  To support extremely high data rates, reconfigurable intelligent surface
(RIS)-assisted terahertz (THz) communication is considered to be a promising
technology for future sixth-generation networks. However, due to the typical
employment of hybrid beamforming architecture in THz systems, as well as the
passive nature of RIS which lacks the capability to process pilot signals,
obtaining channel state information (CSI) is facing significant challenges. To
accurately estimate the cascaded channel, we propose a novel low-complexity
channel estimation scheme, which includes three steps. Specifically, we first
estimate full CSI within a small subset of subcarriers (SCs). Then, we acquire
angular information at base station and RIS based on the full CSI. Finally, we
derive spatial directions and recover full-CSI for the remaining SCs.
Theoretical analysis and simulation results demonstrate that the proposed
scheme can achieve superior performance in terms of normalized
mean-square-error and exhibit a lower computational complexity compared with
the existing algorithms.
