Reliability-based Partial Parallel Interference Cancellation and Iterative Decoding for DS-CDMA over Fading Channels

Multistage partial parallel interference cancellation (PPIC) is proposed for turbo-encoded DS-CDMA systems where the partial cancellation factors are derived based on the log-likelihood ratios (LLR´s) of each user´s bit. The LLR is obtained based on an enhanced estimate of the variance of the multiple- access interference (MAI) and residual MAI (RMAI) plus noise. The (R)MAI terms are modeled as conditionally Gaussian with a time-dependent variance that is a function of the user cross-correlations and fading coefficients. Significant performance gains due to the improved LLR´s are realized over PPIC schemes using fixed variance estimates in frequency non-selective Rayleigh fading channels. The performance gap is greater when two stages of the improved PPIC are used. Reduced complexity forms of the variance estimates and LLR´s are proposed and also shown to give a performance advantage.