## 2 Fronthaul Solutions for 4G/5G
### Combined Fiber-Optic and FSO-Based 5G Fronthaul  
The combined fiber-optic and FSO-based 5G fronthaul followed by wireless access extension is described in a block diagram in Figure 4a. The system encompasses RoF, FSO, and wireless technologies in a single architecture that takes advantage of a hybrid RoF/FSO link for fronthaul integration followed by dual-band 5G NR wireless access. The FSO link enables last-mile applications when there are restrictions to the placement of a fiber-optic connection, thereby increasing system flexibility. After photodetection, further reach is achieved by wireless access. For the experiments, we simultaneously transmitted two RF signals in the hybrid configuration in accordance with the frequency bands standardized by 3 GPP Release 15. A 100 MHz bandwidth 5G NR signal at 3.5 GHz was generated using the VSG\({}_{1}\), whereas a 400 MHz bandwidth \(M\)-QAM signal was generated in baseband using an AWG and upconverted to 26 GHz by means of the VSG\({}_{2}\). Finally, a diplexer combined both signals, resulting in 3 dBm electrical power at the optical modulator. An OS-TL-D-C-50-200-1-S-FA DFB laser from Goligit generated a 13 dBm optical carrier at 1550 nm, which was modulated by the RF-driven signals discussed above using a single-drive MZM. The modulated optical carrier was transmitted throughout a 12.5 km fiber-optic fronthaul to the collimator, and then, as a proof-of-concept, it followed a 1 m long FSO fronthaul in such a way as to assure an eye-safe FSO link. At the receiver side, an optical lens with a 5-fold collimation factor coupled the free-space optical beam into an optical patch cord. A 3-axis micropositioner was used to minimize vibration and misalignments losses. Next, the optical signal was amplified by an EDFA, whereas a variable optical attenuator (VOA) and an optical power monitor (OPM) were used for controlling and monitoring the optical power at the input of the PIN broadband photodetector from u2. It is worth highlighting that the EDFA noise was considered in the approach as there was no filter before the photodetector. If the EDFA amplified spontaneous emission (ASE) had been filtered out, an enhanced performance could have been achieved. After photodetection, the resulting RF signals were amplified by the electrical amplifier EA\({}_{1}\) by about 24 dB. A diplexer was used to separate the 3.5 and 26 GHz signals. Two 5 dBi gain log-periodic antennas were used for transmitting and receiving the 5G NR signal at 3.5 GHz. In parallel, two 25 dBi gain horn antennas enabled 8 m of mm-wave wireless link at 26 GHz aimed at indoor applications. At the receiver side, the 5G signal was amplified (EA\({}_{2}\)) by 20 dB and evaluated employing an FSW-8351 VSA (VSA\({}_{1}\)). Meanwhile, the 400 MHz bandwidth \(M\)-QAM signal was amplified (EA\({}_{3}\)) by 35 dB and evaluated using a DSAZ632A high-frequency oscilloscope (VSA\({}_{2}\)). The system performance was investigated by assessing the hybrid architecture in two distinct scenarios in terms of EVM\({}_{\text{RMS}}\) and in accordance with 3 GPP Release 15 specifications. The first step was to evaluate the hybrid RoF/FSO fronthaul at the photodetector  
Figure 4: Combined fiber-optic and PSO-based 5G fronthaul architecture. (**a**) Block diagram; (**b**) performance analysis of 5G NR and \(M\)-QAM signals in the optical-wireless system. The constellations from the RoF/FSO/wireless approach show 16- and 64-QAM received symbols suitable for demodulation. output, whereas the second step consisted of actually analyzing the dual-band 5G NR wireless system employing the hybrid RoF/PSO fronthaul, as depicted in Figure 4b. It is worth mentioning that the experimental results were achieved using 3 dBm optical power at the photodetector input for both scenarios. In particular, for the 5G NR signal at 3.5 GHz, we selected 64-QAM, which resulted in 578 Mbit/s and measured EVM\({}_{\text{RMS}}\) of around 2.4%, thereby satisfying the 3 GPP requirements (8%) by a wide margin.