Deep-sea long-range underwater acoustic channels have the characteristics of multipath time-delay and non-minimum phase. When the spreading gain is not enough to suppress multi-path interference, channel equalization is necessary. In this paper, bi-directional decision feedback equalization (BiDFE) technique is applied to M-ary spread spectrum underwater acoustic communication system to reduce the error propagation probability and improve the output signal-to-noise ratio of the conventional symbol decision feedback equalizer. The despreading results of orthogonal M-ary spread spectrum are used to compare the qualities of the forward and reverse channels, then the diversity and weight factor is adjusted. Compared with the traditional BiDFE method of combining equal gain with forward and backward equalizing output, the proposed algorithm further improves the received processing gain. The validity of the algorithm is verified by the simulation results of deep-sea long-distance underwater acoustic channel.
