# A Survey on Applications of Reinforcement Learning in Flying Ad-Hoc Networks
## 3 Fundamentals of FANET
### Characteristics of FANET  
FANETs have some unique characteristics that make them different from other ad-hoc networks. Some of the major characteristics are given as follows:  
* **Node mobility and model:** There are different types of aerial vehicles which can move at an average speed of 6-500 km/h [9]. Thus, node mobility is the most important distinguishable factor which makes the FANET different from other ad-hoc networks. Furthermore, node mobility results in several challenges in communication designing. In FANET, UAVs can move freely at any direction and speed, depending on the task on its own. By contrast, other ad-hoc networks have regular, low, predefined, and controlled mobility [27]. Moreover, high mobility in FANET results in frequent changes in network topology compared to other ad-hoc networks.
* **Node density:** In wireless ad-hoc networks, node density is a crucial parameter for selecting data routing path. In FANET, node density mostly depends on the type of UAV, objective, UAV speed, and communication range. As UAVs can be speedy and have a long communication range, the number of UAV per unit area can decrease [30]. In other ad-hoc networks, such as VANETs, SANETs, WSNs, and SPANs, the node density is high compared to FANET [31].
* **Localization:** In ad-hoc network, global positing system (GPS) is widely used to locate the nodes. However, owing to the high speed mobility, FANETs use low latency GPS system to locate the UAVs such as network-based positioning [32], height-based positioning [33], differential GPS (DGPS) [34], and assisted GPS (AGPS) [35]. Moreover, localization is a major factor in flight trajectory and routing path selection.
* **Radio propagation:** When it comes to radio propagation model, FANETs have a great advantage of line-of-sight (LoS) over other ad-hoc networks. In FANET, UAVs can have a clear LoS among them due to their free mobility in the air. By contrast, in other ad-hoc networks, there is little or no LoS between the source and the destination owing to the geographical structure of the terrain.
* **Energy Constraint:** Energy limitation is one of the major design issues in ad-hoc networks. In FANET, it depends on the size of the UAV. Most of the large UAVs are not power-sensitive, whereas energy limitation is a concern for mini-UAVs [9]. In other ad-hoc networks, it varies from type to type as shown in Table 1. Table 1 summarizes the differences among different ad-hoc networks [36; 37; 38; 39; 40].  
Figure 8: Multigroup topology of FANET.  
\begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Characteristics** & **FANETs** & **VANETs** & **SANETs** & **RANETs** & **SPANs** & **WSNs** \\ \hline Node mobility & Random & Regular & Predefined & Controlled & Regular & Static or regular \\ Node Speed & Upto 500 km/h & Upto 150 km/h & Upto 130 km/h & Upto 25 km/h & Upto 6 km/h & Upto 8 km/h \\ Node Density & Low & High & Medium & Low & Medium & Varies with applications \\ Localization & DGFS/AGPS/Net/Height & GPS & GPS & GPS & GPS & GPS \\ Radio propagation & In air and & On ground & On water & On ground & On ground & On ground \\  & high LoS & and low LoS & and high LoS & and low LoS & and low LoS & and low LoS \\ Energy Limitation & Depends on the UAV & Low & Low & High & High & High \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparative analysis of different ad-hoc networks.