A metasurface-based stepped frequency modulation radar transmitter

Abstract

This invention discloses a step-frequency radar transmitter based on a spatiotemporally coded metasurface, belonging to the field of artificial electromagnetic metasurface technology. The transmitter includes: 1) a spatiotemporally coded metasurface composed of subwavelength units, each unit integrating a varactor diode and a surface-mount capacitor, with continuous 0-2π phase modulation of the reflection phase achieved through a bias voltage applied by an FPGA; 2) a step-frequency modulation signal generation method, generating a step-frequency modulated reflected echo by designing a metasurface reflection coefficient encoding vector and a time-varying incident signal; 3) a waveform purity analysis model, quantifying the impact of amplitude and phase errors on signal purity; and 4) a novel radar transmitter architecture. This invention can simultaneously realize radar waveform generation and beam scanning functions, integrating the waveform generator and T / R component functions of a traditional transmitter into a single device. Compared to conventional architectures, this invention significantly reduces hardware complexity and integration costs, and supports broadband waveform generation capabilities, enabling accurate acquisition of target distance and velocity information.

Description

Technical Field

[0001] This invention relates to a step-frequency modulated radar transmitter based on a spatiotemporally coded metasurface, which can simultaneously realize the generation of broadband step-frequency modulated signals and dynamic beamforming, and belongs to the field of novel artificial electromagnetic metasurface technology. Background Technology

[0002] As a core piece of equipment for all-weather target detection, the performance of radar largely depends on the design of its transmitter system. Traditional transmitters typically employ a modular architecture, where waveform generation relies on fixed hardware modules, limiting the flexibility of signal reconstruction. Beamforming, on the other hand, depends on phased array antennas, requiring each radiating element to be equipped with a high-precision T / R component, causing system cost and size to increase exponentially with the number of radiating elements. More critically, existing solutions employ a physically separate architecture for waveform generation and beamforming, resulting not only in low equipment integration and increased maintenance difficulty but also becoming a core technological bottleneck restricting the further development of radar technology.

[0003] Coordinated metasurfaces, as artificial electromagnetic structures, achieve dynamic modulation of the electromagnetic wavefront through the periodic arrangement of subwavelength units. Their spatial beamforming capability can replace phased array antennas, significantly reducing system complexity and cost. Spatiotemporally coded metasurfaces further introduce a time-domain modulation dimension, achieving waveform generation through dynamic spectrum manipulation. Experiments show that they can generate frequency-modulated / phase-modulated signals using time-series coding, meeting diverse radar detection requirements. However, due to the limited modulation speed of the control module and the inherent defects of metasurfaces, traditional spatiotemporally coded metasurfaces are typically only suitable for processing narrowband signals; reduced bandwidth means reduced radar detection accuracy. Therefore, there is an urgent need to develop spectrum spreading methods based on metasurface frequency response characteristic compensation to construct a wideband spatiotemporally coded metasurface radar transmitter architecture to break through the technical limitations of existing metasurface radar transmitters. Summary of the Invention

[0004] Technical issues:

[0005] The problem this invention aims to solve is that traditional spatiotemporally coded metasurface architectures do not support broadband signal processing. Insufficient signal generation bandwidth leads to degraded system detection accuracy, substantially hindering the large-scale engineering application of this technology in radar transmitters. Specifically, the invention achieves this by configuring the frequency domain parameters of the incident signal from the spatiotemporally coded metasurface and designing the time coding of the control signal to generate a stepped frequency-modulated signal. Based on the principle of frequency step synthesis, the signal generated by the spatiotemporally coded metasurface is phase-aligned and bandwidth-synthesized to reconstruct the frequency-modulated signal with equivalent bandwidth expansion. Simultaneously, through spatial domain coding design, the system achieves directional radiation characteristics while generating the stepped frequency-modulated signal.

[0006] Technical solution:

[0007] To address the aforementioned technical problems, this invention proposes a spatiotemporally coded metasurface capable of generating stepped frequency-modulated signals. The core structure of this invention is a spatiotemporally coded metasurface, which is a two-dimensional ultrathin artificial electromagnetic structure constructed from M×L subwavelength-scale units arranged parametrically. Each functional unit integrates a varactor diode as a reconfigurable load, and a control loop is built based on a field-programmable gate array (FPGA). By dynamically adjusting the bias voltage, the electromagnetic wave reflection characteristics are synergistically controlled: while maintaining high reflection efficiency, a continuously adjustable phase response range of 0-2π is obtained.

[0008] Among them, the metasurface structure unit uses F4B double-sided copper-clad dielectric substrate (dielectric constant 2.65, loss tangent 0.001) as dielectric substrate, and uses PCB process to etch 8 sets of rectangular patch arrays with different sizes on the top layer of the unit.

[0009] Among them, the top layer of the metasurface structure unit integrates four SMV-2019 varactor diodes and four 0.1pF chip capacitors. Based on the field programmable gate array (FPGA), voltage is applied across the varactor diodes, which enables dynamic control of the spatiotemporally encoded electromagnetic properties of the metasurface.

[0010] When the control voltage is switched between any value in the range of 0V to 14V, the metasurface can achieve full-cycle coverage of the reflection phase from 0 to 2π in the operating frequency band of 4.0GHz to 4.26GHz.

[0011] Among them, the spatiotemporally encoded metasurface generates a time-coded voltage signal through an FPGA controller, which drives the switching of the reflection coefficient of the metasurface unit to complete the phase and frequency modulation of the incident signal.

[0012] By synchronously controlling the incident signal frequency and the control coding timing, the spatiotemporal coded metasurface can generate a stepped frequency modulation signal and synthesize a frequency modulation signal with an equivalent bandwidth of 256MHz, which meets the technical requirements of radar systems for broadband signal generation and waveform agility.

[0013] This invention also proposes an analytical model for the factors affecting the purity of spatiotemporally coded metasurface waveforms, and reveals the influence mechanism of frequency modulation signal purity through a quantitative evaluation method.

[0014] This model constructs a reflection coefficient encoding framework for a spatiotemporally encoded metasurface, aiming to achieve waveform generation and energy concentration at specific frequencies. In practical applications, due to the non-ideal characteristics of the hardware, amplitude and phase errors are introduced: the former originates from the nonlinear fluctuation of the reflection amplitude during dynamic phase modulation of the metasurface unit, and the latter is caused by the transient response delay of parasitic reactance due to the switching of control signals.

[0015] Rapid timing switching during high-frequency signal generation exacerbates the nonlinear accumulation of phase errors, leading to a significant decrease in signal waveform purity. In contrast, amplitude errors are insensitive to changes in the modulation rate, exhibiting a constant amplitude deviation. This model, by decoupling the physical mechanisms of these two types of errors, provides a theoretical basis for optimizing metasurface parameters and improving signal purity.

[0016] This invention further proposes a step-frequency modulated radar transmitter architecture based on a spatiotemporally coded metasurface.

[0017] This architecture utilizes the time-coding characteristics of spatiotemporally encoded metasurfaces to construct a dynamic encoding mechanism for step-frequency modulation signals with adaptive parameter matching, thereby achieving efficient generation of high-purity step-frequency modulation signals.

[0018] Simultaneously, by integrating spatial domain coding technology, while maintaining the ability to generate waveforms from spatiotemporal coded metasurfaces, beam scanning with full spatial domain coverage is extended.

[0019] Meanwhile, by working in collaboration with the high-speed data acquisition module and the back-end processing unit, and by integrating signal feature extraction algorithms, the target's three-dimensional parameters of "distance-velocity-angle" can be retrieved in real time.

[0020] Beneficial effects:

[0021] 1. Evaluation model of factors affecting the generation of signals by metasurfaces: For the first time, a dynamic evolution model of the reflection coefficient of spatiotemporally coded metasurfaces is established, breaking through the limitations of traditional qualitative analysis. By separating the coupling effects of amplitude error and phase error, a quantitative evaluation of the signal purity degradation mechanism is achieved, providing a mathematical modeling benchmark for the optimization of metasurface parameters.

[0022] 2. High-precision, high-degree-of-freedom, and low-complexity radar transmitter: This invention constructs an integrated architecture of "signal modulation-beamforming", which realizes directional radiation while generating stepped frequency modulation signals. All processes are completed directly on the metasurface, which provides high module integration and eliminates the complex radio frequency links in traditional radar systems. Attached Figure Description

[0023] Figure 1 This is a schematic diagram illustrating the working principle of the step frequency modulation radar transmitter based on a spatiotemporally coded metasurface in this invention.

[0024] Figure 2 This is a unit structure diagram of the spatiotemporal coded metasurface in this invention;

[0025] Figure 3 The relationship between the reflection amplitude and reflection phase of the spatiotemporally encoded metasurface in this invention and the bias voltage at 4 GHz to 4.26 GHz is shown.

[0026] Figure 4The experimental setup for this invention includes: (a) a physical image of the designed spatiotemporally encoded metasurface sample; (b) a schematic diagram of the laboratory ranging test scenario layout; and (c) a schematic diagram of the ranging test scenario layout based on a radar echo simulator.

[0027] Figure 5 This paper compares the signal quality of broadband FM signals generated directly using a spatiotemporally coded metasurface and those generated using the method described in this invention. In this paper, (ab) represents the time-frequency analysis results of the signals generated by the two methods, and (cd) shows the purity of the signals generated by the two methods, as well as the relationship between amplitude error and phase error as a function of frequency.

[0028] Figure 6 The results of target detection using the step-frequency modulated radar transmitter based on a spatiotemporally coded metasurface designed in this invention are shown in the following diagrams: (ab) Ranging results of single and multiple targets in laboratory tests; (cd) Ranging results of single and multiple targets based on a radar echo simulator; (e) Range resolution test results; (f) Target velocity measurement results.

[0029] Figure 7 The results of beam scanning of the spatiotemporally encoded metasurface in this invention. Detailed Implementation

[0030] This invention provides a method for generating stepped frequency-modulated signals based on a spatiotemporally coded metasurface. This method combines the beamforming properties of the metasurface to construct a novel radar transmitter architecture. Its conceptual diagram is shown below. Figure 1 As shown in the figure. This embodiment designs a 12×16 all-phase modulation reflective metasurface sample, the unit structure of which is as follows. Figure 2 As shown. This metasurface sample adopts a three-layer structure design: a top metal patterned layer, a middle dielectric base layer (F4B, ε r =2.65, tanδ=0.0015) and a metal ground. A varactor diode (SMV-2019) and a surface-mount capacitor (0.1pF) are integrated on the surface of the metasurface unit. By adjusting the voltage across the varactor diode, the phase of electromagnetic wave reflection can be precisely controlled.

[0031] Figure 3 (ab) shows the test results of the designed spatiotemporally coded metasurface in the 4 GHz-4.26 GHz frequency band, including the reflection amplitude as a function of the control voltage. Figure 3 a) and the trend of reflection phase with control voltage ( Figure 3 b). Experimental results show that the metasurface unit can achieve continuous adjustment of the reflection phase in the range of 0-2π while maintaining a strong reflection amplitude, and has the ability to generate frequency-modulated signals.

[0032] Build as Figure 4 The illustrated step-frequency modulated radar transmitter system employs a voltage waveform generator (PXIe-5433) with high-speed DAC conversion capability to precisely control the spatiotemporally coded metasurface, and uses a vector signal transceiver (PXIe-5841) for signal transmission and reception. In the experiment, a pyramidal horn antenna transmits a single-frequency signal corresponding to the specified frequency towards the metasurface along its normal direction. The waveform generator generates a periodic voltage control signal, which, after conversion, forms a continuous voltage waveform, driving the metasurface to achieve periodic switching of the transmission phase. Under this configuration, the spectrum of the reflected echo will fully carry the spectral information of the metasurface's reflection coefficient. By inputting the control voltage code corresponding to a specific radar signal, the metasurface's reflected echo can accurately output the required radar waveform, thus fully meeting the performance requirements of the radar transmitter.

[0033] Furthermore, this invention proposes an analytical model for the factors influencing the waveform purity of metasurfaces. Since the reflection coefficient of a metasurface modulation is composed of discrete digital codes, the modulation period of the spatiotemporally coded metasurface is divided into M time slots, with the reflection coefficient of each time slot being Γ. m Then the time-varying reflection coefficient function Γ(t) can be encoded by this set of time slots Γ m Discrete representation is performed, and the result is expressed through Fourier series expansion as follows:

[0034]

[0035] In the formula f n A represents the frequency of the nth harmonic. n This represents the harmonic coefficient. When the frequency is f... c When a single-frequency signal is incident on a metasurface, the reflected electric field after being modulated by the spatiotemporally encoded metasurface is expressed in the frequency domain as follows:

[0036]

[0037] Assuming the +1st harmonic of a spacetime encoded metasurface is used for signal generation, to ensure the purity and efficiency of the generated signal, A is required to... n Satisfy: |A +1 |>0 and A n =0, n≠1, to ensure the concentration of effective energy and the purity of the signal. The reflection coefficient is then solved and encoded as follows:

[0038]

[0039] Formula (2) can be simplified to:

[0040]

[0041] Where f +1 That is, f n(n = +1). It can be seen that the interaction between the incident electromagnetic wave and the metasurface effectively achieves frequency modulation and harmonic synthesis of the electromagnetic wave; the reflected wave only occurs at frequency f. c +f +1 There are frequency components at that location, which can be used to generate signals.

[0042] Although theoretical analysis shows that metasurfaces possess the ability to generate pure signals, errors are unavoidable in practical operation. Based on their sources, these errors can be categorized as amplitude errors ε. A and phase error ε Φ Regarding amplitude error, when analyzing phase-modulated spatiotemporally coded metasurfaces, it is usually assumed that the reflection amplitude A = 1 for simplified derivation. However, in reality, the reflection amplitude corresponding to different reflection phases is not constant, but varies with the phase. Specifically, there is a dependency between the reflection amplitude and the phase; therefore, the reflection amplitude should be considered as a function of the phase, and the error caused by amplitude fluctuations should also be evaluated as a function of the phase, denoted as . in This represents the reflection phase of the spatiotemporally encoded metasurface. As the fluctuations in reflection amplitude intensify, the impact of amplitude error becomes increasingly significant.

[0043] Regarding phase error, the manipulation of spatiotemporally coded metasurfaces typically relies on introducing tunable active devices into the structural design. This involves embedding adjustable factors such as variable resistors and reactance elements within the unit structure to precisely regulate its electromagnetic characteristics. However, these additional adjustable factors often lead to deviations between the theoretical control signal and the actual response. Taking a varactor diode as an example, due to the charging and discharging effect of its tuning capacitor, when the control voltage switches rapidly, the actual control voltage across the diode lags behind the theoretical expectation. The higher the switching rate, the more significant the resulting phase delay and distortion. In a phase-modulated spatiotemporally coded metasurface system, there is a strict functional correspondence between the control signal and the reflection phase; therefore, the theoretical reflection phase difference between adjacent time slots... It can be used to quantify the drasticness of control signal switching. Based on this, phase error can be considered as... The function, denoted as

[0044] Taking all factors into consideration and The reflection coefficient of the actual spatiotemporally encoded metasurface is expressed as:

[0045]

[0046] Because two error terms are introduced, A is now... nNo longer meeting the above requirements not only inevitably reduces the efficiency of the +1st harmonic but also introduces other higher-order harmonic components, negatively impacting the efficiency of signal generation by the spatiotemporally coded metasurface. Therefore, exploring the actual impact of error factors on signal generation and evaluating the signal generation capability of the spatiotemporally coded metasurface at different frequencies is crucial. Amplitude error yes The function of f is determined solely by the degree of fluctuation in the reflected amplitude as a function of phase, and is independent of the harmonic frequency f. +1 Magnitude is irrelevant. In contrast, although phase error yes The function, but because with f +1 There is a correlation:

[0047]

[0048] Where f s This represents the sampling rate. When f s When it remains unchanged, f +1 and They are directly proportional, which means Both are f +1 The higher the harmonic frequency, the more significant the phase error it introduces.

[0049] To quantify the waveform purity of the generated waveform, harmonic efficiency η is defined as follows:

[0050]

[0051] Where P main P represents the power of the dominant frequency component. total This represents the total power. According to the IEEE Standard for Waveform Generators, the device must ensure that at least 90% of the power is concentrated at the dominant frequency, meaning the energy must be concentrated in the spatiotemporally encoded metasurface-generated waveform discussed above. +1 Based on the above analysis, and The presence of will lead to a decrease in η of the waveform generated by the spatiotemporally encoded metasurface. And due to f +1 The increase will lead to The rapid increase in f directly limits the current spatiotemporal coding metasurface technology for generating high-f... +1 The ability to generate broadband signals. A key characteristic of broadband signals is that the waveform frequency needs to cover a sufficiently large frequency range; therefore, the bandwidth of the generated signal and f... +1There is a direct positive correlation between the maximum tunable range and the frequency dynamic range of the spatiotemporally coded metasurface. The frequency dynamic range of the spatiotemporally coded metasurface can only cover narrow-band requirements, which prevents it from achieving the high-resolution target detection capabilities required by modern radar systems. Therefore, it is crucial to explore methods to improve the signal bandwidth generated by the spatiotemporally coded metasurface while ensuring that η meets performance standards.

[0052] Based on the analysis model of factors affecting the purity of metasurface waveforms, this invention proposes a step-frequency modulation signal generation theory based on spatiotemporally encoded metasurfaces: for generating signals whose frequency varies with time, the constant f in formula (4) +1 The function f that needs to be modified to time +1 (t). With pulse width as T p The frequency of the modulation is K r Taking a linear frequency modulated signal as an example, its corresponding harmonic modulation frequency is expressed as:

[0053] f +1 (t)=K r t, 0 ≤ t < T p (9)

[0054] The corresponding reflection coefficient code has been modified to in M(t) = f s / f +1 (t), This indicates rounding down. When the generated waveform frequency is stepped, the carrier frequency is modified as follows:

[0055]

[0056] Where Δf represents the single frequency step size, T r This represents the pulse repetition time (PRT) of the frequency-modulated signal. Combining this with formula (4), the frequency domain representation of the reflected echo from the spatiotemporally coded metasurface is as follows:

[0057]

[0058] The instantaneous frequency of the generated signal is expressed as f. c ′(t)+f +1 (t). In summary, the instantaneous frequency expression of the linear frequency modulated signal emitted by the spatiotemporally coded metasurface is:

[0059]

[0060] Where rect{·} is a rectangular window used to limit the pulse width T of the transmitted waveform. p .

[0061] Simultaneously, leveraging the beam scanning capability of the metasurface, a stepped frequency modulated radar transmitter system was further constructed: Assuming the element width of each column of the metasurface is d, and that elements within the same column share the same control signal, only the initial phase between different columns was changed in the experiment. Otherwise, each column's control signal had the same modulation scheme and modulation period. Let the initial reflection phase of the nth column be φ. n =φ1+Δφ n (i>1), where φ1 represents the initial reflection phase of the first column element, and the phase difference of the +1th harmonic coefficient between the nth column element and the first column element at this time. for:

[0062]

[0063] That is, the phase difference between the signals generated by the two spatiotemporally coded metasurfaces with the same switching mode and modulation period depends only on their initial phases. Therefore, according to antenna theory, when the azimuth angle of the outgoing beam is θ, the phase φ of the nth column on the corresponding metasurface... n It can be represented as:

[0064]

[0065] In the experiment, due to f c ′(t)>>f +1 (t), therefore, to simplify the problem analysis, we can approximate f r (t)≈f c Therefore, the initial phase distribution of each column of cells on the metasurface simplifies to:

[0066]

[0067] To enhance the rigor of the theoretical derivation, three sets of test cases are provided.

[0068] Example 1: Generation Signal Quality Assessment

[0069] Figure 5 (a) Time-frequency analysis results of linear frequency modulated (LFM) signals in the frequency range of 0-30 MHz directly from the spatiotemporally encoded metasurface sample are presented. Since the frequency of the LFM signal increases linearly with time, time-frequency analysis can effectively present the purity of the generated waveform at different frequencies. The results show that as the frequency of the generated waveform gradually increases, the energy of the fundamental and higher harmonics increases accordingly, while the efficiency of the +1 harmonic decreases significantly. Figure 5(c) This paper demonstrates the harmonic efficiency of the signal generated by the spatiotemporally encoded metasurface and the relationship between amplitude error and phase error on the generation efficiency in the theoretical analysis. The results are consistent with the theoretical analysis; the amplitude error remains relatively stable as the generation frequency increases, while the phase error increases rapidly with increasing generation frequency. Therefore, there is a clear upper frequency limit when generating waveforms using the spatiotemporally encoded metasurface sample. Based on this, the equivalent bandwidth of the waveform generated by the spatiotemporally encoded metasurface is extended using the stepping frequency technique proposed in this invention. In the experiment, the single-pulse signal generated by the spatiotemporally encoded metasurface has a bandwidth of B = 4MHz and a step size of 4MHz, which is a linear frequency modulated signal. The time-frequency analysis results of the synthesized signal are as follows: Figure 5 As shown in (b). Compared to Figure 5 (a) At this point, the higher-order harmonic components of the waveform generated by the spatiotemporally encoded metasurface are significantly reduced. The harmonic efficiency and error effects of the signal are as follows: Figure 5 As shown in (d), the results indicate that the harmonic efficiency remains above 95%, generating a relatively pure frequency-modulated signal. Therefore, the scheme of generating stepped frequency-modulated signals using a spatiotemporally encoded metasurface can be considered feasible.

[0070] Example 2: Target range / velocity detection

[0071] A set of classic stepped-frequency radar parameters was selected for the experimental design. The transmitter's single-pulse signal bandwidth was set to B = 4MHz, the step size to 4MHz, and the number of pulses to N = 64. A stepped linear frequency modulated signal with a bandwidth of 256MHz was synthesized, with a frequency range from 4GHz to 4.256GHz. Based on the set radar waveform parameters, the corresponding range resolution can be expressed as:

[0072]

[0073] Where c represents the speed of light, c = 3 × 10 8 m / s. Experimental scenario as follows: Figure 4 As shown in (b), a corner reflector is selected to characterize the target. Figure 6 (a) shows the ranging results of the radar transmitter constructed by bandwidth synthesis and pulse compression of the target echo signal when only the Target I corner reflector is present. The actual corner reflector distance is 6.5 m, and the measurement error is within the allowable range resolution error range. Interpolation of values ​​at nearby points yields the Sinc function image as shown below. Figure 6As shown in (e), the distance Δr = 0.545m corresponding to the 3dB width of the function is basically consistent with the theoretical range resolution ΔR. This indicates that the display resolution after pulse compression meets the expected standard, verifying the accuracy of the ranging experiment results and the effectiveness of the proposed scheme. To better illustrate the broadband operating characteristics of the designed radar transmitter based on the spatiotemporally coded metasurface, a second target, Target II, was placed d = 1.5m in front of Target I. The range measurement results are as follows: Figure 6 As shown in (b), it can be seen that while reacting to the target position, the proposed radar transmitter distinguishes the distance difference between the two targets by means of the synthesized wide operating bandwidth, which verifies the effectiveness and feasibility of the designed transmitter system.

[0074] To enhance the persuasiveness of the experimental setup, a simulated radar environment experiment was further designed and implemented. The experimental scenario was as follows: Figure 4 As shown in (c). In this setup, the signal generated by the metasurface is input to the radar echo simulator via the receiving antenna, processed, and then returned to the radar receiving antenna. The radar echo simulator is used to simulate the radar measurement environment. By setting virtual target information and noise and interference types, it can generate echo signals similar to actual radar targets, thereby effectively reproducing the radar environment. Figure 6 (c) and (d) show the processing results of the radar echo simulator output signal in single-target measurement scenarios at a distance of 1025m and multi-target measurement scenarios at distances of 1200m and 1775m, respectively. To verify the feasibility of the proposed scheme, real radar measurement data was introduced for comparison. Although the radar measurement results based on the spatiotemporally coded metasurface have a lower signal-to-noise ratio compared to real radar, they still maintain a high signal-to-noise ratio, and the measurement results of both are consistent, accurately reflecting target information. Furthermore, based on the ranging experiment, the target velocity was measured. Figure 6 (e) shows the velocity measurement results of a target with a velocity of 23 m / s at a distance of 1.3 km from the spatiotemporally encoded metasurface. The measurement results are accurate, proving that the transmitter architecture proposed in this paper has the ability to replace a real radar transmitter.

[0075] Example 3: Beam Scanning

[0076] Based on formula (14), a spatiotemporal coding strategy for spatiotemporal coded metasurfaces was designed. Figure 7 The test results are presented under vertical incidence conditions, with different frequency harmonics and different exit angles. The measured results show only slight fluctuations compared to the theoretical values, indicating that the proposed radar system possesses good beam scanning capability within a scanning angle range of ±40°. The test results demonstrate the beam scanning capability of the novel radar system and verify its feasibility and reliability in practical applications.

[0077] In summary, this invention achieves the generation of stepped frequency modulated signals by designing a simple periodic control voltage signal and utilizing a spatiotemporally coded metasurface, thereby constructing a stepped frequency modulated radar transmitter using the spatiotemporally coded metasurface. This radar transmitter features high integration, simple reception, and low cost, possessing broad application prospects and significant research value, and is suitable for multiple fields such as radar, communication, and sensing.

[0078] The core technical concepts, key features, and outstanding advantages of this invention have been fully elaborated above. According to common understanding in the technical field, this invention should not be limited to the implementations listed herein; these exemplary examples and related descriptions in the technical documents are merely specific examples illustrating the core design ideas of this invention. While maintaining the basic tenets and scope of this invention, any equivalent substitutions, adaptive adjustments, or optimizations based on the technical essence of this invention fall within the valid protection scope of this invention's patent rights. Ultimately, the protection boundary of this invention should be determined by the specific content of each claim appended to this patent document and its legally equivalent scope.

Claims

1. A method for generating stepped frequency modulated signals based on a spatiotemporally coded metasurface, characterized in that, Includes the following steps: The reflection coefficient encoding vector of the designed spatiotemporally encoded metasurface is: ； in, This indicates the frequency modulation frequency of the frequency-modulated signal generated by the metasurface. Indicates the signal pulse width. Indicates rounding down. Indicates the sampling rate; The designed incident signal frequency is: ； in, Indicates the starting frequency of the frequency modulation signal. Indicates the frequency step size per cycle. Indicates the pulse repetition time of the frequency modulation signal; After the incident signal interacts with the spatiotemporally coded metasurface, the reflected echo from the metasurface is a stepped frequency modulated signal, and the mathematical model is as follows: ； Where N is the number of signal cycles involved in bandwidth synthesis; The spatiotemporal encoded metasurface employs an M-row, L-column basic unit structure to form a three-dimensional reflective architecture. Each column unit achieves synchronous control of the excitation signal via a column select bus. Each basic unit consists of a stacked dielectric substrate, a metal radiating layer, and a ground plane. The metal radiating layer integrates a voltage-controlled diode device, whose capacitance parameters are controlled by applying a bias voltage to achieve continuous adjustment of the reflection phase, covering a phase adjustment range from 0 to... Full lifecycle; The bias voltage signal required for the time-coded metasurface is generated by the FPGA. The FPGA generates control signals in real time to ensure that the time-coded metasurface can generate time-varying signals. The step-frequency radar transmitter based on the aforementioned step-frequency signal generation method and a spatiotemporally encoded metasurface integrates an FPGA and DSP collaborative control architecture. It synchronously realizes step-frequency signal generation and beam scanning through metasurface spatiotemporal encoding. It eliminates the need for discrete T / R components and waveform generators, and generates a frequency-modulated signal with an equivalent bandwidth of 256MHz through integrated spatial domain phase modulation and time domain signal modulation. The beam scanning is achieved in the following way: (a) Set the target beam azimuth angle and operating wavelength ; (b) According to the relation Calculate the initial reflection phase of the nth column element, where d is the element width. This indicates the initial reflection phase of the first column cell; except for the initial reflection phase difference, all column cells use the same control signal modulation method and modulation period; (c) The bias voltage code corresponding to the initial phase of each column is generated by the FPGA, and the metasurface is driven to achieve directional beam scattering.

2. A spatiotemporal coded metasurface waveform purity analysis method based on the step frequency modulation signal generation method of claim 1, characterized in that, Includes the following steps: 1) Based on the measured electromagnetic response of the metasurface unit, determine the nonlinear fluctuation function of the reflection amplitude caused by the change in the reflection phase φ. ; 2) Based on the switching rate of the control signal, establish a mapping function between the theoretical reflection phase difference Δφ between adjacent time slots and the parasitic reactance delay. ; 3) Constructing an actual reflection coefficient model: ； Where M is the number of discrete time slots in the spatiotemporal coding period of the metasurface unit. Characterizing the time slot sequence index; 4) The measured function and Substitute into the model to calculate the harmonic efficiency. ;in Indicates the power of the dominant frequency component. Indicates total power; 5) Through The degree of signal purity degradation is quantified to optimize metasurface structure parameters and control timing.

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