# Cooperative UAV Scheme for Enhancing Video Transmission and Global Network Energy Efficiency
## 1 Introduction  
Flying ad hoc networks (FANETs) [1] are composed of a group of Unmanned Aerial Vehicles (UAVs), which are autonomous aircrafts without the need of a pilot to be on board during the flight [2]. Compared to a single UAV system, a FANET is more robust and can cover a larger area of interest [3; 4]. In this context, FANETs have been employed in many kinds of new smart city scenarios, such as disaster recovery, environmental monitoring, cooperative safety and security, and others. In these scenarios, multimedia content plays an important role in helping ground rescue teams to take appropriate decisions based on detailed and rich visual information [5]. Video dissemination over FANET for such applications requires Quality of Experience (QoE) support to deliver the content with a minimal quality level based on the user perspective [6]. This is because users expect to receive live videos without ghosting, blocking, pixelization, freeze frame in order to enable the rescue team in the control center to take action to explore a hazardous area based on rich video information [7]. In this way, there is a demand for video with low frame loss rate, tolerable end-to-end delay, and low jitter to provide QoE support [8]. The UAVs will be the humans' eyes in the sky, and thus they must be flying and monitoring events for a long period of time [9]. In this sense, energy efficiency is a key factor in FANETs in order to keep the UAVs cooperating as much as possible during a mission [10]. It is also important to maintain the delivery of videos with a better QoE while avoiding interruption periods. In general, UAVs are equipped with batteries with limited capacity, which spends energy on computing, flying, and transmitting. In many cases, it is not recommended to use solar panel or different power sources in UAVs because it can increase the payload. In order to avoid UAV loss due to the energy issue, UAVs must return to the launch point to replace or recharge their batteries. This is because UAV loss reduces the number of UAVs in the scenario, and causes route failure and void area as well as network partitioning. In this sense, effective energy-aware deployment mechanisms are needed for timely onboard energy replenishment without interruption of the video services [2]. Specifically, UAV must leave the monitoring area for battery revamping, during which the service gap is temporarily filled by neighboring UAVs that adjust their positions without the interruption of video services. In addition, the energy-efficient operation must be taken into account through smart energy management, that is, accomplishing missions with minimum energy consumption [2]. For instance, decision-making must take into account the energy consumption associated with UAV movements since they are generally quite energy-intensive. An intelligent coordination protocol must control the behavior of multiple UAVs, maximizing the benefits of a FANET [11]. This UAV controlling protocol manages both the UAV communication (i.e., routing protocols), as well as the UAV movements. In this context, Software-Defined Networking (SDN) [12; 13] is a promising approach for FANET controlling task, since it introduces complete network programmability by separating the control plane and data plane. For instance, the UAV programmability means to perform UAV replacement, to improve the application performance, to determine the data routing paths due to performance or energy reasons. SDN also considers a centralized SDN controller responsible for all control functions, such as routing, UAVs replacement, collision avoidance, and other services. In this sense, the controller node is aware of the global network information to optimize the network operations. This article proposes a cooperative UAV scheme for enhancing video transmission and global energy efficiency, called VOEI.