Enhancing signal coverage for UHF RFID systems

The low signal coverage problem has long been one of the most crucial problems in UHF RFID systems. In this thesis, the problem is tackled using a combination of antenna design, mathematical modelling and algorithm development. A novel UHF RFID reader antenna with 2D beam-scanning ability, a wide axial ratio (AR) beamwidth and tunable polarisation performance is designed, simulated, measured and tested in Chapter 3 of this thesis. It is experimentally verified that the antenna's AR minima follow its gain maxima, generating an ultra-wide 3 dB AR beamwidth of 136°. It is demonstrated that the ratio of inventoried tags within 120 seconds increased from 65% with a static beam configuration to 96% with 1D beam-scanning and further to 100% with 2D beam-scanning. By manually degrading the proposed antenna's AR to 4 dB without affecting other parameters, a maximum 18% degradation in the coverage ratio is observed, verifying the coverage contribution from the proposed antenna's improved AR performance. To further improve the signal coverage of RFID systems, an optimised reader antenna deployment is necessary. A 3D ray-tracing model tailored for RFID applications is proposed, constructed and verified in Chapter 4 of this thesis. Compared with ray-tracing models in the literature, the proposed model considers practical antennas' irregular beam patterns, polarisations, arbitrary orientations and variable AR. The proposed model is verified against commercial MoM simulation software and has shown comparable accuracy but orders of magnitude less time and memory consumption. It is also verified using experiments and shows a good match between the prediction and measurement. In the case study presented, the proposed model demonstrates the ability to consider practical antennas' variable AR, predicting a 13% coverage difference between the AR-optimised antenna and its AR-degraded version, matching the measured value at 10%. Due to the higher power requirement of sensor tags over conventional passive tags, a fast and effective beam-forming algorithm is desirable. A scalable, computationally-light beam-forming algorithm for multi-antenna RFID systems is proposed and experimentally verified in Chapter 5. The proposed algorithm does not rely on iterative phase searching methods, avoiding slow convergence speed and can be implemented with low computational complexity. It is experimentally demonstrated that the proposed algorithm can achieve an RSSI improvement of up to 18 dB using three transmitting antennas. For the signal coverage problem in near-field RFID applications, a simple yet effective near-field antenna design is proposed and verified in Chapter 6 using full-wave simulations. The proposed antenna's signal nulls can be dynamically moved using two sources with suitable phase differences. Moreover, an almost uniform field distribution along the strip-line can be achieved by setting the phase difference of two sources to be 90° or 270°. Furthermore, by adding a new track of strip-line and setting a proper phase difference, a side-power leakage suppression of up to 20 dB can be achieved with the vertical detection range extended from 43 cm to 65 cm at 30 dBm input power. This theory is further verified by changing the phase difference of S1 and S2 to π, in which the side-leakage gets greatly strengthened as predicted by theory.