A Comparison of Forward-Only and Bi-Directional Fixed-Lag Adaptive SISOs

Several structures for fixed-lag (FL) soft-in/soft-out (SISO) algorithms in the case of a perfectly known channel are well-known. These forward-only and bi-directional fixed-lag SISOs have recently been described with the bi-directional version shown to be preferred. Adaptive iterative detection using adaptive SISOs (A-SISOs) have also recently been demonstrated to provide significant performance gains for time-varying channels. However, these impressive results have been obtained with fixedinterval, bi-directional A-SISOs and training signals at both ends of the data packet. We combine these recent results to develop and compare various adaptive, fixed-lag SISOs. Among several reasonable options considered, the preferred A-SISO algorithm is found to be bi-directional with forward-only channel estimation.