Coded response method and system for frequency diversity electromagnetic tag based on spatiotemporal modulation metasurface

Abstract

The present invention discloses a coded response method and system for a frequency-diversity electromagnetic tag based on a spatiotemporal modulation metasurface. Specifically, the method comprises the following steps: using the spatiotemporal modulation metasurface as the electromagnetic tag for response, establishing a mapping code library between modulation frequency combinations and response codes; generating metasurface timing control codes with modulation frequencies corresponding to the response codes based on the code library; using space-frequency domain joint modulation, importing timing control codes with different modulation frequencies into different regions to achieve frequency diversity and modulate the incident signal; performing matched filtering on the modulated linear frequency modulation echo signal to obtain an amplitude spectrum, determining the modulation frequency scheme used by the tag based on the peak position, and searching the code library based on the mapping relationship to obtain the response code value. As a response method for electromagnetic tags, the present invention has the advantages of low power consumption, low cost, and high coding capacity.

Description

Technical Field

[0001] The present invention relates to the technical field of electromagnetic functional materials, and in particular to a coding response method and system for a frequency diversity electromagnetic tag based on a spatiotemporal modulation metasurface. Background Art

[0002] Target recognition, a crucial component of information processing, is the process of analyzing a target's characteristics to determine its category, state, or function in radar detection technology. The continuous advancement of electromagnetic technology, coupled with the increasing diversity of electromagnetic interference and complexity of the electromagnetic environment, places higher demands on electromagnetic tags' recognition and response capabilities. They must address this ever-changing environment while minimizing costs and improving anti-interference capabilities.

[0003] Research is underway on spatiotemporal modulation metasurfaces, which can precisely modulate incident electromagnetic waves. Reconfigurable basic metasurface units can independently apply time-varying phases to echo signals, changing the echo's spectral characteristics. This allows for the creation of spectral signatures tailored to identification requirements, enabling flexible response in diverse environments. Spatiotemporal modulation metasurfaces employ frequency diversity to implement electromagnetic tag encoding, offering the advantages of low power consumption, low cost, and high coding capacity. For example, Reference 1 (C. Hilton and JA Nanzer, "Narrowband Passive RF Tags for Frequency-Selective Harmonic Doppler Radar Tracking," in IEEE Transactions on Antennas and Propagation, vol. 71, no. 2, pp. 1216-1222, Feb. 2023, doi:10.1109 / TAP.2022.3220489.) proposes a design for frequency-selective RF tags for harmonic Doppler radar tracking. Harmonic RFID tags receive microwave signals and retransmit signals at harmonics of the original signal frequency (typically the second harmonic), thereby improving detection capabilities. Using a narrowband split-ring printed antenna, the tag receives signals only within a specified narrowband range. A diode generates the second harmonic, which is retransmitted by a standard dipole antenna. This design achieves narrowband filtering at the fundamental frequency, eliminating the need for additional circuitry to distinguish between multiple tags. However, this electromagnetic tag has a fixed structure and cannot be reconfigured, resulting in only a single modulation mode and a single response format. Summary of the Invention

[0004] The purpose of the present invention is to provide a coding response method and system for a frequency diversity electromagnetic tag based on a spatiotemporal modulation metasurface, which can reduce power consumption and cost and greatly improve coding capacity, thereby changing the coding in real time according to the response requirements and interference environment, and realizing accurate spectrum coding response tags.

[0005] The technical solution to achieve the purpose of the present invention is: a coded response method for a frequency-diversity electromagnetic tag based on a spatiotemporal modulation metasurface, which uses the spatiotemporal modulation metasurface as an electromagnetic tag for response, specifically comprising the following steps:

[0006] Step 1: Establish a theoretical model of space-frequency modulation. The space-time modulation metasurface uses a time modulation sequence to perform double-sideband modulation on the incident signal, generating harmonics corresponding to the modulation frequency, and using the frequency distribution of the harmonics as the encoding information.

[0007] Step 2: Combine the structural parameters of the spatiotemporal modulation metasurface to establish a space-frequency modulation model of the spatiotemporal modulation metasurface;

[0008] Step 3: Change the size and spatial combination of the modulation frequency to establish a mapping code library between the modulation frequency and the response code;

[0009] Step 4: Generate a timing control code of the modulation frequency corresponding to the response code according to the mapping code library to control the scattering state of the metasurface;

[0010] Step 5: Divide the spatiotemporal modulation metasurface into multiple regions. Based on space-frequency domain joint modulation, introduce timing control codes of different modulation frequencies into different regions, so that each region carries a different modulation frequency to achieve frequency diversity.

[0011] Step 6: The radar transmits a linear frequency modulation signal to different areas, and different areas modulate the incident signal differently to generate multiple harmonic signals that are orthogonal to each other;

[0012] Step 7: Perform matched filtering on the modulated linear frequency modulation echo signal to obtain an amplitude spectrum, obtain the modulation frequency used by the electromagnetic tag according to the peak position, and obtain the response code value by searching in the mapping code library based on the mapping relationship.

[0013] A coded response system for a frequency-diversity electromagnetic tag based on a spatiotemporal modulation metasurface, the system being used to implement the coded response method for a frequency-diversity electromagnetic tag based on a spatiotemporal modulation metasurface, comprising a theoretical model construction module, a space-frequency modulation model construction module, a mapping code library construction module, a scattering state control module, a frequency diversity module, a harmonic signal generation module, and a coded response module, wherein:

[0014] Theoretical model construction module, establishes the space-frequency modulation theoretical model. The space-time modulation metasurface uses a time modulation sequence to perform double-sideband modulation on the incident signal, generating harmonics corresponding to the modulation frequency, and uses the frequency distribution of the harmonics as the encoding information;

[0015] The space-frequency modulation model construction module combines the structural parameters of the space-time modulation metasurface to establish the space-frequency modulation model of the space-time modulation metasurface;

[0016] The mapping code library construction module changes the size and spatial combination of the modulation frequency to establish a mapping code library between the modulation frequency and the response code;

[0017] The scattering state control module generates a timing control code of a modulation frequency corresponding to the response code according to the mapping code library to control the scattering state of the metasurface;

[0018] The frequency diversity module divides the spatiotemporal modulation metasurface into multiple regions. Based on space-frequency domain joint modulation, the timing control codes of different modulation frequencies are introduced into different regions, so that each region carries a different modulation frequency, thus achieving frequency diversity.

[0019] Harmonic signal generation module: The radar transmits linear frequency modulation signals to different areas, and different areas modulate the incident signals differently to generate multiple harmonic signals that are orthogonal to each other;

[0020] The coding response module performs matched filtering on the modulated linear frequency modulation echo signal to obtain the amplitude spectrum, obtains the modulation frequency used by the electromagnetic tag according to the peak position, and searches the mapping code library based on the mapping relationship to obtain the response code value.

[0021] A mobile terminal comprises a memory, a processor and a computer program stored in the memory and runnable on the processor. When the processor executes the program, the coding response method of the frequency diversity electromagnetic tag based on the spatiotemporal modulation metasurface is implemented.

[0022] Compared with the prior art, the present invention has the following significant advantages:

[0023] (1) Design based on the spatiotemporal modulation metasurface, using the spatiotemporal modulation metasurface as an electromagnetic tag, which has the advantages of low power consumption and low cost of the electromagnetic metasurface;

[0024] (2) An effective space-frequency modulation theoretical model was established to analyze the generation of harmonics. It has high flexibility and accurate modulation results, and is simple and easy to implement for metasurfaces.

[0025] (3) By using frequency diversity, the modulation frequencies of the basic units of the spatiotemporal modulation metasurface are flexibly allocated, which greatly increases the types of modulated echo signals, that is, the coding capacity;

[0026] (4) For broadband radar signals, the demodulation process can be achieved through matched filtering of the receiver without the need for complex processing of the echo signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a flow chart of the coding response method of the frequency diversity electromagnetic tag based on the spatiotemporal modulation metasurface of the present invention.

[0028] Figure 2 Schematic diagram of a 1-bit phase time modulation sequence in an embodiment of the present invention.

[0029] Figure 3 This is a harmonic distribution diagram obtained by Fourier transforming a 1-bit phase time modulation sequence in an embodiment of the present invention.

[0030] Figure 4 This is a time-frequency diagram of a linear frequency modulation signal after 1-bit modulation in an embodiment of the present invention.

[0031] Figure 5 In the embodiment of the present invention, the modulation frequency is The corresponding time modulation sequence diagram.

[0032] Figure 6 In the embodiment of the present invention, the modulation frequency is Spectrum diagram after modulation.

[0033] Figure 7 FIG. 4 is a time domain diagram of a linear frequency modulation signal in an embodiment of the present invention.

[0034] Figure 8 In the embodiment of the present invention, the LFM signal is modulated at a frequency of Result diagram of matched filtering after modulation.

[0035] Figure 9 In the embodiment of the present invention, the LFM signal is modulated at a frequency of Result diagram of matched filtering after modulation.

[0036] Figure 10 In the embodiment of the present invention, the LFM signal is modulated at a frequency of Result diagram of matched filtering after modulation.

[0037] Figure 11 In the embodiment of the present invention, the LFM signal is modulated at a frequency of Result diagram of matched filtering after modulation.

[0038] Figure 12 In the embodiment of the present invention, the LFM signal is modulated at a frequency of Result diagram of matched filtering after modulation. DETAILED DESCRIPTION

[0039] It is easy to understand that, based on the technical solution of the present invention, those skilled in the art can imagine various embodiments of the present invention without changing the essential spirit of the present invention. Therefore, the following specific embodiments and drawings are merely illustrative of the technical solution of the present invention and should not be regarded as the entire invention or as limiting or defining the technical solution of the present invention.

[0040] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that unless otherwise specifically stated, the relative arrangements of components and steps, numerical expressions and values described in these embodiments do not limit the scope of the present invention.

[0041] Technologies, methods, and equipment known to ordinary technicians in the relevant art may not be discussed in detail, but where appropriate, the technologies, methods, and equipment should be considered part of the specification.

[0042] Combine Figure 1 The present invention provides a coding response method for a frequency diversity electromagnetic tag based on a spatiotemporal modulation metasurface. The method uses the spatiotemporal modulation metasurface as an electromagnetic tag for response, and specifically includes the following steps:

[0043] Step 1: Establish a theoretical model of space-frequency modulation. The space-time modulation metasurface uses a time modulation sequence to perform double-sideband modulation on the incident signal, generating harmonics corresponding to the modulation frequency, and using the frequency distribution of the harmonics as the encoding information.

[0044] Step 2: Based on the structural parameters of the spatiotemporal modulation metasurface, a space-frequency modulation model of the spatiotemporal modulation metasurface is established;

[0045] Step 3: Change the size and spatial combination of the modulation frequency to establish a mapping code library between the modulation frequency and the response code;

[0046] Step 4: Generate a timing control code of the modulation frequency corresponding to the response code according to the mapping code library to control the scattering state of the metasurface;

[0047] Step 5: Divide the spatiotemporal modulation metasurface into multiple regions. Based on space-frequency domain joint modulation, introduce timing control codes of different modulation frequencies into different regions, so that each region carries a different modulation frequency to achieve frequency diversity.

[0048] Step 6: The radar transmits a linear frequency modulation signal to different areas, and different areas modulate the incident signal differently to generate multiple harmonic signals that are orthogonal to each other;

[0049] Step 7: Perform matched filtering on the modulated linear frequency modulation echo signal to obtain an amplitude spectrum, obtain the modulation frequency used by the electromagnetic tag according to the peak position, and obtain the response code value by searching in the mapping code library based on the mapping relationship.

[0050] As a specific example, the spatiotemporal modulation metasurface is composed of a plurality of basic units arranged periodically, and can realize two scattering states of 0° phase and 180° phase, i.e., 1-bit periodic phase;

[0051] The spatiotemporal modulation metasurface has the characteristic of electrically adjustable scattering state. By generating and importing control signals corresponding to the two scattering states, the scattering state of the metasurface is controlled to change with time, thereby achieving precise regulation of the scattering field in the time domain.

[0052] As a specific example, in step 1, a space-frequency modulation theoretical model is established as follows:

[0053] The time modulation sequence used is a 1-bit phase sequence, Expressed as:

[0054] (1)

[0055] like Figure 2 As shown, For time, is the base of natural logarithms, is the imaginary unit, is pi; represents the modulation period, is the modulation frequency; Indicates the time modulation cycles, time modulation sequences That is, a periodic cycle between 0 and A temporal modulation sequence switching between two states;

[0056] Will After Fourier expansion, we get:

[0057] (2)

[0058] in, Indicates the harmonic order, h=±1, ±2, ±3...; express Amplitude of the order harmonics, spectrum Expressed as:

[0059] (3)

[0060] Spectrum results like Figure 3 As shown, represents the Dirac function.

[0061] According to the above derivation, the frequency domain expression of 1-bit modulation phase is: the signal modulated by the metasurface will produce The echo signal is the sum of the multi-order harmonic signals (h=±1, ±2, ±3...), and each order harmonic is independent of each other. Figure 3 As shown, since the energy is concentrated in the positive and negative first-order harmonics, the positive and negative first-order harmonics with the highest energy are taken. The harmonics discussed below are all positive and negative first-order harmonics. , that is, changing the modulation frequency of the spatiotemporal modulation metasurface ; By dividing the space-time modulation metasurface into multiple areas, each area carrying a different modulation frequency, space-frequency modulation of the incident signal is achieved.

[0062] As a specific example, in step 2, the structural parameters of the spatiotemporal modulation metasurface are combined to establish a space-frequency modulation model of the spatiotemporal modulation metasurface, as follows:

[0063] Step 2.1: Assume that the metasurface is controlled by columns. There are N columns of basic units in the x-direction. The spacing between each column of basic units is d. Each column of units is arranged along the y-direction. Each column has the same response in the y-direction. When the center frequency is The electromagnetic wave with an incident angle The incident light is incident on the metasurface, and the reflection angle is , at this time the scattered field modulated by the metasurface is Expressed as:

[0064] (4)

[0065] in is the time modulation sequence of the basic unit in the nth column; is the wave number of the electromagnetic field, is the wavelength.

[0066] According to the scattered field formula (4), the incident angle , the reflection angle is The change of and the size of the array only affect the overall amplitude of the scattered field, and only the time modulation sequence It is a time-related variable that determines the generation of harmonics.

[0067] The electromagnetic tag response scenario is set to a single radar station observation scenario, with normal incidence and only one modulation frequency, that is, , the incident point frequency signal After the spatiotemporal modulation of the metasurface, only the ±1st harmonic with the highest energy is taken, and the scattering signal is:

[0068] (5)

[0069] in, is the harmonic amplitude;

[0070] Theoretically, when the areas between the regions are the same, that is, the number of metasurface units is equal, the amplitudes of different modulation frequencies are Approximately equal, that is, the first-order harmonic energy of different modulation frequencies is similar, which can be used as a basis for screening high-order harmonics to improve decoding accuracy. In other words, by analyzing the spectrum of the scattered echo modulated by the spatiotemporal modulation metasurface, the modulation frequency of the spatiotemporal modulation metasurface can be extracted. ;

[0071] Step 2.2: Under far-field conditions, the electromagnetic wave incident on the time-space modulation metasurface is equivalent to a plane wave. When the time-space modulation metasurface is divided into M regions and different regions are equipped with different modulation frequencies, the electromagnetic waves irradiating different regions are parallel to each other and have the same incident angle.

[0072] The echoes in different regions are modulated to produce different orthogonal modulation frequency harmonics, so the modulation frequency corresponding to the i-th region is The corresponding scattered signal Expressed as:

[0073] (6)

[0074] Where i is the region number, which is a positive integer;

[0075] When the areas between regions are the same, that is, the number of basic units is equal, the amplitude of different modulation frequencies Approximately equal, that is, the energy of the first-order harmonics of different modulation frequencies is similar, which is used as a basis for screening high-order harmonics.

[0076] As a specific example, in step 3, the magnitude and spatial combination of the modulation frequency are changed to establish a mapping code library between the modulation frequency and the response code, as follows:

[0077] Changing the modulation frequency , and according to The size and combination of define the response code;

[0078] Define a set of mappings to establish the relationship between the modulation frequency and the response code used by the spatiotemporal modulation metasurface, forming a mapping code library;

[0079] The modulation frequency used by the known spatiotemporal modulation metasurface , get the corresponding response code according to the mapping code library.

[0080] Furthermore, the modulation frequency The size and combination of are selected according to the actual performance of the receiving device. According to formula (1), the receiver sampling frequency is satisfied. Greater than twice the modulation frequency ,Right now Only when the two states in one modulation cycle are correctly sampled can the harmonic frequency be correctly extracted.

[0081] Furthermore, the appropriate modulation frequency and number of coding regions are determined, combined, and mapping is established based on the combination to construct a coding library, such as Corresponding code value 0, Corresponding code value 1. As long as the modulation frequency used by the metasurface can be obtained , we can get the corresponding encoding based on this set of mappings. The modulation frequency scheme in the encoding library is used to generate the metasurface control signal, which is then imported into the metasurface to complete the transmission of tag information.

[0082] As a specific example, in step 6, the radar transmits a linear frequency modulation signal to different areas, and different areas modulate the incident signal differently to generate multiple harmonic signals that are orthogonal to each other, as follows:

[0083] The analysis is carried out using a typical periodic radar linear frequency modulation (LFM) signal. A single LFM signal pulse is written as:

[0084] (7)

[0085] Where t is time, Indicates the pulse width, is the frequency modulation slope and , is the signal bandwidth, is the center frequency; is a rectangular window function, is equal to 1 during the time range and 0 otherwise.

[0086] Without loss of generality, there is only one modulation frequency when the electromagnetic wave is incident normally and the metasurface In the case of LFM signal incident on the metasurface, the temporal modulation sequence is modulated by the temporal modulation of the metasurface. After reflection and modulation, according to Equation (2), the reflected wave of the LFM signal on the spatiotemporal modulation metasurface is expressed as:

[0087] (8)

[0088] The harmonic space-frequency diagram corresponding to formula (8) is as follows: Figure 4 As shown, Indicates the harmonic order (h=±1, ±2, ±3...), Represents the h-order harmonic amplitude. Analyzing the reflected wave expression, it can be obtained that the modulated reflected wave is expressed as the sum of infinite-order LFM signal harmonics, and the frequency difference between adjacent harmonic signals is the modulation frequency of the time modulation sequence carried. , each order harmonic is independent of each other.

[0089] As a specific example, in step 7, the modulated linear frequency modulation echo signal is subjected to matched filtering to obtain an amplitude spectrum, and the modulation frequency used by the electromagnetic tag is obtained according to the peak position. The response code value is searched in the mapping code library based on the mapping relationship, as follows:

[0090] Matched filtering is performed on the harmonic signal. Since the harmonics of the LFM signal after reflection are independent of each other, the amplitude coefficients between the harmonics of different orders are obtained by matched filtering. The amplitude spectrum output after the matched filter is expressed as:

[0091] (9)

[0092] in, represents convolution, is the impulse response of the matched filter, Indicates signal conjugation of; It is used for distance measurement delay, setting ; The function is defined as ;

[0093] According to formula (9), combined with the frequency formula of the linear frequency modulation signal , the frequency resolution of the amplitude spectrum obtained after matched filtering is , when using matched filtering for decoding, the interval between different modulation frequencies is greater than ;

[0094] (10)

[0095] in is the time interval between two sampling points;

[0096] After the LFM signal is modulated by 1-bit on the metasurface, multi-order harmonics with positive and negative symmetry are generated. After matched filtering, each order harmonic generates a peak at the corresponding spectrum position according to the harmonic energy. The energy of the harmonics generated by 1-bit phase modulation is concentrated on the positive and negative first-order harmonics. So the first harmonic is the encoding; according to the amplitude spectrum after matched filtering, the modulation frequency used is obtained .

[0097] Since the energy of high-order harmonics is very low, even if high-order harmonic components between different modulation frequencies are superimposed, they will not exceed the first-order harmonic and will not affect the decoding effect. Therefore, there is no need to consider the superposition problem between high-order harmonics when selecting the modulation frequency.

[0098] The present invention also provides a coding response system for a frequency diversity electromagnetic tag based on a time-space modulation metasurface, which is used to implement the coding response method for a frequency diversity electromagnetic tag based on a time-space modulation metasurface. The system includes a theoretical model construction module, a space-frequency modulation model construction module, a mapping code library construction module, a scattering state control module, a frequency diversity module, a harmonic signal generation module and a coding response module, wherein each 0:

[0099] Theoretical model construction module, establishes the space-frequency modulation theoretical model. The space-time modulation metasurface uses a time modulation sequence to perform double-sideband modulation on the incident signal, generating harmonics corresponding to the modulation frequency, and uses the frequency distribution of the harmonics as the encoding information;

[0100] The space-frequency modulation model construction module combines the structural parameters of the space-time modulation metasurface to establish the space-frequency modulation model of the space-time modulation metasurface;

[0101] The mapping code library construction module changes the size and spatial combination of the modulation frequency to establish a mapping code library between the modulation frequency and the response code;

[0102] The scattering state control module generates a timing control code of a modulation frequency corresponding to the response code according to the mapping code library to control the scattering state of the metasurface;

[0103] The frequency diversity module divides the spatiotemporal modulation metasurface into multiple regions. Based on space-frequency domain joint modulation, the timing control codes of different modulation frequencies are introduced into different regions, so that each region carries a different modulation frequency, thus achieving frequency diversity.

[0104] Harmonic signal generation module: The radar transmits linear frequency modulation signals to different areas, and different areas modulate the incident signals differently to generate multiple harmonic signals that are orthogonal to each other;

[0105] The coding response module performs matched filtering on the modulated linear frequency modulation echo signal to obtain the amplitude spectrum, obtains the modulation frequency used by the electromagnetic tag according to the peak position, and searches the mapping code library based on the mapping relationship to obtain the response code value.

[0106] The present invention also provides a mobile terminal, including a memory, a processor, and a computer program stored in the memory and runnable on the processor. When the processor executes the program, the coding response method of the frequency diversity electromagnetic tag based on the spatiotemporal modulation metasurface is implemented.

[0107] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0108] Example

[0109] This embodiment simulates and processes the radar linear frequency modulation signal (LFM signal) after 1-bit modulation by the spatiotemporal modulation metasurface. According to step 1, the time modulation sequence phase of the ideal 1-bit spatiotemporal modulation metasurface is set to , the simulation results are as follows Figure 2 As shown, the modulation period variable. After Fourier transform, the spectrum is as follows Figure 3 As shown, changing the modulation period , the corresponding harmonic frequencies also change accordingly.

[0110] According to steps 2 and 3, the spectrum simulation of the frequency diversity results is performed. Next, we take the example of dividing the spatiotemporal modulation metasurface into three equal-area regions. Figure 5 As shown, take , the corresponding modulation frequency is , different modulation frequencies correspond to The amplitudes of are equal, both are 1. The phase of the time modulation sequence is as follows Figure 7 As shown, after Fourier transform, the spectrum is as follows Figure 6 shown.

[0111] The radar linear frequency modulation signal parameters are as follows: pulse width , signal bandwidth , center frequency , sampling frequency , the simulation results are as follows Figure 7 As shown. After adding the above time modulation sequence phase to the linear frequency modulation signal according to step 4, the modulated signal is subjected to matched filtering. The result is as follows Figure 8 shown.

[0112] According to the above process, the linear frequency modulation signal is modulated by taking the following combinations in Table 1 respectively. At the same time, the mapping relationship between the coding library and the modulation frequency value is established. The matched filtering results are as follows: Figure 9 、 Figure 10 、 Figure 11 、 Figure 12 shown.

[0113] Table 1

[0114] Based on the matched filtering results, the metasurface's corresponding modulation frequency can be accurately demodulated. Then, based on the designed mapping, the encoding information sent by the electromagnetic tag can be determined by indexing it in the encoding library. This validates the feasibility of the proposed method for designing frequency-diversity electromagnetic tag coded responses based on a spatiotemporally modulated metasurface.

[0115] The above are only preferred embodiments of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A coding response method for a frequency diversity electromagnetic tag based on a spatiotemporal modulation metasurface, characterized in that: The method uses a spatiotemporal modulation metasurface as an electromagnetic tag for response, and specifically includes the following steps: Step 1: Establish a theoretical model of space-frequency modulation. The space-time modulation metasurface uses a time modulation sequence to perform double-sideband modulation on the incident signal, generating harmonics corresponding to the modulation frequency, and using the frequency distribution of the harmonics as the encoding information. Step 2: Based on the structural parameters of the spatiotemporal modulation metasurface, a space-frequency modulation model of the spatiotemporal modulation metasurface is established; Step 3: Change the size and spatial combination of the modulation frequency to establish a mapping code library between the modulation frequency and the response code; Step 4: Generate a timing control code of the modulation frequency corresponding to the response code according to the mapping code library to control the scattering state of the metasurface; Step 5: Divide the spatiotemporal modulation metasurface into multiple regions. Based on space-frequency domain joint modulation, introduce timing control codes of different modulation frequencies into different regions, so that each region carries a different modulation frequency to achieve frequency diversity. Step 6: The radar transmits a linear frequency modulation signal to different areas, and different areas modulate the incident signal differently to generate multiple harmonic signals that are orthogonal to each other; Step 7: Perform matched filtering on the modulated linear frequency modulation echo signal to obtain an amplitude spectrum, obtain the modulation frequency used by the electromagnetic tag according to the peak position, and obtain the response code value by searching in the mapping code library based on the mapping relationship.

2. The coded response method of the frequency diversity electromagnetic tag based on the spatiotemporal modulation metasurface according to claim 1 is characterized in that: The spatiotemporal modulation metasurface is composed of a plurality of basic units arranged periodically, and can realize two scattering states of 0° phase and 180° phase, i.e., 1-bit periodic phase; The spatiotemporal regulation metasurface has the characteristic of electrically adjustable scattering state. By generating and importing control signals corresponding to the two scattering states, the scattering state of the metasurface is controlled to change with time, thereby realizing regulation of the scattering field in the time domain.

3. The coded response method of the frequency diversity electromagnetic tag based on the spatiotemporal modulation metasurface according to claim 2 is characterized in that: In step 1, a space-frequency modulation theoretical model is established as follows: The time modulation sequence used is a 1-bit phase sequence, Expressed as: （1） in, For time, is the base of natural logarithms, is the imaginary unit, is pi; represents the modulation period, is the modulation frequency; Indicates the time modulation cycles, time modulation sequences That is, a periodic cycle between 0 and A temporal modulation sequence switching between two states; Will After Fourier expansion, we get: （2） in, represents the harmonic order, express Amplitude of the order harmonics, spectrum Expressed as: （3） in represents the Dirac function; The signal modulated by the spatiotemporal metasurface will produce The sum of multiple harmonics of , each order harmonic is independent of each other; the energy is concentrated in the positive and negative 1st order harmonics, then the positive and negative 1st order harmonics with the highest energy are taken Encoded response as coded information; change the modulation period of 1-bit phase sequence , that is, changing the modulation frequency of the spatiotemporal modulation metasurface ; By dividing the space-time modulation metasurface into multiple areas, each area carrying a different modulation frequency, space-frequency modulation of the incident signal is achieved.

4. The coded response method of the frequency diversity electromagnetic tag based on the spatiotemporal modulation metasurface according to claim 3 is characterized in that: In step 2, the space-frequency modulation model of the spatiotemporal modulation metasurface is established by combining the structural parameters of the spatiotemporal modulation metasurface, as follows: Step 2.1: Assume that the spatiotemporal modulation metasurface has N columns of basic units in the x-direction, the spacing between each column of basic units is d, each column of basic units is arranged along the y-direction, and each column has the same response in the y-direction. When the center frequency is The electromagnetic wave with an incident angle The incident light hits the space-time modulation metasurface, and the reflection angle is , at this time, the scattering field after the temporal and spatial modulation of the metasurface is Expressed as: （4） in is the time modulation sequence of the nth column of basic units; is the wave number of the electromagnetic field, is the wavelength; According to formula (4), the incident angle , the reflection angle is The change of and the size of the array only affect the overall amplitude of the scattered field, and only the time modulation sequence Affects the distribution of harmonics; The electromagnetic tag response scenario is set to a single radar station observation scenario, with normal incidence and only one modulation frequency, that is, , the incident point frequency signal After the spatiotemporal modulation of the metasurface, only the ±1st harmonic with the highest energy is taken, and the scattering signal is: （5） in, is the harmonic amplitude; By analyzing the spectrum of the scattered echo after the spatiotemporal modulation metasurface, the modulation frequency of the spatiotemporal modulation metasurface can be extracted. ; Step 2.2: Under far-field conditions, the electromagnetic wave incident on the time-space modulation metasurface is equivalent to a plane wave. When the time-space modulation metasurface is divided into M regions and different regions are equipped with different modulation frequencies, the electromagnetic waves irradiating different regions are parallel to each other and have the same incident angle. The echoes in different regions are modulated to produce different orthogonal modulation frequency harmonics, so the modulation frequency corresponding to the i-th region is The corresponding scattered signal Expressed as （6） Where i is the region number, which is a positive integer; When the areas between regions are the same, that is, the number of basic units is equal, the amplitude of different modulation frequencies Approximately equal, that is, the energy of the first-order harmonics of different modulation frequencies is similar, which is used as a basis for screening high-order harmonics.

5. The coded response method of the frequency diversity electromagnetic tag based on the spatiotemporal modulation metasurface according to claim 4 is characterized in that: In step 3, the magnitude and spatial combination of the modulation frequency are changed to establish a mapping code library between the modulation frequency and the response code, as follows: Changing the modulation frequency , and according to The size and combination of define the response code; Define a set of mappings to establish the relationship between the modulation frequency and the response code used by the spatiotemporal modulation metasurface, forming a mapping code library; The modulation frequency used by the known spatiotemporal modulation metasurface , get the corresponding response code according to the mapping code library.

6. The coded response method of the frequency diversity electromagnetic tag based on the spatiotemporal modulation metasurface according to claim 5 is characterized in that: The modulation frequency The size and combination of are selected according to the actual performance of the receiving device. According to formula (1), the receiver sampling frequency is satisfied. Greater than twice the modulation frequency ,Right now Only when the two states in one modulation cycle are correctly sampled can the harmonic frequency be correctly extracted.

7. The coded response method of the frequency diversity electromagnetic tag based on the spatiotemporal modulation metasurface according to claim 6 is characterized in that: In step 6, the radar transmits a linear frequency modulation signal to different areas. Different areas modulate the incident signal differently to generate multiple harmonic signals that are orthogonal to each other, as follows: For the analysis of periodic radar linear frequency modulation signal LFM, a single LFM signal pulse is written as: （7） Where t is time, Indicates the pulse width, is the frequency modulation slope and , is the signal bandwidth, is the center frequency; is a rectangular window function, is equal to 1 within the time range of , and 0 otherwise; There is only one modulation frequency when the electromagnetic wave is normally incident and the metasurface In the case of LFM signal incident on the metasurface, the temporal modulation sequence is modulated by the temporal modulation of the metasurface. After reflection and modulation, according to Equation (2), the reflected wave of the LFM signal on the spatiotemporal modulation metasurface is expressed as: （8） The modulated reflected wave is represented by the sum of infinite-order LFM signal harmonics. The frequency difference between adjacent harmonic signals is the modulation frequency of the time modulation sequence. , each order harmonic is independent of each other.

8. The coded response method of the frequency diversity electromagnetic tag based on the spatiotemporal modulation metasurface according to claim 7 is characterized in that: In step 7, the modulated linear frequency modulation echo signal is subjected to matched filtering to obtain the amplitude spectrum. The modulation frequency used by the electromagnetic tag is obtained according to the peak position. The response code value is searched in the mapping code library based on the mapping relationship, as follows: The harmonics of the LFM signal after reflection are independent of each other, so the amplitude coefficients between the harmonics of different orders are obtained by matched filtering, and the reflected signal The amplitude spectrum output after the matched filter is expressed as: （9） in, represents convolution, is the impulse response of the matched filter, Indicates signal conjugation of; It is used for distance measurement delay, setting ; The function is defined as ; According to formula (9), combined with the frequency formula of the linear frequency modulation signal , the frequency resolution of the amplitude spectrum obtained after matched filtering is , when using matched filtering for decoding, the interval between different modulation frequencies is greater than ; （10） in is the time interval between two sampling points; After the LFM signal is modulated by 1-bit on the metasurface, multi-order harmonics with positive and negative symmetry are generated. After matched filtering, each order harmonic generates a peak at the corresponding spectrum position according to the harmonic energy. The energy of the harmonics generated by 1-bit phase modulation is concentrated on the positive and negative first-order harmonics. So the first harmonic is the encoding; according to the amplitude spectrum after matched filtering, the modulation frequency used is obtained .

9. A coded response system based on a frequency diversity electromagnetic tag with spatiotemporal modulation metasurface, characterized in that: The system is used to implement the coded response method of the frequency-diversity electromagnetic tag based on the time-space modulation metasurface according to any one of claims 1 to 8, and the system includes a theoretical model construction module, a space-frequency modulation model construction module, a mapping code library construction module, a scattering state control module, a frequency diversity module, a harmonic signal generation module and a coded response module, wherein: Theoretical model construction module, establishes the space-frequency modulation theoretical model. The space-time modulation metasurface uses a time modulation sequence to perform double-sideband modulation on the incident signal, generating harmonics corresponding to the modulation frequency, and uses the frequency distribution of the harmonics as the encoding information; The space-frequency modulation model construction module combines the structural parameters of the space-time modulation metasurface to establish the space-frequency modulation model of the space-time modulation metasurface; The mapping code library construction module changes the size and spatial combination of the modulation frequency to establish a mapping code library between the modulation frequency and the response code; The scattering state control module generates a timing control code of a modulation frequency corresponding to the response code according to the mapping code library to control the scattering state of the metasurface; The frequency diversity module divides the spatiotemporal modulation metasurface into multiple regions. Based on space-frequency domain joint modulation, the timing control codes of different modulation frequencies are introduced into different regions, so that each region carries a different modulation frequency, thus achieving frequency diversity. Harmonic signal generation module: The radar transmits linear frequency modulation signals to different areas, and different areas modulate the incident signals differently to generate multiple harmonic signals that are orthogonal to each other; The coding response module performs matched filtering on the modulated linear frequency modulation echo signal to obtain the amplitude spectrum, obtains the modulation frequency used by the electromagnetic tag according to the peak position, and searches the mapping code library based on the mapping relationship to obtain the response code value.

10. A mobile terminal comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein: When the processor executes the program, the coding response method of the frequency diversity electromagnetic tag based on the spatiotemporal modulation metasurface is implemented as described in any one of claims 1 to 8.

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