Near-optimal convolutionally coded synchronous CDMA with iterative multiuser detection/decoding

We consider in this paper a convolutionally coded synchronous DS-CDMA system with iterative multiuser detection and decoding. Each user's bits are convolutionally encoded, interleaved, and differentially encoded before being transmitted over the multiuser CDMA channel. In contrast to previous work in this area, a differential encoder is inserted to effect an interleaver gain. The receiver consists of a soft-in soft-out multiuser detector followed by a bank of iterative decoders matched to the convolutional code-differential encoder combination. Dramatic gains over systems which contain no differential encoder are demonstrated. We also show that there exists an optimal code rate under a bandwidth constraint. The gains and optimal code rates are demonstrated via bit error rate simulations and density evolution analyses.