Spatial-Wave Optimization for UAV-Mounted SIM-Aided Cell-Free Massive MIMO Systems

Current cell-free massive MIMO (CF-mMIMO) systems relying on fixed terrestrial access points (APs) and single-layer surfaces have limited deployment flexibility and beamforming capability. To address this limitation, we propose a novel unmanned aerial vehicle (UAV)-mounted stacked intelligent metasurface (SIM)-aided CF-mMIMO system that maximizes the sum rate through wave-domain beamforming and adaptive hovering-position optimization. We formulate a joint non-convex optimization problem over AP transmit power, multi-layer SIM phase shifts, and collision-free UAV deployment positions. To tackle this problem, we develop an alternating optimization (AO)-based iterative algorithm that combines a power allocation scheme inspired by maximum ratio transmission, gradient ascent, and successive convex approximation. Simulation results show that the proposed approach outperforms conventional schemes with fixed AP deployment, indicating that the integration of UAV mobility and SIM can effectively enhance the throughput of future wireless networks.