[Paper Review] Digital RIS (DRIS): The Future of Digital Beam Management in RIS-Assisted OWC Systems

Alain R. Ndjiongue, Telex M. N. Ngatched+2 more|arXiv (Cornell University)|Dec 18, 2021

Underwater Vehicles and Communication Systems37 references25 citations

TL;DR

This paper proposes Digital Reconfigurable Intelligent Surfaces (DRIS) for optical wireless communication (OWC) systems, enabling digital signal processing (DSP) techniques like space-time coding (STC) directly within physical metasurfaces. By using programmable liquid crystal (LC)-based elements controlled via binary codes, DRIS achieves precise beamforming with tunable phase shifts and reflection coefficients, demonstrating a 4-element DRIS prototype with controllable beam directions at 0°, 90°, 180°, and 270° and corresponding signal powers from -0.9151 dBm to -5.6864 dBm.

ABSTRACT

Reconfigurable intelligent surfaces (RIS) have been recently introduced to optical wireless communication (OWC) networks to resolve skip areas and improve the signal-to-noise ratio at the user's end. In OWC networks, RIS are based on mirrors or metasurfaces. Metasurfaces have evolved significantly over the last few years. As a result, coding, digital, programmable, and information metamaterials have been developed. The advantage of these materials is that they can enable digital signal processing (DSP) techniques. For the first time, this paper proposes the use of digital RIS (DRIS) in OWC systems. We discuss the concept of DRIS and the application of DSP methods to the physical material. In addition, we examine metamaterials for optical DRIS with liquid crystals serving as the front row material. Finally, we present a design example and discuss future research directions.

Motivation & Objective

• To address signal blockage and low SNR in OWC systems due to line-of-sight (LoS) obstructions.
• To overcome limitations of analog RIS by enabling digital signal processing (DSP) techniques within physical metasurfaces.
• To propose a novel DRIS architecture that treats RIS elements as binary-coded sequences for dynamic beam control.
• To demonstrate a practical LC-based DRIS prototype with controllable beam steering and power levels for OWC applications.

Proposed method

• Proposes DRIS as a RIS defined by code (X, k, ρ₀), where X is the number of elements, k is the number of bits per element, and ρ₀ is the initial reflection coefficient.
• Uses liquid crystal (LC) metasurfaces with blazed grating structures to control light deflection via electrically tunable birefringence and refractive index.
• Employs a 2D array of LC cells, each assigned a 2-bit digital code ('00', '01', '10', '11') to steer light to four distinct angles: 0°, 90°, 180°, and 270°.
• Applies the grating equation na(sinϕr + sinϕi) = mλ to model diffraction, with m = ±1 for first-order diffraction and ϕr = ±ϕi + 2α.
• Uses matrix modeling (Eq. 4) to simulate reflection coefficient dependence on incidence angle and refractive index.
• Designs a 4-element DRIS prototype using A4907 LC panels with 1.3 V, 1.6 V, 1.8 V, and 2.6 V control voltages to generate beams at different power levels.

Experimental results

Research questions

• RQ1 Can digital signal processing (DSP) techniques such as space-time coding (STC) be effectively implemented in physical metasurfaces for OWC systems?
• RQ2 How can liquid crystal (LC)-based metasurfaces be engineered to enable programmable beam steering with controllable phase and amplitude?
• RQ3 What is the impact of incident angle and refractive index on the reflection coefficient in LC-based DRIS elements?
• RQ4 How does the beam pattern and power distribution vary across different digital codes in a 2-bit DRIS configuration?
• RQ5 What are the key design parameters for achieving high directivity and efficient beamforming in optical DRIS?

Key findings

• A 4-element DRIS prototype was successfully designed using liquid crystal (LC) panels, with each element controlled by a 2-bit digital code to steer light to four distinct directions: 0°, 90°, 180°, and 270°.
• The DRIS achieved beam powers of -0.9151 dBm (0°), -2.0066 dBm (90°), -3.4679 dBm (270°), and -5.6864 dBm (180°), corresponding to control voltages of 1.3 V, 1.6 V, 1.8 V, and 2.6 V, respectively.
• The reflection coefficient decreased with increasing incidence angle and refractive index, with values dropping from 0.81 to 0.27 across different LC cell types.
• The beam pattern remained stable across varying incident angles due to the blazed grating design, which maintained high main lobe gain and directivity.
• The use of LCs enabled tunable birefringence and phase shifts via applied voltage, allowing dynamic reconfiguration of beam direction and amplitude.
• The DRIS architecture successfully enabled DSP techniques such as STC in physical materials, offering spatial diversity and improved performance in fading environments.