OTFS for Underwater Acoustic Communications: Practical System Design and Channel Estimation

Recently, orthogonal time frequency space (OTFS), a two-dimensional modulation technique designed in delay-Doppler domain, has received significant attention. Generally, OTFS converts signals in the time-varying and doubly-spread channels into the slow time-varying and sparse channels in delay-Doppler domain. Compared with OFDM system, OTFS is efficient to alleviate the inter-carrier-interference (ICI) for the time-varying and doubly-spread channels. This fact motivates us the use of OTFS in the time-varying and doubly-spread underwater acoustic (UWA) channels. In this paper, we design a practical OTFS system for the UWA communication. To alleviate the Doppler effect in UWA communication, a Doppler compensation step is proposed for the UWA OTFS system. Moreover, we provide a practical channel estimation for the UWA OTFS system to avoid the problem caused by fractional Doppler shifts. We demonstrate performance of the proposed OTFS system. The simulation results show that the UWA OTFS scheme outperforms the OFDM in UWA channels.