Design and Development of Novel Radio-Frequency Front Ends for Backscatter-Based Radio Systems

This thesis has addressed the design and development of key building blocks for backscatter-based radio frequency systems, particularly the development of novel chipless Radio-Frequency Identification (RFID) reader architectures, phasedarray antenna systems based on two-way array factor, and ultrawideband microstrip bandpass filters. The backscatter-based radio frequency systems are widely used in radar and RFID applications, where overall performance is governed by the interaction between the transmitted electromagnetic signal, the scattering characteristics of the target or engineered tag, and the capability of the receiver to extract relevant information. However, practical implementations of such systems are subject to several critical challenges, including weak backscattered signals, limited bandwidth, high sidelobe levels, poor angular resolution, and increased hardware cost and complexity. These challenges impose fundamental trade-offs between overall system performance and hardware feasibility, which remain insufficiently addressed in the existing state-of-the-art solutions. In this context, the first research contribution of the dissertation focuses on the development of a novel chipless RFID reader architecture aimed at enhancing the spectral width of the interrogating signal while maintaining a cost-effective implementation. In conventional frequency-domain RFID readers, the bandwidth of the interrogating signal is directly limited by the bandwidth of the voltage-controlled oscillator, which significantly increases system cost when ultrawideband operation is required. The proposed reader approach overcomes this limitation by enabling a substantial expansion of the interrogating signal bandwidth using low-frequency voltage-controlled oscillators and RF mixing techniques. The proposed novel RFID reader configuration has been analytically modelled and validated through simulation results by considering a 24-bit chipless RFID tag lumped circuit model. Finally, the proposed solutions confirm an effective interrogation of ultrawideband tags with a substantial reduction in hardware costs and implementation complexity compared to existing state-of-the-art solutions. The second major contribution of this thesis concerns the development of advanced phased-array antenna architectures based on the concept of the two-way array factor. In backscatter-based RF systems, such as radar and RFID, the overall radiation performance is governed by the combined effect of the transmit and backscattered electromagnetic signals. Consequently, the analysis and optimization of the two-way array factor are of primary importance. The key performance limitations in such systems include high sidelobe levels near the main radiating beam, poor directivity, limited angular resolution, as well as increased hardware cost and implementation complexity associated with beamforming network design. Conventional state-of-the-art sidelobe level reduction techniques, which are intended to suppress unwanted radiation outside the main beam and hence reduce interference, such as amplitude or phase tapering and optimization-based approaches, often lead to reduced directivity, degraded angular resolution, poor taper efficiency, and increased feeding network complexity. To address these limitations, this thesis develops a systematic array design methodology for three advanced phased array antenna configurations based on a two-way array factor that enables the best trade-off among the key antenna performance parameters, hardware cost, and complexity. The first array design solution is aimed at achieving low sidelobe levels without significantly compromising the directivity or gain using a two-weight amplitude distribution, which is important for maintaining strong radiation performance and reliable target identification or detection in practical RF sensing systems. The second proposed array design solution was primarily aimed at reducing hardware cost and implementation complexity through the intentional deactivation of a subset of antenna array elements, while preserving competitive radiation performance. This is particularly important in modern backscatter-based sensing and identification platforms, where reliable operation, compact design, and cost-effective implementation are often required simultaneously. The final proposed array design solution was developed to jointly achieve low sidelobe levels, reduced hardware cost, and lower implementation complexity through a subarray-based architecture in which the number of active elements is reduced, and a single-phase shifter is assigned to each subarray rather than to each individual element. This design strategy is particularly essential in practical array-based sensing and identification systems, where stringent performance requirements must be met while simultaneously drastically reducing hardware complexity. The final research contribution of this thesis focuses on the design and development of an ultrawideband bandpass filter based on a multi-mode resonator topology. The overall performance of RF front-end systems, such as radar and RFID, is strongly affected by the characteristics of the bandpass filter, especially in terms of bandwidth, insertion loss, return loss, selectivity, and group delay flatness. Conventional ultrawideband filter design solutions often rely on multilayer structures or complex coupling mechanisms, which eventually increase fabrication complexity and implementation cost. In contrast, the proposed bandpass filters introduce a compact and practically realisable filter design approach that enables ultrawideband operation along with enhanced in-band performances while maintaining a simple planar implementation. Overall, the research presented in this dissertation establishes a unified design perspective for backscatter-based electromagnetic systems by addressing the key fundamental building blocks that are important in determining overall system performance. The proposed solutions collectively demonstrate that it is possible to achieve significant improvements in key performance parameters while simultaneously reducing hardware cost and implementation complexity. These contributions are particularly relevant for emerging applications in the Internet of Things, smart sensing environments, airborne sensing systems, healthcare monitoring, and low-cost wireless identification platforms, where compactness, energy efficiency, and scalability are critical requirements.