## 2 System and Channel Model  
At the transmitter of the hybrid FSO/RF parallel system, the signal is separated into two signals after binary modulation (on-off keying, OOK; multilevel phase shift keying, M-PSK; multilevel quadrature amplitude modulation, M-QAM) and sent to FSO and RF subsystems, respectively. In the FSO subsystem, the transmitter sends the modulated signal light to the FSO channel by using intensity modulation, and the receiver can demodulate the received optical signal in two ways: Direct detection and heterodyne detection. In the RF  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Ref.** & **Switch/Combination** & **Modulation** & **Detection** & **FSO Channel** & **RF Channel** & **PE** & **Eavesdropper** & **Metrics** \\ \hline \([22]\) & hard switching & FSK & DD & Log-normal & Nakagami-m & \(\mathbf{\times}\) & \(\mathbf{\times}\) & BER, OP, ECC \\ \([23]\) & hard switching & OOK, 16-QAM & DD & Gamma–Gamma & Rician & \(\checkmark\) & \(\mathbf{\times}\) & BER, OP \\ \([25]\) & hard switching & M-PSK & HD, DD & Malaga & \(\alpha-\eta-\kappa-\mu\) & \(\checkmark\) & \(\mathbf{\times}\) & SER, OP, ECC \\ \([15]\) & SC & M-PSK & DD & Gamma–Gamma & Rayleigh & \(\mathbf{\times}\) & \(\mathbf{\times}\) & BER, OP \\ \([29]\) & SC & M-PSK & DD & Gamma–Gamma & Nakagami-m & \(\mathbf{\times}\) & \(\mathbf{\times}\) & BER, OP \\ \([30]\) & MRC, SC & PSK & DD & Gamma–Gamma & Rician & \(\mathbf{\times}\) & \(\mathbf{\times}\) & BER \\ \([38]\) & MRC, SC & OOK, M-PSK, M-QAM & HD,DD & Gamma–Gamma & \(\kappa-\mu\) & \(\mathbf{\times}\) & \(\mathbf{\times}\) & BER, OP \\ \([26]\) & SC & PSK, FSK & HD, DD & Gamma–Gamma & Nakagami-m & \(\checkmark\) & \(\mathbf{\times}\) & BER, OP, ECC \\ \([28]\) & AC & \(\mathbf{\times}\) & HD,DD & Gamma–Gamma & \(\kappa-\mu\) & \(\checkmark\) & \(\mathbf{\times}\) & ECC \\ \([27]\) & SC & PSK, FSK & DD & Malaga & \(\eta-\mu\) & \(\checkmark\) & RF & BER, OP, SOP \\ \([35]\) & Modified switching & BPSK & DD & Gamma–Gamma & Exponential & \(\checkmark\) & RF & OP, OC, SOP \\ \([36]\) & SC & \(\mathbf{\times}\) & HD,DD & Malaga & Nakagami-m & \(\mathbf{\times}\) & RF & SOP, SPSC, ASC \\ \([37]\) & MRC & \(\mathbf{\times}\) & DD & Gamma–Gamma & Nakagami-m & \(\mathbf{\times}\) & FSO, RF, hybrid & SPSC \\ This & MRC & OOK, M-PSK, M-QAM & HD, DD & Malaga & \(\alpha-\mu\) & \(\checkmark\) & FSO,RF & BER, OP, ECC, SOP, SPSC \\ \hline \end{tabular}
\end{table}
Table 1: Comparison of research contents between the relevant literature and this paper on the hybrid FSO/RF parallel transmission system.  
subsystem, the transmitter upconverts the modulated signal to the millimeter wave (MMW) RF frequency of 60 GHz, and then sends the MMW signal to the RF channel. In addition, the receiver will down-convert and demodulate the MMW signal and retrieve it to the original signal. Finally, the received signals of the FSO and RF links are combined in a certain proportion at the receiving end of the hybrid system, that is the MRC scheme. Because the hybrid FSO/RF parallel transmission system will transmit the same information on two different physical links at the same time, it may encounter two different eavesdropping modes: the FSO link eavesdropping and the RF link eavesdropping, as shown in Figure 1 At the transmitter of the hybrid FSO/RF parallel system, the signal is separated into two signals after binary modulation and subcarrier pre-modulation and modulated to optical and RF carriers, respectively. These carriers are then sent to FSO and RF communication links through the transmitter for transmission, and the received signals of the FSO and RF links are combined in a certain proportion at the receiving end of the hybrid system. For the communication system in which two different links transmit the same information at the same time, there may be two eavesdropping modes: the FSO link eavesdropping and the RF link eavesdropping.