| Wireless sensor networks (WSN) has been identified as the research hotspot in the field of information nowadays, and there are a lot of key technologies to be discovered and researched, such as network topology control, network security, time synchronization, localization technology, routing technology and so on. In WSN, the data that the nodes collect must be integrated with the location information that is measured. As a service for other applications, precise localization results can provide strong technical support to smart home, traffic management, ocean exploration, and so on. In addition, because sensor nodes are tiny embedded devices, its energy, computing power, storage capacity and communications capabilities are limited. Therefore, it is important to extend the life cycle to design a simple and efficient localization algorithm for sensor networks.The research significance and the research status at home and abroad of localization algorithms are fully studied, and then evaluations of node localization algorithms are analyzed in WSN. These evaluations are interrelated, and are often required to make compromise between them to design appropriate localization algorithms. Moreover, three methods to calculate the node location and the classification of localization algorithms are introduced. Then, the localization algorithms of range-based and range-free are analyzed in details.The basic thought of DV-Hop algorithm and its problems are analyzed in details. Because the error of the average per-hop distance for DV-Hop algorithm is great and the overall localization accuracy is not very high, a new algorithm for RHDV-Hop algorithm is proposed. In the first step, the anchor nodes broadcast information packets that contain their own location information. Then, all the nodes receive and forward packets. Meanwhile, all the nodes record the minimum hops away from the anchor node. In the second step, the anchor nodes compute the average per-hop distance, and broadcast it to the network in the form of information packets. The unknown nodes compute the distance away from the anchor node according to the average per-hop distance and the hops information. Meanwhile, the one hop distance away from the anchor node is instead by using RSSI ranging technique. In the third step, the unknown nodes adopt the following two kinds of methods to estimate their own location when they obtain three or more distance information away from the anchor node. (1) If the unknown nodes get three or more RSSI ranging values, they estimate their own location adopting 2-D Hyperbolic algorithm through using these RSSI ranging values. (2) If the unknown nodes get less than three RSSI ranging values, they estimate their own location adopting 2-D Hyperbolic algorithm through using all the obtained distance information.Then, The principle, simulation methods and general processes of NS2 network simulator are introduced, and the basic framework of the Otcl script is analyzed. NS2 is a discrete event simulator and its core is a discrete event simulation engine. The underlying simulation engine in NS2 is written by C++ and it makes use of the object-oriented Otcl language as the simulation command and configuration interface language. C++ and Otcl call each other through the TCLCL toolkit and this method is known as classification object model. NS2 simulation is divided into two levels. In the first level, it is simply to prepare Otcl script to simulate using the existing elements in NS2; in the second level, it is necessary to extend elements, add the required classes, and write Otcl script to simulate in NS2.In order to the comparative analysis of the improved algorithm, DV-Hop algorithm, RDV-Hop algorithm, HDV-Hop algorithm and RHDV-Hop algorithm are simulated and compared. The simulation experiments are carried out in the region of 500*500 which is distributed randomly 100 nodes in NS2. Then, the average value of three sets of data is analyzed. Meanwhile, the comparison chart between the localization error and the ratio of anchor nodes, the comparison chart between the localization error and the average connectivity and the comparison table of four kinds of localization algorithms are obtained. The results show that RHDV-Hop algorithm is better than DV-Hop algorithm and some other improved DV-Hop algorithms in localization accuracy in the case of no additional hardware cost. Because some of the nodes which participates RHDV-Hop algorithm involve in HDV-Hop algorithm and RDV-Hop algorithm, the effectiveness of experimental results of RHDV-Hop algorithm is not the simple sum of the utility of HDV-Hop algorithm and RDV-Hop algorithm. However, the effect is small.A new algorithm for TDV-Hop algorithm is also proposed because the DV-Hop algorithm has more localization error in the case of the lower ratio of anchor nodes and the less average connectivity. In the first step, the anchor nodes broadcast information packets that contain their own location information. Then, all the nodes receive and forward packets. Meanwhile, all the nodes get all the paths away from the anchor node. In the second step, each anchor node computes separately the distance value and the hop value which is the number of triangles between two nodes away from the anchor node, and then the sum of distance value, the sum of hop value and the information of itself are broadcasted to the network. When an anchor node gets the information that all other anchor nodes send, it computes the average per-hop distance, and broadcast it to the network. The unknown nodes compute the distance away from the anchor node according to the average per-hop distance and the hop value which is the number of triangles between the unknown node and the anchor node. In the third step, the unknown nodes adopt maximum likelihood estimation algorithm to estimate their own location when they obtain three or more distance information away from the anchor node.In order to the comparative analysis of the improved algorithm, DV-Hop algorithm and TDV-Hop algorithm are simulated and compared. The simulation experiments are carried out in NS2 in the region of 500*500 which is deployed three kinds of topology: mesh topology, X-type topology and O-type topology. Then, the comparison chart between the localization error and the ratio of anchor nodes, the comparison chart between the localization error and the average connectivity and the comparison table of two kinds of localization algorithms are obtained in the three kinds of topological structures. The results show that TDV-Hop algorithm is better than DV-Hop algorithm in localization accuracy in the case of the lower ratio of anchor nodes, the less average connectivity and no additional hardware cost. |
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