On channel coding and multiuser detection for DS-CDMA

Two issues concerning direct-sequence code-division multiple access (DS-CDMA) multiuser detection are addressed: Reliability estimation (i.e., soft-output detection), and a complexity reduction technique to simplify the optimum (maximum-likelihood) multiuser detector. The soft-outputs are not only suitable for improving the performance of a next stage (in this study an outer soft-decision Viterbi decoder), but also for reducing the complexity of the multiuser detector itself. In this way, a family of suboptimum multiuser detectors with adjustable performance/complexity tradeoff is obtained in a structured manner from the optimum multiuser detector. Reliability generation is applied by post-processing hard decisions of an auxiliary (single or multiuser) detector delivering tentative decisions given matched filter outputs. Reduced-complexity multiuser detection is based on exhaustively searching only in a subset of unreliable decisions given the reliability estimates. A decision-feedback mechanism is shown to be included. In particular it is indicated that the decision-feedback multiuser detector is outperformed on a Gaussian channel, and that almost all multiuser interference is cancelled out in a coded multiuser system in Rayleigh fading, i.e., in a severe near-far situation. The latter result corresponds to a gain on the order of 4 dB at a bit error rate of 10 compared to conventional detection, or a loss of only about 1 dB compared to single user system.