Turbo Multiuser Detection for MC-CDMA Downlink Systems with Transmit Diversity

Multi-carrier code division multiple access (MC-CDMA) and space-time block coding (STBC) are two promising techniques to fulfill the high-speed and spectral efficiency requirements of fourth generation wireless communication systems. In this contribution we present a turbo iterative receiver for the downlink of STBC MC-CDMA systems, that aims to mitigate the effects of multiple access interference (MAI) by jointly optimizing multiuser detection and channel decoding. Our proposal is a parallel interference canceller (PIC), where soft information in the form of log-likelihood ratios (LLRs) is exchanged between the multiuser detector (MUD) and individual convolutional decoders for successive refinement. To reduce complexity, a simple linear space-time combiner eliminates co-antenna interference (CAI) before delivering its output to the cancellation loop. Thus, the MUD is adapted to the equivalent channel as seen after space-time combining, and is designed according to the MMSE criterion, modified to take into account a priori information derived from previous iterations. Several suboptimum variants of this scheme are also discussed and compared in terms of complexity and performance.