Soft direct-Adaptation based bidirectional turbo equalization for MIMO underwater acoustic communications

This paper proposes a soft direct-Adaptation based bidirectional turbo equalizer for multiple-input multiple-output underwater acoustic communication systems. Soft, rather than hard, direct-Adaptation based equalizer combined with the fast self-optimized least mean square algorithm is employed to achieve a faster convergence rate, and the second-order phase-locked loop is embedded into the equalizer to track the time-varying channel. Meanwhile, by utilizing a weighted linear combining scheme, the conventional soft direct-adaptation based equalizer is combined with the time-reversed soft direct-Adaptation based equalizer to exploit bidirectional diversity and mitigate error propagation. Both the simulation and experimental results demonstrate that the soft direct-Adaptation based bidirectional turbo equalizer outperforms the single-direction soft direct-Adaptation based turbo equalizer, and achieves a faster convergence rate than the hard direct-Adaptation based bidirectional turbo equalizer.