## 2 UAV Network Communication Model and Architecture
### Decentralized Communication Architecture
#### 2.2.1 Single Swarm Ad Hoc Network  
In the \"single-group swarm ad hoc network\" (Figure 4), the internal communication of the swarm is independent of the infrastructure. Communication between the swarm and the infrastructure is established through a single-point link. There is a specific UAV in the swarm assigned to perform this link called the gateway UAV. The other UAVs are relay nodes that forward data within the swarm. The UAVs in the swarm can share information in real time to optimize collaborative control and improve efficiency. Similarly, the mutual communication between the gateway UAV and the infrastructure also allows swarms to upload and download information such as guidance, task assignment, or control information. For the UAV gateway, it has two additional types of transceivers. One is to communicate with other UAVs at low power and short range. The other one is to communicate with the infrastructure at high power and long range. Thus, other UAVs in the swarm only need to carry low-cost short-range transceivers. They are more convenient for moving and performing tasks. Thanks to such architectures, the communication range of the network is expanded. The ability to coordinate among different types of UAVs increases in the same swarm. This not only significantly expands the communication range for large coverage requirements but also makes small UAVs with smaller payloads more useful [31]. However, to ensure that the UAV swarm is consistently connected, the \"single-group ad hoc network\" architecture requires all UAVs in the swarm to have the same flight patterns (speed and direction). Therefore, this architecture is suitable for small UAVs. At the same time, it fully exploits the capabilities of small UAVs in complex tasks that they normally cannot complete. For each specific task, in each different phase, the communication architecture in the swarm can change [32]. The change in the communication architecture in the UAV swarm should be based on mission requirements (e.g., wide-area surveillance, centralized data collection, surveillance tracking) or on the mission phase (e.g., initial deployment, mission execution, emergency); environmental conditions (e.g., weather, interference); and system status (e.g., number of UAVs, UAVs losing connection, remaining energy of each UAV). In addition, real-time surveillance can be combined with adaptive algorithms to ensure efficiency and reliability. Figure 5a-c shows three common communication architectures in the swarm. Figure 4: Schematic illustration of a single swarm ad-hoc network. Figure 5a shows an example of a ring architecture. The UAVs in the ring architecture form a closed communication loop through bidirectional connections. Any UAV can act as a gateway node for the swarm. When the direct link between two adjacent UAVs fails, the target UAV can reconnect through the communication loop. Therefore, the ring architecture has a certain stability. However, this architecture lacks the scalability for other UAVs to join the swarm or coordinate with other swarms. In Figure 5b, the communications are organized in a star-shaped architecture. The gateway UAV is in the middle, and it communicates not only with the infrastructure but also with the entire UAV swarm. The star architecture has the inherent disadvantage of SPOF (if the gateway node fails, it will cause the entire system to fail). The mesh architecture is a combination of ring and star architectures, and it has the advantages of both systems (Figure 5c). All UAV nodes in the swarm have the same capabilities, with both terminal and routing functions. Information flow, from one node to another, can be implemented in many different forms, and any UAV can act as a gateway node for the swarm. The mesh architecture has now become the standard for intra-swarm communication systems [32]. However, practical applications often require swarms to use not only small UAVs but also medium and large UAVs. Therefore, the above \"single-group swarm ad hoc network\" architecture makes it difficult to meet these needs.