Research on Maximizing Multi-User Spectrum Efficiency in RIS-UAV mmWave System Based on Alternating Optimization

Reconfigurable Intelligent Surface (RIS) has grown rapidly due to its performance improvement for wireless networks, and the RIS mounted unmanned aerial vehicle (RIS-UAV) system, has been increasingly attracted widespread attention. In this paper, the multi-user millimeter waves (mmWave) system that requires RIS-UAV assistance is considered. By employing the RIS element grouping (EG) strategy, the non-convex optimization challenge of maximizing spectral efficiency (SE) is decomposed into three manageable sub-problems: interference minimization, useful signal power maximization, and signal to interference plus noise ratio (SINR) maximization. These sub-problems are systematically solved using block diagonalization (BD), phase alignment, and block coordinate descent (BCD) techniques, respectively. Subsequently, we propose an alternating optimization algorithm that jointly designs hybrid beamforming and phase shift. Numerical results demonstrate the superior performance of our proposed algorithm compared to existing benchmarks.