Adaptive Digital Down-Conversion to Compensate Uniform and Nonuniform Doppler Shifts and to Reduce Processing Delay

In underwater acoustic communication, Doppler shifts can cause changes in the received time of signals and frequency shifts due to slow propagation speed. Specifically, uniform Doppler shifts cause a constant deviation in the interval to be downsampled in addition to a carrier frequency offset. In vertical communication with a small vessel, the roll, pitch and heave motions cause the time-varying Doppler shifts with periods of several seconds. In that case, the uniform Doppler shift estimation methods using only a preamble possibly yields a result different from that throughout the signal because it provides a point estimate of the Doppler scale. On the other hand, methods using the measured signal length requires that demodulation procedures begin after the postamble has been received. Recently, adaptive downsampled time modification processes have been proposed to track temporal changes caused by nonuniform Doppler shifts. It adaptively adjusts the downsampling time based on estimates of phase shifts caused by these Doppler shifts. In this study, it was confirmed that the adaptive digital down-conversion, which is one of the adaptive processing methods, can compensate for temporal changes caused by the uniform as well as nonuniform Doppler shifts in demodulation without the signal length measurement. This findings indicate the potential of adaptive methods to reduce the processing delay by beginning demodulation without the postamble detection.