JASA logo

Time reversal optimization for underwater communications

A. Silva, asilva@ualg.pt
S.M. Jesus
, sjesus@ualg.pt
Institute for Systems and Robotics, University of Algarve
Campus de Gambelas, PT-8005-139 Faro, Portugal
J.P. Gomes,
jpg@isr.ist.utl.pt
Institute for Systems and Robotics, IST
Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal

Comments: download file (pdf )
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.

ACKNOWLEDGMENT: The authors would like to thank the NATO Undersea Research Centre (NURC) for the loan of the acoustic sound source, the support of Enrico Muzzi and the NRP D. Carlos I crew during the INTIFANTE’00 sea trial. This work was financed by FCT, Portugal, under NUACE project, contract POSI/CPS/47824/2002, ATOMS project, contract POCTI/P/MAR/15296/1999, and the Portuguese Ministery of Defense under the LOCAPASS project.