# Effect of Temperature on Channel Compensation in Optical Camera Communication
## 7 Conclusions  
The effects of temperature on light sources are well studied, showing a direct relationship between peak wavelength and spectral width concerning the p-n junction temperature. Nonetheless, these spectral variations are not usually considered when designing multi-wavelength links based on WDM or CSK, for instance. At least under reduced mobility conditions, these kinds of systems rely on the assumption that CSI does not change as the link geometry does not vary. Nevertheless, as has been demonstrated in this paper, CSI changes due to temperature effects. The experimental part of this work was based on obtaining the emission spectra of an RGB LED at different temperatures. An experimental setup comprising a thermal camera for measuring the p-n junction temperature, an integrating sphere, and a spectrometer was implemented. The LED's stabilized p-n junction temperature was obtained after a settling period because the devices were under normal ambient conditions, and only the driving current affected the LED to produce significant changes in its temperature. However, in some scenarios where the light source would be exposed to high temperatures,  
Figure 17: BER at different temperatures, compensating the channel using the CSI corresponding to 27 °C and with a low SNR value (10 dB). (**a**) Sony IMX249 and (**b**) Foveon X3. Dashed line, dotted line, and dash-dot line correspond to red, green, and blue, respectively. Figure 18: BER at different temperatures, compensating the channel using the CSI corresponding to 60 °C and with a low SNR value (10 dB). (**a**) Sony IMX249 and (**b**) Foveon X3. Dashed line, dotted line, and dash-dot line correspond to red, green, and blue, respectively. the operating LED temperature can change rapidly, causing undesired and unforeseen outcomes. No heat sink was employed in this experiment as the main objective was to analyze the temperature effect on the LEDs, and the use of such a device would have reduced the consequence caused by high temperature. The LEDs performance adding a heat sink to the test condition would improve since the LED spectral features would have been less affected. In this work, the harmful effect of compensating an OCC RGB channel using obsolete CSI in terms of temperature has been explored. After a comprehensive characterization of the light sources at different measured p-n junction temperatures, the associated CSI were obtained by simulating the response of Commercial-Off-The-Shelf (COTS) CMOS cameras. The resulting SINR after the zero-forcing compensation was calculated. It was observed that as the temperature difference between the reference and current CSIs increased, the resulting SINR penalty was higher (lower SINR values). Specifically, the blue channel was profoundly affected, reaching unpractical SINR values below 10 dB. Red and green channels presented small sensitivity to temperature-induced spectral variations in the IMX249, while in the X3 the decrease in SINR was higher. The reason for this is that the blue spectrum was significantly red-shifted, intensifying its intersection with the green component of the Bayer and Foveon filters. The decrease in the blue signal is also caused by the lower response of the blue component in both filters due to the use of a silicon-based CMOS device. Although the red and green spectra were also distorted by temperature, the SINR was not impaired in those bands in the IMX249. However, the decrease mentioned above in SINR on red and green channels might be interpreted as a result of the overlapping between the RGB filters in the X3 sensor. On the other hand, it was clearly observed that the temperature effect was masked as the input SNR level decreased. Besides, the noise affected more the X3 sensor than IMX249 due to the greater overlapping of the first one's spectral responses (Figure 1). The cross-talk between components makes the Foveon X3's associated channel matrix less robust to noisy inputs than the Bayer-based image sensors, which are better conditioned.