This paper presents a 2d localization system for UHF RFID tags. By measuring the phase between the transmitted continuous wave and received backscatter from the tag at different frequencies, it is possible to estimate the distance between the reader and tag. By determining distance estimates to three antennas it is possible to determine a location of a tag with the help of trilateration. The location of the antennas has to be known and any error in distance measurement influences the location estimate. Mainly to overcome the expected impact of the environment on the distance estimate, we opt for the use of reference tags and the K-Nearest Neighbor (KNN) algorithm to derive a location. An experiment is done and results are compared with an KNN algorithm based on received signal strength. Furthermore some experiments are done to verify the use of phase measurements for the detection of stationary tags in a portal application.
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Huiting et al. (2013) studied this question.