A High Precision Time Synchronization Scheme For Wireless Sensor Network

With the development of low-power wireless communication technology, mini-sensor technology, embedded technology and integrated circuit technique, the widespread use of the wireless sensor network has been realized. As a new method of information collecting and analyzing, wireless sensor network has been already used in military affairs, industrial production and civilian inordinately.Time Synchronization is one of the basic support techniques for wireless sensor network. Through a large number of low-cost sensor node, wireless sensor network collects various information people interested in from the under testing area and gathers them together by wireless communication technique, at last, it will integrate and analyze these data collected. To merge the data together, it requires all the information should have a unify time standard in order to identify the sequence of the information. Time synchronization could guarantee the standard of the same time. Compare with the traditional network, the main restriction of the wireless sensor network is the capability of calculation and energy. It's necessary to considerate about the calculation ability and energy sufficiently when design the related technique of the wireless sensor network. Therefore, the traditional technique of time synchronization cannot be used in the wireless sensor network directly. It needs to find a new-method of the time synchronization on account of the characteristics of the WSN.In order to design a scheme of time synchronization which are applicable to wireless sensor network, this paper has done the following things:Firstly, it researches the clock model of WSN node and some frequently-used source clocks, getting the fundamental principles of the time synchronization. Analyze the reasons of the time difference and main influence factors on the time synchronization accuracy. It classifies the existing calculation of WSN time synchronization into three classes, that is. calculation based on the Receiver-Receiver, one-way calculation method based on Sender-Receiver and both-way calculation method based on the Sender-Receiver. It focuses on researching and analyzing one or two representative calculation of each method. This paper analyzes the advantages and disadvantages as well as the restriction of application from the four aspects of precision, scope, power consumption and complexity of calculation.Secondly, after the analysis of the current technique, it proposals a high-precision scheme of time synchronization in order to achieve synchronous effect of microsecond-level by the support of hardware equipment. Time synchronization can be divided into three parts: time correction of reference node, synchronism calculation of nodes and diffusion way of the synchronization in the network. As for the correction of node, it revises the crystal frequency-error of the reference node by using the pulse signal of the GPS receiving equipment. Then through the UTC time provided by GPS. it revises the starting time of the node for timing service. Through the correction, the node can provide a standard logistical clock source for entire network. Through the one-way communication mode, the basic idea of the time synchronization between nodes is. Sender sends two Synchronous Messages SYNC1 and SYNC2 and record the sending time T1 and T2. receiver records receiving time C1 and C2:then the sender sends the recorded T1 and T2 to receiver through the third message SYNC3. receiver adjusts its own logical time to sender's through these four values of T1,T2,C1 and C2. It avoids the influences of MAC and above on the synchronization calculation by using time trapping technology of SFD on the physical layer to record the T1,T2,C1 and Co-This can ensure the accuracy of the calculation. As for the general application of the wireless sensor network, it can adopt flooding way to synchronize the logical time of the reference node to other nodes in the network. As the application of hierarchical structure, network topology could performance better by the way of layered diffusion similar with TPSN. For this reason, the method of synchronization can be selected according to different adaptive feature.Thirdly. On the basis of time synchronization designed before, it realizes the synchronization calculation primarily and verifies the possibility of the scheme by using M-87 GPS receiving equipment and CC2530 development board. With the feature of CC2530 development board, it realizes the logical clock model of node and the function of time capturing based on SFD signal, designs and realizes the data structure used in synchronization. It verifies the Simultaneous Effects of calculation through the time synchronization experiment between sensor nodes, estimates the delays of node communication in the physical layer is about 3.5 microseconds based on actual measure and the Synchronous precision of the calculation is about 0.7 microseconds through the data analysis.In the last, the thesis has a summary on the work and looks into the prospect of future research work.