## 4 Numerical Simulation  
To evaluate the performance of irregular \(\theta\) in overcoming DPF, a comparison of the C/N penalty curve from the RoF link with ODSB modulation, OSSB, scheme C4 (best performing scheme) irregular \(\theta\) and scheme C2 (worst performing scheme) irregular \(\theta\) is carried out using OptiSystem software. An evaluation is carried out at \(m=1\) with \(\lambda_{c}=1550\) nm (D = 17 ps/nm.km) and \(f_{m}=60\) GHz. This simulation circuit is shown in Figure 13, and the parameter setting of LiNb-MZM is shown in Table 5.  
Figure 11: DF value of irregular \(\theta\) for all schemes. Figure 12: RoF links the C/N penalty with an irregular \(\theta\) for all schemes at modulation index \(m=1\). Figure 13: OptiSystem simulation system to validate irregular \(\theta\) performance in overcoming DPF, (**a**) RoF link with ODSB and OSSB modulations; (**b**) RoF system with an irregular \(\theta\) for scheme C2; and (**c**) scheme C4. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Value** & **Units** \\ \hline Extinction ratio & 60 & dB \\ Switching bias voltage & 4 & V \\ Switching RF voltage & 4 & V \\ Insertion loss & 0 & dB \\ Normalize electrical signal & Unchecked & - \\ Bias voltage1 & 0 & V \\ Bias voltage2 & 2 & V \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Setting parameter of LiNb-MZM. Figure 13a is a simulation system used to measure the C/N penalty of the RoF link with ODSB and OSSB modulation. The circuit consists of a sine generator, fork 1 \(\times\) 2, electrical phase shift, continuous wave (CW) laser, MZM, optical fiber, photodetector PIN, bandpass rectangle filter and electrical power meter. The sine generator functions to generate the pure RF signal. The frequency of the sine generator is set to 60 GHz. Because the switching voltage \(V_{\pi}\) used in the simulation is 4 V, the sine generator voltage \(V_{m}\) is regulated at 1.274 V to obtain \(m=1\). The sine generator output is then duplicated using fork 1 \(\times\) 2. The first fork output is forwarded to the EPS and is used as an input of the MZM upper arm. The second fork output is directly used as the input of the MZM lower arm. EPS is used to shift the RF signal phase by \(\theta\). To generate ODSB modulation, the phase shift value of EPS is regulated at 180 deg and 90 deg to produce OSSB modulation. The type MZM used is LiNb-MZM. The MZM is set with parameters as depicted in Table 2. The MZM optical input is obtained from the CW laser. The frequency of the CW laser is set to 1550 nm, the power is set at 0 dBm, and the linewidth is set to 10 MHz. The output of the MZM is therefore transmitted through a single-mode optical fiber. The optical fiber is thus configured using the parameters shown in Table 6.  
In this simulation, the fiber attenuation effect is ignored. At the receiver, the optical signal is detected using photodetector PIN under the parameter of responsivity = 1 A/W and dark current = 10 nA. Because the output of the photodetector consists of an electrical signal with frequencies of 0, 60, 120 GHz, etc., it is filtered by means of a bandpass rectangle filter. To obtain an RF signal at 60 GHz, the parameter filter is used with a frequency of 60 GHz, bandwidth of 10 MHz, insertion loss of 0 dB and depth of 100 dB. The power of the recovered RF signal is measured using an electrical power meter. The power measurement is performed for fiber lengths of 0 to 5 km with a step of 0.1 km. The power value in the simulation is gauged in dBm. The C/N penalty value from the simulation result is obtained by subtracting \(P_{rec}(L)\) at length \(L\) from \(P_{rec}(L)\) at length 0. Figure 13b is a circuit of simulations to measure the C/N penalty of the RoF link with an irregular \(\theta\) on scheme C4. The components used in this circuit are approximately similar to the simulation circuit used to measure the C/N penalty of the RoF link with ODSB and OSSB modulations.