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

The SARFID technique for discriminating tagged items moving through a UHF-RFID gate

The discrimination of tagged items moving along a conveyor belt from other tagged items that are present in the scenario is investigated, when a UHF-RFID gate is installed at a conveyor section. Indeed, tagged items that are static or randomly moving in the scenario (nomad tags) around the reader antenna could be detected even if they are not on the conveyor (false positive readings). The classification procedure here proposed exploits the SARFID phase-based technique used to localize tags on a conveyor belt, which takes advantage of the fact that the tagged items move along a conveyor, whose path and instantaneous speed are both known. The latter can be implemented with only a firmware upgrade, in any conveyor belt scenario already equipped with an RFID system, without any modification of the system infrastructure and additional (reference tags/multiple antennas) or ad hoc hardware. From experimental results in a real scenario, the discrimination between moving tags from static/nomad tags can be obtained with an overall accuracy greater than 99.9%, by employing only one reader antenna

A phase-based technique for localization of uhf-rfid tags moving on a conveyor belt: Performance analysis and test-case measurements

A new phase-based technique for localization and tracking of items moving along a conveyor belt and equipped with ultrahigh frequency-radio frequency identification (UHF-RFID) tags is described and validated here. The technique is based on a synthetic-array approach that takes advantage of the fact that the tagged items move along a conveyor belt whose speed and path are known apriori. In this framework, a joint use is done of synthetic-array radar principles, knowledge-based processing, and efficient exploitation of the reader-tag communication signal. The technique can be easily implemented in any conventional reader based on an in-phase and quadrature receiver and it does not require any modification of the reader antenna configurations usually adopted in UHF-RFID portals. Numerical results are used to investigate the performance analysis of such methods, and also to furnish system design guidelines. Finally, the localization capability is also demonstrated through a measurement campaign in a real conveyor belt scenario, showing that a centimeter-order accuracy in the tag position estimation can be achieved even in a rich multipath environment

Progettazione e caratterizzazione di array a microstriscia focalizzati in campo vicino

In questo lavoro di tesi, ci siamo occupati della progettazione e della caratterizzazione di array a microstriscia focalizzati in campo vicino, che lavorano nella banda del GSM1800 e del GSM900. Il nostro scopo è quello di concentrare il fascio dell’antenna in una regione ben delimitata dello spazio, nel campo vicino dell’antenna stessa. Per fare questo possiamo utilizzare anche un’antenna a tromba con delle lenti, ma abbiamo dei vantaggi nell’usare un’array a microstriscia, per cui abbiamo preferito questa soluzione. Il progetto dell’antenna è stato fatto tramite il software commerciale Ansoft Designer, che implementa il Metodo dei Momenti (MoM); grazie alla sua struttura è stato possibile progettare separatamente i patch e la linea di alimentazione. Per applicazioni nella banda del GSM1800 l’array è formato da 8x8 elementi, mentre per applicazioni nella banda del GSM900 da 4x4 elementi. Dopo aver progettato il singolo elemento, per la simmetria del problema ci siamo concentrati su un quarto della struttura. Per ottenere la focalizzazione è stata realizzata opportunamente la linea di alimentazione; i valori delle fasi delle correnti di alimentazione di ogni elemento, ottenuti dalle simulazioni, sono in buon accordo con quelli teorici. Abbiamo inoltre mostrato i risultati delle simulazioni per i due array completi. Infine è stato realizzato il prototipo dell’array a 1800 MHz, e su di esso sono state fatte delle misure, per verificarne il corretto funzionamento

Experimental validation of a SAR-Based RFID localization technique exploiting an automated handling system

The synthetic aperture radar (SAR) approach has attracted a considerable interest in the context of phase-based techniques for the localization of UHF-RFID passive tags. In this letter, the results of an extensive experimental activity are presented, when the reader antenna is moved in front of a set of static passive UHF-RFID tags by means of a planar handling system. Measured performance is evaluated with respect to several system parameters: tag number and typology, tag reciprocal distance and orientation, trajectory and speed of the moving reader antenna. It is shown that the SAR processing can guarantee an accurate two-dimensional localization of multiple tags, with a localization error comparable to the size of typical commercial tags, by using a single reader antenna and without the need for any reference tag

Numerical analysis of wireless power transfer in near-field UHF-RFID systems

A preliminary numerical analysis of the power transfer efficiency (PTE) for the forward link of near-field (NF) ultra high frequency (UHF)-radio frequency identification (RFID) systems is addressed in this paper, by resorting to an impedance matrix approach where the matrix entries are determined through full-wave simulations. The paper is aimed to quantify the NF-coupling effects on the PTE, as a function of the distance between the reader and tag antennas. To allow for a PTE comparison between different reader and tag antenna pairs, a benchmarking tag-loading condition has been assumed, where the tag antenna is loaded with the impedance that maximizes the PTE. In a more realistic loading condition, the load impedance is assumed as equal to the conjugate of the tag antenna input impedance. Full-wave simulations use accurate antenna models of commercial UHF-RFID passive tags and reader antennas. Finally, a “shape-matched antenna” configuration has been selected, where the reader antenna is assumed as identical to the tag antenna. It is shown that the above configuration could be a valuable compact solution, at least for those systems where the relative orientation/position between the tag and reader antennas can be controlled, and their separation is of the order of a few centimeters or less

Technologies for Near-Field Focused Microwave Antennas

This paper provides a review spanning different technologies used to implement near-field focused antennas at the microwave frequency band up to a few tens of GHz: arrays of microstrip patches and printed dipoles, arrays of dielectric resonator antennas, reflectarrays, transmitarrays, Fresnel zone plate lenses, leaky-wave antennas, and waveguide arrays

Dome-Shaped Ellipsoidal Reflector Antenna for UHF-RFID Readers with Confined Near-Field Detection Region

This letter proposes and demonstrates the concept of ellipsoidal reflector antennas for radio frequency identification reader applications at UHF band. The antenna can be potentially integrated with environmental structures to confine the reader detection region. The energy bounding characteristics result from the dual-focus feature of an ellipsoidal reflector in its near-field region, as the feed located at one of the two foci can create a focused field distribution around the other focus. An axial energy focusing is, thus, formed to confine the energy in a restricted region (near-field beam focusing), also minimizing the interference effects outside of the targeted area. Both numerical simulations and experimental results are presented to demonstrate the feasibility of this antenna concept

Near-field-focused microwave antennas: Near-field shaping and implementation

Focusing the electromagnetic field radiated by an antenna at a point in the antenna near-field (NF) region is a wellknown technique to increase the electromagnetic power density in a size-limited spot region close to the antenna aperture. This article encompasses the basic working principles and the applications of the NF-focused (NFF) microwave antennas as well as the synthesis procedures suggested for the NF shaping around the focal point and the technologies currently used for their implementation