A basic research on the long horizontal active time reversal communication

We have researched on the application of time reversal waves to the underwater acoustic technology. Time reversal waves enable a desired signal to converge to the focus, even in unknown and inhomogeneous media. As time reversal waves converge to a "point" at any range, their focusing property is quite different from that of the conventional beam forming method. In this work, the results of the basic research on active time reversal communication are described. The "active" implies that time reversal waves are transmitted in practice in the ocean, and implies the communication from an array to a point. By using time reversal, the reflected and refracted waves are converged to the focus, the signal level received at the focus increased and the symbol interferences are reduced. However, in order to ensure the communication channel only with time reversal, it is necessary to install many transducers of time reversal array. So, in this study, it is proposed to combine the focusing effect of time reversal waves and conventional adaptive filter. After most of the multipath waves are utilized by using time reversal with a sparse array, unfocused waves, which appear like "noise", are removed by adaptive filter. The simulations in shallow water are performed with various ranges. It is concluded that this proposed method makes high speed communication to the long distance possible.