Vector sensors for underwater acoustic communications

Abstract

Acoustic vector sensors measure acoustic pressure and directional components separately. A claimed advantage of vector sensors over pressure-only arrays is the directional informa- tion in a collocated device, making it an attractive option for size-restricted applications. The employment of vector sensors as a receiver for underwater communications is rela- tively new, where the inherent directionality, usually related to particle velocity, is used for signal-to-noise gain and intersymbol interference mitigation. The fundamental ques- tion is how to use vector sensor directional components to benefit communications, which this work seeks to answer and to which it contributes by performing: analysis of acoustic pressure and particle velocity components; comparison of vector sensor receiver structures exploring beamforming and diversity; quantification of adapted receiver structures in dis- tinct acoustic scenarios and using different types of vector sensors. Analytic expressions are shown for pressure and particle velocity channels, revealing extreme cases of correla- tion between vector sensors’ components. Based on the correlation hypothesis, receiver structures are tested with simulated and experimental data. In a first approach, called vector sensor passive time-reversal, we take advantage of the channel diversity provided by the inherent directivity of vector sensors’ components. In a second approach named vector sensor beam steering, pressure and particle velocity components are combined, re- sulting in a steered beam for a specific direction. At last, a joint beam steering and passive time-reversal is proposed, adapted for vector sensors. Tested with two distinct experimental datasets, where vector sensors are either positioned on the bottom or tied to a vessel, a broad performance comparison shows the potential of each receiver struc- ture. Analysis of results suggests that the beam steering structure is preferable for shorter source-receiver ranges, whereas the passive time-reversal is preferable for longer ranges. Results show that the joint beam steering and passive time-reversal is the best option to reduce communication error with robustness along the range.