| Summary: | 博士 === 淡江大學 === 資訊工程學系博士班 === 96 === A wireless sensor network (WSN) consists of numbers of sensors deployed in a sensing field in an ad hoc or prearranged fashion for the purposes of sensing, monitoring, or tracking environmental events. Unlike ad hoc networks, a WSN is application-specific, data-centric, and energy-constrained in essence. With the limitation in battery energy, all sensors have to cooperatively work to cover the sensing field of the interest. Thus, to develop efficient schemes beneficial to coverage but less energy waste is very important in WSNs. Basically, the coverage preserving protocols nowadays are proposed for area or target coverage. The impacts of the network connectivity on coverage preserving are not carefully considered. Clearly, even though one scheme can obtain maximal sensing coverage, without ensuring the connectivity of sensors to the sink, it is also useless. Therefore, in this dissertation, the coverage preserving issues for area and target coverage in more complicated network condition are discussed.
Recently, mobile sensors have been widely used in a variety of applications in WSNs to achieve the requirement of network coverage. Based on the mobile capability of the sensor, the dissertation proposes a distributed partition avoidance lazy movement (PALM) protocol for mobile sensor networks (MSNs). As mentioned above, connectivity and coverage are two major factors to the success of a sensor network. Therefore, PALM takes both connectivity and coverage into account to avoid network partition and keep high sensing quality. Since sensor movement is the major source of energy consumption, thus, in order not to cause frequent movement, PALM triggers sensor movement only when the network has a risk of partition, but not when coverage holes appear. The dissertation proposes a sufficient condition of keeping a network connected. Based on the condition, PALM adopts the lazy movement policy for a sensor to determine when to move and uses the principles of an effective movement for a sensor to decide where to move. Accordingly, PALM can keep the network connected and can make the effective coverage as large as possible to maintain high sensing quality.
On the other hand, the connected target coverage (CTC) problem in wireless heterogeneous sensor networks (WHSNs) with multiple sensing units, termed MU-CTC problem is considered in the dissertation. MU-CTC problem is firstly reduced to a connected set cover problem and further formulated as integer linear programming (ILP) constraints. However, the ILP problem is an NP-complete problem. Therefore, two heuristic but distributed schemes, REFS (Remaining Energy First Scheme) and EEFS (Energy Efficient First Scheme,) are proposed. In REFS, each sensor considers its remaining energy and neighbors'' decisions to enable its sensing units and communication unit such that all targets can be covered by required attributes, and the sensed data can be delivered to the sink. The advantages of REFS are its simplicity and less communication overhead incurred. However, to utilize sensors'' energy efficiently, EEFS is proposed as well. A sensor in EEFS considers its contribution to the coverage and the connectivity to make a better decision. To our best knowledge, this dissertation is the first one to consider target coverage and connectivity jointly for WHSNs with multiple sensing units.
In general, the issues mentioned above are actually essential and important in both area and target coverage. Overall, the protocols to avoid network partition and to schedule sensor’s activity are respectively proposed for the mobile sensor network and the heterogeneous stationary sensor network with multiple sensing units. In comparison with the related work, PALM can reduce the energy consumption and further extend the network lifetime in mobile sensor network due to the lazy movement policy and the principles of an effective movement. Simulation results verify the advantages of the proposed protocol. For the MU-CTC problem, simulation results show that REFS and EEFS can prolong the network lifetime effectively. Furthermore, EEFS outperforms against REFS in network lifetime.
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