# Effect of Temperature on Channel Compensation in Optical Camera Communication
## 6 Results
### High SNR Case  
Figures 7 and 8 show the SINR for an input SNR of 30 dB, which is labeled as high. For both IMX249 and X3 sensors, the penalty of compensating the channel with obsolete CSI affected the blue band reducing its SINR to unpractical values (below \(\sim\)10 dB) regarding the reference temperature. The red and green bands were also affected, especially in the X3 device.  
From Figure 9b, it can be noted a steep step beginning approximately at 62 \({}^{\circ}\)C. Two reasons mainly caused this for the X3 sensor: first, the blue receiver's spectral response is noticeably wider in X3 than in IMX249 (Figure 1). Second, above 62 \({}^{\circ}\)C, the blue LED spectra are broadened (Figure 5). Therefore, the combination of both phenomena leads to greater interference and, consequently, to an abrupt decrease in the SINR from 62 \({}^{\circ}\)C. Moreover, in Figure 8a, the blue LED showed a band-pass characteristic because of the compensation at 60 \({}^{\circ}\)C. The SINR level is theoretically at its maximum when the CSI  
Figure 5: Emission spectra at different p-n junction temperatures.  
Figure 6: Examples of images of the blue light-emitting diode (LED) driven at different currents, taken by a thermographic camera. (**a**) LED at 10 mA and (**b**) LED at 100 mA. Both X and Y axes correspond to pixels.  
employed for compensation corresponds to an LED at a temperature equal to the LED's stationary temperature regime. Thus, when the LED temperature of the CSIs deviates from the CSI reference temperature, the SINR decreases. The same effect can be seen in Figure 7, in which the maximum level is at 27 \({}^{\circ}\)C (the corresponding temperature of the CSI used to compensate).  
It can be observed from Figure 5 that as temperature increased, the blue spectrum was significantly red-shifted, intensifying its intersection with the green component of the Bayer and Foveon filters. Although the red and the green LED emission presented a similar behavior with respect to temperature increments, its contributions to the other components of the filters remained at low levels in the case of the IMX249. These effects were translated into a rapid decrease in the SINR at the blue wavelength, while both red and green components remained more stable. For the X3, due to the overlapping of its filters, red and green presented a higher decrease in SINR.  
Figure 8: SINR at different temperatures, compensating the channel using the CSI corresponding to 60 \({}^{\circ}\)C and with a high SNR value (30 dB). (**a**) Sony IMX249 and (**b**) Foveon X3. Dashed line, dotted line, and dash-dot line correspond to red, green, and blue, respectively.  
Figure 7: Signal-to-Interference-plus-Noise Ratio (SINR) at different temperatures, compensating the channel using the Channel State Information (CSI) corresponding to 27 \({}^{\circ}\)C and with a high SNRvalue (30 dB). (**a**) Sony IMX249 and (**b**) Foveon X3. Dashed line, dotted line, and dash-dot line correspond to red, green, and blue, respectively.  
Figures 9 and 10 depict the BER performance. For a high input SNR, the BER was below the Forward Error Correction (FEC) limit [40] for most temperature differences. Although the BER increased with the temperature difference, there were values above this limit only while using the CSI corresponding to 27 \({}^{\circ}\)C to compensate CSIs corresponding to temperatures higher than roughly 80 \({}^{\circ}\)C. In Figures 9a and 10a, the BER performance of the red and green bands were well under \(10^{-10}\). As expected, the BER was worse in the blue band than in the red and green due to the spectra red-shifting and the RGB filters overlapping, as seen in the SINR cases. These results suggest that in those links that require high SNR levels to reach higher data rates, the temperature effect would be more significant and must be taken into consideration.