A Covert Communication Method and Device Based on Orthogonal Time-Frequency-Space Technology

Abstract

The present invention relates to the field of communications, and particularly to a covert communication method and device based on orthogonal time-frequency-space technology. The present invention simultaneously employs a conventional signal masking technique and a hopping code spread spectrum technique, and parasitizes a non-stationary covert signal using hopping code and direct spread spectrum on a stationary conventional signal with obvious characteristics using traditional direct spread spectrum in a manner of differential power combination. Furthermore, a joint design is carried out on the two. The covert signal adopts hopping code spread spectrum controlled by a random sequence, and directly uses the data information carried on the conventional signal (which can be approximately regarded as a random sequence) as the hopping code control sequence of the covert signal, thereby hiding the important service data of the covert signal in the conventional service data of the conventional signal. While ensuring the waveform covert performance, it can effectively improve the adaptability of the signal waveform to high-dynamic communication scenarios, and can be used to realize the transmission of non-stationary covert signals for high-mobility communications.

Description

Technical Field

[0001] The present invention relates to the field of communications, and provides a covert communication method and device based on orthogonal time-frequency-space technology. Background Art

[0002] At present, covert communication technology based on integrated spread spectrum technology occupies a dominant position in current physical layer signal covert technology. Existing spread spectrum covert communication technologies mainly focus on the time domain, frequency domain, code domain, and power domain. Time-domain spread spectrum covert communication technologies mainly include time hopping technology and short-time burst communication technology. Frequency-domain spread spectrum covert communication technologies mainly include direct sequence spread spectrum technology and frequency hopping technology. Code-domain spread spectrum covert communication technologies mainly include code hopping spread spectrum technology and chaotic sequence spread spectrum technology. The power domain is mainly conventional signal masking technology. Generally speaking, in order to improve the covert communication ability, it is inclined to fuse and use multiple covert communication technologies in signal waveform design to achieve multi-domain signal concealment. However, the dynamic adaptability of the current main covert communication methods is limited and not suitable for application in high-dynamic communication scenarios.

[0003] Currently, the emerging communication technology in high-dynamic communication scenarios is orthogonal time-frequency-space technology. Orthogonal Time Frequency Space (OTFS) technology is a multi-carrier modulation and demodulation technology that can convert signals / data symbols between the delay-Doppler domain and the time-frequency domain. Its greatest feature is to utilize the relative stability of the communication physical channel in the delay-Doppler domain, fundamentally improving the problem that it is difficult to effectively perform channel estimation and equalization for the rapidly changing time-frequency doubly selective channel faced by traditional time-frequency domain waveforms in high-dynamic fast time-varying communication scenarios. In addition, OTFS technology has strong compatibility and can be applied to the general signal processing framework under the current communication system. Its precoding unit can be added after the modulator of single / multi-carrier waveforms, and its corresponding decoding processing unit is added before the demodulator of the corresponding receiving end, so that it has strong ability to cascade design with other signal waveforms. However, this technology has poor concealment and cannot achieve covert communication.

[0004] Therefore, the present invention proposes a covert communication waveform regulation method and device based on orthogonal time-frequency-space, which can effectively improve the adaptability of the signal waveform to high-dynamic communication scenarios while ensuring the waveform concealment performance, and can be used to realize the transmission of non-stationary covert signals for high-mobility communication. Summary of the Invention

[0005] The purpose of the present invention is to overcome the problem that most existing covert communication technologies are not suitable for application in high-dynamic communication scenarios, and provide a covert communication method and device based on orthogonal time-frequency-space technology.

[0006] To achieve the above-mentioned invention object, the present invention provides the following technical solutions:

[0007] A covert communication transmitting method based on orthogonal time-frequency-space technology, comprising the following steps:

[0008] S1: Perform LDPC coding and row-column interleaving on the covert data bits to be transmitted to obtain a coded bit sequence of the covert signal; the covert data bits to be transmitted are data streams that have completed mode adaptation and stream adaptation;

[0009] S2: Perform random hopping code spreading on the coded bits of the covert signal to obtain a hopping code spread sequence of the covert signal; and perform fixed sequence spreading on the corresponding conventional signal bit sequence of the coded bits of the covert signal to be transmitted to obtain a fixed spread sequence of the conventional signal;

[0010] Wherein, the hopping code control information for the random hopping code spreading is intercepted from the conventional signal bit sequence;

[0011] S3: Respectively perform BPSK modulation on the hopping code spread sequence of the covert signal and the fixed spread sequence of the conventional signal, and then perform non-equal power combination to generate a combined symbol stream;

[0012] S4: Perform orthogonal time-frequency-space transformation and shaping filtering on the combined symbol stream to generate a baseband transmission signal; the orthogonal time-frequency-space transformation includes vector-matrix transformation, inverse symplectic-Fourier transform, Heisenberg transform, and matrix-vector transformation processing.

[0013] As a preferred solution of the present invention, the expression for the random hopping code spreading in S2 is:

[0014]

[0015] Wherein, b y [n] represents the hopping code spread sequence of the covert signal, is the exclusive OR operator, d y [n] represents the coded bit sequence of the covert signal, p y [n] represents the pseudo-random sequence for the random hopping code spreading, p y [n] has a bit rate that is k y times that of d y times, and k y is the spreading factor of the covert signal.

[0016] As a preferred solution of the present invention, the expression for the fixed sequence spreading in S2 is:

[0017]

[0018] Wherein, b d[n] represents the fixed spreading sequence of the regular signal, is the exclusive OR operator, d d [n] represents the regular signal bit sequence corresponding to the coded bit of the covert signal to be transmitted, p d [n] represents the pseudo-random sequence spread by the fixed sequence, p d The bit rate of p d [n] is k d times, k d is the spreading factor of the regular signal.

[0019] As a preferred embodiment of the present invention, the expression of the merging operation in S3 is:

[0020] x B [n] = A d x d [n] + A y x y [n],

[0021] wherein, x B [n] represents the merged symbol stream, x d [n] represents the symbol sequence of the regular signal after BPSK modulation, x y [n] represents the symbol sequence of the covert signal after BPSK modulation, A d represents the amplitude of the regular symbol sequence, A y represents the amplitude of the covert symbol sequence.

[0022] As a preferred embodiment of the present invention, S4 includes the following steps:

[0023] S41: The merged symbol stream is segmented into a group of vectors with a length of MN. Each group of vectors will pass through a vector-matrix transformation to place MN symbols on an M×N-dimensional time-delay-Doppler domain complex matrix, and output this matrix;

[0024] S42: Perform an inverse symplectic-finite Fourier transform on the time-delay-Doppler domain complex matrix to obtain a time-frequency domain complex matrix;

[0025] S43: Perform a Heisenberg transform and a transmitter shaping filter on the time-frequency domain complex matrix to obtain a time-domain matrix signal;

[0026] S44: Perform a matrix-vector transformation on the time-domain matrix signal to generate a baseband transmission signal.

[0027] A covert communication receiving method based on orthogonal time-frequency-space technology, for the receiving end, and the receiving method matches any one of the above-mentioned covert communication transmitting methods based on orthogonal time-frequency-space technology. The method includes the following steps:

[0028] S5: Perform matched filtering and orthogonal time-frequency-space demodulation on the baseband received signal after synchronization, and then perform linear channel equalization based on the equivalent channel state information to output a received combined symbol stream;

[0029] S6: Based on the received combined symbol stream, perform despreading and demodulation of the conventional signal using a fixed spreading sequence to obtain the hopping code control information, and according to the hopping code control information and the received combined symbol stream, perform hopping code despreading and demodulation on the covert signal to output a covert signal coded bit sequence;

[0030] S7: Perform deinterleaving and LDPC channel decoding on the covert signal coded bit sequence to obtain covert data bits.

[0031] As a preferred solution of the present invention, S5 includes the following steps:

[0032] S51: Split the synchronized digital baseband received signal into a group of vectors with a length of MN. Each group of vectors will be transformed through a vector-matrix transformation, and MN symbols will be placed on an M×N-dimensional complex matrix in the time domain, and output this matrix;

[0033] S52: Perform shaping matched filtering and Wigner transform on the time domain matrix signal to obtain a time-frequency domain matrix;

[0034] S53: Perform a symplectic-finite Fourier transform on the time-frequency domain matrix to obtain a delay-Doppler domain matrix;

[0035] S54: Perform a matrix-vector transformation on the delay-Doppler domain matrix to obtain an OTFS demodulation signal;

[0036] S55: Perform linear channel equalization on the OTFS demodulation signal based on the equivalent channel state information to output a received combined symbol stream.

[0037] A covert communication transmitting device based on orthogonal time-frequency-space technology for any one of the above-mentioned covert communication transmitting methods based on orthogonal time-frequency-space technology, used for the transmitting end. The device includes: an LDPC encoding module, a row-column interleaving module, a hopping code spreading module, a fixed sequence spreading module, a BPSK modulation module, a non-equal power combining module, an orthogonal time-frequency-space modulation module, a digital-to-analog converter, and a radio frequency transmission module;

[0038] The LDPC encoding module is used to perform LDPC channel encoding on the covert data bits to be transmitted and send them to the row-column interleaving module;

[0039] The row-column interleaving module is used to perform row-column interleaving processing on the data output by the LDPC encoding module to generate a covert signal coded bit sequence;

[0040] The hopping code spread spectrum module is used to perform random hopping code spread spectrum on the coded bits of the covert signal to generate a covert signal hopping code spread spectrum sequence;

[0041] The fixed sequence spread spectrum module is used to perform fixed sequence spread spectrum on the conventional signal bit sequence corresponding to the coded bits of the covert signal to be transmitted, generating a conventional signal fixed spread spectrum sequence;

[0042] The BPSK modulation module is used to perform BPSK modulation processing on the covert signal hopping code spread spectrum sequence and the conventional signal fixed spread spectrum sequence respectively, and send them to the unequal power combining module;

[0043] The unequal power combining module is used to perform unequal power combining on the data output by the BPSK modulation module to generate a combined symbol stream;

[0044] The orthogonal time-frequency-space modulation module is used to perform orthogonal time-frequency-space transformation and transmitter shaping filtering on the combined symbol stream to generate a digital baseband transmission signal;

[0045] The digital-to-analog converter is used to convert the digital baseband transmission signal into an analog baseband transmission signal and send it to the radio frequency transmission module;

[0046] The radio frequency transmission module performs a series of radio frequency signal processing on the analog baseband transmission signal and then transmits it through an antenna.

[0047] A covert communication receiving device based on orthogonal time-frequency-space technology for performing the above-mentioned covert communication receiving method based on orthogonal time-frequency-space technology, which is used at the receiving end. The device includes: a radio frequency processing module, an analog-to-digital converter, a timing synchronization module, an orthogonal time-frequency-space demodulation module, a channel estimation and channel equalization module, a despreading and demodulation module, and a deinterleaving and LDPC channel decoding module;

[0048] The radio frequency processing module is used to receive the transmitted signal and obtain an analog baseband received signal;

[0049] The analog-to-digital converter is used to convert the analog baseband received signal into a digital baseband received signal;

[0050] The timing synchronization module performs time synchronization on the received signal to obtain a synchronized baseband received signal, ensuring that a series of subsequent receiving processing modules can start processing the received signal in sequence at an appropriate moment;

[0051] The orthogonal time-frequency-space demodulation module is used to perform shaping matched filtering and orthogonal time-frequency-space demodulation on the synchronized digital baseband received signal, and output an OTFS demodulation signal;

[0052] The channel estimation and channel equalization module is used to perform linear channel equalization on the OTFS demodulated signal based on the equivalent channel state information, and output the received combined symbol stream;

[0053] The despreading and demodulation module includes a conventional signal despreading and demodulation module and a covert signal despreading and demodulation module; the conventional signal despreading and demodulation module is used to perform fixed-sequence despreading and demodulation on the conventional signal to obtain the hopping code control information; the covert signal despreading and demodulation module performs hopping code despreading and demodulation on the covert signal according to the hopping code control information, and outputs the covert signal coded bit sequence; the deinterleaving and LDPC channel decoding module is used to perform deinterleaving and LDPC decoding processing on the covert signal coded bit sequence to obtain the covert data bits.

[0054] Compared with the prior art, the beneficial effects of the present invention are:

[0055] The present invention simultaneously adopts the conventional signal masking technology and the hopping code spread spectrum technology, and parasitizes the non-stationary covert signal using hopping code and direct spread spectrum on the stationary conventional signal using traditional direct spread spectrum with obvious characteristics in a differential power combination manner. Then, a joint design is carried out for the two. The covert signal adopts hopping code spread spectrum controlled by a random sequence, and directly uses the data information carried on the conventional signal (which can be approximately regarded as a random sequence) as the hopping code control sequence of the covert signal, so as to hide the important service data of the covert signal in the conventional service data of the conventional signal. There are differences in the design objectives and ideas of the conventional signal and the covert signal. Specifically, when designing the conventional signal, its core requirements are better demodulation error performance and more significant stationary characteristics (with a certain ability to assist in maintaining synchronization) to achieve the correct demodulation of the control parameters of the covert signal; when designing the covert signal, the concealment of signal transmission is mainly considered, and for this purpose, multi-domain non-stationary processing is adopted. At the same time, in order to adapt to high-dynamic communication scenarios, the present invention comprehensively spreads the communication waveform through OTFS modulation. The OTFS transmitter modulation is carried out in a standard manner by cascading the inverse-symplectic finite Fourier transform (ISFFT) and the Heisenberg transform. Then, digital-to-analog conversion is performed for radio frequency transmission. After the signal passes through the high-dynamic link, at the receiver, through corresponding radio frequency reception processing, system synchronization, OTFS demodulation, equivalent channel estimation and equalization, despreading and demodulation of the non-stationary waveform, and channel decoding, the received covert data information is obtained. Description of the Drawings

[0056] Figure 1 It is a schematic flow chart of a covert communication transmission method based on orthogonal time-frequency-space technology according to Embodiment 1 of the present invention;

[0057] Figure 2Schematic block diagram of the design of a covert communication transmission method based on orthogonal time-frequency-space technology described in Embodiment 2 of the present invention;

[0058] Figure 3 Schematic diagram of the joint design of conventional signals and covert signals in a covert communication transmission method based on orthogonal time-frequency-space technology described in Embodiment 2 of the present invention;

[0059] Figure 4 Schematic diagram of the structure of a random hopping code direct sequence spread spectrum signal in a covert communication transmission method based on orthogonal time-frequency-space technology described in Embodiment 2 of the present invention;

[0060] Figure 5 Curve graph of time-delay related intercept detection in a covert communication transmission method based on orthogonal time-frequency-space technology described in Embodiment 2 of the present invention;

[0061] Figure 6 Curve graph of high-order statistic intercept detection in a covert communication transmission method based on orthogonal time-frequency-space technology described in Embodiment 2 of the present invention;

[0062] Figure 7 Curve graph of cyclic spectrum intercept detection in a covert communication transmission method based on orthogonal time-frequency-space technology described in Embodiment 2 of the present invention;

[0063] Figure 8 Flow chart of a covert communication reception method based on orthogonal time-frequency-space technology described in Embodiment 3 of the present invention;

[0064] Figure 9 Schematic block diagram of the design of a covert communication reception method based on orthogonal time-frequency-space technology described in Embodiment 4 of the present invention;

[0065] Figure 10 Schematic diagram of the structure of a covert communication transmission device based on orthogonal time-frequency-space technology described in Embodiment 5 of the present invention;

[0066] Figure 11 Schematic diagram of the structure of a covert communication reception device based on orthogonal time-frequency-space technology described in Embodiment 6 of the present invention. Detailed implementation manners

[0067] The present invention will be further described in detail below in conjunction with test examples and specific implementation manners. However, it should not be understood that the scope of the above-mentioned subject matter of the present invention is limited to the following embodiments. Any technology implemented based on the content of the present invention belongs to the scope of the present invention.

[0068] Before specifically describing the content of the present invention, the application prerequisite conditions of the present invention are first described as follows:

[0069] (1) The present invention is limited to the physical layer baseband waveform design, and the mode adaptation and flow adaptation of data signals as well as the radio frequency processing part of signals are not within the scope of consideration of the present invention. That is, it is assumed that the input data has been adapted, and the output signal will be sent after appropriate radio frequency processing.

[0070] (2) The power of the non-stationary covert signal cannot exceed the power of the masking signal used to mask its signal characteristics.

[0071] (3) The spreading factor of the conventional signal is smaller than that of the covert signal to facilitate the transmission of non-stationary control information.

[0072] (4) The conventional signal used to mask its signal characteristics is a cooperative signal and can be jointly designed with the covert signal.

[0073] Embodiment 1

[0074] As Figure 1 shown, a covert communication transmitting method based on orthogonal time-frequency-space technology includes the following steps:

[0075] S1: Perform LDPC coding and row-column interleaving on the covert data bits to be transmitted to obtain a covert signal coded bit sequence; the covert data bits to be transmitted are data streams that have completed mode adaptation and flow adaptation.

[0076] S2: Perform random hopping code spreading on the covert coded bit sequence to obtain a covert signal hopping code spreading sequence; and perform fixed sequence spreading on the bit sequence of the conventional signal corresponding to the covert coded bits to be transmitted to obtain a conventional signal fixed spreading sequence.

[0077] Among them, the hopping code control information for the random hopping code spreading is intercepted from the conventional signal bit sequence.

[0078] S3: Respectively perform BPSK modulation on the covert signal hopping code spreading sequence and the conventional signal fixed spreading sequence, and then perform non-equal power combination to generate a combined symbol stream.

[0079] S4: Perform orthogonal time-frequency-space transformation and shaping filtering on the combined symbol stream to generate a baseband transmission signal; the orthogonal time-frequency-space transformation includes vector-matrix transformation, inverse symplectic-Fourier transform, Heisenberg transform, and matrix-vector transformation processing.

[0080] Embodiment 2

[0081] This embodiment is a specific implementation manner of the covert communication transmitting method based on orthogonal time-frequency-space technology described in Embodiment 1. As Figure 2 shown, it includes the following steps:

[0082] S1: LDPC encode and row-column interleave the covert data bits to be transmitted to obtain a coded bit sequence of the covert signal; the covert data bits to be transmitted are data streams that have completed mode adaptation and stream adaptation.

[0083] S2: Perform random frequency hopping spread spectrum on the coded bit sequence of the covert signal to obtain a frequency hopping spread spectrum sequence of the covert signal; and perform fixed sequence spread spectrum on the bit sequence of the conventional signal corresponding to the coded covert data bits to be transmitted to obtain a fixed spread spectrum sequence of the conventional signal.

[0084] Among them, the frequency hopping control information for the random frequency hopping spread spectrum is intercepted from the conventional signal bit sequence.

[0085] The expression for the random frequency hopping spread spectrum is:

[0086]

[0087] where b y [n] represents the frequency hopping spread spectrum sequence of the covert signal, d y [n] represents the coded bit sequence of the covert signal, p y [n] represents the pseudo-random sequence for the random frequency hopping spread spectrum, p y [n] has a bit rate that is k y times that of d y times, and k y is the spreading factor of the covert signal.

[0088] The expression for the fixed sequence spread spectrum is:

[0089]

[0090] where b d [n] represents the fixed spread spectrum sequence of the conventional signal, d d [n] represents the bit sequence of the conventional signal corresponding to the coded covert signal bits to be transmitted, p d [n] represents the pseudo-random sequence for the fixed sequence spread spectrum, p d [n] has a bit rate that is k d times that of d d times, and k d is the spreading factor of the conventional signal.

[0091] Specifically, the principle of the conventional signal controlling the random frequency hopping spread spectrum is as follows:

[0092] Assume that the system simultaneously transmits N y coded covert data bits and N d conventional data bits within the same time. Since the concealment of the signal needs to be considered in this embodiment, it is necessary to make the code rate of the covert signal equal to the code rate of the conventional signal, that is, N y·k y = N d ·k d , while making k y = k·k d (k > 1 and k is an integer). That is, while transmitting 1 concealed data coding bit, k regular data bits are transmitted. These k regular data bits are called 1 regular bit group in this embodiment (as shown in Figure 3 ), and these k data bits have 2 k different situations, and these 2 k different situations exactly correspond to 2 k different spreading sequences in the hopping code spread spectrum sequence library / set. The concealed signal is based on this to select which spreading sequence to spread the concealed coding bit at the corresponding position (as shown in Figure 3 ).

[0093] The non-stationary concealed signal waveform adopts the design method of random hopping code spread spectrum to realize the construction of non-stationary signals in the time domain, frequency domain and code domain. The specific process is as follows:

[0094] Let the rate of the concealed data coding bit be The concealed data coding bit is spread by a spreading code with a chip rate of and a spreading factor of k y , that is The hopping code spread spectrum signal adopted by the concealed signal can be expressed as:

[0095]

[0096] Among them, bpskmod[·] is the BPSK modulation operator. t is the time variable, n is a natural number for counting, x y (t) is the concealed signal, d y (t) is the concealed data bit with a duration of T dy , represents the binary concealed information bit stream; p y (t) is the spreading code sequence with a chip duration of T py , represents the binary pseudo-random sequence. At the same time, d y (t) and p y (t) are independent of each other.

[0097] In Equation (3), by changing the selection method of the sequence to achieve hopping code, the signal period (cyclostationary) characteristic in the signal x y (t) will be destroyed. In the present invention, a concealed signal constructed by the random hopping code spread spectrum method is adopted, such asFigure 4 As shown. Each PN code for spread spectrum is selected from the hopping code spread spectrum code group library Ω = {PN1, PN2,..., PN v} according to the control sequence, that is, the regular data carried on the regular signal is simultaneously used as the control parameter for the random hopping code of the covert signal. Ω is a set of spread spectrum codes for the hopping code spread spectrum of the covert signal, and the PN codes therein have good autocorrelation and cross-correlation. Each PN code has its own number, and the process of randomly extracting the PN code is controlled by a random sequence transmitted in advance on the regular signal. The hopping code spread spectrum sequence needs to follow the following basic principles: the code lengths and code rates of the spread spectrum codes within the code group are the same. If they are not the same, it will significantly affect the correlation performance of the spread spectrum code sequence, and the despreading and demodulation performance of the covert signal by the cooperation party will deteriorate severely; at the same time, the code rate of the covert signal is the same as that of the regular signal, otherwise it will also increase the probability of its being identified by various common non-cooperative detection techniques based on spectral analysis.

[0098] According to the above construction method of the random hopping code direct spread signal, the autocorrelation function of the covert signal will not have periodic pulse peaks, showing non-stationary characteristics while maintaining good signal autocorrelation. Theoretically speaking, no matter how long the delay correlator is used to detect the cyclostationarity of the covert signal, its effectiveness will not be improved. Because, for the non-cooperating party, when the random hopping code spread signal adopted by the covert signal is unknown for each pseudo-random code in the hopping code library, the possibility of its being able to demodulate the covert signal normally is extremely small. At this time, the interception difficulty of the non-cooperating party increases sharply, and the anti-interception performance of the covert signal is significantly enhanced; at the same time, the control information of the randomly extracted PN code depends on the data information of the regular signal, and the mapping rule between the two is only known to the legitimate transceiver ends, so the control information of the random hopping code can be basically regarded as pre-known to the cooperation party and will not overly affect the normal reception of the cooperation party.

[0099] The control parameter of the covert signal is carried in the regular signal for the transmission of regular service data, as Figure 3 shown. The theoretical analysis process for the design of the regular signal is similar to that of the covert signal. Let the regular data bit rate be After spreading the regular data bits with a spreading code with a chip rate of and a spreading factor of k d , that is The expression of the regular signal is as follows:

[0100]

[0101] where bpskmod[·] is the BPSK modulation operator. x d (t) is the regular signal, dd (t) is a regular data bit with a duration of T dd , represents a binary regular information bit stream; p d (t) is a spreading code sequence with a chip duration of T pd , represents a binary pseudo-random sequence. At the same time, d d (t) and p d (t) are independent of each other.

[0102] A regular signal using a regular spread spectrum design has significant cyclostationary characteristics. Coupled with its significant power advantage, it can effectively mask the characteristics of the covert signal.

[0103] S3: Perform BPSK modulation on the covert signal hopping spread spectrum sequence and the regular signal fixed spread spectrum sequence respectively, and perform differential power combination to generate a combined symbol stream.

[0104] The expression of the combination operation is:

[0105] x B [n]=A d x d [n]+A y x y [n] (5)

[0106] Among them, x B [n] represents the combined symbol stream, x d [n] represents the regular signal fixed spread spectrum symbol sequence (the discretization of x y (t)), x y [n] represents the covert signal hopping spread spectrum symbol sequence (the discretization of x d (t)), and its combined power ratio is A d 2 / A y 2 (this parameter is set and controlled by the transmitter, and the default setting is 0-10 dB), A d represents the amplitude of the regular symbol sequence, A y represents the amplitude of the covert symbol sequence.

[0107] S4: Perform orthogonal time-frequency-space transformation and shaping filtering on the combined symbol stream to generate a baseband transmission signal; the orthogonal time-frequency-space transformation includes vector-matrix transformation, inverse symplectic-Fourier transform, Heisenberg transform, and matrix-vector transformation processing. The specific steps are as follows:

[0108] S41: Split the combined symbol stream into a set of vectors of length MN. Each set of vectors will undergo a vector-matrix transformation to place MN symbols on a complex matrix in the time-delay - Doppler domain with dimensions M×N, and output the complex matrix in the time-delay - Doppler domain;

[0109] Specifically, split the modulated combined symbol stream x B [n] into a set of vectors of length MN. Hereinafter, one of the vectors will be used for illustration. This vector is a complex column vector containing MN symbols. Through the vector & matrix transformation, place the MN symbols on a complex matrix in the time-delay - Doppler domain with dimensions M×N, that is

[0110] S42: Perform an inverse symplectic - finite Fourier transform (ISFFT) on the complex matrix in the time-delay - Doppler domain to obtain a complex matrix in the time - frequency domain;

[0111] Through ISFFT, transform the combined symbol stream from the time-delay - Doppler domain to the time - frequency domain. The expression of ISFFT is as follows:

[0112]

[0113] where x[l,k] is the (l,k)-th element in the matrix in the time-delay - Doppler domain, n = 0,..., N - 1, m = 0,..., M - 1.

[0114] After matrix - formulating ISFFT, the ISFFT algorithm is as follows:

[0115]

[0116] where is the matrix for transforming the signal from the time-delay - Doppler domain to the time - frequency domain, F M and F N are the normalized FFT matrices of dimensions M and N respectively, is the conjugate transpose of the F N matrix.

[0117] S43: Perform a Heisenberg transform and transmitter shaping filtering on the complex matrix in the time - frequency domain to obtain a time - domain matrix signal;

[0118] Through the Heisenberg transform, transform the two - dimensional signal in the time - frequency domain into a one - dimensional signal in the time domain, and then perform transmitter shaping filtering. The principle formula is as follows:

[0119]

[0120] where g tx ​(t - nT) is the time - domain response function of the transmitter shaping filter; e j2πmΔf(t-nT) can be regarded as a kind of discrete Fourier transform. Specifically, e is the exponential function, Δf is the bandwidth of the frequency - domain sampling interval, and T is the time - domain sampling interval (similar to the sampling interval and sub - carrier interval in OFDM modulation).

[0121] After matrix - formulating the Heisenberg transform and the transmitter shaping filter, its algorithm is as follows:

[0122]

[0123] Among them, is the matrix for transforming the signal from the time - frequency domain to the time domain, and G tx is the equivalent matrix of the transmitter shaping filter.

[0124] S44: Perform matrix - vector transformation on the time - domain matrix signal to generate the baseband transmitted signal.

[0125]

[0126] Among them, is the vector obtained by discretizing the time - domain baseband transmitted signal, corresponding to correspondingly.

[0127] The covert communication waveform after OTFS modulation passes through the baseband equivalent high - dynamic channel. Modeling the high - dynamic channel in the delay - Doppler domain, the channel can be expressed as:

[0128]

[0129] Among them, P represents the total number of multipaths, τ i is the delay value of the i - th path, v i is the Doppler frequency value of the i - th path, h i is the coefficient of the i - th multipath, and δ(v) is the impulse function.

[0130] The following are the experimental results of detecting the baseband transmitted signal using the existing commonly used delay - related intercept detection method:

[0131] 1) Delay - related intercept detection:

[0132] The delay - related intercept detection technology is based on the characteristic differences between the signal waveform and the noise in the autocorrelation function to perform non - cooperative detection of the signal. Due to the inevitable potential time - domain correlation in the waveform design of most signals, some of its periodic characteristics can be determined and represented by a time function. While noise has random uncertainty and cannot be determined and represented by a time function.

[0133] In the delay-related intercept detection, some parameters are as follows:

[0134] Spreading factor of the conventional signal: k d = 512;

[0135] Spreading factor of the covert signal: k y = 4096;

[0136] Combined power ratio of the conventional signal and the covert signal: [A d 2 / A y 2 dB = 10;

[0137] The intercept detection curve using the delay-related method is as Figure 5 shown. For the conventional signal with a fixed spreading sequence, SNR = -14 dB is the lower limit for it to be detected by the delay correlation method; while for the covert signal with random frequency hopping spreading, due to the destruction of the cyclostationary characteristics of the signal, the presence of the covert signal can hardly be detected by the delay correlation method under low signal-to-noise ratio conditions.

[0138] 2) Higher-order statistic intercept detection:

[0139] Higher-order statistics refer to statistics higher than the second order. Higher-order moments, higher-order moment spectra, higher-order cumulants, and higher-order cumulant spectra all belong to higher-order statistics. The higher-order statistic method is used for intercept detection. Specifically:

[0140] In the fourth-order statistic intercept detection, some parameters are as follows:

[0141] Spreading factor of the conventional signal: k d = 512;

[0142] Spreading factor of the covert signal: k y = 4096;

[0143] Combined power ratio of the conventional signal and the covert signal: [A d 2 / A y 2 dB = 10;

[0144] The intercept detection curve using the higher-order statistic method is as Figure 6 shown. By Figure 6 ​​It can be seen that, in terms of signal presence detection, SNR = -16 dB can be regarded as the lower limit for it to be detected by the high-order statistics algorithm; for a conventional signal with a fixed spreading sequence, SNR = -14 dB is the lower limit for it to be detected by the high-order statistics method; while for a covert signal with random frequency hopping spreading, due to the destruction of the cyclic stationary characteristics of the signal, in the case of low signal-to-noise ratio, the presence of the covert signal can hardly be detected by the high-order statistics method.

[0145] 3) Cyclic spectrum interception detection:

[0146] The cyclic spectrum is the Fourier transform of the cyclic autocorrelation, and the cyclic autocorrelation refers to the periodic correlation at different times in a time-domain random signal. The cyclic spectrum and cyclic autocorrelation methods are one of the commonly used non-cooperative signal interception detection algorithms. Generally speaking, some signals that do not have obvious first-order cyclic stationarity or generalized stationary characteristics (most random signals belong to this category) may often have higher-order periodic characteristics, called second-order periodic characteristics or cyclic stationary characteristics, and these characteristics can be well extracted through cyclic autocorrelation and cyclic spectrum.

[0147] In the cyclic spectrum interception detection, some parameters are as follows:

[0148] Spreading factor of the conventional signal: k d = 512;

[0149] Spreading factor of the covert signal: k y = 4096;

[0150] Combined power ratio of the conventional signal and the covert signal: [A d 2 / A y 2 dB = 10;

[0151] Cyclic spectrum generation algorithm: Time smoothing accumulation algorithm (FAM).

[0152] The interception detection curve using the cyclic spectrum method is as Figure 7 shown. From Figure 7 it can be seen that from the cyclic spectrum interception detection curve, for signal presence, SNR = -14 dB is the lower limit for it to be detected by the cyclic spectrum method; and the lower limit for it to distinguish the presence of a conventional signal is SNR = -10 dB; while for a covert signal with random frequency hopping spreading, due to the destruction of the cyclic stationary characteristics of the signal, in the case of low signal-to-noise ratio, the presence of the covert signal can hardly be detected by the cyclic spectrum method.

[0153] In summary, the baseband transmission signal generated by the covert communication transmission method based on orthogonal time-frequency-space technology described in this embodiment has good covert performance. ​

[0154] Embodiment 3

[0155] As Figure 8 shown, a method for receiving covert communication based on orthogonal time-frequency-space technology includes the following main steps:

[0156] S5: Perform matched filtering and orthogonal time-frequency-space demodulation on the synchronized baseband received signal, and then perform linear channel equalization based on the equivalent channel state information to output a received combined symbol stream;

[0157] S6: Based on the received combined symbol stream, perform despreading and demodulation of the conventional signal using a fixed spreading sequence to obtain hop code control information, and perform hop code despreading and demodulation on the covert signal according to the hop code control information and the received combined symbol stream to output a covert data coded bit sequence.

[0158] S7: Perform deinterleaving and LDPC channel decoding processing on the covert signal coded bit sequence to obtain covert data bits.

[0159] Embodiment 4

[0160] This embodiment is a specific implementation of the method for receiving covert communication based on orthogonal time-frequency-space technology described in Embodiment 3. As Figure 9 shown, it includes the following steps:

[0161] S5: Perform matched filtering and orthogonal time-frequency-space demodulation on the synchronized baseband received signal to output a received combined symbol stream; the orthogonal time-frequency-space demodulation includes successively performing vector-matrix transformation, matched filtering and Wigner transformation, symplectic-finite Fourier transform, matrix-vector transformation, and channel estimation and equalization.

[0162] Among them, the baseband received signal r(t) is obtained by superimposing Gaussian white noise on the transmitted signal through the channel, and its expression is:

[0163]

[0164] where w(t) is Gaussian white noise.

[0165] After discretizing r(t), we can obtain:

[0166] r = Hs + w (13)

[0167] where is the vector obtained by discretizing the time-domain received signal. The construction of the equivalent channel matrix of the channel in the time domain can be expressed as: where l i and k iThey are the time delay and Doppler discretization positions of the i-th path respectively. The specific steps are as follows:

[0168] S51: Vector-matrix transformation: Convert the digital baseband received signal into a time-domain matrix signal.

[0169] First, transform the received signal vector r into a time-domain matrix signal:

[0170] R = vec -1 (r) (14)

[0171] where is the time-domain received signal matrix.

[0172] S52: Matched filtering and Wigner transformation: Perform shaped matched filtering and Wigner transformation on the time-domain matrix signal to generate a time-frequency domain matrix.

[0173] The Wigner transformation is the inverse transformation of the Heisenberg transformation, and its principle expression is:

[0174]

[0175] where t is time, f is frequency, Δf is the bandwidth of the frequency-domain sampling interval, T is the time-domain sampling interval (similar to the sampling interval and subcarrier interval in OFDM modulation); n is the time / Doppler domain index of OTFS modulation (n = 0, 1,..., N - 1), m is the frequency / delay domain index of OTFS modulation (m = 0, 1,..., M - 1), which is consistent with the meaning in formula (6). is the cross ambiguity function, and its expression is:

[0176]

[0177] g rx is the time-domain response function of the shaped matched filtering.

[0178] After matrixizing the matched filtering and Wigner transformation, its algorithm is as follows:

[0179] Y tf = F M G rx R (17)

[0180] where is the matrix for transforming the signal from the time domain to the time-frequency domain, G rx is the equivalent matrix of the receiver shaping filter, and R is the time-domain received signal matrix.

[0181] S53: Symplectic-finite Fourier transform (SFFT): Perform symplectic-finite Fourier transform on the time-frequency domain matrix to obtain a delay-Doppler domain matrix.

[0182] The frequency domain and the delay domain, and the time domain and the Doppler domain form two Fourier transform pairs. The frequency domain can be transformed to the delay domain through the inverse fast Fourier transform, and the time domain can be transformed to the Doppler domain through the Fourier transform. Therefore, the time-frequency domain signal can return to the time-delay-Doppler domain through the symplectic finite Fourier transform. The principle formula of the SFFT is as follows:

[0183]

[0184] where y[l,k] is the (l,k)-th element in the time-delay-Doppler domain matrix, n = 0, 1,..., N - 1, m = 0, 1,..., M - 1.

[0185] After matrixifying the SFFT, the SFFT algorithm is as follows:

[0186]

[0187] where, is the matrix for transforming the signal from the time-frequency domain to the time-delay-Doppler domain.

[0188] S54: Matrix-vector transformation: Perform a matrix-vector transformation on the time-delay-Doppler domain matrix to generate an OTFS demodulation signal.

[0189] Finally, perform a matrix & vector transformation at the receiving end, and then obtain the symbol after OTFS demodulation:

[0190] y = vec(Y dd ) (20)

[0191] S55: Channel estimation and equalization: Based on the equivalent channel state information obtained from channel estimation, perform linear channel equalization on the OTFS demodulation signal and output a received combined symbol stream.

[0192] According to the properties of the vec(·) operator (i.e., the matrix-vector transformation operator) and the Kronecker product, we can obtain that the linear transformation relationship between the symbols (x and y) at the transmitting and receiving ends can be expressed as:

[0193] y = H eff x + w eff (21)

[0194] where, is the equivalent time-delay-Doppler domain channel matrix of the channel (for channel estimation and equalization), is the equivalent time-delay-Doppler domain noise vector of the channel.

[0195] S6: Based on the received and combined symbol stream, perform despreading and demodulation of the fixed spreading sequence on the conventional signal to obtain the hopping code control information, and perform hopping code despreading and demodulation on the covert signal according to the hopping code control information and the received and combined symbol stream, and output the covert signal coding bit sequence.

[0196] After OTFS demodulation and channel equalization, perform despreading and demodulation processing corresponding to non-stationary spreading at the transmitter on the received signal, as Figure 9 shown. Perform despreading and demodulation processing on the conventional signal and despreading, demodulation and decoding processing on the covert signal respectively. The despreading and demodulation of the conventional signal are the same as those of the traditional direct spread signal, and the hopping code control information carried on the conventional signal will be used for the despreading of the non-stationary random hopping code direct spread signal of the subsequent covert signal, and output the estimated covert signal coding bit sequence at the receiver.

[0197] S7: Perform deinterleaving and LDPC channel decoding processing on the covert signal coding bit sequence to obtain the covert data bit data.

[0198] Embodiment 5

[0199] A covert communication transmitting device based on orthogonal time-frequency-space technology for performing any one of the covert communication transmitting methods based on orthogonal time-frequency-space technology described in Embodiment 1 or 2, which is used for the transmitter, as Figure 10 shown, includes: an LDPC coding module, a row-column interleaving module, a hopping code spreading module, a fixed sequence spreading module, a BPSK modulation module, a non-equal power combining module, an orthogonal time-frequency-space modulation module, a digital-to-analog converter, and a radio frequency transmission module.

[0200] The LDPC coding module is used to perform LDPC coding processing on the covert data bits to be transmitted and send them to the row-column interleaving module.

[0201] The row-column interleaving module is used to perform row-column interleaving processing on the data output by the LDPC coding module to generate a covert signal coding bit sequence.

[0202] The hopping code spreading module is used to perform random hopping code spreading on the covert signal coding bits to generate a covert signal hopping code spreading sequence.

[0203] The fixed sequence spreading module is used to perform fixed sequence spreading on the conventional signal bit sequence corresponding to the covert signal coding bits to be transmitted to generate a conventional signal fixed spreading sequence.

[0204] The BPSK modulation module is used to perform BPSK modulation processing on the covert signal hopping code spreading sequence and the conventional signal fixed spreading sequence respectively and send them to the non-equal power combining module.

[0205] The unequal power combining module is used to perform unequal power combining on the data output by the BPSK modulation module to generate a combined symbol stream.

[0206] The orthogonal time-frequency-space modulation module is used to perform orthogonal time-frequency-space modulation and transmitter shaping filtering on the combined symbol stream to generate a digital baseband transmission signal.

[0207] The digital-to-analog converter, i.e., DAC, is used to convert the digital baseband transmission signal into an analog baseband transmission signal and send it to the radio frequency transmission module.

[0208] The radio frequency transmission module performs a series of radio frequency signal processing on the analog baseband transmission signal and transmits it through an antenna.

[0209] Embodiment 6

[0210] A covert communication receiving device based on orthogonal time-frequency-space technology for implementing the covert communication receiving method based on orthogonal time-frequency-space technology according to any one of Embodiments 3 or 4, for the receiving end, as Figure 11 shown, includes: a radio frequency processing module, an analog-to-digital converter, a timing synchronization module, an orthogonal time-frequency-space demodulation module, a channel estimation and channel equalization module, a despreading and demodulation module, and a deinterleaving and LDPC channel decoding module.

[0211] The radio frequency processing module is used to receive the transmitted signal and obtain an analog baseband received signal.

[0212] The analog-to-digital converter is used to convert the analog baseband received signal into a digital baseband received signal.

[0213] The timing synchronization module is used for time synchronization of the received signal to obtain a synchronized digital baseband received signal, so as to ensure that a series of subsequent receiving processing modules can start to process the received signal in sequence at an appropriate moment;

[0214] The orthogonal time-frequency-space demodulation module is used to perform shaping matched filtering and orthogonal time-frequency-space demodulation on the synchronized digital baseband received signal and output an OTFS demodulation signal.

[0215] The channel estimation and channel equalization module is used to perform linear channel equalization based on the equivalent channel state information on the OTFS demodulation signal and output a received combined symbol stream.

[0216] The despreading and demodulation module includes a conventional signal despreading and demodulation module and a covert signal despreading and demodulation module. The conventional signal despreading and demodulation module is used to despread the fixed spreading sequence of the conventional signal to obtain the hopping code control information. The covert signal despreading and demodulation module despreads the hopping code spreading sequence of the covert signal according to the hopping code control information and outputs the encoded bit sequence of the covert signal.

[0217] The deinterleaving and LDPC channel decoding module is used to perform deinterleaving and LDPC decoding on the encoded bit sequence of the covert signal to obtain the received covert data bit data.

[0218] The above are only the preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A covert communication transmission method based on orthogonal time-frequency-space technology, characterized in that, It includes the following steps: S1: Perform LDPC coding and row-column interleaving on the covert data bits to be transmitted to obtain a coded bit sequence of the covert signal; the covert data bits to be transmitted are data streams that have completed pattern adaptation and stream adaptation; S2: Perform random hopping code spreading on the coded bits of the covert signal to obtain a hopping code spread sequence of the covert signal; and perform fixed sequence spreading on the corresponding conventional signal bit sequence of the coded bits of the covert signal to be transmitted to obtain a fixed spread sequence of the conventional signal; wherein, the hopping code control information for the random hopping code spreading is intercepted from the conventional signal bit sequence; S3: Respectively perform BPSK modulation on the hopping code spread sequence of the covert signal and the fixed spread sequence of the conventional signal, and then perform non-equal power combination to generate a combined symbol stream; S4: Perform orthogonal time-frequency-space transformation and shaping filtering on the combined symbol stream to generate a baseband transmission signal; the orthogonal time-frequency-space transformation includes vector-matrix transformation, inverse symplectic-Fast Fourier transform, Heisenberg transform, and matrix-vector transformation processing.

2. The method for a covert communication transmitter based on orthogonal time-frequency-space technology according to claim 1, characterized in that The expression for random hopping code spreading in S2 is: Among them, b y [n] represents the covert signal hopping code spread spectrum sequence, is the exclusive OR operator, d y [n] represents the encoded bit sequence of the covert signal, p y [n] represents the pseudo-random sequence of random hopping code spread spectrum, p y The bit rate of [n] is d y [n] of k y times, k y is the spreading factor of the covert signal.

3. A covert communication transmission method based on orthogonal time-frequency-space technology according to claim 1, characterized in that The expression for fixed sequence spreading in S2 is: Among them, b d [n] represents the fixed spreading sequence of the regular signal, is the exclusive OR operator, d d [n] represents the regular signal bit sequence corresponding to the coded bit of the hidden signal to be transmitted, p d [n] represents the pseudo-random sequence spread by the fixed sequence, p d [n] has a bit rate of d d [n] of k d times, k d is the spreading factor of the regular signal.

4. A method for transmitting covert communication based on orthogonal time-frequency-space technology according to claim 1, characterized in that, The expression for the combination operation in S3 is: x B [n] = A d x d [n] + A y x y [n], Among them, x B [n] represents the combined symbol stream, and x d [n] represents the symbol sequence after the conventional signal is modulated by BPSK, and x y [n] represents the symbol sequence after the covert signal is modulated by BPSK. A d represents the amplitude of the conventional symbol sequence, and A y represents the amplitude of the covert symbol sequence.

5. A covert communication transmitting method based on orthogonal time-frequency-space technology according to claim 1, characterized in that, S4 includes the following steps: S41: Divide the combined symbol stream into a group of vectors with a length of MN. Each group of vectors will pass through vector-matrix transformation, place MN symbols on an M×N-dimensional complex matrix in the time-delay-Doppler domain, and output the matrix; S42: Perform inverse symplectic-Fast Fourier transform on the time-delay-Doppler domain complex matrix to obtain a time-frequency domain complex matrix; S43: Perform Heisenberg transform and transmitter shaping filtering on the time-frequency domain complex matrix to obtain a time-domain matrix signal; S44: Perform matrix-vector transformation on the time-domain matrix signal to generate a baseband transmission signal.

6. A method for receiving covert communication based on orthogonal time-frequency-space technology, characterized in that For the receiving end, and the receiving method matches any one of the covert communication transmission methods based on orthogonal time-frequency-space technology described in claims 1-5. The method includes the following steps: S5: Perform matched filtering and orthogonal time-frequency-space demodulation on the baseband received signal after synchronization, and then perform linear channel equalization based on the equivalent channel state information to output a received combined symbol stream; S6: Based on the received combined symbol stream, perform fixed spread sequence despreading and demodulation on the conventional signal to obtain the hopping code control information, and according to the hopping code control information and the received combined symbol stream, perform hopping code despreading and demodulation on the covert signal to output a coded bit sequence of the covert signal; S7: Perform deinterleaving and LDPC channel decoding processing on the coded bit sequence of the covert signal to obtain covert data bits.

7. A covert communication receiving method based on orthogonal time-frequency-space technology according to claim 6, characterized in that S5 includes the following steps: S51: Divide the synchronized digital baseband received signal into a group of vectors with a length of MN. Each group of vectors will pass through vector-matrix transformation, place MN symbols on an M×N-dimensional complex matrix in the time domain, and output the matrix; S52: Perform shaping matched filtering and Wigner transform on the time-domain matrix to obtain a time-frequency domain matrix; S53: Perform a symplectic - finite Fourier transform on the matrix in the time - frequency domain to obtain a time - delay - Doppler domain matrix; S54: Perform a matrix - vector transform on the time - delay - Doppler domain matrix to obtain an OTFS demodulated signal; S55: Perform linear channel equalization on the OTFS demodulated signal based on the equivalent channel state information and output the received combined symbol stream.

8. A covert communication transmitting device based on orthogonal time-frequency-space technology for performing any one of the covert communication transmitting methods based on orthogonal time-frequency-space technology described in claims 1-5, characterized in that, For the transmitter, the device includes: an LDPC encoding module, a row - column interleaving module, a hopping code spreading module, a fixed - sequence spreading module, a BPSK modulation module, a non - equal - power combining module, an orthogonal time - frequency - space modulation module, a digital - to - analog converter, and a radio - frequency transmission module; The LDPC encoding module is used to perform LDPC channel encoding on the concealed data bits to be transmitted and send them to the row - column interleaving module; The row - column interleaving module is used to perform row - column interleaving processing on the data output by the LDPC encoding module to generate a concealed signal encoded bit sequence; The hopping code spreading module is used to perform random hopping code spreading on the concealed signal encoded bits to generate a concealed signal hopping code spreading sequence; The fixed - sequence spreading module is used to perform fixed - sequence spreading on the conventional signal bit sequence corresponding to the concealed signal encoded bits to be transmitted to generate a conventional signal fixed spreading sequence; The BPSK modulation module is used to perform BPSK modulation processing on the concealed signal hopping code spreading sequence and the conventional signal fixed spreading sequence respectively and send them to the non - equal - power combining module; The non - equal - power combining module is used to perform non - equal - power combining on the data output by the BPSK modulation module to generate a combined symbol stream; The orthogonal time - frequency - space modulation module is used to perform orthogonal time - frequency - space modulation and transmitter shaping filtering on the combined symbol stream to generate a digital baseband transmit signal; The digital - to - analog converter is used to convert the digital baseband transmit signal into an analog baseband transmit signal and send it to the radio - frequency transmission module; The radio - frequency transmission module performs a series of radio - frequency signal processing on the analog baseband transmit signal and transmits it through an antenna.

9. A covert communication receiving device based on orthogonal time-frequency-space technology for performing the covert communication receiving method based on orthogonal time-frequency-space technology according to claim 7, characterized in that, For the receiver, the device includes: a radio - frequency processing module, an analog - to - digital converter, a timing synchronization module, an orthogonal time - frequency - space demodulation module, a channel estimation and equalization module, a despreading and demodulation module, and a de - interleaving and LDPC channel decoding module; The radio - frequency processing module is used to receive the transmitted signal and obtain an analog baseband received signal; The analog - to - digital converter is used to convert the analog baseband received signal into a digital baseband received signal; The timing synchronization module is used to perform time synchronization on the received signal to obtain a synchronized digital baseband signal; The orthogonal time - frequency - space demodulation module is used to perform shaping matching filtering and orthogonal time - frequency - space demodulation on the synchronized digital baseband received signal and output an OTFS demodulated signal; The channel estimation and channel equalization module is used to perform linear channel equalization on the OTFS demodulated signal based on the equivalent channel state information and output a received combined symbol stream; The despreading and demodulation module includes a conventional signal despreading and demodulation module and a covert signal despreading and demodulation module; the conventional signal despreading and demodulation module is used to perform fixed sequence despreading and demodulation on the conventional signal to obtain the hopping code control information; the covert signal despreading and demodulation module performs hopping despreading and demodulation on the covert signal according to the hopping code control information and outputs the encoded bit sequence of the covert signal. The deinterleaving and LDPC channel decoding module is used to perform deinterleaving and LDPC decoding processing on the encoded bit sequence of the covert signal to obtain the covert data bit data.

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