Power Eficient Target Coverage in Wireless Sensor Networks

A Wireless Sensor Network (WSN) can be used in a variety of applications, such as in environmental monitoring and in battlefield surveillance in military applications (Akyildiz et al., 2002). A WSN consists of hundreds or thousands of sensors and, depending on the application, the node deployment and placement can be realised either in a deterministic way or randomly. In hostile environments, for example, sensors may be dropped from an aeroplane, resulting in a random placement, where the likely node density requirements cannot be guaranteed; some areas may contain more sensors than others. Each sensor can collect data by monitoring a usually small area that it is in its sensing range. We say that the sensor provides coverage to this area. A sensor collects data periodically or continuously depending on the nature of the application and forwards the data to a node called the Base Station (BS) or sink which provides the necessary connections to infrastructure networking. A sensor node is equippedwith a radio device that supports connectivity between two nodes or between a node and the BS. One of the fundamental problems in WSNs is the coverage of the targets in conjunction with energy efficiency constraints. The problem of coverage in wireless sensor networks has been studied from many different aspects. In (Li et al., 2003; Megerian et al., 2005), the coverage problem is described as a quality of service problem, where the objective is to find how well, in terms of the quality of monitoring data, the field is monitored by the sensors. In (Berman et al., 2004; Cardei & Du, 2005; Cardei, Thai, Li & Wu, 2005; Slijepcevic & Potkonjak, 2001; Zhang & Hou, 2005; Zorbas et al., 2007), the problem is formulated as the maximisation of the network lifetime under the area or target coverage constraint. In the former formulation (see Figure 1left), the whole area (e.g. a big square region) must be monitored by the sensors, while in the latter the sensors must cover a set of points (targets) lying in the field (see Figure 1right). (Berman et al., 2004; Slijepcevic & Potkonjak, 2001; Wang & Kulkarni, 2008; Zhang & Hou, 2005; Zhong et al., 2002) deal with the area coverage problem. This chapter focuses on the target coverage problem, but it often refers to important works about other types of coverage that can help in the solution of the target coverage problem. The most important challenge in a WSN is to efficiently manage the battery consumption of the sensors, sinceWSNs are characterized by limited energy resources and low computational capabilities. Managing the energy consumption in an efficient way can lead to an extension of the total network lifetime. In the case of the deterministic node placement this is translated as an optimal deployment of a set of sensors, where all the targets are covered. When the sensors 15