Performance comparison of turbo-coded DS-CDMA, MC-CDMA and OFDM with frequency-domain equalization and higher-level modulation

For high-speed data communications, channel coding and high-level modulation will be inevitable. In this paper, we compare the turbo coded performance of DS-CDMA and MC-CDMA which are the major contenders for the next generation wireless signaling technique. Minimum mean square frequency-domain equalization (MMSE-FDE) is assumed for both DS-CDMA and MC-CDMA. The log-likelihood ratio (LLR) computation is used to generate the soft decision value needed for turbo decoding. It is found that the DS-CDMA performance is the same for all spreading factors (SF) and equivalent to a fully spread MC-CDMA for all modulation levels and coding rates. For an uncoded system, DS-CDMA is better than MC-CDMA for smaller SF due to a larger frequency diversity gain. However, with turbo coding, MC-CDMA with smaller SF provides better performance due to larger coding gain owing to better frequency interleaving and severe orthogonality destruction for larger SF and DS-CDMA wherein a symbol is spread over the entire bandwidth.