Bidirectional Soft-Decision Feedback Equalization for Robust MIMO Underwater Acoustic Communications

An iterative bidirectional soft decision feedback equalizer (Bi-SDFE) is proposed for robust multiple-input multiple-output (MIMO) underwater acoustic (UWA) communications. The proposed Bi-SDFE use a time-reversed soft decision feedback equalizer (SDFE) in conjunction with a normal SDFE to lower the error propagation in severe inter-symbol interference (ISI) channels. The extrinsic information at the equalizer outputs are combined to improve the reliability and reduce the probability of catastrophic performance. Besides, the normalized least mean squares (NLMS) algorithm is adopted to iteratively estimate the MIMO UWA channel for the equalizer. The performance of the proposed algorithm is verified by the SPACE08 experiment. The processing results show that the iterative Bi-SDFE can effectively improve system performance and data efficiency in time varying MIMO UWA channel.