  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.
