Cooperative HARQ-Aided Multiple UAVs in Optical Aerospace Backhaul Networks

With the growing demand for high-speed connectivity and global coverage in future 6G networks, free-space optics (FSO)-based aerospace integrated networks, incorporating low Earth orbit (LEO) satellites, high-altitude platforms (HAP), and unmanned aerial vehicles (UAV), have recently attracted research efforts worldwide. Nevertheless, critical challenges on FSO links include weather conditions, atmospheric turbulence, and pointing misalignment. This paper addresses the design of error-control protocols for reliable FSO-based aerospace backhaul networks, when multiple UAVs are deployed as flying base stations (BSs). Specifically, we introduce a design proposal of a cooperative hybrid automatic repeat request (C-HARQ)-based frame allocation mechanism (FAM)/rate adaptation. The design proposal guarantees the latency fairness constraints among multiple UAVs experiencing varying turbulence channel conditions. An analytical channel model for HAP-aided relaying LEO satellite to the emerging UAV-mounted BS FSO links is provided. Moreover, we develop a comprehensive analytical framework taking into account the imperfect channel state information (CSI) to assess system performance metrics, including throughput, average frame delay, and energy efficiency. Numerical results confirm the effectiveness of our design proposal by comparing it with the conventional approach without FAM for various turbulence channel conditions and quality of service (QoS) requirements. Additionally, we offer a design guideline for the proper selection of parameters that can be helpful for the practical design of reliable FSO-based aerospace backhaul networks. Finally, the theoretical results are verified by Monte-Carlo simulations, along with some in-depth discussions.