# Energy Aware Cluster-Based Routing in Flying Ad-Hoc Networks
## 1 Introduction  
A FANET is a derived form of vehicular ad-hoc network (VANET) and mobile ad-hoc network (MANET). In FANETs, UAVs are network nodes. FANETs follow the same peer to peer communication. Apart from many common characteristics, there are also some differences between FANETs, MANETs and VANETs. FANETs have fast mobility, rapid topological changes, 3D environmental conditions, different mobility patterns and terrain structure [1]. The highly dynamic topology and the harsh FANET environment produce many challenges for networking and communication [2]. As the nodes fly in the air, they have clear line-of-sight and a relatively large distance from each other. Because of design limitations, UAVs are equipped with a small battery. This small battery can sustain a flight time of about 30 min [3, 4]. Apart from the limited battery energy, UAVs have other limited resources such as computational power and channel bandwidth. These scarce resources hinder the wide application of UAVs. These resources can be utilized efficiently by devising a communication mechanism among UAVs such that it has minimum routing overhead, maximum throughput and low computational complexity [5]. Several techniques have been used for communication in an ad-hoc network. These techniques can be categorized into proactive routing, reactive routing and cluster-based routing. In proactiverouting, routing tables are maintained which contain routes to the other nodes in the network. Routing tables are updated through the exchange of periodic messages. On the other hand, reactive routing protocols don't maintain the routing table. Whenever a node requires sending data, it finds a new route. Upon the disruption of the current route, it finds a new route again. In the highly dynamic environments where nodes change their position very quickly and frequently, maintaining the latest routing table or finding a new route, again and again, represents a huge communication overhead. This communication overhead reduces network throughput, introduces additional delays and wastes the limited battery energy of UAVs [3]. One remedy for this scarce resources problem is clustering. Clustering is an approach for arranging nodes having same geographical neighborhood, into multiple groups. It helps to make the network more scalable, reduce routing overhead and maximize the throughput [6]. This low routing overhead will save the UAVs' energy as well. Although clustering has its own overhead, such as cluster formation and maintenance, this overhead is much less as compared to the other two. In clustering, the whole network is divided into multiple logical groups or clusters. Each cluster has a leader or cluster head (CH), which is responsible for inter-cluster and intra-cluster communication. For the purpose of routing, CH acts as a first hop node and it is responsible for delivering the message to the ultimate destination. Regardless of the potential benefits of node clustering in FANETs, the cluster formation and maintenance demand explicit exchange of control messages [6; 7]. A portion of the computational network radio resources is used for cluster formation and maintenance. In the cluster formation phase, nodes exchange some local information (e.g., node ID, location and residual energy, etc.). This information is exchanged through control messages. Therefore, a portion of the network radio resources is used for cluster formation. During network clustering, nodes perform computations in order to arrange nearby located nodes into clusters and a CH is elected from each cluster. The process of cluster formation and electing CHs is very important in maintaining the cluster structure. Subsequent changes in the relative positions of cluster members (CMs) can modify the cluster structure. In order to track changes in cluster structure, CHs should always periodically broadcast their existence to its CMs, and each CM should reply back its status to the CH. This signaling for cluster maintenance also uses a part of the network radio resources.