A secure transmission method for non-orthogonal multiple access communication systems based on cooperative interference strategy

Abstract

The present invention discloses a secure transmission method for a non-orthogonal multiple access communication system based on a coordinated interference strategy, which is mainly divided into three time slots. The first time slot mainly includes: a base station sends a superimposed signal, a far user sends an interference signal to suppress the eavesdropping ability of an untrusted relay, and a near user and an untrusted relay receive and decode the target signal; in the second time slot, the near user and the far user send an uplink non-orthogonal multiple access signal, and the base station and the untrusted relay decode the target signal; in the third time slot, the untrusted relay forwards the signals received in the first two time slots, the near user sends an uplink new signal, and the base station and the far user decode the target signal. The method of the present invention improves the spectrum efficiency of the system, and utilizes a coordinated interference strategy, and uses a user to send artificial noise to suppress the decoding ability of the eavesdropping node, i.e., the untrusted relay, without adding a dedicated interference terminal, to ensure that the system achieves high spectrum efficiency while achieving secure transmission.

Description

Technical Field

[0001] The present invention belongs to the technical field of wireless communication networks, and in particular to a wireless communication network security transmission scheme of direct transmission and relay cooperative transmission and non-orthogonal multiple access technology. Background Art

[0002] With the rapid development of the Internet of Things and wireless communication technology, higher requirements are placed on wireless communication systems in terms of data rate, latency, spectrum efficiency, communication coverage, and massive device connections. Non-orthogonal multiple access, as a technology with great potential, has greatly alleviated the pressure on wireless communication systems caused by scarce spectrum resources and massive device access. The present invention specifically refers to power domain non-orthogonal multiple access, which will not be described in detail below. Direct transmission and relay cooperative transmission technology not only improves the coverage of the network, but also makes full use of prior information to delete the interference of untrusted relay forwarding links to nearby users without affecting the performance of the system, thereby achieving the purpose of parallel transmission of communication links and further improving spectrum efficiency. Physical layer security is an information encryption technology developed on the basis of information theory. It is a supplementary technology that is different from traditional encryption methods such as cryptography. Specifically, the capacity difference between the main channel and the eavesdropping channel is increased to achieve the purpose of secure transmission. In theory, it is possible to achieve absolute security in information transmission.

[0003] As attached Figure 1 As shown in the figure, consider a downlink non-orthogonal multiple access communication model in which a near user and a far user form a pair of non-orthogonal multiple access users. N and U F The base station S uses superposition coding, the principle is: the signals of two users are superimposed and sent with different powers. N For a near user, the channel condition is better than that of a far user U F , the power allocated to it is less than that of the distant user U F , serial interference elimination is performed during decoding, first U N The signal is regarded as interference and decoded U F signal, remove U from the superimposed signal F signal, and then solve its own signal. F For weak users in non-orthogonal multiple access systems, U N The signal of the receiver is regarded as interference and its own signal is decoded. Non-orthogonal multiple access technology uses superposition coding at the transmitting end to superimpose the user information with weak channel conditions on the user information with strong channel conditions at a low code rate to achieve multi-user information superposition transmission. Serial interference cancellation technology is used at the receiving end to eliminate co-channel interference and improve the decoding signal-to-noise ratio.

[0004] The combination of direct transmission and relay cooperative transmission technology and non-orthogonal multiple access not only improves the coverage of the communication system, but also makes full use of prior information to remove interference, realizes parallel transmission of communication links, and further improves spectrum efficiency. Figure 2 As shown, the system consists of a base station S, a near user U N , a remote user U F and an untrusted relay node. Specific process: In the first time slot, the base station broadcasts a downlink non-orthogonal multiple access signal. N Based on serial interference cancellation, U is first decoded F The signal is removed and then decoded by the untrusted relay. The untrusted relay decodes the target signal x2 in the form of a far user. In the second time slot, the untrusted relay forwards the far user signal, and the base station sends a new signal to the near user. At this time, the near user has already decoded the far user signal and is not disturbed by the untrusted relay forwarding. The far user does not receive interference from the new signal of the base station, and the communication link is parallel. The system is described as a pair of near users directly served by the base station and a far user who needs the untrusted relay to assist in communicating with the base station in the wireless cellular network. By using the prior information obtained by the near user in the first time slot, the interference caused by the untrusted relay forwarding can be deleted, so as to achieve the purpose of parallel transmission without interfering with each other.

[0005] Physical layer security is based on the complex spatiotemporal characteristics of wireless channels, such as randomness and time-varying properties, and uses signal processing technology to achieve secure transmission of information. When the channel condition of the main channel is better than that of the eavesdropping channel, when information is transmitted between the sending and receiving ends of the legitimate user, there must be a coding method that can achieve an arbitrary small probability of information transmission error, and the eavesdropper cannot obtain all the useful information, thereby achieving the purpose of secure transmission. In the study of physical layer security, whether the channel state information of the eavesdropping node can be obtained has a significant impact on the security performance of the system. Generally speaking, the scenario in which the source node has the channel state information of the eavesdropping node is called active eavesdropping, otherwise it is called passive eavesdropping. In the active eavesdropping scenario, the source node can dynamically adjust the sending strategy based on the channel state information of the eavesdropping node to obtain a relatively secure communication scenario.

[0006] In the eavesdropping channel, due to the existence of eavesdropping nodes, the reliability and security of information transmission must be guaranteed at the same time. At this time, there is an upper limit to the information transmission rate of the system. The maximum information transmission rate of the system is the security rate, and its mathematical expression is as follows:

[0007]

[0008] where [x] + =max(0,x),C B and C E Represent the channel capacity of the main link and the eavesdropping link respectively, γ B and γE are the signal-to-noise ratios of the main link and the wiretap link, respectively. Commonly used performance indicators in the field of physical layer security research include traversal security rate and security interruption probability. The traversal security rate is defined as:

[0009]

[0010] in, To find the expectation operator, f(γ B ,γ E ) represents the joint probability density function of the signal-to-noise ratio of the destination node and the eavesdropping node, and the ergodic security rate is represented as the statistical average of the system security rate.

[0011] The communication system is designed by combining non-orthogonal multiple access, direct transmission and relay cooperative transmission technology with physical layer security theory. It not only achieves the advantages of high speed, massive connections and high spectrum efficiency, but also introduces security performance considerations, which will show great application prospects in the field of modern communications. Summary of the invention

[0012] For non-orthogonal multiple access systems with direct transmission and relay cooperative transmission, while pursuing higher system spectrum efficiency, more serious information leakage may occur. The present invention designs communication links based on physical layer network coding to improve system spectrum efficiency. At the same time, it uses a cooperative interference strategy to suppress the decoding ability of eavesdropping nodes by using artificial noise sent by users without adding dedicated interference terminals. The secure transmission scheme ensures that the system achieves high spectrum efficiency while achieving secure transmission.

[0013] In view of this, the technical solution adopted by the present invention is: a secure transmission method for a non-orthogonal multiple access communication system based on a coordinated interference strategy, including three time slots, and the specific steps are as follows:

[0014] Step S1: In the first time slot, based on the downlink non-orthogonal multiple access protocol, the base station S uses the superposition coding technology to superimpose the target signal x1 of the near user and the target signal x2 of the far user and then broadcasts them; at the same time, the far user U F Send the friendly interference signal z1 in the first time slot to suppress the eavesdropping node, that is, the untrusted relay; near user U N Based on the serial interference cancellation technology, x2 is decoded and removed, and x1 is decoded without interference; the untrusted relay eavesdrops on the user's target signals x1 and x2 based on the parallel interference cancellation technology;

[0015] Step S2: In the second time slot, based on the uplink non-orthogonal multiple access protocol, the near user U N The superposition signal of the first target signal x3 of the base station and the interference signal z2 in the second time slot is transmitted to the remote user U FThe base station sends the second destination signal x4; the base station removes the interference signal z2 and decodes x3, and the untrusted relay eavesdrops on the base station's destination signals x3 and x4 based on the parallel interference cancellation technology;

[0016] Step S3: In the third time slot, the untrusted relay R amplifies all received signals and forwards them to the nearest user U. N The superimposed signal of sending x3 and the third destination signal x5 of the base station is first decoded by the base station based on the serial interference elimination technology, and then decodes the signal x4 forwarded by the untrusted relay.

[0017] The beneficial effects of the present invention are mainly: proposing a novel non-orthogonal multiple access secure transmission method that integrates four technologies, namely direct transmission and relay cooperative transmission, non-orthogonal multiple access, network coding and physical layer security, while ensuring the high spectrum characteristics of the system and using a collaborative interference strategy to reduce the probability of confidential information being eavesdropped, and using the half-duplex shielding effect of an untrusted relay to ensure a positive ergodic security rate (x5) of the system to achieve secure transmission.

[0018] The beneficial effects of the present invention specifically come from the following three aspects:

[0019] (1) Use non-orthogonal multiple access and network coding to improve spectrum efficiency. In order to alleviate the shortage of spectrum resources, make full use of non-orthogonal multiple access technology to transmit multiple user information in the same resource block at the same time; use physical layer network coding to save communication time slots and improve system throughput. Figure 5 The Monte Carlo simulation results in show that the present invention can significantly enhance the system security performance compared with the benchmark scheme 1 which does not adopt network coding.

[0020] (2) Adopt cooperative interference and inter-user interference strategies. The inter-user interference strategy and friendly interference signals are used to improve the system security performance and reduce the probability of information being eavesdropped. It is worth noting that the use of superposition coding and idle users to send interference signals does not require the establishment of dedicated interference terminals. Figure 6 It is proved that the cooperative interference strategy proposed in the present invention enhances the security performance of the system compared with the traditional untrusted relay forwarding strategy.

[0021] (3) Balance the security and reliability of the system. The interference signal in the second time slot suppresses the decoding quality of the eavesdropping end on the one hand, and on the other hand, it divides the power of the useful signal and causes a decrease in communication reliability. Therefore, it is necessary to balance the security and effectiveness of the system. Figure 7 The safety and reliability of the trade-off system proposed in the present invention are proved. With the increase of the power allocation coefficient, the overall safety performance of the system increases first and then decreases, that is, there is an optimal power allocation coefficient that makes the safety performance of the system optimal. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a downlink non-orthogonal multiple access system;

[0023] Figure 2 It is a non-orthogonal multiple access system model for direct transmission and relay cooperative transmission;

[0024] Figure 3 It is a system model of direct transmission and relay cooperative transmission-non-orthogonal multiple access-physical layer network coding;

[0025] Figure 4 Exact solutions and lower bounds for ergodic safety and rate;

[0026] Figure 5 is the power allocation coefficient a S Impact on traversal safety and rate;

[0027] Figure 6 is the power allocation factor Impact on traversal safety and rate;

[0028] Figure 7 is the optimal power allocation coefficient

[0029] Figure 8 The overall implementation process of direct transmission and relay cooperative transmission-non-orthogonal multiple access-physical layer network coding transmission is provided. DETAILED DESCRIPTION

[0030] like Figure 3 As shown, the present invention considers a non-orthogonal multiple access hybrid communication network for uplink and downlink based on physical layer network coding, including a base station S as a source node, a near user U N , a remote user U F and an untrusted untrusted relay node R that implements the amplification and forwarding strategy. The so-called untrusted untrusted relay means that the node is trustworthy at the service level but untrustworthy at the data level. (If the untrusted relay node R adopts the decoding and forwarding strategy, the confidential information has actually been decoded, which is impractical for security reasons.) Assume that in this communication system, due to the obstruction of obstacles and severe fading, S and U F There is no direct transmission link, and the communication with the base station needs to be assisted by an untrusted relay node R. The channel coefficient, channel gain and average channel power between node i and node j are expressed as h ij 、|h ij | 2 and λ ij , where i,j∈(S,U N ,U F ,R) and i≠j. All channels are set to independent and identically distributed Rayleigh fading channels and satisfy hij =h ji ;h ji represents the channel coefficient between node j and node i. Assume and λ SR , and They are respectively represented as the average channel power between the base station and the untrusted relay, the base station and the near user, the near user and the untrusted relay, and the near user and the far user. The transmission power of the untrusted relay and the user is set to P U , the base station's transmission power is set to P S , the power of the signal is set to the normalized power. Each transmission time slot is equal and continuous.

[0031] like Figure 8 As shown, the overall implementation process of direct transmission and relay cooperative transmission-non-orthogonal multiple access-physical layer network coding transmission is mainly divided into three time slots. The first time slot mainly includes: the base station S sends a superimposed signal, the far user sends an interference signal, and the near user and the untrusted relay receive and decode the target signal; in the second time slot, the near user and the far user send an uplink non-orthogonal multiple access signal, and the base station and the untrusted relay decode the target signal; in the third time slot, the untrusted relay forwards the signals received in the first two time slots, the near user sends an uplink new signal, and the base station and the far user decode the target signal. Specifically, it includes the following steps:

[0032] The embodiment of the present invention is based on network coding technology and physical layer security technology. It uses physical layer network coding to improve the spectrum efficiency of the system. It adopts a cooperative interference strategy without adding a dedicated interference terminal. It uses the user to send artificial noise to suppress the decoding ability of the eavesdropping node. The core is to use the cooperative interference scheme to increase the channel capacity difference between the legitimate terminal and the eavesdropping terminal. The specific steps are as follows:

[0033] Step S1: Estimate the channel state information in the communication network by sending a pilot sequence (Reference [1]: L. Lv, H. Jiang, Z. G. Ding, L. Yang, J. Chen. Secrecy-enhancing design for cooperative downlink and uplink NOMA with an untrusted relay [J]. IEEE Transactions on Communications, 2020, 68(3): 1698-1715.) Specifically, the channel state information of the link between the base station and the untrusted relay and the link between the untrusted relay and the user.

[0034] Step S2: In the first time slot, based on the downlink non-orthogonal multiple access protocol, the base station S superimposes the target signal x1 of the near user and the target signal x2 of the far user through superposition coding and then broadcasts them, which is expressed as where a S is the power allocation coefficient of signal x1, and The transmission power of the base station is P S In order to suppress untrusted relay nodes, the remote user U F The interfering end sends a friendly interference signal z1 in the first time slot. The friendly interference signal can be a Gaussian pseudo-random sequence, or a deterministic waveform similar to the expected signal structure. (Reference [2]: Z. Rongqing, S. Lingyang, H. Zhu, J. Bingli. Physical layer security for two-way untrusted relaying with friendly jammers [J]. IEEE Transactions on Vehicular Technology, 2012, 61 (8): 3693-3704.) Therefore, the friendly interference signal only affects the receiving signal-to-noise ratio of the eavesdropping end. At this time, the received signal expression of the destination end is:

[0035]

[0036] In the formula, d∈{U N ,R}, It is represented as an additive Gaussian white noise at node d with power σ 2 .h Sd and They are respectively represented as the channel coefficients between the base station and the remote user and node d, P U is the user's transmission power, near user U N Based on serial interference cancellation, the distant user signal is first decoded. After removing the interference, the signal-to-noise ratio of decoded x1 is expressed as in represents the channel coefficient between the base station and the near user. At this time, the untrusted relay, as a potential eavesdropping node, decodes the confidential signal based on parallel interference cancellation, and the decoding signal-to-noise ratio is expressed as in h SR and represent the channel coefficients between the base station and the distant user and R respectively.

[0037] Step S3: In the second time slot, based on the uplink non-orthogonal multiple access protocol, the near user U N The superposition signal of the target signal x3 of the sending base station and the interference signal z2 in the second time slot is expressed as in is the power allocation coefficient of signal x3, and At the same time, remote user U F The base station sends the target signal x4 (the signal x4 is different from x3 at this time, the sending end is different and the signal content is also different). At this time, the received signal of the base station S and the untrusted relay R is expressed as

[0038]

[0039]

[0040] in and They are respectively represented as the additive white Gaussian noise at the base station and the untrusted relay in the second time slot, and Respectively represent the channel coefficients between the base station and the untrusted relay and the near user. Similar to step S2, the corresponding decoding signal-to-noise ratio: the decoding signal-to-noise ratio of the base station decoding the target signal x3 is expressed as The decoding signal-to-noise ratio of the untrusted relay is and in and denote the channel coefficients between the near user and the far user and the untrusted relay, respectively.

[0041] Step S4: In the third time slot, network coding and non-orthogonal multiple access protocols are used to jointly design forwarding and cooperative links to improve the spectrum efficiency of the system. At this time, the untrusted relay R forwards all the signals received in the first and second time slots after amplification. The amplification factor is expressed as in λ SR , and represents the average channel power between the base station and the untrusted relay, the untrusted relay and the distant user, and the untrusted relay and the near user respectively; (Reference [3]: L.Lv, Z.Li, H.Ding, J.Chen. Secure NOMA and OMA coordinated transmission schemes in untrusted relay networks [J]. Science China Information Sciences, 2021, 64 (10).) Near user U N Send the superposition signal of new signal x5 and x3, expressed as (Reference [4]: ​​HWLiu, ZGDing, KJKim, KSKwak, HVPoor. Decode-and-forward relaying for cooperative NOMA systems with directlinks [J]. IEEE Transactions on Wireless Communications, 2018, 17(12): 8077-8093.) Among them is the power allocation coefficient of signal x3, and For half-duplex untrusted relays, signal x5 cannot be eavesdropped and is perfectly safe. The purpose of transmitting x3 is to linearly cancel the far user U F The interference of the received untrusted relay forwarding signal. The corresponding received signal expression is:

[0042]

[0043]

[0044] In the formula and It is expressed as the received signal expression of the untrusted relay in the first two time slots, and It is represented by the additive Gaussian white noise of the untrusted relay in the first two time slots, and represents the additive Gaussian white noise of the base station and the remote user in the third time slot; Where steps a and b represent the destination eliminating known or decoded signals, and ω0 represents the remote user U F The overall expression related to the signal x3 is expressed as Base station S decodes the target signal based on the serial interference cancellation technology. The decoding order is x5→x4. The corresponding decoding signal-to-noise ratio is expressed as

[0045] In order to remove the signal x3 from the distant user U F The interference must satisfy ω0=0, and the resulting constraint is In order to determine the feasibility of the offset, the expected result of the constraint is:

[0046]

[0047] It is worth noting that It is not necessarily guaranteed To meet the power constraint, we set At this time, in order to obtain better performance, the decoding order of the far user is x5→x2, and the corresponding signal-to-noise ratio is

[0048]

[0049] Step S5: Compared with near users, far users are more likely to be attacked by eavesdropping nodes. To enhance the performance of far users, remove x3 for far user U F interference, need to meet Where G represents the amplification factor of the untrusted relay, represents the channel coefficients of the untrusted relay and the distant user, represents the channel coefficients of the untrusted relay and the near user, represents the channel coefficient between the near user and the far user, represents the power allocation coefficient of the second time slot near user x3, P U represents the user's transmit power, represents the power allocation coefficient of the near user x3 in the third time slot, and ω0 represents the power allocation coefficient of the far user U F The overall expression related to the signal x3 can only ensure that x3 is available to the remote user U when the overall expression value is zero. F No interference is generated. At the same time, the distant user decodes x5 and removes it before decoding its own target signal x2 to obtain better performance.

[0050] Step S6: Determine signal x j ,j∈(1,2,3,4,5) instantaneous safety rate is expressed as in Respectively represent the signal x j The signal-to-noise ratio of the legitimate end and the eavesdropping end.

[0051] Step S7: Determine two benchmark solutions to verify the superior performance of the proposed invention. Benchmark solution 1 does not use physical layer network coding to jointly design uplink and downlink links, and transmits 5 signals in four time slots; Benchmark solution 2 does not use a coordinated interference strategy.

[0052] The following is an analysis of the security performance of the system in the embodiment of the present invention. The traversal security and rate of the system are expressed as

[0053]

[0054] In the formula, Represented as signal x j The traversal safety rate, Using Jensen's inequality, the lower bound of the ergodic safety and rate is expressed as Based on probability theory, the ergodic rate is expressed as Where F γ (x) and fγ (x) is expressed as the cumulative distribution function and probability density function of the signal-to-noise ratio γ.

[0055] According to the definition of traversal rate and a series of mathematical derivations, the traversal safety and rate of the system are expressed as

[0056]

[0057] In the formula, φ2(x,y)=φ1(x)-φ1(y),

[0058] C=0.577215 is Euler's constant.

[0059] Define the function:

[0060]

[0061] like Figure 4 , the approximate gap between the exact solution and the lower bound of ergodic safety and rate. From the figure, we can see that the approximate gap between the exact solution and the lower bound of ergodic safety and rate is very small, which shows that it is reasonable and feasible to use the lower bound to approximate the exact solution.

[0062] like Figure 5 , power allocation coefficient a S The impact on traversal security and speed. From the figure, we can see that as a S As the number of users increases, the overall security rate of the system increases. This is because near users obtain better performance based on serial interference cancellation, while far users have limited performance due to inter-user interference and poor channel conditions. At this time, the rate at which the security rate of near users increases is much greater than the rate at which the far users decrease, so the security performance of the overall system is improved.

[0063] like Figure 6 , power allocation coefficient The impact on traversal security and speed. From the figure, we can see that as As the safety rate of the system increases, it increases first and then decreases, that is, there is an optimal This increases the safety performance of the overall system. As the signal x3 increases, the legitimate end capacity and the eavesdropping end capacity increase synchronously. The rate of increase of the eavesdropping end capacity has exceeded the rate of increase of the legitimate end, resulting in a decrease in the security performance of the system.

[0064] like Figure 7 : Optimal power allocation coefficient This is because the interference signal improves the safety performance of the system on the one hand, but reduces the transmission power of the useful signal on the other hand, reducing the reliability of the system. Figure 7 This shows that there is an optimal To balance the security and reliability of the system.

Claims

1. A secure transmission method for a non-orthogonal multiple access communication system based on a cooperative interference strategy, characterized in that: It includes three time slots, and the specific steps are as follows: Step S1: In the first time slot, based on the downlink non-orthogonal multiple access protocol, the base station S uses the superposition coding technology to superimpose the target signal x1 of the near user and the target signal x2 of the far user and then broadcasts them; at the same time, the far user U F Send the friendly interference signal z1 in the first time slot to suppress the eavesdropping node, i.e. the untrusted relay; Near User U N Based on the serial interference cancellation technology, x2 is decoded and removed, and x1 is decoded without interference; the untrusted relay eavesdrops on the user's target signals x1 and x2 based on the parallel interference cancellation technology; Step S2: In the second time slot, based on the uplink non-orthogonal multiple access protocol, the near user U N The superposition signal of the first target signal x3 of the base station and the interference signal z2 in the second time slot is transmitted to the remote user U F The base station sends the second destination signal x4; the base station removes the interference signal z2 and decodes x3, and the untrusted relay eavesdrops on the base station's destination signals x3 and x4 based on the parallel interference cancellation technology; Step S3: In the third time slot, the untrusted relay R amplifies all received signals and forwards them. The amplification factor is expressed as in σ 2 is the power, λ SR , and They represent the average channel power between the base station and the untrusted relay, the untrusted relay and the far user, and the untrusted relay and the near user, respectively. S Represents the transmission power of the base station, P U Indicates the transmission power of an untrusted relay or user; U N The superposition signal of sending x3 and the third destination signal x5 of the base station is expressed as in is the power allocation coefficient of signal x3, and Based on the serial interference cancellation technology, the base station first decodes the signal x5 sent by the near user and then decodes the signal x4 forwarded by the untrusted relay. The corresponding received signal expression is: in and The expression of the received signal of the untrusted relay in the first two time slots is, and It is represented by the additive Gaussian white noise of the untrusted relay in the first two time slots, They are represented as the additive white Gaussian noise at the base station and the remote user in the third time slot, Step a and step b represent the destination eliminating known or decoded signals, ω0 represents the overall expression related to the distant user and x3, which is expressed as Base station S decodes the target signal based on the serial interference cancellation technology. The decoding order is x5→x4. The corresponding decoding signal-to-noise ratio is expressed as To remove x3 from the remote user U F interference, need to meet Where G represents the amplification factor of the untrusted relay, represents the channel coefficients of the untrusted relay and the distant user, represents the channel coefficients of the untrusted relay and the near user, represents the power allocation coefficient of the near user in the second time slot allocated to x3, and P U represents the user's transmit power, represents the power allocation coefficient of the near user x3 in the third time slot, and ω0 represents the power allocation coefficient of the far user U F The overall expression related to the signal x3 is, represents the channel coefficient between the near user and the far user, h SR represents the channel coefficient between the base station and the untrusted relay, a S is the power allocation coefficient of signal x1, and Represents the channel coefficient between the base station and the nearby user.

2. The secure transmission method of a non-orthogonal multiple access communication system based on a cooperative interference strategy according to claim 1, characterized in that: The friendly interference signal is a Gaussian pseudo-random sequence, or utilizes a deterministic waveform similar to the expected signal structure. The friendly interference signal only affects the receiving signal-to-noise ratio of the eavesdropping end.

3. The secure transmission method of a non-orthogonal multiple access communication system based on a cooperative interference strategy according to claim 2, characterized in that: The received signal expression of the destination end in the first time slot is: In the formula, d∈{U N ,R}, represents the additive white Gaussian noise at node d, with power σ 2 ,h Sd represents the channel coefficient between the base station and d, represents the channel coefficient between the distant user and d, P U The user's transmission power, z1 represents the friendly interference signal near user U in the first time slot N Based on serial interference cancellation, the distant user signal is first decoded. After removing the interference, the signal-to-noise ratio of decoded x1 is expressed as in It is represented as the channel coefficient between the base station and the near user, a S is the power allocation coefficient of signal x1, and The decoding signal-to-noise ratios of untrusted relay decoding x1 and x2 are expressed as in h SR and They are represented as the channel coefficients between the base station and the distant user and the untrusted relay, respectively.

4. The secure transmission method of a non-orthogonal multiple access communication system based on a cooperative interference strategy according to claim 1, characterized in that: The received signals of the base station S and the untrusted relay R in step S2 are expressed as in and They are respectively represented as the additive white Gaussian noise at the base station and the untrusted relay in the second time slot, represents the channel coefficient between the near user and the untrusted relay, and the corresponding decoding signal-to-noise ratio: the decoding signal-to-noise ratio of the base station decoding the target signal x3 is expressed as The decoding signal-to-noise ratios of untrusted relay decoding x3 and x4 are and 5. The secure transmission method of a non-orthogonal multiple access communication system based on a cooperative interference strategy according to claim 1, characterized in that: In order to remove the signal x3 from the distant user U F The interference must satisfy ω0=0, and the resulting constraint is In order to determine the feasibility of the offset, the expected result of the constraint is: To meet the power constraint, set At this time, in order to obtain better performance, the decoding order of the far user is x5→x2, and the corresponding signal-to-noise ratio is 6. The secure transmission method for a non-orthogonal multiple access communication system based on a coordinated interference strategy according to any one of claims 1 to 5, characterized in that: Signal x j ,j∈(1,2,3,4,5) instantaneous safety rate is expressed as in Respectively represent the signal x j The signal-to-noise ratio of the legitimate end and the eavesdropping end.

Images