Research On Techniques Of Coverage Enhancement And Topology Control For Wireless Multimedia Sensor Networks

On the basis of Wireless Sensor Networks(WSNs),Wireless Multimedia Sensor Networks(WMSNs)are a new type of sensor networks which enable the perception and processing of multimedia content such as video and audio streams,and images.They come from WSNs and inherit the common ground such as large scale,restricted resources,self-organization,dynamic topology,multi-hop data transmission and strong correlation with applications.However,WMSNs show some outstanding characteristics like directional sensing,large heterogeneity and significant amounts of sensing data.Coverage enhancement and topology control are two key issues in the target monitoring applications based on WMSNs.They adopt coverage performance or network lifetime as the main standard for evaluating the quality of service(QoS).For the sake of promoting the overall monitoring capacity of WMSNs,they optimize network deployment and topological structure.Thus,the two issues are directly related to the effective monitoring quality and time span that WMSNs can provide for specific monitoring tasks.At present,existing research works have the problems like ignoring the heterogeneity of network,converging on local extrema and narrow application scope.Therefore,to design proper coverage enhancement algorithms and topology control protocols,the characteristics of WMSNs and specific application scenarios should be taken into account.This thesis centers on the key problems like sensing models,coverage enhancement and topology control in WMSNs.The main work involves the research on the characteristics of sensing models,designing coverage enhancement algorithms for area coverage,barrier coverage and crossed barrier coverage,and designing clustering protocols in topology control.Major work and innovative points are summarized as follows.1.This thesis has studied the intersecting characteristics of two-dimensional(2D)sector-shaped sensing model and presented a fitting formula,which utilizes the angle difference of sensing directions and the Euclidean distances between nodes to fast estimate the joint coverage rate.Applying the fitting formula can simplify the calculation process of the joint coverage rate,while guaranteeing a certain accuracy of coverage rate.Thus,the problems of repetitive calculation and large computation complexity,caused by the conventional methods that calculate the joint coverage rate by using discrete grids,are avoided.2.Based on the three-dimensional(3D)pinhole camera sensing model and visual coordinate transformation,this thesis has proposed a method that can estimate the visual correlation between sensor nodes quickly.The method measures the visual correlation between sensor nodes,by calculating and comparing the variation degree of reference vectors on the imaging planes of different camera nodes.Compared with the conventional correlation evaluation methods which are based on the joint coverage rate,the proposed method can effectively evaluate the visual correlation difference brought by the variation of sensing view.3.Aiming at the scenario where the target region has a belt-like shape and coverage priorities in the area coverage issue of WMSNs,the thesis introduces the problem of "effective coverage",and then proposes a coverage-enhancing algorithm based on the improved virtual potential field to solve it.By introducing the concept of"virtual attractive force",the coverage rates of the areas with high priorities increases.To overcome the deficiency of local extrema which can reduce the coverage optimization effect in the traditional algorithms based on virtual potential field,the proposed algorithm improves the functions of virtual repulsion force,by taking the joint coverage rate of neighbor nodes into consideration to assist the adjustments to sensing directions of sensor nodes.Compared with the traditional coverage-enhancing algorithms based on virtual potential field,the proposed algorithm can eliminate the sensing overlapping regions and coverage holes effectively,thus enhancing the coverage rates of both the prioritized areas and the whole area.4.For area coverage in WMSNs,this thesis has proposed a coverage-enhancing algorithm based on the improved particle swarm optimization(PSO).To optimize the coverage rate of WMSNs,the PSO with constriction coefficient is adopted in determining the sensing directions of sensor nodes.Meanwhile,the simulated annealing algorithm is introduced and combined with PSO to overcome the deficiency of local optimum,which can reduce coverage optimization effect and lead to sub-optimal convergence.The mechanisms of search speed control and probabilistic leap are employed to ensure the search accuracy and achieve better global search ability,which will enhance the optimization effect of coverage rate.Simulation results show that,compared with the typical coverage-enhancing algorithms,the proposed algorithm achieves better coverage enhancement performance.5.In order to solve the problem of maximizing the strong barrier number in lifetime-heterogeneous WMSNs,the thesis proposes using an extended directional barrier graph to model the adjacent relations between sensors' sensing regions and the lifetime constraints of sensor nodes.Based on this graph,we present an integer linear programming(ILP)formulation of the optimization problem.Then,we propose four heuristic algorithms to build strong barriers,and evaluate their performance by conducting simulations under different parameters.Simulation results show that the Modified Disjoint Path Algorithm(MDPA)with the strategies of multi-round execution and least-hop path selection outperforms the other three algorithms and achieves the strong barrier number that is close to the optimal solution by ILP.6.In order to solve the security vulnerability of traverse intrusion in strong barrier coverage,this thesis innovatively introduces the concept of "crossed barrier coverage"and the problem of maximizing crossed barrier number.Then,we prove that the problem of finding the maximum number of crossed barriers is NP-hard in computation complexity.ILP is used to formulate the optimization problem and the branch-and-bound(B&B)algorithm is adopted to solve the formulations.In addition,we also propose an effective Multi-round Shortest Path Algorithm(MSPA)and several heuristic algorithms to solve the optimization problem.Finally,extensive simulations demonstrate the effectiveness of MSPA in realizing crossed barrier coverage.7.The thesis proposes a novel distributed energy-efficient clustering protocol named DCE for heterogeneous WMSNs,based on a Double-phase Cluster-head Election scheme.In DCE,the procedure of cluster head(CH)election is divided into two phases.In the first phase,tentative CHs are elected with the probabilities decided by the relative levels of initial and residual energy.Then,in the second phase,tentative CHs are replaced by randomly chosen cluster members with more residual energy,forming the final CH set.By designing the functions of election probabilities and introducing an extra CH substitution phase,WMSNs can avoid selecting low-energy nodes as CHs and realize more balanced energy consumption load.Through extensive analysis and simulations,the thesis shows that DCE outperforms other typical clustering algorithms in terms of stability period,and can prolong the stability period of a network effectively.