Joint Trajectory and Phase Shift Design for UAV–RIS Enhanced Passive IoT in Nonterrestrial Networks

In non-terrestrial networks (NTN), passive Internet of Things (IoT) systems typically face challenges including unstable energy supply, limited signal coverage, and complex channel environments. This paper proposes an innovative optimization framework that jointly designs the trajectory and phase configuration of unmanned aerial vehicle (UAV) mounted reconfigurable intelligent surfaces (RIS) based on trust region policy optimization (TRPO) algorithms. The framework encompasses two key technical innovations: first, a transmission protocol based on orthogonal time-slot allocation that maximizes backscatter communication performance while ensuring efficient energy harvesting; second, a position-phase decoupled optimization strategy that employs a weighted geometric center method to determine UAV position, followed by TRPO optimization of RIS phase configuration, effectively overcoming learning convergence difficulties caused by position-dependent channel variations. Simulation results demonstrate that our scheme exhibits excellent performance across various scenarios, providing an efficient and practical solution for passive IoT applications in dynamic NTN.