Coherent multiuser detection for CDMA mobile communication

Abstract

Among various detection schemes for direct sequence code division multiple access (DS-CDMA) systems, decorrelating detectors have been studied by many researchers because they have low complexity linear in the number of users and significantly outperform the conventional receiver when multiple access interference is high or near/far effect is dominant. The application of the decorrelating detectors to asynchronous DS-CDMA when the signal phase and energy are unknown at the receiver owing to channel fading has been addressed by several researchers and is the problem we address in this dissertation work. Previously proposed noncoherent decorrelating detection schemes exhibit an irreducible error floor at high SNR over Rayleigh fading channels with non-zero Doppler spread and have SNR loss compared with the ideal coherent detection. In this dissertation work, we propose practical coherent decorrelating detection arrangements for infinite sequence length data over asynchronous up-link DS-CDMA channels of star-topology mobile radio networks. For frequency-nonselective Rayleigh fading channels, we propose a scheme to provide a decorrelating multiuser detector with channel phase estimates by applying pilot symbol insertion techniques. Closed form expressions for the bit error rate (BER) of the proposed scheme and that of the noncoherent differential detection scheme are derived. Simulation and analysis results show that the proposed method has SNR gain of about 1 dB at low SNRs and exhibits no error floor at high SNRs compared to differential decorrelator. Furthermore the proposed scheme is near-far resistant since it has the same order of BER at very high SNR as that of the ideal coherent decorrelating scheme which is near-far resistant. We apply a coherent diversity combining technique to multiuser decorrelating detection for frequency-selective Rayleigh fading channels. The multipath fading channel in mobile radio environments is well represented by the tapped dela...