# Effect of Phase Noise on the Optical Millimeter-Wave Signal in the DWDM-RoF System
## 4 Results and Discussion  
In the DWDM-RoF system, the eight wavelengths were modulated using a single MZM scheme. Figure 4a shows the individual wavelength at 1550 nm and Figure 4b shows the eight wavelengths modulated together and spaced by 0.8 nm. The mm-wave signal was generated using a single MZM scheme in the DWDM-RoF system. The peak power of the generated 60 GHz mm-wave was -33 dBm, as shown in Figure 5. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline CW laser wavelength & 1550 nm \\ CW laser power & 10 dBm \\ RF signal generator & 30 GHz \\ Data transmitted & 2.5 Gb/s \\ Fiber length & 0–40 km \\ Dispersion & 16.75 ps/mm/km \\ Attenuation & 0.2 dB/km \\ PIN responsivity & 0.7 A/W \\ Cutoff frequency in LPF & 0.75 \\ Attenuator of antenna & 5 dB \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameter settings. Figure 4: The optical spectrum analyzer for transmitted signals: (**a**) first signal and (**b**) 8 signals modulated together. In the long-haul coherent optical fiber communication systems, the phase noise affected the electronic dispersion compensation (EDC) module where the equalization-enhanced phase noise (EEPN) occurred due to the interplay between laser phase noise and EDC module. It significantly degraded the performance of the uncompensated long-haul coherent optical fiber communication systems. For further discussion, the EEPN was reported in [34; 35; 36]. The phase noise of the mm-wave signal in the DWDM-RoF system was monitored through the signal analyzer in the frequency domain. A high phase noise resulting from the phase imbalance of the mm-wave signal at the MZM was observed. To lower the phase noise, the phase imbalance should be controlled. In our study, the phase imbalance could be decreased using the PS. In the architecture, the single MZM reduced the insertion losses by decreasing the phase imbalance from one side. From the other side, the phase imbalance was best controlled using a single MZM in the system to avoid multiphase imbalance from multiple MZMs. Therefore, we used a single MZM to control the phase imbalance of the optical mm-wave signal. In the system, the phase imbalance was calculated at the output of the MZM and controlled by adjusting the phases at the PS. The controlling process used phases from 0\({}^{\circ}\) to 75\({}^{\circ}\), which is equivalent to 0 to 5\(\pi\)/12 radian. Figure 6 shows the phase imbalance vs. the applied phases at the PS. The phase imbalance was controlled and decreased from 0.43\({}^{\circ}\) to 0.1\({}^{\circ}\). To quantify the performance of the mm-wave signal, a constellation diagram was used. In general, a signal sent by an ideal transmitter should result in a constellation diagram with all constellation points precisely at the ideal locations. However, high phase noise and phase imbalance may cause deviation of the constellation points from the ideal locations. The total distance between the points' locations and the ideal locations represents a corresponding measure of phase noise or error phase in the signal. Figure 7 demonstrates the constellation diagram at different phases and the different responses of constellation points at phases from \(\pi\)/12 to 5\(\pi\)/12 over a 10 km fiber length with a dispersion coefficient of 16.75 ps/nm/km. We observed that the minimum distance between the points' locations occurred at the phase of 5\(\pi\)/12. The constellation diagram shows a high location deviation of constellation points at the phase of \(\pi\)/12, while the minimum deviation of constellation points at the phase of 5\(\pi\)/12. We found that the phase of 5\(\pi\)/12 had minimal phase error and lower phase noise than the rest of the phases. This minimal phase error in the mm  
Figure 5: The 60 GHz mm-wave in the DWDM-RoF system. wave signal was considered as a reasonable performance of the mm-wave signal in the system due to a dispersion across 10 km of fiber. Figure 6: The phase imbalance vs. the applied phases at PS.