Experimental Validation of Reflective Near-Field Beamfocusing using a b-bit RIS

TL;DR

This work addresses the practical validation of reflective near-field beamfocusing using a RIS in a millimeter-wave indoor setting. It develops analytical expressions for the depth and angular width of the focal region for a $b$-bit RIS and experimentally confirms these predictions at 28 GHz with a $1$-bit, $1024$-element RIS. The results show that the peak focusing occurs near the intended focal point and that the beam shape, governed by Fresnel-like behavior, aligns with the theoretical model, even in the presence of nonidealities and multipath. The findings demonstrate the viability of RIS-enabled near-field focusing for capacity enhancement and interference management in practical RIS-assisted wireless communications.

Abstract

This paper presents the first experimental validation of reflective near-field beamfocusing using a reconfigurable intelligent surface (RIS). While beamfocusing has been theoretically established as a key feature of large-aperture RISs, its practical realization has remained unexplored. We derive new analytical expressions for the array gain achieved with a $b$-bit RIS in near-field line-of-sight scenarios, characterizing both the finite depth and angular width of the focal region. The theoretical results are validated through a series of measurements in an indoor office environment at 28 GHz using a one-bit 1024-element RIS. The experiments confirm that near-field beamfocusing can be dynamically achieved and accurately predicted by the proposed analytical model, despite the presence of hardware imperfections and multipath propagation. These findings demonstrate that near-field beamfocusing is a robust and practically viable feature of RIS-assisted wireless communications.

Figures (6)

• Figure 1: The receiver is located at $(\varphi_{\textrm{r}},\theta_{\textrm{r}},d_{\textrm{r}})$. We explore how the channel gain changes when the focal point of the RIS reflection is mismatched in either distance or angle.
• Figure 2: The setups used for experimental validation of reflective near-field beamfocusing from a dynamic RIS. The measurements were made at the KTH Royal Institute of Technology.
• Figure 3: The array gain in Setup 1 with $d_{\textrm{r}}=0.6$ m.
• Figure 4: The array gain in Setup 1 with $d_{\textrm{r}}=1.2$ m.
• Figure 5: The array gain in Setup 2 when varying both the focal distance and azimuth angle around the receiver.