Code Division Multiple Access in Centimeter Accuracy Harmonic RFID Locating System

In conventional passive UHF radio frequency identification (RFID) systems, time-division multiple access (TDMA) protocols are designed to counter inter-tag collision and multi-path interference for digital ID retrieval. However, high-accuracy (centimeter-level and below) localization to track multiple tags simultaneously still awaits a feasible solution. Overlap on the same carrier frequency of the downlink (from the reader to tags) and uplink (from tags to the reader) leads to severe self-jamming and reader collision, which makes tag localization especially difficult. The present narrow bandwidth also limits the localization accuracy and reliability. Starting from the polling process and synchronization by TDMA, we propose the code division multiple access within the broadband harmonic backscatter RFID system to enable the simultaneous real-time (15 Hz) tag localization with centimeter accuracy and millimeter deviation. We investigate the impacts of the number of tags with respect to the sampling rate, tag power consumption, and locating errors originated from the inter-tag collision, and then present an experimental prototype for verification.