# Demonstration of a Hybrid B5G System Integrating VLC and RF-Based Technologies with Access Networks
## 3 Experimental Setup  
First, we present the light source characterization, which consisted of a power LED with white emission light Model XQ-A from Cree-Led (XQ-A, Cree-Led). Curve \"A\" in Figure 2 depicts the LED experimental frequency response. Figures 3 and 4a report the experimental setup photographs and the schematic diagram of the proposed VLC system, respectively. The VLC transmitter uses a bias-tee (BT) and a power supply (PS) to energize the LED with 3 V and 170 mA, in addition to coupling the RF signal to be transmitted. A concentrator lens (L1) is used to collimate the light from the LED. Curve \"B\" in Figure 2 exhibits the frequency response of the complete VLC transmitter, i.e., the set composed of L1, LED, BT, electrical amplifier (EA)4, and the low pass filter (LPF), in which a wide 8 dB bandwidth from 8 to 55 MHz can be observed. At the receiver side, an achromatic lens (L2) focuses light onto the photodetector PD2 (ET-2030, EOT). The distance between these lenses (VLC link) was 1.3 m. In the optical-to-electrical (O/E) conversion process performed by PD2, a DC component is generated at the PD2 electrical output since it was biased with an external PS at 12 Vdc in addition to the RF signal. In order to avoid damage to our vector signal analyzer (VSA) input port, a BT was employed for blocking the DC component using a 50-\(\Omega\) load connected to its DC port. The recovered RF signal is then made available in BT RF output. In the VLC receiver, the PD2 photodetector has a bandwidth greater than 1.2 GHz, making the VLC transmitter the limiting factor in the frequency response of the complete VLC system. Therefore, curve \"B\" in Figure 2 also represents the frequency response of the entire VLC system (transmitter plus receiver). Figure 2: Frequency response of the LED with white cold light emission (A) and of the complete VLC transmitter (B). An important contribution of this work is in the network topology, i.e., the system employs several technologies that converge into a unified architecture, which uses a hybrid radio-over-fiber (RoF)/FSO link to integrate the midhaul and fronthaul of the network, while the access link is provided by a VLC link operating at 25 MHz (RF1) along with two wireless links operating at 2.5 GHz (RF2) and 26 GHz (RF3). It is important to emphasize that the current work is a proof-of-concept demonstration, exploring the future applicability of these technologies in midhaul/fronthaul and access networks. A photograph of the experimental setup is depicted in Figure 4b. The CO is composed mainly of three vector signal generators (VSGs) (VSG1, VSG2, and VSG3), a tunable light source (TLS) (Golight, Shenzhen, China), and an MZM (FTM7939 EK, Fujitsu, Tokyo, Japan). VSG1 (EXG N5172B, Keysight, Santa Rosa, CA, USA), VSG2 (BTS MT8222B, Anritsu, Atsugi, Japan), and VSG3 (PSG E8267D, Keysight, Santa Rosa, CA, USA) are used to generate RF signals centered at RF1, RF2, and RF3, respectively. An electrical combiner (SC1) and a diplexer (DPX1) are used to join these three RF signals. The TLS supplies an optical carrier laser centered at 1550 nm with 8 dBm optical power. A polarization controller (PC) is used to maximize the optical power input at the MZM. The latter is used to modulate the optical carrier with the combination of the RF1, RF2, and RF3 electrical signals. MZM bias is provided by a PS. VSG1 is used to generate a generic vector signal without standard transmission protocols. VSG2 generates an LTE signal. In parallel, VSG3, together with an arbitrary waveform generator (AWG) (M8190A, Keysight), is used to supply 5G NR signals. The base-band 5G NR signal is designed using Keysight Signal Studio software (version Pro for 5G NR 2020, update 1.0) and generated by the AWG. The test model 3.1 (TM 3.1) generated by the software was defined by 3 GPP Release 18 [27], which specifies that the 5G NR signal in FR2 operates with bandwidths of up to 400 MHz and with Q-PSK, 16-quadrature amplitude modulation (QAM), and 64-QAM modulation schemes.