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Ref.: PhD Thesis, IST - Technical University of Lisboa, March 2007
Abstract:
The present thesis aims at the development of an environmental-based equalizer for shallow
water coherent communications. In recent years time-reversal aroused as a viable option
for underwater communications since its focusing property allows for a significant signal to
noise ratio enhancement and inter-symbolic interference reduction. In order to use time-
reversal in an operational modem the main drawbacks were identified as the performance
loss due to the source-vertical-line-array geometric mismatch (i.e. source-array relative
range and depth variations) during the data transmission and the optimization concerning
the multipath spread of underwater channel impulse responses in a noisy environment.
For the time-reversal environmental geometric mismatch compensation a physical model
based on waveguide invariants of the acoustic channel was developed. It makes use of the
frequency/range invariant and of the frequency/depth invariant. With such a physical-
model in hand an environmental-based equalizer was developed. The multipath spread that
guarantees the maximum of the signal to noise ratio is given
by the time-reversal overall impulse response maximum power that can be computed
using channel impulse response estimates. Such optimum signal to noise ratio results in a
suboptimum inter-symbolic interference compensation with, however, values close to the
optimum. In parallel with the scientific objectives, the development of a surface buoy prototype -
the Acoustic Oceanographic Buoy (AOB) - was carried out. The AOB is an advanced
sonobuoy with a long and dense acoustic/oceanographic vertical-line-array and with
additional processing capabilities. The AOB was tested in six sea trials where its telemetry
capabilities where successfully proven, and was used to acquire the real data used to test
the developed environmental-based equalizer.
The time-reversal optimization concerning the multipath spread was validated with real
data at 400 and 2000 bits per second, as well as the time-reversal environmental-base
equalizer that showed a mean squared error gain up to 5.5 dB over the non equalized
time-reversal data.
Keywords: Underwater acoustic communication, matched field processing, time-reversal,
waveguide invariants, environmental-based equalizer.