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Ref.: submitted to the Journal of the Acoustical Society of America.
Abstract: Passive time reversal is one of the variants of
time reversal applicable to digital underwater communications. In
passive time reversal a probe-signal is transmitted ahead of the
data-signal in order to estimate the channel impulse response for later
use as a replica signal in a time reversal mirror fashion. In practice
the received probesignal must be captured in a time window and, after
correlation with the transmitted probe-signal, give a noisy estimation
of the channel impulse response. Therefore, the output signal to noise
ratio (SNR) and the detection rate of passive time reversal will
strongly depend of the starting time and on the duration of such time
window. Typically the beginning and the duration of that time window
should depend on the transit time and the dispersion of the acoustic
channel. Heuristic reasoning would suggest that if a short time window
fails to include all significant multipath it will result in an
imperfect focusing, while a too long time window will reduce the
efficiency of the communication system by introducing additional noise
in the passive time reversal system. That problem clearly calls for an
optimization. In order to bring the time reversal capabilities to a
practical modem the time window automatic optimization engineering
problem must be solved. In this paper, the maximization of the passive
time reversal output SNR relative to the probe time window was obtained
in a closed form. Theoretical results are found to be in full
coincidence with simulations and with results obtained on experimental
data taken during the INTIFANTE’00 sea trial.