## 2 Fronthaul Solutions for 4G/5G
### 4G/5G NR Fiber-Wireless (FiWi) Fronthaul  
The long-term evolution-advanced (LTE-A) and 5G NR signals have similar physical channel configurations, including primary (P-SS) and secondary synchronization signal (S-SS), physical transmission channel (PBCH), demodulation reference (DMRS), and shared physical data channel (PDSCH). Different modulation formats are used for the data and control channels. For instance, data signals might be modulated in 64-QAM format, while binary phase shift keying (BPSK) modulation is applied for modulating the P-SS and S-SS channels. In addition, QPSK and 64-QAM are utilized for PBCH-DMRS and PDSCH signals, respectively [19]. The EVM\({}_{\text{RMS}}\) performance of the FiWi system was investigated for the eRAC scenario operating in the 700 MHz frequency range. Figure 3a displays a block diagram of the experimental arrangement of the proposed FiWi system, which employs an A-RoF configuration based on external modulation and direct detection followed by a link access for increased reach. An EXG generator (N5171B) from Keysight generated the LTE-A signal (VSG\({}_{2}\)) at 778 MHz, which was configured offline using the Keysight Signal Studio software for generating signals with 20 MHz bandwidth and 64-QAM. The 20 MHz baseband 5G NR signal was also configured by means of the Signal Studio software in conjunction with an arbitrary waveform generator (AWG M9505A). Next, the PSG E8267D converted the baseband 5G NR signal to the 759 MHz band (VSG\({}_{1}\)). This approach allowed the transmission of the LTE-A and 5G NR signals coexisting in adjacent channels, thereby making use of the non-standalone transmission mode proposed by 3 GPP in Release 15 [3]. A dual-drive Mach-Zehnder modulator (DD-MZM) modulated an optical carrier provided by a laser at 1560 nm with two distinct RF signals. The optical beam was then launched into a single-mode fiber with 0 dBm of optical power. Nonlinearities were not observed in our approach as the optical power level in the SMF did not reach the power thresholds leading to nonlinear effects [26]. Regarding the electrical power levels, RF\({}_{1}\) and RF\({}_{2}\) signals were transmitted at \(-5\) dBm, while the PIN photodetector (EOT ET-5000F) received a fixed optical power of \(-3\) dBm. After photodetection, the signals in the electrical domain were amplified by about 44 dB (EA\({}_{1}\)) to compensate for the overall system losses. Two log-periodic broadband antennas with 5 dBi gain were used to transmit and receive  
Figure 3: The 4G/5G NR fiber-optic-based fronthaul architecture. (**a**) Block diagram; (**b**) coexistence analysis between 4G and 5G technologies in the optical-wireless system. both RF\({}_{1}\) and RF\({}_{2}\), and a 10 m wireless transmission was performed as a proof-of-concept. At the receiver side, a Keysight MXA N9020A vector signal analyzer (VSA\({}_{1}\)) was used to demodulate the received signals and provide the EVM\({}_{\text{RMS}}\) measurement results. In the experiments, a 19 MHz frequency offset was employed between RF\({}_{1}\) and RF\({}_{2}\). This offset was determined as a preliminary step in order to find the minimum frequency offset allowing seamless coexistence between the RF\({}_{1}\) and RF\({}_{2}\) signals. Specifically, we measured the EVM\({}_{\text{RMS}}\) as a function of the central frequency offset between the LTE-A and 5G NR signals. The LTE-A central frequency was kept at 778 MHz central frequency, whereas the 5G NR central frequency was varied from 760 to 758 MHz. For a 18 MHz frequency offset, there was a spectral overlap of 2 MHz between the signals, resulting in EVM\({}_{\text{RMS}}\) higher than 19%. This exceeds the limits specified by 3 GPP [27], making the system unsuitable for 64-QAM transmission. In contrast, the 19 MHz offset was enough to assure an EVM\({}_{\text{RMS}}\) value below 3%. For this offset, there was still a spectral overlap of 1 MHz between the signals.