Parallel interference cancellation (PIC) receivers for turbo-encoded direct-sequence code-division multiple-access (DS-CDMA) systems with random spreading are considered. The residual multiple-access interference (RMAI) that remains in each user's signal after PIC is modeled as Gaussian with time- dependent variance conditioned on the user cross-correlations. An enhanced estimate of the conditional variance of the RMAI is used in the log-likelihood ratio (LLR) computation for the Turbo decoders of each user. Significant performance gains reaching 2 dB are demonstrated on the frequency non-selective Rayleigh fading channel over the conventional LLR which assumes that the RMAI is Gaussian with a fixed variance.
['Ayman Elezabi']
Improved Turbo Decoding and Interference Cancellation for DS-CDMA with Random Spreading