## 3 4G/5G NR VLC-Based Access Network  
VLC has emerged as an alternative for transmitting the 5G NR standard or other wireless signals in environments where electromagnetic interferences must be minimal, such as in hospitals, petrochemical industrial plants, and airplanes. Furthermore, LED-based VLC systems can take advantage of environment lighting to provide free-space data transmission using wavelengths in the visible part of the optical spectrum. The LED is directly modulated, and the optical link operates in IM-DD mode at the photodiode [28]. By using an LED, the wide light beam divergence allows the desired area to be fully eliminated, thereby enabling simultaneous access for multiple devices and/or patterns of data traffic [21]. In addition, carrier aggregation strategies can be employed to increase the effective user throughput.  
Figure 5a depicts the experimental set-up diagram of the LED-based VLC access point for simultaneous transmission of the 5G NR standard signal and an LTE-A three-band signal. RGBA (red, green, blue, and amber) LEDs were considered as the light source because they provide larger modulation bandwidth in comparison to white commercial LEDs [29]. In particular, the final selection of red LED (623 nm optical carrier) was dictated by the manufacturer's datasheet, which indicates that this wavelength offers the best trade-off between output power and quantum efficiency. In the experiments, the 5G NR signal was generated using a SMBV100B vector signal generator (VSG\({}_{1}\)), while a PSGE8267D vector signal generator (VSG\({}_{2}\)) and a M9505A arbitrary waveform generator (AWG) were employed to generate the LTE-A three-band signal. The RF signals were electrically combined and directly modulated the red LED by means of a bias tee. The LED light beam directivity was enhanced by a reflector and launched in a free-space link of 0.75 m.  
At the receiver side, a plane convex lens with the diameter and focal distance of 25.4 and 35.0 mm, respectively, was used to focus the incident beam at the input of the PINphotodetector (EOT ET-2030). Next, an RLC equalizer similar to the one described in [29] was implemented to compensate for the LED frequency response. A DC blocker ensured that the DC component from the OE conversion did not reach the VSA. Finally, a cascade of four electrical amplifiers (EAs) assured a 50 dB gain in the received signal.  
The system performance was evaluated by EVM\({}_{\text{RMS}}\) measurements. The 5G NR signal was generated with a bandwidth of 10 MHz, 64-QAM, centered at 46 MHz. The configuration of the LTE-A signal represented the aggregation of three intra-band carriers in order to triplicate the effective throughput of a single user. Specifically, the LTE-A signal was generated with three sub-bands of 12 MHz, 64-QAM, at 57, 69, and 81 MHz, named as 64-QAM\({}_{1}\), 64-QAM\({}_{2}\), and 64-QAM\({}_{3}\), respectively.  
Figure 5b shows the received spectrum as well as the measured constellations and EVM\({}_{\text{RMS}}\) values. The received signal power decreased as the operation frequency increased on account of the LED frequency response, degrading both the signal-noise ratio (SNR) and the EVM\({}_{\text{RMS}}\) value. The constellations in Figure 5b illustrate these performance issues by considering the four transmitted bands. The measured EVM\({}_{\text{RMS}}\) values were 4.0%, 4.15%, 6.18%, and 6.45% for 5G NR, 64-QAM\({}_{1}\), 64-QAM\({}_{2}\), and 64-QAM\({}_{3}\), respectively, which are all within the 3 GPP limit of 8% for the 64-QAM format [27]. The effective throughputs measured for the 5G NR signal and the aggregation of the three 64-QAM signals were 38 and 180 Mbit/s, respectively, rates which are attractive for a VLC access point.