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).