Experimental demonstration of multiuser communication in deep water using time reversal.

Multiuser communication is demonstrated using experimental data (450-550 Hz) collected in deep water, south of Japan. The multiple users are spatially distributed either in depth or range while a 114-m long, 20-element vertical array (i.e., base station) is deployed to around the sound channel axis (~1000 m). First, signals received separately from ranges of 150 km and 180 km at various depths are combined asynchronously to generate multiuser communication sequences for subsequent processing, achieving an aggregate data rate of 300 bits/s for up to three users. Adaptive time reversal is employed to separate collided packets at the base station, followed by a single channel decision feedback equalizer. Then it is demonstrated that two users separated by 3 km in range at ~1000 m depth can transmit information simultaneously to the base station at ~500 km range with an aggregate data rate of 200 bits/s.