Switched beam array antenna for 2.45 GHz RFID localisation

Nguyen, Dinh Loi

November 2011

Thesis or dissertation

© 2011 Dinh Loi Nguyen. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.

Most people are familiar with Radio Frequency Identification (RFID) technology as its applications are around us in many systems, such as anti-theft devices in shops, supermarkets and libraries, building access systems. The number of applications and devices has developed explosively in the past decade and has become one of the fastest growing sectors of the radio technology industry. RFID systems allow information to be attached to items such as products, animals or even people. This information may be read, and in some cases changed, by an RFID reader some distance away. RFID localisation systems are an active area of research as they add further capabilities to current RFID systems. Localisation allows the position of tagged items to be determined from the RFID communications signal. Current systems can confirm that a specific item is within the reader read range but finding the item requires a systematic search.

The primary objective of this thesis is to build a new, economical and versatile 2.45 GHz localisation system for active and passive RFID tags. Alternative systems rely upon signal strength measurements and yield large uncertainties due to the unknown orientation of the tags. The proposed system reader consists of a microstrip switched beam array antenna. The array antenna allows the estimation of the angular position of the tag. The use of two or more readers provides the tags position by triangulation. The array antenna provides other advantages such as improved read range and mitigation against interference and multipath fading.

In the process of designing the antenna array, several new, compact, slotted, circularly polarised, square patch antennas have been produced. A range of technologies have been applied to achieve miniaturisation of the microwave circuit elements require to drive the antenna array, the Butler matrix (BM). Fabricated circuits achieve significant size area reduction with similar performances compared to the conventional ones. These developments will have wide application beyond the RFID localisation system.

Finally, accuracy of the system varies with angle due to the complicated relationships between Angle of Arrival (AoA) and port output powers. The localisation algorithm along with measured power inside the standard anechoic room show that the standard deviations of the AoA estimation errors less than 1.6° for most angles in the operational range from -90 degrees to 90 degree and a standard error less than 0.6o for AoA angles with 20o of the perpendicular at AoA with power ratios perturbed by additive noise of standard deviation 0.5 dB.

Department of Engineering, The University of Hull
Riley, N. G.; Paulson, Kevin S.
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