## 3 Results
### Electro-Optical Measurements of the Optical Links
#### 3.3.2 Data Transmission Parameters of the Visible Link  
This section examines the data transmission parameters achieved by an implementation of a UHD video transmission using the electro-optical interface proposed in this work to send the data through visible light. This implementation is only one of many applications where the interface can be used, and the parameters depend not only on the optoelectronics interfaces but also on the modulation and coding formats, which depend on the VHDL development. Bit Rate of the Visible Link  
To transmit UHD-quality video, a minimum transmission speed of 5 Mbps is required, depending on the video encoding. Throughout the development of this system, various modulations and encodings were tested. To measure the transmission speed, the VLC link was configured to operate at its maximum speed, and the time taken by the VLC system to send 1 MB was recorded. The results are presented in Table 5. These results demonstrate that UHD-quality video can be transmitted at the minimum required speed, provided the video is correctly encoded. The correction factor R = 1 indicates that all transmitted bits are information bits. In contrast, \(R=\frac{P}{q}\) indicates that for every q bit sent, p bits are information and (p\(-\)q) bits are redundancy bits used for error correction. Bit Error Rate (BER) and Packet Error Rate (PER) of the Visible Link  
To ensure perfect fluidity of multimedia content, it is essential to achieve zero bit errors and zero packet errors, meaning no packet losses. To evaluate the system's performance regarding these criteria, a test was conducted involving the consecutive transmission of 256 random packets, with each packet containing 4096 bytes. In total, 8,388,608 random bits were transmitted during this test. These tests were performed at various distances ranging from 62.5 cm to 102.5 cm, as detailed in Table 6. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Modulation and Codification** & **MB/s** & **Mbps** \\ \hline OOK (8b10b) & 1.91 & 15.25 \\ OFDM (BPSK, R = 1) & 0.71 & 5.69 \\ OFDM (QPSK, R = 1) & 1.41 & 11.25 \\ OFDM (QPSK, \(R=\frac{5}{6}\)) & 1.23 & 9.83 \\ OFDM (QPSK, \(R=\frac{3}{4}\)) & 1.05 & 8.44 \\ OFDM (QPSK, \(R=\frac{1}{2}\)) & 0.71 & 5.69 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Bits rates of the VLC link. At a distance of 102.5 cm, the system was capable of receiving almost all packets, achieving a very satisfactory packet error rate (PER), although it was not possible to evaluate this numerically. As expected, the bit error rate (BER) increased with the distance. Despite this, all the BER values obtained were less than \(3.8\times 10^{-3}\)[54; 55], which is known as 'error free' in optical transport networks, allowing for the application of Forward Error Correction (FEC) techniques at all distances. When the BER is below this threshold, FEC techniques can be employed to correct errors, preventing any data loss. Conversely, if the BER surpasses this value, FEC techniques are unable to correct the errors, leading to data loss. The effect of this on transmission depends on the type of data being transmitted. By implementing an error correction system, the bit error rate was minimized, ensuring the optimal performance of the VLC system for video transmission with the necessary quality. Optical Link Length of the Visible Link  
The maximum distance at which that the system could maintain UHD video without losing was measured using an optical table and rails, which allowed for increasing the distance between the receiver and transmitter without altering other factors such as the direction of the light beam or the distance between the receiver and the lens, as shown in Figure 34.  
Measurements were taken of the number of errors per packet at various distances. As the receiver moved farther from the transmitter, more errors appeared in the packets. This result was expected because the noise level at the receiver remained nearly constant while the signal intensity decreased with increasing distance.