The student Jokin Cifuentes Antxia an EXCELLENT CUM LAUDE with mention INTERNATIONAL DOCTORATE

The student Jokin Cifuentes Antxia an EXCELLENT CUM LAUDE with mention INTERNATIONAL DOCTORATE

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• Thesis title: Design, EM Simulation, Manufacturing and Performance Evaluation of a Varactor-Based RIS

Court:

• Presidency: Iñigo Ederra Urzainqui (Universidad Pública de Navarra)
• Vocal: Alessio Zappone (Universitá degli Studi di Cassino e del Lazio Meridionale)
• Secretary: Maitane Barrenechea Carrasco (Mondragon Unibertsitatea)

Abstract:

The sixth generation of wireless communication (6G) is expected to outperform 5G by providing ultra-high data rates, extremely low latency, and ubiquitous connectivity, while improving energy efficiency and sustainability. Among the key enabling technologies to meet these demands are Reconfigurable Intelligent Surfaces (RIS), which are passive devices composed of a large number of unit cells whose reflective response can be reconfigured. By adjusting these cells, the RIS can modify how the surface reflects incoming signals, thereby improving wireless channel conditions such as signal strength, coverage, and interference. Despite their theoretical potential, RIS implementations in real-world scenarios remain limited, with existing studies often relying on idealized assumptions and lacking detailed design insights. This thesis addresses this gap by designing, fabricating, and experimentally validating a realistic RIS prototype based on BST varactors operating at 9.6 GHz. To facilitate the control of the surface, as well as to reduce the number of discrete components and simplify the control network routing on the PCB, a clustering-based strategy is adopted. In this approach, groups of unit cells are jointly reconfigured using a single control signal and varactor, leading to an 80% reduction in the number of components and control lines, with a quantified worst-case performance loss of 1.72 dB. 

The study includes the design and implementation of four different metasurfaces, aimed at experimentally validating and deepening the understanding of RIS operating principles. Full-wave electromagnetic simulations were performed, incorporating non-idealities such as component tolerances, mutual coupling effects, and realistic S-parameter models, showing strong correlation with experimental results. Furthermore, system-level evaluations reveal the performance gap between idealized and practical RIS models, quantifying the limitations introduced by discretization and reflection angle dependence. Ultimately, this work provides a practical methodology and design guide for the development of RIS, with special emphasis on electromagnetic simulations and the challenges encountered during fabrication. These results help advance the practical implementation of RIS by complementing existing theoretical research with experimentally validated design and fabrication methods.