## 2 Fronthaul Solutions for 4G/5G
### The 5G NR Dual Wireless and FSO-Based Fronthaul  
Figure 2a depicts a block diagram of the dual wireless and FSO-based fronthaul architecture, in which the two parallel data transmission paths enable link dynamic selection according to transmission performance at a given instant, while the other link serves as a backup. The FSO and RF links were implemented at 1550 nm and 38 GHz, respectively. The hybrid FSO/RF system was analyzed under the 5G NR standard operating with 400 MHz bandwidth and 120 kHz between orthogonal frequency division multiplexing (OFDM) subcarriers for quadrature phase shift keying (QPSK) and 16- and 64-quadrature amplitude modulation (QAM). Figure 2: The 5G NR dual wireless and FSO-based fronthaul. (**a**) Block diagram of the proposed hybrid wireless-FSO system. From the left, the signal generated by the vector signal generator (VSG) SMVB100A reaches the FSW signal and spectrum analyzer via the two different links. (**b**) Dependence of hybrid FSO/RF EVM\({}_{\text{RMS}}\) as a function of rain precipitation rate. (**c**) Dependence of hybrid FSO/RF EVM\({}_{\text{RMS}}\) as a function of visibility. In the experiments, a SMVB100A vector signal generator (VSG) was responsible for generating 5G NR signal at 1 GHz with 400 MHz bandwidth for the QPSK, 16-QAM, and 64-QAM schemes. In the FSO link, the modulated RF signal was inserted into a single-drive Mach-Zehnder modulator (MZM) based at the quadrature point. A distributed feedback (DFB) laser at 1550 nm provided the optical carrier, which fed the MZM after passing through a polarization controller (PC). The MZM output was coupled to a collimator to prepare the optical beam for transmission in free space. The transmitted optical beam was then captured by a M-5X objective lens and coupled to a single-mode fiber (SMF). The received signal was amplified by an Erbium-doped fiber amplifier (EDFA) and then photodetected by a \(p\)-intrinsic-\(n\) (PIN) broadband photodetector (u2t XPDV2120 RA). Before demodulation in the electrical spectrum analyzer (FSW), the RF signal was amplified by a low-noise amplifier (LNA, EA\({}_{1}\)). As in references [24] and [25], an FSO link of a few meters was taken as a proof-of-concept for 5G networks to assure an eye-safe indoor environment. As shown below, real-life performance was emulated by varying the optical power reaching the optical receiver. Meanwhile, for the RF link, the same 5G NR signal, once again generated by a SMVB100A VSG, was used to drive the PSG E8267D equipment, which upconverted the 5G NR frequency to 38 GHz. Posterior, its output was connected to a 25 dBi horn antenna, and an identical antenna was used at the receiver side with the same height and polarization. A two-stage amplification (EA\({}_{2}\) and EA\({}_{3}\)) scheme was employed before demodulation. The performance of the hybrid FSO/RF is presented in Figure 2b,c through EVM\({}_{\text{RMS}}\) measurements at different power levels at the receiver, emulating equivalent attenuation levels according to visibility and rain conditions. These power levels at the receiver ranged from \(-41.9\) to \(-7.4\) dBm. Regarding the effect of rain attenuation, the attenuation levels used for the FSO link ranged from 0 to 56 mm/h, while the values ranged from 0 to 102 mm/h for the wireless link. For the specific case of 64-QAM, the 3 GPP EVM\({}_{\text{RMS}}\) limit was reached at about the same precipitation intensity for both FSO and 38 GHz wireless links (36 mm/h, Figure 2b). However, as shown in Figure 2b, while operating in a rainy environment, the wireless link suffered less significant performance degradation as the rain intensity increased. Figure 2c depicts the dependance of EVM regarding visibility for the FSO link for three distinct modulation formats. In every case, successful transmission within the 3 GPP standards required a visibility above 780 m. In contrast, the visibility issue was much less severe for the wireless link.