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Adaptive spatial combining for passive time-reversed communications

J.P. Gomes,
Institute for Systems and Robotics, IST
Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal
A. Silva,
S.M. Jesus
Institute for Systems and Robotics, University of Algarve
Campus de Gambelas, PT-8005-139 Faro, Portugal

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Ref.: Journal of the Acoustical Society of America, 124(2), pp. 1038-1053, August.

Abstract: Passive time reversal has aroused considerable interest in underwater communications as a computationally inexpensive means of mitigating the intersymbol interference introduced by the channel using a receiver array. In this paper the basic technique is extended by adaptively weighting sensor contributions to partially compensate for degraded focusing due to mismatch between the assumed and actual medium impulse responses. Two algorithms are proposed, one of which restores constructive interference between sensors, and the other one minimizes the output residual as in widely-used equalization schemes. These are compared with plain time reversal and variants that employ post-equalization and channel tracking. They are shown to improve the residual error and temporal stability of basic time reversal with very little added complexity. Results are presented for data collected in a passive time-reversal experiment that was conducted during the MREA’04 sea trial. In that experiment a single acoustic projector generated a 2/4-PSK stream at 200/400 baud, modulated at 3600 Hz, and received at a range of about 2Km on a sparse vertical array with 8 hydrophones. The data were found to exhibit significant Doppler scaling, and a resampling-based preprocessing method is also proposed here to compensate it.

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.