Space-time modulation metasurface-based frequency diversity electromagnetic tag coding response method and system

Abstract

The invention discloses a space-time modulation metasurface-based frequency diversity electromagnetic tag coding response method and system, and the method specifically comprises the steps: taking a space-time modulation metasurface as an electromagnetic tag for response, and building a modulation frequency combination and response code mapping coding library; generating a metasurface sequential control code of a modulation frequency corresponding to the response code according to a code library; based on space-frequency domain joint modulation, time sequence control codes of different modulation frequencies are imported into different regions to realize frequency diversity, and an incident signal is modulated; and performing matched filtering processing on the modulated linear frequency modulation echo signal to obtain an amplitude spectrum, obtaining a modulation frequency scheme adopted by a label according to a peak value position, and searching in a coding library according to a mapping relation to obtain a response coding value. The response method of the electromagnetic tag 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 of a frequency diversity electromagnetic tag based on a spatiotemporal modulation metasurface. Background Art

[0002] Target recognition is the process of determining the target category, state or function by analyzing the characteristics of the target in radar detection technology. It is an important part of information processing. Nowadays, electromagnetic technology is constantly developing, electromagnetic interference methods are more diverse, and the electromagnetic environment is more complex. This has put forward higher requirements for the recognition and response of electromagnetic tags. They need to cope with the ever-changing environment and complete the recognition and response tasks while minimizing costs and improving anti-interference capabilities.

[0003] The spatiotemporal modulation metasurface is studied. The spatiotemporal modulation metasurface can be used to precisely modulate the incident electromagnetic wave. The reconfigurable basic metasurface unit can independently apply a time-varying phase to the echo signal to change the spectral characteristics of the echo, thereby constructing the required spectral characteristics according to the identification requirements and flexibly completing the response task in different environments. The spatiotemporal modulation metasurface realizes electromagnetic tag encoding through frequency diversity, which has the advantages of low power consumption, low cost and large 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 of frequency-selective RF harmonic tags for harmonic Doppler radar tracking. Harmonic RFID tags receive microwave signals and generate and re-transmit signals at harmonics of the original signal frequency (usually the second harmonic), thereby improving detection capabilities. Through a narrowband split-ring printed antenna, the tag receives the signal only within a specified narrowband range and generates a second harmonic through a diode, which is re-transmitted by a standard dipole antenna. This design implements narrowband filtering at the fundamental frequency and does not require additional circuitry to distinguish multiple tags. However, this electromagnetic tag has a fixed structure and cannot be reconstructed. It can only implement one modulation mode and a single response form. 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 the 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 coding response method of a frequency diversity electromagnetic tag based on a spatiotemporal modulation metasurface, which uses the spatiotemporal modulation metasurface as an electromagnetic tag for response, and specifically includes 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 to generate harmonics corresponding to the modulation frequency, and uses 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 of the modulation frequency and the spatial combination mode, and establish a mapping code library between the modulation frequency and the response code;

[0009] Step 4: Generate 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;

[0010] Step 5: Divide the spatiotemporal modulation metasurface into multiple regions, and based on space-frequency domain joint modulation, import the 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 search in the mapping code library according to the mapping relationship to obtain the response code value.

[0013] A coding response system of a frequency diversity electromagnetic tag based on a time-space modulation metasurface, the system is used to implement the coding response method of the frequency diversity electromagnetic tag based on the 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:

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

[0015] The space-frequency modulation model building 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] A mapping code library construction module changes the size of the modulation frequency and the spatial combination mode, and establishes a mapping code library between the modulation frequency and the response code;

[0017] A scattering state control module generates a timing control code of a modulation frequency corresponding to the response code according to a 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 the joint modulation of the space-frequency domain, the timing control codes of different modulation frequencies are introduced into different regions, so that each region is equipped with a different modulation frequency to achieve 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 to obtain the response code value based on the mapping relationship.

[0021] A mobile terminal comprises a memory, a processor and a computer program stored in the memory and executable 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 electromagnetic metasurfaces;

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

[0025] (3) By means of frequency diversity, the modulation frequency of the basic units of the spatiotemporal modulation metasurface is 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 The present invention is a flow chart of a coding response method of a frequency diversity electromagnetic tag based on a spatiotemporal modulation metasurface.

[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 It is a time-frequency diagram of the 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 by a frequency of Result diagram of matched filtering after modulation.

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

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

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

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

[0039] It is easy to understand that according to the technical solution of the present invention, without changing the essential spirit of the present invention, a person skilled in the art can imagine various implementation methods of the present invention. Therefore, the following specific implementation methods and drawings are only exemplary descriptions of the technical solution of the present invention, and should not be regarded as the whole of the present invention or as a limitation or restriction of the technical solution of the present invention.

[0040] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that unless otherwise specifically stated, the relative arrangement of components and steps, numerical expressions and numerical 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 as part of the specification.

[0042] Combination Figure 1 The present invention provides a coding response method of a frequency diversity electromagnetic tag based on a time-space modulation metasurface. The method uses the time-space 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 to generate harmonics corresponding to the modulation frequency, and uses the frequency distribution of the harmonics as the encoding information.

[0044] Step 2: Combine the structural parameters of the spatiotemporal modulation metasurface to establish a space-frequency modulation model of the spatiotemporal modulation metasurface;

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

[0046] Step 4: Generate 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;

[0047] Step 5: Divide the spatiotemporal modulation metasurface into multiple regions, and based on space-frequency domain joint modulation, import the 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 search in the mapping code library according to the mapping relationship to obtain the response code value.

[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 regulation metasurface has the characteristic of electrically adjustable scattering state. By generating and importing control signals corresponding to two scattering states, the scattering state of the metasurface is controlled to change with time, thereby realizing 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, It is expressed as:

[0054] (1)

[0055] like Figure 2 As shown, For time, is the base of natural logarithms, is an imaginary unit, is the ratio of pi; represents the modulation period, is the modulation frequency; Indicates time modulation cycle, time modulation sequence That is, a 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 harmonics, spectrum It is 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 of harmonics 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 space-frequency modulation model of the space-time modulation metasurface is established by combining the structural parameters of the space-time modulation metasurface, as follows:

[0063] Step 2.1: Assume that the metasurface is regulated 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 has 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 It is expressed as:

[0064] (4)

[0065] in is the time modulation sequence of the nth column of basic units; 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 temporal modulation sequence It is a time-related variable that determines the generation of harmonics.

[0067] The electromagnetic tag response scenario is set as a single radar station observation, 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 amplitude of different modulation frequencies 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 the incident angles are the same;

[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 It is expressed as:

[0073] (6)

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

[0075] When the areas between the regions are the same, that is, the number of basic units is equal, the amplitude of different modulation frequencies Approximately equal, that is, the first-order harmonic energy 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 size of the modulation frequency and the spatial combination mode 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, establish the relationship between the modulation frequency and the response code used by the spatiotemporal modulation metasurface, and form 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 can be sampled correctly and the harmonic frequency can be extracted correctly.

[0081] Further, determine the appropriate modulation frequency and the number of coding regions, combine them, establish a mapping based on the combination, and construct a coding library, such as The corresponding code value is 0. The corresponding code value is 1. As long as the modulation frequency used by the metasurface can be obtained , the corresponding encoding can be obtained according to this set of mappings. The modulation frequency scheme in the encoding library is used to generate a metasurface control signal, and the control signal is imported into the metasurface to complete the transmission of the tag information.

[0082] As a specific example, in step 6, the radar transmits a linear frequency modulation signal to different regions, and different regions modulate the incident signal differently to generate multiple harmonic signals 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, represents 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 time-space modulation 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 of adjacent harmonic signals is the modulation frequency of the time modulation sequence carried. , each order of harmonics 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, the modulation frequency used by the electromagnetic tag is obtained according to the peak position, and the response code value is obtained by searching in the mapping code library according to the mapping relationship, as follows:

[0090] The harmonic signal is processed by matched filtering. 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 the delay used for distance measurement. ; 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 1bit on the metasurface, multi-order harmonics with positive and negative symmetry are generated. After matched filtering, each order of harmonics generates a peak value at the corresponding spectrum position according to the harmonic energy. The energy of the harmonics generated by 1bit 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 the 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 of a frequency diversity electromagnetic tag based on a time-space modulation metasurface, which is used to implement the coding response method of the frequency diversity electromagnetic tag based on the time-space modulation metasurface, 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 coding response module, wherein each 0:

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

[0100] The space-frequency modulation model building 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] A 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] A scattering state control module generates a timing control code of a modulation frequency corresponding to the response code according to a 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 the joint modulation of the space-frequency domain, the timing control codes of different modulation frequencies are introduced into different regions, so that each region is equipped with a different modulation frequency to achieve 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 to obtain the response code value based on the mapping relationship.

[0106] The present invention also provides a mobile terminal, including a memory, a processor, and a computer program stored in the memory and executable 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 is 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 be The simulation results are as follows: Figure 2 As shown, the modulation period variable. After Fourier transform, the spectrum is Figure 3 As shown, changing the modulation period , the corresponding harmonic frequency also changes accordingly.

[0110] According to step 2 and step 3, the spectrum simulation of the frequency diversity result is performed. Next, the spatiotemporal modulation metasurface is divided into three equal-area regions as an example. 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 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 After the linear frequency modulation signal is superimposed on the phase of the above time modulation sequence according to step 4, the modulated signal is subjected to matched filtering, and the result is as follows Figure 8 shown.

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

[0113] Table 1

[0114] According to the result of matched filtering, the corresponding modulation frequency of the metasurface can be accurately demodulated. Then, according to the designed mapping concern, the coding information sent by the electromagnetic tag can be determined by indexing in the coding library. The feasibility of the frequency diversity electromagnetic tag coding response design method based on the spatiotemporal modulation metasurface in the present invention is verified.

[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 the 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 to generate harmonics corresponding to the modulation frequency, and uses the frequency distribution of the harmonics as the encoding information. Step 2: Combine the structural parameters of the spatiotemporal modulation metasurface to establish a space-frequency modulation model of the spatiotemporal modulation metasurface; Step 3: Change the size of the modulation frequency and the spatial combination mode, and establish a mapping code library between the modulation frequency and the response code; Step 4: Generate 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; Step 5: Divide the spatiotemporal modulation metasurface into multiple regions, and based on space-frequency domain joint modulation, import the 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 search in the mapping code library according to the mapping relationship to obtain the response code value.

2. The coding 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 that the scattering state is electrically adjustable. 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 the regulation of the scattering field in the time domain.

3. The coding 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, It is expressed as: （1） in, For time, is the base of natural logarithms, is an imaginary unit, is pi; represents the modulation period, is the modulation frequency; Indicates time modulation cycle, time modulation sequence That is, a 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 harmonics, spectrum It is 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, each order of harmonics is independent of each other; the energy is concentrated in the positive and negative 1st order harmonics, so the positive and negative 1st order harmonics with the highest energy are taken Encoded response as coded information; change the modulation period of 1bit 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 coding 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 has an incident angle When incident on the space-time modulation metasurface, the reflection angle is , at this time, the scattering field after the temporal and spatial modulation of the metasurface is It 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 temporal modulation sequence Affects the distribution of harmonics; The electromagnetic tag response scenario is set as a single radar station observation, 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 the incident angles are the same; 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 area number, which is a positive integer; When the areas between the regions are the same, that is, the number of basic units is equal, the amplitude of different modulation frequencies Approximately equal, that is, the first-order harmonic energy of different modulation frequencies is similar, which is used as a basis for screening high-order harmonics.

5. The coding 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 size of the modulation frequency and the spatial combination mode 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, establish the relationship between the modulation frequency and the response code used by the spatiotemporal modulation metasurface, and form 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 coding 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 can be sampled correctly and the harmonic frequency can be extracted correctly.

7. The coding 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, and different areas modulate the incident signal differently to generate multiple harmonic signals that are orthogonal to each other, as follows: For the analysis of the periodic radar linear frequency modulation signal LFM, a single LFM signal pulse is written as: （7） Where t is time, represents 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 in 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 time-space modulation 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 expressed as the sum of infinite-order LFM signal harmonics. The frequency difference between adjacent harmonic signals is the modulation frequency of the carried time modulation sequence. , each order of harmonics is independent of each other.

8. The coding 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 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 according to 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 through 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 the delay used for distance measurement. ; 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 1bit on the metasurface, multi-order harmonics with positive and negative symmetry are generated. After matched filtering, each order of harmonics generates a peak value at the corresponding spectrum position according to the harmonic energy. The energy of the harmonics generated by 1bit 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 of frequency diversity electromagnetic tag based on spatiotemporal modulation metasurface, characterized in that: The system is used to implement the coding 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 coding response module, wherein: Theoretical model construction module, establishes the space-frequency modulation theoretical model, the space-time modulation metasurface uses the time modulation sequence to perform double-sideband modulation on the incident signal, generates harmonics corresponding to the modulation frequency, and uses the frequency distribution of the harmonics as the encoding information; The space-frequency modulation model building module combines the structural parameters of the space-time modulation metasurface to establish the space-frequency modulation model of the space-time modulation metasurface; A 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; A scattering state control module generates a timing control code of a modulation frequency corresponding to the response code according to a 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 the joint modulation of the space-frequency domain, the timing control codes of different modulation frequencies are introduced into different regions, so that each region is equipped with a different modulation frequency to achieve 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 to obtain the response code value based on the mapping relationship.

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

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