Performance Analysis of OFDM-CDMA Systems with Doppler Spread

Multi carrier modulation are very sensitive to rapid time-varying multi-path channel characterized by Doppler spread. Although progress has been made in the description of the time variation, there is still considerable gaps in its effect especially on diversity gain acquired by time selectivity. This paper models a general case of time-varying channel effect on the OFDM-CDMA performance. This performance is measured through the Signal to Interference and Noise Ratio SINR at the output of the detector and the Bit Error Rate BER at the output of the channel decoder. The originality of the paper is twofold. First, we propose a simple tool to evaluate an analytical expression of the SINR independently on the spreading codes while taking into account their orthogonality. Second, we adapt a new technique to predict the BER at the output of the channel decoder from the link level simulation expressed in terms of the SINRs. We show by simulation the validity of our analytical models. We show also that the time variation of the channel would be favourable for system performance in MC-DS-CDMA system and QPSK constellation however it is destructive with other simulation assumptions.