MULTI-phase: A passive RFID positioning system based on multi-frequency phase difference

Accurate and rapid indoor positioning is essential for realizing the Internet of Things. Although satellite-based positioning works well outdoors, it often fails indoors due to signal blockage. Radio Frequency Identification (RFID) positioning technology has attracted attention due to its low cost, ease of deployment, and contactless operation. However, traditional methods based on Received Signal Strength Indicator (RSSI) are vulnerable to environmental interference, reducing accuracy. This paper proposes a passive RFID indoor positioning method that uses multi-frequency phase differences ranging. A Gauss-Kalman filter is applied to reduce the influence of environmental noise on phase data. A multi-frequency phase difference distance model is established, and the Chinese Remainder Theorem with a closed-form solution is used to solve phase ambiguity. Finally, a weighted Levenberg-Marquardt algorithm refines the target position, improving convergence and accuracy. Experiments conducted in different indoor environments demonstrate that the proposed method achieves an average positioning error of 0.38 m and a maximum error of 0.5 m, meeting the requirements of indoor positioning applications.