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Recently the technological advancements, a new grooving field of wireless ad-hoc network (WANET) is called FANETs. The FANETs is a WANET specifically designed for the communication between small, portable and flexible devices such as UAVs, often called drones. In this network UAVs as smart nodes which are equipped with special devices such as sensors, camera, computing device, GPS etc. capable of flying and working in 3D space and operate without carrying any human being. Recently, these UAVs become more popular due to their verity of application abilities in military as well as civilian missions (Bekmezci, I. et al., 2013). In FANETs, multiple UAVs such as U1, U2, U3, U4, U5, U6, andU7, can communicate directly with each other or through other UAVs via UAV to UAV (U2U) communication for the creation of ad-hoc network (Figure 1). UAVs can use short as well as long range communication according to the data needed and the rate of transmission of information. Another is called UAV-to-Infrastructure (U2I) communication. Whether single or more UAVs link to the infrastructure (base station or satellite) for the transmitting or receiving data on specific operation (Kumar S., 2020).
As more UAVs participate in FANET, activities can be parallelised which in effect decreases the mission’s completion period.However, the routing technology is a key technology for every ad-hoc network like MANET, VANET and FANET. But the other hand, communication between devices (UAVs) is greater challenging issue for FANETs routing technologies. In FANETs, efficient routing is still a major investigate area for research community due to its versatile nature like high-speed movement of UAV node and very frequent changes in network topology. In FANETs, routing is simply a process to find out an efficient path to deliver data between sources to destination with the help of mini-UAVs. Efficient path can be calculated in term of less packet delivery time, high packet delivery ratio, less number of relay hops, less number of control messages etc. To formulate the communication between UAV nodes in FANETs, routing protocols are broadly classified into two main categories: (i) Topology-based routing (TBR) protocols, and (ii) Position-based routing (PBR) protocols. TBR protocol depends on the information about existing links, and this link information is used to transmit the packets between the UAV nodes. It requires the IP addresses of the source UAV as well as the destination UAV. Furthermore, the PBR protocols which are also called geographical routing protocols, requires the actual position or location about the moving nodes (UAVs or drones) through the GPS system, which is based on location services (latitude and longitude of the moving UAVs). The GPS information is used to recognize the route for forwarding the packets from the source UAV to destination UAV node. Last few years wide range of TBR protocols proposed for FANETs. In addition, the network between UAV nodes in FANETs can be damaged due to the high movement of UAV nodes. Therefore TBR protocols for a moment can be failing when there is a highly dynamic nature of network. To avoid these problems, now a day’s researchers are more focusing on PBR protocols. The PBR protocols are more efficient and scalable for dynamic nature of network when there is a mobility of UAV node is very high like FANETs. Also, PBR protocols greatly reduce the requirements of topology storage (like TBR protocols) and provide suitable and more flexible environment for dynamic behaviour of FANETs.
Recently, some comprehensive survey works on topology-based routing and position-based routing for UAV networks have been reported as summarized in Table 1. To the best of the authors’ knowledge, however, there is no survey on cluster-based routing in the literature.