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.

中文翻译：

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水下声学通信的频-时域涡轮均衡

混合涡轮均衡是一种新颖且有效的通信系统方法，因为它可以在迭代过程的不同阶段受益于两个涡轮均衡器。尽管混合涡轮均衡具有显着的性能改进潜力，但很少在实验中得到验证，目前还不太了解其在水声通信中的可行性。在本文中，提出了一种混合频时域涡轮均衡器（FTD-TEQ），用于水声通信，并进行了全面的实验研究。与传统的基于直接自适应的 Turbo 均衡器 (DA-TEQ) 相比，DA-TEQ 对所有迭代使用一种类型的 Turbo 均衡器，并且可能会遇到收敛慢的问题，FTD-TEQ 在第一次迭代中采用基于频域信道估计的 Turbo 均衡器来提供良好的初始化，并在后续迭代中利用时域 DA-TEQ 来保持低复杂度的实现，旨在从两者中受益turbo 均衡器，而不会产生高计算负担。同时，可以将第一次迭代得到的高质量软判决符号和相对最优的滤波系数初始化传递给下一次迭代，从而提高瓶颈区域的符号检测性能，实现更快的收敛速度. 这种混合均衡机制的有效性已通过数值模拟得到很好的解释，并通过高阶8相移键控调制和16正交调幅的海底通信实验验证。结果表明，FTD-TEQ不仅表现出更优异的性能，而且在实验数据配置方面比传统的DA-TEQ具有更低的计算复杂度。