Research on Physical Layer Security Performance of Hybrid RF/FSO System Based on RIS Assistance under FSO Eavesdropping Environment

This paper proposes ahybrid RF/FSO system based on RIS reflection to interfere with eavesdroppers in FSO eavesdropping scenarios, and analyzes and optimizes the physical layer security performance of this communication system. In this work, the RF link experiences Nakagama-m distribution, there is a single-antenna eavesdropper E1 trying to eavesdrop on the RF link information, and the RIS reflection friendly interference signal is used to interfere with the RF link. The FSO link follows the Málaga (M) distribution. There is a single-antenna passive eavesdropper with optical eavesdropping equipment near the destination node to capture the optical signal sent by the relay and eavesdrop on the information. Based on the above communication model, we derive the Strict Positive Secrecy Capacity (SPSC) and Secure Outage Probability (SOP) under double eavesdropping, and use the Monte Carlo method to verify the correctness of the expressions. The impact of various parameters in the system on safety performance was analyzed. The results showed that the interference signal-to-noise ratio, time switching factor and energy conversion efficiency of the wireless power supply jammer, the increase in the number of RIS reflective elements, good weather conditions and better detection methods, etc. All influencing factors can improve physical layer security performance, which also lays a theoretical foundation for the application of hybrid RF/FSO systems where eavesdropping exists in both RF links and FSO links.

and anti-electromagnetic interference, and is easy to deploy and low in construction cost.The use of FSO communication technology can provide an alternative solution for communication in special occasions where radio frequency transmission is prohibited or restricted.However, FSO communication quality is easily affected by the weather environment, resulting in limited transmission distance [1].In order to give full play to the advantages of FSO communication, extend the distance of FSO communication and improve the system performance degradation of the FSO system due to optical attenuation, the academic community has proposed a hybrid RF/FSO communication system [2].However, due to the broadcasting and openness of radio frequency (RF) link wireless communication systems, potential eavesdroppers existing between two RF communication users can easily eavesdrop on legitimate communications, and communication security is seriously threatened, resulting in the leakage of transmitted data [3], [4].In order to avoid data leakage and ensure communication security, physical layer security technology is proposed using the random characteristics of time-varying wireless channels, which is used in communication systems such as hybrid RF/FSO as a potential solution to resist eavesdropping [5], [6].The physical layer security of the dual-hop RF-UOWC system was studied in the presence of multiple eavesdroppers [7], [8].Based on the existing research results, this paper further explores the feasibility and influencing factors of FSO signal eavesdropping in hybrid RF/FSO systems, especially when there are eavesdroppers.
Due to the high directedness of laser, FSO is considered to be a link less prone to eavesdropping than RF link.However, it is pointed out in [9], [10], [11], [12] that there is a risk of eavesdropping in FSO link, so the eavesdropping of FSO link is still one of the factors affecting security that cannot be ignored.FSO eavesdropping is usually regarded as passive eavesdropping, that is, the eavesdropper exists covertly in the middle of the legal FSO channel or near the legitimate communication user, and passively receives the optical signal emitted by the transmitter [12].For example, in 2024 Wang, X et al. discussed the security of FSO links in different environments and its impact on the SAGIN system [9].In 2016 D. Zou et al. pointed out the possibility of eavesdropping on the FSO link when the eavesdropper, as the intended receiver, is outside the laser beam.Their research found that the eavesdropper can capture FSO link transmissions through non-line-of-sight scattering channels.information, which seriously affects the secure transmission of information in the FSO communication system [10].In 2015, F. J. Lopez-Martinez et al. studied the communication between two line-of-view legitimate nodes with external eavesdroppers in FSO communication from the perspective of physical layer security.In this paper, the probability of strict confidentiality capacity was used as a performance index to discuss the feasible mechanism of eavesdropping communication.That is, the influence of eavesdropper near two different positions of transmitter and receiver on system performance index [11].In 2017, Mohammad Javad Saber and others studied the existence of eavesdropping in FSO line-of-sight communication between two legal nodes, and analyzed the physical layer security of the system using average confidentiality capacity, secrecy outage probability and strict positive confidentiality capacity as security performance indicators, and studied the impact of changes in atmospheric turbulence on safety performance indicators [12].
Since FSO links have the potential risk of being eavesdropped, this security factor must also be taken into consideration when studying the security performance of hybrid RF/FSO systems.For example, in 2020 Vivek K. Dwivedi et al. studied a hybrid FSO/RF system based on two-way relay (TWR).It was assumed that there were eavesdroppers near the transmitter, relay station and destination node.It was considered that the eavesdropper only attacked the RF link and only the RF link.When studying the physical layer confidentiality performance of the system, three scenarios need to be considered: attack on the FSO link, eavesdropping attack, and both occurring simultaneously.In addition, the impact of pointing errors and different types of detection techniques on system performance should also be evaluated.According to the number of activated eavesdroppers during data transmission, the closed expressions of the security outage probability and confidentiality throughput of the hybrid system in these three different scenarios were analyzed and calculated, and they were compared.Since the scenario where both links have eavesdropping attacks has the largest number of eavesdroppers, the system security of this scenario is significantly worse than the first two scenarios [13].Currently, there is only the above article that considers the existence of eavesdropping in both RF links and FSO links and studies the security performance of hybrid RF/FSO systems.
The RIS-assisted hybrid RF/FSO system can significantly improve system performance.In hybrid RF/FSO systems, using RIS reflections to disrupt eavesdroppers can further enhance system security.Our team conducted research on this in 2023.It is assumed that the eavesdropper is trying to eavesdrop on the RF link information.When the RF interference link has non-line-of-sight, RIS is used to reflect friendly interference signals to interfere with the eavesdropper's eavesdropping.We use the security interruption probability and average confidentiality capacity as security indicators, analyze the impact of changes in time switching factor, energy conversion efficiency and average interference signal-to-noise ratio on the system security interruption probability, and compare it with the hybrid RF/FSO system without RIS assistance.By comparison, it can be found that the security performance of the system is improved more significantly by using the RIS reflection interference eavesdropping solution in the RF link of the hybrid RF/FSO system.At the same time, the influence of RIS related parameters such as the number of reflected components, link distance before and after reflection, and fading severity parameters on the safety performance of RIS-assisted hybrid RF/FSO system is also considered [14].
Based on the above, we propose a hybrid RF/FSO communication system based on RIS reflection to interfere with the eavesdropper in the FSO eavesdropping scenario.At the same time, security threats at both ends of the RF link and the FSO link are comprehensively considered.First, the cumulative distribution function and probability density function of the eavesdropper's interference signal-to-noise ratio under the influence of the interference signal reflected by RIS are derived.The Meijer-G formula and the generalized Laguerre integral method are used to further derive the strict positive confidentiality capacity of the system (SPSC) and Secrecy Outage Probability (SOP) closed expressions, and Monte Carlo was used to verify the accuracy of the expressions.Through simulation analysis, the influence of the jammer's average interference signal-to-noise ratio, time switching factor, energy conversion efficiency, number of RIS reflection units, eavesdropping signal-to-noise ratio of the FSO link, atmospheric turbulence, pointing error and optical signal detection method on the system confidentiality performance was analyzed.

II. SYSTEM MODEL
As shown in Fig. 1, a hybrid RF/FSO system based on RIS reflection to interfere with the eavesdropper in an FSO eavesdropping environment is proposed.The system includes a radio frequency source (S), receiving antenna relay (R) and destination node (D).The number of antennas is M S = M R = M D = 1 respectively, a multi-receiving antenna wireless power-friendly jammer (J) and a reflection element with N(N ∈ (1, ∞) blocks.number of pieces RIS (R ).The configuration of multiple antennas can improve the robustness of the jammer in complex environments.The RF link and FSO link follow Nakagami-m fading and M fading distributions, respectively.A single-antenna eavesdropper (E 1 ) is located in the RF link and attempts to eavesdrop on the channel information of the RF link while Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.
being affected by the jammer's transmitted jammer signal.Interferenced by RIS reflection, an eavesdropper located near the destination node attempts to eavesdrop on the optical signal of the FSO link (E 2 ).The entire process of the source node sending radio frequency signals to friendly jammers uses the time switching protocol in SWIPT technology to collect energy and use it for information transmission.In each period T when collecting energy, λT collects and stores energy, and (1 − λ)T performs information transmission [16].Therefore, the energy W J received by J can be expressed as: λ is the time switching factor, which ρ represents the energy conversion efficiency of the jammer in converting radio frequency signals into direct current.d SJ represents the distance between the information source and the jammer, τ is the path loss.P J indicates that the transmitted power of the jammer is in the information transmission stage, and the energy collected and stored is greater than the energy consumed for information transmission.The energy consumed by the jammer transmitting signal cannot exceed the energy collected and stored in the previous stage.Combining (1), it can be seen that the transmitting power of the jammer must also satisfy In the reflection interference link with RIS, respectively represent the channels of the J→R and R'→E links related to the i-th reflection element, α i , θ i respectively correspond to the amplitude and phase of h i , β i , ϕ i represent the amplitude and phase of g i respectively.The signal received by the eavesdropper E 1 can be expressed as: Where x J is the signal emitted by the jammer, represents the corresponding amplitude, n E 1 represents the additive Gaussian white noise at the relay with a mean of zero and a variance of σ 2 E 1 .In the FSO link, the signal obtained by the eavesdropper E 2 can be expressed as: P R is the signal transmission power of the relay node R, x R represents the signal transmitted by R, d RE 2 and h RE 2 represent the distance and channel coefficient of R→E 2 respectively, τ represents the path loss index, and n E 2 is the additive Gaussian white noise at E 2 whose mean is 0 and variance is σ 2 E 2 .

A. RF Channel Model
All RF links and interference eavesdropping links follow the Nakagami-m fading distribution, which can better summarize the channel fading situation.According to [17, Eq. ( 5) (6)], the instantaneous signal in the RF link The PDF and CDF of the noise ratio are: , , They are respectively the average power channel gain of each link, and δ = J→E 1 , Because obstacles such as bushes block the line of sight (No Line Of Sight, NLOS), the CDF of the instantaneous signal-to-noise ratio of the interference link is obtained: By deriving and simplifying formula (6), the PDF of the instantaneous signal-to-noise ratio of the interfering link can be obtained: Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.

B. FSO Channel Model
As can be seen from Fig. 1, R-D uses M fading distribution for the FSO link.This distribution is more commonly used because it can well reflect the channel fading situation.In the literature [18, Eq. ( 4), (5)], this link The expressions of the PDF and CDF of the signal-to-noise ratio under the road can be expressed as: In the above equation, γRD is the average signal-to-noise ratio of the FSO link and α represents a positive parameter related to the effective number of large-scale units in the scattering process, β represents the natural number related to the diffraction effect caused by small-scale eddy currents,ξ represents the ratio between the equivalent beam radius at the receiver and the standard deviation of the pointing error shift outside the receiver, g represents the average power of the received scattered component, and Ω 0 average power contributed to coherence, G m,n p,q [.] represents Meijer G-function.In the above formula, the other unknown variables are: .r represents the type of detection technology, r = 1 represents heterodyne detection, and r = 2 represents IM/DD detection.
. The instantaneous signal-to-noise ratio of the FSO link is , r = 2 .

A. Strictly Confidential Capacity Analysis
To make communications more secure, SPSC is one of the basic parameters to consider the security capacity of the system.According to [19], it can be defined as: The two probability expressions appearing in the above formula can be expressed as: Where + , represent the instantaneous confidentiality capacity of the RF link and FSO link respectively, [x] + = max{x, 0}, Put ( 11), ( 12) into the formula (10) to get: Substitute ( 5), ( 7), ( 8) and ( 9) into ( 13) to calculate and simplify (14) shown at the bottom of the next page: In the above equation, x j is the j-th root of the generalized Laguerre polynomial L

B. Security Outage Probability Analysis
Security outage probability is an important and key performance indicator of confidentiality capacity in wireless communication systems.It represents a parameter when the instantaneous security capacity is lower than the target security capacity C S .The lower bound expression of the security outage probability of double eavesdropping in a hybrid RF/FSO system can be expressed as For [20]: Where, θ = 2 2C S , Put (5), ( 7), ( 8) and ( 9) into (15) to get, calculate and simplify to get (16) shown at the bottom of the next page:

IV. NUMERICAL RESULTS AND ANALYSIS
In this section, the simulation results of the hybrid RF/FSO system under different parameter conditions are shown, and the accuracy of the simulation numerical results is further Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.
verified by Monte Carlo simulation.In the RF link, the following needs to be explained: the instantaneous signal-to-noise ratio of the link γ SR = 15 dB; in the interference link: the number of reflective surfaces N = 3, the fading severity parameter m = 3,Ω = 1,λ = ρ = 0.4, d JR = d R E 1 = 10m, the average interference signal-to-noise ratio is 2 dB; what needs to be clear in the FSO link is: d RD = d RE 2 = 1 km,Wavelength is 785 nm, optical wave number k = 2π/λ 1 ,atmospheric refractive index structure constant C 2 n = 2.1 × 10 −14 ,instantaneous signal-to-noise ratio γ RD = 20 dB.Other parameters include: RF link eavesdropping instantaneous signal-to-noise ratio M j = 2, γFSO = 20 dB, ξ = 6.7, λ 1 = 0.8, τ = 1, α = 8, β = 4,target confidentiality rate C S = 0.01nat/s.In the following Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.simulation curve diagrams, unless otherwise specified, the above parameter values are used.When calculating the generalized Laguerre orthogonal numerical integration method, the series converges when j is 30.In order to verify the validity of the analytical expression, we performed a Monte Carlo simulation.
The numerical results are in good agreement with the simulation results, verifying the accuracy of the analytical expression.
When the instantaneous secrecy capacity is lower than the target secrecy rate R S , an SOP event occurs.Therefore, the lower bound expression of the SOP for the mixed system [16] is Fig. 2 shows the simulation diagram of the relationship between the SPSC of the hybrid RF/FSO system and the instantaneous signal-to-noise ratio λ SR of the RF link under the influence of different average interference signal-to-noise ratios of RIS in an FSO eavesdropping environment.It can be seen from the figure that SPSC increases with the increase of γ, showing an overall upward trend.In particular, γ = 9 dB the curve at that time was significantly higher than the other curves.It can be seen that when RIS reflection is used in the RF interference link to interfere with eavesdroppers, when the average interference signal-to-noise ratio of the interference link is increased, the interference effect of the interference signal on eavesdropping users becomes better, and the restriction on eavesdropping activities is enhanced, thereby ensuring System communication security.when, λ SR = 35 dB,From the above figure, it can be concluded that the SPSC values are approximately: 0.72, 0.75, 0.77, 0.79.SPSC increases with the increase of the average interference signal-to-noise ratio, indicating that the interference effect is better if the average interference signal-to-noise ratio is properly increased, so SPSC will gradually increase, that is, the communication system can obtain better security performance.
Fig. 3 shows a simulation diagram of the relationship between the SPSC of the hybrid RF/FSO system and the instantaneous signal-to-noise ratio λ SR of the RF link under the influence of time switching factor λ and energy conversion efficiency ρ in an FSO eavesdropping environment.It can be seen from the figure that as the values of the two influencing factors of energy collection increase, it shows that the proportion of time used for  energy collection increases and the efficiency of final conversion into energy increases, and the quality of the interference signal emitted by the jammer is better.Therefore, the interference effect reflected by RIS is also gradually enhanced, and the SPSC of the hybrid system shows an upward trend, so the confidentiality performance is improved.when λ SR = 35 dB, the SPSC value of λ = 0.5, ρ = 0.5 is 0.58, Keeping λ constant and increasing ρ to 0.8, the SPSC of the hybrid system is 0.78; Similarly, when λ = 0.8 and ρ are 0.5 and 0.8 respectively, the SPSC of the system are 0.81 and 0.87 respectively.It is obvious that the SPSC increases, so the value of either element of λ or ρ remains unchanged and the other increases.The value of each element has a significant improvement in safety performance, and increasing the values of these two energy collection elements at the same time has a greater effect on optimizing safety performance.link when the eavesdropper E 2 is affected by different eavesdropping signal-to-noise ratios in a hybrid RF/FSO system.That is, when γFSO = 35 dB, γ RE 2 = 3, 5, 7, 9 dB, the SPSC of the system are approximately 0.96, 0.93, 0.91, and 0.88 respectively.When the RF link uses RIS reflected interference signals to limit eavesdropping, the RF link eavesdropper E 1 is affected by the interference signal, and the eavesdropping signal-to-noise ratio is reduced, which reduces its impact on the RF link communication of the hybrid RF/FSO communication system.The strictly positive secrecy capacity of the RF channel increases, and the relay transmits more optical signal data per unit time while the electro-optical conversion efficiency remains unchanged.Since the optical signal has strong directionality and anti-interference, even if there is eavesdropping on the FSO link Moreover, due to the adoption of the RIS anti-interference solution in the RF link, the hybrid RF/FSO system can also achieve an ideal level of confidential communication in a complex eavesdropping environment.
Fig. 5 analyzes the simulation diagram of the relationship between the SPSC of the hybrid RF/FSO system and the instantaneous signal-to-noise ratio γFSO of the FSO link in the FSO eavesdropping environment under strong and weak turbulence and using different detection methods (r = 1 is the heterodyne detection method, r = 2 is the IM/DD detection method).The optical signal transmission process will be affected by turbulence and pointing errors.Turbulence will affect the quality of the optical signal.A better detection method can better detect the optical signal transmitted in the FSO link channel.When, γFSO = 35 dB,The SPSC of r = 1 and r = 2 under strong turbulence are approximately 0.82 and 0.75 respectively; while the SPSC of r = 1 and r = 2 under weak turbulence are approximately 0.94 and 0.88 respectively.Through the analysis of curve trends and data, it can be seen that different detection methods have a significant impact on the SPSC safety performance of the system.The heterodyne detection method is obviously better than the IM/DD detection method.When weather conditions are better, the transmission loss of optical signals is low, and the information received by the destination node is more Fig. 6.The relationship between the SOP of E 2 under the influence of different eavesdropping signal-to-noise ratios in the FSO eavesdropping environment and the FSO instantaneous signal-to-noise ratio γFSO .complete.Of course, when conditions permit, more suitable weather conditions and detection methods can be considered to provide higher security for the hybrid RF/FSO system.
The above figure shows the simulation diagram of the relationship between the SOP and A of the hybrid system E 2 under the influence of different eavesdropping signal-to-noise ratios in the FSO eavesdropping environment.Fig. 6 shows the simulation diagram of the relationship between SOP and γFSO of the hybrid system when E 2 is affected by different eavesdropping signalto-noise ratios in the FSO eavesdropping environment.It can be seen from the figure that with the increase of γFSO , the value of SOP generally shows a downward trend.With the decrease of eavesdropping signal-to-noise ratio, the probability of security interruption of the hybrid system gradually decreases and the security is enhanced.When γFSO = 35 dB and γ SE 2 is 35,79 dB respectively,the values of SOP are approximately 0.0079, 0.0087, 0.0091, 0.0097.It can be seen that when the eavesdropping signal-to-noise ratio increases, that is, the optical link of the hybrid RF/FSO system is more severely eavesdropped, the risk of optical information leakage in the FSO link segment increases, resulting in reduced security and a corresponding increase in SOP.However, since the radio frequency link section adopts the RIS reflection interference scheme to interfere with eavesdroppers, adjusting the strategy of radio frequency links to resist eavesdropping can provide a certain guarantee for the security of the entire hybrid system as a whole.Fig. 7 presents a simulation diagram of the relationship between SOP and λ SR of a hybrid RF/FSO system affected by the number of different reflective elements in RIS in an FSO eavesdropping environment.As can be seen from the above figure, as λ SR increases, the safety interruption probability of the system generally shows a downward trend, but as λ SR increases, increasing the number of reflective elements has a smaller impact on SOP.When a fixed λ SR value is taken, as the number of reflective elements of RIS increases, the SOP value gradually decreases, indicating that when N is appropriately increased, the SOP can be effectively reduced and the confidentiality performance of the system can be improved.When, λ SR = 35 dB, The SOP values for N of 2, 4, 6 and 8 are approximately: 0.0054, 0.0048, 0.0044 and 0.0041 respectively.The simulation results also verify the above conclusion, that is, when we appropriately select a larger number of reflective elements, the RIS in the RF link section can receive and reflect more friendly interference signals from jammers, which can interfere with E 1 eavesdropping activities to a greater extent, thereby ensuring the security of the entire system to a certain extent.
Fig. 8 shows the simulation diagram of the relationship between the SOP of the hybrid RF/FSO system and the instantaneous signal-to-noise ratio γFSO of the FSO link in the FSO eavesdropping environment under strong and weak turbulence and using different detection methods.Strong and weak turbulence affect the signal quality during optical signal transmission, and different r values characterize the detection method of optical signal transmission.Affected by the above two factors, as γFSO increases, the SOP overall shows a downward trend.When, γFSO = 35 dB,in the case of strong turbulence, the SOP of r = 1 and 2 are approximately: 0.008 and 0.01 respectively.Similarly, in the case of weak turbulence, the SOP of r = 1 and 2 are approximately: 0.004 and 0.006 respectively.It can be found that whether it is strong turbulence or weak turbulence, the SOP of r = 1 is lower than the SOP of r = 2, so the optical signal can be better detected using the heterodyne detection method, and a more complete optical signal can be received at the destination node.Information.In addition, transmitting optical signals under good weather conditions can minimize the transmission loss of optical signals in the FSO link section.
Fig. 9 shows the simulation diagram of the relationship between the SOP of the hybrid system and the instantaneous signalto-noise ratio γFSO of the FSO link affected by the pointing error factor ξ in the FSO eavesdropping environment.The pointing error experienced by the FSO link can be seen from the figure above.As ξ increases, the SOP value of the system shows an overall downward trend, indicating that the safety performance of the system is better.When, λ SR = 35 dB,The SOP with ξ of 1.1 and 6.7 are approximately 0.014 and 0.008 respectively, indicating that the SOP when the pointing error factor ξ is 6.7 is lower and the confidentiality performance is better.

V. CONCLUSION
This paper studies the security performance of the physical layer in a hybrid RF/FSO system.The presence of eavesdroppers is considered in the optical link, and RIS is used in the RF link segment to reflect signals to interfere with the eavesdroppers.Through theoretical derivation and simulation, the strict positive confidentiality capacity and security interruption probability of the system are analyzed, and the correctness of the theoretical expression is verified using Monte Carlo simulation.The focus of this study is to explore the impact of factors such as time switching factor, energy conversion efficiency, RIS average interference signal-to-noise ratio, turbulence and pointing error, FSO link eavesdropping signal-to-noise ratio and the number of reflecting surfaces on the system confidentiality performance, and conduct a detailed analysis.The study found that increasing Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.
the time switching factor and energy conversion efficiency, as well as improving the average interference signal-to-noise ratio, can improve the jamming effect by increasing the transmission energy of the friendly jammer and improving the signal-to-noise ratio of the friendly jammer signal.Therefore, the strict positive secrecy capacity of the hybrid RF/FSO system increases accordingly.Therefore, the research results show that in a hybrid RF/FSO communication system where eavesdroppers exist at the same time, increasing the values of the three factors of time switching factor, improving energy conversion efficiency and increasing the average interference signal-to-noise ratio can effectively improve the physical layer security performance of the entire system.
In the hybrid RF/FSO system, we also studied the impact of the increase in the eavesdropping signal-to-noise ratio on the system security performance.As the eavesdropping signal-tonoise ratio increases, the eavesdropping activities of E 2 in the optical link will intensify, resulting in more serious information leakage, posing a threat to the security performance of the entire system.Therefore, in order to ensure the communication security of the system, the eavesdropping signal-to-noise ratio needs to be reduced as much as possible.As the number of RIS reflective elements increases, more interference signals can be reflected, which can reduce the eavesdropper's receiving end rate to a greater extent and improve security.Therefore, the strict confidentiality capacity increases and the security interruption probability decreases.Under suitable weather conditions, the use of heterodyne detection technology can ensure that the destination node receives more complete optical information, which also helps to improve the confidentiality performance of the entire communication system.In actual engineering applications, according to the demand for confidentiality performance, the above appropriate parameter indicators can be selected when conditions permit.This can improve the physical layer security performance of the double-eavesdropping hybrid RF/FSO system and provide engineering implementation.Good theoretical basis.In particular, it has broad application prospects in the fields of military communications, satellite communications, drone communications, smart cities, financial data transmission and emergency communications.

Fig. 1 .
Fig. 1.Schematic diagram of a hybrid RF/FSO system based on RIS reflection to interfere with eavesdroppers in an FSO eavesdropping environment.

Fig. 2 .
Fig. 2. In the FSO eavesdropping environment, the eavesdropper E 1 is subject to different average interference γ from RIS.The relationship between SPSC and λ SR under the signal-to-noise ratio.

Fig. 3 .
Fig. 3.The relationship between SPSC and λ SR under the influence of different time switching factors and energy conversion efficiency of E 1 in FSO eavesdropping environment.

Fig. 4 .
Fig. 4. The relationship between SPSC and γF SO of eavesdropper E 2 under the influence of different eavesdropping signal-to-noise ratio in FSO eavesdropping environment.

Fig. 4
describes the simulation diagram of SPSC changing with the instantaneous signal-to-noise ratio γFSO of the FSO Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.

Fig. 5 .
Fig. 5.The relationship between SPSC and γFSO under the influence of different detection methods and strong and weak turbulence in FSO eavesdropping environment.

Fig. 7 .Fig. 8 .
Fig. 7.The relationship between SOP and λ SR when E 1 is affected by the number of different reflective elements in RIS in FSO eavesdropping environment.

Fig. 9 .
Fig. 9.The relationship between SOP and γF SO under the influence of pointing error factor ξ in FSO eavesdropping environment.