Relative localisation of passive UHF-tags by phase tracking

In this paper a solution is presented that realise the detection of order of moving UHF-RFID (Ultra High Frequency - Radio Frequency Identification) tags. Besides pure in-field detection of tags, further information corresponding to the physical distance between tag and interrogator such as signal strength or timing information can be evaluated. The method used in this work exploits phase values together with time stamps. In this way the reader behaves like a multi sensor platform. To solve the system inherent problem of ambiguities caused by phase wrapping Kalman Filters are used to recover the unwrapped phase for every tag. Based on this information the algorithm then retrieves the tag's location incorporating knowledge about the principle arrangement and movement of the tags. To validate the robustness of our approach various test series with an industrial ready reader have been performed also discussing the limits of the given approach.