## 3 Experiments and Results  
The experiments were divided into two parts. The proposed MPS-OOK modulation was verified without the scintillation effect. Furthermore, the proposed SOA-based MPS-OOK detection was compared to the conventional OOK detection under different scintillation effects. Figure 2a shows the experimental setup of the MPS-OOK modulation. The wavelength of the LD was set to 1550 nm. The SOP was set to 45\({}^{\circ}\) using PC1. A PBS was used to obtain optical beams with orthogonal polarizations. Data1\(d\)\(\underline{1}[t]\) and data2\(d\)\(\underline{2}[t]\) were imported into the radio-frequency (RF) ports of MZM1 and MZM2, respectively. The bias voltages of MZM1 and MZM2 were set to the quadrature points in order to have large ERs. An optical delay line (ODL) was used to match the bit synchronization between the optical signals from MZM1 and MZM2. VOA1 was set to 15 dB to have the optimized ER and SOPs for the MPS-OOK signal [9]. An MPS-OOK signal was generated by combining two signal branches using the PBC. Figure 2b depicts the experimental setup of the SOP measurement. PC2 was used to match the optical axes between the MPS-OOK and the PBS. VOA2 and VOA3 were applied to measure the average powers of orthogonal polarizations. The PC2 was altered to provide equal values for VOA2 and VOA3 under the transmission of the MPS-OOK with a single SOP of 45\({}^{\circ}\) in order to match the optical axes between the MPS-OOK and the PBS. The SOPs of the modulated MPS-OOK were calculated using the power ratio between VOA2 and VOA3 via \(\text{SOP}=\arctan(\text{VOA2}/\text{VOA3})\times 180^{\circ}/\pi\). Figure 2c illustrates the experimental setup of the SOA-based MPS-OOK detection. An MZM-based fading simulator was introduced to emulate the scintillation effect [9, 12]. The modeled time-varying intensity fluctuation signal and MZM were used to establish the fading simulator. Based on the Kolmogorov power-law spectrum and Taylor frozen turbulence hypotheses,  
Figure 1: Block diagram of the proposed technique. CW: continuous wavelength, LD: laser diode, LP: linear polarizer, M-FTD: multilevel fixed-threshold decision, MPS-OOK: multilevel polarization shift on-off keying, MZM: Mach–Zehnder modulator, PBC: polarization beam combiner, PBS: polarization beam splitter, PC: polarization controller, PD: photodiode, SOA: semiconductor optical amplifier, SOP: state of polarization, VOA: variable optical attenuator. the time-varying intensity fluctuations were generated from the temporal spectrum of the log-amplitude fluctuations [13]. Then, the random phase information was modulated into the temporal spectrum. Then, it was transformed into the time domain using an inverse Fourier transform. Finally, the normalized time-varying intensity fluctuations signal was modeled using first-order Rytov approximations [9; 14]. The modeled signal was imported into the RF port of the MZM to accommodate the scintillation effect into the traveling optical signal. The degree of the scintillation effect was measured in terms of the scintillation index \(\sigma_{I}^{2}=\left\langle I^{2}\right\rangle/\left\langle I\right\rangle^{2}-1\), where \(\sigma_{I}^{2}\) indicates the normalized variance of the intensity fluctuations [14]. Figure 3a illustrates the MZM-based fading-simulator-emulated scintillation effect with a \(\sigma_{I}^{2}\) of 0.11 in the time domain. A time-varying intensity fluctuation signal was observed. Figure 3b illustrates the frequency domain of the accommodated scintillation effect, which was dominated by the low-frequency components. Figure 3c shows that the accommodated scintillation effect fit well with the lognormal distribution, which is the widely used statistical model for weak turbulence. Therefore, the scintillation effect was effectively emulated by the MZM-based fading simulator. Since SOPs keep a stable state in the turbulence channel, the MPS-OOK signal suffers the scintillation-effect-induced intensity fluctuations.