Spatial diversity in passive time reversal communications

Abstract

A time reversal mirror exploits spatial diversity to achieve spatial and temporal focusing, a useful property for communications in an environment with significant multipath. This paper presents the impact of spatial diversity on passive time reversal communications between a single probe source and a vertical receive array using at-sea experimental data, while the probe source is either fixed or moving at about 4 knots. The performances of two different approaches are compared: (1) time reversal alone and (2) time reversal combined with adaptive channel equalization. In the presence of source motion, the motion-induced Doppler shift is coarsely estimated using a decision-feedback phase-locked loop with a training sequence and then the received time series is resampled prior to the demodulation process. The time-varying channel responses due to source motion require an adaptive channel equalizer such that time reversal combined with the equalizer outperforms time reversal alone by up to 13 dB as compared to 5 dB for a fixed source case. The experimental results around 3 kHz with a 1-kHz bandwidth illustrate that even two or three receivers (i.e., 2- or 4-m aperture) can provide resonable performance at 4.2- and 10-km ranges in a 118-m deep water.