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Advanced Wireless Networks

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  • 1. ADVANCED WIRELESS NETWORKS Cognitive, Cooperative and Opportunistic 4G Technology Second Edition Savo Glisic Beatriz Lorenzo University of Oulu, Finland A John Wiley and Sons, Ltd., Publication
  • 2. ADVANCED WIRELESS NETWORKS
  • 3. ADVANCED WIRELESS NETWORKS Cognitive, Cooperative and Opportunistic 4G Technology Second Edition Savo Glisic Beatriz Lorenzo University of Oulu, Finland A John Wiley and Sons, Ltd., Publication
  • 4. This edition first published 2009 C 2009 John Wiley & Sons Ltd., Registered office John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, United Kingdom For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at www.wiley.com. The right of the author to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. Library of Congress Cataloging-in-Publication Data Glisic, Savo G. Advanced wireless networks : 4G technologies / Savo Glisic, Beatriz Lorenzo Veiga. – 2nd ed. p. cm. Includes bibliographical references and index. ISBN 978-0-470-74250-1 (cloth) 1. Wireless communication systems. I. Veiga, Beatriz Lorenzo. II. Title. TK5103.2.G553 2009 621.384—dc22 2009001817 A catalogue record for this book is available from the British Library. ISBN 978-0-470-74250-1 (H/B) Typeset in 9/11 Times by Laserwords Private Limited, Chennai, India Printed in Singapore by Markono Print Media Pte Ltd
  • 5. To our families
  • 6. Contents Preface to the Second Edition xix 1 Fundamentals 1.1 4G Networks and Composite Radio Environment 1.2 Protocol Boosters 1.2.1 One-element error detection booster for UDP 1.2.2 One-element ACK compression booster for TCP 1.2.3 One-element congestion control booster for TCP 1.2.4 One-element ARQ booster for TCP 1.2.5 A forward erasure correction booster for IP or TCP 1.2.6 Two-element jitter control booster for IP 1.2.7 Two-element selective ARQ booster for IP or TCP 1.3 Green Wireless Networks References 1 1 7 9 9 9 9 10 10 11 11 11 2 Opportunistic Communications 2.1 Multiuser Diversity 2.2 Proportional Fair Scheduling 2.3 Opportunistic Beamforming 2.4 Opportunistic Nulling in Cellular Systems 2.5 Network Cooperation and Opportunistic Communications 2.5.1 Performance example 2.6 Multiuser Diversity in Wireless Ad Hoc Networks 2.6.1 Multiple-output and multiple-input link diversity 2.6.2 Localized opportunistic transmission 2.6.3 Multiuser diversity-driven clustering 2.6.4 Opportunistic MAC with timeshare fairness 2.6.5 CDF-based K-ary opportunistic splitting algorithm 2.6.6 Throughput 2.6.7 Optimal opportunistic MAC 15 15 16 19 20 22 25 27 29 30 31 34 34 37 37
  • 7. viii CONTENTS 2.7 2.8 2.6.8 Contention resolution between clusters 2.6.9 Performance examples Mobility-Assisted Opportunistic Scheduling (MAOS) 2.7.1 Mobility models 2.7.2 Optimal MAOS algorithm 2.7.3 Suboptimum MAOS algorithm 2.7.4 Mobility estimation and prediction 2.7.5 Estimation of Lagrange multipliers 2.7.6 Performance examples Opportunistic and Cooperative Cognitive Wireless Networks 2.8.1 The system model 2.8.2 The outage probability 2.8.3 Cellular traffic shaping 2.8.4 User mobility modeling 2.8.5 Absorbing Markov chain system model 2.8.6 Throughput analysis 2.8.7 Collision resolution 2.8.8 Opportunistic transmission with intercell interference awareness 2.8.9 Performance examples References 38 40 46 48 49 51 51 52 52 53 53 57 58 59 61 62 65 65 68 70 3 Relaying and Mesh Networks 3.1 Relaying Strategies in Cooperative Cellular Networks 3.1.1 The system model 3.1.2 System optimization 3.1.3 Relay strategy selection optimization 3.1.4 Performance example 3.2 Mesh/Relay Networks 3.2.1 The system model 3.2.2 Exhaustive sleep 3.2.3 Practical applications 3.2.4 Performance example 3.3 Opportunistic Ad Hoc Relaying For Multicast 3.3.1 The system model 3.3.2 Proxy discovery and route interference 3.3.3 Near-optimal multicast and approximations 3.3.4 Performance examples References 73 73 73 75 79 84 85 86 88 94 95 97 98 99 101 103 107 4 Topology Control 4.1 Local Minimum Spanning Tree (LMST) Topology Control 4.1.1 Basics of MST topology control 4.1.2 Performance examples 4.2 Joint Topology Control, Resource Allocation and Routing 4.2.1 JTCR algorithm 4.3 Fault-Tolerant Topology 4.3.1 The system model 4.3.2 Fault-tolerant topology design 4.3.3 Þ-Approximation algorithms 4.3.4 Performance examples 113 115 115 118 118 121 123 124 124 127 132
  • 8. CONTENTS 4.4 ix Topology Control in Directed Graphs 4.4.1 The system model 4.4.2 Minimum-weight-based algorithms 4.4.3 Augmentation-based algorithms 4.4.4 Performance examples Adjustable Topology Control 4.5.1 The system model 4.5.2 The r -neighborhood graph Self-Configuring Topologies 4.6.1 SCT performance References 132 133 133 135 138 138 140 142 143 145 148 5 Adaptive Medium Access Control 5.1 WLAN Enhanced Distributed Coordination Function 5.2 Adaptive MAC for WLAN with Adaptive Antennas 5.2.1 Description of the protocols 5.3 MAC for Wireless Sensor Networks 5.3.1 S-MAC protocol design 5.3.2 Periodic listen and sleep 5.3.3 Collision avoidance 5.3.4 Coordinated sleeping 5.3.5 Choosing and maintaining schedules 5.3.6 Maintaining synchronization 5.3.7 Adaptive listening 5.3.8 Overhearing avoidance and message passing 5.3.9 Overhearing avoidance 5.3.10 Message passing 5.4 MAC for Ad Hoc Networks 5.4.1 Carrier sense wireless networks 5.4.2 Interaction with upper layers References 157 157 160 160 166 167 168 168 169 169 170 170 172 172 172 174 176 179 180 6 Teletraffic Modeling and Analysis 6.1 Channel Holding Time in PCS Networks References 183 183 191 7 Adaptive Network Layer 7.1 Graphs and Routing Protocols 7.1.1 Elementary concepts 7.1.2 Directed graph 7.1.3 Undirected graph 7.1.4 Degree of a vertex 7.1.5 Weighted graph 7.1.6 Walks and paths 7.1.7 Connected graphs 7.1.8 Trees 7.1.9 Spanning tree 7.1.10 MST computation 7.1.11 Shortest path spanning tree 7.2 Graph Theory 193 193 193 193 194 194 195 195 195 196 197 199 201 212 4.5 4.6
  • 9. x CONTENTS 7.3 7.4 Routing with Topology Aggregation Network and Aggregation Models 7.4.1 Line segment representation 7.4.2 QoS-aware topology aggregation 7.4.3 Mesh formation 7.4.4 Star formation 7.4.5 Line-segment routing algorithm 7.4.6 Performance measure 7.4.7 Performance example References 214 215 217 220 220 221 222 224 225 228 8 Effective Capacity 8.1 Effective Traffic Source Parameters 8.1.1 Effective traffic source 8.1.2 Shaping probability 8.1.3 Shaping delay 8.1.4 Performance example 8.2 Effective Link Layer Capacity 8.2.1 Link-layer channel model 8.2.2 Effective capacity model of wireless channels 8.2.3 Physical layer vs link-layer channel model 8.2.4 Performance examples References 235 235 237 238 238 241 243 244 246 249 251 254 9 Adaptive TCP Layer 9.1 Introduction 9.1.1 A large bandwidth-delay product 9.1.2 Buffer size 9.1.3 Round-trip time 9.1.4 Unfairness problem at the TCP layer 9.1.5 Noncongestion losses 9.1.6 End-to-end solutions 9.1.7 Bandwidth asymmetry 9.2 TCP Operation and Performance 9.2.1 The TCP transmitter 9.2.2 Retransmission timeout 9.2.3 Window adaptation 9.2.4 Packet loss recovery 9.2.5 TCP-OldTahoe (timeout recovery) 9.2.6 TCP-Tahoe (fast retransmit) 9.2.7 TCP-Reno fast retransmit, fast (but conservative) recovery 9.2.8 TCP-NewReno (fast retransmit, fast recovery) 9.2.9 Spurious retransmissions 9.2.10 Modeling of TCP operation 9.3 TCP for Mobile Cellular Networks 9.3.1 Improving TCP in mobile environments 9.3.2 Mobile TCP design 9.3.3 The SH-TCP client 9.3.4 The M-TCP protocol 9.3.5 Performance examples 257 257 258 259 260 261 262 262 263 264 264 265 265 265 265 265 265 266 267 267 268 269 270 272 273 275
  • 10. CONTENTS 9.4 9.5 Random Early Detection Gateways for Congestion Avoidance 9.4.1 The RED algorithm 9.4.2 Performance example TCP for Mobile Ad Hoc Networks 9.5.1 Effect of route recomputations 9.5.2 Effect of network partitions 9.5.3 Effect of multipath routing 9.5.4 ATCP sublayer 9.5.5 ATCP protocol design 9.5.6 Performance examples References 10 Network Optimization Theory 10.1 Introduction 10.2 Layering as Optimization Decomposition 10.2.1 TCP congestion control 10.2.2 TCP Reno/RED 10.2.3 TCP Vegas/Drop Tail 10.2.4 Optimization of the MAC protocol 10.2.5 Utility optimal MAC protocol/social optimum 10.3 Crosslayer Optimization 10.3.1 Congestion control and routing 10.3.2 Congestion control and physical resource allocation 10.3.3 Congestion and contention control 10.3.4 Congestion control, routing and scheduling 10.4 Optimization Problem Decomposition Methods 10.4.1 Decoupling coupled constraints 10.4.2 Dual decomposition of the basic NUM 10.4.3 Coupling constraints 10.4.4 Decoupling coupled objectives 10.4.5 Alternative decompositions 10.4.6 Application example of decomposition techniques to distributed crosslayer optimization 10.5 Optimization of Distributed Rate Allocation for Inelastic Utility Flows 10.5.1 Nonconcave utility flows 10.5.2 Capacity provisioning for convergence of the basic algorithm 10.6 Nonconvex Optimization Problem in Network with QoS Provisioning 10.6.1 The system model 10.6.2 Solving the nonconvex optimization problem for joint congestion–contention control 10.7 Optimization of Layered Multicast by Using Integer and Dynamic Programming 10.7.1 The system model 10.7.2 Lagrangian relaxation for integer programs 10.7.3 Group profit maximization by dynamic programming 10.8 QoS Optimization in Time-Varying Channels 10.8.1 The system model 10.8.2 Dynamic control algorithm 10.9 Network Optimization by Geometric Programming 10.9.1 Power control by geometric programming: high SNR 10.9.2 Power control by geometric programming: low SNR 10.10 QoS Scheduling by Geometric Programming xi 276 276 277 280 280 280 280 281 282 287 287 289 289 290 290 291 292 292 295 298 298 301 303 306 307 307 308 310 310 313 315 319 319 322 323 323 325 326 327 329 329 331 331 332 337 338 340 340
  • 11. xii CONTENTS 10.10.1 Optimization of OFDM system by GP 10.10.2 Maximum weight matching scheduling by GP 10.10.3 Opportunistic scheduling by GP 10.10.4 Rescue scheduling by GP References 344 344 345 345 346 11 Mobility Management 11.1 Introduction 11.1.1 Mobility management in cellular networks 11.1.2 Location registration and call delivery in 4G 11.2 Cellular Systems with Prioritized Handoff 11.2.1 Channel assignment priority schemes 11.2.2 Channel reservation – CR handoffs 11.2.3 Channel reservation with queueing – CRQ handoffs 11.2.4 Performance examples 11.3 Cell Residing Time Distribution 11.4 Mobility Prediction in Pico- and MicroCellular Networks 11.4.1 PST-QoS guarantees framework 11.4.2 Most likely cluster model Appendix: Distance Calculation in an Intermediate Cell References 351 351 353 355 374 377 377 378 382 383 388 390 391 398 403 12 Cognitive Radio Resource Management 12.1 Channel Assignment Schemes 12.1.1 Different channel allocation schemes 12.1.2 Fixed channel allocation 12.1.3 Channel borrowing schemes 12.1.4 Simple channel borrowing schemes 12.1.5 Hybrid channel borrowing schemes 12.1.6 Dynamic channel allocation 12.1.7 Centralized DCA schemes 12.1.8 Cell-based distributed DCA schemes 12.1.9 Signal strength measurement-based distributed DCA schemes 12.1.10 One-dimensional cellular systems 12.1.11 Reuse partitioning (RUP) 12.2 Dynamic Channel Allocation with SDMA 12.2.1 Single-cell environment 12.2.2 Resource allocation 12.2.3 Performance examples 12.3 Packet-Switched SDMA/TDMA Networks 12.3.1 The system model 12.3.2 Multibeam SDMA/TDMA capacity and slot allocation 12.3.3 SDMA/TDMA slot allocation algorithms 12.3.4 SDMA/TDMA performance examples 12.4 SDMA/OFDM Networks with Adaptive Data Rate 12.4.1 The system model 12.4.2 Resource allocation algorithm 12.4.3 Impact of OFDM/SDMA system specifications on resource allocations 12.4.4 Performance examples 12.5 Intercell Interference Cancellation – SP Separability 407 407 409 410 410 411 412 414 415 417 419 420 422 426 426 430 435 435 437 439 441 445 446 446 448 450 453 454
  • 12. CONTENTS 12.6 12.7 12.8 12.9 12.5.1 Channel and cellular system model 12.5.2 Turbo space–time multiuser detection for intracell communications 12.5.3 Multiuser detection in the presence of intercell interference 12.5.4 Performance examples Intercell Interference Avoidance in SDMA Systems 12.6.1 The BOW scheme 12.6.2 Generating beam-off sequences 12.6.3 Constrained QRA-IA Multilayer RRM 12.7.1 The SRA protocol 12.7.2 The ESRA protocol Resource Allocation with Power Preassignment (RAPpA) 12.8.1 Resource assignment protocol 12.8.2 Analytical modeling of RAPpA Cognitive and Cooperative Dynamic Radio Resource Allocation 12.9.1 Signal-to-interference ratio 12.9.2 System performance 12.9.3 Multicell operation 12.9.4 Performance examples Appendix 12A: Power Control, CD Protocol, in the Presence of Fading Appendix 12B: Average Intercell Throughput References 13 Ad Hoc Networks 13.1 Routing Protocols 13.1.1 Routing protocols 13.1.2 Reactive protocols 13.2 Hybrid routing protocol 13.2.1 Loop-back termination 13.2.2 Early termination 13.2.3 Selective broadcasting (SBC) 13.3 Scalable Routing Strategies 13.3.1 Hierarchical routing protocols 13.3.2 Performance examples 13.3.3 FSR (fisheye routing) protocol 13.4 Multipath Routing 13.5 Clustering Protocols 13.5.1 Introduction 13.5.2 Clustering algorithm 13.5.3 Clustering with prediction 13.6 Cashing Schemes for Routing 13.6.1 Cache management 13.7 Distributed QoS Routing 13.7.1 Wireless links reliability 13.7.2 Routing 13.7.3 Routing information 13.7.4 Token-based routing 13.7.5 Delay-constrained routing 13.7.6 Tokens 13.7.7 Forwarding the received tokens 13.7.8 Bandwidth-constrained routing xiii 455 457 459 460 461 467 468 468 470 471 473 475 476 479 484 486 488 491 492 494 498 499 505 505 507 512 524 526 527 528 531 531 533 534 537 539 539 541 542 549 549 558 558 558 559 559 560 561 562 562
  • 13. xiv CONTENTS 13.7.9 Forwarding the received tickets 13.7.10 Performance example References 562 564 567 14 Sensor Networks 14.1 Introduction 14.2 Sensor Networks Parameters 14.2.1 Pre-deployment and deployment phase 14.2.2 Post-deployment phase 14.2.3 Re-deployment of additional nodes phase 14.3 Sensor networks architecture 14.3.1 Physical layer 14.3.2 Data link layer 14.3.3 Network layer 14.3.4 Transport layer 14.3.5 Application layer 14.4 Mobile Sensor Networks Deployment 14.5 Directed Diffusion 14.5.1 Data propagation 14.5.2 Reinforcement 14.6 Aggregation in Wireless Sensor Networks 14.7 Boundary Estimation 14.7.1 Number of RDPs in P 14.7.2 Kraft inequality 14.7.3 Upper bounds on achievable accuracy 14.7.4 System optimization 14.8 Optimal Transmission Radius in Sensor Networks 14.8.1 Back-off phenomenon 14.9 Data Funneling 14.10 Equivalent Transport Control Protocol in Sensor Networks References 573 573 575 576 576 577 577 578 578 581 585 586 587 590 591 593 593 596 598 598 599 600 602 606 607 610 613 15 Security 15.1 Authentication 15.1.1 Attacks on simple cryptographic authentication 15.1.2 Canonical authentication protocol 15.2 Security Architecture 15.3 Key Management 15.3.1 Encipherment 15.3.2 Modification detection codes 15.3.3 Replay detection codes 15.3.4 Proof of knowledge of a key 15.3.5 Point-to-point key distribution 15.4 Security management in GSM networks 15.5 Security management in UMTS 15.6 Security architecture for UMTS/WLAN Interworking 15.7 Security in Ad Hoc Networks 15.7.1 Self-organized key management 15.8 Security in Sensor Networks References 623 623 625 629 631 635 637 637 637 637 638 639 643 645 647 651 652 654
  • 14. CONTENTS xv 16 Active Networks 16.1 Introduction 16.2 Programable Networks Reference Models 16.2.1 IETF ForCES 16.2.2 Active networks reference architecture 16.3 Evolution to 4G Wireless Networks 16.4 Programmable 4G Mobile Network Architecture 16.5 Cognitive Packet Networks 16.5.1 Adaptation by cognitive packets 16.5.2 The random neural networks-based algorithms 16.6 Game Theory Models in Cognitive Radio Networks 16.6.1 Cognitive radio networks as a game 16.7 Biologically Inspired Networks 16.7.1 Bio-analogies 16.7.2 Bionet architecture References 659 659 661 662 662 665 667 670 672 673 675 678 682 682 684 686 17 Network Deployment 17.1 Cellular Systems with Overlapping Coverage 17.2 Imbedded Microcell in CDMA Macrocell Network 17.2.1 Macrocell and microcell link budget 17.2.2 Performance example 17.3 Multitier Wireless Cellular Networks 17.3.1 The network model 17.3.2 Performance example 17.4 Local Multipoint Distribution Service 17.4.1 Interference estimations 17.4.2 Alternating polarization 17.5 Self-Organization in 4G Networks 17.5.1 Motivation 17.5.2 Networks self-organizing technologies References 693 693 698 699 702 703 704 708 709 711 711 713 713 715 717 18 Network Management 18.1 The Simple Network Management Protocol 18.2 Distributed Network Management 18.3 Mobile Agent-Based Network Management 18.3.1 Mobile agent platform 18.3.2 Mobile agents in multioperator networks 18.3.3 Integration of routing algorithm and mobile agents 18.4 Ad Hoc Network Management 18.4.1 Heterogeneous environments 18.4.2 Time varying topology 18.4.3 Energy constraints 18.4.4 Network partitioning 18.4.5 Variation of signal quality 18.4.6 Eavesdropping 18.4.7 Ad hoc network management protocol functions 18.4.8 ANMP architecture References 721 721 725 726 728 728 730 735 735 735 736 736 736 736 736 738 743
  • 15. xvi CONTENTS 19 Network Information Theory 19.1 Effective Capacity of Advanced Cellular Networks 19.1.1 4G cellular network system model 19.1.2 The received signal 19.1.3 Multipath channel: near–far effect and power control 19.1.4 Multipath channel: pointer tracking error, rake receiver and interference canceling 19.1.5 Interference canceler modeling: nonlinear multiuser detectors 19.1.6 Approximations 19.1.7 Outage probability 19.2 Capacity of Ad Hoc Networks 19.2.1 Arbitrary networks 19.2.2 Random networks 19.2.3 Arbitrary networks: an upper bound on transport capacity 19.2.4 Arbitrary networks: lower bound on transport capacity 19.2.5 Random networks: lower bound on throughput capacity 19.3 Information Theory and Network Architectures 19.3.1 Network architecture 19.3.2 Definition of feasible rate vectors 19.3.3 The transport capacity 19.3.4 Upper bounds under high attenuation 19.3.5 Multihop and feasible lower bounds under high attenuation 19.3.6 The low-attenuation regime 19.3.7 The Gaussian multiple-relay channel 19.4 Cooperative Transmission in Wireless Multihop Ad Hoc Networks 19.4.1 Transmission strategy and error propagation 19.4.2 OLA flooding algorithm 19.4.3 Simulation environment 19.5 Network Coding 19.5.1 Max-flow min-cut theorem (mfmcT) 19.5.2 Achieving the max-flow bound through a generic LCM 19.5.3 The transmission scheme associated with an LCM 19.5.4 Memoryless communication network 19.5.5 Network with memory 19.5.6 Construction of a generic LCM on an acyclic network 19.5.7 Time-invariant LCM and heuristic construction 19.6 Capacity of Wireless Networks Using MIMO Technology 19.6.1 Capacity metrics 19.7 Capacity of Sensor Networks with Many-to-One Transmissions 19.7.1 Network architecture 19.7.2 Capacity results References 747 747 749 750 752 753 755 757 757 761 762 764 765 768 769 773 773 775 776 776 777 778 779 780 783 784 784 787 788 789 792 793 794 794 795 798 800 805 805 807 809 20 Energy-efficient Wireless Networks 20.1 Energy Cost Function 20.2 Minimum Energy Routing 20.3 Maximizing Network Lifetime 20.4 Energy-efficient MAC in Sensor Networks 20.4.1 Staggered wakeup schedule References 813 813 815 816 821 821 823
  • 16. CONTENTS xvii 21 Quality-of-Service Management 21.1 Blind QoS Assessment System 21.1.1 System modeling 21.2 QoS Provisioning in WLAN 21.2.1 Contention-based multipolling 21.2.2 Polling efficiency 21.3 Dynamic Scheduling on RLC/MAC Layer 21.3.1 DSMC functional blocks 21.3.2 Calculating the high service rate 21.3.3 Heading-block delay 21.3.4 Interference model 21.3.5 Normal delay of a newly arrived block 21.3.6 High service rate of a session 21.4 QoS in OFDMA-Based Broadband Wireless Access Systems 21.4.1 Iterative solution 21.4.2 Resource allocation to maximize capacity 21.5 Predictive Flow Control and QoS 21.5.1 Predictive flow control model References 827 827 829 831 831 832 835 837 838 840 841 841 842 842 846 848 849 850 854 Index 859
  • 17. Preface to the Second Edition Although the first edition of the book was not published long ago, a constant progress in research in the field of wireless networks has resulted in a significant accumulation of new results that urge the extension and modification of its content. The major additions in the book are the following new chapters: Chapter 1: Fundamentals, Chapter 2: Opportunistic Communications, Chapter 3: Relaying and Mesh Networks, Chapter 4: Topology Control, Chapter 10: Network Optimization and Chapter 12: Cognitive Radio Resource Management. OPPORTUNISTIC COMMUNICATIONS Multiuser diversity is a form of diversity inherent in a wireless network, provided by independent time-varying channels across the different users. The diversity benefit is exploited by tracking the channel fluctuations of the users and scheduling transmissions to users when their instantaneous channel quality is near the peak. The diversity gain increases with the dynamic range of the fluctuations and is thus limited in environments with little scattering and/or slow fading. In such environments, the multiple transmit antennas can be used to induce large and fast channel fluctuations so that multiuser diversity can still be exploited. The scheme can be interpreted as opportunistic beamforming and true beamforming gains can be achieved when there are sufficient users, even though very limited channel feedback is needed. Furthermore, in a cellular system, the scheme plays an additional role of opportunistic nulling of the interference created on users of adjacent cells. This chapter discusses the design implications of implementing this scheme in a wireless system. RELAYING AND MESH NETWORKS In a wireless network with many source–destination pairs, cooperative transmission by relay nodes has the potential to improve the overall network performance. In a distributed multihop mesh/relay network (e.g. wireless ad hoc/sensor network, cellular multihop network), each node acts as a relay node to forward data packets from other nodes. These nodes are often energy-limited and also have limited buffer space. Therefore, efficient power-saving mechanisms (e.g. sleeping mechanisms) are
  • 18. xx PREFACE TO THE SECOND EDITION required so that the lifetime of these nodes can be extended while at the same time the quality of service (QoS) requirements (e.g. packet delay and packet loss rate) for the relayed packets can be satisfied. In Chapter 3, a queuing analytical framework is presented to study the tradeoffs between the energy saving and the QoS at a relay node as well as relaying strategies in cooperative cellular networks. In addition integrated cellular and ad hoc multicast, which increases multicast throughput through opportunistic use of ad hoc relays, is also discussed. NETWORK TOPOLOGY CONTROL Energy efficiency and network capacity are perhaps two of the most important issues in wireless ad hoc networks and sensor networks. Topology control algorithms have been proposed to maintain network connectivity while reducing energy consumption and improving network capacity. The key idea to topology control is that, instead of transmitting with maximal power, nodes in a wireless multihop network collaboratively determine their transmission power and define the network topology by forming the proper neighbour relation under certain criteria. The topology control affects network spatial reuse and contention for the medium. A number of topology control algorithms have been proposed to create a power-efficient network topology in wireless multihop networks with limited mobility. In Chapter 4, we summarize existing work in this field. Some of the algorithms require explicit propagation channel models, while others incur significant message exchanges. Their ability to maintain the topology in the case of mobility is also rather limited. The chapter will discuss the tradeoffs between these opposing requirements. NETWORK OPTIMIZATION Network protocols in layered architectures have traditionally been obtained on an ad hoc basis, and many of the recent crosslayer designs are also conducted through piecemeal approaches. Network protocol stacks may instead be systematically analyzed and designed as distributed solutions to some global optimization problems. Chapter 10 presents a survey of the recent efforts toward a systematic understanding of layering as optimization decomposition, where the overall communication network is modelled by a generalized network utility maximization problem, where each layer corresponds to a decomposed subproblem and the interfaces among layers are quantified as functions of the optimization variables coordinating the subproblems. There can be many alternative decompositions, leading to a choice of different layering architectures. This chapter will survey the current status of horizontal decomposition into distributed computation and vertical decomposition into functional modules such as congestion control, routing, scheduling, random access, power control and channel coding. Key results are summarized and open issues discussed. Through case studies, it is illustrated how layering as optimization decomposition provides a common language to modularization, a unifying, top-down approach to design protocol stacks and a mathematical theory of network architectures. COGNITIVE RADIO RESOURCE MANAGEMENT Network optimization, including radio resource management, discussed in Chapter 10, provides algorithms that optimize system performance defined by a given utility function. In Chapter 12, we present suboptimum solutions for resource management that include high level of cognition and cooperation to mitigate intercell interference. An important segment of this topic dealing with the
  • 19. PREFACE TO THE SECOND EDITION xxi flexible spectra sharing is covered in another of our books on Advanced Wireless Communications focusing more on the physical layer, published by John Wiley & Sons, Ltd in 2007. In addition to the new chapters, which represent about 40 % of the book, other chapters have been also updated with latest results. Savo Glisic Beatriz Lorenzo
  • 20. 1 Fundamentals 1.1 4G NETWORKS AND COMPOSITE RADIO ENVIRONMENT In the wireless communications community we are witnessing more and more the existence of the composite radio environment (CRE ) and as a consequence the need for reconfigurability concepts based on cognitive, cooperative and opportunistic algorithms. The CRE assumes that different radio networks can be cooperating components in a heterogeneous wireless access infrastructure, through which network providers can more efficiently achieve the required capacity and quality of service (QoS) levels. Reconfigurability enables terminals and network elements dynamically to select and adapt to the most appropriate radio access technologies for handling conditions encountered in specific service area regions and time zones of the day. Both concepts pose new requirements on the management of wireless systems. Nowadays, a multiplicity of radio access technology (RAT) standards are used in wireless communications. As shown in Figure 1.1, these technologies can be roughly categorized into four sets: ž Cellular networks that include second-generation (2G) mobile systems, such as Global System for Mobile Communications (GSM) [1], and their evolutions, often called 2.5G systems, such as enhanced digital GSM evolution (EDGE), General Packet Radio Service (GPRS) [2] and IS 136 in the US. These systems are based on TDMA technology. Third-generation (3G) mobile networks, known as Universal Mobile Telecommunications Systems (UMTS) (WCDMA and cdma2000) [3] are based on CDMA technology that provides up to 2 Mbit/s. Long-term evolution (LTE) [4–12] of these systems is expected to evolve into a 4G system providing up to 100 Mbit/s on the uplink and up to 1 Gbit/s on the downlink. The solutions will be based on a combination of multicarrier and space–time signal formats. The network architectures include macro, micro and pico cellular networks and home (HAN) and personal area networks (PAN). ž Broadband radio access networks (BRANs) [13] or wireless local area networks (WLANs) [14] which are expected to provide up to 1 Gbit/s in 4G. These technologies are based on OFDMA and space–time coding. ž Digital video broadcasting (DVB) [15] and satellite communications. ž Ad hoc and sensor networks with emerging applications. Advanced Wireless Technologies: Cognitive, Cooperative & Opportunistic 4G Technology Second Edition Savo G. Glisic C 2009 John Wiley & Sons, Ltd.
  • 21. 2 FUNDAMENTALS Sensor networks (self configur ation) PLMN PSTN Ad hoc networks IP Network (mobile user agents) Private Network Cellular multihop network macro/micro/ Pico/PAN/BAN Space-time– frequency coding (100Mb) Cellular multihop network Access Network Reconfiguration & Dynamic Spectra Allocation BRAN/ WLAN/mesh Access Space-timefrequency coding, (1Gbit) DVB satellite Reconfigurable Mobile Terminals (Cognitive, Cooperative and Opportunistic) Figure 1.1 Composite radio environment in cognitive, cooperative and opportunistic 4G networks. In order to increase the spectral efficiency further, besides the space–time frequency coding in the physical layer, the new paradigms like cognitive [16–20], cooperative [21–32] and opportunistic [33–38] solutions will be used. Although 4G is open for new multiple access schemes, the CRE concept remains attractive for increasing the service provision efficiency and the exploitation possibilities of the available RATs. The main assumption is that the different radio networks, GPRS, UMTS, BRAN/WLAN, DVB and so on, can be components of a heterogeneous wireless access infrastructure. A network provider (NP) can own several components of the CR infrastructure (in other words, can own licenses for deploying and operating different RATs), and can also cooperate with affiliated NPs. In any case, an NP can rely on several alternate radio networks and technologies, for achieving the required capacity and QoS levels, in a cost-efficient manner. Users are directed to the most appropriate radio networks and technologies, at different service area regions and time zones of the day, based on profile requirements and network performance criteria. The various RATs are thus used in a
  • 22. 4G NETWORKS AND COMPOSITE RADIO ENVIRONMENT 3 complementary manner rather than competing each other. Even nowadays a mobile handset can make a handoff between different RATs. The deployment of CRE systems can be facilitated by the reconfigurability concept, which is an evolution of a software-defined radio [39, 40]. The CRE requires terminals that are able to work with different RATs, and the existence of multiple radio networks offering alternate wireless access capabilities to service area regions. Reconfigurability supports the CRE concept by providing essential technologies that enable terminals and network elements dynamically (transparently and securely) to select and adapt to the set of RATs that are most appropriate for the conditions encountered in specific service area regions and time zones of the day. According to the reconfigurability concept, RAT selection is not restricted to those that are pre-installed in the network element. In fact, the required software components can be dynamically downloaded, installed and validated. This makes it different from the static paradigm regarding the capabilities of terminals and network elements. The networks provide wireless access to IP (Internet protocols)-based applications and service continuity in the light of intrasystem mobility. Integration of the network segments in the CR infrastructure is achieved through the management system for the CRE (MS-CRE) component attached to each network. The management system in each network manages a specific radio technology; however, the platforms can cooperate. The fixed (core and backbone) network will consist of public and private segments based on IPv4- and IPv6-based infrastructures. A mobile IP (MIP) will enable the maintenance of IP-level connectivity regardless of the likely changes in the underlying radio technologies used that will be imposed by the CRE concept. Figures 1.2 and 1.3 depict the architecture of a terminal that is capable of operating in a CRE context. The terminals include software and hardware components (layer 1 and 2 functionalities) for operating with different systems. The higher protocol layers, in accordance with their peer entities in the network, support continuous access to IP-based applications. Different protocol busters can further enhance the efficiency of the protocol stack. There is a need to provide the best possible IP performance over wireless links, including legacy systems. Within the performance implications of link characteristics (PILC) of the IETF group, the concept of a performance-enhancing proxy Terminal management system • Network discovery support • Network selection • Mobility management intersystem (vertical) handover • QoS monitoring • Profile management user preferences, terminal characteristics Application Enhanced for TMS interactions and Information flow synchronization Transport layer TCP/UDP Network layer protocol boosters & conversion IP Mobile IP bandwidth reasignment GPRS support protocol Layers 2/1 UMTS support protocol Layers 2/1 WLAN/BRAN Support protocol Layers 2/1 DVB-T Support protocol Layers 2/1 Figure 1.2 Architecture of a terminal that operates in a composite radio environment.
  • 23. 4 FUNDAMENTALS Application Enhanced for TMS interactions and Information flow synchronization Terminal management System • Network discovery support • Network selection • Mobility management intersystem (vertical) handovers • QoS monitoring • Profile management • Functionality for software download, instalation, validation • Security, fault/error recovery Transport layer TCP/UDP protocol boosters & conversion Network layer IP, Mobile IP Active configurations Repository Reconfigurable modem Interface • Reconfiguration commands • Monitoring information Software components for communication through The selected RATs bandwidth reasignment Rat-specific and generic software components an parameters Figure 1.3 Architecture of a terminal that operates in the reconfigurability context. (PEP) [41–44] has been chosen to refer to a set of methods used to improve the performance of Internet protocols on network paths where native TCP/IP performance is degraded due to characteristics of a link. Different types of PEPs, depending on their basic functioning, are also distinguished. Some of them try to compensate for the poor performance by modifying the protocols themselves. In contrast, a symmetric/asymmetric boosting approach, transparent to the upper layers, is often both more efficient and flexible. A common framework to house a number of different protocol boosters provides high flexibility, as it may adapt to both the characteristics of the traffic being delivered and the particular conditions of the links. In this sense, a control plane for easing the required information sharing (cross-layer communication and configurability) is needed. Furthermore, another requirement comes from the appearance of multihop communications, as PEPs have been traditionally used over the last hop, so they should be adapted to the multihop scenario. Most communications networks are subject to time and regional variations in traffic demands, which lead to variations in the degree to which the spectrum is utilized. Therefore, a service’s radio spectrum can be underused at certain times or geographical areas, while another service may experience a shortage at the same time/place. Given the high economic value placed on the radio spectrum and the importance of spectrum efficiency, it is clear that wastage of radio spectrum must be avoided. These issues provide the motivation for a scheme called dynamic spectrum allocation (DSA), which aims to manage the spectrum utilized by a converged radio system and share it between participating radio networks over space and time to increase overall spectrum efficiency, as shown in Figures 1.4 and 1.5. Composite radio systems and reconfigurability, discussed above, are potential enablers of DSA systems. Composite radio systems allow seamless delivery of services through the most appropriate
  • 24. 4G NETWORKS AND COMPOSITE RADIO ENVIRONMENT Time or region RAN3 RAN1 RAN3 RAN2 RAN3 RAN3 RAN2 RAN2 RAN1 RAN2 RAN1 RAN3 RAN3 Time or region RAN1 RAN1 RAN2 RAN1 RAN3 RAN2 RAN1 RAN1 RAN1 RAN1 RAN1 Frequency RAN2 RAN2 RAN2 RAN2 RAN1 RAN2 RAN1 RAN2 RAN2 RAN1 RAN1 RAN2 RAN1 RAN2 RAN2 RAN1 Fragmented Frequency RAN2 RAN1 Frequency RAN2 Contiguous RAN1 Fixed 5 Time or region Figure 1.4 Fixed spectrum allocation compared to contiguous and fragmented DSA. 2483 2400 2200 1900 1880 1710 960 880 854 470 230 217 WLAN UMTS GSM GSM Analog TV and DVB-T DAB Contiguous DSA (a) Contiguous DSA WLAN Analog TV and DVB-T GSM GSM UMTS WLAN Analog TV and DVB-T DAB ••• Fragmented DSA (b) Contiguous DSA WLAN GSM UMTS WLAN GSM GSM UMTS Analog TV and DVB-T DAB Fragmented DSA (c) Figure 1.5 DSA operation configurations: (a) static (current spectrum allocations); (b) continuous DSA operations; (c) discrete DSA operations.
  • 25. 6 FUNDAMENTALS access network, and close network cooperation can facilitate the sharing not only of services but also of spectrum. Reconfigurability is also a very important issue, since with a DSA system a radio access network could potentially be allocated any frequency at any time in any location. It should be noted that the application layer is enhanced with the means to synchronize various information streams of the same application, which could be transported simultaneously over different RATs. The terminal management system (TMS) is essential for providing functionality that exploits the CR environment. On the user/terminal side, the main focus is on the determination of the networks that provide, in a cost-efficient manner, the best QoS levels for the set of active applications. A first requirement is that the MS-CRE should exploit the capabilities of the CR infrastructure. This can be done in a reactive or proactive manner. Reactively, the MS-CRE reacts to new service area conditions, such as the unexpected emergence of hot spots. Proactively, the management system can anticipate changes in the demand pattern. Such situations can be alleviated by using alternate components of the CR infrastructure to achieve the required capacity and QoS levels. The second requirement is that the MS-CRE should provide resource brokerage functionality to enable the cooperation of the networks of the CR infrastructure. Finally, parts of the MS-CRE should be capable of directing users to the most appropriate networks of the CR infrastructure, where they will obtain services efficiently in terms of cost and QoS. To achieve the above requirements the MS architecture shown in Figure 1.6 is required. The architecture consists of three main logical entities: ž Monitoring, service-level information and resource brokerage (MSRB). ž Resource management strategies (RMS). ž Session managers (SMs). Short-term operation Mid-term operation MS-CRE Profile and service-level information Resource brokerage Session manager Management Plane interface Service configuration traffic distribution Netwotk configuration Status monitoring MSRB Management Plane interface Mobile terminal Managed network (component of CR infrastructure)— Legacy element and network management systems User and control plane interface Figure 1.6 Architecture of the MS-CRE. RMS Management Plane interface
  • 26. PROTOCOL BOOSTERS Session manager MSRB RMS 7 MS-CRE 1. Identification of new condition in service area 2. Extraction of status of Network and of SLAs 3a. Offer request 3b. Offer request 3c. Offer request 4a. Optimization request 4b. Determination of new service provision pattern (QoS levels, traffic distribution to networks) Computation of Tentative reconfigurations 4c. Reply 5. Solution acceptance phase. Reconfiguration of managed Network and managed components Figure 1.7 MS-CRE operation scenario. The MSRB entity identifies the triggers (events) that should be handled by the MS-CRE and provides corresponding auxiliary (supporting) functionality. The RMS entity provides the necessary optimization functionality. The SM entity is in charge of interacting with the active subscribed users/terminals. The operation steps and cooperation of the RMS components are shown in Figures 1.7 and 1.8, respectively. In order to gain an insight into the scope and range of possible reconfigurations, we review the network and protocol stack architectures of the basic CRE components as indicated in Figure 1.1. 1.2 PROTOCOL BOOSTERS As pointed out in Figure 1.2, an element of the reconfiguration in 4G networks are protocol boosters. A protocol booster is a software or hardware module that transparently improves protocol performance. The booster can reside anywhere in the network or end systems, and may operate independently (one-element booster) or in cooperation with other protocol boosters (multielement booster). Protocol boosters provide an architectural alternative to existing protocol adaptation techniques, such as protocol conversion. A protocol booster is a supporting agent that by itself is not a protocol. It may add, delete or delay protocol messages, but never originates, terminates or converts that protocol. A multielement protocol booster may define new protocol messages to exchange among themselves, but these protocols are originated and terminated by protocol booster elements, and are not visible or
  • 27. 8 FUNDAMENTALS MSRB Service configuration traffic distribution Network configuration 1. Optimization request 2. Service configuration and traffic distribution: Allocation to QoS and networks 3a. Request for checking the feasibility of solution 3b. Computation of Tentative network reconfiguration 3c. Reply on feasibility of solution 4. Selection of best Feasible solution 5. Reply 6. Solution acceptance phase 7. Network configuration Figure 1.8 Cooperation of the RMS components. Protocol messages Host X Booster A Booster B Host Y Booster messages Figure 1.9 Two-element booster. meaningful external to the booster. Figure 1.9 shows the information flow in a generic two-element booster. A protocol booster is transparent to the protocol being boosted. Thus, the elimination of a protocol booster will not prevent end-to-end communication, as would, for example, the removal of one end of a conversion (e.g. a TCP/IP header compression unit). In what follows we will present examples of protocol busters.
  • 28. PROTOCOL BOOSTERS 9 1.2.1 One-element error detection booster for UDP UDP has an optional 16-bit checksum field in the header. If it contains the value zero, it means that the checksum was not computed by the source. Computing this checksum may be wasteful on a reliable LAN. On the other hand, if errors are possible, the checksum greatly improves data integrity. A transmitter sending data does not compute a checksum for either local or remote destinations. For reliable local communication, this saves the checksum computation (at the source and destination). For wide-area communication, the single-element error detection booster computes the checksum and puts it into the UDP header. The booster could be located either in the source host (below the level of UDP) or in a gateway machine. 1.2.2 One-element ACK compression booster for TCP On a system with asymmetric channel speeds, such as broadcast satellite, the forward (data) channel may be considerably faster than the return (ACK) channel. On such a system, many TCP ACKs may build up in a queue, increasing round-trip time and thus reducing the transmission rate for a given TCP window size. The nature of TCP’s cumulative ACKs means that any ACK acknowledges at least as many bytes of data as any earlier ACK. Consequently, if several ACKs are in a queue, it is necessary to keep only the ACK that has arrived most recently. A simple ACK compression booster could ensure that only a single ACK exists in the queue for each TCP connection. (A more sophisticated ACK compression booster allows some duplicate ACKs to pass, allowing the TCP transmitter to get a better picture of network congestion.) The booster increases the protocol performance because it reduces the ACK latency and allows faster transmission for a given window size. 1.2.3 One-element congestion control booster for TCP Congestion control reduces buffer overflow loss by reducing the transmission rate at the source when the network is congested. A TCP transmitter deduces information about network congestion by examining acknowledgments (ACKs) sent by the TCP receiver. If the transmitter sees several ACKs with the same sequence number, then it assumes that network congestion caused a loss of data messages. If congestion is noted in a subnet, then a congestion control booster could artificially produce duplicate ACKs. The TCP receiver would think that data messages have been lost because of congestion, and would reduce its window size, thus reducing the amount of data it injects into the network. 1.2.4 One-element ARQ booster for TCP TCP uses ARQ to retransmit data unacknowledged by the receiver when a packet loss is suspected, such as after a retransmission timeout expires. If we assume the network of Figure 1.9 (except that Booster B does not exist), then an ARQ booster for TCP will: (a) cache packets from Host Y; (b) if it sees a duplicate acknowledgment arrive from Host X and it has the next packet in the cache; then it deletes the acknowledgment and retransmits the next packet (because a packet must have been lost between the booster and Host X); and (c) delete packets retransmitted from Host Y that have been acknowledged by Host X. The ARQ booster improves performance by shortening the retransmission path. A typical application would be if Host X were on a wireless network and the booster were on the interface between the wireless and wireline networks.
  • 29. 10 FUNDAMENTALS 1.2.5 A forward erasure correction booster for IP or TCP For many real-time and multicast applications, forward error correction coding is desirable. The two-element FZC booster uses a packet forward error correction code and erasure decoding. The FZC booster at the transmitter side of the network adds parity packets. The FZC booster at the receiver side removes the parity packets and regenerates missing data packets. The FZC booster can be applied between any two points in a network (including the end systems). If applied to an IP, then a sequence number booster adds sequence number information to the data packets before the first FZC booster. If applied to TCP (or any protocol with sequence number information), then the FZC booster can be more efficient because: (1) it does not need to add sequence numbers and (2) it could add new parity information on TCP retransmissions (rather than repeating the same parities). At the receiver side, the FZC booster could combine information from multiple TCP retransmissions for FZC decoding. 1.2.6 Two-element jitter control booster for IP For real-time communication, we may be interested in bounding the amount of jitter that occurs in the network. A jitter control booster can be used to reduce jitter at the expense of increased latency. At the first booster element, timestamps are generated for each data message that passes. These (a) (b) Figure 1.10 Three-dimensional amplitude patterns of a two-element uniform amplitude array for d D 2½, directioned towards (a) Â0 D 0Ž , (b) Â0 D 60Ž .
  • 30. GREEN WIRELESS NETWORKS (a) (c) (b) 11 (d) Figure 1.11 Three-dimensional amplitude patterns of a ten-element uniform amplitude array for d D ½=4, directioned towards (a) Â0 D 0Ž , (b) Â0 D 30Ž , (c) Â0 D 60Ž , (d) Â0 D 90Ž . timestamps are transmitted to the second booster element, which delays messages and attempts to reproduce the intermessage interval that was measured by the first booster element. 1.2.7 Two-element selective ARQ booster for IP or TCP For links with significant error rates using a selective ARQ protocol (with selective acknowledgment and selective retransmission) can significantly improve the efficiency compared to using TCP’s ARQ (with cumulative acknowledgment and possibly go-back-N retransmission). The two-element ARQ booster uses a selective ARQ booster to supplement TCP by: (1) caching packets in the upstream booster, (2) sending negative acknowledgments when gaps are detected in the downstream booster and (3) selectively retransmitting the packets requested in the negative acknowledgments (if they are in the cache). 1.3 GREEN WIRELESS NETWORKS 4G wireless networks might be using a spatial notching (angle Þ) to suppress completely antenna radiation towards the user, as illustrated in Figures 1.10 and 1.11. These solutions will be referred to as ‘green wireless networks’ for obvious reasons. In order to ensure the connectivity in the case when the antenna lobe is not directed towards the access point, a multihop communication, with the possibility of relaying, is required. In addition, to reduce the overall transmit power a cooperative transmit diversity, discussed in Section 19.4, and adaptive MAC protocol, discussed in Chapter 5 can be used. REFERENCES [1] M. Mouly and M.-B. Pautet, The GSM System for Mobile Communications, Palaiseau, France, 1992.
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  • 33. 14 FUNDAMENTALS [40] J. Mitola III and G. Maguire Jr, Cognitive radio: making software radios more personal, IEEE Pers. Commun., vol. 6, no. 4, August 1999, pp. 13–18. [41] J. Border et al., Performance enhancing proxies intended to mitigate link-related degradations, RFC 3135, June 2001. [42] D. C. Feldmeier et al., Protocol boosters, IEEE J. Selected Areas Commun., vol. 16, no. 3, April 1998, pp. 437–444. [43] M. Garc´a et al., An experimental study of snoop TCP performance over the IEEE 802.11b ı WLAN, in 5th Int. Symp. on Wireless Personal Multimedia Commun., vol. III, Honolulu, HA, October 2002, pp. 1068–1072. [44] L. Mu˜ oz et al., Optimizing Internet flows over IEEE 802.11b wireless local area networks: n a performance enhancing proxy based on forward error correction, IEEE Commun. Mag., vol. 39, no. 12, December 2001, pp. 60–67.
  • 34. 2 Opportunistic Communications As pointed out in Chapter 1, opportunistic signaling will be used in 4G networks to increase further the spectral efficiency of these systems. In this chapter we discuss a number of different solutions that are based on that principle. 2.1 MULTIUSER DIVERSITY Multiuser diversity is provided in wireless networks by independent time-varying channels across the different users. The diversity benefit is exploited by tracking the channel fluctuations of the users and scheduling transmissions to users when their instantaneous channel quality is highest. The diversity gain increases with the dynamic range of the fluctuations and is thus limited in environments with slow fading. In such environments, multiple-transmit antennas can be used to induce large and fast channel fluctuations so that multiuser diversity can be improved [1]. The scheme can be interpreted as opportunistic beamforming and beamforming gains can be achieved when there are sufficient users. In a cellular system, the scheme plays an additional role of opportunistic nulling of the interference created on users of adjacent cells. Let us assume a simple model of the downlink of a cellular wireless communication system with a base station (transmitter) having a single antenna communicating with K users (receivers). The time-slotted block-fading channel model in baseband is given by yk (t) D h k (t)x(t) C zk (t); T k D 1; 2; : : : ; K (2.1) T In Equation (2.1), x(t) 2 C is the vector of T transmitted symbols, yk (t) 2 C is the vector of T received symbols of user k, h k (t) 2 C is the fading channel gain from the transmitter to receiver k and fzk (t)gt is an independent and identically distributed (i.i.d.) sequence of zero mean circularsymmetric Gaussian random vectors CN (0; ¦ 2 IT ). Here we assume that the channel is constant over time slots of length T samples and that the transmit power level is P D const at all times, i.e. E[kx(t)k2 ] D P T . Advanced Wireless Technologies: Cognitive, Cooperative & Opportunistic 4G Technology Second Edition Savo G. Glisic C 2009 John Wiley & Sons, Ltd.
  • 35. 16 OPPORTUNISTIC COMMUNICATIONS Total Throughput in bps/Hz 2,2 AWGN Channel Rayleigh Fading 2 1,8 1,6 1,4 1,2 1 0,8 0 2 4 6 8 10 Number of Users 12 14 Figure 2.1 Sum capacity of two channels, Rayleigh fading and AWGN, with average SNR D 0 dB. We can view this downlink channel as a set of parallel Gaussian channels, one for each fading state. The sum capacity of this channel, defined by the maximum achievable sum of long-term average data rates transmitted to all the users, can be achieved by a simple time division multiple access (TDMA) strategy: at each fading state, transmit to the user with the strongest channel [2]. The sum capacity of the downlink channel is presented in Figure 2.1, as a function of the number of users, for the case when users undergo independent Rayleigh fading with average received signal-to-noise ratio (SNR) D 0 dB. One can see that the sum capacity increases with the number of users in the system. On the other hand, the sum capacity of a nonfaded downlink channel, where each user has a fixed additive white Gaussian noise (AWGN) channel with SNR = 0 dB, is constant irrespective of the number of users. In a system with many users with independently varying channels, it is likely that at any time there is a user with a channel much stronger than the average SNR. By transmitting to users with strong channels at all times, the overall spectral efficiency of the system can be made high, significantly higher than that of a nonfaded channel with the same average SNR. In order to exploit such multiuser diversity it is necessary that: ž Each receiver should track its own channel SNR through, say, common downlink pilot, and feed back the instantaneous channel quality to the base station. ž The base station has the ability to schedule transmissions among the users as well as to adapt the data rate as a function of the instantaneous channel quality. 2.2 PROPORTIONAL FAIR SCHEDULING The concept of multiuser diversity brings about two issues: fairness and delay. When users’ fading statistics are the same, the strategy above maximizes not only the total capacity of the system but also the throughput of individual users. When the statistics are not symmetrical, the multiuser diversity concept provides maximum long-term average throughputs. In practice, there are latency requirements, in which case the average throughputs and limited delay is the performance metric of interest. In the sequel the objective will be to address these issues while at the same time exploiting the multiuser diversity gain inherent in a system, with users having independent, fluctuating channel conditions. In a further discussion the feedback of the channel quality of user k in time slot t
  • 36. PROPORTIONAL FAIR SCHEDULING 17 Requested rates in bps/Hz 1 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0 50 100 150 200 250 Time Slots Requested rates in bps/Hz Figure 2.2 For symmetric channel statistics of users, the scheduling algorithm reduces to serving each user with the largest requested rate. 1,1 1 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0 50 100 150 200 250 Time Slots Figure 2.3 In general, with asymmetric user channel statistics, the scheduling algorithm serves each user when it is near its peak within the latency time scale tc . to the base station will be expressed in terms of a requested data rate Rk (t). This is the data rate that the kth user’s channel can support at the time. The scheduling algorithm keeps track of the average throughput Tk (t) of each user in a past window of length tc , and in time slot t the scheduling algorithm transmits to the user k Ł with the largest Rk (t)=Tk (t) among all active users in the system. The average throughputs Tk (t) can be updated using an exponentially weighted low-pass filter Tk (t C 1) D (1 (1 1=tc )Tk (t) C Rk (t)=tc ; 1=tc )Tk (t); k D kŁ : k 6D k Ł (2.2) The operation of the algorithm is illustrated in Figures 2.2 and 2.3. The sample paths of the requested data rates of two users are plotted as a function of time slots (each time slot is 1.67 ms). In Figure 2.2, the two users have identical fading statistics. If the scheduling time scale tc is much larger than the correlation time scale of the fading dynamics, then by symmetry the throughput of each user Tk (t) converges to the same quantity. The scheduling algorithm reduces to always
  • 37. OPPORTUNISTIC COMMUNICATIONS Average Throughput in bps/Hz 18 2,2 2 1,8 1,6 1,4 1,2 Mobile Fixed Equal time scheduling 1 0,8 0 5 10 15 Number of Users 20 25 Figure 2.4 Multiuser diversity gain in fixed and mobile environments. picking the user with the highest requested rate. Thus, each user is scheduled when its channel is good and at the same time the scheduling algorithm is perfectly fair in the long term. In Figure 2.3, one user’s channel is much stronger than the other user’s on the average, although both channels fluctuate due to multipath fading. Always picking the user with the highest requested rate means giving all the system resources to the statistically stronger user and would be unfair. Under the scheduling algorithm defined by Equation (2.2), users compete for resources not only based on their requested rates but after normalization by their respective average throughputs. The user with the statistically stronger channel will have a higher average throughput. Thus, the algorithm schedules a user when its instantaneous channel quality is high relative to its own average channel condition over the period tc . In other words, data are transmitted to a user when the channel is near its own peaks. Multiuser diversity benefit can still be exploited because channels of different users fluctuate independently so that if there is a sufficient number of users in the system, there is likely to be a user near its peak at any one time. The parameter tc is tied to the latency time scale of the application. Peaks are defined with respect to this time scale. If the latency time scale is large, then the throughput is averaged over a longer time scale and the scheduler can afford to wait longer before scheduling a user when its channel hits a really high peak. The theoretical properties of this scheduling algorithm are further explored later in this chapter. It will be shown that this algorithm guarantees a fairness property called proportional fairness. Figure 2.4 gives some insights into the issues involved in realizing multiuser diversity benefits in practice. The plot shows the total throughput of the downlink under the proportional fair scheduling algorithm in the following two simulated environments: ž Fixed. Users are static but there are movements of objects around them (2 Hz Rician, def Ä D E direct =E specular D 5). ž Mobile. Users move at walking speeds (3 km/h, Rayleigh). The total throughput increases with the number of users in both the fixed and mobile environments, but the increase is more dramatic in the mobile case. While the channel fades in both cases, the dynamic range and the rate of the variations is larger in the mobile environment than in the fixed one. This means that over the latency time scale (1.67 s in these examples), the peaks of the channel fluctuations are likely to be higher in the mobile environment, and the peaks are what determines the performance of the scheduling algorithm. Thus, the inherent multiuser diversity is more limited in the fixed environment.
  • 38. OPPORTUNISTIC BEAMFORMING 2.3 19 OPPORTUNISTIC BEAMFORMING The effectiveness of multiuser diversity depends on the rate and dynamic range of channel fluctuations. In environments where the channel fluctuations are small, the multiuser diversity gain can be increased by inducing faster and larger fluctuations. In Reference [1] multiple-transmit antennas at the base station are used for these purposes, as illustrated in Figure 2.5. For such a system with N transmit antennas at the base station, let h nk (t) be the complex channel gain from antenna n to the kth user in time slot t. In time slot t, the same block of symbols x(t) is p transmitted from all of the antennas except that it is multiplied by a complex number Þn (t)e jÂn (t) N at antenna n, for n D 1; : : : ; N , such that nD1 Þn (t) D 1, preserving the total transmit power. The received signal at user k is given by N Þn (t)e jÂn (t) h nk (t) x(t) C zk (t) yk (t) D (2.3) nD1 Thus, the overall channel gain seen by receiver k is now N Þn (t)e jÂn (t) h nk (t) h k (t) :D (2.4) nD1 In Equation (2.4), the Þn (t) denote the fractions of power allocated to each of the transmit antennas and the Ân (t) the phase shifts applied at each antenna to the signal. By varying over time Þn (t) from 0 to 1 and Ân (t) from 0 to 2³, fluctuations in the overall channel can be induced even if the physical channel gains h nk (t) do not fluctuate much. Each receiver k reports back the value of x (t) a(t) 1−a(t)e jq(t) h2k (t) h1k (t) User k Figure 2.5 The same signal is transmitted over the two antennas with time-varying phase and powers.
  • 39. 20 OPPORTUNISTIC COMMUNICATIONS Average Throughput in bps/Hz 2,2 2 1,8 1,6 1,4 Mobile Fixed, Opp, BF Fixed Equal time scheduling 1,2 1 0,8 0 5 10 15 20 25 30 Number of Users Figure 2.6 Amplification in multiuser diversity gain with opportunistic beamforming in a fixed environment. SNRjh k (t)j2 =¦ 2 of its own channel to the base station and the base station schedules transmissions to users accordingly. The rate of variation of fÞn (t)g and fÂn (t)g in time is a design parameter of the system. On one side it should be as fast as possible to provide full channel fluctuations within the latency time scale of interest. On the other hand, the variation should be slow enough to allow the channel to be reliably estimated by the users and the SNR information feedback. Further, the variation should be slow enough to ensure that the channel seen by the users does not change abruptly and thus maintains stability of the channel tracking loop. To illustrate the performance of this scheme, we now consider the fixed environment of Figure 2.5 with two antennas of equal and constant (over time) power split and phase rotation over [0; 2³ ] (with one complete rotation in 30 ms as in Reference [1]). Figure 2.6 plots the improved performance as a function of number of users. This improvement is due to the fact that the channel is changing faster and the dynamic range of variation is larger over the time scale of scheduling (1.67 s in this example). 2.4 OPPORTUNISTIC NULLING IN CELLULAR SYSTEMS For wide-band cellular systems with full frequency reuse, it is important to consider the effect of intercell interference on the performance of the system, particularly in interference-limited scenarios. In a cellular system, the channel quality of a user is measured by the signal-to-interference plus noise ratio (SINR). In a fading environment, the energies in both the received signal and the received interference fluctuate over time. Since the multiuser diversity scheduling algorithm allocates resources based on the channel SINR (which depends on both the channel amplitude and the amplitude of the interference), it automatically exploits both the fluctuations in the energy of the received signal as well as that of the interference: the algorithm tries to schedule resource to a user whose instantaneous channel is good and the interference is weak. Thus, multiuser diversity naturally takes advantage of the time-varying interference to increase the spatial reuse of the network. From this point of view, power and phase randomization at the base station transmit antennas plays an additional role: it increases not only the amount of fluctuations of the received signal
  • 40. OPPORTUNISTIC NULLING IN CELLULAR SYSTEMS 21 to the intended users within the cells, but also the amount of the fluctuations of the interference the base station causes in adjacent cells. Hence, opportunistic beamforming has a dual benefit in an interference-limited cellular system. In fact, opportunistic beamforming performs opportunistic nulling simultaneously, while randomization of power and phase in the transmitted signals from the antennas allows near-coherent beamforming to some user within the cell; it will create near nulls at some other user in adjacent cells. This in effect allows interference avoidance for that user if it is currently being scheduled. In a slow flat fading scenario under power and phase randomization at all base stations, the received signal of a typical user being interfered by J adjacent base stations is given by J g j (t)u j (t) C z(t) y(t) D h(t)x(t) C (2.5) jD1 where x(t) is the signal of interest, u j (t) is the interference from the jth base station and z(t) is additive Gaussian noise. All base stations have the same transmit power P and N transmit antennas and are performing power and phase randomization independently; h(t) and g j (t) are the overall channel gains from the base stations N Þn (t)e jÂn (t) h n h(t) :D (2.6) nD1 N þn j (t) D e j g j (t) :D n j (t) gn j (2.7) nD1 where h n and gn j are the slow fading channel gains to the user from the nth transmit antenna of the base station of interest and the interfering base station j, respectively. Averaging over the signal x(t) and the interference u j (t), the (time-varying) SINR of the user can be computed to be Pjh(t)j2 SINR(t) D J P (2.8) jg j (t)j2 C ¦ 2 jD1 The SINR varies because of both the variations of the overall gain from the base station of interest as well as those from the interfering base station. In a system with many other users, the N proportional fair scheduler will serve this user while its SINR is at its peak P nD1 jh n (t)j2 =¦ 2 , i.e. when the received signal is the strongest and the interference is completely nulled out. Thus, the opportunistic nulling and beamforming technique has the potential to shift a user from a low-SNR interference-limited regime to a high-SNR noise-limited regime. How close the performance of opportunistic beamforming and nulling in a finite-size system is to this asymptotic limit depends on the probability that the received signal is near beamformed and all the interference is near null. In the interference-limited regime when P=¦ 2 × 1, the performance depends mainly on the probability of the latter event. This probability is larger when there are only one or two base stations contributing most of the interference, as is typically the case. In contrast, when there is interference from many base stations, interference averaging occurs and the probability that the total interference is near null is much smaller. Interference averaging, which is good for CDMA networks, is actually unfavorable for the opportunistic scheme described here, since it reduces the likelihood of the nulling of the interference and hence the likelihood of the peaks of the SINR. In a typical cell, there will be a distribution of users, some closer to the base station and some closer to the cell boundaries. Users close to the base station are at high SNR and are noise-limited; the contribution of the intercell interference is relatively small. These users benefit
  • 41. 22 OPPORTUNISTIC COMMUNICATIONS mainly from opportunistic beamforming (diversity gain plus a 3 dB power gain if there are two transmit antennas). Users close to the cell boundaries, on the other hand, are at low SNR and are interference-limited; the average interference power can be much larger than the background noise. These users benefit both from opportunistic beamforming and from opportunity nulling of intercell interference. Thus, the cell-edge users benefit more in this system than users in the interior. This is rather desirable from a system fairness point of view, as the cell-edge users tend to have poorer service. This feature is particularly important for a system without soft handoff (which is difficult to implement in a packet data scheduling system). To maximize the opportunistic nulling benefits, the transmit power at the base station should be set as large as possible, subject to regulatory and hardware constraints. 2.5 NETWORK COOPERATION AND OPPORTUNISTIC COMMUNICATIONS In this section we consider a network architecture with B cooperating and B noncooperating base stations (BSs). The cluster of cooperating BSs cooperate to improve the capacity with fairness to the users in the network. The noncooperating BSs do not cooperate with BSs in this cluster and cause interference to the users, which cannot be mitigated. As an example, in Figure 2.7 [3], there are three BSs (marked with circles), which cooperate and provide service to the users in the shaded region. The other BSs cause interference to these users. In the sequel we focus only on the activities of the cooperating BSs. During every time slot each coordinating BS selects one beam out of the set of L beams available to it and services one user. In this way, the cluster of B cooperating BSs supports B users simultaneously. The system operates in two steps. In the first step, each BS transmits a pilot signal using a certain beam and collects the SINR reports from the users. The BS then transmits these reports to a centralized unit which, in the second step, schedules users based on their current and past SINR reports for data packet transmission. This process is repeated for the entire time period over which data are transmitted. The number of users K being serviced during this time period is assumed constant. Figure 2.7 Three-base intercell coordination scenario.
  • 42. NETWORK COOPERATION AND OPPORTUNISTIC COMMUNICATIONS 23 In a given timeslot n during the first step, each BS transmits a pilot signal using one of the L beams. The received signal power at user i from the BS b on the lth beam is Si [n] D P(i;b) jh (i;b;l) [n]j2 , where h (i;b;l) [n] is the channel gain from BS b to the user i on beam l and P(i;b) is the received signal power at the user i with the combined effect of path loss and shadowing. The interference plus noise power is INi [n] D Ii [n] C Noi [n], where I (Ð) is the interference power from the noncooperating BSs and No(Ð) is the thermal noise power. The user measures the received SINR, (i;b;l) [n] D Si [n]=INi [n] and transmits this SINR value to the BS. The centralized scheduler collects the SINR reports from all the BSs and obtains the matrix K ðB , where the (i; b)th element represents the SINR feedback by user i on the beam from BS b. In the second phase the centralized scheduler using these data opportunistically schedules B users on each of the B BSs, and each BS then transmits to the scheduled user. The data rate R(i;b;l) , a BS b, provides a given user i on beam l and is calculated by    R(i;b;l) D log2 1 C Si [n]   D log2 1 C B  I Ni [n] C (i;b;l) [n] 1 C 2 P(i;b) h (i;b;l) [n]  ˜ ˜ ˜ ˜ b6Db  B ˜ ˜ (i;b;l) [n]  (2.9) ˜ b6Db In a scenario where the BSs cooperate and opportunistically beamform and schedule users the main steps are [3]: (1) Choose U users to serve: (kU k D K u ; B < K u < K ) using one of the following criteria: a) Select users that experience large inter-packet delay, ¼i [n]. b) Select users who have low short-term throughput Ti [n]. c) Select users who have the largest ¼i [n]=Ti [n] ratio. (2) The centralized scheduler using the user-base-beam selection algorithm described below selects, a subset of B users from the above chosen group of K u users, and the corresponding BSs and beams on which they may be serviced. This algorithm generates a set of B triplets where the triplet (i j ; b j ; l j ) represents the case where user i j is best served by BS b j on the beam l j ; j 2 f1 Ð Ð Ð Bg: (3) Each BS generates the above assigned beam and collects the SINR reports from all the users. All the BSs transmit these reports to the centralized scheduler. (4) The centralized scheduler using the K ðB SINR matrix and the short-term throughput vector T schedules B users for service. Steps 1,2 and 3 constitute the first phase of the system and step 4 constitutes the second phase. In the user-base-beam selection (UBBS) algorithm, mentioned above, B users from the initially chosen set of K u users and the indices of the BSs and the beams on which these users may be serviced are chosen. The beams must be chosen such that they least interfere with the B selected users. One possible approach to attain this objective is through the instantaneous system throughput, which is the sum of the achievable user rates. For B D 3 we state this formally as arg max i 1 ;i 2 ;i 3 2f1ÐÐÐK u g;b1 ;b2 ;b3 2f1ÐÐÐBg;l1 ;l2 ;l3 2f1ÐÐÐLg f (i; b; l) (2.10)
  • 43. 24 OPPORTUNISTIC COMMUNICATIONS with i D fi 1 ; i 2 ; i 3 g, b D fb1 ; b2 ; b3 g, l D fl1 ; l2 ; l3 g and f (Ð) is the sum of the rates of the three users. By using Equation (2.9) we have f (i; b; l) D log2 1C (i 1 ;b1 ;l1 ) 1C ð 1C (i 1 ;b2 ;l2 ) C 1C (i 1 ;b3 ;l3 ) (i 2 ;b2 ;l2 ) 1C (i 2 ;b1 ;l1 ) C (i 2 ;b3 ;l3 ) (i 3 ;b3 ;l3 ) 1C (i 3 ;b1 ;l1 ) C (i 3 ;b2 ;l2 ) In the absence of current SINRs, the short-term averaged SINRs ˜(i;b;l) can be used instead of in Equation (2.10). The short-term SINR is computed as ˜(i;b;l) [n] D n 1 tD0 n t 1 (i;b;l) [t]Ž(Â(i;b) [t]; l)a n 1 n t 1 tD0 Ž(Â(i;b) [t]; l)a (i;b;l) (2.11) where Â(i;b;l) [t] represents the index of the beam on which user i was serviced by base station b at time instant t. Thus, this alternative criterion for selecting the set of triplets is stated as arg max i 1 ;i 2 ;i 3 2f1ÐÐÐK u g;b1 ;b2 ;b3 2f1ÐÐÐBg;l1 ;l2 ;l3 2f1ÐÐÐLg ˜ (i; b; l) f (2.12) with ˜ (i; b; l) given by Equation (2.10) using ˜(i;b;l) instead of (i;b;l) . f The optimization of Equation (2.12) is practically infeasible when B and L are large. The following, computationally less intensive, modified UBBS algorithm was proposed in Reference [3] for scenarios where B and L are large. A group of users U is initially chosen and arranged in order of preference for service using one of the criteria stated earlier. The main steps of this modified procedure are: (1) Choose the first user i 1 and obtain the BS and beam index fb1 ; l1 g on which the user i 1 experiences maximum throughput. Formally these indices are obtained as (b1 ; l1 ) D arg max ˜(i;b;l) (2.13) b2f1ÐÐÐBg;l2f1ÐÐÐLg Ä1 D ˜(i1 ;b1 ;l1 ) (2.14) (2) Choose the second user i 2 from the remainder of the set (U the beam and BS using the following criterion: arg max i 1 ), and the indices for ˜2 (i; b; l); f (2.15) b2 2f1ÐÐÐBg;l2 2f1ÐÐÐLg with i D fi 1 ; i 2 g and b D fb1 ; b2 g; l D fl1 ; l2 g and ˜2 (i; b; l) D f 1C ˜(i1 ;b1 ;l1 ) 1 C ˜(i1 ;b2 ;l2 ) ð 1C ˜(i2 ;b2 ;l2 ) 1 C ˜(i2 ;b1 ;l1 ) f Ä2 D ˜2Ł (i; b; l) In this way the newly chosen user i 2 causes minimal interference to user i 1 and vice versa. The terms Ä1 , Ä2 represent the virtual SINR of the system as the system is loaded with users and BSs. We also use the constraint that Ä2 > Ä1 to ensure that the addition of a new user does not decrease the overall virtual SINR of the system. In the same way, the remaining set of users and the beams on which they are serviced by the BSs are chosen. When this constraint of increasing the virtual SINR of the system fails, the beams are chosen at random. The scheduler obtains the terms (i;b;l) , which are the SINR reports of the users from the BSs on the beams given by the UBBS algorithm. Then the rate R(i;b;l) that user i obtains from BS b using
  • 44. NETWORK COOPERATION AND OPPORTUNISTIC COMMUNICATIONS 25 beam l is computed. Using this information, B users are chosen whose weighted sum of rates is maximum, with the weights given by the reciprocal of the user’s short-term throughputs T [Ð]. This criterion is also computationally intensive, so the following alternative is considered. The users are ordered according to their weighted rates as (R=T )i1 ;b1 ½ (R=T )i2 ;b2 Ð Ð Ð ½ (R=T )i K ;bk and the first B users are chosen while maintaining the constraint that only one user is scheduled from a BS. 2.5.1 Performance example We will illustrate the performance of the above methods for two different hexagonal cellular networks, as shown in Figures 2.7 and 2.8. The number of coordinating BSs B in these two networks are 3 and 9 respectively. The hexagonal cells are divided into three 120Ž sectors and each of these sectors are covered by a BS with a smart antenna system. The BSs are located at the centre of each of the hexagonal cells and the users are distributed randomly in the shaded regions. The received signal at the user is modelled as in Section 2.5 to take into account path loss, shadow fading and correlated Rayleigh fading effects. The path loss is based on the Hata urban propagation model with a path loss coefficient of 3.5. A lognormal shadow fading process with zero mean and a standard deviation of 8 dB was used to characterize the variations in the signal due to environmental clutter. Two different correlated Rayleigh fading channels corresponding to Doppler rates with user velocities of 1 m/s and 8 m/s are analyzed. The antennas at the BSs consist of a uniform linear array (ULA) of antenna elements. Each of these elements is fed equal currents and only their phases are varied. Eleven such equispaced radiation patterns were generated using a four-element array. The normalized array factor considered in the simulations is j A( )j D jsin(N =2)= sin( =2)j=N , where N is the number of antenna array elements and is the azimuth in radians. The radiation pattern of each element is shown by the dashed line in Figure 2.9. The interference terms due to the noncoordinating BSs were simulated by randomly generating beams from these BSs in their assigned sectors. The other system parameters used in the simulations are the same as in Reference [3]: cell radius 1 km, transmit base power 40 dBm, carrier frequency 2 GHz, sampling frequency 500 Hz, number of users per sector 20 and the forgetting factor a used in Equation (2.11) is 0.998. Figure 2.8 Nine-base intercell cooperation scenario.
  • 45. 26 OPPORTUNISTIC COMMUNICATIONS Single Antenna Linear Array 0 Gain in dB −4 −8 −12 −16 −20 100 140 180 220 Azimuth in Degrees 260 Figure 2.9 Single antenna and linear array antenna radiation pattern. The following algorithms using methods described in Section 2.5 have been simulated for a comparative study. (a) In the case of 3 BS cooperation, K u D 15 is chosen initially and using the USBBA the three sets of users and the indices of beams on which they are best serviced are obtained. In the case of cooperation with nine BSs, the modified USBBA is used to choose these set of indices, BSs and beams. (b) In this case the beams are chosen randomly. The cooperation here is limited to choosing the users who are to be serviced by the BSs according to the PF (proportional fair) criterion. (b1) This is the (b) algorithm where beams are sequentially chosen at each BS. (c) In these methods, the users are randomly deployed in the sectors and are tied to the BSs to which they are geographically close. In these scenarios the BSs do not cooperate with the neighboring BSs either for generating beams or scheduling users. The BSs implement an independent algorithm (a) while in (c1) an independent algorithm (b) is executed. (d) In this method, the BSs have a single conventional 120Ž sectored antenna and the users are scheduled according to the PF algorithm. In our simulations, for a fair comparison between these methods and the above proposed methods, it was ensured that the BSs transmit at the same power in their respective sectors. In Figures 2.10 and 2.11, the interpacket delay is plotted versus the total system throughput statistics when user velocity v D 1 m/s in the three-BS and nine-BS coordination scenario. In Figures 2.12 and 2.13, the tradeoff when v D 8 m/s is shown. For the three-BS coordination scenario with v D 1 m/s, simulation results show small gains in terms of throughput using the described algorithms. In the nine-BS coordination scenario the throughput and interpacket delay tradeoffs with (a) and (b1) are almost the same. Significant gains with v D 8 m/s are observed as expected due to the rapidly varying channel conditions. With v D 8 m/s, the throughput delay tradeoff patterns are similar to the case with v D 1 m/s.
  • 46. MULTIUSER DIVERSITY IN WIRELESS AD HOC NETWORKS 27 6 System Throughput bps/Hz a) 5 4 3 2 1 0 d) a) b1) b) c) c1) 10−4 10−3 10−2 10−1 Probability(Inter-Packet Delay > 80 Timeslots) Figure 2.10 Base coordination, interpacket delay versus total system throughput statistics with user velocity of 1 m/s. System Throughput in bps/Hz 6 Tc=60 5 Tc=80 Tc=100 Tc=40 4 3 2 1 a) b) c) c1) d) 10−4 10−3 10−2 10−1 Probability(Inter-Packet Delay > 80 Timeslots) Figure 2.11 Nine-base coordination, interpacket delay versus total system throughput statistics with user velocity of 1 m/s. 2.6 MULTIUSER DIVERSITY IN WIRELESS AD HOC NETWORKS In this section, we discuss a scheme to utilize the shared medium in IEEE 802.11-based ad hoc networks efficiently by taking advantage of multiuser diversity, distributed scheduling and adaptability. In a heterogeneous ad hoc network or a mesh network, some nodes may need to communicate with multiple one-hop nodes. The scheme allows such a node with a certain number of links to function as a clusterhead to coordinate multiuser communications locally. A CDF (cumulative distribution function)-based K-ary opportunistic splitting algorithm and a distributed stochastic scheduling algorithm are used to resolve intra- and intercluster collisions, respectively. Fairness is formulated and solved in terms of social optimality within and across clusters. Analytical and simulation results, which will be presented in this section, show that such a scheme can significantly improve communication efficiency while providing social fairness.
  • 47. 28 OPPORTUNISTIC COMMUNICATIONS System Throughput in bps/Hz 6 Tc=60 Tc=40 Tc=30 Tc=80 Tc=100 5 4 3 2 a) b1) b) c) c1) d) 1 10−6 10−5 10−4 10−3 Probability(Inter-Packet Delay > 80 Timeslots) 10−2 Figure 2.12 Three-base coordination, interpacket delay versus total system throughput statistics with user velocity of 8 m/s. 5.5 System Throughput in bps/Hz Tc=60 Tc=80 Tc=100 Tc=40 5 4.5 4 3.5 3 a) b) c) c1) d) 10−2 10−4 10−3 Probability(Inter-Packet Delay > 80 Timeslots) Figure 2.13 Nine-base coordination, interpacket delay versus total system throughput statistics with user velocity of 8 m/s. The diversity techniques considered so far in this chapter may not be applicable in multihop ad hoc networks [4–22] because there is no base station to act as the central controller and no dedicated control channel to feed back the channel state in a timely fashion. In addition, in ad hoc networks, the medium access control is distributed and each node randomly accesses the shared medium without prior channel information. Most of the work on diversity in ad hoc networks is limited to multipath diversity, as described in Chapter 10 on crosslayer optimization and References [16], [17], [18], [19] and [20], i.e. using multiple paths to opportunistically forward packets to enhance end-to-end reliability. Research on multi-output link diversity and multi-input link diversity is still open. The multi-output link diversity is the output multiuser diversity from one to many (from a node to multiple neighbors). A corresponding case of the multi-output link diversity in ad hoc networks is the downlink diversity in the cellular networks. Similarly, the multi-input link diversity is the input multiuser diversity
  • 48. MULTIUSER DIVERSITY IN WIRELESS AD HOC NETWORKS 29 from many to one (from multiple neighbors to a node), which corresponds to uplink diversity in the cellular networks. Having this in mind, in this section we present an 802.11-based MAC protocol that exploits these two kinds of multiuser diversity to improve channel and energy efficiency. The scheme is designed to work for multihop ad hoc networks, especially heterogeneous ad hoc networks and mesh networks, where multiple nodes need to communicate with a relatively powerful node in the distributed manner. Since opportunistic medium access may easily lead to unfairness, in this section the fairness is addressed in terms of social optimality. The operation of the scheme is based on the following steps. Each node with a certain number of links is enabled to form a cluster and function as the clusterhead to coordinate multiuser communications locally. In each cycle of data transmissions, the clusterhead initiates medium access (with certain probability in a multicluster scenario to ensure intercluster fairness), and then the cluster members (which will be referred to as users in the following) make medium access decisions in a distributed way based on the observed instantaneous channel conditions. A CDFbased K-ary opportunistic splitting algorithm is used to guarantee that only the user with the best normalized instantaneous channel quality will win the channel. After successful collision resolution, a rate-adaptation technique is employed to achieve the highest reliable data rate for the selected user. In this way, the exploitation of link diversity is facilitated during the process of collision avoidance. In some other works [12, 13], MAC enhancement is discussed to utilize output link diversity. The scheme discussed in this section [4] differs from that work in its throughput scaling and social optimality. Two papers mostly related to this approach are References [14] and [15]. They presented a channel-aware Aloha protocol and a binary opportunistic splitting algorithm, which is one of the first schemes that addresses the utilization of multiuser diversity in a distributed way. The solution presented in this section first of all targets the IEEE 802.11-based multihop ad hoc networks, the design of which is very different from that of Aloha-based single-hop networks. Second, the CDF-based binary opportunistic splitting algorithm is an extension to the weighted CDF-based K-ary opportunistic splitting algorithm so that general optimization goals can be achieved in a more efficient way. Third, a distributed intercluster collision resolution scheme is used to achieve the system-wide social optimality in multihop ad hoc networks. This design is in line with the 802.11 (DCF mode) standard in that the scheme inherits similar mechanisms to probe the channel and avoid collisions and uses a similar idea to resolve collisions. Thus, the scheme can be easily incorporated into future 802.11 standards. Theoretical analysis and simulation results demonstrate that the scheme can significantly improve communication efficiency while providing social fairness and can significantly increase energy efficiency and throughput. 2.6.1 Multiple-output and multiple-input link diversity The model of ad hoc networks used in this section is quite general. It includes homogeneous and heterogeneous ad hoc networks, and wireless mesh networks. In all these networks, the channel quality of a link is normally time-varying due to fading, shadowing, noise and interference. Different links usually experience independent instantaneous channel qualities. This phenomenon was referred to in the previous sections as the multiuser diversity. For example, in the case of ad hoc networks, as shown in Figure 2.14(a), node 1 is interfered by ongoing transmission of node 5 and the link of 0 ! 2 suffers deep fading or shadowing. The link of 0 ! 4 has an instantaneous quality to support basic data rate transmission. The link quality of 0 ! 3 happens to be ‘on-peak’. Since only one of these links is allowed to communicate at a time, it is better for node 0 to transmit data to node 3 or 4 rather than node 1 or 2 at the current time. This is referred to as the multi-output link diversity. Similarly, the multi-input link diversity is demonstrated in the example shown in Figure 2.14(b).
  • 49. 30 OPPORTUNISTIC COMMUNICATIONS Cluster 1 3 Cluster 1 4 0 2 4 3 0 2 1 1 Interference range Ongoing transmission 5 Cluster 2 Carrier sensing range Ongoing transmission 5 6 6 (a) Cluster 2 (b) Figure 2.14 (a) An illustration of multi-output link diversity. (b) An illustration of multi-input link diversity. 2.6.2 Localized opportunistic transmission If there is a global scheduler who knows the system-wide channel information and topology with little cost, the efficiency will be maximally exploited by considering both link quality and space reuse of all active links. In practice, global multiuser scheduling is impossible in multihop ad hoc networks, where no centralized scheduler is available and complete channel information and topology information are hard to obtain. It was shown that it is still interesting to utilize the multiuser diversity locally [12, 13]. Multiuser diversity gain can be significant without too many links in participation. Even two links can produce substantial diversity gain. Based on the above observation, we limit multiuser scheduling to a set of links with the same sending node or the same receiving node. Therefore, a node satisfying certain degree requirements can locally coordinate multiuser scheduling among its own input links or output links. The set of links with the same sending node or the same receiving node will be defined as a cluster and the common sending node or receiving node as the clusterhead and others as cluster members or users. Since multiple clusters may share the same medium, intercluster contention resolution is necessary. In the protocol discussed in this section, the clusterhead represents the whole cluster to resolve intercluster channel contention. If a clusterhead successfully captures the channel floor, it immediately coordinates the multiuser diversity-based transmission within the cluster for a certain period. As illustrated in Figure 2.14, nodes 0–4 form one cluster and nodes 5–6 form another cluster. Node 0 coordinates the opportunistic transmissions to (from) nodes 1–4. Node 5 coordinates the transmission to node 6. Further details on the cluster formation and maintenance will be discussed in the next section. One should notice that this link-layer diversity-driven clustering is very different from the network-layer clustering discussed later in Chapter 13 on ad hoc networks and, for example, in References [21], [22] and [23]. The network-layer clustering is designed to improve routing scalability and simplify network management; the dimension of network-layer clustering may be across several hops and the cost to establish and maintain such network clusters is one of the major design issues. The link-layer clustering is a logical organization to utilize multiuser diversity
  • 50. MULTIUSER DIVERSITY IN WIRELESS AD HOC NETWORKS 31 locally, under which each cluster member is directly associated with a clusterhead, and neighboring clusters do not need to exchange information between each other. The first challenge in this concept is the MAC design for multiuser diversity-based transmissions within a cluster. One straight approach is that the clusterhead schedules transmissions based on the channel and queue information collected from all users. Although this approach is feasible in cellular networks where dedicated control channels (one for each user) are available, the cost of collecting channel and queue information can be too high in single-channel ad hoc networks, especially when the number of users is large. Another approach is that each user makes a transmission decision in a distributed way based on its own instantaneous channel quality (known to each user by observation) and past channel quality distribution. The user with the higher normalized channel quality is granted higher priority, e.g. with a smaller IFS (interframe space) to access the medium. In this chapter, a MAC based on the latter approach will be designed since it does not require the collection of channel information from others, so that the overhead can be significantly reduced. Under this distributed approach, the challenge will be how to make the user with the highest normalized channel quality win the channel in a more efficient way. The objective in utilizing multiuser diversity is to improve the system performance without sacrificing social fairness. Intracluster fairness and intercluster fairness will be used to characterize the fairness within and across clusters, respectively. The intracluster problem occurs when links in a cluster have different channel quality distributions. Opportunistic medium access may easily lead to unfairness when the channel states of different users are statistically heterogeneous. For example, in the saturated scenario with two users, the user with a poor average SNR (signal-to-noise-plus-interference ratio) may be severely starved, while the one with a good average SNR may occupy the medium for most of the time. The intracluster problem becomes more complicated when links have different weights in terms of traffic and utility. The intercluster fairness problem always exists since intercluster channel contentions are normally location-dependent and clusters may carry different weights in terms of aggregate traffic and utility. The intercluster fairness problem is hard to solve because the traffic/topology/channel information of a cluster is normally unknown to another cluster. In this section, we will discuss both intracluster and intercluster fairness problem in terms of social optimality within and across clusters. 2.6.3 Multiuser diversity-driven clustering When a node has more than one output link, it will form an output cluster so that each output link joins the output cluster. When a node has more than one input link that is not associated with an output cluster, it will form an input cluster so that each input link that is not associated with an output cluster joins the input cluster. Each cluster has a unique cluster ID. The cluster ID is comprised of the MAC address of the clusterhead, the direction flag (1 indicates input, 0 indicates output) and a sequence number. Each cluster member is allocated a local user ID for identification within the cluster. The cluster ID and the local user ID are sent to the cluster member as a global identity. This process is called the association process. After a cluster is created, the clusterhead periodically or reactively checks the connectivity of associated links. If a link is disconnected, the clusterhead removes the link from its cluster and returns the local user ID to the user ID pool. This is referred to as the deassociation process. Similarly, if a new link is established to be connected with the clusterhead, the clusterhead will allocate a new local user ID from the user ID pool to the new user and run the association process. Depending on the change of out-degree at the sender side and in-degree at the receiver side, a directed link may leave an input cluster to join an output cluster, and vice versa, by following the same rule as the cluster formation introduced earlier in this section.
  • 51. 32 OPPORTUNISTIC COMMUNICATIONS 2.6.3.1 Intracluster medium access and collision resolution In each transmission cycle, the clusterhead initiates medium access, while cluster members make medium access decisions in a distributed way based on the observed instantaneous channel conditions. In the multi-output scenario, the clusterhead is the sender of data traffic, so sender-initiated medium access control (SI-MAC) is used. The handshake before data transmission in senderinitiated medium access control includes an RTS (request-to-send) and CTS (clear-to-send) in sequence. The unicast RTS can be extended to the multicast RTS and CTS enhanced with channel awareness capability so that the handshake can probe the channel among multiple users as well as avoiding or resolving collisions within and outside the cluster. In the multi-input scenario, the clusterhead is the receiver of data traffic, so the receiver-initiated medium access control (RIMAC) [24, 25] is used. The handshake before data transmission in the receiver-initiated medium access control includes RTR (ready-to-receive), RTS and CTS in sequence. In Reference [4] multicast RTR and channel-aware RTS (followed by CTS) using multi-input link diversity as well as collision avoidance and resolution is discussed. For the RI-MAC, it is better for the clusterhead to know the traffic information. In this section it is assumed that the clusterhead knows this information either by service level agreement on the flow level or the piggyback in the users’ transmitted/received packets on the packet level. Related discussions can be found in References [24] and [25]. When the clusterhead has packets backlogged for several users (in the SI-MAC) or wants to receive packets from one of the backlogged users (in the RI-MAC), it will multicast RTS or RTR with the cluster ID to the chosen candidate users. To notify a user as to whether it is chosen or not, an array of n bits is included in the RTS or RTR, where n is the total number of cluster members. The ith bit in the array corresponds to the user whose user ID is equal to i. If a bit is marked as 1, this means that the corresponding user is chosen to compete for data reception or transmission; otherwise, it is not within the candidate list in the given transmission cycle. When all the users in the cluster are candidates for data reception or transmission, the clusterhead may send RTS or RTR in the manner of groupcast without using the bit array to notify individual users. The groupcast frames are similar, but without the bit marking the array. The noise power included in the RTR frame indicates the noise plus interference power level at the clusterhead. Recall that, in the multi-input scenario, data traffic is transmitted from users to the clusterhead. It is the received SNR at the clusterhead that determines the achievable data rate. However, the medium access decisions are made in a distributed way by the users. Therefore, it is necessary that the users know the noise power level at the clusterhead. Assuming that the instantaneous channel gains between two nodes are identical in either direction, a candidate user can derive the channel gain. From the derived channel gain and informed noise power level at the clusterhead, users can estimate the received SNR and make the appropriate MAC decision. Since an RTS (RTR) has to be sent at the beginning of each cycle of data transmission for collision avoidance and channel probing and is normally sent at the basic rate in multirate ad hoc networks, the overhead of the RTS (RTR) is one of the major factors affecting the channel utilization ratio. With the clustering technique introduced above, the length of the RTS (RTR) is quite small and scalable to a very large group (one additional bit with one additional member). In addition, a large group may be partitioned into several smaller groups so that the scalability is still maintained. Anyone except the candidate receivers who receive the multicast RTS (RTR) should tentatively keep silent to avoid possible collisions before the clusterhead receives the collision-free CTS (RTS). After a qualified user is selected and the transmission duration is determined and announced by the CTS, the sender will include the duration in the subheader of DATA. The subheader is referred to as the reservation subheader (RSH), which has already been employed in the MAC header of data packets in IEEE 802.11e. RSH is sent at the basic rate so that all overhearing nodes can decode.
  • 52. MULTIUSER DIVERSITY IN WIRELESS AD HOC NETWORKS 33 Upon receiving a multicast RTS (RTR), each user checks the bit-marking array. If the corresponding bit is set to 1 and the observed instantaneous SNR is above the threshold indicated in the RTS (RTR) message, it is allowed to compete for the channel. For groupcast RTS (RTR), every user needs to evaluate the channel and prepare to compete for the channel if the observed channel condition is good enough. If there is only one user who has the desired channel condition, the user captures the channel without a collision within the cluster. If there is no qualified user, the group head defers for a certain time and sends a multicast RTS (RTR) again. In case there is more than one qualified user, collisions may happen. Thus, a collision resolution scheme is required to find a qualified user. A scheme termed the CDF-based K-ary opportunistic splitting algorithm is discussed later. This scheme can quickly determine the user with the best normalized channel quality. The basic CDF-based K-ary splitting algorithm guarantees timeshare fairness among users and a generalized weighted CDF-based K-ary opportunistic splitting algorithm optimizes local system performance. The basic principle and necessary procedures to utilize multi-input link diversity are similar to that of multi-output link diversity. Thus, in the following discussion, we will focus on the multi-output link diversity. In each cycle of contention resolution plus data transmission, it is assumed that, once a user captures the shared medium, it is allowed to transmit data up to TXOP (transmission opportunity) within which a user can consecutively transmit data without contending for the channel. TXOP can be represented as the total transmission time T . For low mobility, the channel is modelled as a block-fading channel and it is assumed that the noise plus interference power does not change much during one cycle, although it may change significantly when a user captures the channel in another cycle. Therefore, the SNR is stable in each cycle but may randomly change from one cycle to another. Let h i (t) D h i D SNR of user i, which is considered to be independent for different users. If the large-scale path loss changes very slowly in comparison with the instantaneous channel gain and noise power, h i can be considered ergodic during a sufficiently long system observation period. In the multihop scenario, the interference may partly depend on which set of nodes are actually transmitting. In other words, the instantaneous interference each user gets is spatially correlated to some extent, even though fading effects make the interference power random. However, since a multihop intercluster collision is avoided and resolved based on carrier sensing, the interference can be kept low. The simulation results validate this assumption. It is assumed that h i is a random variable with the probability density function f Hi (n). In practice, each user may know the distribution of its own SNR but not those of other nodes. Similarly, each user can determine its own instantaneous SNR just at the beginning of each cycle of data transmission, but not those of other nodes. The instantaneous channel state can be measured during the handshake of the collision avoidance process. The long-term SNR distribution can be derived via the iterative approach. If SNRs required to support the lowest rate and the highest rate are h min and h max , respectively, h min C j=M(h max h min )(0 Ä j Ä M) is quantized channel quality, where (h max h min ) / M represents the quantization interval, k is the iteration index and P j is the PMF (probability mass function) that the quantized channel quality equals h min C j=M(h max h min ); then P j can be updated as P jkC1 D (1 (1 k k k )Pm C )P jk ; k ; j Dm j 6D m where k (0 Ä k Ä 1) is the step size to update the PMF. An appropriate step-size sequence to balance the convergence speed and smoothness (used also in the simulation presented later) is k D 1= min(k; 1000). Rate adaptation is based on the instantaneous SNR h evaluated at the beginning of each cycle of data transmission. Once the data rate is set, it will not be changed during the whole data
  • 53. 34 OPPORTUNISTIC COMMUNICATIONS transmission period. In the sequel, R(h) will denote the rate at which a user can reliably transmit if the instantaneous SNR evaluated at the beginning of data transmission is h. 2.6.4 Opportunistic MAC with timeshare fairness If channel quality of each user follows i.i.d. distribution, we can directly use the SNR value as the criterion for MAC in each cycle of data transmission. Timeshare fairness is naturally preserved due to the statistical properties of the SNR. However, it fails to guarantee timeshare fairness if the SNRs are heterogeneously distributed among users. One possibility to guarantee timeshare fairness, while exploiting multiuser diversity, is to use the normalized channel quality as the threshold for MAC and as the criteria to determine who will win the channel access. In the sequel we will use the complementary cumulative probability 1 pi D FHi (h i ) D f Hi (h)dh or pi D FHi (h i ) D hi M jDM(h i h min )=(h max h min ) Pj where P j is the PMF and takes some p(0 < p Ä 1) as the MAC threshold, which means that users with the instantaneous SNR higher than FHi1 ( p) are allowed medium access. The lower p corresponds to the higher channel quality for medium access, which limits the number of users involved in the competition for the channel. Considering FHi (h i ) as a random variable, we easily find that FHi (h i ) is i.i.d across users with the normalized uniform distribution. This means that each user has the same probability to access the medium for any medium access threshold p. In addition, the policy that the user with the lowest instantaneous FHi (h i ) wins the channel will guarantee that each user has the same probability of capturing the channel. This scheme will be referred to as the basic opportunistic medium access control (BO-MAC). The remaining question is how to design a distributed collision resolution algorithm to find the user with the best normalized instantaneous quality. In the following, we discuss a fast carrier sensing-based splitting algorithm, namely the K-ary opportunistic splitting algorithm, to resolve the collisions among the qualified users originally described in Reference [4]. The K-ary opportunistic splitting algorithm can be considered the extension of the binary-tree collision resolution algorithm introduced in Reference [15, 26]. 2.6.5 CDF-based K-ary opportunistic splitting algorithm When user i, with channel quality h i > FHi1 ( p), receives an RTS (ready to send) it is allowed to compete for the channel and transmit a CTS (clear-to-send) at the beginning of minislot m 1;i if there is no transmission in the previous m 1;i 1 minislots. Carrier sensing is used for detection of transmission of other users without the need to decode the transmitted packet. The carrier sensing range is usually more than twice the transmission range. In addition, CTS (as well as RTS) is applied with sufficient channel error coding and sent at the basic data rate. Therefore, even with fading, a large carrier sensing range still allows users in the same cluster to check if the channel is busy or not. The minislot used in this chapter for the performance example is aSlotTime (20 µs in 802.11b with DSSS), as defined in the 802.11 standard and also used in Reference [4]. The round of competition following the RTS is the first round of competition. If there are at least two users in the above process who send CTS simultaneously, it goes to the second round of competition. The users involved with collisions can detect collisions by observing that there is no data transmission one minislot after the CTS. Let m j (1 Ä m j Ä K ) denote the number of minislots at the beginning of which collisions occur in the jth round of competition. User i will participate in the second round of competition if it participated in the first round of competition, i.e. had channel quality better than FHi1 (m 1 p=K ). It will transmit CTS at the
  • 54. MULTIUSER DIVERSITY IN WIRELESS AD HOC NETWORKS 35 beginning of minislot m 2;i if there is no transmission in the previous m 2;i 1 minislots after it detects collisions. In the jth( j ½ 3) round of competition, user i involved with the ( j 1)th round of j 2 competition, i.e. user i with channel quality better than FHi1 ( kD1 (m k 1) p=K k C m j 1 p=K j 1 ), will participate in the competition again and transmit a CTS at the beginning of minislot m j;i if there is no data transmission in the previous m j;i 1 minislots after it detects collisions, where  j D1  FHi (h i )=( p=K ) ;  j 1 (2.16) m j;i D  FHi (h i ) (m k 1) Kpk =( p=K j ) ; j ½ 2  kD1 An example of intracluster collision resolution with five users, medium access threshold p D 0:8 and K D 4 is shown in Figure 2.15 [4]. In the first round, those users with channel quality between 0 and 0.2 are expected to transmit CTS at the time SIFS (short interframe space) after receiving RTS. Those users with channel quality between 0.2 and 0.4 are expected to transmit CTS at the time SIFS C aSlotTime after receiving RTS and so on. Since no user is with channel quality less than 0.2, users 1 and 2 take the opportunity to transmit CTS at the time SIFS C aSlotTime after receiving RTS. User 3 and user 4 may transmit CTS at SIFS C 2 * aSlotTime and SIFS C 3 * aSlotTime, respectively, after receiving RTS, but observe that the channel is busy and, thus, they yield opportunity to user 1 and user 2. User 5 is not qualified and thus will not prepare to transmit CTS. User 1 and user 2 detect collision and enter the second round of channel contention. Since the channel quality of user 1 falls between 0.2 and 0.25, user 1 transmits CTS at the beginning of the first minislot after detecting collision. User 2 may transmit CTS at the beginning of the second minislot if it observes that the channel stays idle after collision. Since user 1 has better quality and takes the opportunity, user 2 gives up. Now, the clusterhead receives collision-free CTS and starts DATA transmission. When two of the best quality users, say user a and user b, have very close SNRs, which means that jFHa (h a ) FHb (h b )j ! 0, it may take a large number of competition rounds to resolve collisions. With the limitation of quantization in SNR distribution, it may be impossible to tell which one is better than the other. Besides, it is not worth finding the best one since not much SNR gain can be achieved even if the best one is found. The following algorithm can be used to resolve collisions after certain rounds, say Þ, of competition. Collision detection SIFS Cluster Head RTS 0 ~ 0.2 aSlotTime User 1 FH1 (h1) = 0.23 User 2 FH2 (h2) = 0.35 0.2 ~ 0.4 User 3 FH3 (h3) = 0.45 User 4 FH4 (h4) = 0.78 User 5 FH5 (h5) = 0.86 aSlotTime CTS collision CTS SIFS DATA CTS 0.2 ~ 0.25 Note: p = 0.8, K = 4 Figure 2.15 An example of an intracluster collision resolution. (Reproduced by permission of IEEE [4]).
  • 55. OPPORTUNISTIC COMMUNICATIONS Average time required to resolve collision (ms) 36 1,3 1,2 1,1 1 0,9 0,8 Analytical bound Simulation result 0,7 0,6 0 10 20 30 40 Number of involved users 50 Figure 2.16 Average time required to resolve collision. Table 2.1 Parameter setting Parameter p0 aSlotTime (µs) Tid (µs) Tini (µs) Tcr s (µs) Tcr f (µs) Value 0.9 20 20 300 300 320 In the jth( j ½ Þ) round of competition, user i involved with the ( j 1)th round of competition will randomly select a minislot, say m i; j , among K minislots to transmit a CTS again after it detects collisions if there is no transmission in the previous m i; j 1 minislots in the jth(j ½ Þ) round of competition. In practice, the maximal number of rounds to resolve collisions should not be too large; otherwise, the channel condition may change significantly after a user wins out. For this reason the window size of opportunistic collision resolution, i.e. the total resolution time, should be limited to Tic , which is a system parameter set by the clusterhead according to the number of backlogged users and channel coherence time. In practice, the average number of rounds of competition needed is very small, about O(log K (n)), where n is the number of qualified users. If EXn denotes the expected time required to resolve a collision with n involved users in SIMAC by the carrier sense opportunistic splitting algorithm with K -dimensional trees, then we have [4] EXn Ä Tini C logk (n)Tcr f C (logk (n) C K =2)Tid C Tcr s for any K ½ 2 and n ½ 1, where Tid D aSlotTime, Tini D RTS C SIFS, Tcr s D CTS C SIFS and Tcr f D CTS C aSlotTime. Figure 2.16 shows the analytical bound and simulation result. The parameter settings for the simulation shown in Table 2.1 are the same as in Reference [4]. In addition, we use B D 1 MHz, T D 6 ms, K D 4, Þ D 4, Tic D 2 ms and ¾ D 106 . Q The lower bound of the expected time, denoted by EXn , required to resolve a collision by the Q Q algorithm described in Reference [15] is EXn ½ Tini C log2 (n)Tcr f C Tcr s for any n ½ 1, where Q Tcr f denotes the round-trip time required for a user to transmit a small reservation packet and Q detect if a collision occurs. Tcr f is no less than Tcr f . Since Tcr f is normally tens of aSlotTime, it can be seen that the K-ary carrier sensing splitting algorithm reduces a lot of collision-resolution overhead when K is larger than 2.
  • 56. MULTIUSER DIVERSITY IN WIRELESS AD HOC NETWORKS 37 2.6.6 Throughput Let us assume that a user transmits data with fixed T after successfully capturing the medium. For simplicity it is assumed that the packet transmitted in T is pure data. Then the achievable throughput of user i, S B ( p; n; i), and the total achievable throughput, SB ( p; n), with n independent backlogged users and the medium access threshold p for the pure output-link scenario with the BO-MAC and the K-ary splitting tree algorithm, are [4] S B ( p; n; i) ½ [R( p; n; i)T =n]=[T0 ( p; n; K ) C T ] (2.17) and n R( p; n; i)T =n =[T0 ( p; n; K ) C T ] S B ( p; n) ½ (2.18) iD1 with timeshare fairness, where n kD1 To ( p; n; K ) D (1 p n k R( p; n; i) D pk (1 p)n k 0 R FHi1 (t) (k= p)(1 p)n ) (Tini C logk fnp=[1 (1 C (logK fnp=[1 (1 (1 t= p)k 1 dt (2.19) p)]gTcr f p) ] C K =2gTid C Tcr s ) n (2.20) R(h) is the transmission rate when SNR is h, Tid D aSlotTime, Tini D RTS C SIFS, Tcr s D CTS C SIFS and Tcr f D CTS C aSlotTime. 2.6.7 Optimal opportunistic MAC In the last section, the opportunistic medium access algorithm guarantees each user the same probability of accessing the channel. In other words, each user gains the same time fraction of T . A modification of the algorithm (MA or MO-MAC) can provide a different proportion of service time to different users. By adjusting the weight vector, we can obtain the desired rate vector that optimizes the local system performance. If w D fw1 ; : : : ; wk ; : : : ; wn g denotes the weight vector, where 0 < wi < 1(8i) and iD1;n wi D 1, the MA is defined as [4] i Ł D arg max 1 i FHi (h) 1=wi (2.21) The probability that user i is selected for transmission, given that h i D h, is Prfi Ł D ijh i D hg D [1 FHi (h)](1 wi )=wi and the average probability that user i can win the channel is wi . To prove these statements we start with Prfi Ł D ijh i D hg D Prf[1 D [1 FH j (h j )]1=w j Ä [1 FHi (h)]1=wi ; 8 j 6D ig FHi (h)]w j =wi D [1 FHi (h)](1 wi )=wi (2.22) j6Di and 1 Prfi Ł D ig D Prfi Ł D ijh i D hg f Hi (h) dh 0 1 D (1 FHi (h))(1 wi )=wi d(1 FHi (h)) D wi (2.23) 0 In the sequel, we discuss the modified opportunistic splitting algorithm (MOSA). The threshold pis now set to 1, which means each user is allowed to compete for the channel regardless of
  • 57. 38 OPPORTUNISTIC COMMUNICATIONS its instantaneous channel condition. Similarly to the basic K-ary splitting algorithm discussed in Section 2.6.5, in the first round of channel competition, user i will transmit a CTS at the beginning of minislot m 01;i if there is no transmission in the previous m 01;i 1 minislots after receiving RTS. User i will participate in the jth( j ½ 2) round of competition if it participated in the last round of competition and collided with others. It will transmit a CTS at the beginning of minislot m 01;i if there is no transmission in the previous m 01;i 1 minislots after it detects collisions. Parameter m 01;i is calculated as follows:  j D1  K (1 (1 FHi (h i ))1=nwi ) ;  Ł j 1 (2.24) m j;i D  K j 1 (1 FH (h i ))1=nwi (m k 1)=K k ; j ½ 2  i kD1 where n is the number of backlogged users. We note that, when each user has equal weight, i.e. wi D w j (8i 6D j), the MOSA is reduced to the basic one given the medium access threshold p D 1. Generally speaking, the expected time to resolve collision in each cycle of data transmission will depend on the weight vector w. However, it would be a good approximation if we used the upper bound of EXn specified in Section 2.6.5 to characterize the expected time to resolve collision by C the MOSA. If EXn denotes the upper bound of EXn and the transmission opportunity time (TXOP) is T , then the service rate of user i is S E (n; i; wi ) D D 1 T CT R(h) Prfi Ł D ijh i D hg f Hi (h) dh C EXn 0 1 T CT FHi (h))(1 R(h)(1 C EXn wi )=wi f Hi (h) dh (2.25) 0 where R(h) is the transmission rate when SNR is h. SE (n; i; wi ) is concave in wi and can be optimized. To prove it we start with SE (n; i; wi ) D D T CT C EXn T CT C EXn 1 R(h)(1 FHi (h))(1 wi )=wi f Hi (h) dh 0 1 wi R(1) wi (1 FHi (h))1=wi R 0 (h) dh (2.26) 0 Since nonnegative weighted sums and integrals preserve concavity, it is sufficient to show 1=w that f (wi ) Á wi ai i is concave in wi , given 0 < wi Ä 1 and 0 Ä ai Ä 1. Since f 00 (wi ) D a 1=wi (log a)2 =wi3 Ä 0, it completes the proof. To optimize the system performance by choosing an appropriate weight vector we define Ui (x) as the utility function of user i in terms of average service rate x. Suppose it is strictly increasing, concave, differentiable and additive. The optimal weight vector can be achieved by solving the optimization problem n Ui (SE (n; i; wi )) Maximize z D (2.27) iD1 n wi Ä 1 and wi ½ 0; 8i subject to (2.28) iD1 For analytical tools to solve the optimization problem (2.28) see Chapter 12. 2.6.8 Contention resolution between clusters Within a cluster, as discussed in the previous sections, opportunistic splitting algorithms (OSA) can be used to resolve collisions among qualified users. For collision resolution between clusters,
  • 58. MULTIUSER DIVERSITY IN WIRELESS AD HOC NETWORKS 39 an exponential backoff algorithm can still be used but may not provide system-wide fairness. In order to optimize system-wide performance, a persistent-based contention resolution algorithm was introduced in References [4] and [27] to resolve collision between clusters. The system-wide optimization problem is now formulated as nj Ui (r j S E (n j ; i; wi j )) Maximize Z D j2 A iD1 nj wi j Ä 1; 8 j 2 A subject to (2.29) iD1 wi j ½ 0; r j ½ 0; 8i; j r j Ä 1; 8k 2 A; j2 Ak In Problem (2.29), j; k are indices of clusters, A is the set of all clusters, Ak is a subset of A, which includes cluster k plus the clusters who share the channel with cluster k, r j is the channel allocation rate (i.e. time fraction) for j, n j is the number of associated users (active directed links) in cluster j and wi j is the weight for user i in cluster j. Problem (2.29) can be solved using a convex optimization technique similar to that shown in Chapter 12, either in a centralized or a distributed way. Here we focus on fully distributed channel allocation. Each cluster is supposed to control its channel allocation rate, which is adjusted in response to the feedback of contention with neighboring clusters. The optimization function of each cluster is nj Ui (r j S E (n j ; i; wi j )) Maximize J (r j ) D ½( j )r j iD1 nj wi j Ä 1; subject to wi j ½ 0; 8(i; j) (2.30) iD1 In Problem (2.30),  j is the perceived contention loss probability between cluster j and its neighbouring clusters. The higher r j results in larger  j . Parameter ½( j ) is the price in terms of contention loss probability. The price function should be strictly increasing and convex in Â. The above problem can also be represented as nj Ui (r j S E (n j ; i; wiŁj (r j ))) Maximize J (r j ) D ½( j )r j (2.31) iD1 where wiŁj (r j ) denotes the optimal wi j given r j . The optimal weight vector wŁ is obtained by j solving the following problem: nj Ui (r j S E (n j ; i; wi j )) Maximize z D iD1 nj wi j Ä 1; wi j ½ 0; 8(i; j ) subject to (2.32) iD1 j2 A J (r j ) is maximized when each individual cluster j maximizes its own objective function J (r j ) [4]. As the price becomes large with the increase of contention loss probability, the fully distributed solution also converges to a channel allocation scheme that maximizes the aggregate utility over all the clusters. The concave objective function J (r j ) is maximized when J 0 (r j ) D 0, resulting in nj 0 Ł Ł Ł ½( Ł ) D 0, where r Ł is the optimal channel allocation rate of j j iD1 Ui (r j S E (n j ; i; wi j (r j )))
  • 59. 40 OPPORTUNISTIC COMMUNICATIONS cluster j and  Ł is the perceived contention loss probability when cluster j and its neighborj ing clusters access the medium with an optimal channel allocation rate. A time-averaging stochastic approximation algorithm with feedback can be used to update the channel allocation rate: nj r kC1 D r kj C ¾ vk j Ui0 r kj S E n j ; i; wiŁj r kj k ½ Âj iD1 r kj D (1  (1 k Âj D (1 þk )r kj 1 C þk r k j k 1 Žk ) j C Žk ; k 1 Žk ) j ; collision happens in cycle k no collision in cycle k where ¾ , vk , þk and Žk are adjusting parameters. Using the standard methodology described in Reference [28], we can show that r k converges to r Ł with probability 1. j j 2.6.9 Performance examples In this section, we take ¾ as 106 and take vk ; þk and Žk as 1=k as in Reference [4] and show by simulations that the convergence speed is quick. The state diagram of a clusterhead is shown in Figure 2.17. At the initial state, k, r j and  j are, respectively, set to 100, 1/6 and 0. However, the setting of initial values for r j and  j is quite flexible. With r j optimized iteratively, the statistical approximation algorithm guarantees that r j converges to r Ł with any initial state. The optimal j Begin k = 100; rj = 1 / 6; θj = 0 No No Is channel idle? Yes Wait K-DIFS Is channel idle? Yes Rand(0, 1) < = rj Update θj, rj, Wj*, k No Transmit RTS Receive intented CTS withinTic Transmit DATA & ACK up to T Wait a Slot Time > rj Update θj, rj, Wj*, k Yes Figure 2.17 State diagram of clusterhead. (Reproduced by permission of IEEE [4]).
  • 60. MULTIUSER DIVERSITY IN WIRELESS AD HOC NETWORKS 41 weight vector wŁ will be derived iteratively by Problem (2.29) with updated r j . K-DIFS (K-ary j DCF interframe space) is equal to aSlotTime. Tic , as defined earlier, denotes the window size for intracluster opportunistic collision resolution. A local area ad hoc network with only one clusterhead is considered first. All the traffic is from the clusterhead to the users. The clusterhead can immediately initiate a new cycle of data transmission after the previous one completes. It is assumed that each packet can be correctly received once it is transmitted at the appropriate data rate and that ACK is not used. The protocol operation is set in this way to facilitate the comparative study on the channel efficiency of different schemes. The channel is modeled as Rayleigh fading. If h i D SNR then we have f Hi (h) D e h=h i h i , where h i is the average SNR of user i. It is assumed that the link qualities at different nodes are i.i.d. The achievable data rate of each link is formulated as R(h) D B log2 [1 C min(h; h max )], where h max is the upper bound related to T (h max is set as 100). The basic algorithm (BO-MAC) is compared with the round-robin scheduler and the ideal scheduler. The ideal scheduler has full knowledge of the channel information prior to scheduling so that it can target the best quality user without overhead. For the analytical results, the formula to calculate the throughput of BO-MAC has been introduced in the previous sections. The formula to calculate the throughput of the round-robin scheduler and the ideal scheduler are similar to that of BO-MAC and are omitted here. The case with the homogeneous channel condition and h i equal to 1 for each user is considered first. Figure 2.18 shows one set of analytical and simulation results in which the medium access threshold for BO-MAC is 0.9. The simulation results are quite close to the analytical results. The BO-MAC performs much better than the round-robin scheduler (channel efficiency 0.8 and independent of the number of users) and can scale well with the number of backlogged users. The channel efficiency of the BO-MAC can approach approximately 90 % of that for the ideal scheduler when TXOP is T . When the medium access thresholds for BO-MAC are set with different values, the performance is affected. However, the throughput scaling effects and the relationship between the theoretical results and simulation results are also observed. Heterogeneous channel conditions were considered in Reference [4]. Table 2.2 shows the channel condition and throughput result of each user. The simulation results match the analytical results very well. Almost every user of the BO-MAC scheme obtains twice the throughput of the roundrobin scheme. For general optimization [4] considers two cases. In the first case, the utility function of user i equals Ui (xi ) D vi log(xi ), where xi is the achievable throughput (bit/s). This kind of utility function will be used most often in Chapter 12. In the second case, the utility function of user i is equal to 2,2 2 1,8 Ideal-Simulation Ideal-Analysis BO MAC-Simulation BO MAC-Analysis 1,6 1,4 1,2 0 10 20 30 40 Number of Users Figure 2.18 Channel efficiency with timeshare fairness. 50
  • 61. 42 OPPORTUNISTIC COMMUNICATIONS Table 2.2 Throughput with timeshare fairness in the heterogeneous case (BO-MAC D OMAR). (Reproduced by permission of IEEE [4]) Flow ID hi Ideal-Anl (bit/s) Ideal-Sim (bit/s) OMAR-Anl (bit/s) OMAR-Sim (bit/s) RR-Anl (bit/s) RR-Sim (bit/s) 0 1 2 3 4 5 6 7 8 Total 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 139323 153259 165952 177609 188388 198413 207785 216584 224876 1672189 140834 154745 167252 177787 187636 197637 207066 216356 223877 1673190 128551 141410 153121 163877 173822 183073 191720 199838 207490 1542902 132708 146138 157953 168142 177599 187697 197382 206643 213307 1587569 60516 67753 74529 80904 86929 92644 98082 103271 108235 772863 59938 67820 73932 80858 86931 93228 98136 103030 108196 772069 Ui (xi ) D vi Ł xi =1000, where vi is the assigning utility weight. As shown in Table 2.3, the higher the utility weight vi , the higher the throughput and the higher the achieved utility. In the first case, even with log(Ð) effects, the aggregate utility is still improved significantly in comparison with the round-robin scheme. For the second case, since the utility function is linear, the aggregate utility has been significantly improved. In the case of multicluster without mobility we discuss the intercluster collision resolution. Global fairness is achieved by distributedly optimizing social optimality. Figure 2.19 shows the simulated topology. There are three clusters with five, one and three directed link flows, respectively. Suppose that the traffic on each link flow is greedy. The utility function of each flow is Ui (xi ) D vi log(xi ), where vi is the utility weight and xi is the throughput (bit/s). In the first cluster, the clusterhead node 6 coordinates the RI-MAC to exploit the multi-input link diversity. In cluster 3, node 7 coordinates the SI-MAC to exploit the multi-output link diversity. The channel model and achievable rate function are formulated in the same way as the single-cluster case. The stochastic approximation algorithm is used to achieve the optimal channel allocation rate for each cluster in a distributed way. The pricing in terms of perceived collision probability is given by ½(Â) D 0:001Â . Figure 2.20 shows the convergence speed of the time-averaged stochastic approximation with feedback. It takes only about 200 cycles, i.e. 1.5 s, to reach stability. It was also found that the slope of the pricing function does not affect the convergence speed much as long as it is sufficiently large. The channel allocation rate of each cluster decreases (proportionally with each other) when the pricing increases to a very large value. However, the throughput and utility are not affected much. The reason is that each clusterhead persistently accesses medium at the channel allocation rate, minislot by minislot, whenever a channel is free. Having in mind the fact that the minislot is relatively short to the TXOP, the wasted time for collision resolution is small. Throughput and utility are used to compare the performance gain with 802.11. Table 2.4, shows that the aggregate utility and aggregate throughput have been improved by 6 % and 54 %, respectively. In 802.11, the throughputs are equally shared. In BO-MAC (OMAR), more timeshares are given to the user with a higher utility function; thus the aggregate utility is increased. Furthermore, as BO-MAC exploits multiuser diversity, the throughput of each user has been increased. For the analysis of a multicluster with mobility a heterogeneous ad hoc network as shown in Figure 2.21 is used. Each high-power (HP) node forms a cluster and functions as the clusterhead. Each low-power (LP) node associates with the nearest HP node. In the beginning, both HP nodes
  • 62. 0 1:0 1:0 0:071 11:7 11:3 0:014 29:8 78:2 OMAR (W Ł ) OMAR-utility RR-utility OMAR (W Ł ) OMAR-utility RR-utility case 1 case 2 Parameter Flow ID hi ¹i 0:027 58:1 86:1 0:078 13:0 12:4 1 1:0 1:1 0:043 94:8 93:0 0:084 14:2 13:5 2 1:0 1:2 0:063 145:1 102:2 0:091 15:5 14:6 3 1:0 1:3 0:084 197:8 109:8 0:097 16:8 15:8 4 1:0 1:4 0:106 256:5 116:8 0:103 18:0 16:9 5 1:0 1:5 0:130 321:4 126:5 0:110 19:3 18:0 6 1:0 1:6 0:153 388:7 132:9 0:116 20:6 19:3 7 1:0 1:7 0:178 461:2 140:0 0:122 21:9 20:3 8 1:0 1:8 9 1:0 1:9 0:202 536:6 148:8 0:128 23:2 21:4 Table 2.3 Case study of general optimization (BO-MAC D OMAR), (Reproduced by permission of IEEE [4]) 1.0 2490.0 1134.4 1.0 174.0 163.4 Aggregate MULTIUSER DIVERSITY IN WIRELESS AD HOC NETWORKS 43
  • 63. 44 OPPORTUNISTIC COMMUNICATIONS 4 5 Cluster 1 3 2 1 F2 F4 F3 F1 F0 6 Cluster 2 F5 7 Cluster 3 F8 F7 10 9 F6 8 ( F = Flow ) Figure 2.19 Multicluster topology. Channel allocation rate (%) 50 Cluster 1 Cluster 3 Cluster 2 45 40 35 30 25 20 0 200 400 600 Cycle number 800 1000 Figure 2.20 Convergence speed of the channel allocation rate with the stochastic approximation algorithm. and LP nodes are evenly distributed over a 400 m ð 600 m area (but each LP node is away from the nearest HP node by 70 m). Later on, LP nodes move randomly, following the random-way point mobility model with minimal speed larger than 0.1 m/s. Each LP node estimates its SNR distribution iteratively, as discussed in Section 2.6.3. For simplicity, it is assumed that all the traffic is from HP nodes to associated LP nodes. The traffic of each link is saturated. The utility function of each link is Ui (xi ) D log(xi ), where xi is the throughput (bit/s). Under this utility function, each user can obtain equal opportunity to access the channel in a cluster; i.e. timeshare fairness within a cluster can be achieved by BO-MAC. For 802.11, round-robin scheduling is used to guarantee the timeshare fairness among the output links. The achievable data rate, in terms of the distance d and the fading factor h, is R(d; h) D B log2 f1 C [250= max(25; d)]4 [min(h; h max )=156]g where h follows the Rayleigh distribution with expectation 1 and h max is equal to 100. The simulation time of each scene is 2000 s. The result of each scenario is averaged over 10 simulation results.
  • 64. 0 1.0 1.0 111665 11.6 108151 11.6 Flow ID hi ¹i OMAR-throughput OMAR-utility 802.11-throughput 802.11-utility 108150 11.6 111229 11.6 1 1.0 1.0 108151 11.6 116085 11.7 2 1.0 1.0 108152 11.6 113279 11.6 3 1.0 1.0 108150 11.6 110363 11.6 4 1.0 1.0 108151 57.9 172492 60.3 5 1.0 5.0 36050 21.0 137754 23.7 6 1.0 2.0 36051 21.0 145348 23.8 7 1.0 2.0 Table 2.4 Optimal intercluster collision resolution. (Reproduced by permission of IEEE [4]) 36050 21.0 145591 23.8 8 1.0 2.0 757057 178.9 1163807 189.7 Aggregate MULTIUSER DIVERSITY IN WIRELESS AD HOC NETWORKS 45
  • 65. 46 OPPORTUNISTIC COMMUNICATIONS Powerful node Common node Cluster Figure 2.21 The simulated heterogeneous ad hoc network. The total number of nodes is varied from six to 60 such that the average number of users in each cluster is varied from one to 10. Each LP node moves at the average speed of 1 m/s. As shown in Fig. 2.22(a), the aggregate throughput can be significantly improved by utilizing multiuser diversity even though the number of users in each cluster is small, e.g. two. The performance gain steadily increases as the number of users increases. When the average number of users in each cluster is 10, the throughput by BO-MAC is about twice that of 802.11. For the mobility study the average number of users in each cluster is five. As shown in Figure 2.22(b), both BO-MAC and 802.11 increase throughput as the average speed increases. This should be expected. In the beginning, each node is put 70 m away from the nearest HP node. Later on, some nodes get much closer to the associated HP node even though some nodes become further away. However, no matter how far away, each link is in a cluster and gets a similar opportunity to access the channel. The aggregate throughput will increase as the speed increases from 0.5 m/s to 5 m/s. It is worthy to note that the performance gain of BO-MAC over 802.11 is affected when speed increases, mainly due to increasing clustering overhead and the channel estimation error. However, the performance gain of BO-MAC over 802.11 is still substantial. BO-MAC achieves higher aggregate throughput without sacrifice of individual fairness. Figure 2.22 shows the throughput of each user in the scenario with 30 LP nodes moving at 1 m/s on average. Users are sorted according to throughput. Each user of BO-MAC achieves much higher throughput than that under 802.11. The reason that some nodes get higher throughput than others under the same scheme is mainly because of their shorter distances to HP nodes after they randomly move. C 2.7 MOBILITY-ASSISTED OPPORTUNISTIC SCHEDULING (MAOS) In this section we present optimal (OOSA) and suboptimal (SOOSA) opportunistic scheduling algorithms that combine channel fluctuation and user mobility information in their decision rules. The algorithms are based on the opportunistic scheduling framework with dynamic constraints for fairness. These fairness constraints adapt according to the user mobility. The adaptation of constraints in the algorithms implicitly results in giving priority to the users that are in the most favourable locations. The optimal algorithm precomputes constraint values according to a known
  • 66. Aggregate throughput (Mbit/s) MOBILITY-ASSISTED OPPORTUNISTIC SCHEDULING (MAOS) 1,6 1,5 1,4 1,3 1,2 1,1 1 0,9 0,8 0,7 47 BO MAC 802.11 1 2 3 4 5 6 7 8 Average number of users per cluster 9 10 Aggregate throughput (Mbit/s) (a) 1,6 1,5 1,4 1,3 1,2 1,1 1 0,9 0,8 0,7 BO MAC 802.11 0,5 1 1,5 2 2,5 3 3,5 Average speed (m/s) 4 4,5 5 Throughput (kbit/s) (b) 55 50 45 40 35 30 25 20 15 BO MAC 802.11 0 5 10 15 User index 20 25 30 (c) Figure 2.22 (a) Throughput versus node density. (b) Throughput versus node mobility. (c) Throughput fairness. mobility model. The suboptimal algorithm relies on the future prediction of the user mobility locations in time. Let us assume that there are N users in a cell with the feasible data rate vector at a generic scheduling timeslot R D (R1 ; : : : ; R N ) (the rate at which user i can be served if scheduled). It is also assumed that E(Ri 1fQ(R)g ) is the average scheduled data rate of user i achieved by the
  • 67. 48 OPPORTUNISTIC COMMUNICATIONS scheduling policy Q, where 1fg is the indicator function. The overall system data rate is E(R Q(R) ) D N iD1 E(Ri 1fQ(R)Dig ). The temporal fairness measure considers time as a resource and shares it among multiple users according to the given fairness constraints. In other words, if PfQ(R) D ig is the probability of scheduling user i by policy Q and rl is the long-term minimum temporal requirement for user N i, where ri ½ 0, " D iD1 ri Ä 1, then the LCS (Liu, Chong, Shroff) algorithm [29] solves the following stochastic optimization problem: max E(R Q(R) ); Q subject to PfQ(R) D ig ½ ri ; i D 1; 2; ::; N (2.33) The LCS algorithm defines the optimal policy as Q Ł (R) D arg maxi (Ri C viŁ ) (2.33a) viŁ where are given Lagrange multipliers that satisfy the constraints in problem (2.33). The algorithm has a polynomial complexity of O(N ). As an illustration let us assume that there are 10 users sharing a single channel with fairness constraints given as ri D 1=12. These constraints mean that each user expects an allocation of at least 8.33 % of the time slots for itself. This is an average fairness measure and considers only a fraction of the total time. In other words, the position of the allocated time slots or their order is not important. 2.7.1 Mobility models For the mobility, a finite-state semi-Markov model is used. The model divides cell geography into a set of nonoverlapping topological spaces S D f1; : : : ; Mg called mobility states. The model can, for example, represent regular concentric rings [30] or irregular cell areas depending on the accessibility and geography of the cell. The rings or spaces are characterized by the average SINR, ²(m), where m 2 S. The average SINR depends on ym (the distance between BS and mobility state m). User mobility among these states follows a semi-Markov process with (known) dwell times with distribution function Pd (m) and mean d(m). The stationary distribution of the underlying process is found as follows [31]: ³(m) D ³ e (m)d(m) 6m 0 2S ³ e (m 0 )d(m 0 ) (2.34) where ³ e (m) is the stationary distribution of the embedded Markov chain. The SINR is a function of path loss, interference and fading processes. Path loss is a function of the distance from the BS and interference is a function of the distance from the neighboring cells. Because our mobility model allows user mobility over large areas, the average SINR will change over time, resulting in a nonstationary channel model. To cope with the nonstationarity and to have a simpler model that allows analysis, we employ a piecewise stationary channel model and assume that the path loss and interference remains constant within a mobility state. Let mobility state m be at distance ym from the BS. By using a simple propagation model, the path loss at distance ym from the BS is given as 0(m) D 1; ym Ä y0 (y0 =ym )Þ ; ym > y0 (2.35) In Equation (2.35), y0 is the maximum distance from the BS up to where full transmitted power Pt is received and Þ is the path loss exponent. In the network where the central cell is surrounded
  • 68. MOBILITY-ASSISTED OPPORTUNISTIC SCHEDULING (MAOS) 49 by six interfering cells the interference from the neighboring cells is approximated as [30] I (m) D PI ð 0 (2Y ym ) C 20 (Y ym )2 C 3Y 2 (Y C ym )2 C 3Y 2 C 0 (2Y C ym ) C20 (2.36) where PI is the transmit power of the interfering cell and Y is the cell radius. For small-scale fading, a flat fading channel is assumed, valid for systems where, the feed back delay is relatively short compared to the fading frequency. If user l is in mobility state m at a generic time slot then the received SINR is given as [30]: Z l (m) D l Pt ð 0(m) ; Á C I (m) (2.37) where l is the time-dependent fading process, Pt is the transmitted power, 0(m) and I (m) are the path loss and interference experienced by the user and Á is the background noise. The fading processes l are independent and identically distributed (i.i.d.) (exponentially distributed stationary process with unit mean). Therefore, Z is also an exponentially distributed random process with mean ²(m) D Pt ð 0(m)=[Á C I (m)] and, for z ½ 0, its distributions are given as p Z (z; m) D e z=²(m) =²(m). If H D fZ 0 ; Z 1 ; : : : ; Z H 1 g represents the set of discrete SINR levels in ascending order, the fading channel takes level h if the received SINR falls in the interval [Z h ; Z hC1 ) and the corresponding steady-state probability of level h in mobility state m is Z hC1 p(h; m) D p Z (z; m)dz D e zh = p(m) e zhC1 =²(m) (2.38) Zh The state transition probabilities are computed according to the procedure given in References [32] and [33]. The physical layer coding and modulation schemes map fading levels to corresponding data rates. The overall system model employs a hierarchical time scale with two levels. The mobility slot operates on a wider timeslot. This timeslot allows the algorithm to take advantage of the slow time path loss variations in the feasible data rate. At the mobility slot level, the fast time-fading variations in the feasible rate can be represented by their expected values. The scheduling slot handles scheduling decisions and has a narrower timeslot. The scheduling slot allows the algorithm to benefit from the fast time-fading fluctuations. At the scheduling slot, the slow time path loss variations can be modelled as a constant path loss experienced at the centre of the corresponding mobility state. The presence of two time scales helps to accommodate user mobility, which evolves relatively slowly compared to the scheduling decisions. 2.7.2 Optimal MAOS algorithm At the time scale of user mobility, the fading experienced by a user in a mobility state can be abstracted by the corresponding expected feasible rate. In order to improve the system data rate, it is imperative to give preference to users when they are in the most favorable locations, i.e. having high expected feasible rates. Since the algorithm considers time as a resource to be allocated, if time fractions are distributed such that they maximize their product with the expected feasible rates of the corresponding users for all aggregate mobility states, then the algorithm can achieve its stated objectives of improving the system data rate. Therefore, this stage of the MAOS algorithm determines optimum time fractions that maximize the expected feasible rates of the users under the constraints of providing fairness and preventing starvation. For N users and M mobility states per user, the set of possible aggregate states is an N -dimensional space given by S D S1 ð Ð Ð Ð ð S N , where ð denotes the Cartesian product and
  • 69. 50 OPPORTUNISTIC COMMUNICATIONS Sl is the state space of user l defined earlier. Therefore, the aggregate mobility model contains M N states (jSj D M N ), and a state is identified by a vector m D (m 1 ; : : : ; m N ) at a generic mobility slot. Assuming independence among users, the stationary distribution of state m 2 S equates to the N product of the individual stationary distributions, P(m) D lD1 ³(m l ). In the sequel, we assume that the scheduler knows the above mobility model, expected feasible rates R(m; l)8m and l, and can track the user state accurately. The MAOS algorithm first computes the optimal time-fraction allocations for every mobility state m that maximizes its product with the expected feasible rate summed over all states and for N all users. If rl is the long-term minimum temporal requirement for user l and " D lD1 rl Ä 1, the MAOS algorithm solves the following linear program (LP): N R(m; i)r (m; i) max (2.39a) m2S iD1 N r (m; i) Ä P(m); 8m 2 S subject to (2.39b) iD1 r (m; i) D ri ; i D 1; : : : ; N (2.39c) m2S r (m; i) ½ Âri ; 8m 2 S; 8i and 0 Ä Â Ä Âmax jSj (2.39d) where r (m; i) are optimization variables and  is a parameter. LP (2.39) maximizes the weighted sum of the expected feasible rate and complies with the stationary distribution of user mobility (2.39b), satisfies long-term temporal fairness constraints (2.39c) and prevents long-term starvation (2.39d). The time fraction ri behaves as a resource and the above LP optimally distributes it among all mobility states accessible to the user i. With  > 0 in (2.39d) the long-term starvation of users is prevented. With  D 0, the resulting solution of (2.39) is a greedy solution where users are denied access in mobility states with weak channels. This denial results in the starvation of such users. Therefore,  > 0 guarantees that a minimum fraction of rl , distributed over all mobility states, is assigned to every user, even in mobility states with bad channels. In this way, the MAOS algorithm avoids long-term starvation. For the feasibility of LP,  is upper bounded by Âmax D min(P(m))jSj=". m If r Ł (m; i) maximizes the objective function (2.39a) it is still only a fraction of ri because of the construction of LP (2.39). When " D 1, then ri behaves like the probability of allocation of user i. The resulting r Ł (m; i) is the joint probability of two events: user i 0 s allocation and the occurrence of mobility state m. Thus, user i 0 s allocation probability given mobility state m can be found by dividing r Ł (m; i) by the corresponding probability of the state. For a more general case when " < 1 and considering the feasibility of the constraint (2.39b), the resulting allocation probability, called the normalized time fraction r (:), is ˆ r (m; i) D "r Ł (m; i)= ˆ N iD1 r Ł (m; i); 8m 2 S (2.40) The algorithm gains also from the fast time-fading fluctuations in the feasible data rate. For this purpose, it uses the LCS methodology for the normalized time fractions found in the first stage. It maximizes the expectation of the scheduled (system data) rate for each mobility state and satisfies the respective time-fraction allocation. For this objective, the algorithm solves the following stochastic optimization problem after modifying problem (2.33): max E(R Q(R(m)) ) Q subject to PfQ(R(m)) D ig ½ r (m; i); i D 1; 2; ::; N ˆ (2.41)
  • 70. MOBILITY-ASSISTED OPPORTUNISTIC SCHEDULING (MAOS) 51 The problem has dynamic constraint values that are functions of m. The optimal scheduling policy is found by adapting the solution (2.33a) for state m: Q Ł (R) D arg maxi [Ri C viŁ (m)] (2.41a) In Equation (2.41a), viŁ (m) is the given Lagrange multiplier that satisfies the corresponding constraint in problem (2.41). Therefore, MAOS can be considered as a piecewise LCS algorithm, i.e. one for every m state. 2.7.3 Suboptimum MAOS algorithm The optimal MAOS algorithm has exponential complexity in the number of users because it precomputes time fractions for all aggregate mobility states. The first stage of the suboptimum MAOS finds r (t C 1; i); 8i and 1 D 1; : : : ; − , as a solution to the following LP: ˆ − N R(t C 1; i)r (t C 1; i) max (2.41b) 1D1 iD1 N r (t C 1; i) D "; 81 subject to (2.41c) iD1 1 − − r (t C 1; i) D ri ; i D 1; : : : ; N ; (2.41d) 1D1 where R(t C 1; i) is the average feasible data rate of user i at the t C 1 (mobility) timeslot in the future in accordance with the predicted mobility state. The constraint (2.41b) ensures that, for every 1, the time fraction resource " is distributed among users, and constraint (2.41c) makes sure that, for every − , the assigned fraction satisfies the long-term fairness requirement. LP (2.41) is solved after every − mobility slot. If r (t C 1; i) maximize the objective function (2.41) in the second stage, these values are used ˆ to modify and solve problem (2.33). The resulting stochastic optimization problem maximizes the expectation of the scheduled (system data) rate and satisfies the new fairness constraints max E(R Q(R(tC1)) ) Q subject to PfQ(R(t C 1)) D ig ½ r (t C 1; i) ˆ (2.42) i D 1; 2; : : : ; N ; 1 D 1; : : : ; − Parameter r (t C 1; i) changes only when a user makes a transition to a new mobility state. After ˆ that transition, it remains constant for several mobility timeslots until the next transition. LP (2.41) can be modified by lower bounding r (t C 1; :) like (2.39d) in order to avoid possible long-term starvation. The solution of problem (2.42) is a modified LCS policy that selects a user at every (scheduling) timeslot according to Q Ł (P) D arg maxi [Ri C viŁ (t C 1)] (2.42a) In Equation (2.43),vlŁ (t C 1) is the Lagrange multiplier for the constraint active at (mobility) time t C 1. The resulting subopimum MAOS algorithm has a complexity of O(N − ). 2.7.4 Mobility estimation and prediction The optimal MAOS algorithm only needs to estimate the current mobility state of every user. This is not difficult because the mobility model considers a large area within a cell as a state.
  • 71. 52 OPPORTUNISTIC COMMUNICATIONS A user anywhere in that area is considered to be in that state. Monitoring the respective radio signal strength or alternative technologies, like the angle of arrival (AoA) and time delay of arrival (TDoA), may be used for location estimation [34, 35]. The presence of two time scales helps in reducing the errors in the estimation process. The suboptimum MAOS algorithm also needs to predict the mobility state in the future. An alternative technique is suggested in Reference [4]. Straight-line movement on highways mostly follows deterministic mobility with constant speed and velocity [36–38]. Therefore, the future trajectory can be accurately predicted. For stochastic user mobility, the approximate MAOS algorithm can employ a hierarchical mobility model. The higher layer of this model will have a discrete-state space like the one presented in this section. The lower layer of the model will have a continuous-state space modelled by some linear dynamical system. This continuous-state-space model will help to monitor not only the state but also the fine-grain location information in the form of coordinates, velocity and acceleration [37, 38]. The fine-grain mobility information will help in predicting the future discrete states to be visited by a user. For example, Reference [37] showed a fairly accurate prediction of the next state several seconds ahead of time. With the help of fingerprinting of the road network, the future prediction of mobility states can be further improved. 2.7.5 Estimation of Lagrange multipliers Both the optimal and suboptimal MAOS algorithms use a stochastic approximation technique for the estimation of Lagrange multipliers. The optimal MAOS sets the initial value of v(:) D 08i and m and maintains a database for every mobility state m, where it holds the most recent estimated values of v(:) and supporting parameters used in its computation. Whenever the system enters into mobility state m, the algorithm retrieves the stored values from the database. As long as the system remains in state m, it updates the values of v(:) for every scheduling slot k C 1 as (see Chapter 12 and Reference [29]) vikC1 (m) D max[(vik (m) Ž (1fQ k (R(m)Di)g r (m; i)); 0)] ˆ (2.43) A small constant Ž is used as the step size because the corresponding LCS algorithm, being indifferent to the user mobility information, views the piecewise stationary channel as a nonstationary process. For nonstationary processes, the stochastic approximation technique recommends a constant step size [29]. The presence of a database to store the values of v(:) and supporting parameters for future use and the difference in time scales between mobility transitions and scheduling decisions guarantees the saturation of Equation (2.43). The suboptimum MAOS cannot store the values of v(:)into a database because it lacks the complete knowledge of the state space. Therefore, whenever a new state is visited, the algorithm resets v(:) and the supporting parameters. Then it uses an expression similar to (2.43) for subsequent scheduling and mobility slots until the next mobility transition. The time scale difference between mobility transitions and scheduling may allow the suboptimum MAOS to achieve saturation. 2.7.6 Performance examples In Reference [39] a performance example for MAOS is presented for a hexagonal cell that has a maximum coverage distance of 5 km from the central BS to a mobile user. The service area is divided into two concentric rings (m D f1; 2g). The innermost ring covers an area of up to 3 km from the BS. The BS’s maximum output power is 15 W and 80 % of this power is used for the shared data channel. The system provides a data service with feasible data rates of f2457.6, 1843.2, 1228.8, 921.6, 614.4, 307.2, 204.8, 153.6, 102.6, 76.8, 38.4g Kbps and corresponding SINRs of f9.5, 7.2, 3.0, 1.3, 1.0, 4.0, 5.7, 6.5, 8.5, 9.5, 12.5gdB. The parameter ²(m), defined in Section 2.7.1, at the centre of the rings is found from the path loss model for Þ D 4; y0 D 1 km, and
  • 72. OPPORTUNISTIC AND COOPERATIVE COGNITIVE WIRELESS NETWORKS 53 Á D 1 W. The steady-state channel distribution is determined using Equation (2.38) for a speed of 60 km/h. The resulting expected feasible rate values are 1438 Kbps in the inner ring and 116 Kbps in the outer ring. The scheduling decisions are made at every 1.67 ms. For the two concentric rings, the coarse-grained mobility model is learned through the simulation of a user following a fine-grained mobility model. For our simulation, random-walk mobility is considered with a constant speed of 60 km/h to model fine-grained user movement. Travel intervals are randomly distributed between 8 to 12 minutes. After completing one travel interval, the user selects a new direction randomly from 0 to 2³. When the user moves out of the service area, it wraps around and re-enters the service area. Mobility slots are of one second duration. The choice of identical mobility and channel distributions is only for ease of simulation. The algorithm can handle nonidentical cases too. Half of the users require (minimum) fairness requirements of "/6 and the other half expect at least an "/12 fraction. In Equation (2.43), Ž D 0:05 is used. Figure 2.23(a) compares the average data rates achieved by eight users under the MR, LCS and MAOS algorithms. Because the first four users have higher fairness requirements, their data rates are higher than the remaining four users. On the other hand, LCS suffers performance losses in order to support constant fairness constraints. These constant fairness constraints force the LCS algorithm to schedule users when they are in an unfavorable location. Although the channel information is considered in scheduling, this information only allows it to benefit from the fast time fading while slow time path loss variations remain untapped. Thus, LCS ends up satisfying fairness constraints for users when they have weak channels. The performance improvements seen in the case of MAOS have been achieved without sacrificing the long-term fairness requirements, as shown in Figure 2.23(b). This figure compares the temporal fairness requirements for all users to their actual allocations performed by the scheduling algorithms. The MR algorithm, which is a greedy algorithm without any regard to fairness, failed to support the required fairness measure. It gave equal access to all users, not because of any conscious decision on the part of the algorithm, but because of identical mobility and channel distributions used in the simulation. The MAOS and LCS are able to satisfy the minimum fairness requirements of r1;:::;4 D "/6 and r5;:::;8 D "/12. 2.8 OPPORTUNISTIC AND COOPERATIVE COGNITIVE WIRELESS NETWORKS Cognitive wireless networks benefit from context awareness, which is integrated in the decisionmaking process in different layers. In this section, we discuss a system where the MAC layer transmission permission probability is modified, for elastic traffic users, in such a way as to discourage the transmissions from/to the users at the border of the cell where transmissions cause significant interference in adjacent cells. This will be referred to as spatial traffic shaping. By using the absorbing Markov chain theory, we provide, for such a concept, analytical models to analyze system performance, mainly the probability of successful channel assignment and message delivery delay. The analysis shows that in the cellular network, with the channel spatial reuse factor equal to one, the probability of successful channel assignment close to one can be achieved with an acceptable message delivery delay. 2.8.1 The system model We assume that the beamforming has generated M spatial channels that can be reused across each cell. Within each spatial channel, N additional channels are generated by using either a timeslot (TDMA), frequency beam (OFDM) or code (CDMA). In the sequel we model adjacent cell interference for the case when the cochannel adjacent cell interference is caused by one interfering user (TDMA or OFDMA system) or users located in a relatively small area (CDMA). In this section we focus on the downlink although extension to the uplink is straightforward [42].
  • 73. 54 OPPORTUNISTIC COMMUNICATIONS Normalized Individual Data Rates (x 2457.6 kbps) 0.15 LCS MAOS MR 0.12 0.09 0.06 0.03 0 1 2 3 4 5 6 7 8 User index (a) Fraction of Time Assignments Fairness 0.18 LCS MaOS MR 0.15 0.12 0.09 0.06 0.03 0 1 2 3 4 5 6 7 8 User index (b) Figure 2.23 Comparison of MAOS with the LCS and MR algorithms: (a) individual data rates for " D 0:99 and (b) fairness. (Reproduced by permission of IEEE [40]).
  • 74. OPPORTUNISTIC AND COOPERATIVE COGNITIVE WIRELESS NETWORKS 55 C Interference x x 60° A q x r R d ∆r r b B j x Interference a Figure 2.24 Size 3 cell cluster scenario. 2.8.1.1 Signal to adjacent cell interference ratio Suppose mobile stations a and b are in two adjacent cells, as shown in Figure 2.24. Different simplified models for a signal-to-adjacent cell interference ratio are used for specific sets of assumptions. (1) Assuming no power control and no fading, with the same transmit power P. Let I (b ! a) represent the interference power level at location a of the signal transmitted to station b, by base B. This power is defined by the set of equations P P P D pa ; Þ D pb ; I (b ! a) D Þ ra rb (rb C d)Þ SIR D pa P(rb C d)Þ DyD D Þ I (b ! a) ra P rb C d ra Þ (2.44) where Þ is the propagation constant, P is the base station transmit power and pa and pb are the useful signal powers received at the locations a and b transmitted from their base stations A and B, respectively. From the geometry shown in Figure 2.24 we also have ra =R D g (2R ra cos Â)2 C (ra sin Â)2 D (d C rb )2 (2R=ra cos  )2 C (sin  )2 D y1 D (2=g cos  )2 C (sin Â)2 d C rb ra 2 (2.44a) Þ=2 i 1 D 1=y1 (2) Assuming no power control in the presence of the fading. If ¾ represents Rayleigh fading, þ lognormal shadowing and Þ the propagation constant, the previous relation becomes ¾b þb P ¾bCd þbCd P ¾a þa P D pa ; D pb ; I (b ! a) D Þ Þ ra rb (rb C d)Þ SIR D y D ¾a þa P(rb C d)Þ ¾a þa D Þ ra P¾bCd þbCd ¾bCd þbCd rb C d ra Þ
  • 75. 56 OPPORTUNISTIC COMMUNICATIONS ra =R D g (2R ra cos Â)2 C (ra sin Â)2 D (d C rb )2 cos  )2 C (sin  )2 D (2R=ra ¾a þa (2=g ¾bCd þbCd i 2 D 1=y2 y2 D d C rb ra (2.45) 2 cos Â)2 C (sin Â)2 Þ=2 (3) Power control, arbitrary user location and no fading. In this case the received signal power P is constant and we have Pb Pb Pa D P; Þ D P; I (b ! a) D Þ ra rb (rb C d)Þ SIR D y5 D P(rb C d)Þ D Pb rb C d rb Þ D rb =R C d=R rb =R Þ D h C d=R h Þ h D rb =R a ) (x; y) ) (r;  ) (2R x)2 C y 2 D (d C rb )2 dD rb C (2R x)2 C y 2 x D r cos  r 2 D x 2 C y2 (2.46) dD rb C (2R r cos  )2 C y 2 dD rb C 4R 2 4Rr cos  C r 2 dD rb C R 4 d=R D SIR D d=R D hC 4 d=R C h h 1CgC 4(r=R) cos  C (r=R)2 Þ 4g cos  C g 2 4 4g cos  C g 2 g; h 2 [0; 1] arbitrary independent values In the case of a size 3 cluster scenario (cells A, B and C), as shown in Figure 2.24, the angle between directions AB and AC is ' D 60Ž . If the distance is calculated for cells A and B by using (1) then for cells A and C we use (1) with Â1 D  C ' D  C 60Ž . The extension of the previous two models to the case with power control, arbitrary user location and channel with fading is straightforward. (4) sir (snir) control. In order to keep the equations simple and representative we will ignore the presence of Gaussian noise in the snir control loop and focus our attention on sir only. In the case when  is such that the users a and b mutually interfere with each other the above equations become Þ Þ Pa =ra D pa ; Pb =rb D pb ; I (b ! a) D Pb =(rb C d)Þ ; I (a ! b) D Pa =(ra C d)Þ Pa Pb Pb Þ sir (b) D pa =I (a ! b) D Pb (ra C d)Þ =Pa rb D Pa Þ sir (a) D pa =I (b ! a) D Pa (rb C d)Þ =Pb ra D rb C d ra ra C d rb Þ Þ (2.47)
  • 76. OPPORTUNISTIC AND COOPERATIVE COGNITIVE WIRELESS NETWORKS 57 If the required sir is Y , we have sir (a) D sir (b) ! Y and the question is whether or not we can guarantee this value across the entire network for all mutual positions of the terminals given that Pa ; Pb Ä Pmax . This can also be defined in terms of an equivalent power constraint Pmin Ä Pa ; Pb Ä Pmax or equivalently the power ratio Pr D Pa =Pb ; Pr min Ä Pr Ä Pr max . With d defined in the previous discussions we have to find solutions where the above conditions are satisfied. From the required sir we have rb C d ra ½ Y ! Pr ½ Y Þ ra C d rb 1 Pr Þ rb C d ra Pr ½ Y ! Pr Ä Y ra C d rb 1 Þ Þ (2.48) This should be within the limits Pr D Pa =Pb ; Pr min Ä Pr Ä Pr max . In the case when only one user interferes with the other (as in Figure 2.24) the simplification of the above discussion is straightforward. 2.8.2 The outage probability The outage probability is Pout (x; Â ) D P(i > ") D 1 P(i < ") D 1 P ¾bCd þbCd i 0 (x; Â ) < " ¾b þb (2.49) In general there will be a correlation between the spatial samples of ¾bCd and ¾b as well as between þbCd and þb . The amount of correlation will depend on both the beam width and the mutual distance d between interfering users. Both larger distance and larger beamwidth will result in less correlation. Although the modeling of the channel resemblance (correlation) for the two interfering users is outside the scope of this book we can make some suggestions that can be used in further evaluation of the protocols. For a very narrow beam we can use the approximation ¾bCd D ¾b ¾d and for d ! 0, ¾bCd ! ¾b . For larger beamwidths we can use the approximation ¾bCd D (1 ²)¾ b C ²¾b , where ¾ b represents the portion of the signal received in point b C d not seen by point b, and the resemblance factor ² can be represented as ²('; d) D (1 d=d0 )(1 '='0 ). For a precise analysis of the Pout we need joint probability distribution functions of ¾bCd and ¾b . However, for a relatively wide beam and d large enough (e.g. d D R for the CD protocol described in Chapter 12) we can assume that the fading at two mutually interfering terminal locations is independent. In this case, if þ is lognormal variable with zero mean and variance ¦ then in this calculus we can use the n bCd n b D 10n is also lognormal variable with zero mean and variance fact p that þ D þbCd =þb D 10 2¦ . Therefore, we have Pout (x; Â ) D P(i > ") D 1 1 D1 P(i < ") D 1 ¾bCd i 0 (x; Â ) < " ¾b 1 1 P(þ)dþ 0 P þ P(¾bCd )d¾bCd 0 P(¾b )d¾b (2.49a) i 0 þ¾bCd =" In the sequel we use the notation S(k * ) for the set of all possible combinations of k channels where i > " and S(k + ) for the set of all possible complementary combinations of N k channels where i < ". For each element with index s in S(k + ) there is one corresponding element in S(k + ).
  • 77. 58 OPPORTUNISTIC COMMUNICATIONS The probability of having k channels with i > " can be presented as (i) pout pout (k) D s i2s2S(k*) (1 ( j) pout ) (2.50) j2s2S(k+) (i) where pout is the channel outage probability given by Equation (2.49). 2.8.3 Cellular traffic shaping Cognition may help to stimulate users to transmit from the locations where they induce minimum interference to other users in the network, especially to reduce intercell interference. The conventional approach for service pricing so far includes only charging the user based on the required bit rate, with additional possible requirements on delay, without taking into account how much the user was interfering with others and so reducing the overall available capacity for others. An introduction of an effective network usage parameter may stimulate the user, having delayed elastic traffic, to transmit/receive from/at the positions that are preferable. Storing the message and waiting for such a position may be a way to go. For the theoretical modeling of such systems game theory may offer interesting solutions. This will be further elaborated later. For illustration purposes suppose that there is effectively one cochannel interfering signal sent to the user in the adjacent cell per beam. We consider two scenarios: (1) Unregulated traffic. In this case the user is charged based on a bandwidth independent of its location in the cell. Therefore, its position in the moment of transmission/reception is uniformly distributed within the cell of radius R D 1. (2) Spatially regulated traffic. In this case users are encouraged to transmit/receive at the position closer to the BS (further away from the cell border where they produce a lot of interference) by using the cost function c D r , where r is the normalized distance from the BS. Based on this we assume that a rational user will transmit with probability f (r ) D 1 r (2.51) given that it has traffic to transmit. The number of additional options for f (r ) will also be discussed. This protocol will be referred to as position aware multiple access (PAMA) as a part of a more general class of protocols that will be referred to as context aware multiple access (CAMA). CAMA may include additional cognition when calculating transmission probabilities f (r ). The most relevant information would be the terminal speed. If the terminal is static there would not be a possibility to postpone the transmission because there would be no chance that the terminal would assume a more preferable position in the near future. If v M AX is the maximum speed of the terminal a possible modification of the transmission probability could be f (r; v) D (1 v=v M AX ) C (v=v M AX ) f (r ) (2.51a) If more sophisticated cognition is available to provide information about the direction of terminal movement, then we can define direction D as DD 2j1Âj j1Â j D ³=2 ³ (2.51b)
  • 78. OPPORTUNISTIC AND COOPERATIVE COGNITIVE WIRELESS NETWORKS 59 where 1 is the angle between the direction mobile base and the terminal velocity vector. Now we can use the transmission probability as f (r; v; 1Â) D 1; D > 1 D C (1 D) f (r; v); D < 1 (2.51c) An alternative definition of the directivity which is convenient to specify the Markov chain model appropriately is q DD Dq pCq Cs where q can be estimated from the sojourn angle 1 as q D (360 1Â)=360. If q is represented as q D kq 0 , where k is an integer 0 Ä k Ä K max , then the transmission probability function can be generated as f (r; k) D (1 r )k or f (r; k) D 1 rk Parameter K max is the design parameter and k is obtained from the observed value of 1 in the optimization process. Depending on the type of service, the performance measure (pm) to be optimized may be either throughput S for data or message transmission delay − for voice or a joint parameter pm D S=− . Parameter k is chosen to maximize pm as k D maxk pm and can be either pretabulated based on the results shown later or dynamically adjusted based on the observation of the pm. For delay constrained traffic we may use f (r ) as f (r ) D f (r; v; 1Â); − < −max 1; otherwise (2.51d) where −max is the maximum delay that can be tolerated. For fair scheduling we can use f (r ) D − f (r; v; 1 ); − < −max 1; otherwise (2.51e) where users with longer starvations (time − since the last transmission) are given higher priority. The above algorithms will be collectively referred to as traffic shaping multiple access (TSMA). When applied to multicarrier systems these protocols will be referred to as context aware broadband access (CABA). 2.8.4 User mobility modeling For further analysis of the system we model user mobility by a Markov random walk model, shown in Figure 2.25(a), with states 0 and N modeled as reflective states (see below). The system is observed in discrete time intervals T seconds (packet or message length) apart and the state of a user represents its normalized distance r from the base station. In the mobility model we are not interested in the information on the angular position  from Figure 2.24, because the decision on whether or not to transmit will depend only on r . On the other hand, the outcome of the transmission (success or collision) will also depend on  and could be averaged out with respect to  in the appropriate range. The state transition probabilities are given as p D P(21 > dr > 1); q D P( 21 < dr < 1); s D P(0 Ä jdr j Ä 1) (2.52) Suppose that the cell radius R D 1 is divided into R=1 D N segments. Therefore the user movement across the cell is characterized by discrete state n, representing its current distance from the
  • 79. 60 OPPORTUNISTIC COMMUNICATIONS s p s p n−1 q s p q r = (n − 1)∆ q r = n∆ (a) 1 1 s(1− fn) s(1− fn+1) t t t fn fn−1 p(1− fn−1) r = (n − 1)∆ q r = (n + 1)∆ 1 s(1− fn−1) n−1 p n+1 n n q(1− fn) r = n∆ fn+1 p(1− fn) n+1 q(1− fn+1) r = (n+1)∆ (b) 1 s(1− f0) 1 s(1− fN−1) t t f1 (p+q)(1− fn) 1 s(1− fN) t f0 0 1 s(1− f1) t fN fn−1 N−1 1 p(1− fN−1) N (p+q)(1− fn) q(1− f1) (c) (d) Figure 2.25 User mobility modeling: (a) reflecting Markov chain model; (b) to (d) absorbing Markov chain model. base station r D n1, the state transition probability (N C 1) ð (N C 1) matrix P and initial state probability vector row p(0) , where P D k pi j ki; j D 0; 1; : : : ; N (0) (0) (0) p(0) D ( p0 ; p1 ; p2 ; : : : ; p(0) ) N p(1) D p(0) P (k) p (k 1) Dp (2.53) (0) k PDp P If we now introduce the row vector of transmission probabilities as f D ( f0 ; f1 ; f2 ; : : : ; f N ) (2.54) then the probability to transmit at instant tk D kT is given by recursion: pt(0) D p(0) fT pt(1) D (1 . . . pt(0) )p(1) fT D (1 pt(k) pt(k 1) )p(k) fT D (1 p(0) fT )p(0) PfT (2.55) D (1 pt(k 1) )p(0) Pk fT
  • 80. OPPORTUNISTIC AND COOPERATIVE COGNITIVE WIRELESS NETWORKS 61 1 s(1−fN−1) 1 t fn−1 N−1 p(1−fN−1) N Figure 2.26 Handoff modeling. The average transmission delay is given as D D TEk (kpt(k) ) (2.56) We point out once again that for the above analysis states 0 and N are modeled as reflective states. For more information on mobility modeling see Reference [40]. 2.8.5 Absorbing Markov chain system model Although the above approach is straightforward it becomes impractical for producing numerical results. For this reason, in the sequel we present an alternative way to model the problem by using absorbing Markov chains, shown in Figure 2.25(b) to (d), where each state is also connected to an absorbing state t representing transmission. The probability that the user leaves the cell (handoff) without transmission can be calculated by modeling state N and state t (transmission) as absorbing and state 0 as the reflecting state, with state transition probabilities defined as indicated in Figures 2.25 and 2.26. If f 0 D 1, state 0 also becomes the absorbing state: P D k pi j k pi j ! pi j (1 f i ) for transient states pit D f i (2.57) p01 D ( p C q)(1 pN N 1 f0 ) D0 Next we reorganize the transmission matrix into (N C 2) ð (N C 2) matrix of the form [41] PŁ D I 0 R Q (2.58) where I is a 2 ð 2 diagonal unitary matrix, 0 is a 2 ð N all-zero matrix, R is an N ð 2 matrix of transition probabilities from transient states to absorbing state t and Q is an N ð N matrix of transition probabilities between the transient states. By using the notation N D (I Q) 1 , the mean time for the process to reach any absorbing state starting from transient state i is [41] τ D (−0 ; −1 ; : : : ; − N 1 )T D T (I Q) 1 1 D T N1 (2.59) and if the dwell time Ti D T for each state i is the same otherwise, then τ D (−0 ; −1 ; : : : ; − N 1 )T D (I η D column vec fTi g Q) 1 η D Nη (2.59a)
  • 81. 62 OPPORTUNISTIC COMMUNICATIONS where 1 is an N ð 1 column vector of all once. In general, the variance of that time is var τ D 2(I Q) 1 η C (I Q) 1 TQ(I Q) 1 (ηsq ) ((I Q) 1 η)sq T D diag matrix fTi g (2.60) and if the dwell times for each state is the same then var τ D [(2N I)N1 (N1)sq ]T 2 (N1)sq D square of each component of N1 (2.61) The average time to reach absorbing state is −a D fτ (2.62) where f is a row and τ is a column vector. The probability that the Markov process starting in a transient state i ends up in an absorbing state j is bi j where B D [bi j ] D (I Q) 1 R (2.63) The average probabilities of handoff and transmission from the cell are given as a D (a1 ; a2 ) D ( ph ; pt ) D p(0) B (2.64) If we are not interested from which cell the information is transmitted then state N should be modeled as a reflective state and only state t would be an absorbing state. By modifying the equations accordingly the average time to transmit or average waiting time would be calculated in the same way with the new model, and vector a would have only one element. 2.8.6 Throughput analysis In general the amount of interference between the users will depend on their mutual distance. Therefore, for the analysis of the throughput we need to model joint mobility of the two interfering users belonging to two different cells. The model is based on a two-dimensional random walk process with states (i; j) representing the joint distance (ra ; rb ) of the two users with respect to their corresponding BSs. As explained earlier, only distance from the base station has an impact on the decision to transmit. For a given transmission the performance (success or collision) will depend on angle Â, shown in Figure 2.24, and performance could be averaged with respect to this parameter. 2.8.6.1 Low-mobility (or wide-beam) model In this case we assume that during the transmission waiting period the two interfering users do not leave the interfering area. The walk of the process along I (i) and J ( j) axes is independent and both of these walks can be represented by the model from Figure 2.25 with separate t states; call them t(I ) and t(J ). The state transition probabilities now have the form pi1 i2 ; j1 j2 D P((i 1 ; j1 ) ) (i 2 ; j2 )) (2.65) which is the probability that the process moves from state (i 1 ; j1 ) to the state (i 2 ; j2 ) and the matrix P has the form P D k pi1 i2 ; j1 j2 k: (2.66) To analyze the throughput, states t(I ) and t(J ) should be converted into states S (successful transmission) if only one user was transmitting. An additional state C (collision) will be introduced
  • 82. OPPORTUNISTIC AND COOPERATIVE COGNITIVE WIRELESS NETWORKS 63 to represent the event when both users transmit simultaneously, producing a collision, which happens with probability Pout (i; j). These probabilities can be represented as   s(1 f i1 )s(1 f j1 ); i 1 D i 2 ; j1 D j2    s(1 f i1 ) p(1 f j1 ); i 1 D i 2 ; j2 D j1 C 1     s(1 f i )q(1 f j ); i 1 D i 2 ; j2 D j1 1  1 1    s(1 f ) f D p  i1 j1 i 1 i 2 ; j1 s (state S); i 1 D i 2 ; j2 D t(J )        p(1 f )s(1 f ); i D i C 1; j D j   i1 j1 1 2 1 2    p(1 f i ) p(1 f j ); i 1 D i 2 C 1; j2 D j1 C 1  1 1    p(1 f i )q(1 f j ); i 1 D i 2 C 1; j2 D j1 1   1 1   p(1 f ) f D p   i1 j1 i 1 i 2 ; j1 s (state S); i 1 D i 2 C 1; j2 D t(J )   (2.67) pi1 i2 ; j1 j2 D  q(1 f i1 )s(1 f j1 ); i 1 D i 2 1; j1 D j2     q(1 f i1 ) p(1 f j1 ); i 1 D i 2 1; j2 D j1 C 1    q(1 f )q(1 f ); i D i  1; j2 D j1 1 i1 j1 1 2      q(1 f i1 ) f j1 D pi1 i2 ; j1 s (state S); i 1 D i 2 1; j2 D t(J )        f i s(1 f j ) D pi s; j j ; i 1 D t(I ); j1 D j2   1 1 1 1 2   f p(1 f ) D p  i1 j1 i 1 s; j1 j2 ; i 1 D t(I ); j2 D j1 C 1     f i1 q(1 f j1 ) D pi1 s; j1 j2 ; i 1 D t(I ); j2 D j1 1     f i f j (1 Pout (i 1 ; j1 )) D pi s; j s (state S); i 1 D t(I ); j2 D t(J )  1 1  1 1  f i1 f j1 Pout (i 1 ; j1 ) D pi1 c; j1 c (state C); i 1 D t(I ); j2 D t(J ) By modeling states S and C as absorbing states we can calculate the probability of successful transmission and throughput by using Equation (2.61). In the case of cell cooperation, only the states with mutual distance R would exist and matrix P should be modified accordingly. The rest of the analysis is the same. The model can be further elaborated by having states Ci j and Si j from each state (i; j) and transitions back to state (i; j). For low mobility the transitions would be with probability one to state (i; j) and for high mobility with equal probability 1/N to any other state. Any other mobility can be included to make more probable transitions closer to (i; j). Now the steady-state solutions of the system would give the steady-state probabilities of being in state Si j . More precisely, Si j represents the throughput of a user in state i given that the interfering user is in state j. Similarly, Si D Si j (2.68) j represents the average throughput of a user in state i and SD Si D i Si j i (2.69) j represents the average throughput of the user. The modified function f (r; v), defined by Equation (2.51a) can be directly included in the model in Figure 2.25. Modifications that include directivity defined by (2.51b) and (2.51c) can be included by additionally modifying probabilities p, q and s accordingly, as discussed in the derivation of (2.51c). To include modifications defined by (2.51d) and (2.51e) the model has to be extended into the two-dimensional Markov model (−; r ), which also includes state dependency on the discrete value of − , representing the number of times the message transmission has been postponed from/to the terminal, as shown in Figure 2.27.
  • 83. 64 OPPORTUNISTIC COMMUNICATIONS 0,n−1 fn−1 t fn 0,n t q(1−fn) 1,n−1 q(1−fn+1) s(1−fn) s(1−fn−1) fn−1 p(1−fn−1) t fn 1,n s(1−fn−1) p(1−fn−1) q(1−fn) 0,n+1 s(1−fn) p(1−fn) t fn+1 t s(1−fn+1) 1,n+1 fn+1 t p(1−fn) q(1−fn+1) s(1−fn+1) s(1−fn−1) q(1−fn) p(1−fn−1) s(1−fn) q(1−fn+1) p(1−fn) s(1−fn+1) t,n−1 1 1 t,n t t t,n+1 1 t Figure 2.27 Two-dimensional Markov model of the opportunistic scheduling with delay constrained. 2.8.6.2 High-mobility (or narrow-beam) model Due to higher mobility (or narrow beams), during the transmission waiting period the two users may leave the interfering area and interact with different users as interferers. The new interferer may have any position with respect to the reference user, which can be modeled by allowing the second user to transfer to any state with the same probability. It may also happen that in the new situation the reference user will not be jammed due to angular isolation. In a cognitive system this information may be available and the BS is free to transmit to the user at this moment so that f (r ) becomes one. A number of additional models are also possible, but due to the limited space we cannot go into further details. 2.8.6.3 Performance measure and system utility function The concept discussed so far requires criteria for optimal transmission probability f n . In this section we use the utility function to optimize location dependent f n . One should be aware that we use the utility function only as a performance measure and do not use utility function decomposition techniques to implement and optimize the MAC protocol, as discussed later in Chapter 10. The utility function can be defined as Un D (1 UD 2Pout (n)) f n Un n f n D max U fn (2.70) f n 2 = D f f n (k)g The set of feasible values for function f n 2 = D f f n (k)g can be generated from the function f n ! f n (k) ! f (r; k) D f (n1r; k) D (1 r )k D (1 n1r )k ; k D 1; 2; ::; K max , and r Ä 1, as discussed in Section 2.3. The first term [1 2Pout (n)] 2 [0; 1] in the expression for the utility function tries to minimize the errors. The second term is the transmission probability. Increasing f n would increase U , but at the same time would increase the outage probability, which would tend to reduce U .
  • 84. OPPORTUNISTIC AND COOPERATIVE COGNITIVE WIRELESS NETWORKS 65 The previous expression does not include the delay. The delay D can be included in the optimization process in two ways: (1) f n D max(ÞU fn þ D); f n 2 = D f f n g (2) f n D max U ; f n 2 = D f f n g fn (2.71) subject to D Ä Dmax An alternative approach would be to define the utility function in state n as Un D Sn UDS (2.72) f n D max U ; f n 2 = D f f n g fn where Sn and S are given by Equations (2.68) and (2.69) with i ! n. 2.8.7 Collision resolution 2.8.7.1 Joint master/slave base MAC protocol The network controller allocates the status M (master) or S (slave) to the bases. After the first transmission the neighboring cells find out the channels with collision, say k out of N . In the next transmission M basis use all channels and S basis only N k with no collision. The cognition is used to allocate M and S status, which may alternate depending on traffic. The average intercell throughput is S D (2N k)=(2N ) D 1 k=(2N ) (2.73) 2.8.7.2 Joint master/slave mobile MAC protocol The terminals are classified as M (master with lower processing capabilities, e.g. a phone) or S (slave with higher processing capabilities, e.g. a PC). After the first transmission the neighboring cells find out the channels with collision, say k out of N . In the next transmission M terminals use the channels with no collisions and S terminals use the channels with collision and have to engage sp (signal processing)–separability (interference cancellation). The throughput now is 1. 2.8.8 Opportunistic transmission with intercell interference awareness The concept discussed so far will achieve high probability of successful transmission per user when transmission occurs. Unfortunately the overall throughput (channel allocation efficiency per user) will be relatively low because of a significant percentage of time the user might have to wait to move into the position where the opportunity will arise to transmit with such high probability of success. From the network point of view (social optimum) it would be of interest to constantly use the resource so that a channel is always allocated to the user who has an opportunity to tra