Reconfigurable Intelligent Surface Aided Hybrid Beamforming: Optimal Placement and Beamforming Design

We consider reconfigurable intelligent surface (RIS) aided sixth-generation (6G) terahertz (THz) communications for indoor environment in which a base station (BS) wishes to send independent messages to its serving users with the help of multiple RISs. For indoor environment, various obstacles such as pillars, walls, and other objects can result in no line-of-sight signal path between the BS and a user, which can significantly degrade performance. To overcome such limitation of indoor THz communication, we firstly optimize the placement of RISs to maximize the coverage area. Under the optimized RIS placement, we propose 3D hybrid beamforming at the BS and phase adjustment at RISs, which are jointly performed at the BS and RISs via codebook-based 3D beam scanning with low complexity. Numerical simulations demonstrate that the proposed scheme significantly improves the average sum rate compared to the cases of no RIS and randomly deployed RISs and also outperforms several benchmark schemes. It is further shown that the proposed codebook-based 3D beam scanning efficiently aligns analog beams between BS-user links or BS-RIS-user links and, as a consequence, achieves the average sum rate close to that of coherent beam alignment requiring global channel state information.