A low-interception waveform design and processing method based on composite encryption

Abstract

The application discloses a low-interception waveform design and processing method based on composite encryption, applied to the technical field of radar detection, and aims at the problem that the existing radar transmission waveform is easily intercepted by enemy electronic jammers; the method adopts a new encrypted waveform encrypted by a pseudo-random binary code, and the encrypted waveform effectively disguises a conventional waveform by presenting chaotic parameters; then, since the encrypted signal can have a large bandwidth, a special decryption method based on local detection is proposed, which helps to reduce the signal-to-noise ratio drop in the extended target detection to the maximum extent; finally, the coherent processing is completed within one pulse repetition time, and the position and speed information of the target are obtained.

Description

Technical Field

[0001] This invention belongs to the field of radar detection and low intercept waveform design technology, and specifically relates to a radar waveform design and processing technology. Background Technology

[0002] With the continuous development of electronic warfare, the need for electromagnetic environment awareness has led to the emergence of a series of advanced reconnaissance technologies. The waveforms emitted by traditional radars are becoming increasingly easier for enemy electronic reconnaissance equipment to intercept and sort, posing a significant threat to the survivability of traditional radar systems. In recent decades, low probability of intercept (LPI) radar has been proposed and has become a current trend in radar design. LPI radar can be implemented through low probability of intercept beamforming design and low probability of intercept waveform design. Compared with beamforming design, low probability of intercept waveform design has high versatility and ease of implementation, and therefore has attracted much attention.

[0003] In the paper "X. Liu, T. Zhang, X. Yu, Q. Shi, G. Cui, and L. Kong, 'LPI waveform design for radar system against cyclostationary analysis intercept processing,' Signal Process., vol. 201, 2022," low probability of intercept (LPI) waveform design methods are mainly divided into two categories: anti-interception and anti-identification. The first method primarily achieves counter-reconnaissance and low probability of intercept by reducing the power of the transmitted waveform to prevent it from exceeding the detection threshold of electronic reconnaissance equipment. However, low transmission power reduces the radar echo signal-to-noise ratio, leading to a decrease in radar detection performance. Therefore, the literature "Liu Jiafang. Research on Broadband Low Probability of Interception and Random Polarization Radar Signal Model [D]. University of Chinese Academy of Sciences (National Space Science Center, Chinese Academy of Sciences), 2019" mentions that current low probability of intercept waveforms often use long time-width product waveforms, such as LFM (Linear Frequency Modulation) and NLFM (Non-linear Frequency Modulation). These waveforms, due to their high coherence integral gain, can achieve a high signal-to-noise ratio after echo processing while using lower transmit power. However, these waveforms usually exhibit regular characteristics in the time or frequency domain, making them easily sorted and identified by electronic reconnaissance equipment. The second method mainly achieves low probability of intercept by designing waveforms in the time or frequency domain. These low probability of intercept waveforms mainly consist of various phase-coded waveforms, such as pseudo-random phase sequences. With the development of electronic reconnaissance technology, advanced time-frequency analysis techniques can be used to identify traditional low probability of intercept waveforms. Summary of the Invention

[0004] To address the problem that traditional low probability of intercept (LPI) waveforms are becoming increasingly easy to intercept, this invention proposes a novel LPI radar waveform design and processing method based on composite encryption. At the transmitting end, a specific sequence is used to encrypt the transmitted detection waveform to achieve low intercept. At the receiving end, a target detection method based on local detection is introduced to ensure detection performance.

[0005] One of the technical solutions adopted in this invention is: a low-interception waveform design method based on composite encryption, comprising:

[0006] A1. The signal source generates an LFM signal as the signal before encryption;

[0007] A2. Use the pseudo-random binary code as the encryption sequence, multiply it with the waveform generated in step A1 to encrypt it and form a low intercept waveform.

[0008] A3. Upconvert the encrypted waveform from step A2 and use it as the radar transmission signal.

[0009] The second technical solution adopted in this invention is: a low-interception waveform processing method based on composite encryption, comprising:

[0010] B1. The receiving end uses a local detection-based extended target processing method to downconvert and decrypt the received echo signal.

[0011] B2. Perform pulse compression and moving target detection (MTD) processing on the decrypted target echo signal to extract the target's range and Doppler information.

[0012] The beneficial effects of this invention are as follows: First, this invention uses pseudo-random binary phase codes to encrypt the detection signal, resulting in a chaotic signal with disordered parameters in both the time and frequency domains. Then, a processing method corresponding to the encrypted waveform is proposed, along with a special decryption method based on local detection, which helps to minimize the signal-to-noise ratio (SNR) drop in extended target detection. Finally, coherent processing is completed within one pulse repetition time to obtain the target's position and velocity information. Simulation results show that the waveform designed in this invention has low interception characteristics and can detect the target without sacrificing detection performance. This invention, through its proposed decryption method based on local detection, effectively reduces the SNR loss during extended target detection. Attached Figure Description

[0013] Figure 1 This is a radar system framework that employs composite encrypted waveforms.

[0014] Figure 2 This is a flowchart of the extended target decryption process based on local detection.

[0015] Figure 3 The time-domain waveform and spectrum of the composite encryption waveform in an embodiment of the present invention;

[0016] Where (a) is the time-domain waveform and (b) is the frequency spectrum.

[0017] Figure 4 This is a comparison of the spectra of the composite encrypted waveform and the narrowband LFM signal in an embodiment of the present invention.

[0018] Figure 5 The time-frequency diagrams of the embodiments of the present invention and narrowband LFM signals are compared to reflect the improvement in low intercept performance;

[0019] Wherein, (a) is the time-frequency diagram of this embodiment, and (b) is the time-frequency diagram of the narrowband LFM signal.

[0020] Figure 6 This is a comparison chart of the pulse compression results for point target detection using embodiments of the present invention and narrowband LFM signals;

[0021] Wherein, (a) is the pulse compression result of point target detection in this embodiment, and (b) is the pulse compression result of point target detection of narrowband LFM signal.

[0022] Figure 7 This is a comparison chart of MTD results for point target detection using embodiments of the present invention and narrowband LFM signals;

[0023] Wherein, (a) is the MTD result of point target detection in this embodiment, and (b) is the MTD result of point target detection of narrowband LFM signal.

[0024] Figure 8 The diagram shows the target detection results of the embodiment of the present invention and the extension of narrowband LFM and broadband LFM signals.

[0025] Figure 9 This is a comparison chart of the echo processing results of the local detection-based decryption method in this invention and the traditional decryption method. Detailed Implementation

[0026] To facilitate understanding of the technical content of this invention by those skilled in the art, the following description, in conjunction with the accompanying drawings, further illustrates the invention.

[0027] The main technical framework and signal processing flow of this invention are attached. Figure 1 As shown, it specifically includes two parts: the low intercept waveform design process of the sending end based on composite encryption, and the corresponding processing process of the receiving end on the received encrypted waveform;

[0028] The low-interception waveform design process for the transmitting end based on composite encryption includes the following steps:

[0029] S1. The signal source generates an LFM signal as the signal before encryption;

[0030] S2. Use the pseudo-random binary code as the encryption sequence, multiply it with the waveform generated in step S1 to encrypt it and form a low intercept waveform.

[0031] S3. Upconvert the encrypted waveform from step S2 and use it as a radar transmission signal.

[0032] The receiving end's processing of the received encrypted waveform includes the following steps:

[0033] S4. The received echo signal is down-converted and decrypted using an extended target processing method based on local detection.

[0034] S5. Perform pulse compression and MTD processing on the decrypted target echo signal to extract the target's range and Doppler information.

[0035] Step S1 is as follows:

[0036] S11 and LFM signals can be represented as

[0037] s(t)=exp{jπμt 2} (1)

[0038] Where t∈[0,T] p ], T P Indicates the signal pulse width, μ = B / T P B represents the frequency modulation slope of the LFM signal, and B is the signal bandwidth.

[0039] Step S2 specifically includes the following sub-steps:

[0040] S21. First, given a uniformly distributed pseudo-random sequence c(t), where the sequence follows a uniform distribution between (-1, 1), binarize the sequence, where values ​​greater than 0 are 1, and values ​​less than or equal to 0 are -1. Describe the binarized encrypted sequence c in the form of code chips. B (t) can be represented as:

[0041]

[0042] Among them, c n Represents a binary confidential sequence c B The nth chip of (t), T c N represents the duration of each chip. c This represents the number of chips. g(t) represents a rectangular function, which can be expressed as:

[0043]

[0044] S22. An arbitrary baseband signal s(t) generated by a radar signal source is represented in chip form as follows:

[0045]

[0046] Among them, s n T represents the nth chip of the LFM signal s(t). c This represents the duration of each chip, consistent with the meaning and value described above. s(t) is then compared with the binarized encryption sequence c generated in step S21. B Multiplying (t) together, we obtain the encrypted waveform d(t), which can be expressed as:

[0047] d(t)=s(t)c B (t) (4)

[0048] Step S3 is as follows:

[0049] S31. Assume the radar pulse repetition time is T and the transmitted signal carrier frequency is f. c The transmitted signal is represented as

[0050] x(t)=d(t)exp{j2πf c t} (3)

[0051] Step S4 is as follows:

[0052] S41. First, model the echo of the extended target. Assuming there are M pulses within a coherent processing interval, the slow-time intermediate frequency echo model of the m-th pulse received by the radar can be expressed as:

[0053]

[0054] Among them, △T m = (m-1)T, where T is the pulse repetition time, consistent with the previous definition and value, τ p -2v(t+△T m ) / c represents the time delay corresponding to the m-th pulse, P represents the total number of scattering points of the extended target, and A p ,τ p Let R represent the scattering coefficient (including amplitude and phase) and the corresponding time delay of the p-th scattering point, respectively. Assume the relative distance between the p-th scattering point of the target and the radar is R. p Therefore, at t=0, the time delay of the p-th scattering point of the target relative to the radar is τ. p =2R p / c, where c is the speed of light, and it is assumed that the velocity of each scattering point is v;

[0055] S42. Down-convert the intermediate frequency signal r(t,m) to obtain the baseband echo signal r. d(t,m), denoted as

[0056]

[0057] Among them, A p ′=A p exp{j2πf c τ p} represents the constant term; f d =2vf c / c represents the Doppler frequency of the target;

[0058] S43. The baseband echo signal r in step S32 is processed using an extended target processing method based on local detection. d The flowchart of the extended target processing method for (t,m) decryption and local detection is attached. Figure 2 As shown. Assuming the radar is in tracking mode, it only needs to focus on the ranging interval of interest, i.e., several ranging units near the target. The division of ranging units is related to the sampling rate, signal bandwidth, and wave position, and is often set to the time width of the two pulses before and after the target pulse location. The echo delay corresponding to the ranging resolution unit of the narrowband waveform is set to the size of the local detection window in the echo. At the same time, decryption is applied to each segment in the local detection window, and the length of each segment is equal to the pulse width.

[0059] The specific decryption process can be summarized as follows: First, determine the echo delay corresponding to the range interval of interest, and divide the local detection window according to the range resolution unit of the narrowband waveform; then, use the same c(t) as the transmitter to traverse each segment in the time domain that may be the target echo. All echo segments are included in the local detection window. Before decryption, zero-padded each window is applied to make its length equal to the length of the local detection window; assuming the radar is in tracking mode, the corresponding distance of each window where the target may exist is known. Therefore, only a few points around the peak value after correlation with s(t) are taken, and all peak values ​​within the local detection window are summed; finally, threshold detection is performed on the summation value. The threshold is the sampling points near the target in tracking mode, and the average of the correlated peak values ​​is taken. The presence of a target in the local detection window can be determined by threshold detection.

[0060] The decrypted echo signal after threshold detection can be represented as:

[0061]

[0062] Among them, t p,m =τ p -2v(t+△T m ) / c.

[0063] Simulation verification and analysis

[0064] Simulation parameters:

[0065] The narrowband LFM signal is the signal before encryption, with a bandwidth B = 5MHz and a time width T. p =10μs, sampling frequency f s =600MHz, pulse repetition period T r =300μs, carrier frequency f c =1GHz, the number of pulses within one CPI is 64; in the extended target scenario, the bandwidth B of the broadband LFM w =200MHz, with the other parameters being the same as the narrowband, and the energy of the broadband LFM being consistent with that of the narrowband LFM.

[0066] In the single-target scenario, the target distance is R = 120km and the target speed is v = 100km / s; in the extended target scenario, the number of target scattering points is 5, the scattering coefficient is 1, and the signal-to-noise ratio (SNR) is 20dB.

[0067] The encryption sequence uses pseudo-random binary code with a symbol width T. B =1 / f s The encryption coefficient is 10.

[0068] The time-frequency plot was calculated using STFT with a Hanning window function, a window length of 128, a HopSize of 4, an Overlap of 124 points, and an FFT of 1024 points. Pulse compression and MTD results were not windowed.

[0069] Simulation analysis:

[0070] Appendix Figure 3 The time-domain waveform and spectrum of the composite encrypted signal after narrowband LFM encryption are shown in the attached figure. Figure 4 To compare the spectra of the composite encrypted signal after narrowband LFM encryption with those of the narrowband LFM signal, combined with the attached... Figure 3 and attached Figure 4 It can be seen that the designed composite encryption signal has a larger bandwidth and a higher degree of confusion compared to the original LFM signal. (See attached image) Figure 5 By comparing the time-frequency diagrams of the composite encrypted signal after narrowband LFM encryption with those of the narrowband LFM signal, it can be concluded that after encryption, general time-frequency analysis cannot verify the signal characteristics, and the designed encrypted signal has low interception characteristics. (Appendix) Figure 6 and attached Figure 7 By comparing the pulse compression and MTD results of the composite encrypted signal after narrowband LFM encryption and the point target detection results of narrowband LFM, it can be concluded that correctly decrypting the encrypted signal does not affect the point target detection performance. (Appendix) Figure 8The image shows a comparison between the designed composite encrypted signal after narrowband LFM encryption and a broadband LFM signal with the same bandwidth. It can be seen that due to the decryption step, even if a broadband encrypted signal is transmitted, the decrypted signal will still become narrowband LFM, resulting in decreased resolution and accumulation of multiple scattering points. (Attached) Figure 9 The graph shows a comparison of pulse compression results between the traditional decryption method and the local detection-based decryption method proposed in this invention. It can be seen that the proposed decryption method can effectively reduce the signal-to-noise ratio loss when dealing with extended targets.

[0071] Those skilled in the art will recognize that the embodiments described herein are intended to help the reader understand the principles of the invention, and should be understood that the scope of protection of the invention is not limited to such specific statements and embodiments. Various modifications and variations can be made to the invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the invention should be included within the scope of the claims of the invention.

Claims

1. A low-interception waveform design and processing method based on composite encryption, characterized in that, include: A1. The signal source generates an LFM signal as the signal before encryption; A2. Using the pseudo-random binary code as the encryption sequence, multiply it with the waveform generated in step A1 to encrypt and form a low-interception waveform; the implementation process of step A2 specifically includes the following sub-steps: A21. Generate a pseudo-random sequence. The sequence follows The uniform distribution between them is used to binarize the sequence, and the binarized pseudo-random sequence is represented in chip form: ； in, Represents a binary pseudo-random sequence The nth chip, This indicates the duration of each chip. Indicates the number of chips. Represents a rectangular function; A22. The LFM signal generated by the signal source in step A1 is represented in chip form as follows: ； in, Indicates signal The nth chip, Indicates the duration of each chip; A23, will The product generated in step A21 Multiply to obtain the encrypted waveform. , represented as: ; A3. Upconvert the encrypted waveform from step A2 and use it as a radar transmission signal; A4. The transmitter sends the radar signal obtained in step A3; A5. The receiving end uses a local detection-based extended target processing method to down-convert and decrypt the received echo signal; the implementation process of step A5 specifically includes the following sub-steps: A51. First, model the echo of the extended target. Assuming there are M pulses in a coherent processing interval, the slow-time intermediate frequency echo model of the m-th pulse received by the radar is expressed as: ； in, This represents the encrypted waveform. Indicates the carrier frequency of the transmitted signal. At the speed of light, , The pulse repetition time, For the goal of The time delay of each scattering point relative to the radar, The velocity of the scattering point, Indicates the first The time delay corresponding to each pulse This represents the total number of scattering points from the extended target. They represent the first The scattering coefficients and corresponding time delays at each scattering point; A52, Yes Down-conversion yields the baseband echo signal. , represented as ； in, For constant terms; Indicates the Doppler frequency of the target; A53. Utilize the extended target processing method based on local detection to process the baseband echo signal in step A52. Decryption A6. Perform pulse compression and MTD processing on the decrypted target echo signal to extract the target's range and Doppler information.

2. The low-interception waveform design and processing method based on composite encryption according to claim 1, characterized in that, The implementation process of step A53 includes the following steps: A531. First, determine the echo delay corresponding to the distance interval of interest, and divide the local detection window according to the range resolution unit of the narrowband waveform; A532, then utilize Traverse every segment in the time domain that could be the target echo. All echo segments are contained in the local detection window. Before decryption, zero-padded each window is made so that its length is equal to the length of the local detection window. A533. Assuming the radar is in tracking mode, the corresponding distance to the target in each possible window is known; therefore, only the distance to the target within the specified window is considered. The correlation peak is observed at several points around it, and all peaks within the local detection window are summed. A534. Finally, a threshold test is performed on the summation value to determine whether a target exists in the local detection window.

3. The low-interception waveform design and processing method based on composite encryption according to claim 2, characterized in that, The decrypted echo signal is represented as follows: ； in, For any baseband signal generated by a radar signal source, .

4. The low-interception waveform design and processing method based on composite encryption according to claim 1, characterized in that, , For the goal of The relative distance between the location of each scattering point and the radar.

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