Scalable mm-Wave Liquid Crystal Reconfigurable Intelligent Surfaces based on the Delay Line Architecture

TL;DR

The paper presents scalable, broadband mm-wave reconfigurable intelligent surfaces based on a delay-line architecture that decouples the phase-shifting LC layer from the radiating plane, enabling continuous phase control exceeding $360^ ext{$0$}$ with a micrometer-thin LC layer around $4.6\ \mu\mathrm{m}$. Two prototypes with $120$ and $750$ elements operating near $60\mathrm{GHz}$ demonstrate beam steering up to $\pm60^ ext{$0$}$ and $-3\mathrm{dB}$ bandwidths exceeding $9\%$, while per-unit-cell power consumption remains nanowatt-scale; however, measured aperture efficiencies are lower than simulations due to fabrication tolerances and layer nonuniformities. The work analyzes steering, power use, response times, and aperture efficiency, and discusses the impact of fabrication tolerances, providing a detailed comparison with conventional LC-RIS approaches. The results support the scalability and potential advantages of DLA-based LC-RIS—namely, very thin LC layers, broad bandwidth, and favorable trade-offs between bandwidth, efficiency, and response time—while highlighting practical manufacturing challenges that must be addressed for large-scale deployment.

Abstract

This paper presents the design, fabrication, and characterization of broadband liquid crystal (LC) reconfigurable intelligent surfaces (RIS) operating around 60 GHz and scaling up to 750 radiating elements. The RISs employ a delay line architecture (DLA) that decouples the phase shifting and radiating layer, enabling wide bandwidth, continuous phase control exceeding 360°, and fast response times with a micrometer-thin LC layer of 4.6 micrometer. Two prototypes with 120 and 750 elements are realized using identical unit cells and column-wise biasing. Measurements demonstrate beam steering over +-60° and -3 dB bandwidths exceeding 9% for both apertures, confirming the scalability of the proposed architecture. On top of a measured nanowatt power consumption per unit cell, aperture efficiencies above 20% are predicted by simulations. While the measured efficiencies are reduced to 9.2% and 2.6%, a detailed analysis verifies that this reduction can be attributed to technological challenges in a laboratory environment. Finally, a comprehensive comparison between the applied DLA-based LC-RIS and a conventional approach highlights the superior potential of applied architecture.

Figures (7)

• Figure 1: Exemplary outdoor scenario for a RIS neuder_a_broadband_750_2025. The different colors in the RIS qualitatively indicate different reflected phase at the radiating elements. LoS: Line of sight. RIS: Reconfigurable intelligent surface.
• Figure 2: Design and simulation results of the proposed - neuder_a_broadband_750_2025. a) Operation principle of the delay line architecture, the unit cell layout and the side view of the LC-RIS. b) Performance of the phase shifter in terms of losses (FoM) and compactness. The red dotted line indicates a compactness of 360°/λ_0. c) Simulated aperture efficiency $\mathrm{\eta_{Co}}$ in a 768 ($\mathrm{32 \times 24}$) element RIS. LC: Liquid crystal. RIS: Reconfigurable intelligent surface. FoM: Figure of Merit. $\theta_\mathrm{r}$: reflected angle.
• Figure 3: Measurement of the proposed - neuder_a_broadband_750_2025. a) Front and back view of the - with 120 and 750 elements and the measurement setup. b) Normalized measurement of the beam-steering capabilities of the 120 element . c) Normalized measurement of the beam-steering capabilities of the 750 element . The dashed line in the right heat map indicates the impact of beam-squinting. : Reconfigurable intelligent surface. : Liquid crystal. Tx: Transmitting antenna. Rx: Receiving antenna. $\theta_\mathrm{r}$: Reflected angle. VNA: Vector network analyzer.
• Figure 4: Aperture efficiency results of the RIS prototype with 120 elements.
• Figure 5: Aperture efficiency results of the RIS prototype with 750 elements.