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Multi-Carrier Code Division Multiple Access

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Multi-Carrier Code Division Multiple Access Richard Stirling-Gallacher 1vJ S -I A thesis subm itted for the degree of Doctor of Philosophy. The University of Edinburgh. - August t. 4.t I Abstract
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Multi-Carrier Code Division Multiple Access Richard Stirling-Gallacher 1vJ S -I A thesis subm itted for the degree of Doctor of Philosophy. The University of Edinburgh. - August 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 guidance. 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 interesting summer placements. Roke Manor Research Ltd. and EPSRC for for providing me with financial support. 11 Contents List of Figures List of Tables Abbreviations List of Symbols VI Ix X A 1 Introduction Cellular systems Main research areas Thesis structure Background Spread spectrum Frequency hopping Chirp Time hopping Direct sequence Code division multiple access Pseudo noise (PN) sequences Proposed CDMA systems Mobile radio channel Orthogonal frequency division multiplexing History of Multi-carrier modulation and basic principles Principles of OFDM Digital broadcasting Summary Review Introduction One PN sequence per carrier One PN chip per carrier One PN sequence per OFDM multiplex Summary Multi-carrier CDMA Introduction System description...30 fm Contents 4.3 BPSK modulation BPSK DS-CDMA RAKE BPSK performance evaluation DPSK modulation DPSK per data bit DPSK per carrier (Frequency domain RAKE) DPSK DS-CDMA RAKE DPSK performance evaluation Adaptive receiver summary Calculated MMSE Adaptive algorithm Adaptive summary Chapter summary Orthogonal frequency divison multiplexing CDMA Introduction System description Independence assumption Detection techniques Detection summary Channel coding Convolutional coding Convolutionally coded OFDM-CDMA /2 rate convolutionally coded DS-CDMA Alternative coding schemes for OFDM-CDMA Summary Combination of channel coding and different detection techniques MLD with punctured convolutional coding (PCC) Interference cancellation with convolutional coding Interference cancellation with orthogonal convolutional coding Chapter summary Synchronisation and channel estimation Introduction Synchronisation Frequency synchronisation Timing acquisition Synchronisation conclusions Channel estimation Conclusions Conclusions Summary of the work Summary of main points Suggestions for further work iv Contents Cellular analysis Power amplifier non-linearities Improvement in combined canceller/decoder Synchronisation Channel estimation Other multi-carrier CDMA systems 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) General mobile radio channel Multi-carrier transmitter The reduction of ISI from multi-carrier transmission Overlapping sinc functions (composite sum shown as continuous line) OFDM transmitted symbols (Only two carriers shown for clarity, N carriers are present) Multi-carrier CDMA from Sourour and Nakagawa [25] Spectrum of transmitted signal from multi-carrier CDMA shown in Figure 3.1 [25] Multi-carrier CDMA from Sourour and Nakagawa [26] Multi-carrier CDMA from Sourour and Nakagawa [27] Spectrum of transmitted signal from multi-carrier CDMA system shown in Figure 3.4 [27] (P=3, S=3) Spectrum of transmitted signal from Kondo and Milstein multi-carrier CDMA [30] OFDM-CDMA from Fazel [5] Multitone DS-CDMA from Wiel and Vandendrope [43] MC-CDMA transmitter MC-CDMA digital transmitter MC-CDMA receiver DS-CDMA BPSK RAKE receiver path channel Block A for Figure DS-CDMA MRC RAKE 4-paths DS-CDMA EGC RAKE 4-paths MC-CDMA MRC 4-paths MC-CDMA EGC 4-paths DPSK per data bit MC-CDMA receiver DPSK per carrier MC-CDMA receiver DS-CDMA DPSK RAKE receiver DPSK per data bit MC-CDMA and DPSK DS-CDMA receivers in Gaussian noise channel DPSK per carrier MC-CDMA receiver in a Gaussian noise channel DS-CDMA RAKE - 4-path Rayleigh fading MC-CDMA DPSK per data bit receiver - 4-path Rayleigh fading MC-CDMA DPSK per carrier - 4-path Rayleigh fading MC-CDMA BPSK MMSE MC-CDMA BPSK Non-optimal MMSE MC-CDMA Adaptive BPSK receiver LMS convergence for MC-CDMA with 30 users (p = ) Eb/NO = 0 db LMS convergence for MC-CDMA with 30 users (p = ) Eb/NO = 10 db RLS convergence for MC-CDMA with 30 users (.A = 1.0) Eb/No = 0 db RLS convergence for MC-CDMA with 30 users = 1.0) Eb/NO = 10 db MC-CDMA receiver BER after 1000 iterations using the LMS (p = 1 x 10) MC-CDMA receive BER after 1000 iterations using the RLS (A = 1.00) Block training structure RLS convergence for MC-CDMA with 30 users (A = 0.99), Eb/NO = 0 db RLS convergence for MC-CDMA with 30 users (A = 0.99), E/No = 10 db RLS convergence for MC-CDMA with 30 users (A = 0.99), Eb/NO = 20 db VI List of Figures 4.32 Adaptive MC-CDMA receiver BER in multipath with RLS algorithm X = 0.99) OFDM-CDMA system type OFDM-CDMA system type OFDM-CDMA receiver Single user OFDM-CDMA system modelled in the frequency domain BER performance for single user with EGC for different channels Equal gain combining (EGC) Maximal ratio combining (MRC) Zero forcing (ZF) Controlled equalisation (CE), threshold = Optimal MMSE Non-optimal MMSE Receiver with single stage interference cancellation One stage interference cancellation Two stage interference cancellation Maximum likelihood detection (MLD) Spectral efficiency for different detection schemes General constraint length K, k/n rate convolutional coder Viterbi decoder trellis OFDM-CDMA receiver with channel decoding BER results for 1/2 rate coder in Gaussian noise BER results for 1/2 rate coder (K = 3) in multipath BER results for 1/2 rate coder (K = 7) in multipath Transmision scheme for DS-CDMA for downlink BER results for 1/2 rate (K = 7) in multipath Spectral efficiency of coded DS-CDMA and coded OFDM-CDMA systems BER results for 3/4 rate punctured convolutional coder in Gaussian noise BER results for 3/4 rate punctured convolutional in multipath Spectral efficiency for 3/4 rate punctured convolutional code in multipath Orthogonal coder (K = 3) BER performance for orthogonal coder (K = 3) in multipath Super orthogonal coder (K = 5) BER performance for super orthogonal coder (K = 5) in multipath Spectral efficiency for the orthogonal and super orthogonal coder BER results for MLD receiver with punctured convolutional coding (PCC) Receiver with convolutional decoding and interference cancellation BER results for 3/4 rate punctured convolutional coding with interference cancellation Spectral efficiency for 1.3 MHz bandwidth BER results for orthogonal convolutional coding with interference cancellation BER results for super orthogonal convolutional coding with interference cancellation Spectral efficiency results for 1 MHz bandwidth BER against frequency offset for the OFDM-CDMA system in a Gaussian channel and the 8 path channel at 4 db Eb/NO Transmission frame structure from Nogami [65] Pilot tone arrangement in data portion of frame (18 % overhead) Pilot tone arrangement in data portion of frame (13 % overhead) Frequency shifted pilot tones Estimated against actual frequency offset for 31 pilot tones spaced 16 carriers apart Estimated against actual frequency offset for 31 pilots modulated by length 31 m- sequence (frequency step 2kHz) Estimated against actual frequency offset for 31 pilots modulated by length 31 insequence (frequency step 500 Hz) Frequency estimation variance against Eb/NO Error signal E against frequency for system with length 18 guard interval Error signal E against frequency for system with length 64 guard interval vii List of 6.12 Linear correlator Correlation against timing offset for linear correlator Probability of detection for linear correlator in a Gaussian channel (threshold is normalised to maximum value of linear correlator function.) Sign only correlator Probability of detection for sign-only correlator (threshold is normalised to maximum value of sign-only correlator function) Probability of false alarm for sign-only correlator OFDM-CDMA receiver BER with EGC with perfect and estimated channel response NA = BER with EGC with perfect and estimated channel response NA = BER with EGC with perfect and estimated channel response NA = viii List of Tables 2.1 Valid rn-sequences Different DAB modes Summary of different MC-CDMA systems Parameters Performance of different detection schemes Switch positions for orthogonal coder (K=3) B.1 BU-12 Delay profile B.2 Delay profile ix Abbreviations ACF AMPS AWGN BER BPSK CC CCF CD CDMA DAB DAPSK DAT DECT DV!' DPC DPSK DS DS-CDMA DS-SS DQPSK DVB EGC ETSI FH FM FDM FDMA auto correlation function advanced mobile telephone system additive white Gaussian noise bit error rate binary phase shift keying convolutional code cross correlation function compact disc code division multiple access digital audio broadcasting differential amplitude phase shift keying digital audio tape digital European cordless telephone discrete Fourier transform differential phase combining differential phase shift keying direct sequence direct sequence code division multiple access direct sequence spread spectrum differential quaternary phase shift keying digital video broadcasting equal gain combining European Telecommunications Standards Institute Frequency hopping frequency modulation frequency division multiplexing frequency division multiple access x Abbreviations FFT GSM HSDL IC ICI IDFT 1FF'!' IS! LFSR LMS MC-CDMA MCM MLD MMSE MPEG MRC MSE NMT OFDM OFDM-CDMA 0-QAM PCC PCM PDF PG PN QAM QASK RDS RLS SFN SNR SQAM TACS TDMA TH UMTS W-CDMA fast Fourier transform Groupe Speciale Mobile (or Global system for mobile communications) high speed digital subscriber line Interference cancellation Inter carrier interference Inverse discrete Fourier transform Inverse fast Fourier transform Inter symbol interference linear feedback shift register least mean square multi carrier code division multiple access multi carrier modulation maximum likelihood detection minimum mean square error motion pictures experts group maximum ratio combining mean square error nordic mobile telephone orthogonal frequency division multiplexing orthogonal frequency division multiplexing code division multiple access offset quaternary amplitude modulation punctured convolutional code pulse coded modulation probability density function processing gain pseudo noise quaternary amplitude modulation quaternary amplitude shift keying radio data system recursive least square single frequency network signal to noise ratio staggered quadrature amplitude modulation total access communication system time division multiple access time hopping Universal mobile telecommunication system Wideband - CDMA xi List of principal symbols Ok1 ai ak (n) B Bin B 33 BW 1 BW m bm bkm C complex gain for the lth path of the kth symbol real part of received sampled complex received signal xi before FFT (OFDM-CDMA) equaliser coefficient for symbol k on carrier n (MC-CDMA) Bandwidth message bandwidth spread spectrum bandwidth bandwidth of one carrier from MC-CDMA system (Kondo and Milstein) bandwidth of m carriers from MC-CDMA system (Kondo and Milstein) data bit for user m (OFDM-CDMA) /cth transmitted symbol for user m (MC-CDMA) Channel capacity C1 ear (y) linear correlator function as a function of offset y C39 (y) Cm D D' D3 d dmin F E Id fd,,,,. Ioffset b//set sign only correlator function as a function of offset y spreading code vector for user m Number of multipath components Frequency diversity Delay spread differentialy encoded data bit for user m minimum free distance of convolutional code guard interval length (in seconds) Error signal for frequency tracking loop Doppler frequency maximum Doppler frequency frequency offset frequency offset estimate frequency offset for carrier i G diagonal equaliser matrix (OFDM-CDMA) AII List of principal symbols 91 Equaliser coefficent for the lth row and lth column of matrix G (OFDM-CDMA) H h i J K L L error L1 M M rn N diagonal channel matrix (OFDM-CDMA) channel coefficent for the lth row and lth column of matrix H (OFDM-CDMA) spacing of sub-carriers MC-CDMA constriant length spreading sequence length (OFDM-CDMA) number of guard interval samples used for tracking number of pilot tones in the frequency domain size of serial to parallel converter (OFDM-CDMA) Hadamard matrix mobile user index spreading sequence length = number of carriers (MC-CDMA) N. spreading sequence length (MC-CDMA, one sequence per carrrier type a) Nb spreading sequence length (MC-CDMA, one sequence per carrier type b) N spreading sequence length (MC-CDMA, one sequence per carrier type c) N N N9 n P P(d) number of users maximum number of users guard interval length (in samples) received noise vector number of parallel data streams (MC-CDMA) probability that the wrong path at distance d is selected P puncturing period for punctured convolutional coding Plhres Q threshold for controlled equalisation size of sub-system (OFDM-CDMA) = k/n code rate of convolutional code r S received signal vector after the FF1' (OFDM-CDMA) parallel streams (MC-CDMA) S. (t) continous time representation of baseband signal produced by the rnth user S. (i) discrete time representation of baseband signal produced by the mth user Spilots S Tb T Tdata number of data symbols between pilot tones sent signal vector (OFDM-CDMA) data bit duration chip duration (DS-CDMA) chip duration (MC-CDMA) useful data section for adaptive algorithm (MC-CDMA) xlii List of principal symbols T3 Tt rain T symbol duration training time for adaptive algorithm (MC-CDMA) useful part of transmitted symbol (OFDM-CDMA) t (1) value of transmitted pilot tones on carrier j for symbol 1 U Vi V W X(t) Xi xk (n) Xlk Y Yk (ii) Zi received signal vector after equalisation (OFDM-CDMAIMC-CDMA) soft output from MLD despreader (OFDM-CDMA) possible sent sequence vector (OFDM-CDMA) length of window for Viterbi deocder received signal before FF1' (OFDM-CDMA) sampled received signal before FF1' (OFDM-CDMA) sampled received signal before FF1' for data bit k (MC-CDMA) deccorelated symbol for the kth symbol on the lth path (DS-CDMA) offset for timing composite transmitted signal (DS-CDMA) imaginary part of sampled received xi before FF1' (OFDM-CDMA) 18, power of path I 5J 3 (zf) ('f), ('.f),2 (zf) distance between received sequence r and sent sequence v difference in Euclidean distance between the ith possible and the chosen jth MLD sequence. coherence bandwidth coherence bandwidth (Prasad) coherence bandwidth (Proakis) coherence bandwidth (Lee) 71 delay of path 1 step size for LMS algorithm wd (At), or 2 distance spectra of convolutional code at distance d coherence time frequency estimation variance forgetting factor of RLS algorithm xiv Chapter 1 Introduction This thesis will consider a communication system based on combining multi-carrier modulation techniques with code division multiple access (CDMA). These two different techniques have historically evolved from different fields. CDMA is a technique for providing communication for multiple users using spread spectrum techniques and is used for high capacity commercial cellular communication systems such as IS-95 [1]. Multi-carrier modulation has evolved from frequency division multiplexing in the 1950s to its use in digital television and audio broadcasting systems of today. This chapter will commence by summarising the current state of the art in wireless technology. It will examine cellular technology and explain why the proposed multi-carrier code division multiple access system is of interest. Following this, a brief summary of the main areas of research covered in the thesis will be presented and the thesis structure described. 1.1 Cellular systems In recent years, there has been an explosion of wireless communication services. This has occurred due to the demand of communicating without the constraints of a fixed network and the advances in microelectronics to provide such a system. A wireless communication system provides a flexible link using different forms of data. The most widely used wireless communication networks are personal communications networks (PCN) which are based on cellular networks. Other wireless communication networks include wireless area networks (both local and wide area), paging systems, cordless system and satellite systems. Each of these will be described before discussing in more detail cellular networks. Wireless area networks are designed for low data rate communication and are based on low mobility operation in an office (local area) or a mobile user (wide area). Cordless systems allow the user limited mobility by providing a communication link between the users terminal and a base station connected
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