# Demonstration of a Hybrid B5G System Integrating VLC and RF-Based Technologies with Access Networks
## 4 Results and Discussion  
All experimental investigations were performed considering the EVM\({}_{\text{RMS}}\) parameter. The BER performance metric could have also been employed. However, we have opted for EVM\({}_{\text{RMS}}\) because it was adopted by the 3 GPP Release 18 [27]. The EVM metric describes the effective distance of the received complex symbol from its ideal position in the constellation diagram. Therefore, EVM\({}_{\text{RMS}}\) is defined as the root mean square of all error vector magnitudes between the received symbols and their closest ideal constellation symbol location. This metric provides an assessment of the signal modulation properties by indicating distortions in its waveform and it is usually measured in dB or percentage [30]. The first investigation consisted of evaluating the fronthaul system performance--in other words, evaluating the performance of the RoF followed by the FSO link. For this purpose, from VSG3, a 5G NR signal employing a single modulation scheme of 64-QAM was transmitted and received at the photodetector PD1 output. On the other side, the optical power was varied by changing the gain in the EDFA. The optical power attenuation can be utilized to emulate atmospheric turbulence such as rain, fog, and visibility, enabling the analysis of their impact on FSO links. Figure 5 depicts the performance of the received signal as a function of the measured EVM\({}_{\text{RMS}}\) regarding the optical power at the photodetector input PD1 for bandwidths of 50, 100, 200, and 400 MHz. Particularly for a bandwidth of 50 MHz, the proposed RoF/FSO system easily met the 3 GPP specifications (\(\leqslant\)8% for 64-QAM) for optical power levels higher than \(-\)8 dBm. To increase the data throughput of the system, the bandwidth was increased to 100 MHz, requiring an additional 2 dB of optical power level at the reception. For signals with bandwidths of 200 and 400 MHz, the optical power at the reception must be increased to values greater than \(-\)5 dBm and \(-\)3 dBm, respectively. The best performance point was around 2.5 dBm optical power, where the EVM\({}_{\text{RMS}}\) values were 3.2%, 3.5%, 4.0%, and 4.6% for 50, 100, 200, and 400 MHz bandwidth, respectively. Data throughput in 5G NR is used to evaluate network performance and can be determined either through calculations or practical tests. In our case, we referred to the 3 GPP technical document TS 38.306 [31] to calculate throughput in 5G NR systems, as it provides a comprehensive and standardized framework defining the radio access capabilities of user equipment (UE). Thus, based on the data depicted in Figure 5, a maximum theoretical throughput of 1.616 Gbps was achieved. Figure 5: RoF/FSO system digital performance as a function of the received optical power at the photodetector PD1 using 5G NR with 64-QAM signals. It is important to highlight that the results involving system performance were obtained while all transmissions were performed simultaneously. In this context, the next step was to find the best transmission scheme for each access network, i.e., the best modulation scheme and the best injected RF power to the system. Therefore, the second investigation was to evaluate the transmission of the mm-wave link, i.e., in RF3 5G NR signals, defined in TM 3.1, with different modulation schemes and bandwidths. The 64-QAM modulation scheme exhibited the best performance. Figure 6 presents results for this particular modulation order. The 3 GPP FR2-1 determines four bandwidths: 50, 100, 200, and 400 MHz to transmit 5G NR signals. In this context, Figure 6 displays the electric spectrum received at the VSA3 receptor for these bandwidths, all centered on RF3. Notice that these plots show the reception power levels. The TM 3.1 test model was employed to generate these signals, setting the TDD mode and spacing of 120 kHz between the subcarriers. Consequently, maximum theoretical data throughputs of 202, 404, 808, and 1616 Mbps were achieved, respectively, using [31].