A method for signal frequency demodulation of a photon sub-nyquist folded receiver

Abstract

This invention discloses a method for frequency deambiguation of a photonic sub-Nyquist folded receiver signal, belonging to the field of microwave photonic signal processing technology. The method includes: inputting multi-mode time-frequency domain overlapping radio frequency signals into a first photonic folded link, a second photonic folded link, and a third photonic folded link respectively to obtain three multi-mode time-frequency domain overlapping signals; using an independent component extraction method with maximum negative entropy, processing the three multi-mode time-frequency domain overlapping signals in parallel for signal separation; and finally measuring the frequency ambiguity number of the three independent signals after signal separation. This invention separates multi-mode time-frequency domain overlapping signals based on a photonic sub-Nyquist folded receiver and performs frequency deambiguation measurement on the separated signals. This invention solves the current problem of difficulty in measuring signal parameters under multi-mode time-frequency domain overlap, which is beneficial for improving the target recognition rate of the signal and enhancing subsequent tracking performance.

Description

Technical Field

[0001] This invention belongs to the field of microwave photonic signal processing technology, and in particular relates to a method for deambiguing the signal frequency of a photonic sub-Nyquist folded receiver. Background Technology

[0002] With the rapid development of radar, communication, and electronic countermeasures systems, new signal types such as low-intercept radar and pulse-to-pulse waveform transform radar are becoming increasingly diverse, and the electromagnetic environment is becoming more complex and variable. Reconnaissance receivers used for electronic support are also gradually developing towards wider operating frequency bands, smaller weight and size, and higher processing speeds. Traditional reconnaissance receivers, mainly based on superheterodyne or channelized systems using radio frequency electronics, are limited by electronic bandwidth bottlenecks when simultaneously receiving and processing signals across a wide instantaneous frequency range, making them increasingly unable to meet the demands of future electronic warfare.

[0003] Therefore, microwave photonics receiving technology, with its advantages of large instantaneous bandwidth, resistance to electromagnetic interference, and low transmission loss, is expected to break through the bottleneck of broadband signal reception and processing in electronic reconnaissance receiving systems, and significantly improve the instantaneous operating frequency range of reconnaissance receivers. In microwave photonics-based receiving systems, photonic sub-Nyquist folded receivers, composed of sampling analog photonic links, have promising application prospects in future electronic warfare for situational awareness in complex electromagnetic spectrums due to their large instantaneous frequency coverage and link structure that directly samples radio frequency signals at different frequencies.

[0004] Signal parameter measurement is a crucial aspect of radar reconnaissance receivers. Measuring parameters such as the radar's PDW (Pulse Description Word) allows for the determination of the radiation source's characteristics, type, and location. For photonic Nyquist folded receivers, the spectral folding principle introduces ambiguity in carrier frequency measurement, making accurate signal frequency measurement a significant challenge. Furthermore, the expansion of modern communication systems into wider frequency bands leads to overlap with commonly used radar frequency bands. For example, when a photonic Nyquist folded receiver with a 1 GHz repetition frequency passively receives radar signals from traditional frequency bands (2-18 GHz), it may also receive signals from other communication systems. Moreover, signals from various mixed systems are down-converted to the 0-500 MHz range after passing through the photonic Nyquist folded receiver, potentially causing overlap in both the time and frequency domains. Therefore, to address the signal frequency ambiguity issue in photonic Nyquist folded receivers, the primary challenge is separating overlapping signals in the time and frequency domains. Failure to do so will affect the accurate measurement of the radiation source frequency, consequently impacting subsequent target detection and identification.

[0005] Frequency deambiguity algorithms for photonic sub-Nyquist folded receivers typically employ a method of solving a system of equations by reading signal frequencies through three parallel links with different sampling rates. However, this method is ineffective when dealing with broadband signals or scenarios where multiple signals overlap in time and frequency.

[0006] In summary, the key problem to be solved is how to perform frequency deambiguation on overlapping time-frequency signals of multiple systems under the photonic sub-Nyquist folded receiver system. Summary of the Invention

[0007] To address the problems existing in the prior art, the purpose of this invention is to provide a method for deambiguing the signal frequency of a photonic sub-Nyquist folded receiver, thereby improving the ability of the photonic sub-Nyquist folded receiver to measure signal frequency parameters in complex electromagnetic environments.

[0008] To achieve the above objectives, the present invention provides a method for deambiguing the signal frequency of a photonic sub-Nyquist folded receiver, comprising the following steps: A three-channel photonic sub-Nyquist folded receiver, consisting of three photonic folded links with different sampling rates, is used to simultaneously input multi-mode time-domain overlapping RF signals into the three photonic folded links for down-conversion processing, and output three multi-mode time-frequency domain overlapping intermediate frequency signals respectively. The three-channel multi-system time-frequency domain overlapping intermediate frequency signals are sent to the signal separation module. The independent component extraction method based on the maximum negative entropy is used to remove the correlation between signal vectors and separate the multi-system time-frequency domain overlapping intermediate frequency signals to obtain the separated independent signals. The separated independent signals are subjected to cross-correlation spectrum detection to obtain the abscissa frequency corresponding to the peak value of the cross-correlation spectrum. The abscissa frequency is then divided by the sampling rate difference between the two photonic folding links involved in the cross-correlation to calculate the frequency ambiguity number of the corresponding independent signal.

[0009] Furthermore, the three photonic folded links are the first photonic folded link, the second photonic folded link, and the third photonic folded link. Each photonic folded link includes an optical frequency comb, an electro-optic intensity modulator, a photodetector, and a low-pass filter connected in sequence. The sampling rate of each photonic folded link is determined by the frequency repetition interval of the optical frequency comb teeth in the link.

[0010] Furthermore, the specific steps for outputting the three-channel multi-mode time-frequency domain overlapping intermediate frequency signals include: The multi-mode time-domain overlapping radio frequency signals are respectively input to the radio frequency input terminal of the electro-optic intensity modulator in each photonic folding link to convert the electrical signal into an optical modulation signal. Each optical modulation signal is converted into an electrical signal by a photodetector in its respective link, and then down-converted to an intermediate frequency state through a beat frequency process to obtain three multi-mode time-frequency domain overlapping intermediate frequency signals.

[0011] Furthermore, before using the independent component extraction method based on the maximum negative entropy to remove the correlation between signal vectors, the process also includes the steps of centering and whitening the intermediate frequency signal.

[0012] Furthermore, the specific steps of centralized processing and whitening processing include: The mathematical expectation of the intermediate frequency signal with overlapping time and frequency domains of multiple systems is calculated. The original signal is then centered by subtracting the mathematical expectation, resulting in a centered signal vector with a mean of zero. Calculate the eigenvalues ​​and eigenvectors of the covariance matrix of the centered signal vector, and construct the whitening matrix based on the diagonal matrix composed of the eigenvalues ​​and the eigenvectors; A linear transformation is performed on the centered signal vector using a whitening matrix to obtain a whitened signal vector free of correlation.

[0013] Furthermore, the specific steps for obtaining the separated independent signals using the independent component extraction method based on maximizing negative entropy include: Construct an objective function that maximizes negative entropy. The objective function uses a non-quadratic function to measure the non-Gaussianity of the product of the standardized separation matrix and the whitened signal vector. The objective function is solved using a quasi-Newton iterative method to update and obtain the separation matrix; The final separated signal is obtained by calculating the separation matrix and the whitened signal vector.

[0014] Furthermore, in the process of updating the separation matrix using the quasi-Newton iteration method, the steps of orthogonalizing and standardizing the separation matrix generated in each iteration are also included, and it is determined whether the iteration result meets the preset convergence accuracy. If it does, the final separation matrix is ​​output; otherwise, the iteration continues.

[0015] Furthermore, the specific steps for performing cross-correlation spectrum detection on the separated independent signals include: Each independent signal after separation is processed by envelope detection in parallel. Perform a Fast Fourier Transform on the detected signal to extract the spectral information of each signal. Cross-correlation calculations are performed on the spectral information of the output signals of any two photonic folded links to generate cross-correlation spectra.

[0016] Furthermore, a Fast Fourier Transform (FFT) is performed on the detected signal. Specifically, a sliding window FFT method is adopted, which involves horizontally shifting a time window function of a preset length on the time axis and performing a FFT on the discrete time sequence signal within the time window to obtain continuous spectral information.

[0017] Furthermore, after performing the Fast Fourier Transform and before performing the cross-correlation calculation, a step is included to accumulate the continuous spectral information obtained by sliding the time window, in order to enhance the spectral signal strength.

[0018] The present invention has the following technical effects: This invention can solve the problem of frequency deambiguity when separating overlapping time-frequency signals in a three-channel photonic sub-Nyquist folded receiver. It can effectively obtain the frequency ambiguity number of independent signals, thereby obtaining signal frequency information and improving its ability to detect and identify targets in complex electromagnetic environments.

[0019] To address the issue of overlapping time-frequency domain signals in photonic Nyquist folded receivers, this invention separates signals based on maximizing negative entropy, solving the problem of frequency de-ambiguity in current photonic Nyquist folded receivers when multiple signal types overlap. By separating the overlapping time-frequency domain signals to obtain the independent waveforms of each signal, the frequency ambiguity number of each independent signal can be obtained based on a cross-correlation spectrum detection method. This is beneficial for subsequent target detection and tracking of radar signals, improving radar reconnaissance performance. This invention solves the frequency measurement problem of overlapping time-frequency domain signals using a three-channel photonic Nyquist folded receiver with the advantage of large instantaneous bandwidth reception. It can separate multiple signal types, such as radar communication signals, and obtain signal frequency parameter information, improving the accuracy of target signal identification. Attached Figure Description

[0020] Figure 1 This is a schematic diagram illustrating the process of receiving multi-mode time-frequency domain overlapping signals based on a photonic sub-Nyquist folded receiver in an embodiment of the present invention. Figure 2 This is a flowchart of the signal separation algorithm based on a photonic sub-Nyquist folded receiver in an embodiment of the present invention; Figure 3 This is a schematic diagram illustrating the principle of ambiguity number measurement based on the signal frequency of a photonic sub-Nyquist folded receiver in an embodiment of the present invention. Detailed Implementation

[0021] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0023] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0024] The following combination Figures 1-3 Specific embodiments of the present invention will be described in detail below. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the present invention.

[0025] This invention discloses a method for deambiguing the signal frequency of a photonic sub-Nyquist folded receiver. Specifically, it processes intermediate frequency (IF) signals received and output from three photonic sub-Nyquist folded receivers with different sampling rates. First, signal separation is used to separate the overlapping time-frequency domain signals of multiple systems, obtaining waveform information of the independent signals. Then, a cross-correlation spectrum detection method is employed to resolve the sub-Nyquist ambiguity problem in the signal frequency domain, aiming to improve the ability of photonic sub-Nyquist folded receivers to measure signal frequency parameters in complex electromagnetic environments.

[0026] This invention provides a method for deambiguing the signal frequency of a photonic sub-Nyquist folded receiver, comprising the following steps: A three-channel photonic sub-Nyquist folded receiver architecture is adopted, consisting of three photonic folded links with different sampling rates: a first photonic folded link, a second photonic folded link, and a third photonic folded link. Multi-mode time-domain overlapping RF signals are simultaneously input to the first, second, and third photonic folded links, respectively. After down-conversion processing by these three links, three multi-mode time-frequency domain overlapping intermediate frequency signals are output.

[0027] The obtained three-channel multi-mode time-frequency domain overlapping intermediate frequency signals are sent to the signal separation module. Parallel computation is performed on each signal data channel. Utilizing the independent component separation algorithm, the obtained multi-mode time-frequency domain overlapping intermediate frequency signals are first centered and whitened to remove correlations between signal vectors, resulting in a whitened signal. Secondly, combining operations research optimization principles, an independent component extraction method based on maximizing negative entropy is used to obtain the separated signal.

[0028] The three separated signals are subjected to cross-correlation spectrum detection. The frequency of the horizontal axis corresponding to the peak value of the detected spectrum is divided by the difference in the sampling rate of the two cross-correlation links to obtain the frequency ambiguity number of the corresponding signal (i.e., the sub-Nyquist zone to which the original input RF signal belongs).

[0029] The first, second, and third photonic folded links each include: an optical frequency comb, an electro-optic intensity modulator, a photodetector, and a low-pass filter. The sampling rates of the three photonic folded links are slightly different, and the sampling rate is determined by the repetition frequency of the optical frequency comb (the repetition frequency refers to the frequency repetition interval between each tooth of the optical frequency comb).

[0030] The repetition frequencies of the three photon folding links are denoted as follows: The three conditions must be distinguishable and the difference must be within a preset small deviation range to ensure that the corresponding folding frequency mappings do not overlap.

[0031] The range of sampling frequency difference values ​​is constrained as follows: Assume the repetition frequencies (sampling rates) of the three photon undersampling links are respectively... And satisfy ,in For system frequency resolution, It is a positive integer. Pairwise coprime, and Not less than the highest frequency of the radio frequency signal under test And the difference in sampling rates between any two paths The value should be greater than twice the upper limit of the frequency measurement error to ensure that the folding factor can be stably rounded. (Indicates the least common multiple).

[0032] The specific steps for obtaining multi-mode time-domain overlapping RF signals by inputting them into the first photonic folding link, the second photonic folding link, and the third photonic folding link, respectively, include: The multi-mode time-domain overlapping radio frequency signals are respectively input to the radio frequency input terminals of the electro-optic intensity modulators of the first, second, and third photonic folded links. In the first, second, and third photonic folded links, the electrical signals are converted into optical signals by the electro-optic intensity modulators to obtain three optical modulation signals. The three optical modulation signals are respectively converted into electrical signals by photodetectors in their respective links, and the signals are down-converted to intermediate frequencies through a beat frequency process to obtain three multi-mode time-frequency domain overlapping intermediate frequency signals.

[0033] The specific steps for removing correlations between signals through operations such as centering and whitening include: Let the overlapping intermediate frequency signal in the time-frequency domain of the multi-system be... After being centralized, it becomes After centering, the vector satisfies the following mean: . The covariance is The vector after whitening is Whitening matrix ,in For the reason A diagonal matrix composed of the eigenvalues ​​of . for eigenvectors.

[0034] The specific steps for obtaining the separated signal based on the independent component extraction method with the maximum negative entropy include: First, establish the objective function that maximizes negative entropy. In the formula It has zero mean and and Gaussian vectors with the same covariance It is not a quadratic function. This is the signal after whitening.

[0035] Secondly, the separation matrix is ​​solved using the quasi-Newton method. Finally, the separated signal is calculated. .

[0036] The specific steps for performing cross-correlation spectrum detection on a signal to obtain the ambiguity number of the signal frequency include: First, envelope detection and FFT are performed on each separated signal to obtain the signal spectrum. Second, cross-correlation is performed on the spectra of each pair of links. The frequency value of the horizontal axis corresponding to the maximum value of the cross-correlation spectrum is read and divided by the difference in the sampling rate of the two links to obtain the frequency ambiguity number of the signal.

[0037] The present invention will now be described in further detail with reference to the accompanying drawings: This invention provides a method for deambiguing the signal frequency of a photonic sub-Nyquist folded receiver, comprising the following steps: Step 1: Input the multi-mode radio frequency signals into the first photonic folding link, the second photonic folding link, and the third photonic folding link respectively to obtain three time-frequency domain overlapping intermediate frequency signals. Then, perform analog-to-digital conversion through ADCs to obtain three multi-mode time-frequency domain overlapping digital signals. Step 2: Using a parallel computing approach, based on the independent component extraction method with the maximum negative entropy, the multi-mode time-frequency domain overlapping digital signals obtained in Step 1 are separated to obtain the signal separation result. Step 3: Using the cross-correlation spectrum detection method, the spectra of the three signals are accumulated, and the cross-correlation spectrum between the three signals is calculated to obtain the frequency ambiguity number of the signal.

[0038] Specifically, the first photonic folding link, the second photonic folding link, and the third photonic folding link all include: an optical frequency comb generated by a mode-locked laser, an electro-optic intensity modulator, a photodetector, and a low-pass filter.

[0039] like Figure 1 As shown in the specific example embodiment of the present invention, a method for deambiguing the signal frequency of a photonic sub-Nyquist folded receiver is provided, comprising the following steps: 1) Based on the principle of photonic sub-Nyquist folded receiver, three photonic folded links with slightly different sampling rates are constructed; each of the three photonic folded links includes a mode-locked laser, an electro-optic intensity modulator, a photodetector, and a low-pass filter.

[0040] 2) Input the multi-mode signals to the RF end of the electro-optic intensity modulator of the three photonic folding links respectively; in each photonic folding link, the electrical signal is down-converted to the intermediate frequency by the beat frequency of the photodetector after double-sideband modulation.

[0041] 3) The three intermediate frequency signals output in step 2) are converted from analog to digital by ADC to obtain three digital discrete intermediate frequency signals.

[0042] In the technical solution provided by the embodiments of the present invention, a three-channel photonic sub-Nyquist folded receiver is constructed using three photonic sampling links.

[0043] A multi-mode time-frequency domain overlapping signal is separated using an independent component extraction method based on maximum negative entropy. When building the model based on this method, a non-Gaussianity metric can be used to enhance the separability of the target signal from the interference signal. The specific algorithm steps are as follows: Figure 2As shown. The algorithm steps for separating overlapping time-frequency signals from multiple systems are as follows: 1) Centralized processing. Zero-mean processing is performed on the overlapping time-frequency domain signals of multiple systems. ; Original signal The average value; 2) Whitening treatment. The purpose of this step is to remove the correlation between signals. ,in This is a signal after whitening. This is the whitening matrix; 3) Initialize parameter settings. Select a non-quadratic function. Its derivative is Commonly used nonlinear functions have the following three forms: , , , The independent variable is a nonlinear function. For radar pulse signals, the preferred method is... In this format, the maximum number of iterations, convergence threshold, and separation matrix are set. ; 4) Solve the separation matrix using the quasi-Newton method. ; A model is established based on the independent component extraction method with the maximum negative entropy. ,maximize By finding the separation matrix , making The non-Gaussianity is maximized; To and Gaussian distributed signals with the same variance; According to the KKT conditions, let the first derivative of the constrained optimization problem be zero, that is: ,in Let be the Lagrange multipliers. The above formula is solved iteratively using Newton's method. It can be approximately replaced by The updated formula is obtained as follows: ; for The second derivative; 5) Orthogonalization and standardization of the separation matrix ; Because it extracts a single signal component, directly through Normalization is sufficient. Let be the separation matrix of the k-th iteration. The separation matrix in the (k+1)th iteration; 6) Determine if convergence has occurred by calculating the change in the vector between the two iterations and determining if the convergence condition is met. ( As the convergence threshold, we can take... If convergence is achieved, the separation matrix is ​​output. Otherwise, return to step 4) and continue iterating; 7) Final output separation signal .

[0044] like Figure 3 As shown in the embodiment of the present invention, a method for measuring the frequency ambiguity number of a signal after signal separation based on a photonic sub-Nyquist folded receiver includes the following steps: 1) Perform envelope detection processing on each independent signal after separation and extract its main spectral peak.

[0045] 2) The sliding window fast Fourier transform method is used to move a time window of a preset length on the time axis and perform FFT on the discrete time series within the window to obtain continuous spectrum information.

[0046] 3) Perform cross-correlation calculation on the spectral information of any two signals and accumulate the cross-correlation spectra to improve peak stability.

[0047] 4) Estimate the signal frequency ambiguity number based on the ratio of the peak value of the cross-correlation spectrum to the difference in the corresponding sampling rate.

[0048] The technical advantages of this invention are: 1) The technical solution of this invention is based on a photonic sub-Nyquist folded receiver to separate overlapping time-frequency signals of multiple systems, which can improve the target distinguishability in complex scenarios.

[0049] 2) Introducing a cross-correlation spectrum detection algorithm after signal separation to perform frequency deambiguity is beneficial to improving the stability and repeatability of ambiguity number measurement.

[0050] 3) By combining the maximization of negative entropy with the accumulation of sliding window spectrum, both separation effect and engineering implementation efficiency can be taken into account.

[0051] In summary, this invention proposes a signal frequency deambiguation method based on a photonic sub-Nyquist folded receiver, which can effectively solve the frequency measurement problem of overlapping time-frequency signals from multiple systems.

[0052] Any process or method described in the flowcharts of this invention or otherwise herein can be understood as representing a module, segment, or portion of code comprising one or more executable instructions for implementing a particular logical function or process, which can be implemented in any computer-readable medium for use by an instruction execution system, apparatus, or device. The computer-readable medium can be any medium containing a program for storage, communication, propagation, or transmission for use by the execution system, apparatus, or device, including read-only memory, magnetic disks, or optical disks.

[0053] In the description of this specification, references to terms such as "embodiment," "example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, those skilled in the art can combine or combine the different embodiments or examples described in this specification and the features therein without causing contradiction.

[0054] While embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions, and alterations to the above embodiments within the scope of the present invention.

Claims

1. A method for deambiguing the signal frequency of a photonic sub-Nyquist folded receiver, characterized in that, Includes the following steps: A three-channel photonic sub-Nyquist folded receiver, consisting of three photonic folded links with different sampling rates, is used to simultaneously input multi-mode time-frequency domain overlapping radio frequency signals into the three photonic folded links for down-conversion processing, and output three multi-mode time-frequency domain overlapping intermediate frequency signals respectively. The three-channel multi-system time-frequency domain overlapping intermediate frequency signals are sent to the signal separation module. The independent component extraction method based on the maximum negative entropy is used to remove the correlation between signal vectors and separate the multi-system time-frequency domain overlapping intermediate frequency signals to obtain the separated independent signals. The separated independent signals are subjected to cross-correlation spectrum detection to obtain the abscissa frequency corresponding to the peak value of the cross-correlation spectrum. The abscissa frequency is then divided by the sampling rate difference between the two photonic folding links involved in the cross-correlation to calculate the frequency ambiguity number of the corresponding independent signal.

2. The method for deambiguing the signal frequency of a photonic sub-Nyquist folded receiver according to claim 1, characterized in that, The three photonic folded links are the first photonic folded link, the second photonic folded link, and the third photonic folded link. Each photonic folded link includes an optical frequency comb, an electro-optic intensity modulator, a photodetector, and a low-pass filter connected in sequence. The sampling rate of each photonic folded link is determined by the frequency repetition interval of the optical frequency comb in the link.

3. The method for deambiguing the signal frequency of a photonic sub-Nyquist folded receiver according to claim 2, characterized in that, The specific steps for outputting the overlapping intermediate frequency signals in the time and frequency domains of three multi-mode systems include: The multi-mode time-frequency domain overlapping radio frequency signals are respectively input to the radio frequency input terminal of the electro-optic intensity modulator in each photonic folding link, and the electrical signals are converted into optical modulation signals. Each optical modulation signal is converted into an electrical signal by a photodetector in its respective link, and then down-converted to an intermediate frequency state through a beat frequency process to obtain three multi-mode time-frequency domain overlapping intermediate frequency signals.

4. The method for deambiguing the signal frequency of a photonic sub-Nyquist folded receiver according to claim 1, characterized in that, Before using the independent component extraction method based on the maximum negative entropy to remove the correlation between signal vectors, the process also includes centering and whitening the intermediate frequency signal.

5. The method for deambiguing the signal frequency of a photonic sub-Nyquist folded receiver according to claim 4, characterized in that, The specific steps of centralization and whitening processes include: The mathematical expectation of the intermediate frequency signal with overlapping time and frequency domains of multiple systems is calculated. The original signal is then centered by subtracting the mathematical expectation, resulting in a centered signal vector with a mean of zero. Calculate the eigenvalues ​​and eigenvectors of the covariance matrix of the centered signal vector, and construct the whitening matrix based on the diagonal matrix composed of the eigenvalues ​​and the eigenvectors; A linear transformation is performed on the centered signal vector using a whitening matrix to obtain a whitened signal vector free of correlation.

6. The method for deambiguing the signal frequency of a photonic sub-Nyquist folded receiver according to claim 5, characterized in that, The specific steps for obtaining the separated independent signals using the independent component extraction method based on maximizing negative entropy include: Construct an objective function that maximizes negative entropy. The objective function uses a non-quadratic function to measure the non-Gaussianity of the product of the standardized separation matrix and the whitened signal vector. The objective function is solved using a quasi-Newton iterative method to update and obtain the separation matrix; The final separated signal is obtained by calculating the separation matrix and the whitened signal vector.

7. The method for deambiguing the signal frequency of a photonic sub-Nyquist folded receiver according to claim 6, characterized in that, The process of updating the separation matrix using the quasi-Newton iteration method also includes steps of orthogonalizing and standardizing the separation matrix generated in each iteration, and determining whether the iteration result meets the preset convergence accuracy. If it does, the final separation matrix is ​​output; otherwise, the iteration continues.

8. The method for deambiguing the signal frequency of a photonic sub-Nyquist folded receiver according to claim 1, characterized in that, The specific steps for performing cross-correlation spectrum detection on the separated independent signals include: Each independent signal after separation is processed by envelope detection in parallel. Perform a Fast Fourier Transform on the detected signal to extract the spectral information of each signal. Cross-correlation calculations are performed on the spectral information of the output signals of any two photonic folded links to generate cross-correlation spectra.

9. The method for deambiguing the signal frequency of a photonic sub-Nyquist folded receiver according to claim 8, characterized in that, The detected signal is subjected to Fast Fourier Transform (FFT), specifically using the sliding window FFT method. This involves horizontally shifting a time window function of a preset length along the time axis and performing FFT on the discrete time series signal within that time window to obtain continuous spectral information.

10. The method for deambiguing the signal frequency of a photonic sub-Nyquist folded receiver according to claim 9, characterized in that, After performing the Fast Fourier Transform and before performing the cross-correlation calculation, the method also includes a step of accumulating the continuous spectral information obtained by sliding the time window to enhance the spectral signal strength.

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