OTFS-Based Underwater Acoustic Communication System: Turbo Receiver and Field Experiment

Orthogonal time-frequency space (OTFS) modulation is widely applied to address the nonuniform Doppler shifts issue while still modeling the Doppler effect as different frequency shifts in various paths. This work formulates a practical zero-padding-OTFS system for underwater acoustic (UWA) communications. Taking the impact of time compression/dilation into account, an effective delay-Doppler domain representation is utilized to describe UWA channel response. A basis expansion model (BEM) is employed to capture delay and Doppler multipath features of UWA channel with a weighted combination of basis functions. Leveraging the sparse Bayesian learning (SBL) inference, BEM coefficients can be reconstructed to accomplish the channel estimation. Combined with a decision feedback scheme, the BEM-based SBL turbo receiver is proposed exchanging soft information among channel estimation, equalization, and decoding, achieving coding and iteration gains. The proposed scheme has been verified through the underwater trial data collected from an experiment conducted in the Guishan Island Sea area, Zhuhai, China, in January 2024. Its effectiveness has been evaluated against other benchmarks in terms of bit error rate.