In underwater acoustic communication channels, Doppler shifts can severely affect received signals due to the slow propagation speed. A characteristic feature of such channels is that the time-varying Doppler shift of a dominant signal introduces nonuniform time compression or dilation, as well as nonlinear phase shifts. As a result, the optimal timing for down-sampling dominant signal varies in a nonlinear manner. Furthermore, the other multipath signals exhibit a variety of Doppler shifts depending on their directions of arrival, resulting in Doppler spreading and associated phase distortions. Addressing these phase shifts is challenging, as each multipath signals requires individual phase compensation. This study demonstrates that adaptive digital down-conversion (ADDC) can effectively mitigate the impact of the nonuniform Doppler shift associated with the dominant signal. In addition, it shows that equalization designed to compensate for Doppler-induced phase shifts in the multipath signals leads to improved demodulation performance. The latter signal processing is based on a digital phase-locked loop applied to each feedback filter tap in a decision feedback equalizer (FBDPLL-DFE). Therefore, both equalization techniques can be easily integrated into conventional underwater acoustic communication systems. Work supported by JSPS KAKENHI Grant No. JP24K17463.
Deguchi et al. (Wed,) studied this question.