Spatial diversity in passive time reversal communications

A time reversal mirror exploits spatial diversity to achieve spatial and temporal focusing, a useful property for communications in an environment with significant multipath. Taking advantage of spatial diversity involves using a number of receivers distributed in space. This paper presents the impact of spatial diversity in passive time reversal communications between a probe source (PS) and a vertical receive array using at-sea experimental data, while the PS is either fixed or moving at about 4knots. The performance of two different approaches is compared in terms of output signal-to-noise ratio versus the number of receiver elements: (1) time reversal alone and (2) time reversal combined with adaptive channel equalization. The time-varying channel response due to source motion requires an adaptive channel equalizer such that approach (2) outperforms approach (1) by up to 13dB as compared to 5dB for a fixed source case. Experimental results around 3kHz with a 1kHz bandwidth illustrate that as few as two or three receivers (i.e., 2 or 4m array aperture) can provide reasonable performance at ranges of 4.2 and 10km in 118m deep water.