# A Coalition Formation Game Approach for Efficient Cooperative Multi-UAV Deployment
## 1 Introduction  
The past few decades have witnessed rapid development of application of unmanned aerial vehicles (UAVs), popularly known as drones. Given the characteristics of small volume, high mobility and low energy consumption, UAVs have been widely utilized in public, civil and military applications [1; 2; 3]. In particular, with proper deployment and operation, UAVs can provide reliable and efficient wireless communication solutions for various real-world scenarios. Furthermore, UAVs can act like self-organizing sensor nodes, and efficiently carry out the pre-assigned tasks [4]. For instance, in public safety scenarios (earthquake, volcano, military attack, etc. ), UAVs can be utilized as flying base stations, and replace damaged communication infrastructure and guarantee information dissemination for UAV-assisted sensor networks [5; 6]. Note that single UAV often fails to meet the need of complicated missions such as area coverage, communication connectivity and relay deployment. To address this issue, the large scale of UAVs are required to form groups (formations) to execute missions under different requirements. In [7], a multi-UAV cooperative model was presented to work out search and surveillance issues. Authorsin [8] considered the information consensus under multi-UAVs communication noise, and constructed it as a signal-to-noise ratio model for UAV cooperative target tracking. The authors in [9] proposed a cooperative detection model which consisted of target identification and time control. UAV area coverage problems were studied in [10; 11; 12; 13; 14]. Under the circumstance of cooperative multi-UAV coverage deployment, missions such as investigation, communication and mapping can be accomplished well. The most important part of the UAV area coverage problem is the evaluation of coverage ability. In [10], the authors described the ratio of coverage area and given area as the evaluation of UAV coverage ability, which is one of the most common evaluations. Notably, characteristics of mobility and line-of-sight (LOS) enable UAVs to support terrestrial networks for information dissemination and connectivity enhancement [15; 16; 17]. Given the fact that the ground communication infrastructure will be destroyed during natural disasters, it is critical for UAVs to perform efficient coverage deployment and execute searching and rescue operations. In the above scenarios, UAVs arrive at the designated position according to task demand and perform coverage tasks such as data collection. Next, the information (data, etc.) will be forwarded to the central UAV through the UAV-to-UAV link. Note that UAVs can carry limited energy, and the energy consumed by information transmission must be minimized based on mission requirements. Therefore, the data transmission mechanism should be well constructed in UAV communication. In general terms, as the transmitting distance grows, the energy consumption of UAVs will increase exponentially. To avoid high energy consumption, an effective technical solution is proposed in [18], utilizing some UAVs as relays. However, it remains challenging to deploy UAVs and design transmission mechanism due to the complex cooperative behaviors among UAVs. Motivated by the design of the cooperative mechanism, with which authors significantly reduce the overhead of spectrum acquisition in [19; 20], we adopt the coalition formation game (CFG) to describe the relationships between UAVs. Further work about data transmission mechanism design is introduced in [21]. However, unlike the studies above, the coalition is constituted by considering coverage reward and corresponding energy consumption among UAVs in this work. The object of CFG is to study the network coalition structure to optimize performance [22], which can be the basis for investigating cooperative behaviors of UAVs. Research has been done to study CFG in UAV networks.