Realistic Frequency-Coded Linear and Nonlinear Chipless RFID: System Model and Clutter Mitigation

The future of identification technology is expected to shift from barcode to Chipless Radio Frequency Identification (RFID) due to its advanced non-line-of-sight (NLOS) scanning, extended reading range, and parallel readout method. However, chipless RFID technology is still in its early stages and faces several challenges that hinder practical implementation. This dissertation’s research focuses on three crucial elements of the frequency-coded (FC) chipless RFID system.

This dissertation first explores the impact of backscattered signals on the frequency spectrum in chipless RFID tags containing encoded information bits produced by resonators. A notch model is developed to analyze the effect of the notch position and quality factors. The study also considers incident wave polarization and orientation in the calculation of tag radar cross section (RCS) for detection performance analysis with polarization and orientation mismatches. The limitations and errors of tag measurements are explained, and approaches for RCS measurement and retransmission-based tags are presented. The maximum read range is calculated and verified, taking into account FCC regulations for UWB. The experiments and simulations consider a monostatic configuration with one antenna for transmission and reception.

The second topic covered in this thesis is how clutter and multipath reflections affect linear chipless RFID tags, as well as a mitigation method. To thoroughly investigate the effects of clutter reflected signals and multipath, a detailed mathematical model of the linear RFID system based on the bi-static reader is first developed. The model takes into account reflections from clutter and multipath objects, the angle at which the incoming signal arrives and the angle at which the reflected signal departs, the orientation of the tag, and the reader and tag’s polarization and spatial gain properties. Two co-polarized RCS-based chipless tags (dipole array and square patch) are investigated to evaluate the established analytical model and investigate detectability in an interference environment using a matched-filter detection technique. When the tag is measured and simulated in both a disturbance-free and a disturbance-affected environment, there is a noticeable close agreement between the measured and analytically generated results, demonstrating the mathematical model’s effectiveness. The simulation and measurement results also revealed that when a flat metal reflector (30.5 cm × 22.5 cm) was placed behind the tag, unwanted notches appeared and the genuine notch vanished, indicating that the interfering signal completely overlaps the ID of the co-polarized tag and severely limits the tag’s detectability. As a result, a solution based on reading the tag in cross-polarization mode by etching a diagonal slot in the square patch tag was proposed. The proposed tag has excellent environmental immunity, allowing detection even in the presence of interfering items (dielectric and metallic). The dynamic range of the received peak signal, however, decreases as the reflector gets closer to the tag. The deployment of the cross-polarization-based tag may also be hampered by environmental conditions that change polarization.

The third topic covered in this dissertation focuses on the improvement of a nonlinear chipless RFID tag to eliminate clutter effects and reduce the background multipath effect. A biasfree diode is integrated into the tag structure to create nonlinearity and the tag is interrogated at a fundamental frequency, responding with its unique ID at a harmonic frequency. The performance of the non-linear system is examined using a state-of-the-art nonlinear tag, which performed well in an interference-free environment but showed a drop in second harmonic power in front of a metal reflector. A mathematical model and ray tracing simulation were developed to understand the cause of the drop and validate the measurement results. The simulation and measurement results show that the existing nonlinear tag with an omnidirectional radiation pattern at the transmitting antenna is not able to fully eliminate the background noise. As a result, the main objective of eliminating background noise was not achieved. To improve the performance, a new tag design was introduced with a directional antenna system and improved impedance matching, resulting in a 13 dBm higher second harmonic received power and increased read range, as well as better harmonic performance and reduced background multipath.



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