Asymmetric underwater acoustic communication using a time-reversal mirror

Phase conjugation using a time-reversal mirror is an array processing technique that allows accurate focusing of waves in poorly characterized media. When used for underwater communication, phase conjugation requires sending a probe signal from the desired focal point to the mirror, providing local templates for subsequent waveform generation that creates signals with low intersymbol interference at the focus. In spite of the simplicity of this method, and the potential reduction of receiver complexity that it can afford, frequent transmission of probes is undesirable because it forces the channel to remain almost idle during relatively long periods. In this paper, pulse shapes are estimated from incoming data blocks received at the mirror, thus eliminating the need for explicit channel probes. If a phase-conjugate array is used for bidirectional communication, pulse estimation can be an integral part of multichannel equalization algorithms, and its use in transmit mode entails virtually no increase in computational requirements. Simulation results illustrate significant performance improvements at low signal to noise ratios relative to direct pulse measurement, as averaging in estimation algorithms greatly limits the impact of noise. Formatting techniques that allow the mirror to change the data rate and pulse shape of the transmitted signal are also considered.