The remotely powered underwater acoustic sensor networks (RPUASN) paradigm is introduced, whereby sensor nodes harvest and store the power supplied by an external acoustic source, indefinitely extending their lifetime. Necessary source characteristics are determined. Feasibility is illustrated with realistic examples, and open research issues are pointed out. Performance of RPUASN is directly related to the sensing coverage and communication connectivity over the field the sensor nodes are deployed. The required number of RPUASN nodes and the volume which is guaranteed to be covered by the nodes are analyzed in terms of electrical power, range, directivity, and transmission frequency of the external acoustic source, and node power requirements. Index Terms-Power harvesting, sensing coverage, underwater acoustic sensor networks (UASN), wireless sensor networks.

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We study the performances of several computationally efficient and simple techniques for estimating direction of arrival (DOA) of an underwater acoustic source using a single acoustic vector sensor (AVS) in shallow water. Underwater AVS is a compact device, which consists of one hydrophone and three accelerometers in a packaged form, measuring scalar pressure and three-dimensional acceleration simultaneously at a single position. A very controlled experimental setup is prepared to test how well-known techniques, namely, arctan-based, intensity-based, time domain beamforming, and frequency domain beamforming methods, perform in estimating DOA of a source in different circumstances. Experimental results reveal that for almost all cases beamforming techniques perform best. Moreover, arctan-based method, which is the simplest of all, provides satisfactory results for practical purposes.

Wireless sensor networks (WSN) are event-based systems based on the collaboration of several microsensor nodes. Due to the limited supply of energy at sensor nodes, energyefficient configuration of WSN has become a major design goal to improve the lifetime of the network. Many clustering algorithms have been proposed as energy-efficient, however, the existing classical pre-event clustering solutions form clusters in the entire network unnecessarily that brings significant overheads in maintaining the network configuration. Unlike preevent clustering, energy-efficient operation of WSN requires the event-to-sink directed clustering notion, which forms clusters when and where they are needed and in the direction of data flow from event location to the sink. To the best of our knowledge, energy-efficient clustering in WSN has not been studied from this perspective before. In this paper, we propose a novel Eventto-Sink Directed Clustering (ESDC) protocol for WSN. ESDC realizes energy efficiency in sensor network configuration by employing two techniques: (1) clustering of the nodes only within the event-to-sink data flow corridor to avoid unnecessary cluster formation, (2) directional clustering to minimize the number of hops for data forwarding. The directional clustering process in ESDC also sets up the routing path of the event flows over the clusters. Performance results reveal that the ESDC protocol achieves the energy-efficiency objectives and outperforms the existing conventional pre-event clustering approaches.

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