Wideband Reconfigurable Intelligent Surfaces

Abstract

Reconfigurable intelligent surfaces (RISs) have emerged as one of the most significant innovations in wireless communications, offering a novel approach to meeting the escalating demand for higher data rates, seamless coverage, and energy-efficient connectivity in next-generation networks. Unlike conventional wireless systems that rely on active and power-hungry components, RIS leverages nearly passive reflecting elements arranged in a planar array, whose electromagnetic responses can be dynamically reconfigured. By enabling programmable control over the incident wavefront, RIS introduces a new paradigm in which the wireless environment itself becomes a controllable entity. This capability not only enhances system performance but also reduces overall power consumption and hardware complexity, positioning RIS as a key enabler for sixth-generation (6G) and beyond communication technologies.

This thesis provides a comprehensive investigation into the principles, design methodologies, and system-level benefits of RIS technology. The research begins with an in-depth review of the current state of the art, highlighting both theoretical foundations and practical implementations of RIS. Building on this foundation, the work develops novel design strategies for reconfigurable unit cells intended for RIS applications. Several geometries are explored with the goal of achieving tunable reflection phase profiles, wide operational bandwidth, and multi-polarization capability. The designs integrate semiconductor switches and micro-electromechanical systems (MEMS) actuators, demonstrating the feasibility of programmable reconfigurability while addressing practical fabrication challenges .

To validate the proposed concepts, the designed unit cells are extended into array structures, where their performance is evaluated through both simulation and experimental testing. A practical prototype of a 1-bit reflectarray is fabricated and tested in an anechoic antenna chamber. The prototype demonstrates the key required functionalities, including beam steering, wideband operation, and dual-polarization control. These results confirm the potential of RIS to dynamically manipulate electromagnetic propagation in real-world scenarios. Furthermore, the thesis addresses critical implementation issues related to the scalability of RIS, the integration of control circuitry, and the trade-offs between design complexity and achievable performance.

The findings presented in this research underscore the innovative role of RIS in reshaping the architecture of wireless communication systems. By turning the propagation environment into an intelligent and programmable medium, RIS has the potential to significantly improve spectral efficiency, energy utilization, and overall network adaptability. The contributions of this thesis extend the understanding of RIS operation, provide novel unit cell structures, and deliver practical insights for prototyping and implementation. In doing so, this work not only advances academic knowledge in the field but also offers practical guidelines for industrial adoption of RIS in future wireless systems. Ultimately, this research highlights the promise of RIS as a cornerstone technology for realizing the vision of 6G and beyond.