Experimental Approach to Adaptive RIS-Based Beam Tracking and User Localization in 5G mmWave Networks

The utilization of Reconfigurable Intelligent Surfaces (RIS) offers a promising avenue for realizing electronic and software-driven beam control within wireless communication setups. In this paper, a beam management technique based on the user-controlled-RIS approach is presented, where the user feedback directs the operation of the RIS. This paper introduces a novel scheme for user equipment (UE) localization supported by RIS. The proposed approach focuses on accurately determining the UE's position and dynamically adjusting the beam direction as the user moves. To achieve this, we assume that the UE can transmit signaling information, such as received signal strength (RSS), to the RIS via a low-frequency side link. In our design, the RIS leverages its ability to control the beamwidth, considering the number of its unit cells, thereby enhancing both positioning/tracking accuracy and communication efficiency. The proposed technique is implemented in a mmWave RIS-aided transmission system operating at 28 GHz. Measurement results showcase precise tracking capabilities, with angle mismatches ranging from 1° to 2°. The implemented algorithms facilitate flexible beam refinement through rapid beam management techniques, thereby minimizing tracking latency to near real-time levels. Fisher Information (FI) and Cramér-Rao lower bound (CRLB) analysis are derived to investigate the accuracy of the positioning and tracking system.