# Non-Terrestrial Networks with UAVs: A Projection on Flying Ad-Hoc Networks
## 2 Structure of NTN and its Key Components  
This section introduces the key components of NTN along with its structure. It also explains the significance of UAV networks, i.e., FANET, in comparison with other NTN components. In conjunction with terrestrial networks (such as cellular networks), a structure of NTN is illustrated in Figure 4 with geosynchronous Equatorial orbit (GEO), medium Earth orbit (MEO), and low Earth orbit (LEO) satellites. Furthermore, there are high altitude platforms (HAP) easing the communication in lower layers close to where airplanes fly. In the altitudes closer to the Earth, UAVs are supposed to be dominant flying objects facilitating real-time communication in both non-terrestrial and terrestrial networks. In Table 2, the technical specifications of these key NTN components are summarized while they are discussed in the following. Figure 4: Structure of NTN and its key components. UAVs play a vital role to facilitate real-time and reliable communication in both non-terrestrial and terrestrial networks. These flying networks have a great potential to leverage current mobile communication networks to deliver advanced IoT services in the future. Figure 3: Organization of the paper. * **GEO Satellites:** GEO satellites are launched to orbit at an angular speed that is equivalent to that of Earth. Moreover, they are assigned to orbit along a route parallel to Earth's rotation (also referred to as geostationary or stabilized satellites because they appear stationary to the user on the ground), thus mostly delivering coverage to a defined and fixed area [49]. Deploying GEOs is typically very expensive mainly due to their launching costs. This is because they are allocated to orbit at altitudes higher than 35,000 km [50] (common altitude: 35,786 km). GEO satellites are mainly used for TV broadcasts and in some cases to relay communications between spacecrafts, including the space shuttle, the Hubble space telescope, and Earth-based control centers. * **MEO Satellites**: MEOs, also referred to as intermediate circular orbit (ICO), are satellites that orbit Earth between altitudes of 2000 km and 35,780 km (common altitude: 20,000 km). MEOs orbit Earth at faster angular speeds than GEO satellites due to their proximity to Earth. Indeed, as satellites are closer to Earth, the gravitational attraction becomes greater, and the satellites move faster [51]. Usually, it takes 2 to 24 h for one MEO satellite to complete a full orbit around Earth. They are mostly used for navigation systems, such as global positioning systems (GPS) [52]. * **LEO Satellites**: LEO satellites are designed to orbit Earth at much lower altitudes, usually between 200 km and 2000 km. This enables LEO satellites to provide satellite services at relatively low delays, but at the expense of deploying more satellites [53]. However, since they are closer to Earth, they orbit much faster, i.e., (\(>\)25,000 km/h, and their orbit period varies over a range of 40-120 min. This means that each LEO experiences at least 12 and up to 36 morning and night periods in only 24 h [54]. Hence, a constellation of LEO satellites is proposed to compensate for and offer continuous, worldwide coverage for high-speed broadband communication as well as imaging and communication backhaul [53; 55]. * **HAPs**: Contrary to satellites and at lower altitudes of 17 to 50 km (stratospheric layer), HAPs (Additionally, known as high altitude aeronautical platforms (HAAPs))can be used to provide broadband communication services as well as broadcasting services by either unmanned airships, e.g., balloons, or airplanes [56]. HAP-based communication is suitable for large geographical areas where HAPs can move more freely and flexibly compared to satellites [57]. They are mainly powered by solar technology and non-polluting fuel cells. * **UAVs**: The use of UAVs is anticipated to be essential in 6G and beyond, thanks to their widespread and rising use in a variety of applications [58].