## 4 Evaluation of the Logarithmic Transimpedance Circuit
### Experimental Evaluation in Long Range Outdoor Conditions
Thus, during these testes, the VLC receiver is exposed to bright sunlight having the irradiance up to 67,250 \(\upmu\)W/cm\({}^{2}\) (Figure 7b), whereas as the sun is close to sunset, it is directly facing the VLC sensor. Consequently, one can see that these situations are very representative for the worst case VLC scenario as they imply long range communications (i.e., 50 m), outdoor conditions, strong light, and direct sun exposure. Accordingly, in order to maintain connectivity in such harsh conditions, the FOV of the receiver has been narrowed to 15\({}^{\circ}\) (\(\pm\)7.5\({}^{\circ}\)), whereas as the sun went toward the sunset and it was directly facing the VLC receiver, the FOV has been further narrowed to 7.5\({}^{\circ}\) (\(\pm\)3.75\({}^{\circ}\)). Figure 7c clearly illustrates the experimental setup showing the VLC receiver simultaneously facing the VLC emitter and the sun, whereas Figure 8 illustrates the shape of the received signal and its reconstruction process at the VLC receiver level. One can see that in strong sun exposure, the logarithmic transimpedance circuit prevents the saturation  
Figure 7: Picture showing the experimental setup for the 50 m VLC tests: (**a**) outdoor tests performed in moderate background lighting conditions; (**b**) strong sunlight conditions with the sun positioned left of the VLC receiver; (**c**) VLC evaluation in strong sunlight conditions: The VLC receiver is facing the LED traffic light VLC emitter. The positions of the VLC emitter and of the VLC receiver are static whereas the sun position is changing. During the afternoon, the sun is directly facing the VLC receiver. of the PIN photodiode and it is able to maintain the communication link. Nevertheless, in these conditions the logarithmic transimpedance circuit provides a signal whose amplitude does not exceeds 2-3 mV and which is very affected by noise. However, due to an adequate signal processing plan, the VLC receiver is able to reconstruct the signal and to accurately decode the data. The summary of the experimental evaluation in bright sun conditions is also presented in Table 7, whereas the BER as a function of the estimated SNR is illustrated in Figure 9. A discussion concerning the results is provided in Section 5. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sunlight Irradiance at** **Sensor Level [\(\mu\)W/cm\({}^{2}\)]** & **VLC Receiver FOV** & **Average BER** & **Uncontrolled Outdoor Conditions** \\ \hline
1500 & 30\({}^{\circ}\) & \(<\)10\({}^{-6}\) & Cloudy afternoon \\ \hline
4000 & 30\({}^{\circ}\) & \(\approx\)10\({}^{-4}\) & Bright day, with no direct visibility of the sun. See Figure 7a \\ \hline
12,000 & 7.5\({}^{\circ}\) & \(<\)10\({}^{-6}\) & The sun sets and its irradiance is gradually decreasing. \\ \hline
25,000 & 7.5\({}^{\circ}\) & \(10^{-4}\)–10\({}^{-6}\) & Bright sun conditions, with no clouds. The sun is located right above the VLC emitter, directly facing the VLC receiver. Worst case scenario for VLC. See Figure 7c \\ \hline
63,000 & 15\({}^{\circ}\) & \(10^{-3}\)–10\({}^{-5}\) & In this setup, the sun is not directly facing the VLC receiver. The sun is on the same axis with the VLC systems, facing the receiver. See Figure 7b \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Summary of the experimental evaluation of the VLC system in various outdoor conditions. Figure 8: Scope screen 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 signal which will be used in the data decoding process.