Underwater Acoustic Communication Using a Time-Reversal Mirror Approach

A. Silva asilva@ualg.pt
EST - Universidade do Algarve, Faro, Portugal
J.P. Gomes jpg@isr.ist.utl.pt
ISR - Instituto Superior Tecnico, Lisboa,Portugal
S.M. Jesus sjesus@ualg.pt
UCEH - Universidade do Algarve, Faro, Portugal
V. Barroso vab@isr.ist.utl.pt
ISR - Instituto Superior Tecnico, Lisboa,Portugal

Comments: download pdf file .
Ref.: Proc. ECUA 2000, (ISBN 92-828-9530-0), pp. 531-536, Lyon, France, June 2000.

Abstract : The communication between an underwater autonomous vehicle (AUV) and a fixed station (FS), generally requires two kinds of links: a low data rate link, to send/receive commands/status from/to FS and the AUV, and a high data rate link to send data from the AUV to the FS. The former is generally implemented with a robust incoherent modulation technique, while the later requires the use of coherent modulation, a multi-receiver array and a much higher requirement for online computation signal processing. The time-reversal mirror (TRM) principle has recently been shown to provide a robust temporal and spatial refocus in the original source position. Spatial refocus means, in terms of underwater communications, that the TRM undoes the channel multipath, which is mostly significant for reducing the duration of the channel impulse response and therefore significantly reducing the signal ISI. Reducing signal ISI implies smaller (and faster) signal equalizers what, in turn, allows for higher data rates. This paper introduces a ''virtual TRM'' implemented as the electronic TRM of the channel response to a pulse shape narrowband impulse sent from the AUV prior to establishing communication. The output of the virtual TRM's for the all receiving array then serves as input to an adaptive equalizer. Since there is a time lag between the channel response measurement and the actual message, the question that is addressed in this paper is whether a source/receiver position mismatch can impact on the coherence of the virtual TRM for multipath recombination. Or, in other words,  until which degree of realistic mismatch is it worth use such a virtual-TRM for high data rate links. Computer simulations, using a normal mode range-dependent propagation model in a real shallow water scenario, show that, even in presence of high incertitude  about AUV and the hydrophone's relative position, the virtual mirror can strongly reduce multipath structure, allowing for the use of an equaliser with a reduced number of coefficients. In practical terms system's performance can be quantified by the much faster convergence rate of such equalizer when compared to that without the virtual TRM combiner.

ACKNOWLEDGMENT: this work was partially supported by project ATOMS from FCT (Portugal).