# Reinforcement Learning-Based Routing Protocols in Flying Ad Hoc Networks (FANET): A Review
## 4 Flying Ad Hoc Networks
### Unmanned Aerial Vehicles (UAVs)  
Progress in communication technologies, sensors, and electronic equipment has facilitated the manufacture of unmanned aerial vehicles (UAVs). These aerial systems, also known as drones, can fly automatically and pilot without human intervention. A UAV includes various equipment such as a power system, control system, different sensors, and communication module [40,41]. These components are shown in Figure 3.  
* **Power system:** This system is responsible for supplying the energy needed for data processing, sending and receiving data, flying, and especially controlling rotors through one or more batteries [42,43]. * **Control system:** This component is responsible for managing flight operations such as changing the flight height by rotating rotors based on the defined commands [42,43]. * **Sensor:** This module is responsible for sensing the environment and sending the collected data to the control system [42,43]. * **Communication module:** This module is responsible for receiving and sending information through radio signals or WiFi [42,43]. Figure 3: Components of a UAV. Figure 2: Various types of communication in the flying ad hoc network. Today, various types of drones have been designed and manufactured. They have unique features and are suitable for specific applications. For example, drones can be divided into two categories based on configuration:  
* **Rotary-wing UAVs (RW-UAVs):** These drones can be fixed in the air and perform vertical take-off and landing. As a result, they are more stable and more suitable for indoor areas. However, these drones have higher energy restrictions, slower speeds, and a low capacity compared to fixed-wing UAVs. These features affect their flight time because this time depends on various factors such as path plan, speed, weight, and energy source. RW-UAV is represented in Figure 4a [44,45]. * **Fixed-wing UAVs (FW-UAVs):** These drones have a longer flight duration, higher flight speed, and aerodynamic design compared to rotary-wing UAVs. These drones can be used for aerial surveillance. They include one body and two wings and are made in different sizes (small and large). However, their weight is more than rotary-wing UAVs. They are similar to traditional aircraft, and cannot be fixed in the air. Therefore, these drones are not suitable for fixed applications. FW-UAVs are displayed in Figure 4b [44,45]. Moreover, UAVs are divided into two groups based on autonomy level: remote control-based and fully automatic. * **Remote control-based UAV:** These drones are piloted directly by a pilot in line of sight (LoS) or based on feedback received from UAV sensors [45]. * **Fully automatic UAV:** They perform the flight operations completely independently and without human intervention and can complete their mission when faced with unforeseen operational and environmental conditions [45]. In addition, drones are categorized into two classes based on operating height: low-altitude and high-altitude. * **Low-altitude UAVs:** These drones are almost small, lightweight, and cheap. One feature of these drones is that they can easily deploy in an area and fly at a low flight altitude (from 10 meters to a few kilometers). Their speed is very high. However, they suffer from energy restrictions, and therefore, have a short flight duration. Due to fast and easy deployment, these drones are suitable for time-sensitive applications and can be used as an aerial station to provide a high data rate and wireless services at high-risk areas, sports events, and festivals [46]. * **High-altitude UAVs:** These drones are almost heavy and large and perform their flight operations at a high altitude (more than 10 km). Compared to low-altitude drones, high-altitude UAVs have a longer flight duration and can cover a wide area. However, they are more expensive than low-altitude drones and their deployment is more difficult. They are suitable for applications, which need a longer flight duration and more area coverage.