Improved single-user decoder metrics for two-stage detectors in DS-CDMA

Among the advantages of subtractive interference cancellation (IC), or multistage detectors for CDMA (code-division multiple-access) systems is that they facilitate feedback between the multiuser detection and decoding stages in a CDMA receiver. We propose modified branch metrics for the single-user Viterbi decoders used in conjunction with two-stage detectors for CDMA systems with long (random) spreading sequences. The modifications are based on modeling the residual multiple-access interference (RMAI) as conditionally Gaussian with time-dependent variance where the conditioning is on the time-varying user cross-correlations. A novel estimate of the variance of the total RMAI is presented, and put to use in the modified branch metrics. Significant performance gains are demonstrated over the Euclidean branch metric of the standard Viterbi decoder, and further improvements are discussed.