## 3 Current Status of Research  
### Air-To-Ground Optical Communications  
In 2013, a Korea Electronics and Telecommunications Research Institute (KETRI) team developed a specialized optical communication terminal for UAVs and ground terminals. This device uses integrated optical technology to simultaneously emit beacon and signal light. In the experiment, this terminal successfully constructed a full-duplex optical link with a transmission distance of 50 m and a rate of 1.25 Gb/s without error code transmission. The realization of this technology brought a breakthrough in the field of UAV communication and promoted the application and development of UAVs in various fields [2]. In 2015, the Institute for Communication and Navigation of the German Aerospace Center (DLR), in cooperation with Airbus Space and Defense Europe, promoted two projects, ALCOS and DOD Fast. The ALCOS project aims at wireless optical communication between UAVs and satellites, with a target communication rate of 1.8 Gb/s and a communication range of up to thousands of kilometers. The DOD fast project focuses on UAVs and ground-based wireless optical communication, with an expected communication rate of 1.25 Gb/s and a maximum communication distance of 79 km. These projectswill bring significant breakthroughs and advancements for future UAV communication technology [3]. In 2016, [PERSON] and his team designed a novel tracking system for air-to-ground laser communication experiments. As shown in Figure 4, the system consists of two key components: a coarse-tracking module constructed by a CCD camera and an exemplary tracking module consisting of a spatial light modulator and a position sensor (PSD). These two modules work synergistically to improve the tracking accuracy and reliability significantly. In the air-to-ground laser communication test, a 2 Mb/s optical communication link was successfully established with a communication distance of 300 m. In this test, the BER of the system was about 2 \(\times\) 10\({}^{-4}\), proving its good stability while maintaining good communication quality. This research has significantly improved the performance of laser communication systems [4]. In 2017, Facebook partnered with Germany's Mynaric to test an air-to-ground laser communications link. In this test, a Cessna 310 aircraft was equipped with a 70 mm-diameter laser communication terminal, and the ground station was fitted with a 200 mm-diameter laser communication terminal. The plane flew at a speed of 206 km/h during the test, 1.8 times higher than the speed tested in 2016. However, the quality of the communication was affected by the aircraft's high speed, as well as vibrations in the body and atmospheric turbulence. The communication rate reached 10 Gb/s, and the communication distance was 9 km. This experimental result clearly shows that factors such as the fast flight of the airborne platform, the uncertain trajectory, and random errors from attitude changes cause significant difficulties in the short and stable establishment of the laser link [5]. In 2017, the Thirty-fourth Research Institute of China Electronics Technology Group Corporation (CETC) developed an air-to-ground wireless laser communication system for small uncrewed aerial vehicles (UAVs) and proposed a complete system design. The system is based on a small rotary-wing UAV platform and can carry out stable signal transmission at a communication distance of 6.7 km and a communication rate of 1.25 Gb/s. In addition, the institute did an outfield test on signal transmission quality for in-depth testing and analysis. In the following year, CLP 34 successfully conducted a laser communication test on an eight-rotor UAV. The system achieved a communication rate of 100 Mbps at an air-to-ground communication distance of 6.7 km. Under reasonable weather conditions, the system's packet loss rate was only 0.3%, and it took only 10 s to establish a communication connection, demonstrating its high efficiency and reliability.