# Free Space Optical Communication: An Enabling Backhaul Technology for 6G Non-Terrestrial Networks
### Airborne Nodes  
In addition to space nodes, aerial NTN nodes in the form of UAVs and HAPSs can be deployed based on the intended coverage and operation altitude requirements. Rotary-wing UAVs in different form factors are already available from different manufacturers and cover lower altitudes, typically up to a few kilometers. For example, base stations can be mounted on rotary-wing UAVs to provide wireless access, e.g., Nokia F-Cell [40] and AT&T's Cell on Wings (COW) [41]. The Nokia F-Cell system consists of a 64-antenna massive Multiple-Input Multiple-Output (MIMO) system that is used to create eight beams. It can handle multiple channels simultaneously and achieve a system throughput rate of about 1 Gbps over existing Long-Term Evolution (LTE) networks [40]. AT&T's COW is a tethered UAV that provides cellular coverage across around 100 km\({}^{2}\) area [42]. Their primary purpose is to serve as a cellular and data bridge to the internet for AT&T's First Responder \"FIRSTNET\" network, ensuring critical connectivity in disaster areas [43]. Advances in autonomous avionics and composite materials have also made possible the development of lightweight and long-endurance HAPSs for operation in the stratospheric layer, at altitudes of 17-22 km. In comparison to LEO satellites, they can provide a much lower latency and have the potential for a higher data rate capacity delivered per unit area. HAPS can be also seen as more environmentally friendly solutions compared to satellites. For example, it was discussed in [44] that the increasing number of LEO mega-constellations increases the risk of collisions and may result in space debris. By virtue of their operation, HAPS avoid such disadvantages associated with LEO satellites. HAPSs also have lower launch and recovery costs than satellite systems and are more cost-effective solutions. By achieving higher data rate capacities per unit area, HAPS are able to deliver enhanced communication and data transmission capabilities to specific target regions, due to their smaller footprints. Moreover, HAPSs provide a lower latency by being much closer to the Earth's surface, reducing signal travel times and allowing faster responses for real-time applications. Recent examples of aerostatic HAPS include the Thunderhead Balloon [45], Google's Loon [46], and Scheye [47]. Raven Aerostar has developed the Thunderhead Balloon, which is an aerostatic HAPS. These are polyethylene-made, pumpkin-shaped super-pressure balloons available in multiple sizes. They have a volume ranging from 1812 to 11,327 m\({}^{3}\) and can achieve flight altitudes between 15 and 20 km for the smaller models and between 22 and 28 km for the larger ones [48]. They have a payload capacity of around 55 kg, i.e., they can typically carry communication equipment, cameras, and other sensors for various monitoring and data-gathering purposes. For maintaining a stable position in the stratosphere, the balloon is equipped with sophisticated navigation and control systems. As another example, the Loon project (developed by Google X) was intended to provide internet access to remote and rural areas by means of balloons at an average altitude of around 20 km. The payload of Loon is in the range of 10 kg and it has a flying time that surpasses 100 days [49; 50; 51]. These balloons were maneuvered by adjusting their altitude in the stratosphere based on wind data. Project Loon was terminated in January 2021, due to technical and operational challenges. Renewed interest has arisen in the development of solar-powered aerodynamic HAPS with extended flight endurance. Recent examples include Facebook's Aquila [52], Airbus's Zephyr [53], Softbank's Sunglider [54], Korea Aerospace Research Institute (KARI) EAV-3 [55], High Altitude Long Endurance Unmanned Aerial System (HALE-UAS) [56], High Altitude Long Endurance (HALE)-Persistent High Altitude Solar Aircraft (PHASA) (PHASAA-35) [57], and Skydweller Aero [58], These are powered by solar energy harvested through solar panels attached to the top of their wings.