## 3 Airborne Segment  
As stated in the introduction, the airborne segment, with its marked properties of flexibility, mobility and versatility, has been considered as an indispensable technology for enabling extremely high data rates and global wireless coverage [91; 92; 45]. In addition, they represent a more cost-effective solution than satellite layers or the network densification technique applied to the ground level [93]. Furthermore, wireless communication assisted by airborne segments could have many advantages with respect to space segments such as lower transmit power and reduced propagation delay, key features for many applicative scenarios [93].  
The idea to exploit flying platforms to reach ubiquitous connectivity is not completely new since the first attempts date back to the 1990s [94; 95; 96]. However, owing to the recent advances in autonomous vehicles, phased array technology, solar panel efficiency as well as battery UAVs have regained a tremendous attention for both researchers and the industry. For instance, some recent projects focusing on the deployment of UAV platforms for wireless connectivity are reported in [97; 98; 99].  
A straightforward UAV classification belonging to the airborne layer can be performed according to their operating altitude. Specifically, they can be classified into two categories: Low Altitude Platform (LAP) and High Altitude Platform (HAP). However, it is possible to achieve a more detailed classification of these flying platforms according to their size, mission endurance, engine type, take-off and landing methods and wing loading as reported in [100; 101].  
LAPs can fly at an altitude of tens of meters up to a few kilometers (km), and their greatest strengths are essentially fast movements as well as their extreme flexibility [102; 103]. Therefore, they can easily recharge or be replaced if needed. On the contrary, HAPs consist of flying platforms such as gas-filled balloons, airships or aircraft operating in the stratosphere at an altitude of around 20 km [104]. Due to the absence of clouds, thunderstorms and any weather disturbance at these altitudes, solar energy can be effectively utilized and can turn out to be a fundamental asset for HAPs. In general, they are more dedicated to longer missions as well as for providing a wider wireless footprint coverage [105]. Tethered Balloons (TBs) represent another promising technology solution within the framework of LAP flying platforms for delivering wireless communications [106]. TBs consist of inflated balloons, usually filled with helium to lift in air ad hoc equipment, operating in the troposphere at an altitude of around 200-500 m and are tethered to the ground by several ropes [47]. These flying platforms have the potential to deliver a wide variety of wireless communication services by overcoming various challenges faced in terrestrial or satellite segments such as limited coverage area, delay and lack of Line of Sight (LoS) links [107]. Their usage has been exploited in different scenarios spanning from emergency communication systems, observation, military applications and antenna radiation pattern measurements [107; 108; 109]. Other examples where TBs have also been exploited deal with fronthaul Wi-Fi access by using high gain omnidirectional antennas [110] as well as WiMAX connectivity [107]. Furthermore, in [111], TB has been employed for improving the achievable end-to-end data rate of ground users.