Acoustic Communication in the High Doppler Regime: Synchronization Revisited

Underwater acoustic communications are prone to motion-induced Doppler effects whereby the received signal experiences both phase and delay drifting. In single-carrier communications, which are the focus of the present study, a phase locked loop (PLL) is engaged in tandem with an equalizer to track the phase drift, while delay drifting is often neglected as its effect is minor over typical data packet lengths. Although justified in many situations, ignoring the delay drift is not an option in extreme Doppler regimes. To address this issue, we propose an algorithm for simultaneous delay and phase tracking, in which an estimate of the time-varying delay is derived from the PLL output, and adaptive bit-by-bit delay synchronization (resampling) is coupled with equalization or channel estimation. The algorithm is demonstrated in two case studies: one that focuses on long-term channel estimation, and another that focuses on communication in highly mobile scenarios with speeds on the order of 10 m/s. Very good results are observed in both cases. Moreover, adaptive delay tracking is shown to allow for complete elimination of front-end frequency synchronization and resampling.