# Review and Comparison of Emerging Routing Protocols in Flying Ad Hoc Networks
###### Abstract
In [21], a more comprehensive classification method is proposed by [PERSON] et al., which divides the routing protocols into two parts: based on network structure and based on data transmission. The author also summarizes five main research problems and provides popular research directions like link disconnection and hybrid metrics etc. Similarly, [PERSON] et al. [22] considered the energy factor in FANET and listed four routing protocols which based on energy, such as EPLA [23], IMRL [24], etc. This paper not only describes the network architecture of FANET in detail, but also compares the mobility models for flying nodes, and then analyzes FANET application scenarios. In [25], the author introduced the classification of UAVs from two aspects: configuration and operating height. Further, the application of communication architectures in future wireless networks are presented. Routing protocols are then classified and compared based on whether base station is involved or not. However, this paper lacks the research on clustering algorithm. Previous studies have summarized the development of FANET routing, but almost all routing classification methods are the same, such as static routing, proactive routing, reactive routing etc. The classification method they used is based on the characteristics of traditional ad hoc networks. However, they ignore the instability of the FANET network topology and the stratification of the network. In this paper, we study FANET in a more realistic scenario, and the routing protocols listed are all designed for the specific environments. From this point, other surveys are not fresh as this paper. In addition, in all the surveys we mentioned above, the most of the routing protocols surveyed are the variants of the routing algorithms designed for traditional ad hoc networks, and protocols specially for FANETs are very limited. Compared with the mobile devices on the ground, the UAV flying in the air has a disadvantage that the energy resources are used for more than communication, actually it takes more energy to support UAVs flying, so the energy-saving routing scheme needs more attention. However, when we look at the survey articles mentioned above, little consideration is given to the UAV energy during classifying routing protocols. Therefore, we summarized a kind of the clustering method to help saving energy in UAV swarm in this paper. In fact, many researches have been done by scholars on FANET routing particularly, and many new routing protocols including various technologies have been proposed. Thus, in order to make up for the insufficiencies of the above articles in these aspects, focusing on the routing algorithms specifically designed for FANETs, we newly conduct a more extensive and in-depth study on the routing protocols, and survey them from a totally different perspective. A comparison of existing surveys and the one of ours is presented in Table 1.  
In summary, the main contributions of our work in this article can be described as follows. * We studies various newly developed FANET routing protocols and conducts extensive investigations on their characteristics, techniques, advantages, and disadvantages. * The routing protocols surveyed are classified according to the core technology of the routing protocols and their respective characteristics. * We give an overview of quality of service (QoS) parameters for performance evaluation of existing FANET routing protocols. * The open issues and future research directions related to individual routing protocol type have been discussed. The remainder of this paper is organized as follows. In Section 2, we give the differences between FANET and traditional ad hoc networks. In Section 3, the common communication architectures of FANET are introduced. In Section 4, we review the state-of-the-art routing protocols and compare the performance of different technologies for each category of protocols. Finally, we discuss the future directions in Section 5, and conclude this paper in Section 6.