A transmit diversity scheme using space-time spreading for DS-CDMA systems in Rayleigh fading channels
Walaa Hamouda, Mohamed AlJerjawi · 2006
The application of space-time block coding (STBC) in the downlink of a communication system has shown to improve the quality of the received signal at the mobile user by placing the spatial diversity at the base station. In this paper, we propose a simple space-time spreading scheme for code-division multiple-access (CDMA) systems over fast-fading channels. The proposed transmit diversity scheme is based on two transmit and one receive antenna and is suitable for Rayleigh fast-fading channels. In this we employ orthogonal spreading codes to exploit the time diversity introduced by the channel, and hence a two-fold of the diversity order obtained using existing space- time spreading schemes is achieved. Nevertheless, for slowly- fading channels, we show that the proposed coding scheme reduces to existing schemes introduced in the literature with no performance degradation. Finally, we examine the effect of using nonorthogonal spreading codes on the receiver performance. Our results show that using a simple adaptive decoder, based on the minimum mean-squared error (MMSE) criterion, the diversity order is still maintained and only a small loss in the signal-to- noise ratio is incurred relative to the ideal case with orthogonal codes.