Reconfigurable Intelligent Surfaces for THz: Hardware Impairments and Switching Technologies

TL;DR

The paper addresses the challenge of deploying Reconfigurable Intelligent Surfaces (RIS) for THz communications by enforcing physics-based bounds on performance, rather than relying on idealized models. It conducts a link-budget analysis at $140$ GHz for indoor ($20$ m) and outdoor ($100$ m) scenarios, deriving required RIS sizes, switching element counts, and the maximum bandwidth and capacity achievable given angular coverage, phase quantization, and insertion losses. The study reveals that beam squint, finite aperture, and non-ideal aperture efficiency substantially constrain THz RIS performance, especially as RIS size and frequency increase, and highlights the critical role of practical unit-cell design and switch technology. By showcasing a D-band unit-cell example and surveying conventional and emerging switch technologies, the work provides a concrete pathway for hardware-aware RIS design and fosters collaboration between hardware developers and communications researchers for realistic RIS deployments. The findings have practical implications for balancing RIS coverage, bandwidth, and energy efficiency in next-generation 6G/6G-plus networks.

Abstract

The demand for unprecedented performance in the upcoming 6G wireless networks is fomenting the research on THz communications empowered by Reconfigurable Inteligent Surfaces (RISs). A wide range of use cases have been proposed, most of them, assuming high-level RIS models that overlook some of the hardware impairments that this technology faces. The expectation is that the emergent reconfigurable THz technologies will eventually overcome its current limitations. This disassociation from the hardware may mask nonphysical assumptions, perceived as hardware limitations. In this paper, a top-down approach bounded by physical constraints is presented, distilling from system-level specifications, hardware requirements, and upper bounds for the RIS-aided system performance. We consider D-band indoor and outdoor scenarios where a more realistic assessment of the state-of-the-art solution can be made. The goal is to highlight the intricacies of the design procedure based on sound assumptions for the RIS performance. For a given signal range and angular coverage, we quantify the required RIS size, number of switching elements, and maximum achievable bandwidth and capacity.

Figures (6)

• Figure 1: A typical wireless communications scenario includes two types of RISs: a transmissive RIS for efficient beamforming at the Base Station (BS) side and a reflective RIS to extend the coverage towards a mobile terminal. The reconfigurability of the RIS functionality is managed by a dedicated controller.
• Figure 2: Full-wave evaluation of a RIS composed by $30\times30$ unit cells ($300\times300~\text{mm}^{2}$) operating at $30$ GHz with $1$-bit phase reconfigurability (PIN-diode) when illuminated by a near-field source.
• Figure 3: Numerical evaluation of an idealized RIS aperture with diameter $111$ mm operating at $140$ GHz that emulates an outgoing tilted plane wave ($45^{\circ})$ generated with different phase quantization steps (1/2/3 bits).
• Figure 4: Radar cross-section of a $30\times30$ RIS with $1$-bit phase resolution at $30$ GHz designed to reflect and tilt a normal incident plane wave to $45^{\circ}$ (this corresponds to $135^{\circ}$ in the adopted frame for the simulation model).
• Figure 5: Photo of the fabricated $1$-bit RIS prototype at sub-THz in Eucap2024RISfab.