OTFS-Based Underwater Acoustic Communication System: Modeling and Channel Estimation

Orthogonal time frequency space (OTFS) modulation has been introduced to underwater acoustic (UWA) communications. However, due to the wideband nature of UWA channels, the Doppler effect cannot be formulated with a uniform frequency offset. In view of this, we pursue a UWA OTFS model and corresponding channel estimation scheme in this paper, considering the Doppler-introduced time compress/dilation effect. By analyzing the characterization of UWA OTFS input-output relationships, the formulation of effective delay-Doppler (DD) domain UWA channel response is provided at first, where we also demonstrate the generation of inter-path interference (IPI) among the DD grids. Further, we show that the effective channel matrix can be formulated by a low-dimensional weighted combination of basis functions based on the basis-expansion-model (BEM) concept, thus constructing the channel estimation as a sparse coefficients recovery problem. To address this issue, we then propose a BEM-based iterative sparse Bayesian learning (SBL) channel estimation algorithm leveraging the framework of SBL and the iterative decision feedback scheme. Finally, simulation results validate the performance advantages of our proposed channel estimation algorithm by comparing it to the typical benchmarks.