Bidirectional least-symbol-error-rate turbo equalization for underwater acoustic faster-than-Nyquist system

Faster-than-Nyquist (FTN) technology enhances transmission rates effectively, but its application in underwater acoustic communication is limited due to the inter-symbol interference (ISI) introduced artificially. In this paper, an FTN-ISI cancellation algorithm based on the linear minimum mean square error criterion is proposed for FTN communication to reduce the impact of ISI induced artificially. Meanwhile, a bidirectional direct-adaptive based turbo equalizer (DA-TEQ) algorithm based on the minimum-symbol-error-rate (MSER) criterion is proposed for single-input multiple-output underwater acoustic FTN communication. The proposed adaptive algorithm exploits the gain of bidirectional diversity to reduce the impact of error propagation. Furthermore, the MSER criterion is utilized to update the filter coefficient to further decrease the BER of the FTN communication system directly, leading to better performance compared with the traditional minimum-mean-squared-error criterion. To verify the effectiveness of the proposed algorithm, we performed deep-sea experiments in the South China Sea near southeastern Sanya in April 2023 and lake experiments in Qiandao Lake. The results of the experiments show that the proposed algorithm is robust against the ISI introduced by underwater acoustic channels and FTN.