# A Review of Flying Ad Hoc Networks: Key Characteristics, Applications, and Wireless Technologies
## 3 Main Characteristics of FANETs  
As shown in Figure 3, the main characteristics of FANETs are node density, node mobility, changing network topology, communication range, radio propagation model, localization, power consumption, frequency band, cost efficiency, versatility, agility, and network lifetime [65; 66; 67; 68], as detailed below.  
* _Node Density:_ The average number of UAVs per unit volume is the node density. The node density of UAVs in FANETs is less than in other ad hoc networks such as MANET and VANET. The node density can be varied according to the objective of the UAVs mission.
* _Node Mobility:_ Node mobility is one of the main features of FANETs; it is very high compared to VANET and MANET. UAV speed varies from 30 to 460 km/h depending on the type of UAV. This can causes issues, including disruptions, link failure, and more [69].
* _Changing network topology:_ The network topology in FANETs undergoes frequent changes due to the rapid movements of UAVs. Possible FANET topologies under conditions of frequent topological fluctuation include star topology, in which all UAVs directly communicate with the ground control station (GCS), and mesh topology, in which dynamic routing is necessary. Both the star and mesh network topologies have advantages and disadvantages; for example, with star network topologies, the dedicated link between each UAV and GCS fluctuates due to the high speed of UAVs, which can affects data exchange [66; 70; 71].
* _Radio propagation model:_ A crucial element when designing and simulating any communications system is the radio propagation model employed in the network. The simplest and most popular propagation model used in simulation tests is the Friis free space model. This model only uses the distance and frequency of the signal, which has corresponding limitations [72]. A UAV-to-Ground (U2G) communication channel is a widely used channel model in the literature. The different types of propagation models can be categorized [73] as follows:  
Figure 2: UAV Classification based on application domain: delivery, construction, military, agriculture, wireless coverage, and environmental monitoring.  
* _Theoretical models_: These models provide a detailed propagation model of U2G or U2U channels for UAV network scenarios.
* _Empirical models_: These models are obtained from a series of measurements made in various rural or urban scenarios.
* _Semi-empirical models_: These models are initiated as theoretical models and then varied according to a set of measures to match reality.
* _Well-known models_: These models attempt to verify the sufficiency of already-known propagation models in UAV network scenarios.
* _Localization_: FANETs take advantage of low-latency global positing (GPS) to locate UAVs to compensate for their high speed and mobility and the resulting network topology changes. Localization in FANETs can be based on network positioning, height, assisted GPS (AGPS), or differential GPS (DGPS) [66;74].
* _Power consumption and network lifetime_: Energy constraints represent a critical issue in ad hoc networks. Power consumption in FANETs depends on the size of the UAVs, the distances involved, the communication hardware of the FANET and the link, and other hindrances. Sensor and actuator nodes play a vital role in the power consumption of FANETs; lowering the requirements of power-sensitive devices in FANETs can directly improve network lifetime and reduce network breakdowns [66;74].
* _Frequency band_: Unlicensed bands such as 0.9 GHz and 2.4 GHz are widely used in UAV communication systems. However, using these bands can cause congestion. The frequency of 5 GHz integrated with IEEE 802.11a provides the best result for UAV-to-Ground links. Avoiding interference with other bands is best at 5.9 GHz with IEEE 802.11p [66].