# Routing Schemes in FANETs: A Survey
## 1 Introduction  
Flying ad hoc network (FANET) is a branch of networking that provides communication among flying nodes, particularly unmanned aerial vehicles (UAVs), with base station (BS) [1]. Flying UAVs are autonomous nodes capable of making decisions (e.g., changing speed and direction) in the air in a distributed manner rather than receiving decisions made from the ground in a centralized manner [2; 3]. FANET has distinguishing features compared to existing ad hoc networks such as mobile ad hoc networks (MANETs) [4] and vehicular ad hoc networks (VANETs) [5]. In general, the flying nodes are prone to frequent link disconnections and network partitions due to the following:  
* Movement in the three-dimensional space (against movement that is confined to roads and highways in VANETs [6]). * Flying speed ranges from 30 to 460 km/h [7] (against driving speed that ranges from 10 to 120 km/h [8; 9]). * Low node density (against high node density in urban areas in VANETs). In addition, the flying nodes, which are generally equipped with batteries that provide limited energy, have high energy consumption [10]. This means that frequent disconnections cannot be addressed by increasing the transmission power to provide long-range communication in FANETs. Hence, establishing long-term, reliable, and robust connections and routes is essential to increase route lifetime and to improve quality of service (QoS) (e.g., low latency and route setup time), yet it is challenging in FANETs. Nevertheless, the flying nodes naturally have a) a large coverage and b) minimal effects of obstacles (e.g., buildings and trees) and bad weather due to its elevated look angle [7]. Figure 1 shows two scenarios, namely the single-UAV and multi-UAV scenarios. In the single-UAV scenario, a single UAV establishes a connection to a radio access infrastructure (i.e., FANET base station (BS), cellular BS, and satellite). In the multi-UAV scenario, multiple UAVs cooperate and establish an ad hoc network. Compared to the single-UAV scenario, the multi-UAV scenario provides three main advantages [11]. Firstly, FANETs increase network scalability as multiple UAVs can increase coverage [12; 13; 14]. Secondly, FANETs increase network survivability (or network robustness and reliability) in a dynamic operating environment (e.g., due to poor weather condition) as multiple UAVs can either connect with each other to form an ad hoc network or to connect to radio access infrastructure directly [15]. The presence of a large swarm of UAVs (or multi-UAV swarm) [16; 17] can cater to the failure of a UAV during operations [18; 19]. Thirdly, FANETs can distribute payload among UAVs, which helps to reduce the weight of each UAV and, hence, the required energy for the UAV to reach and maintain a targeted altitude, leading to a longer route lifetime [20]. The multi-UAV scenario can be extended to multi-UAV swarm, whereby a large swarm of autonomous, small-sized, and lightweight UAVs are deployed. The multi-UAV swarm can coordinate and prevent collisions among themselves while completing tasks that require a large swarm of UAVs, such as surveillance [16] and search and rescue missions during catastrophe [17] and gathering a swarm of UAVs at a target location [21]. The multi-UAV swarm can be used to upload a huge amount of data collected in a distributed manner (e.g., data collected from cellular users) to the base station. In Reference [22], a multi-UAV swarm with high mobility uses a store-carry-forward approach to capture and transfer images and videos from a post-disaster area to the base station. In Reference [23], a multi-UAV swarm senses and allocates subchannels for UAV-to-X communication in a collaborative manner in order to maximize the sum-rate of uplinks while optimizing the speed of UAVs. In Reference [24], a multi-UAV swarm, which consists of flexible aerial nodes, is deployed to form an emergency network that can recover communication rapidly in a catastrophic area.