## 3 Experiments and Results
Thus, it was sufficient to emulate the turbulence channel using the MZM-based fading simulator in this study. An erbium-doped fiber amplifier (EDFA) was applied to improve the received optical power. The polarization-independent SOA was applied to optically mitigate the scintillation effect and equalize the intensities of the various SOPs in the deep gain saturation region. PC3 was deployed to match the optical axis between the MPS-OOK and the LP (Fiber-Optic In-Line Polarizer, POL-001, General Photonics, CA, USA). The LP was used to convert the various SOPs into different intensity levels. The transformed MPS-OOK signal was detected using a single PD and decided using an M-FTD. The symbol rate was set to 1.25 G  
Figure 3: MZM-based fading-simulator-emulated scintillation effect with a \(\sigma_{I}^{2}\) of 0.11: (**a**) time domain, (**b**) frequency domain, and (**c**) distribution. Figure 2: Experimental setup of the (**a**) MPS-OOK modulation, (**b**) SOP measurement, and (**c**) SOA-based MPS-OOK detection. Tables 2 and 3 illustrate the measurement of SOPs of the modulated three-level polarization shift on-off keying (3 PS-OOK) and 4 PS-OOK. \(d\_1[t]\) and \(d\_2[t]\) were coded according to Equations (1) and (2). The SOPs of the modulated 3 PS-OOK and 4 PS-OOK were calculated using the power ratio between VOA2 and VOA3. Regarding 3 PS-OOK, the high-intensity level was modulated with SOPs of \(0^{\circ}\) and the low-intensity level was modulated with SOPs of \(60^{\circ}\) and \(85^{\circ}\). Regarding 4 PS-OOK, the high-intensity level was modulated with SOPs of \(0^{\circ}\) and the low-intensity level was modulated with SOPs of \(48.2^{\circ}\), \(70.5^{\circ}\), and \(85^{\circ}\). Figure 4 shows the eye diagrams of the proposed 3 PS-OOK and 4 PS-OOK transmissions. The experiments were conducted without the scintillation effect to provide a clear measurement of the eye patterns. The average input power at the PD was set to \(-3\) dBm using VOA5 to reduce the PD noise. The eye patterns of the OOK were observed for the modulated 3 PS-OOK and 4 PS-OOK. The average input power at the SOA was configured to 2 dBm using VOA4 in order to have a deep gain saturation. The eye diagrams after the SOA show that the intensities of the various SOPs were effectively equalized by the gain-saturated SOA. The eye diagrams of the 3 PS-OOK and 4 PS-OOK were measured after the LP. Two and three levels of nearly balanced vertical eye-opening were observed for the 3 PS-OOK and 4 PS-OOK, respectively. Thus, the knowledge of the SOPs was effectively transformed into the various intensity levels. Therefore, the 3 PS-OOK and 4 PS-OOK were effectively modulated and detected using the proposed technique. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline Wavelength & 1550 nm \\ \hline Bias voltage of the MZM & Quadrature point \\ \hline Turbulence channel & MZM-based fading simulator \\ \hline \(\sigma_{l}^{2}\) & 0.11, 0.59 \\ \hline VOA1 & 15 dB \\ \hline Symbol rate & 1.25 \(\mathrm{Gsymbol/s}\) \\ \hline \end{tabular}
\end{table} 
Table 1: Parameters used in the experiments. \begin{table}
\begin{tabular}{c c c c} \hline \hline \(d\_1[t]\) & 1 & 0 & 0 \\ \hline \(d\_2[t]\) & 0 & 0.15 & 1 \\ \hline SOP & \(0^{\circ}\) & \(60^{\circ}\) & \(85^{\circ}\) \\ \hline \end{tabular}
\end{table} 
Table 2: SOPs of three-level polarization shift on–off keying (3 PS-OOK). \begin{table}
\begin{tabular}{c c c c} \hline \hline \(d\_1[t]\) & 1 & 0 & 0 \\ \hline \(d\_2[t]\) & 0 & 0.1 & 0.25 & 1 \\ \hline SOP & \(0^{\circ}\) & \(48.2^{\circ}\) & \(70.5^{\circ}\) & \(85^{\circ}\) \\ \hline \end{tabular}
\end{table} 
Table 3: SOPs of the four-level polarization shift on–off keying (4 PS-OOK). Figure 5 shows the bit error rate (BER) performance of the OOK, two-level polarization shift on-off keying (2 PS-OOK), 3 PS-OOK, and 4 PS-OOK transmissions under \(\sigma_{I}^{2}\)'s of 0.11 and 0.59. The scintillation effect was accommodated using the MZM-based fading simulator.