Study On Clock Synchronization Of Wireless Sensor Networks For Traffic Surveillance

Wireless sensor networks (WSNs) are usually deployed in a monitoring area of a largenumber of tiny sensor nodes, which can form a multiple-hop and self-organizing distributedsystem by wireless communication. In recent years, WSNs have provided a new method tosolve some critical problems in traffic surveilliance. However, time synchronization inwireless sensor networks for traffic surveillance (WSN-TS) has played a vital role to realizelink layer or network layer protocols that are based on time division mechanism,collaboration awareness for traffic conditions and information fusion of traffic sensing data.In this paper, several key issues for time synchronization in WSN-TS are discussed further,and the main work is described as follows:(1) An algorithm for greedy synchronization topology based on two-hop neighbors(GST-TN) is proposed to reduce synch overhead in a non tree-like hierarchical structure.Firstly, the layer discovery based on the flood mechanism is improved to build a non tree-likehierarchical structure. Then, a selection strategy for greedy broadcast tuples is proposed withthe foundation of two-hop neighbors’ adjacent information. Finally, the proposed method isverified with the same simulation conditions in MATLAB, and experimental results show thatGST-TN can achieve energy efficient in terms of sync message packets, compared to GPA,DMSP and TPSN, respectively.(2) An algorithm for greedy synchronization topology based on one-hop neighbors(GST-ON) is put forward to synchronize the nodes in a tree-like hierarchical structure withless sync message overhead. Firstly, a tree-like hierarchical structure, in which any nodesexcept the root node must have the unique father node, is constructed by the ClusteringRouting Protocols (CRPs) and the Neighbors Handshake Protocol (NHP). Then, a selectionstrategy for greedy broadcast tuples is proposed with the foundation of single-hop neighbors’adjacent information. Finally, the proposed method is verified in MATLAB, and theexperimental results have shown that GST-ON can achieve good effectiveness in terms ofsync packets, compared to TPSN, ERTSP, CCS and HRTS, respectively.(3) A WSN time synchronization algorithm based on Gaussian delay model is brought forward to estimate clock deviations between any two nodes in a greedy synchronizationtopology (GST). Firstly, local timestamps are shared by the broadcast communication ofgreedy broadcast tuples. Secondly, the clock interdependency in a greedy synchronizationtopology is set up due to the linear clock model with the first order. Then, the clockdeviations of the nodes whether they are all in a greedy broadcast tuple (inside the layer) ornot (outside the layer), are calculated based on the Gaussian delay model. Finally, theproposed method is verified in MATLAB, and the experimental results have shown that theproposed algorithm can achieve good sync accuracy. Moreover, the synch accuracy of clockdeviations that are inside the layer is improved by22.51s,22.51,14.64and1.5,compared to RBS, TPSN, RTSP and HRTS, respectively. And the synch error of clockdeviations that are outside the layer is less by5.29, which is compared to TPSNalgorithm.(4) A WSN clock synchronization algorithm based on component decoupling fusion(CSA-CDF) is proposed to optimize the compatibility of higher sync accuracy and lowersync overhead in a greedy synchronization topology. Firstly, the clock deviations of the nodesinside the layer are estimated by two different ways, which are based on the interdependentrelationship of local timestamps. Then, the clock deviations of the nodes inside layer areoptimized by using the component decoupling fusion. And the weighted parameters in thecomponent decoupling fusion are derived by the principle of minimum mean square error.Finally, the experimental results have shown that the proposed algorithm can improve theprecision of time synchronization without the additional synch packets consumption.Moreover, with the sync packets of40, the synchronization accuracy of the proposedalgorithm is improved by4.02,4.52,7.19,13.8and25.48, compared toTSP-GDM-IL, PBS, ERTSP, TPSN and RBS, respectively.(5) A time synchronization test system is designed to verify or improve clocksynchronization algorithms that will be executed in WSN-TS. Firstly, the overall structure ofthe time synchronization test system is described. Then, the time synchronization topology,the online and offline synchronization software are realized based on the ideas of GST-TN,GST-ON, TSP-GDM and TSP-CDF. Finally, synch message overhead and synch accuracy areobtained and analyzed in a representative traffic surveillance scenario. The experimental results have shown that the greedy synchronization topology is energy efficient, and thesynchronization method based on the Gaussian delay model can achieve high synch accuracyin our test scenario.In summary, the dissertation has solved some key problems of clock synch in WSN-TSfrom four aspects, that is, energy efficiency, synch accuracy, balance between energyconsumption and synch accuracy, and synch extensibility, by studying synch topologyconstruction algorithms, clock deviations estimation strategy, and fusion model of clockdeviations. The study results for realizing the time synchronization of WSN-TS is ofimportant reference value.