Experimental Study of Acoustic Channel Reciprocity in the Shallow Ocean

The theoretical proof of channel reciprocity assumes static propagation medium and boundary conditions. Since the assumption makes little sense in the case of dynamic shallow-ocean environments, it brings up a question: how well does channel reciprocity hold in the shallow ocean? To answer this important question, a sea-going experiment was recently conducted to study the shallow-water channel reciprocity experimentally. In the experiment, two custom-built software-defined acoustic modems were deployed to probe the shared channel from two opposite directions. The channel reciprocity was evaluated by comparing the two probed channels directly. To minimize the effects of channel dynamics, a novel handshaking protocol was developed to synchronize the two-way channel probing in spite of the time asynchronism between the two modems. A set of handshaking signals was also designed to facilitate real-time channel probing while overcoming the limitations posed by half-duplex acoustic modems. Our experimental results have shown that while the shallow-ocean channels experienced in our study are not fully reciprocal in terms of channel impulse responses, they are partially reciprocal in terms of the arrival time of the dominant paths at a high probability. This article provides the details of this experimental study, including its design, setups, execution, and results.