Toward continuous underwater acoustic communications

The paper addresses the technical challenges and approaches for semi-continuous underwater acoustic communications, i.e., transmission of long packets under different environmental conditions without periodic training. The packet length is normally constrained by the channel coherence time for a given bandwidth, beyond which channel tracking using an adaptive decision feedback equalizer becomes difficult. But if the channel is known for each block of coherence time, communications can proceed in principle continuously without time limitation. Using the iterative correlation-based equalizer where decision symbols are assumed to be the true symbols to estimate the channel condition, it is shown based on at-sea data that long packets of data can be transmitted with minimal errors even in harsh environments where the channel coherence time is very short (a fraction of the packet length).