Sum Rate Maximization of Uplink Active RIS and UAV-assisted THz Mobile Communications

Terahertz (THz) band communication is considered as a key technology for 6G and beyond. THz communication can boost plethora of promising applications. Wireless communication systems for unmanned aerial vehicle (UAV) promises to provide cost-effective wireless connectivity without infrastructure coverage. As a supporting element, an active reconfigurable intelligent surface (RIS) can be used such that both phase shifts and amplitudes of its elements are tuned to create favorable beam steering, such that undesired interference from other devices is mitigated. In this paper, we consider a UAV equipped with an active RIS operating in THz band, where a cellular user as well as a number of co-existing device-to-device (D2D) links are considered. We assume that direct paths between each two communicating nodes are not available. Only the RIS-reflected paths are considered. We propose a joint power allocation and active RIS precoding matrix optimization scheme to maximize the sum rate. The proposed scheme is evaluated in terms of the achieved sum rate and compared with passive RIS and multiantenna amplify-and-forward (AF) relaying. Numerical results show that the proposed active RIS-aided approach is superior to the other schemes, achieving the highest sum rate.