Space-time spreading and diversity in asynchronous CDMA systems over frequency-selective fading channels

In this paper, the performance of space-time spreading (STS) introduced in the literature is analyzed for asynchronous direct sequence code division multiple access (DS-CDMA) systems in frequency-selective slowly-fading environment. The transmit diversity scheme is based on two transmit and one receive antenna. The bit-error-rate performance of the underlying system is investigated for an uplink transmission where a decorrelator detector is used at the base station receiver. The proposed receiver has the advantage of mitigating the multipath fading effects and suppressing the multiuser interference (MUI). Both simulation and analytical results show significant performance gains over the conventional STS receiver. Different from the decorrelator detector, the achieved diversity order of the conventional detector is shown to deteriorate as a function of the number of users.