Enabling Localization in WSNs with Solar-Powered End Devices Rushi Vyas, Vasileios Lakafosis, Manos Tentzeris School of Electrical and Computer Engineering, Georgia Institute of Technology Atlanta, Georgia, USA Rushi.Vyas@gatech.edu Abstract—A novel remote localization system utilizing renewable solar energy to power and trigger communication between end devices and a wireless network is presented. The novelty of the system is the unique node design that allows for battery less operation using ambient solar energy. Hardware and software design considerations involved in making the localization system operational inspite of the limitations of solar power are outlined. The final functioning localization system results as an end application are presented. A remote localization system with very good accuracy utilizing this end node communicating with a WSN is showcased as an end application with tremendous potential. Keywords- Solar; RF; Power Scavenging; WSN; localization; Ambient power; microcontroller; RFID; wireless sensor node I. INTRODUCTION This paper presents a fully operational localization system that tracks the position of a remotely placed end device through lateration. This service has been found to be very helpful in large parking lot environments; not only does this localization service offered help the user, for example car auction dealer, to be able to directly find any vehicle fast and accurately, but it also enables the user to exploit the location data for optimization and cost reduction of its everyday operation. The goal of the work presented in this paper is the provision of location information of a solar powered battery-less RFID tag placed on the dashboard of a vehicle in large scale parking lots to customers, such as car auction dealers. The localization technique deployed is the RSSI (Received Signal Strength Indicator)-based lateration. II. SYSTEM Lateration is the method of estimating the position of an end device by using its distance from 3 or more anchor points whose exact location coordinates are known in advance. The reference points were wireless sensor network (WSN) nodes made up of Crossbow's MICA2 wireless nodes. The distance of the end device from the anchor nodes was estimated from the strength of the wireless signal transmitted from the end device as measured by Received Signal Strength Indicator (RSSI) on the TI CC1000 transceiver [2] that the MICA2 motes relied upon. RSSI measurements from the anchor node were pooled together on a central computer where they were selectively parsed for computations that would estimate the position of the end device. The novelty of the system was that the end node not only used ambient light to power itself, but also used it as a triggering mechanism for initiating communication between itself and the WSN nodes thereby allowing for a completely stand-alone, battery-less asynchronous communication link from itself to the WSN used in the lateration [2]. The drawback of course would be that such a localization system would only be operable during day time. The elimination of the need to replace batteries every few months does makes such a system ideally suited for large scale item level tracking in many commercial applications. Section III describes the Power scavenging hardware design of the end node; Section IV describes considerations taken to establish a robust wireless communication link between the end node and WSN given the power limitations and Section V covers the wireless front end namely the amplifier characterization and antenna design of the end node; section VI describes the localization techniques used; the final results of the localization estimation using a commercial mapping software are covered in section VII. III. HARDWARE The hardware system for the end node that was to be remotely tracked by the WSN was designed to meet the following criteria for successful lateration: constant RF Power output during each transmission, high rate of communication, long range, omni-directionality, wireless connectivity. The fundamental problem with using a finite sized solar cell array was its scarce output power. The palm-sized solar cell array that was stacked in a parallel configuration was capable of generating a maximum of 15mW. The most power hungry portion of the end node which was the wireless front-end consumed a peak power of 48 mW in transmit mode. A comparison of the 2 numbers showed that while it was not possible to continuously power the end node with the solar cell array, in a relatively short period of time enough solar energy could be harnessed from the environment to supply the end node with just enough power for communication for a short period of time. A system level diagram of the end node is shown in Fig.1. 2010 IEEE International Conference on Sensor Networks, Ubiquitous, and Trustworthy Computing 978-0-7695-4049-8/10 $26.00 © 2010 IEEE DOI 10.1109/SUTC.2010.52 155