Millimeter-Wave Reconfigurable Intelligent Surface With Independent and Continuous Amplitude-Phase Control: Unit Cell Design and Circuit Model

This paper presents a reconfigurable intelligent surface (RIS) unit cell (UC) with independent amplitude-phase control, crucial for complex field shaping in RIS-assisted over-the-air testing. Unlike conventional phase-only tuning UCs, the proposed design enables continuous control from reflective to absorptive states for advanced field synthesis. To quantify reconfigurability, we introduce a generalized complex-plane representation, termed Γ-coverage, mapping both amplitude and phase over all bias states. For the first time at millimeter-wave frequencies, a loop-embedded end-folded UC integrating a forward-biased <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i>-<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</i>-<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> diode and a reverse-biased varactor diode is developed to maximize Γ-coverage. The UC achieves a 0–0.5 amplitude tuning range (equivalent to a controllable loss from complete attenuation to 6 dB) and a −180°–+180° phase tuning range at 28 GHz. The maximum incident angle reaches 45° within ±48° phase fluctuations, extending to 60° with full-phase tuning under a 12 dB loss criterion. We derive empirical circuit models for both diodes to account for high-frequency parasitic effects and formulate a semi-analytical UC equivalent circuit model. The UC prototype is evaluated using a waveguide simulator. The operational bandwidths for reconfigurability are 25.8–28.0 GHz with a relative Γ-coverage area exceeding 25%, and 26.5–27.7 GHz for 360° phase control with 8.4–12 dB losses. The instantaneous bandwidth for stable operation spans 27.3–27.7 GHz, maintaining ±50° phase fluctuations within 5.6–13.4 dB losses. The UC is analyzed in an RIS-assisted near-field plane-wave generation scenario for a compact antenna test range (CATR), achieving high field uniformity (< 0.6 dB and < 2.5° errors). Despite intrinsic UC losses, the RIS-based CATR reduces total system loss by 22.8 dB compared to a far-field test range.