Research Article Geometric Range-Free Localization Algorithm Based on Optimal Anchor Node Selection in Wireless Sensor Networks Hyunjae Woo and Chaewoo Lee Ajou University, 206 World cup-ro, Yeongtong-gu, Suwon, Gyeonggi-do 443-749, Republic of Korea Correspondence should be addressed to Chaewoo Lee; cwlee@ajou.ac.kr Received 11 November 2013; Revised 4 February 2014; Accepted 9 February 2014; Published 10 April 2014 Academic Editor: Long Cheng Copyright © 2014 H. Woo and C. Lee. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In range-free localization scheme of wireless sensor networks, estimating the distance to the anchor nodes having the actual location is common to compute the position of unknown node. Since the range-free scheme is based on the topology information, the accuracy of distance estimation is considerably afected by node density or node deployment. In this paper, we propose a geometric range-free localization algorithm which estimates the unknown positions geometrically by topological information without considering the distance estimation. To achieve this, we propose an optimal anchor node selection algorithm which selects the anchor nodes connected topologically well for the geometrical location estimation. Simulation results show that the proposed algorithm ofers considerably an improved performance compared to the other existing studies. 1. Introduction Wireless sensor networks have gained attention in recent years, because they can be applied in various felds such as environmental monitoring, medical care, military monitor- ing, and disaster relief. Since most of these applications need the physical position of wireless sensor nodes, the localiza- tion/positioning has been an important issue for wireless sensor networks [1]. Range-free scheme is one of the exist- ing localization techniques, which estimates sensor node’s unknown position with the relative connectivity information (e.g., hop count between the sensor nodes) [2]. Generally, the sensor nodes with their known positions are called as anchor nodes, while the others are called unknown nodes. In this scheme, unknown nodes calculate their position by utilizing the topology information such as the hop count of the shortest hop path between the anchor nodes and between the anchor node and unknown node and the position of anchor nodes. Most of the existing range-free algorithms estimate the Euclidean distance to the anchor nodes in order to obtain unknown node’s position. Afer estimating the distance, each sensor node starts to calculate its location by multilateration technique [3]. Hence, the most important issue in range- free algorithm is to precisely estimate the Euclidean distance between the anchor node and the unknown node (called as “2 distance” in this paper). DV-Hop [4] is the well- known range-free algorithm, which utilizes a metric (called as “average hop length” in this paper) to estimate the 2 distance. It estimates the 2 distance by multiplying the average hop length with the hop count of the corresponding shortest hop path. In the DV-Hop, the average hop length is obtained by considering the entire network. Hence, it will cause a lot of errors when estimating the distance, if the shortest hop path has a form which is diferent from the average. Later, there have been a lot of studies to improve the accuracy of 2 distance estimation. References [5– 7] proposed an algorithm which improves the accuracy of average hop length. Te authors of [5] calculated the average hop length stochastically by considering the number of neighboring nodes. Te authors of [6] computed the optimal average hop length by minimizing the sum of squares of the distance errors between all anchor nodes. Te authors of [7] recalculated the average hop length by considering the number of neighboring nodes. And the authors of [8, 9] proposed the refnement algorithms using optimization algorithms. References [10, 11] proposed a scheme which estimates the specifc anchor nodes that ofer the well-estimated 2 Hindawi Publishing Corporation International Journal of Distributed Sensor Networks Volume 2014, Article ID 509892, 10 pages http://dx.doi.org/10.1155/2014/509892