A signal covert transmission method based on time-varying fractional Fourier transform

Abstract

The present invention proposes a signal covert transmission method based on time-varying fractional Fourier transform, which is applicable to the field of communication technology. The traditional fractional Fourier transform converts the constellation diagram of the wireless signal into the constellation diagram of other modulation methods or noise by adjusting different parameters to achieve signal hiding. On this basis, the present invention adds a time function, sets different conversion parameters at different times of signal transmission, converts into different constellations or noise, and achieves covert signal transmission. During the signal transmission process, after the non-cooperative receiver receives the signal, the addition of the time function increases the difficulty of parsing the signal, making the signal transmission more covert.

Description

Technical Field

[0001] The invention relates to a signal concealment transmission method based on time-varying fractional Fourier transform, which is applicable to the technical field of communication. Background Art

[0002] Fractional Fourier transform is a new type of signal transform developed on the basis of traditional Fourier transform, and is a generalized Fourier transform. It breaks through the limitation of traditional Fourier transform that can only analyze signals in the time domain or frequency domain, and can analyze and process signals in the fractional domain between the time domain and the frequency domain. Through signal processing in the fractional Fourier domain, the modulation characteristics of the communication signal can be changed, and the modulation characteristics of the original signal can be simulated as the modulation characteristics of the target signal, thereby reducing the risk of signal interception, and providing new solutions for signal camouflage, modulation domain encryption and other means in electronic countermeasures.

[0003] By adding the fractional Fourier algorithm and adjusting different parameters, the distribution of the signal constellation diagram rotates and diverges, disguising it as noise or other modulation methods, thereby concealing the signal and improving the signal's anti-detection capability. This causes the non-target receiver to be unable to correctly demodulate the signal by automatically demodulating according to the characteristics of the constellation diagram. Assuming that the non-target receiver knows that the system uses the fractional Fourier transform technology, the parameters of the fractional Fourier transform are identified by enumeration detection. When there are many fractional Fourier transform parameters, it is generally impossible to restore the correct constellation diagram.

[0004] With the development of quantum computing technology, there is a risk of cracking the signal after it is intercepted by the method of performing fractional Fourier transform using a certain set of specific parameters. Therefore, it is necessary to propose a fractional Fourier transform method with variable transformation parameters to enhance the signal's anti-cracking ability. Summary of the invention

[0005] The purpose of the present invention is to design a fractional Fourier transform method with variable parameters, so as to avoid the problem that when a set of specific transform parameters is used for signal transmission, a non-target receiver uses powerful computing resources to identify the transform parameters by enumeration detection.

[0006] The technical solution adopted by the present invention is:

[0007] A signal covert transmission method based on time-varying fractional Fourier transform specifically comprises the following steps:

[0008] (1) After the data source at the transmitting end is baseband modulated, it is forward transformed using the fractional Fourier algorithm. The fractional Fourier algorithm selects transformation parameters that vary with time and changes the modulation characteristics of the signal in real time.

[0009] (2) Send out the signal after processing;

[0010] (3) After receiving the signal, the receiver performs filtering and then uses the fractional Fourier algorithm to inversely transform the signal to restore the original modulated signal characteristics; the inverse transformation parameters are consistent with the forward transformation parameters and change synchronously with time;

[0011] (4) The inverse transformed signal is demodulated at baseband to restore the original data.

[0012] Furthermore, the forward transformation parameters and the inverse transformation parameters in step (1) and step (3) include the transformation order and a scale vector V, where The value range of is (0,4], V = [ M V , N V ] , M V = [ m 0 , m 1 , m 2 , m 3 ] , N V = [ n 0 , n 1 , n 2 , n 3 ] , and the parameter of the scale vector is an integer, is the number of weighted items, =4;

[0013] Transformation order :

[0014]

[0015] Scale vector V:

[0016]

[0017] in, Indicates rounding towards 0. For time.

[0018] Furthermore, the time when the sending end and the receiving end are synchronized is set to time 0.

[0019] The beneficial effects of the present invention are:

[0020] When using fractional Fourier transform to change the modulation characteristics of a signal, since the transformation parameters are changing and not a certain set of fixed values, it is very difficult for a non-cooperative receiver to parse the parameters at different times. The present invention can further increase the difficulty of signal cracking for a non-cooperative receiver by changing the frequency of parameter changes, thereby achieving secure transmission of the signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the information transmission process of the present invention.

[0022] Figure 2 It is a schematic diagram of signal modulation characteristics at different moments during the specific implementation of the present invention. DETAILED DESCRIPTION

[0023] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0024] A signal covert transmission method based on time-varying fractional Fourier transform is based on the classic 4-term multi-parameter fractional Fourier transform (WFRFT, Weighted-type Fractional Fourier Transform). When using fractional Fourier transform to transmit signals, a time function is added, and different parameter values ​​are selected at different times, so that the modulation characteristics of the signal change dynamically over time, increasing the difficulty of analysis for non-cooperative receivers and improving the concealment of the signal. Figure 1 As shown, the specific steps include:

[0025] (1) After the data source at the transmitting end is baseband modulated, it is forward transformed using the fractional Fourier algorithm. The fractional Fourier algorithm selects transformation parameters that vary with time and sets a function that varies with time. Different function output values ​​correspond to different transformation parameters. The signal is fractionally transformed, making the signal present different modulation characteristics. The parameter transformation frequency is variable, which can further enhance the signal security performance.

[0026] like is any complex sequence, They are The 0~3 discrete Fourier transform can define the multi-parameter WFRFT as follows:

[0027] F 4 W α , V [ X 0 ] = w 0 X 0 + w 1 X 1 + w 2 X 2 + w 3 X 3

[0028] in, is the weighting coefficient, =0,1,2,3, yes and V, defined as follows:

[0029] w l ( α , V ) = 1 M ∑ k = 0 M − 1 exp { ± 2 π i M [ ( Mm k + 1 ) α ( k + Mn k ) − lk ] }

[0030] in is the transformation order, the value range is (0,4], V = [ M V , N V ] is the scale vector, M V = [ m 0 , m 1 , m 2 , m 3 ] , N V = [ n 0 , n 1 , n 2 , n 3 ] , and the parameter of the scale vector is an integer, is the number of weighted items, =4.

[0031] The control parameters of multi-parameter WFRFT transform domain modulation include transform order 𝛼 and scale vector V. Therefore, multi-parameter WFRFT modulation has a total of nine control parameters. Through the optimization design of the above nine parameters, the fission and fuzzy changes of the weighted signal constellation characteristics can be achieved, and the modulation characteristics of the signal can be changed. The time-varying transform parameter formula is as follows.

[0032] Transformation order:

[0033]

[0034] Scale vector:

[0035]

[0036] Since the scale vector parameter is an integer, Indicates rounding towards 0. Indicates time.

[0037] (2) The positive transformed signal is sent out after D / A conversion, filtering and up-conversion.

[0038] (3) After receiving the signal, the receiver performs down-conversion, filtering and A / D conversion, and then uses the fractional Fourier algorithm to inversely transform the signal to restore the original modulated signal characteristics. The inverse transformation parameters are consistent with the forward transformation parameters and change synchronously with time. In order to ensure consistency with the transformation parameters of the transmitter, the time when the two ends are synchronized is taken as time 0 to ensure time alignment. The same time function is used to select the transformation parameters to achieve signal restoration.

[0039] (4) The inverse transformed signal is demodulated at baseband to restore the original data.

[0040] like Figure 2 As shown, taking the original signal as QPSK modulation as an example, the constellation diagram after fractional Fourier transform at different times generates different transformation parameters at different times, resulting in a time-varying constellation diagram, which increases the concealment of the signal.

[0041] In summary, based on the classical weighted fractional Fourier transform, the present invention adds a set of time-varying functions, so that the transformation parameters change with time, generating a time-varying constellation diagram, thereby increasing the concealment of signal transmission; at the same time, by changing the unit of the time parameter, the frequency of the constellation diagram transformation can be changed, thereby further increasing the difficulty of analysis for non-cooperative receivers and achieving secure signal transmission.

Claims

1. A signal covert transmission method based on time-varying fractional Fourier transform, characterized in that: The specific steps include: (1) After the data source at the transmitting end is baseband modulated, it is forward transformed using the fractional Fourier algorithm. The fractional Fourier algorithm selects transformation parameters that vary with time and changes the modulation characteristics of the signal in real time. (2) Send out the signal after processing; (3) After receiving the signal, the receiver performs filtering and then uses the fractional Fourier algorithm to inversely transform the signal to restore the original modulated signal characteristics; the inverse transformation parameters are consistent with the forward transformation parameters and change synchronously with time; (4) Demodulate the inverse transformed signal through baseband to restore the original data; The forward transformation parameters and the inverse transformation parameters in step (1) and step (3) include the transformation order and a scale vector V, where The value range of is (0,4], , , and the parameter of the scale vector is an integer; Transformation order : ； Scale vector V: ； in, Indicates rounding towards 0. For time.

2. The signal concealment transmission method based on time-varying fractional Fourier transform according to claim 1 is characterized in that: Set the synchronization time between the sender and the receiver to time 0.

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