Uplink MIMO Communications With RIS-Integrated Base Station: Modeling and Experiments

Reconfigurable intelligent surface (RIS) has gained significant momentum as a cost-effective and energy-efficient technology to enable the next generation of mobile communications. In this article, we propose an RIS-integrated base station (BS) by deploying an RIS sufficiently close to the base station antennas (BAs), within its radiative near-field range. In the proposed RIS-integrated BS system, we utilize RIS as a passive array to reconfigure incoming signals from user equipments (UEs) without experiencing substantial path loss. The near-field channel model between the RIS and BAs is analyzed and applied to the RIS beam control model. Furthermore, we develop a practical algorithm to maximize the performance of the proposed RIS-integrated BS-based uplink multiple-input-multiple-output (MIMO) system with the aim of maintaining reasonable complexity. This goal is achieved by combining the two proposed algorithms, beam search and path-antenna pairing algorithms. The beam search algorithm identifies the radio paths of all UEs to the RIS by sweeping the beams of the RIS. Then, the path-antenna pairing algorithm allocates the strongest radio path between each UE and the RIS to one of the BAs by controlling an RIS beam to direct the signal from that path to the BA. Finally, we have built a real-time testbed to experiment with the proposed RIS-integrated BS-based uplink MIMO system. Numerical results, including experimental data, validate the effectiveness of the proposed RIS-integrated BS-based uplink MIMO system.