Trade-off between channel coding and spreading in multi-carrier CDMA systems

The efficiency of multi-carrier code division multiple access (MC-CDMA) systems for mobile radio communications depends on the processing gain due to bandwidth expansion and on the gain offered by channel coding. This paper shows how to evaluate the optimal ratio between spreading code length and channel coding rate for MC-CDMA systems in the downlink. The investigations are carried out for conventional MC-CDMA detection with minimum mean square error (MMSE) equalization and joint detection with maximum likelihood sequence estimation (MLSE). The results obtained with MC-CDMA systems are compared with those achieved by multi-carrier frequency division multiple access (MC-FDMA) and by multi-carrier time division multiple access (MC-TDMA) systems. Furthermore, for channel coding using convolutional codes, simple approximations of the log-likelihood ratio for the Viterbi decoding algorithm are presented in the case of MC-CDMA.