We consider unmanned aerial vehicle (UAV)-enabled wireless systems where downlink communications between a multi-antenna UAV and multiple users are assisted by a hybrid active-passive reconfigurable intelligent surface (RIS). We aim at a fairness design of two typical UAV-enabled networks, namely the static-UAV network where the UAV is deployed at a fixed location to serve all users at the same time, and the mobile-UAV network which employs the time division multiple access protocol. In both networks, our goal is to maximize the minimum rate among users through jointly optimizing the UAV's location/trajectory, transmit beamformer, and RIS coefficients. The resulting problems are highly nonconvex due to a strong coupling between the involved variables. We develop efficient algorithms based on block coordinate ascend and successive convex approximation to effectively solve these problems in an iterative manner. In particular, in the optimization of the mobile-UAV network, closed-form solutions to the transmit beamformer and RIS passive coefficients are derived. Numerical results show that a hybrid RIS equipped with only 4 active elements and a power budget of 0 dBm offers an improvement of 38%-63% in minimum rate, while that achieved by a passive RIS is only about 15%, with the same total number of elements.

翻译：本文考虑在混合有源-无源可重构智能表面（RIS）辅助下，多天线无人机与多用户间下行链路通信的无人机无线系统。我们针对两种典型无人机网络的公平性设计展开研究：静态无人机网络（无人机部署于固定位置同时服务所有用户）和移动无人机网络（采用时分多址协议）。在两种网络中，目标均是通过联合优化无人机位置/轨迹、发射波束成形器以及RIS系数，最大化用户间的最小速率。由于变量间存在强耦合，所构建的问题呈现高度非凸性。我们基于块坐标上升法和逐次凸近似技术开发了高效算法，以迭代方式有效求解这些问题。特别地，在移动无人机网络优化中，我们推导出发射波束成形器与RIS无源系数的闭式解。数值结果表明：配备仅4个有源元件且功率预算为0 dBm的混合RIS，在最小速率上可获得38%-63%的提升，而相同总数元件的无源RIS仅提升约15%。