Reconfigurable Intelligent Surfaces for Over-the-Air Testing of Wireless Systems

Licentiatavhandling, 2025

Extensive research into reconfigurable intelligent surface (RIS) technology for manipulating electromagnetic (EM) propagation environments has enabled a wide range of emerging applications. This thesis explores the application of RISs in developing efficient and cost-effective over-the-air (OTA) testing methods and platforms for future wireless systems—a use case that demands advanced RIS architectures capable of complex field synthesis. Departing from conventional phase-only RIS designs, this work focuses on the development of amplitude-phase controllable RISs to provide enhanced control flexibility. A comprehensive framework encompassing RIS unit cell (UC) design, modeling, analysis, and experimental validation is established, addressing key technical challenges encountered at millimeter-wave (mmWave) frequencies.
This study presents a complete workflow from component-level evaluation to system-level demonstration. Tunable components—specifically p-i-n and varactor diodes—are experimentally characterized, and improved diode circuit models (DCMs) and EM models (EMMs) are developed to capture high-frequency parasitics and bias-dependent junction impedances. Two mmWave UCs are designed to address device-specific constraints: the varactor-based UC achieves continuous phase tuning exceeding 330.7°, despite a limited capacitance tuning ratio of 1.8; the p-i-n-based UC enables continuous amplitude tuning from 0 to 0.8, while revealing distinct nonlinear phase shift.
To realize simultaneous, independent, and continuous amplitude-phase control, a loop-embedded, end-folded RIS UC is proposed, integrating a forward-biased p-i-n diode and a reverse-biased varactor diode. A generalized complex-plane representation, referred to as Γ-coverage, is introduced to visualize UC reflection properties across the full bias space. Two performance metrics, A_cov and Γ_max, are defined to quantify reconfigurability. At 28 GHz, the UC achieves a 0–0.5 amplitude tuning range and full 360° phase control, with maximized Γ-coverage. The design employs a rigorous EM-circuit co-design methodology, incorporating DCMs, EMMs, and a semi-analytical equivalent circuit model (ECM) for accurate performance prediction under varying bias conditions and incidence angles.
The proposed RIS UC is further validated in a proof-of-concept OTA scenario, serving as a near-field plane wave (PW) generator for a compact antenna test range (CATR). The RIS-assisted configuration demonstrates high field uniformity and over 20 dB improvement in dynamic range compared to conventional far-field test ranges. These results highlight the strong potential of RIS technology for integration into next-generation OTA testing platforms.