## 4 Discussion and Conclusions  
This work thus illustrates that utilizing a VLC system to facilitate the transmission of UHD content in commercial vehicles is feasible and scalable for mass transit applications, as demonstrated in the following link: [[https://www.youtube.com/watch?v=GH5](https://www.youtube.com/watch?v=GH5) htUZf-Uw]([https://www.youtube.com/watch?v=GH5](https://www.youtube.com/watch?v=GH5) htUZf-Uw) (accessed on 5 September 2024). This video can be downloaded in the Supplementary Materials section. The implemented system comprises two parts. The first part is the 'LiFi router', which is made up of a stack of three boards, a visible LED, and an infrared receiver. It operates on a supply voltage of 12 V with a current draw of 920 mA. There are two power sources it can utilize: (i) a 12 V AC/DC adapter with a jack power connector or (ii) a 12 V battery. The ultra-white LED, driven by the implemented driver on one of the boards, achieves a bandwidth of 14.45 MHz, while the IR PD driver attains a bandwidth exceeding 12.27 MHz. The second part is the USB dongle, which also consists of a stack of three boards, an IR LED, and a visible-light receiver. Operating with a 5 V supply voltage and drawing 1.12 A, this part can be powered in two ways: (i) via a 5 V AC/DC adapter with a jack power connector or (ii) via a USB Type-C connector. The infrared LED, accompanied by the implemented driver, achieves a bandwidth exceeding 20 MHz, whereas the visible photodiode (PD) driver, powered by the interface board at +5 V and \(-\)5 V, attains a bandwidth surpassing 11.66 MHz. Additionally, the LED driver is designed for high-power white LEDs with a power consumption of approximately 1.5 W, replicating the reading lamps commonly found in mass transit. The first two stages of the driver consist of equalization amplification circuits designed to enhance the bandwidth of the LED. Each of these stages incorporates phase-advanced equalizers, resulting in a bandwidth of 14.45 MHz, marking a significant improvement (almost a decade) from the initial bandwidth of 1.8 MHz. The final stage encompasses a current source, comprising a voltage-biased N-MOSFET transistor that superimposes the modulated signal on the DC level necessary for the LED to achieve a luminous intensity of 75,000 lx. Conversely, the developed PD driver, comprising three gain stages, attains a bandwidth of 11.66 MHz for the visible PD driver and 12.27 MHz for the IR PD driver. While these are modest values, they are sufficient to transmit UHD content as required. In summary, the bandwidth of the visible downlink is 11.66 MHz, while the IR uplink achieves a bandwidth of 12.27 MHz. In [10], a bandwidth of 8 MHz and a link distance of 8 m were reported. However, that system employed an APD as the photodetector. Considering that APDs require high bias voltages such as 65 V, and given that the receiver needs to be powered by a 5 V supply, using an APD is not viable. To avoid potential spurious data, the visible optical link was configured to operate at a speed of 10 MHz, a submultiple of the 40 MHz sampling frequency determined by the ADC and DAC. Moreover, the maximum optical length achieved is 110 cm, accomplished by using a Fresnel lens at the receiver side with a focal length of 10 mm and a diameter of 13 mm. The system's noise level amplitude is approximately 45 mV\({}_{\text{pp}}\). Likewise, the infrared optical link operates at 10 MHz, in harmony with the 40 MHz sampling frequency. It also achieves an operating distance of 110 cm, utilizing the same Fresnel lens. The system's noise level amplitude is approximately 45 mV\({}_{\text{pp}}\). To ensure the system's correct performance, the minimum signal-to-noise ratio (SNR) for both optical links is set at 10.74 dB. The visible optical channel was modulated using a multicarrier modulation scheme, with each carrier modulated using Quadrature Phase Shift Keying (QPSK).