Ambiguity-Free 3D Millimeter-Precision RFID Tag Localization Inside Building Materials

Phase-based 3D radio-frequency identification (RFID) tag localization has the advantage of high accuracy, but the inherent phase-range ambiguity needs to be resolved by either frequency or spatial diversity. We propose a reliable 3D tag localization method that exploits spatial diversity to achieve millimeter-precision inside building materials with heavy multi-path interferences. We first obtain the functional relationship between differential phase and differential distance using polynomial fitting and optimization for reference tags and then evaluate the 3D positions of new tags given only phase measurements. A novel ambiguity-free algorithm is devised to identify the correct tag location from multiple candidates by leveraging redundant channel resources with spatial diversity. We prototyped the tag localization system on Universal Software Radio Peripheral (USRP) devices and harmonic backscatter RFID tags and demonstrated millimeter-level localization accuracy at 1.8 GHz second-harmonic carrier frequency inside building materials.