A covert communication method based on communication and perception integration in downlink NOMA system

Abstract

This invention relates to a covert communication method based on integrated communication and perception in a downlink NOMA system. Based on the characteristics of the downlink NOMA system, it combines the dual-function signals (perception signal and communication signal) transmitted by the radar. This method not only detects the target but also enables communication with legitimate users while resisting eavesdroppers. To optimize the waveform emitted by the radar transmitter, the invention uses a continuous convex approximation algorithm to solve the convex optimization problem of the beamforming signal and balances the value of the concealment factor to improve system performance. This technology can more effectively ensure system security and reliability, while achieving higher spectrum utilization and better addressing spectrum shortage issues.

Description

Technical Field

[0001] The present invention relates to the field of wireless communication technologies, and in particular to a covert communication method based on communication perception integration in a downlink NOMA system. Background Art

[0002] With the continuous development of mobile communication technology, the increasing coupling and promotion between air interface capabilities and new business needs are driving the development of the sixth generation mobile communication system (6G), the next generation mobile communication network for 2030, towards the construction of a multi-dimensional capability system. Communication and perception integration technology enables the system to have both communication and perception capabilities, improving system spectrum efficiency and hardware resource utilization, and reducing application costs. As one of the key enabling technologies of 6G, communication and perception integration can enable mobile cellular networks to have new perception capabilities such as speed measurement, ranging, positioning, target imaging and recognition, meeting the new needs of intelligent scenarios such as smart transportation, drone monitoring, autonomous driving environmental perception, and robot interaction.

[0003] Due to the large-scale device connectivity, integrated communication and awareness systems are likely to encounter spatially correlated channels and user overload in future wireless networks. Conventional multi-antenna technologies, when channels are highly correlated or spatial degrees of freedom are insufficient, communication users will suffer from severe inter-user interference. In a NOMA system, communication users in the power domain can be reused and inter-user interference can be mitigated by utilizing sequential interference cancellation techniques, providing additional degrees of freedom. Furthermore, compared to traditional multiple access technologies, NOMA can serve more users, achieving higher spectrum efficiency and improving the performance of integrated communication and awareness systems.

[0004] With the rapid increase in wireless communication transmission rates, information security is gaining increasing attention. However, traditional information encryption and physical layer security technologies cannot completely address privacy concerns, as eavesdroppers can obtain critical encrypted information by analyzing eavesdropped traffic data. Covert communication, also known as low-probability-of-detection communication, allows for concealed information transmission between communicating parties, preventing malicious eavesdroppers from detecting the communication signal. This process, known as signal concealment, makes it difficult for malicious users to confirm the signal's presence and carry out further illegal activities. Therefore, researchers are committed to exploring new technologies to enhance the security and reliability of covert communication, aiming to achieve more secure transmission. Covert communication is an important technology that can meet the needs of user groups with specific security and reliability requirements, such as government agencies, military organizations, and financial institutions.

[0005] For the next-generation mobile communication network (6G) expected in 2030, covert communication based on integrated communication and perception in the downlink NOMA system has become a key research technology. As mobile communications develop, spectrum resources are becoming increasingly scarce. NOMA technology can improve spectrum utilization efficiency while meeting user experience requirements. Furthermore, covert communication can more effectively transmit information in wireless signal transmission environments. Therefore, covert communication technology based on integrated communication and perception in NOMA systems will become even more important in this context. Summary of the Invention

[0006] The purpose of the present invention is to provide a covert communication method based on integrated communication and perception in a downlink NOMA system, so that the radar can communicate with legitimate users under the monitoring of the listener and perceive the target at the same time, thereby improving the spectrum utilization of the system.

[0007] To achieve the above-mentioned purpose, the technical solution of the present invention is: a covert communication method based on integrated communication and perception in a downlink NOMA system, a covert communication system with integrated communication and perception in a downlink NOMA system is designed, including a transmitter radar, legal user 1, legal user 2, a target and a listener, the transmitter radar combines the communication signal and the perception signal to form a dual-function signal, and transmits it to the target, legal user 1, and legal user 2. At the same time, the listener can also obtain the dual-function signal; based on the covert communication system, the method optimizes the waveform transmitted by the transmitter radar and weighs the value of the system concealment factor so that the transmitter radar communicates with the legal user 1 and the legal user 2 under the monitoring of the listener, and perceives the target at the same time.

[0008] In one embodiment of the present invention, the method is specifically implemented in the following steps:

[0009] Step S1: Construct a corresponding communication model based on the characteristics of the downlink NOMA system and describe its communication expression and communication rate expression;

[0010] Step S2: Build a perception model based on the characteristics of the communication model, and build a monitoring model at the same time, describe the monitoring expression, and give the error detection probability;

[0011] Step S3: Improve the performance of the system by optimizing the beamforming vector of the transmitter radar and the value of the weight coefficient concealment factor.

[0012] In one embodiment of the present invention, in step S1, the constructed communication model is expressed as:

[0013]

[0014] Where y c is the signal received by the communication user, and the transmitter radar transmits the signal xi =w i s i (i=1,2), w i (i=1,2) represents the beamforming vector of the transmitter radar when transmitting, s i (i=1,2) represents the signal transmitted by the transmitter before radar beamforming, and satisfies Where E represents the expected operation, and P aui =||w i || 2 (i=1,2) to represent the transmission power of the transmitter radar to the legal user 1 and the legal user 2 respectively, where ||·|| 2 It means to find the square of the two norm, is the channel vector between the transmitter radar and the legitimate user i, H represents the conjugate transpose operation, n b is the additive Gaussian white noise at the legitimate user, which has a mean of 0 and a variance of σ b 2 The complex Gaussian distribution of

[0015] In the downlink NOMA system, after the serial interference cancellation technology, the actual achievable rates R1 and R2 of legal user 1 and legal user 2 are obtained as follows:

[0016]

[0017]

[0018] where |·| 2 is the modular square operation.

[0019] In one embodiment of the present invention, step S2 is specifically implemented as follows:

[0020] The communication waveform is used to perceive the target. The perception model is:

[0021]

[0022] Where R t is the covariance matrix, K is the number of legal users;

[0023] At the same time, the detection power P(θ m )for:

[0024] P(θ m )=a H (θ m )R t a(θ m )

[0025] in For the target direction, is the perception area, a(θ m ) is the steering vector;

[0026] The cross-correlation expression of different user waveforms is expressed as:

[0027] C(θ1,θ2)=|a H (θ1)R t a(θ2)|

[0028] Where |·| represents the modulo operation;

[0029] Next, in the downlink NOMA system, legitimate user 1 is a weak user and legitimate user 2 is a strong user. For the communication between the listener and legitimate user 1, a random power transmission scheme is adopted to construct a monitoring model, describe the binary detection expression, and derive the false detection probability, which is specifically:

[0030]

[0031] Transmit power at Continuous uniform distribution on is the maximum power of the transmitter radar signal transmitted to the legal user 1, is the probability density function of the transmitted power; and α1, α2 are power allocation factors;

[0032] The signal y received by the listener w for:

[0033]

[0034] h aw is the channel vector between the transmitter radar and the listener, n w is the additive Gaussian white noise at the legitimate user, which has a mean of 0 and a variance of σ w 2 The complex Gaussian distribution of H0 means that the transmitter radar does not send a signal to the legitimate user 2, so the listener does not receive the signal. H1 means that the transmitter radar sends a signal to the legitimate user 2, so the listener receives the signal.

[0035] The listener's false detection probability ξ is:

[0036]

[0037] where τ represents the threshold for radar detection of the transmitter, At the same time, the minimum detection probability α1 is obtained at τ=ρ1.

[0038] In one embodiment of the present invention, step S3 is specifically implemented as follows:

[0039] Considering the radar's transmission power and the system's minimum transmission rate, we construct the optimization equation:

[0040]

[0041] stR1≥R min

[0042]

[0043]

[0044] C≤δ

[0045] α1≥1-ε

[0046] ρ c ≥0,ρ r ≥0 is the orthogonalization parameter, R min , P0 represent the minimum transmission rate and total transmission power of the covert communication system based on the downlink NOMA system, N is the number of antennas, δ is the upper bound, and ε is the concealment factor;

[0047] The above optimization equation solves the rank-one problem through a continuous convex approximation algorithm and a method of converting constraint terms into penalty terms. Finally, the optimization equation is converted into a convex problem, and the optimal solution is obtained through convex optimization.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] (1) The present invention considers the integrated communication and sensing system under the downlink NOMA system, transmits dual-function signals for communication and sensing at the same time, and reduces the interference between users by utilizing the serial interference cancellation technology, thereby providing additional degrees of freedom and improving spectrum utilization efficiency.

[0050] (2) The present invention uses covert communication in downlink NOMA to assist in proposing a communication perception integrated system, in which the security and reliability of the system can be more effectively guaranteed.

[0051] (3) The present invention considers using the false detection probability to evaluate the performance of the radar in the radar perception model in the downlink NOMA system. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 It is a schematic diagram of the system structure of the present invention.

[0053] Figure 2 3 is a comparison chart of simulation results and numerical results for both false alarm rate and missed detection rate in one embodiment of the present invention.

[0054] Figure 3 This is a simulation diagram of system perception power and communication rate when the concealment factor takes different values ​​in one embodiment of the present invention. DETAILED DESCRIPTION

[0055] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings.

[0056] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present application belongs.

[0057] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0058] according to Figure 1 We propose a covert communication system based on integrated communication and perception in the downlink NOMA system, which consists of a transmitter radar, legitimate user 1, legitimate user 2, target and listener; the transmitter radar combines the communication signal and the perception signal to form a dual-function signal, and transmits it to the target and the legitimate receiver user. At the same time, the listener can also obtain this dual-function signal.

[0059] The radar system is equipped with N antennas. The radar hopes to minimize the probability of being detected by eavesdroppers and to communicate covertly with legitimate receiver users.

[0060] The communication model from the radar to the legitimate user can be expressed as:

[0061]

[0062] Where y c is the signal received by the communication user, and the radar transmission signal is x i =w i s i (i=1,2), w i (i=1,2) represents the beamforming vector when the radar transmits, s i (i=1,2) represents the signal transmitted before radar beamforming, and satisfies Where E represents the expected operation, and P aui =||w i ||2 (i=1,2) to represent the transmission power of the transmitter radar to the legal user 1 and the legal user 2, where ||·|| 2 It means to find the square of the two norm. is the channel vector between the radar and the legitimate user i, H represents the conjugate transpose operation, n b is the additive Gaussian white noise at the legitimate user, which has a mean of 0 and a variance of σ b 2 The complex Gaussian distribution of

[0063] In the downlink NOMA system, after using the serial interference cancellation technology, the actual achievable rate for legitimate user 1 and legitimate user 2 is:

[0064]

[0065]

[0066] where |·| 2 is the modular square operation.

[0067] NOMA is a 5G wireless access technology that improves the performance of next-generation cellular communications. Compared to orthogonal frequency division multiple access (OFDMA), NOMA achieves better spectrum efficiency, higher cell-edge throughput, relaxed channel feedback, and low transmission latency.

[0068] The basic idea of ​​serial interference cancellation technology is to adopt a step-by-step interference cancellation strategy. In the received signal, users are judged one by one. After amplitude recovery, the multi-access interference generated by the user signal is subtracted from the received signal. The remaining users are judged again, and the operation is repeated until all multi-access interference is eliminated.

[0069] Through the radar perception model, communication waveforms can be used to perceive targets:

[0070]

[0071] Where R t is the covariance matrix, and K is the number of legal users.

[0072] At the same time, the detection power in the target direction is:

[0073] P(θ m )=a H (θ m )R t a(θ m )

[0074] in For the target direction, is the perception area, a(θ m ) is the steering vector.

[0075] The cross-correlation expression of different user waveforms is expressed as:

[0076] C(θ1,θ2)=|a H (θ1)R t a(θ2)|

[0077] where |·| represents the modulo operation.

[0078] The system model of the present invention involves two legitimate communication users, and uses communication waveforms for target perception. The smaller the cross-correlation between the two communication waveforms, the smaller the interference.

[0079] Next, in the NOMA system, legitimate user 1 is set as a weak user and legitimate user 2 is set as a strong user. For the communication between the listener and legitimate user 1, a random power transmission scheme is adopted, and the transmission power probability density function is:

[0080]

[0081] Transmit power at Continuous uniform distribution on The maximum power of the radar signal transmitted to the legitimate user 1, while satisfying α1, α2 are power allocation factors.

[0082] By building a monitoring model, the signal received by the listener is:

[0083]

[0084] where h aw is the channel vector between the radar and the listener, n w is the additive Gaussian white noise at the legitimate user, which has a mean of 0 and a variance of σ w 2 The complex Gaussian distribution of

[0085] H0 indicates that the radar did not transmit a signal to legitimate user 2, and therefore the listener did not receive the signal. H1 indicates that the radar transmitted a signal to legitimate user 2, and therefore the listener received the signal. Listener detection can result in two error conditions: a false alarm, in which the listener mistakenly determines that a signal was transmitted, even though the radar transmitter and legitimate user 2 did not actually transmit a signal. A missed detection, in which the radar transmitter and legitimate user 2 actually transmitted a signal, but the listener mistakenly determines that a signal was not transmitted. These two error probabilities are the false alarm rate and the missed detection rate. The false detection probability is equal to the sum of the false alarm rate and the missed detection rate, and can be used to measure the system's stealth performance.

[0086] The probability of false detection by the listener is:

[0087]

[0088] Where τ represents the threshold of radar detection, At the same time, the minimum detection probability ξ is obtained at τ = ρ1 * =α1.

[0089] Generally speaking, the eavesdropper attempts to find an optimal detector that can detect the transmission between the radar transmitter and the legitimate user 2 with the minimum probability of false detection. Correspondingly, the goal of ensuring communication concealment is: no matter what strategy the eavesdropper adopts, for any given small concealment factor ε, it must always satisfy ξ. * ≥1-ε.

[0090] Taking all the strategies mentioned above into account, and taking into account the radar's transmit power and the system's minimum transmission rate, we can get the optimization equation:

[0091]

[0092] stR1≥R min

[0093]

[0094]

[0095] C≤δ

[0096] α1≥1-ε

[0097] where ρ c ≥0,ρ r ≥0 is the orthogonalization parameter, R min represents the minimum transmission rate of the system, P0 is the total system power, N is the number of antennas, δ is the upper bound of correlation, and ε is the concealment factor.

[0098] The above beam optimization problem can be solved by using a continuous convex approximation algorithm and a method of converting constraint terms into penalty terms to solve the rank-one problem. Finally, the optimization problem is converted into a convex problem, and the optimal solution can be obtained by a convex optimization solver (such as CVX).

[0099] Continuously convex approximation algorithms transform non-convex optimization problems into continuously convex ones through continuous convexification. These algorithms then find their approximate solutions using convex optimization algorithms such as gradient descent and first-order Taylor expansion. Specifically, when solving non-convex optimization problems, continuous convex approximation algorithms transform the problem into a continuously convex one and gradually approach the optimal solution to the original problem by continuously transforming the objective function. "Continuously convexification" here refers to approximating the original objective function through a series of convex functions, resulting in a continuous convex function within a certain interval. For example, continuous convexification can be achieved through piecewise linear approximation or piecewise quadratic approximation.

[0100] In summary, an optimal beamforming method can be obtained, and then the beamforming method can be deployed in the radar to achieve the optimal communication solution.

[0101] After careful research, this paper proposes a covert communication method based on integrated communication and perception in the downlink NOMA system. By combining the communication signal and perception signal emitted by the radar, the dual purposes of communication and perception can be achieved while resisting eavesdropping. This paper also proposes a continuous convex approximation algorithm to solve the convex optimization problem of beamforming signals, thereby optimizing the radar waveform. In summary, the covert communication method based on integrated communication and perception in the downlink NOMA system proposed in this paper can more effectively achieve higher spectrum utilization while ensuring system security and reliability, reduce interference between legitimate users, and help achieve secure and efficient communication.

[0102] Figure 2 This is a comparison chart of the simulation results and numerical results for the false alarm rate and missed detection rate in the examples of the present invention. The missed detection rate and the detection probability add up to 1. The numerical results are the results derived from mathematical models and mathematical methods, which can be used to describe the characteristics of the system. The simulation results, on the other hand, are the results of research conducted by simulating the actual behavior of the system. The numerical results are more accurate, but often require more calculations, while the simulation results are more practical but are limited by the accuracy of the simulation. Based on this figure, it can be seen that the formulas for the false alarm rate and detection probability in the examples of the present invention are consistent with the simulation results.

[0103] Figure 3 This figure shows simulations of system perception power and communication rate for different values ​​of the concealment factor in an example of the present invention. When the radar transmitter power is constant, the system performs both communication and target perception. As the communication rate increases, the target perception power initially rises and then decreases, which is consistent with actual conditions. As the communication rate increases, for a short period of time, part of the system power is used for communication and part for target perception. Once the communication rate reaches a certain value, the total system transmit power remains constant, so while communication is guaranteed, the power used for target perception decreases.

[0104] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other manner. Any person skilled in the art may utilize the above-disclosed technical content to modify or modify the present invention into equivalent embodiments. However, any simple modifications, equivalent variations, and modifications to the above embodiments that do not depart from the technical content of the present invention and are based on the technical essence of the present invention remain within the scope of protection of the present invention.

[0105] The above are preferred embodiments of the present invention. Any changes made according to the technical solution of the present invention, as long as the resulting functions and effects do not exceed the scope of the technical solution of the present invention, shall fall within the scope of protection of the present invention.

Claims

1. A covert communication method based on communication perception integration in a downlink NOMA system, characterized in that: Design a covert communication system that integrates communication and perception in the downlink NOMA system, including a transmitter radar, legal user 1, legal user 2, a target, and an observer. The transmitter radar combines the communication signal and the perception signal to form a dual-function signal, which is transmitted to the target, legal user 1, and legal user 2. At the same time, the observer can also obtain the dual-function signal. Based on the covert communication system, the method optimizes the waveform transmitted by the transmitter radar and balances the value of the system's concealment factor, allowing the transmitter radar to communicate with legitimate user 1 and legitimate user 2 under the monitoring of the listener, while also sensing the target. The specific implementation steps of the method are as follows: Step S1: Construct a corresponding communication model based on the characteristics of the downlink NOMA system and describe its communication expression and communication rate expression; Step S2: Build a perception model based on the characteristics of the communication model, and build a monitoring model at the same time, describe the monitoring expression, and give the error detection probability; Step S3: improving the performance of the system by optimizing the beamforming vector and the value of the weight coefficient concealment factor of the transmitter radar; Step S2 is specifically implemented as follows: The communication waveform is used to perceive the target. The perception model is: Where R t is the covariance matrix, K is the number of legal users, w i represents the beamforming vector of the transmitter radar when transmitting, i=1,2; At the same time, the detection power P(θ m )for: P(θ m )=a H (i m )R t a(θ m ) in For the target direction, is the perception area, a(θ m ) is the steering vector; The cross-correlation expression of different user waveforms is expressed as: C(θ1,θ2)=|a H (θ1)R t a(θ2)| Where |·| represents the modulo operation; Next, in the downlink NOMA system, legitimate user 1 is a weak user and legitimate user 2 is a strong user. For the communication between the listener and legitimate user 1, a random power transmission scheme is adopted to construct a monitoring model, describe the binary detection expression, and derive the false detection probability, which is specifically: Transmit power at Continuous uniform distribution on is the maximum power of the transmitter radar signal transmitted to the legal user 1, is the probability density function of the transmitted power; and α1, α2 are power allocation factors, P au1 、P au2 They represent the transmission power of the transmitter radar to the legal user 1 and the legal user 2 respectively; The signal y received by the listener w for: h aw is the channel vector between the transmitter radar and the listener, n w is the additive Gaussian white noise at the legitimate user, which has a mean of 0 and a variance of σ w 2 The complex Gaussian distribution of H0 means that the transmitter radar does not send a signal to the legitimate user 2, so the listener does not receive the signal. H1 means that the transmitter radar sends a signal to the legitimate user 2, so the listener receives the signal. The listener's false detection probability ξ is: where τ represents the threshold for radar detection of the transmitter, At the same time, the minimum detection probability α1 is obtained at τ=ρ1.

2. A covert communication method based on integrated communication and perception in a downlink NOMA system according to claim 1, characterized in that: In step S1, the constructed communication model is expressed as: Where y c is the signal received by the communication user, and the transmitter radar transmits the signal x i =w i s i , w i represents the beamforming vector of the transmitter radar when transmitting, s i Represents the signal transmitted by the transmitter before radar beamforming, and satisfies Where E represents the expected operation, and P aui =||w i || 2 To represent the transmission power of the transmitter radar to legal user 1 and legal user 2, where ||·|| 2 It means to find the square of the two norm, is the channel vector between the transmitter radar and the legitimate user i, H represents the conjugate transpose operation, i = 1, 2, n b is the additive Gaussian white noise at the legitimate user, which has a mean of 0 and a variance of σ b 2 The complex Gaussian distribution of In the downlink NOMA system, after the serial interference cancellation technology, the actual achievable rates R1 and R2 of legal user 1 and legal user 2 are obtained as follows: where |·| 2 is the modular square operation.

3. A covert communication method based on integrated communication and perception in a downlink NOMA system according to claim 1, characterized in that: The specific implementation in step S3 is as follows: Considering the transmission power of the transmitter radar and the minimum transmission rate of the system, the optimization equation is constructed: s.t.R1≥R min C≤δ α1≥1-ε ρ c ≥0,ρ r ≥0 is the orthogonalization parameter, R min , P0 represent the minimum transmission rate and total transmission power of the covert communication system based on the downlink NOMA system, N is the number of antennas, δ is the upper bound, and ε is the concealment factor; The above optimization equation solves the rank-one problem through a continuous convex approximation algorithm and a method of converting constraint terms into penalty terms. Finally, the optimization equation is converted into a convex problem, and the optimal solution is obtained through convex optimization.

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