F.A. Bozzi fabozzi(at)ualg.pt
ans
S.M. Jesus sjesus(at)ualg.pt
LarSys, Universidade do Algarve, Campus de Gambelas, PT-8005-139 Faro, Portugal.
Comments: download pdf.
Ref.: in Proceedings of Meetings on Acoustics (POMA), Vol.44(1), 070011, October 2021.
Abstract
The performance of a vector sensor array (VSA) receiving communication signals in
the Makai experiment is quantified. Makai experiment was a four-week field
experiment, which took place off the coast of Kauai Island, Hawai, in 2005. The
data analyzed here refers to the communication test carried out on September
23rd. The VSA is composed of four accelerometer-based vector sensors, which was
tied to a drifting research vessel in a shallow water area. A moored bottom source
was used to transmit signals from 8k to 14kHz frequency range.
The present study explores the VS beam steering method. In this method, pressure
and particle velocity channels are weighted-combined leading to directional gain.
DoA estimation uses the Minimum Variance Distortionless Response (MVDR)
beamforming as a pre-processing for the VS beam steering. Noise normalization
uses the Maximum Likelihood Estimation (MLE) for denoising. Then, the
communications chain for coherent Phase Shift keying (PSK) modulation is
composed of synchronization and Doppler compensation, and a single Decision
Feedback Equalizer. Pressure and particle velocity components of VS have been
used in beamforming or diversity techniques. These techniques act somehow in
opposite ways, depending on the channels cross-correlation assumption. In this
work, a beamforming technique for communications is addressed.
Communication results are analyzed by the bit error rate (BER) for the pressure-
only array, a single VS, and the VSA. The ranges analyzed (230m and 910m) present
delay-Doppler spread that can achieve 20 symbol intervals and 10Hz. These
characteristics make synchronization and Doppler tracking a challenge in coherent
modulation. Thus, velocity channels are used for improvement. Bearing fluctuation
issue is noticed in the operating frequency. Fluctuation of 90 degrees is verified,
and its impact on demodulation is high-lighted. Denoising has shown benefits since
the VSA is under the ship noise and the channel-gain of the acquisition system is
unknown. The study shows that even with the bearing fluctuation, a single VS can
estimate the source direction with an accuracy of 50 degrees. It is shown that a
single VS provides similar communication performance comparing to four pressure
sensors. The BER for the shorter range varies from 1% to 5%, depending on the
number of sensors used. For the long-range, BER does not achieve values lower
than 9%. It is noticed that in a multipath environment, steering to the DoA may not
lead to the lowest error for communications.