STAR-RIS-based RSMA wireless communication system secure transmission method

Abstract

The invention discloses an RSMA wireless communication system secure transmission method based on STAR-RIS, and belongs to the technical field of wireless communication. Based on a related Rayleigh fading channel model, a cascade channel average gain from a base station to a user through STAR-RIS is firstly deduced, a simplified analysis method is provided, the average channel gain is directly used for deducing a closed expression of a system security outage probability (SOP), and the secure diversity gain characteristic of the system is further determined by analyzing the asymptotic SOP of a high signal-to-noise ratio area; the security performance of the method is obviously better than that of an STAR-RIS assisted NOMA scheme and an STAR-RIS assisted OMA scheme, and the flexible resource allocation mechanism of the method has more advantages in interference control and secure transmission guarantee; according to the method, the power distribution coefficients of the public information and the private information of the RSMA are optimally designed, the rate and power ratio of the two types of information streams can be dynamically and flexibly adjusted according to the actual situation, and the appropriate balance of the transmission security of the two types of information streams is realized.

Description

Technical Field

[0001] This invention relates to the field of wireless communication technology, and more specifically to a secure transmission method for an RSMA wireless communication system based on STAR-RIS. Background Technology

[0002] In the evolution from 5G to 6G, the core requirements of ultra-high speed, ultra-low latency, and ultra-high density connectivity present three major bottlenecks for wireless communication systems: scarce spectrum resources, complex channel environments, and information security threats. Reconfigurable Intelligent Surfaces (RIS), as one of the core technologies for overcoming these bottlenecks, can optimize signal transmission by controlling electromagnetic response. However, traditional RIS only supports unidirectional reflection, limiting coverage to the same half-space as the base station, making it difficult to meet the full-space communication requirements of 6G. Therefore, Simultaneous Transmitting and Reflecting Reconfigurable Intelligent Surfaces (STAR-RIS) have emerged. Through a special metasurface structure, they achieve synchronous transmission and reflection control of incident signals, achieving 360° full-space coverage and providing the system with a higher degree of freedom (DoF), effectively solving the coverage limitations of traditional RIS. To address the ultra-high density connectivity demands of 5G / 6G, multiple access technologies have evolved from traditional Orthogonal Multiple Access (OMA) to non-orthogonal technologies, with Rate-Splitting Multiple Access (RSMA) demonstrating significant advantages. Compared to Non-Orthogonal Multiple Access (NOMA), which relies on power reuse and is prone to strong interference due to differences in user channels, RSMA dynamically adjusts the rate and power ratio of user information streams by splitting user information into a common stream (decoding shared by all users) and a private stream (decoding dedicated to the target user). This flexible resource allocation mechanism retains the spectral efficiency advantages of non-orthogonal multiplexing while precisely managing multi-user interference, providing crucial support for performance balance in 5G / 6G heterogeneous user coexistence scenarios.

[0003] In cellular systems, Orthogonal Multiple Access (OMA) technology has long been widely used in cellular networks because it effectively reduces interference and provides stable communication services to users through orthogonal resource allocation. However, with the continuous connection of massive numbers of terminals such as smartphones, wearable devices, and Internet of Things (IoT) devices to the network, the existing OMA technology, limited by its principles and characteristics, is struggling to meet the new demands of large-scale connectivity, revealing its limitations.

[0004] Faced with this challenge, non-orthogonal technologies have gradually become a new direction for research and application, aiming to overcome the limitations of existing technologies and better adapt to future network development. Non-orthogonal multiple access (NOMA) technology is a typical example. It improves spectrum utilization to a certain extent by allowing multiple users to share the same time and frequency resources to transmit data, especially when user channel conditions differ. However, NOMA technology also has drawbacks. For example, it requires complex signal processing at the receiver, and its performance tends to degrade when a large number of users participate in communication simultaneously, which adversely affects its practical application. Summary of the Invention

[0005] To address the issues of low spectrum utilization and weak interference management capabilities in existing multiple access technologies, as well as poor security performance caused by the lack of consideration for channel correlation in traditional RIS-assisted systems, this invention provides a secure transmission method for RSMA wireless communication systems based on STAR-RIS, achieving the goals of improving system spectrum efficiency, reducing the probability of security outages, and enhancing channel adaptability.

[0006] To achieve the above objectives, the technical solution of this invention is: a secure transmission method for an RSMA wireless communication system based on STAR-RIS, the specific steps of which are as follows:

[0007] Step 1: Construct a STAR-RIS-assisted RSMA wireless communication system model, including a single-antenna base station (BS) and a single-antenna reflective user (U). r Located in the STAR-RIS reflection region), single-antenna transmission user (U t A downlink communication system consisting of a single-antenna eavesdropping user (Eve, located in the STAR-RIS transmission area) and a STAR-RIS (containing N independently adjustable reflection / transmission phase shift units). Assume a reflecting user U... r There is a direct connection channel between it and the base station, U t Eve is blocked by obstacles or is too far away, and has no direct channel with the base station, so it can only communicate with STAR-RIS.

[0008] Step 2: Based on the excellent performance of the Gamma distribution in the STAR-RIS equivalent channel approximation task and the universality of the moment matching method in adapting to any target distribution, we carry out approximate modeling of the STAR-RIS equivalent channel.

[0009] Step 3: Clarify the transmission mechanism and function of public information, namely, public information is transmitted through a shared channel and can be decoded by all users, thereby improving the overall communication efficiency of the system to a certain extent. Define the system interruption judgment criterion, that is, when the secure transmission rate of two legitimate users is lower than their respective preset secure transmission rate thresholds, it is judged as an interruption; based on this criterion, the expression for the user's secure interruption probability (SOP) is derived.

[0010] Step 4: Define the analysis scenario and core object of this section, namely, for scenarios where the signal-to-noise ratio approaches infinity, focus on the reflection user U. r and transmission user U t The analysis focuses on the progressive security interruption probability, while also incorporating the special case where Eve can take any signal-to-noise ratio when eavesdropping on legitimate user information. Based on the above scenario and object settings, U is given... r and U t The asymptotic security outage probability derivation results are used to complete the asymptotic analysis of the security performance of the system in the high signal-to-noise ratio region.

[0011] Step 5: Monte Carlo simulation was used to verify the accuracy of the theoretical results and to verify the significant security advantages of the RSMA system compared with other multiple access schemes such as NOMA and OMA. At the same time, the impact of channel correlation on the security performance of the STAR-RIS assisted RSMA system was analyzed.

[0012] Beneficial effects:

[0013] (1) The security performance of the STAR-RIS-assisted RSMA scheme is significantly better than that of the STAR-RIS-assisted NOMA scheme and the STAR-RIS-assisted OMA scheme. Its flexible resource allocation mechanism has more advantages in interference control and secure transmission guarantee.

[0014] (2) In order to ensure the overall security performance of the system, the present invention optimizes the power allocation coefficient of public information and private information of RSMA. It can dynamically and flexibly adjust the rate and power ratio of the two types of information streams according to the actual situation, and achieves a proper balance that takes into account the transmission security of the two types of information streams. It not only improves the spectrum efficiency, but also accurately controls the interference between multiple users.

[0015] (3) This invention provides solid support for achieving a good balance of performance in the scenario of coexistence of 5G / 6G heterogeneous users, enabling it to demonstrate strong adaptability and competitiveness in dealing with ultra-high density connection scenarios. Attached Figure Description

[0016] Figure 1 This is a system model diagram of the present invention.

[0017] Figure 2 It is the curve showing the change in the security interruption probability of legitimate users as a function of the signal-to-noise ratio.

[0018] Figure 3 It is a curve showing the change of the security interruption probability of legitimate users with the signal-to-noise ratio under different channel correlations.

[0019] Figure 4 It is the curve showing the asymptotic security outage probability of legitimate users in high signal-to-noise ratio areas as a function of SNR.

[0020] Figure 5 This is a curve showing the change in the security interruption probability of legitimate users with the number of STAR-RIS units.

[0021] Figure 6 It is the curve showing the change in the system security interruption probability as a function of the public information power allocation coefficient.

[0022] Figure 7 It is a curve showing the change in the system security interruption probability as a function of the private information power allocation coefficient. Detailed Implementation

[0023] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. This embodiment is implemented based on the technical solution of the present invention, and provides detailed implementation methods and specific operation processes.

[0024] This invention relates to a STAR-RIS, a base station BS, and two legitimate users U. r U t And a user named Eve who was eavesdropping. The system uses the RSMA scheme, which divides user information into public and private parts to achieve more flexible resource allocation and higher spectrum efficiency.

[0025] Step 1: Construct a STAR-RIS-assisted RSMA wireless communication system model.

[0026] like Figure 1 As shown, the communication system constructed in this invention includes a single-antenna base station BS and a single-antenna reflector user U. r A single antenna transmits to user U t A single-antenna eavesdropping user Eve and a STAR-RIS for auxiliary signal transmission, wherein the STAR-RIS contains N independent units, and the horizontal length of a single unit is d. H The vertical length is d V Therefore, the physical area of ​​a single unit is A = d. H dV Furthermore, STAR-RIS's reflection and transmission functions are achieved through independent phase shift matrices, and the two types of phase shift matrices are uniformly represented as follows: Where i = r represents the reflection phase shift matrix, i = t represents the transmission phase shift matrix, and θ i,n and β i Let be the phase shift and amplitude coefficient of the nth unit of STAR-RIS in the corresponding functional mode, respectively, and satisfy β. t ,β r ∈[0,1] and β t +β r =1 constraint relationship.

[0027] Assuming U is located in the reflection region r The distances between the base station BS and STAR-RIS are d0 and d, respectively. r U, located in the transmission region t Because of obstacles or long transmission distances, the eavesdropping user Eve cannot establish a direct communication connection with the base station (BS) and can only transmit signals via STAR-RIS. Assume U... t The distances between Eve and STAR-RIS are d and d respectively. t and d e The distance between base station BS and STAR-RIS is fixed at d. Based on the above distance parameters, base station BS travels via STAR-RIS to U r U t The large-scale fading of Eve's cascaded channels is represented as follows: and In the formula Λ r =Λ t =Λ e The average path loss intensity per unit distance is represented by α, which is the path loss exponent of the channel between the base station (BS) and STAR-RIS. r α t and α e From STAR-RIS to U r U t The path loss exponent between the channel and Eve; similarly, the path loss exponent from base station BS to U. r Large-scale fading of direct-connect channels can be expressed as Where α0 represents the distance from base station BS to U r The path loss index of the direct connection channel between them.

[0028] The small-scale fading of the channel between the base station (BS) and STAR-RIS can be expressed as:

[0029] h = (h1, h2, ..., h N ) T (1)

[0030] And h~CN(0,R). STAR-RIS and U r U t The small-scale fading of the channel between Eve and Eve is expressed as follows:

[0031] g r =(g r,1 ,g r,2 ,...,g r,N (2)

[0032] g t =(g t,1 ,g t,2 ,...,g t,N (3)

[0033] g e =(g e,1 ,g e,2 ,...,g e,N (4)

[0034] And g r ~CN(0,R), g t ~CN(0,R) and g e ~CN(0,R). Where R represents the correlation matrix, and each element can be represented as:

[0035]

[0036] in, Representing the The distance between the first element and the τth element, where λ represents the wavelength.

[0037] In the system model under consideration, the base station (BS) uses RSMA technology to divide each user's information into public and private information. r Public information and U t Public information is encoded through a shared codebook, forming a public stream, while U r and U t Private information is independently encoded into each user's private stream and decoded only by the target user. Let P... s a represents the total transmission power available to the base station (BS). c and a i ,i∈{r,t} represent the information allocated to public information and user U, respectively. i Power allocation coefficients for private information, where i = r represents the reflecting user U r i = t represents the transmission user U t Therefore, the signal broadcast by the base station can be represented as:

[0038]

[0039] The power allocation coefficient satisfies 0 < a c ,a r ,a t <1 and a c +a r +a t The relationship is equal to 1. Therefore, U r U t The received signals at points E and Eve can be represented as follows:

[0040]

[0041] in, and Representing U r U t Additive white Gaussian noise at Eve. Assuming the base station BS and the legitimate user are unaware of the eavesdropping user's presence, the STAR-RIS phase shift is designed to ensure the signal reaching the legitimate user is aligned, i.e., θ... r,n =∠h0-∠g r,n -∠h n θ t,n =-∠g t,n -∠h n , where h n Let g represent the nth element of h, and g represent the nth element of h. j,n Then it represents g j The nth element of {r,t,e}. Under this configuration, the STAR-RIS phase shift maximizes the signal-to-interference-plus-noise ratio at the legitimate user. Due to the eavesdropping users Eve and U... t Both are located in the transmission region of STAR-RIS, so Φ e =Φ t Assuming that due to STAR-RIS phase shift, h and g e Due to the combined effect of phase, the signal reaches the eavesdropping user in a random manner.

[0042] First, all users treat all private information as interference when decoding public information. Therefore, the SINR of a legitimate user decoding public information is:

[0043]

[0044] When i = r When i = t as well as

[0045] Assuming the eavesdropping user possesses significant eavesdropping capabilities, able to directly decode the desired information without interference from other information, Eve's SNR for decoding public information is:

[0046] γ c,e =ρ e a c X e (11)

[0047] in,

[0048] After successfully decoding the public information, the public information is first removed from the received signal. Then, the private information of other users is treated as noise, and the user decodes their own private information. Therefore, the SINR of a user decoding their own private information can be expressed as:

[0049]

[0050] Where i = r Eve's SNR for eavesdropping on legitimate users' private information is:

[0051] γ p,e→i =ρ e a i X e (13)

[0052] Step 2: Use the moment matching method to perform approximate modeling of the STAR-RIS equivalent channel.

[0053] Existing research has demonstrated that the Gamma distribution performs well in approximating the STAR-RIS equivalent channel, and moment matching is applicable to any target distribution. Therefore, the moment matching method will be used to apply it to X. i Perform approximate modeling.

[0054] Base station BS and U r Equivalent channel X between r The cumulative distribution function and expectation are as follows:

[0055]

[0056] in, k r and θ r These represent the shape and scale parameters of the Gamma distribution, respectively. Represents the gamma function, while This represents an incomplete gamma function. And the parameter k... r and θ r The following formulas are given respectively:

[0057]

[0058] Base station BS and U t Equivalent channel X between t The cumulative distribution function and expectation are as follows:

[0059]

[0060] in

[0061] Regarding the SNR of the eavesdropping user and The probability density functions can be expressed as follows:

[0062]

[0063] Among them, Λ e,t =a t β r η e and Λ e,r =a r β r η e .

[0064] Similar to the method used to process the statistical characteristics of legitimate user channels, moment matching is also used to approximate the distribution of Y as a Gamma distribution, thus obtaining the probability density function of Y as follows:

[0065]

[0066] in and These represent shape parameters and scale parameters, respectively.

[0067] Equivalent channel X between base station BS and Eve e The expectation is:

[0068]

[0069] Step 3: Define system interruption determination criteria and derive the expression for the probability of safe interruption for legitimate users.

[0070] In the STAR-RIS-RSMA system proposed in this invention, public information is transmitted via a shared channel and can be decoded by all users, which improves the overall communication efficiency of the system to a certain extent. An interruption is defined as occurring when the secure transmission rates of two legitimate users are each lower than their respective preset secure transmission rate thresholds; based on this, the secure interruption probability (SOP) of a legitimate user can be expressed as:

[0071] P out,i =Pr{C i <R th,i},i∈{r,t} (24)

[0072] Among them, C i Representing U i Safe transmission rate, R th,i Representing U i The secure rate transmission threshold. Based on formula (10), the achievable rate for a legitimate user to decode public information can be expressed as follows:

[0073] R c,r =log2(1+γ) c,r (25)

[0074] R c,t =log2(1+γ) c,t (26)

[0075] To ensure U r and U t All were able to successfully decode the public stream, and the overall transmission rate of the public information should be subject to U. r and U t The minimum achievable rate limit, i.e., R c =min{R c,r ,R c,t Based on the above analysis, U r and U t Secure transmission rate C r and C t They can be represented as:

[0076] C r = [m1 log2(1+γ c )+log2(1+γ p,r )-m1 log2(1+γ c,e )-log2(1+γ p,e→r )] + (27)

[0077] C t = [m2 log2(1+γ c )+log2(1+γ p,t )-m2 log2(1+γ c,e )-log2(1+γ p,e→t )] + (28)

[0078] Where, [x] + =max{x,0}, γ c =min{γ c,r ,γ c,t}, m1 and m2 represent U r and Ut The proportion of public information in the public flow satisfies the relationship 0≤m1, m2≤1, and m1+m2=1. To evaluate the security performance of the system, U will be given in the following two theorems. r and U t The SOP closed expression.

[0079] In the STAR-RIS-assisted RSMA system, U r The SOP can be represented as:

[0080]

[0081] in,

[0082] m3=min{ρ r (η0+η r tr(Ψ r )),ρ t η t tr(Ψ t )}.

[0083] In the STAR-RIS-assisted RSMA system, U t The SOP can be represented as:

[0084]

[0085] in,

[0086] In the STAR-RIS-RSMA system, an interrupt event can be defined as the system interrupting when any legitimate user causes an interrupt. Therefore, the system's Standard Operating Procedure (SOP) can be expressed as:

[0087] P out =1-(1-P) out,r (1-P) out,t (31)

[0088] Step 4: For scenarios where the signal-to-noise ratio approaches infinity, analyze the asymptotic safety interruption probability for reflecting and transmitting users, and provide U after incorporating special cases. r and U t The derivation of the asymptotic safety outage probability.

[0089] Special case: Eve's SNR for eavesdropping on legitimate user information can take any value.

[0090] When U r When the signal-to-noise ratio of the decoded information approaches infinity, i.e., ρ r →∞,U r The asymptotic SOP can be expressed as:

[0091]

[0092] When U t When the signal-to-noise ratio of the decoded information approaches infinity, i.e., ρ t →∞,U t The asymptotic SOP can be expressed as:

[0093]

[0094] Where, k r and k t U r and U t The safety diversity gain, as can be seen in the above derivation, is k r and k t The values ​​are all affected by the number of STAR-RIS units and channel correlation.

[0095] Step 5: Simulation Experiment and Analysis:

[0096] This invention uses Monte Carlo simulation to verify the accuracy of theoretical results. On one hand, it confirms the significant security advantages of RSMA systems compared to other multiple access schemes such as NOMA and OMA. On the other hand, it analyzes the impact of channel correlation on the security performance of the STAR-RIS-assisted RSMA system. The simulation uses a correlated Rayleigh fading channel model and sets the wavelength to λ = 0.1m. Furthermore, it is assumed that STAR-RIS consists of N units, with each unit having a horizontal and vertical width represented by d... H and d V It means that d is recorded. H =d V =d HV Therefore, the area of ​​each STAR-RIS unit is A = d. H d V The reflection amplitude coefficient and transmission amplitude coefficient of the STAR-RIS unit are respectively set to β. r =0.5 and β t =0.5. In a STAR-RIS-assisted RSMA system, set the common information power allocation coefficient, U r Private information power allocation coefficient and U t The private information power allocation coefficients are respectively a c =0.1, a r =0.3 and a t =0.6, while in the comparison scheme STAR-RIS-assisted NOMA system, U is set r and U t The power allocation coefficients are respectively a r =0.2 and at =0.8. Base station BS and U r The distance between STAR-RIS and U is set to 20m. r U t The distances between BS and Eve are 8m, 12m, and 30m, respectively. The path loss factor between BS and STAR-RIS, and between STAR-RIS and the legitimate user, is set to 2.5, while the path loss factor between STAR-RIS and the eavesdropping user Eve is set to 3, and the power attenuation for the reference distance is set to 0dB.

[0097] Figure 2 It's U r and U t The curves showing the SOP as a function of signal-to-noise ratio in the STAR-RIS-RSMA, STAR-RIS-NOMA, STAR-RIS-OMA, and STAR-RIS-RSMA schemes with random phase shift are shown in this figure. Other parameters are set as follows: d HV =λ3, N=9, R th,r =0.7BPCU,R th,t =0.8BPCU, and m=0.5. From the figure, we can see that: (1) The Monte Carlo simulation results and the theoretical expression are in perfect agreement, which verifies the correctness and reliability of the theoretical analysis. (2) As the signal-to-noise ratio increases, the probability of security interruption decreases under all schemes, which shows that improving the signal-to-noise ratio can effectively enhance the security performance of the system. (3) The STAR-RIS-RSMA scheme achieves the lowest SOP under all signal-to-noise ratio conditions, which shows that it has a significant advantage in terms of security. This is mainly because the RSMA scheme can effectively use channel state information for power allocation and rate matching, thereby effectively improving the security of communication. In contrast, the STAR-RIS-NOMA scheme has a higher SOP under low signal-to-noise ratio conditions, but its security gradually improves as the SNR increases. The STAR-RIS-OMA scheme has a higher SOP under all signal-to-noise ratio conditions, which shows that it is not as secure as the RSMA and NOMA schemes. (4) The security performance of the STAR-RIS-RSMA scheme under random phase shift is far lower than that of the STAR-RIS-RSMA scheme under optimal phase shift. This is because random phase shift increases the uncertainty of the signal transmission environment, which may cause the signal sent to the legitimate user to be directed to the eavesdropping user, seriously damaging the security performance of the system.

[0098] Figure 3 U under the influence of different channel correlations r and U t The curve of SOP versus signal-to-noise ratio was plotted and compared with a scenario without a direct link. Other parameter settings are as follows: N=9, Rth,r =0.7BPCU,R th,t =0.8BPCU, and m=0.5. From the figure, we can know that: (1)U r and U t The security performance of both decreases with the increase of channel correlation. This is because channel correlation limits the multipath diversity gain provided by STAR-RIS, reduces the system's anti-fading capability, and also limits the degree of freedom of STAR-RIS beamforming, resulting in a decrease in beamforming performance and an inability to effectively focus signal energy, thus negatively impacting the system's security performance. (2) In scenarios without direct links, U r The security performance is far lower than in scenarios with direct links. r The security performance indicates that the presence of a direct link can significantly improve the security of the system. (3) In scenarios with a direct link, channel correlation affects U r The SOP has a relatively small impact, while in scenarios without direct links, channel correlation has a greater impact on U. r The impact of SOPs increases significantly. This is because direct links provide an additional signal path, significantly enhancing signal reliability and stability. In the absence of direct links, the system relies more heavily on the STAR-RIS reflection path, leading to a significant increase in the impact of channel correlation.

[0099] Figure 4 In the high signal-to-noise ratio (SNR) region, the curves showing the asymptotic SOP of legitimate users as a function of SNR are presented, demonstrating the impact of different channel correlations and different numbers of smart reflector units on diversity gain. In this case, consider setting the SNR of the eavesdropping user to a constant value, ρ. e =20dB, and other parameter settings are as follows: R th,r =1.1BPCU,R th,t =1.2BPCU, and m=0.5. From the figure, we can see that: (1) In the high signal-to-noise ratio region, as the SNR increases, U r and U t(1) The asymptotic SOP curves gradually approach their respective SOP curves, a phenomenon that verifies the accuracy and reliability of the theoretical analysis under high signal-to-noise ratio conditions. (2) As the number of STAR-RIS units increases, the SOP of legitimate users shows a downward trend, and the slope of the asymptotic SOP curve also increases, which means that the diversity gain of users increases with the increase of the number of STAR-RIS units. (3) It also verifies the impact of channel correlation on secure diversity gain. Specifically, as channel correlation decreases, the secure diversity gain of users is improved. These conclusions are consistent with the conclusions in Note 4.2 and reveal the important role of the number of STAR-RIS units in enhancing the security and reliability of communication systems, as well as the key role of channel correlation in optimizing communication system design.

[0100] To further reveal the important role of the number of STAR-RIS cells in enhancing the security of communication systems, Figure 5 The SOP (Sum of Operation) of legitimate users under different signal-to-noise ratios was plotted as a function of the number of STAR-RIS cells N. Other parameter settings are as follows: R th,r =0.7BPCU,R th,t =0.8BPCU, and m=0.5. From the figure, we can see that: (1) as N increases, U r and U t The SOP curves all show a downward trend, indicating that increasing N effectively enhances the security performance of legitimate users. This is because increasing the number of STAR-RIS units effectively enhances the channel gain of the cascaded link from the base station to the user, improving the signal transmission quality and thus reducing the probability of interruption. (2) Under the same number of STAR-RIS units N, the SOP of legitimate users decreases with the increase of SNR, and the rate of decrease gradually accelerates. This observation is consistent with... Figure 2 This is consistent. The reason is that as the SNR increases, the system's anti-interference capability is enhanced, and even in the presence of noise, the system can decode signals more reliably.

[0101] Figure 6 It is assigned to U in RSMA r Private information power allocation coefficient a r With the system's SOP remaining unchanged, the power allocation factor a allocated to common information by RSMA changes accordingly. c The curve of change and the display of U r and U t The different proportions of public information. Specific parameter settings are as follows: R th,r =1.2BPCU,R th,t =1.3BPCU,ρ r =ρ t =40dB, N=20, ar =0.1, a t =0.9-a c From the figure, it can be observed that: (1) the system's SOP changes with a c The increase in U first decreases and then increases. This is because as the public information allocation rate increases, more power is allocated to public information, which can enhance the signal quality of public information, thereby improving the signal reception quality of all users and increasing the total capacity of the system. However, as the public information allocation rate further increases, the power allocation for private information will decrease accordingly, making it impossible for legitimate users to successfully decode private information, thus affecting their communication security. (2) By further comparing U r and U t The different proportions of public information reveal that when a c When the size is small, the system's security performance is mainly determined by U. r Decision, and when a c When the value is large, the system's security performance is mainly determined by U. t Decision. (3)U r and U t Different proportions of public information, i.e., different values ​​of m1 and m2, will affect the optimal a. c The choice, specifically, when m1 is large and m2 is small, a c It should be set smaller. This is mainly because a larger m1 will increase U... r The performance of [the system / system] will be improved, but at the same time, the performance of [the system / system] will be reduced t The performance, therefore, will a c Setting it to a smaller value allows for allocation to U t The rate of private information allocation is increased to compensate for the performance degradation caused by the small proportion of public information.

[0102] Figure 7 In RSMA, the power allocation factor a is assigned to the public information. c Without changing, the system's SOP is allocated to U along with RSMA. r Power allocation coefficient a for private information r The variation curves were plotted, and the performance was compared with that of STAR-RIS with N cell numbers. Specific parameter settings are as follows: R th,r =1.2BPCU,R th,t =1.3BPCU,ρ r =ρ t =40dB, a c =0.1, a t =0.9-a r From the figure, it can be observed that: (1) the system's SOP changes with a r The increase also shows a trend of first decreasing and then increasing, which is mainly due to the fact that when a is fixed... cAt that time, with a r The increase of U r The security performance has been significantly improved, however when a r When it increases to a certain extent, a t It will become very small, causing U t The safety performance of the system is significantly weakened, which in turn affects the overall safety performance of the system. This trend indicates that when designing an RSMA system, a balance needs to be found between the power allocation coefficients to ensure the safety and efficiency of the entire system. (2) As the number of STAR-RIS units N increases, the safety performance of the system increases significantly. Specifically, when N=12, the SOP of the system is approximately 100 and 1000 times that when N=16 and N=20, respectively. This result shows that increasing the number of STAR-RIS units can effectively reduce the probability of system safety interruption, thereby significantly improving the safety and reliability of the system.

[0103] The above description is only a preferred embodiment of the present invention. The present invention is not limited to the above embodiments. If various modifications or variations of the present invention do not depart from the spirit and scope of the present invention and fall within the scope of the claims and equivalent technologies of the present invention, the present invention also intends to include these modifications and variations.

Claims

1. A secure transmission method for a RSMA wireless communication system based on STAR-RIS, characterized in that, The method comprises the following steps: Step 1: Construct a model of a STAR-RIS assisted RSMA wireless communication system; Step 2: Perform approximate modeling on an equivalent channel of the STAR-RIS by using a moment matching method; Step 3: Define a system outage criterion, and deduce an expression of a user safety outage probability (SOP); Step 4: For the case that the signal-to-noise ratio tends to infinity, the asymptotic outage probability of reflective users and transmissive users is analyzed, and the special case is taken into account to give the asymptotic outage probability derivation results of U r and U t ​ Step 5: Verify the accuracy of the theoretical result by using Monte Carlo simulation.

2. The STAR-RIS-based RSMA wireless communication system secure transmission method of claim 1, wherein, The model in step 1 includes: a single-antenna base station BS and a single-antenna reflection user U. r Single antenna transmission user U t Single-antenna eavesdropping users Eve and STAR-RIS; the single-antenna reflecting user U r Located in the STAR-RIS reflection region, the single antenna transmits to user U t The user Eve, who is eavesdropping on a single antenna, is located in the transmission region of the STAR-RIS, which contains N independently adjustable reflection / transmission phase shift units; assuming U r There is a direct connection channel between U and the base station (BS). t Due to obstacles or long distances, Eve has no direct channel with the base station (BS) and can only communicate through STAR-RIS. The model construction method is as follows: Small-scale fading of a channel between a base station BS and a STAR-RIS is represented as: h = (h1, h2,..., h N ) T (1) and h ~ CN(0, R); STAR-RIS with U r , U t The small-scale fading of the channel between Bob and Eve is denoted by g r = (g r,1 ,g r,2 ,...,g r,N ) (2) g t = (g t,1 ,g t,2 ,...,g t,N ) (3) g e = (g e,1 ,g e,2 ,...,g e,N ) (4) and g r ~ CN(0, R), g t ~ CN(0, R) and g e ~ CN(0, R); where R represents a correlation matrix, where each element is represented as: The base station BS applies RSMA technique to divide each user's information into common information and private information, U r The common information of U t is encoded by a shared codebook to form a common stream, while the private information of U r and U t is independently encoded into each user's private stream and decoded only by the target user; let P s be the total transmission power available to the base station BS, a c and a i , i∈{r,t} represent the power allocation coefficients allocated to the common information and the private information of user U i , where i=r represents the reflecting user U r and i=t represents the transmitting user U t , so that the signal broadcasted by the base station is represented as: where the power allocation factor a satisfies 0 < a < 1 c ,a r ,a t < 1 and a c + a r + a t = 1, so that U r , U t and the received signals at Eve are expressed as: wherein, respectively represent U r , t and additive white Gaussian noise at Eve, assuming that the base station BS and the legitimate user do not know the existence of the eavesdropping user, therefore the purpose of designing the STAR-RIS phase shift is to make the signal reaching the legitimate user is aligned, i.e. r,n = ∠h0- ∠g r,n - ∠h n , θ t,n = - ∠g t,n - ∠h n , wherein h n represents the nth element of h, and g j,n represents the nth element of g j , i∈{r,t,e}, under this setting, the STAR-RIS phase shift makes the signal-to-interference-plus-noise ratio at the legitimate user maximum, since the eavesdropping user and U t are located in the transmission area of the STAR-RIS, so Φ e = Φ t , assuming that due to the joint influence of STAR-RIS phase shift, h and g e phase, the signal reaches the eavesdropper in a random way; First, all users decode common information by regarding all private information as interference, so that the SINR of decoding common information by a legal user is: when i = r, when i = t, and Suppose that a eavesdropping user has a large eavesdropping capability, and can directly decode information to be eavesdropped without interference from other information, so that the SNR of decoding common information by Eve is: gamma c,e = rho e alpha c X e (11) wherein, After successfully decoding common information, the common information is first removed from received signals, and then private information of other users is regarded as noise to decode private information of the user, so that the SINR of decoding private information of the user is: where i = r The SNR of Eve eavesdropping the private information of the legitimate user is gamma p,e→i = rho e alpha i X e (13).

3. The STAR-RIS-based RSMA wireless communication system secure transmission method of claim 1, wherein, The specific method in step 2 is: The base station and the UE r have an equivalent channel X r The cumulative distribution function and the expectation of X are respectively: where k r and θ r represent the shape and scale parameters of the Gamma distribution, respectively, represents the Gamma function, and represents the incomplete Gamma function, and the parameters k r and θ r are given by the following equations: The base station and the UE t between the base station and the UE t The cumulative distribution function and the expectation of the equivalent channel X wherein The probability density function of the SNR for an eavesdropper, and are expressed as where Λ e,t = a t β r η e and Λ e,r = a r β r η e ; Similar to the processing method of the statistical characteristics of the channel of the legal user, the distribution of Y is approximated to a Gamma distribution by using moment matching, so that the probability density function of Y is: wherein respectively represent a shape parameter and a scale parameter; The equivalent channel X between the base station BS and Eve e The expectation is that:

4. The STAR-RIS-based RSMA wireless communication system secure transmission method of claim 1, wherein, The specific step 4 is: When safety transmission rates of two legal users are respectively lower than preset safety transmission rate thresholds, it is determined that outage occurs; based on this, the safety outage probability (SOP) of the legal user is represented as: P out,i = Pr{C i < R th,i}, i e {r, t} (24) where C i represents the security transmission rate of U i th,i represents the security rate transmission threshold of U i Based on formula (10), the achievable rates of the legitimate users decoding the common information are respectively represented as:​ R c,r = log2(1 + γ c,r ) (25) R c,t = log2(1 + γ c,t ) (26) To ensure U r and U t can successfully decode the common stream, the transmission rate of the whole common information should be limited by the minimum rate that U r and U t can achieve, i.e., R c = min{R c,r , R c,t}. Based on the above analysis, the safe transmission rates C r and C t for U r and U t are respectively given by: C r = [m1log2(1+γ c )+log2(1+γ p,r )-m1log2(1+γ c,e )-log2(1+γ p,e→r )] + (27) C t = [m2log2(1+γ c )+log2(1+γ p,t )-m2log2(1+γ c,e )-log2(1+γ p,e→t )] + (28) wherein [x] + = max{x, 0}, y c = min{y c,r , y c,t}, m1 and m2 represent the proportion of the public information of U r and U t in the public stream respectively and satisfy the relationship of 0≤m1, m2≤1 and m1+m2=1. U r The SOP for S is represented as: wherein, m3 = min{p r (η0+η r tr(Ψ r )), p t η t tr(Ψ t )}; U t The SOP for S is represented as: wherein The outage event of the system is defined as that the system is out of service when outage occurs for any legal user, so that the SOP of the system is represented as: P out = 1 - (1 - P out,r )(1 - P out,t ) (31).

5. The STAR-RIS-based RSMA wireless communication system secure transmission method of claim 1, wherein, The specific step 4 is: The special case is that the SNR of eavesdropping common information of the legal user by Eve can take any value; When the signal-to-noise ratio of the decoded information tends to infinity, i.e. p r →∞, the asymptotic SOP of U r r is given by​ When the signal-to-noise ratio of the decoded information tends to infinity, i.e. p t →∞, the asymptotic SOP of U t t is given by:​ where k r and k t are the safety diversity gains of U r and U t , respectively. In the derivation above, it is found that the values of k r and k t are affected by the number of STAR-RIS elements and the channel correlation.

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