Two-stage Receiver Structures for Convolutionally-encoded Ds-cdma Systems

We consider alternative structures for subtractive interference cancellation (IC) detectors with single-user Viterbi decoding for convolutionally encoded code-division multiple-access (CDMA) systems on synchronous additive white Gaussian noise (AWGN) and time-uncorrelated Rayleigh fading channels. In the first structure interference cancellation with undecoded decisions (ICUD) is performed followed by single-user decoding of each user. In the second structure post-decoding interference cancellation (PDIC) is applied followed by a second bank of single-user decoders. As expected, the PDIC scheme usually outperforms the ICUD but we find some special cases where the performance of the ICUD is better. This behavior is analyzed and conditions for the existence of an error probability floor for ICUD receivers are derived. Furthermore, a multiuser interleaving scheme, in which each user is assigned a distinct interleaving pattern (DIP), is proposed for the PDIC scheme to overcome a problem caused by decoding prior to IC. The performance gain due to DIP is significant on the AWGN channel and, for a small number of users, on the time-uncorrelated fading channel. By analyzing the residual multiple-access interference (RMAI), we project that the performance gain due to DIP will be significant on time-correlated channels with slow fading even for a large number of users. The complexity of the proposed DIP scheme is analyzed and its applicability to the asynchronous AWGN channel and multipath fading environments is discussed. Finally, a family of approximations to the bit error rate (BER) and some exact results are derived for the various receiver structures. In particular, a novel expression for the variance of the RMAI in the ICUD receiver is derived and shown to improve the accuracy of the BER approximation. CDMA systems using both deterministic and random spreading sequences are considered. ∗This research was supported by NSF grants CCR-9725271 and CCR-981-5002 and ARO grant DAA-19-01-1-0638