EurOCEAN 2000 - Vlaams Instituut voor de Zee

PROJECT ACHIEVEMENTS
SHALLOW WATER ACOUSTIC CHANNEL MODELING
As acoustic vectors for communication systems, shallow seas are particularly complex
channels, because of unavoidable multiple interactions with rough boundaries: the corrugated
sea-bed, and the moving rough water-to-air surface; the resulting signals are corrupted by the
phenomenon classically called ″fading″, in a way that seems unfortunately more complex than
in classical fading channels (like Hertzian waves in the lower atmosphere) : the random fading
part of the response is quite generally combined, particularly for short propagation ranges (less
than a few km’s), with more stable contributions, that travel directly from source to receiver,
with no surface interaction, and that may fluctuate, at their own different rate (generally
slower), due to other phenomena (movements of the source and receiver, tidal oscillations and
internal waves in the water column, etc.). Such channels can not be understood and analysed
within the classical frame of ″Rayleigh channels″; a minimal modelling has to be the far less
common ″Rician″ channel. Our “ambitious” objective is to derive an operational model for
such complex fading channels.
A version of the channel stochastic simulator was developed, finalised and distributed to the
partners for evaluation. This latest version include the modelling of spatial correlation between
different receivers at different locations, and gives the user the opportunity to generate the
signals that may be simultaneously collected on several transducers, i.e. to simulate arrays with
correct inter-correlation. The user is also given the opportunity to evaluate synthetic scales,
more grossly, but with a direct interest for quantifying the capabilities of channel, or for
comparing different configurations (correlation lengths or times, channel spreading, energetic
balance between random and deterministic parts of signals, etc.). The model was played in the
SWAN experimental configuration planned in May 2000 in order to produce some
recommendations for the sea trial concerning the receiving array geometry and the
environmental measurements to be carried out in order to bring adequate elements for model
validation.
Investigations of the ROBLINKS data collected during the common SWAN-ROBLINKS
experimental campaign of May 1999 displayed interesting fluctuations, which could be
interpreted as arising from an unexpected "direct" effects of the swell: in very shallow waters
(about 20.m), the variations of water height associated with swell or wind-waves produce
fluctuations in static pressure and horizontal water velocity, that generate fast horizontal
fluctuations of the sound-speed profile. A model was developed for evaluating the orders of
magnitude and observability of this unexpected phenomenon, negligible in most
configurations, but important in very shallow waters like those involved in the experiment and
in most coastal areas; a salient property of this effect is its very high sensibility on the angle
between the source-to-receiver axis and the propagation direction of swell.
BLIND AND TRAINED SIGNAL PROCESSING
1. Decision directed equalization
Multiple access communication has been an area of active research for Underwater Acoustics
(UWA) channels over the recent years. However, an UWA channel is characterised as a
multipath channel due to signal reflections from the surface and the bottom of the sea and also
volume scattering. Because of sea-wave motion, the signal multipath components undergo
time-varying propagation delays that result in signal fading. In addition, there is frequency
dependent attenuation, which is approximately proportional to the square of the signal
frequency. These result in the effect of inter symbol interference (ISI) in the received stream.
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