Optimal detection when combining OFDM-CDMA with convolutional and turbo channel coding

Orthogonal frequency division multiplexing (OFDM)-CDMA is an interesting multiple access technique for future mobile radio systems. Especially in the down-link OFDM-CDMA enables low complexity mobile receivers since OFDM can prevent intersymbol interference (ISI), and thus reduce the complexity of a RAKE receiver in a multipath channel. In this paper an OFDM-CDMA system combined with convolutional and turbo channel coding is investigated for the down-link. We derive the optimal soft value-the so called log-likelihood ratio-for the Viterbi decoding algorithm, applied in the OFDM-CDMA system. Based on this derivation we present the performance of an OFDM-CDMA system with channel coding. The investigations are carried out for conventional detection with minimum mean square error (MMSE) equalization and for joint detection with maximum likelihood sequence estimation (MLSE). The various combinations between detection techniques and decoding schemes enable a comparison between achievable system performance and necessary system complexity