Research On Node Localization System And Algorithm For Wireless Sensor Networks

Wireless sensor networks (WSNs) are self-organized networks composed of a great deal of small nodes randomly distributed in the sensing area. A certain kind of sensor, a data processing unit and a communication module are the common components of a node in the WSNs. In the recent few years, WSNs have accessed rapid development and reveal vast application prospects in many fields, such as military affairs, industry, agriculture, environment, medical treatment, etc. As a brand-new research field, WSNs pose many challenging topics to the research workers. The problem of node localization, that is, determining where a given node is physically or relatively located in a network, is one of the challenging tasks and yet extremely crucial for many applications. The self-localization of nodes in WSNs has been the basis for most of the applications, such as taget recognization, monitoring, and tracking. The node localization has been an important and critical research direction in the study of WSNs.Because of the large and random deployment of the sensor nodes in WSNs and some restrictive conditions of the sonsor nodes themselves, such as short battery life, low reliability, and limited wireless communication distance, WSNs pose high demands on the techniques and algorithms which are used for the node localization. Some routine localization methods, such as GPS and surveying method, are not suited for the node localization in WSNs. The node localization systems for WSNs are commonly requied self-orgornized, robust, energy-efficient, and computation-distributed.Our study is focused on the node localization in WSNs, and the main object is to solove the node localization problem from the aspect of algorithm design, under the current hardware conditions of sensor nodes. The main contribution of this dissertation includes: 1. In many WSNs'applications, for computation simplicity and ease of presentation, the sensing areas are commonly assumed flat and the node localization problem is solved in two-dimensional (2D) space only. In this paper, we present a distributed node localization system for WSNs, which fits for the application scenarios where the localization environment can be simplified as a 2D space. The system includes three common steps:â‘ determine node-beacon distances,â‘¡compute node positions, andâ‘¢refine the positions. Compared with other current distributed node localization systems, our system innovates in the second step and the third step.(1) An algorithm combination Min-max+LI is proposed to be the position derivation algorithm. Min-max is a representative node position derivation algorithm for WSNs, and LI is a control point densification algorithm used in engineering survey field originally. The combination usage of the two algorithms is an optimisation to let them make up each other in terms of their respective advantages and disadvantages in the localization computation.(2) SD method is presented for the refinement, and thus, the refinement procedure is transformed into a solution procedure for a nonlinear equation system. The feasibility and effectivity of our sytem are demonstrated through analysis in theory and simulation. Results show that our proposed sytem can perform better than some representative distributed node localization schemes presented in previous researches in terms of the trade-off among accuracy, coverage, computation cost, and communication overhead.2. Most previous approaches on node localization are designed and evaluated considering only 2D applications, but sometimes it is unreasonable to just simplify the node localization problem to 2D level due to the complexity of the actual application scenarios. The necessity of solving the node localization problem considering three-dimensional (3D) environments is discussed, and some innovative work are done on the system model design and algorithm application of 3D node localization in WSNs.(1) The advantages of using the mobile beacon mechanism are demonstrated, and a 3D node localization system using an aircraft-carried mobile beacon for the localization signal coverage is proposed. The working modes of the system is that the unknown nodes in the sensing area receive the UWB signals from the mobile beacon passively and measure the distance to the mobile beacon using TOA technique, and then, the nodes localize themselves locally.(2) SDI is proposed as the 3D node position derivation algorithm, and it is a spatial control point densification algorithm used in engineering survey field originally.Some features of our proposed system are discussed, and the feasibility and effectivity of using SDI as the position derivation algorithm are demonstrated through analysis in theory and simulation. Based on the results, mainly two conclusions are obtained. One is that SDI is sensitive to the node-beacon distance error, and hence, it is a suitable 3D node position derivation algorithm when the distance measurement is precise. Since UWB TOA technique can provide precise distance measurements, our system can perform well in general. The other is that SDI performances much better than the other two representative algorithms, Min-max and Lateration, in terms of the vertical positioning error under all the range situations while Min-max has the best performance on horizontal positioning when the range precision is poor. Therefore, in the cases where only corse ranging results can be obtained (e.g., using RSSI for ranging), both SDI and Min-max should be considered for 3D node position derivation, in which, Min-max is used to derive the horizontal position of a node while SDI is used to drive the vertical position.