## 2 Materials and Methods  
### Description of the Proposed System  
The communication system is bidirectional, featuring both a downlink and an uplink. The downlink is based on VLC technology, meaning it employs white light to transmit audio-visual content from the LiFi router to the universal serial bus (USB) dongle. Conversely, the uplink uses infrared light to send control commands, similar to those found in TV remote controls, from the end user's device to the LiFi router. The infrared light does not interfere with the white light used in the downlink, as they operate at different wavelengths [17; 18; 19]. Both the LiFi router and the USB dongle utilize this bidirectional communication, making use of both the downlink and uplink for efficient data transmission in both directions. The van wirelessly receives data streaming through packets from the content server via a mobile network or WiFi. The LiFi router then takes these packets, which are digital signals, and converts them into analog signals. The analog signals modulate the white-light LED, which emits optical signals [20]. Thus, the visible-light emitter serves a dual purpose: (i) providing illumination to the indoor vehicle and (ii) transmitting modulated information to the visible-light receiver located at the USB dongle. The USB dongle receiver detects the visible optical signals, which performs the reverse function of the LiFi router by transforming the visible optical signals back into analog signals, and then the analog signals are converted into digital signals and these digital signals are joint to form the packets which are transmitted through the USB port to the end user device, such as a laptop, tablet, or smartphone. In the reverse direction, the user can send commands such as rewind, forward, play, pause, or stop to the LiFi router through  
Figure 1: The LiFi router, located on the vehicle’s interior roof, connects to the content server via a wireless Internet connection. The USB dongle, connected to the user’s portable device, receives and plays the audio-visual content as described. the infrared uplink. This separation of the downlink and uplink into different wavelengths prevents interference [17; 21], establishing a bidirectional communication system. The block diagram of the system is shown in Figure 2. The diagram includes the LiFi router and the USB dongle, which both consist of optical components such as an LED, PD, electronic drivers, and interfaces for the uplink and downlink, all depicted in blue, yellow and orange colors, respectively. Additionally, the advanced RISC machine (ARM) and FPGA blocks are integrated into a Trenz Electronic TE0720 system on a chip (SoC) module [22]. The ARM block handles firmware development, while the FPGA block encompasses virtual hardware. The ARM block is responsible for memory management processes and implementing physical (PHY) and medium access control (MAC) layers to efficiently manage data flow and its corresponding frames. The FPGA block manages the encoding and decoding of visible and infrared light signals, as well as their modulation and demodulation. Although the ARM and FPGA blocks contribute to the overall application, they are not part of the hardware development of this work. On the router LiFi side, the FPGA connects to the visible LED driver through the FPGA/LED driver interface, and the IR PD driver connects to the FPGA through the PD driver/FPGA interface. The router LiFi connects to the content server via a mobile network or WiFi. The server provides the UHD audio-visual content to the system. On the USB dongle side, the visible PD driver connects to the FPGA through the PD driver/FPGA interface, and the FPGA connects to the IR LED driver through the FPGA/LED driver interface. The USB dongle connects to the user's device via the USB port of its mobile device. The visible/IR LED drivers establish the bias point of the LED and stimulate it with modulated signals for emission.