Spatial Focusing and Intersymbol Interference in Multiple-Input-Single-Output Time Reversal Communication Systems

In this paper, we study a multiple-input-single-output (MISO) underwater communication system that applies time reversal (TR) to transmit signals so that they focus spatially and compress temporally on the intended receiver. Our simulations model an underwater acoustic channel as a waveguide, and we investigate the cases of a waveguide both with and without random inhomogeneities. We investigate physical TR metrics and communications related performance indicators. The results of our simulations show that spatial focusing depends strongly on the delay spread (DS), as has been seen in experiments. This physical property of TR could be exploited in communication systems where signal coherence is desired only at the receiver location. However, in the simulations, we find that while spatial compression increases with DS in a robust way (i.e., even when inhomogeneities exist), time compression does not increase with DS. Moreover, physical measures of the temporal compression (temporal pealk-to-sidelobe ratio) do not improve with waveguide inhomogeneities. Nevertheless, TR reduces intersymbol interference (ISI) at the receiver as DS increases for both types of waveguides, which is an important effect for efficient, high-speed communication. In addition to TR, preequalization at the transmitter can ideally eliminate ISI without significantly affecting spatial compression. However, this preequalization causes a reduction of received power, which may be acceptable when the signal-to-noise ratio (SNR) at the receiver is high.