CN109890031B - Multi-relay non-orthogonal multiple access system safe transmission method based on artificial noise - Google Patents

Abstract

The present invention proposes a multi-relay non-orthogonal multiple access system security transmission scheme based on artificial noise; the transmission system includes a source node, two users, an eavesdropping node and multiple relay nodes; in the present invention, the entire communication process is divided into Four stages: preparation stage, multi-relay decoding stage, relay selection stage and user decoding stage; system communication is completed in two time slots: in the first time slot, the source S broadcasts the message to all relay nodes. Using the message mapping strategy, the relay receives the signal and decodes it; in the second time slot, the system performs dual relay selection, selects the optimal relay to forward the user signal from the set of decodable relays, and selects the optimal relay to forward the user signal from the set of non-decodable relays. The optimal relay to send artificial noise; the relay selection strategy can effectively reduce the complexity of the system and the probability of security interruption while maintaining the diversity gain of the multi-relay system; artificial noise scrambling can further reduce the signal-to-noise ratio of the eavesdropping end and improve the System security features.

Description

A Secure Transmission Method for Multi-relay Non-Orthogonal Multiple Access System Based on Artificial Noise

technical field

The invention belongs to the technical field of wireless communication networks, in particular to a wireless communication network adopting cooperative non-orthogonal multiple access technology, and belongs to the field of physical layer security.

Background technique

In the past two decades, mobile communication technology has experienced rapid development. At the same time, the explosive growth of mobile data volume brought about by the popularization of intelligent terminals has put forward higher requirements on the rate, delay, and signal coverage of wireless communication networks. Compared with traditional orthogonal multiple access technology, non-orthogonal multiple access technology has attracted extensive attention in the industry and academia due to its high bandwidth efficiency, high user fairness, ultra-high connectivity and compatibility. The cooperative communication technology can not only improve the coverage of the network, but at the same time, because the receiver adopts the diversity technology, the system performance can also be significantly improved. The security of the physical layer does not require a key, and the time-varying encryption communication system of the wireless channel can theoretically achieve absolute security of information transmission. As the computing power of the terminal is gradually enhanced, the ability of the eavesdropper to decipher the information is also gradually improving. Traditional cryptography is facing increasing pressure in the field of information encryption. The physical layer security is based on information theory and realizes information security in the physical layer, which is a very promising encryption method in the field of communication.

A general downlink non-orthogonal multiple access communication network model is shown in FIG. 1 . There are two main technologies of non-orthogonal multiple access: using superposition coding at the transmitting end to transmit multiple user information on the same frequency at the same time, and using serial interference cancellation technology at the receiving end to ensure that the ) can serve multiple users. The basic steps of serial interference cancellation are: 1. First, solve the signal of the user with the worst channel condition in the superimposed signal (in the non-orthogonal multiple access system, the power of the signal sent to the user is the strongest); 2 .Delete the signal from the superimposed signal; 3. Perform the same steps for users with the next worst channel conditions, and solve the signals of all users one by one in this way.

The cooperative communication technology can not only improve the coverage of the network, but also significantly improve the system performance because the receiver adopts the diversity technology. The combination of cooperative communication and non-orthogonal multiple access technology can further improve the performance of the non-orthogonal multiple access communication system. As shown in Figure 2: The common non-orthogonal multiple access cooperative communication system mainly has two architectures: 1. Dedicated relay system, the source node and the destination node cannot communicate directly due to the existence of deep fading, so set up Dedicated relay, which forwards the message of the base station for two users; 2. User cooperation, since the strong user needs to decode the weak user signal first, the strong user can forward the information to the weak user through short-distance communication (such as Bluetooth, WiFi, etc.). This cooperative approach can reduce system redundancy and improve diversity gain. There are two commonly used relay protocols: amplify-and-forward protocol and decode-and-forward protocol. The present invention is mainly aimed at the decoding and forwarding relay system.

Physical layer security is a security theory based on information theory. It utilizes the time-varying nature of wireless channels and combines channel coding and encryption technology to ensure that information cannot be deciphered by eavesdroppers. C.E.Shannon in 1945 in the classic literature (see document [1]: Shannon C E. Communication theory of secrecy systems [J]. The Bell SystemTechnical Journal, 1949, 28(4): 656-715.) from the perspective of information theory Proof: To achieve absolute security of the message, the communication process must use the "one-time-one-pad" encryption method, that is, one bit of data should have one bit of key. This condition is too harsh and difficult to apply in the engineering field. A. Wyner first established a noise-containing eavesdropping channel model on the basis of Shannon's research in 1975 (see document [2]: Wyner A D. The wire-tap channel [J]. The Bell SystemTechnical Journal, 1975, 54 (8 ):1355-1387.), as shown in Figure 3. Wyner proved that when the channel condition of the main channel is better than that of the eavesdropping channel, when the source and the legitimate receiver transmit information, there must be a coding method that can make the probability of transmitting information error arbitrarily small. At this time, the eavesdropping end No useful information can be obtained, and the system is completely secure. After decades of development, the physical layer security theory has gradually matured. The use of physical layer security to realize the secure transmission of wireless communication systems has increasingly become a research hotspot in academia and industry. The measurement indicators of the security of the physical layer of the system mainly include traversal security capacity and security interruption probability. Among them, the security interruption probability refers to the probability that the instantaneous security capacity of the system is less than the given threshold value. The mathematical expression is:

R_s表示安全速率阈值，F_D(x)表示主信道信噪比的累积分布函数。f(γ_E)表示γ_E的概率密度函数。In the formula, γ _D and γ _E represent the signal-to-noise ratio of the destination end and the eavesdropping end,

R _s represents the safe rate threshold, and F _D (x) represents the cumulative distribution function of the signal-to-noise ratio of the main channel. f(γ _E ) represents the probability density function of γ _E.

The downlink cooperative communication system based on the combination of non-orthogonal multiple access, cooperative communication and physical layer security theory will show great application prospects in the 5G era that pursues high speed, large integration and large connection.

SUMMARY OF THE INVENTION

(1) Problems solved by the present invention

Aiming at the system security problem that eavesdroppers may exist in the two-hop transmission of the downlink cooperative multi-relay non-orthogonal multiple access system, the present invention provides an artificially scrambled multi-relay cooperative non-orthogonal multiple access system security transmission scheme. The dual-relay selection strategy is adopted to effectively reduce the complexity of the system while maintaining the diversity gain of the multi-relay communication network; at the same time, it effectively interferes with the eavesdropping terminal to ensure the safe transmission of the non-orthogonal multiple access system.

(2) Technical scheme of the present invention

表示，其中

表示发送端，

表示接收端。假设所有信道均服从独立同分布的Nakagami-m衰落。因此源节点υ和目的节点l间链路的信道增益的概率密度函数和累积分布函数可表示为：As shown in FIG. 4 , the communication system according to the embodiment of the present invention includes K+3 nodes in total, where K is the number of half-duplex relay nodes R, and K≧1 is satisfied. In addition, the system also includes source node S, two user nodes U ₁ and U ₂ (wherein U ₁ and U ₂ represent strong users and weak users, respectively), and eavesdropping node E. There is no direct transmission link between the source node S and the two users U ₁ and U ₂ due to deep fading, the communication needs multi-relay assistance, and all nodes are equipped with a single antenna. The channel power gain is

means that the

represents the sender,

Indicates the receiver. It is assumed that all channels obey IID Nakagami-m fading. Therefore, the probability density function and cumulative distribution function of the channel gain of the link between the source node v and the destination node l can be expressed as:

Γ(·)是伽马函数。

表示值为正整数的衰落系数，

表示平均信道功率增益。为了简化分析，假定两跳链路均为独立同分布。亦即

且

in,

Γ(·) is the gamma function.

represents a fading coefficient whose value is a positive integer,

represents the average channel power gain. In order to simplify the analysis, it is assumed that the two-hop links are both independent and identically distributed. that is

and

The embodiments of the present invention are based on the physical layer security technology, and consider the correlation between the security capacities of two users in a multi-relay downlink non-orthogonal multiple access system. An artificially scrambled multi-relay cooperative non-orthogonal multiple access system security transmission strategy is disclosed. The core of the strategy is the optimal security relay selection based on physical layer security technology. The specific implementation steps of the strategy are as follows:

Step S1: System initialization. The source node first broadcasts the training sequence to the downlink multi-relay non-orthogonal multiple access system. This step has two purposes: (1) Estimate the channel state information of the main channel, including the channel state information of SR _k ,k∈[1:K], R _k -U _i ,i∈{1,2}; (2) Estimate the channel state information of the eavesdropping channel. That is, the channel state information of the R _k -E channel. The acquisition of the channel state information of each link can be achieved by monitoring the transmission of each port or using some complex channel estimation algorithms. in wireless communications using diversity techniques [J]. IEEE Network, 2015, 29(1): 42-48.) I will not repeat them in this article.

其中“×”表示笛卡尔乘积，

是W_j,j＝0,1,2的有限码簿。通过训练序列，所有节点均可解码采用消息映射策略发送的信息。因此第一时隙有R₀＝R₁+R₂恒成立，其中R_i为消息W_i,i＝0,1,2的数据速率；Step S2: In the first time slot, the source S uses the message mapping strategy (see document [4]: Xu P, Yang Z, Ding Z, et al. Optimal relay selection schemes for cooperative non-orthogonal multiple access [J] .IEEE Transactions on Vehicular Technology, 2018, 67(8):7851-7855. The text is omitted) to superimpose the broadcast signal, and the superimposed signal can be expressed as:

where "×" represents the Cartesian product,

is a finite codebook of W _j , j=0,1,2. Through the training sequence, all nodes can decode the information sent using the message mapping strategy. Therefore, in the first time slot, R ₀ =R ₁ +R ₂ is always established, where R _i is the data rate of the message Wi , _i =0, 1, 2;

其中，P_S是基站功率，s₀表示叠加信号的码字，

表示方差为δ²的加性高斯白噪声。随后中继采用解码转发协议解码转发信源的广播信息；Step S3: In the second time slot, the relay receives the broadcast information of the source S, and the signal received by the kth relay is:

Among them, P _S is the base station power, s ₀ represents the codeword of the superimposed signal,

represents additive white Gaussian noise with variance δ ² . Then the relay uses the decoding and forwarding protocol to decode and forward the broadcast information of the source;

Step S4: Establish a positively decodable relay combination Φ. Obviously, after experiencing the channel fading of SR _k , some relays may not be able to decode the information normally. This step selects the relay set Φ that can be decoded normally from all K relays. The mathematical expression is:

是由于系统传输信息需要两个时隙，ρ_S表示S的发送信噪比，

是U_i的数据速率门限。in the formula

It is because the system needs two time slots to transmit information, ρ _S represents the transmission signal-to-noise ratio of S,

is the data rate threshold for U _i .

式中，

α_i(i＝1,2)表示第k个中继的功率分配因数，α₁+α₂＝1，α₁＞α₂，P_R表示第k个中继的发送功率，σ²表示噪声功率，α_J表示人工噪声的功率分配因数。对于第m个中继，定义：Step S5: Select the optimal relay to forward the signal from Φ. The optimal relay selection principle will now be described in detail. According to the basic principle of serial interference cancellation, U _{1 first} uses its weak signal as interference through serial interference cancellation, solves the signal of U ₂ , then subtracts the signal of U ₂ from the mixed signal, and finally solves the signal of U ₁ . U ₂ directly uses the U ₁ signal as interference and solves its signal. The SINRs of the two users can be expressed as:

In the formula,

α _i (i=1,2) represents the power distribution factor of the kth relay, α ₁ +α ₂ =1, α ₁ >α ₂ , _PR represents the transmit power of the kth relay, σ ² represents the noise Power, α _J denotes the power distribution factor of the artificial noise. For the mth relay, define:

为了最大化系统的安全性能，选择中继的标准为

即选择Φ中使X_m取值最大的中继。基于以上分析，可知该中继选择策略可以最小化多中继协作非正交多址接入系统的安全中断概率。where δ ₃ (x)=l+θ ₁ x,

In order to maximize the security performance of the system, the criteria for selecting relays are

That is, select the relay that maximizes the value of X _m in Φ. Based on the above analysis, it can be known that the relay selection strategy can minimize the security interruption probability of the multi-relay cooperative non-orthogonal multiple access system.

中选择发送人工噪声的最优中继。为了进一步增强多中继协作非正交多址接入系统的安全性能，从不能正确解码的中继集合

中，选择到窃听节点E的信道增益最大的中继k^*发送人工噪声。选择标准表示为

其中

表示Φ的补集。为了将本发明策略和传统多中继转发策略进行公平比较，所有中继的总功率限制为P_R。Step S6: from

Select the optimal relay for sending artificial noise. In order to further enhance the security performance of the multi-relay cooperative non-orthogonal multiple access system, the set of relays that cannot be decoded correctly

, select the relay k ^* with the largest channel gain to the eavesdropping node E to send artificial noise. The selection criteria are expressed as

in

represents the complement of Φ. In order to make a fair comparison between the strategy of the present invention and the traditional multi-relay forwarding strategy, the total power of all the relays is limited to P _R .

其中，

表示人工加扰中继的发送信噪比，

Step S7: _The users U1 and U2 receive and decode the message forwarded by the relay m ^* , the eavesdropping terminal E simultaneously eavesdrops _on the signal forwarded by the relay, and the relay k ^* sends artificial noise to interfere with the eavesdropping terminal E. It is assumed that α _J (0≦α _J <1) is a power distribution factor for transmitting artificial noise. According to the above analysis, the signal-to-noise ratios of users U ₁ , U ₂ and the eavesdropping terminal E can be expressed as:

in,

represents the transmit signal-to-noise ratio of the artificially scrambled relay,

同理，第m个中继发送信号时，U₂的安全中断概率可表示为：The security performance of the system according to the embodiment of the present invention is analyzed below. According to the expression of safe outage probability and some simple algebraic operations, the safe outage probability of U ₁ when the mth relay sends a signal can be obtained as:

Similarly, when the mth relay sends a signal, the safe interruption probability of U ₂ can be expressed as:

时，U₂将安全中断。根据安全中断概率的定义和一系列数学推导，该策略下系统的安全中断概率表达式为：Obviously, when

, _U2 will safely interrupt. According to the definition of safe outage probability and a series of mathematical derivations, the safe outage probability expression of the system under this strategy is:

In the formula,

In the formula, the g and h functions are complex integrals that cannot find closed-form solutions, and can be approximated by the Gaussian Chebyshev expansion as follows:

In the formula, N is the number of expansion terms, S _i =t _i +1, t _i is the ith zero point of the Lagrangian polynomial, w _i is the Gaussian weight,

(3) Beneficial effects of the present invention

The beneficial effects of the present invention are mainly: considering the correlation of the security capacity of the two users, when the security rate thresholds of the two users are different, the dual-relay selection strategy can effectively reduce the multi-relay system diversity gain while maintaining the multi-relay system diversity gain. The complexity and safety interruption probability of relay cooperative non-orthogonal multiple access system improve the safety performance of the system.

The beneficial effects of the present invention come from the following three aspects:

(1) Multi-relay cooperative communication is adopted. Due to the time-varying nature of the wireless channel, the attenuation of the signal changes drastically during transmission. Cooperative techniques provide additional diversity gain to the communication system, thus increasing the coverage of the network. At the same time, because the receiver adopts diversity technology, the system performance can also be significantly improved.

(2) Adopt relay selection strategy. The relay selection strategy can effectively reduce the complexity of the system while maintaining the diversity gain of the multi-relay communication network. In addition, different from the general relay selection scheme, the present invention considers the correlation of the security capacity of the two users, and is completely applicable to the situation where the security rate thresholds of the two users are different. The influence of different safety rate thresholds on the probability of system safety interruption is shown in Figure 5. The Monte Carlo simulation results in FIG. 5 show that the present invention can significantly enhance system security compared with other forwarding strategies.

(3) Adopt artificial noise addition strategy. Different from the previous single-relay selection strategy, the present invention proposes a dual-relay selection strategy. Select one of the relays that cannot be decoded normally to send artificial noise to interfere with the eavesdropping terminal. FIG. 6 proves that the artificial noise addition strategy proposed by the present invention enhances the security performance of the system compared with the single-relay selection strategy and the traditional multi-relay forwarding strategy.

Description of drawings

Figure 1: Model of a generic non-orthogonal multiple access system. In the figure, S is the source node, and D ₁ and D ₂ are the strong and weak users, respectively. The source node adopts superposition coding, the principle is: the signals of two users are superimposed and sent at the same frequency and different powers at the same time. Since D ₁ is a strong user, the system allocates less power to it, and performs serial interference cancellation during decoding. First, the signal of D ₁ is regarded as interference, the signal of D ₂ is decoded, and the signal of D ₂ is deleted from the superimposed signal. Resolve your own signal. D ₂ is a weak user in the non-orthogonal multiple access system. When decoding, the signal of D ₁ is regarded as interference, and the signal of D ₂ is decoded.

Figure 2: Two types of cooperative non-orthogonal multiple access systems. Figure (a) is a dedicated relay scenario, where the source node and two users are in deep fading due to the blocking of mountains or dense buildings. At this time, a dedicated relay is set up, and after receiving the signal of the source node, the superimposed message is forwarded to the two users. Figure (b) shows user cooperation. Since the strong user performs serial interference cancellation, the weak user's signal needs to be solved first. At this time, the strong user can forward information to the weak user through short-distance communication (such as Bluetooth, WiFi, etc.). User cooperation reduces redundancy and improves the diversity gain of the system.

Figure 3: Wyner eavesdropping channel model. The eavesdropping model is an improvement of the Shannon model. In this model, Wyner pointed out that when the channel condition of the main channel is better than that of the eavesdropping channel, there must be a coding method when the source and the legitimate receiver transmit information, which can make the The probability of transmitting information errors is arbitrarily small, and the eavesdropping terminal cannot obtain any useful information at this time, and the system achieves absolute security. And from the perspective of information theory, the physical layer security is defined by using the source entropy.

Figure 4: Multi-relay cooperative non-orthogonal multiple access system model. The present invention considers a multi-relay downlink cooperative non-orthogonal multiple access system. Among them, S represents the base station, R ₁ . . . R _K are decoding forwarding and half-duplex relays with a total number of K, U ₁ and U ₂ represent strong users and weak users respectively, and E is the eavesdropping terminal. Assuming that there is no direct transmission link between the base station and the _two users U1 and U2 due to deep fading, the communication needs multi _- relay assistance, and all nodes are equipped with a single antenna.

对协作非正交多址接入系统安全中断概率的影响。参数设置如下：

，K＝2，m_U＝m_E＝m_R＝2，α₁＝0.2，α_J＝0.5，Ω₁＝12dB，Ω_R＝Ω₂＝10dB，Ω_E＝-5dB。其中“ODRS”表示本发明中提出的双中继选择策略，“OSRS”代表单中继选择策略(只转发不加噪)，“TMRF”代表传统多中继转发策略(所有中继均转发信号，用户和窃听端采用最大比合并策略合并信号)，“Sim”表示蒙特卡洛仿真结果，“Analysis”表示理论分析结果。本图以蒙特卡洛仿真证明了理论分析的正确性。显然，提高安全速率门限会使无线通信系统的安全性能恶化。在发送信噪比较大时，本发明提出的双中继选择策略的安全性能明显优于其他两种策略。Figure 5: Different Safe Rate Thresholds

Influence on Safe Outage Probability of Cooperative Non-Orthogonal Multiple Access System. The parameter settings are as follows:

, K=2, m _U = m _E = m _R =2, α ₁ =0.2, α _J =0.5, Ω ₁ =12dB, Ω _R =Ω ₂ =10dB, Ω _E =-5dB. "ODRS" represents the dual-relay selection strategy proposed in the present invention, "OSRS" represents the single-relay selection strategy (only forwarding without adding noise), and "TMRF" represents the traditional multi-relay forwarding strategy (all relays forward signals , the user and the eavesdropping terminal use the maximum ratio combining strategy to combine the signals), "Sim" represents the Monte Carlo simulation result, and "Analysis" represents the theoretical analysis result. This figure proves the correctness of the theoretical analysis by Monte Carlo simulation. Obviously, increasing the security rate threshold will deteriorate the security performance of the wireless communication system. When the transmission signal-to-noise ratio is large, the security performance of the dual relay selection strategy proposed by the present invention is obviously better than the other two strategies.

Figure 6: Influence of artificial noise allocation parameter α _J and relay number K on the probability of system safety interruption. The parameters are set as: R ₁ =0.1 nat, R ₂ =0.2 nat, ρ _S =ρ _R =10dB, m _U =m _E =m _R =m,α ₁ =0.2,Ω ₁ =12dB,Ω _R =Ω ₂ =10dB, Ω _E =-5dB. "ODRS" represents the dual-relay selection strategy proposed in the present invention, "OSRS" represents the single-relay selection strategy (only forwarding without adding noise), "Sim" represents the Monte Carlo simulation result, and "Analysis" represents the theoretical analysis result . This figure proves the correctness of the theoretical analysis by Monte Carlo simulation. Obviously, increasing the number of relays can improve the diversity gain of the system, thereby enhancing the security performance of the system. In addition, the simulation results show that the dual-relay selection strategy can further improve the system security compared with the single-relay selection strategy. The performance improvement becomes more and more obvious as the number of relays increases.

Figure 7: The implementation process of the secure transmission scheme of the multi-relay cooperative non-orthogonal multiple access system. It is mainly divided into two time slots. The first time slot mainly includes: the base station sends the superimposed signal and the relay receives and decodes the signal; the second time slot is the relay decision and user decoding, and the optimal relay is selected to forward the two user signals. The relay with the strongest channel gain at the eavesdropping end sends artificial noise, the weak user decodes the signal of the strong user as interference, and the strong user decodes based on serial interference cancellation.

Figure 8: The whole communication process in the present invention is divided into four stages: preparation stage, multi-relay decoding stage, relay selection stage and user decoding stage.

Detailed ways

(1) System default

The non-orthogonal multiple access communication system according to the embodiment of the present invention includes K+3 nodes in total, where K is the number of half-duplex relay nodes R, and K≥1 is satisfied. In addition, the system also includes source node S, two user nodes U ₁ and U ₂ (wherein U ₁ and U ₂ represent strong users and weak users, respectively), and eavesdropping node E. There is no direct transmission link between the source node S and the two users U ₁ and U ₂ due to deep fading, the communication needs multi-relay assistance, and all nodes are equipped with a single antenna. It is assumed that all channels obey IID Nakagami-m fading.

(2) Implementation process

The specific implementation process of the present invention is shown in FIG. 7 . The implementation process of the present invention is divided into seven steps:

同时，训练序列使中继和目的节点获知消息映射策略信息；Step S1: System initialization; send a training series in the multi-relay downlink cooperative non-orthogonal multiple access system, estimate the channel state information of each channel through a channel estimation algorithm, and obtain the mth relay to U ₁ , U ₂ and the channel gains of E are:

At the same time, the training sequence enables the relay and the destination node to learn the message mapping policy information;

Step S2: in the first time slot, the base station sends a superimposed signal to all K relays based on superposition coding and a message mapping strategy according to the basic principle of non-orthogonal multiple access;

Step S3: in the second time slot, decode and forward the received signal of the relay and decode it, and then use the dual-relay option to forward the message to the user terminal and add noise;

其中Φ定义为：

ρ_S表示源节点S的发送信噪比，

是U_i的安全速率门限；

为Φ的补集，为不能正常解码的中继集合。可以正确解码的中继转至步骤S5：不能正确解码的中继转至步骤S6；Step S4: The system establishes two sets according to the relay decoding situation: a set of decodable relays Φ and a set of relays that cannot be decoded

where Φ is defined as:

ρ _S represents the transmission signal-to-noise ratio of the source node S,

is the safe rate threshold of U _i ;

is the complement of Φ, and is the set of relays that cannot be decoded normally. The relay that can be decoded correctly goes to step S5; the relay that cannot be decoded correctly goes to step S6;

Step S5: In Φ, for the mth relay, define

其中

为第m个中继到 U₁和U₂的信道增益，δ₃(x)＝l+θ₁x，

Select the relay that maximizes the value of X _m in Φ, that is, select

in

is the channel gain of the mth relay to U ₁ and U ₂ , δ ₃ (x)=l+θ ₁ x,

中，选择到窃听端信道增益最大的中继发送人工噪声。即选择

Step S6: in

, select the relay with the largest channel gain to the eavesdropping end to transmit artificial noise. i.e. choose

Step S7: User decoding, wherein the strong user U ₁ performs serial interference cancellation, preferentially decodes the signal of the weak user and deletes it from the superimposed signal, and then decodes its own signal, and the weak user U ₂ directly regards the signal of U ₁ as noise. , which decodes its own signal.

Claims (1)

1. a multi-relay non-orthogonal multiple access system security transmission method based on artificial noise, is characterized in that, comprises double relay selection and artificial scrambling strategy: namely select optimal relay in decodable relay Forward the signal, and select the optimal relay among the non-decodable relays to send artificial noise. The specific implementation process is as follows: Step S1: System initialization; send a training series in the multi-relay downlink non-orthogonal multiple access system, estimate the channel state information of each channel through a channel estimation algorithm, and obtain the mth relay to U ₁ , U ₂ and E The channel gains of are:

At the same time, the training sequence enables the relay and the destination node to know the message mapping policy information; U ₁ and U ₂ represent two user nodes, and E represents the eavesdropping node; Step S2: in the first time slot, the base station sends a superimposed signal to all K relays based on superposition coding and a message mapping strategy according to the basic principle of non-orthogonal multiple access; Step S3: in the second time slot, decode and forward the received signal of the relay and decode it, and then use the dual relay selection to forward the message to the user terminal and add noise; specifically: the relay receives the broadcast information of the source S, and the kth in the Following the received signal is:

where P _S is the base station power,

represents the channel gain from the source node S to the kth relay, s ₀ represents the codeword of the superimposed signal,

Represents additive white Gaussian noise with a variance of δ ² ; then the relay uses the decoding and forwarding protocol to decode and forward the broadcast information of the forwarding source; Step S4: The system establishes two sets according to the relay decoding situation: a set of decodable relays Φ and a set of relays that cannot be decoded

where Φ is defined as:

ρ _S represents the transmission signal-to-noise ratio of the source node S,

is the decoding rate threshold of U _i ;

is the complement of Φ, and is the set of relays that cannot be decoded normally, the relays that can be decoded correctly go to step S5, and the relays that cannot be decoded correctly go to step S6; Step S5: In Φ, for the mth relay, define

Select the relay that maximizes the value of X _m in Φ, that is, select

in

is the channel gain of the mth relay to U ₁ and U ₂ , δ ₃ (x)=l+θ ₁ x,

ρ _R represents the transmit signal-to-noise ratio of R,

represents the security rate threshold of the ith user, α _i , i=1, 2 represents the power allocation factor of the ith user; Step S6: in

, select the relay with the largest channel gain to the eavesdropping end to send artificial noise, that is, select

Step S7: User decoding, wherein the strong user U ₁ performs serial interference cancellation, preferentially decodes the signal of the weak user and deletes it from the superimposed signal, and then decodes its own signal, and the weak user U ₂ directly regards the signal of U ₁ as noise. , which decodes its own signal.

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