Joint Optimization in UAV-ground Communications Empowered by Multiple Aerial RISs

Being capable of controlling the radio environment well, the contemporary advanced technique, namely reconfigurable intelligent surface (RIS), has been expected to facilitate unmanned aerial vehicle (UAV) communications. The existing studies mainly focus on terrestrial RIS to realize communication performance improvement, which can achieve only half-space reflection and may experience serious signal attenuation brought by several reflections. Motivated by this reason, this paper proposes to install multiple aerial RISs on the balloons to cooperatively boost the received signal power at ground users. Based on this architecture, we jointly optimize UAV trajectory, active beamforming, and passive beamforming, so as to maximize the average system sum rate. Considering that the composite channel gain becomes a complex function of UAV trajectory in the presence of multiple RISs, the block coordinate descent method is developed to address the formulated optimization problem. The mathematical treatment involves three stages, which can be executed by an iterative algorithm. Finally, numerical results verify the competitive superiorities of multiple aerial RISs and the excellence of joint optimization design compared with other benchmark ones.