Reconfigurable Intelligent Surface Aided Non-Coaxial OAM Transmission for Capacity Improvement

The reconfigurable intelligent surface (RIS) is expected to be a key enabler for future wireless systems, as it can control the propagation environment and overcome the harmful effects of the wireless channel. In this paper, the orbital angular momentum (OAM) beam generated by a holographic multiple-input multiple-output (MIMO) is employed to utilize the OAM mode orthogonality and improve the capacity performance of the line-of-sight base station-RIS channel. The pre-transmitting and pre-receiving matrices are designed to guarantee the coaxial requirement of OAM transmission. Multiple beams are generated towards different reflecting RISs, and various pre-receiving matrices are deployed to adapt to varying movements and rotation conditions, ensuring spatial multiplexing gain and transmission robustness. Moreover, the RIS-user equipment (UE) phase shift matrix of the reflecting RIS is designed for both far-field and near-field UEs with different wavefront. Simulation results are provided to validate the pre-transmitting, pre-receiving matrix designs and the orthogonality of OAM beams with distinct modes, as well as demonstrate the capacity improvement of the proposed OAM beam-empowered multi-RIS transmission.