Localization of Passive UHF RFID Tags Based on a Frequency-Stepped Continuous-Wave Approach

Abstract

This paper introduces a 2-D position measurement system for passive UHF RFID tags based on a frequency-stepped continuous-wave approach. The main application of the system is the localization of objects tagged with RFID transponders. By using this method, phase-ambiguity can be avoided by evaluating the backscattered transponder signals using multiple transmit frequencies of the interrogator signal. A multiple channel system in which, sequentially, each frontend is configured to work as a transmitter while the remaining frontends serve as receivers is used to enable position estimation. For proof of concept, a local position measurement system demonstrator was built comprising conventional passive EPCglobal Class-1 Gen-2 UHF RFID tags, a commercial off-the-shelf RFID reader, eight transceiver fron-tends, baseband hardware, and signal processing. Measurements were carried out in an indoor office environment where a measurement zone of 5.0 m x 3.5 m was surrounded by drywalls, concrete floor and ceiling. The experimental results showed accurate localization with a root-mean-square error of 1.57 cm and a median absolute error of 1.29 cm. These results were confirmed by simulations, which were performed to determine the limits of the system.