## 3 Experimental Results  
To reduce the impact of atmospheric conditions on the datalink performance, this experiment was conducted in a stable and clear atmospheric condition at a 50 m datalink distance. In addition, to increase the margin of QPD-based tracking, the beam reaching the counterpart FSO terminal should be as large as possible. The position of the light source on the EMS at each FSO terminal was initially set to 0 mm, making the beam size maximum at the counterpart FSO terminal. Figure 4 shows the variation of \(V\), X, and Y of the QPD, and the current value of APD depending on the pan angle, when the incident beam size matches the size of the QPD. When the X value of the QPD approaches zero, the \(V\) value of the QPD and the output current (\(I\)) of the APD are also maximized. Due to the 2D symmetry of the Gaussian beam incident to the QPD, the output of QPD changes identically for the same change in the pan and tilt angles. Therefore, experiments were conducted only for the pan angle variations. Figure 3: (**a**) FSO terminal on PTU, (**b**) inside view of FSO terminal, and (**c**) custom-designed quadrature photodiode (QPD) sensor board and main board for precision PAT. Figure 5 shows the output of the X value of the QPD with respect to the change in the pan angle of the PTU for various incident beam sizes on the QPD. Simulations and experiments were performed based on three conditions between the incident beam diameter on the QPD and the size of the QPD. The light source used in the simulation had an optical power of 200 mW and Gaussian beam profile. The following attributions were defined: \"Beam size 1 mm\" corresponds to the case when the incident beam size on the QPD is half the size of QPD; \"Beam size 2 mm\" to the case when both sizes are the same; \"Beam size 4 mm\" to the case when the incident beam size on the QPD is twice the size of QPD. To change the incident beam size on the QPD, the focus was adjusted via the \(z\)-axis of the three-axis manual stage of the QPD. The size of the incident beam was confirmed by scanning the output value of the QPD through the \(x\) and \(y\)-axes travel of the three-axis manual stage. When the incident beam size on the QPD was approximately equal to the size of the QPD, the angle control points of the PTU that control the incident beam on the QPD to the center of the QPD had a linear resolution. In addition, these results produced similar results for the Y value of QPD. When the incident beam size became smaller than the size of QPD, the span of the pan angle became narrower, reducing the dynamic range of the angle control of the PTU in tracking control. When the incident beam size was larger than the size of QPD, the flat dead zone was formed on the X value of QPD output, making it difficult to control the angle of the PTU, which was responsible for controlling the position of the incident beam to the center of the QPD. As shown in Figure 5, it was confirmed that the outputs of QPD according to the incident beam size in the simulation and experimental results are similar. Figure 6 shows the tracking time depending on the incident beam size on the QPD obtained through the experiment. The case of \"Beam size 2 mm\" is the fastest to reach the desired goal compared to the other incident beam sizes on the QPD. On the other hand, when the beam sizes were smaller/larger as indicated by the red/blue curves, the measurement took 2.5/4.25 times longer. These results show that when \"Beam size 1 mm\" and \"Beam size 2 mm\" are based on Figures 5 and 6, the QPD generates a linearized output for the received beam and provides linear resolution for the control points of the QPD, helping to reach the desired goal region during tracking control. Among them, in the case of \"Beam size 2 mm\", the dynamic range (i.e., control point) is relatively wider than in  
Figure 4: Result of \(I\) value of QPD, X value of QPD, Y value of QPD, and current value of APD with pan angle variation of PTU (incident beam size on the QPD = QPD size).