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Ref.: Proc. OCEANS MTS/IEEE ' 2007,
(ISBN:), p., Vancouver, Canada, 2007. (to appear)
Abstract: In recent years active Time Reversal (TR) has
received particular attention from the
scientific community. After practical underwater demonstration of
its spatial-temporal
focusing capabilities several applications of active TR (aTR),
from tomography to
communications, have been suggested. The passive version of TR
(pTR) uses a receive
only array, and the source sends a probe signal ahead of the data,
for Impulse Response
(IR) channel estimation. Such IR estimate is then used as a
synthetic channel for
temporal focusing of the data signal, which is equivalent
to deconvolving the multipath
generated by the real channel.
When applied to underwater digital coherent communications, due to
the IR estimation
error and the time variability of the channel, the achieved TR
focus is not perfect
resulting in uncompensated intersymbol interference (ISI).
That problem is more
relevant when applied to communications with a moving source
and/or receiver. In such case it is intuitive that a rapid
degradation of the passive TR temporal focusing
capability will occur, due to the increased mismatch between
the assumed and the actual channel. In order to guarantee a
longer stability of the focal spot, threesolutions
with advantages and disadvantages are usually suggested:
one is to send probe signals
more frequently; another is to use a high complexity
adaptive algorithm to track the IR
from the initial probe signal IR estimation; and finaly a third
alternative is to use a
low complexity equalizer for residual uncompensated ISI. A
performance comparison
between those adaptive pTR versions is presented in [1].
In the present paper, a physics-based adaptive algorithm for
IR tracking is suggested.
Such adaptive algorithm is based on the waveguide invariant
properties of the shallow
water channel. The waveguide invariant property has been applied
to change the aTR
range focus in [2], and to interpret a model for performance
prediction of a time
reversal communication system [3]. In [4] the waveguide
invariant property was
extended to the compensation of geometric mismatch. This
novel waveguide invariant
property interpretation states that changes on geometric
characteristics of the
acoustic channel, such as source-receiver range, source depth
and array depth, can be
compensated by a frequency shift in the channel frequency
response. The Frequency Shift compensation of the pTR (FSpTR),
will guarantee a maximization of pTR output
power and that results in a minimization of the mean square
error between the detected
and the transmitted symbol sequences. That will result in
an underwater communications physics-based equalizer that is able
to detect the transmitted data sequence and to
simultaneously estimate the source-array range, source
depth and array depth.
The reliability of the physics-based waveguide invariant pTR
equalizer is demonstrated using experimental data obtained
during the MREA 2004 sea trial, where binary PSK signals at
a data rate of 400 bits per second were transmitted with a
carrier frequency of 3.6 kHz. Results obtained with the FSpTR,
after Doppler compensation of the received signals, shows a long
term compensation of the channel mismatch with the mean squared
error between the transmitted and the received data symbols
keeping stable up to a range mismatch of 40 m in presence of
source depth varying between 71.6 m and 72.3 m and an array depth
oscillation of approximately 0.63 m.
References
[1] J. Gomes, A. Silva, S.M. Jesus. Spatial combining for passive
time-reversed communications. Submitted to J. Acoust. Soc. Am.,
18 March 2007.
[2] H. C. Song, W. A. Kuperman, and W. S. Hodgkiss. Time-reversal
mirror with
variable range focusing. J. Acoust. Soc. Am., 103(6): 3234-3240, June
1998.
[3] D. Rouseff. Intersymbol interference in underwater acoustic
communications using time-reversal signal processing. J. Acoust.
Soc. Am., 117(2),
February 2005.
[4] A. Silva, S.M. Jesus, J. Gomes. Passive time-reversal
geometric mismatch
compensation by using waveguide invariant properties. To be
submitted to J. Acoust.
Soc. Am.