# Development and Testing of a Two-UAV Communication Relay System
## 2 Related Works  
Numerous researches have been performed in UAV communication relay scheme. In the conceptual study of UAV based military communication relays, [PERSON] et al. [8] described a multi-year airborne communication range extension program plan which was primarily used to support the Battlefield Information Transmission System. The main objective of the program was to provide the beyond line of sight (BLOS) communications capabilities without the help of scarce satellite resources. Their range of coverage can vary from 50 to 100 miles for UAVs of interest. [PERSON] et al. [9] developed a multipurpose aerial platform using a UAV for tactical surveillance of maritime vessels. They concluded that the communication relay using a UAV system is a low-cost application which could provide air dominance with satisfied communication range with lost cost resources. In [PERSON] et al. [10], the use of the self-organizing paradigm to design efficient UAV relay networks to support military operations was proposed. Abundant previous studies have focused on the algorithms for vehicle position assignment and network configuration. [PERSON] et al. [11] developed guidance laws to optimize the position of a  
Figure 1: Application scenarios of UAV communication relay system. Micro-UAV which relayed communication and video signals when the other UAV is out of radio contact range with the base. [PERSON] et al. [12] investigated a communication system in which UAVs were used as relays between ground terminals and a network base station. Their work mainly focused on developing the algorithm for optimizing the performance of the relay system. In [13; 14], [PERSON] and coworkers proposed a novel dynamic approach to the relay chain concept to maintain communication between vehicles in the long-range communication relay infrastructure. Their path planning method was based on the artificial potential field. Many other researchers used simulation methods to propose and test the relay systems. [PERSON] et al. [15] conducted 2D simulations of the design on a swarm system which does not make use of positioning information or stigmergy for application in aerial communication relay. [PERSON] and [PERSON] [16] simulated the dynamic UAV relay positioning for multiple mobile ground nodes and multi-antenna UAVs. In [17], a relay network composed of UAVs keeping the Wireless Sensor Network connected was proposed. They also used cooperative multiple input multiple output (MIMO) techniques to support communication. [PERSON] et al. [18] provided a simulation-based comparison between two strategies for an autonomous mobile aerial node communication relay between two mobile BLOS ground nodes. [PERSON] et al. [19] proposed an airborne relay-based regional positioning system to address pseudolite system limitations. They conducted simulations and demonstrated that the proposed system guarantees a higher accuracy than an airborne-based pseudolite system. For experimental studies, most of the previous works were based on IEEE 802.11b (Wi-Fi) standard. [PERSON] et al. [20; 21] implemented a wireless mobile ad hoc network with radio nodes mounted at fixed sites, ground vehicles, and UAVs. One of their scenarios was to extend small UAVs' operational scope and range. They measured detailed data on network throughput, delay, range, and connectivity under different operating regimes and showed that the UAV has longer range and better connectivity with the ground nodes. [PERSON] et al. [22] also built a mobile ad hoc network (MANET) of light UAVs. The system has implemented a line of sight dynamic routing in the network. [PERSON] et al. [23] described the implementation and characterization of a mobile ad hoc network for light flying robots. Their UAVNet prototype was able to autonomously interconnect two end systems by the airborne relay. [PERSON] et al. [24; 25] conducted simulation and experimental study of image relay transmission for UAVs. [PERSON] et al. [26] proposed a simple antenna extension to 802.11 devices for aerial UAV nodes.