ofdm and mc-cdma...comms-s liaison to ieee press, mostafa hashem sherif british library cataloguing...

OFDM and MC-CDMA

OFDM and MC-CDMA

A Primer

L. HanzoUniversity of Southampton, UK

T. KellerAnalog Devices Ltd., Cambridge, UK

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John Wiley & Sons, Ltd

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IEEE Communications Society, SponsorCOMMS-S Liaison to IEEE Press, Mostafa Hashem Sherif

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We dedicate this monograph to the numerous contributors ofthis field, many of whom are listed in the Author Index

Contents

About the Authors xv

Other Wiley and IEEE Press Books on Related Topics xvi

Acknowledgements xix

1 Introduction 11.1 Motivation of the Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Orthogonal Frequency Division Multiplexing History . . . . . . . . . . . . . 5

1.2.1 Early Classic Contributions and OFDM Standards . . . . . . . . . . 51.2.2 Peak-to-Mean Power Ratio . . . . . . . . . . . . . . . . . . . . . . . 61.2.3 Synchronisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.2.4 OFDM/CDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.2.5 Decision-Directed Channel Estimation . . . . . . . . . . . . . . . . . 81.2.6 Uplink Detection Techniques for Multi-User SDMA-OFDM . . . . . 131.2.7 OFDM Applications . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.3 Outline of the Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161.4 Chapter Summary and Conclusion . . . . . . . . . . . . . . . . . . . . . . . 18

I OFDM System Design 19

2 Introduction to Orthogonal Frequency Division Multiplexing 212.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212.2 Principles of QAM-OFDM . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.3 Modulation by DFT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252.4 Transmission via Bandlimited Channels . . . . . . . . . . . . . . . . . . . . 292.5 Basic OFDM Modem Implementations . . . . . . . . . . . . . . . . . . . . . 322.6 Cyclic OFDM Symbol Extension . . . . . . . . . . . . . . . . . . . . . . . . 352.7 Decision-Directed Adaptive Channel Equalisation . . . . . . . . . . . . . . . 362.8 OFDM Bandwidth Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . 382.9 Chapter Summary and Conclusion . . . . . . . . . . . . . . . . . . . . . . . 39

vii

viii CONTENTS

3 OFDM Transmission over Gaussian Channels 413.1 Orthogonal Frequency Division Multiplexing . . . . . . . . . . . . . . . . . 423.2 Choice of the OFDM Modulation . . . . . . . . . . . . . . . . . . . . . . . . 423.3 OFDM System Performance over AWGN Channels . . . . . . . . . . . . . . 423.4 Clipping Amplification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3.4.1 OFDM Signal Amplitude Statistics . . . . . . . . . . . . . . . . . . 433.4.2 Clipping Amplifier Simulations . . . . . . . . . . . . . . . . . . . . 44

3.4.2.1 Introduction to Peak-Power Reduction Techniques . . . . . 453.4.2.2 BER Performance Using Clipping Amplifiers . . . . . . . 463.4.2.3 Signal Spectrum with Clipping Amplifier . . . . . . . . . . 47

3.4.3 Clipping Amplification – Summary . . . . . . . . . . . . . . . . . . 503.5 Analogue-to-Digital Conversion . . . . . . . . . . . . . . . . . . . . . . . . 503.6 Phase Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

3.6.1 Effects of Phase Noise . . . . . . . . . . . . . . . . . . . . . . . . . 533.6.2 Phase Noise Simulations . . . . . . . . . . . . . . . . . . . . . . . . 54

3.6.2.1 White Phase Noise Model . . . . . . . . . . . . . . . . . . 543.6.2.1.1 Serial Modem . . . . . . . . . . . . . . . . . . . 553.6.2.1.2 OFDM Modem . . . . . . . . . . . . . . . . . . 55

3.6.2.2 Coloured Phase Noise Model . . . . . . . . . . . . . . . . 573.6.3 Phase Noise – Summary . . . . . . . . . . . . . . . . . . . . . . . . 60

3.7 Chapter Summary and Conclusion . . . . . . . . . . . . . . . . . . . . . . . 60

4 OFDM Transmission over Wideband Channels 614.1 The Channel Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

4.1.1 The Wireless Asynchronous Transfer Mode System . . . . . . . . . . 624.1.1.1 The WATM Channel . . . . . . . . . . . . . . . . . . . . . 624.1.1.2 The Shortened WATM Channel . . . . . . . . . . . . . . . 64

4.1.2 The Wireless Local Area Network . . . . . . . . . . . . . . . . . . . 644.1.2.1 The WLAN Channel . . . . . . . . . . . . . . . . . . . . . 65

4.1.3 UMTS System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 654.1.3.1 The UMTS Type Channel . . . . . . . . . . . . . . . . . . 65

4.2 Effects of Time-Dispersive Channels on OFDM . . . . . . . . . . . . . . . . 664.2.1 Effects of the Stationary Time-Dispersive Channel . . . . . . . . . . 674.2.2 Non-Stationary Channel . . . . . . . . . . . . . . . . . . . . . . . . 68

4.2.2.1 Summary of Time-Variant Channels . . . . . . . . . . . . 704.2.3 Signalling over Time-Dispersive OFDM Channels . . . . . . . . . . 70

4.3 Channel Transfer Function Estimation . . . . . . . . . . . . . . . . . . . . . 704.3.1 Frequency Domain Channel Transfer Function Estimation . . . . . . 70

4.3.1.1 Pilot Symbol-Assisted Schemes . . . . . . . . . . . . . . . 714.3.1.1.1 Linear Interpolation for PSAM . . . . . . . . . . 714.3.1.1.2 Ideal Lowpass Interpolation for PSAM . . . . . . 734.3.1.1.3 Summary . . . . . . . . . . . . . . . . . . . . . 75

4.3.2 Time Domain Channel Estimation . . . . . . . . . . . . . . . . . . . 784.4 System Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

4.4.1 Static Time-Dispersive Channel . . . . . . . . . . . . . . . . . . . . 784.4.1.1 Perfect Channel Estimation . . . . . . . . . . . . . . . . . 78

CONTENTS ix

4.4.1.2 Differentially Coded Modulation . . . . . . . . . . . . . . 814.4.1.3 PSAM Aided Channel Transfer Function Estimation . . . . 83

4.4.2 Slowly Varying Time-Dispersive Channel . . . . . . . . . . . . . . . 884.4.2.1 Perfect Channel Estimation . . . . . . . . . . . . . . . . . 894.4.2.2 Pilot Symbol-Assisted Modulation Summary . . . . . . . . 90

4.5 Intersubcarrier Interference Cancellation . . . . . . . . . . . . . . . . . . . . 904.5.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 904.5.2 The Signal Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 944.5.3 Channel Estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . 964.5.4 Cancellation Schemes . . . . . . . . . . . . . . . . . . . . . . . . . 974.5.5 ICI Cancellation Performance . . . . . . . . . . . . . . . . . . . . . 994.5.6 Conclusions on ICI Cancellation . . . . . . . . . . . . . . . . . . . . 100


5 OFDM Time and Frequency Domain Synchronisation 1035.1 System Performance with Frequency and Timing Errors . . . . . . . . . . . . 103

5.1.1 Frequency Shift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1035.1.1.1 The Spectrum of the OFDM Signal . . . . . . . . . . . . . 1045.1.1.2 Effects of Frequency Mismatch on Different Modulation

Schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . 1085.1.1.2.1 Coherent Modulation . . . . . . . . . . . . . . . 1085.1.1.2.2 Pilot Symbol Assisted Modulation . . . . . . . . 1085.1.1.2.3 Differential Modulation . . . . . . . . . . . . . . 1095.1.1.2.4 Frequency Error - Summary . . . . . . . . . . . 109

5.1.2 Time Domain Synchronisation Errors . . . . . . . . . . . . . . . . . 1105.1.2.1 Coherent Demodulation . . . . . . . . . . . . . . . . . . . 1105.1.2.2 Pilot Symbol-Assisted Modulation . . . . . . . . . . . . . 1115.1.2.3 Differential Modulation . . . . . . . . . . . . . . . . . . . 112

5.1.2.3.1 Time Domain Synchronisation Errors - Summary 1145.2 Synchronisation Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . 114

5.2.1 Coarse Frame and OFDM Symbol Synchronisation Review . . . . . 1155.2.2 Fine Symbol Tracking Review . . . . . . . . . . . . . . . . . . . . . 1165.2.3 Frequency Acquisition Review . . . . . . . . . . . . . . . . . . . . . 1165.2.4 Frequency Tracking Review . . . . . . . . . . . . . . . . . . . . . . 1165.2.5 Synchronisation based on Auto-Correlation . . . . . . . . . . . . . . 1175.2.6 Multiple Access Frame Structure . . . . . . . . . . . . . . . . . . . . 117

5.2.6.1 The Reference Symbol . . . . . . . . . . . . . . . . . . . 1175.2.6.2 The Correlation Functions . . . . . . . . . . . . . . . . . . 119

5.2.7 Frequency Tracking and OFDM Symbol Synchronisation . . . . . . . 1205.2.7.1 OFDM Symbol Synchronisation . . . . . . . . . . . . . . 1205.2.7.2 Frequency Tracking Studies . . . . . . . . . . . . . . . . . 120

5.2.8 Frequency Acquisition and Frame Synchronisation Studies . . . . . . 1225.2.8.1 Frame Synchronisation Studies . . . . . . . . . . . . . . . 1225.2.8.2 Frequency Acquisition Studies . . . . . . . . . . . . . . . 1225.2.8.3 Block Diagram of the Synchronisation Algorithms . . . . . 122

5.2.9 Frequency Acquisition Using Pilots . . . . . . . . . . . . . . . . . . 123

x CONTENTS

5.2.9.1 The Reference Symbol . . . . . . . . . . . . . . . . . . . 1245.2.9.2 Frequency Acquisition . . . . . . . . . . . . . . . . . . . . 1245.2.9.3 Performance of the Pilot-Based Frequency Acquisition in

AWGN Channels . . . . . . . . . . . . . . . . . . . . . . 1265.2.9.4 Alternative Frequency Error Estimation for Frequency Do-

main Pilot Tones . . . . . . . . . . . . . . . . . . . . . . . 1315.3 Comparison of the Frequency Acquisition Algorithms . . . . . . . . . . . . . 1335.4 BER Performance with Frequency Synchronisation . . . . . . . . . . . . . . 1375.5 Chapter Summary and Conclusion . . . . . . . . . . . . . . . . . . . . . . . 1385.6 Appendix: OFDM Synchronisation Performance . . . . . . . . . . . . . . . 139

5.6.1 Frequency Synchronisation in an AWGN Channel . . . . . . . . . . 1395.6.1.1 One Phasor in AWGN Environment . . . . . . . . . . . . . 139

5.6.1.1.1 Cartesian Coordinates . . . . . . . . . . . . . . . 1395.6.1.1.2 Polar Coordinates . . . . . . . . . . . . . . . . . 139

5.6.1.2 Product of Two Noisy Phasors . . . . . . . . . . . . . . . 1405.6.1.2.1 Joint Probability Density . . . . . . . . . . . . . 1405.6.1.2.2 Phase Distribution . . . . . . . . . . . . . . . . . 1415.6.1.2.3 Numerical Integration . . . . . . . . . . . . . . . 141

6 Adaptive Single- and Multi-user OFDM Techniques 1456.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

6.1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1456.1.2 Adaptive Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . 146

6.1.2.1 Channel Quality Estimation . . . . . . . . . . . . . . . . . 1476.1.2.2 Parameter Adaptation . . . . . . . . . . . . . . . . . . . . 1486.1.2.3 Signalling the AOFDM Parameters . . . . . . . . . . . . . 148

6.1.3 System Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1506.2 Adaptive Modulation for OFDM . . . . . . . . . . . . . . . . . . . . . . . . 150

6.2.1 System Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1506.2.2 Channel Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1516.2.3 Channel Transfer Function Variations . . . . . . . . . . . . . . . . . 1516.2.4 Choice of the Modulation Modes . . . . . . . . . . . . . . . . . . . 152

6.2.4.1 Fixed Threshold Adaptation Algorithm . . . . . . . . . . . 1526.2.4.2 Sub-Band BER Estimator Adaptation Algorithm . . . . . . 155

6.2.5 Constant Throughput Adaptive OFDM . . . . . . . . . . . . . . . . 1566.2.6 AOFDM Mode Signalling and Blind Detection . . . . . . . . . . . . 158

6.2.6.1 Signalling . . . . . . . . . . . . . . . . . . . . . . . . . . 1586.2.6.2 Blind Detection by SNR Estimation . . . . . . . . . . . . . 1596.2.6.3 Blind Detection by Multi-Mode Trellis Decoder . . . . . . 161

6.2.7 Sub-Band Adaptive OFDM and Turbo Channel Coding . . . . . . . . 1646.2.8 Effects of the Doppler Frequency . . . . . . . . . . . . . . . . . . . 1646.2.9 Channel Transfer Function Estimation . . . . . . . . . . . . . . . . . 167

6.3 Adaptive OFDM Speech System . . . . . . . . . . . . . . . . . . . . . . . . 1686.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1686.3.2 System Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

6.3.2.1 System Parameters . . . . . . . . . . . . . . . . . . . . . . 169

CONTENTS xi

6.3.3 Constant Throughput Adaptive Modulation . . . . . . . . . . . . . . 1706.3.3.1 Constant-Rate BER Performance . . . . . . . . . . . . . . 171

6.3.4 Multimode Adaptation . . . . . . . . . . . . . . . . . . . . . . . . . 1736.3.4.1 Mode Switching . . . . . . . . . . . . . . . . . . . . . . . 173

6.3.5 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 1746.3.5.1 Frame Error Results . . . . . . . . . . . . . . . . . . . . . 1746.3.5.2 Audio Segmental SNR . . . . . . . . . . . . . . . . . . . 176

6.4 Pre-equalisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1766.4.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1766.4.2 Pre-equalisation with Sub-Band Blocking . . . . . . . . . . . . . . . 1796.4.3 Adaptive Modulation with Spectral Predistortion . . . . . . . . . . . 181

6.5 Comparison of the Adaptive Techniques . . . . . . . . . . . . . . . . . . . . 1846.6 Near-Optimum Power- and Bit Allocation in OFDM . . . . . . . . . . . . . 186

6.6.1 State of the Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1866.6.2 Problem Description . . . . . . . . . . . . . . . . . . . . . . . . . . 1866.6.3 Power and Bit Allocation Algorithm . . . . . . . . . . . . . . . . . . 187

6.7 Multi-User AOFDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1916.7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1916.7.2 Adaptive Transceiver Architecture . . . . . . . . . . . . . . . . . . . 192

6.7.2.1 An Overview . . . . . . . . . . . . . . . . . . . . . . . . . 1926.7.2.2 The Signal Model . . . . . . . . . . . . . . . . . . . . . . 1936.7.2.3 The SMI Algorithm . . . . . . . . . . . . . . . . . . . . . 1936.7.2.4 The Adaptive Bit-Assignment Algorithm . . . . . . . . . . 1946.7.2.5 The Channel Models . . . . . . . . . . . . . . . . . . . . . 194

6.7.3 Simulation Results - Perfect Channel Knowledge . . . . . . . . . . . 1956.7.3.1 General Remarks . . . . . . . . . . . . . . . . . . . . . . 1956.7.3.2 Two-Branch Maximum-Ratio Combining . . . . . . . . . . 1956.7.3.3 SMI Co-Channel Interference Suppression . . . . . . . . . 195

6.7.4 Pilot-Based Channel Parameter Estimation . . . . . . . . . . . . . . 1986.7.4.1 System Description . . . . . . . . . . . . . . . . . . . . . 1986.7.4.2 Simulation Results . . . . . . . . . . . . . . . . . . . . . . 200


II OFDM versus MC-CDMA Systems 203

7 OFDM versus MC-CDMA 2057.1 Amalgamating DS-CDMA and OFDM . . . . . . . . . . . . . . . . . . . . . 205

7.1.1 The DS-CDMA Component . . . . . . . . . . . . . . . . . . . . . . 2057.1.2 The OFDM Component . . . . . . . . . . . . . . . . . . . . . . . . 208

7.2 Multi-Carrier CDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2117.2.1 MC-CDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2117.2.2 MC-DS-CDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2147.2.3 MT-CDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215

7.3 Further Research Topics in MC-CDMA . . . . . . . . . . . . . . . . . . . . 2167.4 Chapter Summary and Conclusion . . . . . . . . . . . . . . . . . . . . . . . 217

xii CONTENTS

8 Basic Spreading Sequences 2198.1 PN Sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219

8.1.1 Maximal Length Sequences . . . . . . . . . . . . . . . . . . . . . . 2198.1.2 Gold Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2218.1.3 Kasami Sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . 222

8.2 Orthogonal Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2238.2.1 Walsh Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2238.2.2 Orthogonal Gold Codes . . . . . . . . . . . . . . . . . . . . . . . . 2248.2.3 Multi-Rate Orthogonal Gold Codes . . . . . . . . . . . . . . . . . . 226


9 MC-CDMA Performance in Synchronous Environments 2319.1 The Frequency Selective Channel Model . . . . . . . . . . . . . . . . . . . . 2329.2 The System Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2339.3 Single User Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235

9.3.1 Maximal Ratio Combining . . . . . . . . . . . . . . . . . . . . . . . 2369.3.2 Equal Gain Combining . . . . . . . . . . . . . . . . . . . . . . . . . 2399.3.3 Orthogonality Restoring Combining . . . . . . . . . . . . . . . . . . 241

9.4 Multi-User Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2429.4.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2429.4.2 Maximum Likelihood Detection . . . . . . . . . . . . . . . . . . . . 2439.4.3 Concatenated Space-Time Block Coded and Turbo Coded Symbol-

by-Symbol Adaptive OFDM and Multi-Carrier CDMA . . . . . . . . 2449.5 Chapter Summary and Conclusion . . . . . . . . . . . . . . . . . . . . . . . 250

III Advanced Topics: Multi-User OFDM Systems 251

10 Maximum-Likelihood Enhanced Sphere Decoding of MIMO-OFDM 25310.1 Classification of Smart Antennas . . . . . . . . . . . . . . . . . . . . . . . . 25310.2 Introduction to Space-Time Processing . . . . . . . . . . . . . . . . . . . . . 25510.3 SDM-OFDM System Model . . . . . . . . . . . . . . . . . . . . . . . . . . 259

10.3.1 MIMO Channel Model . . . . . . . . . . . . . . . . . . . . . . . . . 25910.3.2 SDM-OFDM Transceiver Structure . . . . . . . . . . . . . . . . . . 260

10.4 Optimised Hierarchy Reduced Search Algorithm-Aided SDM Detection . . . 26210.4.1 OHRSA-Aided ML SDM Detection . . . . . . . . . . . . . . . . . . 26310.4.2 Search Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267

10.4.2.1 Generalisation of the OHRSA-ML SDM Detector . . . . . 26910.4.3 Bitwise OHRSA ML SDM Detection . . . . . . . . . . . . . . . . . 272

10.4.3.1 Generalisation of the BW-OHRSA-ML SDM Detector . . . 27710.4.4 OHRSA-Aided Log-MAP SDM Detection . . . . . . . . . . . . . . 27910.4.5 Soft-Output OHRSA-Aided Approximate Log-MAP Detection . . . . 290

10.4.5.1 Complexity Analysis. . . . . . . . . . . . . . . . . . . . . 29310.4.5.2 Performance Analysis . . . . . . . . . . . . . . . . . . . . 297


CONTENTS xiii

11 Genetic Algorithm Aided Joint Channel Estimation and MUDfor SDMA OFDM 30311.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30311.2 SDMA MIMO Channel Model . . . . . . . . . . . . . . . . . . . . . . . . . 30511.3 System Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30611.4 GA-Aided Iterative Joint Channel Estimation and Multi-User Detection . . . 306

11.4.1 Pilot-Aided Initial Channel Estimation . . . . . . . . . . . . . . . . . 30911.4.2 Generating Initial Symbol Estimates . . . . . . . . . . . . . . . . . . 31111.4.3 GA-Aided Joint FD-CHTF and Data Optimisation Providing Soft

Outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31311.4.3.1 Extended GA Individual Structure for MIMO Systems . . . 31311.4.3.2 Initialisation . . . . . . . . . . . . . . . . . . . . . . . . . 31411.4.3.3 Joint Genetic Optimisation . . . . . . . . . . . . . . . . . 315

11.4.3.3.1 Cross-Over Operator . . . . . . . . . . . . . . . 31511.4.3.3.2 Mutation Operator . . . . . . . . . . . . . . . . . 31611.4.3.3.3 Comments on the Joint Optimisation Process . . 317

11.4.3.4 Generating the GA’s Soft Outputs . . . . . . . . . . . . . . 31711.5 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319

11.5.1 Effects of the Maximum Mutation Step Size . . . . . . . . . . . . . . 32011.5.2 Effects of the Doppler Frequency . . . . . . . . . . . . . . . . . . . 32311.5.3 Effects of the Number of GA-JCEMUD Iterations . . . . . . . . . . 32411.5.4 Effects of the Pilot Overhead . . . . . . . . . . . . . . . . . . . . . . 32511.5.5 Joint Optimisation versus Separate Optimisation . . . . . . . . . . . 32511.5.6 Comparison of GA-JCEMUDs Having Soft and Hard Outputs . . . . 32711.5.7 MIMO Robustness . . . . . . . . . . . . . . . . . . . . . . . . . . . 327


12 Multi-User OFDM Employing Genetic Algorithm Aided Minimum Bit ErrorRate Multi-User Detection 33112.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331

12.1.1 Minimum Bit Error Ratio Detection of OFDM . . . . . . . . . . . . 33212.2 System Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332

12.2.1 Space Division Multiple Access . . . . . . . . . . . . . . . . . . . . 33212.2.2 Error Probability of a BPSK System . . . . . . . . . . . . . . . . . . 33512.2.3 Exact MBER Multi-User Detection . . . . . . . . . . . . . . . . . . 336

12.3 Genetic Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33812.3.1 Overview of GAs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33912.3.2 Employing GAs in the MBER MUD Aided SDMA OFDM System . 341

12.4 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34212.4.1 Performance of a Four-User and Four-Receiver Antenna Scenario . . 34212.4.2 Performance of the Four-Antenna Scenario versus the Number

of Users . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34312.5 Complexity Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34612.6 Chapter Summary and Conclusion . . . . . . . . . . . . . . . . . . . . . . . 346

xiv CONTENTS

13 Conclusion and Further Research Problems 35113.1 Summary and Conclusions of Part I . . . . . . . . . . . . . . . . . . . . . . 351

13.1.1 Summary of Part I . . . . . . . . . . . . . . . . . . . . . . . . . . . 35113.1.2 Conclusions of Part I . . . . . . . . . . . . . . . . . . . . . . . . . . 352

13.2 Summary and Conclusions of Part II . . . . . . . . . . . . . . . . . . . . . . 35313.2.1 Summary of Part II . . . . . . . . . . . . . . . . . . . . . . . . . . . 35313.2.2 Conclusions of Part II . . . . . . . . . . . . . . . . . . . . . . . . . 353

13.3 Summary and Conclusions of Part III . . . . . . . . . . . . . . . . . . . . . 35413.3.1 Near-ML Enhanced Sphere Detection of MIMO-OFDM . . . . . . . 35413.3.2 GA-Aided Joint MUD and Channel Estimation . . . . . . . . . . . . 35513.3.3 GA-Aided MBER MUD . . . . . . . . . . . . . . . . . . . . . . . . 355

13.4 Closing Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356

Glossary 359

Bibliography 363

Subject Index 395

Author Index 401

About the AuthorsLajos Hanzo, Fellow of the Royal Academy of Engineering, receivedhis first-class degree in electronics in 1976 and his doctorate in 1983. In2004 he was awarded the Doctor of Sciences (DSc) degree by the Uni-versity of Southampton, UK. During his career in telecommunicationshe has held various research and academic posts in Hungary, Germanyand the UK. Since 1986 he has been with the Department of Electronicsand Computer Science, University of Southampton, UK, where he holdsthe chair in telecommunications. He has co-authored 12 books, totalling9000 pages on mobile radio communications, published in excess of 600

research papers, has acted as TPC Chair of numerous major IEE and IEEE conferences, pre-sented various keynote lectures and has been awarded a number of distinctions. Currentlyhe heads an academic research team, working on a range of research projects in the fieldof wireless multimedia communications sponsored by industry, the Engineering and Physi-cal Sciences Research Council (EPSRC) UK, the European IST Programme and the MobileVirtual Centre of Excellence (VCE), UK. He is an enthusiastic supporter of industrial andacademic liaison and he offers a range of industrial courses. Lajos is also an IEEE Distin-guished Lecturer of both the Communications as well as the Vehicular Technology Society,a Fellow of both the IEEE and the IEE. He is an editorial board member of the Proceedingsof the IEEE and a Governor of the IEEE VT Society. For further information on research inprogress and associated publications, please refer to http://www-mobile.ecs.soton.ac.uk

Thomas Keller studied Electrical Engineering at the University ofKarlsruhe, Ecole Superieure d’Ingenieurs en Electronique et Elec-trotechnique, Paris, and the University of Southampton. He graduatedwith a Dipl.-Ing. degree in 1995. Between 1995 and 1999 he was withthe Wireless Multimedia Communications Group at the University ofSouthampton, where he completed his PhD in mobile communications.His areas of interest include adaptive OFDM transmission, widebandchannel estimation, CDMA and error correction coding. Following thecompletion of his PhD he joined Ubinetics, Cambridge, UK, where he

was involved in the research and development of third-generation wireless systems. Follow-ing a reorganization, he was part of a team that was transferred to Analog Devices, Cam-bridge, UK. Dr. Keller has co-authored three monographs and about 30 various researchpapers.

xv

Other Wiley and IEEE PressBooks on Related Topics1

• R. Steele, L. Hanzo (Ed): Mobile Radio Communications: Second and Third Gener-ation Cellular and WATM Systems, John Wiley and IEEE Press, 2nd edition, 1999,ISBN 07 273-1406-8, 1064 pages

• L. Hanzo, F.C.A. Somerville, J.P. Woodard: Voice Compression and Communications:Principles and Applications for Fixed and Wireless Channels; IEEE Press and JohnWiley, 2001, 642 pages

• L. Hanzo, P. Cherriman, J. Streit: Wireless Video Communications: Second to ThirdGeneration and Beyond, IEEE Press and John Wiley, 2001, 1093 pages

• L. Hanzo, T.H. Liew, B.L. Yeap: Turbo Coding, Turbo Equalisation and Space-TimeCoding, John Wiley and IEEE Press, 2002, 751 pages

• J.S. Blogh, L. Hanzo: Third-Generation Systems and Intelligent Wireless Networking:Smart Antennas and Adaptive Modulation, John Wiley and IEEE Press, 2002, 408pages

• L. Hanzo, C.H. Wong, M.S. Yee: Adaptive Wireless Transceivers: Turbo-Coded,Turbo-Equalised and Space-Time Coded TDMA, CDMA and OFDM Systems, JohnWiley and IEEE Press, 2002, 737 pages

• L. Hanzo, L-L. Yang, E-L. Kuan, K. Yen: Single- and Multi-Carrier CDMA: Multi-User Detection, Space-Time Spreading, Synchronisation, Networking and Standards,John Wiley and IEEE Press, June 2003, 1060 pages

• L. Hanzo, M. Munster, T. Keller, B-J. Choi, OFDM and MC-CDMA for BroadbandMulti-User Communications, WLANs and Broadcasting, John Wiley and IEEE Press,2003, 978 pages

• L. Hanzo, S-X. Ng, T. Keller and W.T. Webb, Quadrature Amplitude Modulation:From Basics to Adaptive Trellis-Coded, Turbo-Equalised and Space-Time CodedOFDM, CDMA and MC-CDMA Systems, John Wiley and IEEE Press, 2004, 1105pages

1For detailed contents and sample chapters please refer to http://www-mobile.ecs.soton.ac.uk

xvii

AcknowledgementsWe are indebted to our many colleagues who have enhanced our understanding of the subject,in particular to Prof. Emeritus Raymond Steele. These colleagues and valued friends, too nu-merous to be mentioned, have influenced our views concerning various aspects of wirelessmultimedia communications. We thank them for the enlightenment gained from our collab-orations on various projects, papers and books. We are grateful to Steve Braithwaite, JanBrecht, Jon Blogh, Marco Breiling, Marco del Buono, Sheng Chen, Peter Cherriman, Stan-ley Chia, Joseph Cheung, Sheyam Lal Dhomeja, Dirk Didascalou, Lim Dongmin, StephanErnst, Peter Fortune, Eddie Green, David Greenwood, Hee Thong How, Ee Lin Kuan, W. H.Lam, C. C. Lee, Xiao Lin, Chee Siong Lee, Tong-Hooi Liew, Vincent Roger-Marchart, JasonNg, Michael Ng, M. A. Nofal, Jeff Reeve, Redwan Salami, Clare Somerville, Rob Stedman,David Stewart, Jurgen Streit, Jeff Torrance, Spyros Vlahoyiannatos, William Webb, StephanWeiss, John Williams, Jason Woodard, Choong Hin Wong, Henry Wong, James Wong, Lie-Liang Yang, Bee-Leong Yeap, Mong-Suan Yee, Kai Yen, Andy Yuen, and many others withwhom we enjoyed an association.

We also acknowledge our valuable associations with the Virtual Centre of Excellence(VCE) in Mobile Communications, in particular with its chief executive, Dr Walter Tuttlebee,and other leading members of the VCE, namely Dr Keith Baughan, Prof. Hamid Aghvami,Prof. Ed Candy, Prof. John Dunlop, Prof. Barry Evans, Prof. Peter Grant, Prof. MikeWalker, Prof. Joseph McGeehan, Prof. Steve McLaughlin and many other valued colleagues.Our sincere thanks are also due to the EPSRC, UK for supporting our research. We wouldalso like to thank Dr Joao Da Silva, Dr Jorge Pereira, Dr Bartholome Arroyo, Dr BernardBarani, Dr Demosthenes Ikonomou, Dr Fabrizio Sestini and other valued colleagues from theCommission of the European Communities, Brussels, Belgium.

Without the kind support of Mark Hammond, Sarah Hinton, Jennifer Beal and their col-leagues at the Wiley office in Chichester, UK this monograph would never have reached thereaders. Finally, our sincere gratitude is due to the numerous authors listed in the AuthorIndex — as well as to those whose work was not cited owing to space limitations — for theircontributions to the state of the art, without whom this book would not have materialised.

Lajos Hanzo and Thomas KellerSchool of Electronics and Computer Science

University of Southampton, UK

xix

Chapter 1Introduction

1.1 Motivation of the Book

Whilst the concept of Orthogonal Frequency Division Multiplexing (OFDM) has been knownsince 1966 [1], it only reached sufficient maturity for employment in standard systems duringthe 1990s. OFDM exhibits numerous advantages over the family of more conventional serialmodem schemes [2], although it is only natural that it also imposes a number of disadvan-tages. The discussion of the associated design tradeoffs of OFDM and Multi-Carrier CodeDivision Multiple Access (MC-CDMA) systems constitutes the topic of this monograph andin this context our discussions include the following fundamental issues:

1) A particularly attractive feature of OFDM systems is that they are capable of operatingwithout a classic channel equaliser, when communicating over dispersive transmissionmedia, such as wireless channels, while conveniently accommodating the time- andfrequency-domain channel quality fluctuations of the wireless channel.

Explicitly, the channel SNR variation versus both time and frequency of an indoorwireless channel is shown in a three-dimensional form in Figure 1.1 versus both timeand frequency, which suggests that OFDM constitutes a convenient framework for ac-commodating the channel quality fluctuations of the wireless channel, as will be brieflyaugmented below. This channel transfer function was recorded for the channel impulseresponse of Figure 1.2, by simply transforming the impulse response to the frequencydomain at regular time intervals, while its taps fluctuated according to the Rayleighdistribution.

These channel quality fluctuations may be readily accommodated with the aid of sub-band-adaptive modulation as follows. Such an adaptive OFDM (AOFDM) modem ischaracterised by Figure 1.3, portraying at the top a contour plot of the above-mentionedwireless channel’s signal-to-noise ratio (SNR) fluctuation versus both time and fre-quency for each OFDM subcarrier. We note at this early stage that these channel qualityfluctuations may be mitigated with the aid of frequency-domain channel equalisation,

OFDM and MC-CDMA: A Primer L. Hanzo and T. Kellerc© 2006 John Wiley & Sons, Ltd

2 CHAPTER 1. INTRODUCTION

Figure 1.1: Instantaneous channel SNR for all 512 subcarriers versus time, for an average channelSNR of 16 dB over the channel characterised by the channel impulse response (CIR) ofFigure 1.2.

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1.1. MOTIVATION OF THE BOOK 3

as will be detailed throughout the book, but nonetheless, they cannot be entirely eradi-cated.

More specifically, as can be seen in Figure 1.1, that when the channel is of high quality— as for example in the vicinity of the OFDM symbol index of 1080 — the sub-band-adaptive modem considered here for the sake of illustration has used the samemodulation mode, as the identical-throughput conventional fixed-rate OFDM modemin all subcarriers, which was 1 bit per symbol (BPS) in this example, as in conventionalBinary Phase Shift Keying (BPSK). By contrast, when the channel is hostile — forexample, around frame 1060 — the sub-band-adaptive modem transmitted zero bitsper symbol in some sub-bands, corresponding to disabling transmissions in the low-quality sub-bands. In order to compensate for the loss of throughput in this sub-band,a higher-order modulation mode was used in the higher quality sub-bands.

In the centre and bottom subfigures of Figure 1.3 the modulation mode chosen foreach 32-subcarrier sub-band is shown versus time for two different high-speed wirelessmodems communicating at either 3.4 or 7.0 Mbps, respectively, again, correspondingto an average throughput of either 1 or 2 BPS.

However, these adaptive transceiver principles are not limited to OFDM transmissions.In recent years the concept of intelligent multi-mode, multimedia transceivers (IMMT)has emerged in the context of a variety of wireless systems [2–7]. The range of variousexisting solutions that have found favour in already operational standard systems hasbeen summarised in the excellent overview by Nanda et al. [5]. The aim of theseadaptive transceivers is to provide mobile users with the best possible compromiseamongst a number of contradicting design factors, such as the power consumption ofthe hand-held portable station (PS), robustness against transmission errors, spectralefficiency, teletraffic capacity, audio/video quality and so forth [4].

2) Another design alternative applicable in the context of OFDM systems is that the chan-nel quality fluctuations observed, for example, in Figure 1.1 are averaged out with theaid of frequency-domain spreading codes, which leads to the concept of Multi-CarrierCode Division Multiple Access (MC-CDMA). In this scenario typically only a fewchips of the spreading code are obliterated by the frequency-selective fading and hencethe chances are that the spreading code and its conveyed data may still be recoverable.The advantage of this approach is that in contrast to AOFDM-based communications,in MC-CDMA no channel quality estimation and signalling are required. ThereforeOFDM and MC-CDMA will be comparatively studied in Part II of this monograph.Part III will also consider the employment of Walsh-Hadamard code-based spreadingof each subcarrier’s signal across the entire OFDM bandwidth, which was found tobe an efficient frequency-domain fading counter-measure capable of operating withoutthe employment of adaptive modulation.

3) A further technique capable of mitigating the channel quality fluctuations of wirelesschannels is constituted by space-time coding, which will also be considered as an at-tractive anti-fading design option capable of attaining a high diversity gain. Space-timecoding employs several transmit and receive antennas for the sake of achieving diver-sity gain and hence an improved performance.


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Figure 1.3: The micro-adaptive nature of the sub-band-adaptive OFDM modem. The top graph is acontour plot of the channel SNR for all 512 subcarriers versus time. The bottom two graphsshow the modulation modes chosen for all 16 32-subcarrier sub-bands for the same periodof time. The middle graph shows the performance of the 3.4 Mbps sub-band-adaptivemodem, which operates at the same bit rate as a fixed BPSK modem. The bottom graphrepresents the 7.0 Mbps sub-band-adaptive modem, which operated at the same bit rate asa fixed QPSK modem. The average channel SNR was 16 dB.

1.2. ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING HISTORY 5

4) By contrast, in Part III of the book we employ multiple antennas at the base-stationfor a different reason, namely for the sake of supporting multiple users, rather than toachieving transmit diversity gain. This is possible, since the users’ channel impulseresponses (CIR) or channel transfer functions are accurately estimated and hence thesechannel transfer functions may be viewed as unique user signature sequences, whichallow us to recognise and demultiplex the transmissions of the individual users, in asimilar fashion to the unique user-specific spreading codes employed in CDMA sys-tems. We note, however, that this technique is only capable of reliably separating theusers communicating within the same bandwidth, if their CIRs are sufficiently differ-ent. This assumption is typically valid for the uplink, although it may have a limitedvalidity, when the base station receives from mobile stations in its immediate vicinity.By contrast, different techniques have to be invoked for downlink multi-user transmis-sions.

Our intention with the book is:

1) First, to pay tribute to all researchers, colleagues and valued friends, who contributedto the field. Hence this book is dedicated to them, since without their quest for bettertransmission solutions for wireless communications this monograph could not havebeen conceived. They are too numerous to name here, hence they appear in the authorindex of the book. Our hope is that the conception of this monograph on the topic willprovide an adequate portrayal of the community’s research and will further fuel thisinnovation process.

2) We expect to stimulate further research by exposing open research problems and bycollating a range of practical problems and design issues for the practitioners. Thecoherent further efforts of the wireless research community is expected to lead to thesolution of the range of outstanding problems, ultimately providing us with flexiblewireless transceivers exhibiting a performance close to information theoretical limits.

1.2 Orthogonal Frequency Division Multiplexing History

1.2.1 Early Classic Contributions and OFDM Standards

The first OFDM scheme was proposed by Chang in 1966 [1] for dispersive fading channels.During the early years of the evolution of OFDM research the contributions due to the effortsof Weinstein, Peled, Ruiz, Hirosaki, Kolb, Cimini, Schussler, Preuss, Ruckriem, Kalet etal. [1, 8–20] have to be mentioned. As unquestionable proof of its maturity, OFDM wasstandardised as the European digital audio broadcast (DAB) as well as digital video broadcast(DVB) scheme. It constituted also a credible proposal for the recent third-generation mobileradio standard competition in Europe. Finally, OFDM was recently selected as the highperformance local area network’s (HIPERLAN) transmission technique as well as becomingpart of the IEEE 802.11 Wireless Local Area Network (WLAN) standard.

The system’s operational principle is that the original bandwidth is divided into a highnumber of narrow sub-bands, in which the mobile channel can be considered non-dispersive.Hence no channel equaliser is required and instead of implementing a bank of sub-channel


modems they can be conveniently implemented with the aid of a single Fast Fourier Trans-former (FFT), as it will be outlined in Chapter 2.

These OFDM systems - often also termed frequency division multiplexing (FDM) ormulti-tone systems - have been employed in military applications since the 1960s, for exam-ple by Bello [21], Zimmermann [8], Powers and Zimmerman [22], Chang and Gibby [23] andothers. Saltzberg [24] studied a multi-carrier system employing orthogonal time–staggeredquadrature amplitude modulation (O-QAM) of the carriers.

The employment of the discrete Fourier transform (DFT) to replace the banks of si-nusoidal generators and the demodulators was suggested by Weinstein and Ebert [9] in1971, which significantly reduces the implementation complexity of OFDM modems. In1980, Hirosaki [20] suggested an equalisation algorithm in order to suppress both intersym-bol and intersubcarrier interference caused by the channel impulse response or timing andfrequency errors. Simplified OFDM modem implementations were studied by Peled [13]in 1980, while Hirosaki [14] introduced the DFT-based implementation of Saltzberg’s O-QAM OFDM system. From Erlangen University, Kolb [15], Schussler [16], Preuss [17] andRuckriem [18] conducted further research into the application of OFDM. Cimini [10] andKalet [19] published analytical and early seminal experimental results on the performance ofOFDM modems in mobile communications channels.

More recent advances in OFDM transmission were presented in the impressive state-of-the-art collection of works edited by Fazel and Fettweis [25], including the research byFettweis et al. at Dresden University, Rohling et al. at Braunschweig University, Vandendorpat Loeven University, Huber et al. at Erlangen University, Lindner et al. at Ulm University,Kammeyer et al. at Bremen University and Meyr et al. [26, 27] at Aachen University, butthe individual contributions are too numerous to mention. Important recent references are thebooks by van Nee and Prasad [28] as well as by Vandenameele, van der Perre and Engels [29].

As a summary of this section, we outline the milestones and the main contributions foundin the OFDM literature in Table 1.1, which culminated in the ratification of numerous OFDM-based standards in recent years.

While OFDM transmission over mobile communications channels can alleviate the prob-lem of multipath propagation, recent research efforts have focused on solving a set of inherentdifficulties regarding OFDM, namely the peak-to-mean power ratio, time and frequency syn-chronisation, and on mitigating the effects of the frequency selective fading channel. Theseissues are addressed below with reference to the literature, while a more in-depth treatmentis given throughout the book.

1.2.2 Peak-to-Mean Power Ratio

It is plausible that the OFDM signal - which is the superposition of a high number of mod-ulated sub-channel signals - may exhibit a high instantaneous signal peak with respect tothe average signal level. Furthermore, large signal amplitude swings are encountered, whenthe time domain signal traverses from a low instantaneous power waveform to a high powerwaveform, which may results in a high out-of-band (OOB) harmonic distortion power, unlessthe transmitter’s power amplifier exhibits an extremely high linearity across the entire signallevel range. This then potentially contaminates the adjacent channels with adjacent channelinterference. Practical amplifiers exhibit a finite amplitude range, in which they can be con-

1.2. ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING HISTORY 7

Year Milestone

1966 First OFDM scheme proposed by Chang [1] for dispersive fading channels.1967 Saltzberg [24] studied a multi-carrier system employing Orthogonal QAM (O-QAM) of the

carriers.1970 U.S. patent on OFDM issued [30].1971 Weinstein and Ebert [9] applied DFT to OFDM modems.1980 Hirosaki designed a subchannel-based equalizer for an orthogonally multiplexed QAM sys-

tem [20].Keasler et al. [31] described an OFDM modem for telephone networks.

1985 Cimini [10] investigated the feasibility of OFDM in mobile communications.1987 Alard and Lasalle [32] employed OFDM for digital broadcasting.1991 ANSI ADSL standard [33].1994 ANSI HDSL standard [34].1995 ETSI DAB standard [35]: the first OFDM-based standard for digital broadcasting systems.1996 ETSI WLAN standard [36].1997 ETSI DVB-T standard [37].1998 ANSI VDSL and ETSI VDSL standards [38, 39].

ETSI BRAN standard [40].1999 IEEE 802.11a WLAN standard [41].2002 IEEE 802.11g WLAN standard [42].2004 ETSI DVB-H standard [43].

IEEE 802.16 WMAN standard [44].Candidate for IEEE 802.11n standard for next generation WLAN [45].Candidate for IEEE 802.15.3a standard for WPAN (using MB-OFDM) [46].

2005 Candidate for 4G standards in China, Japan and South Korea (CJK) [47].

Table 1.1: Milestones in the history of OFDM.

sidered almost linear. In order to prevent severe clipping of the high OFDM signal peaks- which is the main source of OOB emissions - the power amplifier must not be driven tosaturation and hence they are typically operated with a certain so-called back-off, creating acertain ”head room” for the signal peaks, which reduces the risk of amplifier saturation andOOB emission. Two different families of solutions have been suggested in the literature, inorder to mitigate these problems, either reducing the peak-to-mean power ratio, or improvingthe amplification stage of the transmitter.

More explicitly, Shepherd [48], Jones [49], and Wulich [50] have suggested differentcoding techniques which aim to minimise the peak power of the OFDM signal by employingdifferent data encoding schemes before modulation, with the philosophy of choosing blockcodes whose legitimate code words exhibit low so-called crest factors or peak-to-mean powerenvelope fluctuation. Muller [51], Pauli [52], May [53] and Wulich [54] suggested differentalgorithms for post-processing the time domain OFDM signal prior to amplification, whileSchmidt and Kammeyer [55] employed adaptive subcarrier allocation in order to reduce thecrest factor. Dinis and Gusmao [56–58] researched the use of two-branch amplifiers, whilethe clustered OFDM technique introduced by Daneshrad, Cimini and Carloni [59] operateswith a set of parallel partial FFT processors with associated transmitting chains. OFDMsystems with increased robustness to non-linear distortion have been proposed by Okada,


Nishijima and Komaki [60] as well as by Dinis and Gusmao [61]. These aspects of OFDMtransmissions will be treated in substantial depth in Part II of the book.

1.2.3 Synchronisation

Time and frequency synchronisation between the transmitter and receiver are of crucial im-portance as regards the performance of an OFDM link [62, 63]. A wide variety of tech-niques have been proposed for estimating and correcting both timing and carrier frequencyoffsets at the OFDM receiver. Rough timing and frequency acquisition algorithms relying onknown pilot symbols or pilot tones embedded into the OFDM symbols have been suggestedby Claßen [26], Warner [64], Sari [65], Moose [66], as well as Bruninghaus and Rohling [67].Fine frequency and timing tracking algorithms exploiting the OFDM signal’s cyclic exten-sion were published by Moose [66], Daffara [68] and Sandell [69]. OFDM synchronisationissues are the topics of Chapter 5.

1.2.4 OFDM/CDMA

Combining multi-carrier OFDM transmissions with code division multiple access (CDMA)allows us to exploit the wideband channel’s inherent frequency diversity by spreading eachsymbol across multiple subcarriers. This technique has been pioneered by Yee, Linnartzand Fettweis [70], by Chouly, Brajal and Jourdan [71], as well as by Fettweis, Bahai andAnvari [72]. Fazel and Papke [73] investigated convolutional coding in conjunction withOFDM/CDMA. Prasad and Hara [74] compared various methods of combining the two tech-niques, identifying three different structures, namely multi-carrier CDMA (MC-CDMA),multi-carrier direct sequence CDMA (MC-DS-CDMA) and multi-tone CDMA (MT-CDMA).Like non-spread OFDM transmission, OFDM/CDMA methods suffer from high peak-to-mean power ratios, which are dependent on the frequency domain spreading scheme, as in-vestigated by Choi, Kuan and Hanzo [75]. Part II of the book considers the related designtrade-offs.

1.2.5 Decision-Directed Channel Estimation

In recent years numerous research contributions have appeared on the topic of channel trans-fer function estimation techniques designed for employment in single-user, single transmitantenna-assisted OFDM scenarios, since the availability of an accurate channel transfer func-tion estimate is one of the prerequisites for coherent symbol detection with an OFDM re-ceiver. The techniques proposed in the literature can be classified as pilot-assisted, decision-directed (DD) and blind channel estimation (CE) methods, as detailed in the extended versionof this monograph [90].

In the context of pilot-assisted channel transfer function estimation a subset of the avail-able subcarriers is dedicated to the transmission of specific pilot symbols known to the re-ceiver, which are used for ”sampling” the desired channel transfer function. Based on thesesamples of the frequency domain transfer function, the well-known process of interpolationis used for generating a transfer function estimate for each subcarrier residing between thepilots. This is achieved at the cost of a reduction in the number of useful subcarriers avail-able for data transmission. The family of pilot-assisted channel estimation techniques was

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