In this paper, a second stage for algorithms applying iterative soft decision interference cancellation (ISDIC) [1], [2], [3] for multiuser detection is proposed performing error search and correction. Analysis by simulations shows for the considered downlink scenario that the bit error ratio of a matched filter (MF) ISDIC can be lowered by a factor of nearly 100 for high signal-- to--noise ratios with the subsequent second stage approaching the performance of a more complex minimum mean--squared error (MMSE) ISDIC. Furthermore, the MMSE ISDIC can be improved by the proposed second stage by up to 0.5 dB. It is shown that the ISDIC scheme with additional second stage can approach the matched filter bound up to 1 dB for the analyzed channel and 4QAM transmission.

A well--known receiver strategy for CDMA transmission is iterative soft decision interference cancellation. For calculation of soft estimates, the distribution of residual interference is commonly assumed to be Gaussian. In this paper, we analyze matched--filter based iterative soft decision interference cancellation (MF ISDIC) when utilizing the actual probability density function (pdf) of residual interference. Additionally, a hybrid scheme is proposed, which approximates the actual pdf by considering only the strongest residual interferers accurately while the Gaussian assumption is applied on the weak residual interferers. It turns out, that the bit error ratio decreases already noticeably when only a small number of residual interferers is regarded according to its actual pdf. In fact, for the considered CDMA transmission we are able to lower the bit error ratio by 80% for high signal--to--noise ratios when modelling all residual interferers but the strongest 3 to be Gaussian distributed.