Performance of turbo-codes in time-synchronous BPSK/CDMA systems and Rayleigh fading channels

In this paper, the performance of turbo-codes for time-synchronous BPSK/CDMA systems in Rayleigh fading channels is investigated. A single cell scenario is adopted where there is perfect channel side information. The forward link of the BPSK/CDMA is considered where the fading amplitudes of all the received signals are equal. The turbo decoder is modified to take into account the exact expression for the multiuser interference term. It is shown that the inputs of the turbo decoder which are affected by multiuser interference are statistically independent, and thus the log-likelihood ratio at the output of the decoder can be separated into three terms as in the case of AWGN channels. The performance of the modified turbo encoding/decoding scheme is investigated by simulations in the low signal-to-noise ratio region for different values of processing gain and different number of users