OTFS underwater acoustic communications based on passive time reversal

Orthogonal time-frequency-space (OTFS) is a new two-dimensional (2D) modulation technique that offers reliable communications over time-varying channels. Due to the characteristics of underwater acoustic (UWA) channels, the computational complexity of traditional OTFS receivers is too high to implement in practice. In this paper, a 2D passive time reversal (PTR) structure receiver is designed in the Delay-Doppler domain, achieving spatial, temporal, and frequency focusing. Compared with existing methods, the proposed method could achieve similar performance with a lower complexity. Meanwhile, we also propose an adaptive channel estimation method based on the improved proportionate normalized least mean squares (IPNLMS) algorithm to reduce the complexity of channel estimation for the OTFS UWA communications system. The performance is evaluated under statistical simulated channels and measured channels from real-world experiments. The results verify the proposed 2D PTR receiver and demonstrate it could achieve good BER performance with a lower complexity.