Antennas and Propagation Models in advanced RFID Systems

The demand for RFID (Radio Frequency IDentification) systems in real-time identification, location-based services, asset tracking, intelligent transportation, security surveillance, item level tagging and many other authentication and management processes is continuously increasing. Since RFID systems are based on a wireless radio link between the reader and the transponders (tags), antennas for both readers and tags are recognized to be crucial elements of the whole system, together with channel propagation phenomena as multipath, clutter and path loss. Moreover, radio-localization techniques can take advantage of low-cost and low-size of RFID tags which can be mounted on the object/person to be tracked, as well as of easy-deployment of an infrastructure of networked readers. Therefore, a proliferation of RFID tag location and tracking solutions and their integration in commercial RFID readers or ad-hoc data processing systems is expected in the near future. In this context, during her PhD studies the author has dealt with some antenna and propagation issues arising in modern RFID systems. Part of the work has been devoted to Near-Field (NF) focused antennas for UHF/microwave RFID systems. Indeed, Near-Field UHF RFID systems are expected to combine typical advantages of UHF systems (high reading rate and data rate) with those of HF systems (robustness to the environment and multipath); communication occurs in the antennas near-field zone, like in the HF systems, but through an electromagnetic coupling. A Near-Field focused antenna for fixed reader antenna suitable for RFID portals has been designed and characterized. When compared to an equal-phase array, the NF focused array is able to radiate a higher power in the near-field region but with a lower power in the far-field region (the latter property being useful to better satisfy the EIRP limits). These advantages are obtained without increasing antenna cost and complexity with respect to a conventional far-field focused array. The design criteria of near-field focused planar arrays have been widely investigated and design curves have been carried out as a function of array size, inter-element distance and focal distance. The radio-link for Near-Field UHF RFID systems have also been investigated by referring to a significant set of real commercial antennas, through a careful numerical analysis of the impedance matrix associated to the system made of the reader antenna coupled to the tag antenna. Estimation of the power transfer efficiency, the far-field boundary and the impedance mismatching loss for practical RFID configurations have been obtained. A set of polarization alignments between the reader and the tag antennas have also been considered, as well as the “matched-antenna” case where two identical tag antennas are used at the reader and tag sides. Finally, location techniques for RFID tags have been examined and a new phase-based technique have been proposed (patent pending). It takes advantages of the fact that the tagged items move along a conveyor belt, whose instantaneous speed and path are known a priori. Algorithm performance is shown thorough a numerical model as a function of standardized UHF-RFID electrical parameters. The location algorithm can be implemented in a conventional reader that is based on an I-Q receiver, so the proposed technique does not require any modification of the reader antenna configuration usually adopted in RFID applications and it allows a simultaneous detection and tracking of all tags moving along the conveyor belts.