Performance analysis of a DS-CDMA wireless communication system with space time block code (STBC)

Abstract

To enhance the future wireless system further research should: be carried out on correlation between different codes in a CDMA system and appropriate diversity techniques should be formulated to acquire an MAl free system. The performance limitation of a DS-CDMA system due to multi-path fading can be improved by the use of space time block coding (STBC). A number of research work have been reported on the CDMA system with STBC having performance results evaluated by simulation. It is important to find the bit error performance results analytically in presence of MAl and multi-path fading for a CDMA system with STBC. Space-time coding refers to channel coding techniques for transmission with multiple transmit and receive antennas. In this thesis the analytical method has been used to study a generic STBC-CDMA system as already reported in the literature. A comprehensive analysis has been carried out to derive the bit error rate performance expression of such a system under MAl and flat fading, considering maximal ratio combining scheme at the receiving side. The system has been extended with Nakagami-m fading channel. The performance has been also been evaluated with considering a 15 digit code with its time shifted code sequence. It is observed that as the number of simultaneous user increases, the BER goes high in m-sequence and gold code but not in OC code. To obtain diversity by STBC the delay, which depends on the length of the space-time-block-code frame has also been considered. Here optimum value of user capacity, diversity antenna are studied and these values are compared with the OC code, M-sequence and Gold code sequence. It is seen.. that an STBC-CDMA with the OC codes performs much better than t.h.at with .'ei. ther' Gold codes or m-sequences. Since gold code has a relatively well controlled crosscorrelation values and m-sequence does not have a regular cross-correlation.. :vai~e-,:.".t,.he STBC-CDMA system with the m-sequences is very sensitive to the change. in user population.