Study On Several Problems In Ultra High Frequency Radio Frequency Identification

This dissertation focuses on theoretical investigations and design practices of ultra-high-frequency radio frequency identification (UHF RFID) systems, which are carried out in four research areas as follows: a) Design of the RF front-ends of long range passive backscatter UHF RFID readers; b) Source pulse waveforms optimizations for Ultra-wideband (UWB) RFID systems; c) Miniaturized antenna designs for UHF RFID readers; d) Design optimizations for UHF RFID tag antennas.This dissertation provides mathematically formulated theoretical models and practical engineering designs for developing the RF front-ends of UHF-band long range passive backscatter RFID readers, which fill the gap in the relevant research area. The operational principles of the reader's RF front-end are investigated and the signals in the system are described and analyzed in detail. The reader's structure is proposed and particularly the design details of its RF front-end are presented. Through evaluating the time-averaged absorption power of the tag and the demodulation output signal-noise-ratio of the receiver, the mathematical models of the read range are figured out. Based on the models, several key design suggestions are concluded to improve the performance of read range. This dissertation also presents the detailed hardware implementations and engineering designs for the RF front-ends of two readers, of which one works at 915 MHz and the other one at 2.45 GHz. The prototype readers are manufactured and measured, which achieve the read range of 11.8 meters and are comparable to the first-class readers in the international markets. The measurement results also show good agreement with the calculated ones, which validate the proposed mathematical models and design methods well.This dissertation creatively provides mathematically formulated theoretical models and design methods for optimizing the source pulse waveforms of UWB RFID systems. The basic operational principles of UWB RFID systems that are based on the delay-reflection-pulse-sequence passive tags are investigated and itssignals are described and analyzed in detail. Through establishing the functional of the source pulse which is based on the concept of "usable energy" in the received pulse and then extremizing the functional by means of the calculus of variations, the mathematical models of the optimized source pulse waveforms are shown to be associated with the eigenfunctions of a homogeneous Fredholm linear integral equation of the second kind. An efficient algorithm is developed for numerically solving the integral equation models. A design example of UWB pulse transmission system is also demonstrated and the optimized source pulse waveforms are figured out. Specially, the transmission performance of the optimized pulse is compared with that of several common pulse waveforms, which validates the proposed optimization models and design methods well. Based on the proposed mathematical model, through optimizing the interrogating pulse waveforms of the reader, its read range can be maximized to efficiently improve the overall performance of UWB RFID systems.This dissertation develops a 2.45 GHz miniaturized planar printed-dipole-type UHF RFID reader antenna having necessary directive gain. The novel V-shaped ground plane reflection structure is proposed, which increases the antenna's impedance bandwidth and improve its radiation directivity. The antenna's dimensions are optimized by means of electromagnetic simulation and the finalized antenna's size is only 51 mm x 40 mm x 1.6 mm. The measurement results show that the antenna's percent impedance bandwidth is 33%, the peak antenna gain is more than 2 dBi, and the backward radiation suppression is beyond 7 dB, which all demonstrate better performance than the traditional antenna without such a V-shaped reflection structure.This dissertation develops a 2.45 GHz miniaturized planar printed-dipole-type UHF RFID tag antenna. The novel impedance transformer is proposed to boost the resistance of the short dipole and simultaneously compensate its reactance inductively, which contributes to miniaturizing the tag antenna efficiently. The antenna's dimensions are optimized by means of electromagnetic simulation and thefinalized antenna's size is only 33.55 mm × 8.54 mm × 0.50 mm. The antenna achieves good conjugated-impedance matching with its terminated microchip, its radiation pattern has the ideal omni-directionality and the peak antenna gain reaches -0.2 dBi, which all meet the design requirements for the miniaturized tag antennas well.