## 5 Experimental Results
### Experimental Waveforms  
The prototype was experimentally evaluated as a VLC driver to prove the concept, generating a communication signal following the 64-QAM modulation scheme. During the tests, the efficiency of the converter was measured, and the most significant waveforms were obtained to illustrate communication performance. The efficiency calculation involves the measurement of the both the input and output power. The input power was directly measured by high-precision multimeters, measuring input voltage and input current provided by V\({}_{1}\). The output power needed to be measured using the oscilloscope because of the high frequency of the communication signal. Both the output voltage and the output current were measured, stored and post-processed to calculate the output power. The output voltage was directly measured by an oscilloscope probe and the output current was measured using a high-precision low-inductance shunt resistor with high thermal stability. The high thermal stability in the shunt is mandatory, since it is connected in series and close to the HB-LED load. The low stray inductance and the proximity of the shunt are also required, due to the nature of the high frequency of f\({}_{\text{sig}}\). During regular operation, when maximum communication power was processed (the worst case, because the amplitude of the communication signal could be at a maximum in certain symbols), the efficiency achieved was 94%, with the output power being 7 W. Figure 19 shows the most significant waveforms from the converter during its operation as a VLC driver (i.e., v\({}_{\text{o}}\)(t) and i\({}_{\text{o}}\)(t)). The average value of the output current, I\({}_{\text{o}}\), was kept at 0.25 A by the action of the output voltage loop of the auxiliary Buck DC/DC converter. The instantaneous value of v\({}_{\text{o}}\)(t) (with a small portion of DC) was controlled by the TIBuck DC/DC converter according to v\({}_{\text{sig}}\)(t). The output current was never 0 A. Therefore, the HB-LEDs do not work outside their linear regions. The voltage v\({}_{\text{sw}}\)(t) is the switching node voltage of the TIBuck DC/DC converter and it is a square waveform modulated in PWM that varies between V\({}_{1}\) and V\({}_{2}\), as was expected. The top value V\({}_{1}\) is constant, while V\({}_{2}\) changes according to the current loop control. At room temperature, the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Li-1** & **C\({}_{\text{ti-1}}\)** & **Li-2** & **C\({}_{\text{ti-2}}\)** & **Li-3** & **C\({}_{\text{ti-3}}\)** \\ \hline
1.7 uH & 9.9 nF & 2.2 uH & 9.9 nF & 1.9 uH & 5.72 nF \\ \hline \hline \end{tabular}
\end{table} 
Table 2: List of components of the auxiliary TIBuck DC/DC converter. Figure 19: Most significant waveforms of the prototype acting as a VLC driver: output voltage, v\({}_{\text{o}}\)(t), output current, io(t), received light, v\({}_{\text{rx}}\)(t) and switching node voltage, v\({}_{\text{sw}}\)(t). average voltage of this HB-LED load would be 28 V (data extracted from the datasheet). According to Figure 19, the average voltage \(\mathrm{V_{o}}\) was 24 V, which is 14% lower. This value is on the same page as the correction made at the beginning of the design when selecting the voltage \(\mathrm{V_{1}}\). This allows partial counteracting of the temperature effect on the resolution by designing the voltage \(\mathrm{V_{1}}\) closer to the operation value rather than at room temperature. To validate the correct conversion of the communication signal into variations of light, the emitted light was measured. A high-speed and wide-bandwidth optical receiver Thorlabs PDA10A-E was placed at 0.4 m in front of the HB-LED load. The voltage \(\mathrm{v_{rx}(t)}\) is the output of the receiver, which was proportional to the intensity of light, checking that the HB-LED load is fast enough and its bandwidth was wide enough to allocate the communication scheme. Figure 19 shows a 12-symbol transmission of the 64-QAM modulation scheme.