Trajectory Planning and Transmission Scheduling for UAV-Borne RIS Assisted Energy-Efficient Uplink Transmissions

Mounted on an uncrewed aerial vehicle (UAV), UAV-borne reconfigurable intelligent surfaces (RISs) can enjoy flexibility with a considerable probability of line-of-sight (LoS) channels, and assist communications between terrestrial nodes. Challenges arise in designing UAV-borne RIS (U-RIS) assisted communications, such as the influence of non-LoS, dynamic RIS configuration, and energy consumption of the UAV. To tackle these challenges, we put forth a new U-RIS assisted uplink transmission framework, in which the direct links from the ground users to the corresponding ground base station are blocked. We optimize the energy efficiency of the network by collectively devising the RIS configuration, user scheduling, power allocation, and UAV trajectory, under a probabilistic LoS channel model. Through alternating optimization and successive convex approximation, an efficient approach is established. Simulations validate that our approach is able to significantly enhance energy efficiency by 78% in comparison with its benchmarks.