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

We consider in this paper a convolutionally coded asynchronous 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. We view the asynchronous CDMA channel as a periodically time-varying ISI channel. The receiver jointly decodes the differential encoders and the CDMA channel with a combined trellis, and shares soft output information with the convolutional decoders in an iterative (turbo) fashion. Dramatic gains over conventional convolutionally coded systems are demonstrated via simulation. Near single-user turbo performance is obtained with a moderate number of users. We also show that there exists an optimal code rate under a bandwidth constraint. The performance and optimal code rates are also demonstrated via density evolution analyses.