# Flickering-Free Distance-Independent Modulation Scheme for OCC
## 4 Modulation/Demodulation Scheme
### Modulation's Advantages  
The flickering free, distance independent modulation scheme proposed in this work overcomes two of the main constrains for OCC modulation techniques: limited distance range and high algorithm's complexity. Since at least three consecutive frames are required for transmitting two bits symbols, the data rate is limited to 2 \(f_{\text{CAM}}/3\). These data rate is greater than the value obtained by other similar modulations which reach transmissions of \(f_{\text{CAM}}/2\). However, a 2 \(f_{\text{CAM}}/3\) transmission is considered slow but usable for IoT applications. Therefore, the data rate issue is partially addressed.  
Nevertheless, the proposed procedure increases the regular distance range of the modulations schemes, and is intended for its implementation for short, medium, and long links. This modulation is defined as a rolling shutter effect-based modulation. The distance constraint for this particular type of modulation is that the number of recovered bands decreases with the distance. However, for this technique, the data extraction requires a minimum number of light bands. Therefore, it can perform without problems even for long distances where only two bands are recovered.  
Additionally, the modulation is based on signals changing the switching frequency for different symbols (similar to a frequency shift keying modulation, FSK). The demodulation process relies on the simple calculation of the PCC value from consecutive frames, which requires a low complexity and computational cost processing. The use of the PCC value also eliminated the necessity of an adaptive thresholding algorithm for obtaining the correct light and black bands from the transmitted signals. Similarly, the pixel's intensity value is irrelevant due to the frame normalization performed at the beginning of the demodulation process. Therefore, an initial calibration is not required, neither is the system affected by changes in the environmental conditions. On the other hand, the data are coded using square signals with different frequencies. Those frequencies are high enough to assure no flickering, and the square shape maintains the duty cycle constant, which reduces even more the possible flickering effect.  
Due to these advantages, the proposed modulation scheme can be easily implemented for a wide range of OCC applications independently of the distance between the emitters and the camera. Since the complexity is minor, non-specialized hardware can be used for the deployment of the OCC systems without impacting its performance. Therefore, this modulation scheme is suitable for IoT applications in urban environments where the distances varies from less than one meter to several hectometers.  
The previous system explanation does not consider additional channel effects because they have a low impact on the system performance, or they can be compensated with further processing independently to the reception stages. For example, cross-talk interference between RGB emissions can be reduced by channel compensation techniques. Multi-path phenomena have a negligible effect in this work due to the small used bandwidth compared to the channel capacity. Source motion aspects are not considered in this paper. Still, they can be addressed by implementing light source discovery and tracking processesto maintain the emitter source into the camera's field of view. Finally, the blurring effect does not change the bands on the image since they depend only on the time variation of the data signal nor on the shape of the picture.  
For observing the performance of the proposed systems, simulations and experimental implementations have been prepared. The main results are presented in the next sections.