Evaluating And Improving Coverage Performance For Wireless Sensor Networks

Wireless sensor network (WSN), as an integrated network which can perform information gathering, processing and delivering, can connect the real world and logistic information world. It is greatly changing the interaction between people and nature. There are wide potential applications for wireless sensor network, such as industry, agriculture, military affairs, environment monitoring, biomedicine, city managing and disaster succoring.After sensor nodes are deployed and a connected wireless network is formed by communicating with one another either directly or indirectly, in WSN there are two fundamental issues: 1) how to evaluate the coverage performance of monitoring region or an escapee; and 2) how to improve the coverage performance by changing the situations of mobile sensor nodes or deploying some additional nodes. They directly influence the energy-efficiency, topology-control, and process-ability which are all kernel terms in WSN. They also determine the improvement of the quality of service (QoS) of apperception, surveillance, sense and communication in WSN.The sensing model of a sensor is the foundation of coverage control. The most existing works are based on sensing model of circle or sector. However, the sensing area of a sensor node with different sensing capability in each direction is highly irregular in the real environment. The main reason is the impact of self-causation or environment, such as insufficient hardware calibration, the existence of obstacles and radio irregularity depending on the direction of propagation. Thus, the previous research results which can provide valuable high-level guidelines could cause wrong estimation or limited application of system performance in the real world.In this dissertation, according to the irregular sensing area of sensor nodes, we deeply research the following important problems: how to achieve the realistic sensing area of sensor node, and how to evaluate or improve the coverage performance of wireless sensor network for the monitoring region or an escapee. The following are the main research of the dissertation.Firstly, we survey the recent domestic and overseas research on coverage control for wireless sensor networks. And the challenging problems are considerably addressed. Wireless sensor networks are introduced briefly from history, architecture, applications and research contents. Based on the analysis about the correlative coverage problem, sensing model of sensor node and coverage taxonomy, we review the related works for the coverage problem. The recent novel and representative idea is summarized, and the significant protocols and algorithms of coverage problem are explored.Secondly, aimed at the sensing irregularity of sensor node, we study the problem how to how to achieve the realistic sensing area of sensor node. Assume the event on a position of monitoring region is detected by sensor, and then the position is in the sensing area of sensor. Based on this observation, we can obtain all detecting events of each sensor node, and conclude the sensing area of sensors by generating the events in all positions of the monitoring region where sensor nodes are deployed. For simplification, we can divide the monitoring region as a grid, and the sensing situation of the center point of a grid denotes this grid. By generating the event at the center of every grid by mobile object such as mobile sensor network and people, this dissertation proposed a carpet mobile algorithm. Furthermore, by the motivation of the observation that the boundary of the sensing area can denote the sensing area of sensor node, we devise a heuristic mobile algorithm requiring less number of events. The heuristic mobile algorithm generates the events around the sensing boundary of sensor node. However, it only saves the changing position from detecting to disappearing or from disappearing to detecting. Thus, the heuristic algorithm can reduce cost as the carpet mobile algorithm. Simulation results show the effect on coverage control of the irregular sensing area and the validity of carpet and heuristic mobile algorithms achieving the sensing area of sensor node.Thirdly, aimed at the evaluation of coverage performance for wireless sensor network in the real environment, we study the problem how to determine the coverage degree of the region coverage or barrier coverage. Based on computational geometry and regional closeness, the dissertation prove the coverage degree of a monitoring region is equal to the minimum coverage degree of all coverage intersections which include: 1) the intersecting points or the two end points of intersecting lines of any two boundaries of sensors; or 2) the intersecting points or the two end points of intersecting lines between any boundary of sensor and the boundary of coverage region. We also prove the barrier coverage degree is equal to the maximal coverage degrees of sub-boundary of the least coverage path on which each point is covered by the least sensor nodes. Consequently, we propose the algorithms evaluating the coverage degree of region and barrier by using graph theory respectively. Therefore, in the abstract, the evaluation of coverage degree of a monitoring region is transformed from continuous space difficultly tackling to discrete space easily resolving. The boundary of sensing area is dynamic change and very difficult to achieve in real environment. Based on the grid division of the monitoring region, we transform the evaluation of coverage degree from all points of region to the center point of a grid, and the evaluating problem of barrier coverage degree from all paths to finding a connected grid-path with the minimum coverage degree. Furthermore, we propose the relevant approximate algorithms of evaluating coverage degree for wireless sensor network respectively. Simulation results show the approximate grid-algorithm can deal with the coverage-evaluating problem validly.Lastly, aimed at the no-uniformity of coverage (e.g. the dense or sparse distribution inducing the sensing overlapping regions and coverage holes) deriving random deployment of huge scale of sensor nodes, we study the problems how to improve the coverage performance of the region coverage, barrier coverage or point coverage by adjusting the situations of mobiling sensor nodes and how to deploy additional sensor nodes. The dissertation introduces the concept of the set of contributed nodes, overlapping region, blinding region and critical coverage region. We construct the model of virtual potential field between overlapping region and the set of contributed nodes and between critical coverage region and the set of contributed nodes, and devise a coverage-improving technique by changing distributions of sensor nodes. A distributed algorithm is proposed based on virtual potential field.We prove the relationship between the superset which includes any two or three region of all blinding region in monitoring region and the number and position of additional nodes. We translate the pending problem into a broadly-studied easy graph coloring problem, and propose a centrally approximate greedy algorithm. Simulation results show that the coverage-improving algorithm can evidently enhance the coverage performance, and the greedy algorithm can perfectly resolve the deployment problem of addition nodes.In summary, this dissertation has addressed the essential issues implementing wireless sensor network in realistic environment. We studied the problems about achieving the realistic sensing area of sensor node and evaluating and improving the coverage performance. In succession, we propose the models and algorithms resolving the relevant problems. And the simulation results are analyzed. These works have academic and practical value for advancing the theory and practicability of coverage control in wireless sensor networks.