## 4 Experimental Evaluation of the VLC System and Experimental Results
### Experimental Setup No. 3--Long Communication Distance in Low SNR Conditions  
The purpose of the next experimental setup is to evaluate the behavior of the proposed system at longer communication distances. In this case, the communication distance was increased up to 40 m and the behavior of the communications has been analyzed for the two modulation techniques. At this distance, the power of the received signal is 0.11 \(\upmu\)W/cm\({}^{2}\). The picture of the experimental setup is available in Figure 6. Figure 7a illustrates the signal reconstruction process within the receiver at the level of the different signal processing blocks for this distance. The SNR level was further reduced, as the VLC receiver is still able to properly reconstruct the data signal. Thus, a perturbing white LED was placed at 47 cm toward the VLC receiver. In these conditions, the power of the parasitic LED light measured at the sensor level was 120 \(\upmu\)W/cm\({}^{2}\) (in addition to the power of the natural light--see Table 1). The effect of the parasitic light on the VLC receiver can be observed in Figure 7b. One can see that the logarithmic transimpedance circuit automatically degreases its gain in order to prevent the photosensitive element saturation. Thus, the amplitude of the voltage that is produced by the logarithmic transimpedance circuit becomes two to three times lower (Figure 7b). Furthermore, the strong perturbing light also affects the shape of the signal, as it can be observed at the output of the filtering block (Figure 7b--Channel 3). Hence, the signal loses its square shape, whereas the period of the data bits is slightly affected. Nevertheless, the data message can still be decoded, as the frequencies involved are quite low, whereas the tolerances are reasonably high. Table 4 presents the summary of the experimental results for this setup. The use of the DSSS modulation enables the system to maintain a BER lower than \(10^{-6}\). This BER is also achieved for OOK modulation when data rates of 11 or 24 kb/s are selected. However, the BER decreased for both Manchester and Miller codes when the data rate was increased to 50 kb/s. In conclusion, in the current configuration, the 50 kb/s data rate should only be used for low priority data, in high SNR conditions or in conjunction with error correcting codes. Figure 5: Oscilloscope screens illustrating the signal reconstruction process at the level of the different blocks of the VLC sensor: Channel 1 (orange) shows the output of the logarithmic transimpedance circuit; Channel 2 (blue) shows the output of the transimpedance circuit after a 24\(\times\) amplification; Channel 3 (purple) shows the output of the filtering block; Channel 4 (green) shows the reconstructed data signal. The figure shows the response of the VLC receiver as the power of the incident parasitic light is increasing: (**a**). diffuse natural light having a power of 150 \(\upmu\)W/cm\({}^{2}\); (**b**). direct LED light having a power of 400 \(\upmu\)W/cm\({}^{2}\); (**c**). direct LED light having a power of 800 \(\upmu\)W/cm\({}^{2}\). One can see the response of the VLC receiver: as the SNR deteriorates the logarithmic transimpedance circuit reduces its sensibility in order to prevent the photoelectron saturation. Figure 6: Picture of the experimental setup with the VLC emitter placed 40 m away from the VLC receiver, a LED noise source placed at 47 cm from the VLC sensor, and the oscilloscope showing the received signal.