Outage-Aware Deployment in Multi-AAV-RIS Aided Networks With Imperfect CSI

In comparison to the unmanned aerial vehicle-mounted base station (BS), the unmanned aerial vehicle-mounted reconfigurable intelligent surface (UAV-RIS) is a potential technology for future high-quality communications due to its high gain and flexibility. At the same time, the inaccuracy of channel state information (CSI) estimation can cause fluctuations in rates. This poses a challenge for the deployment optimization of UAV-RIS. In this paper, we propose an outage-aware transmission scheme in multi-UAV-RIS assisted networks, where the outage probability of all users is constrained and the three-dimensional (3D) position optimization of UAV-RIS is taken into consideration. Specifically, by utilizing the Bernstein-Type inequality, the challenging outage probability constraint is approximated into a manageable explicit expression. We then decouple the resulting total power minimization problem and solve three sub-problems separately: beamforming at the BS, phase shifts at RISs, and 3D positions of UAV-RISs. Furthermore, we first consider the imperfect cascaded channels and assume perfect direct channels. By solving three sub-problems through techniques such as equivalent matrix transformation, semi-definite relaxation, and first-order Taylor expansion, an alternating optimization algorithm is formed. Subsequently, we extend the proposed algorithm to the case where direct channels are also inaccurate, and the complexity and convergence of the proposed algorithm are rigorously proven. Simulation results show that a reasonable configuration of UAV-RIS can effectively improve system performance. Compared to the baseline scheme, 3D position optimization of UAV-RIS and outage awareness can reduce the transmit power by 9.6 dB.