Concentric Anchor-Beacons (CAB) Localization for Wireless Sensor Networks Vijayanth Vivekanandan and Vincent W.S. Wong Department of Electrical and Computer Engineering, The University of British Columbia, Vancouver, Canada e-mail: {vijayv, vincentw}@ece.ubc.ca Abstract— Many applications in wireless sensor networks re- quire sensor nodes to obtain their absolute or relative geographi- cal positions. Although various localization algorithms have been proposed recently, most of them require nodes be equipped with range-determining hardware to obtain distance information. In this paper, we propose a concentric anchor-beacons (CAB) localization algorithm for wireless sensor networks. CAB is a range-free approach and uses a small number of anchor nodes. Each anchor emits beacons at different power levels. From the information received by each beacon heard, nodes determine which annular ring they are located within each anchor. Each node uses the approximated center of intersection of the rings as its position estimate. Simulation results show that the estimation error reduces by half when anchors transmit beacons at two different power levels instead of at a single level. CAB also gives a lower estimation error than other range-free localization schemes (e.g., Centroid, APIT) when the anchor-to-node range ratio is less than four. I. I NTRODUCTION Significant advances in hardware technology have led to the miniaturization of devices capable of communication with each other. Wireless sensor networks consist of hundreds or thousands of tiny nodes that are deployed to monitor and gather data in a target geographical area. These nodes have limited processing capabilities and energy in which to operate. Wireless sensor networks are envisioned to allow for ease of deployment through redundancy and ad hoc placement. Applications such as remote surveillance or habitat monitoring require sensor nodes to obtain their absolute or relative geo- graphical positions. When an event occurs (or a stimulus being detected), the sensor nodes can forward the data information along their coordinates. Various centralized [1][2] and distributed [3][4] localiza- tion algorithms have been proposed recently. With respect to robustness and energy efficiency, distributed algorithms are preferred over centralized schemes. The localization algo- rithms can further be divided into range-based [4][5], angle- based [6][7], and range-free [8][9] approaches. Range-based schemes assume that sensor nodes have the ability to obtain distance estimates to other nodes. In angle-based schemes, the relative angular information between nodes is required. Range-free approaches assume that no specialized angle or range-determining hardware is necessary for the sensor nodes. To determine the absolute geographical location, most of the localization algorithms also assume the use of special anchor nodes. Each anchor may equip with a GPS (Global Positioning System) receiver to obtain its absolute position information. Although both range-based and angle-based approaches pro- vide a lower estimation error than the range-free approaches, they require special hardware for sensor nodes to obtain relatively accurate distance (or angle) measurements to other nodes and anchors. This may not be cost-effective for appli- cations which require hundreds of sensor nodes over a large coverage area. Our work focuses on improving distributed range-free algorithms with higher accuracy. In this paper, we propose a concentric anchor-beacons (CAB) localization algorithm for wireless sensor networks [10]. CAB is a distributed range-free approach and uses a small percentage of anchor nodes. Each anchor emits beacon signals at different power levels. Each beacon carries information including the anchor’s position, its power level, and the esti- mated maximum distance the beacon can travel. Nodes listen and record which anchors they can hear the beacons from and at which power levels. From the information received by each beacon heard, nodes determine which annular ring they are located within each anchor. Each node uses the approximated center of intersection of the rings as its position estimate. CAB requires no specialized range-determining hardware in the sensor nodes, and relies only on node-anchor commu- nication to localize the nodes. This simplifies the distributed operation and is more energy-efficient since neighboring sen- sor nodes do not need to exchange information. Simulation experiments are conducted to evaluate the per- formance of CAB localization algorithm by varying the num- ber of anchors heard, anchor-to-node range ratio, and radio pattern degree of irregularity. Simulation results show that the estimation error reduces by half when anchors transmit beacons at two different power levels periodically instead of a single power level. In addition, we also compare CAB with two other range-free localization algorithms: Centroid [8] and APIT (Approximated Point-In-Triangulation) [9]. Results show that CAB provides a higher accuracy than Centroid. CAB gives a lower estimation error than APIT when the anchor-to-node range ratio is less than four. This paper is organized as follows. The related work is summarized in Section II. The CAB localization algorithm is described in Section III. The performance evaluation of CAB as well as the comparisons with APIT and Centroid are presented in Section IV. Conclusions are given in Section V. This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE ICC 2006 proceedings. 1-4244-0355-3/06/$20.00 (c) 2006 IEEE 3972