# Review of Unmanned Aerial Vehicle Swarm Communication Architectures and Routing Protocols
## 4 Conclusions and Discussions  
With the development of UAV technology and network communication, the application mode of UAVs is also constantly developing. The UAV swarm is an SI-based solution proposed for the expansion of UAV application functions and multi-UAV cooperative operations. UAV swarms have been used in many scenarios, such as in disaster management, forest fire monitoring, and border monitoring. Therefore, it is of great practical significance to further pursue research on UAV swarm technology. UAV swarms are highly mobile and their topology frequently changes, which requires communication networks with low latency and high reliability. Therefore, it is necessary to design communication architectures and routing protocols with good applicability, high efficiency and stability for consistent UAV swarm communication. In this article, we first introduced four communication architectures that can be deployed in UAV swarms, and also introduced three common intra-swarm communication architectures. Through the final summary table, we can conclude that the multi-layer architecture combined with the meshed intra-swarm architecture is currently the most applicable communication architecture. Subsequently, we presented a comprehensive classification of existing routing protocols for UAV communications, namely topology-based, geographic/position-based, and SI-based. These routing protocols were investigated, and their characteristics were described in detail. We also presented a summary table for comparative analysis. It is proved that topology-based routing cannot cope with UAV networks. SI-based routing and geographic/position-based routing are more suitable for UAV networks. Although the existing theoretical research and experiments have solved some technical problems in UAV swarm communication architectures and routing protocols, there are still many interesting, important and yet challenging open issues deserving further investigation. Some of them are briefly summarized as follows:  
1. Multi-layer architecture can better adapt to the characteristics of the UAV swarm communication, but it also brings new challenges. Because of the importance of gateway UAVs in swarm communication, it is necessary to have the ability to detect the failure of gateway UAVs. Further, if malfunctions have occurred, there should be a reliable algorithm to select the next UAV to act as the gateway. At the same time, the data stored in fault gateway UAV should be able to synchronize to the standby one. 2. The fast-moving characteristic of UAVs and the frequent change of network topology may cause the swarm communication to be intermittently connected. It has always been one of the important issues with routing protocols. Therefore, the solution to the problem of intermittent connectivity will remain the focus of research in the future. 3. Currently, most proposed routing protocols focus on performance improvement. However, security is an important content that cannot be ignored in any communication network. Therefore, it is necessary to propose new routing solutions including security components. 4. Energy efficiency plays an important role in UAV networks with energy constraints. The concerns regarding power saving in the UAV networks are in some ways similar to that in mobile ad hoc networks and wireless sensor networks. Many energy-saving routing protocols have been tried for UAV networks, but the applicability of these protocols in different drone network scenarios has yet to be proven. 5. In addition, the UAV communication network should be compatible with other networks in specific mission scenarios. Therefore, the design of the new routing protocols should be able to effectively support the different transmission requirements in the diverse mission scenarios.