Characterization of Indoor RIS-Assisted Channels at 304 GHz: Experimental Measurements, Challenges, and Future Directions

TL;DR

This study investigates indoor THz channels augmented by static RIS prototypes at $304\,\mathrm{GHz}$, designed with 1-, 2-, and 3-bit phase quantization to achieve non-specular reflections toward a $30^{\circ}$ direction. It combines full-wave EM simulations with a $304\,\mathrm{GHz}$ channel-sounding campaign to validate end-to-end path gains and the angular-profile of MPCs, revealing that near-field beamforming errors must be accounted for to match measurements. The results show good agreement with bistatic radar-based path-gain formulas once near-/far-field corrections using the RIS effective aperture are included, and demonstrate that higher phase-resolution RISs provide greater angular diversity and richer MPC signatures than lower-resolution or PEC backflips. The paper also outlines critical challenges in THz RIS fabrication, multi-functional RIS deployments, and the development of efficient, physics-informed channel models and time-domain RIS-aware propagation frameworks for robust RIS-enabled communications and sensing at THz frequencies.

Abstract

Reconfigurable Intelligent Surfaces (RISs) are expected to play a pivotal role in future indoor ultra high data rate wireless communications as well as highly accurate three-dimensional localization and sensing, mainly due to their capability to provide flexible, cost- and power-efficient coverage extension, even under blockage conditions. However, when considering beyond millimeter wave frequencies where there exists GHz-level available bandwidth, realistic models of indoor RIS-parameterized channels verified by field-trial measurements are unavailable. In this article, we first present and characterize three RIS prototypes with $100\times100$ unit cells of half-wavelength inter-cell spacing, which were optimized to offer a specific non-specular reflection with $1$-, $2$-, and $3$-bit phase quantization at $304$ GHz. The designed static RISs were considered in an indoor channel measurement campaign carried out with a $304$ GHz channel sounder. Channel measurements for two setups, one focusing on the transmitter-RIS-receiver path gain and the other on the angular spread of multipath components, are presented and compared with both state-of-the-art theoretical models as well as full-wave simulation results. The article is concluded with a list of challenges and research directions for RIS design and modeling of RIS-parameterized channels at THz frequencies.

Figures (5)

• Figure 1: The fabricated static RISs at $304$$\rm GHz$ with $100 \times 100$ unit cells of half-wavelength inter-cell spacing and $1$-, $2$-, and $3$-bit phase quantization designed to offer a non-specular reflection from a normal incidence to a $30^{\circ}$ outgoing plane.
• Figure 2: Normalized radiation pattern of the designed $100 \times 100$ static RIS with $3$-bit phase resolution with respect to the RCS of the main beam for different distances $r$ from the RIS center.
• Figure 3: Schematic view of the indoor channel measurements setups at $304$ GHz incorporating the designed static RISs.
• Figure 4: Comparison of the measured and theoretical (both formulas \ref{['bistatic radar equation']} and \ref{['bistatic radar equation adjusted']}) gains of the end-to-end Tx-RIS-Rx path with the designed $3$-bit RIS for the Setup 1 in Fig. \ref{['fig: Schematic view of P2P measurement']} versus the RIS-Rx distance $d_2$.
• Figure 5: Power angular profile measurements for the Setup 2 illustrated in Fig. \ref{['fig: Schematic view of angular sweep measurement']} considering three different cases for the RIS.