# Review and Comparison of Emerging Routing Protocols in Flying Ad Hoc Networks
###### Abstract  
With the development of Unmanned Air Vehicle (UAV) communication, Flying Ad Hoc Network (FANET) has become a hot research area in recent years, which is widely used in civil and military fields due to its unique advantages. FANET is a special kind of networks which are composed of UAV nodes, and can be used to implement data transfer in certain unique scenarios. To achieve reliable and robust communication among UAVs, a routing algorithm is the key factor and should be designed elaborately. Because of its importance and usefulness, this topic has attracted many researchers, and various routing protocols have also been put forward to improve the quality of data transmission in FANETs. Thus, in this paper, we give a survey on the state-of-the-art of routing protocols proposed in recent years. First, an in-depth research of the routing in FANETs recently has been brought out by absolutely differentiating them based on their routing mechanism. Then, we give a comparative analysis of each protocol based on their characteristics and service quality indicators. Finally, we propose some unsolved problems and future research directions for FANET routing. f [table]cappositionposition=top  
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Compared with the traditional wireless ad hoc network, the distribution of UAV nodes in the FANET is asymmetric. Obviously, the nodes have the characteristic of asymmetric information transfer when they are moving. Therefore, data transmission between UAVs faces serious challenges due to the unique characteristics of FANET communication. In recent years, the researches on FANET mainly focus on the design of routing protocols, and many schemes and algorithms have been proposed. Meanwhile, some summary work on this topic has also been done. To the best of our knowledge, [PERSON] et al. first provided a comprehensive survey for FANET in [15]. The main motivation of this paper is to define FANET as a separate ad hoc network family and introduce unique challenges and design constraints. The application scenarios of FANET and the problems during designing are discussed in detail. The routing protocols are introduced from five aspects: physical layer, MAC layer, network layer, transport layer, and cross-layer structure.This article greatly encourages researchers to conduct follow-up studies on the open question of FANET. In [16], [PERSON] et al. summarized the challenges of FANETs compared with traditional ad hoc networks. They divided existing FANET routing protocols into six categories, such as static protocols, proactive protocols, reactive protocols, hybrid protocols, position/geographic-based protocols, and hierarchical protocols, and compared them according to different performance standards. Finally, the author gives six main problems to be solved in FANET. [PERSON] et al. [17] conducted a comprehensive survey of the various categories of position-based routing protocols. What is original about this survey is that it complements existing surveys on the same subject and provides more details about various metrics. [PERSON] et al. [18] made a review for communication architectures and routing protocols of FANETs. They compare the characteristics of different communication architectures and discuss their advantages and disadvantages. However, the classification and summary of routing protocols are not novel enough. In [19], [PERSON] et al. proposed five categories of FANET routing protocols: static, reactive, proactive, hybrid, and hierarchical, and present a comparison of routing strategies based on certain criteria. The key problems in FANET research are presented, and the main challenges are introduced from three perspectives of routing, path planning, and transportation quality. [PERSON] [20] also studied the routing protocols for FANET. In this article, he divided routing protocols into six categories, which is same as in [16]. Moreover, he made a comprehensive comparison of the protocol from many aspects, such as security, data transmission mode, and transmission delay.