## 2 Materials and Methods
### Definition and Design of the System's Components  
#### 2.2.1 LED Driver  
The LUW-CN7N-KYLX-EMKM OSRAM LED [23] was selected for the visible-light downlink. This LED was bought on [[https://eu.mouser.com/](https://eu.mouser.com/)]([https://eu.mouser.com/](https://eu.mouser.com/)) (accessed on 5 September 2024). This high-power LED boosts the detection of the transmitted signal, offering improved optical link reach distance compared to other LEDs, like the LUXEON 3020 LED [24]. The bias point of the LUW-CN7N-KYLX-EMKM LUW OSRAM LED was established at a forward voltage of 3.27 V and a forward current of 120 mA, setting an illumination level of around 75,000 lx. The LED's cut-off frequency at \(-3\) dB measured a value of f\({}_{\text{\text{\text{\text{--3 dB}}}}}\) = 1.8 MHz. Given the application requires around a 10 MHz of bandwidth for the visible downlink, it is critical to obtain the LED's equivalent electric circuit to try to enhance its bandwidth. The procedure detailed in [25] was conducted to characterize the ultra-white LUW-CN7N-KYLX-EMKM OSRAM LED in alternating current (AC) [23]. This was carried out to determine the parameters of the simplified equivalent electric circuit of the LED, yielding the following values: R\({}_{\text{d}}\) = 81 m\(\Omega\), R\({}_{\text{s}}\) = 26.88 \(\Omega\), and C\({}_{\text{j}}\) \(\simeq\) 1.09 uF. The simplified equivalent electric circuit of the LUW-CN7N-KYLX-EMKM OSRAM LED, displayed in Figure 3, was simulated using the LTSpice XVII simulator software for the analog electronic circuit simulator. The simulation results showed that the cut-off frequency at \(-3\) dB was f\({}_{\text{\text{\text{--3 dB}}}}\) = 1.78 MHz, which closely aligns with the measured bandwidth of f\({}_{\text{\text{\text{--3 dB}}}}\) = 1.8 MHz, thereby validating the proposed simplified equivalent circuit shown in Figure 3.  
Before the design of the LED stage to increase its bandwidth, the LED driver was implemented based on a voltage division bias circuit developed through an n-type MOSFET transistor, as illustrated in Figure 4. The alternating current controls the transistor gate, thereby switching the LED. Figure 4: LED driver based on a division voltage for n-type MOSFET. Figure 3: Equivalent simplified circuit of the LUW-CN7N-KYLX-EMKM OSRAM LED for AC. The n-type MOSFET TN0110N3-G [26] was chosen to control the current flow through the LED using the resistor R\({}_{\text{S}}\). This transistor was manufactured by Microchip Technology and bought on [[https://eu.mouser.com/](https://eu.mouser.com/)]([https://eu.mouser.com/](https://eu.mouser.com/)) (accessed on 5 September 2024). To achieve this, the MOSFET must be in the active region. This can be ensured by confirming that the voltages V\({}_{\text{GS}}\) and V\({}_{\text{DS}}\) exceed certain threshold values defined in the MOSFET's datasheet, specifically, V\({}_{\text{GS}}\) > 2 V and V\({}_{\text{DS}}\) > 2 V. It is worth noting that the LED driver can be tuned to adapt if the LED is replaced with another model. To set the bias point of the circuit, a DC source defined as V\({}_{\text{dd}}\) is required to supply voltage to the LED and maintain the MOSFET in the active region. V\({}_{\text{in}}\) is linked with the AC generator signal, which is short-circuited for the DC analysis performed for the bias point study. After analyzing the voltage division bias circuit for the MOSFET transistor TN0110N3-G with a DC voltage supply of V\({}_{\text{dd}}\) = 12 V, the LED's bias point was determined as I\({}_{\text{LED}}\) = 120 mA and V\({}_{\text{LED}}\) = 3.27 V. The values of the passive elements in the circuit were eventually calculated as R\({}_{\text{I}}\) = 10 k\(\Omega\), R\({}_{\text{2}}\) = 5 k\(\Omega\), and R\({}_{\text{S}}\) = 5.5 \(\Omega\). Equalization aims to balance the signal energy in specific frequency bands to stabilize the transfer function and enhance bandwidth. By adding an equalizer block, the objective is to accomplish a response as uniform as possible within the band of interest.