Free Space Spot-Beamforming for IoT Multi-User Near-Orthogonal Overlay Communications Enhanced by OAM Waves and Reconfigurable Meta-Surface

The rapid proliferation of Internet of things (IoT) devices has driven an unprecedented demand for efficient multi-user communication solutions. Traditional methods for multiplexing, such as frequency-division and code-division multiplexing, are struggling to accommodate the increasing number of users and the limited spectrum resources available. To address these challenges, this paper presents an innovative wireless communication system that leverages reconfigurable intelligent surface (RIS) and orbital angular momentum (OAM) technologies to enhance spectral efficiency, reduce interference, and meet the high data rate requirements of modern IoT applications. The proposed system integrates a multi-mode OAM transmitter to generate signals carrying multiplexed data streams, which are subsequently directed to a metasurface-based RIS. The RIS is designed to receive the incident OAM beams, demultiplex the data, and focus it in real-time to distinct spatial regions, achieving a high signal-to-noise ratio (SNR) and minimizing interference for efficient multi-user transmission. A practical 2-bit transmissive RIS design is employed, which enables dynamic control over OAM modes, focusing energy at different user locations in the near-field, thereby realizing flexible and independent control of each unit. To generate the multi-mode OAM beams, a simplified microstrip antenna working at 10 GHz is utilized, incorporating a multi-layer printed circuit board (PCB) isolation design to minimize coupling interference between modes, ensuring reliable and efficient mode generation. To validate the effectiveness of the proposed system, we conduct full-wave simulation experiments, and build a complete communication testing environment, covering the entire communication process from the multi-mode OAM transmitter to the RIS-based communication link. Experimental results demonstrate that the proposed solution can effectively achieve near-field spot-beam focusing through the RIS and enable multi-user, same-frequency data separation at each focal point. This provides a novel and efficient solution for high-spectral-efficiency, low-interference multi-user data interaction in IoT networks, offering new insights for enhancing multi-user access and data transmission efficiency in various IoT scenarios, including smart factories, logistics centers, and in-vehicle communications.