A Study on Spatial Impedance Adjustment Techniques for Cost-effective Beamforming Surface Design

Abstract

This thesis focuses on a cost-effective beamforming surface design method based on spatial impedance adjustment techniques. In order to simplify multi-variants three-dimensional electromagnetic analysis, spatial impedance adjustment techniques are proposed and exemplified at millimeter wave (mmWave) spectrum. First, a wide-beam coverage lens antenna is presented for enlarging the antenna effective aperture while maintaining high gain and low sidelobe level with highly compatible to the conventional phased array antennas. In this work, a new accurate and efficient design methodology is proposed for complex integrated lens antenna (ILA), to achieve wide-angle beam coverage with scan loss mitigation at mmWave spectrum. A two-dimensional (2-D) ray-tracing model is used to estimate the refractions on the elliptically curved boundaries based on geometrical optics. The computational memory for optimizing the geometrical parameters of the ILA is reduced by a factor of 10,000 by using the proposed Geometric Optics-based Multiple Scattering (GOMS) algorithm. The measurement results for the performance of the fabricated prototype of the ILA validate the wide-angle scanning with scan loss mitigation. Second, in order to compensate the undesired blockage effect, scattering mitigation frequency selective surface (FSS) is proposed without modifying the obstacle. A new concept of cloaking, which renders the obstacle electromagnetically-transparent, is exemplified by using a single substrate FSS at mmWave. The devised FSS mitigates the undesired forward scattering and reconstructs the distorted far-field radiation pattern. Based on superposition and linearity, spatial impedance distribution is analyzed and the proposed impedance mapping is validated by full-wave simulations. In addition, the measured radiation patterns confirm the blockage compensation capabilities for both TMz- and TEz-polarization. This work is expected to be applicable in inevitable blockage scenarios for both indoor wireless channel and outdoor propagation channel model. Third, liquid crystal-driven sustainable reconfigurable intelligent surface (RIS) is proposed for managing random wireless propagation channels. Reconfigurable beam scanning is achieved by varying the permittivity of nematic liquid crystal. In addition, a passive lens-based beamforming circuit is employed for detecting the locations of transmitter. According to the channel state information, the devised RIS operates the self-sustainable feedback loop for providing optimal line-of-sight propagation channel link. The fabricated prototype is validated by link-level test including throughput, constellation and error vector magnitude (EVM).