Application of adaptive digital down-conversion to underwater acoustic communication with nonuniform Doppler shift

In underwater acoustic communication, nonuniform Doppler shift can severely degrade demodulation performance. Nonuniform Doppler shift causes nonuniform phase shifts and variation of received time. In recent studies, a new type of processing was proposed to suppress the effects of the nonuniform time variation. In this paper, we show mathematically how the nonuniform time variation affects communication in conventional demodulation, and describe how the proposed processing works against the time variation. Additionally, we use a simulation to investigate how the proposed processing improves demodulation performance. The simulation is carried out under numerous motional conditions at various symbol rates. The results clearly show the improvement by the proposed processing and clarify the conditions under which the new processing improves performance. (C) 2020 The Japan Society of Applied Physics