## 1 Introduction  
The rapid scientific and technological development, and the growing network demand are overwhelming the radio frequency spectrum resources. Compared with traditional radio frequency (RF) communication, FSO (Free Space Optics) is a well-established wireless optical transmission technology. FSO communication has attracted a lot of interest as a potential solution to the short-range 5G wireless communication problem [1, 2]. FSO communication has the advantages of being license-free and highly secure, as well as a low deployment cost and high traffic rate transmission, which is often used to solve the \"last-kilometer\" problem [3, 4]. FSO provides a wide range of applications in outdoor and indoor services, for example, wireless video surveillance, data centers, terrestrial transmission, LAN connectivity, mobile cellular networks, last-mile solutions, space communications, radio astronomy, remote sensing, etc. [5, 6]. However, FSO links are easily affected by directional errors, weather conditions, atmospheric turbulence and other factors, resulting in reduced communication quality or even interruption of communication. Therefore, although it has strong directivity and high security, it is not suitable for long-distance communication [7, 8]. RF and FSO communication have their own advantages and disadvantages. Therefore, there are many technologies proposed to solve these problems. Using both RF and FSO technology is a very effective solution. This combination includes two possible configurations. The first one is the hybrid RF/FSO, in which an RF link is incorporated in parallel with an FSO link. The second combination is the mixed RF/FSO communication, in which RF communication is used at one hop and FSO communication at the other in dual-hop or relay-assisted networks [9]. We choose the mixed RF/FSO communication system to solve the aforementioned problems. The combination of RF communication reliability and FSO communication security under bad weather conditions can lead to better safety communication and also extend the information transmission distance [10]. The RF communication can be an easy target of eavesdropping by illegal eavesdroppers, because the RF link in the mixed communication system is a broadcast link. This problem seriously affects the security performance of the communication system; therefore, we further consider the mixed system's physical layer security (PLS) performance. Secure communication can be achieved by just using the physical characteristics of wireless channels because the PLS does not need to rely on an enciphering algorithm. The PLS also has a low rate of complexity. Therefore, the mixed RF/FSO system's PLS has received a significant amount of attention [11; 12]. Compared with a single user, multiple users are usually more suitable for practical communication scenarios. The multiple users can improve the system throughput fault tolerance and reliability of RF links. For example, [PERSON] et al. studied a multi-user mixed RF/FSO link model with a single input and multiple outputs. In particular, the FSO link is based on the Gamma-Gamma distribution, and the maximum proportional combination and selective combination schemes are used in the multi-antenna repeater, where the system selects the worst user to interfere with the eavesdropper [13]. The authors also studied the influence of RF channel co-frequency interference on the security performance of multi-user mixed RF/FSO communication systems under eavesdropping attack conditions [14]. [PERSON] et al. studied the optimization problem based on the secrecy rate in multi-user mixed RF/FSO spectrum sharing networks [15]. The aforementioned studies are the only ones that consider multiple users in the PLS of mixed RF/FSO communication systems. The optimal user transmission and worst user interference schemes are used in these studies. The optimal user transmission is actually a single user transmission scheme, which can be easily affected by the external electromagnetic wave interference and the co-channel interference.