[Paper Review] Double-Sided Beamforming in OWC Systems Using Omni-Digital Reconfigurable Intelligent Surfaces

Alain R. Ndjiongue, Telex M. N. Ngatched+2 more|arXiv (Cornell University)|Mar 8, 2022

Advanced Wireless Communication Technologies4 citations

TL;DR

This paper proposes omni-digital reconfigurable intelligent surfaces (omni-DRIS) for optical wireless communications (OWC), enabling simultaneous reflection and refraction of light via digitally controlled elements. The system achieves double-sided beamforming, improving coverage and achievable rate, with performance peaking at an optimal number of elements and increasing with higher phase shift resolution.

ABSTRACT

In this paper, we introduce a variant of reconfigurable intelligent surfaces (RISs) called omni-digital-RISs (DRISs), which allow multiple physical processes, with application to optical wireless communications systems. The proposed omni-DRIS contains both reflectors and refractive elements, as well as elements that perform both simultaneously. We describe and explain the concept of omni-DRIS, suggest and analyze an omni-DRIS coding structure, discuss metamaterials to be used, and provide a design example. Furthermore, we demonstrate that the achievable rate of an omni-DRIS system depends on the number of omni-DRIS elements, bits per phase shift, and the number of unused elements. In addition, we show that the achievable rate upper bound is related to the number of omni-DRIS elements, and conclude by discussing future research directions.

Motivation & Objective

• Address the limitation of conventional RIS in covering users on both sides of the surface by enabling simultaneous reflection and refraction.
• Overcome the blind spot issue in traditional RIS deployments where users behind the surface are not served.
• Develop a digital, reconfigurable surface that supports both reflective and transmissive wavefront control via programmable elements.
• Design a coding structure and material framework (e.g., liquid crystals) to enable multi-mode operation of omni-DRIS elements.
• Investigate the impact of element count, phase shift resolution, and unused elements on system performance in OWC.

Proposed method

• Propose omni-DRIS as a hybrid RIS that integrates reflectors, refractors, and dual-function elements in a single surface.
• Design a four-state coding protocol where each omni-DRIS element can be in reflection, refraction, simultaneous reflection/refraction, or inactive state.
• Utilize liquid crystal (LC)-based metamaterials to enable dynamic reconfiguration of phase shifts and transmission/reflection coefficients.
• Implement double-sided beam management to steer light beams toward users on both sides of the omni-DRIS surface.
• Model the OWC channel considering double-pass transmission in reflective elements and single-pass in transmissive ones.
• Use achievable rate as a performance metric, analyzing its dependence on total elements, active elements, and bits per phase shift.

Experimental results

Research questions

• RQ1 How does the integration of reflection and refraction in a single RIS element improve coverage in OWC systems?
• RQ2 What is the optimal number of omni-DRIS elements that maximizes the achievable rate in a VLC system?
• RQ3 How does increasing the number of bits per phase shift affect the system’s achievable rate and where is the performance saturation point?
• RQ4 What is the impact of unused or inactive omni-DRIS elements on system throughput and beamforming efficiency?
• RQ5 How do material properties, such as those of liquid crystals, influence the feasibility and performance of omni-DRIS in OWC?

Key findings

• The achievable rate of an omni-DRIS-assisted OWC system depends on the number of active elements, the number of bits per phase shift, and the number of unused elements.
• There exists an optimal number of omni-DRIS elements that maximizes the achievable rate, beyond which performance degrades due to increased phase quantization overhead.
• Increasing the number of bits per phase shift improves the achievable rate up to a point, after which the rate begins to decrease due to diminishing returns and system complexity.
• The achievable rate is inversely proportional to the number of active elements for a fixed total number of elements, indicating a trade-off between resolution and resource utilization.
• Liquid crystal-based omni-DRIS designs can support all four operational states (reflective, transmissive, dual, inactive) through bottom-layer tuning, enabling full digital control.
• Double-sided beamforming with omni-DRIS enables 360° user coverage, significantly enhancing network connectivity in indoor visible light communication environments.