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Environmental-based underwater communications

António J. Silva, asilva(at)
Institute for Systems and Robotics, University of Algarve,
8005-139 Faro, Portugal.

Comments: download file (pdf)
Ref.: PhD Thesis, IST - Technical University of Lisboa, March 2007

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.