Time-reversal communication in deep ocean — Results of recent experiments

We have conducted the basic research on long horizontal communication using time reversal in the deep ocean, which will be applied for communicating with an autonomous underwater vehicle (AUV) in the future. To achieve such communication, time reversal is an attractive solution. Because intersymbol interferences (ISI) are removed and signal noise ratio is improved. In our previous studies, a method of combining time reversal and adaptive equalization was proposed. To confirm the effectiveness of the method, at-sea experiments were executed in these years. In our latest trial, we executed three kinds of experiments of passive time-reversal communication as follow. First, the experiment at the range of 1,000 km was performed. This is the first implementation of time-reversal communication at such a range. As results, demodulation could be achieved well by the proposed method. The second was the experiment of the communication from the source near the sea-bottom to the receiver at the axis of the SOFAR channel at ranges up to 180 km. The results show that it is possible to realize communication in transmitting from outside the SOFAR channel. The third is the experiment in which the two sources transmit signals simultaneously, that is, a kind of experiment of multi-user communication. By the simple frequency division multiplexing, it was demonstrated that communication from the two sources were accomplished. In this paper, the results in these experiments are described.