Setup-Independent Phase-Based Sensing by UHF RFID

By processing the phase of the backscattered field from a radio frequency identification ultrahigh-frequency tag antenna, physical information can be derived about the change of the materials and boundary conditions around a sensor-less tag. The phase-based sensing, however, is strongly dependent on the specific setup arrangement, and huge artifacts would arise in successive independent measurements involving temporary (handheld) readers at different distances. This letter introduces a multiport tag arrangement suitable to achieve distance-independent measurements. The method exploits the use of sensitive and reference tags. Their phase responses are combined within a double phase difference metric suitable for robust data inversion. The method is demonstrated through an experiment with a displacement-sensing system.