Energy Efficiency Maximization in RISs-Assisted UAVs-Based Edge Computing Network Using Deep Reinforcement Learning

Edge Computing (EC) pushes computational capability to the Terrestrial Devices (TDs), providing more efficient and faster computing solutions. Unmanned Aerial Vehicles (UAVs) equipped with EC servers can be flexibly deployed, even in complex terrains, to provide mobile computing services at all times. Meanwhile, UAVs can establish an air-to-ground line-of-sight link with TDs to improve the quality of communication link. However, the UAV-to-TD link may be obstructed by ground obstacles such as buildings or trees, leading to sub-optimal data transmission rates. To surmount this issue, Reconfigurable Intelligent Surfaces (RISs) emerge as a promising technology capable of intelligently reflecting signals to enhance communication quality between UAVs and TDs. In this paper, we consider the RISs-assisted multi-UAVs collaborative edge Computing Network (RUCN) in urban environment, in which we study the computational offloading problem. Our goal is to maximize the overall energy efficiency of UAVs by jointly optimizing the flight duration and trajectories of UAVs, and the phase shifts of RISs. It is worth noting that this problem has been formally established as NP-hard. Therefore, we propose the Deep Deterministic Policy Gradients based UAV Trajectory and RIS Phase shift optimization algorithm (UTRP-DDPG) to solve this complex optimization challenge. The results of extensive numerical experiments show that the proposed algorithm outperforms the other benchmark algorithms under various parameter settings. Specially, the UTRP-DDPG algorithm improves the UAV energy efficiency by at least 2% compared to DQN algorithm.