# The Necessity of LED to Ambient Light Ratio Optimization for Vehicular Optical Camera Communication
## 3 LED to Ambient Light Ratio Optimization
### Constraint Optimization Problem of VOCC System  
#### 3.4.1 The Idea behind VOCC System Optimization  
As explained above, the LED spacing and the L2A ratio are the parameters with decisive influence on the LED bit detection accuracy of VOCC systems. The former has a great impact on the LED bit detection. As the LED spacing increases, the interference between the On and Off LEDs reduces, the B2W variance ratio increases, and thus the LED bit detection accuracy increases. Therefore, one can reduce the BER by increasing the LED spacing. From Equation (16), this can be achieved by reducing the communication distance or increasing the lens focal length and the LED interval. It is obvious that communication distance is not the parameter to be changed as the vehicles communicate at any distance. The lens focal length is also not a parameter that can be increased freely since it will narrow down the field of view of the camera. LED interval is the parameter that has the best chance to be increased to increase the LED spacing. However, as the physical area of a LED panel in the vehicle is limited, the number of LEDs in the panel must be reduced to increase the LED interval. This leads to a decrease in data rate. Consequently, LED interval should not be considered as variable to be changed to increase the LED bit detection accuracy but a soft constraint with varying values given from each VOCC system.  
Even though the L2A ratio has less impact on LED bit detection compared to LED interval, the change of this parameter does not affect the data rate of the VOCC system. Therefore, the L2A ratio is the only variable chosen in this paper for the optimization of the VOCC system.  
As a optimization variable, the value of the L2A ratio needs to be optimized to achieve the highest LED bit detection accuracy. The interesting thing about the L2A ratio, which is also the thing that make the L2A ratio optimization worth researching, is its contradicting effects on LED bit detection as shown in Figure 8.  
Figure 8: Contradictory effect of the light to ambient light (L2A) ratio on extinction ratio interference. (**a**) Direct strong sunlight. (**b**) Direct mild sunlight. (**c**) Indirect sunlight. (**d**) Overcast day. (**e**) Twilight.  
Figure 8 shows the captured images of two LEDs at various conditions of ambient light. In all images, the left LED is Off and the right LED is On. The luminance of the On LED remains the same in all images. Since the illuminiance of the ambient light decreases from the left to the right images, the L2A ratio increases accordingly. Under the direct strong sunlight, the L2A ratio is low and thus both the extinction ratio and interference are low. As the L2A ratio increases, both the extinction ratio and interference increase. Note that in Figure 8e, the illuminance in twilight condition is too weak compared to the LED and thus the L2A ratio becomes too high. To human eyes, such a high L2A ratio would cause serious flare. To the camera, such a high L2A ratio causes unacceptable blooming interference which makes the On and Off LED completely indistinguishable. As mentioned earlier, such a high L2A must be avoided in VOCC for the sake of driving safety and thus needs not to be further examined in this paper.  
As the L2A ratio increases, the increase of the extinction ratio and interference results in the increase of both Bvar and Wvar. This suggests that there is an optimal L2A ratio where the B2W ratio is at maximum as shown in Figure 9. This is the idea behind the optimization of VOCC systems in this paper.