Achievable Rate Optimization of the Multi-layer RIS-Aided Terahertz Ultra-massive MIMO Communications

THz communication, as one of the candidate technologies for 6G, suffers a large path loss, and reconfigurable intelligent surface (RIS) has been regarded as an effective assistant. In this paper, a multi-layer RIS (ML-RIS) aided terahertz (THz) ultra-massive MIMO communication system is proposed. Based on the proposed ML-RIS, we formulate a problem to maximize the achievable rate for ML-RIS-assisted THz communication. Aiming at the non-convexity caused by the RIS multi-layer structure, we design an efficient iterative algorithm to solve the resulting non-convex problem. Numerical simulation results demonstrate that the proposed schemes can greatly enhance the achievable data rate compared to traditional single layer RIS systems.