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Ref.: PhD Thesis, University of Algarve, August 2017
Abstract:
Advances in underwater acoustic communications require the development of methods to
accurately compensate channels that are prone to severe double spreading of time-varying
multipath propagation, fading and signal phase variations. Assuming the environmental
information as a key issue, this work aims to improve communications performance of
single-input-multiple-output links in such channels through the enhancement of their
estimates used for equalization. The acoustic propagation physical parameters of the
environment between the source and the receivers are considered in the process. The approach is
to mitigate noise effects in channel identification for Passive Time-Reversal (PTR), which
is a low complexity probe-based refocusing technique to reduce time spreading and inter-symbol
interference. The method Environmental-based PTR (EPTR) is proposed that,
inspired by matched field inversion, inserts physics of acoustic propagation in the channel
compensation procedure through ray trace modeling and environmental focalization
processing. The focalization is the process of tweaking the environmental parameters to
obtain a noise-free numerical model generated channel response that best matches the
observed data. The EPTR performance is tested and compared to the pulse-compressed
PTR and to the regularized `1-norm PTR. The former is based on classical `2-norm chan-
nel estimation and the latter, inspired by compressive sensing, uses weighted `1-norm into
the `2-norm estimation problem to obtain improved estimates of sparse channels. Exper-
imental results succeed with the proposed method for signals containing image messages
transmitted at 4 kbits/sec from a source to a 16-hydrophones vertical array at 890 m
range during the UAN'11 experiment conducted off the coast of Trondheim (Norway).
The scientific contributions of this work are (i) the understanding obtained on the process
of employing physical modeling and environmental focalization to equalize and retrieve
received messages in underwater acoustic communications, thus exploiting the sensitivity
of environmental parameters in order to adapt a communications system to the scenario
where it is used; and (ii) the presentation of a new PTR-based method that focuses
environmental parameters to model suitable noise-free channel responses for equalization and
whose real data results succeeded for a set of coherent signals collected in experiment at
sea. The proposed method contributes in a step forward to better understand how to insert
physical knowledge of the environment for equalization in digital underwater acoustic
communications.
Keywords: Underwater acoustic communications, passive time reversal, equalization,
coherent communications, physical modelling, environmental focalization.