# Free Space Optical Communication: An Enabling Backhaul Technology for 6G Non-Terrestrial Networks
## 3 FSO-Based Backhauling for NTNs
### Self-Sustainability  
A major design criterion in airborne backhauling is the capability for self-sustainable operation, to enable extended operation periods. Airborne nodes, such as UAVs and HAPSs, often face challenges in terms of a limited energy supply. To address this, solar energy harvesting is commonly used [130; 131; 132; 133; 134]. By harvesting energy, airborne nodes can reduce their reliance on traditional power sources, such as batteries or fuel cells with limitations in capacity and weight. Solar power offers a sustainable and continuously available energy supply (during the daytime), making it an attractive option for extending the operational lifespan and capabilities of UAVs and HAPSs. Photovoltaic (PV) cells are installed on UAVs and HAPSs, to convert sunlight into electrical energy. At high altitudes above the troposphere, solar light can directly reach the HAPSs without atmospheric attenuation. This is because the transmittance above the troposphere is almost unity. This unobstructed access to sunlight allows PV cells to generate more efficient and consistent energy. Integrating solar cells into the structure of UAVs and HAPSs involves careful design considerations [133; 134]. For example, the PV cells can be placed on the wide-span wings of light-weight HAPS, maximizing the surface area available for solar energy harvesting. To further optimize energy efficiency, advanced techniques, such as Maximum Power Point Tracking (MPPT) can be deployed. MPPT algorithms ensure that the PV cells operate at their maximum power output, adapting to varying solar conditions and increasing the energy harvesting efficiency. Another energy harvesting method is laser power beaming as depicted in Figure 14, which uses a dedicated laser source for wireless power transfer [135; 136]. A high-power long-distance laser is used for energizing the airborne nodes, eliminating the need for these nodes to return to a charging station for recharging. The laser source can be positioned on the ground or on an LEO satellite that captures solar energy. Alternatively, they could be mounted on another HAPS or balloon. In the literature, various studies have explored energy harvesting for airborne nodes and energy-efficient communications. For example, [PERSON] et al., in [137], considered energy harvesting for stratospheric HAPS, to support long-endurance mobile wireless communication services. They utilized PV cells to harvest solar energy during the day, ensuring self-sufficiency for propulsion and communication. They also proposed energy-efficient flight trajectories and power allocation, to optimize energy usage for communication and aerodynamics, emphasizing the importance of joint design for resource allocation. In [138], [PERSON] et al. considered a three-layer heterogeneous network model integrating space-air-ground resources for optimization of the sum-rate of Internet of Things (IoT) devices in 6G networks. They developed an algorithm for task scheduling, resource optimization, yielding improved energy efficiency and sum-rate compared to existing methods. [PERSON] et al., in [139], examined a scenario where an airborne energy transmitter broadcasts wireless power to recharge two Machine-Type Devices (MTDs). Later on, this work was expanded by the same authors, in [140], to involve multiple MTDs. [PERSON] et al. [141] studied a UAV-based wireless system with energy harvesting, focusing on energy-efficient communication and path planning. They addressed the problem through trajectory optimization using dual methods and successive convex approximation. Optimal data transmission parameters were derived, and efficient solutions for energy minimization were provided and validated through simulations. [PERSON] et al. [142] considered a scenario where an UAV charges ground users using RF wireless power transfer, enabling uplink data transmission with harvested energy. They optimized the UAV trajectory and wireless resources, to enhance user throughput during the UAV's flight duration.