Energy-Efficient UAV-Relayed High-altitude Platform to Ground User Communication

Unmanned aerial vehicles (UAVs) are increasingly pivoted in emerging Internet-of-Things (IoT) applications, serving as aerial base stations to provide reliable communication links. These UAVs enable ground users (GUs) to access remote areas where high-altitude platforms (HAPs) are exploited to connect with GUs, UAV relays and form multi-layer non-terrestrial networks to cooperate with other systems. However, UAV-based wireless networks have faced significant challenges in terms of energy consumption and operational duration of aerial devices. To address these challenges, we investigate the optimal resource allocation to maximize energy efficiency in a time-constrained HAP-to-GU communication network via assistance from a UAV relay. We propose a joint optimization method for the UAV’s location, transmit powers of HAP and UAV, and transmission time allocation to maximize energy efficiency while meeting the work time and power budget constraints. Our low-complexity iterative alternating optimization algorithm exploits the individual generalized convexity of the time allocation, power control, and UAV location problems to obtain the individually global optimal solutions. We also derive closed-form expressions for optimal time and power allocation variables to obtain analytical design insights. Finally, we conduct simulations to provide design insights and compare the performance of the jointly optimal and semi-adaptive algorithms against benchmarks.