## 4 Experimental Evaluation of the VLC System and Experimental Results
### Experimental Setup No. 1--Medium Communication Distance in High SNR Conditions  
The aim of the next set of experiments was to evaluate the functionality of the system in low noise conditions and to also evaluate the performances of the two modulation techniques. In the first experimental setup, the VLC emitter and the VLC sensor were placed 18 m away from each other within a corridor of the laboratory. The power of the emitted light measured at 1 m is 190 \(\mu\)W/cm\({}^{2}\). As any electromagnetic radiation, the power of the transmitted light decreases exponentially as the distance is increasing. Furthermore, the power reaching the receiver is also influenced by the angle between emitter and receiver. Thus, the measured power for the received signal is 0.56 \(\mu\)W/cm\({}^{2}\), whereas the power of the natural light is between 100-300 \(\mu\)W/cm\({}^{2}\).  
Data sets of about 10 million-bit have been transmitted by repeating a predefined 72-bit text message in order to determine the BER. After reconstructing the data signal (analog signal processing), it is then transformed into a digital message that is processed by the 180 MHz microcontroller. The microcontroller generates an interruption on every rising and falling edge of the digital signal. Thus, the difference between two consecutive interruptions determines the pulse width. Based on this width, and by knowing the code used in the message coding (this info is extracted from the message header), the microcontroller is able to establish the bit value (i.e., 1 or 0). This value is stored in the microcontroller memory and it is than compared to the bits that it supposed to receive. After each comparison, a counter is incremented for correct or incorrect bits. The microcontroller does not use the serial connection while it is processes the data in order not to perturb the data decoding process and enable real-time BER processing. The data have been transmitted using DSSS modulation and SIK coding, OOK modulation and Manchester coding and OOK modulation, and Miller coding. For the data that are transmitted using DSSS, a modulation frequency of 22 kHz has been selected. For OOK modulation, the data have been transmitted using three different data rates 11 kb/s, 24 kb/s, and 50 kb/s. The VLC emitter was placed at a height of 190 cm, whereas the VLC receiver at a height of 85 cm. Table 2 presents the summary of the experimental results. The experimental results confirm the suitability of the proposed design. Thus, for both modulation techniques BERs lower than 10\({}^{-6}\) have been achieved. These BER results have been determined for the physical layer, without using any error correcting methods.  
\begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Feature/Value** \\ \hline Emitter type & 200 mm standard complying LED traffic light having an measured \\  & optical power of 190 \(\mu\)W/cm\({}^{2}\) (at one meter) \\ Receiver type & \(\pm\)45\({}^{\circ}\)/\(\pm\)15\({}^{\circ}\) PIN photodiode-based receiver using a logarithmic \\  & transimpedance configuration \\  & 190 cm \\  & Receiver height & 85 cm \\   Noise power induced by the natural light & 100–300 \(\mu\)W/cm\({}^{2}\) \\   Noise power induced by LED light & 0–800 \(\mu\)W/cm\({}^{2}\) \\   Received signal power & 190 \(\mu\)W/cm\({}^{2}\)-0.076\(\mu\)W/cm\({}^{2}\) \\   Emitter–receiver distance & 1–50 m \\  & \(\bullet\) & DSSS modulation with SIK coding: 3 kb/s \\   Modulations, coding techniques and data rates & \(\bullet\) & OOK modulation with Manchester coding: 11, 24 and 50 kb/s \\  & \(\bullet\) & OOK modulation with Miller coding: 11, 24 and 50 kb/s \\ \hline \end{tabular}
\end{table}
Table 1: Summary of the experimental parameters.