A method for measuring multiple signal frequencies in a microwave photonic folding receiver

Abstract

This invention discloses a multi-signal frequency measurement method for microwave photonic folding receivers, belonging to the field of microwave photonic signal processing technology. The method includes: inputting analog microwave signals into a photonic folding link to obtain three-channel intermediate frequency signals; converting these signals to obtain digital discrete signals; performing Hilbert transform and signal separation on the digital discrete signals to obtain baseband I / Q data for each signal in the three channels; performing an FFT on the baseband I / Q data to obtain the leading edge and trailing edge frequencies of the signals; calculating the frequency difference between the baseband I / Q data of the three channels; measuring the ambiguity number of the signals; and estimating the original frequency by combining the ambiguity number with the leading and trailing edge frequencies. This invention solves the frequency aliasing problem faced by microwave photonic sub-Nyquist folding receivers, effectively addressing the frequency ambiguity problem under multi-signal time-frequency aliasing, and improving the real-time signal capture and spectrum monitoring capabilities in complex electromagnetic environments.

Description

Technical Field

[0001] This invention belongs to the field of microwave photonic signal processing technology, and relates to a method for measuring multiple signal frequencies in a microwave photonic folding receiver. Background Technology

[0002] Traditional broadband receivers based on radio frequency electronics are limited by the sampling rate and bit depth of electronic analog-to-digital converters (ADCs), gradually failing to meet the requirement of full-probability reception of instantaneous broadband signals. Microwave photonics technology, with its advantages of high frequency, large bandwidth, low transmission loss, resistance to electromagnetic interference, and parallel processing, has gradually become one of the effective development paths to overcome the bottlenecks in broadband signal reception and processing of electronic receiver systems, significantly improving the frequency performance of broadband receivers. In broadband receiver systems based on microwave photonics technology, compared to channelized structures, photonic sub-Nyquist folded receivers, due to their large instantaneous frequency coverage and relatively simplified link structure, show promising application prospects in complex electromagnetic spectrum situational awareness.

[0003] The basic structure of a microwave photonic sub-Nyquist folded receiver link is a sampled analog photonic link. A single-channel sampled analog photonic link consists of an optical frequency comb, an electro-optic intensity modulator (MZM), a photodetector (PD), and a low-pass filter (LPF). From the time domain perspective, the ultrashort optical pulses generated by the optical frequency comb can undersample high-frequency microwave signals. From the frequency domain perspective, high-frequency microwave signals can modulate the optical frequency comb generated by the optical frequency comb through electro-optic conversion, and then downconvert the modulated optical signal to intermediate frequency / baseband through photoelectric conversion.

[0004] However, when the sampling frequency of the optical frequency comb used... When the frequency is much smaller than that of a microwave signal (undersampling), according to the Nyquist sampling theorem, the frequency of the signal higher than... The frequency components will be Because the mirror point is folded into the low-frequency region, spurious frequencies appear in the sampled signal that cannot be distinguished from the true low frequencies, resulting in frequency ambiguity. Therefore, the spectral folding principle of the sampled analog photonic link causes signals from different frequency bands received simultaneously to alias into the first sub-Nyquist region (frequency range 0~). This leads to signal frequency ambiguity.

[0005] Applying fine modulation to an optical frequency comb can resolve frequency ambiguity issues using a single-channel link; however, this method is computationally complex, relies heavily on the accuracy of modulation sideband power measurement, and has a very limited applicable signal frequency range. A dual-channel sampling analog link composed of two optical frequency combs with a slight repetition frequency difference (the link sampling rate equals the optical frequency comb repetition frequency) can also alleviate these problems; however, its ambiguity-free range is at most on the order of the least common multiple of the two sampling rates, and requires the two sampling rates to be coprime or satisfy a certain relationship. A three-channel sampling analog photonic link composed of three optical frequency combs with a slight repetition frequency difference (coprime repetition frequencies) can measure signal frequencies without dead zones. However, current microwave photonic sub-Nyquist folded receivers based on three-channel sampling analog photonic links require solving equations or using the Chinese remainder theorem to deambiguate signal frequencies, and the applicable signal patterns and number of signals are very limited. Summary of the Invention

[0006] To address the problems existing in the prior art, the present invention aims to provide a multi-signal frequency measurement method for microwave photonic folding receivers, thereby resolving the frequency ambiguity issue. The technical solution provided by this invention, compared to existing three-channel frequency ambiguity resolution methods, is applicable to various signal types and can be used in scenarios where multiple signals arrive simultaneously, facilitating its application in complex electromagnetic spectrum sensing.

[0007] To achieve the above objectives, the present invention provides a multi-signal frequency measurement method for a microwave photonic folding receiver, comprising the following steps: Acquiring multi-signal intermediate frequency data: The received mixed signal containing multiple radio frequency signals is input into three-channel photonic folding links with different repetition frequencies (sampling rates) through a power divider, and photoelectric modulation and down-conversion are performed to acquire three-channel intermediate frequency signals; Analog-to-digital conversion and complex signal extraction: Analog-to-digital conversion is performed on the three-channel intermediate frequency signals to obtain digital discrete signals, and the digital discrete signals are transformed. The baseband complex signal sequence in each channel is extracted by performing Hilbert transform on the intermediate frequency signals. Multi-signal time-frequency separation: Based on the time-frequency domain characteristics of the signal, time-frequency transformation and connected component identification are performed on the baseband complex signal sequence to separate the mixed signal into independent signal sequences corresponding to different single signal sources in the time-frequency domain; Frequency feature extraction: For each independent signal sequence after separation, extract its leading edge frequency and trailing edge frequency in each channel; Calculate the frequency ambiguity number and estimate the original frequency: Perform pairwise cross-correlation on the independent signal sequences separated from the three channels to obtain the frequency difference between the channels. Based on the frequency difference and the sampling rate difference between the three channels, enumerate and calculate to obtain the sub-Nyquist folding region and frequency folding factor of each signal sequence. Finally, combine the leading edge frequency and trailing edge frequency to estimate the original frequency of the RF signal.

[0008] Furthermore, the specific structure of the photonic folding link includes: Each photonic folding link is cascaded with an optical frequency comb, an electro-optic intensity modulator, a photodetector, and a low-pass filter; The mixed signal is input to the radio frequency input terminal of the electro-optic intensity modulator in the three-channel link through the power divider, and the electrical signal is converted into an optical signal; the signal is down-converted to intermediate frequency by the photodetector in each link through beat frequency.

[0009] Furthermore, the step of extracting the baseband complex signal sequence in each channel specifically includes: Perform Fourier transforms on the three-channel intermediate frequency real signals respectively, and set the negative frequency part to zero; The amplitude of the positive frequency portion after setting the negative frequency to zero is multiplied by 2, and then the intermediate frequency complex signal of each channel is obtained by inverse Fourier transform (IFFT). The real part of the intermediate frequency complex signal is the original real signal, and the imaginary part is the Hilbert transform result of the original real signal.

[0010] Furthermore, the steps of multi-signal time-frequency separation specifically include: A short-time Fourier transform is performed on the baseband complex signal sequence, and the two-dimensional time spectrum of the signal is obtained after taking the modulus. The time-spectrum image is converted into a grayscale image and binarized according to a preset energy threshold. Pixels below the energy threshold are considered background noise, and pixels above the energy threshold are considered target signals. In a binary image, a connected component labeling algorithm is used to identify connected target signal pixels, and pixels belonging to the same connected component are classified as the same independent signal source. Time-frequency masking is designed based on the location information of connected components to extract target signals, and short-time inverse Fourier transform is performed on each separated target signal to convert it back into a one-dimensional time series signal.

[0011] Furthermore, the steps of extracting the leading edge frequency and trailing edge frequency in each channel specifically include: Perform Fourier transform on the complex signal data of each independent signal sequence after separation; The maximum frequency obtained from the first Fourier transform is extracted as the leading edge frequency, and the maximum frequency obtained from the last Fourier transform is extracted as the trailing edge frequency.

[0012] Furthermore, the step of obtaining the frequency difference between channels by performing pairwise cross-correlation operations on the independent signal sequences separated from the three channels specifically includes: The baseband complex signals of the same time series in the three channels are subjected to pairwise conjugate multiplication to obtain three sets of related signals; The frequency of each group of relevant signals is extracted as the frequency difference between the corresponding two channels.

[0013] Furthermore, the steps of calculating the frequency ambiguity number and estimating the original frequency specifically include: Divide the frequency difference between each pair of channels extracted at each time step by the repetition frequency difference (sampling rate difference) of the optical frequency comb between the corresponding two channels to obtain the folded spectrum value at each time step. Perform histogram-based cumulative statistics on the folded spectral values ​​at all times within each independent signal sequence; The folded spectrum value with the highest statistical frequency in the histogram is selected, rounded up, and used as the frequency folding factor for the independent signal sequence.

[0014] Furthermore, the step of estimating the original frequency of the radio frequency signal by combining the leading edge frequency and the trailing edge frequency specifically includes: Based on the determined signal folding region, frequency folding factor, and extracted leading and trailing frequencies, the true frequency of each radio frequency signal is calculated by reverse deduction according to the Nyquist folding principle.

[0015] Furthermore, the optical frequency combs contained in the three-channel photonic folded link have different repetition frequencies, and the repetition frequencies are pairwise coprime.

[0016] Furthermore, the electro-optic intensity modulator employs a quadrature bias point modulation method to perform double-sideband modulation on the signal under test.

[0017] The beneficial effects of this invention are as follows: This invention can solve the frequency aliasing problem faced by microwave photonic sub-Nyquist folded receivers, effectively address the problem of frequency ambiguity under time-frequency aliasing of multiple signals, and improve its ability to capture signals in real time and monitor the spectrum in complex electromagnetic environments. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the overall process of the frequency measurement method based on a microwave photonic sub-Nyquist folded receiver in an embodiment of the present invention. Figure 2 This is a schematic diagram of the process for time-frequency domain separation of multiple signals based on a microwave photonic sub-Nyquist folded receiver in an embodiment of the present invention. Figure 3 This is a schematic diagram of the process for measuring ambiguity number based on a microwave photonic sub-Nyquist folded receiver in an embodiment of the present invention. Detailed Implementation

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] This invention discloses a multi-signal frequency measurement method for microwave photonic folded receivers, specifically a method for measuring the multi-signal frequencies of a three-channel microwave photonic sub-Nyquist folded receiver in scenarios where multiple signals arrive simultaneously. The three-channel sampled analog photonic link is referred to as a "microwave photonic sub-Nyquist folded receiver," and each link of the three-channel sampled analog photonic link includes an optical frequency comb, an electro-optic intensity modulator, a photodetector, and a low-pass filter.

[0024] Taking a single channel as an example, assuming the radio frequency signal frequency is... The frequency folding factor is ( If the integer is non-negative, then the signal frequency after being folded by the photon subnyquist folding receiver is... The expression is as follows:

[0025] See Figure 1 The present invention provides a multi-signal frequency measurement method for microwave photonic folding receivers, comprising the following steps: Step 1: Acquire multi-signal intermediate frequency data Multiple radio frequency signals of unknown frequency are input into the first photonic folding link, the second photonic folding link, and the third photonic folding link respectively to obtain three-channel intermediate frequency signals; The repetition frequencies (sampling rates) of the three photon folding links are denoted as follows: The three factors must be distinguishable and their differences must be within a preset small deviation range to ensure that the corresponding folded frequency mappings do not overlap. The sampling frequency difference range is constrained as follows: assuming the sampling rates of the three-channel photonic undersampling link 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).

[0026] Step 2: Analog-to-Digital Conversion and Complex Signal Extraction The obtained three-channel intermediate frequency signals are converted from analog to digital by the ADC of each channel to obtain three-channel digital discrete signals; here the ADC sampling rate satisfies the Nyquist theorem, that is, it is greater than twice the maximum frequency of the intermediate frequency signal.

[0027] Step 3: Time-frequency domain separation of multiple signals Hilbert transforms are performed on the three-channel digital discrete signals to obtain the I / Q complex signal data of each channel, and then the complex signals are separated. Due to the sub-Nyquist folding principle, multiple signals may exhibit aliasing and overlap in the frequency domain, making methods for signal separation only in the time or frequency domains highly inapplicable. Therefore, multiple signals can be separated in the time-frequency domain using methods such as Short-Time Fourier Transform (STFT) and connected component processing. By treating the time-frequency graph of the signal as an image, and utilizing the continuity of the signal in the time-frequency domain, a connected component labeling algorithm is used to identify and separate energy blocks belonging to different signals in the graph. Then, by performing an inverse Fourier transform (ISTFT) on each separated signal, it can be converted from the time-frequency domain into a separated one-dimensional time series signal, thus obtaining the baseband I / Q data of each individual signal sequence in the three channels.

[0028] Step 4: Frequency Feature Extraction For each independent signal sequence after separation, the obtained baseband I / Q data is subjected to Fourier transform (FFT) to measure the leading edge frequency and trailing edge frequency of each signal sequence, and the leading edge frequency and trailing edge frequency in the three channels are extracted respectively.

[0029] Step 5: Calculate the frequency ambiguity number and estimate the original frequency. The frequency difference between the three channels is obtained by pairwise correlation of the baseband I / Q data between the three channels: the baseband complex signals of each time series of the three channels are conjugately multiplied to obtain three sets of signals. The frequency of each set of signals represents the frequency difference between the two channels, and the phase represents the phase difference between the two signals.

[0030] Divide the frequency difference between each pair of channels at each time step by the repetition frequency difference between the corresponding two channels to obtain the folded spectrum value. The frequency folding region is calculated by enumeration, that is, the folded spectrum values ​​at all times in each signal sequence are accumulated and statistically analyzed. The frequency histogram of the folded spectrum values ​​at different times is calculated, and the highest value is the folding factor (fuzzy number) of the signal.

[0031] Finally, the original frequency is estimated by combining the measured ambiguity number with the calculated leading edge frequency and trailing edge frequency.

[0032] To further explain, analog microwave or radio frequency signals are input into the first photonic folding link, the second photonic folding link, and the third photonic folding link, respectively. The specific steps to obtain the three-channel intermediate frequency signal include: Analog microwave or radio frequency signals are input to the radio frequency input terminals of the electro-optic intensity modulators of the first, second, and third photonic folded links, respectively. 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-channel optical modulation signals. The three-channel optical modulation signals are then converted into electrical signals by photodetectors in their respective links, and the signals are down-converted to intermediate frequencies by beat frequency conversion to obtain three-channel intermediate frequency signals.

[0033] The specific process for obtaining three-channel complex signal data through Hilbert transform is as follows: After performing Fourier Transform (FFT) on the three-channel intermediate frequency real signals, the negative frequencies are set to zero, and the amplitude of the resulting spectral data is doubled. Then, the inverse Fourier Transform is performed to obtain the intermediate frequency complex signals of each channel. The real part of the complex signal is the original real signal, and the imaginary part is the Hilbert transform result of the original signal.

[0034] like Figure 2 As shown, the specific process of separating multiple signals through STFT transformation and connected components is as follows: The first step is to perform a short-time Fourier transform on the mixed signal to obtain a two-dimensional complex matrix containing time and frequency information. After taking the modulus, the two-dimensional time spectrum of the signal can be obtained.

[0035] The second step is to convert the time-spectrum image into a grayscale image format suitable for computer processing. By setting an energy threshold, the image is binarized. Pixels below the threshold are considered background noise (value 0), and pixels above the threshold are considered target signals (value 1), thus obtaining a binarized image.

[0036] The third step involves labeling all pixels with a value of 1 on the binary image using connected component markers, and grouping connected pixels into the same "connected component." Each component typically corresponds to an independent signal source. Once the connected components containing all signal components are found, a "time-frequency mask" can be designed based on their location information in the time-frequency map, allowing only energy from that region to pass through, thereby separating the target signal.

[0037] The fourth step is to perform an inverse short-time Fourier transform (ISTFT) on each separated signal component, converting it from the time-frequency domain representation back to the original one-dimensional time series signal, thereby separating multiple signals into independent signal sequences in the time-frequency domain.

[0038] See Figure 3 The principle behind solving for the frequency difference by pairwise conjugate multiplication of the three-channel intermediate frequency complex signal time-domain sequence data is as follows: Pairwise conjugate multiplication of the three-channel intermediate frequency complex signal time-domain sequences essentially eliminates the common phase term of the same radio frequency signal in two channels, retaining only the difference term of the folded intermediate frequency between the two channels. Therefore, the slope of the phase change of the conjugate product over time corresponds to the intermediate frequency difference between the two channels.

[0039] Suppose the same radio frequency signal passes through the first Road and the After the photonic link is folded, the resulting intermediate frequency complex signals are as follows: ;

[0040] in: They represent the first Intermediate frequency IQ complex signal; These represent the amplitudes of the two signals, respectively. The intermediate frequency after folding the two paths respectively; These represent the initial phases of the two channels, including the link phase offset; It is a time variable.

[0041] Multiplying the two signals by their conjugates yields: ; Substituting, we get: ; Conjugate product It is still a complex exponential signal with the following frequency:

[0042] The frequency of the new complex signal obtained by conjugate multiplication of two intermediate frequency complex signals is the difference between the two folded intermediate frequency frequencies. The slope of the phase change of the conjugate product signal over time corresponds to the difference between the two intermediate frequency frequencies.

[0043] For a three-channel photonic folded link, the conjugate multiplication between each pair yields three sets of frequency differences: ; These three sets of frequency differences, along with the three sampling rate differences, are used together to solve for the folding factor. If the... The intermediate frequency of the circuit satisfies: ; No. The intermediate frequency of the circuit satisfies: ; in The frequency of the radio frequency signal to be measured is... This is the frequency folding factor. For the first Road sampling rate, For the first If the sampling rate is different for each path, then subtracting the two paths will give the result: ; therefore: ; The equivalent is written as: ; in, The intermediate frequency difference is obtained by multiplying the conjugates of the two channels. The difference in sampling rates between the two channels.

[0044] Due to noise and estimation errors in actual measurements, engineering practices typically involve rounding and consistency checks on the three sets of results. ; Finally, by enumerating all possible frequencies within the monitoring frequency range... The value is determined by histogram statistics, and the final fuzzy number (frequency folding factor) is determined by combining the preceding and following edge frequencies to calculate the frequency.

[0045] Specifically, this invention provides a method for measuring multiple signal frequencies in a microwave photonic folding receiver, which may include the following steps: 1) Based on the principle of microwave photonic sub-Nyquist folded receiver, a three-channel photonic folded link with slightly different sampling rates is constructed; each of the three photonic folded links includes an optical frequency comb, an electro-optic intensity modulator, a photodetector, and a low-pass filter.

[0046] 2) The radio frequency signals are respectively input to the radio frequency input terminal of the electro-optic intensity modulator of the three-channel photonic folding link; in each photonic folding link, the electrical signal is converted into an optical signal by the electro-optic intensity modulator and modulated on both sides of each optical frequency comb (the modulation method in this embodiment can be Q-point modulation, i.e., double-sideband modulation), and the three-channel optical modulation signals are down-converted to intermediate frequency by the beat frequency of the photodetector and output; 3) The three-channel intermediate frequency signals output in step 2) are converted from analog to digital by a three-channel low-speed ADC (interpretive, sampling rate greater than half the repetition frequency) to obtain three-channel digital discrete intermediate frequency signals.

[0047] 4) Convert the intermediate frequency signal to the time-frequency domain and separate the signal through the connected components to separate multiple signals into a single signal sequence.

[0048] 5) Perform Hilbert transform on each separated intermediate frequency data to obtain baseband I / Q data.

[0049] 6) The obtained baseband I / Q data is used to perform FFT to measure the leading edge frequency and trailing edge frequency of each signal in the three channels. The overall processing flow is as follows: Figure 2 As shown, the leading and trailing frequencies (i.e., the maximum frequency of the first and last FFTs) are measured by performing an FFT on the baseband I / Q data, thereby extracting the leading and trailing frequencies of the three channels.

[0050] This invention mainly uses "frequency difference measurement + enumeration statistics" for frequency measurement, and has the following technical advantages: 1) The technical solution of this invention is based on a microwave photonic sub-Nyquist folded receiver to perform frequency unambiguity measurement of signals by using methods such as time-domain signal conjugate multiplication and frequency difference statistics. This solves the problem of signal frequency measurement when multiple signals arrive, and is beneficial to improving the performance of target detection of subsequent signals.

[0051] 2) The technical solution of this invention adopts the method of "frequency difference measurement + enumeration statistics" when solving frequency ambiguity, which provides a new idea for solving frequency and the algorithm has strong real-time performance.

[0052] 3) The technical solution of this invention, the frequency deambiguation method of the three-channel microwave photonic sub-Nyquist folded receiver, makes up for the shortcomings of multi-signal aliasing and frequency measurement caused by the advantages of the high-frequency and large instantaneous bandwidth signal processing capabilities of the photonic analog front-end, so as to effectively improve the engineering application value of the microwave photonic sub-Nyquist folded receiver.

[0053] The present invention has the following beneficial effects: This invention effectively acquires signal frequency information, thereby improving its real-time signal capture and spectrum monitoring capabilities in complex electromagnetic environments. Addressing the frequency ambiguity problem caused by frequency folding in microwave photonic Nyquist folded receivers, this invention first performs a Hilbert transform on the three-channel intermediate frequency (IF) signals received by the microwave photonic Nyquist folded receiver to obtain three-channel IQ data. Then, it performs STFT and connected component analysis on the three-channel IQ data to separate multiple signals. Next, it performs FFT on the IQ data of each separated signal to measure the IF frequency of each signal in the three channels. Then, it uses a "frequency difference measurement + enumeration statistics" method to calculate the ambiguity number of the signal. Finally, it combines the measured ambiguity number and the measured IF frequency to estimate the original frequency. This invention overcomes the challenge of frequency measurement for multiple simultaneously arriving signals using a three-channel microwave photonic Nyquist folded receiver, which has the advantage of receiving signals with a large instantaneous bandwidth.

[0054] In summary, the embodiments of the present invention propose a method for frequency measurement of microwave photonic sub-Nyquist folded receivers, which can solve the frequency aliasing problem caused by microwave photonic sub-Nyquist folded receivers, effectively acquire frequency information, and effectively improve the reconnaissance performance of radar.

[0055] 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.

[0056] 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.

[0057] 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 measuring multiple signal frequencies in a microwave photonic folding receiver, characterized in that, Includes the following steps: Acquiring multi-signal intermediate frequency data: The received mixed signal containing multiple radio frequency signals is input into a three-channel photonic folding link with different repetition frequencies, and photoelectric modulation and down-conversion are performed to obtain the three-channel intermediate frequency signal; Analog-to-digital conversion and complex signal extraction: Analog-to-digital conversion is performed on the three-channel intermediate frequency signals to obtain digital discrete signals, and the digital discrete signals are transformed to extract the baseband complex signal sequences in each channel; Multi-signal time-frequency separation: Based on the time-frequency domain continuity characteristics of signals, time-frequency transformation and connected component identification are performed on baseband complex signal sequences to separate the mixed signals into independent signal sequences corresponding to different single signal sources in the time-frequency domain; Frequency feature extraction: For each independent signal sequence after separation, extract its leading edge frequency and trailing edge frequency in each channel; Calculate the frequency ambiguity number and estimate the original frequency: Perform pairwise cross-correlation on the independent signal sequences separated from the three channels to obtain the frequency difference between the channels. Based on the frequency difference, enumerate and calculate to obtain the sub-Nyquist folding region and frequency folding factor of each signal sequence. Finally, combine the leading edge frequency and trailing edge frequency to estimate the original frequency of the RF signal.

2. The multi-signal frequency measurement method according to claim 1, characterized in that, The specific structure of the photonic folding link includes: Each photonic folding link is cascaded with an optical frequency comb, an electro-optic intensity modulator, a photodetector, and a low-pass filter; The mixed signals are input to the radio frequency input of the electro-optic intensity modulator in the three-channel link, converting the electrical signals into optical signals; the signals are then down-converted to intermediate frequency by the photodetectors in each link through beat frequency.

3. The multi-signal frequency measurement method according to claim 1, characterized in that, The steps for extracting the baseband complex signal sequence in each channel specifically include: Perform Fourier transforms on the three-channel intermediate frequency real signals respectively, and set the negative frequency part to zero; The amplitude of the spectrum data after the negative frequency is set to zero is multiplied by a preset factor, and then the intermediate frequency complex signal of each channel is obtained by inverse Fourier transform. The real part of the intermediate frequency complex signal is the original real signal, and the imaginary part is the Hilbert transform result of the original real signal.

4. The multi-signal frequency measurement method according to claim 1, characterized in that, The steps of multi-signal time-frequency separation specifically include: A short-time Fourier transform is performed on the baseband complex signal sequence, and the two-dimensional time spectrum of the signal is obtained after taking the modulus. The time-spectrum image is converted into a grayscale image and binarized according to a preset energy threshold. Pixels below the energy threshold are considered background noise, and pixels above the energy threshold are considered target signals. In a binary image, a connected component labeling algorithm is used to identify connected target signal pixels, and pixels belonging to the same connected component are classified as the same independent signal source. Time-frequency masking is designed based on the location information of connected components to extract target signals, and short-time inverse Fourier transform is performed on each separated target signal to convert it back into a one-dimensional time series signal.

5. The multi-signal frequency measurement method according to claim 1, characterized in that, The steps for extracting the leading edge frequency and trailing edge frequency in each channel include: Perform Fourier transform on the complex signal data of each independent signal sequence after separation; The maximum frequency obtained from the first Fourier transform is extracted as the leading edge frequency, and the maximum frequency obtained from the last Fourier transform is extracted as the trailing edge frequency.

6. The multi-signal frequency measurement method according to claim 1, characterized in that, The steps to obtain the frequency difference between the three channels by performing pairwise cross-correlation operations on the independent signal sequences separated from the three channels specifically include: The baseband complex signals of the same time series in the three channels are subjected to pairwise conjugate multiplication to obtain three sets of related signals; The frequency of each group of relevant signals is extracted as the frequency difference between the corresponding two channels.

7. The multi-signal frequency measurement method according to claim 1, characterized in that, The steps for calculating the frequency ambiguity number and estimating the original frequency include: Divide the frequency difference between each pair of channels extracted at each time point by the repetition frequency difference of the optical frequency comb between the corresponding two channels to obtain the folded spectrum value at each time point. Perform histogram-based cumulative statistics on the folded spectral values ​​at all times within each independent signal sequence; The folded spectrum value with the highest statistical frequency in the histogram is selected, rounded up, and used as the frequency folding factor for the independent signal sequence.

8. The multi-signal frequency measurement method according to claim 7, characterized in that, The steps for estimating the original frequency of a radio frequency signal by combining the leading edge frequency and the trailing edge frequency specifically include: Based on the determined signal folding region, frequency folding factor, and extracted leading and trailing frequencies, the true frequency of each radio frequency signal is calculated by reverse deduction according to the Nyquist folding principle.

9. The multi-signal frequency measurement method according to claim 1, characterized in that, The optical frequency combs contained in the three-channel photonic folded link have different repetition frequencies, and the repetition frequencies are pairwise coprime.

10. The multi-signal frequency measurement method according to claim 2, characterized in that, The electro-optic intensity modulator uses a quadrature bias point modulation method to perform double-sideband modulation on the signal under test.

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