# Laser-Based Mobile Visible Light Communication System
## 3 Results and Discussion  
Considering the bandwidth limitations of the components within the system, we transmitted an OOK signal at a rate of 500 Mbps. Initially, we examined the variations in the BER when the SiPM remained stationary at different positions along the track, as illustrated in Figure 6. The BER values represented in the graph are the averages of at least twenty measurements taken at specific distances and positions, with different color data points indicating the communication performances at varying distances between the transmitter and receiver. The left, center, and right positions correspond to track coordinates of 0 m, 0.4 m, and 0.8 m, respectively. Firstly, when observing the variation in the BER with distance, it generally followed the pattern that the BER increased as the optical transmission distance lengthened. Taking the example of the BER with the receiver positioned in the middle, the BER was 1.4 \(\times\) 10  
Figure 5: Experimental setup and flowchart of the system, including: **(a)** Light source shining on mobile VLC track. **(b)** Receiving end SiPM on mobile platforms. **(c)** Overall VLC system. Figure 6: Comparison of BER at different reception locations for 500 Mbps data sent at rest at 1 m, 2 m, 3 m, and 4 m.  
at one meter, 1.7 \(\times\) 10\({}^{-4}\) at two meters, 2.8 \(\times\) 10\({}^{-4}\) at three meters, and 6.9 \(\times\) 10\({}^{-4}\) at four meters. This was owing to the reduced received optical power with a longer transmission distance. An exception occurred at distances of 1 m at both ends, where the BER at 1 m was higher than that at 2 m. At two meters, the BER at both ends was only 1.7 \(\times\) 10\({}^{-4}\), whereas at one meter, it was 2.4 \(\times\) 10\({}^{-4}\). The analysis suggests that this abnormal phenomenon can be attributed to the 22-degree FOV of the light source. Consequently, at a distance of 1 m, the diameter of the light spot was around 0.8 m, meaning that the measurement points on either side of this distance were already at the edges of the light emitted by the source. The quality of the light at the very edges deteriorated, and with rapid power attenuation, this led to an increase in the BER. Upon observing the variation in the BER at the same distance but different positions, it was evident that the BER at the central position was lower than that at the lateral positions. The difference became nearly negligible at 2 m, indicating that, at a distance of 2 m, the edge light intensity was already sufficient to support data transmission. Being directly in front of the light source may potentially have saturated the SiPM, hence, the BER at the central position did not significantly decrease further. At 1 m, as mentioned previously, due to being at the edges of the FOV of the light source, the BER on the sides noticeably increased again. Following the analysis of stationary conditions, we proceeded to compare the performance of the receiver when in motion to when it was stationary. The stationary data include measurements taken at fixed positions at track coordinates of 0 m, 0.4 m, and 0.8 m. The dynamic data were sampled at random positions during motion, including various motion speeds. Taking into account the influence of a one-meter coverage, the experiment contrasts the BER distribution at stationary and mobile states for distances of two meters, three meters, and four meters, as depicted in Figure 7.  
At a distance of 2 m, Figure 7a reveals that the distribution of the BER in both stationary and mobile VLC systems was essentially consistent, varying from 1.0 \(\times\) 10\({}^{-4}\) to 3.5 \(\times\) 10\({}^{-4}\), with the BER range in mobile VLC slightly exceeding that of stationary VLC. This indicates that, at this juncture, the movement of the receiver did not significantly impact the system, allowing for stable transmission under any condition. However, as the distance extended to 3 m and 4 m, the BER was significantly higher, and there were a small number of data that were far off, as illustrated in Figure 7b,c.