# Review of Unmanned Aerial Vehicle Swarm Communication Architectures and Routing Protocols
## 1 Introduction  
When first developed, Unmanned Aerial Vehicles (UAVs) were utilized for military applications, and their applications primarily comprised search and destroy missions [1], border monitoring [2] and close air support (CAS) [3]. In addition to military applications, UAVs also play an important role in commerce and public service applications, such as disaster relief [4], forest fire management [5; 6], wind assessment [7], civil safety and security [8], agricultural remote sensing [9], traffic monitoring and management [10], and relay networks [11; 12]. For example, Drone technology has been utilized for disaster relief and management in the cases of some globally known disasters such as Katrina, the L'Aquila earthquake, Typhoon Morakot, Tohoku Earthquake, and Haiti earthquake [13]. Owing to their advantage of wide applicability, research and development of UAVs has been growing rapidly over the past few years. More and more UAVs appear in formation or in swarm. In 2015, the United States initiated a few UAV swarm research projects, such as the \"elves\" project, which is part of the Defense Advanced Research Projects Agency and the Low-cost UAV Swarm Technology Project. In 2016, Jungle Wolf UAVs were exhibited at the Farnborough Air Show. China Electronics Technology Group Corporation announced its swarm technology in 2017 when it deployed more than a hundred UAVs [14]. Compared with the single-UAV system, the swarm or formation formed by multi-UAVs has obvious advantages. Table 1 summarizes performance comparisons of single and multi-UAV systems. A detailed explanation of the different criterion is as follows:* **Survivability:** In a single-UAV system, it is considered a failure if a single-UAV is shot down during the mission. However, for multi-UAV systems, a single out of control UAV is nothing serious because other UAVs will continue to operate. * **Scalability:** Using large UAVs for single-UAV systems only increases coverage to a certain point. By contrast, multi-UAV systems can easily increase the range of operations [15]. * **Speed of mission:** Research shows that missions can be completed faster when multi-UAV systems are used [16]. This is especially true for search tasks, as multi-UAV systems can process tasks in parallel, thereby speeding up the time it takes to complete tasks [17]. * **Autonomy:** For single-UAV systems, the typical mode of operation is that the pilot on the ground has direct real-time control of all aircraft systems. For most multi-UAV systems, the onboard automation ensures controlled flight in accordance with flight plans and other directives received from infrastructures [18]. * **Cost:** Research shows that missions can be completed at lower costs when multi-UAV systems are used. * **Communication needs:** Single-UAV systems need to maintain communication with the ground pilots or infrastructures at all times. By contrast, a multi-UAV system has only one specific UAV that communicates with the ground and forwards the message to other UAVs. * **Radar cross-section:** For military applications, multi-UAV systems produce only a small radar cross-section, which enhances the security of military operations [19]. Actually, a swarm of UAVs is a set of aerial robots that work together for a specific goal. The flight of the UAVs is controlled either manually, i.e., by remote control operations, or autonomously by using processors deployed on the drones [20]. Both operations require communication for support. In addition, a high-performance and reliable communication plays an important role in meeting the challenges of a UAV swarm, including coordination and collaboration strategies [21; 22; 23], control mechanisms [24; 25; 26], security [27; 28; 29], mission planning algorithms [30; 31; 32], and more. The communications in UAV swarm include not only UAV-to-UAV(U-T-U) communication but also UAV-to-Infrastructure(U-T-I) communication [33; 34].