A comparison of iterative demapping with iterative soft decision interference cancellation employing the distribution of interference

A well-known receiver strategy is iterative soft decision interference cancellation (ISDIC) which can be used for CDMA transmission as well as PAM transmission over dispersive channels. ISDIC can also be adopted with a decoding unit in the loop yielding a turbo multiuser detector or a turbo channel equalizer. The distribution of residual interference (RI) after cancellation is commonly assumed to be Gaussian. Recently, an analysis of matched-filter based ISDIC (MF ISDIC) when utilizing the actual probability density function (pdf) of RI has been given. Another well-known receiver strategy is iterative demapping of non-Gray mapped symbols. We analytically compare both receiver strategies. It first turns out, that the cancellation step in the MF ISDIC receiver is redundant when considering the actual pdf of RI for calculation of improved a posteriori probabilities of symbols. Secondly, it can be shown that NW ISDIC employing the actual pdf of interference and iterative demapping are equivalent as the calculations needed for computation of improved a posteriori information supplied to a channel decoder in each iteration have the same structure. Additionally, it is shown that both schemes coincide exactly if the mapping can be written as a weighted superposition of interfering bits.