UHF RFID Differential Sensing via the Backscatter Coupling Between the Reader-Tag Antennas

Enhanced by the simultaneous wireless power and data transfer, the radio frequency identification (RFID) techniques create new chances for the Internet of Things (IoT), which is used to integrate sensitive elements for different sensing purposes. However, the sensor-augmented RFID tags and chip-less RFIDs suffer from the limited power supply and the impact of ambient environments, respectively. This investigation explores the commercial RFID-based sensing techniques to obtain the physical parameters of the RFID-labeled objects with differential sensing via the backscatter coupling between the reader-tag antennas. The main contributions of this investigation include: 1) analysis of RFID backscatter coupling and the establishment of the relational model of measured parameters, antenna parameters, and RFID received signal strength indicator (RSSI)/phase; 2) simulation studies of the electromagnetic coupling between the RFID tag and material under test (MUT) to reveal the relationship of the MUT parameters with antenna impedance; 3) design of an RFID tags differential sensing scheme to compensate for the environment impact using a shielding layer; and 4) determination of sensitive frequencies with principal component analysis (PCA) and measurand reconstruction method with Levenberg–Marquardt (LM) curve fitting. Finally, the experimental studies are conducted with NaCl solution sensing. The results show that the presented techniques can effectively obtain the change in the solution’s concentration, which demonstrates the effectiveness of the proposed techniques.