This is the author's accepted version of:U. Kumbul, A. Farsaei, A. Sheikh, and G. Dolmans, "IndoorLocalization with Passive RFID System Based on Phase Differenceof Displacement," Proc. 19th European Conf. on Antennas andPropagation (EuCAP), Glasgow, UK, Mar. 2025.doi: 10.23919/EuCAP63536.2025.10999484The final version is available at:https://doi.org/10.23919/EuCAP63536.2025.10999484 Abstract-The radio frequency identification (RFID) system isinvestigated for indoor localization. The phase difference betweenthe relative displacement of the tag is used to estimate therange towards each anchor. The updated range information ateach anchor is then used to compute the coordinates of thetag using analytical multilateration positioning. The localizationperformance of this multilateration method is demonstratedexperimentally and compared with maximum likelihood positioning,which serves as an upper bound for localization performance.The experiments show that the passive RFID system with analyticalmultilateration positioning can achieve localization errorsbetween 0.06 m and 0.18 m for the chosen trajectory. Index Terms-RFID, Positioning, Phase-based Ranging, IoT,Phase Difference of Arrival, Indoor Localization.