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

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 beamfocusing can be dynamically achieved and accurately predicted by the proposed simplified 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.

💡 Research Summary

This paper delivers the first experimental demonstration of reflective near‑field beamfocusing using a reconfigurable intelligent surface (RIS). While theoretical works have highlighted the ability of large‑aperture RISs to focus reflected waves in the Fresnel (radiative near‑field) region, no practical validation had been reported. The authors first derive closed‑form expressions for the array gain of a b‑bit RIS in line‑of‑sight (LOS) near‑field links, explicitly characterizing the depth (range) and angular width of the focal region.

The system model assumes a single‑antenna transmitter and receiver linked via an N‑element RIS. The optimal continuous‑phase configuration maximizes the end‑to‑end channel gain, but practical RIS hardware can only realize a finite set of phase values. By modeling the quantization error for each element as an independent uniform random variable in the interval (