Frequency-Time Domain Turbo Equalization for Underwater Acoustic Communications

Hybrid turbo equalization is a novel and effective approach for communication systems as it can benefit from two turbo equalizers at different stages of iterative process. Despite its notable potential for performance improvement, hybrid turbo equalization has rarely been validated experimentally and its feasibility in underwater acoustic communications is presently not well understood. In this paper, a hybrid frequency-time domain turbo equalizer (FTD-TEQ) is proposed for underwater acoustic communications with comprehensive experimental investigations. In contrast to the traditional direct-adaptation-based turbo equalizers (DA-TEQs), which use one type of turbo equalizer for all iterations and may suffer from slow-convergence problem, the FTD-TEQ employs the frequency-domain channel-estimation-based turbo equalizer in the first iteration to provide a good initialization and utilizes the time-domain DA-TEQ in the subsequent iterations to maintain low-complexity implementation, aiming at benefiting from both turbo equalizers without incurring high computational burden. In the meantime, the high-quality soft-decision symbols and the relatively optimal initialization of the filter coefficients derived from the first iteration can be delivered to the next iteration, so as to improve symbol detection performance in the bottleneck region and achieve faster convergence rate. The effectiveness of this hybrid equalization mechanism has been well interpreted by numerical simulations, and also has been verified through an undersea communication experiment with high-order 8-phase shift keying modulation and 16-quadrature amplitude modulation. The results demonstrate that the FTD-TEQ not only shows more excellent performance, but also has lower computational complexity than the traditional DA-TEQ for the experimental data configuration.