Estimate of Passive Time Reversal Communication Performance in Shallow Water

Time reversal processes have been used to improve communication performance in the severe underwater communication environment characterized by significant multipath channels by reducing inter-symbol interference and increasing signal-to-noise ratio. In general, the performance of the time reversal is strongly related to the behavior of the q-function, which is estimated by a sum of the autocorrelation of the channel impulse response for each channel in the receiver array. The q-function depends on the complexity of the communication channel, the number of channel elements and their spacing. A q-function with a high side-lobe level and a main-lobe width wider than the symbol duration creates a residual ISI (inter-symbol interference), which makes communication difficult even after time reversal is applied. In this paper, we propose a new parameter, Eq, to describe the performance of time reversal communication. Eq is an estimate of how much of the q-function lies within one symbol duration. The values of Eq were estimated using communication data acquired at two different sites: one in which the sound speed ratio of sediment to water was less than unity and one where the ratio was higher than unity. Finally, the parameter Eq was compared to the bit error rate and the output signal-to-noise ratio obtained after the time reversal operation. The results show that these parameters are strongly correlated to the parameter Eq.