Multi-Carrier Code Division Multiple Access Richard Stirling-Gallacher 1vJ -I S A thesis subm itted for the degree of Doctor of Philosophy. The University of Edinburgh. - August 1997 - t. 4.t• I Abstract The topic of this thesis is the use of multi-carrier modulation with code division multiple access (CDMA). The motivation of this work is to establish if the combination of multi-carrier modulation with CDMA has a performance advantage over a conventional direct sequence CDMA (DS-CDMA) communication system. In this thesis three types of multi-carrier CDMA are identified and the main work is concentrated on one particular combination, which is referred to as a one chip per carrier multi-carrier CDMA system. This system itself, however, can be split into different variations and an examination of two of these is made. The first of these one chip per carrier multi-carrier CDMA systems utilises the same number of carriers as the spreading sequence length. The carriers overlap and adjacent chips of the spreading sequence modulate adjacent carriers. There is no guard interval and therefore intercarrier interference occurs. If the receiver is synchronised and has a perfect estimate of the channel, it is shown that this multi-carrier CDMA system has comparable performance to a DS-CDMA system of the same bandwidth. It is further shown that it is simple to compute the minimum mean square error criteria as the equaliser consists of N one tap equalisers, where N is the number of carriers. The second system utilises many overlapping low data rate orthogonal carriers. The orthogonality of the carriers is maintained due to a cyclically extended guard interval and the number of carriers is much higher than the spreading sequence length. After spreading, the data streams are interleaved onto the carriers to maximise diversity. A practical form of maximum likelihood detection for 64 users is described. It is shown from simulation results that when the system is used in conjunction with 1/2 rate (constraint length 7) coding and equal gain combining the system can support 64 users at 6 dB Eb/No for a bit error rate of 2 x iO. This compares with an equivalent DS-CDMA system which can only support 16 users for the same bit error rate and Eb/NO. These results assume perfect channel knowledge and synchronisation. It is further shown that to provide high spectral efficiency in a coded system a high rate convolutional coding scheme is needed. A combined decoder/canceller is also presented. Finally, techniques to achieve synchronisation and channel estimation algorithms are presented. These algorithms are considered in conjunction with the second system. In the framework of synchronisation, methods are presented for frequency and timing synchronisation. For channel estimation, simulation results are presented for a simple channel estimator. Declaration of originality This thesis was composed entirely by myself. The work reported herein was conducted exclusively by myself in the Department of Electrical Engineering at the University of Edinburgh. Richard Stirling-Gallacher August 1997 Acknowledgements I would like to thank the following people for their invaluable assistance during the course of this PhD: . Dr. Gordon Povey and Prof. Peter Grant, my supervisors, for their continous support and guid- ance. Also for reading this thesis. • Everyone in the Signals and System group for their tremendous support throughout my time at Edinburgh. I would like in to thank in particular the people who read my thesis, including Ian Band, lain Scott, Gunther Auer, Rudy Tanner, John Thompson and Sascha Spangenberg. • Dr. Ed Warner currently at DRA for much help and assistance at the beginning of my PhD. • Peter Hulbert and Steve Wales of Roke Manor Research Ltd. for providing me with two interest- ing summer placements. • Roke Manor Research Ltd. and EPSRC for for providing me with financial support. 11 Contents List of Figures VI List of Tables Ix Abbreviations X List of Symbols A 1 Introduction 1 1.1 Cellular systems ......................................1 1.2 Main research areas ....................................2 1.3 Thesis structure ......................................3 2 Background 4 2.1 Spread spectrum ...................................... 4 2.1.1 Frequency hopping ................................ 5 2.1.2 Chirp ....................................... 6 2.1.3 Time hopping ................................... 6 2.1.4 Direct sequence .................................. 6 2.1.5 Code division multiple access .......................... 7 2.1.6 Pseudo noise (PN) sequences ........................... 8 2.1.7 Proposed CDMA systems ............................ 10 2.2 Mobile radio channel ................................... 10 2.3 Orthogonal frequency division multiplexing ....................... 12 2.3.1 History of Multi-carrier modulation and basic principles ............ 13 2.3.2 Principles of OFDM ............................... 14 2.3.3 Digital broadcasting ............................... 16 2.4 Summary ......................................... 19 3 Review 20 3.1 Introduction ........................................ 20 3.2 One PN sequence per carrier ............................... 20 3.3 One PN chip per carrier .................................. 25 3.4 One PN sequence per OFDM multiplex ......................... 27 3.5 Summary ......................................... 28 4 Multi-carrier CDMA 30 4.1 Introduction ........................................30 4.2 System description ....................................30 fm Contents 4.3 BPSK modulation 31 4.3.1 BPSK DS-CDMA RAKE ............................33 4.3.2 BPSK performance evaluation ..........................34 4.4 DPSK modulation .....................................38 4.4.1 DPSK per data bit ................................39 4.4.2 DPSK per carrier (Frequency domain RAKE) ..................40 4.4.3 DPSK DS-CDMA RAKE . 41 4.4.4 DPSK performance evaluation ..........................42 4.5 Adaptive receiver summary ................................47 4.5.1 Calculated MMSE ................................47 4.5.2 Adaptive algorithm ................................49 4.5.3 Adaptive summary ................................58 4.6 Chapter summary ..................................... 59 5 Orthogonal frequency divison multiplexing CDMA 60 5.1 Introduction ........................................60 5.2 System description ....................................60 5.2.1 Independence assumption ............................62 5.3 Detection techniques ...................................64 5.3.1 Detection summary ................................73 5.4 Channel coding ......................................74 5.4.1 Convolutional coding ...............................74 5.4.2 Convolutionally coded OFDM-CDMA .....................76 5.4.3 1/2 rate convolutionally coded DS-CDMA ....................80 5.4.4 Alternative coding schemes for OFDM-CDMA .................82 5.4.5 Summary .....................................89 5.5 Combination of channel coding and different detection techniques ...........89 5.5.1 MLD with punctured convolutional coding (PCC) ................89 5.5.2 Interference cancellation with convolutional coding ...............90 5.5.3 Interference cancellation with orthogonal convolutional coding .........91 5.6 Chapter summary ..................................... 95 6 Synchronisation and channel estimation 97 6.1 Introduction ........................................97 6.2 Synchronisation ......................................97 6.2.1 Frequency synchronisation ............................97 6.2.2 Timing acquisition ................................106 6.2.3 Synchronisation conclusions ...........................111 6.3 Channel estimation ....................................112 6.4 Conclusions ........................................115 7 Conclusions 117 7.1 Summary of the work ...................................117 7.2 Summary of main points .................................119 7.3 Suggestions for further work ...............................120 iv Contents 7.3.1 Cellular analysis 120 7.3.2 Power amplifier non-linearities .......................... 120 7.3.3 Improvement in combined canceller/decoder .................. 120 7.3.4 Synchronisation ................................. 121 7.3.5 Channel estimation ................................ 121 7.3.6 Other multi-carrier CDMA systems ....................... 121 References 122 A Original Publications 127 B Coherence bandwidth 150 C Probability of false alarm 153 V List of Figures 2.1 Linear feedback shift register (LFSR) ..........................9 2.2 General mobile radio channel ...............................11 2.3 Multi-carrier transmitter .................................13 2.4 The reduction of ISI from multi-carrier transmission ...................14 2.5 8 Overlapping sinc functions (composite sum shown as continuous line) ........15 2.6 OFDM transmitted symbols (Only two carriers shown for clarity, N carriers are present) 15 3.1 Multi-carrier CDMA from Sourour and Nakagawa [25] ................. 21 3.2 Spectrum of transmitted signal from multi-carrier CDMA shown in Figure 3.1 [25] 21 3.3 Multi-carrier CDMA from Sourour and Nakagawa [26] ................. 22 3.4 Multi-carrier CDMA from Sourour and Nakagawa [27] ................. 23 3.5 Spectrum of transmitted signal from multi-carrier CDMA system shown in Figure 3.4 [27] (P=3, S=3) ...................................... 23 3.6 Spectrum of transmitted signal from Kondo and Milstein multi-carrier