# The Necessity of LED to Ambient Light Ratio Optimization for Vehicular Optical Camera Communication
## 5 Conclusions  
In a VOCC system, the digital bits 1 and 0 are represented by the On and Off status of LEDs. The On and Off statuses of LEDs in a received image need to be determined in order to obtained the bits transmitted from one vehicle to another. Lowering the BER is obviously the goal of all VOCC system designers. One might intuitively assume that increasing the LED power would make the On and Off LEDs more distinguishable and thus reduce the BER. However, this paper shows that it is not that simple for VOCC. Increasing the LED luminance is known to reduce the BER because the LED extinction ratio increases. However, due to the nature of image acquisition, a higher LED luminance also leads to higher LED interference, which has negative impact on BER. Consequently, an optimal ratio of LED to ambient light (L2A) exists in which the BER is at its lowest. Through theoretical analysis, this paper elucidates that besides the L2A ratio, LED interval is another parameter with a decisive effect on BER. Furthermore, this paper reveals that the lowest BER can be achieved through maximizing the between-class to within-class (B2W) variance ratios of the On and Off LED features in received images. Based on these analyses, we formulated a constrained optimization method of finding the optimal L2A ratio to maximize the B2W variance ratio subject to different LED intervals. Through simulations, it is verified that by applying these optimal L2A ratios, much lower BERs of a VOCC system can be achieved without changing the data rate. VOCC system designers might wonder if the optimal L2A ratios reported in this paper are what should be directly applied to their system. The answer is no. While the optimal L2A ratios suggested in this paper are likely to provide better performance than a random one, they might not necessarily be the optimal ratios for their system. Even with the exact LED intervals in this paper, the LED interference might be slightly different in their system due to different LED coatings, different LED panels, different auto-ISO mechanisms of cameras, or other reasons. However, it is worth noting that the most important contributions of this paper are not the optimization results themselves but the whole idea of the necessity of L2A optimization, the theoretical analysis of the effect of the L2A ratio on bit detection accuracy, and the means to obtain the optimal L2A ratio in the VOCC system. To the best of our knowledge, this paper is the first work in the literature to discuss the necessity of LED dimming, the proper auto exposure mechanism of a camera, and the effect of L2A ratios in VOCC. Due to many fundamental difficulties, especially the wireless synchronization between multiple cameras and LEDs, studies on VOCC are still focusing on the simplest version of the system, which involves the communication between two vehicles with one transmitting the LED panel and one receiving the camera. In such a simple system, the LED luminance can be fixed, the camera exposure can be manually adjusted to the LED luminance and thus the L2A ratio problem can be neglected. In a real VOCC system  
Figure 22: BER at different communication distances. where the communication happens between multiple vehicles with various cameras and LED panels, the camera exposure and the LED luminance need to be adjusted to the ambient light and the L2A ratio will become a deciding factor for the system performance. This paper not only introduces the idea of L2A ratio optimization to the research community but also provides mathematical analysis to elucidate the effect of the L2A ratio on the bit detection accuracy of the VOCC system. Through this paper, VOCC system designers would know that instead of trying to use more powerful LEDs, they need to find the optimal L2A ratio to achieve the lowest BER. This task can be achieved by using the proposed optimization method through maximizing the B2W variance ratio as described in the paper. [PERSON]-H.D. proposed the idea, performed the analysis and wrote the manuscript.