## 2 Materials and Methods
### Definition and Design of the System's Components
#### 2.2.2 PD Driver  
In designing the PD driver, it was necessary to ensure that a bandwidth of at least 10 MHz was achieved to comply with the VLC channel requirement. We selected the OSRAM PIN BP 104 S photodiode [31] due to its optimal balance between the active area and response time. This PD was bought on [[https://mouser.com/](https://mouser.com/)]([https://mouser.com/](https://mouser.com/)) (accessed on 5 September 2024). The circuit was implemented using a trans-impedance amplifier with the LTC6268-10 amplifier [32], which was chosen for its 4 GHz gain-bandwidth (GBW) product and suitable supply voltage range. The receiver circuit was powered by a 5 V voltage supply, slightly below the maximum allowable voltage supply of the LTC6268-10 amplifier, which is 5.5 V. The PIN BP 104 S OSRAM photodiode [31] was reverse-biased at V\({}_{\text{PD}}\) = V\({}_{-}\) = \(-\)2.5 V to operate in the photoconductive mode. Additionally, a voltage divider was incorporated at the amplifier's positive input to set the voltage to V\({}_{+}\) = 2.5 V. As the amplifier's input voltage range depends on the input voltages V\({}_{+}\) and V\({}_{-}\), the input voltage range is from \(-\)2.7 V to 2.7 V.  
Thus, by adding a voltage divider at the positive input of the amplifier, the input signal range permitted by the amplifier is increased. The gain of the first stage is 15 k\(\Omega\), while the gains of the second and third stages are 5 and 9, respectively. The second and third stages are based on the LCT6253 amplifier due to its GBW of 720 MHz [33]. The schematic design of the PD driver is depicted in Figure 11. The components selected for the PD driver are listed in Table 2. The datasheets of the LTC6253 amplifier can be found in [33]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Amplification Stage 1** & **Amplification Stage 2** & **Amplification Stage 3** \\ \hline R\({}_{1}\) & 5 k\(\Omega\) & R\({}_{4}\) & 200 \(\Omega\) & R\({}_{8}\) & 900 \(\Omega\) \\ R\({}_{2}\) & 5 k\(\Omega\) & C\({}_{5}\) & 100 nF & R\({}_{9}\) & 100 \(\Omega\) \\ C\({}_{4}\) & 56 nF & R\({}_{7}\) & 1 k\(\Omega\) & Amplifier & LTC6253 \\ R\({}_{7}\) & 15 k\(\Omega\) & R\({}_{5}\) & 16 k\(\Omega\) & & \\ C\({}_{3}\) & 0.25 pF & R\({}_{6}\) & 1 k\(\Omega\) & & \\ Amplifier & LTC6253 & C\({}_{6}\) & 100 nF & & \\ Amplifier & LTC6253 & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Components of the PD driver. Figure 9: (a) Set-up implemented to take the frequency response measurement of the LUW-CN7N-KYLX-EMKM OSRAM LED and its LED driver based on 2 equalization and 2 amplification stages; (b) frequency response measurement of the LUW-CN7N-KYLX-EMKM OSRAM LED and its LED driver based on 2 equalization and 2 amplification stages. Subsequently, the three-stage amplification PD driver was simulated in LTSpice, resulting in a simulated bandwidth of 44.25 MHz, as shown in Figure 12.  
Figure 11: PD driver implemented with three amplifier stages. Figure 10: Frequency response measurement of the IR HSDL-4250 LED and its LED driver based on 2 equalization and 2 amplification stages. The frequency and phase response of the visible optical channel were measured using the Moku frequency response analyzer. The measurement was conducted at a distance of 50 cm between the LUW-CN7N-KYLX-EMKM OSRAM LED and the three-stage PD driver, without utilizing any lens on the implemented PD driver, as depicted in Figure 13a. Additionally, the measurement was performed with an illuminance level of 500 lx on the PIN BP 104 S OSRAM PD, as recorded using the MAVOLUX 5032C USB luxmeter. The transfer function of the three-stage PD driver is illustrated in Figure 13b, demonstrating that the measured bandwidth attained 11.66 MHz when using the LUW-CN7N-KYLX-EMKM OSRAM LED. The measurements confirmed that the receiver circuit had an appropriate gain and aligned with the expected values.