###### Abstract  
Energy-efficient routing has become a critical issue for advanced energy-hungry unmanned aerial vehicles (UAVs). Routing in a flying ad hoc network is always challenging and becomes even more critical when a small number of UAVs must cover a large area. The routing protocols based on the delay-tolerant network (DTN) are best suited for such scenarios. However, traditional DTN-based routing protocols depend on data dissemination to offer a better packet delivery ratio, leading to congestion and excess transmissions, causing heavy and unnecessary energy consumption. We propose a location estimation-based congestion-aware routing protocol (LECAR) to balance these two issues. Considering outdated location information, LECAR takes advantage of the mobility model to estimate the current location of the destination. In addition, LECAR routes a packet by considering both the distance to destination and buffer occupancy of the neighboring UAVs. Simulation results show that LECAR could ensure both a high packet delivery ratio and low energy consumption. Moreover, LECAR could provide a minimal number of transmissions, while minimizing the number of copies per packet at a time. DTN; DTN-based routing protocol; energy-efficient routing protocol; energy-efficient UAV; FANET; location estimation-based routing protocol; LECAR +
Footnote †: journal: Sensors  
reaches the destination [12; 13; 14]. Consequently, this data flooding consumes a significant amount of network resources, leading to a network where the nodes are highly active to deal with the data. Moreover, multiple copies of the same packet exist in the network, causing high congestion in the buffer of almost all UAVs, leading to frequent packet drops. Although some recent research on DTN-based routing focuses on maintaining a small number of copies, this research is underdeveloped and poses several shortcomings [15]. The multiple copies generate many transmissions between the UAVs, which can become a significant concern as it keeps the resources busy dealing with the duplicate packets. Thus, this strategy consumes a substantial amount of energy. On the other hand, UAVs often have a shortage of energy. Especially for small UAVs, energy shortage is a paramount concern. If the small UAVs must cover a larger area, they have minimal scope to allow any unnecessary energy consumption. In such a situation, if the routing protocol consumes excessive energy, then it becomes a significant problem. However, at the same time, it is vital to establish an efficient network where UAVs can successfully exchange the acquired information with the command-and-control station to fulfill the mission successfully, leading to a great bargain. Although the FANET comprises enormous challenges, it also has its advantages. In general, while involved in a mission, UAVs cooperate among themselves. They usually follow a path-planning mechanism based on a map shared among themselves. Implementing a common logic known to every UAV; the UAVs choose their path from the map. They can also estimate/plan their consecutive moves from this path mapping (i.e., which direction a UAV takes). If location information for the other UAVs is present, a UAV can estimate the path of any other UAV by applying the same path-planning logic. Even though the location estimation might not be exact, it can provide the best idea about any other UAVs' current and next possible locations. This information can be of great advantage for the routing protocols in solving the connectivity issues without losing much power. In this paper, we propose a novel location estimation-based congestion-aware routing protocol (LECAR) that incorporates the location estimation functionality of a path-planning mechanism in making the routing decision. Based on the location map and path-planning mechanism, LECAR estimates the location of a UAV. Then, considering the estimated location, LECAR routes the packet to a neighbor UAV, which moves closer to the destination and has enough space in the buffer to handle the packets.