# Formation Control Algorithm of Multi-UAV-Based Network Infrastructure
## 1 Introduction  
As one of the promising solutions to the increasing wireless network traffic on the ground, a dynamic network infrastructure utilizing a number of Unmanned Air Vehicles (UAVs) was introduced [1; 2]. The graphical representation of the network infrastructure is shown in Figure 1. The infrastructure is designed by two components: an ad hoc network among the UAVs, and a wireless access networks per UAV. The UAVs are spread in an area, and they establish an ad hoc wireless network with Ground Control Station (GCS) which is connected with the backbone network. At the same time, each UAV acts as an Access Point (AP) of the access network and connects the terrestrial mobile devices to the backbone network (i.e., Internet). Through the two-folded network architecture, the terrestrial mobile devices nearby the UAVs can connect to the gateway operating at the GCS. The network infrastructure has a number of unique features as follows. * Mobility. UAV becomes one of the most versatile devices cooperating with various actuators such as camera [3; 4], mechanic arms [5], and even weapons [6]. This trend is originated from the numerous studies in the network of the embedded computing board [7] and the UAV control research domain. While the previous infrastructures statically install a number of the base stations or APs on the ground, the UAVs can act as an AP and instantly deployed at the aerial space to provide the network to the ground. Providing network service by the multiple UAVsis a promising form of the Delay-Tolerant Network (DTN) concept described in RFC 4838 [8]. UAVs and their ad hoc network can act as a bundle layer, and provide functionality of the gateway to deliver the packets on the disrupted environments, such as a disaster area [9], wireless sensor networks [10] or highly congested traffic regions. * Scalability. The fact that the UAVs are connected by an ad hoc network suggests the high flexibility of the network coverage. While the existing base stations or terrestrial APs should be plugged by wires that guarantee the connection to the backbone network, the multi-UAV-based network infrastructure can connect a new UAV so as to enlarge the service area of the access network. Also, with the possibility that the pre-installed network infrastructure could be act as a GCS, UAVs can be sparsely deployed in large area while augmenting the network-providing service. * Wireless medium extensibility. Due to the immobility of the existing APs and base stations, the majority of the mobile devices share the wireless medium nearby the ground. Although Wireless LAN (WLAN) and cellular network use wired connection except one-hop wireless transmission, increment of the network traffic in a single AP or base station causes drastic decrement of the network throughput of the devices. Since UAVs are flying on the air, the wireless medium of the UAVs cannot be interfered by the channel of the existing networks. By extending the available wireless medium via multiple UAVs, the infrastructure has the potential to enhance the overall network throughput. Compared to the multi-UAV-based network infrastructure having explicit possibilities, there has been lack of research for improving the system performance of the architecture. The reason for the lack of interest is mainly the doubt about the network efficiency of the infrastructure. Wireless ad hoc network among the UAVs grants the large scalability of network, but the limited range of communication and the throughput decrement of network are noticeable. However, it should be clarified that the multi-UAV-based network infrastructure is a non-replaceable solution in the situation when the previously deployed network infrastructure on the ground is disabled or degraded. To seamlessly provide the network by multiple UAVs in the emergency situations, their formation should be elaborately and instantly controlled in real time. In this paper, we aim to tune up the locations of the UAVs to efficiently provide the network traffic to the terrestrial mobile devices.