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1

Novel equalizer for shallow water acoustic communications

Yoon Y., Zielinski A.

Hydroacoustics

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2000

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Vol. 3

185--190

EN

The signal transmitted through an underwater acoustic channel is corrupted by multipath propagation caused by surface and bottom reflections. This delay spread causes intersymbol interference (ISI). The surface waves produced by wind affect the signal delay spread at the receiver. The delay spread is large for a calm sea and decreases with rough sea agitated by winds. To compensate for distortion introduced by the multipath channel, adaptive equalizers have been utilized to improve the performance of a communication system. These equalizers have been designed to handle the worst case of a signal delay spread condition associated with a calm sea and have a suitable and fixed number of taps. However, it is known that the power consumed by the processor increases with the number of computations performed per time unit. In this paper, we propose a novel equalizer with a variable number of taps which adaptively changes depending on channel conditions in order to conserve power. The proposed equalizer was tested by computer simulations using a model of an underwater acoustic channel. Results indicate savings in computational load up to 54% for a selected case. Power savings were also obtained when a directional receiver is used together with the proposed equalizer. Lower power consumption is particularly desirable for battery operated systems.

2

Design considerations for shallow water acoustic communication systems

Yoon Y., Zielinski A.

Hydroacoustics

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1999

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Vol. 2

179--184

EN

Relevant characteristics of the underwater channel used for acoustic communications are briefly described. There is a trade-off between achievable transmission range and data throughput. Transmission range of several kilometers with carrier frequency of 50kHz and several tens of kilometers with frequency less than 10 kHz might be possible using only 75 watts of acoustic power. Also, we investigated the required acoustic power for certain transmission ranges at given signal-to-noise ratio (SNR) values and the effect of wind speed. A shallow water channel model is proposed to study signal attenuation and arriving angles of the multipath. As the signal time delay increases, the arriving angle of the reflected signals becomes larger, allowing us to limit the number of multipath signals using a directional receiver. We will see that using directional receiver is better suited for a channel with a small range-to-depth ratio (RDR) and that equalization methods are better suited for a channel with a large RDR. Finally hardware complexity for designing shallow water acoustic communication systems is studied using a currently available digital signal processing (DSP) technology.

3

Coherent underwater acoustic communications - A review

Yoon Y., Zielinski A.

Hydroacoustics

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1997

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Vol. 1

279--284

EN

Digital communications through the underwater acoustic channel has been an active area of research in recent years. Applications include data transmission from bottom instrumentation, control of autonomous underwater vehicles (AUVs), digital voice and video transmission, etc. The effects of multipath propagation, Doppler frequency shifts due to relative motion of transmitter and receiver, and channel time and space variability which cause intersymbol interference and phase fluctuations of signals impose unique requirements for system design. Most research has been focused on the development of algorithms to cope with intersymbol interference and phase fluctuations. Development of coherent communication systems has improved bandwidth efficiency and reliability. In this paper, the trends and results of recent research on underwater communications, including channel models, equalization, diversity and synchronization, are reviewed. Some of our own research results are presented to illustrate the feasibility and effectiveness of proposed transmission schemes.