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. 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.

翻译：本文研究面向室内环境的可重构智能表面辅助第六代（6G）太赫兹通信系统，其中基站需通过多个RIS的帮助向服务用户发送独立消息。在室内环境中，立柱、墙体及其他障碍物会导致基站与用户间无视距信号路径，严重降低通信性能。为克服室内太赫兹通信的这一局限，我们首先优化RIS的布设位置以最大化覆盖范围。基于优化后的RIS布设，我们提出了基站侧的三维混合波束赋形与RIS侧的相位调整联合设计方案，通过基于码本的低复杂度三维波束扫描在基站与RIS间实现协同操作。数值仿真表明，与无RIS及随机布设RIS的场景相比，所提方案显著提升平均和速率。进一步研究表明，所提基于码本的三维波束扫描能有效对准基站-用户链路或基站-RIS-用户链路的模拟波束，最终使平均和速率接近需要全局信道状态信息的相干波束对准方案。