Multiple Access Protocols for Mobile Communications - GPRS, UMTS and Beyond

Multiple Access Protocols for Mobile Communications - GPRS, UMTS and Beyond

MULTIPLE ACCESS PROTOCOLS FOR MOBILE COMMUNICATIONS

Multiple Access Protocols for Mobile Communications

GPRS, UMTS and Beyond

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A catalogue record for this book is available from the British Library

British Library Cataloguing in Publication Data

Brand, Alex

Multiple access protocols for mobile communications: GPRS, UMTS and beyond/

Alex Brand, Hamid Aghvami

Typeset in 10/12pt Times by Laserwords Private Limited, Madras, India.

Printed and bound in Great Britain by Antony Rowe Ltd, Chippenham, Wiltshire.

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To M o n i c a

1.1.1 The Cellular Concept 1 1.1.2 Propagation Phenomena in Cellular Communications 2 1.1.3 Basic Multiple Access Schemes 3 1.1.4 Cell Clusters, Reuse Factor and Reuse Efficiency 6 1.1.5 Types of Interference and Noise Affecting Communications 6

1.2 The Emergence of the Internet and its Impact on Cellular

1.3 The Importance of Multiple Access Protocols in Cellular

1.4.1 Why Combine CDMA and PRMA? 12 1.4.2 Hybrid CDMA/TDMA Multiple Access Schemes 14 1.4.3 Literature on Multiple Access Protocols for Packet CDMA 15 1.4.4 Access Control in Combined CDMA/PRMA Protocols 15 1.4.5 Summary 21

2.2.1 Analogue First Generation Cellular Systems 25 2.2.2 Digital Second Generation Systems 25

2.3.1 Requirements for 3G 27 2.3.2 Evolution of 2G Systems towards 3G 29 2.3.3 Worldwide 3G Standardisation Efforts 31 2.3.4 The Third Generation Partnership Project (3GPP) 32 2.3.5 The Universal Mobile Telecommunications System (UMTS) 33 2.3.6 The Spectrum Situation for UMTS 35 2.3.7 UTRA Modes vs UTRA Requirements 36

3.3.1 Why Medium Access Control is Required 57 3.3.2 Medium Access Control in GSM 58

3.4.1 On the Importance of Multiple Access Protocols 59 3.4.2 Medium Access Control in CDMA 60 3.4.3 Conflict-free or Contention-based Access? 62

3.5.1 Random Access Protocols: ALOHA and S-ALOHA 64 3.5.2 Increasing the Throughput with Splitting or Collision

Resolution Algorithms 68 3.5.3 Resource Auction Multiple Access 69 3.5.4 Impact of Capture on Random Access Protocols 70 3.5.5 Random Access with CDMA 72 3.5.6 Protocols based on some Form of Channel Sensing 72 3.5.7 Channel Sensing with CDMA 74 3.5.8 A Case for Reservation ALOHA-based Protocols 75

3.6 Packet Reservation Multiple Access: An R-ALOHA Protocol

3.6.1 PRMA for Microcellular Communication Systems 76 3.6.2 Description of ‘Pure’ PRMA 77 3.6.3 Shortcomings of PRMA 79 3.6.4 Proposed Modifications and Extensions to PRMA 81 3.6.5 PRMA for Hybrid CDMA/TDMA 84

3.7.1 3G Requirements Relevant for the MAC Layer 86 3.7.2 Quality of Service Requirements and the MAC Layer 89 3.7.3 A few R-ALOHA Design Options 92 3.7.4 Suitable R-ALOHA Design Choices 94

4.1.1 The GSM System 99

CONTENTS ix

4.1.2 GSM Phases and Releases 101 4.1.3 Scope of this Chapter 104 4.1.4 Approach to the Description of the GSM Air Interface 105

Multiframes 115 4.2.5 Parameters describing the Physical Channel 115

4.3.1 Traffic Channels 115 4.3.2 Signalling and Control Channels 116 4.3.3 Mapping of TCH and SACCH onto the 26-Multiframe 120 4.3.4 Coding, Interleaving, and DTX for Voice on the TCH/F 120 4.3.5 Coding and Interleaving on the SACCH 124 4.3.6 The Broadcast Channel and the 51-Multiframe 124

4.4.1 Purpose of the RACH 126 4.4.2 RACH Resources in GSM 127 4.4.3 The Channel Request Message 127 4.4.4 The RACH Algorithm 128 4.4.5 Contention Resolution in GSM 131 4.4.6 RACH Efficiency and Load Considerations 132

4.5.1 How to Increase Data-rates 134 4.5.2 Basic Principles of HSCSD 135 4.5.3 Handover in HSCSD 136 4.5.4 HSCSD Multi-slot Configurations and MS Classes 137 4.5.5 Enhanced Circuit-Switched Data (ECSD) 139

4.6.1 When are Resources Used and for What? 140 4.6.2 How to Assess Resource Utilisation 143 4.6.3 Some Theoretical Considerations —

The Erlang B Formula 144 4.6.4 Resource Utilisation in Blocking-limited GSM 145 4.6.5 Resource Utilisation in Interference-limited GSM 152

4.7.1 The Purpose of GPRS: Support of Non-real-time

Packet-data Services 155 4.7.2 Air-Interface Proposals for GPRS 157 4.7.3 Basic GPRS Principles 158 4.7.4 GPRS System Architecture 160 4.7.5 GPRS Protocol Stacks 161 4.7.6 MS Classes 163 4.7.7 Mobility Management and Session Management 163

4.8 GPRS Physical and Logical Channels 164

4.8.1 The GPRS Logical Channels 164 4.8.2 Mapping of Logical Channels onto Physical Channels 165 4.8.3 Radio Resource Operating Modes 168 4.8.4 The Half-Rate PDCH and Dual Transfer Mode 169

4.9.1 Services offered and Functions performed by the Physical

Link Layer 171 4.9.2 The Radio Block Structure 171 4.9.3 Channel Coding Schemes 171 4.9.4 Theoretical GPRS Data-Rates 172 4.9.5 ‘Real’ GPRS Data-rates and Link Adaptation 175 4.9.6 The Timing Advance Procedure 177 4.9.7 Cell Reselection 179 4.9.8 Power Control 179

4.10.1 Services offered and Functions performed by MAC and RLC 180 4.10.2 The RLC Sub-layer 181 4.10.3 Basic Features of the GPRS MAC 181 4.10.4 Multiplexing Principles 183 4.10.5 RLC/MAC Block Structure 184 4.10.6 RLC/MAC Control Messages 185 4.10.7 Mobile Originated Packet Transfer 188 4.10.8 Mobile Terminated Packet Transfer 194

4.11.1 Why a New Random Access Scheme for GPRS? 197 4.11.2 Stabilisation of the Random Access Algorithm 198 4.11.3 Prioritisation at the Random Access 206 4.11.4 The GPRS Random Access Algorithm 207

4.12.1 EGPRS Coding Schemes and Link Quality Control 212 4.12.2 Other EGPRS Additions and Issues 216 4.12.3 EDGE Compact 218 4.12.4 Further Evolution of GPRS 220

5.1.1 What to Account For and How? 221 5.1.2 Using Approximations for Error Performance Assessment 222 5.1.3 Modelling the UTRA TD/CDMA Physical Layer 223 5.1.4 On Capture and Required Accuracy of Physical Layer

Modelling 225

5.2.1 On Gaussian Approximations for Error Performance

Assessment 225 5.2.2 The Standard Gaussian Approximation 227 5.2.3 Deriving Packet Success Probabilities 228 5.2.4 Importance of FEC Coding in CDMA 229

CONTENTS xi

5.2.5 Accounting for Intercell Interference 231 5.2.6 Impact of Power Control Errors 236

5.3.1 TD/CDMA as a Mode for the UMTS Terrestrial Radio

5.3.2 The TD/CDMA Physical Layer Design Parameters 238 5.3.3 In-Slot Protocols on TD/CDMA 240

5.5.1 Choice of Model 242 5.5.2 Description of the Chosen Source Model 244 5.5.3 Model of Aggregate Voice Traffic 245

5.6.1 Data Terminals 246 5.6.2 The UMTS Web Browsing Model 247 5.6.3 Proposed Email Model 250 5.6.4 A Word on Traffic Asymmetry 252 5.6.5 Random Data Traffic 253

Acknowledgements 262 6.2.7 Resource Allocation Strategies for Different Services 263 6.2.8 Performance Measures for MD PRMA 263

6.3.1 Approaches to Time-Division Duplexing 264 6.3.2 TDD with Alternating Uplink and Downlink Slots 266 6.3.3 MD FRMA for TDD with a Single Switching-Point per

6.4.1 The Concept of Channel Access Functions 267 6.4.2 Downlink Signalling with Load-based Access Control 269 6.4.3 Load-based Access Control in MD PRMA vs Channel Load

Sensing Protocol for Spread Slotted ALOHA 269

6.5.1 Stabilisation of Slotted ALOHA with Ternary Feedback 270 6.5.2 Pseudo-Bayesian Broadcast for Slotted ALOHA 270 6.5.3 Bayesian Broadcast for Two-Carrier Slotted ALOHA 271

6.5.4 Bayesian Broadcast for MD PRMA with Orthogonal

Code-Slots 272 6.5.5 Accounting for Acknowledgement Delays 273 6.5.6 Bayesian Broadcast for MD FRMA 274 6.5.7 Estimation of the Arrival Rate 274 6.5.8 Impact of MAI on Backlog Estimation 275

7.1.1 System Definition and Simulation Approach 279 7.1.2 Choice of Design Parameters 280

7.2.1 Description of the Random Access Protocol 281 7.2.2 Analysis of the Random Access Protocol 282 7.2.3 Analysis vs Simulation Results 283 7.2.4 On Multiplexing Efficiency with RAP 284

7.3.1 The Minimum-Variance Benchmark 288 7.3.2 The ‘Circuit-Switching’ Benchmark 291 7.3.3 Access Control based on Known Backlog 291

7.4.1 The Heuristic Approach 293 7.4.2 Semi-empirical Channel Access Functions 293

7.5.1 Simulation Results vs Benchmarks 298 7.5.2 Benefits of Fast Voice Activity Detection 301 7.5.3 Interpretation of the Results and the

‘Soft Capacity’ Issue 302

7.6 Impact of Power Control Errors and the Spreading Factor on

8.1.1 System Definition and Choice of Design Parameters 311 8.1.2 Simulation Approach, Traffic Parameters and Performance

8.1.3 Analysis of MD PRMA 313

8.2.1 Simulation Results, No Interleaving 314 8.2.2 Performance Comparison and Impact of Interleaving 316

CONTENTS xiii

8.3.1 Impact of Acknowledgement Delays on MD PRMA

Performance 317 8.3.2 MD FRMA vs MD PRMA 320 8.3.3 Performance of MD FRMA in TDD Mode 321 8.3.4 Impact of Voice Model Parameters on MD PRMA

Performance 322

8.4.1 Accounting for Multiple Access Interference 323 8.4.2 Performance of Combined Load- and Backlog-based Access

9.2.1 Bayesian Scheme with Two Priority Classes and

Proportional Priority Distribution 331 9.2.2 Bayesian Scheme with Two Priority Classes and

Non-proportional Priority Distribution 332 9.2.3 Bayesian Scheme with Four Priority Classes and

Semi-proportional Priority Distribution 332 9.2.4 Bayesian Scheme with Four Priority Classes and

Non-proportional Priority Distribution 333 9.2.5 Priority-class-specific Backlog Estimation 334 9.2.6 Algorithms for Frame-based Protocols 335

9.3.1 System Definition 336 9.3.2 Simulation Approach 337 9.3.3 Traffic Scenarios Considered 338

9.4.1 Voice and a Single Class of Web Traffic 339 9.4.2 Voice and Two Classes of Web Traffic 340

9.5.1 Performance with Unlimited Allocation Cycle Length 341 9.5.2 Impact of Limiting Allocation Cycle Lengths 342

9.6.1 Equal Share of Data Traffic per Priority Class 344 9.6.2 Unequal Share of Data Traffic per Priority class 344

10.1.1 UTRAN Architecture 349 10.1.2 Radio Interface Protocol Architecture 350 10.1.3 3GPP Document Structure for UTRAN 352 10.1.4 Physical Layer Basics 352 10.1.5 MAC Layer Basics 356 10.1.6 RLC Layer Basics 357

10.2 UTRA FDD Channels and Procedures 358

10.2.1 Mapping between Logical Channels and Transport Channels 358 10.2.2 Physical Channels in UTRA FDD 358 10.2.3 Mapping of Transport Channels and Indicators to Physical

10.2.4 Power Control 363 10.2.5 Soft Handover 364 10.2.6 Slotted or Compressed Mode 365

10.3.1 RACH Procedure and Packet Data on the RACH 366 10.3.2 The Common Packet Channel 370 10.3.3 Packet Data on Dedicated Channels 374 10.3.4 Packet Data on the Downlink Shared Channel 377 10.3.5 Time-Division Multiplexing vs Code-Division Multiplexing 378

10.4.1 Mapping between Logical and Transport Channels 380 10.4.2 Frame Structure and Physical Channels in UTRA TDD 381 10.4.3 Random Access Matters in UTRA TDD 382 10.4.4 Packet Data on Dedicated Channels 384 10.4.5 Packet Data on Shared Channels 384

10.5.1 Adaptive Modulation and Coding, Hybrid ARQ 386 10.5.2 Fast Cell Selection 388 10.5.3 MIMO Processing 388 10.5.4 Stand-alone DSCH 388 10.5.5 And What About Increased Data-rates on the Uplink? 389

11.2.1 Payload Optimisation 395 11.2.2 VoIP Header Overhead 396 11.2.3 How to Reduce the Header Overhead 396

11.3.1 Adoption of UMTS Protocol Stacks for GERAN 399 11.3.2 Shared or Dedicated Channels? 399 11.3.3 Proposals for Shared Channels 401 11.3.4 Likely GERAN Solutions 402

11.4 Summarising Comments on Multiplexing Efficiency and Access

11.4.1 TDMA Air Interfaces 404 11.4.2 Hybrid CDMA/TDMA Interfaces 405 11.4.3 CDMA Air Interfaces 407

Hamid Ahgvami, the director of the Centre for Telecommunications Research at King’sCollege London, who supervised numerous research projects on third generation (3G)mobile communication systems at his centre, was intrigued by both code-division multipleaccess (CDMA) and packet reservation multiple access (PRMA) In the early 1990s, thesewere two prime multiple access candidates for 3G systems, the latter essentially enhancing

an air interface using time-division multiple access (TDMA) as a basic multiple accessscheme When Alex Brand arrived at King’s in 1994 as an exchange student to carry out

a project in conclusion of his studies at the Swiss Federal Institute of Technology (ETH)

in Zurich, his brief was simple: try to combine CDMA and PRMA

What started as a five-month research project resulted in several publications, a Ph.D.thesis, and quite a few follow-on publications by other researchers on the subject ofcombined CDMA/PRMA protocols, both at King’s and elsewhere In the following, we

refer to these protocols as so-called multidimensional PRMA (MD PRMA) protocols, an

umbrella term, which accommodates also ‘non-CDMA environments’ Nevertheless, weare mostly looking at ‘CDMA environments’

The Ph.D thesis, while naturally focussing on specific research contributions related

to PRMA-based protocols, embedded these results in a quite thorough discussion ofmultiple access protocols for mobile cellular communications in general, which are themain topic of this book Accordingly, its starting point was the Ph.D thesis It was thensubstantially expanded to cover multiple access in GSM/GPRS and in UMTS, as well

as latest trends in the industry towards the merging of wireless communications and based data communications, including their impact on multiple access strategies for mobilecommunications Indeed, in tune with the increasing importance of IP technologies, themain focus of this book is on the support of packet-voice and packet-data traffic on the airinterface Topics of particular interest in this context include matters related to resourceutilisation and multiplexing efficiency and probabilistic access control used for accessarbitration at the medium access control (MAC) layer

IP-From an OSI layering perspective, the generic term ‘multiple access’ spans often bothlayer 1, the physical layer, and the lower sub-layer of layer 2, the MAC (sub-)layer We

associate ‘basic multiple access schemes’ with the physical layer, and ‘multiple access

protocols’ with the MAC (sub-)layer Unlike the few books dealing exclusively with

multiple access protocols, a key concern for the present book is the wider framework (ofmobile communications) in which they have to operate Apart from issues associated withthe physical layer and with layers above the MAC sub-layer, this includes also generaldesign principles and constraints of mobile communication systems, which have an impact

on these protocols

In its present shape, this book hopes to provide a good balance between specific researchresults, some disseminated already by the authors in scientific journals and conferenceproceedings, others published here for the first time, and useful considerations on mobilecommunications from 2G to evolved 3G systems The latter includes a discussion of theevolutionary path of the dominant 2G standard, GSM, first to a 2.5G system (mainlythrough the addition of GPRS), then to the first release of a ‘full’ 3G system in the shape

of UMTS, and finally to subsequent releases adopting more and more IP-based gies Possible 4G scenarios are also discussed This book is therefore a valuable source ofinformation for anybody interested in the latest trends in mobile communications, which

technolo-is accessible in Chapters 1 to 4, 10 and 11 without having to delve into lots of maths.Chapters 5 to 9 are more geared towards researchers and designers of multiple accessprotocols and other aspects of air interfaces for mobile communication systems Accord-ingly, some of these chapters feature a few mathematical formulas, mostly in the area ofprobability theory

Chapter 1 introduces first the main concepts related to mobile cellular communicationsystems It then discusses the importance of multiple access protocols and the impact

of the emergence of the Internet on cellular communications Finally, it summarises thespecific research contributions of the authors documented in detail in further chapters.They are mainly related to access control in the context of the multiple access protocolsinvestigated

Expanding on this introductory chapter, Chapter 2 provides more insight into currentand future cellular communication systems from 1G to 4G In particular, it discussesinitial requirements on which the design of 3G systems was based, how 2G systems can

be evolved to meet 3G requirements, and what drives the further evolution of 3G towards4G systems The latter includes a possible convergence between cellular communications,the Internet and the broadcast world The role which wireless local area networks (WLAN)are expected to play in such scenarios is also reviewed

In Chapter 3, MAC strategies for cellular communication systems, which help meetthe requirements for 3G and beyond, are examined in the context of the general problem

of multiple access in cellular communications A considerable effort is invested in posing PRMA-based strategies with possible alternatives, assessing the respective advan-tages and disadvantages qualitatively and/or quantitatively

juxta-Chapter 4 traces the evolution of the GSM air interface standards from the first systemrelease through to release 1999 of the standards, that is from a system designed primarilyfor voice to one which offers sophisticated support for packet data through an enhancedversion of GPRS The air interface spans roughly the first three OSI layers Naturally,our main interest lies at the MAC layer, but its description is embedded into an in-depthdiscussion of physical and logical channels defined for GSM In fact, for ‘plain GSM’, theMAC layer is a relatively minor matter anyway, of certain limited relevance for issuessuch as radio resource utilisation, another topic of interest on which we also presentsome research results From a MAC perspective, GPRS is much more interesting andthus featured more prominently than ‘plain GSM’ The GPRS MAC layer, in particularthe random access protocol, is explained in considerable detail The description of thelatter is complemented by research results we fed into the GPRS standardisation process.The release 1999 additions which are discussed include incremental redundancy and theso-called EDGE COMPACT mode

PREFACE xvii

To investigate the performance of multiple access protocols, it is necessary to modelsomehow the physical layer, on the services of which the MAC layer depends In addition,one has to be aware of what services higher layers expect from the MAC layer For ourresearch on PRMA-based protocols, as far as higher layers are concerned, the main focus

is on the traffic generated, which will be handed down to the MAC layer and which it has

to transfer making best possible use of the available physical link Appropriate models forphysical layer performance assessment and traffic generation are discussed in Chapter 5.Chapter 6 will define MD PRMA in detail, and introduce the two fundamental approa-ches considered to probabilistic access control at the MAC, namely load-based andbacklog-based access control As mentioned previously, access control is one of theresearch topics to which we devote particular attention

In Chapter 7, results of investigations on the benefit of load-based access control in MDPRMA are reported To this end, for voice-only traffic, the performance of MD PRMA

in the presence of intracell and intercell interference is compared with that of a randomaccess protocol and with various benchmarks The impact of power control errors is alsodiscussed

In Chapter 8, backlog-based access control for MD PRMA is treated in detail and

an assessment of its advantages compared to fixed permission probabilities is provided.This includes a comparison of the multiplexing efficiencies achieved in TDMA-only,hybrid CDMA/TDMA, and CDMA-only environments The impact of acknowledgementdelays and a protocol mode for time-division duplex are discussed The combination ofbacklog-based and load-based access control is studied Again, only packet-voice traffic

is considered

Chapter 9 provides a discussion of approaches to prioritisation at the random access

It then presents simulation results for the chosen prioritised pseudo-Bayesian algorithm,tested in a mixed traffic environment consisting of voice, Web browsing, and email traffic

In Chapter 10, after having introduced basic concepts of the UMTS air interface and theradio access network architecture, UTRA FDD channels and procedures are reviewed Themain effort is invested in exploring how packet-data traffic can be supported on UTRAFDD according to release 1999 of the standards Packet access in UTRA TDD is alsoreviewed Further, we discuss to what extent some of the research results on access controldocumented in the previous chapters can be applied to UTRA FDD and UTRA TDD.Finally, the nature of possible enhancements beyond release 1999 providing high-speedpacket access is explained

Chapter 11 concludes the main body of this book It introduces architectural ments to the UMTS packet-switched core network for the support of real-time IP-basedtraffic, the new IP multimedia subsystem, and enhancements to the GPRS/EDGE radioaccess network which allow the latter to be connected to the UMTS core network Theseendeavours can be viewed as an important step towards ‘all-IP’ Challenges relating tothe support of real-time IP services over cellular air interfaces are discussed and possiblesolutions are outlined on how to overcome problems such as the spectral inefficiencyassociated with standard voice over IP over radio Enhancements to the GPRS/EDGEair interface, enabling it to support real-time packet-data services, are reviewed The lastsection provides summarising comments on multiplexing efficiency and access control,two key topics dealt with extensively throughout this book

enhance-Each chapter is preceded by a short outline of the topics to be treated Chapters aredivided into a number of sections (e.g Section 4.2), which may in turn be split into

several subsections (such as Subsection 4.2.3) With two exceptions, chapters close with

a summary and some intermediate conclusions Readers may find the list of abbreviationsuseful, and also another list containing the symbols used throughout this book They can

be found after the acknowledgements following this preface Care was taken to choose thesymbols in a manner so that ambiguities are avoided At times, this required the choice of

symbols other than those used in previous publications, possibly untypical ones, such as X for the spreading factor instead of the widely used N , because N is also commonly used

to denote the number of time slots per TDMA frame As far as acronyms are concerned,

we made an effort to write them out in full whenever they occurred first in each chapter,but exceptions include regularly recurring acronyms and cases where they are used inpassing first, and explicitly introduced soon after Finally, following a list of references,

an appendix provides some useful information on GSM and UMTS standard documents

The authors would like to thank the many people at King’s College London and elsewherewho either acted in a supporting role, for instance by maintaining the necessary computerinfrastructure, or contributed directly to the research efforts documented in this book Thelatter include Celia Fresco Diez, David Sanchez, Francois Honore and Jean-ChristophSindt, all M.Sc or exchange students involved in research projects relating to topicstreated in this book H C Perle and Bruno Rechberger, at the time with the Swiss FederalInstitute, Zurich, provided useful input to the early stages of our research efforts, asmentioned in the text Special thanks go to John Pearson, who reviewed all of our earlierjoint publications and Alex Brand’s Ph.D thesis; in part these documents are incorporated

in this book We would also like to express our gratitude to Dr Mark Searle and Prof.Lajos Hanzo for the useful input provided, and to our industrial partners in the LINKACS research project, NEC, Plextek and Vodafone, for the many stimulating discussions

It was thanks to the involvement in this project that Alex Brand could attend SMG2 GPRSstandardisation meetings from 1995 to 1997 as an academic A special mention goes toJohn Wiley & Sons, Ltd, particularly for the considerable flexibility shown regarding thesubmission deadline for this book

Alex Brand would like to thank his parents for all the support provided, not the leastfor having wired their house throughout, providing an excellent computing infrastructurewhich, while visiting them in Switzerland, accelerated the completion of the book, and theparents in Italy for the continuous moral support provided He would also like to mentionhis colleagues at BT Wireless, for instance Fred Harrison, Steve Hearnden, Kevin Holleyand Steve Mecrow, with whom he had many useful discussions on topics related to thisbook Discussions held with Richard Townend proved particularly valuable Apart fromproviding some key input to Chapter 10, they have helped by shifting away from thesometimes misleading notion of ‘packet-switching vs circuit-switching over the air’, andinstead focus on dedicated vs common or shared channels

One person has to be singled out, Monica Dell’Anna, Alex Brand’s wife Among hermany roles, she was an invaluable technical consultant, mainly on physical layer issues,

a guinea pig as a reader of the text, judging it both in terms of content and presentation,and she acted as an illustrator She also helped with some of the more tedious jobs, such

as text formatting, and besides all this, she ran the household with little support from herhusband Simply put, this book would not have been possible without her Words cannotexpress enough gratitude

A

Associated Control CHannel (GSM)

physical channel)

Broadcast CHannel (UTRA transport channel)

C

Channel Assignment in UMTS (for CPCH operation)

(UTRA FDD physical channel)

FDD indicator)

T´el´ecommunications (European Conference of Postal andTelecommunications Administrations)

ABBREVIATIONS xxiii

D

DTCH Dedicated Traffic CHannel (UTRA logical channel)

E

F

Fast Cell Selection (for UMTS HSDPA)

G

ABBREVIATIONS xxv

Global System for Mobile Communications

H

(in EGPRS) Header Check Sequence

I

or Idle Sense Multiple Access

J

L

LINK ACS A collaborative research project on Advanced Channel Structures

for mobile communications funded by the UK government, whichwas part of phase II of the LINK personal communicationsprogramme

M

N

ABBREVIATIONS xxvii

O

P

Physical Random Access CHannel (UTRA physical channel)

PS Puncturing Scheme (EGPRS)

Q

R

technologies in Europe

S

S-RACH Short RACH (i.e using short access bursts and optionally mini-slots)

ABBREVIATIONS xxix

T

W

Z

C f [t] Number of unsuccessfully contending users in time-slot t

simultaneously

C s [t] Number of successfully contending users in time-slot t

C x [t] Number of collision slots from time-slot t − x to time-slot t − 1

F X (x) Cumulative distribution function (cdf) of the random variable X

throughput S)

respective RV)

K , K[t] Users simultaneously accessing the channel (e.g in a particular

time-slot)

K
Calls assigned to each time-slot (in case of circuit-switching)

Kmax
Maximum value of K
at a certain admissible packet error rate (P pe )max

K pemax Maximum value of K at a certain admissible packet error rate (P pe )max

M 0.01 Maximum number of conversations at Pdrop= 0.01 or Ploss= 0.01

S-ALOHA system

selected

N f Reuse factor or cluster size (the inverse is the reuse efficiency)

appropriate index

size of c

P K (k) Probability distribution function of a discrete random variable K

(Ploss)max Maximum admissible packet-loss ratio

P pe [K] Packet error probability due to MAI in a given slot carrying K packets

(P pe )max Maximum admissible packet error rate

Pslot Probability of selecting a particular time-slot in the TDMA frame

Q pe [K] Probability of packet success, 1− P pe [K]

number of cells and random number for PRACH procedure in GPRS

R [t] Reserved slots in time-slot t or reservation-mode users accessing

time-slot t

Rcell Total (aggregate) user bit-rate sustained in one cell

system

power-controlled users

a k (t) Spreading, signature, or direct-sequence of user k

f X (x) Probability density function (pdf) of the continuous random variable X

functions

parameter specifying a probability value (that is, an input value,

while P is an output probability value or ratio)

p( 1) Initial permission probability (in transmission backoff schemes)

p v [R] Average p v values classified according to R

Random number (from 0 to 1)

each MS

delay

z Prioritisation parameter (z = z1+ z2); also realisation of Z

SYMBOLS xxxv

propagation attenuation, ornormalised sensing (inhibit) delay in CSMA (ISMA)

ηmux Multiplexing efficiency relative to perfect statistical multiplexing

σ Probability that a talk gap ends in a certain time-slot; also generic

standard deviation

time-slot

σ pc Standard deviation of the log-normally distributed power control error

ε(t)

τ Propagation delay

INTRODUCTION

This book focuses on issues related to multiple access for cellular mobile communications,with a specific interest in access arbitration through multiple access protocols situated atthe lower sub-layer of the second OSI layer, namely the medium access control (MAC)layer

In this chapter, first an introduction to cellular mobile communication systems isprovided This introduction will be further expanded upon in Chapter 2, particularly withrespect to the features which distinguish the different generations of mobile communica-tion systems, from analogue first generation (1G) systems to possible fourth generation(4G) scenarios Next, it is discussed what impact the emergence of the Internet mayhave on cellular communication systems The importance of multiple access protocols isalso examined, particularly in the context of packet-based systems, and packet reserva-tion multiple access (PRMA) is considered as a case study Finally, together with somebackground information, an overview of our own research efforts related to PRMA-basedprotocols is provided These efforts are mainly concerned with how to combine PRMAwith code-division multiple access (CDMA), when such a combination is beneficial, andmore generally with different approaches to access control at the MAC layer and theirrespective benefits They are documented in detail in later chapters

The first land mobile communication systems were based on wide area transmission [1].Each base station had to provide coverage for large autonomous geographical zones Calls

of customers leaving a zone had to be dropped and re-established in a new zone [2] Suchsystems suffered from low-capacity and high-transmit power requirements for mobiletransceivers, shortcomings that would not have allowed us to witness the tremendousgrowth in mobile communications in the past few years, with penetrations now exceeding

70% in many countries Only the introduction of cellular mobile communication systems

in the late 1970s made this development possible, by enabling frequencies, used in onecell, to be reused under certain conditions in other cells to increase capacity Nowadays,

mobile communication systems are almost by implication cellular communication systems

as well We use either of these two terms interchangeably, sometimes also the full term,

namely cellular mobile communication systems.

Cellular mobile communication systems are designed to provide moving users (frompedestrians to travellers in high-speed trains) with a means of communication In contrast

to (basic) cordless telephones, cellular telephones (also referred to as mobile phones,

mobile stations, mobile terminals or sometimes simply handsets) are not attached to aparticular base station, but may make use of any one of the base stations provided bythe company that operates the corresponding network Each of these base stations covers

a particular area of the landscape, called a cell The ensemble of base stations should

cover the landscape in such a way that the user can travel around and carry on a phonecall without interruption, possibly making use of more than one base station, as shown in

Figure 1.1 The procedure of changing a base station at cell boundaries is called handover

or handoff We prefer the first term, since it implies (unlike the second term) that an effort

is made to sustain a call across cell boundaries Obviously, these systems can also servestationary users, and do so increasingly, as fixed telephones are more and more substituted

by wireless phones

Communication from the mobile station (MS) to the base station (BS) takes place on

the uplink channel or reverse link and from BS to MS on the downlink channel or forward

link (Figure 1.1) To enable communication, some resources need to be allocated to the

base station (these may be frequency bands, time-slots, sets of codes, or any combination

of the three), which in turn may assign a portion of them to individual calls to supportcommunication on both uplink and downlink channels The amount of resources allocated

to users will depend on the current resource availability and the particular requirements

of each requesting user As the base station must be able to assign individual portions of

its resources to support multiple communications, basic multiple access techniques (such

as frequency-, time-, or code-division multiple access, with FDMA, TDMA, and CDMA

as their respective acronyms) are required together with multiple access protocols, which

govern access to these resources The basic multiple access schemes are briefly describedfurther below in this section, and the importance of the multiple access protocols isexamined in Section 1.3

The design of cellular communication systems is particularly challenging because of theadverse propagation conditions experienced on the radio channel Without discussing thecomplex underlying physical mechanisms, for which the reader may consult a mobilecommunications handbook such as that in Reference [3] or a book dedicated to radio

Figure 1.1 (a) Basic principle of cellular communications (b) Uplink and downlink channel

1.1 AN INTRODUCTION TO CELLULAR COMMUNICATION SYSTEMS 3

propagation such as that in Reference [4], three main propagation effects are usuallydistinguished These are the pathloss, slow fading or shadowing, and fast fading or multi-

path fading The pathloss describes the average signal attenuation as a function of the

distance between transmitter and receiver, which includes the free-space attenuation as onecomponent, but also other factors come into play in cellular communications, resulting

in an environment-dependent pathloss behaviour Shadowing or slow fading describes

slow signal fluctuations, which are typically caused by large structures, such as big

build-ings, obstructing the propagation paths Fast or multipath fading is caused by the fact

that signals propagate from transmitter to receiver through multiple paths, which can add

at the receiver constructively or destructively depending on the relative signal phases.The received signal is said to be in a deep fade when the paths add destructively in amanner that the received signal level is close to zero Fades occur roughly once everyhalf wavelength [3] Given that we are dealing with wavelengths of 30 cm and less incellular communication systems, it is clear that multipath fading can result in relativelyfast signal fluctuations; exactly how fast depends on the speed of the mobile station and

on the dynamics of the surrounding environment

When designing cellular communication systems and particularly when planning thedeployment of such systems (e.g choosing suitable base station locations), one willhave to account for these propagation phenomena appropriately One way to do this

is to use deterministic propagation tools such as ray tracing tools, which will late experienced signal levels for every specific location of the planned system coveragearea, taking into account every structure which could affect signal levels Another way

calcu-is to resort to statcalcu-istical models, which have to be establcalcu-ished by analysing tion measurements performed in suitably chosen environments, e.g classified as denseurban, typical urban, suburban and rural propagation environments, to name just a few.For the purposes of some of our investigations, we will deal with distance-independentpathloss coefficients and a so-called lognormal shadowing model, as outlined in detail inChapter 5

For reasons discussed in detail in Chapter 3, we make a distinction between basic multiple

access schemes, such as FDMA, TDMA, and CDMA, associated with the physical layer

(PHY) on the air interface of a mobile communications system, and multiple access

protocols, situated at the medium access control (MAC) layer above the PHY Roughly

speaking, the basic schemes provide the capability of dividing the total resources available

to a base station into individual portions, which can be assigned to different users, andthe protocols govern access to these resource portions, e.g provide access arbitration.Analogue first generation cellular communication systems made use of FDMA as a basicmultiple access scheme In digital 2G systems, TDMA is predominant, but a CDMA-basedsystem exists as well CDMA is the most commonly used form of multiple access for thirdgeneration systems, in some cases complemented by a hybrid CDMA/TDMA scheme

In FDMA, each communication is carried over one or two (depending on the duplexscheme, see below) narrowband frequency channels The channel bandwidth and themodulation scheme determine the gross bit-rate that can be sustained Because of non-ideal