## 2 UAV Communication
### Characterizing the UAV Network
#### 2.1.1 Multi UAV Network  
The utilization of a single large UAV for a mission was common in the early days of UAV use. As a result, in these systems, the UAV-based communication network only had one aerial node and one or more terrestrial nodes. The UAVs in a multi UAV system are smaller and less expensive and they have the ability to work together. multi UAV systems may now be used to carry out most public and civic applications more efficiently [36]. The communication network, which ensures communication between UAVs and between UAVs and ground nodes, constitutes a key component in most multi UAV systems. These UAVs can be designed to cooperate together to provide services and function as relays to extend network coverage. The mobility of UAVs is determined by the application. For example, in order to provide communication in an earthquake-stricken area [37], UAVs would hover over the operation area and the linkages would be slow and dynamic. The fact that the UAVs may go out of service due to failure or battery drain demonstrates the dynamic nature of the network setup and connectivity. Agricultural and forest monitoring applications, on the other hand, demand UAVs to move across a vast region, with links breaking and reestablishing often. This is also true for UAVs that must hover over an area for extended periods of time. To take their place, new unmanned aerial vehicles must be launched. Some of the UAVs may be pulled out of operation to save power until a more appropriate time comes. As a result, it would be a requirement that the linkages immediately reconfigure themselves in all such circumstances. Multi UAV systems, while beneficial in many ways, complicate the UAV communication network. Reliability and survivability through redundancy are two significant advantages of multi UAV systems. When a single UAV fails in a multi UAV system, the network must reorganize and retain communication through other nodes. In a single UAV system, this would be impossible. However, in order to gain the full benefits of numerous UAVs operating together in a multi UAV topology, the protocols in place must address concerns such as power limits, mobility and changing topology [38]. Figure 3 illustrates the multi UAV system; each UAV struggles to address mobility and varying link' quality dividends in an ever changing topology during the flying role. As per interference requirements, the UAV link selection and topology should be dynamically optimized. The phenomenon is a governing element in terms of authentication protocol implementation in a multi UAV network. A single UAV system would have to maintain communication linkages with the control station(s), servers and base stations as well as provide access for functionality. The limited battery power and bandwidth are severely hampered as a result. In a multi UAV system, one or two UAVs can be utilized for the provision of feed to other UAVs in the network. Moreover, some can connect to control the network [39]. In order to provide access ability for voice, data and video, the UAV network will have to retain the mesh architecture. Additionally, maintenance and operational cost of multi UAV systems is relatively less expensive than their legacy equivalents. In this study [40], it is stated that the communication umbrella range provided by multi UAV infrastructure can be easily enhanced by adding more UAVs to the mesh. Missions carried out through small UAV systems are usually cost effective compared to solo UAVs. Moreover, missions are completed with even better efficiency and speed [36]. The authors of [41] have elucidated the results of the successful accomplishment of missions by explaining multi UAV networks based on PRoPHET routing protocol attaining the capability of finding a path, even if two end points are not directly connected. In the study by [42], the authors present their research on multi UAV systems and explain how multi UAV systems are reliable and even robust to loss.