Frequency-Dependent Beamforming for RIS-Assisted Wideband Terahertz Systems.

To support extremely high data rates in 6G wireless networks, reconfigurable intelligent surface (RIS)-assisted terahertz (THz) communications have gained much attention. By controlling the phase shifts of reflecting elements, RIS can proactively modify the wireless THz channel, thereby enhancing the achievable data rate significantly. One major challenge of the wideband THz communication is the severe array gain loss caused by the beam split effect that the path components split into different spatial directions at different subcarrier frequencies. Therefore, the conventional phase shift control and beamforming techniques cannot be directly applied to wideband THz systems. In this paper, we propose a RIS-assisted frequency-dependent beamforming (R-FDB) technique maximizing the average data rate of the RIS-assisted wideband THz systems. Key idea of R-FDB is to alternately optimize the analog beamforming vector and the RIS phase shift vector by properly designing the parameters of the R-FDB network such that the average data rate of the wideband THz system is maximized. We demonstrate from the numerical evaluations that R-FDB achieves a significant data rate gain over the conventional schemes.