SPC11-1: A Soft-Output Tree/Trellis Multiuser Equalizer for an Iterative FEC-coded DS-CDMA system in Frequency Selective Fading

This paper presents a reduced-complexity soft- output trellis/tree multiuser equalizer for an iterative DS-CDMA system undergoing Rayleigh frequency selective fading. The algorithm first expands the equalizer-trellis to an equivalent trellis/tree structure. Then it applies the M-algorithm to the equivalent structure twice, once to reduce the number of states in the trellis, and the other to reduce the number of branches emanating from each state. To compute soft-information, the algorithm utilizes not only those fully-extended paths reaching the end of the trellis, but also paths that are traversed and discarded in the pruned trellis. We show a simple and effective method of using these discarded paths without having to store them. BER performance is presented for convolutional-coded CDMA system employing random spreading sequences. Our results demonstrate that the proposed algorithm is capable of achieving single-user performance with a much reduced complexity.