Abstract— We propose a new algorithm for estimating the location of an outdoor RFID node, based on the received signal strength (RSS) of a transmitted message by a set of readers with known locations. The algorithm supposes omni-directional RFID readers that are prevalent for outdoor long range RFID systems. The method does not require knowledge of the Effective Isotropic Power (EIPR) of the transmitting tag, and bounds the position of the node within a small delimited candidate area. Our simulation results demonstrate that the Received Signal Compensation Based Localization (CBL) method outperforms other compared RSS-based methods in improving the precision of the bounded area. Index Terms—Localization, RFID, RSS I. INTRODUCTION Radio Frequency Identification (RFID) technology is increasingly pervasive. Passive RFID tags are inexpensive and thus are the most prevalent form of RFID technology. Passive RFID poses, however, a number of limitations. The radio range of the reader does not exceed a few meters. Communication can be disturbed by materials such as metal, human body, walls, water, etc…Tags are parasitically powered by the energy from RFID readers, and usually employ linearly polarized antennas. If the angle between a reader and a tag is not within certain values, it may result in false negative readings. Most exiting works on RFID-based localization suppose a directional reader/tag antenna. But recently, omni-directional antennas for passive RFID are gaining much interest [1]. Active or semi-active RFID technology uses battery powered tags and can reach distances up to 200m. Typically, long-range active RFID technology uses omni-directional antennas and is targeted towards longer range indoors applications. For example, Montreal city has recently adopted an active RFID-based solution where RFID- tagged buses and RFID-enabled bus stops provide increased transit visibility [2]. Transit managers have a real-time view on bus dispositions and riders can wait sheltered from frigid weather temperatures while viewing real-time information about their bus expected time-of-arrival. In this work we focused on network-based location estimation mechanisms for outdoor RFID nodes assuming omni-directional RFID readers. Given an RFID message emitted by a tag, the Received Signal Strength (RSS) collected at readers situated within the radio range of the tag is used to pinpoint the position of the emitter in a bounded zone with a certain confidence. The RSS value at each RFID reader is used to estimate a probable range of distances of the emitting tag from each reader. The new algorithm, called Compensation Based Localization (CBL) is based on distance ratios of a tag from two or more readers. The algorithm does not need assistance from the active tag in estimating its position. Neither does it require knowledge of the Effective Isotropic Power (EIRP) of the transmitting tag that may be affected, for example, with the state of its battery. The rest of the paper is organized as follows: the next section discusses some related work. Section III presents the proposed approach. In section IV we present the simulation results and their analysis. Section V concludes the paper. II. RELATED WORKS Generally, radio frequency localization can be implemented by following one of two approaches. In the first one, most employed, the mobile node is the emitter and its location is computed at a central setup. In the second approach, the mobile is the receiver and its location is computed at its own level, like it is the case for GPS-equipped devices. Three methods are commonly used with RFID-based localization; Triangulation/Trilateration [3], Fingerprinting [4] and Landmark [5]. The majority of triangulation techniques use the first approach. Fingerprinting may use one or the other. However, most of RFID-based localization schemes presume a directional antenna model for the reader [6]. Ferret for example uses the location and directionality of RFID readers to infer the locations of nearby tags [7]. If we presume an omni-directional antenna model for RFID nodes, different works have been proposed to locate general wireless nodes based on some collected signal information. Time-of-Arrival based approaches such as in [8] need that all transmitters and receivers be precisely synchronized and have fast processors to eliminate the delay due to processing. This might not be feasible for RFID tags. Some relative RSS localization mechanisms were also described [9], [10] where a node collects the RSS values it receives to calculate its position. This again might not be practical in the case of RFID tags. RSS variations are taken into account in other works to construct a minimum and maximum distance annulus between the receiver and the emitter but the methods require knowledge of the emitter EIRP [11], [12] which might not be feasible if there are variations in the transmitting tag power. Received Signal Compensation-Based Position Estimation of Outdoor RFID nodes Mohamed Amine Abid and Soumaya Cherkaoui Department of Electrical and Computer Engineering Université de Sherbrooke, Quebec, Canada Mohamed.Amine.Abid@usherbrooke.ca, Soumaya.Cherkaoui@usherbrooke.ca 978-1-4244-5637-6/10/$26.00 ©2010 IEEE This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE Globecom 2010 proceedings.