Joint Trajectory–Energy Design for Integrated Satellite–Terrestrial Networks With Hybrid Light/RF Energy Harvesting

Integrated satellite–terrestrial networks (ISTNs) are envisioned as a key component of 6G to support global connectivity for massive Internet of Things (IoT) devices. However, while terminals operate under strict energy and computational constraints and face additional challenges from satellite dynamics, current designs rarely integrate orbital information with device energy states in a unified system design, leaving significant room for improvement. Motivated by this, we propose a joint trajectory- and energy-aware framework for ISTNs operating in dynamic environments, supported by a low-complexity algorithm tailored for energy-constrained IoT devices. Specifically, we first develop a system model centered on a day–night separated hybrid energy-harvesting (DNEH) mechanism, where each device harvests light energy during the daytime and radio frequency (RF) energy from the gateway (GW) at night. Meanwhile, the GW broadcasts simplified orbital information to assist device operations, thereby forming an integrated communication–energy transfer framework that enhances system sustainability. Analytical characterizations are subsequently derived for the joint coverage probability, and simulations capture orbital dynamics such as the real-time distance variations. Building on the semi-analytical model, the proposed drift-regularized per-frame (DRPF) algorithm determines transmission opportunities that optimize the average Age of Information (AoI) by capturing the coupling between trajectory dynamics and energy availability. Numerical evaluations confirm the theoretical advantages of the proposed framework, showing substantial performance gains over the Immediate and Minimum Distance transmission baselines. Moreover, the optimized TD-SWIPT configuration and transmission scheduling translate these findings into actionable design guidelines for energy-constrained ISTNs.