## 3 Experimental Results
the case of \"Beam size 1 mm\", so it is possible to obtain a linearized output for all control positions. It is thus confirmed that the desired goal region can be reached fastest when a tracking algorithm is performed. This strongly indicates that the incident beam size on the QPD should be considered for the optimized QPD-based PAT system. Figure 5: Result of X value of QPD based on incident beam size on QPD with respect to the change of pan angle of PTU. The dotted and solid line represent the simulation and experimental results, respectively. Figure 6: Tracking time depending on the incident beam size on QPD. Figure 7 shows the measured bit error rate (BER) of the FSO communication link between OGT and OAT at a 50 m distance. BER was measured with the non-return to zero (NRZ) format pseudorandom binary sequence (\(2^{7}-1\)) data stream using pulsed pattern generator (Anritsu MP1763C) and error detector (Anritsu MP1764C). The measured laser output power at the output fiber and the sensitivity of the APD were approximately 200 mW and \(-32\) dBm, respectively. The 1.25 Gbps error-free (BER \(<10^{-12}\)) data communication was verified with 1.25 Gbps at the 50 m distance. Various PAT systems have been reported for mobile FSO communication as compared in Table 1. The MRR-based mobile FSO communication system employs TRx only on the ground because it uses simplex method. Accordingly, the platform mounted on the UAV has the advantages of a small size, lightweight, and simple structure [16; 17; 18], but the ground terminal carries the burdens caused by the separated optical communication paths and beacon beam tracking. In addition, it has limitations in the datalink distance and modulation speed. Recent studies have reported 500 Mbps at the distance of 560 m between UAVs and the ground [18]. The German aerospace center (DLR) performed 1.25 Gbps FSO communication between the airborne platform flying at a maximum speed of Mach 0.7 and the ground [24]. However, it required a large number of optical components due to the use of separate optical paths for data communication and beacon beam tracking, simplex methods, and the hybrid PAT system. The naval research laboratory (NRL) in the United States conducted full-duplex 1 Gbps FSO communication using a five-cell combinational detector capable of tracking and communication with one detector [25]. However, the control complexity and the number of optical components increased by using the hybrid PAT and separated optical path for transmitter and receiver [25]. The national institute of information and communication (NICT) in Japan performed the first statistical analysis on 100 m simplex communication channels between the ground and UAV equipped with corner-cube retro-reflector and camera [26]. Since transceivers share optical paths in this  
Figure 7: Bit error rate (BER) measurement of FSO link between OGT and OAT at 50 m distance: (a) OGT to OAT and (b) OAT to OGT. system, the number of the required optical components is smaller than that of MRR-based mobile FSO systems. Our mobile FSO system, sharing optical paths for data transmission and beacon beam tracking and using QPD-based PAT systems, performed 1.25 Gbps full-duplex communication with the least optical components compared to other studies. The performance was evaluated in a stable and clear atmospheric condition at 50 m datalink distance. The effect of the realistic atmospheric environment and vibration of unwanted UAVs on the performance of FSO links will be analyzed in the future to optimize the PAT system.