## 7 Comparison and Roadmap of Multi-UAV Architectures  
The rapid emergence of UAVs and the diversity of their applications has led to a variety of communication architectures. Each one has its advantages and disadvantages. Table 3 summarizes the key performance of the four communication architectures seen in this paper. Multi-level and multi-group architectures are the major players. These are interoperable systems that support various types of UAVs. Their communication range extends over long distances, which allows them to ensure permanent connectivity in the network. They are also suitable for complex missions involving several types of UAVs that collaborate in real time. The robustness of these two architectures guarantees the scalability, fault tolerance, and stability of the system. They are currently implemented in the military domain for coordinated attack and defense missions [2; 3; 66]. The single-group architecture is suitable for small and medium UAVs. It is not interoperable and therefore does not allow the deployment of various types of UAVs. Its communication range is relatively dependent on the distance between UAVs. The connectivity becomes unstable when the UAVs move away from each other. It is appropriate for simple collaborative missions in limited environments. Bidirectional communication  
Figure 13: Multi-UAV communication architecture based on multi-level. (air-to-ground) between the GCS and the group of UAVs is achieved through the gateway UAV. When the gateway fails, the system shuts down. Most of the projects [13; 14; 57; 15] related to the carrying of payloads implement this architecture since it offers peer-to-peer communication. Finally, the centralized architecture is less efficient than the other three. It also does not support interoperability and its communication scheme is centralized on the GCS. This leads to latency in air-to-ground communications. Intra-UAV communication is also not supported in this type of architecture. Therefore, it is not suitable for collaborative mission planning. The only point of failure is the GCS: if it fails, then the whole system is paralyzed. Generally speaking, this architecture is used for very basic missions with relatively small UAVs [26; 27; 48; 57]. The choice of an architecture is a delicate task during the conceptual phase of a multi-UAV system because it depends on the type and size of the UAVs, the area of the environment to explore, and the complexity of the mission to plan. If the choice is made out of context, a robust architecture could be inefficient on some types of UAVs and vice versa. Therefore, it is very important to define the type of mission and then fix the evaluation parameters seen in Table 3 before selecting an appropriate architecture. Figure 14 shows a roadmap for multi-criteria decision support. It provides information on how to choose an architecture that is well adapted to the context. For simple missions including very small and small UAVs, it is preferable to opt for classical architectures. Ideally, if all operations are centralized on the GCS, then it is advisable to deploy a centralized architecture. If the mission involves sharing resources or exchanging information between UAVs, it is better to opt for a single-group architecture. It is important to ensure that each UAV remains within range of adjacent UAVs to maintain connectivity in the network. For complex missions, the ideal would be to use decentralized architectures. In this sense, multi-group and multi-layer architectures remain the best choices. The robustness of the multi-level architecture makes it an appropriate choice for mission planning involving several different tasks [57].