Coverage And Energy Conservation Problems In Wireless Sensor Networks

Sensor networks have emerged as promising platforms for many applications, such asenvironmental monitoring, battlefield surveillance, and health care. A sensor network mayconsist of a large number of small sensor nodes that are composed of sensing, processing andcommunicating components. On one hand, as sensors may be distributed arbitrarily, one ofthe fundamental issues in wireless sensor networks is the coverage problem. The coverageof a sensor network, represents how well the interested region or targets are monitored. Onthe other hand, power conservation is another important issue in wireless sensor networksdue to the following reasons. First, most sensor nodes have limited power sources and arenon-rechargeable. Also, the batteries of the sensor nodes are hard to replace due to hostileor inaccessible environments in many scenarios. In this dissertation, we mainly consider thefollowing problems.Firstly, we define and solve the problem of finding a cover set, called directional coverset problem (DCS), in target-oriented directional sensor networks. Unlike convectionalomni-directional sensors that always have an omni-angle of sensing range, directional sen-sors may have a limited angle of sensing range due to technical constraints or cost consid-erations. A directional sensor network consists of a number of directional sensors, whichcan switch to several directions to extend their sensing ability to cover all the targets in agiven area. Because a directional sensor has a smaller angle of sensing range than an omni-directional sensor or even does not cover any target when it is deployed, we need to schedulesensors in the network to face to certain directions to cover all the targets. The DCS is tofind a cover set that is a subset of directions of the sensors, in which the directions cover allthe targets. We prove the DCS to be NP-complete and propose two algorithms for the DCS.We also prove the correctness of the proposed algorithms. Simulation results are presentedto demonstrate the performance of these algorithms. Secondly, we define and solve the problem of finding non-disjoint cover sets and al-locating the work time for each of them to maximize the network lifetime, called multipledirectional cover sets problem (MDCS), in target-oriented directional sensor networks. InMDCS, we organize the directions of sensors into non-disjoint subsets, each of which is acover set, and allocate the work time for each cover set. We alternately activate only onecover set at any time. When one cover set is activated, each sensor that has a direction in thiscover set is in the active state and works in this direction, while all the other sensors are inthe sleep state. We prove the MDCS to be NP-complete and propose several algorithms forthe MDCS. We compare the performance of these algorithms by simulations exhaustively.Thirdly, we propose a precise and energy-aware coverage control algorithm, namedarea-based collaborative sleeping (ACOS) algorithm, for area-oriented omni-directional sen-sor networks. This algorithm precisely controls the mode of sensors to maximize the areacoverage and minimize the energy consumption based on the net sensing area of a sensor.The area coverage of a sensor network is measured by the fraction of the region covered.The net sensing area of a sensor is the area of the region exclusively covered by the sensoritself. If the net sensing area of a sensor is less than a given threshold, the sensor will goto sleep. Collaboration is introduced to the algorithm to balance the energy consumptionamong sensors. Performance study shows that ACOS has better coverage of the surveillancearea while waking fewer sensors than other state-of-the-art sleeping algorithms.