Design and performance of a low-complexity iterative multiuser joint decoder based on Viterbi decoding and parallel interference cancellation

We propose a low-complexity multiuser joint parallel interference cancellation (PIC) decoder for direct-sequence CDMA. An estimate of the multiple-access interference (MAI) signal is formed by weighting the hard decisions produced by hard-output Viterbi decoders. Such MAI interference estimate is subtracted from the received signal in order to improve decoding in the next iteration. By using asymptotic performance analysis of random-spreading CDMA, we optimize the feedback weights at each iteration. Then, we consider two performance limitation factors: the bias of residual interference and the ping-pong effect. We propose then a modification of the basic PIC algorithm, which allows higher channel load and/or faster convergence to the single-user performance. The main conclusion of this work is that, in most practical cases, SISO decoders are not needed to attain very high spectral efficiency, and simple conventional Viterbi decoding suffices for most practical settings.