# Non-Terrestrial Networks with UAVs: A Projection on Flying Ad-Hoc Networks
## 2 Structure of NTN and its Key Components
UAVs have a substantial advantage over other NTN components because of the free and flexible mobility of drones and their remarkable adaptability. In addition, they have several applications, \begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multirow{2}{*}{**Type**} & **Altitude** & **Speed** & **Uses** & **RTT [ms]** & **Coverage** \\  & **[km]** & **[km/s]** & **Uses** & **RTT [ms]** & **Ranking** \\ \hline GEO & 35–800 & \multirow{2}{*}{\(\sim\)3} & \multirow{2}{*}{Relay, BB} & \multirow{2}{*}{\(\sim\)500} & \multirow{2}{*}{1} \\ Sat. & [\(\times\)10\({}^{3}\)] & & & & \\ \hline MEO & 2–35 & \multirow{2}{*}{\(\sim\)4.2} & Navigation, relay, & \multirow{2}{*}{\(\sim\)200} & \multirow{2}{*}{2} \\ Sat. & [\(\times\)10\({}^{3}\)] & & & backhauling & \\ \hline LEO & 180–2000 & \multirow{2}{*}{\(\sim\)8} & High speed BB, & \multirow{2}{*}{\(\sim\)40} & \multirow{2}{*}{3} \\ Sat. & & & imaging, backhauling & & \\ \hline HAP & \(\sim\) 20 & \(<\)0.3 & \multirow{2}{*}{\
\begin{tabular}{c} Fixed/mobile BB, \\ Short/midterm backhauling. \\ \end{tabular} } & \multirow{2}{*}{0.13–0.33} & \multirow{2}{*}{4} \\ UAVs & \(<\)0.5 & \(<\)0.07 & \multirow{2}{*}{
\begin{tabular}{c} Communication, sensing, \\ relay, high resolution imaging \\ \end{tabular} } & \multirow{2}{*}{\(\sim\)1} & \multirow{2}{*}{5} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Key NTN components and their characteristics. UAVs are the lowest altitude platforms in the NTN that are advantageous in terms of cost, quick deployment, and flexibility in flight manoeuvres compared to other NTN components. RTT: Round trip time, BB: Broadbandincluding expanding cellular coverage, agriculture, civil, military, industry, search and rescue, and fire monitoring, among others [19]. With the increasing adoption of the UAVs (The global commercial market of UAVs in 2020 was valued at USD 13.44 billion [59] mainly in industries such as media and entertainment, delivery and logistics, energy, agriculture, real estate, and construction, security and law enforcement, and so on. Because corporate use cases for UAVs have grown significantly in recent years, industry participants, such as UAV manufacturers and telecommunication industry partners, are continually inventing, testing, and enhancing solutions for UAVs network markets. From 2021 to 2028, the UAVs share market is expected to expand at a compound annual growth rate (CAGR) of 57.5% [59]. Therefore, based on what has been elaborated above and the significant growth expected in the future of FANETs, in the following, we narrow down the focus of this work to UAV networks and FANETs while the detailed investigation of other NTN components is deferred to our future work), their role in NTNs is becoming more vital since they are flying closer to the Earth compared to satellites in higher orbits, e.g., LEOs, MEOs, and GEOs or even HAPs. Therefore, they can provide shorter delays and reduce the round-trip-time (RTT) of NTN communications. Furthermore, UAVs can provide better spatial, temporal, or spectral resolution than satellites by providing complementary dimensions [60]. Moreover, UAVs have better ASE than other NTN components because other NTN components cover larger areas with a restricted finite bandwidth, limiting their use to largely unpopulated areas. In addition, since satellites are constantly moving, and the pathloss of signals is significantly high, communication and synchronization between satellites are far more challenging than in UAVs. Overall, UAVs may outperform all other NTN components in some NTN use cases determined by 3 rd generation partnership project (3 GPP) such as (i) service reliability and (ii) ubiquity to provide NTN connectivity in the event of terrestrial network failure; and (iii) capacity to scale up service to meet peak demand for traffic from terrestrial networks [10; 61].