A chirp signal index modulation covert communication method

Abstract

The present application provides a chirp signal index modulation covert communication method, which transmits signals through digital beamforming DBF technology to provide physical layer communication security at a specific azimuth angle, and uses intelligent reflective surface auxiliary communication technology to improve channel gain while enhancing the communication concealment of directional transmission. Simulation results show that compared with existing covert communication schemes, the modulation covert communication method of the present invention has better bit error rate BER performance and higher communication concealment gain.

Description

Technical Field

[0001] The present application relates to the technical field of integrated communication and perception, and in particular to a frequency-shifted chirp spread spectrum index modulation covert communication method of a dual-function radar communication system assisted by an intelligent reflective surface. Background Art

[0002] Communication and perception integration is a new information processing technology that realizes the coordination of communication and perception functions based on software and hardware resource sharing or information sharing. It is one of the key technologies that can potentially enable 6G. Among them, military or civilian Internet of Vehicles, Internet of Things, drone communication and other technologies have widely realized communication and radar. Although communication and radar have similar implementation principles, in traditional design schemes, communication and radar rely on two sets of software and hardware equipment to realize their respective functions, resulting in high hardware costs and low resource utilization of communication or radar.

[0003] When realizing the collaboration between radar and communication, the ones with the least impact on radar operation can be divided into three categories, namely beam mode modulation, index modulation IM and fast time modulation. Among them, IM has the advantages of low power consumption, low cost and simple hardware implementation, and has been widely studied in communication systems. In the research on communication perception integration, especially communication radar integration, IM can transmit additional information by transmitting DFRC inherent or achievable physical resources, such as spatial modulation through different antenna activation states, and orthogonal frequency division multiple access index modulation OFDM-IM by selecting different subcarriers to send. However, most IMs cannot be used to transmit radar signals, or additional hardware equipment for IM transmission entities is required in the system that transmits radars.

[0004] In addition, in traditional digital mobile communications, broadcast communications are often used to transmit communication signals in all spatial directions, which will create hidden dangers in physical layer communication security. In covert communication schemes, channel state information needs to be obtained in advance for digital beamforming DBF, which will generate a large amount of pilot overhead and other resource consumption. Summary of the invention

[0005] The present application provides a chirp signal index modulation covert communication method, which involves dual-function radar communication system technology, digital beamforming, index modulation technology and RIS-assisted communication technology. The dual-function radar communication system DFRC designed based on this method sends the same form of frequency-shifted chirp spread spectrum index modulation FSCSS-IM signal in both radar and communication working modes, which greatly reduces the hardware cost while realizing the integration of communication perception. The present application adopts the following technical solutions:

[0006] A chirp signal index modulation covert communication method, the method comprising the following steps:

[0007] Step 1: constructing a dual-function radar communication system RIS-DFRC assisted by an intelligent reflective surface, wherein the operating state of the dual-function radar communication system includes two working modes: radar working and communication working;

[0008] Step 2: In the radar working mode, the dual-function radar communication system DFRC base station obtains the azimuth and radial distance of the user target by frequency-shift chirp spread spectrum index modulation FSCSS-IM signal, and transmits specific communication bits to the user target;

[0009] Step 3: In the communication working mode, the position information of the user target measured in the radar working mode is used to obtain channel estimation, and the bits of FSCSS-IM signal transmission communication are sent to the user target, and covert communication is achieved through RIS auxiliary communication and digital beamforming method.

[0010] Further, in step 2, the working process of the radar working mode of the dual-function radar communication system is as follows:

[0011] Step 201: Group the input bit stream of the radar working mode into groups B 1 bit, used to select the FSCSS-IM joint mapping codebook S = {s 1 ,s 2 ,K,s q ,K,s Q The transmitted signal s in q ,in It is represented by the number of bits in each group after the bit stream is grouped, q = 1, 2, ···, Q represents the index value of the transmitted signal in S, It is expressed as the number of combinations of selecting L out of W chirp signals for superposition, represents the binomial coefficient, Indicates rounding down;

[0012] Step 202: In radar working mode, generate a FSCSS-IM joint mapping codebook S;

[0013] Step 203: Select the transmission signal s in the joint mapping codebook S according to the input bits. q (t), s q (t) is s q Expressions in the time domain;

[0014] Step 204: In the radar working mode, the radar signal is transmitted through the two paths of "base station-user target-base station" and "base station-RIS-user target-base station", and the echo signal received by the DFRC base station Expressed as

[0015]

[0016] Among them, α is the reflection factor of the user target, ω is the weight vector of digital beam forming, and ω is set to a dimension of N in the radar working mode. B ×1 full 1 matrix; V B (t) for obedience Complex Gaussian distributed noise function; is the RIS reflection matrix, which is set to a unit matrix with dimension N×N in the radar working mode; s q (t-τ 1 ) and s q (t-τ 2 ) is the transmitted signal s m (t) respectively after a time delay τ 1 and τ 2 The signal after Indicates the delay caused by the signal passing through the "base station-user target-base station" path, represents the delay of the signal through the path of “base station-RIS-user target-base station”, ξ BR is the distance between the DFRC base station and the RIS, ξ RT is the distance between RIS and the user's target, ξ BT is the distance between the DFRC base station and the user target;

[0017] Step 205: Perform zero forcing detection at the receiving end of the user target, and the calculation formula is as follows:

[0018]

[0019] Among them, s i is the signal in the joint mapping codebook S, Functions are used to calculate The maximum value of s i Index

[0020] Further, in step 204, the Rice fading channel between the DFRC base station and the RIS Expressed as

[0021]

[0022] H LOS is the direct path of the Rice fading channel H, H NLOS is the indirect path of the Ricean fading channel H, H NLOS The channel elements obey CN(0 ,1) Complex Gaussian distribution; ξ BT Angle θ RT ,θ BR ,θBT are departure angles, θ RT Defined as the angle between RIS and the target; θ BR Defined as the angle between the DFRC base station antenna array and the RIS antenna array; θ BT Defined as the angle between the DFRC base station antenna array and the user target; angle θ RB ,θ TB is the angle of arrival, θ RB Defined as the angle between the RIS antenna array and the DFRC base station antenna array; θ TB It is defined as the angle between the DFRC user target and the base station antenna array; for a r (θ TB ) is the transpose of; in the dual-function radar communication system, θ RB =θ BR ; a t (θ RT ),a t (θ BR ),a t (θ BT ) are all steering vectors, which respectively indicate the direction to θ RT ,θ BR ,θ BT The weights of the transmitted beams in the direction are expressed as

[0023]

[0024] a r (θ RB ) and a r (θ TB ) are respectively expressed as

[0025]

[0026] where a r (θ RB ), a r (θ TB ) are response vectors, a r (θ RB ) represents the receiving response of the RIS array to the signal transmitted by the DFRC base station, a r (θ RB ) represents the receiving response of the DFRC base station to the user target reflected echo signal.

[0027] Further, in step 204, when the time delay difference of the radar signal passing through the two paths is less than the radar range gate, that is, The delay difference between the two paths can be ignored. After removing the sampling time index, the discrete time signal matrix Y received by the user target end isT Simplified expression:

[0028] Y T =(H d +(GΘH))ωs q +V T

[0029] =a t (θ BT )ωs q +a t (θ RT )ΘHωs q +V T (9)

[0030] It is represented as the direct path channel between the DFRC base station and the user target, set to H d =a t (θ BT ); G represents the direct path channel between RIS and the user target, set to G = a t (θ BT );s q is the discrete time matrix of the transmitted signal, V T It is represented as the noise discrete time matrix of the user target receiver; To obey The complex Gaussian noise vector, f s is the sampling rate of the digital signal.

[0031] Further, in step 3, the working process of the communication working mode of the dual-function radar communication system is as follows:

[0032] Step 301: Use the azimuth of the user target obtained by the radar working mode to estimate the direct channel, use digital formation technology to achieve covert communication, and share the radar information to the RIS end to enhance the communication concealment;

[0033] Step 302: In the communication working mode, generate a FSCSS-IM joint mapping codebook X;

[0034] Step 303: In the communication working mode, the baseband signal x m Transmitted to the user target through the direct path channel of "DFRC base station-user target" and the RIS reflection path channel of "DFRC base station-RIS-user target";

[0035] Y T =(H d +(GΘH))ωx m +V T (15)

[0036] Among them, H d , G, H are consistent with the channel of the radar working mode. The azimuth of the user target relative to the DFRC base station measured by the radar working mode is The corresponding estimated channel matrix is Set to

[0037]

[0038] Θ n is the nth element on the main diagonal of Θ, expressed as

[0039] Θ n =exp(jarg(g n h n ω)) (17)

[0040] Among them, g n G = [g 1 ,g 2 ,K,g n ,Kg N ]The nth element of the vector, h n is H = [h 1 ,h 2 ,K,h n ,Kh N ] T The nth row vector of the matrix; arg(·) is the function that calculates the argument of a complex number, exp(·) is the exponential function, is the plural symbol;

[0041] Step 304: Calculate the maximum likelihood detection of the receiving end of the legitimate user target, and the calculation formula is as follows:

[0042]

[0043] Since the signal at the legitimate user is not distorted, a detection method with lower complexity is used for decoding. The calculation formula is as follows:

[0044]

[0045] Step 305: At the eavesdropper, the received signal is decoded by maximum likelihood detection;

[0046] The received signal is calculated as follows:

[0047] Y eve =(H d,eve +(G eve ΘH))ωx m +V T (20)

[0048] The calculation formula for decoding by maximum likelihood detection is as follows:

[0049]

[0050] in It is represented as the direct path channel between the DFRC base station and the eavesdropper, set to H d,eve =a t (θ BT,eve );G eve It is represented as the direct path channel between RIS and the user target, set to G eve =a t (θ BT,eve ).

[0051] Further, in step 302, the FSCSS-IM joint mapping codebook X is generated as follows:

[0052] Step 3021: In the communication working mode, simplify the FSCSS signal into a communication codeword, and generate W orthogonal FSCSSs. The calculation formula is as follows:

[0053]

[0054] Step 3022: Select L signals from W orthogonal FSCSSs for superposition, with a total of There are combinations of these methods, and the corresponding binomial coefficient index combination set is p = {p 1 ,p 2 ,K,p Q}, where p 1 ,p 2 ,K,p Q is the combination of signal indices. If W = 4 and L = 2, then the corresponding p = {{1,2},{1,3},{1,4},{2,3},{2,4},{3,4}}. The obtained FSCSS-IM signal is expressed as

[0055]

[0056] Step 3023: multiply the FSCSS-IM signal by an M-PSK constellation symbol u to obtain a transmitted baseband signal x. m , expressed as

[0057] x m (n) = s q (n)u (14).

[0058] Further, in step 3023, the baseband signal x m The generation method is as follows:

[0059] Step 30231, select a FSCSS-IM signal s respectively q (n) and an M-PSK constellation symbol, and the candidate codebook is obtained based on all the results generated after multiplication. i and j are the indices of candidate transmission signals;

[0060] Step 30232: Calculate any two different candidate transmitted signals x' in the candidate codebook i With x' j The corresponding Euclidean distance d ij =tr((x' i -x' j )(x' i -x' j ) H );

[0061] Step 30233: All the calculated d ij Sort in descending order and count the d in the first half of the order. ij The frequency of the corresponding candidate codebook index appearing in all candidate codebook indexes;

[0062] Step 30234: The 2 with the highest frequency B2 All candidate signals in X' corresponding to the indices are selected into the joint mapping codebook X.

[0063] A dual-function radar communication system assisted by an intelligent reflective surface, the system is used to implement the above method, characterized in that the dual-function radar communication system comprises N units of intelligent reflective surfaces RIS and N B A DFRC base station for a dual-function radar communication system with array elements.

[0064] Furthermore, an N B An antenna linear uniform array ULA with N elements is deployed near the DFRC base station. The array has the function of digital beamforming DBF. The element spacing of the array is d, the carrier wavelength of communication or radar is λ, and d≤λ / 2.

[0065] Furthermore, in the communication working mode and the radar working mode, when the DFRC base station transmits a signal, the DFRC base station and the RIS are both in a fully activated state; in the radar working mode, when the DFRC base station does not transmit a signal, the DFRC base station and the RIS are both in a fully closed state; the DFRC base station only sends communication signals to a single legitimate user target at the same time, while the eavesdropper attempts to monitor the signal of the legitimate user target outside the detection range of the DFRC base station.

[0066] Through the embodiments of the present application, the following technical effects can be obtained:

[0067] The frequency-shifted chirp spread spectrum index modulation concealed RIS-DFRC-FSCSS-IM communication method of the dual-function radar communication system assisted by the intelligent reflective surface of the present application includes the design of a collaborative mode of integrated communication perception and the design of communication and radar baseband signals. The present application proposes to integrate communication and radar in the same DFRC system, and both send the same type of baseband signal in the working mode, which can realize the sharing of software and hardware resources between radar and communication and greatly reduce the cost of software and hardware. Compared with the existing directional concealed communication scheme, the simulation results show that the RIS-DFRC-FSCSS-IM proposed in the present invention has better bit error rate performance than CDIDM and can provide higher directional concealment and security performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] In order to more clearly illustrate the technical solutions in the embodiments of the present application, a brief introduction will be given below to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0069] Figure 1 It is the block diagram of the RIS-DFRC system;

[0070] Figure 2 Schematic diagram of simulation results for different covert communication schemes;

[0071] Figure 3 A flowchart of a covert communication method. DETAILED DESCRIPTION

[0072] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0073] The covert communication method of the present application is applied to the dual-function radar communication system RIS-DFRC assisted by the intelligent reflective surface, and the system performs frequency shift chirp spread spectrum index modulation FSCSS-IM. Figure 1 This is the block diagram of the RIS-DFRC system.

[0074] The dual-function radar communication system includes N units of intelligent reflective surfaces RIS and N BA DFRC base station for a dual-function radar communication system with 10 elements;

[0075] like Figure 1 As shown, an N B An antenna linear uniform array ULA with N elements is deployed near the DFRC base station. The array has the function of digital beamforming DBF. The array element spacing is d, the carrier wavelength of communication or radar is λ, and d≤λ / 2 is set.

[0076] In the communication working mode and radar working mode, when the DFRC base station transmits signals, the DFRC base station and RIS are both in a fully activated state; in the radar working mode, when the DFRC base station does not transmit signals, the DFRC base station and RIS are both in a fully closed state; the DFRC base station only sends communication signals to a single legitimate user target at the same time, while the eavesdropper attempts to monitor the signals of the legitimate user target outside the detection range of the DFRC base station.

[0077] The dual-function radar communication system DFRC is used to detect the position information of a low-speed moving or quasi-static single-user target in real time and communicate with the user target. The dual-function radar communication system adopts M-PSK constellation modulation, and the user target perceived by the communication has only a single receiving antenna; the working mode of the dual-function radar communication system includes a radar working mode and a communication working mode, and the frequency-shifted chirp spread spectrum index modulation FSCSS-IM signal is used as the baseband signal in both working modes. The target position information measured in the radar working mode is used for covert communication in the auxiliary communication working mode. Figure 3 The figure is a flow chart of a covert communication method, which comprises the following steps:

[0078] Step 1: constructing a dual-function radar communication system RIS-DFRC assisted by an intelligent reflective surface, wherein the operating state of the dual-function radar communication system includes two working modes: radar working and communication working;

[0079] Step 2: In the radar working mode, the dual-function radar communication system DFRC base station obtains the azimuth and radial distance of the user target by frequency-shift chirp spread spectrum index modulation FSCSS-IM signal, and transmits specific communication bits to the user target.

[0080] Step 3: In the communication working mode, the position information of the user target measured in the radar working mode is used to obtain the channel estimate, and the bit B of the FSCSS-IM signal transmission communication is sent to the user target. 1 RIS-assisted communication and digital beamforming methods are used to achieve covert communication.

[0081] In step 2, the workflow of the radar working mode of the dual-function radar communication system is as follows:

[0082] Step 201: Group the input bit stream of the radar working mode into groups B 1 bit, used to select the FSCSS-IM joint mapping codebook S = {s 1 ,s 2 ,K,s q ,K,s Q} in which a signal s is emitted q ,in It is represented by the number of bits in each group after the bit stream is grouped, q = 1, 2, ···, Q represents the index value of the transmitted signal in S, It is expressed as the number of combinations of selecting L out of W chirp signals for superposition, represents the binomial coefficient, Indicates rounding down;

[0083] Step 202: In radar working mode, generate FSCSS-IM joint mapping codebook s m ;

[0084] The FSCSS-IM jointly maps the codebook s m The generation method is as follows:

[0085] Step 2021: Calculate the chirp signal at time t, and the calculation formula is as follows:

[0086]

[0087] in, is the complex symbol, T r is the pulse width of the chirp signal pulse, f 0 The center frequency set for the chirp signal, μ = B / T r is the chirp signal linear modulation frequency, B is the bandwidth occupied by the radar signal in the spectrum, is a rectangular function;

[0088] Step 2022: The pulse width T of the chirp signal pulse r Divide the chirp signal into W equal parts, delay the chirp signal equally in the time domain, and obtain a signal set F of W orthogonal frequency-shifted chirp spread spectrum FSCSS signals. FSCSS ={f FSCSS (1,t),f FSCSS (2,t),K,f FSCSS (W,t)}, the FSCSS signal f in the signal set FSCSS (w,t) is expressed as:

[0089]

[0090] Step 2023, in F FSCSS Select L different FSCSS signals from the signal set and superimpose them in the time domain to obtain There are combinations of these methods, and the binomial coefficient index combination set corresponding to each combination is generated as p = {p 1 ,p 2 ,K,p Q}. Among them, p 1 ,p 2 ,K,p Q is the combination of signal indices. If W = 4 and L = 2, then the corresponding p = {{1,2},{1,3},{1,4},{2,3},{2,4},{3,4}}. The resulting FSCSS-IM signal set is F FSCSS-IM ={f FSCSS-IM (p 1 ,t),f FSCSS-IM (p 2 ,t),K,f FSCSS-IM (p Q ,t)}, the FSCSS-IM signal in the signal set is expressed as:

[0091]

[0092] Step 2024: Sample the FSCSS-IM signal. The sampled signal is expressed as:

[0093]

[0094] where f s Indicates the signal sampling frequency;

[0095] Step 2025: Group the input bit stream in the radar working mode into groups B 1 bits, where For selecting the joint mapping codebook S = {s 1 ,s 2 ,K,s q ,K,s 2B1}Select one of the signals s q .

[0096] The method for selecting the joint mapping codebook S is as follows:

[0097] Step 20251. In F FSCSS-IM Select the FSCSS-IM signal and get the candidate codebook i and j are the indices of candidate transmission signals;

[0098] Step 20252: Calculate any two different candidate transmitted signals s' in the candidate codebook i With s' j The Euclidean distance d of the corresponding digital sampling signal ij =tr((s' i -s' j )(s' i -s' j ) H );

[0099] Step 20253: All the calculated d ij Sort in descending order and count the d in the first half of the order. ij The frequency of the corresponding candidate codebook index appearing in all candidate codebook indexes;

[0100] Step 20254: The 2 with the highest frequency B1 All candidate signals in S' corresponding to the indexes are selected into S to generate a joint mapping codebook S;

[0101] Step 203: Select the transmission signal s in the joint mapping codebook S according to the input bits. q (t), s q (t) is s q Expressions in the time domain;

[0102] Step 204: In the radar working mode, the radar signal is transmitted through the two paths of "base station-user target-base station" and "base station-RIS-user target-base station", and the echo signal Y received by the DFRC base station is B (t) is expressed as

[0103]

[0104] Among them, α is the reflection factor of the user target, ω is the weight vector of digital beam forming, and ω is set to a dimension of N in the radar working mode. B ×1 full 1 matrix; V B (t) for obedience Complex Gaussian distributed noise function; is the RIS reflection matrix, which is set to a unit matrix with dimension N×N in the radar working mode; s q (t-τ 1 ) and s q (t-τ 2 ) is the transmitted signal s m (t) respectively after a time delay τ 1 and τ 2 The signal after Indicates the delay caused by the signal passing through the "base station-user target-base station" path, represents the delay of the signal through the path of “base station-RIS-user target-base station”, ξ BR is the distance between the DFRC base station and the RIS, ξ RT is the distance between RIS and the user's target, ξ BT is the distance between the DFRC base station and the user target;

[0105] In step 204, the Rice fading channel between the DFRC base station and the RIS Expressed as

[0106]

[0107] H LOS is the direct path of the Rice fading channel H, H NLOS is the indirect path of the Ricean fading channel H, H NLOS The channel elements of obey CN(0,1) complex Gaussian distribution; BT Angle θ RT ,θ BR ,θ BT are departure angles, θ RT Defined as the angle between RIS and the target; θ BR Defined as the angle between the DFRC base station antenna array and the RIS antenna array; θ BT Defined as the angle between the DFRC base station antenna array and the user target; angle θ RB ,θ TB is the angle of arrival, θ RB Defined as the angle between the RIS antenna array and the DFRC base station antenna array; θ TB Defined as the angle between the DFRC user target and the base station antenna array; a r T(θ TB ) is a r (θ TB ) is the transpose of; in the dual-function radar communication system, θ RB =θ BR ; a t (θ RT ),a t (θ BR ),a t (θ BT ) are all steering vectors, which respectively indicate the direction to θ RT ,θ BR ,θ BT The weights of the transmitted beams in the direction are expressed as

[0108]

[0109] a r (θRB ), a r (θ TB ) are response vectors, a r (θ RB ) represents the receiving response of the RIS array to the signal transmitted by the DFRC base station, a r (θ RB ) represents the receiving response of the DFRC base station to the user target reflected echo signal, a r (θ RB ) and a r (θ TB ) are respectively expressed as

[0110]

[0111] In step 204, when the time delay difference of the radar signal passing through the two paths is less than the radar range gate, that is, The delay difference between the two paths can be ignored. After removing the sampling time index, the discrete time signal matrix Y received by the user target end is T Simplified expression:

[0112] Y T =(H d +(GΘH))ωs q +V T

[0113] =a t (θ BT )ωs q +a t (θ RT )ΘHωs q +V T (9)

[0114] It is represented as the direct path channel between the DFRC base station and the user target, set to H d =a t (θ BT ); G represents the direct path channel between RIS and the user target, set to G = a t (θ BT );s q is the discrete time matrix of the transmitted signal, V T It is represented as the noise discrete time matrix of the user target receiver; To obey The noise vector of complex Gaussian distribution; f s is the sampling rate of the digital signal.

[0115] Step 205: Perform zero forcing detection at the receiving end of the user target, and the calculation formula is as follows:

[0116]

[0117] Among them, s i is the signal in the joint mapping codebook S, Functions are used to calculate The maximum value of s i Index

[0118] In step 3, the workflow of the communication working mode of the dual-function radar communication system is as follows:

[0119] Step 301: Use the azimuth of the user target obtained by the radar working mode to estimate the direct channel, use digital formation technology to achieve covert communication, and share the radar information to the RIS end to enhance the communication concealment;

[0120] In the communication working mode, the input bit stream of the frequency shift chirp spread spectrum index modulation RIS-DFRC-FSCSS-IM of the dual-function radar communication system assisted by intelligent reflecting surface grouping is grouped, and each group is divided into B bits. bit is used to select the joint mapping codebook Select Send signal x m , It is represented by the number of combinations of selecting L out of W chirp signals for superposition, and M is the number of M-PSK constellation symbols;

[0121] Step 302: In the communication working mode, generate a FSCSS-IM joint mapping codebook X;

[0122] The FSCSS-IM joint mapping codebook X is generated as follows:

[0123] Step 3021: In the communication working mode, simplify the FSCSS signal into a communication codeword, and generate W orthogonal FSCSSs. The calculation formula is as follows:

[0124]

[0125] Step 3022: Select L signals from W orthogonal FSCSSs for superposition, with a total of There are combinations of these methods, and the corresponding binomial coefficient index combination set is p = {p 1 ,p 2 ,K,p Q}. Among them, p 1 ,p 2 ,K,p Qis the combination of signal indices. If W = 4 and L = 2, then the corresponding p = {{1,2},{1,3},{1,4},{2,3},{2,4},{3,4}}. The obtained FSCSS-IM signal is expressed as

[0126]

[0127] Step 3023: multiply the FSCSS-IM signal by an M-PSK constellation symbol u to obtain a transmitted baseband signal x m , expressed as

[0128] x m (n) = s q (n)u (14)

[0129] Baseband signal x m The generation method is as follows:

[0130] Step 30231, select a FSCSS-IM signal s respectively q (n) and an M-PSK constellation symbol, and the candidate codebook is obtained based on all the results generated after multiplication. i and j are the indices of candidate transmission signals;

[0131] Step 30232: Calculate any two different candidate transmitted signals x' in the candidate codebook i With x' j The corresponding Euclidean distance d ij =tr((x' i -x' j )(x' i -x' j ) H );

[0132] Step 30233: All the calculated d ij Sort in descending order and count the d in the first half of the order. ij The frequency of the corresponding candidate codebook index appearing in all candidate codebook indexes;

[0133] Step 30234: The 2 with the highest frequency B2 All candidate signals in X' corresponding to the indexes are selected into the joint mapping codebook X;

[0134] Step 303: In the communication working mode, the baseband signal x m Transmitted to the user target through the direct path channel of "DFRC base station-user target" and the RIS reflection path channel of "DFRC base station-RIS-user target";

[0135] Y T =(Hd +(GΘH))ωx m +V T (15)

[0136] Among them, H d , G, H are consistent with the channel of the radar working mode. The azimuth of the user target relative to the DFRC base station measured by the radar working mode is The corresponding estimated channel matrix is Set to

[0137]

[0138] Θ n is the nth element on the main diagonal of Θ, expressed as

[0139] Θ n =exp(jarg(g n h n ω))(17)

[0140] Among them, g n G = [g 1 ,g 2 ,K,g n ,Kg N ]The nth element of the vector, h n is H = [h 1 ,h 2 ,K,h n ,Kh N ] T The nth row vector of the matrix; arg(·) is the function that calculates the argument of a complex number, exp(·) is the exponential function, is the plural symbol;

[0141] Step 304: Calculate the maximum likelihood detection of the receiving end of the legitimate user target, and the calculation formula is as follows:

[0142]

[0143] Since the signal at the legitimate user is not distorted, a less complex correlation detection method can be used for decoding. The calculation formula is as follows:

[0144]

[0145] Step 305: At the eavesdropper, the received signal is decoded by maximum likelihood detection;

[0146] The received signal is calculated as follows:

[0147] Y eve =(H d,eve +(Geve ΘH))ωx m +V T (20)

[0148] The calculation formula for decoding by maximum likelihood detection is as follows:

[0149]

[0150] in It is represented as the direct path channel between the DFRC base station and the eavesdropper, set to H d,eve =a t (θ BT,eve );G eve It is represented as the direct path channel between RIS and the user target, set to G eve =a t (θ BT,eve ).

[0151] In order to demonstrate the superiority of the proposed RIS-DFRC-FSCSS-IM scheme, Figure 2 Monte Carlo simulation results are used to evaluate the BER performance of the proposed modulation scheme. The RIS-DFRC-FSCSS-IM scheme is compared with the existing covert communication modulation scheme CDIDM scheme under the ML detector. In order to make a fair comparison, the transmitting antenna N is set. t =4, receiving antenna N r =1, spectrum efficiency B = 0.8125 bit / s / Hz. The azimuth angle of the location of the legal user DFRC base station is θ BT =30, the azimuth angle of the legal user relative to the location of the DFRC base station is θ BT,eve =60. CDIDM uses Hadamard coding with a code length of 16 and an M-PSK modulation order of 32. The DFRC-FSCSS-IM scheme refers to a covert communication scheme without a RIS link, and the RIS-DFRC-FSCSS-IM scheme is a dual communication channel link scheme with a RIS link. The number of RIS elements in RIS-DFRC-FSCSS-IM is set to N=16, and the Ricean factor of the Ricean channel between the RIS and DFRC base stations is set to K= 10 From the simulation results, it can be seen that under the above simulation conditions, the BER performance of the legitimate user and the eavesdropper of FSCSS-IM is better than that of the CDIDM scheme, but the concealment of the communication is reduced. -4When the directional concealment gain of DFRC-FSCSS-IM is 6dB, the directional concealment gain of CDIDM is 10dB, and the directional concealment gain of RIS-DFRC-FSCSS-IM assisted by RIS is 17dB. In addition, when the BER of the legitimate user drops to 10 -4 When BERDFRC-FSCSS-IM is used, the BER performance of BERDFRC-FSCSS-IM differs from that of CDIDM by 3 dB, while the BER performance of RIS-DFRC-FSCSS-IM differs from that of CDIDM by 13 dB.

[0152] In summary, the present application provides a very promising solution that uses a dual-function radar communication system DFRC for joint design, integrating the functions of communication and radar on the same antenna array. This joint design improves performance by promoting the coexistence of dual functions, and helps to reduce the number of antennas, system size, weight and power consumption. In addition, in the IM mode, the frequency-shifted chirp spread spectrum index modulation technology can not only meet the needs of transmitting communication signals, but also the chirp signal itself is widely used in the baseband waveform design of radars. Considering the use of the same type of baseband signal in radar and communication systems will greatly reduce the hardware cost of the DFRC system. After sharing the hardware resources of DFRC, it is natural to consider using the digital beamforming DBF function of the DFRC system to assist IM and realize the covert communication of IM. Among the various existing IM schemes, the intelligent reflective surface RIS auxiliary communication technology is widely used to enhance the channel gain, spectrum efficiency, and communication reliability of IM. In addition, RIS itself has the function of passive DBF, which can realize the active and passive reciprocity of IM, or consider enhancing the covert communication of IM based on DBF.

[0153] Finally, it should be noted that the above preferred embodiments are only used to illustrate the technical solution of the present application rather than to limit it. Although the present application has been described in detail through the above preferred embodiments, those skilled in the art should understand that various changes can be made in form and details without departing from the scope defined by the claims of the present application.

Claims

1. A chirp signal index modulation covert communication method, It is characterized in that The method comprises the following steps: Step 1: constructing a dual-function radar communication system RIS-DFRC assisted by an intelligent reflective surface, wherein the operating state of the dual-function radar communication system includes two working modes: radar working and communication working; Step 2: In the radar working mode, the dual-function radar communication system DFRC base station obtains the azimuth and radial distance of the user target by frequency-shift chirp spread spectrum index modulation FSCSS-IM signal, and transmits specific communication bits to the user target; Step 3: In the communication working mode, the position information of the user target measured in the radar working mode is used to obtain channel estimation, and the bits of FSCSS-IM signal transmission communication are sent to the user target, and covert communication is achieved through RIS auxiliary communication and digital beam forming method; In step 2, the workflow of the radar working mode of the dual-function radar communication system is as follows: Step 201: Group the input bit stream of the radar working mode into groups, each group is divided into Bit, used to select the FSCSS-IM joint mapping codebook The transmitted signal in ,in It is expressed as the number of bits in each group after the bit stream is grouped. express The index value of the transmitted signal in Expressed as from Chirp signal selection The number of combinations to be superimposed, represents the binomial coefficient, Indicates rounding down; Step 202: Generate FSCSS-IM joint mapping codebook in radar working mode ; Step 203: According to the input bits, in the joint mapping codebook Select the transmit signal , for Expressions in the time domain; Step 204: In the radar working mode, the radar signal is transmitted through the two paths of "base station-user target-base station" and "base station-RIS-user target-base station", and the echo signal received by the DFRC base station It is expressed as: （5） in, is the reflection factor of the user target, is the weight vector for digital beamforming, In the radar working mode, the dimension is set to A matrix of all 1s; For obedient Complex Gaussian distributed noise function; is the RIS reflection matrix, which is set to have dimensions of The unit array of and It is the transmitting signal After time delay and The signal after Indicates the delay caused by the signal passing through the "base station-user target-base station" path, Indicates the delay caused by the signal passing through the "base station-RIS-user target-base station" path, is the distance between the DFRC base station and the RIS, is the distance between RIS and the user's target, is the distance between the DFRC base station and the user target; Step 205: Perform zero forcing detection at the receiving end of the user target, and the calculation formula is as follows: （10） in, is the joint mapping codebook The signal in Functions are used to calculate The maximum value corresponding to Index ; In step 204, the Rice fading channel between the DFRC base station and the RIS It is expressed as: （6） Rician fading channel The direct path of Rician fading channel The indirect path of The channel elements obey Complex Gaussian distribution; angle , , are departure angles, It is defined as the angle between RIS and the target; Defined as the angle between the DFRC base station antenna array and the RIS antenna array; Defined as the angle between the DFRC base station antenna array and the user target; Angle , is the angle of arrival, Defined as the angle between the RIS antenna array and the DFRC base station antenna array; It is defined as the angle between the DFRC user target and the base station antenna array; for The transpose of; In the dual-function radar communication system, ; , , are steering vectors, which respectively indicate the direction , , The weights of the transmitted beams in the direction are expressed as: （7） and Respectively expressed as: （8） in , are response vectors, Indicates the receiving response of the RIS array to the signal transmitted by the DFRC base station. Indicates the reception response of the DFRC base station to the user target reflected echo signal.

2. The method according to claim 1, It is characterized in that In step 204, when the time delay difference of the radar signal passing through the two paths is less than the radar range gate, that is, , then the delay difference between the two paths can be ignored. After removing the sampling time index, the discrete time signal matrix received by the user target end is Simplified expression: （9） It is represented as the direct path channel between the DFRC base station and the user target, which is set as ; It is represented as the direct path channel between RIS and the user target, set as ; is the discrete time matrix of the transmitted signal, It is represented as the noise discrete time matrix of the user target receiver; To obey The complex Gaussian noise vector is is the sampling rate of the digital signal.

3. The method according to claim 1, It is characterized in that In step 3, the workflow of the communication working mode of the dual-function radar communication system is as follows: Step 301: Use the azimuth of the user target obtained by the radar working mode to estimate the direct channel, use digital formation technology to achieve covert communication, and share the radar information to the RIS end to enhance the communication concealment; Step 302: Generate FSCSS-IM joint mapping codebook in communication working mode ; Step 303: In the communication working mode, the baseband signal Transmitted to the user target through the direct path channel of "DFRC base station-user target" and the RIS reflected path channel of "DFRC base station-RIS-user target"; （15） in, , , The channels are consistent with the radar working mode. The azimuth of the user target relative to the DFRC base station measured by the radar working mode is , the corresponding estimated channel matrix is , Set to: （16） for The first elements, represented by: （17） in, yes The vector elements, yes The matrix row vector; is a function that calculates the argument of a complex number, is an exponential function, is the plural symbol; Step 304: Calculate the maximum likelihood detection of the receiving end of the legitimate user target, and the calculation formula is as follows: （18） Since the signal at the legitimate user is not distorted, a detection method with lower complexity is used for decoding. The calculation formula is as follows: （19） Step 305: At the eavesdropper, the received signal is decoded by maximum likelihood detection; The received signal is calculated as follows: （20） The calculation formula for decoding by maximum likelihood detection is as follows: （21） in It is represented as the direct path channel between the DFRC base station and the eavesdropper, which is set to ; It is represented as the direct path channel between RIS and the user target, set as .

4. The method according to claim 3, It is characterized in that In step 302, the FSCSS-IM jointly maps the codebook The generation method is as follows: Step 3021: In the communication working mode, simplify the FSCSS signal into a communication codeword and generate The calculation formula for the orthogonal FSCSS is as follows: （12） Step 3022: Choose from the orthogonal FSCSS signals are superimposed, There are combinations of binomial coefficients, and the corresponding binomial coefficient index combination set is ,in is a combination of signal indices, if , , then the corresponding , the obtained FSCSS-IM signal is expressed as: （13） Step 3023: Combine the FSCSS-IM signal with an M-PSK constellation symbol. Multiply to get the transmitted baseband signal , expressed as: （14）。 5. The method according to claim 4, It is characterized in that In step 3023, the baseband signal The generation method is as follows: Step 30231: Select a FSCSS-IM signal and an M-PSK constellation symbol, and obtain the candidate codebook according to all the results generated after multiplication , and is the index of the candidate transmission signal; Step 30232: Calculate any two different candidate transmission signals in the candidate codebook and The corresponding Euclidean distance ; Step 30233: All the calculated Sort in descending order, and count the top half of the rankings after sorting. The frequency of the corresponding candidate codebook index appearing in all candidate codebook indexes; Step 30234: The corresponding index All candidate signals in the joint mapping codebook are selected middle.

6. A dual-function radar communication system assisted by an intelligent reflective surface, the system being used to implement the method of any one of claims 1 to 5, It is characterized in that The dual-function radar communication system includes The intelligent reflective surface RIS of each unit and A DFRC base station for a dual-function radar communication system with array elements.

7. The system according to claim 6, It is characterized in that A DFRC base station was deployed A linear uniform array (ULA) with 4 elements is deployed near the DFRC base station. The array has the function of digital beamforming DBF. The array element spacing is , the carrier wavelength for communication or radar is , and set .

8. The system according to claim 7, It is characterized in that In the communication working mode and radar working mode, when the DFRC base station transmits signals, the DFRC base station and RIS are both in a fully activated state; in the radar working mode, when the DFRC base station does not transmit signals, the DFRC base station and RIS are both in a fully closed state; the DFRC base station only sends communication signals to a single legitimate user target at the same time, while the eavesdropper attempts to monitor the signals of the legitimate user target outside the detection range of the DFRC base station.

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