3G CDMA2000 Wireless System Engineering

3G CDMA2000 Wireless System Engineering

For a listing of recent titles in the Artech House Mobile Communications Library, turn to the back of this book.

3G CDMA2000 Wireless System Engineering Samuel C. Yang

Artech House, Inc. Boston • London www.artechhouse.com

Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the U.S. Library of Congress.

British Library Cataloguing in Publication Data Yang, Samuel C. 3G CDMA2000 wireless system engineering.—(Artech House mobile communications library) 1. Wireless communication systems. 2. Code division multiple access I. Title 621.3'845 ISBN

1-58053-757-x

Cover design by Yekaterina Ratner © 2004 ARTECH HOUSE, INC. 685 Canton Street Norwood, MA 02062 All rights reserved. Printed and bound in the United States of America. No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without permission in writing from the publisher. All terms mentioned in this book that are known to be trademarks or service marks have been appropriately capitalized. Artech House cannot attest to the accuracy of this information. Use of a term in this book should not be regarded as affecting the validity of any trademark or service mark. International Standard Book Number: 1-58053-757-x 10 9 8 7 6 5 4 3 2 1

To my wife Jenny and my son Daniel

.

Contents Preface

xiii

Acknowledgments

xvii

CHAPTER 1 Introduction to 3G CDMA 1.1 1.2 1.3 1.4 1.5

Third Generation Systems Protocol Architecture Other Elements of Protocol Architecture Spreading Rate 1 and Spreading Rate 3 Differences Between IS-2000 and IS-95 1.5.1 Signaling 1.5.2 Transmission 1.5.3 Concluding Remarks References

1 1 2 3 5 7 7 8 8 9

CHAPTER 2 Physical Layer: Forward Link

11

2.1 Introduction 2.2 Radio Configurations 2.3 Signaling Channels 2.3.1 Forward Dedicated Control Channel (F-DCCH) 2.3.2 Quick Paging Chanel (F-QPCH) 2.3.3 Forward Common Control Channel (F-CCCH) 2.3.4 Broadcast Control Channel (F-BCCH) 2.3.5 Common Assignment Channel (F-CACH) 2.3.6 Common Power Control Channel (F-CPCCH) 2.3.7 Pilot Channels 2.4 User Channels 2.4.1 Forward Fundamental Channel (F-FCH) 2.4.2 Forward Supplemental Channel (F-SCH) 2.5 Channel Structure 2.6 Modulation 2.7 Capacity Gain: Forward Link References Selected Bibliography

11 14 15 15 16 19 20 21 22 24 26 26 27 31 32 34 35 35

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Contents

CHAPTER 3 Physical Layer: Reverse Link

37

3.1 Introduction 3.2 Radio Configurations 3.3 Signaling Channels 3.3.1 Reverse Dedicated Control Channel (R-DCCH) 3.3.2 Reverse Common Control Channel (R-CCCH) 3.3.3 Enhanced Access Channel (R-EACH) 3.3.4 Reverse Pilot Channel (R-PICH) 3.4 User Channels 3.4.1 Reverse Fundamental Channel (R-FCH) 3.4.2 Reverse Supplemental Channel (R-SCH) 3.5 Channel Structure 3.6 Modulation 3.7 Capacity Gain: Reverse Link References Selected Bibliography

37 39 40 40 41 42 45 49 50 50 50 51 52 53 53

CHAPTER 4 Medium Access Control

55

4.1 4.2 4.3 4.4

Introduction Primitives Multiplex Sublayers Radio Link Protocol (RLP) 4.4.1 Overview of Layer 2 Protocols 4.4.2 llustration of the RLP 4.4.3 Concluding Remarks 4.5 Signaling Radio Burst Protocol (SRBP) 4.6 System Access 4.6.1 Basic Access Mode 4.6.2 Reservation Access Mode 4.6.3 Power Controlled Access Mode 4.6.4 Designated Access Mode References

55 55 57 60 60 61 62 63 64 65 65 67 68 68

CHAPTER 5 Signaling Link Access Control

71

5.1 Introduction 5.2 LAC Sublayers 5.2.1 Authentication and Addressing Sublayers 5.2.2 ARQ Sublayer 5.2.3 Utility Sublayer 5.2.4 Segmentation and Reassembly Sublayer 5.3 Sublayer Processing 5.3.1 Common Signaling: Forward Link 5.3.2 Common Signaling: Reverse Link 5.3.3 Dedicated Signaling: Forward Link

71 71 71 73 73 74 74 74 76 77

Contents

ix

5.3.4 Dedicated Signaling: Reverse Link 5.4 Interaction of Layer and Sublayers 5.4.1 Transmit Side 5.4.2 Receive Side References

80 80 81 82 83

CHAPTER 6 Signaling: Upper Layers

85

6.1 Overview 6.2 State Transitions: Call Processing 6.2.1 Initialization State 6.2.2 Mobile Station Idle State 6.2.3 System Access State 6.2.4 Mobile Station Control on the Traffic Channel State 6.3 Mode Transitions: Packet Data Transmission 6.3.1 Active Mode 6.3.2 Control Hold Mode 6.3.3 Dormant Mode 6.3.4 Transitions 6.4 Channel Setup 6.4.1 Example 1: Base Station-Originated Voice Call 6.4.2 Example 2: Mobile Station-Originated Voice Call 6.4.3 Example 3: Mobile Station-Originated Packet Data Call 6.4.4 Example 4: Supplemental Channel Request During a 6.4.4 Packet Data Call 6.4.5 Concluding Remarks References

85 87 88 89 91 94 96 96 96 96 97 97 98 99 100 101 104 104

CHAPTER 7 Power Control

107

7.1 Introduction 7.2 Power Control of the Forward Link 7.2.1 Inner Loop and Outer Loop 7.2.2 Power Control of Multiple Forward Traffic Channels 7.3 Power Control of the Reverse Link: Open Loop 7.3.1 Power Control of Multiple Reverse Channels 7.3.2 Summary 7.4 Power Control of the Reverse Link: Closed Loop 7.4.1 Inner Loop and Outer Loop 7.4.2 Power Control of Multiple Reverse Channels References

107 107 107 110 113 113 116 117 118 119 121

CHAPTER 8 Handoff

123

8.1 Introduction 8.2 Soft Handoff 8.2.1 Active Set

123 123 124

x

Contents

8.3

8.4 8.5

8.6 8.7

8.2.2 Candidate Set 8.2.3 Neighbor Set 8.2.4 Remaining Set 8.2.5 Set Transitions 8.2.6 Example: Soft Handoff Idle Handoff 8.3.1 Active Set 8.3.2 Neighbor Set 8.3.3 Private Neighbor Set 8.3.4 Remaining Set 8.3.5 Idle Handoff Process Access Entry Handoff Access Handoff 8.5.1 Active Set 8.5.2 Neighbor Set 8.5.3 Remaining Set 8.5.4 Access Handoff Process Access Probe Handoff Concluding Remarks References

127 128 129 129 129 133 133 134 134 134 134 134 135 136 136 136 136 138 139 140

CHAPTER 9 System Performance

141

9.1 Introduction 9.2 Channel Supervision 9.2.1 Forward Link: Traffic Channel 9.2.2 Forward Link: Common Channel 9.2.3 Reverse Link 9.3 Code Management 9.3.1 Generation of Walsh Codes 9.3.2 Assignment of Walsh Codes: Forward Link 9.3.3 Quasi-Orthogonal Functions 9.3.4 Assignment of Walsh Codes: Reverse Link 9.4 Turbo Codes 9.5 Transmit Diversity 9.5.1 Orthogonal Transmit Diversity 9.5.2 Space Time Spreading 9.5.3 Concluding Remarks References Selected Bibliography

141 141 141 142 142 142 143 144 147 147 150 152 152 154 156 156 157

CHAPTER 10 System Design: Coverage

159

10.1 Introduction 10.2 Forward Pilot Channel 10.3 Forward Fundamental Channel 10.4 Forward Supplemental Channel

159 161 162 163

Contents

10.5 10.6 10.7 10.8 10.9 10.10

xi

Upper Bounds of Interference: Forward Link Reverse Fundamental Channel Reverse Supplemental Channel Upper Bounds of Interference: Reverse Link Eb/N0 and Receiver Sensitivity Concluding Remarks Reference

165 165 167 168 169 169 170

CHAPTER 11 System Design: Capacity

171

11.1 Introduction 11.2 Mathematical Definitions 11.2.1 Received Signal Power 11.2.2 Loading Factor 11.3 Reverse Link 11.3.1 Capacity 11.3.2 Capacity Improvements in IS-2000 11.3.3 Capacity Improvements in a System 11.4 Forward Link 11.4.1 Capacity 11.4.2 Capacity Improvements in IS-2000 11.4.3 Capacity Improvements in a System References

171 171 171 173 174 174 176 177 178 179 182 183 185

CHAPTER 12 Network Architecture

187

12.1 Introduction 12.2 2G Network 12.2.1 Network Elements 12.2.2 Protocols 12.3 3G Network 12.3.1 Network Elements 12.3.2 Protocols 12.4 Simple IP 12.5 Mobile IP 12.6 Concluding Remarks References

187 187 187 189 189 190 191 192 193 196 197

CHAPTER 13 1xEV-DO Network

199

13.1 Introduction 13.2 1xEV-DO Network 13.3 Protocol Architecture 13.3.1 Application Layer 13.3.2 Stream Layer 13.3.3 Session Layer 13.3.4 Connection Layer

199 201 202 204 205 205 206

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Contents

13.3.5 Security Layer 13.3.6 Concluding Remarks References

210 210 211

CHAPTER 14 1xEV-DO Radio Interface: Forward Link

213

14.1 Introduction 14.2 MAC Layer 14.2.1 Forward Traffic Channel MAC Protocol 14.2.2 Control Channel MAC Protocol 14.3 Physical Layer 14.3.1 Pilot Channel 14.3.2 Forward Traffic Channel/Control Channel 14.3.3 MAC Channel 14.3.4 Time Division Multiplexing 14.3.5 Modulation 14.4 Concluding Remarks References Selected Bibliography

213 213 214 215 215 215 216 219 221 225 226 226 226

CHAPTER 15 1xEV-DO Radio Interface: Reverse Link

227

15.1 Introduction 15.2 MAC Layer 15.2.1 Reverse Traffic Channel MAC Protocol 15.2.2 Access Channel MAC Protocol 15.3 Physical Layer 15.3.1 Reverse Traffic Channel 15.3.2 Access Channel 15.3.3 Modulation 15.4 Reverse Power Control 15.4.1 Open-Loop Power Control 15.4.2 Closed-Loop Power Control References Selected Bibliography

227 227 227 228 229 231 236 238 239 239 240 240 240

About the Author

241

Index

243

Preface Over the past few years, many fundamental changes have taken place in wireless communications that will influence the future of this dynamic field. One phenomenon driving these changes has been the integration of wireless communication devices in people’s lives. While the 1990s were the years when wireless voice telephony became popular, the 2000s should be the time when wireless data applications are truly un-tethered from homes and offices. As more people adopt wireless communication devices and applications effected by these devices, the demand on wireless networks will continue to grow. Although code division multiple access (CDMA) has become an integral part of the ensemble of third generation (3G) standards, many wireless network operators have found the implementation of IS-2000 affords a good balance between cost and performance of providing 3G services, especially if an operator evolves its network from IS-95 to IS-2000. As such, IS-2000 has become a popular choice of 3G for operators around the world, notably in Asia and the Americas. This book has been written to address the technical concepts of IS-2000. The focus is on basic issues, and every effort has been made to present the material in an expository and interesting fashion. One strategy is to utilize examples not to offer proofs (as they cannot) but to help the reader grasp the fundamental issues at hand. In this regard, mathematical details and models have an important role but serve as means to an end. While CDMA is by nature theory-intensive, every attempt is made to strike a balance between theory and practice. In addition, to minimize the duplication of foundational material of spread spectrum communications and IS-95, this book does not describe those introductory concepts (e.g., synchronization of PN codes) in detail and assumes that the reader is familiar with basic material such as those found in CDMA RF System Engineering (Samuel Yang, Artech House, 1998). Furthermore, this book assumes that the reader is familiar with the layered frameworks of the Internet Model and OSI Model. In 3G, the system requires the full participation of not only the physical layer but also medium access control, link access control, and upper layers to provide not only circuit voice call but also packet data call functions. Hence in 3G, one needs to focus on the entire system rather than just on a particular layer. To that end, the book starts with a layer-by-layer treatment of IS-2000. In Chapters 1 to 6, it follows the protocol layer framework and describes IS-2000 from Layers 1 to 3. Chapter 1 introduces basic concepts and requirements of 3G and highlights key differences between IS-2000 and IS-95. Chapters 2 and 3 describe physical layers of forward and reverse links, respectively. The channel structure and functions of different channels are described in these two chapters. Chapter 4 covers medium access

xiii

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Preface

control and focuses on radio link protocol, signaling radio burst protocol, and system access. Then, Chapter 5 goes into link access control; this chapter first reviews the functions of the sublayers of link access control, then it illustrates sublayer processing on both forward and reverse links. Chapter 6 goes over Layer 3 or upper layer signaling of IS-2000; the emphasis here is on call processing, state transition, and mode transitions. After building the foundation of the structure of an IS-2000 system, the book proceeds to the systems aspects of IS-2000 in Chapters 7 to 12. Since IS-2000 contains power control and handoff functions that are superior to those in IS-95-A, Chapters 7 and 8 describe in detail power control and handoff functionalities, respectively. Chapter 9 then proceeds to cover system performance and describes those features adopted by IS-2000 to increase performance such as code management, turbo codes, and transmit diversity. Since a CDMA system essentially trades off coverage versus capacity, these design aspects are presented in Chapters 10 and 11. In particular, Chapter 10 covers coverage, and Chapter 11 covers capacity. These two chapters contain systematic developments of key concepts, and necessary mathematical developments are included where necessary to clarify the material. Chapter 12 is on network architecture and serves as a capstone on all the chapters presented thus far. It describes the IS-2000 architecture from a network perspective and shows how a 3G network differs and evolves from a 2G network. This chapter introduces how IS-2000 works and interacts with other elements in the network. Advanced concepts such mobile IP are also introduced here. The last three chapters concern a special topic that is of particular interest—1xEV-DO (1x Evolution for Data Optimized), which has gained popularity in recent years and is designed to work with an IS-2000 system. The topics related 1xEV-DO are included to make the book a more complete reference. Specifically, Chapter 13 focuses on the top five layers of 1xEV-DO (i.e., application, stream, session, connection, and security), and Chapters 14 and 15 cover medium access control and physical layers of forward and reverse links, respectively. Without a loss generality, this book emphasizes Spreading Rate 1 at 1.25 MHz. The discussions on Spreading Rate 1 can be readily extended to direct-spread or multiple-carrier options of wider bandwidths. In addition, throughout the book we cite specific examples of radio configurations instead of exhaustively describe the details of every radio configuration. These selective descriptions serve to illustrate more fully the reason for a particular implementation. Overall, the emphasis of the book is on the conceptual understanding of the salient points, focusing on the “how” and “why” instead of the “what.” It is hoped that the mastery of the material presented will serve as a strong foundation from which readers can further explore the technology. This book is intended as a reference for radio frequency (RF) and system engineers, technical managers, and short-course students who desire to quickly get up to speed on the essential technical issues of IS-2000. The material covered in the book is broad enough to serve students of various backgrounds and interests and to allow teachers much flexibility in designing their course material. As such, this book should be a good complement to advanced undergraduate or first-year graduate level courses in wireless communications as well.

Preface

xv

Finally, the material presented in this book is given for informational purpose and instructional value and is not guaranteed for any particular purpose. The publisher or the author does not offer any warranties or representations and does not accept any liabilities with respect to the material presented in this book. Furthermore, as technical information changes quickly, the purchaser of the book or user of the information contained in this book should seek updated information from other sources. The publisher or the author assumes no obligation to update or modify the information, nor does the publisher or the author undertake any obligation to notify the purchaser of the book or user of the information contained in the book of any update. The purchase of the book or the use of the information contained in the book signifies the purchaser’s or user’s agreement to the above.

.

Acknowledgments As always, the completion of a book would not be possible without the support of many people. I would like to thank Barry Pasternack who has given me encouragement during this project as well as guidance in other areas, Mabel Kung who has spent many hours giving me support and words of wisdom, Paul Minh who has given me advice during the writing of this book, and Joseph Sherif who is always willing to make himself available for conversations. I appreciate Samir Chatterjee who often meets with me to discuss various technical topics, and Lorne Olfman who has continued to give me guidance out of his busy schedule. I also thank the reviewer whose suggestions have made this a better book. I am also grateful to the editors at Artech: Mark Walsh who has given me much valuable feedback in the initial formulation of this project, and Barbara Lovenvirth who has done a great job managing the project and keeping me on track. I also thank Jill Stoodley and the staff at Artech for their help in the production of the book. No acknowledgment will be complete without mentioning my wife, Jenny, who has supported all my endeavors with a gentle spirit and has always encouraged me. I can always count on her for being there, and I am very much thankful for her. Last and not the least, I would like to mention my son, Daniel, who has been a source of my joy; his laughter and cheerful spirit have always given me strength during challenging parts of this project, and this book is also dedicated to him.

xvii

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CHAPTER 1

Introduction to 3G CDMA 1.1

Third Generation Systems While there are several wireless standards and systems that qualify as third generation (3G) systems, this book specifically deals with the IS-2000 implementation of 3G. In the mid-1990s, the International Telecommunication Union (ITU) initiated an effort to develop a framework of standards and systems that will provide wireless and ubiquitous telecommunications services to users anywhere at anytime. Subsequently, International Mobile Telecommunications-2000 (IMT-2000), a subgroup of the ITU, published a set of performance requirements of 3G. It is useful to review the performance requirements of a 3G wireless system, which are as follows (for both packet-switched and circuit-switched data): • • •

A minimum data rate of 144 Kbps in the vehicular environment; A minimum data rate of 384 Kbps in the pedestrian environment; A minimum data rate of 2 Mbps in the fixed indoor and picocell environment.

In addition, in all environments the system must support same data rates for both forward and reverse links (symmetric data rates), as well as support different data rates for both forward and reverse links (asymmetric data rates) [1]. Some standards and systems such as Universal Mobile Telephone System (UMTS) are implemented in the new 3G spectrum (e.g., in Europe). While other standards and systems such as IS-2000 can introduce 3G services in spectrums already used by second generation (2G) systems (e.g., in North America). The latter case takes into account those investments already deployed in the field where useful and necessary [2]. The correction in the valuation of high-technology assets in early 2000 underscores the importance of making calculated infrastructure investment while taking into account the market demand for these services. This consideration is one reason why IS-2000 has gained popularity in the initial deployment of 3G [3]. In addition, as will be seen in later chapters of this book, IS-2000 is backward compatible with existing 2G IS-95 systems. This backward compatibility gives IS-2000 two important advantages. First, IS-2000 is able to support the reuse of existing IS-95 infrastructure equipment and hence requires only incremental investment to provide 3G services. Second, because IS-2000 represents a natural technical evolution from its predecessor, there is a lower implementation risk when transitioning to 3G.

1

2

1.2

Introduction to 3G CDMA

Protocol Architecture One architectural difference between the IS-2000 standard and the IS-95 standard is that IS-2000 calls out explicitly the functions of four different protocol layers. These layers are the physical layer, medium access control, signaling link access control, and upper layer. Physical layer (Layer 1) [4]: The physical layer is responsible for transmitting and receiving bits over the physical medium. Since the physical medium in this case is over the air, the layer would have to convert bits into waveforms (i.e., modulation) to enable their transmission through air. In addition to modulation, the physical layer also carries out coding functions to perform error control functions at the bit and frame levels. Medium access control (MAC) sublayer (Layer 2) [5]: The MAC sublayer controls higher layers’ access to the physical medium that is shared among different users. In this regard, MAC carries out analogous functions as a MAC entity that controls a local area network (LAN). Whereas a LAN MAC controls different computers’ access to the shared bus, the IS-2000 MAC sublayer manages the access of different (low-speed voice and high-speed data) users to the shared air interface. Signaling link access control (LAC) sublayer (Layer 2) [6]: The LAC sublayer is responsible for the reliability of signaling (or overhead) messages that are exchanged. Recall that the over-the-air medium is extremely error-prone, and information messages are at times received (and accepted) with errors. On the other hand, since signaling messages provide important control functions, these messages have to be reliably transmitted and received. The LAC sublayer performs a set of functions that ensure the reliable delivery of signaling messages. Upper layer (Layer 3) [7]: The upper layer carries out the overall control of the IS-2000 system. It exercises this control by serving as the point that processes all and originates new signaling messages. The information (both data and voice) messages are also passed through Layer 3. Recall that the IS-95 standard does not explicitly and separately describe the functions of each layer. However in IS-95 those functions that are carried out by the layers do exist. For example, in IS-95 mobile access is logically a function of the MAC sublayer, but its descriptions are lumped together with the other functions within a single standard. At this point the reader may ask why the layered architecture was not employed in IS-95 but now used in IS-2000. The layered architecture is now used in IS-2000 because it brings the system into conformance with the 3G architecture delineated in IMT-2000. The IMT-2000 framework calls for different networks to cooperate to provide services to end users, and the level and extent of these cooperation are more clearly organized if viewed from the perspective of the layered architecture. Welldefined layer functions provide modularity to the system. As long as a layer still performs its functions and provides the expected services, the specific implementation

1.3

Other Elements of Protocol Architecture

3

of its functions can be modified or replaced without requiring changes to the layers above and below it [8]. Figure 1.1 shows the structure of the protocol architecture used by IS-2000. Without a loss of generality, this figure is shown from the perspective of the mobile station; a similar figure can also be drawn from the perspective of the base station by reversing the direction of some arrows and changing the placement of some entities. Figure 1.1 is a rather important figure and we will refer to it from time to time throughout the book. For now, note the three different layers (Layers 1, 2, and 3), the two sublayers in Layer 2 (MAC and LAC), the entities in the layers [e.g., Signaling Radio Burst Protocol (SRBP)], and the communication paths among the layers and entities. Also note that the layer structure shown in Figure 1.1 resembles that of the Open Systems Interconnection (OSI) Reference Model [9].

Other Elements of Protocol Architecture In addition to the individual layers themselves, other important elements of the protocol architecture are described as follows:

R-ACH R-EACH R-CCCH F-SYNCH F-CPCCH F-CACH F-PCH F-CCCH F-BCCH

r-dtch voice f-dtch voice

r-dtch f-dtch

RLP

Dedicated channel multiplex sublayer

R-FCH R-SCH R-DCCH F-FCH F-SCH F-DCCH

f-csch

LAC PDU SRBP

Common channel multiplex sublayer

Physical layer

Voice services

Signaling LAC

r-csch f-csch

MAC sublayer

Layer 2

Data services

RLP SDU RLP SDU

L3PDU L3PDU

Signaling Data burst

r-dsch f-dsch

Upper layers

Data burst

LAC sublayer

Layer 3

Physical channels: The physical channels are the communication paths between the physical layer and the common/dedicated channel multiplex sublayers. The physical channels are designated by uppercase letters. In the designation, the first

Layer 1

1.3

Reverse link: coding and modulation Forward link: demodulation and decoding

RL FL

Figure 1.1 Structure of the protocol architecture used by IS-2000. (Note that this structure is shown from the perspective of the mobile station. After: [5].)

4

Introduction to 3G CDMA

letter and the dash stand for either forward link (F-) or reverse link (R-), and the last two letters “CH” always stand for “channel.” For example, R-ACH stands for reverse access channel, and F-FCH stands for forward fundamental channel. A list of physical channel names and their designations is shown in Table 1.1; note that legacy IS-95 physical channels are denoted with asterisks. Logical channels: The logical channels are the communication paths between the common/dedicated channel multiplex sublayers and higher layer entities. One can think of logical channels as carrying the logical units of signaling or user information. Contrast those with physical channels which can be thought of as the actual physical vehicles that transport the signaling or user information over the air. The logical channels are designated by lower-case letters. The first letter and the dash stand for either forward link (f-) or reverse link (r-), and the last two letters “ch” always stand for “channel.” For example, r-csch stands for reverse common signaling channel, and f-dtch stands for forward dedicated traffic channel. A list of logical channel names and their designations are shown in Table 1.2. Data unit: The data units are logical units of signaling and user information that are exchanged between SRBP entity/Radio Link Protocol (RLP) entity and higher layer entities. There are two types of data units: payload data units (PDU) and service data units (SDU). PDU is used to designate those data units that are accepted by a Table 1.1

Physical Channel Designations in IS-2000

Forward Link Channel Designation

Channel Name

Reverse Link Channel Designation

F-SCH

Forward supplemental channel

R-SCH

Reverse supplemental channel Reverse supplemental code channel

Channel Name

F-SCCH

Forward supplemental code channel

R-SCCH

F-FCH*

Forward fundamental channel

R-FCH*

Reverse fundamental channel

R-DCCH

Reverse dedicated control channel

R-ACH*

Access channel

R-EACH

Enhanced access channel

R-CCCH

Reverse common control channel

R-PICH

Reverse pilot channel

F-DCCH

Forward dedicated control channel

F-PCH*

Paging channel

F-QPCH

Quick paging channel

F-CCCH

Forward common control channel

F-BCCH

Broadcast control channel

F-CPCCH

Common power control channel

F-CACH

Common assignment channel

F-SYNCH*

Sync channel

F-PICH*

Forward pilot channel

F-TDPICH

Transmit diversity pilot channel

F-APICH

Auxiliary pilot channel

F-ATDPICH

Auxiliary transmit diversity pilot channel

1.4

Spreading Rate 1 and Spreading Rate 3

Table 1.2

5

Logical Channel Designations in IS-2000

Forward Link Channel Designation

Channel Name

Reverse Link Channel Designation

f-csch

Forward common signaling channel

r-csch

Reverse common signaling channel

f-dsch

Forward dedicated signaling channel

r-dsch

Reverse dedicated signaling channel

f-dtch

Forward dedicated traffic channel

r-dtch

Reverse dedicated traffic channel

Channel Name

provider of service from a requester of service, and SDU those data units that are 1 given to a provider of service by a requester of service . The use of PDUs and SDUs is discussed in more detail later in Chapter 4 (medium access control), Chapter 5 (link access control), and Chapter 6 (upper layer signaling). In the MAC sublayer, there are four different entities: SRBP, RLP, common channel multiplex sublayer, and dedicated channel multiplex sublayer. Common channel multiplex sublayer performs the mapping between the logical common channels (channels that are shared among multiple users) and the physical common channels. Dedicated channel multiplex sublayer performs the mapping between the logical dedicated channels (channels that are dedicated to specific users) and the physical dedicated channels. Note that while dedicated channels can be used for both signaling and user data, common channels are only used for signaling. SRBP and RLP are protocol entities in the MAC sublayer. They are described in more detail in Chapter 4. It suffices to say now that SRBP handles common-channel signaling (as opposed to dedicated-channel signaling) and RLP handles user information that is packetized in nature.

1.4

Spreading Rate 1 and Spreading Rate 3 Without a loss of generality, this book will focus on Spreading Rate 1 (also known as “1x”) of IS-2000. Spreading Rate 1 by definition uses one times the chip rate of IS-95 (i.e., 1.2288 Mcps). See Figure 1.2. In addition, the IS-2000 standard also supports Spreading Rate 3 (also known as “3x”). Spreading Rate 3 is used when higher data rates are desired. Spreading Rate 3 has two implementation options: direct spread (DS) or multicarrier (MC). On the forward link, Spreading Rate 3 uses the MC option by utilizing three separate RF carriers, each spread using a chip rate of 1.2288 Mcps. In this case, the user data is multiplexed onto three separate RF carriers that are received by the mobile. On the reverse link, Spreading Rate 3 uses the DS option. The DS option allows the mobile to directly spread its data over a wider bandwidth using a chip rate of 3.6864 Mcps. See Figure 1.3. To harmonize with other 3G systems such as

1.

In the OSI Reference Model, a higher layer entity typically requests services from a lower-layer entity.

6

Introduction to 3G CDMA

1.25 MHz Forward link

Base station

Reverse link

Mobile station

1.25 MHz Figure 1.2 MHz.

Spreading Rate 1. A chip rate of 1.2288 Mcps occupies an RF bandwidth of 1.25

3.75 MHz Forward link

Base station

Reverse link

Mobile station

3.75 MHz Figure 1.3

Spreading Rate 3.

UMTS, a Spreading Rate 3 signal can have 625 kHz of guard band on each side resulting in a total RF bandwidth of 5 MHz. These options for the forward and reverse links are included in the standard in order to reduce the complexity of the mobile’s receiver. As readers may have already noticed, the above-stated configurations mean that the mobile’s receiver only has to receive and demodulate 1x carriers and does not have to receive and demodulate any 3x carrier. Incidentally, a mobile can also receive at Spreading Rate 3 and transmit at Spreading Rate 1. See Figure 1.4. This particular arrangement takes advantage of the fact that data rates required for downstreaming are typically higher than those required for upstreaming. Wider bandwidth options such as 6x, 9x, and 12x are under consideration for even higher data rate applications. As far as 3G systems are concerned, Spreading Rate 3 satisfies all the performance requirements as set forth by IMT-2000.

1.5

Differences Between IS-2000 and IS-95

7

3.75 MHz Forward link

Base station

Reverse link

Mobile station

1.25 MHz Figure 1.4

Spreading Rate 3 on forward link and Spreading Rate 1 on reverse link.

As a final note: The original intention of the IS-2000 family of standards is to evolve progressively to higher data rates using wider bandwidths (i.e., 3x…12x). However, the current trend seems to be one of deploying high data rate solutions that use 1.25 MHz of bandwidth (e.g., 1xEV-DO). There are several advantages of using solutions like 1xEV-DO, one of which is that wireless operators can carve out selected 1.25 MHz carriers dedicated to and optimized for high rate data. 1xEV-DO is covered later in Chapters 13–15.

1.5

Differences Between IS-2000 and IS-95 IS-2000 represents a natural technical extension from its IS-95 predecessor, and this extension can be seen in the fact that IS-2000 users and IS-95 users can coexist in the same carrier. Although IS-2000 is backward compatible with IS-95, there are many differences between IS-2000 and IS-95. We will point out now, by way of introduction, those differences that represent a substantial departure from IS-95. Since the requirement of 3G and IS-2000 is transmitting and receiving at a higher data rate, two types of improvements are needed to enable data rates at or above 144 Kbps: improvements in signaling and improvements in transmission.

1.5.1

Signaling

In order to implement high-rate packet-switched data, IS-2000 needs to dynamically acquire and release air link resources, and efficient signaling is required to perform quick acquisitions and releases of these resources. These new signaling mechanisms include: •

On the forward link, there are new overhead/signaling physical channels. They are quick paging channel (F-QPCH), forward common control channel (F-CCCH), broadcast control channel (F-BCCH), common power control channel (F-CPCCH), and common assignment channel (F-CACH).

8

Introduction to 3G CDMA

•

•

•

On the reverse link, there are new overhead/signaling physical channels. They are reverse dedicated control channel (R-DCCH), enhanced access channel (R-EACH), and reverse common control channel (R-CCCH). On the reverse link, there are shorter signaling messages. IS-2000 can transmit shorter 5-ms frames on the enhanced access channel (R-EACH). This is done to reduce the probability of access collision. On the forward link, IS-2000 can also transmit shorter signaling messages. It can use shorter 5-ms frames (i.e., 1/8 rate) on the forward fundamental channel for this purpose.

In addition, an IS-2000 mobile can now be in one of several modes (e.g., dormant mode) to accommodate bursty packet data transmissions and to conserve air link resources. These modes are described in more detail in Chapter 6 on upper layer signaling. The new overhead/signaling physical channels on the forward link are discussed in Chapter 2, and the new overhead/signaling physical channels on the reverse link are discussed in Chapter 3. 1.5.2

Transmission

A higher air link capacity is obviously needed to implement high-rate data, and various changes are made to improve air link capacity to beyond that of IS-95. These changes are also made to effect a more efficient use of air link resources. Some major changes are listed below: •

•

•

•

Forward supplemental channel (F-SCH) and reverse supplemental channel (R-SCH) are added to transport high-rate user data. Reverse link now has a reverse pilot channel (R-PICH) to support coherent modulation on the reverse link. Forward link now has fast closed-loop power control (compared with the slower power control in IS-95). Power control groups are transmitted on the reverse pilot channel to enable fast closed-loop power control of the forward link. In addition to power controlling the traffic channels, IS-2000 can also power control the signaling channel (i.e., forward dedicated control channel [F-DCCH]).

Supplemental channels are discussed in more detail in Chapter 2 and Chapter 3. IS-2000 power controls are discussed in more detail in Chapter 7. Other transmission improvements include the implementation of a more efficient quadrature phase-shift keying (QPSK) in the modulation stage and the use of more efficient turbo codes for high date rate transmissions. 1.5.3

Concluding Remarks

The differences between IS-2000 and IS-95 are not limited to those introduced above. Throughout the book, we will regularly point out, where appropriate, more

1.5

Differences Between IS-2000 and IS-95

9

differences to which system engineers and planners should pay attention. Noting these differences is important because being aware of them not only facilitates the understanding of 3G IS-2000, but also leverages the experience already gained in operating 2G IS-95 systems.

References [1] [2] [3] [4] [5] [6] [7] [8] [9]

ITU-R Recommendation M.1225, Guidelines for Evaluation of Radio Transmission Technologies for IMT-2000, International Telecommunication Union, 1997. Prasad, R., W. Mohr, and W. Konhauser (eds.), Third Generation Mobile Communication Systems, Norwood, MA: Artech House, 2000, p. 2. The Economist, “Mobile Telecoms: Time for plan B,” Economist, September 28–October 4, 2002, pp. 57–58. TIA/EIA/IS-2000.2-A, Physical Layer Standard for cdma2000 Spread Spectrum Systems, Telecommunications Industry Association, March 2000. TIA/EIA/IS-2000.3-A, Medium Access Control (MAC) Standard for cdma2000 Spread Spectrum Systems, Telecommunications Industry Association, March 2000. TIA/EIA/IS-2000.4-A, Signaling Link Access Control (LAC) Standard for cdma2000 Spread Spectrum Systems, Telecommunications Industry Association, March 2000. TIA/EIA/IS-2000.5-A, Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems, Telecommunications Industry Association, March 2000. Forouzan, B. A., Data Communications and Networking, New York: McGraw-Hill, 2004. ITU-T Recommendation X.210, Information Technology–Open Systems Interconnection–Basic Reference Model: Conventions for the Definition of OSI Services, International Telecommunication Union, 1993.

.

CHAPTER 2

Physical Layer: Forward Link 2.1

Introduction The physical layer is responsible for transmitting and receiving bits (organized in frames) over the physical medium. The physical layer carries out coding functions to enable error correction and detection at the bit and frame levels. Besides coding, the layer would have to convert bits into waveforms (i.e., modulation) and vice versa to enable their transmission over the air. In addition to coding and modulation, the physical layer also carries out the channelization function by which different users of the system can be distinguished from one another. In a shared direct sequence spread spectrum system (such as IS-2000 and IS-95), channelization is done via the use of orthogonal and nearorthogonal codes. This chapter deals with the physical channels that exist on the forward link in the IS-2000 system, and their descriptions are organized into two broad categories: signaling channels and user channels. Signaling channels, described in Section 2.3, are those channels that carry signaling and control information. Signaling channels can be further classified into two types: dedicated and common channels. The F-DCCH is a dedicated signaling channel because this channel, once assigned, is only used by one user. The remaining signaling channels, such as the F-CCCH and F-QPCH are examples of common signaling channels because they are shared among multiple users. User channels, described in Section 2.4, are those channels that carry user information. The user information may be voice, low-rate data (e.g., short message service or SMS), or high-rate data (e.g., video streaming). There are three physical channels primarily used to carry user information: (1) F-FCH which is equivalent to forward traffic channel in IS-95, (2) F-SCCH which is equivalent to forward supplemental code channel in IS-95 (more specifically, IS-95-B [1]), and (3) F-SCH which is a new channel in IS-2000. Figure 2.1 shows the categorization of these forward link channels, both signaling and user. Table 2.1 is a list of physical channels used by the physical layer. Both forward link and reverse link channels and their descriptions are shown for completeness. Also, for each forward link physical channel, its counterparts on the reverse link are shown in the same row for correspondence. Asterisked channel designations show those channels that also exist in IS-95 systems. Note that (forward and reverse) fundamental channels are equivalent to the IS-95 traffic channels. In addition, boldfaced channel names show those channels that are collectively known as the

11

12

Physical Layer: Forward Link Common channels

Paging channel (F-PCH*) Quick paging channel (F-QPCH) Forward common control channel (F-CCCH) Broadcast control channel (F-BCCH) Common assignment channel (F-CACH) Common power control channel (F-CPCCH) Sync channel (F-SYNCH*) Forward pilot channel (F-PICH*) Transmit diversity pilot channel (F-TDPICH) Auxiliary pilot channel (F-APICH) Auxiliary transmit diversity pilot channel (F-ATDPICH)

Dedicated channels

Forward dedicated control channel (F-DCCH)

Signaling channels

User channels

Forward fundamental channel (F-FCH*) Forward supplemental channel (F-SCH) Forward supplemental code channel (F-SCCH*)

Figure 2.1 Categories of forward link physical channels. Legacy IS-95 physical channels are denoted with asterisks.

Table 2.1 Channel

Forward Link Physical Channels and Their Reverse Link Counterparts Channel Name

Description

F-SCH

F-SCCH*

Channel Name

Description

Forward supplemental channel

For transmitting user data while a call is R-SCH active; uses convolutional or turbo coding

Reverse supplemental channel

For transmitting user data while a call is active; uses convolutional or turbo coding

Forward supplemental code channel

For transmitting user data while a call is R-SCCH* active; uses convolutional coding

Reverse supplemental code channel

For transmitting user data while a call is active; uses convolutional coding

F-FCH*

Forward fundamental channel

For transmitting user and signaling data while a call is active; R-FCH* uses convolutional coding

Reverse fundamental channel

For transmitting user and signaling data while a call is active; uses convolutional coding

F-DCCH

Forward dedicated control channel

For transmitting signaling and user data while a call is active

Reverse dedicated control channel

For transmitting signaling and user data while a call is active

F-PCH*

Paging channel

For transmitting MSspecific and system overhead data

Channel

R-DCCH

2.1

Introduction

13

Table 2.1 (continued) Channel

F-QPCH

Channel Name

Description

Quick paging channel

For telling MS (operating in slotted mode while in the idle state) whether or not it should receive F-CCCH or F-PCH starting in the next F-CCCH or F-PCH slot

Forward common control channel

For transmitting signaling data when F-FCH, F-SCCH, F-SCH, or F-DCCH is not active

Broadcast control channel

For transmitting signaling data when F-FCH, F-SCCH, F-SCH, or F-DCCH is not active

F-CPCCH

Common power control channel

For transmitting common power control subchannels (one bit per subchannel) to power-control multiple R-CCCHs and R-EACHs

F-CACH

Common assignment channel

For transmitting signaling data to allocate R-CCCH resources

F-SYNCH*

Sync channel

For providing MS time and frame synchronization

F-PICH*

Forward pilot channel

For assisting MS to acquire initial time synchronization

F-TDPICH

Transmit diversity pilot channel

F-CCCH

F-BCCH

For implementing transmit diversity on the forward link

Channel

Channel Name

Description

R-ACH*

Access channel

For initial communications with BS, i.e., initiating access and responding to pages

R-EACH

For initial communications with BS, i.e., Enhanced access initiating access channel or responding to MS-specific messages

R-CCCH

For transmitting signaling and user data when Reverse common R-FCH, control channel R-SCCH, R-SCH, or R-DCCH is not active

R-PICH

Reverse pilot channel

For assisting BS to detect MS transmission

14

Physical Layer: Forward Link

Table 2.1 (continued) Channel

Channel Name

Description

Channel

F-APICH

Auxiliary pilot channel

For supporting the use of spot beam

F-ATDPICH

Auxiliary transmit diversity pilot channel

For implementing transmit diversity in the spot beam

Channel Name

Description

“IS-2000 traffic channels” (not to be confused with IS-95 traffic channels) since these channels can all carry user traffic data in IS-2000 systems.

2.2

Radio Configurations In IS-2000, each traffic channel (i.e., forward fundamental channel, forward supplemental code channel, forward supplemental channel, and forward dedicated control channel) can assume different configurations to implement different data rates. For any one configuration, the associated coding rate, modulation characteristics, and spreading rate would have to be matched to achieve a specified final transmitted data rate. Table 2.2 shows these different radio configurations [2]. For these radio configurations, the data rates shown in the table are maximum data rates. For a given radio configuration, data rates lower than the maximum are possible. Note that Radio Configuration 1 and Radio Configuration 2 are backward compatible with IS-95 in that they are equivalent to Rate Set 1 and Rate Set 2 of IS-95. For each radio configuration, the table shows the maximum achievable data rate (instead of all possible data rates). For example, for Radio Configuration 1 the system is capable of transmitting at 1.2 Kbps, 2.4 Kbps, 4.8 Kbps, and 9.6 Kbps; only the maximum data rate of 9.6 Kbps is shown. In addition, for each radio

Table 2.2

Radio Configurations on the Forward Link

Radio Configuration

Coding Rate R

Modulation

Spreading Rate

Maximum Data Rate

1

1/2

BPSK

1

9.6 Kbps

2

1/2

BPSK

1

14.4 Kbps

3

1/4

QPSK

1

153.6 Kbps

4

1/2

QPSK

1

307.2 Kbps

5

1/4

QPSK

1

230.4 Kbps

6

1/6

QPSK

3

307.2 Kbps

7

1/3

QPSK

3

614.4 Kbps

1/4 (20 ms)

QPSK

3

460.8 Kbps

QPSK

3

1.0368 Mbps

8

1/3 (5 ms) 9

1/2 (20 ms) 1/3 (5 ms)

2.3

Signaling Channels

15

configuration the coding rate R is normally the same regardless of the size of the frame (20 ms or 5 ms). But for Radio Configurations 8 and 9 (i.e., Spreading Rate 3), the coding rate is dependent on the size of the frame transmitted.

2.3

Signaling Channels One of the key requirements of 3G is high-data rate. In order to meet this requirement one needs to make the physical layer more efficient. Recall that in 2G IS-95, while a call is active signaling information is typically carried by the traffic channel (i.e., fundamental channel). In doing so, signaling bits rob traffic channel’s ability to carry user data bits. 3G IS-2000 deals with this issue by implementing separate signaling channels that carry signaling information. Although signaling data can still be carried by the fundamental channel, IS-2000 has the option of sending signaling data on separate signaling channels. This frees up fundamental channel’s and supplemental channel’s capability to transport more user data. 2.3.1

Forward Dedicated Control Channel (F-DCCH)

The F-DCCH is a unique signaling channel in two respects: •

•

Unlike other signaling channels, the F-DCCH is a dedicated signaling channel. Once assigned, the F-DCCH is only allocated to one designated user. All other signaling channels (to be described later) are common to and shared with other users. Just as the forward fundamental channels can carry signaling data (through dim-and-burst and blank-and-burst), the F-DCCH can carry user data. The kind of user data that the F-DCCH carries is typically low-rate (such as SMS). Such data service requests are sporadic in nature and short in duration. For such transmission requests, instead of expending resources to set up a fullfledge fundamental channel or supplemental channel, the system can choose to temporarily suspend transmitting signal data and start sending user data over the F-DCCH.

In addition, both 20-ms and 5-ms frame formats are supported by the F-DCCH. For example, one 20-ms frame format for the F-DCCH is 192 bits in length consisting of 172 information bits, 12 cyclic redundancy check (CRC) bits, and 8 encoder tail bits. This gives an F-DCCH data rate of (192 bits/20 ms) 9.6 Kbps. See Figure 2.2. Note that in this case, this F-DCCH frame has the same capacity as an IS-95 Rate Set 1 paging channel frame. On the other hand, a 5-ms frame structure for the F-DCCH is 48 bits in length consisting of 24 information bits, 16 CRC bits, and 8 encoder tail bits. This gives an F-DCCH data rate of (48 bits/5 ms) also 9.6 Kbps. Also see Figure 2.2. Note that a 5-ms frame obviously does not have as much datacarrying capacity as a 20-ms frame. The reason why 5-ms frames are necessary is that at times a signaling message is short and cannot fill up the entire (traditional) 20-ms frame, and it would be

16

Physical Layer: Forward Link 20-ms frame (9.6 Kbps)

172 information bits 12 CRC bits

8 encoder tail bits

5-ms frame (9.6 Kbps)

16 24 information CRC bits bits

Figure 2.2

8 encoder tail bits

Examples of 20-ms and 5-ms F-DCCH frames.

inefficient to transmit a short minimessage using a 20-ms frame. Using a 5-ms frame to transport a short signaling message is a more efficient use of the air link resources. An important type of signaling data that the F-DCCH carries is power control bits used to power-control the reverse link. Recall that in IS-95 the power control bits are multiplexed onto the forward traffic channel at 800 bps in power control groups. In a similar fashion, the power control bits can be multiplexed onto the F-DCCH as well. The structure and organization of the power control groups on the F-DCCH is referred to as forward power control subchannel. In effect, a forward power control subchannel exists on the F-DCCH to transport the power control bits. The mobile uses these power control bits to perform closed-loop power control of the reverse dedicated control channel, reverse fundamental channel, and reverse supplemental channel. 2.3.2

Quick Paging Channel (F-QPCH)

The F-QPCH is a new physical channel used in IS-2000 to improve the efficiency of sending page messages. The IS-95 F-PCH, while effective, does have some drawbacks: •

•

In the nonslotted mode the mobile has to monitor continuously the entire paging channel slot, which in IS-95 lasts 80 ms. As a result, the mobile expends a lot of battery power to perform this continuous monitoring. In the slotted mode the mobile monitors only those time slots that are assigned to it. While this does save some battery power, it is still inefficient. From the base station’s perspective, it is inefficient because when the base station has a

2.3

Signaling Channels

17

mobile-specific page to send, it cannot immediately send it. The base station has to wait for the correct slot to come along to send the page. As a result, the mobile often does not receive its designated pages immediately. From the mobile’s perspective, while the mobile saves some battery power by only monitoring its assigned slot, the assigned slot still lasts 80 ms. At the beginning of its assigned slot, the mobile still has to wake up to monitor the entire 80-ms slot, and most of the time there is no page directed at the mobile. In IS-2000, the F-QPCH is added to alleviate the drawbacks cited above. If there is a page directed to a mobile, the base station first uses the F-QPCH to send shorter paging indicator bits to the mobile. The mobile monitors its designated paging indicators. If the paging indicators show that there is no mobile-specific page, then the mobile does nothing. If the paging indicators show that there is a mobile-specific page coming in, then the mobile wakes up and monitors its assigned paging channel slot. Note that in this regard, the F-QPCH works with a paging channel operating in slotted mode. In addition, the F-QPCH can also work with a forward common control channel operating in slotted mode. 2.3.2.1

Paging Indicators

Figure 2.3 shows in more detail how the F-QPCH works in conjunction with the F-PCH. As one can see in the figure, a paging channel slot and a quick paging channel slot both last 80 ms, and quick paging channel slots are offset from (ahead of) paging channel slots by 20 ms. Each quick paging channel is divided into four 20-ms portions. In this case, let’s assume that a mobile’s assigned paging channel slot is slot Y. Instead of always monitoring paging channel slot Y, the mobile would monitor 80-ms Paging channel slot (Y)

X

Z

Y

80-ms Quick paging channel slot (y)

p

y1 20-ms portion

y2

p

y3

y4

z1

z2

z3

z4

p: Paging indicator

Figure 2.3 Channel format: F-QPCH. As an example, the figure shows two paging indicators located in the second and fourth 20-ms portions of the quick paging channel slot (y).

18

Physical Layer: Forward Link

its paging indicators in the quick paging channel slot (y) that comes before the assigned paging channel slot (Y). In a quick paging channel slot, the mobile always monitors two paging indicators. The two paging indicators either fall in the first 20-ms portion and in the third 20-ms portion (e.g., y1 and y3), or fall in the second 20-ms portion and in the fourth 20-ms portion (e.g., y2 and y4). So in our example, if two mobiles are both assigned paging channel slot Y, the first mobile may monitor a paging indicator in y1 and a paging indicator in y3; the second mobile may monitor a paging indicator in y2 and a paging indicator in y4. In actuality, the exact position of a paging indicator in the 20-ms portion is determined by a hash algorithm, the same type of algorithm that determines the assigned paging channel slot for a mobile operating in the slotted mode. 2.3.2.2

Other Indicators

In addition to carrying paging indicators, the F-QPCH also carries two other types of indicators: broadcast indicators and configuration change indicators. The mobile monitors its broadcast indicators to check if it needs to monitor its assigned slot (for broadcast messages) on the forward common control channel or paging channel. Furthermore, all mobiles monitor configuration change indicators; these indicators are used to inform mobiles of a change in configuration parameters (e.g., neighbor list) [2]. The relative positions of broadcast and configuration change indicators are shown in Figure 2.4. As shown in the figure, the number of broadcast and configuration change indicators depends on the data rate of the F-QPCH. 2.3.2.3

Characteristics of Quick Paging Channel

One distinguishing feature of the F-QPCH is that this physical channel has no error protection. This means that the bits sent on the F-QPCH do not have CRC bits 80-ms Quick paging channel slot (y)

bc

y1 20-ms portion

y2

bc

y3

y4

bc

z1

z2

bc

z3

z4

b: Broadcast indicator c: Configuration change indicator b=2 and c=2 if F-QPCH data rate = 2.4 Kbps b=4 and c=4 if F-QPCH data rate = 4.8 Kbps

Figure 2.4

Broadcast and configuration change indicators on the F-QPCH.

2.3

Signaling Channels

19

added, are not convolutionally coded, and are not block-interleaved. The reason for this design choice is that paging indicator bits need to be quickly demodulated at the receiver so a decision can be made quickly regarding whether or not to monitor the paging channel slot that follows. Not needing to check the CRC bits, convolutionally decode all the bits, and deinterleave save a lot of processing time. Note that this is the same reason for not error-protecting the power control bits in IS-95 (and in IS-2000). Power control bits need to be demodulated quickly so that power control decisions can be made quickly to adapt to changing channel conditions. An IS-2000 carrier can have up to three quick paging channels. However, configuration change indicators and broadcast indicators are only used on the first quick paging channel [2]. 2.3.3

Forward Common Control Channel (F-CCCH)

To further improve the signaling efficiency of the link, IS-2000 added two additional physical signaling channels: F-CCCH and F-BCCH. Recall that the functions of the paging channel in IS-95 are to deliver (1) specific messages that are intended for specific mobiles (e.g., channel assignment message), and (2) broadcast messages that are intended for all mobiles (e.g., system parameters message and neighbor list message). Using a single paging channel for these two functions is not very efficient because of the queuing characteristics of these two types of messages. The broadcast messages are sent at more regular intervals, while the specific messages are sent more irregularly on-demand. As a result, mixing two statistically different types of messages on the same channel results in less-than-optimal scheduling of the paging channel. Furthermore, recall that IS-95 allows up to seven paging channels per carrier; since each mobile only monitors one paging channel, if there are more than one paging channels in the carrier then broadcast system messages would have to be duplicated on all paging channels. To alleviate the responsibilities of the paging channel, IS-2000 added two additional channels: F-CCCH and F-BCCH. The F-CCCH is used to transmit specific messages intended for specific mobiles, while the F-BCCH is used to transmit broadcast system messages intended for all mobiles. Note that although the F-CCCH is “common” in the sense that it is shared by many mobiles, its purpose is to carry mobile-specific messages. The broadcast control channel is described in Section 2.3.4. Since the function of the F-CCCH is to carry messages (e.g., channel assignment message) that are previously carried by the paging channel, it is no surprise that the structure of the F-CCCH is similar to that of the paging channel. For example, the F-CCCH consists of F-CCCH slots each lasting 80 ms. What is new in IS-2000 is that it supports three different frame duration: 20 ms, 10 ms, and 5 ms. For example, a 20-ms frame for the F-CCCH may be 192 bits in length consisting of 172 information bits, 12 CRC bits, and 8 encoder tail bits. This gives an F-CCCH data rate of (192 bits/20 ms) 9.6 Kbps. See Figure 2.5. Note that in this case, this F-CCCH frame has the same capacity as an IS-95 Rate Set 1 paging channel frame. Other data rates of 19.2 Kbps and 38.4 Kbps are also supported. Figure 2.5 gives some examples of F-CCCH frame structures.

20

Physical Layer: Forward Link 20-ms frame (9.6 Kbps)

12 CRC bits

172 information bits

8 encoder tail bits

20-ms frame (38.4 Kbps)

744 information bits 16 CRC bits

8 encoder tail bits

10-ms frame (38.4 Kbps)

360 Information bits

16 CRC bits

8 encoder tail bits

5-ms frame (38.4 Kbps)

12 172 information CRC bits bits

Figure 2.5

8 encoder tail bits

Examples of 20-ms, 10-ms, and 5-ms F-CCCH frames.

The F-CCCH can also be used in conjunction with the F-QPCH. Recall that the quick paging channel is a new physical channel used in IS-2000 to improve the efficiency of sending page messages. For example, a mobile does not have to monitor the F-CCCH all the time for page messages intended for it. Rather, by monitoring its paging indicators on the quick paging channel, the mobile knows whether or not it should start receiving the F-CCCH in the next F-CCCH slot [2]. 2.3.4

Broadcast Control Channel (F-BCCH)

As mentioned previously, the purpose of the F-BCCH is transmitting broadcast system messages (e.g., system parameters message and access parameters message) to those mobiles in a base station’s coverage area. Although the F-BCCH performs a function that is previously carried out by the IS-95 paging channel, the structure of the F-BCCH is somewhat different. See Figure 2.6.

2.3

Signaling Channels

21 160-ms F-BCCH slot

80-ms F-BCCH slot

40-ms F-BCCH slot

40-ms frame (19.2 Kbps peak)

744 information bits

Figure 2.6

16 CRC bits

8 encoder tail bits

F-BCCH structure and frame.

As shown in Figure 2.6 instead of using a single slot duration of 80 ms (like the paging channel and the forward common control channel), the F-BCCH can have slots that last 40 ms, 80 ms, or 160 ms. In addition, unlike the paging channel and the forward common control channel the F-BCCH has only one frame format that lasts 40 ms. Therefore, it is obvious that a 160-ms slot always contains four frames, an 80-ms slot always contains two frames, and a 40-ms slot always contains one frame. An F-BCCH frame always lasts 40 ms and always contains 744 information bits, 16 CRC bits, and 8 encoder tail bits, resulting in a total of 768-bits-per-frame. This gives a (peak) F-BCCH data rate of (768 bits/40 ms) 19.2 Kbps. With sequence repetition (similar to symbol repetition in IS-95), this peak data rate of 19.2 Kbps can be throttled down. For example, 2x sequence repetition drops the data rate by half to 9.6 Kbps, and 4x sequence repetition drops the data rate by a quarter to 4.8 Kbps [2]. 2.3.5

Common Assignment Channel (F-CACH)

The function of the F-CACH is for the base station to quickly allocate reverse common control channel (R-CCCH) resources to the different mobiles. As will be discussed in Section 3.3.2, the R-CCCH is used by mobiles to transmit signaling information when the R-DCCH or the R-FCH is not active. The scheduling information for the use of the reverse common control channel is transmitted by the F-CACH. Because the F-CACH is used to control another signaling channel R-CCCH, the F-CACH is really a signaling channel for a signaling channel. In other words, the F-CACH has to quickly transmit signaling information (to the mobile) so that an

22

Physical Layer: Forward Link

R-CCCH resource can be quickly allocated to transmit some other signaling information (back to the base station). As such, the F-CACH uses 5-ms frames exclusively. Figure 2.7 shows the frame format of the F-CACH. As shown in Figure 2.7, the F-CACH frame consists of 48 bits which yield 9.6 Kbps (= 48 bits/5 ms) [2].

2.3.6

Common Power Control Channel (F-CPCCH)

In addition to power controlling the reverse fundamental channel (R-FCH) and the reverse supplemental channel (R-SCH), IS-2000 has the ability to power control signaling channels to further improve air link capacity. The function of the F-CPCCH is to carry signaling information to power control the following reverse link signaling channels: • •

Reverse common control channel (R-CCCH); Enhanced access channel (R-EACH).

The F-CPCCH consists of a stream of (power control) bits that is not errorprotected. This is because power control bits need to be demodulated rapidly so that power control decisions can be made quickly to adapt to changing channel conditions. Not needing to check the CRC bits, convolutionally decode all the bits, and de-interleave save a lot of processing time. Note that this is the same reason for not error-protecting the power control bits in IS-95 (and in IS-2000). Figure 2.8 shows an example format of the F-CPCCH. Here each F-CPCCH frame (which lasts 20 ms) consists of 16 power control groups. Each power control group lasts 1.25 ms, hence the transmission rate of the power control groups is 800-times-per-second (= 1 / 1.25 ms). Each power control group contains 12 power control bit positions. This gives a total of 192 power control bit positions per 20-ms frame. In IS-2000, the forward link uses QPSK modulation which consists of two paths: the in-phase (I) path and the quadrature (Q) path. The F-CPCCH is structured in such a way that each path contains separate and distinct power control bits. As Figure 2.8 shows, in the I path the first bit position of each power control group is used to transmit power control subchannel 0; the second bit position is used to 5-ms frame (9.6 Kbps)

32 information bits Figure 2.7

F-CACH frame.

8 CRC bits

8 encoder tail bits

2.3

Signaling Channels

23 20-ms frame (9.6 Kbps)

1.25-ms PCG 1

PCG 15

PCB 11

PCB 2

PCB 0 PCB 1

PCG 0

Power control subchannels

0 I (path)

11

12

Q (path)

PCG 0

Figure 2.8

PCB 11

PCB 2

PCB 0 PCB 1

23

PCG 1

PCG 15

F-CPCCH: 16 power control groups per 20-ms frame.

transmit power control subchannel 1, and so on. In the Q path the first bit position of each power control group is used to transmit power control subchannel 12, and the last bit position in the same power control group is used to transmit power control subchannel 23. Therefore, in this example each power control group is capable of carrying 24 power control subchannels (0–23). Since the same 24 power control subchannels are replicated in subsequent common power control groups1, the transmission rate of the power control bits for each subchannel is 800-times-per-second. 1.

IS-2000 refers to a power control group in the I path and its corresponding power control group in the Q path as a common power control group.

24

Physical Layer: Forward Link

In actuality, the power control bit positions (and their corresponding power control subchannels) are not arranged sequentially in a power control group. Rather, they are arranged pseudorandomly in a power control group. The long code mask and the long code generator are used to pseudorandomize the bit positions in a power control group. Since the mobile also possesses the same long code mask and the same long code generator, the exact position of a given power control subchannel is known perfectly to the mobile. In addition to supporting 16 power control groups per 20-ms frame, IS-2000 can also support 8 power control groups per 20-ms frame and 4 power control groups per 20-ms frame. It is an easy exercise then to calculate the number of power control subchannels supported by these formats: •

•

For 8 power control groups per 20-ms frame, the F-CPCCH can support a total of 48 power control subchannels. For 4 power control groups per 20-ms frame, the F-CPCCH can support a total of 96 power control subchannels.

For a F-CCCH, a CDMA carrier can have a maximum of 32 R-CCCHs and a maximum of 32 R-EACHs. Plus if an F-CACH is also active, the same carrier can have another (maximum) set of 32 R-CCCHs. For each carrier, this gives a maximum total of 96 reverse common control channels and enhanced access channels. These 96 channels can all be power controlled using the 96 power control subchannels provided by the 4 power control groups per 20-ms frame on the F-CPCCH [2]. Note that each common power control subchannel is used to control the power of a single mobile. 2.3.7

Pilot Channels

In IS-2000, there are actually four types of pilot channels on the forward link. They are: • • • •

Forward pilot channel (F-PICH); Transmit diversity pilot channel (F-TDPICH); Auxiliary pilot channel (F-APICH); Auxiliary transmit diversity pilot channel (F-ATDPICH).

2.3.7.1

Forward Pilot Channel (F-PICH)

The F-PICH is equivalent to the IS-95 pilot channel. This channel is identified by Walsh code w 128 0 . It contains no baseband information in that the baseband sequence is a stream of 1s that are spread by Walsh code w 128 0 , which is also a 2 sequence of 1s . The resulting sequence (still all 1s) is then multiplied by a pair of quadrature PN codes. Thus the forward pilot channel is effectively the PN code itself. As in IS-95, the forward pilot channel provides the mobile with timing and phase reference. Each base station sector has only one forward pilot channel.

2.

See Chapter 9 on the generation of Walsh codes.

2.3

Signaling Channels

2.3.7.2

25

Transmit Diversity Pilot Channel (F-TDPICH)

The F-TDPICH is a new signaling channel in IS-2000. This channel is identified by Walsh code w 128 16 . It also carries no baseband information in that the baseband sequence is a stream of 1s that are spread by Walsh code w 128 16 . The transmit diversity pilot channel works with the forward pilot channel to support transmit diversity on the forward link (see Chapter 9). Each base station sector can have at most one transmit diversity pilot channel. If there is one, then the transmit diversity pilot channel is transmitted continuously at a power level that is the same as or lower than that of the forward pilot channel. 2.3.7.3

Auxiliary Pilot Channel (F-APICH)

The F-APICH is a new signaling channel in IS-2000. This channel can be identified by a Walsh code or by a quasi-orthogonal function . It carries no baseband information in that the baseband sequence is a stream of 1s that are spread by its assigned Walsh code or quasi-orthogonal function. The auxiliary pilot channel supports the use of spot beam on the forward link. If a base station sector has a spot beam (formed by a single antenna or an array antenna) within the coverage area defined by its forward pilot channel, then that spot beam must have an auxiliary pilot channel. The auxiliary pilot channel is optional in that IS-2000 does not specify how many auxiliary pilot channels (or spot beams) a base station sector can have. If there is one, then the auxiliary pilot channel is transmitted continuously. In addition, if there are more than one spot beams then each spot beam is assigned a different auxiliary pilot channel (which is in turn spread by a different Walsh code or quasiorthogonal function). From the mobile’s perspective, the mobile reports the number of auxiliary pilots that it sees using the NUM_AUX_PILOTS field in the origination message and page response message. For each reported auxiliary pilot, the mobile also reports parameters such as the phase (PILOT_PN_PHASE), strength (PILOT_STRENGTH), and Walsh code (PILOT_WALSH) if a Walsh code is used to spread the auxiliary pilot. If a quasi-orthogonal function is used to spread the auxiliary pilot, then the mobile reports that quasi-orthogonal function (QOF)3. In addition, the mobile can report information regarding its received auxiliary pilots in the supplemental channel request message and extended pilot strength measurement message. From the base station’s perspective, the base station specifies information regarding the auxiliary pilots in messages such as the extended channel assignment message, general neighbor list message, extended neighbor list update message, and candidate frequency search request message. 2.3.7.4

Auxiliary Transmit Diversity Pilot Channel (F-ATDPICH)

A spot beam itself can also support its own transmit diversity to increase forward link gain. If it does, then the spot beam uses the F-ATDPICH in addition to its 3.

See Chapter 9 for more details on quasi-orthogonal functions.

26

Physical Layer: Forward Link

auxiliary pilot channel. The auxiliary transmit diversity pilot channel can be identified by a Walsh code or by a quasi-orthogonal function. It carries no baseband information in that the baseband sequence is a stream of 1s that are spread by its assigned Walsh code or quasi-orthogonal function. The auxiliary transmit diversity pilot channel works with the auxiliary pilot channel to provide orthogonal transmit diversity to the mobiles using the spot beam. IS-2000 does not specify how many auxiliary pilot channels a base station sector can have. But if a spot beam supports transmit diversity then it has to transmit an auxiliary transmit diversity pilot channel. In addition, if a spot beam uses an auxiliary transmit diversity pilot channel, then the auxiliary transmit diversity pilot channel transmits at a power level that is the same as or lower than that of the auxiliary pilot channel.

2.4

User Channels There are three user channels in IS-2000: F-FCH, F-SCCH, and F-SCH. The F-SCCH is provisioned to provide backward compatibility with IS-95 (more specifically, IS-95-B [1]). Here we focus our discussions on the forward fundamental and supplemental channels. 2.4.1

Forward Fundamental Channel (F-FCH)

2.4.1.1

Functions

The IS-2000 F-FCH is similar to the IS-95 forward traffic channel in that the primary function of the F-FCH is to carry user data. The following lists the main purposes of the IS-95 forward traffic channel: • • •

•

Transmission of voice traffic; Transmission of low-rate data traffic; Transmission of signaling traffic via dim-and-burst and blank-and-burst schemes; Transmission of power control bits to power control the reverse link.

In IS-2000, the F-FCH is also capable of carrying out all the responsibilities listed above, but the main advantage is that the IS-2000 F-FCH does not have to perform all of them. Specifically, the transmission of signaling traffic is optional in that it can be done by the F-FCH or, more efficiently, by other signaling channels provided by IS-2000. For example, the F-DCCH can be used to carry signaling information; therefore, if both the F-FCH and the F-DCCH are active, then signaling information can travel on the F-DCCH, freeing the F-FCH to carry more user traffic. Although in IS-2000 signaling messages can be carried by separate signaling channels, the F-FCH is endowed with improved capabilities to transport signaling messages. One such capability is the support of shorter 5-ms frames in addition to the conventional 20-ms frames. Recall that both F-DCCH and F-CCCH also support 5-ms frames to carry shorter signaling messages. The F-FCH has the same

2.4

User Channels

27

ability to transmit short signaling messages in 5-ms frames as these other signaling channels. 2.4.1.2

Forward Fundamental Channel Frames

In terms of carrying user traffic, in IS-95 the traffic channel can only support two radio configurations (or rate sets) and two peak data rates. At Rate Set 1 or a peak data rate of 9.6 Kbps, the IS-95 traffic channel can support 1.2 Kbps, 2.4 Kbps, 4.8 Kbps, and 9.6 Kbps. At Rate Set 2 or a peak data rate of 14.4 Kbps, the IS-95 traffic channel can support 1.8 Kbps, 3.6 Kbps, 7.2 Kbps, and 14.4 Kbps. In IS-2000, the F-FCH can use one of nine different radio configurations to support rates from 0.75 Kbps to 14.4 Kbps. Regardless of the radio configuration, an IS-2000 F-FCH can support only 20-ms and 5-ms frames. Figure 2.9 shows some examples of F-FCH frames (at peak data rates) for the different radio configurations. Note that Radio Configurations 1 and 2 are equivalent to the IS-95 Rate Sets 1 and 2. Readers are referred to Section 3.1.3.11.2 of [2] for an exhaustive list of radio configurations, corresponding data rates and frame formats supported by the F-FCH. 2.4.2

Forward Supplemental Channel (F-SCH)

2.4.2.1

Functions

The purpose of IS-2000 F-SCH is to serve as a dedicated transport pipe for high-rate packet data. Because of the requirement of transmitting packet data at high rates, the F-SCH has two unique characteristics: •

•

Because it is acting as a high data rate transport pipe, the F-SCH only carries user traffic data and does not carry any signaling traffic. Because it is used to transmit packet data which is bursty in nature, the existence of the F-SCH is itself bursty. This means that the F-SCH is set up and torn down rather quickly.

These two characteristics in turn lead to two obvious implications for the F-SCH. First, because the F-SCH cannot carry any signaling traffic, the F-SCH has to coexist with another physical channel that is able to carry signaling traffic for the F-SCH. In other words, while the F-SCH is active, one of these other physical channels has to be active as well. Some possible operating configurations are: •

•

F-SCH operating with an F-DCCH: Here the most likely scenario is that the mobile is conducting a data-only packet data session, and the forward dedicated control channel is used to carry signaling traffic between the base station and the mobile, while the forward supplemental channel is carrying high-rate traffic data between them. F-SCH operating with an F-FCH: The most likely scenario here is that in addition to conducting a packet data session, the mobile is also having a voice call at the same time. In this scenario the forward supplemental channel is carrying high-rate traffic data, and the forward fundamental channel is carrying both user voice traffic and signaling traffic.

28

Physical Layer: Forward Link 20-ms frame (9.6 Kbps) (Radio Configurations 1, 3, 4, 6, 7)

172 information bits 12 CRC bits

8 encoder tail bits

20-ms frame (14.4 Kbps) (Radio Configurations 2, 5, 8, 9)

268 information bits 12 CRC bits

8 encoder tail bits

5-ms frame (9.6 Kbps) (Radio Configurations 3, 4, 6, 7 and 5, 8, 9)

24 information bits

16 CRC bits

8 encoder tail bits

Figure 2.9 Examples of 20-ms and 5-ms F-FCH frames. Note that in the 20-ms frame (14.4 Kbps), the 268 information bits include one reserved bit.

The readers should now recognize the merits of using shorter signaling messages that can be carried by small 5-ms frames. In order to support the transmission of packet data which is bursty in nature, the F-SCH has to be set up and torn down quickly. These fast assignment and de-assignment of F-SCH resources are done by quick exchanges of these short signaling messages that last no longer than 5 ms. The necessity to quickly set up and tear down the F-SCH is why both the forward fundamental channel and the forward dedicated control channel support 5-ms frames (for quick signaling). 2.4.2.2

Forward Supplemental Channel Frames

In IS-2000, the F-SCH can use one of seven different radio configurations (Radio Configurations 3 through 9) to support rates from 0.6875 Kbps to 1.0368 Mbps.

2.4

User Channels

29

It can also support three different frame duration: 20 ms, 40 ms, and 80 ms. Figure 2.10 shows examples of different 20-ms frames for Radio Configuration 4. Figure 2.11 shows examples of different 40-ms frames for Radio Configuration 4, and Figure 2.12 shows examples of different 80-ms frames for Radio Configuration 4. In Figures 2.10, 2.11, and 2.12, readers may have noticed that for a given radio configuration (e.g., Radio Configuration 4), the longer the frame duration, the lower the peak data rate. For example, the peak data rate supported by a 20-ms frame is 307.2 Kbps; this peak date rate drops to 153.6 Kbps for a 40-ms frame, and down further to 76.8 Kbps for an 80-ms frame. It seems counterintuitive that longer frame duration support lower peak data rates. It turns out that in IS-2000, longer frame duration (e.g., 40 ms and 80 ms) are not used for higher data-carrying capacity but for data protection. As the frame duration increases, the corresponding interleaver size can increase as well. The 20-ms frame (307.2 Kbps)

6,120 information bits

16 CRC bits

8 encoder tail bits

16 CRC bits

8 encoder tail bits

20-ms frame (153.6 Kbps)

3,048 information bits 20-ms frame (76.8 Kbps)

1,512 information bits

16 CRC bits

8 encoder tail bits

20-ms frame (38.4 Kbps)

744 information bits

16 CRC bits

8 encoder tail bits

20-ms frame (19.2 Kbps)

360 information bits

Figure 2.10

16 CRC bits

Examples of 20-ms F-SCH frames: Radio Configuration 4.

8 encoder tail bits

30

Physical Layer: Forward Link 40-ms frame (153.6 Kbps)

6,120 information bits

16 CRC bits

8 encoder tail bits

16 CRC bits

8 encoder tail bits

40-ms frame (76.8 Kbps)

3,048 information bits 40-ms frame (38.4 Kbps)

1,512 information bits

16 CRC bits

8 encoder tail bits

40-ms frame (19.2 Kbps)

744 information bits

Figure 2.11

16 CRC bits

8 encoder tail bits

Examples of 40-ms F-SCH frames: Radio Configuration 4.

80-ms frame (76.8 Kbps)

6,120 information bits

16 CRC bits

8 encoder tail bits

16 CRC bits

8 encoder tail bits

16 CRC bits

8 encoder tail bits

80-ms frame (38.4 Kbps)

3,048 information bits 80-ms frame (19.2 Kbps)

1,512 information bits

Figure 2.12

Examples of 80-ms F-SCH frames: Radio Configuration 4.

2.5

Channel Structure

31

consequence of having a longer interleaver size is that the system can combat fades of longer duration. This results in a better ability to correct burst errors coming from longer-term fades [3]. Given their better error-protection properties, why are 40-ms and 80-ms frames not used in IS-95? The reason is that IS-95 is intended primarily for voice applications, and 20-ms frames are used is because speech statistics are nearly stationary (do not vary much) over a 20-ms interval. In addition, decoding 20-ms frames does not result in much noticeable voice delay. On the other hand, using longer duration frames would result in delays that are noticeable during a conversation. This is why 40 ms and 80 ms are used by supplemental channels for data transmissions only. Readers are referred to Section 3.1.3.12.2 of [2] for an exhaustive list of radio configurations, corresponding data rates and frame formats supported by the F-SCH.

2.5

Channel Structure After a physical channel generates a frame, then the physical layer performs the usual functions such as: •

Adding the CRC bits for detecting frame errors;

•

Coding the bits for correcting bit errors;

•

Interleaving for combating fades.

These functions are similar to those of IS-95. After block interleaving, the symbols undergo long code scrambling, then a gain for that physical channel is applied (the gain is determined by forward power control). In addition, if the physical channel is a forward dedicated control channel or a forward fundamental channel then power control bits can be punctured into the symbol stream. Figure 2.13 shows a general block diagram up to just before modulation for broadcast control channel,

Modulation symbols I Block interleaver

Long code scrambling

Channel gain

Demux Q

Long code mask

Figure 2.13 Conceptual block diagram of broadcast control channel4, common assignment channel, and forward common control channel.

4.

Broadcast control channel has a sequence repetition function between block interleaver and long code scrambler.

32

Physical Layer: Forward Link

common assignment channel, and forward common control channel; Figure 2.14 shows a general block diagram up to right before modulation for forward dedicated control channel, forward fundamental channel, and forward supplemental channel. As shown in Figures 2.13 and 2.14, the demultiplexer converts one input symbol stream into two output symbol streams. It assigns input symbols alternately to the two outputs. As a result, symbols are transmitted using two different paths: I and Q. As we will see later, this quadrature arrangement is one of the reasons why IS-2000 has a capacity higher than that of IS-95. Figures 2.13 and 2.14 show those functions that are unique to IS-2000, and only those channels of Spreading Rate 1 are shown. Note that a forward supplemental channel cannot have power control bits punctured into it.

2.6

Modulation After demultiplexing, the I and the Q symbols are channelized and spread by a Walsh code (or quasi-orthogonal function) assigned to that physical channel. The Walsh code (or quasi-orthogonal function) runs at the chip rate of 1.2288 Mcps for Spreading Rate 1. After channelization, the chip stream undergoes another layer of spreading by the short PN codes. Similar to IS-95, there are two short PN codes (pI and pQ), and they are the same as those used to spread the forward pilot channel (for identifying a specific base station sector). pI and pQ are different short PN codes in that they are produced by two different generator polynomials. However, both codes start at the same time and are offset by the same amount of chips. This is important because each base station sector is uniquely identified by its short PN code offset. Figure 2.15 shows how the I and the Q symbols are channelized and spread in a complex manner. Although Figure 2.15 may look complex at first, it is really just a way to diagrammatically implement the multiplication of two complex numbers. Because

Modulation symbols I Block interleaving

Long code scrambling

Long code mask

Channel gain

Power control puncturing*

Demux Q

Power control bits *Only active for F-DCCH and F-FCH

Figure 2.14 Conceptual block diagram of forward dedicated control channel, forward fundamental channel, and forward supplemental channel5.

5.

Forward supplemental channel does not have punctured power control bits.

2.6

Modulation

33 wiIpI + I

wiIpI−wiQpQ

wiI

BF

pI − wiQpQ

cos(2πfct) Y(t)

wi or QOF

pQ wiIpQ +

Q

wiIpQ+ wiQpI

wiQ

BF

pQ + wiQpI pI

Figure 2.15 shown.

sin(2πfct)

BF: Baseband filter

Complex modulation: Forward link. For the sake of simplicity, Walsh rotation is not

there are now two baseband symbol streams I and Q, these two symbol streams can be succinctly represented by one complex number I + jQ

After multiplying with the Walsh code (or quasi-orthogonal function), the complex number becomes w i (I + jQ) = w i I + jw i Q

(2.1)

To spread the above using the two short PN codes, we multiply the above complex number (wi I + jwi Q) by another complex number made up of the two short PN codes (pI + jpQ), (w i I + jw i Q)(pI + jpQ ) = w i IpI + jw i IpQ + jw i QpI − w i QpQ = w i (IpI − QpQ ) + jw i (IpQ + jQpI )

(2.2)

Note that the real part of (2.2) corresponds to the input into the in-phase branch of the QPSK modulator, and the imaginary part of (2.2) corresponds to the input into the quadrature branch of the QPSK modulator (see Figure 2.15). As a last note, it is easy to see that if Q is zero (as in the case of IS-95), the modulation in Figure 2.15 collapses back to that of IS-95. See Figure 2.16. In this case, I is the only symbol stream, which is duplicated on both the in-phase and the quadrature branches of the QPSK modulator. In fact, Radio Configurations 1 and 2 are implemented in IS-2000 by zeroing out the Q symbol stream.

34

Physical Layer: Forward Link wiIpI + I

wiI

BF

pI

cos(2πfct) Y(t) wi or QOF

pQ

+

wiIpQ

BF

sin(2πfct)

Figure 2.16 Collapsed complex modulation as in the cases of Radio Configuration 1 and Radio Configuration 2.

2.7

Capacity Gain: Forward Link One reason IS-2000 has a higher physical layer capacity than IS-95 is because IS-2000 uses QPSK whereas IS-95 uses BPSK6. Both QPSK and BPSK have the same bandwidth because both have the same transmission symbol rate. However, QPSK encodes two chips-per-symbol whereas BPSK encodes only one chip-per-symbol. By transmitting two independent symbol streams in two dimensions, QPSK can double the data rate using the same bandwidth. To take advantage of this higher physical layer capacity on the forward link, IS-2000 also increased the maximum length of its Walsh codes from 64 to 128 (Spreading Rate 1). The increased length means that there are now more Walsh codes available, and more available Walsh codes means that the forward link can now support more physical channels at the same time. With the increased number of available codes, an IS-2000 base station likely would become interference limited (exhaust its power resources at the base station) first before it becomes code limited (exhausts its available codes). However, an IS-2000 base station likely would become code limited first if a user demands high data rate and thus requires a short Walsh code (i.e., low spreading factor). See Chapter 9 for more details on the constraints of Walsh code assignment.

6.

Although IS-95 also has a QPSK modulator, its in-phase and quadrature branches carry identical symbols. Thus strictly speaking, IS-95 uses two identical BPSK modulators.

2.7

Capacity Gain: Forward Link

35

References [1] [2] [3]

TIA/EIA-95-B, Mobile Station-Base Station Compatibility Standard for Wideband Spread Spectrum Cellular Systems, Telecommunications Industry Association, March 1999. TIA/EIA/IS-2000.2-A, Physical Layer Standard for cdma2000 Spread Spectrum Systems, Telecommunications Industry Association, March 2000. Yang, S. C., CDMA RF System Engineering, Norwood, MA: Artech, 1998.

Selected Bibliography TIA-97, Recommended Minimum Performance Standards for cdma2000 Spread Spectrum Base Stations, Telecommunications Industry Association, February 2003. TIA-98, Recommended Minimum Performance Standards for cdma2000 Spread Spectrum Mobile Station, Telecommunications Industry Association, February 2003. Garg, V. K., IS-95 CDMA and cdma2000: Cellular/PCS Systems Implementation, Upper Saddle River, NJ: Prentice Hall PTR, 2001.

.

CHAPTER 3

Physical Layer: Reverse Link 3.1

Introduction As described in Chapter 2, the physical layer is responsible for transmitting and receiving bits (organized in frames) over the physical medium. As such, the layer performs functions such as coding, modulation, and channelization. This chapter deals with the physical channels that exist on the reverse link of the IS-2000 system, and their descriptions are organized into two broad categories: signaling channels and user channels. Like their forward link counterparts, the signaling channels can also be classified into two types, dedicated and common channels, depending on whether or not the channel is assigned to only one user or can be used by multiple users. The R-DCCH is an example of a dedicated signaling channel. On the other hand, the R-CCCH, the R-ACH, and the R-EACH are examples of common signaling channels. The signaling channels are described in Section 3.3. On the reverse link, user channels are used to carry user information from the mobile back to the base station. There are three physical channels used primarily for this purpose: (1) R-FCH which is equivalent to the reverse traffic channel in IS-95, (2) R-SCCH which is equivalent to the reverse supplemental code channel in IS-95 (more specifically, IS-95-B [1]), and (3) R-SCH which is a new channel in IS-2000. Figure 3.1 shows the categorization of these reverse link channels.

Common channels

Access channel (R-ACH*) Enhanced access channel (R-EACH) Reverse common control channel (R-CCCH)

Dedicated channels

Reverse pilot channel (R-PICH) Reverse dedicated control channel (R-DCCH)

Signaling channels

User channels

Reverse fundamental channel (R-FCH*) Reverse supplemental channel (R-SCH) Reverse supplemental code channel (R-SCCH*)

Figure 3.1 Categories of reverse link physical channels. Legacy IS-95 physical channels are denoted with asterisks.

37

38

Physical Layer: Reverse Link

Table 3.1 shows a list of physical channels used by the physical layer. This table is identical to Table 2.1 and is reproduced here for completeness. Note that asterisked channel designations show those channels that also exist in IS-95. Also, boldfaced channel names show those channels that are collectively known as the IS-2000 traffic channels since these channels can all carry user traffic data in IS-2000 systems.

Table 3.1

Reverse Link Physical Channels and Their Forward Link Counterparts

Channel

Channel Name

Description

F-SCH

Forward supplemental channel

Channel Name

Description

For transmitting user data while a call is active; uses convolu- R-SCH tional or turbo coding

Reverse supplemental channel

For transmitting user data while a call is active; uses convolutional or turbo coding

F-SCCH*

Forward supplemental code channel

For transmitting user data while a call is R-SCCH* active; uses convolutional coding

Reverse supplemental code channel

For transmitting user data while a call is active; uses convolutional coding

F-FCH*

Forward fundamental channel

For transmitting user and signaling data while a call is active; R-FCH* uses convolutional coding

Reverse fundamental channel

For transmitting user and signaling data while a call is active; uses convolutional coding

F-DCCH

Forward dedicated control channel

For transmitting signaling and user data R-DCCH while a call is active

For transmitting Reverse dedicated signaling and user data while a call is control channel active

F-PCH*

Paging channel

For transmitting MS-specific and system overhead data

Quick paging channel

For telling MS (operating in slotted mode while in the idle state) whether or not it should receive F-CCCH or F-PCH starting in the next F-CCCH or F-PCH slot

F-QPCH

Channel

R-ACH*

R-EACH

Access channel

Enhanced access channel

For initial communications with BS, i.e., initiating access and responding to pages For initial communications with BS, i.e., initiating access or responding to MS-specific messages

3.2

Radio Configurations

39

Table 3.1 (continued) Channel

Channel Name

Description

F-CCCH

For transmitting signaling data when Forward common F-FCH, F-SCCH, R-CCCH control channel F-SCH, or F-DCCH is not active

F-BCCH

For transmitting signaling data when Broadcast control F-FCH, F-SCCH, channel F-SCH, or F-DCCH is not active

F-CPCCH

Common power control channel

For transmitting common power control subchannels (one bit per subchannel) to powercontrol multiple R-CCCHs and R-EACHs

F-CACH

Common assignment channel

For transmitting signaling data to allocate R-CCCH resources

F-SYNCH*

Sync channel

For providing MS time and frame synchronization

F-PICH*

Forward pilot channel

For assisting MS to acquire initial time synchronization

F-TDPICH

For implementing Transmit diversity transmit diversity on pilot channel the forward link

F-APICH

Auxiliary pilot channel

Channel

R-PICH

Channel Name

Description

Reverse common control channel

For transmitting signaling and user data when R-FCH, R-SCCH, R-SCH, or R-DCCH is not active

Reverse pilot channel

For assisting BS to detect MS transmission

For supporting the use of spot beam

Auxiliary transmit For implementing transmit diversity in F-ATDPICH diversity pilot the spot beam channel

3.2

Radio Configurations Similar to the forward link, the reverse link traffic channels (i.e., reverse fundamental channel, reverse supplemental code channel, reverse supplemental channel, and reverse dedicated control channel) can also assume different configurations to implement different data rates. But the difference here is that the number of radio configurations on the reverse link is less than those on the forward link. While the forward link can have up to nine different radio configurations, the reverse link can only have up to six. Table 3.2 shows these different radio configurations [2]. For these radio configurations, the data rates shown in the table are maximum

40

Physical Layer: Reverse Link Table 3.2

Radio Configurations on the Reverse Link

Radio Configuration

Coding Rate R

Modulation

Spreading Rate

Maximum Data Rate

1

1/3

64-ary

1

9.6 Kbps

2

1/2

64-ary

1

14.4 Kbps

Orthogonal Orthogonal 3

1/4

BPSK

1

153.6 Kbps

1/2

BPSK

1

307.2 Kbps

4

1/4

BPSK

1

230.4 Kbps

5

1/4

BPSK

3

153.6 Kbps

1/3

BPSK

3

614.4 Kbps

1/4

BPSK

3

460.8 Kbps

1/2

BPSK

3

1.0368 Mbps

6

data rates. For a given radio configuration, data rates lower than the maximum are possible. Radio Configuration 1 and Radio Configuration 2 are backward compatible with IS-95 in that they are equivalent to Rate Set 1 and Rate Set 2 of IS-95. In fact, Radio Configurations 1 and 2 differ from the rest of the radio configurations in that Radio Configurations 1 and 2 use the 64-ary orthogonal modulation to be compatible with IS-95, while Radio Configurations 3 through 6 use the coherent binary phase shift keying (BPSK). The use of coherent modulation is now possible in IS-2000 because of the addition of the R-PICH. For each radio configuration, the table shows the maximum achievable data rate (instead of all possible data rates). For example, for Radio Configuration 4 the system is capable of transmitting at 1.8 Kbps, 3.6 Kbps, 7.2 Kbps, 14.4 Kbps, 28.8 Kbps, 57.6 Kbps, 115.2 Kbps, and 230.4 Kbps; only the maximum data rate of 230.4 Kbps is shown.

3.3

Signaling Channels This section describes the physical signaling channels (both dedicated and common) that exist in an IS-2000 system. 3.3.1

Reverse Dedicated Control Channel (R-DCCH)

The R-DCCH is a dedicated signaling channel in that once assigned, it is only allocated to one designated user. In addition, just as the reverse fundamental channels can carry signaling data (through dim-and-burst and blank-and-burst), the R-DCCH can carry user data (i.e., low-rate data such as SMS). The R-DCCH supports both 20-ms and 5-ms frame formats. Examples of 20-ms and 5-ms frame formats are shown in Figure 3.2. Readers may notice that these frame formats are similar to those for the F-DCCH shown in Figure 2.2. In fact, the

3.3

Signaling Channels

41 20-ms frame (9.6 Kbps)

12 CRC bits

172 information bits

8 encoder tail bits

5-ms frame (9.6 Kbps)

24 information bits

Figure 3.2

16 CRC bits

8 encoder tail bits

Examples of 20-ms and 5-ms R-DCCH frames.

frame formats are symmetrical between the forward link and the reverse link (i.e., the frame formats of the R-DCCH are identical to those of the F-DCCH1) [2]. 3.3.2

Reverse Common Control Channel (R-CCCH)

In IS-95, if a mobile wants to send information back to the base station when a call is not active (i.e., when there is no traffic channel assigned), the mobile has to use the only other physical channel that is available–the access channel. But the problem is that the access channel is a random-access channel. This means that mobiles transmit on the access channel more or less randomly; if there is a collision, then the messages are not received by the base station, and the colliding mobiles would have to transmit again. Further complicating the situation is that some full-fledge messages, such as the authentication challenge response message and the origination message, are sent on the access channel. The transmission of these longer signaling messages is best done over a channel whose resources are scheduled rather than over one whose accesses are contention-based. To minimize the problem of random collisions of long signaling messages on the access channel, IS-2000 added two new signaling physical channels–the R-CCCH

1.

Since the forward link and the reverse link have asymmetrical radio configurations, some mapping of radio configurations are needed. The frame structures are identical if we map forward link Radio Configurations 3, 4, 6, and 7 to reverse link Radio Configurations 3 and 5, and forward link Radio Configurations 5, 8, and 9 to reverse link Radio Configurations 4 and 6.

42

Physical Layer: Reverse Link

and the R-EACH. The R-EACH is described in the next section. The R-CCCH is “common” in that it is shared among many mobiles, but the key difference here is that the use of this channel is scheduled among the different mobiles2. Note that the R-CCCH is only used when there is no R-DCCH or no R-FCH active on the reverse link. As soon as a reverse dedicated control channel and/or reverse fundamental channel become active, signaling messages are then sent on either one or both of these channels. In a given CDMA carrier, there is a one-to-one correspondence between an F-CCCH and an R-CCCH. Therefore, it is not surprising that the frame formats supported by the R-CCCH are exactly the same as its forward link counterpart. Similar to the forward common control channel, the R-CCCH supports three different frame duration (20 ms, 10 ms, and 5 ms) as well as three different data rates (9.6 Kbps, 19.2 Kbps, and 38.4 Kbps). See Figure 3.3. Despite the identical frame formats used by both the F-CCCH and the R-CCCH, the transmission of the R-CCCH does differ from its forward link counterpart. First, before transmitting the actual R-CCCH frame(s) the mobile transmits a preamble. This is done to help the base station in acquiring the R-CCCH. Second, because the R-CCCH is coherently modulated, a reverse pilot is transmitted at the same time to aid in its coherent demodulation at the base station. Figure 3.4 illustrates the transmission of the R-CCCH. As shown in Figure 3.4, the preamble preceding the R-CCCH frame(s) is nothing more than a series of gated reverse pilot channel transmissions. In other words, gated reverse pilot channel transmissions are used as the preamble for R-CCCH. The duration of the gated transmissions t1, the duration of the off time t3, the duration of the last gated transmission t2, the total number of gated transmissions (N + 1), and the total duration of the preamble Tpreamble are completely specified by the appropriate R-CCCH parameters. Note that to further facilitate the acquisition of the R-CCCH, the mobile transmits the reverse pilot during the preamble at a higher power level [2]. 3.3.3

Enhanced Access Channel (R-EACH)

Although scheduling mobile transmissions on the R-CCCH (when the reverse fundamental channel and/or reverse dedicated control channel are not active) solves the problem of collisions when transmitting large signaling messages, there is still the issue of a mobile’s very first transmission of its access request. This issue is important because the mobile does not (yet) have any assigned channels or resources when it first transmits its access request. To transmit its access request, the mobile uses the R-EACH. The R-EACH is similar to the IS-95 access channel (R-ACH) in that the mobiles’ R-EACH transmissions are randomized. Although collisions are still possible, a mobile’s R-EACH transmission is typically shorter in duration than a mobile’s R-ACH transmission3. Hence the probability of collisions is smaller on the R-EACH. 2. 3.

In reservation access mode. In reservation access mode, only a short 5 ms “header” frame and its preamble are transmitted on the R-EACH.

3.3

Signaling Channels

43 20-ms frame (9.6 Kbps)

12 CRC bits

172 information bits

8 encoder tail bits

20-ms frame (38.4 Kbps)

744 information bits 16 CRC bits

8 encoder tail bits

10-ms frame (38.4 Kbps)

360 Information bits

16 CRC bits

8 encoder tail bits

5-ms frame (38.4 Kbps)

172 information bits

Figure 3.3

12 CRC bits

8 encoder tail bits

Examples of 20-ms, 10-ms, 5-ms R-CCCH frames.

In terms of frame formats, the frame formats supported by the R-EACH are identical to those supported by the R-CCCH with one important exception. In addition to supporting 20-ms, 10-ms, and 5-ms data frames, the R-EACH also supports a special 5-ms header frame which is used to transmit these short initial access requests4. Figure 3.5 shows the format of this 5-ms header frame and some examples of the 20-ms, 10-ms, and 5-ms data frames.

4.

In reservation access mode.

44

Physical Layer: Reverse Link Tpreamble

R-PICH

t1

t1

t1

t2

1

2

N

N+1

t3

t3

R-CCCH

Preamble

Figure 3.4

R-CCCH frame(s)

R-CCCH transmission.

Similar to the R-CCCH transmission, the R-EACH also transmits a preamble before transmitting the actual R-EACH frame(s). This is done to help the base station in acquiring the R-EACH. In addition, because the R-EACH is coherently modulated, a reverse pilot is transmitted at the same time to aid in its coherent demodulation at the base station. Figures 3.6, 3.7, and 3.8 show the different types of R-EACH transmissions. Similar to the R-CCCH transmission, the R-EACH preamble consists of a series of gated reverse pilot channel transmissions. The mobile transmits the reverse pilot during preamble at a higher power to facilitate acquisition of the R-EACH. For the preamble, the duration of the gated transmissions t1, the duration of the off time t3, the duration of the last gated transmission t2, the total number of gated transmissions (N + 1), and the total duration of the preamble Tpreamble are completely specified by the appropriate R-EACH parameters. Note that in the IS-2000 standard, each R-EACH transmission shown in Figures 3.6 through 3.8 is referred to as an R-EACH probe [2]. As illustrated in Figures 3.6 through 3.8, the actual R-EACH transmission depends on the operative access mode at the time. In reservation access mode (where the mobile first transmits a short message on the R-EACH and then gets assigned an R-CCCH), the R-EACH transmission consists of only the preamble and the 5-ms header frame (see Figure 3.6). In basic access mode (where the mobile immediately transmits access messages), the R-EACH transmission consists of the preamble and the longer data frames (see Figure 3.7). In power controlled access mode (where the base station power controls the R-EACH), the R-EACH transmission consists of the preamble, the header frame, and the data frames (see Figure 3.8). These different access modes are described in more detail in Chapter 4 on medium access control.

3.3

Signaling Channels

45 5-ms header frame (9.6 Kbps)

32 information bits

8 CRC bits

8 encoder tail bits

20-ms data frame (9.6 Kbps)

172 information bits

12 CRC bits

8 encoder tail bits

16 CRC bits

8 encoder tail bits

20-ms data frame (38.4 Kbps)

744 information bits

10-ms data frame (38.4 Kbps)

360 Information 16 CRC 8 encoder bits bits tail bits 5-ms data frame (38.4 Kbps)

172 information bits

Figure 3.5

3.3.4

12 CRC bits

8 encoder tail bits

R-EACH header frame and examples of 20-ms, 10-ms, 5-ms R-EACH data frames.

Reverse Pilot Channel (R-PICH)

The R-PICH is a new physical channel used in IS-2000 to help the base station in detecting the reverse link. Recall that in IS-95, the access channel (R-ACH) and the reverse traffic channel (R-FCH) both use 64-ary modulation that does not require coherent demodulation. To improve the signal-to-noise ratio performance, IS-2000 uses BPSK for the new IS-2000 channels (i.e., reverse dedicated control channel, reverse supplemental channel, etc.). The R-PICH is used to help coherently demodulate these new channels. Similar to the forward pilot channel (F-PICH), the R-PICH is an unmodulated signal. But the R-PICH differs from the forward pilot channel in some important aspects:

46

Physical Layer: Reverse Link Tpreamble

R-PICH

t1

t1

t1

t2

1

2

N

N+1

t3

t3 5 ms

R-EACH

Preamble

Figure 3.6

R-EACH header frame

R-EACH transmission: Reservation access mode.

Tpreamble

R-PICH

t1

t1

t1

t2

1

2

N

N+1

t3

t3

R-EACH

Preamble

Figure 3.7

R-EACH transmission: Basic access mode.

R-EACH data frame(s)

3.3

Signaling Channels

47 Tpreamble

R-PICH

t1

t1

t1

t2

1

2

N

N+1

t3

t3

R-EACH

Preamble

Figure 3.8

•

•

R-EACH header frame

R-EACH data frame(s)

R-EACH transmission: Power controlled access mode.

Unlike the forward pilot channel, which does not carry any information, the R-PICH does carry information. This information is power control feedback that the mobile sends back to the base station. The base station uses this feedback to power control the forward link. To reduce interference, the R-PICH can transmit in gated mode in which some parts of the pilot stream are turned off while others turned on.

These differences are described in the following sections. 3.3.4.1

Power Control Subchannel

The R-PICH is used to carry power control information. The power control signaling information carried by the R-PICH helps power control the forward link. But here lies a major difference between IS-95 and IS-2000. In IS-95 Rate Set 1, forward link power control is done by using information contained in the power measurement report messages transmitted by the mobile. Since these messages are carried in-traffic on the reverse traffic channel, the frequency of these message transmissions and hence of forward link power control updates is low. In IS-95 Rate Set 2, forward link power control is done by using the erasure indicator bits contained in each reverse traffic channel frame. While also a form of in-traffic signaling, the erasure indicator bits are transmitted at a higher rate (i.e., at the rate of transmission of each reverse traffic channel frame); this is once every 20 ms, or 50 times a second.

48

Physical Layer: Reverse Link

In IS-2000 (specifically Radio Configurations 3 through 6), the R-FCH and the R-SCH have no facility provisioned to carry any power control feedback back to the base station. Therefore, power control signaling, in the form of power control bits, is carried entirely by the R-PICH. One area of improvement in IS-2000 is that the frequency of power control update has increased from that in IS-95. In Radio Configurations 3 through 6, the power control bits carried by the R-PICH are transmitted at a maximum rate of 800-times-per-second, enabling the rate of forward link power control to increase from a maximum of 50-times-per-second (in IS-95) to a maximum of 800-times-per-second. Figure 3.9 shows the format of the R-PICH transmission. The power control subchannel can be thought of as a structure provided by the R-PICH in which power control bits can be transported. This structure is shown in Figure 3.9. An R-PICH can be divided into 16 segments called power control groups. Each power control group lasts 1.25 ms. It can then be inferred that for Spreading Rate 1, each power control group contains 1,536 chips (=1.2288 Mcps × 1.25 ms). IS-2000 specifies that the first 1,152 chips of a power control group constitute the pilot signal, and the last 384 chips of the same power control group constitute a power control bit. As Figure 3.9 shows, if the power control bit is 0 then the last 384 chips of the power control group are all zeros. If the power control bit is 1 then the same chips are all ones. Note that in Figure 3.9 since the R-PICH is being transmitted continuously, the rate of feedback of power control bits in this case is 1/1.25 ms, or 800times-per-second. 3.3.4.2

Gating

To reduce interference to other reverse physical channels, the R-PICH can be gated. If the parameter PILOT_GATING_USE_RATE = 1 then the R-PICH operates in gated mode. In this case the gating rate is specified by the parameter PILOT_GATING_RATE. When gating rate = 1, all power control groups are on. When gating rate = 1/2, half of all power control groups are on. When gating rate = 1/4, only one quarter of all power control groups are on. Figure 3.10 shows three possible gating rates. 16 power control groups

1.25 ms

0 all 0s

1 all 0s or all 0s all 1s

2 all 0s or all 1s

1,152 384 chips chips Figure 3.9

R-PICH format.

all 0s

15 all 0s or all 1s

all 0s

all 0s or all 1s

3.4

User Channels

49 16 power control groups

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Gating rate = 1

1

3

5

7

9

11

13

15

Gating rate = 1/2

3

7

11

15

Gating rate = 1/4 Figure 3.10

R-PICH gating.

On the other hand, if PILOT_GATING_USE_RATE = 0 then the R-PICH operates in nongated mode and there is no gating on the R-PICH. In this case the system ignores the PILOT_GATING_RATE parameter. It is important to note that the gating rate does not necessarily determine the rate of power control feedback. For example, even when the gating rate = 1 the system has the option of combining all 16 power control bits in a 20-ms period to yield one single power control feedback. In this case, the rate of feedback of power control is 1/20 ms, or 50-times-per-second. The rate of power control feedback is specified by the parameter FPC_MODE (forward power control operating mode) [2]. See Chapter 7 for more details on power control.

3.4

User Channels There are three user channels on the IS-2000 reverse link: R-FCH, R-SCCH, and R-SCH. The R-SCCH is provisioned to provide backward compatibility with IS-95 (more specifically, IS-95-B [1]). The purposes and structures of the reverse

50

Physical Layer: Reverse Link

fundamental channel and reverse supplemental channel are similar to their counterparts on the forward. Therefore only important features are described in the following sections. 3.4.1

Reverse Fundamental Channel (R-FCH)

The IS-2000 R-FCH is similar to the IS-95 reverse traffic channel in that its primary function is to carry user data. The following are the main functions of the IS-2000 R-FCH: • • •

Transmission of voice traffic; Transmission of low-rate data traffic; Transmission of signaling via dim-and-burst and blank-and-burst schemes.

The R-FCH does not carry power control bits (in Radio Configurations 3 through 6). The R-FCH can carry signaling traffic and supports the shorter 5-ms frames in addition to the conventional 20-ms frames. Readers are referred to Section 2.1.3.7.2 of [2] for an exhaustive list of radio configurations, corresponding data rates, and frame formats supported by the R-FCH. 3.4.2

Reverse Supplemental Channel (R-SCH)

The purpose of IS-2000 R-SCH is to serve as a dedicated transport pipe for high-rate packet data. In performing this function, the R-SCH has two unique characteristics: •

•

Because it is acting as a high data rate transport pipe, the R-SCH only carries user traffic data and does not carry any signaling traffic. Because it is used to transmit packet data which is bursty in nature, the existence of the R-SCH is itself bursty. This means that the R-SCH is set up and torn down rather quickly.

Because the R-SCH does not carry any signaling, it has to coexist with another physical channel that is able to carry signaling traffic for the R-SCH (i.e., reverse fundamental channel or reverse dedicated control channel). In other words, while the R-SCH is active, one of these other physical channels has to be active as well. Since both the reverse fundamental channel and reverse dedicated control channel support 5-ms frames, signaling traffic can be transmitted on these channels quickly to set up and tear down the R-SCH. The R-SCH can use one of four different radio configurations (Radio Configurations 3 through 6) to support rates from 0.6875 Kbps to 1.0368 Mbps. It can also support three different frame duration: 20 ms, 40 ms, and 80 ms. Readers are referred to Section 2.1.3.8.2 of [2] for an exhaustive list of radio configurations, corresponding data rates, and frame formats supported by the R-SCH.

3.5

Channel Structure After a physical channel generates a frame, then the physical layer performs the usual functions such as:

3.6

Modulation

• • •

51

Adding the CRC bits for detecting frame errors, Coding the bits for correcting bit errors, Interleaving for combating fades,

These functions are similar to those of IS-95. After block interleaving, the symbols of a physical channel undergo a channel gain. The gain is determined by reverse power control. Figure 3.11 depicts a general block diagram up to right before modulation for enhanced access channel, reverse common control channel, reverse dedicated control channel, reverse fundamental channel, and reverse supplemental channel. The figure shows the function that is unique to IS-2000 and corresponds to Spreading Rate 1.

3.6

Modulation After being applied a channel gain, the symbol stream of a physical channel (except the reverse pilot channel) is multiplied and spread by its assigned Walsh code for channelization. The Walsh code runs at the chip rate of 1.2288 Mcps for Spreading Rate 1. After channelization, the chip streams of the reverse pilot channel, reverse dedicated control channel, and the second reverse supplemental channel are added together, and the chip streams of the reverse fundamental channel and enhanced access channel, reverse common control channel, or the first reverse supplemental channel are added together. See Figure 3.12. These two summations undergo another layer of spreading by a pair of spreading codes: sI and sQ. These two spreading codes are derived from the long PN code, which in turn is derived from the mobile’s unique identity5. To reiterate, the Walsh codes provide channelization of the different physical channels transmitted by the mobile, while the long PN code provides identification of the mobile to the base station. Figure 3.12 depicts how the two summations of the physical channels are spread and fed into two separate BPSK modulators. Modulation symbols

Block interleaver

Channel gain

Z

Figure 3.11 Conceptual block diagram of enhanced access channel, reverse common control channel, reverse dedicated control channel, reverse fundamental channel, and reverse supplemental channel.

5.

These spreading codes are derived from the long PN code, in contrast to the spreading codes used on forward link that are short PN codes.

52

Physical Layer: Reverse Link (R-PICH)

+ (R-DCCH)

Z

−

wi for R-DCCH

(R-SCH 2)

BF

sI

cos(2πfct) sQ

Z

Y(t) wi for R-SCH 2

(R-FCH)

Z +

wi for R-FCH

BF

sQ (R-EACH), (R-CCCH), or (R-SCH 1)

+ sin(2πfct)

Z wi for R-EACH, R-CCCH, or R-SCH 1

sI BF: Baseband filter

Figure 3.12 Modulation: Reverse link. (After: [2]. See Chapter 9 for the assigned Walsh codes of physical channels.)

Figure 3.12 shows that the chip stream of any particular physical channel is always fed into both BPSK modulators. For example, the reverse dedicated control channel is transmitted by both BPSK signals. The two BPSK signals are two independent signals separated by phase. This method of using two independent BPSK signals to transmit a single physical channel is similar to that used by IS-95 forward link.

3.7

Capacity Gain: Reverse Link One reason IS-2000 has a higher physical layer capacity than IS-95 is because IS-2000 uses coherent BPSK whereas IS-95 uses 64-ary orthogonal modulation. See Chapter 5 of [3] for a review of 64-ary orthogonal modulation. Because IS-2000 now has a pilot on the reverse link, it is able to coherently demodulate BPSK signals. For a given probability of bit error, coherent BPSK requires less Eb/N0 than 64-ary orthogonal modulation, and a lower required Eb/N0 means higher RF capacity in a direct sequence spread spectrum system. In addition, the existence of a pilot on the reverse link allows Walsh codes to be used for channelization. This is because the reverse pilot channel enables the base station to synchronize the Walsh codes so that they are orthogonal to each other

3.7

Capacity Gain: Reverse Link

53

with respect to a common start time; the maintenance of orthogonality is essential for channelization. The use of Walsh codes for channelization also means that IS-2000 can now have multiple channels active at the same time (whereas IS-95 can only have one channel active at a time) on the reverse link.

References [1] [2] [3]

TIA/EIA-95-B, Mobile Station-Base Station Compatibility Standard for Wideband Spread Spectrum Cellular Systems, Telecommunications Industry Association, March 1999. TIA/EIA/IS-2000.2-A, Physical Layer Standard for cdma2000 Spread Spectrum Systems, Telecommunications Industry Association, March 2000. Yang, S. C., CDMA RF System Engineering, Norwood, MA: Artech, 1998.

Selected Bibliography TIA-97, Recommended Minimum Performance Standards for cdma2000 Spread Spectrum Base Stations, Telecommunications Industry Association, February 2003. TIA-98, Recommended Minimum Performance Standards for cdma2000 Spread Spectrum Mobile Station, Telecommunications Industry Association, February 2003.

.

CHAPTER 4

Medium Access Control 4.1

Introduction The MAC sublayer serves as an interface between the physical layer below it and LAC sublayer and upper layers above it. In this regard, the MAC sublayer controls higher layers’ access to the physical medium (i.e., the air link) that is inevitably shared among different users. The IS-2000 MAC sublayer acts in the same way as a MAC entity controlling a LAN. Whereas a LAN MAC controls different computers’ access to the shared cable medium, the IS-2000 MAC sublayer controls different users’ access to the shared air medium. The MAC sublayer logically belongs to Layer 2 of the OSI Reference Model [1]. Figure 4.1 shows that there are four different entities in the MAC sublayer: common channel multiplex sublayer, dedicated channel multiplex sublayer, SRBP, and radio link protocol (RLP). The primary function of the MAC sublayer is to multiplex (transmitted) logical channels onto different physical channels and to demultiplex (received) physical channels into different logical channels. This is done by the two multiplex sublayers of the MAC. On the other hand, RLP handles user packet data, and SRBP handles common-channel signaling which is inserted into the air interface using radio burst techniques.

4.2

Primitives Before describing the entities in the MAC sublayer, we need to review how the MAC sublayer as a whole communicates with the LAC sublayer above it and with the physical layer below it. The layers communicate using messages. In fact, these layer/sublayers behave like objects in an object-oriented computer program in that they pass messages amongst themselves to communicate. Primitives are a form of these communication messages that travel back and forth between the layer/sublayers. A primitive contains both payload information and control information and resembles a procedure call made by one computer program to another computer program. In the case of IS-2000, a primitive may be used by a layer/sublayer (i.e., service requester) to request a service or resource from another layer/sublayer (i.e., service provider). Or a primitive may be used by a layer/sublayer (i.e., service provider) to indicate to another layer/sublayer (i.e., service requester) that an event has occurred. Two widely used types of primitives are:

55

Upper layers

Medium Access Control

Data burst

f-dtch

R-DCCH F-FCH F-SCH F-DCCH

F-CCCH F-BCCH

Dedicated channel multiplex sublayer

R-FCH R-SCH

f-csch

r-csch

Common channel multiplex sublayer

F-CACH F-PCH

R-ACH

R-EACH

RLP

r-dtch

SRBP

r-dsch f-dsch

f-csch

LAP PDU

f-dtch (voice)

Voice services

r-dtch (voice)

L3 PDU

RLP SDU

Data services

RLP SDU

Data burst

Signaling LAC

MAC sublayer Physical layer

Layer 1

Layer 2

LAC sublayer

L3 PDU

Signaling

R-CCCH F-SYNCH F-CPCCH

Layer 3

56

RL Reverse link: coding and modulation Forward link: demodulation and decoding FL

Figure 4.1 Structure of the protocol architecture in IS-2000 as shown from the perspective of the mobile. (After: [2].)

•

•

Request: It is sent from a service requester to a service provider. A service requester uses request primitives to request a service or a resource. Indication: It is sent from a service provider to a service requester. A service provider uses indication primitives to indicate that an event for the service requester has occurred.

A primitive can be written in the form: Layer/Sublayer-Primitive_Name.Primitive_Type (Parameters) A primitive has four parts which are: •

•

Layer/Sublayer: It shows the service provider associated with the primitive. It can be either PHY (for physical layer) or MAC (for MAC sublayer) Primitive_Name: It is the name of the specific primitive call.

4.3

Multiplex Sublayers

•

•

57

Primitive_Type: It shows the type of the primitive. It can be either a request or indication. Other primitives types are confirm and response. Parameters: They contain the actual parameters carried by the primitive. The parameters may include the actual user/signaling data and the size of the data.

For example, when the MAC sublayer wants the physical layer to transmit some signaling data on the F-CCCH, the MAC sublayer sends a request primitive to the physical layer: PHY-FCCCH.Request (sdu,…, num_bits)

where the designation PHY shows that it is the physical layer that is performing the service (of transmitting the signaling data over the air interface). FCCCH.Request is the name and the type of the primitive used to request data transmission. The parameters of the primitive include the actual signaling data (sdu) and the size of the data (num_bits) [2]. On the other hand, when the physical layer delivers signaling data to the MAC sublayer, the physical layer sends a primitive to the MAC sublayer. For example, when the physical layer delivers signaling data to the MAC sublayer on the F-CCCH, the physical layer sends an indication primitive to the MAC sublayer: PHY-FCCCH.Indication (sdu,…, num_bits, frame_quality)

where the designation PHY shows that it is the physical layer that is performing the service (of delivering the signaling data over the air interface). FCCCH.Indication is the name and the type of the primitive used to indicate that data is received and delivered. The parameters of the primitive include the actual signaling data (sdu), the size of the data (num_bits), and frame quality (frame_quality) [2].

4.3

Multiplex Sublayers The multiplex sublayers, both common channel and dedicated channel, are responsible for the mapping between logical channels and physical channels. Table 4.1 shows this mapping on the forward link. For example, the f-dsch can use both the forward dedicated control channel and forward fundamental channel for physical transport. This is so because both are designed to carry dedicated signaling data. On the other hand, both the f-dsch and the f-dtch can use the forward dedicated control channel for physical transport. This is so because the forward dedicated control channel can carry both dedicated user and signaling data. Table 4.2 shows the mapping between logical channels and physical channels on the reverse link. Needless to say, logical channel data should be reliably delivered from the mobile (or base station) to the base station (or mobile). In executing reliable delivery, the MAC sublayer assembles data received from higher layers and passes the assembled data to the physical layer for transmission. The MAC sublayer also receives data from the physical layer, disassembles the data and passes the disassembled data to higher layers. Figure 4.2 illustrates these functions of the multiplex

58

Medium Access Control

Table 4.1

Mapping Between Logical Channels and Physical Channels: Forward Link

Logical Channels Channel Designation

Forward common signaling channel

f-csch

Forward dedicated signaling channel

f-dsch

Forward dedicated traffic channel

f-dtch

Table 4.2

Physical Channels Channel Designation

Channel Name

F-SYNCH

Sync channel

F-PCH

Paging channel

F-CCCH

Forward common control channel

F-BCCH

Broadcast control channel

F-CPCCH

Common power control channel

F-CACH

Common assignment channel

F-DCCH

Forward dedicated control channel

F-FCH

Forward fundamental channel

F-DCCH

Forward dedicated control channel

F-FCH

Forward fundamental channel

F-SCH

Forward supplemental channel

Mapping Between Logical Channels and Physical Channels: Reverse Link

Logical Channels Channel Designation r-csch

Channel Name

Channel Name Reverse common signaling channel

r-dsch

Reverse dedicated signaling channel

r-dtch

Reverse dedicated traffic channel

Physical Channels Channel Designation

Channel Name

R-ACH

Access channel

R-EACH

Enhanced access channel

R-CCCH

Reverse common control channel

R-DCCH

Reverse dedicated control channel

R-FCH

Reverse fundamental channel

R-DCCH

Reverse dedicated control channel

R-FCH

Reverse fundamental channel

R-SCH

Reverse supplemental channel

sublayer. On the transmit side, the MAC sublayer assembles data blocks (received from a higher layer) into an SDU and delivers the SDU to the physical layer for transmission. On the receive side, the MAC sublayer receives an SDU, dissembles the SDU into data blocks, and delivers them to higher layers. While the multiplex sublayer can only interact with the physical layer below, the multiplex sublayer can interact with four entities above it: RLP, voice services, LAC, and SRBP.

Multiplex Sublayers

59 Higher layer entities

Higher layer entities

Data blocks

Data blocks

Multiplex sublayer (common channel or dedicated channel)

SDUs

SDUs

Physical layer

Figure 4.2

Inputs and outputs of the multiplex sublayer.

Figure 4.3 shows an example of the assembling operation of the multiplex sublayer at the mobile. Here the mobile is transmitting. The multiplex sublayer receives two different data blocks at the same time from two higher layer entities: LAC and RLP. The LAC entity (which handles signaling) passes a signaling data block to the multiplex sublayer on the r-dsch. The RLP entity (which handles user

Header

Data block

R-DCCH

Dedicated channel multiplex sublayer

Data block

Physical layer

Figure 4.3

Assembling an R-DCCH SDU.

Data block

Data block

SDU

r-dtch

RLP

LAC

r-dsch

4.3

60

Medium Access Control

data) passes a user data block to the multiplex sublayer on the reverse r-dtch. The multiplex sublayer assembles the two data blocks and adds a header to form an R-DCCH SDU. It then passes the R-DCCH SDU to the physical layer for transmission on the R-DCCH. The R-DCCH is used here because it can carry both user and dedicated signaling data. Note that the MAC sublayer (of which the multiplex sublayer is a part) passes its SDUs down to the physical layer as request primitives, and the MAC sublayer delivers data blocks up to higher layer entities over the appropriate logical channels. In delivering data blocks to higher layer entities, the MAC sublayer chooses the appropriate logical channels based on the mapping described in Tables 4.1 and 4.2. Although IS-2000 defines the mapping between logical channels and physical channels, the standard does not specify any rule regarding service priority [2]. For example, IS-2000 does not specify any prioritization rules to resolve contention when multiple data blocks simultaneously arrive at the multiplex sublayer competing for limited physical channel resources (i.e., SDUs). Therefore, the equipment vendors are free to implement their own prioritization rules.

4.4

Radio Link Protocol (RLP) 4.4.1

Overview of Layer 2 Protocols

The RLP is a Layer 2 [1] protocol that is responsible for the delivery and receipt of user packet data [2]. As packets are transported from the transmitter to the receiver, some will be received in error. Thus, an important function of a Layer 2 entity is to control errors introduced by the physical layer. In general, a Layer 2 entity uses one of several mechanisms to control packet errors [3]: •

•

•

Positive acknowledgment. For error-free packet(s) received, the receiver sends to the transmitter an acknowledgment indicating that packet(s) has/have been received successfully. This positive acknowledgment is sometimes referred to as an “ACK.” Negative acknowledgment. For packet(s) received in error, the receiver sends to the transmitter an acknowledgment indicating that packet(s) has/have not been received successfully. This negative acknowledgment is sometimes known as a “NAK.” Retransmission. The transmitter may retransmit packet(s) if, (1) it has not received a positive acknowledgment after a predetermined amount of time, or (2) it has received a negative acknowledgment.

Some examples of Layer 2 protocols (also known as data link control protocols) are the logical link control (LLC) protocol and the link access protocol-balanced (LAPB). LLC is used as a part of the IEEE 802 family of standards for operating over a LAN, and LAPB is used as part of the X.25 standard to connect a device to a packet-switching network [3]. These protocols use a combination of positive and negative acknowledgments and retransmission to provide reliable delivery of packets.

Radio Link Protocol (RLP)

4.4.2

61

llustration of the RLP

The IS-2000 standard makes use of the RLP for the delivery of user packet data. The RLP is a data link control protocol that is designed especially for use over an air interface [4]. The RLP can be viewed as a scheme that “shields” the errors and problems of the air interface from higher layer entities (e.g., TCP/IP). Without the RLP, higher layer entities such as TCP/IP would be rendered useless if they interfaced directly with the error-prone air interface. Since the air link is inherently error-prone, the RLP does not attempt to provide a guaranteed delivery of packets over the air link because doing so would cost too many retransmissions and render Layer 2 inefficient. Instead, the RLP provides a best effort delivery in that it will attempt to deliver a packet up to a point, then give up. Note that in adopting the best effort strategy, the RLP (and the system) is implicitly relying on error-control mechanisms at higher layers to guarantee the delivery of user data, if such a quality of service (QoS) is required. To further minimize the transmission of control packets (e.g., ACK and NAK) over the air link, the RLP uses only negative acknowledgment (NAK) and retransmission mechanisms. Figure 4.4 shows an example of the negative acknowledgment and retransmission processes.

Transmitter

Receiver

a1

SEQ 0

a2

SEQ 1

a3

b1

SEQ 2

a4

b2

SEQ 3

b3

SEQ 1

b4

NAK 1 a5 D

D

b5 a6

Time

4.4

a7

SEQ 0

a8

b6

SEQ 1

a9

b7

SEQ 2 SEQ 3

b8

a10

b10

NAK 2 a11

a12 a13 D

a14 a15

b9

D

SEQ 2 NAK 2 NAK 2

b11 b12

SEQ 2

b13

SEQ 2

b14

D

SEQ 0 SEQ 1 SEQ 2 SEQ 3

Figure 4.4

Example of using negative acknowledgment and retransmission.

62

Medium Access Control

In Figure 4.4, the transmitter bursts a series of four packets, numbered with sequence numbers 0, 1, 2, and 31. These four packets are transmitted at times a1, a2, a3, and a4, respectively. The receiver receives sequence 0 (SEQ 0) at time b1, sequence 2 at time b3, and sequence 3 at time b4, but it does not receive the packet with sequence 1. At time b3, the receiver realizes it has missed the packet with sequence 1 because it has received the packet with sequence 2 (out of sequence). So at time b3, the receiver sends a negative acknowledgment NAK 1 back to the transmitter indicating that the receiver did not receive sequence 1. At time a5, the transmitter receives NAK 1 and retransmits the packet with sequence number 1. The receiver receives sequence 1 successfully at time b5. After a roundtrip time delay D, the transmitter does not receive any NAK from the receiver, so the transmitter infers that the receiver must have received sequence 1 correctly. Then the transmitter proceeds to burst a new set of four packets; here the sequence counter resets, and the sequence number is cycled from 0 to 3 again. This time sequence 2 is lost. At b10, the receiver realizes that it has received sequence 3 out of sequence and it has missed sequence 2. So at b10, the receiver sends NAK 2 back to the transmitter. At a11, the transmitter receives NAK 2 and retransmits sequence 2. Unfortunately, sequence 2 is again lost. At b11, the receiver should have received the retransmission of sequence 2 but did not, so it sends negative acknowledgment again. But this time it sends two copies of negative acknowledgment NAK 2 (at b11 and b12) to increase the probability of successful reception. At a12 and a13, the transmitter receives both copies of NAK 2 and resends two copies of sequence 2. At b14, the receiver realizes that it should have received two copies of sequence 2 by now, but it does not. So it decides to stop sending retransmission requests and does not send any more negative acknowledgment. At a15, the transmitter does not receive any negative acknowledgment from the receiver. The transmitter assumes that everything is fine and bursts out a new set of four packets. Note that at a6 (after roundtrip delay D), the transmitter does not immediately transmit a new set of packets. Rather it waits for another incremental time step until a7. This is so because the transmitter expects that if there is a packet error then it should receive two NAK copies from the receiver. This is why the transmitter waits until a7 to start transmitting the new sequences. 4.4.3

Concluding Remarks

It is important to note that the above example serves only to illustrate NAK and retransmission processes, not to describe detailed operations of the RLP. In actuality, the RLP uses three classes of frames: control frames, retransmitted data frames, and new data frames. Understandably, control frames carrying control information have the highest priority; old data frames that need to be retransmitted are second in priority, then new data frames that have not been transmitted have the lowest priority [4]. In addition, RLP (more specifically, RLP 3) uses 8-bit and 12-bit sequence numbers (instead of the 2-bit sequence number used in the above example).

2.

The handoff capability is actually for the soft handoff of common power control channels that power control the mobile’s transmission of R-CCCH.

4.5

Signaling Radio Burst Protocol (SRBP)

63

The RLP family of standards [4] includes three types of RLPs. RLP 3 is the RLP that implements packet data service over IS-2000 traffic channels; it is used for data rates up to 2 Mbps. Other types of RLPs include RLP 1 and RLP 2. RLP 1 implements packet data service over IS-95-A traffic channel; it operates over the fundamental channel at 9.6 or 14.4 Kbps. RLP 2 implements packet data service over IS-95-B traffic channels; it operates over IS-95-B fundamental and supplemental code channels.

4.5

Signaling Radio Burst Protocol (SRBP) Whereas as the RLP controls the processing of user packet data that travel on dedicated user channels, the SRBP controls the processing of signaling messages that travel on the common signaling channels. The SRBP controls the processing of the following common signaling channels: • • • • • • • •

F-SYNCH; F-PCH; F-CACH; F-CCCH; F-BCCH; R-ACH; R-EACH; R-CCCH.

For example, during a mobile’s access attempt [5] on the access channel, the SRBP is the entity that computes the power level of each successive access probe, performs the persistence test, and calculates the randomization delay for each access subattempt. In addition to generating and computing parameters needed for the transmission and reception of common signaling messages, the SRBP also assembles SDUs for the physical layer to transmit on the physical channels, as well as pass the received SDUs from the physical layer to the LAC sublayer. Figure 4.5 shows an example of the processing of the F-CCCH at the base station. As shown in Figure 4.5, when the SRBP is ready to process the F-CCCH, the SRBP sends to the LAC sublayer the following primitive: MAC-Availability.Indication (channel_type, max_size, system_time)

where channel_type is set to “F-CCCH frame,” max_size is the maximum number of information bits that can be transmitted in the next F-CCCH frame (i.e., FCCCH_FRAME_SIZE), and system_time specifies the time that the physical layer will start transmitting the next F-CCCH frame (i.e., “departure time”). This shows the LAC that SRBP is ready to receive more information for transmission on the F-CCCH. The LAC sublayer then sends to the MAC sublayer the following primitive: MAC-Data.Request (channel_type, data, size)

where channel_type is set to “F-CCCH frame,” data contains the actual data to be transmitted on the F-CCCH, and size is the length of the actual data (in bits). Here

64

Medium Access Control PHY

MAC

SRBP

MAC-Availability.Indication

MAC-Data.Request

(Processing)

SRBP

LAC

(Time)

PHY-FCCCH.Request

Figure 4.5

CCM

PHY: Physical layer MAC: Medium access control sublayer LAC: Link access control sublayer SRBP: Signaling radio burst protocol CCM: Common channel multiplex sublayer

Example of SRBP processing at the base station: Forward common control channel.

the LAC sublayer is requesting the SRBP to assemble an SDU to be transmitted on the F-CCCH. After receiving the MAC-Data.Request primitive, the SRBP examines its size parameter and appends (FCCCH_FRAME_SIZE – size) zeros to the end of data to form an SDU to be transmitted on the F-CCCH. Then the common channel multiplex sublayer sends the following primitive to the physical layer: PHY-FCCCH.Request (sdu, fccch_id, frame_duration, num_bits)

where sdu is the assembled F-CCCH SDU, frame_duration is set to 20 ms, 10 ms, or 5 ms depending on the data rate and the number of information bits in the frame, and num_bits is the number of bits in the sdu. After receiving the PHY-FCCCH.Request primitive, the physical layer proceeds to transmit the F-CCCH frame over the air interface.

4.6

System Access The IS-2000 standard has the ability to operate in one of four different access modes: basic access mode, reservation access mode, designated access mode, and power controlled access mode. These four access modes are described in the following sections.

4.6

System Access

4.6.1

65

Basic Access Mode

The access procedure in the basic access mode is similar to that used in IS-95. Namely, the mobile keeps transmitting access probes at increasing power levels until it gets a response back from the base station. Mobiles also transmit pseudorandomly in their attempts to gain access. In addition, a mobile can transmit access probes on the R-EACH instead of the R-ACH. Furthermore, to improve the probability of probe detection, a mobile cannot transmit the probe on the R-EACH unless the primary sector’s Ec/I0 exceeds the parameter EACH_ACCESS_THRESH. As shown in Figure 4.6, each R-EACH probe contains a preamble and enhanced access data. The preamble consists of a series of gated reverse pilot channel transmissions, which are used to facilitate the acquisition of the R-EACH by the base station. The reverse pilot channel transmission also continues during the transmission of enhanced access data. In the basic access mode, each R-EACH probe carries an entire set of enhanced access data, which may require several 20-ms, 10-ms, or 5-ms frames to transport (recall that the R-EACH supports 20-ms, 10-ms, and 5-ms frame formats). In other words, the enhanced access data portion of the probe may last a long time as compared to, say, a single 5-ms frame. Therefore, in the basic access mode the R-EACH still has the issue of random collisions of long probes, an issue similar to that in IS-95. 4.6.2

Reservation Access Mode

In the reservation access mode, a mobile transmits a short (5 ms) burst of message on the R-EACH to attempt to “reserve” a space on the R-CCCH. After securing the reservation, the mobile transmits the rest of access data on the R-CCCH whose

Preamble

R-PICH

R-EACH

Figure 4.6

An R-EACH probe: Basic access mode.

Enhanced access data

66

Medium Access Control

resources are scheduled by the base station and are free of collision problems. Furthermore, because the transmitted probe on the R-EACH is now shorter, the collision problem on the R-EACH is minimized as well. To improve the probability of probe detection, a mobile cannot transmit the probe on the R-EACH unless the primary sector’s Ec/I0 exceeds the parameter EACH_ACCESS_THRESH. When a mobile wishes to gain access or has information to send to the base station while there is no active R-DCCH or no R-FCH, it sends a probe on the R-EACH (see Figure 4.7). In the reservation access mode, this probe consists of the preamble and enhanced access header. The header lasts only 5 ms and contains only the following fields [2]: •

•

• •

•

•

HASH_ID or hash identifier. The mobile uses this field to identify itself to the base station. This field is 16-bits long. RATE_WORD or rate and frame size indicator. The mobile uses this field to request the data rate and frame format on the R-CCCH. This field is 3-bits long. MODE_ID or mode identifier. Its default value is 0. HO_REQ_ID or handoff request identifier. The mobile uses this field to request handoff capability on the R-CCCH2 if such a capability is supported and if a candidate sector’s Ec/I0 is greater than a predetermined threshold. This field has a maximum length of 1 bit. NEIGHBOR_PN or neighbor pilot PN offset. The mobile uses this field to report the neighbor pilot PN offset if it is requesting handoff capability on the R-CCCH. This field has a maximum length of 9 bits. RESERVED or reserved bits.

As one can see, all fields excluding the RESERVED field add up to a maximum of 30 bits. Including the RESERVED field brings the total number of bits in the header to 32 bits, which can be easily transmitted in the 5-ms header. After receiving the header, the base station uses the received HASH_ID and RATE_WORD parameters to schedule an R-CCCH resource for the mobile. The base station then transmits an early acknowledgment channel assignment message 5 ms

R-PICH

R-EACH

Preamble

Figure 4.7 1.

Enhanced access header

An R-EACH probe: Reservation access mode.

The sequence number is a part of the control (or header) information that the RLP attaches to the packet it sends. In this simplified example, the sequence number used is a 2-bit field that cycles from 0 to 3.

4.6

System Access

67

(EACAM) back to the mobile on the forward common assignment channel (F-CACH). The EACAM contains the following fields [2]: •

•

•

•

•

•

•

MSG_TYPE or message type. This field is set to “000” to indicate that this message is an EACAM. This field is 3-bits long. HASH_ID or hash identifier. The base station uses this field to retransmit the hash identifier that it received in the enhanced access header. This field is 16bits long. RATE_WORD or rate and frame duration indicator. The base station uses this field to indicate to the mobile the data rate and frame format on the R-CCCH that it grants to the mobile. This field is 3-bits long. RCCCH_ID or reverse common channel identifier. The base station uses this field to let the mobile know the R-CCCH index that it grants to the mobile to transmit. This field is 5-bits long. CPCCH_ID or common power control channel identifier. The base station uses this field to let the mobile know the R-CPCCH index that the mobile should monitor. This field is 2-bits long. HO_FLAG or handoff flag. The base station sets this field to 1 if the mobile had previously requested handoff capability in the header and the base station grants the request. This field is 1-bit long. RESERVED or reserved bits.

As one can see, the EACAM is 32-bits long which can just fit into a 5-ms R-CACH frame (which, readers may recall, contains 32 information bits and 8 CRC bits). The mobile knows that the EACAM is intended for it by examining the HASH_ID field in the EACAM; this HASH_ID should be identical to the one transmitted previously on the enhanced access header. After verifying that the received EACAM is intended for it, the mobile starts to transmit the enhanced access data on the allocated R-CCCH specified by the RCCCH_ID field. In doing so, the mobile uses the data rate and frame duration specified by the RATE_WORD field. In addition, the mobile starts to monitor the F-CPCCH specified by the CPCCH_ID field. The base station uses the specified F-CPCCH to send power control bits to power control the mobile’s transmitted power on the R-CCCH. 4.6.3

Power Controlled Access Mode

In the power controlled access mode, the base station uses the F-CPCCH to power control the R-EACH (instead of to power control the R-CCCH as in the case of reservation access mode). The enhanced access probe used in the power controlled access mode also differs from those used in the basic access mode and reservation access mode. Here the probe consists of a preamble, the header, and the enhanced access data. See Figure 4.8. The base station uses the F-CACH to provide a fast acknowledgment to the mobile and to let the mobile know the specific F-CPCCH (i.e., CPCCH_ID) to monitor. The mobile uses the power control bits it receives on the F-CPCCH to adjust the power it transmits on the R-EACH [2].

68

Medium Access Control

Preamble

Enhanced access header

Enhanced access data

R-PICH

R-EACH

Figure 4.8

4.6.4

An R-EACH probe: Power controlled access mode.

Designated Access Mode

The designated access mode is a mode of operation where the mobile, in response to requests received on the F-CCCH from the base station, responds using the R-CCCH. Similar to the reservation access mode, the R-CCCH is power controlled in the designated access mode. Here the base station uses the F-CPCCH to power control the R-CCCH. However, there are differences between the designated access mode and the reservation access mode. First and foremost, in the designated access mode the mobile does not initiate access or autonomously send access request, rather the mobile responds to requests received on the F-CCCH from the base station (hence the term designated access). For example, the mobile may receive a status request message on the F-CCCH that requires a response. In this case, the mobile may respond in the designated access mode on the R-CCCH. Second, the R-EACH is not used at all in designated access mode. Only the R-CCCH is used. Third, while IS-2000 allows for the support of soft handoff of R-CCCH in the reservation access mode, it does not provision for soft handoff of R-CCCH in the designated access mode [2].

References [1]

[2]

ITU-T Recommendation X.210, Information Technology–Open Systems Interconnection–Basic Reference Model: Conventions for the Definition of OSI Services, International Telecommunication Union, 1993. TIA/EIA/IS-2000.3-A, Medium Access Control (MAC) Standard for cdma2000 Spread Spectrum Systems, Telecommunications Industry Association, March 2000.

4.6

System Access [3] [4] [5]

69

Stallings, W., Business Data Communications, Upper Saddle River, NJ: Prentice-Hall, 2005. TIA/EIA/IS-707-A, Data Service Options for Wideband Spread Spectrum Systems, Telecommunications Industry Association, February 2003. Yang, S. C., CDMA RF System Engineering, Norwood, MA: Artech, 1998.

.

CHAPTER 5

Signaling Link Access Control 5.1

Introduction The signaling LAC entity executes a data link protocol; this data link protocol ensures that signaling data generated by upper layers are correctly delivered across the air link (see Figure 5.1). In some ways, LAC is analogous to the RLP entity, which is responsible for the delivery of user packet data across the air interface. Both LAC and RLP implement data link protocols. However, LAC differs from RLP in one important respect: while RLP provides a best effort transport of user packet data whose delivery is not assured, LAC provides a reliable delivery of signaling data. The reliable delivery of signaling data is needed in any communication network to ensure its smooth operation. Note in Figure 5.1 that the LAC entity interfaces (either directly or indirectly) with both common channel multiplex sublayer and dedicated channel multiplex sublayer. This means that LAC is responsible for delivering signal data reliably over both common and dedicated signaling channels. In many ways, LAC is just an interface between the MAC sublayer and Layer 3. The use of LAC enables software and hardware in the MAC sublayer to be separated from the logical functions of the LAC sublayer. By separating the LAC sublayer from the MAC sublayer, it is simpler to change the MAC software and hardware without affecting the software in Layer 3 [1]. To ensure reliable delivery of signaling data, the LAC entity depends on five sublayers to perform a variety of functions. These sublayers are (1) authentication sublayer, (2) addressing sublayer, (3) automatic repeat request (ARQ) sublayer, (4) utility sublayer, and (5) segmentation and reassembly (SAR) sublayer. Figure 5.2 shows the structure of these sublayers in the LAC entity [2].

5.2

LAC Sublayers 5.2.1

Authentication and Addressing Sublayers

The authentication sublayer’s function is to authenticate mobiles that try to gain access to the network. In doing so, this sublayer processes authentication-related fields. The addressing sublayer’s function is to process addressing information, such as a mobile’s electronic serial number (ESN) and mobile identification number (MIN).

71

r-dtch (voice) f-dtch (voice)

RLP SDU

RLP SDU

Voice services

r-csch f-csch

r-dtch f-dtch

RLP

F-DCCH

F-FCH F-SCH

R-DCCH

R-SCH

R-FCH

Dedicated channel multiplex sublayer

R-ACH R-EACH R-CCCH F-SYNCH F-CPCCH F-CACH F-PCH F-CCCH F-BCCH

Physical layer

r-dsch f-dsch

f-csch

LAC PDU SRBP

Common channel multiplex sublayer

Layer 1

Data services

Signaling LAC

MAC sublayer

Layer 2

Data burst Data burst

L3PDU

L3PDU

Signaling

LAC sublayer

Upper layers

Signaling Link Access Control

Layer 3

72

Reverse link

Reverse Link: coding and modulation Forward Link: demodulation and decoding Forward link

Figure 5.1 Structure of the protocol architecture in IS-2000 as shown from the perspective of the mobile. (After: [1].)

Authentication

LAC sublayer

ARQ

Addressing

Utility

SAR

Figure 5.2

Structure of the LAC sublayer.

5.2

LAC Sublayers

73

One important characteristic of the authentication and addressing sublayers is that they are only active in common signaling1. This is so because authentication is needed only when a mobile is first trying to access the network using common signaling channels2. Once it has access, the mobile has the use of dedicated traffic channels and authentication is no longer required. Similarly, addressing is only needed when a mobile communicates on common signaling channels. If a mobile gains use of a dedicated traffic channel, then it is identified by the assigned code (e.g., Walsh code). 5.2.2

ARQ Sublayer

The ARQ sublayer is the sublayer that is responsible for the reliable delivery of signaling data. ARQ is a term that collectively refers to a mechanism in which a receiver automatically requests for retransmission if it detects an error in the received data. In general, ARQ uses retransmission and positive and/or negative acknowledgment to provide reliable delivery. The ARQ sublayer in LAC can deliver Layer 3 PDUs in two modes: assured delivery and unassured delivery. Layer 3 specifies the type of delivery (using parameters in the request primative) when requesting delivery service from the LAC sublayer. In assured delivery, the transmitting LAC repeatedly sends signaling data at fixed intervals until it receives a positive acknowledgment from the receiving LAC. If after a predetermined number of transmissions the transmitting LAC does not receive any positive acknowledgment, then the transmitting LAC aborts any further transmissions. In unassured delivery, the transmitting LAC sends signaling data, but the receiving LAC does not send any positive acknowledgment. To improve the probability of successful delivery, Layer 3 may request the transmitting LAC to send signaling data multiple times; here, the receiving LAC detects and discards duplicate messages. But no positive acknowledgment is sent by the receiving LAC [2]. 5.2.3

Utility Sublayer

The utility sublayer is best described as one whose function is to perform those miscellaneous functions that are not done by other sublayers in LAC. Some examples of the utility sublayer’s functions are: •

•

1. 2.

Assembling the radio environment report fields and attaching them to the PDU, if required. They are fields such as the Ec/I0 of the active pilot (ACTIVE_PILOT_STRENGTH) and the number of pilots in addition to the active set that the mobile sees (NUM_ADD_PILOTS). This function is only performed by the utility sublayer at the mobile. Padding the PDU to bring it to the required number of bits.

One exception is the sync channel (F-SYNCH), which is technically a forward common signaling channel but does not require authentication and addressing functions. The mobiles typically do not authenticate a base station. Hence authentication is not active in common signaling on the forward link.

74

Signaling Link Access Control

5.2.4

Segmentation and Reassembly Sublayer

On the transmit side, the segmentation and reassembly (SAR) sublayer segments the PDUs (that have been encapsulated by the LAC sublayers) into PDU fragments of sizes that can be transferred by the MAC sublayer. The SAR sublayer may also compute the CRC and append it to the PDU. On the receive side, the SAR sublayer reassembles the encapsulated PDU fragments (received from the MAC sublayer) into encapsulated PDUs. In addition, the SAR may check the CRC to verify valid receipts. The SAR sublayer then presents the encapsulated PDUs to the sublayers above in LAC for further processing [2].

5.3

Sublayer Processing On the transmit side, the LAC entity as a whole accepts Layer 3 PDUs from the upper layers, and the different sublayers perform their functions in sequence and attach their own control information to the data unit. At the end the SAR sublayer segments the PDU (that has been encapsulated by the LAC sublayers) into PDU fragments of sizes that can be transferred by the MAC sublayer. It then outputs these PDU fragments to the MAC sublayer for transport [2]. On the receive side, the reverse process takes place. The following sections describe the processing done by the different sublayers in LAC for four scenarios: (1) common signaling on the forward link, (2) common signaling on the reverse link, (3) dedicated signaling on the forward link, and (4) dedicated signaling on the reverse link. 5.3.1

Common Signaling: Forward Link

Figure 5.3 shows the processing done by the different LAC sublayers (at the base station) when the base station transmits common signaling data to the mobile. In this case, all LAC sublayers are involved with the exception of the authentication sublayer. This is so because while the base station needs to authenticate the mobiles (e.g., to prevent fraud), there is rarely a need for a mobile to authenticate the base station. At the base station, a Layer 3 PDU is first passed on to the ARQ sublayer. The ARQ sublayer adds the appropriate acknowledgment fields, and the addressing sublayer adds the appropriate address parameters [for addressing the mobile(s)]. Then the utility sublayer finishes the assembly of the LAC PDU by performing padding (if necessary) and other relevant functions. After receiving the data unit, the SAR sublayer first calculates the CRC and appends it to the data unit. Then, in communicating with the MAC sublayer (using primatives), the SAR sublayer is notified of whether there is capacity available on the appropriate physical channels (e.g., forward common control channel). Once capacity becomes available, the SAR sublayer transfers the encapsulated PDU fragments to the MAC sublayer for delivery. Note that common signaling data on the forward link logically travels on the f-csch. Figure 5.4 shows the processing done by the LAC sublayers (at the mobile) when the mobile receives common signaling data from the base station. Here the reverse

Sublayer Processing

75

Upper layers

Layer 3 PDU

ARQ

LAC SDU

Addressing

LAC SDU

Partially-formed LAC PDU

Utility

LAC SDU

LAC PDU

LAC SDU

SAR

LAC SDU

Encapsulated PDU

f-csch

Encapsulated PDU fragments

MAC SDUs

MAC sublayer

Figure 5.3

Common signaling-forward link processing: Base station. (After: [2].)

Upper layers

Layer 3 PDU

ARQ

LAC SDU

Addressing

LAC SDU

Utility

SAR

LAC SDU

LAC SDU

LAC PDU

LAC SDU

Encapsulated PDU Encapsulated PDU fragments

f-csch

5.3

MAC SDUs

MAC sublayer

Figure 5.4

Common signaling-forward link processing: Mobile station. (After: [2].)

76

Signaling Link Access Control

process takes place. At the mobile, the SAR sublayer first concatenates the encapsulated PDU fragments received from the MAC sublayer. It strips off the CRC and performs the CRC check. After forming the LAC PDU, it passes the LAC PDU to the utility sublayer. After the utility sublayer performs its functions3, the PDU is passed to the addressing sublayer which processes the address fields for address matching. Then the data unit is passed to the ARQ sublayer. The ARQ sublayer processes the ARQ fields. If the received PDU requires an acknowledgment, then the (receiving) ARQ sublayer notifies its counterpart on the transmit side. Finally, the data unit is delivered to Layer 3. Note that, on the forward link, a mobile logically receives common signaling data on the f-csch. 5.3.2

Common Signaling: Reverse Link

Figure 5.5 shows the processing done by the different LAC sublayers when the mobile transmits common signaling data to the base station. Note that in this case,

Upper layers

Layer 3 PDU

Authentication

LAC SDU

ARQ

LAC SDU

Addressing

LAC SDU

Partially-formed LAC PDU

LAC SDU

LAC PDU

Utility

LAC SDU

SAR

LAC SDU

Encapsulated PDU

r-csch

Encapsulated PDU fragments

MAC SDUs

MAC sublayer

Figure 5.5

3.

Common signaling-reverse link processing: Mobile station. (After: [2].)

For example, if the received PDU contains a universal page message, then the utility sublayer processes the message.

5.3

Sublayer Processing

77

all LAC sublayers are involved. When the mobile transmits on the reverse common signaling channel, there is a definite need for the base station to authenticate the mobile, as well as a need for that mobile to identify itself to the base station using addressing fields. At the mobile, a Layer 3 PDU is first passed on to the authentication sublayer, which adds the authentication fields to the data unit. Afterwards, the ARQ sublayer takes over and appends the acknowledgment fields. Then the addressing sublayer adds the appropriate address fields. After processing of the addressing field, the partially formed LAC PDU is passed to the utility sublayer. Here the utility sublayer adds the radio environment report fields to the PDU, and then it finishes the assembly of the encapsulated PDU by performing padding (if necessary) and other relevant functions. After receiving the data unit from the utility sublayer, the SAR sublayer first calculates the CRC and adds it to the data unit. Then the SAR sublayer, in communicating with the MAC sublayer (using primitives), is notified of whether there is capacity available on the appropriate physical channels (e.g., reverse common control channel). Once capacity becomes available, the SAR sublayer transfers the encapsulated PDU fragments to the MAC sublayer for delivery. Note that common signaling data on the reverse link logically travels on the r-csch. Figure 5.6 shows the processing done by the LAC sublayers when the base station receives common signaling data from the mobile. Here the reverse process takes place. At the base station, the SAR sublayer first concatenates the encapsulated PDU fragments received from the MAC sublayer. It strips off the CRC and performs the CRC check. After forming the PDU, it passes the PDU to the utility sublayer. After the utility sublayer performs its functions4, the PDU is passed to the addressing sublayer which processes the address fields for address matching. Then the PDU is passed to the ARQ sublayer. The ARQ sublayer processes the ARQ fields and passes the data unit to the authentication sublayer. The authentication sublayer processes the authentication fields and tests for authentication. If the authentication passes, the remaining Layer 3 SDU is delivered to Layer 3. Note that, on the reverse link, the base station logically receives common signaling data on the r-csch. 5.3.3

Dedicated Signaling: Forward Link

Figure 5.7 shows the processing done by the different LAC sublayers (at the base station) when the base station transmits dedicated signaling data to the mobile. In this case, only the ARQ, utility, and SAR sublayers are involved; the addressing and authentication layers are not active. The addressing sublayer is not needed because when receiving dedicated traffic channels, mobiles are uniquely identified by their assigned codes (e.g., Walsh codes). The authentication sublayer is not needed because there is no need for mobiles to authenticate the base station. At the base station, the Layer 3 PDU is first passed to the ARQ sublayer. The ARQ sublayer adds the appropriate acknowledgment fields, then the utility 4.

For example, the utility sublayer may interpret the MSG_ID field to see what kind of message is carried by the received PDU (e.g., if MSG_ID = 000110 then the message is an authentication challenge response message).

78

Signaling Link Access Control Upper layers

Layer 3 PDU

Authentication

LAC SDU

ARQ

LAC SDU

Addressing

LAC SDU

Utility

LAC SDU

LAC SDU

SAR

LAC SDU

LAC PDU

Encapsulated PDU

r-csch

Encapsulated PDU fragments

MAC SDUs

MAC sublayer

Figure 5.6

Common signaling-reverse link processing: Base station. (After: [2].)

sublayer performs padding (if necessary) and other relevant functions. After the data unit is passed to the SAR sublayer, SAR calculates the CRC and appends it to the data unit. The SAR sublayer, in communicating with the MAC sublayer (using primitives), is notified of whether there is capacity available on the appropriate physical channels (e.g., forward dedicated control channel). Once capacity becomes available, the SAR sublayer transfers the encapsulated PDU fragments to the MAC sublayer for delivery. Note that dedicated signaling data on the forward link logically travels on the f-dsch. Figure 5.8 shows the processing done at the mobile when the mobile receives dedicated signaling data from the base station. Here the reverse process takes place. At the mobile, the SAR sublayer first concatenates the encapsulated PDU fragments received from the MAC sublayer. It strips off the CRC and performs the CRC check. The completed LAC PDU is then passed to the utility sublayer. After the utility sublayer performs its functions5, the data unit is passed to the ARQ sublayer which processes the ARQ fields and removes them from the data unit.

5.

For example, the utility sublayer may interpret the MSG_TYPE field to see what kind of message is carried by the received PDU (e.g., if MSG_TYPE = 00000010 then the message is an authentication challenge message).

Sublayer Processing Upper layers

79

Layer 3 PDU

ARQ

LAC SDU

Partially-formed LAC PDU

Utility

LAC SDU

LAC PDU

LAC SDU

SAR

LAC SDU

Encapsulated PDU

f-dsch

Encapsulated PDU fragments

MAC SDUs

MAC sublayer

Figure 5.7

Dedicated signaling-forward link processing: Base station. (After: [2].)

Upper layers

Layer 3 PDU

ARQ

LAC SDU

Utility

LAC SDU

LAC PDU

LAC SDU

SAR

LAC SDU

Encapsulated PDU Encapsulated PDU fragments

f-dsch

5.3

MAC SDUs

MAC sublayer

Figure 5.8

Dedicated signaling-forward link processing: Mobile station. (After [2].)

80

Signaling Link Access Control

Note that, on the forward link, a mobile logically receives dedicated signaling data on the f-dsch. 5.3.4

Dedicated Signaling: Reverse Link

The LAC processing of dedicated signaling data on the reverse link is similar and symmetrical to those done for dedicated signaling data on the forward link. For the sake of completeness, Figures 5.9 and 5.10 are included to show the processing done at the mobile and at the base station, respectively. Readers can see that the processing shown in these figures resemble those performed for dedicated signaling data on the forward link (i.e., only the ARQ, utility, and SAR sublayers are active). In this case, the dedicated signaling data travels on r-dsch [2].

Interaction of Layer and Sublayers As readers can see from the above discussion, the LAC sublayer as a whole is in constant communication with Layer 3 above it and with the MAC sublayer below it. In carrying out this communication, Layer 3, LAC, and MAC use primitives to pass data units and control information between Layer 3 and LAC and between LAC and MAC. Note that the actual data unit transferred is simply one of the parameters of the primitive (see Chapter 4).

Upper layers

Layer 3 PDU

ARQ

LAC SDU

Partially-formed LAC PDU

Utility

LAC SDU

LAC PDU

LAC SDU

SAR

LAC SDU

Encapsulated PDU Encapsulated PDU fragments

r-dsch

5.4

MAC SDUs

MAC sublayer

Figure 5.9

Dedicated signaling-reverse link processing: Mobile station. (After: [2].)

Interaction of Layer and Sublayers

81

Upper layers

Layer 3 PDU

ARQ

LAC SDU

Utility

LAC SDU

LAC PDU

LAC SDU

SAR

LAC SDU

Encapsulated PDU Encapsulated PDU fragments

r-dsch

5.4

MAC SDUs

MAC sublayer

Figure 5.10

5.4.1

Dedicated signaling-reverse link processing: Base station. (After: [2].)

Transmit Side

Figure 5.11 shows the interaction of primitives when sending signaling data. When Layer 3 has a PDU to send, Layer 3 requests this service from the LAC sublayer by invoking the L2-Data.Request primitive. L2 is the name of the service provider. From the perspective of Layer 3 the entity that provides the service is the entire Layer 2, not just LAC or MAC. Data is the name of this particular primitive, and Request denotes that this primitive is a request for service. When the LAC sublayer (or specifically the SAR sublayer) wants to send a PDU, the LAC sublayer invokes the MAC-SDUReady.Request primitive. This primitive lets MAC know that there is a PDU ready for transmission. Here MAC is the entity that provides the service. SDUReady is the name of the primitive, and Request denotes that this primitive is a request for service. If space is available for data transfer on the physical channel(s), MAC sends LAC the MAC-Availability.Indication primitive. This primitive lets LAC know that there is space available as well as how much space is available (i.e., how many bits are available on the physical channel(s)). This primitive shows that MAC is the service provider, and Availability is the name of the primitive. This Indication primitive lets the service requester (i.e., LAC) know that some event has occurred (i.e., there is transport space available). In addition, MAC may also invoke the MACAvailability.Indication primitive to “advertise” to the LAC sublayer that there is space available on the physical channel(s).

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Signaling Link Access Control

MAC-Data.Request

MAC-Availability.Indication

MAC-SDUReady.Request

Layer 2

LAC sublayer

Signaling data

L2-Data.Request

Layer 3

Upper layers

MAC sublayer Figure 5.11 Interaction of primitives: Transmit side. Note that primitives used in notifying extraordinary and error conditions are not shown.

After receiving the MAC-Availability.Indication primitive, LAC sends the MAC-Data.Request primitive to MAC. This primitive requests a data transport service from MAC by carrying the actual data (as one of its parameters). Here MAC is the entity that performs the service. Data is the name of the primitive and signifies that the primitive carries actual data. As usual, Request denotes that the primitive is used to request a service, in this case one of data transport [2]. 5.4.2

Receive Side

Figure 5.12 shows the interaction of primitives when receiving signaling data. The MAC sublayer sends the SAR sublayer each encapsulated PDU fragment using the MAC-Data.Indication primitive. In this case, MAC is the entity that performs the service (i.e., receiving signaling data and passing it onto LAC). Data signifies that this is a primitive that carries the actual data, and Indication is a primitive that is sent from a service provider (e.g., MAC) to a service requester (e.g., LAC). MAC uses this primitive to “indicate” to LAC that signaling data has been received by MAC and delivered to LAC. After processing, LAC sends Layer 3 PDU to Layer 3 using the L2-Data.Indication primitive. In this case, L2 (the entire Layer 2) is designated as the service provider. This is because from the perspective of Layer 3, Layer 2 is the entity that provides the service to Layer 3 [2].

5.4

Interaction of Layer and Sublayers

83

MAC-Data.Indication

Layer 2

LAC sublayer

Signaling data

L2-Data.Request

Layer 3

Upper layers

MAC sublayer

Figure 5.12 Interaction of primitives: Receive side. Note that primitives used in notifying extraordinary and error conditions are not shown.

References [1] [2] [3]

Fitzgerald, J., and A. Dennis, Business Data Communications and Networking, New York: John Wiley and Sons, 2004. TIA/EIA/IS-2000.4-A, Signaling Link Access Control (LAC) Standard for cdma2000 Spread Spectrum Systems, Telecommunications Industry Association, March 2000. TIA/EIA/IS-2000.1-A, Introduction to cdma2000 Standards for Spread Spectrum Systems, Telecommunications Industry Association, March 2000.

.

CHAPTER 6

Signaling: Upper Layers Overview

RLP SDU

Voice services

r-dsch f-dsch

r-dtch f-dtch

RLP

r-csch f-csch

F-DCCH

F-FCH F-SCH

R-SCH

R-FCH

R-ACH R-EACH R-CCCH F-SYNCH F-CPCCH F-CACH F-PCH F-CCCH F-BCCH

Dedicated channel multiplex sublayer

R-DCCH

SRBP

f-csch

LAC PDU

Signaling LAC

Common channel multiplex sublayer

Physical layer

Data services

r-dtch (voice) f-dtch (voice)

Data burst Data burst

RLP SDU

L3PDU

Signaling

L3PDU

Upper layers MAC sublayer

Layer 2

LAC sublayer

Layer 3

The signaling entity (see Figure 6.1) is the one that effectively controls the operation of the entire IS-2000 system. In doing so it follows state transitions that have been specified by the IS-2000 standard. In addition, the signaling entity also controls and executes those functions that are necessary for the setup, maintenance, and tear down of a call.

Layer 1

6.1

Reverse link

Reverse link: coding and modulation Forward link: demodulation and decoding Forward link

Figure 6.1 Structure of the protocol architecture in IS-2000 as shown from the perspective of the mobile. (After: [1].)

85

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Signaling: Upper Layers

States and state transitions

Overall, the operation of the signaling entity can be viewed along two dimensions: states and functions. See Figure 6.2. From the perspective of states and state transitions, the signaling entity enters and exits states and substates depending on which stage of the call processing cycle it is in. At the top level, the IS-2000 standard specifies four states: (1) mobile station initialization, (2) mobile station idle, (3) system access, and (4) mobile station control on the traffic channel. These states are similar to those in IS-95. Furthermore, in packet data transmission a mobile may be in one of several modes and transition between these modes. These modes are unique to IS-2000 and are implemented to accommodate bursty packet data transmissions and to conserve air link resources. The dimension of functions provides another perspective on the operation of the signaling entity. Here, the signaling entity effectively controls and executes different functions that are required for call processing. These functions include but are not limited to registration, handoff, and power control. In performing these functions, the signaling entity originates and receives messages. In originating a message, the signaling entity (e.g., at the mobile) requests Layer 2 to deliver the message to its counterpart at the other side (e.g., at the base station). In receiving a message, the signaling entity takes delivery of the message from Layer 2 that was transmitted by the other side. Before we begin, it would be instructive to highlight the difference between the terms “message” and “data unit” (e.g., PDU or SDU). For example when the signaling entity at the mobile sends an origination message, the message is sent as a PDU, and its first physical destination is the LAC sublayer. However, the message itself is really intended logically for the signaling entity across the link at the base station. Therefore, the best way to distinguish between message and data unit is to use message in a logical and semantic context and to use data unit in a physical and protocol context. When referring to interactions between two entities, such as between the signaling entity at the base station and the signaling entity at the mobile, use the term message since a message is logically sent from one entity to another. When referring to interactions between two protocol layers, such as between Layer 3 and Layer 2, use the term data unit (e.g., PDU or SDU) since a data unit sent by a higher layer is successively processed by lower layers for the eventual transmission over the air.

Functions and message processing

Figure 6.2

Two dimensions of signaling operation.

6.2

State Transitions: Call Processing

87

The next section (Section 6.2) describes the states and state transitions related to call processing. Then Section 6.3 focuses on packet data transmission and covers the associated modes and mode transitions. Finally, Section 6.4 solidifies the materials learned on signaling by going through several call flow examples, including both voice calls and packet data calls.

6.2

State Transitions: Call Processing The IS-2000 standard specifies four states for the signaling entity (or Layer 3) at the mobile: (1) mobile station initialization state, (2) mobile station idle state, (3) system access state, and (4) mobile station control on the traffic channel state. Similar to IS-95, the IS-2000 standard largely specifies the states and substates for the mobile station. But it is obvious that whatever functions the base station performs, they must work with the specified mobile states and substates. The infrastructure vendors have flexibility here to implement their own functions at the base station to satisfy specified call processing requirements [2, 3]. Figure 6.3 shows these top-level states and their transitions. After power up, the mobile enters the initialization state. After a call is finished, the mobile returns to the initialization state. Mobile responds to order or message, originates a call, or Mobile is directed performs registration to traffic channel

Mobile has acquired system

Power-down

Power-up

MS initialization state Mobile selects and acquires system

MS idle state Mobile monitors messages on f-csch

Mobile needs to reacquire system

System access state Mobile sends messages on r-csch and receives messages on f-csch

Mobile communicates with base station on traffic channel

Mobile performs registration or responds without making call

Call is terminated

Figure 6.3

MS control on the traffic channel state

State and state transitions at the mobile. (After: [2] and [4].)

88

Signaling: Upper Layers

While in the initialization state, the mobile selects and acquires a system. After the mobile acquires the system, it enters the idle state where the mobile monitors messages on the f-csch. The mobile stays in the idle state until it receives a message (on the f-csch) or originates a call or performs registration (on the r-csch). If any one of these three events occurs, then the mobile enters the system access state where it sends messages on the r-csch and receives messages on the f-csch. If call origination is successful, the mobile is directed to a traffic channel1 by the base station, in which case the mobile enters the mobile station control on the traffic channel state. In this state, the mobile communicates with the base station using the traffic channel [4]. There are events that can cause the mobile to return to a previous state. If a mobile loses the paging channel, forward common control channel, or broadcast control channel, or if it goes from CDMA to AMPS, then the mobile transitions from the idle state back to the initialization state to reacquire the system or to acquire another system. In addition, if a mobile performs a registration without making a call, it returns from the system access state back to the idle state. Note that ideally power down provides an exit from the idle state, but in reality power down can cause an exit from any one of the four states. 6.2.1

Initialization State

After powering up, the mobile enters the initialization state, which contains four substates. The mobile goes through these four substates in sequence: • • • •

System determination substate; Pilot channel acquisition substate; Sync channel acquisition substate; Timing change substate.

These substates are similar to those previously specified by IS-95. 6.2.1.1

System Determination Substate

The system determination substate is the first substate that the mobile enters upon entering the initialization state. In this substate, the mobile selects which system to use (e.g., system A or system B in the cellular band). Alternatively, the mobile can also be redirected to a different system using the information received in the service redirection message, global service redirection message, or extended global service redirection message. 6.2.1.2

Pilot Channel Acquisition Substate

In the pilot channel acquisition substate, the mobile demodulates and acquires the forward pilot channel of the selected system. The mobile has to acquire the pilot within a specific time limit T20m (defined by [4] as 15 seconds). If it does, then it 1.

In terms of logical channels, a “traffic channel” is either forward or reverse dedicated traffic channel (f-dtch or r-dtch) or forward or reverse dedicated signaling channel (f-dsch or r-dsch).

6.2

State Transitions: Call Processing

89

enters the sync channel acquisition substate. If it does not, then it goes back to the system determination substate. 6.2.1.3

Sync Channel Acquisition Substate

In the sync channel acquisition substate, the mobile proceeds to acquire the sync channel and receive the sync channel message. The mobile obtains the system timing information, such as the pilot PN offset (PILOT_PN), the system time (SYS_TIME), and long code state (LC_STATE), from the sync channel message. These enable the mobile to sync up its long PN code [2] and to acquire subsequent common signaling channels (e.g., paging channel or forward common control channel) later. In addition, the mobile also obtains system configuration information, such as the minimum protocol revision level supported by the base station (MIN_P_REV) and whether or not the base station supports broadcast control channel (SR1_BCCH_SUPPORTED), from the sync channel message. In general, a protocol revision level of six or greater shows that the mobile or the system supports 3G capabilities. The mobile needs to receive the sync channel message within a specific time limit T21m (defined by [4] as 1 second) upon entering this substate. If it does not, then the mobile returns to the system determination substate. 6.2.1.4

Timing Change Substate

In the timing change substate, the mobile synchronizes its own timing and long PN code phase to those of the system. It does this by using three parameters obtained from the sync channel message (i.e., PILOT_PN, SYS_TIME, and LC_STATE). After the mobile has fully acquired the system, it enters the mobile station idle state. 6.2.2

Mobile Station Idle State

A mobile typically spends a majority of its time in the mobile station idle state, In this state, the mobile primarily monitors for messages sent on the F-PCH, F-QPCH, F-CCCH, or F-BCCH. The following sections describe different ways the mobile can monitor these channels. 6.2.2.1

Monitoring Paging Channel

If a mobile operates in the IS-95 mode, then it monitors only the F-PCH. The paging channel transmission is divided into slots that are 80 ms in length (see Chapter 2). The mobile monitors the paging channel for two types of messages: (1) specific messages that are intended for specific mobiles (e.g., page message), and (2) broadcast messages that are intended for all mobiles (e.g., system parameters message). There are two ways that the mobile can monitor the paging channel: nonslotted mode or slotted mode. In nonslotted mode, the mobile monitors the paging channel at all times. In slotted mode, the mobile monitors the paging channel only during its assigned paging channel slots. Because the mobile doesn’t have to monitor all the slots all the time, the mobile operating in slotted mode uses less battery power.

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Signaling: Upper Layers

6.2.2.2

Monitoring Quick Paging Channel

In IS-95, using slotted mode on the paging channel does have a couple of disadvantages. From the base station’s perspective, it is inefficient because when the base station has a mobile-specific page to send, it cannot immediately send it. The base station has to wait for the right slot to come along to send the page. As a result, the mobile often does not receive its designated page immediately. From the mobile’s perspective, while the mobile saves some battery power by only monitoring its assigned slot, the assigned slot still lasts 80 ms. At the beginning of its assigned slot, the mobile still has to wake up to monitor the entire 80-ms slot, and most of the time there is no page directed at the mobile in the slot. In IS-2000, the F-QPCH is added to alleviate the drawbacks described above. A quick paging channel slot also lasts 80 ms. But the mobile monitors only its designated (and shorter) paging indicator bits in the quick paging channel slot. The exact positions of a mobile’s paging indicators are determined by a hash algorithm. If the paging indicators show that there is no mobile-specific message, then the mobile does nothing. If the paging indicators show that there is a mobile-specific message coming in, then the mobile wakes up and monitors the paging channel slot that comes after the end of the current quick paging channel slot. In this regard, the F-QPCH works with a paging channel in slotted mode. In addition to working with the paging channel, the F-QPCH can also work with a F-CCCH operating in slotted mode. The use of the F-CCCH is described in more detail in the next section. 6.2.2.3 Monitoring Forward Common Control Channel (F-CCCH )and Broadcast Control Channel (F-BCCH)

In IS-95, a mobile monitors the paging channel for both specific and broadcast messages. But using a single paging channel to transmit these two types of messages is not very efficient because of their different queuing characteristics. The broadcast messages are sent at more regular intervals, while the specific messages are sent ondemand. Mixing two statistically different types of messages on the same channel results in less-than-optimal scheduling of the paging channel. Furthermore, recall that IS-95 allows up to seven paging channels per carrier; since each mobile only monitors one paging channel, if there are more than one paging channels in the carrier then broadcast system messages would have to be duplicated on all paging channels. IS-2000 added two additional channels: F-CCCH and F-BCCH to alleviate the responsibilities of the paging channel. The F-CCCH is used to transmit specific messages intended for particular mobiles, and the F-BCCH is used to transmit broadcast system messages intended for all mobiles. The mobile needs to monitor both the F-CCCH and F-BCCH for both types of messages. In monitoring the F-CCCH, the mobile may operate in either nonslotted mode or slotted mode. In nonslotted mode, a mobile needs to monitor the F-CCCH at all times for the specific message intended for it. In slotted mode, the mobile only monitors the F-CCCH during its assigned forward common control channel slots, each of which also lasts 80 ms. In addition, a quick paging channel may be used in conjunction with the F-CCCH in slotted mode. This way a mobile only has to monitor those F-CCCH slots that actually contain messages intended for it.

6.2

State Transitions: Call Processing

91

In monitoring the F-BCCH, a mobile operating in nonslotted mode would have to monitor the F-BCCH at all times for broadcast messages. In slotted mode, the mobile first monitors its specially assigned slots on the forward common control channel called broadcast slots. The mobile monitors these broadcast slots for enhanced broadcast pages, which are nothing more than general page messages or universal page messages. These broadcast pages then in turn tell the mobile which future broadcast control channel slot to monitor for the broadcast message. 6.2.2.4

Other Functions

In addition to performing those functions described above, the mobile can also perform other functions in the mobile station idle state. These functions include originating a call, transmitting a user message, and performing a registration. When the mobile originates a call, transmits a user message, or performs a registration the mobile transitions from the mobile station idle state to the update overhead information substate of the system access state. In addition, the mobile can also perform idle handoff in the mobile station idle state. Idle handoff is described in more detail in Chapter 8. 6.2.3

System Access State

In the system access state, the mobile sends messages to the base station and receives messages from the base station. If the mobile receives messages on the paging channel only, then it sends messages on the access channel. If the mobile receives messages on the forward common control channel and broadcast control channel, then the mobile sends messages on the enhanced access channel. There are seven substates in the system access state: (1) update overhead information substate, (2) page response substate, (3) mobile station origination attempt substate, (4) registration access substate, (5) mobile station order/message response substate, (6) mobile station message transmission substate, and (7) priority access and channel assignment (PACA) cancel substate. Figure 6.4 shows the nominal transitions among these substates. It is important to note that while in the system access state, the mobile monitors the paging channel or the forward common control channel at all times [4]. 6.2.3.1

Update Overhead Information Substate

In this substate, the mobile monitors for broadcast overhead messages on either the paging channel (IS-95) or the broadcast control channel (3G). These messages include: •

• •

•

System parameters message, extended system parameters message, and ANSI-41 system parameters message; Access parameters message and enhanced access parameters message; Neighbor list message, extended neighbor list message, general neighbor list message, private neighbor list message, and universal neighbor list message; CDMA channel list message and extended CDMA channel list message.

92

Signaling: Upper Layers

IS Cancels PACA call

IS Generates data burst message

IS MS idle state

Update overhead information substate

Receives general page message IS

Performs registration access

PACA cancel substate

Receives general page message

MS message transmission substate Receives general page message Page response substate Receives general page message

MS control on the traffic channel state

Registration access substate Originates call

MS origination attempt Originates call substate or reoriginates PACA call MS order/ IS message response substate Receives order IS

or message that requires response

Figure 6.4

IS: Idle state

Nominal transitions among substates in the system access state. (After: [4].)

If the mobile receives a general page message, then it transitions from this substate to the page response substate. 6.2.3.2

Page Response Substate

The mobile moves to this substate after receiving a general page message. While in this substate, the mobile transmits to the base station a page response message [4]. After receiving the page response message, the base station may send a channel assignment message or extended channel assignment message to the mobile. In IS-2000, the extended channel assignment message contains parameters that the mobile uses to start receiving on a forward traffic channel. These parameters include information such as the forward and reverse traffic channel radio configurations (FOR_RC and REV_RC), CDMA_FREQ (frequency assignment), FRAME_OFFSET (frame offset), CODE_CHAN_FCH (fundamental channel code channel), and QOF_MASK_ID_FCH (quasi-orthogonal function mask). The mobile also receives in the extended channel assignment message some power control parameters. After updating its internal parameters, the mobile then enters the

6.2

State Transitions: Call Processing

93

traffic channel initialization substate of the mobile station control on the traffic channel state. 6.2.3.3

Mobile Station Origination Attempt Substate

In this substate, the mobile transmits to the base station an origination message [4]. After receiving the origination message, the base station may send a channel assignment message or extended channel assignment message to the mobile. In IS-2000, the extended channel assignment message contains parameters that the mobile uses to start receiving on a forward traffic channel. The mobile also receives in the extended channel assignment message some power control parameters. After updating its internal parameters, the mobile then enters the traffic channel initialization substate of the mobile station control on the traffic channel state. 6.2.3.4

Registration Access Substate

In this substate, the mobile transmits to the base station a registration message [4]. The mobile typically enters this substate from the update overhead information substate. For example, in performing a power-up registration (without making a call) a mobile would transition from mobile station initialization state to mobile station idle state, then to update overhead information substate and finally to registration access substate. After sending the registration message, it would then return to mobile station idle state. In addition, while in the idle state the mobile may receive a registration request order from the base station. In performing this ordered registration, the mobile would transition to update overhead information substate and onto registration access substate. After sending the registration message, the mobile would return to mobile station idle state. 6.2.3.5

Mobile Station Order/Message Response Substate

In this substate, the mobile transmits to the base station a response or acknowledgment in response to an order or message received. An example of an order from the base station that requires a response is the base station challenge confirmation order. This order tells the mobile to update the shared secret data (SSD) as part of the authentication procedure. After updating, the mobile then sends back to the base station a response called SSD update confirmation order indicating a successful SSD update [4]. Other examples include a status request message from the base station; the mobile responds to this message by nominally sending a status response message back to the base station. After sending the required response, the mobile returns to the mobile station idle state. 6.2.3.6

Mobile Station Message Transmission Substate

In this substate, the mobile transmits to the base station a data burst message. The support of this substate is optional.

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Signaling: Upper Layers

6.2.3.7

PACA Cancel Substate

In this substate, the mobile transmits to the base station a PACA cancel message. A PACA call is a priority call originated by the mobile for which no traffic channel is available, so the call is queued for a priority channel assignment [4]. After receiving an origination message, the base station may send a PACA message to the mobile. This serves to inform the mobile of the queue position of the PACA call or to instruct the mobile to reoriginate the PACA call. PACA is supported for protocol revision levels greater than four. While in queue, the user may direct the mobile to cancel the PACA call, in which case the mobile proceeds to PACA cancel substate through update overhead information substate. After sending the PACA cancel message to the base station, the mobile returns to mobile station idle state. 6.2.4

Mobile Station Control on the Traffic Channel State

In this state, the mobile exchanges user information with the base station using forward traffic channels and reverse traffic channels. This state contains three substates: (1) traffic channel initialization substate, (2) traffic channel substate, and (3) release substate. The mobile nominally goes through these three substates sequentially, and it exits from release substate back to system determination substate of mobile station initialization state [4]. 6.2.4.1

Traffic Channel Initialization Substate

In this substate, the mobile checks that it can correctly receive the forward traffic channel and starts to transmit on the reverse traffic channel. Upon entering this substate, the mobile first initialize its variables. For example, the mobile sets its forward traffic channel power control variables (e.g., TOT_FRAMES, BAD_FRAMES, DCCH_TOT_FRAMES, DCCH_BAD_FRAMES, SCH_TOT_FRAMES, and SCH_BAD_FRAMES) to zero. While still monitoring all the pilots in the active set, the mobile tunes to the assigned CDMA channel and the assigned forward traffic code channel, as well as sets its forward and reverse traffic channel frame offsets to those that are assigned [4]. If the mobile can receive consecutive good frames for a period of 40 ms (2 × 20 ms) within one second after entering traffic channel initialization substate, then the mobile proceeds to transmit the traffic channel preamble, which is really a sequence of all-zero frames on the reverse traffic channel. The traffic channel preamble is used to help the base station acquire the reverse traffic channel. At the same time, if Layer 3 at the mobile receives a forward dedicated channel acquired indication from Layer 2 within two seconds after receiving those consecutive good frames, then Layer 3 enters the traffic channel substate. 6.2.4.2

Traffic Channel Substate

In this substate, the mobile actively exchanges user and/or signaling information with the base station. In doing so, the mobile exchanges user information on the traffic channel and processes dedicated signaling messages. It also performs

6.2

State Transitions: Call Processing

95

functions such as traffic channel supervision, pilot monitoring and reporting, power control, and handoff processing. Recall that in IS-95, the mobile can only transmit on one physical channel at a time. It transmits on the access channel to respond to messages or to access the system, then it transmits on the traffic channel when a call becomes active. But in IS-2000, the mobile can transmit on more than one physical channel at a time. It can transmit and receive on the fundamental channel during a voice call, and it can, at the same time, initiate a packet data call by using the dedicated control channel or supplemental channel. This means that IS-2000 can support multiple simultaneous sessions. Each one of these calls (which may occur simultaneously) is characterized by its service option connection, which is nothing more than a logical set of parameters that characterizes that particular call or session. Each service option connection is identified by a: •

•

•

•

Service option connection reference (CON_REF), which is a unique identifier for a specific service option connection. Service option (SERVICE_OPTION), which specifies the kind of service/application that is used by this service option connection. For example, the service option in use may be a voice service using enhanced variable rate coder (service option 3) or high-speed packet data service (service option 33). SERVICE_OPTION is a 16-bit parameter. For a complete list of service options, consult [5]. Forward traffic channel traffic type (FOR_TRAFFIC), which specifies the type of forward traffic channel traffic used to support the service option. This can be either primary traffic or secondary traffic. Reverse traffic channel traffic type (REV_TRAFFIC), which specifies the type of reverse traffic channel traffic used to support the service option. This can be either primary traffic or secondary traffic.

Supporting simultaneous sessions means that there can be more than one service option connection at any given time. If Layer 3 at the mobile receives a request to release a call, then one of two things can occur. First, if the corresponding service option connection is not the only one connected (e.g., there is another service option connection connected), then the mobile requests the release of this service option connection by sending a resource release request message, resource release request minimessage, or service request message. Second, if the corresponding service option connection is the only one connected, then the mobile enters the release substate [4]. 6.2.4.3

Release Substate

In this substate, the mobile releases the call and releases its occupation of the physical channel(s). It does so by sending a release order, extended release response message, or extended release response minimessage back to the base station. After releasing the call, the mobile nominally returns to system determination substate of mobile station initialization state.

96

6.3

Signaling: Upper Layers

Mode Transitions: Packet Data Transmission The state transitions described above are used to manage over-the-air resources in an IS-2000 system. In particular, it is in the traffic channel substate (of the mobile station control on the traffic channel state) when the system actively exchanges user data over the air. A distinguishing feature of IS-2000 is that it can exchange highrate packet data over the air. Packet data transmissions are bursty in nature with transmissions followed by quiet periods, and it is not desirable to hold on to overthe-air resources during quiet periods. IS-2000 accommodates intermittent packet data transmissions by using two modes while the mobile is in the traffic channel substate: active mode and control hold mode2 [4]. 6.3.1

Active Mode

In the active mode, there is an active exchange of user packet data and dedicated signaling data between the mobile and the base station. As such, the reverse pilot channel is not gated (i.e., PILOT_GATING_USE_RATE = 0). User data can only be exchanged in the active mode, and user data is exchanged using the supplemental channel and could be simultaneously exchanged using the dedicated control channel (or fundamental channel) with the supplemental channel [6]. In addition, dedicated signaling data is exchanged using the dedicated control channel. 6.3.2

Control Hold Mode

In the control hold mode, there is no exchange of user packet data between the mobile and the base station, and the reverse pilot channel may be gated to minimize interference on the air link (i.e., PILOT_GATING_USE_RATE = 1 and PILOT_GATING_RATE ≠ ‘00’). However, the dedicated control channel is active in this mode. Here, the system maintains MAC control and power control (through the dedicated control channel) and saves the parameters (e.g., Walsh code) related to this call. This is done to allow the next packet data (burst) transmission to begin immediately with no delay (due to power control stabilization and call setup) [7]. To further minimize interference, the dedicated control channel may operate in discontinuous (DTX) mode, which allows its transmissions to be gated [6]. 6.3.3

Dormant Mode

In addition to active and control hold modes, there is the dormant mode. In the dormant mode, there is no exchange of user packet data and no exchange of signaling data between the mobile and the base station. In other words, no over-the-air resources are assigned. However, information regarding the user’s packet data service registration and PPP connection is still kept [7]. Since no air link resources are assigned in this mode, the dormant mode by definition should coincide with the mobile station idle state. 2.

The packet option that uses both of these modes is known as packet option P2. Packet option P1 has no control hold mode [6].

6.4

Channel Setup

6.3.4

97

Transitions

The mobile enters the active mode if there is packet data to send. Signaling messages that effect the transition to the active mode include the extended supplemental channel assignment message, forward supplemental channel assignment minimessage, or reverse supplemental channel assignment minimessage. The message assigns a supplemental channel for packet data burst and moves the mobile into the active mode (see Section 6.4.4). The mobile may exit the active mode to the control hold mode through timeout, which triggers the necessary signaling (e.g., extended release message or extended release minimessage). There exists a timer setting the maximum time the mobile may be in the active mode after sending a packet data transmission [6]. If the timer expires, the mobile must give up its supplemental channel and transitions to the control hold mode. The mobile may exit from the control hold mode to the dormant mode also through timeout which triggers the associated signaling (e.g., extended release message, extended release minimessage, or release order). There exists a timer setting the maximum time the mobile may spend in the control hold mode (and keep the dedicated control channel) after first entering the control hold mode [6]. If the timer expires, the mobile must give up its dedicated control channel and transitions to the dormant mode. Figure 6.5 depicts the different modes and the transitions between them.

6.4

Channel Setup In this section we provide another perspective on call processing and packet data transmission by going through four different call setup examples. The first example is a standalone voice call that is initiated by the base station. The second example is a standalone voice call that is initiated by the mobile. The third example is a standalone packet data call that is initiated by the mobile. Since the base station rarely initiates a standalone packet data call by itself, the mobile-initiated example is the only standalone packet data call setup we describe. Finally, the fourth example deals with how the mobile and the base station request for supplemental channel resources during a packet data call.

Timeout Active mode

Timeout Control hold mode

There is data to send

There is data to send Figure 6.5

Active, control hold, and dormant modes and transitions.

Dormant mode

98

Signaling: Upper Layers

6.4.1

Example 1: Base Station-Originated Voice Call

When initiating a call that is terminated at the mobile, the base station first sends a general page message on the F-PCH or F-CCCH. See Figure 6.6. The general page message specifies the service option for voice call through the SERVICE_OPTION field (e.g., service option 3 for enhanced variable rate voice service). The mobile responds by sending a page response message on the R-ACH or R-EACH. Upon receiving the page response message, the base station sends the base station acknowledgment order on the F-PCH or F-CCCH. At the same time, the base station sets up the traffic channel and starts to send null traffic channel data on the F-FCH. Then the base station sends a channel assignment message or extended channel assignment message on F-PCH or F-CCCH. The mobile also sets up the traffic channel using the information it receives in the channel assignment message or extended channel assignment message. It first verifies that it can receive N5m consecutive good frames, then it begins transmitting the traffic channel preamble and null traffic channel data on the R-PICH. After

Base station

General page message

Mobile station

F-PCH or F-CCCH Page response message R-ACH or R-EACH Base station acknowledgment order F -PCH or F -CCCH Set up traffic channel

Null traffic channel data F-FCH

Channel assignment message or extended channel assignment message F-PCH or F-CCCH Set up traffic channel Receive N5m consecutive good frames Traffic channel preamble and null traffic channel data R-PICH Acquire reverse traffic channel Base station acknowledgment order F-FCH Service connect message F-FCH Service connect completion message R-FCH User voice data F-FCH and R-FCH

Figure 6.6

Example 1: Setup of voice call initiated by the base station.

6.4

Channel Setup

99

receiving the traffic channel preamble, the base station sends the base station acknowledgment order on the F-FCH. The service parameters are finalized when the base station transmits the service connect message on the F-FCH. The mobile responds with a service connect completion message on the R-FCH, after which the mobile and the base station begin to actively exchange user voice data over the air link. The user voice data may include voice traffic, as well as in-traffic messages sent over the F-FCH and R-FCH such as alert with information message (i.e., ring) and connect order [4]. 6.4.2

Example 2: Mobile Station-Originated Voice Call

As shown in Figure 6.7, after the user presses the SEND key the mobile transmits the origination message on the R-ACH or R-EACH. The origination message specifies the service option for voice call through the field SERVICE_OPTION (e.g., service option 3 for enhanced variable rate voice service). The base station responds by sending a base station acknowledgment order on the F-PCH or F-CCCH. At the same time, the base station sets up the traffic channel and starts to send null traffic Base station

Mobile station

Origination message F-ACH or R-EACH Base station acknowledgment order F-PCH or F-CCCH

Set up traffic channel

Null traffic channel data F-FCH

Channel assignment message or extended channel assignment message F-PCH or F-CCCH Set up traffic channel Receive N5m consecutive good frames Traffic channel preamble and null traffic channel data R-PICH Acquire reverse traffic channel Base station acknowledgment order F-FCH Service connect message F-FCH Service connect completion message R -FCH User voice data F-FCH and R-FCH

Figure 6.7

Example 2: Setup of voice call initiated by the mobile station.

100

Signaling: Upper Layers

channel data on the F-FCH. Then the base station sends a channel assignment message or extended channel assignment message on F-PCH or F-CCCH. The mobile also sets up the traffic channel using the information it receives in the channel assignment message or extended channel assignment message. It first verifies that it can receive N5m consecutive good frames, then it begins transmitting the traffic channel preamble and null traffic channel data on the R-PICH. After receiving the traffic channel preamble, the base station sends the base station acknowledgment order on the F-FCH. The service parameters are finalized when the base station transmits the service connect message on the F-FCH. The mobile then responds with a service connect completion message on the R-FCH, after which the mobile and the base station begin to actively exchange user voice data over the air link [4]. 6.4.3

Example 3: Mobile Station-Originated Packet Data Call

The setup of a packet data call is similar to that of the voice call. See Figure 6.8. The mobile first transmits an origination message on the R-ACH or R-EACH. The origination message specifies the service option for a high-speed packet data call (e.g., service option 33); unlike the origination message for a voice call, the origination message for a packet data call contains no dialed digits. The base station responds by Base station

Origination message

Mobile station

F-ACH or R-EACH Base station acknowledgment order F-PCH or F-CCCH Set up PDSN resources

Null traffic channel data F-FCH or F-DCCH

Channel assignment message or extended channel assignment message F-PCH or F-CCCH Set up traffic channel Traffic channel preamble and null traffic channel data R-PICH Acquire reverse traffic channel Base station acknowledgment order F-FCH or F-DCCH Service connect message F-FCH or F-DCCH Service connect completion message F-FCH or F-DCCH User packet data F-FCH and R-FCH, or F-DCCH and R-DCCH

A

Figure 6.8

Example 3: Packet data call setup.

6.4

Channel Setup

101

sending a base station acknowledgment order on the F-PCH or F-CCCH. At the same time, the base station initiates the setup of packet data service node (PDSN) resources and starts to send null traffic channel data on the F-FCH or F-DCCH. The base station then sends a channel assignment message or extended channel assignment message on F-PCH or F-CCCH. The mobile sets up the traffic channel using the information it receives in the channel assignment message or extended channel assignment message. It begins transmitting the traffic channel preamble and null traffic channel data on the R-PICH. After receiving the traffic channel preamble, the base station sends the base station acknowledgment order on the F-FCH or F-DCCH. The service parameters are finalized when the base station transmits the service connect message on the F-FCH or F-DCCH. The mobile responds with a service connect completion message on the R-FCH or R-DCCH, after which the mobile and the base station begin to exchange user packet data traffic on the forward and reverse fundamental channels or dedicated control channels. 6.4.4

Example 4: Supplemental Channel Request During a Packet Data Call

If the base station and mobile need to exchange bursts of high-rate packet data during a packet data call, either the base station or the mobile may initiate the request for supplemental channel resources. The following two sections describe each of these two cases: base station-initiated request and mobile station-initiated request. 6.4.4.1

Base Station-Initiated Request of Supplemental Channels

If the base station wishes to send a burst of high-rate packet data during a packet data call, it may request a temporary use of the supplemental channel. The base station sets up supplemental channel resources by sending an extended supplemental channel assignment message (ESCAM) on the F-FCH or F-DCCH. The ESCAM is a rather large message that, among other things, updates the supplemental channel code list (SCCL). The SCCL specifies a list of F-SCH configurations that can be used. The ESCAM may also include forward and reverse supplemental channel assignments that are to be immediately used. For the R-SCH, the ESCAM may specify fields such as: • •

•

•

REV_SCH_ID (reverse supplemental channel identifier); REV_SCH_START_TIME (start time of the assignment of the reverse supplemental channel); REV_SCH_DURATION (duration of the assignment of the reverse supplemental channel); REV_SCH_NUM_BITS_IDX (index of the reverse supplemental channel number of bits per frame).

These tell the mobile that it may transmit supplemental channel REV_SCH_ID starting at time REV_SCH_START_TIME for a period of REV_SCH_DURATION using the configuration specified by REV_SCH_NUM_BITS_IDX.

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Signaling: Upper Layers

For the F-SCH, the ESCAM may specify fields such as: • •

•

•

FOR_SCH_ID (forward supplemental channel identifier); FOR_SCH_START_TIME (start time of the assignment of the forward supplemental channel); FOR_SCH_DURATION (duration of the assignment of the forward supplemental channel); SCCL_INDEX (index of the supplemental channel code list).

These tell the mobile that it may receive supplemental channel FOR_SCH_ID starting at time FOR_SCH_START_TIME for a period of FOR_SCH_DURATION using the configuration specified by SCCL_INDEX. After the successful reception of ESCAM, the subsequent assignments of the F-SCH and R-SCH can be done by the use of forward supplemental channel assignment mini message (FSCAMM) and reverse supplemental channel assignment mini message (RSCAMM). These messages are very short, each containing only four fields totaling 14 bits. For assigning the R-SCH, the RSCAMM contains only those four R-SCH assignment fields mentioned above (i.e., REV_SCH_ID, REV_SCH_START_TIME, REV_SCH_DURATION, and REV_SCH_NUM_ BITS_IDX). For assigning the F-SCH, the FSCAMM contains only those four F-SCH assignment fields mentioned above (i.e., FOR_SCH_ID, FOR_SCH_START_TIME, FOR_SCH_DURATION, and SCCL_INDEX). These short minimessages enable a very quick allocation of supplemental channels. In fact, the base station and the mobile do not even have to send “release orders” for supplemental channels because their assignments only last FOR_SCH_DURATION and REV_SCH_DURATION. Hence using minimessages quickens supplemental channel assignments and conserves air link resources. Figure 6.9 shows an example of the assignment of supplemental channels that is first initiated by the base station. The base station first sends an ESCAM to the

A

Base station

Mobile station Extended supplemental channel assignment message F-FCH or F-DCCH High-rate user packet data F-SCH and/or R-SCH

Forward supplemental channel assignment minimessage and/or Reverse supplemental channel assignment minimessage F-FCH or F-DCCH High-rate user packet data F-SCH and/or R-SCH

Figure 6.9

Supplemental channel assignment initiated by the base station.

6.4

Channel Setup

103

mobile. The ESCAM updates the SCCL that specifies a list of F-SCH configurations that can be used. The mobile later can quickly refer to which F-SCH configuration to use by referencing the SCCL_INDEX. In the same ESCAM, the base station may elect to include those F-SCH assignment fields (e.g., FOR_SCH_START_TIME) and R-SCH assignment fields (e.g., REV_SCH_START_TIME) that are to be used immediately for an exchange of high-rate packet data. If the ESCAM contains assignment fields that are to be immediately used, then the mobile uses those parameters to exchange bursts of high-rate packet data with the base station on the F-SCH and R-SCH. After this initial exchange, there may be a quiet period when there is no high-rate data exchange. During this time, the physical supplemental channel resources are released and assigned to other users in the same carrier. However, if after a period of time the base station elects to transmit another burst of high-rate packet data, it sends the FSCAMM and/or RSCAMM to quickly assign forward and/or reverse supplemental channels. Then the base station and the mobile may again exchange high-rate packet data on the assigned supplemental channels. 6.4.4.2

Mobile Station-Initiated Request of Supplemental Channels

Figure 6.10 shows an example of mobile-initiated request for supplemental channels. If during a packet data call the mobile wishes to set up supplemental channel resources for the first time (i.e., it has not received an ESCAM), then the mobile first

A

Base station

Mobile station Supplemental channel request minimessage R-FCH or R-DCCH Extended supplemental channel assignment message F-FCH or F-DCCH High-rate user packet data F-SCH and/or R-SCH

Supplemental channel request minimessage R-FCH or R-DCCH Forward supplemental channel assignment minimessage and/or Reverse supplemental channel assignment minimessage F-FCH or F-DCCH High-rate user packet data F-SCH and/or R-SCH

Figure 6.10

Supplemental channel assignment initiated by the mobile station.

104

Signaling: Upper Layers

transmits a supplemental channel request mini message (SCRMM). This minimessage contains only one field that is 16-bits long. The field is REQ_BLOB or the reverse supplemental channel request block of bytes. REQ_BLOB contains information specifying the characteristics of the reverse supplemental channel requests [4]. The base station responds by transmitting an ESCAM. The ESCAM updates the SCCL that contains a list of F-SCH configurations that can be used. The mobile can later quickly identify a specific F-SCH configuration to use by referencing the SCCL_INDEX. If the ESCAM contains the F-SCH assignment information (e.g., FOR_SCH_START_TIME) and/or R-SCH assignment information (e.g., REV_SCH_START_TIME) that is to be immediately used, then the mobile and the base station starts exchanging high-rate packet data for a period of FOR_SCH_DURATION and/or REV_SCH_DURATION. The mobile’s subsequent requests for supplemental channels can be accomplished by sending another SCRMM. The base station responds by transmitting the FSCAMM and/or RSCAMM. The FSCAMM and RSCAMM contain assignment information for F-SCH and R-SCH, and the mobile can use this information to again exchange high-rate packet data for a given period. 6.4.5

Concluding Remarks

Although the above examples describe voice and packet data calls that are standalone, the IS-2000 standard supports simultaneous voice and packet data calls. For example, if during a voice call the mobile wishes to initiate a packet data call, the mobile may send an enhanced origination message (typically as an in-traffic message on the R-FCH) to request a simultaneous packet data call. After the base station responds with a service connect message, a separate packet data call becomes active. In addition, the IS-2000 standard also supports the power control of supplemental channels that is independent of fundamental channels. When the base station initially transmits the (large) ESCAM, the ESCAM also contains power control fields that are used to power control supplemental channels. These fields include FPC_SCH_FER (the target frame error rate of supplemental channel), FPC_SCH_MIN_SETPT (the minimum outer loop Eb/N0 set point of supplement channel), and FPC_SCH_MAX_SETPT (the maximum outer loop Eb/N0 set point of supplemental channel) [4].

References [1] [2] [3] [4] [5]

TIA/EIA/IS-2000.1-A, Introduction to cdma2000 Standards for Spread Spectrum Systems, Telecommunications Industry Association, March 2000. Yang, S. C., CDMA RF System Engineering, Norwood, MA: Artech House, 1998. Garg, V. K., K. Smolik, and J. E. Wilkes, Applications of CDMA in Wireless/Personal Communications, Upper Saddle River, NJ: Prentice Hall, 1997. TIA/EIA/IS-2000.5-A, Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems, Telecommunications Industry Association, March 2000. TIA/EIA/TSB-58, Administration of Parameter Value Assignments for cdma2000 Spread Spectrum Standards, Telecommunications Industry Association, January 2002.

6.4

Channel Setup [6] [7]

105

Yoon, S. Y., and S. W. Lee, “Forward Link Performance of Medium Access Control for IS-2000,” Proceedings of Vehicular Technology Conference, 2001, pp. 2759–2762. Knisely, D. N., et al., “Evolution of Wireless Data Services: IS-95 to cdma2000,” IEEE Communications, October 1998, pp. 140–149.

.

CHAPTER 7

Power Control 7.1

Introduction The IS-2000 standard has added a host of new features to the function of power control, all aimed at improving its speed and accuracy, which have direct consequences on capacity and quality. In terms of capacity, fast and accurate power control minimizes the variances of powers received from multiple transmitters, and minimizing the variances of received powers in the same band allows more users to be on the system. In terms of quality, fast and accurate power control ensures that each user has its share of power resources that maintains adequate link quality. These important benefits warrant a separate chapter on power control. In this chapter, we focus on those power control topics that are unique to IS-2000. The IS-95 power control had two limitations. First, the system could only power control one physical channel (i.e., traffic channel on the forward link and traffic channel on the reverse link). Second, power control rate was asymmetric between forward and reverse directions. While it could power control (closed loop) reverse traffic channel at a rate of 800-times-per-second, it could only power control (closed loop) forward traffic channel at a peak rate of 50-times-per-second (for Rate Set 2). The IS-2000 standard improves upon these two limitations. First, the system can power control multiple physical channels on the forward and reverse links. Second, the system can now power control (closed loop) both the forward link and the reverse link at 800-times-per-second.

7.2

Power Control of the Forward Link Since IS-2000 can now also power control the forward link at 800-times-persecond, it is no surprise that power control of the forward link is approximately a mirror image of the IS-95 reverse link. Note that since Radio Configurations 1 and 2 in IS-2000 correspond to Rate Sets 1 and 2 in IS-95, forward link power control at a maximum rate of 800-times-per-second is only available for Radio Configurations 3 through 6. 7.2.1

Inner Loop and Outer Loop

Once the mobile starts to receive the forward traffic channel and begins to communicate with the base station, the mobile continuously monitors the forward link and

107

108

Power Control

measures its link quality. If the link quality starts to get worse, then the mobile will request the base station, via the reverse link, to power up. If the link quality becomes too good, then there is excess signal power on the forward link. In this case, the mobile will request the base station to power down. Ideally, FER is a good indication of link quality, but it takes a long time for the mobile to accumulate enough bits to calculate FER. Thus some signal-to-noise ratio measure (e.g., Eb/N0 that can be quickly calculated) can be used as an indicator of forward link quality [1, 2]. The closed-loop power control of the forward link is done as follows: 1. The mobile continuously monitors Eb/N0 on the forward link. 2. If Eb/N0 is too high (i.e., if it exceeds a certain threshold), then the mobile requests the base station to decrease the base station transmit power. 3. If Eb/N0 is too low (i.e., if it drops below a certain threshold), then the mobile requests the base station to increase the base station transmit power. The mobile requests the base station to power up and down using power control bits (PCBs) that are multiplexed onto the R-PICH. Hence these PCBs are not error protected. Recall that the reason why PCBs are not error protected is to reduce delays that are inherent in decoding error protected bits. Since PCBs are used to combat fast Rayleigh fading, PCBs are not error protected so that the base station can quickly recover them and adjust its transmit power accordingly. The above describes the inner loop of the closed-loop power control of the forward link. The assumption of the inner loop is that there exists a predetermined Eb/N0 threshold by which power-up and power-down decisions are made. But because in a mobile environment the relationship between Eb/N0 and FER is constantly changing, the Eb/N0 threshold has to be dynamically adjusted to maintain an acceptable FER. This dynamic adjustment of the Eb/N0 threshold (i.e., Eb/N0 setpoint) is referred to as the outer loop of the closed-loop power control [3]. Figures 7.1 and 7.2 show a conceptual illustration of the closed-loop forward link power control. Figure 7.1 depicts those functions carried out by the mobile. Here the mobile implements the entire outer loop and part of the inner loop. The mobile receives the forward link from the base station. The mobile first demodulates the signal and estimates the FER of the forward link. This information on the forward link quality is fed into the outer loop. The outer loop, using current FER and Eb/N0 estimates, dynamically computes the new Eb/N0 setpoint required to maintain an acceptable FER. The new Eb/N0 setpoint and the current Eb/N0 estimate are then compared. If the estimate is greater than the setpoint, then the link Eb/N0 is higher than what is necessary to maintain a good FER; a PCB of 1 is thus sent to the base station to request power down. If the estimate is less than the setpoint, then the link Eb/N0 is lower than what is necessary; a PCB of 0 is then sent to request the base station to power up. The PCBs are multiplexed onto the reverse pilot channel and transmitted to the base station at a maximum rate of 800-times-per-second. Figure 7.2 shows the corresponding illustration of the closed-loop forward link power control as carried out by the base station. Here the base station has the rest of the inner loop. The base station receives the reverse pilot channel signal. It recovers the PCB and, based on the PCB, makes a decision to power up or to power down [3].

7.2

Power Control of the Forward Link

109

Rx Antenna

Tx Antenna

Mobile station R-PICH

Transmitter

PCB Eb/N0 estimate Eb/N0 estimate RCVR

OUTER LOOP

Eb/N0 estimate

Demod

FER estimate

Adjust Eb/N0 setpoint

Forward link power control functions carried out by the mobile.

Tx Antenna Transmitter

Power control decisions

Figure 7.2

If estimate< setpoint then PCB=0

Eb/N0 setpoint

FER estimate

Figure 7.1

Decision: If estimate> setpoint then PCB=1

Rx Antenna

Base station Forward traffic channel

Decision: If PCB=0 then power down If PCB=1 then power up

PCB

Demod

RCVR

Forward link power control functions carried out by the base station.

Note that in Figure 7.1, the outer loop(s) can monitor all forward traffic channels that the mobile is receiving (i.e., forward fundamental channel, forward dedicated control channel, and forward supplemental channel). If the outer loop is monitoring the F-FCH, the outer loop uses as its inputs current Eb/N0 and FER estimates of the F-FCH. In addition, the outer loop also takes into account the following parameters: •

FPC_FCH_FER or the target frame error rate of the F-FCH;

•

FPC_FCH_INIT_SETPT or the initial setpoint of the F-FCH;

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Power Control

• •

FPC_FCH_MIN_SETPT or the minimum setpoint of the F-FCH; FPC_FCH_MAX_SETPT or the maximum setpoint of the F-FCH.

Obviously, the new Eb/N0 setpoint computed by the outer loop has to be bound by FPC_FCH_MIN_SETPT and FPC_FCH_MAX_SETPT. These parameters are part of the nonnegotiable service configuration parameters to be used by the mobile. They are sent in messages like service connect message, general handoff direction message, and universal handoff direction message. They can also be sent to the mobile in extended channel assignment message and power control message. If the outer loop is monitoring the F-DCCH, in addition to using as its inputs current Eb/N0 and FER estimates of the F-DCCH the outer loop also takes into account the following: • • • •

FPC_DCCH_FER or the target frame error rate of the F-DCCH; FPC_DCCH_INIT_SETPT or the initial setpoint of the F-DCCH; FPC_DCCH_MIN_SETPT or the minimum setpoint of the F-DCCH; FPC_DCCH_MAX_SETPT or the maximum setpoint of the F-DCCH.

The new Eb/N0 setpoint of the F-DCCH computed by the outer loop has to be bound by FPC_DCCH_MIN_SETPT and FPC_DCCH_MAX_SETPT. These parameters are part of the nonnegotiable service configuration parameters to be used by the mobile and are sent in messages like service connect message, general handoff direction message, and universal handoff direction message. They can also be sent to the mobile in extended channel assignment message and power control message. If the outer loop is monitoring the F-SCH, in addition to using as its inputs current Eb/N0 and FER estimates of the F-SCH the outer loop also considers the following parameters1: • • • •

FPC_SCH_FER or the target frame error rate of the F-SCH; FPC_SCH_INIT_SETPT or the initial setpoint of the F-SCH; FPC_SCH_MIN_SETPT or the minimum setpoint of the F-SCH; FPC_SCH_MAX_SETPT or the maximum setpoint of the F-SCH.

The new Eb/N0 setpoint of the F-SCH calculated by the outer loop has to be bound by FPC_SCH_MIN_SETPT and FPC_SCH_MAX_SETPT. These parameters are sent in extended supplemental channel assignment message and power control message2 [4]. 7.2.2

Power Control of Multiple Forward Traffic Channels

PCBs are carried by the R-PICH using the power control subchannel. The power control subchannel is a structure provided by the R-PICH (or another host physical channel) by which PCBs can be transported (see Chapter 3). In addition, the power control subchannel is structured in such as way so that more than one forward 1. 2.

These parameters can be different for the different forward supplemental channels. Power control message does not contain FPC_SCH_INIT_SETPT.

7.2

Power Control of the Forward Link

111

traffic channels can be power controlled. IS-2000 uses the parameter forward power control operating mode (FPC_MODE) to specify how multiple forward traffic channels are power controlled and the rate of power control feedback. Table 7.1 shows the seven configurations specified by the seven forward power control operating modes when the R-PICH operates in nongated mode. When the R-PICH is not gated, its power control subchannel can be further divided into two streams (i.e., primary power control subchannel and secondary power control subchannel). For example, if FPC_MODE = 000 then the power control subchannel is not divided, and all 16 power control groups are used for a single power control subchannel. Here the rate of power control feedback is 800-times-per-second. However, if FPC_MODE = 001 then the power control subchannel is divided into two subchannels; eight of the 16 power control groups are used for the primary subchannel, and the other eight of the 16 power control groups are used for the secondary subchannel. In this case, the rate of power control feedback is 400-timesper-second for the primary subchannel and 400-times-per-second for the secondary subchannel. If FPC_MODE = 010 then again the power control subchannel is divided into two subchannels. But here only four of the 16 power control groups are used for the primary subchannel, while the other 12 power control groups are used for the secondary subchannel. This division yields a rate of power control feedback of 200times-per-second for the primary subchannel and 600-times-per-second for the secondary subchannel. Table 7.1

Forward Power Control Operating Modes

FPC_ MODE

Primary PC Subchannel on R-PICH

Secondary PC Subchannel on R-PICH

000

PCB @ 800 Hz

001 010

Outer Loop

Primary Inner Loop

Secondary Inner Loop

Not used

F-DCCH; F-FCH; F-SCH

F-FCH or F-DCCH

Not applicable

PCB @ 400 Hz

PCB @ 400 Hz

F-DCCH; F-FCH; F-SCH

F-FCH or F-DCCH

F-SCH

PCB @ 200 Hz

PCB @ 600 Hz

F-DCCH; F-FCH; F-SCH

F-FCH or F-DCCH

F-SCH

110

PCB @ 400 Hz

EIB @ 50 Hz for F-SCH (20 ms); EIB @ 25 Hz for F-DCCH; F-SCH (40 ms); F-FCH EIB @ 12.5 Hz for F-SCH (80 ms)

F-FCH or F-DCCH

Not applicable

011

EIB @ 50 Hz for F-FCH or F-DCCH

Not used

Not applicable

Not applicable

Not applicable

100

QIB @ 50 Hz for F-FCH or Not used F-DCCH

Not applicable

Not applicable

Not applicable

101

EIB @ 50 Hz for F-SCH (20 ms); QIB @ 50 Hz for EIB @ 25 Hz for Not applicable F-FCH or F-SCH (40 ms); F-DCCH EIB @ 12.5 Hz for F-SCH (80 ms)

Not applicable

Not applicable

112

Power Control

Furthermore, the primary and secondary power control subchannels can be used to transmit erasure indicator bits (EIBs) or quality indicator bits (QIBs). In Radio Configurations 3 through 6, the mobile uses the EIB to tell the base station that a received F-FCH or F-DCCH frame is bad. Recall that in IS-95 Rate Set 2 (or IS-2000 Radio Configuration 2), the mobile uses an EIB to tell the base station that a received forward traffic channel (or F-FCH) frame is bad. On the other hand, the mobile uses a QIB to tell the base station that a received F-DCCH frame is bad. An EIB or a QIB is transported by combining different power control groups on the power control subchannel. For example, when FPC_MODE = 011 the system combines all 16 power control groups in a 20-ms period to yield one single EIB, resulting in an EIB feedback rate of 1/20 ms or 50-times-per-second. If FPC_MODE = 000 then the mobile’s outer loop estimates the Eb/N0 setpoint for all forward traffic channels (i.e., F-DCCH, F-FCH, and F-SCH). The mobile transmits PCBs at 800 bps on the primary (and only) power control subchannel on the R-PICH. These PCBs are used to perform inner-loop power control for either the F-FCH or the F-DCCH. If FPC_MODE = 001 then the mobile’s outer loop estimates the Eb/N0 setpoint for all forward traffic channels (i.e., F-DCCH, F-FCH, and F-SCH). The mobile transmits a stream of PCBs at 400 bps on the primary power control subchannel. This stream of PCBs is used to perform inner-loop power control for either the F-FCH or the F-DCCH. In addition, the mobile also transmits a separate stream of PCBs at 400 bps on the secondary power control subchannel. These PCBs are used to perform inner-loop power control for the F-SCH. The case of FPC_MODE = 010 is similar to that of FPC_MODE = 001, except here the PCBs are transmitted at 200 bps on the primary subchannel and at 600 bps on the secondary subchannel. If FPC_MODE = 110 then the mobile’s outer loop estimates the Eb/N0 setpoint for the F-DCCH and F-FCH. The mobile transmits a stream of PCBs at 400 bps on the primary power control subchannel. This stream of PCBs can be used to perform inner-loop power control for either the F-FCH or the F-DCCH. On the secondary power control subchannel, the mobile sends EIBs to indicate to the base station the quality of received F-SCH frames. Since one EIB is sent for every received F-SCH frame, the rate of EIB feedback is 1/20 ms or 50 bps for 20-ms F-SCH frames, 1/40 or 25 bps for 40-ms SCH frames, and 1/80 ms or 12.5 bps for 80-ms F-SCH frames. The base station uses these EIBs to adjust its transmit power for the F-SCH. If FPC_MODE = 011 then the mobile does not perform any outer loop estimates of the Eb/N0 setpoint. The mobile only transmits EIBs at 50 bps. These EIBs indicate to the base station the quality of the received frames on the F-FCH or the F-DCCH. The case of FPC_MODE = 100 is similar to that of FPC_MODE = 011, except here the QIBs are transmitted instead of the EIBs. Lastly, if FPC_MODE = 101 then the mobile does not perform any outer loop estimates of the Eb/N0 setpoint. On the primary power control subchannel, the mobile transmits QIBs at 50 bps. These QIBs indicate to the base station the quality of the received frames on the F-FCH or the F-DCCH. On the secondary power control subchannel, the mobile sends EIBs to indicate to the base station the quality of received F-SCH frames. The EIBs are sent at 50 bps for 20-ms F-SCH frames, 25 bps

7.3

Power Control of the Reverse Link: Open Loop

113

for 40-ms SCH frames, and 12.5 bps for 80-ms F-SCH frames. The base station uses these EIBs to adjust its transmit power for the F-SCH [3]. In closing, why would a system engineer choose a slower power control feedback rate (e.g., 50 Hz) using EIBs or QIBs when a faster power control feedback rate (e.g., 800 Hz) is available using PCBs? One advantage of using PCBs in outer-loop and inner-loop power controls is obviously the faster feedback rate. But remember that PCBs are not error protected, thus it is possible that many PCBs may be received in error and hence degrade forward power control performance. On the other hand, as many as 16 PCBs may be combined to yield one EIB or QIB. Therefore, one advantage of using EIBs and QIBs is that they have lower error rates (all things being equal). The decision between these two choices reflects a classic tradeoff between data transmission rate (feedback rate) and bit-error probability (feedback accuracy).

7.3

Power Control of the Reverse Link: Open Loop Reverse link power control for IS-2000 Radio Configurations 1 and 2 is identical to that of IS-95 Rate Sets 1 and 2. Readers can refer to references such as [5–7] for its description. For other radio configurations, the IS-2000 power control of the reverse link is conceptually similar to the one in IS-95, with some enhancements. One significant difference in IS-2000 is that the R-PICH now has a prominent role in reverse link power control. We will focus our discussion on those reverse radio configurations (Radio Configurations 3 and 4) that support Spreading Rate 1. The discussions are readily generalizable to the Spreading Rate 3 radio configurations (Radio Configurations 5 and 6). Section 7.3 discusses the open-loop part of the reverse link power control. Section 7.4 discusses the closed-loop part. 7.3.1

Power Control of Multiple Reverse Channels

When a mobile moves around in a cell, the path loss between the mobile and the base station continues to change, hence the received power at the mobile also continues to change. In open-loop power control, the mobile monitors its received power continuously and adjusts its transmit power accordingly. Note that openloop power control is done solely by the mobile and does not involve the base station at all. In IS-2000, the mobile can perform open-loop power control on three types of reverse physical channels: • • •

R-EACH; R-CCCH; Reverse traffic channel, including R-DCCH, R-FCH, and R-SCH.

For each of these channels, the open-loop power control is done in two separate parts. In the first part, the mobile calculates the pilot channel transmit power of the R-PICH, which is almost always active. In the second part, the mobile calculates the code channel transmit power of the reverse channels themselves. See Table 7.2.

114

Power Control

Table 7.2

Open-Loop Power Control of Reverse Physical Channels Enhanced Access Channel

Reverse Common Control Channel

Reverse Traffic Channel

Reverse Pilot Channel Transmit Power

Calculate pt of preamble

Calculate pt of preamble

Calculate pt of reverse pilot

Reverse Code Channel Transmit Power

Calculate pt of R-EACH header and R-EACH data

Calculate pt of R-CCCH data

Calculate pt of reverse traffic channel

7.3.1.1

Enhanced Access Channel (R-EACH)

Recall from Chapter 3 that an R-EACH transmission consists of two parts: (1) preamble, and (2) R-EACH header and/or R-EACH data. The preamble is nothing more than a reverse pilot channel transmission. Therefore during the transmission of R-EACH preamble, the mobile (in performing open-loop power control of the R-PICH) calculates the transmit power of the R-PICH. Here the mobile calculates its R-PICH transmit power based on the following general equation (in decibels): pt,R − RICH = − pr + ( sum of correction factors)

(7.1)

where pt,R-PICH is the open-loop estimate of the transmit power, and pr is the received power at the mobile. IS-2000 has added some new correction factors that are used in calculating the mobile transmit power. The description of these correction factors can be found in Section 2.1.2.3.1 of [3] and is not repeated here. The R-PICH is also active during the transmission of R-EACH header and data. The transmit power of R-PICH during R-EACH header and data transmission is (in decibels) pt,R − PICH = − pr + ( sum of correction factors) + ( sumof all closed loop power control corrections)

(7.2)

The correction factors used here differ slightly from those in (7.1). If the system performs closed-loop power control on R-EACH, then the transmit power of R-PICH also includes the sum of all closed-loop power control corrections. See Section 7.4 for more details on closed-loop power control. As the next paragraph will describe, by controlling the transmit power of R-PICH during R-EACH header and data transmission, the mobile also controls the transmit power of R-EACH header and data transmission itself. The R-EACH header and R-EACH data frames contain data (for signaling). Therefore, during header and data frame transmission, the transmit power of the R-EACH code channel itself needs to be calculated. The transmit power of the R-EACH code channel during header and data frame transmission is (in decibels) pt,R − EACH = − pt,R-PICH + ( sum of gain factors)

(7.3)

where pt,R-PICH is the transmit power of R-PICH during R-EACH header and data frame transmission, shown in (7.2). The gain factors are those factors used to adjust

7.3

Power Control of the Reverse Link: Open Loop

115

the transmit power of the R-EACH code channel based on a variety of parameters, such as data rate and frame length. The description of these gain factors can be found in Section 2.1.2.3.3 of [3] and is not repeated here. As one can see, the transmit power of the R-EACH code channel is clearly dependent on the current transmit power of the R-PICH. In other words, the transmit power of the R-EACH code channel references that of the R-PICH; as the R-PICH transmit power changes, so does the R-EACH transmit power. 7.3.1.2

Reverse Common Control Channel (R-CCCH)

An R-CCCH transmission also consists of two parts: (1) preamble, and (2) R-CCCH data. The R-CCCH preamble is also a reverse pilot channel transmission (see Chapter 3). Therefore, the mobile performs open-loop power control of the R-PICH during the transmission of R-CCCH preamble. Here the mobile calculates its R-PICH transmit power based on the following general equation (in decibels): pt,R − PICH = − pr + ( sum of correction factors)

(7.4)

where pt,R-PICH is the open-loop estimate of the transmit power, and pr is the received power at the mobile. The correction factors here have some that are used specifically for the transmission of R-CCCH preamble. The description of these correction factors can be found in Section 2.1.2.3.1 of [3] and is not repeated here. The R-PICH is also active during the transmission of R-CCCH header and data. The transmit power of R-PICH during R-CCCH header and data transmission is (in decibels) pt,R − PICH = − pr + ( sum of correction factors) + ( sumof all closed loop power control corrections)

(7.5)

The description of the correction factors used here can be found in Section 2.1.2.3.1 of [3] and is not repeated here. If the system performs closed-loop power control on R-CCCH, then the transmit power of R-PICH also includes the sum of all closed-loop power control corrections. See Section 7.4 for more details on closed-loop power control. By power controlling the transmit power of R-PICH during R-CCCH header and data transmission, the mobile also power controls the transmit power of R-CCCH header and data transmission itself. The R-CCCH data frames also contain signaling data. Therefore, during data frame transmission, the transmit power of the R-CCCH code channel itself needs to be calculated. The transmit power of the R-CCCH code channel during data frame transmission is (in decibels) pt,R −CCCH = pt,R-PICH + ( sum of gain factors)

(7.6)

where pt,R-PICH is the transmit power of R-PICH during R-CCCH data frame transmission, shown in (7.5). The gain factors here are identical to those used in the R-EACH [i.e., in (7.3)]. Again, the transmit power of the R-CCCH code channel is dependent on the current transmit power of the R-PICH.

116

Power Control

7.3.1.3

Reverse Traffic Channel

During the transmission of reverse traffic channels (i.e., R-DCCH, R-FCH, and R-SCH), the R-PICH is also active to aid their coherent demodulation. Thus the mobile power controls the R-PICH during the transmission of reverse traffic channels. Here the mobile calculates its R-PICH transmit power based on the following general equation (in decibels): pt,R − PICH = − pr + ( sum of correction factors) + ( sumof all closed loop power control corrections)

(7.7)

The correction factors here are different from those used in either the R-EACH or the R-CCCH. The description of these correction factors can be found in Section 2.1.2.3.1 of [3]. In addition to controlling the transmit power of the R-PICH, the mobile also needs to power control the reverse traffic channels themselves. Since the reverse traffic channels carry user data, the transmit power of the reverse traffic channel code channel itself needs to be calculated. The transmit power of the reverse traffic channel is (in decibels) pt,reverse traffic channel = pt,R-PICH + ( sum of gain factors)

(7.8)

where pt,R-PICH is the transmit power of R-PICH during the transmission of reverse traffic channel, shown in (7.7). The gain factors here are all specific to reverse traffic channels and are different from those used in R-EACH and R-CCCH. The description of these gain factors can be found in Section 2.1.2.3.3 of [3] and is not repeated here. As one can see, the transmit power of the reverse traffic channel references that of the R-PICH; as the R-PICH transmit power changes, so does the reverse traffic channel transmit power. 7.3.2

Summary

Figure 7.3 summarizes the power control discussions thus far. The received power, together with the various correction factors, plays a part in determining the transmit power of the R-PICH. The transmit power of the R-PICH in turn determines the transmit power of R-EACH, R-CCCH, and reverse traffic channel. The closed-loop power control corrections are first applied to the R-PICH and subsequently manifested in the R-EACH, R-CCCH, and reverse traffic channel. By using the received power as a decision metric, the open-loop power control is based on an estimate of the forward path loss. This power control thus compensates for slow-varying and log-normal shadowing effects where there is a correlation between the forward link and reverse link fades. However, since forward and reverse links are at different frequencies, the open-loop power control is too slow to compensate for fast Rayleigh fading (recall Rayleigh fading depends on the frequency). Therefore, closed-loop power control is used in addition to open-loop to combat Rayleigh fading.

7.4

Power Control of the Reverse Link: Closed Loop

Rx Antenna

117

Tx Antenna

Mobile station pt,rev traffic ch Reverse to be used traffic channel

GFs Σ GFs Σ GFs

pt,R-CCCH to be used

R-CCCH Σ

pt,R-EACH to be used

R-EACH

pt,R-PICH

Closed loop corrections pt,R-PICH to be used for R-EACH Σ CFs Closed loop corrections RCVR

pr

Σ

Transmitter

Σ

R-PICH

pt,R-PICH to be used for R-CCCH

CFs Closed loop corrections pt,R-PICH to be used for reverse traffic channel Σ CFs Figure 7.3 factor.

7.4

Reverse link power control. CF stands for correction factor, and GF stands for gain

Power Control of the Reverse Link: Closed Loop For the reverse link, the closed-loop power control of IS-2000 is similar to that of IS-95. Readers can refer to references such as [5–7] for its description, and we will highlight only significant differences here. To power control the reverse link, the base station continuously monitors the reverse link and measures its link quality. If the link quality starts to get worse, then the base station will command the mobile, via the forward link, to power up. If the link quality becomes too good, then there is excess power on the reverse link. In this case, the base station will command the mobile to power down.

118

Power Control

7.4.1

Inner Loop and Outer Loop

The IS-2000 standard (and the IS-95 standard) itself does not explicitly specify how the reverse link closed-loop power control is to be performed. It only states that the base station should ascertain the received signal quality of a mobile’s signal, and that the base station should transmit the PCB based the received signal quality. One implementation is measuring the Eb/N0 of one of the reverse traffic channels to ascertain reverse link quality [8, 9]. For example: 1. The base station continuously monitors Eb/N0 of the reverse traffic channel. 2. If Eb/N0 is too high (i.e., if it exceeds a certain threshold), then the base station commands the mobile to decrease the mobile transmit power. 3. If Eb/N0 is too low (i.e., if it drops below a certain threshold), then the base station commands the mobile to increase the mobile transmit power. The base station commands the mobile to power up and power down using PCBs that are multiplexed onto the forward link. The actual forward physical channel that carries PCBs depends on the reverse physical channel to be power controlled. Regardless of the forward physical channel that carries PCBs, PCBs are not error protected. They are not error protected for the reason that the mobile can quickly recover the PCB and adjust its transmit power accordingly. The above describes the inner loop of the closed-loop power control of the reverse link. The assumption of the inner loop is that there exists a predetermined Eb/N0 threshold by which power-up and power-down decisions are made. The function of the outer loop, then, is to adjust the Eb/N0 threshold (i.e., Eb/N0 setpoint) to maintain an acceptable FER. Figures 7.4 and 7.5 both show a conceptual illustration of the closed-loop reverse link power control. Figure 7.4 depicts those functions carried out by the base station. Here the base station implements the entire outer loop and part of the inner loop. The base station receives the reverse traffic channel from the mobile. The base station first demodulates the signal and estimates the FER. This information on the reverse link quality is fed into the outer loop. The outer loop, using current FER and Eb/N0 estimates, dynamically computes the new Eb/N0 setpoint required to maintain an acceptable FER. The new Eb/N0 setpoint and the current Eb/N0 estimate are then compared. If the estimate is greater than the setpoint, then the link Eb/N0 is higher than what is necessary to maintain a good FER; a PCB of 1 is thus sent to the mobile to command power down. If the estimate is less than the setpoint, then the link Eb/N0 is lower than what is necessary; a PCB of 0 is then sent to command the mobile to power up. PCBs are multiplexed onto the F-CPCCH, F-DCCH, or F-FCH depending on which reverse physical channel is to be power controlled (see Figure 7.5). PCBs are transmitted to the mobile at a maximum rate of 800-times-per-second. Figure 7.5 shows the corresponding implementation of the closed-loop reverse link power control as carried out by the mobile. Here the mobile has the rest of the inner loop. The mobile receives the F-CPCCH, F-DCCH, and F-FCH signals and recovers the PCBs. Based on the PCB, the mobile makes a decision to power-up or to power-down [3]. Note that closed-loop power control corrections applied to reverse physical channels (shown in Figure 7.5) correspond to those shown in Figure 7.3.

Power Control of the Reverse Link: Closed Loop

Rx Antenna

119

Tx Antenna

Base station R-CPCCH

Σ

R-DCCH

Transmitter

7.4

R-FCH PCB PCB PCB Eb/N0 estimate Eb/N0 estimate RCVR

OUTER LOOP

Eb/N0 estimate

Demod

FER estimate

Adjust Eb/N0 setpoint

FER estimate

Figure 7.4

7.4.2

Decision: If estimate> setpoint then PCB=1 If estimate< setpoint then PCB=0

Eb/N0 setpoint

Reverse link power control functions carried out by the base station.

Power Control of Multiple Reverse Channels

IS-2000 has the ability to perform closed-loop power control of R-EACH, R-CCCH, and reverse traffic channels. To adjust the transmit power of these reverse physical channels, the mobile makes use of the different streams of PCBs it receives on the forward link. 7.4.2.1

Enhanced Access Channel (R-EACH)

To further apply closed-loop correction to the transmit power of the R-EACH, the mobile demodulates the F-CPCCH and recover the PCBs assigned. See Chapter 2 for details on how power control groups and power control subchannels are organized on the F-CPCCH. The mobile uses these PCBs to adjust the transmit power of the R-EACH. 7.4.2.2

Reverse Common Control Channel (R-CCCH)

To power control an R-CCCH, the mobile demodulates the F-CPCCH and recovers the PCBs assigned to that particular R-CCCH. The mobile uses these PCBs to apply closed-loop correction to the transmit power of the R-CCCH.

120

Power Control Tx Antenna

Rx Antenna

Mobile station

Transmitter

Decision: Closed-loop If PCB=0 PCB correction then power down for reverse If PCB=1 traffic channel then power up Decision: Closed-loop If PCB=0 PCB correction then power down for reverse If PCB=1 traffic channel then power up Decision: If PCB=0 then power down PCB If PCB=1 then power up

Closed-loop correction for R-CCCH

Decision: If PCB=0 PCB then power down If PCB=1 then power up

DEMUX

Closed-loop correction for R-EACH

Figure 7.5

7.4.2.3

Demod F-FCH

Demod

RCVR

F-DCCH

Demod F-CPCCH

Reverse link power control functions carried out by the mobile.

Reverse Traffic Channels

To apply closed-loop correction to the transmit power of the reverse traffic channel, the mobile demodulates the F-DCCH or F-FCH and recovers the assigned PCBs. The mobile uses these PCBs to adjust the transmit power of the reverse traffic channel. It is worth noting that PCBs are inserted into the F-DCCH and F-FCH by puncturing them into these channels. PCBs carried by the F-DCCH and F-FCH are transmitted at a peak rate of 800 bps if the R-PICH is not gated. If the R-PICH is gated, then the transmission rate of the PCBs is reduced accordingly. If the R-PICH is gated at 1/2 rate, then PCBs are transmitted at 400 bps. IF the R-PICH is gated at 1/4 rate, then PCBs are transmitted at 200 bps. The actual forward physical channel that carries the PCBs is determined as follows: •

•

If the R-PICH is gated, then only the F-DCCH carries PCBs for the closed-loop power control of the reverse traffic channel. If the R-PICH is not gated, then either the F-DCCH or the F-FCH carries PCBs. The parameter FPC_PRI_CHAN specifies exactly which forward physical channel (F-DCCH or F-FCH) carries them.

7.4

Power Control of the Reverse Link: Closed Loop

121

Recall that in IS-95, PCBs are multiplexed onto the forward traffic channel in power control groups. In a similar fashion, IS-2000 multiplexes the PCBs onto the F-DCCH and F-FCH. The structure and organization of the power control groups on the F-DCCH is referred to as the forward power control subchannel. In effect, a forward power control subchannel exists on the F-DCCH and F-FCH to transport the PCBs. The position and duration of these puncturing PCBs (carried by the forward power control subchannel) on the F-DCCH and F-FCH are completely specified [3], so the mobile knows exactly where to recover the PCBs. The parameter FPC_PRI_CHAN is one of the nonnegotiable service configuration parameters to be used by the mobile. They are sent in messages like service connect message, general handoff direction message, and universal handoff direction message. They can also be sent to the mobile in power control message, extended channel assignment message, and extended supplemental channel assignment message.

References [1]

[2]

[3] [4] [5] [6] [7] [8]

[9]

Paranchych, D. W., “On the Performance of Fast Forward Link Power Control in IS-2000 CDMA Networks,” Proceedings of Wireless Communications and Networking Conference, Chicago, IL, September 23–28, 2000, pp. 603–607. Chulajata, T., and H. M. Kwon, “Combinations of Power Controls for cdma2000 Wireless Communications System,” Proceedings of Vehicular Technology Conference, Boston, MA, September 24–28, 2000, pp. 638–645. TIA/EIA/IS-2000.2-A, Physical Layer Standard for cdma2000 Spread Spectrum Systems, Telecommunications Industry Association, March 2000. TIA/EIA/IS-2000.5-A, Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems, Telecommunications Industry Association, March 2000. Yang, S. C., CDMA RF System Engineering, Norwood, MA: Artech House, 1998. Lee, S., Spread Spectrum CDMA: IS-95 and IS-2000 for RF Communications, New York: McGraw-Hill, 2002. Lee, J. S., and L. E. Miller, CDMA Systems Engineering Handbook, Norwood, MA: Artech House, 1998. Lee, W., and N. P. Secord, “Performance of Closed-Loop Power Control for a MultipleChannel Mobile Station in the cdma2000 System,” Proceedings of Wireless Communications and Networking Conference, New Orleans, LA, September 21–24, 1999, pp. 908–912. Chulajata, T., and H. M. Kwon, “Combinations of Power Controls for cdma2000 Wireless Communications System,” Proceedings of Vehicular Technology Conference, Boston, MA, September 24–28, 2000, pp. 638–645.

.

CHAPTER 8

Handoff 8.1

Introduction The IS-2000 standard has some enhanced features for handoff1 all aimed at improving system performance, and in the case of soft handoff system capacity as well. In terms of soft handoff, the speed and accuracy with which it is executed have direct consequences on capacity and quality [2]. Fast and accurate soft handoffs decrease the number of dropped calls and conserve transmit power resources. Fewer dropped calls translate into improved system performance, and conserving transmit power reduces unnecessary interference to other receivers and enables the redirection of power resources to where they are needed. In terms of idle, access entry, access, and access probe handoffs, they all serve to increase a mobile’s chance of successfully receiving or sending a message, and thus to improve system performance. This chapter will focus on those handoff features that are important to the operation of an IS-2000 system. Specifically, Section 8.2 describes soft handoff. Section 8.3 describes idle handoff, which can occur when a mobile is in the mobile station idle state. Sections 8.4 then discusses access entry handoff, which can occur right before a mobile enters the system access state, and Sections 8.5 and 8.6 describe access handoff and access probe handoff, respectively, which can occur in the system access state.

8.2

Soft Handoff Soft handoff is a process in which the mobile actively exchanges traffic channel information with two or more base stations, hence by definition soft handoff can only occur when a mobile is in the mobile station control on the traffic channel state. Readers are no doubt already familiar with the advantage of soft handoff [i.e., it enables diversity combining when a mobile transitions between coverage areas of two base stations (see Figure 8.1)]. In addition, soft handoff can only occur between two base stations transmitting forward traffic channels that are on the same CDMA carrier and at the same frame offset [3]. In managing the soft handoff process, the mobile maintains in its memory four exclusive lists of sectors of base stations. These lists are exclusive in that their

1.

All of the handoff features discussed in this chapter were available in IS-95-B [1], and IS-2000 has more or less adopted them with minimal modifications.

123

124

Handoff Base station controller (BSC)

Source cell (X)

Figure 8.1

Mobile

Target cell (Y)

Soft handoff.

contents do not overlap. In these lists, the sectors are stored in the form of pilot PN offsets of the sectors. The lists are also called sets. Similar to IS-95, the mobile (or more specifically, Layer 3 at the mobile) maintains the active set, candidate set, neighbor set, and remaining set. Although the mobile is the one that maintains these sets, the base station (or more specifically, Layer 3 at the base station) can also influence the makeup of these sets. In general, both the mobile and the base station follow a set of rules when administering the contents of these sets. 8.2.1

Active Set

The active set contains the pilots of those sectors that are actively exchanging traffic channel information with the mobile. If the active set has only one pilot in it, then the mobile is not in soft handoff. If the active set has two or more pilots in it, then the mobile is maintaining connections with those sectors specified by their pilots in the list. When the base station first assigns the forward traffic channel to the mobile, the base station specifies the pilots in the active set through the use of extended channel assignment message or channel assignment message. Subsequent updates to the contents of the active set are done by using the extended handoff direction message, general handoff direction message, or universal handoff direction message. The active set can have a maximum of six pilots. 8.2.1.1

Removing Pilots from the Active Set

Each pilot in the active set has a handoff drop timer associated with that pilot. In IS-95-A, the mobile starts the handoff drop timer for that pilot when its Ec/I0 goes below T_DROP. If the pilot’s Ec/I0 comes back above T_DROP before the handoff drop timer expires (i.e., reaches T_TDROP), then the pilot remains in the active set

8.2

Soft Handoff

125

and the timer is reinitialized. On the other hand, if the pilot remains below T_DROP until the handoff drop timer expires, then a pilot strength measurement message (PSMM) is sent to the base station, and that pilot is moved from the active set to the neighbor set. As we can see, the administration of the pilots in the active set and therefore of the soft handoff process itself is dependent on the drop threshold parameter T_DROP. T_DROP is static in nature and does not change. In IS-2000, the system has a handoff algorithm that uses a pilot drop threshold that is dynamic in nature. In other words, this new drop threshold (which we shall designate as T_DROP*) is a function of time while the mobile is contemplating on removing a pilot from the active set. T_DROP* is given by the larger of T_DROP or  1  1 SOFT_SLOPE ⋅  10 log p j  +   DROP_INTERCEPT   ∑   8 2   j

(8.1)

where pj is the received pilot strength of pilot j, and the summation is over those pilots in the active set that have strengths greater than the strength of the pilot under consideration. Expression (8.1) is called the handoff drop criterion for dropping a pilot [3]. Note that two new parameters are used for the calculation of the handoff drop criterion: SOFT_SLOPE and DROP_INTERCEPT. They are basically the slope and the intercept (in dB) of the handoff drop criterion. Note that given (8.1), the overall T_DROP* is given by   SOFT_SLOPE   DROP_INTERCEPT ,T_DROP  T_DROP* = max  ⋅  10 log ∑ p j  + 2 8   j   (8.2)

What (8.2) tells us is that in assessing whether or not to drop a pilot (e.g., pilot i), the mobile would sum up the strengths of all those pilots that have strengths greater than that of pilot i. The mobile would then convert this sum of pilot strengths into a T_DROP-like variable (i.e., handoff drop criterion) by using SOFT_SLOPE and DROP_INTERCEPT. It would then compare the handoff drop criterion and the original T_DROP. If the original T_DROP is larger than the handoff drop criterion, then the mobile still uses the old T_DROP as the drop threshold. If the handoff drop criterion is larger than T_DROP, then the mobile would use the value given by the handoff drop criterion as the new drop threshold. Note that by choosing the larger of the two (i.e., the handoff drop criterion or the old T_DROP), the mobile essentially implements an overall stricter drop threshold. This has the effect of dropping the pilot more quickly whenever possible. In other words, the mobile will request that the base station drops a pilot that is not value-added relative to the other (stronger) pilots in the active set2. Also note that since the pilot

2.

This request is done implicitly through the PSMM or the EPSMM that is transmitted after the handoff drop timer expires.

126

Handoff

strengths in the handoff drop criterion (8.1) change as a function of time, T_DROP* (8.2) itself also changes as a function of time. 8.2.1.2

Adding Pilots to the Active Set

Adding an additional pilot to the active set means that the mobile is to commence communication with the new sector represented by that pilot. The base station always specifies the contents of a mobile’s active set by using various handoff direction messages (e.g., extended handoff direction message) and thereby directs soft handoff performed by the mobile. A pilot to be added to the active set always comes from the candidate set. In IS-95-A, if the strength of a pilot in the candidate set exceeds the strength of a pilot in the active set by the threshold T_COMP × 0.5 dB, then a PSMM transmission is triggered. Upon receiving the PSMM, the base station would most likely command the mobile to move that pilot from the candidate set to the active set. In IS-2000, the system uses a new pilot detection threshold that is dynamic in nature. This detection threshold (which we shall designate as T_ADD*) consists of the handoff add criterion for adding a pilot [3]. This criterion is given as 1   1 SOFT_SLOPE ⋅  10 log    ∑k pk  +  2  ADD_INTERCEPT  8 

(8.3)

where pk is the received pilot strength of pilot k. In this case, the summation is over all the pilots in the active set regardless of their strengths. SOFT_SLOPE and ADD_INTERCEPT are the slope and intercept of the handoff add criterion equation. Note that SOFT_SLOPE is identical to that shown in (8.1). Since the pilot strengths in (8.3) change as a function of time, the overall handoff add criterion is dynamic and also changes as a function of time. Expression (8.3) states that the criterion depends on the current sum of strengths of all pilots in the active set. The sum is then linearly transformed into the handoff add criterion by using SOFT_SLOPE and ADD_INTERCEPT. In assessing whether or not to add pilot i, the mobile would compare the handoff add criterion with the strength of pilot i. If the strength of pilot i is less than the handoff add criterion, then the mobile does nothing and pilot i remains in the candidate set. On the other hand, if the strength of pilot i exceeds the handoff add criterion, then the mobile sends a PSMM or an EPSMM. Then the base station may command the mobile to move pilot i from the candidate set to the active set. In essence, the strength of a pilot in the candidate set is compared with (a linear transformation of) the sum of strengths of all pilots in the active set (i.e., the handoff add criterion). If the strength of the candidate set pilot exceeds the criterion, then it may be moved from the candidate set to the active set. One last note in this section, the system can also use both the T_COMP criterion and the handoff add criterion in deciding whether or not to move a pilot from the candidate set to the active set. In doing so, the mobile would send a PSMM or an EPSMM if both of the following conditions are met:

8.2

Soft Handoff

127

•

The strength of a pilot in the candidate set exceeds the strength of a pilot in the active set by the threshold T_COMP × 0.5 dB;

•

The strength of a pilot in the candidate set exceeds the handoff add criterion shown in (8.3).

By demanding that the candidate set pilot meets both conditions, the system on balance enforces a stricter standard for elevating a pilot from the candidate set to the active set. 8.2.2

Candidate Set

The candidate set contains the pilots of those sectors whose Ec/I0 are sufficient to make them handoff candidates. In other words, the signals of those sectors in the candidate set can be successfully demodulated by the mobile if necessary. When the base station first assigns the forward traffic channel to the mobile, the mobile’s candidate set is empty. The candidate set can contain a maximum of 10 pilots. 8.2.2.1

Adding Pilots to the Candidate Set

Adding an additional pilot to the candidate set means that the mobile decides that the pilot is a good candidate for soft handoff. In IS-2000, if the strength of a remaining set pilot or a neighbor set pilot exceeds T_ADD, then the mobile autonomously moves that pilot from the remaining set or the neighbor set to the candidate set. Note that the T_ADD used here is the static pilot detection threshold. The dynamic handoff add criterion discussed previously is only applicable when adding a pilot to the active set. The base station can also move pilots to the candidate set via one of the handoff direction messages (e.g., general handoff direction message). More specifically, the base station can implicitly direct the mobile to move a pilot from the active set to the candidate set. The base station does so by not specifying a current active set pilot in its handoff direction message. Upon receiving the message, the mobile will move that pilot from the active set to the candidate set if one of the following two conditions is met: •

•

That pilot’s handoff drop timer has not expired (i.e., has not reached T_TDROP); That pilot’s handoff drop timer has expired, but the strength of that pilot is greater than T_DROP.

8.2.2.2

Removing Pilots from the Candidate Set

Each pilot in the candidate set has a handoff drop timer associated with that pilot. In both IS-95-A and IS-2000, the mobile starts the handoff drop timer for that pilot when its Ec/I0 goes below T_DROP. If the pilot’s Ec/I0 comes back above T_DROP before the handoff drop timer expires, then the pilot remains in the candidate set and the timer is reinitialized. On the other hand, if the pilot remains below T_DROP until the handoff drop timer expires (i.e., reaches T_TDROP), then the mobile

128

Handoff

autonomously removes that pilot from the candidate set. Note that the T_DROP used here is the static drop threshold. The dynamic drop threshold discussed previously is only applicable when dropping a pilot from the active set. Similar to the case of adding pilots, the base station can also remove pilots from the candidate set via a handoff direction message (e.g., general handoff direction message). More specifically, the base station can implicitly direct the mobile to remove a pilot from the candidate set. The base station does so by specifying a current candidate set pilot in its handoff direction message. Upon receiving the message, the mobile will then remove that pilot from the candidate set and add it to the active set. In a situation where the candidate set already has 10 pilots in it and the mobile wishes to add one more, the mobile would prioritize the 10 pilots first according to the statuses of their handoff drop timers and second according to their strengths [3]. The mobile then would delete the pilot that has the lowest priority to make room for the new one, and this pilot is then moved to the neighbor set. Ideally, the mobile would never run into this overflow situation in the candidate set. An overflowing candidate set means that there are 10 usable pilots at the mobile’s location in addition to the pilots in the active set! The system designer should minimize the number of usable pilots in a geographical area to avoid pilot pollution. 8.2.3

Neighbor Set

The neighbor set contains the pilots of those sectors that are likely contenders to which to handoff. Initially, the neighbor set contains those pilots that are sent to the mobile in the general neighbor list message, extended neighbor list message, or neighbor list message. Subsequent updates to the contents of the neighbor set are done by using the extended neighbor list update message or the neighbor list update message. The neighbor set can contain a maximum of 40 pilots. 8.2.3.1

Adding Pilots to the Neighbor Set

In many ways, the neighbor set serves as the reservoir of pilots that have been demoted from the candidate set or from the active set. As discussed in Section 8.2.2.2, the mobile station autonomously moves a candidate set pilot to the neighbor set when that pilot’s handoff drop timer has expired (i.e., has reached T_TDROP) or that pilot is bumped from an overflowing candidate set because of its low priority. In moving a pilot from the active set to the neighbor set, the base station can do so by using one of the handoff direction messages (e.g., general handoff direction message). The base station can implicitly direct the mobile to move a pilot from the active set to the neighbor set. This is done by not specifying a current active set pilot in its handoff direction message. Upon receiving the message, the mobile will move that pilot from the active set to the neighbor set if the following occurs: •

That pilot’s handoff drop timer has expired (i.e., has reached T_TDROP), and the strength of that pilot is less than T_DROP.

8.2

Soft Handoff

8.2.3.2

129

Removing Pilots from the Neighbor Set

In order to keep current all the pilots in the neighbor set, the mobile keeps an aging counter for each pilot in the neighbor set. The counter is initialized to zero when that pilot is moved from the active set or candidate set to the neighbor set. Whenever an extended neighbor list update message or a neighbor list update message is received, that pilot’s counter is incremented. If the counter of a pilot exceeds NGHBR_MAX_AGE, then the mobile autonomously moves that pilot from the neighbor set to the remaining set. There are other ways by which the mobile can autonomously remove a pilot from the neighbor list. As discussed previously in Section 8.2.2.1, if the strength of a neighbor set pilot becomes greater than T_ADD, then the mobile moves that pilot from the neighbor set to the candidate set. In addition, if the neighbor set already has 40 pilots in it and the mobile wishes to add one more, the mobile would prioritize the 40 pilots first according to the statuses of their aging counters, and second according to their strengths [3]. The mobile then would remove the pilot that has the lowest priority to make room for the new one, and the lowest priority pilot is then moved to the remaining set. Of course, the base station can affect the content of the neighbor set by using one of the handoff direction messages (e.g., general handoff direction message). The base station can implicitly direct the mobile to move a pilot from the neighbor set to the active set. This is done by specifying a current neighbor set pilot in its handoff direction message. Upon receiving the message, the mobile will move that pilot from the neighbor set to the active set. 8.2.4

Remaining Set

The remaining set contains all possible pilots in the system for the current CDMA carrier frequency, not including those that are in active, candidate, and neighbor sets. The pilot PN offsets in the remaining set are defined by the parameter pilot increment PILOT_INC. For example, if PILOT_INC is 4, then individual sectors in the system can only transmit pilots with offsets of 0, 4, 8, 12, and so forth. In terms of adding pilots to the remaining set, the mobile moves a pilot from the neighbor set to the remaining set when the pilot’s aging counter exceeds NGHBR_MAX_AGE. A pilot can also enter the remaining set from an overflowing neighbor set if it is one of low priority. In terms of removing pilots from the remaining set, if the strength of a remaining set pilot exceeds T_ADD, then the mobile autonomously moves that pilot from the remaining set to the candidate set. 8.2.5

Set Transitions

Figure 8.2 gives a pictorial representation of the events that trigger and can trigger a pilot’s transitions between the different sets. Note that (Ec/I0)A denotes the pilot strength of an active set pilot. 8.2.6

Example: Soft Handoff

130

Handoff

Initially filled by BS in a channel assignment msg

Active set (max=6)

Later updated by BS in a HO direction msg

Ec/Io>T_ADD* (PSMM or EPSMM transmission) OR Ec/Io>T_ADD* and Ec/Io –(Ec/Io)A> T_COMP x 0.5 dB (PSMM or EPSMM transmission)

Pilot not in a HO direction msg and HO drop timer not expired OR Pilot not in a HO direction msg and HO drop timer expired but Ec/Io>T_DROP

HO drop timer expired (PSMM or EPSMM transmission) Initially filled by BS in a neighbor list msg

Candidate set (max=10)

HO drop timer expired Overflow

Neighbor set (max=40)

Aging counter expired Overflow

Remaining set

Ec /Io>T_ADD

Pilot not in a HO direction msg and HO drop timer expired and Ec/Io
Later updated by BS in a neighbor list updatemsg

Ec /Io>T_ADD

Figure 8.2

Table 8.1

Set transitions. Note that not all transitions are shown here.

Pilot Strengths and Dynamic Thresholds at Different Times

Time

(Ec/I0) for X

(Ec/I0) for Y

T_DROP* for X

T_DROP* for Y

T_ADD*

Pilot X

Pilot Y

1

−3.70

−17.00

—

—

−10.74

Active

Neighbor

2

−3.80

−16.00

—

—

−10.83

Active

Neighbor

3

−3.90

−15.00

—

—

−10.91

Active

Neighbor

4

−4.00

−14.00

—

—

−11.00

Active

Neighbor

5

−4.25

−13.00

—

—

−11.22

Active

Candidate

6

−4.50

−12.00

—

—

−11.44

Active

Candidate

7

−4.75

−11.00

—

—

−11.66

Active

Candidate

8

−5.00

−10.00

—

−10.38

−10.83

Active

Active

9

−5.50

−9.00

—

−10.81

−10.91

Active

Active

10

−6.00

−8.00

—

−11.25

−10.89

Active

Active

11

−7.00

−7.00

—

—

−10.99

Active

Active

12

−8.00

−6.00

−11.25

—

−10.89

Active

Active

13

−9.00

−5.50

−10.81

—

−10.91

Active

Active

14

−10.00

−5.00

−10.38

—

−10.83

Active

Active

15

−11.00

−4.75

−10.16

—

−10.85

Active

Active

16

−12.00

−4.50

−9.94

—

−10.82

Active

Active

17

−13.00

−4.25

—

—

−11.22

Candidate

Active

18

−14.00

−4.00

—

—

−11.00

Candidate

Active

19

−15.00

−3.90

—

—

−10.91

Candidate

Active

20

−16.00

−3.80

—

—

−10.83

Neighbor

Active

21

−17.00

−3.70

—

—

−10.74

Neighbor

Active

Soft Handoff

131

Table 8.1 shows an example of soft handoff between two base stations X and Y (see Figure 8.1). The example is meant to show how the dynamic pilot detection threshold (T_ADD*) and pilot drop threshold (T_DROP*) work. Here the mobile is moving from base station X to base station Y at a constant velocity. When t = 1, the mobile is very close to base station X; at t = 21, the mobile is very close to base station Y. Table 8.1 shows, for each time stamp, the pilot strengths of X and Y at the mobile. It also shows the values of T_DROP* (for pilot X and pilot Y) and T_ADD* at different time stamps. The T_DROP* and T_ADD* values are calculated using SOFT_SLOPE = 7 dB, DROP_INTERCEPT = −12 dB, and ADD_INTERCEPT = −15 dB. In this soft handoff example, T_DROP = −15 dB and T_ADD = −13 dB. Table 8.1 also shows in which sets the pilots are at each time stamp. The boldfaced time stamps denote the interval when the mobile is in soft handoff between base station X and base station Y. Figure 8.3 gives a pictorial representation of this handoff example. The vertical axis denotes the pilot strength in dB, and the horizontal axis denotes the time (stamp) from 1 to 21. Figure 8.3 also shows the time-varying levels of T_DROP* (for pilots X and Y) and T_ADD*. As the mobile gradually leaves base station X and goes toward base station Y, its received Ec/I0 from base station X (i.e., (Ec/I0)X) gradually decreases while the received Ec/I0 from base station Y (i.e., (Ec/I0)Y) increases. The following describes events that occur at some key time stamps. For the purpose of this example, assume that T_TDROP is one time-stamp long. −3.00

−5.00 (Ec/Io) for X (Ec/Io) for Y T_DROP* for X T_DROP* for Y T_ADD*

−7.00

Pilot strength (dB)

8.2

−9.00

−11.00

−13.00

−15.00

−17.00 0

1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18 19 20 21 22

Time (time stamp)

Figure 8.3 Soft handoff example. The mobile is moving from base station X to base station Y at a constant velocity.

132

Handoff

1. At t = 5, (Ec/I0)Y first goes above T_ADD (−13 dB). The mobile moves pilot Y from the neighbor set to the candidate set. 2. At t = 7, (Ec/I0)Y now goes above T_ADD*. This event triggers a PSMM or EPSMM transmission from the mobile. 3. At t = 8, the mobile receives a handoff direction message (i.e., extended handoff direction message, general handoff direction message, or universal handoff direction message). The mobile moves pilot Y from the candidate set to the active set. It is now in soft handoff with both base station X and base station Y. 4. At t = 15, (Ec/I0)X first goes below T_DROP* for pilot X. The mobile starts the handoff drop timer for pilot X. 5. At t = 16, the handoff drop timer for pilot X reaches T_TDROP and expires. This triggers a PSMM or EPSMM transmission from the mobile [3]. 6. At t = 17, the mobile receives a handoff direction message (i.e., extended handoff direction message, general handoff direction message, or universal handoff direction message). The mobile moves pilot X from the active set to the candidate set. It is now communicating only with base station Y. 7. At t = 19, (Ec/I0)X now goes below T_DROP (−15 dB). The mobile starts the handoff drop timer for pilot X. 8. At t = 20, the handoff drop timer for pilot X reaches T_TDROP and expires. The mobile moves pilot X from the candidate set to the neighbor set. Note that in Figure 8.3, T_DROP* for pilot X is not defined from t = 1 through t = 7. This is so because since pilot X is the only pilot in the active set, there is no other active set pilot that has a strength that is greater than that of pilot X, hence the handoff drop criterion cannot be calculated for pilot X. See (8.1). This also makes sense because if there is only one pilot in the active set, there is no other pilot with which to compare and there is no use calculating a dynamic pilot drop threshold for the lone pilot. T_DROP* for pilot X is also not defined from t = 8 through t = 11, but here for a slightly different reason. Starting at t = 8, the mobile is in soft handoff with both base station X and base station Y. But from t = 8 through t =11, (Ec/I0)X is still greater than (Ec/I0)Y, and since there is still no other active set pilot that has a strength greater than that of pilot X, the handoff drop criterion cannot be calculated for pilot X. From t = 17 to t = 21, T_DROP* for pilot X is not defined for the reason that pilot X is no longer in the active set during this interval, and handoff drop criterion is not defined for pilots that are not in the active set. For the sake of completeness, T_DROP* values for pilot Y are shown in Table 8.1. For similar reasons, T_DROP* is only defined for pilot Y from t = 8 through t = 10. This is because from t =1 through t = 7, pilot Y is not in the active set; from t = 11 through t = 16, no other pilot in the active set has a strength greater than that of pilot Y, and from t = 17 through t = 21, pilot Y is the only pilot in the active set. Note that T_ADD* jumps to slightly above −11 dB from t = 8 through t = 16 (i.e., during soft handoff). This is because during soft handoff, there are two pilots (X and Y) in the active set, and the summation in the handoff add criterion (8.3)

8.3

Idle Handoff

133

adds the pilot strengths of all pilots in the active set. Therefore, when a pilot is added to the active set the dynamic pilot detection threshold steps up; when a pilot is removed from the active set, the same threshold steps down. Again, this makes sense because when there are already more than one pilot in the active set, the standard of entry into the active set for an additional pilot should be higher. This has the net effect of not allowing mediocre pilots to enter the active set. The SOFT_SLOPE, ADD_INTERCEPT, and DROP_INTERCEPT values used to generate the previous handoff example (see Table 8.1) are for illustrative purposes only. It is obvious that system designers can adjust these parameters to change the dynamics of T_ADD* and of the different T_DROP*s. By adjusting how and how quickly T_ADD* and the different T_DROP*s change in response to received strengths of different pilots, a system designer can attempt to obtain the desired handoff performance between multiple base stations.

8.3

Idle Handoff Idle handoff occurs when a mobile, while in the mobile station idle state, stops monitoring the F-PCH or F-CCCH/F-BCCH of one base station and starts monitoring the corresponding channel of another base station. Unlike soft handoff, which can only occur in the mobile station control on the traffic channel state, idle handoff can only occur in the mobile station idle state (see Figure 8.4). In order to perform idle handoff, the mobile maintains in its memory four exclusive lists of sectors of base stations. These lists, or sets, are exclusive in that their contents do not overlap one another. Although these sets are similarly named as those used in soft handoff, they are logically different sets. One characteristic that differentiates these idle handoff sets from those used in soft handoff is that they are only defined in the mobile station idle state. They are the active set, neighbor set, remaining set, and private neighbor set [3].

Access entry Idle HO occurs here HO occurs here

Power up

MS initialization state

MS idle state

System access state

Soft HO occurs here

MS control on the traffic channel state

Power down

Figure 8.4 Mobile station states in which idle handoff, access entry handoff, and soft handoff can take place.

134

Handoff

8.3.1

Active Set

The mobile monitors only the F-PCH or F-CCCH of the sector of the base station that is in the active set. Unlike the active set used in soft handoff, this active set can only have one pilot in it. This means that at any given time, the mobile can only monitor the F-PCH or F-CCCH of one single sector. 8.3.2

Neighbor Set

The neighbor set contains those pilots that are potential contenders for idle handoff. The neighbor set contains the pilots of those sectors that have been specified by the universal neighbor list message, general neighbor list message, extended neighbor list message, or neighbor list message. Similar to the neighbor set used in soft handoff, this neighbor set can have a maximum of 40 pilots. 8.3.3

Private Neighbor Set

The private neighbor set contains the pilots of those sectors that are potential candidates to which to handoff in the private system. This set is analogous to the neighbor set, except here the private neighbor set contains only likely handoff candidates in a previous-defined private system. A private system (as perceived by the mobile) can be implemented if the system supports tiered services and has user zones defined. For example, a user zone can be defined in the business district of a city that has a particular set of services available (e.g., high-speed data). This “private system” can then be made available only to selected mobiles that travel in the area. The contents of the private neighbor set are specified by the private neighbor list message. It can also contain a maximum of 40 pilots. 8.3.4

Remaining Set

The remaining set contains all possible pilots in the system for the current CDMA carrier frequency that are not already in the active, neighbor, and private neighbor sets. The contents of this remaining set may be similar (but are not necessarily identical) to the remaining set used in soft handoff. This is so because the pilot PN offsets in this remaining set are also defined by the pilot increment PILOT_INC. 8.3.5

Idle Handoff Process

While in the mobile station idle state, the mobile continues to measure the strengths of pilots that are in its active, neighbor, private neighbor, and remaining sets. In the slotted mode, if the Ec/I0 of a pilot in the neighbor, private neighbor, or remaining set exceeds the Ec/I0 of the active set pilot by 3 dB, then the mobile removes the weaker pilot from the active set and moves the stronger pilot into the active set. In the nonslotted mode, the Ec/I0 of a pilot needs to exceed the Ec/I0 of the active set pilot by 3 dB for a period longer than one second [4]; if this occurs, then the mobile removes the weaker pilot from and moves the stronger pilot into the active set.

8.4

8.4

Access Entry Handoff

135

Access Entry Handoff Similar to idle handoff, an access entry handoff occurs when a mobile stops monitoring the F-PCH or F-CCCH/F-BCCH of one base station and starts monitoring the corresponding channel of another base station. But unlike idle handoff (which can only occur in the mobile station idle state), access entry handoff occurs when the mobile is transitioning into the system access state from the mobile station idle state (see Figure 8.4). The mobile can only perform an access entry handoff if it has received a page or an order/message to which the mobile needs to respond. If it has received such a page or an order/message, then the mobile can determine whether or not it should perform an access entry handoff. The IS-2000 standard actually does not specify what criteria a mobile should use to make that determination, but it would be reasonable to assume that the mobile should not perform an access entry handoff to a neighbor having a pilot strength that is too weak. Once a mobile determines that it should perform an access entry handoff, the mobile actually follows the same procedure as idle handoff to switch the monitoring of F-PCH or F-CCCH/F-BCCH from the current base station to the new base station. If the mobile does perform an access entry handoff, it should do so before entering the update overhead information substate of the system access state [3].

Power-up

MS initialization state

MS idle state

Power down

System access state

Soft HO occurs here

Access HO occurs here

Access entry HO occurs here

Access Handoff

Idle HO occurs here

8.5

MS control on the traffic channel state

Access probe HO occurs here

Figure 8.5 Mobile station states in which access handoff, access probe handoff, idle handoff, access entry handoff, and soft handoff can take place.

136

Handoff

Access handoff occurs when a mobile, after an access attempt, stops monitoring the F-PCH or F-CCCH/F-BCCH of one base station and starts monitoring the corresponding channel of another base station. Since an access handoff occurs after an access attempt, the mobile is in the system access state while performing an access handoff (see Figure 8.5). In the system access state, the mobile maintains in its memory three exclusive lists of sectors of base stations. These lists, or sets, are exclusive in that their contents do not overlap one another. These sets are similarly named but different from those sets used in idle handoff and soft handoff. They are the active set, neighbor set, and remaining set [3]. 8.5.1

Active Set

While in the system access state, the mobile monitors only the F-PCH or F-CCCH of the sector of the base station that is in the active set. Similar to the active set used in idle handoff, this active set can only have one pilot in it. Thus the mobile can only monitor the F-PCH or F-CCCH of one single sector while in the system access state. 8.5.2

Neighbor Set

The neighbor set contains the pilots of those sectors that have been specified by the universal neighbor list message, general neighbor list message, extended neighbor list message, or neighbor list message. The neighbor set pilots are those that are potential contenders for access handoff or access probe handoff. 8.5.3

Remaining Set

The remaining set contains all possible pilots in the system for the current CDMA carrier frequency that are not already in the active and neighbor sets. The contents of this remaining set are generated in a way similar to the generation of the remaining set used in soft handoff (i.e., the pilot PN offsets in this remaining set are also defined by the pilot increment PILOT_INC). 8.5.4

Access Handoff Process

As mentioned above, the mobile can only perform access handoff when it is in the system access state. Recall from Chapter 6 that the mobile enters the system access state typically after some action has been taken, either by the mobile or by the base station. Figure 8.6 shows the substates within the system access state. It turns out that the mobile can only perform access handoff when it is in two of the substates: page response substate and mobile station origination attempt substate. As Figure 8.6 shows, the mobile enters the page response substate after receiving a general page message. While it is in the page response substate, the mobile responds by transmitting a page response message. The page response message may be transmitted on either an R-ACH which is associated with an F-PCH, or an R-EACH which is associated with an F-CCCH. On the other hand, the mobile enters the mobile station origination attempt substate after originating a call. While it is in

8.5

Access Handoff

137

IS

IS

PACA cancel substate

Receives general page message

MS message transmission substate Receives general page message

Update overhead IS information Receives substate general MS idle page state message IS

MS control on the traffic channel state

Page response substate Receives general page message Registration access substate Originates call

MS origination attempt Originates call substate or reoriginates PACA call IS

IS

Figure 8.6

MS order/ message response substate

IS: Idle state

Substates in which access handoff and access probe handoff can take place.

the mobile station origination attempt substate, the mobile transmits an origination message, and the origination message may be transmitted on either an R-ACH which is associated with an F-PCH or an R-EACH which is associated with an F-CCCH. The standard specifies that the mobile must perform an access handoff in the page response substate or in the mobile station origination attempt substate if one of the following occurs [3]: •

•

The mobile loses the F-PCH or F-CCCH while waiting for a response from the base station, and the mobile is not already in the middle of an access attempt. The mobile loses the F-PCH or F-CCCH after receiving a message but before responding to that message, and the mobile is not already in the middle of an access attempt.

Recall that an access attempt is the process of sending a message to the base station and receiving an acknowledgment for the message [5]. In performing an access

138

Handoff

handoff, the mobile chooses as its target sector one with the best pilot strength. In addition, the chosen target sector should have a pilot strength that is greater than T_ADD as well [3]. How a mobile determines whether or not it has lost the F-PCH or F-CCCH is described in Chapter 9.

8.6

Access Probe Handoff Access probe handoff occurs when a mobile, during an access attempt, stops sending access probes to the current base station and starts sending access probes to a new base station. Since an access probe handoff occurs during an access attempt, the mobile is in the system access state while performing an access probe handoff (see Figure 8.5). Similar to access handoff, access probe handoff can only take place in the page response substate or the mobile station origination attempt substate (see Figure 8.6). Typically, a mobile may perform an access probe handoff in the page response substate or in the mobile station origination attempt substate if the following occurs: •

The mobile loses the F_PCH or F-CCCH, and the mobile has not performed more than MAX_NUM_PROBE_HO access probe handoffs during the current access attempt.

The parameter MAX_NUM_PROBE_HO is the maximum number of times that a mobile can perform an access probe handoff. The parameter is meant to prevent the mobile from performing an excessive number of access probe handoffs during an access attempt. In performing an access probe handoff, the mobile first completes the transmission of the current access subattempt to the current base station, and then it begins to transmit the next access subattempt to the new base station [3]. An access subattempt in IS-2000 is equivalent to a complete access attempt in IS-95-A. As Figure 8.7 shows, several access subattempts make up one complete access attempt, and a mobile can transmit one complete access subattempt to one and only one base station. At the end of the current access subattempt, the mobile either performs an access probe handoff (and thereby starts to transmit another access subattempt to a new base station) or terminates the access attempt. Therefore, if the mobile does not perform any access probe handoff, then an access attempt consists of only one access subattempt [6]. Figure 8.7 shows that an access attempt can contain up to (1 + MAX_NUM_PROBE_HO) access subattempts since the mobile cannot perform more than MAX_NUM_PROBE_HO access probe handoffs during an access attempt. Within each access subattempt, there can be up to MAX_RSP_SEQ access probe sequences if the mobile is transmitting a response message, or up to MAX_REQ_SEQ access sequences if the mobile is transmitting a request message. 3.

In basic access mode or reservation access mode.

8.7

Concluding Remarks

139 Access attempt Access subattempt 1

Access subattempt M (M = 1+MAX_NUM_PROBE_HO)

time

Access probe sequence 1

Access probe sequence 2

Access probe sequence N

(N = MAX_REQ_SEQ) or (N = MAX_RSP_SEQ)

time

Access probe 1

Access probe 2

Access probe P

(P = 1+NUM_STEP)

time

Figure 8.7

Access attempt. (After: [5].)

And of course within each access sequence, there can be up to (1 + NUM_STEP) access probes. Each access probe is basically a single R-ACH or R-EACH transmission3 described previously in Chapter 4.

8.7

Concluding Remarks Readers should recognize now that the different handoff features described in this chapter are meant to improve system performance, and in the case of soft handoff system capacity as well. In terms of soft handoff, using the new drop threshold and add threshold enables a mobile to drop pilots that are not and add pilots that are value-added relative to the other pilots in the active set. In terms of idle, access entry, and access handoffs, they afford the mobile an opportunity to switch the monitoring of overhead channels from one base station to another at different points of the state transitions. This serves to minimize the prob-

140

Handoff

ability of missing a message from the base station. In terms of access probe handoff, this feature gives the mobile, in the midst of an access attempt, the ability to transmit an additional set of access subattempt to a different base station having perhaps a stronger pilot; this helps improves the probability of achieving a successful access by the mobile.

References [1]

[2] [3] [4] [5] [6]

ANSI/TIA/EIA-95-B, Mobile Station-Base Station Compatibility Standard for Wideband Spread Spectrum Cellular Systems, Telecommunications Industry Association, March 1999. Yang, S. C., CDMA RF System Engineering, Norwood, MA: Artech House, 1998. TIA/EIA/IS-2000.5-A, Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems, Telecommunications Industry Association, March 2000. TIA/EIA-98-C, Recommended Minimum Performance Standards for Dual-Mode Spread Spectrum Mobile Stations, Telecommunications Industry Association, December 1999. TIA/EIA/IS-2000.3-A, Medium Access Control (MAC) Standard for cdma2000 Spread Spectrum Systems, Telecommunications Industry Association, March 2000. TIA/EIA/IS-2000.4-A, Signaling Link Access Control (LAC) Standard for cdma2000 Spread Spectrum Systems, Telecommunications Industry Association, March 2000.

CHAPTER 9

System Performance 9.1

Introduction This chapter addresses several topics related to system performance, and it will focus on those topics that are specific to the operation of an IS-2000 system. Section 9.2 describes how the system supervises a channel and makes decisions regarding whether or not to continue transmitting and receiving the channel. In addition because IS-2000 has added a host of new physical channels that can operate at different data rates, the administration of orthogonal codes becomes an important issue, and Section 9.3 explains how the different orthogonal codes are allocated to different physical channels. Lastly, Sections 9.4 and 9.5 discuss the codes and transmit diversity used by IS-2000.

9.2

Channel Supervision 9.2.1

Forward Link: Traffic Channel

In Chapter 7 (Section 7.4.2.3), we first mentioned the parameter FPC_PRI_CHAN; this parameter specifies the traffic channel (F-DCCH or F-FCH) that carries the forward power control subchannel. It turns out that FPC_PRI_CHAN is also the traffic channel (F-DCCH or F-FCH) that the mobile monitors in performing forward traffic channel supervision. The mobile performs forward traffic channel supervision in the mobile station control on the traffic channel state because it is in this state that the forward traffic channel is active. While in the mobile station control on the traffic channel state, the mobile monitors each received frame on the traffic channel (F-DCCH or F-FCH). If the mobile decides that the received frame has insufficient frame quality, then it declares that the received frame is a bad frame. Otherwise, the received frame is a good frame [1]. For each frame, the mobile checks the frame quality indicator (also known as CRC) bits in making its decision. If the mobile receives consecutive bad frames for a period of 240 ms (or 12 × 20 ms), then it must shut off its transmitter. The mobile can only turn on its transmitter again if, after receiving consecutive bad frames for a period of 240 ms, it receives consecutive good frames for a period of 40 ms (or 2 × 20 ms).

141

142

System Performance

In addition, the mobile keeps a fade timer for the forward traffic channel (F-DCCH or F-FCH) being monitored. The timer is initialized when the mobile first turns on its transmitter while in the traffic channel initialization substate of the mobile station control on the traffic channel state. The fade timer is set to 5 seconds and then counts down. It is reset to 5 seconds every time the mobile receives consecutive good frames for a period of 40 ms (or 2 × 20 ms). If the timer expires, then the mobile must shut off its transmitter and declare a loss of the forward traffic channel [2]. The mobile performs forward traffic channel supervision while in all the substates (i.e., traffic channel initialization substate, traffic channel substate, and release substate) of the mobile station control on the traffic channel state. If the mobile declares a loss of the forward traffic channel, then the mobile returns to the system determination substate of the mobile station initialization state [2]. 9.2.2

Forward Link: Common Channel

The mobile performs forward common channel supervision in the mobile station idle state and system access state by monitoring the F-PCH, F-CCCH, or F-BCCH. So how does a mobile determine whether or not it has lost the F-PCH, F-CCCH, or F-BCCH? It does so by keeping a timer whenever it starts to monitor the F-PCH, F-CCCH, or F-BCCH. The timer is kept as follows: it is initially set for 3 seconds; the timer is reset to 3 seconds whenever the mobile receives a valid message on the F-PCH, F-CCCH, or F-BCCH; the timer is stopped when the mobile is not monitoring the F-PCH, F-CCCH, or F-BCCH. The mobile decides that it has lost the F-PCH, F-CCCH, or F-BCCH if the timer expires [2]. Note that different equipment vendors can also have their own algorithms for determining whether or not a mobile still has a usable F-PCH, F-CCCH, or F-BCCH. 9.2.3

Reverse Link

The same kind of channel supervision also occurs on the reverse link (i.e., the base station also supervises the reverse traffic channels [e.g., R-DCCH and/or R-FCH] and reverse common channels [e.g., R-EACH and/or R-ACH]). Although the IS-2000 standard specifies that the base station has to supervise the channels, it does not specify how the base station should do so. Each equipment vendor may have different ways of performing reverse traffic and common channel supervisions.

9.3

Code Management Readers will recall that, in a DS-CDMA system, orthogonal (or near-orthogonal) sequences are used to channelize the different users that are in the same RF band. In IS-95, the system uses Walsh codes for channelization on the forward link and pseudorandom noise (PN) codes for channelization on the reverse link. In IS-2000, the system mostly uses Walsh codes for channelization on both the forward link and the reverse link. Therefore, this section focuses on how IS-2000 manages the assignment of Walsh codes.

9.3

Code Management

9.3.1

143

Generation of Walsh Codes

A group of orthogonal Walsh codes is a set of N binary orthogonal sequences, w Nk , where k denotes the kth sequence in the set. Walsh codes can be derived using the Hadamard matrix in that higher order matrixes can be recursively generated from lower order ones [3], H H2N =  N H N

HN  H N 

(9.1)

where H N contains the inverted elements of H N , and the seed matrix is 0 0 H2 =   0 1

(9.2)

To generate a set of four Walsh codes w 40 , w 14 , w 42 , and w 43 , we need to generate a Hadamard matrix of order 4, or H H4 =  2 H 2

0 H 2  0 = H 2  0  0

0 1 0 1

0 0 1 1

0 1 1  0

The four Walsh codes in this Walsh code set are taken from the rows of the matrix H4, and each Walsh code has a length of 4, w 40 w 14 w 42 w 43

= [0 = [0 = [0 = [0

0 1 0 1

0 0 1 1

0] 1] 1] 0]

Another way to generate Walsh codes is using a recursive tree shown in Figure 9.1. Each node at each level of the tree has two branches below it, resulting in two nodes at the next level. The left node at the next level consists of two instances of the node above it. The right node at the next level consists of one instance of the node above it and one inverted instance. The root node of the tree is 0. Figure 9.2 shows an example of generating Walsh codes using a recursive tree. The root of the tree is 0 (at level 0). This results in 0, 0, 0, and 1 at the next level (level 1). When one repeats this procedure, the Walsh code set of order 4 is Level j

C

C Figure 9.1

Recursive tree.

C

C

C

Level j +1

144

System Performance 0

Level 0

0 0

0000

0 1

0011

0101

0110

00000000 00001111 00110011 00111100 01010101 01011010 01100110 01101001

Figure 9.2

2

Level 1

W

Level 2

W4

Level 3

W8

An example of generating Walsh codes using a recursive tree.

generated at level 2. This set of Walsh codes contains the codes [0 0 0 0], [0 0 1 1], [0 1 0 1], and [0 1 1 0]. At level 3, the Walsh code set of order 8 (or 23) is generated. In general, a Walsh code set of order N = 2j is generated at level j of the recursive tree. 9.3.2

Assignment of Walsh Codes: Forward Link

To have effective channelization, the system needs to ensure that all channelization codes in use are orthogonal to each another. In IS-2000, this task is complicated by the fact that at any given time, there may be more than one type of channel operating on the link, and these channels may be operating at different data rates. For example, on the forward link, both the F-DCCH and the F-SCH may be simultaneously active for a user. For Spreading Rate (SR) 1, the final chip rate is fixed at 1.2288 Mcps in order to fit the RF carrier in a 1.25-MHz band. If Radio Configuration (RC) 3 is used, then the F-DCCH can operate at a data rate of 9.6 Kbps, and the F-SCH can operate at 153.6 Kbps (see Chapter 2). To obtain a final chip rate of 1.2288 Mcps, the system needs a processing gain of 128 (= 1.2288 Mcps / 9.6 Kbps) for the F-DCCH, and a processing gain of 8 (= 1.2288 Mcps / 153.6 Kbps) for the F-SCH. Because the processing gain of the F-DCCH is greater than that of the F-SCH, the length of the Walsh code used by the F-DCCH must be longer than that used by the F-SCH. In fact, in this case the F-DCCH uses a Walsh code of length 64, and the F-SCH uses a Walsh code of length 4. Although the above example deals with the forward link, similar situations also exist on the reverse link. Because IS-2000 supports different data rates simultaneously and the final chip rate is fixed at 1.2288 Mcps (at least for SR1), the system must be able to use Walsh codes of different lengths simultaneously (i.e., a lower data-rate channel requires a longer Walsh code, and a higher data-rate channel requires a shorter Walsh code). Therefore, an IS-2000 system employs Walsh codes of different lengths for different channels. Table 9.1 shows the different Walsh code lengths that different forward link channels support. To prevent mutual interference, the system needs to ensure that all Walsh codes in use are orthogonal to each another. Because IS-2000 can use Walsh codes of different lengths simultaneously, the system needs to make sure that active Walsh codes

9.3

Code Management Table 9.1 Channel

145

Lengths of Walsh Codes Used on the Forward Link SR1 (RC1, 2, 3, 4, and 5)

SR3 (RC6, 7, 8, and 9)

F-SCH

128, 64, 32, 16, 8, or 4

256, 128, 64, 32, 16, 8, or 4

F-FCH

128, 64, 32, 16, 8, or 4

256, 128, 64, 32, 16, 8, or 4

F-DCCH

128 or 64

256 or 128

F-CCCH

128, 64, 32, or 16

256, 128, or 64

F-BCCH

64 or 32

128

F-CACH

128 or 64

256

F-CPCCH

128 or 64

128

F-QPCH

128

256

F-SCCH

64

—

F-PCH

64

—

F-SYNCH

64

64

of different lengths are also orthogonal. This requirement places constraints on which specific Walsh codes can be used when one or more Walsh codes are already in use. For example, let’s say that F-SCH 1 is currently active and operating at 307.2 Kbps1. The Walsh code being used by F-SCH 1 is of length 4. This results in a final chip rate of 1.2288 Mcps (= 614.4 Kbps × 4). Now the mobile requests a second F-SCH (or F-SCH 2) to be operated simultaneously with F-SCH 1. F-SCH 2 operates at a lower rate of 153.6 Kbps1, and the Walsh code to be used by F-SCH 2 needs to be of length 8 in order to keep the final chip rate at 1.2288 Mcps (= 153.6 Kbps × 8). Figure 9.3 shows the bit streams and chip streams for F-SCH 1 and F-SCH 2. If a channel uses a Walsh code, then a simultaneous channel cannot use any Walsh code that is related directly or indirectly by branching on the recursive tree. For example, if an active channel is using Walsh code [0101] for channelization, then another channel cannot use [01], [01010101], [01011010], and so on (see Figure 9.2). The reason is that, based on the way a recursive tree is constructed, at least a part of a given Walsh code is always identical to a Walsh code above it; when the two Walsh codes are aligned in the transmission, it is possible that some bits of one of the channels would be multiplied by identical Walsh chips, thus rendering the corresponding bits unrecoverable later on. Figure 9.4 shows an example of this situation. R-SCH 1 is channelized using Walsh code [0101]. R-SCH 2 is channelized using Walsh code [01011010] (which is below [0101] on the recursive tree). One can see in Figure 9.4 that the Walsh chips multiplying the first and fourth bits of F-SCH 1 are identical to the corresponding Walsh chips on F-SCH 2. This will make it impossible to recover the first and fourth bits of F-SCH 1 later on at the receiver. To guarantee orthogonality for all corresponding bit positions among simultaneously active channels, we can only use Walsh codes that are not related directly or indirectly by branching. In our example, if Walsh code [0101] is 1.

This rate would be the modulation symbol rate in either the I path or the Q path right before Walsh code spreading, not the baseband data rate.

146

System Performance F-SCH 1 1 bit 307.2 Kbps 1 chip 1.2288 Mcps 4

use w k

F-SCH 2 1 bit 153.6 Kbps 1 chip 1.2288 Mcps use w k8

Figure 9.3

Example: F-SCH 1 operating at 307.2 Kbps and F-SCH 2 operating at 153.6 Kbps.

F-SCH 1 307.2 Kbps

1st bit 0

1

4th bit 0

1

0

1

0

1

0

1

0

1

0

1

0

1

1

0

1

1

0

1

0

1.2288 Mcps

F-SCH 2 153.6 Kbps

0

1

0

1

1

0

1

0

0

1.2288 Mcps Figure 9.4 An example of nonorthogonal anomaly when two Walsh codes related by direct and indirect branching are used simultaneously.

already in use, then we can assign Walsh code (length 8) [00001111], [00110011], [00111100], [01100110], or [01101001]. Readers should recognize now that, to guarantee orthogonality among all bit positions in all bit streams, the system needs to dynamically assign Walsh codes subjected to the constraints described above. Furthermore, the IS-2000 standard has preassigned Walsh codes to some channels. For example, the F-PCH uses Walsh

9.3

Code Management

147

code w 164 . The existence of these preassigned Walsh codes further reduces the number of freely available Walsh codes on the recursive tree. 9.3.3

Quasi-Orthogonal Functions

Even with these Walsh codes made available by IS-2000, it is still possible to run out of Walsh codes. Exhausting Walsh codes could happen when interference is relatively low and the forward link can sustain more channels than there are available Walsh codes. In this case, the forward link is limited by the number of available Walsh codes (Walsh code limited) instead of limited by interference (interference limited). When the forward link is Walsh code limited, more channelization codes can be generated by multiplying a set of Walsh codes by a specific masking function. In other words, a new set of channelization codes can be derived by the multiplication of a set of Walsh codes and a masking function. This new set of channelization codes (or “masked” Walsh codes) is called a set of quasi-orthogonal functions (QOFs). The derivation of specific masking functions is beyond the scope of this book. It suffices to say that for a set of Walsh codes, there are more than one masking functions. For example, given a set of Walsh codes one can generate two sets of QOFs: one set by multiplying the set of Walsh codes with one masking function, and another set by multiplying the set of Walsh codes with another masking function. Quasi-orthogonal functions drawn from the same set of QOFs are orthogonal, but quasi-orthogonal functions drawn from different sets of QOFs are not completely orthogonal (hence the term quasi-orthogonal functions). In addition, quasiorthogonal functions have constant (hence predictable) and minimal mini-max cross correlation with the set of Walsh codes. Note that if the original set of Walsh codes contains 256 Walsh codes, then the above process has generated 512 additional channelization codes or quasi-orthogonal functions. The base station will use Walsh codes for channelization until they are exhausted. Then the base station will generate a set of QOFs and use quasiorthogonal functions drawn from that set. If that set of QOFs is exhausted, then the base station will use quasi-orthogonal functions from a different set of QOFs. The IS-2000 standard specifies three masking functions. These three masking functions are defined such that quasi-orthogonal functions drawn from different sets of QOFs have minimal cross correlation with each other and with the set of Walsh codes. In addition, the cross correlation of any two quasi-orthogonal functions drawn from different sets of QOFs has constant magnitude [4]. Quasiorthogonal functions are used by the forward dedicated control channel, forward fundamental channel, and forward supplemental channel. 9.3.4

Assignment of Walsh Codes: Reverse Link

The IS-2000 reverse link (for RC3, RC4, RC5, and RC6) is fundamentally different from that of IS-95. On the IS-95 reverse link, only one channel is active at a time (i.e., either the access channel or the traffic channel). Therefore, there is no need to distinguish individual channels transmitted by a mobile since each mobile can only

148

System Performance

transmit one channel at a time. In IS-95, a mobile is distinguished from other mobiles by its long PN code. In IS-2000, a mobile can transmit multiple channels simultaneously. So the base station not only has to distinguish among different mobiles, but also has to distinguish among different channels transmitted by a specific mobile. In IS-2000, the base station still discriminates the mobiles by their individual long PN codes, but after a mobile is identified using its long PN code, the base station demodulates the channels (transmitted by that mobile) using their assigned Walsh codes2. Note that this arrangement is analogous to the forward link where, to a mobile, different base stations are first identified by their short PN codes. After a mobile identifies a specific base station, the mobile then demodulates the channels (transmitted by that base station) using their assigned Walsh codes. What enables the operation of the IS-2000 reverse link is that the mobile now transmits an R-PICH, which allows the base station to perform coherent detection of the mobile’s signal and to lock onto a mobile’s identifying PN code. In IS-2000, the base station does not dynamically assign Walsh codes for a mobile to use. Rather, the IS-2000 standard defines the Walsh code each reverse link channel must use. Table 9.2 shows these predefined Walsh codes and their lengths on the reverse link [1]. To ensure orthogonality, the preassignment of these Walsh codes also meet the constraints described in the previous section. For example, a mobile would sometimes transmit both the R-FCH and the R-DCCH simultaneously. As shown in Figure 9.5, the Walsh code preassigned to the R-FCH ([0000111100001111]) is not directly or indirectly related to the Walsh code preassigned to the R-DCCH ([0000000011111111]). In addition, if the system chooses [01] to channelize R-SCH 1, then it can choose either [0011] or [00111100] to channelize R-SCH 2. If it chooses [0011] to channelize R-SCH 1, then it cannot choose [00111100] to channelize R-SCH 2; in fact, in this case the mobile cannot have an additional R-SCH (i.e., R-SCH 2) because the standard does not have preassigned any usable Walsh code. Figure 9.5 shows the location of these Walsh codes on the recursive tree.

Table 9.2 Walsh Codes Used on the Reverse Link for Both SR1 (RC3 and RC4)3 and SR3 (RC5 and RC6) Channel

2. 3.

Walsh Code 4 2 8 6 16 4 16 8 8 2 8 2

Walsh Code Length 2 1

R-SCH 1

w =[0011] or w =[01]

4, or 2

R-SCH 2

w =[00111100] or w42 =[0011]

8 or 4

R-FCH

w =[0000111100001111]

16

R-DCCH

w =[0000000011111111]

16

R-EACH

w =[00110011]

8

R-CCCH

w =[00110011]

8

IS-2000 calls Walsh codes used on the reverse link Walsh covers. Walsh code lengths for RC1 and RC2 are not shown because Walsh codes are not used for channelization in these radio configurations.

9.3

Code Management

149

0

Level 0

00

0 1 R-SCH1 R-SCH1 0011 or R-SCH2

0000

00000000

00001111

0000000011111111 R-DCCH

0101

00110011 00111100 01010101 R-EACH R-SCH2 or R-CCCH

0110

01011010

01100110

01101001

Level 1

W2

Level 2

W4

Level 3

W8

Level 4

W 16

0000111100001111 R-FCH

Figure 9.5

Location of reverse link Walsh codes on the recursive tree.

At this point, readers may ask that given a predefined Walsh code, how does the system vary the transmission rate on the reverse link. To change the transmission rate, the mobile uses the same Walsh code but repeats it in a given bit. Figure 9.6 illustrates this operation. Suppose that an R-SCH is operating at 614.4 Kbps1 and is using Walsh code [01]. This gives a final chip rate of 1.2288 Mcps (= 614.4 Kbps × 2). Now the system would like to change the rate from 614.4 Kbps down to 153.6 Kbps1 (by a factor of four). What the mobile would do then is to repeat Walsh code [01] four times during a bit, thus still attaining a final chip rate 1.2288 Mcps. Note that repeating a Walsh code does not cause any orthogonality problems because the 1 bit

614.4 Kbps 1 chip 0

1

0

use

2 w1

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

1.2288 Mcps

1 bit

153.6 Kbps 1 chip 0

1

0

1

0

1

0

1

0

1

1.2288 Mcps repeat w 12 four times

Figure 9.6

Example: Changing transmission rate on the R-SCH.

150

System Performance

standard has already predefined the Walsh codes in such a way that they are not related directly and indirectly (by branches), and repeating a Walsh code is equivalent to traversing down the recursive tree along the branches.

9.4

Turbo Codes Turbo codes are relatively new in the family of error-correcting codes. They were first proposed in 1993 [5, 6] and constitute a new way of implementing concatenated codes. IS-2000 makes use of turbo codes because of their ability to achieve low error rates at very-small signal-to-noise ratios (SNR) or Eb/N0. Recall that M∝

(W / R ) (Eb / N 0 )

(9.3)

where M is the number of simultaneous users [3]. Equation (9.3) states that, in general, the capacity of a CDMA system is directly proportional to the processing gain (W/R) and inversely proportional to the required Eb/N0. Therefore, to the extent that the required Eb/N0 can be reduced, the capacity of a CDMA system can be increased (subject to the availability of orthogonal codes, of course). A turbo encoder has some important and distinguishing characteristics: •

A turbo encoder typically contains two convolutional encoders. These convolutional encoders are sometimes referred to as constituent encoders. The constituent encoders are usually identical to each other.

•

Each constituent encoder operates as a recursive systematic convolutional (RSC) encoder. In producing coded bits, an RSC encoder not only linearly combines shifted bits at different stages in the register, but also feeds back these shifted bits to the beginning of the register.

•

Whereas the first constituent encoder codes the message bits as they enter the turbo encoder, the second constituent encoder codes a permuted version of the message bits.

Figure 9.7 shows an example of a turbo encoder with its two constituent encoders arranged in a parallel fashion. In this example, each constituent encoder operates at rate 1/2 and produces two coded bits for every input bit. Turbo codes implemented by turbo encoders shown in Figure 9.7 are also known as parallel concatenated convolutional codes (PCCC) because they are generated by concatenating two convolutional codes in parallel. Figure 9.7 shows that the message bits mi (to be encoded) also form parts of the output bits. Because each constituent encoder is of rate 1/2, Encoder 1 operates on mi and produces coded bits x′i and x″i. Encoder 2 operates on a permuted version of mi, or mi, and produces coded bits y′i and y″i. The interleaver rearranges the bits mi to produce mi in such a way that mi appear completely different from mi. The interleaver typically operates on a block of input bits at a time. In addition, a puncturing function selectively deletes the coded bits (produced by constituent encoders) to arrive at a desired rate for the whole turbo encoder. In the example shown in Figure

Turbo Codes

151 mi

xi' Constituent encoder 1 (rate 1/2)

Puncture x''i

Turbo encoded bits

Interleaver

(

9.4

mi

yi' Constituent encoder 2 (rate 1/2)

Puncture y''i

Figure 9.7

A turbo encoder.

9.7, each constituent encoder operates at rate 1/2 and produces two coded bits for every input bit. If there is no puncturing, then the entire turbo encoder operates at rate 1/5 because a total of five bits (including the input bit itself) are produced for every input bit. The turbo encoder shown in Figure 9.7 happens to be the one used by IS-2000. In fact, both the forward link and the reverse link use the same turbo encoder. By using puncturing, the turbo encoder used in IS-2000 can operate at rates 1/2, 1/3, and 1/4. For example, to achieve rate 1/3, the turbo encoder only outputs mi, x′i, and y′i and lets the puncturing function completely punctures out x″i and y″i. In terms of the interleaver, the interleaver permutes one frame of bits at a time. The interleaver is a key to turbo codes’ good error-correcting performance. Because of the interleaver, the two constituent encoders essentially operate on the same set of bits but in different order. Therefore, at the receiver, erroneous bit sequences that appear correct to one decoder would more likely be rejected by the other decoder [7]. As the previous sentence implies, at the receiver the turbo decoder consists of two convolutional decoders. The decoding of turbo codes is beyond the scope of this book. It suffices to say that the two convolutional decoders work together by exchanging soft decisions between themselves and iteratively arrive at the (hard) decisions on what the correct message bits should be. See [8, 9] for good discussions of turbo decoding. The reason why turbo codes are used in 3G systems is their ability to correct errors at a low SNR. They have been shown to achieve very-low error rates (i.e., around 10–5) but require only an SNR of less than 1 dB above the Shannon’s limit. The cost of such a superior error performance is equally obvious. Turbo coding and decoding are computationally more intensive than conventional convolutional coding and decoding. Delays resulting from such computations render turbo codes unusable in voice applications. This is the reason why, in IS-2000, turbo codes are only used for supplemental channels (forward and reverse) for data applications

152

System Performance

where delays can be tolerated. Research has shown that in IS-2000 turbo codes can achieve gains of 1.3 to 1.5 dB at an FER of 1% [10].

9.5

Transmit Diversity IS-2000 has made much effort to improve the forward link. One reason is that field experience with IS-95 has shown that in many instances the system is forward link limited [11]. Another reason is that many high-rate data applications supported by 3G require higher throughput on the forward link. One enhancement made to improve the forward link is the enhanced forward link power control (see Chapter 7); IS-2000 can now power control the forward link at a rate of 800-timesper-second. Another enhancement IS-2000 made is transmit diversity, which typically uses two transmit antennas. Transmission through two antennas achieves spatial diversity when antennas are spaced sufficiently apart (by several wavelengths). This way two transmitted signals undergo independent fading and are uncorrelated, and the probability that both signals undergo fading at the same time is small. IS-2000 provisions two transmit diversity schemes: orthogonal transmit diversity (OTD) and space time spreading (STS). In OTD, the base station splits the symbol stream into multiple streams and transmits them through multiple antennas. In STS, the base station duplicates multiple copies of the symbol stream and transmits them through multiple antennas. 9.5.1

Orthogonal Transmit Diversity

OTD is a technique by which the base station splits the symbol stream into multiple streams and transmits them through multiple antennas. A common configuration is to split the symbol stream into two streams (odd and even) and to transmit them through two antennas. OTD is a transmit diversity scheme that was proposed in [12] and later adopted by the IS-2000 standard [13]. Figure 9.8 shows the concept of OTD. As shown in Figure 9.8, the interleaved symbol stream is split into two streams: odd and even. The odd symbol stream is channelized by one Walsh code, and the even symbol stream is channelized by another (different) Walsh code to maintain orthogonality between the two streams. Quasi-orthogonal functions can also be used. Both channelized streams are spread by the same short PN code4 and then transmitted by two different antennas. Figure 9.9 shows the implementation of OTD in IS-2000. Here the channel structure is for the forward dedicated control channel, forward fundamental channel, and forward supplemental channel5 (same as that shown in Figure 2.14 of Chapter 2) except that the symbol stream is split into four different streams: Iodd, Ieven, Qodd, and Qeven. The odd streams are fed into a complex modulator which 4. 5.

In actuality there are two short PN codes: pI and pQ. See Chapter 2. Forward supplemental channel does not have punctured power control bits.

Transmit Diversity

153

wi or QOF odd Yodd(t)

Yeven(t)

p

Demux even

w’i or QOF’ Figure 9.8

Othogonal transmit diversity.

Iodd Yodd(t)

Complex modulator

Qodd

wi or QOF

pI

pQ

Ieven Complex modulator

Yeven(t)

Qeven w’i or QOF’ Modulation symbols Block interleaving

Long code scrambling

Iodd Channel gain

Power control puncturing

Demux

9.5

Ieven Qodd Qeven

Long code mask

Figure 9.9

Power control bits

Implementation of OTD in IS-2000. For brevity, symbol repetition is not shown.

154

System Performance

generates the transmit signal Yodd(t) for one antenna. The even streams are fed into another complex modulator which generates the transmit signal Yeven(t) for another antenna. Each complex modulator uses a different Walsh code but the same short PN codes. The internal structure of the complex modulator is identical to that used by IS-2000 forward link (see Figure 2.15 in Chapter 2). To support transmit diversity, the base station sector transmits two pilot channels, one for each of the two transmit antennas. One antenna transmits the original F-PICH, and the other antenna transmits the F-TDPICH (see Chapter 2). These two pilots are used to perform estimation for the two (independent) propagation channels and to aid coherent detection of the two signals. At the receiver, the two signals are despread using the same two Walsh codes (used at the transmitter) and multiplexed and combined to recover the original symbol stream [14]. Studies have shown that OTD outperforms no transmit diversity. Typical gains are between about 1 to 4 dB (e.g., [14]) depending on the mobile’s speed (transmit diversity performs best when the mobile is stationary). 9.5.2

Space Time Spreading

STS is a technique by which the base station duplicates multiple copies of the symbol stream and transmits them through multiple antennas. A common configuration is to duplicate two copies (1 and 2) and to transmit them through two antennas. STS is a transmit diversity scheme that was proposed in [15] and later adopted by the IS-2000 standard [13]. Figure 9.10 shows the concept of STS. As shown in Figure 9.10, the interleaved symbol stream is split into two streams: odd and even. The odd and even streams are combined to form two identical paths 1

wi or QOF

odd Σ

1

Y1(t) p

Demux Σ

2

even wi or QOF Figure 9.10

Space time spreading.

Y2(t)

9.5

Transmit Diversity

155

and 2. Path 1 is channelized by one Walsh code, and path 2 is channelized by another (complementary) Walsh code to maintain orthogonality between the two paths. Quasi-orthogonal functions can also be used. Both channelized paths are spread by a short PN code6 and then transmitted by two different antennas. Figure 9.11 shows the implementation of OTD in IS-2000. Here the channel structure is for the forward dedicated control channel, forward fundamental channel, and forward supplemental channel7 (same as that shown in Figure 2.14 of Chapter 2) except that the symbol stream is split into four different streams: Iodd, Ieven, Qodd, and Qeven. For path 1 (top complex modulator), the combination of Iodd and Ieven streams forms the in-phase input, and the combination of Qodd and Qeven streams forms the quadrature input. For path 2 (bottom complex modulator), the combination of Iodd and Ieven streams forms the in-phase input, and the combination of Qodd and Qeven streams forms the quadrature input. The top complex modulator generates the path 1 transmit signal Y1(t) for one antenna. The bottom complex modulator generates the path 2 transmit signal Y2(t) for another antenna. This way, each path effectively transmits all symbol streams. Each complex modulator uses a complementary Walsh code but the same short PN codes. The internal structure of the complex modulator is identical to that used by IS-2000 forward link (see Figure 2.15 in Chapter 2).

Iodd Ieven

Σ

Qodd Qeven

Σ

wi or QOF Iodd Ieven

Σ

Qodd Qeven

Σ

Y1(t)

Complex modulator

pI

Complex modulator

pQ

Y2(t)

wi or QOF Modulation symbols Long code scrambling

Channel gain

Power control puncturing

Demux

Block interleaving

Iodd Ieven Qodd Qeven

Long code mask

Figure 9.11 6. 7.

Power control bits

Implementation of STS in IS-2000. For brevity, symbol repetition is not shown.

In actuality there are two short PN codes: pI and pQ. See Chapter 2. Forward supplemental channel does not have punctured power control bits.

156

System Performance

To support transmit diversity, the base station sector also transmits two pilot channels (F-PICH and F-TDPICH), one for each transmit antenna, to perform channel estimation for and coherent detection of the two signals. At the receiver, the two signals are despread using the same two Walsh codes (used at the transmitter) and combined to recover the original symbol stream [14]. Studies have shown that STS outperforms OTD by about 0.3 to 2.4 dB (e.g., [13]) depending on the speed of the mobile (transmit diversity performs best when the mobile is stationary). 9.5.3

Concluding Remarks

Based on transmitter structures shown in Figures 9.9 and 9.11, it is clear that both OTD and STS require modifications of the receiver to support these transmit diversity schemes. Therefore, the mobile needs to inform the base station whether or not the mobile’s receiver is capable of supporting transmit diversity. The mobile does so by using OTD_SUPPORTED and STS_SUPPORTED fields in messages such as registration message, origination message, and page response message.

References [1] TIA/EIA/IS-2000.2-A, Physical Layer Standard for cdma2000 Spread Spectrum Systems, Telecommunications Industry Association, March 2000. [2] TIA/EIA/IS-2000.5-A, Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems, Telecommunications Industry Association, March 2000. [3] Yang, S. C., CDMA RF System Engineering, Norwood, MA: Artech House, 1998. [4] Jalloul, L. M. A., and A. Shanbhag, “Enhancing Data Throughput Using Quasi-Orthogonal Functions Aggregation for 3G CDMA Systems,” Proceedings of Vehicular Technology Conference, 2002, pp. 2008–2012. [5] Berrou, C., A. Glavieux, and P. Thitimajshima, “Near Shannon Limit Error-Correcting Coding and Decoding: Turbo Codes,” Proceedings of IEEE International Conference on Communication, 1993, pp. 1064–1070. [6] Berrou, C., and A. Glavieux, “Near Optimum Error Correcting Coding and Decoding: Turbo-Codes,” IEEE Trans. Comm., October 1996, pp. 1261–1271. [7] Rothweiler, J., “Turbo Codes,” IEEE Potentials, February/March 1999, pp. 23–25. [8] Sklar, B., “A Primer on Turbo Code Concepts,” IEEE Communications, December 1997, pp. 94–102. [9] Ryan, W. E., “Concatenated Convolutional Codes and Iterative Decoding,” In J. G. Proakis (Ed.), Wiley Encyclopedia of Telecommunications, Hoboken, NJ: Wiley, 2002. [10] Zhang, P., and P. Luukkanen, “Performance of Turbo Codes in 3rd Generation cdma2000 Mobile System,” Proceedings of Vehicular Technology Conference, 1999, pp. 1674–1677. [11] Walton, R., M. Wallace, and S. Howard, “CDMA Downlink Performance Issues,” Proceedings of the Ninth IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, 1998, pp. 308–312. [12] Weerackody, V., “Diversity for Direct-Sequence Spread Spectrum System Using Multiple Transmit Antennas,” Proceedings of IEEE International Conference on Communication, 1993, pp. 1775–1779. [13] Wu, G., et al., “Performance Evaluation of Space-Time Spreading and Orthogonal Transmit Diversity in CDMA2000,” Proceeding of IEEE Seventh International Symposium on Spread Spectrum Techniques and Applications, 2002, pp. 323–327.

9.5

Transmit Diversity

157

[14] Derryberry, R. T., et al., “Transmit Diversity in 3G CDMA Systems,” IEEE Communications, Vol. 40, No. 4, 2002, pp. 68–75. [15] Papadias, C., et al., “Space-Time Spreading for CDMA Systems,” Proceeding of Sixth Workshop Smart Antennas Wireless Mobile Communications, July 22–23, 1999, Stanford, CA.

Selected Bibliography Hanzo, L., T. H. Liew, and B. L. Yeap, Turbo Coding, Turbo Equalisation and Space-Time Coding for Transmission over Fading Channels, New York: John Wiley & Sons, 2002.

.

CHAPTER 10

System Design: Coverage 10.1

Introduction The purpose of system design is to ascertain whether or not a given model of the system will meet some predetermined requirement. For a wireless system, the requirement is often that a user should perceive his or her radio connection to be one of high quality (i.e., the user is well within the coverage area of a cell or sector). Since coverage has to do with the quality of the radio link, this chapter emphasizes on the analysis of RF part of the system. In performing such an analysis, we are concerned with the classical figures of merit of a CDMA system (i.e., Ec/I0 and Eb/N0). Specifically, we will examine the following more important parameters [1]: • • • • •

Ec/I0 of the forward pilot channel; Eb/N0 of the forward fundamental channel; Eb/N0 of the reverse fundamental channel; Eb/N0 of the forward supplemental channel; Eb/N0 of the reverse supplemental channel.

Calculating the actual Eb/N0 and Ec/I0 ratios essentially involves the accounting of signal and interference terms. Note that throughout this chapter, we refer to the mobile of interest as the probe mobile, and link equations are derived from the perspective of this probe mobile both on the forward link and on the reverse link. Before we proceed, it is instructive to first examine the analytic definitions of Eb/N0 and Ec/I0 as these terms are used often in CDMA. In fact, we are especially interested in the relationship between Eb/N0 and Ec/I0, if any. In terms of Eb/N0, Eb/N0 is the energy-per-bit (Eb) per noise-power density (N0). It can be treated as a proxy of SNR when one recognizes that energy-per-bit is equal to the average signal power allocated to each bit duration, E b = S b Tb

(10.1)

where Sb is the average signal power and Tb is the time duration of each bit. Notice that (10.1) is consistent with dimensional analysis, which states that energy is equivalent to power multiplied by time. Substituting bit rate Rb (which is the inverse of bit duration Tb) into the above equation produces

159

160

System Design: Coverage

Eb =

Sb Rb

(10.2)

Eb/N0 is thus Eb Sb = N 0 Rb N 0

(10.3)

Given that noise power density N0 is just the noise power N divided by the bandwidth W, Eb/N0 becomes Eb S W = b N0 N Rb

(10.4)

Equation (10.4) states that Eb/N0 is the same as the SNR (Sb/N) enhanced by (or multiplied by) the processing gain (W/Rb). In terms of Ec/I0, Ec/I0 is the pilot energy-per-chip (Ec) per total power density (I0). Ec can be written as E c = S c Tc

(10.5)

where Sc is the average pilot power and Tc is the time duration of each chip. Substituting chip rate Rc (which is the inverse of chip duration Tc) into the above equation produces Ec =

Sc Rc

(10.6)

Ec/I0 then is Ec Sc = I 0 Rc I 0

(10.7)

Given that total power density I0 is just the total power I divided by the bandwidth W, Ec/I0 becomes Ec S W = c I0 I Rc

(10.8)

If we make the assumption that Rc (e.g., 1.2288 Mcps in 1x) is approximately the same as W (e.g., 1.25 MHz in 1x), then Ec/I0 is essentially the pilot power (Sc) divided by the total power (I), Ec S ≈ c I0 I

(10.9)

10.2

Forward Pilot Channel

161

Lastly, we are interested in deriving a relationship between Eb/N0 and Ec/I0. Solving the above equation for I and assuming that N is approximately the same as I yield N≈I=

Sc Ec / I 0

(10.10)

Substituting (10.10) into (10.4) produces Eb S W Ec ≈ b N 0 S c Rb I 0

(10.11)

Eb / N 0 S b W ≈ Ec / I 0 S c Rb

(10.12)

Therefore,

Equation (10.12) shows that the ratio between Eb/N0 and Ec/I0 is approximately the product of the ratio of average signal power to average pilot power and the processing gain. If average signal power is the same as average pilot power, then the ratio between Eb/N0 and Ec/I0 is simply the processing gain, Eb / N 0 W ≈ Ec / I 0 Rb

10.2

if S b = S c

(10.13)

Forward Pilot Channel The Ec/I0 is the pilot energy summed over the period of one chip divided by the total power density in the RF band. The Ec/I0 is the figure of merit of the forward pilot channel; it is important in system design because the Ec/I0 effectively determines the coverage area of a sector. It does so in two ways. First, the forward pilot channel serves as a coherent carrier phase reference for the demodulation of other forward physical channels. If a mobile is too far away from the base station, the received Ec/I0 would not be sufficient for the successful demodulation of other forward physical channels. Second, the Ec/I0 is an important parameter in deciding whether or not a handoff should occur. This is especially so for the enhanced algorithms that are now available in IS-2000 (see Chapter 8). The Ec/I0 of the forward pilot channel is given by E c α 0 A 0 (θ 0 )L 0 (θ 0 ,d 0 )G = I0 rN

(10.14)

where •

A0 ( θ 0 ) is the total effective radiated power (ERP) of all available amplifier power of the home sector (sector 0). Because ERP depends on the antenna pattern, which is a function of direction θ 0 , ERP is also a function of direction θ 0 .

162

System Design: Coverage

•

•

• • •

α 0 is the fraction of the total ERP of all available amplifier power of the home sector (sector 0) that is allocated to the F-PICH. L 0 ( θ 0 , d 0 ) is the path loss from the home base station in the direction θ 0 to the probe mobile a distance d0 away. G is the receive antenna gain of the probe mobile; N is the thermal noise power; r is the ratio of total interference power received at the probe mobile (on the forward link) to thermal noise power.

Note that the ERP of the forward pilot channel (α 0 A0 ( θ 0 )) is deterministic and does not change. As defined above, the term rN is then the total interference received on the forward link at the mobile and has three components, rN = rh N + ra N + N

(10.15)

where •

•

rh is the ratio of interference power received at the probe mobile (from the home sector) to thermal noise power. ra is the ratio of interference power received at the probe mobile (from other adjacent sectors) to thermal noise power.

For the sake of brevity, (10.14) (and all subsequent link equations in this chapter) does not include those received interference from other sources of non-CDMA origins (e.g., intentional or unintentional jammers). For the same reason, we also do not include those margins that are typically found in link budgets (e.g., lognormal shadow fading).

10.3

Forward Fundamental Channel In IS-2000, the forward fundamental channel is more frequently used for voice applications. To ascertain whether or not the forward fundamental channel can adequately support voice applications, a system designer must assess the quality of the radio link associated that channel. Eb/N0 is perhaps the most popular figure of merit for assessing the quality of the radio link. This is so because Eb/N0 determines the bit error rate (BER) of the link and ultimately affects the frame error rate (FER) of the channel. In assessing the quality of the link, one ensures that the actual Eb/N0 is at least equal to the required Eb/N0. The Eb/N0 of the forward fundamental channel is given by Eb F ( θ )L ( θ , d )G  W  = 0 0 0 0 0   N 0 (1 − ε)rh N + ra N + N  R F 

where

(10.16)

10.4

Forward Supplemental Channel

•

•

• • •

•

163

F0 ( θ 0 ) is ERP of the F-FCH of the home sector (sector 0). Because ERP depends on the antenna pattern, which is a function of direction θ 0 , ERP is also a function of direction θ 0 . L 0 ( θ 0 , d 0 ) is the path loss from the home base station in the direction θ 0 to the probe mobile a distance d0 away. G is the receive antenna gain of the probe mobile. N is the thermal noise power. W is the final spread bandwidth, which for Radio Configurations 1 through 5 is 1.25 MHz. RF is the data rate of the forward fundamental channel.

The factor (W/RF) is also known as the processing gain. Regarding the interference terms in the denominator, • •

•

ε is the orthogonality factor1; rh is the ratio of interference power received at the probe mobile (from the home sector) to thermal noise power; ra is the ratio of interference power received at the probe mobile (from other adjacent sectors) to thermal noise power.

Equation (10.16) states that the interference contributions consist of those from thermal noise (N), interference received from other adjacent sectors (raN), and interference received from the home sector ((1 − ε) rh N). Interference received from the home sector is essentially the interference power received from the home sector (i.e., rhN) suppressed by the orthogonality factor (i.e.,1 − ε). In other words, interference received from the home sector that is intended for other users is suppressed because of the orthogonal property of channelization codes. Due to power control, F0 ( θ 0 ) is not deterministic. Rather, F0 ( θ 0 ) is constantly changing due to adjustments made by the base station to maintain an acceptable Eb/N0. In that regard, F0 ( θ 0 ) can be treated as a random variable. In addition, for the sake of brevity (10.16) does not include the effects of diversity gain in soft/softer handoffs.

10.4

Forward Supplemental Channel In IS-2000, the forward supplemental channel is only used for data applications. To find out if the forward supplemental channel can adequately support the transmission of data, a system designer evaluates the quality of the radio link associated with the forward supplemental channel. Similar to the forward fundamental channel, Eb/N0 is used as the figure of merit because it determines the BER of the link and ultimately the FER of the channel. The actual Eb/N0 should at least be equal to the required Eb/N0. The Eb/N0 of the forward supplemental channel is given by

1.

Orthogonality factor indicates how much interfering signal power a receiver can reject due to the orthogonal property of channelization codes. Ideally, orthogonality factor is 1 (or 100%) if codes are orthogonal and perfectly aligned.

164

System Design: Coverage

Eb S ( θ )L ( θ , d )G  W  = 0 0 0 0 0   N 0 (1 − ε)rh N + ra N + N  R S 

(10.17)

where •

S 0 ( θ 0 ) is ERP of the F-SCH of the home sector (sector 0). Note that ERP is a function of direction θ 0 .

•

L 0 ( θ 0 , d 0 ) is the path loss from the home base station in the direction θ 0 to the probe mobile a distance d 0 away.

•

G is the receive antenna gain of the probe mobile.

•

N is the thermal noise power.

•

W is the final spread bandwidth, which for Radio Configurations 1 through 5 is 1.25 MHz.

•

RS is the data rate of the forward supplemental channel.

The factor (W/RS) is also known as the processing gain. Because the data rate of the forward supplemental channel (RS) is often greater than that of the forward fundamental channel (RF), the processing gain of the forward supplemental channel is typically less than that of the forward fundamental channel. This is so because W is fixed. The effect of this lower processing gain is described later in this section. Regarding the interference terms in the denominator, •

ε is the orthogonality factor.

•

rh is the ratio of interference power received at the probe mobile (from the home sector) to thermal noise power.

•

ra is the ratio of interference power received at the probe mobile (from other adjacent sectors) to thermal noise power.

Equation (10.17) states that the interference contributions consist of those from three sources. The first two are similar to those shown for the forward fundamental channel [i.e., thermal noise (N) and interference received from other adjacent sectors (raN)]. The third source ((1 − ε) rh N) consists of interference received from the home sector. It is essentially the interference power received from the home sector (i.e., rh N) suppressed further by the orthogonality factor (i.e., 1 − ε). Again in this case, interference received from the home sector that is intended for other users is suppressed because of the orthogonal property of channelization codes. Due to power control, S 0 ( θ 0 ) is constantly changing due to adjustments made by the base station to maintain an acceptable Eb/N0. In that sense, S 0 ( θ 0 ) is random. In addition, as mentioned before, the forward supplemental channel typically has a lower processing gain than the forward fundamental channel. This results in a lower link Eb/N0 if all other parameters remain the same. What this means is that, for a given required Eb/N0, the forward supplemental channel has to transmit at a higher power S 0 ( θ 0 ) to compensate for the lower processing gain (W/RS). Figure 10.1 shows the signal and interference components of the forward link.

10.5

Upper Bounds of Interference: Forward Link

F 0, S0

165

L0

Other BS

G raN

Home BS

(1−ε)rhN

Probe MS

Signal

Other BS

Interference Figure 10.1 Forward link: Signal and interference components where the base station (BS) is the transmitter and the mobile station (MS) is the receiver.

10.5

Upper Bounds of Interference: Forward Link Although rh and ra (and hence r) are typically calculated using system simulations, we can nevertheless derive useful upper bounds for these ratios. For the interference received from the home sector (sector 0), rh ≤

[ A 0 ( θ 0 ) − F 0 ( θ 0 )]L 0 ( θ 0 , d 0 )G N

(10.18a)

rh ≤

[ A 0 ( θ 0 ) − S 0 ( θ 0 )]L 0 ( θ 0 , d 0 )G N

(10.18b)

or

where A0 ( θ 0 ) is the total ERP of all available amplifier power of the home sector (sector 0). Equation (10.18) effectively states that the interference received from the home sector can never be greater than what can be radiated by the available amplifier power of the home sector. For the interference received from other sectors K −1

ra ≤

G ∑ A k ( θ k )L k ( θ k , d k ) k =1

N

(10.19)

where Ak ( θ k ) is the total ERP of all available amplifier power of the kth sector, and L k ( θ k , d k ) is the path loss from the kth sector in the direction θ k to the probe mobile a distance dk away. Note that there are a total of K sectors in the system.

10.6

Reverse Fundamental Channel In IS-2000, the reverse fundamental channel is more frequently used for voice applications. In a similar manner, the system designer determines whether or not the reverse fundamental channel can adequately support voice applications. In doing

166

System Design: Coverage

so, he or she calculates the actual Eb/N0 and ensures that it is at least equal to the required Eb/N0. The Eb/N0 of the reverse fundamental channel is given by Eb F ′ L′( θ 0 , d 0 )G′( θ 0 )  W  = 0    RF ′  N0 r ′N

(10.20)

where •

F0′ is ERP of the R-FCH of the probe mobile (mobile 0); the transmit pattern is assumed to be omnidirectional.

•

L ′( θ 0 , d 0 ) is the path loss from the probe mobile (mobile 0) in the direction θ 0 back to the home base station a distance d0 away. G ′( θ 0 ) is the receive antenna gain of the home sector in the direction θ 0 to the probe mobile (mobile 0). N is the thermal noise power. W is the final spread bandwidth, which for Radio Configurations 3 and 4 is 1.25 MHz. RF′ is the data rate of the reverse fundamental channel.

•

• •

• •

r′ is the ratio of total interference power received at the home sector (on the reverse link) to thermal noise power.

r′ is sometimes known as the reverse link rise. As defined above, the term r′N is the total interference power received on the reverse link at the home sector and has three components, r ′N = rh′ N + ra′ N + N

(10.21)

where •

•

rh′ is the ratio of interference power received at the home sector (from those mobiles in the home sector) to thermal noise power. ra′ is the ratio of interference power received at the home sector (from those mobiles in other adjacent sectors) to thermal noise power.

Even though IS-2000 has a reverse pilot channel, (10.20) and (10.21) nevertheless do not contain the orthogonality factor. This is because each mobile is free to transmit its own reverse pilot channel, and no attempt is made at the base station to align all the channelization codes used by the mobiles in a particular sector. Nevertheless, the reverse pilot channel provides an important benefit as it is used as a reference signal to facilitate the coherent detection and demodulation of other reverse physical channels (e.g., reverse fundamental channel). This benefit shows up as the lower required Eb/N0 for IS-2000 reverse fundamental channels. Due to power control, F′ is not deterministic. Rather, F′ is constantly changing due to adjustments made by the mobile to maintain an acceptable Eb/N0. In that regard, F′ can be treated as a random variable. Similarly, for the sake of brevity (10.20) does not include the effects of diversity gain in soft/softer handoffs.

10.7

10.7

Reverse Supplemental Channel

167

Reverse Supplemental Channel The reverse supplemental channel is solely used for data applications. To determine whether or not the reverse supplemental channel can support data transmission, one calculates its actual Eb/N0 and ensures that it meets the required Eb/N0. The Eb/N0 of the reverse supplemental channel is given by Eb S ′ L′( θ 0 , d 0 )G′( θ 0 )  W  = 0   RS′  N0 r ′N

(10.22)

where •

•

•

• •

• •

S ′0 is ERP of the R-SCH of the probe mobile (mobile 0); the transmit pattern is assumed to be omnidirectional; L ′( θ 0 , d 0 ) is the path loss from the probe mobile (mobile 0) in the direction back to the home base station a distance d0 away; G ′( θ 0 ) is the receive antenna gain of the home sector in the direction θ 0 to the probe mobile (mobile 0); N is the thermal noise power; W is the final spread bandwidth, which for Radio Configurations 3 and 4 is 1.25 MHz; RS′ is the data rate of the reverse supplemental channel; r′ is the ratio of total interference power received at the home sector (on the reverse link) to thermal noise power.

Similar to the Eb/N0 expression of the reverse fundamental channel, the term r′N is the total interference power received on the reverse link at the home sector and is defined in (10.21). Due to power control, S′ is not fixed but constantly changing due to reverse link power control. Also, to simplify the expression, (10.22) does not include the effects of diversity gain in soft/softer handoffs. Figure 10.2 shows the signal and interference components of the reverse link.

r’a N MSs served by other BSs

G’ Home BS

L’

r’hN MSs served by home BS

F’0, S’0 Probe MS

Signal Interference Figure 10.2 Reverse link: Signal and interference components where the mobile station (MS) is the transmitter and the base station (BS) is the receiver.

168

10.8

System Design: Coverage

Upper Bounds of Interference: Reverse Link Similar to their forward link counterparts, rh′ and ra′ (and hence r′) can be determined using system simulations and actual measurements. We can also derive upper bounds for these ratios. For the interference received at the home sector from those mobiles in the home sector, J −1

rh′ ≤

( J − 1)A′ ∑ L′( θ j , d j )G′( θ j ) j =1

(10.23)

N

where •

•

•

A′ is the maximum ERP of the available amplifier power of a single mobile; the transmit pattern is assumed to be omnidirectional. L ′( θ j , d j ) is the path loss from the jth mobile in the direction θ j back to the home base station a distance dj away. G ′( θ j ) is the receive antenna gain of the home sector in the direction θ j to the jth mobile (mobile j).

Note that there are a total of J mobiles in the home sector, including the probe mobile itself. Equation (10.23) effectively states that the interference received at the home sector can never be greater than what can be radiated by the available amplifier power of all the mobiles in the sector. For the interference received from other mobiles in other sectors, K −1

ra′ ≤

∑V

k

k =1

N

(10.24)

where Vk is the total possible interference that can be received (at the home sector) from those mobiles in the