## 7 Conclusions  
In this paper, the PLS performance of a mixed RF/FSO communication system with multi-user SWIPT under optimal user interference was studied. The mixed system's SOP and ASC were researched theoretically and through simulations, with the derived expressions' validity verified using the Monte Carlo method. The impacts of the number of jamming antennas, average jamming SNR, energy conversion efficiency, time allocation factor and atmospheric turbulence on the mixed system's security performance were mainly studied. Simulation results indicated that as the RF link's instantaneous SNR increased, the system's SOP gradually decreased while the ASC increased. When the number of antennas from which the optimal user transmitted the interference signals was enlarged, the mixed system's SOP was reduced. Increasing the number of the optimal user's jamming antennas could enhance the interference effect and reduce the transmit power of each single antenna, which was an effective scheme to enhance the mixed system's security performance. When the optimal user interference SNR increased, the mixed system's SOP decreased and ASC gradually increased. This showed that the interference effect on the eavesdropper could be enhanced by increasing the optimal user interference SNR, which improved the mixed system's security performance. On the other hand, improving the energy conversion efficiency of the optimal user simultaneously enabled the energy harvesting device to store more energy and increase the power of the interference signal transmitted by the energy harvesting device, which in turn enhanced the secrecy ability of the mixed system. By using the SWIPT technology to adjust the time allocation factor, the optimal user energy harvester could prolong the receiving time of the RF energy signal sent by the relay. This phenomenon increased the energy required by the optimal user to transmit the interference signals to the illegal eavesdropper and enhance the mixed system's security performance. Additionally, the system's ASC demonstrates an increase as the FSO link's instantaneous SNR increases with optimal user interference. Notably, when turbulence factors a, b of the \(\mathcal{F}\) distribution are adjusted from strong to weak turbulence, the system's ASC increases, suggesting that improved weather conditions can enhance the PLS performance. It can be seen that the mixed RF/FSO system based on optimal user interference limits the eavesdropping SNR of potential illegal eavesdropors to a large extent, thereby increasing the average security capacity of the mixed system and reducing its security outage probability. This scheme not only improves the communication quality of the mixed system, but also significantly improves the physical layer security of the system, which is of great significance to solve the problem of interception in the mixed system. With the development of science and technology, there may be potential eavesdroppers in the FSO link of mixed system. Therefore, the application of the optimal user jamming scheme into the mixed system with eavesdroppers in both links in the future has important research value to improve the security performance of the system.  
Methodology, Z.S. and J.W.; software, Z.S.; validation, Z.S. and J.W.; formal analysis, Y.W.; writing--original draft preparation, Z.S.; writing--review and editing, Z.S. and Y.W.; supervision, Y.W. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Research Fund of State Key Laboratory of Coal Resources and safe Mining, CUMT (SKLCRSM21 KF010); the National Natural Science Foundation of China (No. 51704267).  
Not applicable.  
The authors declare no conflict of interest.