Millimeter-Wave Reconfigurable Intelligent Surfaces: Enhancing Bandwidth and 3D coverage with Limited Phase Quantization Levels

Overview

Millimeter-wave (mm-Wave) communication systems offer large bandwidths but suffer from considerable propagation loss, blockage sensitivity, and limited diffraction, leading to unreliable links. This can be improved if the propagation environment can be controlled between the transmit and receive systems. Reconfigurable intelligent surfaces (RIS) have emerged as a promising solution to reconfigure electromagnetic (EM) wave propagation and enhance wireless performance by engineering the radio environment. Despite the increasing research interest in reconfigurable intelligent surfaces (RISs) for millimeter-wave (mm-wave) communications, most existing studies focus on theoretical analysis and numerical simulations, while practical hardware demonstrations and experimental validations remain relatively limited. Moreover, practical RIS implementations at mm-Wave frequencies face several critical challenges, including limited bandwidth, degraded beamforming performance due to phase quantization, and restricted spatial coverage of conventional planar configurations. To address these issues, this thesis investigates the design, fabrication, and experimental validation of advanced wideband mm-Wave RIS architectures. First, a wideband mm-Wave RIS operating across the 5G n257 and n258 bands is developed to overcome the narrowband limitation of conventional resonant unit cells, enabling stable reflection characteristics over a wide frequency range. While this design improves bandwidth performance, its limited phase resolution introduces quantization effects that degrade beamforming quality and increase 5 sidelobe levels in the near field as well as the quantization lobe in the far field. To address this limitation, a second RIS design with 2-bit phase quantization is proposed, achieving improved beamforming accuracy and reduced sidelobe levels while maintaining wideband operation. Furthermore, conventional planar RIS structures are limited to a single (planar) surface and hemispherical coverage. To overcome this constraint, a three-dimensional RIS architecture is introduced to enable spatial EM wave control and enhanced coverage. The proposed designs are validated through full-wave simulations, hardware prototyping, and experimental characterization. This thesis also validates their practical utility through system-level demonstrations.