## 3 Experiment Result  
### Analysis of Proposed FC-Coupled Number Plate  
An intensity analysis according to the beam size and incident light position is performed to characterize the influence of the incident beam pattern on the FC-coupled number plate. As shown in Figure 5a, the signal intensity distribution from the blue LD beam (Thorlabs, PL450B) is measured. DC and sinusoidal signals are adjusted to be in the linear driving condition and applied to the blue LD, and the beam diameter (\(\phi\)) is varied to 2.4 cm, 3.8 cm, and 5.4 cm. The three locations (top, middle, and bottom) of the FC are measured separately. At the top, an APD is located to detect the signal. The three locations mean the vertical distance from the APD by 3 cm, 5.5 cm, 8 cm, respectively. The received sinusoidal peak to peak is measured, along with the length, by moving the horizontal position of the incident light with 1 cm interval. The value of 0 for the horizontal position means the center of the concentrator.  
Case (i) in Figure 5a shows the received signal strength distribution when the smallest beam is incident on the top surface of the FC. Case (ii) is intermediate in terms of the beam location and the size, and Case (iii) presents the result from the case with the largest beam size at the bottom. The dots in the graph are the measured points, and the fitted curve is from the Gaussian model. As shown in the graph, the Gaussian fitted curves are in good agreement with data points. The standard deviation for each case is 17, 19.8, and 20.3, respectively. The overall standard deviation analysis from a series of similar measurements is shown in Figure 5b. In the y-axis of the graph, 1, 0, and \(-1\) mean the top, middle, and bottom (which means the farthest point from the APD) of the FC, respectively. As a result, it is confirmed that the shorter the distance from the receiver, the higher the variance of the channel, and the larger the spot diameter, the lower the variance.  
Figure 4: Geometric configuration for top and side views.  
Next, the feasibility of the beam steering method introduced in Section 2.4 is investigated at 5 m link distance. Figure 6a shows the experimental setup. First, the taillights seen from the left and the right camera are tracked as an object that is indicated as 'S=1' in the figure. Using the differential location of the tracked objects, the relative position of the license plate is calculated, which is the information used for steering the mirror. Changing the angle of the transmitter module, the relative signal strength by applying the beam-steering and tracking method is monitored. As shown in Figure 6b, without the beam-steering method ('Non-steering'), the link is abruptly broken as soon as the beam is off the license plate. Consequently, at an angle beyond \(+-3\) degrees, there is no signal detected by APD. However, when activating the beam-steering method, the operational angle is much improved. The maximum beam-steering angle is measured up to \(+-25\) degrees. The  
Figure 5: (**a**) Measured signal strength distribution and Gaussian fit; (**b**) overall standard deviation analysis.  
camera used in this work has a frame rate of 60 frame/s and resolution of 1296 \(\times\) 976; the mirror has an optomechanical movement speed of 0.65 ms/degree; and the computational time is sub-milliseconds. Consequently, the worst-case latency stemming from 50 degrees beam steering (max. steerable angle of the system) is calculated to be -50 ms.