF Internet Engineering Task Force. IETF is a large open international community of network designers, operators, vendors, and researchers concerned with the evolution of the Internet architecture and the smooth operation of the Internet. It is open to any interested individual.

Intermediate frequency. IF is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator.

IF

IFM Interference fade margin. Expressed in decibels.

Incumbent local exchange carrier. The existing local exchange carrier in any given area, as opposed to CLECs.

ILEC

IMA Inverse multiplexing for ATM.

The term used by the International Telecommunications Union for the specification for the projected third-generation wireless services (3G). Formerly referred to as Future Public Land-Mobile Telephone Systems (FPLMTS).

IMT-2000

IN Intelligent network. A capability in the public telecom network environment that allows new services. Also implies a well-developed network infrastructure. INM Integrated Network Management. Internet The name given to the worldwide collection of networks and

gateways using the TCP/IP protocol that functions as a single virtual network.

IP Internet Protocol. See also TCP/IP.

444

Glossary

IPTV IP television. IPTV is defined (per ITU) as multimedia services

such as television/video/audio/text/graphics/data delivered over IP-based networks managed to provide the required level of quality of service and experience, security, interactivity and reliability.

IS-2000 Interim standard for CDMA2000 1X, also known as 1x and 1xRTT, is the core CDMA2000 wireless air interface standard. The designation “1x,” meaning 1 times Radio Transmission Technology, indicates the same RF bandwidth as IS-95 (a duplex pair of 1.25-MHz radio channels). IS-54–TDMA The first North American TDMA cellular system interim standard specification. After a number of revisions, it is known today as IS-136. IS-95 Interim standard 95. The first specification for the Qualcomm’s

CDMA wireless system. The brand name for IS-95 is cdmaOne. IS-95 is also known as TIA-EIA-95.

ISDN Integrated Services Digital Network. A digital public telecommunications network in which multiple services (voice, data, images, and video) can be provided via standard terminal interfaces. Offers 2 × 64 kbps over the landline network. ISM Instructional, Scientific, and Medical; microwave bands (2.4 GHz

and 5.8 GHz) that do not require licensing in the U.S.

ISO International Organization for Standardization. ISO is responsible

for a wide range of standards, including many that are relevant to networking. Their best-known contribution is the development of the OSI reference model and the OSI protocol suite.

An antenna that radiates in all directions (about a point) with a gain of unity (not a realizable antenna, but a useful concept in antenna theory). Used as a 0 dB gain reference in directivity calculation (gain).

Isotropic antenna

ISTO Industry Standards and Technology Organization. A not-for-profit

membership organization, affiliated with IEEE and IEEE Standards Association, the ISTO is a federation of programs that works to standardize technical implementations that span the spectrum of today’s electro-technologies.

International Telecommunications Union. Based in Geneva, the ITU is an organization of the UN that oversees telecommunications standards around the world.

ITU

Glossary

445

ITU-R Recommendations The international technical standards devel-

oped by the Radiocommunication Sector (formerly CCIR) of the ITU. They are the result of studies undertaken by Radiocommunication Study Groups on the use of the radio frequency spectrum in terrestrial and space radio communication including the use of satellite orbits and the characteristics and performance of radio systems. The interconnection of radio systems in public communication networks and the performance required for these interconnections are part of the ITU-T recommendations.

JISC Japanese Industrial Standards Committee. kA Kiloamperes. Unit of measurement for the electrical current. kHz Kilohertz; 1,000 cycles per second. LAN Local area network. A communication network covering a small

physical area, like a home, office, or small group of buildings, such as a school, or an airport.

Land usage The numerical classification of different surface types, e.g.,

water, forest, urban, and so on. Also called clutter.

The amount of time it takes a packet to travel from source to destination. Together, latency (delay) and bandwidth define the speed and capacity of a network.

Latency

LCT Local craft terminal; a PC with a web browser used for setup and configuration of microwave (or any other) terminal hardware. Leased line A permanent telephone connection between two points set up by a telecommunications common carrier. Typically, leased lines are used by businesses to connect geographically distant offices. Unlike normal dialup connections, a leased line is always active. The fee for the connection is a fixed monthly rate. The primary factors affecting the monthly fee are distance between end points and the bandwidth of the circuit. LMDS Local multipoint distribution services in the 28-GHz band. Local loop The general term for the line from a telephone customer’s

premises to the telephone company central office (CO).

Line of sight. An unobstructed radio path or link between the transmitting and receiving antennas of a communications system, i.e.,

LOS

446

Glossary

the transmission path is not established by or dependent upon reflection or diffraction. LTE

Line terminal equipment.

LTS

Laser transmission system. See FSO.

Media access control. MAC is a sublayer of the Data Link Layer specified in the seven-layer OSI model (layer 2).

MAC

The lobe containing the maximum power (exhibiting the greatest field strength). The horizontal radiation pattern is plotted as a function of azimuth around the antenna and is usually specified. The width of the main lobe is usually specified as the angle encompassed between the points where the power has fallen 3 dB below the maximum value. The vertical radiation pattern, plotted as a function of elevation from a specified azimuth, is also of interest, and may be similarly specified. Also called main beam.

Main lobe

MAN A Metropolitan Area Network (MAN) is a large computer network

that spans a metropolitan area or campus. Its geographic scope falls between a WAN and LAN. MANs provide Internet connectivity for LANs in a metropolitan region, and connect them to wider area networks like the Internet. A MAN usually interconnects a number of local area networks (LANs) using a high-capacity backbone technology, such as fiber-optical links, and provides uplink services to wide area networks and the Internet.

MEF Metro Ethernet Forum. MEF was founded in 2001 as a non-profit international industry consortium, dedicated to worldwide adoption of Carrier Ethernet networks and services.

Multiple-input multiple-output; a technique that minimizes signal fading due to path obstructions or atmospheric disturbances.

MIMO

Minimum bending radius The amount of bend that a fiber or copper cable can withstand before experiencing performance problems. MMDS Multichannel multipoint distribution services in the 2.1- and

2.7-GHz bands.

Monsoon Strong, often violent winds that change direction with the season. Monsoon winds blow from cold to warm regions so they blow from the land toward the sea in winter and from the sea toward land in the summer.

Glossary

447

MPLS Multiprotocol label switching. MPLS is a mechanism in high-

performance telecommunications networks that directs and carries data from one network node to the next. MPLS is a highly scalable, protocol agnostic, data-carrying mechanism. MPLS is called multiprotocol because it works with the Internet Protocol (IP), Asynchronous Transport Mode (ATM), and frame relay network protocols. The essence of MPLS is the generation of a short fixed-length label as a representation of an IP packet’s header. MPLS involves setting up a specific path for a given sequence of packets, identified by a label put in each packet, thus saving the time needed for a router to look up the address to the next node to forward the packet to.

MTBF Mean time between failures. MTBF is the predicted (calculated) elapsed time between inherent failures of a component/system during operation. MTBF should not be confused with a component’s useful life, since two concepts are not related in any way. MTSO Mobile telephone switching office. MTSO is the brain of a wire-

less network. It is responsible for assigning frequencies to each call, reassigning frequencies for handoffs, interconnecting calls with the local and long distance landline telephone companies, compiling billing information, etc. It also provides resources needed to efficiently serve a mobile subscriber such as registration, authentication, location updating and call routing. All cellular systems have at least one MTSO.

Mux Multiplexer (multiplexing). Combining two or more signals into a single bit stream that can be individually recovered.

A classification of the information capacity or bandwidth of a communication channel. Narrowband is generally taken to mean a bandwidth of 64 kbps or below.

Narrowband

NATE National Association of Tower Erectors. NATE is a non-profit trade association providing a unified voice for tower erection, maintenance and service companies. NATE is headquartered in Watertown, South Dakota.

National Council for Radiation Protection and Measurement. NCRP was chartered by the U.S. Congress in 1964 as the National Council on Radiation Protection and Measurements.

NCRP

NEBS Network Equipment Building Standards. NEBS covers a large

range of requirements including criteria for personnel safety, protection of property, and operational continuity. NEBS is a major test of

448

Glossary

quality that is extremely valuable for any organization supplying or purchasing network equipment. Originally developed by Bell Telephone Laboratories in the 1970s and expanded by Bellcore, these requirements are known as Network Equipment Building System (NEBS) Requirements. NEC The National Electrical Code (NEC) is a United States standard for the safe installation of electrical wiring and equipment. It is part of the National Fire Codes series (NFPA 70) published by the National Fire Protection Association (NFPA). The code is updated every three years. NEXT Near-end crosstalk. NF Noise figure. NIST National Institute of Standards and Technology. Founded in 1901,

NIST is a non-regulatory federal agency within the U.S. Department of Commerce. NIST’s mission is to promote U.S. innovation and industrial competitiveness by advancing measurement science, standards, and technology.

NIU Network interface unit; test unit installed at the demarcation point. NLOS Non-line-of-sight (NLOS) or near-line-of-sight is a term used to

describe radio transmission across a path that is partially obstructed, usually by a physical object in the Fresnel zone.

NMS Network Management System. NMS is a combination of hardware

and software used to monitor and administer a network. Individual network elements (NEs) in a network are managed by an element management system. Network-to-network interface. Demarcation between carrier Ethernet networks operated by one or more carriers.

NNI

NOC Network operations center. NOC is a location from which control is exercised over a computer, television broadcast, or telecommunications network. Although the term NOC is normally used when referring to telecommunications providers, there is a growing number of other organizations such as public utilities (e.g., SCADA) and private companies that also have such centers, both to manage their internal networks and to provide monitoring services.

Glossary

449

NTIA National Telecommunications and Information Administration. NTIA is an agency in the U.S. Department of Commerce that serves as the executive branch agency principally responsible for advising the President on telecommunications and information policies. NTIA also manages the Federal use of spectrum. NTS Naval Telecommunications System. N-WEST National Wireless Electronic Systems Testbed. OA&MP

Operations, administration, and maintenance provisioning.

OC-n Optical carrier, level n. For example, OC-1 is an optical carrier

level 1 (51.84 Mbps), OC-3 is an optical carrier level 3 (155 Mbps) OC-12 is an optical carrier level 12 (622 Mbps), and OC-48 is an optical carrier level 12 (2.4 Gbps).

OEM Original equipment manufacturer. A company whose products are used as components in another company’s product. The OEM will generally work closely with the company that sells the finished product and customize the designs based on their needs. OFDM

Orthogonal frequency division multiplexing.

OHLOSS Over-the-horizon loss. OM Optical multiplexer. OMC Operations and maintenance center. A location used to operate and maintain a wireless network. See NOC. Omnidirectional antenna An antenna that radiates and receives equally in all directions in azimuth. OOF Out of frame. OOS Out of synchronization or out of service. OPEX Operating expenses. OSHA Occupational Safety and Health Administration. Congress cre-

ated OSHA in 1971, under the Occupational Safety and Health Act, signed by President Richard M. Nixon in 1970. OSHA’s mission is to prevent work-related injuries, illnesses, and deaths.

450

Glossary

OSI Open System Interconnection. A seven-layer architecture model for communications systems developed by ISO and used as a reference model for most network architectures today. OSS Operations support systems. Computer systems (used by telecom-

munications service providers) that support processes such as maintaining network inventory, provisioning services, configuring network components, and managing faults.

OTDR Optical time domain reflectometer. OTDR is an optoelectronic instrument used for measurements on an optical fiber. An OTDR may be used for estimating the fiber’s length and overall attenuation, including splice and mated-connector losses. It may also be used to locate faults, such as breaks, and to measure optical return loss.

Power amplifier. There are different types of power amplifiers for different applications. A transmitter’s RF power amplifier is a type of electronic amplifier used to convert a low-power radio-frequency signal into a larger signal of significant power, typically for driving the antenna of a transmitter. It is usually optimized to have high efficiency, good linearity, good return loss on the input and output, good gain, and optimum heat dissipation.

PA

Packet switching A method of handling high-volume traffic that allows for efficient sharing of network resources, as packets from different sources can all be sent over the same channel in the bitstream. A packetswitched network breaks up the information into digital packets that are addressed and individually routed and then reassembled in the correct sequence at the destination. These networks allow the medium to be shared, so they are more efficient than circuit-switched networks. Packetized voice Any means by which voice traffic is split into packets and then transferred to its end destination. This category includes both IP and Internet telephony. Packets are sent separately over available network resources. The packets have headers denoting their place in the message, and when the destination is reached, the message is reassembled. The process is quick but not yet as quick as traditional telephony, resulting in a delay known as latency. PAM Pulse amplitude modulation. PAM is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses.

Peak-to-average power ratio. The measurement of peak-to-average power is a ratio of the maximum peak envelope power to the average power in a system.

PAPR

Glossary

451

The amount of loss introduced by the propagation environment between a transmitter and receiver.

Path loss

PBB-TE Provider backbone bridging—traffic engineering. PBB-TE is an

approved networking standard, IEEE 802.1Qay-2009. PBB-TE adapts Ethernet technology to carrier class transport networks.

PBX Private branch exchange; a private telephone switching system.

Pulse code modulation. A digital representation of an analog signal; the magnitude of the signal is sampled regularly at uniform intervals and then quantized to a series of symbols in a numeric (usually binary) code.

PCM

PCN Personal communication network. Wireless network in 1.9-GHz

band.

PCS Personal communications service. A generic term for a massmarket mobile personal communications service, independent of the technology used to provide it. PDH Plesiochronous digital hierarchy. PDH is an older technology used in telecommunications networks to transport data over digital transport equipment such as fibre optic and microwave radio systems. PDH is typically being replaced by Synchronous Digital Hierarchy (SDH) or Synchronous optical networking (SONET) equipment in most telecommunications networks. PDV

Packet delay variation.

PEP Peak envelope power. The average power supplied to the antenna transmission line by a radio transmitter during one radio frequency cycle at the crest of the modulation envelope taken under normal operating conditions.

Physical layer. The physical layer consists of the basic hardware transmission technologies of a network. The physical layer is the first and lowest layer in the seven-layer OSI model of computer networking.

PHY

PL Private line; a leased line, not switched. Plenum An air-handling space such as that found above drop-ceiling tiles or in raised floors. Also, a fire-code rating for an indoor cable.

452

Glossary

POI Point of interface.

Point of presence. In telecommunications, a point-of-presence (POP) is a demarcation point or interface point between different communications systems.

POP

POTS

Plain old telephone service.

ppm Parts per million. PPP Point-to-Point Protocol. In networking, PPP is a data link protocol

commonly used to establish a direct connection between two networking nodes. It can provide connection authentication, transmission encryption privacy, and compression. PPP is used over many types of physical media. A measure of the intensity of precipitation expressed by the increase in the height of water reaching the ground per unit time. Rain rate is generally expressed in millimeters (or inches) per hour. Also called rainfall rate or rain rate.

Precipitation rate

Propagation The process an electromagnetic wave undergoes as it radi-

ates from the antenna and spreads out across the physical terrain. The propagation channel is the physical medium of electromagnetic wave propagation between the transmit and receive antennas, and it includes everything that influences the propagation between the two antennas.

PRS Primary reference source. The master clocking source in a network. PSK Phase-shift keying. A digital modulation scheme that conveys information by changing, or modulating, the phase of a carrier wave. PSTN Public switched telephone network. The traditional, wired telephone network. PSU Power supply unit. PTT Post Telephone and Telegraph Company. A governmental agency

in many countries.

PUC Public Utilities Commission. A state regulatory body. QA Quality assurance, or QA for short, refers to planned and systematic production processes that provide confidence in a product’s suitability for its intended purpose.

Glossary

453

Quadrature amplitude modulation. In QAM, the amplitude of two waves, 90 degrees out-of-phase with each other (in quadrature), is changed (modulated or keyed) to represent the information signal. Amplitude modulating two carriers in quadrature can be equivalently viewed as both amplitude modulating and phase modulating a single carrier.

QAM

QoS Quality of service. A term used to characterize network availabil-

ity, quality, and reliability. It is also used to designate different classes of ATM service.

QPSK Quadrature phase-shift keying. PSK is a digital modulation scheme that conveys information by changing, or modulating, the phase of a carrier wave. QPSK is sometimes known as quaternary or quadriphase PSK, 4-PSK, or 4-QAM. It uses four points on the constellation diagram. With four phases, QPSK can encode two bits per symbol. QRSS Quasirandom signal sequence.

A graphical representation in either polar or rectangular coordinates of the spatial energy distribution of an antenna.

Radiation pattern

Radome A radio frequency transparent cover used to protect the antenna from wind load, snow and ice, or dust buildup that would otherwise cause excessive mechanical stress on the antenna and the tower structure and undesirable radiation pattern distortion. Add-on radomes are most often seen on parabolic antennas. RAN Radio Access Network (RAN) is the ground-based infrastructure

required for delivery of third-generation (3G) wireless communications services, including high-speed mobile access to the Internet. The RAN must be able to manage a wide range of tasks for each 3G user, including access, roaming, transparent connection to the public switched telephone network and the Internet, and Quality of Service (QoS) management for data and Web connections.

RBER Residual bit-error rate. RBOC The Regional Bell Operating Companies (RBOCs) are the result of the U.S. Department of Justice antitrust suit against the former American Telephone & Telegraph Company (later known as AT&T Corp.) On January 8, 1982, AT&T Corp. settled the suit and agreed to divest (“spin off ”) its local exchange service operating companies. Effective January 1, 1984, AT&T Corp.’s local operations were split

454

Glossary

into seven independent Regional Bell Operating Companies known as “Baby Bells.” RBOCs were originally known as Regional Holding Companies (RHCs). Receiver sensitivity The minimum RF signal power level required at the

input of a receiver for certain performance (e.g., BER).

Refraction A change in direction of propagating radio energy caused by

a change in the refractive index, or density, of a medium.

Remote Error Indication. Formerly called Far End Block Error (FEBE), an alarm signal used in synchronous optical networking (SONET). It indicates to the transmitting node that the receiver has detected a block error.

REI

REM Radio element manager. Repeater A device used to regenerate an optical or electrical signal (of

any frequency) to allow an increase in the system length.

RF Radio frequency. RF transparent Describes a material that is able to let RF signals pass through with minimal or no losses. Also called RF friendly. RFI Request for information. Request sent to equipment suppliers and/

or service providers to provide initial information for their hardware and/or services.

RFID Radio frequency identification. RFID is the use of miniature elec-

tronic circuits (tags) applied to or incorporated into a product, animal, or person for the purpose of identification and tracking using radio waves. RFID has many applications; for example, it is used in enterprise supply chain management to improve the efficiency of inventory tracking and management.

RFP Request for proposal. Request sent to equipment suppliers and/or service providers to provide complete and detailed solution (including pricing) for the specific item(s) that could be hardware and/or services. RFQ Request for quotation, sent to equipment suppliers and/or service

providers to provide a price for the specific item, which could be hardware and/or services.

RFT Request for tender. Same as RFP.

Glossary

455

Rigger (also called climber) is the member of the radio installation team responsible for installing the antenna and cabling on the transmission tower.

Rigger

RJ-11 Standard four-wire connectors for phone lines. RJ-45

works.

Standard eight-wire connectors for IEEE 802.3 1Base-T net-

RMS error Root-mean-square error. An error index that describes an average or mean error for observations made under the same (or similar) measurement conditions. Also called standard error or standard deviation.

Return on investment. The time it takes to pay back (or recoup) the money invested in a technology or strategy.

ROI

Rollout Implementing (deploying) a product or a system. Synonym with “build-out.” RSL Received signal level. The signal level at the receiver input (from the antenna), usually expressed in dBm.

Received signal strength indicator. RSSI is a measurement of the power present in a received radio signal. RSSI output is often a DC analog level.

RSSI

RSTP Rapid Spanning Tree Protocol. RSTP is an evolution of the STP

(see Spanning tree topology), a link layer network protocol that ensures a loop-free topology for any bridged LAN. RSTP was created to provide faster recovery (convergence time) from topology changes.

RTU Remote telemetry unit. Used for monitoring and reporting to a remote location (over a telephone line) the condition of an associated piece of equipment at predetermined times, when polled and when the equipment condition dictates. RZ Return to zero. RZ describes a line code used in telecommunications

signals in which the signal drops (returns) to zero between each pulse.

SAR Specific absorption rate. A measure of the rate of energy absorbed

by (dissipated in) an incremental mass contained in a volume element of dielectric materials such as biological tissues. SAR is usually expressed in terms of watts per kilogram (W/kg) or milliwatts per gram (mW/g).

456

Glossary

Guidelines for human exposure to RF fields are based on SAR thresholds where adverse biological effects may occur. When the human body is exposed to an RF field, the SAR experienced is proportional to the squared value of the electric field strength induced in the body. SCADA Supervisory control and data; the monitor/control of pipelines, railroads, electrical utility networks, and so on. SCC The Standards Council of Canada (SCC) offers a variety of programs and services to organizations and individuals who deal either directly, or indirectly with standardization issues. The Standards Council of Canada serves as Canada’s World Trade Organization and North America Free Trade Agreement (WTO/NAFTA) Enquiry Point. Scintillation Rapid and random fluctuation in one or more of the characteristics (amplitude, phase, polarization, direction of arrival) of a received signal, caused by refractive index fluctuations of the transmission medium. Also a synonym for a rapid, shallow fading. SD See Space diversity. SDH Synchronous digital hierarchy. SDR Software-defined radio. SDR system is a radio communication system where components that have typically been implemented in hardware (e.g. mixers, filters, amplifiers, modulators/demodulators, detectors. etc.) are instead implemented using software. Such a design produces a radio that can receive and transmit a different form of radio protocol just by running different software. SDRs have significant utility for the military and cell phone services, serving a wide variety of changing radio protocols in real time. SDRM Subrate digital multiplexer. Digital multiplexers, operating at DS1 level, which are used to provide data channels at 64 kbps (DS0 level) and at various subrates such as 2.4, 4.8, 9.6, 19.2, and 56 kbps. SES Severely errored seconds. Seconds with 10 SESR

-3

BER or worse.

Severely errored second ratio.

SF Superframe format; a DS1 framing format of 12 frames. Sidelobes The radiation lobes in any direction other than that of the

main lobe.

Glossary

457

Site plans A set of drawings that show the makeup of a designated area for construction; included are measurements of existing structures, proposed dimensions of new structures, and other needed information about the site.

An action performed to gather information about a proposed area for construction. Also, it can be performed during and after the construction and/or equipment installation.

Site walk

Service level agreement. The contract between a provider and a customer that sets the amount of bandwidth and quality of service, among other things.

SLA

SLC Subscriber loop carrier. It refers to equipment providing central office-like telephone interface functionality. Smart jack Network interface unit with cross-connecting and monitoring capabilities. SMR Specialized mobile radio. A conventional two-way radio system, or trunked radio system, operated by a service in the 800- or 900-MHz bands. Some systems with advanced features are referred to as an Enhanced Specialized Mobile Radio (ESMR). SNMP Simple Network Management Protocol, a set of protocols for man-

aging complex networks. SNMP works by sending messages, called protocol data units (PDUs), to different parts of a network. SNMP-compliant devices, called agents, store data about themselves in management information bases (MIBs) and return this data to the SNMP requesters.

SNR Signal-to-noise ratio, defined as

SNR (dB) = signal level (dBm) – noise level (dBm) SOH SONET overhead channel. SoHo

Small office home office.

SONET Synchronous optical network.

A diversity technique widely used in radio communications since the very beginning. It consists of two receive antennas physically (spatially) separated to provide uncorrelated receive signals.

Space diversity

Spanning tree topology Defined by standard IEEE 802.1d, a scheme used by Layer 2 switches to automatically inactivate certain network

458

Glossary

links so that traffic will have only one path between a specific source and destination and will not travel endlessly in loops. Spanning tree is a self-learning protocol that automatically reconfigures itself if any network link fails to send traffic over another path if possible, although this reconfiguration time can be relatively slow in light of today’s networking speeds. In a spanning tree there is exactly one path from every node to every other node. It uses the least amount of links to connect all nodes but offers no redundancy. See also RSTP, Rapid Spanning Tree Protocol. SRDM Subrate data multiplexing.

Spread spectrum. Spread-spectrum techniques are methods by which electromagnetic energy generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth.

SS

SS7 Signaling system 7. An architecture for performing out-of-band signaling in support of the call-establishment, billing, routing, and information-exchange functions of the public switched telephone network (PSTN). It identifies functions to be performed by a signaling-system network and a protocol to enable their performance. Standard radio atmosphere

tivity gradient.

An atmosphere having the standard refrac-

Statistical Time Division Multiplexing. Time slots are assigned to signals dynamically to make better use of the bandwidth.

STDM

STM Synchronous transfer module. An ITU-defined communications method that transmits a group of time division multiplexed (TDM) streams synchronized to a common reference clock. It reserves bandwidth according to a rigid hierarchy, regardless of actual channel usage.

Synchronous transfer module—Level N. (STM-1 equals 155 Mbps).

STM–N

STP Spanning Tree Protocol. Stratum Level of clock source used to categorize accuracy. STS Synchronous transport signal.

Glossary

459

Subrefraction Refraction for which the refractivity gradient is greater (i.e., positive or less negative) than the standard refractivity gradient.

Refraction for which the refractivity gradient is less (i.e., more negative) than the standard refractivity gradient.

Super-refraction

SWR Standing wave ratio. Synchronous Type of transmission in which the transmission and reception of all data is synchronized by a common clock, and the data is usually transmitted in blocks rather than individual characters. T1 A North-American transmission standard of 1.544 Mbps. T-BERD

A trade name for T1 bit-error rate tester made by TTC.

Telemetry Byte Oriented Serial. TBOS is a communications protocol for the microcomputer link that monitors and controls remote equipment over an RS-422 serial telemetry communication line. E-telemetry systems were developed as a method to monitor and control diverse network elements from a remote, centralized location.

TBOS

TCO

Total cost of ownership.

TCP/IP Transmission Control Protocol/Internet Protocol. The data pro-

tocols used for the Internet.

TDD Time division duplexing. TDD is the application of time-division

multiplexing to separate outward and return signals. It emulates fullduplex communication over a half-duplex communication link. Time division duplex has a strong advantage in the case where the asymmetry of the uplink and downlink data speed is variable.

TDM Time division multiplexing. TDM is a type of digital multiplex-

ing in which two or more signals or bit streams are transferred in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent timeslots of fixed length, one for each sub-channel. TDM is used for circuit-switched communication with a fixed number of channels and constant bandwidth per channel.

TDMA Time division multiple access. A digital transmission technique

used for GSM, D-AMPS (IS-136), and PDC air interfaces. D-AMPS in North America is often just called TDMA.

460

Glossary

TE Terminal equipment. Telcordia Technologies Formerly Bellcore, an SAIC company, this is the world’s largest provider of operations support systems, network software, and consulting and engineering services to the telecommunications industry. TFM Thermal fade margin. Expressed in decibels. TIA Telecommunications Industry Association. The U.S. telecom industry

standards body.

TM Terminal multiplexer. TMN Total/Telecommunications Management Network (ITU-T Recommendation M.3010). TND Transmission (transport) network design. Topographical map A map that describes the elevation of the terrain, its nature, and built-up areas. UAS Unavailable Seconds (UAS) are calculated by counting the number of seconds that the interface is unavailable. The DS1 interface is said to be unavailable from the onset of ten contiguous SESs, i.e. seconds with BER worse than 10-3. UHF Ultra-high frequency. UHF designates a range of electromagnetic

waves with frequencies between 300 MHz and 3 GHz (3,000 MHz).

Underwriters Laboratory. A nonprofit laboratory that examines and tests devices, materials, and systems for safety and has begun to establish safety standards.

UL

ULS Universal licensing system. In an Order adopted September 17, 1998, the Federal Communications Commission facilitates its implementation of the Universal Licensing System (ULS), and consolidates, revises, and streamlines rules governing application procedures for radio services licensed by the Wireless Telecommunications Bureau. ULS simplifies the application and licensing processes and provides secure, world-wide access through the Internet. This results in reduced filing time and financial savings for both customers and the U.S. federal government.

Glossary

461

Universal Mobile Telecommunications System. A 3G mobile technology that delivers broadband information at speeds up to 2 Mbps.

UMTS

UNI User-to-network interface. Physical interface/demarcation between

the service provider, cable operator, carrier, and subscriber.

Unlicensed National Information Infrastructure; a 5-GHz microwave band that does not require licensing in the U.S.

U-NII

Unstructured 1.544 Mbps or 2.048 Mbps link Link without frame, where all bits are used as one single 1.544-Mbps (or 2.048-Mbps) data channel.

Some multipath conditions cause a signal’s amplitude to be increased because signals travelling by different paths arrive at the receiver in phase and become additive to the main signal. As a result, the total signal that reaches the receiver will be stronger than the signal would otherwise have been without the multipath conditions.

Upfade

UPS Uninterruptible power supply, also known as a battery backup,

provides emergency power from a separate source when utility power is not available. It differs from an auxiliary or emergency power system or standby generator, which does not provide instant protection from a momentary power interruption. A UPS, however, can be used to provide uninterrupted power to equipment, typically for up to 30 minutes until an auxiliary power supply can be turned on, utility power restored, or equipment safely shut down.

UPSR Unidirectional path-switched ring. A two-fiber configuration, the unidirectional path switched ring (UPSR) is a simple SONET protection mechanism. All payloads are carried clockwise on a single fiber. The payloads are duplicated counterclockwise on a second (protection) fiber, which is unutilized in the absence of failure. USGS United States Geological Survey. As an unbiased, multi-disci-

plinary science organization that focuses on biology, geography, geology, geospatial information, and water.

UTC Universal time coordinated; an international time standard. This

is the current term for what used to be known as Greenwich mean time (GMT). In-depth information about time and time-related issues can be found at http://tycho.usno.navy.mil.

UTM Universal transverse mercator. UTM coordinate system is a gridbased method of specifying locations on the surface of the Earth.

462

Glossary

UTP Unshielded twisted pair. UTP cable is probably the most popular cable around the world. UTP cable is used not only for networking but also for the traditional telephone (UTP-Cat 1). There are six different types of UTP categories. Category 1/2/3/4/5/6 is a specification for the type of copper wire (most telephone and network wire is copper) and jacks. CAT1 is typically telephone wire, not capable of supporting computer network traffic and is not twisted. CAT2 is used mostly for token ring networks, supporting speeds up to 4 Mbps. For higher network speeds (100 Mbps plus) CAT5 wire is used, but for 10 Mbps CAT3 will suffice. CAT3, CAT4, and CAT5 cable are actually four pairs of twisted copper wires and CAT5 has more twists per inch than CAT3 and therefore can run at higher speeds and greater lengths. The “twist” effect of each pair in the cables will cause reduction in interference from other cables and between pairs within the same cable. CAT3 and CAT4 are both used for Token Ring and have a maximum length of 100 meters. CAT6 wire was originally designed to support gigabit Ethernet (although there are standards that will allow gigabit transmission over CAT5 wire, that’s CAT 5e). It is similar to CAT5 wire, but contains a physical separator between the four pairs to further reduce electromagnetic interference. UWB Ultrawideband. UWB is a radio frequency band that can be used

at very low energy levels for short-range/broadband communications. UWB refers to any radio technology having bandwidth exceeding the lesser of 500 MHz or 20 percent of the arithmetic center frequency, according to Federal Communications Commission (FCC).

VF Voice frequency. A frequency within the audio range. In telephony,

the usable voice frequency band ranges from approximately 300 Hz to 3,400 Hz. The bandwidth allocated for a single voice-frequency transmission channel is usually 4 kHz, including guard bands, allowing a sampling rate of 8 kHz to be used as the basis of the pulse code modulation system used for the digital transmission of voice.

Very high frequency. VHF is the radio frequency range from 30 MHz to 300 MHz.

VHF

VLAN Virtual LAN. VLANs can logically group networks so that the network location of users is no longer so tightly coupled to their physical location. VLANs provide the flexibility to adapt to changes in network requirements and allow for simplified administration.

Voice over IP. A general term for a family of transmission technologies for delivery of voice communications over IP networks such as the Internet or other packet-switched networks.

VoIP

Glossary

463

VRLA Valve-regulated lead-acid battery. VRLA battery is one of many

types of lead-acid batteries. Lead-acid batteries, invented in 1859 by French physicist Gaston Planté, are the oldest type of rechargeable battery. In a VRLA battery the hydrogen and oxygen produced in the cells largely recombine back into water. In this way there is minimal leakage, though some electrolyte still escapes if the recombination cannot keep up with gas evolution. Valve-regulated lead acid batteries cannot spill their electrolyte.

VSWR Voltage standing wave ratio. The voltage standing wave ratio is a measure of how well a load is impedance-matched to a source. The value of VSWR is always expressed as a ratio with number one in the denominator (2:1, 3:1, 10: 1, etc.) VT Virtual tributary. VT is a signal designed for transport and switch-

ing of sub-STS-1 payloads.

WAAS Wide area augmentation system. A system of satellites and ground stations that provide GPS signal corrections, giving better position accuracy. Currently, WAAS satellite coverage is only available in North America.

A wide area network (WAN) is a computer network that covers a broad area and used to connect LANs and other types of networks together.

WAN

WCDMA Wideband CDMA (the Ericsson-Nokia version of CDMA). A technology for wideband digital radio communications of Internet, multimedia, video, and other capacity-demanding applications. WCDMA has been selected for the third generation of mobile telephone systems in Europe, Japan, and the United States. WDM Wavelength division multiplexing. An optical transmission technique in which two or more wavelengths (each carrying its own information) are combined for transmission over a single optical fiber. At the receiving end, the wavelengths are separated and directed to separate receivers. It is used to increase the capacity of data transmission over optical fibers.

A classification of the information capacity or bandwidth of a communication channel. Wideband is generally taken to mean a bandwidth between 64 kbps and 2 Mbps.

Wideband

WiMAX Worldwide for Microwave Interoperability Access. The current standard for Broadband Wireless MAN networks that will provide

464

Glossary

a wireless alternative to cable, DSL, and T1/E1 for last-mile broadband access. WiMAX Forum A nonprofit organization formed to promote the adoption

of WiMAX-compatible products and services.

Loading imposed on an element (communications tower, for example) from the effects of the wind.

Wind load

Wireless communication facilities A land-use facility supporting antennas and microwave dishes that send and/or receive radio frequency signals that provide commercial mobile services, unlicensed wireless services, and common carrier wireless exchange access services. The facilities include structures, towers, and accessory buildings. See pictures at http://gallery.wirelessadvisor.com/.

A system that uses radio transmitters and receivers in place of wire lines; when connected to the evolving public switched network, it provides comprehensive telephone service to customers.

Wireless communications

Wireless Internet Service Provider. WISPs are Internet service providers with networks built utilizing wireless systems either in licensed or license-exempt frequency bands.

WISP

WLAN Wireless local area network. Wireless LAN access is currently

available in three formats, depending on the application needed. The three formats are 900 MHz (best range for in-building LANs with a maximum data rate of 1 Mbps), 2.4 GHz (allows for higher data rates of 11 Mbps), and 5 GHz for the highest data rate of up to 54 Mbps and least range.

WLL Wireless local loop. WLL is a term for the use of a wireless com-

munications link as the “last mile / first mile” connection for delivering plain old telephone service (POTS) and/or broadband Internet to telecommunications customers. Various types of WLL systems and technologies exist.

xDSL Generic digital subscriber line. DSL or xDSL is a family of dif-

ferent technologies that provides digital data transmission over the wires of a local telephone network. The download speed of consumer DSL services typically ranges from 384 kbps to 20 Mbps, depending on DSL technology, line conditions and service-level implementation.

Glossary

465

Typically, upload speed is lower than download speed for the common ADSL (Asymmetric Digital Subscriber Line). XPI Cross-polar interference. The energy received in the wanted (transmitted) polarization from the unwanted (orthogonal) polarization. XPD Cross-polarization discrimination. The ratio between the energy

received in the wanted (transmitted) polarization to that received in the unwanted (orthogonal) polarization.

XPIC Croos-polarization interference canceller. Adaptive coupling circuit

between two orthogonal cofrequency channels or two alternated adjacent channels, on the same link, used to reduce cross-polar interference during adverse propagation conditions.

Zero code suppression The insertion of a 1 bit to prevent the transmis-

sion of eight or more consecutive 0s. Used to guarantee minimum pulse density.

This page intentionally left blank

Index

A

AAL (ATM adaptation layer), 14 AAL2 (ATM adaptation layer 2), 14 above ground level (AGL), 291 above mean sea level (AMSL), 291 absorbent glass mat (AGM), 336 absorption loss, 93 AC power, 333–334 acceptance test. See ATP (acceptance test procedure) active repeaters, 150–151, 155 activities, project management, 357–358, 390 adaptive channel equalizers, 55 adaptive equalizers, 226–227 adaptive modulation, 55, 133–136, 413 adaptive time domain equalizers (ATDE), 227 adjacent-channel interference, 194, 196 adjacent-channel interference fade margin (AIFM), 109–110, 112 ADMs (add/drop multiplexers), 10 aerosols, radio propagation and, 52 AES (Advanced Encryption Standard), 29–30 AGC voltage, 273, 277, 344 AGL (above ground level), 291 AGM (absorbent glass mat), 336 AIFM (adjacent-channel interference fade margin), 109–110, 112 AIS (alarm indication signals), 221–222 alignment, antenna installation, 272–273, 276–277 allocation, of frequency band, 196 allotment, or RF or RF channel, 196–197 alternative energy sources, 338–340 alternative mounting structures, 324 altitude, antenna installation, 273 altitude, geodetic, 291, 312–315 AM (amplitude modulation), 85 American cable stranding, 417–418 Ampere’s Law of Magnetostatics, 34 amplitude, digital modulation of, 84–86 amplitude modulation (AM), 85 AMSL (above mean sea level), 291 angle diversity, 139–140, 147–148

anomalous propagation mechanisms, 48–52 ANSI (American National Standards Institute), 245, 320–321 antenna-coupling unit, 65 Antenna Structure Registration (ASR), 303 antennas, microwave alignment, 276–277 concealment, 406 configuration, 219–220 gain, 61, 262–264 ground reflection protection, 98–99 for harsh environments, 284–288 height, 275–276 installation, 272–275 mounting. See towers orienting to true north, 298 radiation patterns, 263, 266–267 selection, 270–271 site acquisition process, 364 specifications, 261–268 transmitting RF energy, 246 antireflective systems, 148–150 applications, microwave systems, 55–56 approval, project, 362–363 APS (automatic protection switching), 20–21 as-built documentation, 371–372 ASR (Antenna Structure Registration), 303 ATDE (adaptive time domain equalizers), 227 ATDM (asynchronous time-division multiplexing), 256 ATM adaptation layer (AAL), 14 ATM adaptation layer 2 (AAL2), 14 ATM (asynchronous transfer mode) 3G networks using, 164–165 delay caused by switches, 28 effect on microwave link planning, 165–166 microwave point-multipoint system architecture, 76 overview of, 13–15 atmosphere, Earth’s, 6–7, 39–40 ATP (acceptance test procedure), 343–346, 371, 382

467

468

Index

ATPC (automatic transmit power control), 134, 231–232 ATR (available time ratio), 122 attenuation, 6, 53–54. See also loss/ attenuation calculations attenuator, 220 automatic protection switching (APS), 20–21 availability calculations, 112–114 designing for, 413 of different network topologies, 208–212 equipment, 206–208 microwave link objectives, 120–126 network performance and, 22–23 reliability vs., 137 AWG (American Wire Gauge), 417–418 azimuth, 273

B

back-to-back antennas, 155–158 background block error (BBE), 123 background block error ratio (BBER), 122–124 background error rate. See BER (background error rate) backhaul, 15–18, 161–162, 348–349 backhoe fade, 3 bandwidth antenna specifications, 267–268 efficiency, 84 fiber-optic, 3 over-subscription and, 183 T1 systems, 8 base station controller (BSC), 14, 162–163 baseband switching, 142 basic rate, 81 batteries, 318–320, 336–337 baud rate, 97 BBE (background block error), 123 BBER (background block error ratio), 122–124 BCM (block coded modulation), 234–235 BE (Best Effort Service), WiMAX, 83 beam efficiency, antennas, 267 Bellcore objectives, 117 BER (background error rate) ATM requirements, 15 long-term measurements, 352–353 microwave radio configuration, 224 network performance and, 22–23 outages and availability, 113–114 quality and availability calculations, 121–123 receiver sensitivity threshold for TDM traffic, 221 RF requirements, 84 using constellation diagram, 353–355 BERT (bit error rate testing), 346–347 Best Effort Service (BES), WiMAX, 83 bid specification, 367 billboard repeaters, 151–157, 188

binary phase shift keying (BPSK), 87, 232 BIP (bit interleaved parity), 122 bit error rate testing (BERT), 346–347 bit interleaved parity (BIP), 122 bit rate, 87, 220 bits (binary numbers), 29–30 blackout fading, 50, 413 BLER (block error rate), 123 block, 122 block coded modulation (BCM), 234–235 block error rate (BLER), 123 Bluetooth, 79–81 Boltzmann, Ludwig, 34–35 BoM (Bill of Material), 364–366 BPSK (binary phase shift keying), 87, 232 branching losses, 65, 90 Braun, Karl Ferdinand, 35 bridges, Layer 2, 15–16 BSC (base station controller), 14, 162–163 burst test, 350

C

C/I (carrier-to-interference), 110 C/N curve, 224 “c” suffix channels, 11–12 cabinets, equipment, 318 cable stranding, American, 417–418 cabling, and signal termination, 258–261 capacity, 162–163, 177–178, 406 carbon dioxide, 39 carrier Ethernet, 16–18, 24–25 carrier-to-interference (C/I), 110 Cassegrain antenna feed, 269 Causebrook, 97 CCDP (co-channel dual polarized) applications, 228 CCIR Rep. 338, 114 CDMA (code division multiple access), 28, 68 CE mark, 379 cell on wheels (COW), 72–73 cells, 13 CEPT (Conference of Postal and Telecommunications Administration), 7, 9 CFM (composite fade margin), 112 chain/tandem topology, 178, 200–202, 208–209 change orders, 385 channeling plan, 177 channelized links, 12 channels, 197–199, 232–234 circuit emulation, 401–402 Clarke 1866, 289, 293 clear channel, 11, 346 clearance criteria, 60 clearance rules, 42, 58–61 co-channel dual polarized (CCDP) applications, 228 co-channel interference, 194–195 coaxial cables, 3, 277–280

Index code division multiple access (CDMA), 28, 68 code of engineering ethics, 392–394 coded modulation, 234–235 COFDM (coded OFDM), 238 Cofetel, 5 collocation, 191–196, 320, 327 commissioning, 372 compatibility, 245–246 composite fade margin (CFM), 112 concealment, antenna, 406 Conference of Postal and Telecommunications Administration (CEPT), 7, 9 configuration management, 244 connecting blocks, 261 consecutive severely errored seconds (CSES), 123, 225, 401 constellation diagram, 353–355 consultants, 387 contour maps, 289 contractors, 387 controls, project management, 381–382 convolutional codes, 234 coordinate systems, 290–294, 408 coordinated universal time (UTC) frequency, 25 copper lines, 2–3 corrosive environments, antennas for, 284–285 coupling loss, 194 coverage, design for, 162–163 COW (cell on wheels), 72–73 Crane rain outage model, 127–130, 132–133, 306 CRC (cyclic redundancy check), 122 critical gradient, 49 cross-polarization, 18, 227–231, 265 crossband diversity, 134, 145 CSES (consecutive severely errored seconds), 123, 225, 401 customer questionnaire, 160

D

DACS (digital access cross-connects), 21 DADE (diversity antenna delay equalization), 274 daisy-chain topology, 178 Data Encryption Standard (DES), 29–30 data switching, 235, 274 datums, 289–290, 408 dB (decibels), 420 dBm, 420–421 dBW, 420–421 DC rectifiers, 334–335 dedicated DS1 master timing source, 25 dedicated service, 172 delay, 28–29 DEM (digital elevation model) data, 295–296, 409 dense wavelength division multiplexing (DWDM), 3, 12

469

deployment, digital microwave radio, 218–256 basic parameters, 218–224 compatibility and safety, 245–246 configurations, 218 duplexers, 240–242 environmental requirements, 242–243 Ethernet microwave radio, 239 microwave radio installation, 246–247 millimeter-wave. See millimeter-wave point-to-point systems network management system, 243–245 performance. See performance improvement service telephone network, 240 T/I curves, 239–240 deployment, microwave antenna alignment, 276–277 antenna height, 275–276 antenna installation, 272–275 antenna mounting structures. See towers antenna selection, 270–271 antenna specifications, 261–268 antennas for harsh environments, 284–288 cabling and signal termination, 258–261 digital multiplexers, 256–258 digital point-to-point. See PTP (point-to-point) microwave systems field surveys. See field surveys GIS data. See GIS (Geographic Information Systems) data grounding and surge protection, 340–342 housing equipment, 317–320 impedance, 282–284 installation safety and security, 280–282 introduction, 217–218 parabolic antenna feed methods, 268–270 power supply and battery backup, 333–340 radomes and shrouds, 271–272 testing and troubleshooting, 342–355 transmission lines, 277–280 DES (Data Encryption Standard), 29–30 desert areas, multipath in, 411 design process flowchart, 89–90 designing microwave networks frequency planning, 196–203 interference effects and frequency sharing, 189–196 introduction, 185 planning phase. See planning microwave networks rollout phase, 360, 364–366 spectrum management, 185–189 systems engineering, 204–212 tips, hints and suggestions, 212–215 tools, 203, 406

470

Index

deterministic multiplexing, 165 Deygout method, 97 DFM (dispersive fade margin), 105–106, 112 DFS (dynamic frequency selection), 69–70 DGPS (Differential Global Positioning System), 315 Differential Global Positioning System (DGPS), 315 diffraction, 44, 47–48, 95–96 digital access cross-connects (DACS), 21 digital elevation model (DEM) data, 295–296, 409 digital microwave radio deployment. See deployment, digital microwave radio digital modulation, 84–86 digital multiplexers, 256–258 digital point-to-point systems. See PTP (point-to-point) microwave systems digital signal cross-connects (DSX), 7, 260–261 digital terrain model (DTM), 295–296 direct sequence (DS), 67–68 directional power flux density, 61 directive gain, 262 discrimination, antenna, 111, 196, 199–200 dispersive fade margin (DFM), 105–106, 112 diversity angle, 147–148 frequency, 144–146, 415 gain, 140 hybrid, 146–147 improvement factor, 140 media, 148 microwave radio configuration, 219 overview of, 138–140 preventing rain-attenuation fade, 134 propagation impairment and, 55 vs. protection, 400–401 route, 148 space, 99, 140–144, 415 diversity antenna delay equalization (DADE), 274 documentation, 371–372, 382–385 DoD (Department of Defense), 186, 295 double-plane reflectors, 151, 154–155 downfade, 100 drop-insert, 56, 58, 257 DS (direct sequence), 67–68 DS1 circuits, 7–8, 24–26, 126 DS3 circuits, 126 DSX (digital signal cross-connects), 7, 260–261 DTM (digital terrain model), 295–296 dual-homed rings, 20, 171 ducts, 49–52, 54, 413–414 duplex spacing, 197–198 duplexers, 240–242 dust, 52–54 DWDM (dense wavelength division multiplexing), 3, 12

dynamic bandwidth allocation, 76 dynamic frequency selection (DFS), 69–70

E

E band, 249–252 E-fields (electric field lines), 34, 35–36 E1 circuits frequently asked questions, 397–398 overview of, 7–9 ring architecture reliability and, 20 topology and capacity planning, 177 wireless network synchronization scheme, 26 Earth bulge, 275–276 circumference, 311 radius, 42–46 earth electrode, 340 earthquake effects, tower placement, 330 Earth’s atmosphere, 6–7, 39–40. See also radio propagation EB (errored block), 122 ECMWF (European Centre for MediumRange Weather Forecast), 128 EDCs (error detection codes), 122 EGNOS (Euro Geostationary Navigation Overlay Service), 317 EIA-222-G, 320 EIRP (effective isotropic radiated power) 60-GHZ license-exempt band and, 253 microwave radio configuration, 223–224 overview of, 69 quick reference sheet, 421 electric transmission towers, 404–405 electrodynamics, laws of, 33–34 electromagnetic field, 36 electromagnetic radiation (EMR), 33, 35–37, 42 electromagnetic spectrum, 37–39 electromagnetic theory of light, 33 elevation, 312–315 ellipsoid, 312–315 empirical prediction models, 89–90 EMR (electromagnetic radiation), 33, 35–37, 42 encryption, 29–31 end-to-end microwave system, 56 end-to-end testing, 347 Endangered Species Act (ESA), 380 engineering code of ethics, 392–393 for microwave systems, 204–212 millimeter-wave link, 254–256 enhanced data rate, 81 environment antennas for harsh, 284–288 digital microwave radio requirements, 242–243 path surveys in dense urban areas, 306–308 EOW (engineer order wire), 240 EPS (Evolved Packet System), 83

Index Epstain-Peterson method, 97 equipment availability calculations, 206–208 BoM for detailed network design, 365–366 housing, 317–320 installation, 369–371, 399 microwave radio configuration, 219 procurement, 366–369 erosion, and tower placement, 330 error detection codes (EDCs), 122 error performance definitions, 122–123 effect of ATM on MW link planning, 166–167 ITU objectives, 125–126 long-term BER measurements for, 352–353 North American objectives, 126 transmission quality parameters, 346–347 errored block (EB), 122 errorless receiver data switching, 235 ES (errored second), 113, 122 ESA (Endangered Species Act), 380 ESR (errored second ratio), 122–124 EthAIS (Ethernet alarm indication signal), 222 ether, 33 Ethernet backhaul, 15–18 microwave radio, 239 testing in mobile backhaul, 348–349 timing islands created by, 26 using PTP for timing and synchronization, 27 Ethernet alarm indication signal (EthAIS), 222 ethical issues, 392–397 Euro Geostationary Navigation Overlay Service (EGNOS), 317 Eurofix system, 315 European Centre for Medium-Range Weather Forecast (ECMWF), 128 evaporation ducts, 51 EVC (Ethernet virtual connections), 17 Evolved Packet System (EPS), 83

F

F/D ratio, antennas, 268 F.1330, 345 F.1491, 126 F.1493, 126 F.1668, 126 F.1703, 126 FAA (Federal Aviation Administration), 316, 373–375 factory acceptance testing, 342–343 fade margin composite fade margin (CFM), 112 dispersive fade margin (DFM), 105–106 interference, 109–112

471

microwave radio configuration, 224 preventing rain-attenuation fade, 134 quick reference on, 421 radio path link budget and, 65 fade season, 102 fading multipath, 99–106 outages and availability, 112–114 quality and availability, 120–126 rain, 106–108 refraction-diffraction, 108–109 types of, 99 failures in time (FITS), equipment, 206–208 far field, 34, 61–64 far interference, 193–194 Faraday’s Law, 34 fast facility protection (FFP), 21 fault management, 243 FCC (Federal Communications Commission) regulation frequency coordination, 376–378 license-exempt bands, 67 low frequency bands and, 202 Part 101 rules, 187 rapid deployment, 67–72 regulatory issues, 373–375 RF spectrum, 38–39 FDD (frequency-division duplex), 26–27, 77, 82–83 FDM (frequency-division multiplexing), 1, 256 FEC (forward error correction), 29, 227 Federal Aviation Administration (FAA), 316, 373–375 feeder, antenna, 262 FFP (fast facility protection), 21 FHSS (frequency hopping), 68, 71, 80 fiber-optic cables, 260 fiber-optic transmission, 2, 3, 170 field surveys path surveys, 301–308 site surveys, 298–301 using GPS for field measurements, 308–317 first article, 342–343 first Fresnel zones, 58–59 Fish and Wildlife Service (FWS), 380–381 FITS (failures in time), equipment, 206–208 five nines, 22–23 fixed-satellite service, 4 flash method, marking site, 305, 308 flat fading, 102–103, 120–126, 412 flat networks, 19 flooded batteries, 336–337 FM/PM (frequency/phase modulation), 85 fog, 410 Form 601, FCC, 376 forward error correction (FEC), 29, 227 4G wireless networks, 28, 78, 167–171 frame loss test, 350

472

Index

free-space laser communications, 5–7, 398–399 path loss, 91–92, 422 free-space optical (FSO), 5–7 frequency-division duplex (FDD), 26–27, 77, 82–83, 241 frequency-division multiplexing (FDM), 1, 256 frequency (f) Bluetooth bands, 80 coordination, 188, 365, 376–378, 403 designing, 189–196 digital modulation, 84–86 diversity, 139–140, 144–147, 415 gas attenuation vs., 93–94 hopping. See FHSS (frequency hopping) measuring electromagnetic energy, 36–37 planning, 196–203 RF, 4–5 sharing, 189–190 transmit, 219 wavelength and, 419 frequency hopping (FHSS), 68, 71, 80 frequency-selective fading, 103–106 frequency shift keying (FSK), 85 frequently asked questions, project management, 397–416 Fresnel, Augustin, 42 Fresnel zones and clearance rules, 58–61 defined, 42 k-factor and, 45–46 minimum antenna height, 275–276 Friis free-space path loss model, 91 front feed, antennas, 269 front-to-back ratio, antennas, 267 FSK (frequency shift keying), 85 FSO (free-space optical), 5–7 fuel cells, 337–338 FWS (Fish and Wildlife Service), 380–381

G

G-Series Recommendations, 121–122 G.801, 125–126 G.8021, 21 G.8032, 20 G.810, 25 G.821, 125–126 G.826, 122–124, 125–126 G.828, 124, 166 Galileo (global navigation satellite system), 309 gas absorption, 93–94, 253 Gauss’s Law, 34 gel cell batteries, 336 geodetic datums, 289–290 Geographic Information Systems. See GIS (Geographic Information Systems) data geographical coordinates, 291

geoid, 312–315 GigE (Gigabit Ethernet), 18, 239, 349 gin pole, 281 Giovanelli method, 97 GIS (Geographic Information Systems) data coordinate systems, 290–294 datums and geometric Earth models, 289–290 DEM data, 295–296 frequently asked questions, 406–409 GPS, 294–295 magnetic and true north, 296–298 types of maps, 288–289 global Crane model, 127 GNSS (Global Navigation Satellite Systems), 315 governments, and microwave systems, 56 GPR (ground potential rise), 259–260 GPS (global positioning system) for field measurements, 308–317 overview of, 294–295 providing timing, 25 synchronization using, 26 grazing, 95–96 Gregorian method, 269 ground reflection, 98–99, 101, 215 grounding, 317, 340–341 guaranteed bandwidth, 183 guyed tower, 321–322, 404

H

H-fields, 34–36 hailstorms, 409 half/doubling method, 350 half-duplex FDD, 82 half-power beamwidth, 264–265 hardware redundancy, 21–22, 136–138 HD (hybrid diversity), 146–147 height, antenna installation, 273, 275–276 helicopter method, marking site, 306 Hertz, Heinrich Rudolf, 33–34 heterosphere, 39 hexadecimal numbers, 29–30 high latency, 28 high-low technique, 98, 102–103 highway cell sites, 178 hitless receiver data switching, 235 holdover time, GPS, 26 homogeneous atmosphere, 47 homologation, 378–379, 403 homosphere, 39 horn antenna, 268 hot standby protection, 136–137, 225 HRP (hypothetical reference path), 123, 125–126 hub topology, 179–180, 209 Huygens’ principle, 47 hybrid diversity (HD), 146–147 hypothetical reference path (HRP), 123, 125–126

Index

I

I (in-phase), 87 I/Q modulator/demodulator, 86–88 I.356, 166 ICAO (International Civil Aviation Organization), 40 ice load, 288, 321, 333, 402 IDU (indoor unit), 218, 225, 280, 342 IEC 529, 242 IEE 802.11, 78 IEE 802.16, 81–83 IEEE 1588v2, 27–28 IEEE 802.20, 168 IF combining, 142 IFM (interference fade margin), 109–112 IGRF (International Geomagnetic Reference Field), 310 impedance, 282–284 IMT (International Mobile Telecommunications), 163 in-band frequency diversity, 134 in quadrature, signals in, 86–88 index of refraction, 42–43, 60 indoor unit. See IDU (indoor unit) industrial, scientific, and medical (ISM) RF band, 5, 68–69 Industry Canada, 5, 69 infrared transmission systems, 7 insolation, 339 installation antenna, 272–275 coaxial cable, 279–280 equipment, 369–371, 399 microwave radio, 246–247 millimeter-wave link, 256 safety and security, 280–282 site ready for, 369 waveguide, 278–279 interference avoiding, 414 collocation of radio stations, 191–193 in detailed network design, 365 FCC frequency coordination and, 376–378 frequency sharing and, 189–190 managing spectrum, 187 minimizing near and far, 193–196 paths, 190–191 interference fade margin (IFM), 109–112 intermodulation, 194, 404 International Civil Aviation Organization (ICAO), 40 International Fire Code, 338 International Geomagnetic Reference Field (IGRF), 310 International Mobile Telecommunications (IMT), 163 International Organization for Standardization (ISO), 40 international standard atmosphere (ISA), 39–40

473

International Telecommunications Union. see ITU (International Telecommunications Union) intersymbol interference (ISI), 226–227, 235, 238 inverse multiplexers, 258 ionization, 245 IP-based wireless networks, 16, 26 IP traffic, 222 ISA (international standard atmosphere), 39–40 ISI (intersymbol interference), 226–227, 235, 238 ISM (industrial, scientific, and medical) RF band, 5, 68–69 ISO (International Organization for Standardization), 40 ISPs, wireless, 78 ITU (International Telecommunications Union) models, 93 publishing recommendations, 203 zones, 128 ITU-R (International Telecommunications Union Radio Section) defined, 127 ITU-R F.1093, 104 ITU-R M.1457, 83 ITU-R P.452, 45 ITU-R P.530, 128, 144, 167 ITU-R P.617, 74 ITU-R P.618, 128 ITU-R P.837, 128 ITU-R P.838, 128, 130, 133 ITU-R P.843, 75 managing radio at international level, 186 multipath probability model, 117–120 rain outage model, 130–133 ITU-T Recommendations for Telecommunications defined, 203 ITU-T G.821, 166 ITU-T G.826, 166 quality and availability calculations, 121–125

J

J1 systems, 8 Japanese digital hierarchy, 8 jitter, 352

K

k-factor, 42–46 k-type fading, 109 kick-off meeting, microwave rollout, 360–361 knife-edge diffraction, 60, 95–97

474

L

Index

L6 band, 156, 202–203 latency, network, 28–29 latency testing, 351 latitude, 291–293 Law of Conservation of Charge, 34 Layer 2 OSI, 15–16 Layer 3 OSI, 15–16 layered networks, 19 LCT (local craft terminal), 243 leased lines, 171–175 LEO (low Earth orbit) satellites, 4 license-exempt bands 60-GHZ, 252–254 802.11a in 5 GHz band, 78–79 concerns about, 415–416 optical wireless limitations, 6 planning microwave networks, 161 regulatory controls, 70–71 RF spectrum, 5 spread-spectrum communications, 67–70 WiMAX supporting, 82 licensing process, E-band, 250–252 line-interactive UPS, 334 line-of-sight (LOS). See LOS (line-of-sight) line rate, optical carriers, 11 linear 1+1 protection switching, 22 links aggregation, 238–239 availability, 120–126 budget, 64–66, 99 engineering process, 365 failures causing IP network downtime, 23–24 feasibility formula, 223 frequently asked questions, 397–406 radio-propagation questions, 409–412 live data emulation, 346 LO (local oscillator), 219 loading rules, FCC, 202–203 local craft terminal (LCT), 243 local management, 243 local oscillator (LO), 219 logarithms, basic rules of, 419 Long-Term Evolution (LTE), 27, 83–84, 168–169 long-term outages, 113 long-term tests, 347 longitude, 291–293 loopback testing, 347 Loran-C, 315–316 LOS (line-of-sight) conducting path survey, 301–302 conducting site survey, 299–300 microwave point-multipoint system, 77 microwave radio considerations, 40–42 path surveys in dense urban areas, 306–308 using path surveys vs. establishing, 399 loss/attenuation calculations ground reflection, 98–99 microwave radio configuration, 220–221

propagation losses. See propagation losses types of, 90 low Earth orbit (LEO) satellites, 4 low latency, 28 LTE (Long-Term Evolution), 27, 83–84, 168–169

M

M-ary system, 87, 232 M-curve, 105 M13 multiplexer, 257 M.1457, 83 M.2301, 125 magnetic declination, 296–298, 409 magnetic north, 296–298, 407–408 magnitude, signal, 85, 87 maintenance program, 372–373 map study, site survey, 298 maps. See GIS (Geographic Information Systems) data Marconi, Guglielmo, 35 master MW site database, 384 maximum-power combiners, 144 maximum receiver signal, 222–223 Maxwell, James Clerk, 33–36 MBWA (Mobile Broadband Wireless Access), 168 mean sea level (MSL), 291, 313 mean-squared error (MSE), 136 mean time between failures (MTBF), 137, 206–208 mean time to repair (MTTR), 137, 206–208 measurement, units of, 423–426 media diversity, 21, 148, 400 transmission network, 1–7 MEF (Metro Ethernet Forum), 17 MEN (metro Ethernet networks), 17 Mercator, Gerardus, 293 mesh topology, 181–182, 202 metallic cables, and GPR, 259–260 Metro Ethernet Forum (MEF), 17 metro Ethernet networks (MEN), 17 MHSB (monitored hot standby), 137, 225 microwave communications, 33–88 basic terminology, 35–37 Bluetooth, 79–81 brief history of radio, 33–35 digital modulation, 84–86 Earth’s atmosphere and, 39–40 I/Q modulator/demodulator, 86–88 LTE, 83–84 over-the-horizon, 74–75 point-to-multipoint, 75–77 radio propagation. See radio propagation RF requirements, 84 spectrum considerations, 37–39 WiMAX, 81–83 WLANs, 77–79

Index microwave hop, 56–57 microwave link design deployment expense, 397 design process flowchart, 89–90 in difficult areas, 215–216 fading and fade margin. See fade margin; fading frequently asked questions, 397–406 loss/attenuation calculations. See loss/ attenuation calculations multipath probability models, 114–120 performance improvement, 225–226 quality and availability, 120–126 rain attenuation and outage models. See rain attenuation using antireflective systems, 148–150 using diversity improvement, 138–148 using hardware redundancy, 136–138 using repeaters. See repeaters microwave networks deploying. See deployment, microwave designing. See designing microwave networks planning. See planning microwave networks rollout. See project management, microwave rollout Migratory Bird Treaty Act, 380–381 milestones, project scheduling, 390 millimeter-wave bands, 5 millimeter-wave bands, defined, 398 millimeter-wave point-to-point systems, 247–256 60-GHZ license-exempt band, 252–254 70, 80, and 90-GHZ bands, 249–250 about millimeter-wave radios, 247–249 E-band licensing process, 250–252 engineering and installation, 254–256 millimeter waves (MMW), 40, 77 MIMO (multiple-input multiple-output) systems, 168, 235–236 minimum-phase notch, 105 minimum visual impact antenna structures, 324 mirror method, marking site, 305 miscellaneous losses, 90 misinsertion rates, 15 MLCM (multilevel coded modulation), 234–235 MMW (millimeter waves), 40, 77 Mobile Broadband Wireless Access (MBWA), 168 mobile switching center (MSC), 162 modem, 218 modulation adaptive, 55, 133–136, 413 choosing, 411 coded, 234–235 electromagnetic energy and, 37 microwave communications and, 84–86 performance and, 232–234

475

modulation rate, 87 monitored hot standby (MHSB), 137, 225 monopoles, 321, 325, 363 morality, 392 motor generators, 337 MPLS (multiprotocol label switching), 125 MSAS (Multifunctional Satellite Augmentation System), Japan, 317 MSC (mobile switching center), 162 MSE (mean-squared error), 136 MSL (mean sea level), 291, 313 MTBF (mean time between failures), 137, 206–208 MTTR (mean time to repair), 137, 206–208 multichannel protection, 137–138 Multifunctional Satellite Augmentation System (MSAS), Japan, 317 multilayer switching, 16 multilevel coded modulation (MLCM), 234–235 multiline protection, 137–138 multipath fading designing links in difficult areas, 215 dispersive fade margin, 103–106 flat fading, 103 frequency-selective fading, 103–106 outages and availability, 113 overview of, 99–103 probability models, 114–120 radio-propagation and, 410–411 rain attenuation fades and, 133 reducing using antireflection, 148–150 multipath propagation, 54 multiple-input multiple-output (MIMO) systems, 168, 235–236 multiplexing, 1, 256–258 multiprotocol label switching (MPLS), 125 MUX, 20, 257–258 MW (microwaves), 160–161

N

N-unit, 43 N+1, 21–22, 138, 146, 212 NAD27, 290, 407 NAD83, 290, 296, 407 nailed-up circuit, 172 National Electrical Code (NEC), 258 National Elevation Dataset (NED), 296 National Environmental Protection Act (NEPA), 379 National Historic Preservation Act (NHPA), 379–380 National Institute of Standards and Technology (NIST), 30 National Security Agency (NSA), 29, 31 National Spectrum Manager’s Association (NSMA), 110 National Telecommunications and Information Administration. See NTIA (National Telecommunications and Information Administration)

476

Index

National Wilderness Preservation System, 380 National Wildlife Refuge (NWR), 380 NAVD 88 (North American Vertical Datum of 1988), 296, 313 NBS (National Bureau of Standards), 29 NDAs (nondisclosure agreements), 160 near field, 34, 61–64, 412 near-field parameter, 153 near interference, 193–194 near-LOS, WiMAX, 82 NEC (National Electrical Code), 258 NED (National Elevation Dataset), 296 NEMA 4, 242 NEPA (National Environmental Protection Act), 379 network management system (NMS), 21, 243–245 network planning, microwave rollout, 361–362 network-to-network interfaces (NNI), 17 network traffic plan, 177 NGVD 29, 313 NHPA (National Historic Preservation Act), 379–380 night vision, 303 NIST (National Institute of Standards and Technology), 30 nitrogen, 39 NLOS (non-line-of-sight) areas, 77, 82 NMS (network management system), 21 NNI (network-to-network interfaces), 17 noise, 220 non-line-of-sight (NLOS) areas, 77, 82 non-real-time Polling Service (nrtPS), WiMAX, 83 nonionizing radiation, 245–246 nonregenerative repeaters, 58 North American digital hierarchy, 8 North American Vertical Datum of 1988 (NAVD 88), 296, 313 notch depth, 104–105 nrtPS (non-real-time Polling Service), WiMAX, 83 NSA (National Security Agency), 29, 31 NSMA (National Spectrum Manager’s Association), 110 NTIA (National Telecommunications and Information Administration), 146, 186, 250–252, 373–375 null, 100, 267 NWR (National Wildlife Refuge), 380

O

O&M (operation and maintenance), 19, 243–244 obstacle loss calculation, 95–98 OC-3, 10–11, 166

OC-n (optical carrier) levels, 11 ODU (outdoor unit), 280, 324, 333 OEM (original equipment manufacturer), 204, 367 OFDM (orthogonal frequency division multiplexing), 82, 168, 236–238 OFDMA (orthogonal frequency division multiple access), 83, 168 offset feed method, antennas, 269 offset GPS measurement procedure, 310–312 OH (over-the-horizon) microwave systems, 47–48, 74–75 OHLOSS (over-the-horizon loss), 75 on-site interference, 193–194 1+1 hardware redundancy, 21–22 operation and maintenance (O&M), 19, 243–244 OPGW (optical power ground wire), 3 opportunistic spectrum sharing systems, 71–72 optical (or visual) LOS, 41–42 optical power ground wire (OPGW), 3 optical wireless, 5–6 order wire, 240 original equipment manufacturer (OEM), 204, 367 orthogonal frequency division multiple access (OFDMA), 83, 168 orthogonal frequency division multiplexing (OFDM), 82, 168, 236–238 orthophoto maps, 289 OSI (Open System Interconnection Reference Model), 15–16 outage models comparison of, 132–133 Crane rain, 129–130 ITU-R rain, 130–132 rain attenuation and, 127–129 outage time multipath, 103, 229–230 obstructed path, 41 rain, 108, 130 in seconds per year, 115 for selective fading, 106 outages calculations, 112–114 combatting rain fade, 133–136 Vigants multipath probability model, 114–117 outdoor unit (ODU), 280, 324, 333 outsourcing services, 385–387 over-the-horizon loss (OHLOSS), 75 over-the-horizon (OH) microwave systems, 47–48, 74–75 over-the-water paths, 414, 416 overhead, 11 oversubscription testing, 182–183, 350–351 oxygen, 39

Index

P

P.526, 75 P.618, 128 P.833-2, 93 P.837, 128 packet delay variation (PDV), 351–352 Packet Error Ratio (PER), 222 PAL (Protocol Adaptation Layer), 81 PAN (Personal Area Network), 79–80 PAPR (peal-to-average power ratio), 238 parabolic antenna directive gain, 422 parabolic antenna feed methods, 268–270 parity check, 122 partial response signaling, 235 passive repeaters, 58, 150–155 path profile, 45–46, 302 path surveys frequently asked questions, 399, 409, 411, 414 overview of, 301–308 Pathloss, 203, 406 payback, 162, 174–175 PCN (prior coordination notice), 376–378 PCS (personal communications service), 3–4, 78, 186 PDH (Plesiochronous Digital Hierarchy) ITU-T Recommendation G.826, 124 microwave radio systems, 57 network synchronization using, 24 overview of, 9–10 ring architecture, 20 SDH and SONET replacing, 12 PDV (packet delay variation), 351–352 peal-to-average power ratio (PAPR), 238 pencil beam antennas, 249–250, 262 PER (Packet Error Ratio), 222 performance difficulties of evaluating, 26 management, 244 transmission network, 21–24 performance improvement adaptive equalizers, 226–227 automatic transmit power control, 231–232 channel width, spectral efficiency and modulation schemes, 232–234 coded modulation, 234–235 cross-polarization interference canceller, 227–231 forward error correction, 227 link aggregation, 238–239 microwave link protection, 225–226 MIMO, 235–236 OFDM and COFDM, 236–238 receiver data switching, 235 Personal Area Network (PAN), 79–80 personal communications service (PCS), 3–4, 78, 186 phase, digital modulation of, 84–86 phase shift keying (PSK), 232 photovoltaic panels, 338–339 photovoltaic (PV) power, 338–339

477

piconet (Bluetooth), 81 plane reflectors, 150, 151–155 planimetric maps, 288–289 planning, frequency, 196–203 planning microwave networks 3G wireless, 163–167 4G wireless, 167–171 process for, 159–161 replacing leased lines, 171–175 rollout phase, 359–360 topology and capacity planning, 177–183 in utility telecom networks, 176–177 wireless, 161–163 plenums, 258 plesiochronous approach, 24 Plesiochronous Digital Hierarchy. See PDH (Plesiochronous Digital Hierarchy) PMP (point-to-multipoint) systems, 75–77 point-to-multipoint (PMP) systems, 75–77 point-to-point microwave systems. See PTP (point-to-point) microwave systems polar diagrams, 85–86 polarization, 102, 219, 265, 414 pollution, 410 power combiner, 226 power divider, 226 power efficiency, 84 power flux density, 61 power supply, 333–340 Poynting vector, 35 PRBS (pseudorandom bit sequence), 122 precipitation, 54, 94–95, 103. See also rain attenuation Precision Time Protocol (PTP), 27–28 primary reference source (PRS), 25 prior coordination notice (PCN), 376–378 private line, 172 private networks, 55 project, defined, 357 project management changes and modifications, 385 document control, 382–384 ethical issues, 392–397 frequently asked questions, 397–416 outsourcing services, 385–388 project controls and reporting, 381–382 record keeping, importance of, 384 regulatory issues, 373–381 S-curve, 391–392 of time and resources, 388–389 tools, 389–390 project management, microwave rollout acceptance testing, 371 activities, 357–358 as-built documentation, 371–372 commissioning, 372 detailed network design, 364–366 equipment and services procurement, 366–369

478

Index

project management, microwave rollout (continued) equipment installation, 369–371 maintenance program, 372–373 network planning, 361–362 project approval, 362–363 project kick-off meeting, 360–361 site acquisition, 363–364 site ready for installation, 369 three-stage process, 359–360 project manager, 360–362, 381–392 project organization, 381–382 project schedule, 381, 390 project system engineer, 361–362 projections, map, 289 propagation, 35 propagation delay, 28–29 propagation losses, 90–98, 409–416 propagation quiet time, 277 protection agent, 378 protection, microwave links, 400–401 Protocol Adaptation Layer (PAL), 81 PRS (primary reference source), 25 pseudo-wire (PWE), 401–402 pseudorandom bit sequence (PRBS), 122 PSK (phase shift keying), 232 PSTN (public switched telephone network), 1–2, 76 PTP (point-to-point) microwave systems, 55–73 applications, 55–56 COW systems, 72–73 defined, 57–58 Fresnel zones and clearance rules, 58–61 further information on, 406 link budget, 64–66 microwave radio basics, 56–58 near and far fields, 61–64 opportunistic spectrum sharing systems, 71–72 overview of, 66–67 spread-spectrum systems, 67–71 PTP (Precision Time Protocol), 27–28 public switched telephone network (PSTN), 1–2, 76 public transport systems, 56 Pursley, frequency diversity model, 144–145 PV (photovoltaic) power, 338–339 PWE (pseudo-wire), 401–402

Q

Q (quadrature) component, 87 QAM (quadrature amplitude modulation), 71, 85, 88, 233 QoS (Quality of Service) adaptive modulation and, 136 Carrier Grade Ethernet for, 17 for high performance, 22 IP-based wireless network limitations, 16–17 WiMAX and, 82–83

QRSS (quasi-random signal sequence) tests, 346, 403 QPSK (quadrature phase shift keying), 85, 87, 232 quad diversity, 147 quality control, 342 objectives, 120–126

R

radiating near field, 63 radiation defined, 35 efficiency, 261, 263–264 hazards, 275, 399 microwave safety and, 245 patterns, 263, 266–267 radiation pattern envelope (RPE), 266 Radiative Transfer Modeling, 93 radio LOS, 41–42 terminal components, 56 radio access network (RAN), 169 Radio Astronomy Quiet Zones, 39 radio base station (RBS), 14, 76 radio frequency spectrum. See RF (radio frequency) spectrum radio propagation, 40–55 anomalous propagation, 48–52 countermeasures, 54–55 Earth radius and k-factor, 42–46 microwave and millimeter waves, 40 sand and dust and, 52–54 standard propagation, 46–48 Radio Quiet Zone, 398 radio-relay, 40 radomes, 271–272, 288 rain attenuation calculation of, 94–95 Crane method, 129–130, 132–133 designing in difficult areas, 215 effects on microwave propagation, 409–410 fading, 106–108, 121–126 ITU-R rain outage model, 130–132 microwave systems and, 77 outages and availability, 113 overview of, 127–129 propagation and, 53–55 reducing effects, 133–136 RAN backhaul, 169 RAN (radio access network), 169 RBER (residual background error rate), 15, 224, 402 RBS (radio base station), 14, 76 reactive near field, 62–63 Real-Time Polling Service (rtPS), WiMAX, 83 receive antenna, 36, 41 receive passband, 241 receive signal level (RSL), 110, 223 receiver data switching, 235

Index receiver noise, 220 receiver sensitivity threshold, 65 receiver sensitivity threshold (Rx), 221–223, 241–242 receivers optical wireless, 6–7 over-the-horizon, 74 radio path link budget and, 64 sensitivity threshold, 65, 221–222 reciprocity, antenna, 263 rectifiers, DC, 334–335 reference ellipsoid, 313–314 reflection designing links in difficult areas, 215 Fresnel zone and, 60 ground, 98–99 standard propagation, 47 using antireflective systems, 148–150 refraction-diffraction fading, 108–109, 120–126 refractivity gradient (G), 44–45 regenerative repeaters, 58 regenerator section termination (RST) model, 26 registration, tower, 404 regulatory issues, 373–381 reliability, 137 remote management, 243 repeater stations, 57–58 repeaters active, 150 back-to-back antennas, 155–156 billboard, 151–155 calculations, 156–158 passive, 58, 150–151 reporting, project management, 381–382, 390 request for information (RFI), 367 request for pricing (RFP), 367 request for quotation (RFQ), 343, 360, 366–368 residual background error rate (RBER), 15, 224, 402 resonance, 106, 254, 329 resource management, 387–389 responsibility matrix, 161 return loss, 283 revised two-component Crane model, 127 RF radiation. See radiation RF (radio frequency) spectrum communications requirements, 84 electromagnetic, 38 frequency diversity and, 144–146 overview of, 35 quick reference, 419–422 regulating, 5 wireless technology and, 4 RFC 2544, 349 RFI (request for information), 367 RFP (request for pricing), 367 RFQ (request for quotation), 343, 360, 366–368

479

Rijndael algorithm, AES, 30–31 ring topology 4G RAN, 170 availability in, 209–212 choosing for reliability, 20, 405 fiber-optics using, 3 frequency planning and, 200–202 overview of, 180–181 risers, cable placement in, 258 rollout. See project management, microwave rollout route diversity, 134, 148 routers, 15–16, 23 RPE (radiation pattern envelope), 266 RS195B, 270 RS222, 270 RSL (receive signal level), 110, 223 RST (regenerator section termination) model, 26 rtPS (Real-Time Polling Service), WiMAX, 83 rusty fence syndrome, 404 Rx (receiver sensitivity threshold), 221–223, 241–242

S

S-curve, 384, 391–392 SA (selective availability), 296 safety antenna radiation, 275 compatibility and, 245–246 construction standards for towers, 321 equipment, 317–320 grounding, 340–341 installation, 280–282 personnel, 259–260 sampling, 1 sand, 52–54, 411 satellite imagery, and GPS positions, 408–409 SCADA, 56, 176 scattering, 48 scope and delineation list, 160–161 SDH (Synchronous Digital Hierarchy) ITU-T Recommendation G.826 for, 124 master timing source, 26 microwave radio systems, 57 network synchronization using, 24–26 overview of, 10–13 ring architecture, 20 seasonal link performance, 411 secular variation, 296 security, 29–31, 244–245, 280–282 selective availability (SA), 296 selective fading, 120–126 self-healing network configurations, 20, 21 self-supporting towers, 321–325 semiflexible elliptical waveguides, 277, 278–279 service level agreements (SLAs), 16–17 service telephone network, 240

480

Index

services, procurement, 366–369 SES (severely errored second), 113, 115–117, 122 SESR (severely errored second ratio), 122–124 sexagesimal system, 291 shade-relief maps, 289 shelters, equipment, 317–318 short-term outages, 113 SHPO (State Historic Preservation Officer), 380 shrouds, 271–272 signal termination, 258–261 signal-to-noise (SNR or S/N) ratio, 110, 220, 421 single-input single-output (SISO), 168 SISO (single-input single-output), 168 site acquisition, 363–364 site ready for installation, 369 site surveys, 298–301, 364–365 SLAs (service level agreements), 16–17 slip, T1, 24 SM.1046, 234 Smith chart, 283 smog, 410 snow, 410 SNR (signal-to-noise) ratio, 220 software failures, 23 soil testing, tower placement, 330, 333 solar energy, 338–339 SONET DS1 master timing source, 26 SONET (Synchronous Optical Network), 10–13, 20, 24–26, 57 space diversity, 99, 134, 139–144, 146–147, 415 spanning tree protocol (STP), 16 spectral efficiency, 232–234 spectral occupancy test, 220 spectrum considerations, 37–39 electromagnetic, 37–39 FCC and NTIA regulations, 375 increasing efficiency, 412–413 microwave network design, 185–189 microwave radio configuration, 220 millimeter-wave, 40 sweep, 189 specular reflection, 47 spheroid, 289, 313 spillover, 262 spread-spectrum systems, 67–72 spreading code, 68 standard atmosphere, 46–48 standard gradients, 43–44 standard propagation mechanisms, 46–48 standby UPS, 334 standing wave ratio (SWR), 282 star networks, 178–179, 200–202, 209 State Historic Preservation Officer (SHPO), 380 statistical multiplexing, 257

statistical time division multiplexing (STDM), 257 STDM (statistical time division multiplexing), 257 STM (synchronous transfer mode), 10–11, 13–14 STP (spanning tree protocol), 16 stratum clock system, 24–25 STS-1 (synchronous transmission signal, level 1), 10–11, 261 STS (synchronous transmission signal) data rates, SONET, 10–11 subrefraction, 48–49 superrefraction, 49 surface duct, 51 surge arrestors, 342 surge suppressors, 341–342 switches, 15–16, 28 SWR (standing wave ratio), 282 symbol rate, 87, 233 synchronization, network, 24–28 Synchronous Digital Hierarchy. See SDH (Synchronous Digital Hierarchy) synchronous network hierarchies, 10–12, 24–26 Synchronous Optical Network. See SONET (Synchronous Optical Network) synchronous transfer mode (STM), 10–11, 13–14 synchronous transmission signal, level 1 (STS-1), 10–11, 261 synchronous transmission signal (STS) data rates, SONET, 10–11 system gain, 65, 412, 422 systems engineering, microwave, 204–212

T

T/I (threshold-to-interference), 110–112, 239–240 T1 circuits frequently asked questions, 397–398 network synchronization in, 24 overview of, 7–9 ring architecture reliability and, 20 topology and capacity planning, 177 wireless network synchronization scheme, 26 T1.101, 24–25 tactical transportable antenna system (TTAS), 73 tandem topology, 178 taper loss, antennas, 262 tasks, scheduling, 390 TCM (trellis coded modulation), 234–235 TD (threshold degradation), 111 TDD (time-division duplex) Bluetooth using, 81 LTE architecture supporting, 83 PMP using, 77 WiMAX support for, 82 wireless synchronization scheme, 27

Index TDM (time-division multiplexing) Carrier Ethernet providing reliability of, 18 defined, 1 microwave radio configuration, 221 not suited to Ethernet, 16 overview of, 256–258 TDMA network, 77, 182 Teflon, 277 telecommunications, 55, 176–179 Telecommunications Distribution Methods Manual, 259 temperature control, equipment, 317–319 Tesla, Nikola, 35 testing and troubleshooting BERT, 346–347 burst test, 350 factory acceptance testing, 342–343 field acceptance testing, 343–344 frame loss test, 350 latency testing, 351 oversubscription testing, 350–351 testing Ethernet networks, 348–349 testing packet delay variation, 351–352 testing throughput, 349–350 using long-term BER measurements, 352–353 TFM (thermal fade margin), 112 The Complete Laws of Electrodynamics (Maxwell), 33–34 THPO (Tribal Historic Preservation Officer), 380 three-stage process, microwave rollout, 359–360 3GPP (3G Partnership Project), 27, 84, 163 3G wireless networks, 14, 78, 163–167 threshold degradation (TD), 111 throughput test, 349–350 thunderstorms, and rain outages, 134 time-division duplex. See TDD (time-division duplex) time-division multiplexing. See TDM (time-division multiplexing) time management, 387–389 timing island, 26 timing-related problems. See synchronization, network topographic maps, 289 topology, network 4G wireless network, 169–170 availability of different network, 208 chain/tandem, 178 choosing best, 405 frequency planning and, 200–202 mesh, 181–182 over-subscription, 182–183 planning microwave networks, 177–183 ring, 180–181 simple star, 178–179 TDMA transmission network optimization, 182

481

transmission network capacity requirements, 177 utilizing hubs, 179–180 topology, transmission network, 18–21 towers, 320–333 antenna mounting structures, 321–325 conducting site survey for, 300–301 FAA, FCC and NTIA regulations, 374–375 guyed, 321–322 installation safety and security, 280–282 maximum allowed antenna deflection, 325–327 monopole, 321 overview of, 320–321 planning microwave networks, 161 registration of, 404 requirements, 327–333 self-supporting, 321–325 in telecom networks, 176–177 using electric transmission, 404–405 traffic-carrying DS1, 26 transhorizon. See OH (over-the-horizon) microwave systems transition zone, 63 transmission, 2 transmission lines, 277–280 transmission networks ATM systems, 13–15 delays, 28–29 E1 systems, 7–9 Ethernet backhaul, 15–18 free-space laser communications, 5–7 J1 systems, 7–9 media. See media, transmission network PDH systems, 9–10, 12 performance, 21–24 planning process, 160 SDH systems, 10–13 security and encryption, 29–31 SONET, 10–13 synchronization, 24–28 T1 systems, 7–9 topology, 18–21 wireless systems, 3–5 wireline systems, 2–3 Transmission Systems Design Handbook for Wireless Networks (Artech House), 406 transmit antenna, 36, 41 transmit frequencies, 219 transmit (Tx) output power, 218–219, 241–242, 400 transmitters, 6–7, 64, 74 transport, 2 trapping, 50, 413 tree configuration, 200–201 trellis coded modulation (TCM), 234–235 Triple-DES, 29–30 tripods, 269, 324–325, 370 tropopause, 39

482

Index

troposcatter, 48, 74–75 troposphere, 39, 74 tropospheric duct, 50 Tropospheric Ducting Forecasts, 45 true north, 298, 407–408 truncated parabolic dish, 269 TTAS (tactical transportable antenna system), 73 turbines, wind, 339–340, 405 turnkey projects, 386–387, 400 TV broadcast station, close to microwave system, 405 twist/sway, 270, 325–328, 400 two-component Crane model, 127 Tx (transmit) output power, 218–219, 241–242, 400

U

U6, 203, 208–211 UATR (unavailable time ratio), 122 UGS (Unsolicited Grant Service), WiMAX, 83 UHF frequencies, 72 UMTS (Universal Mobile Telecommunications System), 163 unavailability objectives, 125–126, 137 UNI (user-network interface), 10–11, 17 UNII (Unlicensed National Information Infrastructure) band, 5, 69–71 units of measurement, 423–426 unlicensed (illegal) microwave systems, 67 upfade, 100, 215 UPS (uninterruptible power supply), 333–334 urban areas, 306–308, 404 urban canyon problem, 306 USGS (United States Geological Survey), 296, 310, 409 UTC (coordinated universal time) frequency, 25 utility companies, 55 utility telecom networks, 176–177 utilization factor, 177 UTM (Universal Transverse Mercator), 291, 293–294 UTP (unshielded twisted pair), 2

V

vegetation attenuation calculation, 92–93 VHF frequencies, 72 Vigants, 114–117, 144–145 visibility, 40–42, 53–54 visual LOS, 41–42 VoIP (voice over IP), 2 VRD (vertical reference datum), 291 VRLA (valve-regulated lead-acid) batteries, 336–337

VSWR (voltage standing wave radio), 282–283

W

W-curve, 105 WAAS (Wide Area Augmentation System), 316–317 waveguides, 214, 218, 246, 277–278 wavelength (λ), 36–37, 419 Weissberger’s model, 92–93 wet radome loss, 272 WGS 84 (World Geodetic System 1984), 290, 293, 313–314 white space reallocation, 71–72 Wide Area Augmentation System (WAAS), 316–317 Wilderness Act, 379–380 WiMAX (Worldwide Interoperability for Microwave Access), 27, 81–83, 168 wind antenna installation design, 274–275 antennas for high, 285–288 construction standards for towers, 321 load, 287, 333 speed, 287, 333 turbines, 339–340, 405 wireless backhaul, 15–18, 167 wireless transmission, 2–7, 26–28 WLANs (wireless local area networks), 77–79 WMM (World Magnetic Model), 310 World Geodetic System 1984 (WGS 84), 290, 293, 313–314 Worldwide Interoperability for Microwave Access (WiMAX), 27, 81–83, 168 worst month, 102, 107 WSPs (wireless service providers), 162 WTB (Wireless Telecommunications Bureau), 186, 375, 378

X

XPD (cross-polar discrimination), 227–229 XPI (cross-polar isolation), 230 XPIC (cross-polarization interference canceller), 18, 227–231, 401 XPIF (cross-polarization improvement factor), 229–231

Y

Y.1344, 20 Y.1540, 124 Y.1541, 124 Y.1561, 125

Z

zoning, 317, 364