Underwater piezoelectric acoustic metasurface and high-order backscattering modulation and demodulation method

Abstract

This invention discloses an underwater piezoacoustic metasurface and a high-order backscatter modulation and demodulation method. The piezoacoustic metasurface integrates a passive resonant cavity through optimized unit structure, achieving high reflection efficiency and 180-degree phase shift within the operating frequency band. Microsecond-level phase control is achieved through an external electrical load. The modulation and demodulation method significantly reduces the search space of the metasurface codebook by decoupling beamforming and signal modulation targets. An adaptive multi-channel decision feedback equalizer (DFE) is used at the receiver to eliminate inter-symbol interference caused by underwater multipath propagation. The underwater high-reflection-efficiency piezoacoustic metasurface fabrication method of this invention achieves integrated metasurface molding, balancing underwater waterproof protection and acoustic modulation performance, and effectively avoiding the adverse effects of encapsulation on the metasurface's resonant characteristics.

Description

Technical Field

[0001] This application relates to the fields of underwater acoustic communication and acoustic metasurface manipulation technology, specifically to an underwater piezo-acoustic metasurface and a high-order backscatter modulation and demodulation method. Background Technology

[0002] Underwater communication plays a vital role in marine environmental monitoring and seabed infrastructure inspection. These applications typically require equipment to operate continuously underwater for extended periods, placing high demands on the energy consumption of the communication system. Traditional active underwater acoustic communication systems rely on oscillators and amplifiers to generate and amplify signals; these devices consume significant power, which substantially shortens the lifespan of battery-powered equipment.

[0003] To overcome the problem of high energy consumption, underwater acoustic backscatter communication technology has gradually attracted attention. Unlike active transmission methods, backscattering devices transmit information by reflecting acoustic carrier signals from the environment, thus enabling communication with lower energy consumption. However, existing underwater acoustic backscattering systems generally use relatively simple binary modulation methods, such as on / off keying and binary phase-shift keying, which limits the amount of information carried by a single symbol, resulting in low data transmission rates. This low throughput is insufficient to meet the demands of modern marine applications for large data transmission, such as the transmission of underwater images, videos, and complex monitoring data.

[0004] To improve backscatter communication throughput, existing solutions mainly attempt to increase the switching rate or employ multi-carrier modulation. However, increasing the switching rate typically requires a higher frequency oscillator, significantly increasing system power consumption; while multi-carrier schemes have high bandwidth requirements, which are incompatible with the low-frequency narrowband acoustic transducers commonly used in long-distance underwater communication. In contrast, higher-order backscatter modulation increases the data rate by carrying more bits in each symbol, without requiring additional bandwidth expansion or significantly sacrificing energy efficiency, making it more suitable for high-throughput underwater communication scenarios.

[0005] Despite this, achieving high-order acoustic modulation underwater still faces significant challenges. First, existing hardware control capabilities are limited. Compared to radio frequency systems, acoustic systems lack mature devices for finely adjusting reflection states. Existing underwater backscattering devices typically rely on discrete switch arrays, providing only a limited number of discrete reflection states, making it difficult to support higher-order constellation point control. Second, as the number of reflection states increases, the distance between symbols in the constellation diagram decreases, consequently reducing the system's tolerance to noise and distortion. Significant multipath interference and substantial propagation attenuation are prevalent in underwater environments, further reducing the signal-to-noise ratio of the received signal, thus severely limiting the communication distance and practical availability of high-order modulation.

[0006] In recent years, acoustic metasurface technology has provided a new approach to space signal modulation. By designing and controlling the reflection characteristics of metasurface units, reflected beams can be directionally enhanced, thereby increasing the received signal strength in the target direction. However, traditional mechanical metasurfaces typically rely on altering their physical structure to adjust the phase, resulting in slow response speeds and high power consumption, making it difficult to meet the demands of high-speed modulation in communication scenarios. On the other hand, existing piezoelectric metasurfaces are mostly designed for air environments. If directly used underwater, the significant difference in acoustic impedance between the media can easily lead to a decrease in reflection efficiency. Furthermore, the strong inter-unit coupling effect in the underwater environment can cause the phase response to deviate from the theoretical design results, affecting beamforming performance.

[0007] Therefore, there is an urgent need for an underwater communication method that can simultaneously achieve high signal-to-noise ratio beamforming and high-order backscatter modulation under low power consumption conditions, in order to solve the technical problem of long-distance, high-throughput data transmission in underwater IoT scenarios. Summary of the Invention

[0008] To overcome the shortcomings of existing technologies, such as the low data throughput and limited communication distance caused by the reliance on low-order binary modulation in underwater backscatter communication, as well as the severe multipath interference caused by the complex underwater environment and the excessive search space of traditional calculation methods that cannot meet real-time requirements, this application provides an underwater piezo-acoustic metasurface and a high-order backscatter modulation and demodulation method.

[0009] To solve the above problems, the technical solution adopted by the present invention includes:

[0010] An underwater piezo-acoustic metasurface, comprising at least one piezo-acoustic metasurface unit; The piezoelectric metasurface unit is provided with a copper-backed piezoelectric composite structure, which is composed of a piezoelectric sheet and a copper sheet bonded together. The piezoelectric sheet is the sound wave incident side, and the copper sheet is the backing side. A passive resonant cavity shell is provided, and the copper-backed piezoelectric composite structure is inserted inside the passive resonant cavity shell. The copper sheet and the passive resonant cavity shell form a closed air cavity, and the piezoelectric sheet and the passive resonant cavity shell form an open liquid cavity.

[0011] Optionally, the copper-backed piezoelectric composite structure is circular, with the diameter of the copper sheet being larger than the diameter of the piezoelectric sheet, and the thickness of the copper sheet being equal to the thickness of the piezoelectric sheet.

[0012] Optionally, the passive resonant cavity shell is a metal shell; It consists of two covers joined together. The upper cover has an open structure, while the lower cover is a closed structure. The longitudinal cross-section of the inner cavity of both the upper and lower covers is a trapezoidal structure.

[0013] Optionally, an FR4 control substrate is stacked on the contact surface between the copper sheet and the passive resonant cavity shell; an encapsulation layer is provided on the surface of the piezoelectric sheet; The piezoelectric metasurface unit is connected to two electrical loads with different inductance values. The two electrical loads are alternately connected by switching on and off using a low-power electronic switch.

[0014] Optionally, multiple copper-backed piezoelectric composite structures are arranged in an array and uniformly installed inside a passive resonant cavity shell.

[0015] A high-order backscattering modulation and demodulation method for an underwater piezoacoustic metasurface, implemented using any of the underwater piezoacoustic metasurfaces described in this invention, includes: A reference codebook for beamforming is calculated based on the incident signal direction and the target reflection direction. While maintaining the relative phase difference between the various underwater piezo-acoustic metasurfaces, a uniform phase shift is applied to the reference codebook to obtain multiple sets of phase modulation codebooks. By selecting a subset of metasurface units participating in coherent reflection, an optimized target is constructed to optimize the state of the remaining non-beamforming subset of metasurface units to minimize interference with the target amplitude, obtaining different main lobe amplitude levels and generating an amplitude modulation codebook. Based on the standard constellation diagram of the target orthogonal amplitude modulation, a corresponding codebook set is selected from the candidate codebook set composed of the phase modulation codebook and the amplitude modulation codebook to achieve high-order backscatter modulation. The received backscatter signal is demodulated based on the training sequence in the data packet and multi-channel adaptive decision feedback equalization.

[0016] Optionally, the construction of the beamforming reference codebook and the phase modulation codebook specifically involves: Calculate the incident wave at the th Line number Phase delay and target direction at column unit The phase delay of the reflected wave is adjusted to obtain the required compensated phase:

[0017] k represents the wave number, and d represents the spacing between adjacent cells in the metasurface array. These represent the pitch angle and azimuth angle, respectively, in the direction of target reflection. These represent the elevation angle and the azimuth angle, respectively, representing the direction of incidence of the target. Quantize continuous compensated phases to 0 or according to the following rules. Two states generate a 1-bit reference codebook. : ; In the 1-bit reference codebook Based on this, the same constant phase shift is added to all metasurface units. Construct a new translation codebook set This is to achieve a quasi-continuous change in the phase of the synthesized signal.

[0018] Optionally, the optimization objective is used to optimize the metasurface unit states of the remaining non-beamforming subsets, specifically as follows: To obtain the desired main lobe amplitude Select size as A subset of metasurface units is used for beamforming; For the remaining Each unit is based on the actual reflected signal superposition model. Define the objective function and its tolerance threshold constraint as follows: ; ; Where Q is the desired number of modulation phases, This is the tolerance threshold. and These represent the expected magnitude and the actual magnitude in the target direction, respectively. This represents the q-th 1-bit phase codebook.

[0019] Optionally, the step of selecting the corresponding codebook set from the candidate codebook set to achieve higher-order backscatter modulation specifically includes: Let the ideal constellation point set of the target QAM modulation scheme be... P represents the number of constellation points in the QAM modulation scheme, which is any 1-bit codebook. The corresponding complex reflection coefficient is denoted as Select an optimal codebook set from the candidate codebooks. This minimizes the error between its complex reflection coefficient in the I / Q plane and the ideal constellation point. .

[0020] Optionally, the demodulation of the received backscattered signal based on the training sequence in the data packet and the multi-channel adaptive decision feedback equalization specifically includes: Using the training sequence in the packet preamble, a recursive least squares algorithm is applied to each symbol. Update the composite tap vector containing both feedforward and feedback taps.

[0021] ; ; ; Where b represents the current symbol time and a represents the index of the historical symbol time; This represents the multichannel regression vector at the a-th symbol time. The weighted autocorrelation matrix represents the regression vector; This represents the weighted cross-correlation vector between the regression vector and the training symbols; Indicates the forgetting factor; This represents the a-th known training symbol in the preamble training sequence; Indicates conjugate transpose; Indicates complex conjugation; After the preamble training is completed, the converged tap vector is used. Decoding is performed on subsequent data payloads to eliminate inter-symbol interference.

[0022] Compared with the prior art, this application has the following beneficial effects: 1. The underwater high-reflection-efficiency piezoacoustic metasurface of this application achieves high underwater reflection efficiency by optimizing the unit structure and can achieve precise switching of 0 / π phase state. At the same time, it effectively reduces the influence of the substrate on the unit resonance, suppresses the interference of underwater structural disturbance on the reflection phase, and ensures the consistency of the metasurface array and the stability of beamforming.

[0023] 2. The underwater high-reflectivity piezoacoustic metasurface preparation method of this application realizes the integrated molding of the metasurface, takes into account both underwater waterproof protection and acoustic modulation performance, and effectively avoids the adverse effects of encapsulation on the metasurface resonance characteristics. Attached Figure Description

[0024] This application can be better understood by referring to the description given below in conjunction with the accompanying drawings, which, together with the detailed description below, are incorporated in and form part of this specification. In the drawings: Figure 1 A schematic diagram of the basic structure and characteristics of the copper-backed piezoelectric composite structure is shown, where (a) is the basic physical form of the copper-backed piezoelectric composite structure, (b) is the equivalent circuit model of the structure, (c) shows the phase shift characteristics of the unit under different inductive loads, (d) is the reflection efficiency curve of the basic structure, and (e) is a comparison of the acoustic impedance of the basic structure and the water body. , The thickness and diameter of the piezoelectric element, , The thickness and diameter of the copper sheet; Figure 2A schematic diagram of the optimized piezoelectric metasurface unit structure and its characteristics is shown, where (a) is the overall structure of the unit with an integrated passive resonant cavity shell, (b) is the equivalent circuit model of the optimized structure, (c) is the phase shift characteristics of the optimized unit, (d) is the reflection efficiency curve of the optimized structure, and (e) is the acoustic impedance comparison between the optimized structure and water. r is the radius of the resonant cavity neck, and t is the thickness of the resonant cavity neck and the thickness of the lower metal shell. The length of the top edge of the trapezoidal structure of the air cavity, L is the length of the base of the trapezoidal air cavity structure, and L is the height of the trapezoidal air cavity structure. Figure 3 A schematic diagram of a fabricable piezoelectric metasurface unit is shown, where (a) is a side view of the unit and (b) is an exploded view of the unit. , The thickness and diameter of the piezoelectric element, , denoted by , where is the thickness and diameter of the copper sheet; 'r' is the radius of the resonant cavity neck; and 't' is the thickness of the resonant cavity neck and the thickness of the lower metal shell. The length of the top edge of the trapezoidal structure of the air cavity, The length of the base of the trapezoidal structure of the air cavity; Figure 4 A schematic diagram of the fabrication process of piezo-acoustic metasurfaces is shown.

[0025] In the above figures, the meanings of the reference numerals are as follows: 1-passive resonant cavity shell, 2-FR4 control substrate, 3-piezoelectric sheet, 4-air cavity, 5-copper sheet, 6-encapsulation layer; Figure 5 A schematic diagram of the overall architecture of an underwater beamforming and high-order backscattering system based on a piezo-acoustic metasurface according to an embodiment of this application is shown. Figure 6 A schematic diagram of a piezo-acoustic metasurface unit according to an embodiment of the present application is shown, wherein (a) is a cross-sectional structural schematic diagram and (b) is an equivalent circuit schematic diagram; Figure 7 A schematic diagram of the spatial superposition model of acoustic metasurface reflection signals and the beamforming principle according to an embodiment of this application is shown; Figure 8 A schematic diagram showing the relationship between the main lobe amplitude of the reflected signal and the size of the metasurface unit subset participating in beamforming, according to an embodiment of this application, is provided. Figure 9 Standard constellation diagrams for different modulation orders (such as QPSK, 8-QAM, 16-QAM) according to embodiments of this application are shown. Detailed Implementation

[0026] Exemplary embodiments of the present application will be described below with reference to the accompanying drawings. For clarity and brevity, not all features of the actual embodiments are described in the specification. However, it should be understood that many embodiment-specific decisions can be made in the development of any such actual embodiment to achieve the developer’s specific objectives, and these decisions may vary as the embodiments differ.

[0027] It should also be noted that, in order to avoid obscuring this application with unnecessary details, only the device structure closely related to the solution according to this application is shown in the accompanying drawings, while other details that are not closely related to this application are omitted.

[0028] It should be understood that this application is not limited to the described embodiments by virtue of the following description with reference to the accompanying drawings. In this document, embodiments may be combined with each other, features may be substituted or borrowed between different embodiments, and one or more features may be omitted in one embodiment, where feasible.

[0029] Combination Figure 1-4 The underwater piezo-acoustic metasurface of the present invention comprises at least one piezo-acoustic metasurface unit; the piezo-acoustic metasurface unit comprises a copper-backed piezo-acoustic composite structure, which is composed of a piezoelectric sheet 3 and a copper sheet 5 bonded together, with the piezoelectric sheet 3 being the incident side of the sound wave and the copper sheet 5 being the backing side; a passive resonant cavity shell 1 is provided, and the copper-backed piezo-acoustic composite structure is inserted into the passive resonant cavity shell 1, with the copper sheet 5 and the passive resonant cavity shell 1 forming a closed air cavity 4, and the piezoelectric sheet 3 and the passive resonant cavity shell 1 forming an open liquid cavity.

[0030] In this invention, the copper-backed piezoelectric composite structure is circular, and the diameter of the copper sheet 5 is larger than the diameter of the piezoelectric sheet 3, while the thickness of the copper sheet 5 is equal to the thickness of the piezoelectric sheet 3. The copper-backed piezoelectric composite structure is composed of a circular piezoelectric sheet 3 and a circular copper sheet 5 bonded together. The piezoelectric sheet 3 is the sound wave incident side, and the copper sheet 5 is the backing side. The copper-backed piezoelectric composite structure is connected to the passive resonant cavity shell 1 to form a closed air cavity structure. The passive resonant cavity shell 1 and the copper-backed piezoelectric composite structure work together to change the acoustic impedance of the unit, thereby improving the underwater sound wave reflection efficiency.

[0031] In this invention, the passive resonant cavity shell 1 is a metal shell; it is composed of two upper and lower covers, the upper cover having an open structure and the lower cover having a closed structure, and the longitudinal section of the inner cavity of both the upper and lower covers is a trapezoidal structure. For example, a single piezoelectric metasurface unit and a copper-backed piezoelectric composite structure are installed inside a passive resonant cavity shell 1; or multiple copper-backed piezoelectric composite structures can be installed together in an integrated metal shell. The metal shell is made of a high acoustic impedance metal material, and the metal shell is sealed to the epoxy resin encapsulation layer and the FR4 control substrate to form an overall sealed structure, suitable for complex underwater working environments.

[0032] In this invention, an FR4 control substrate 2 is stacked on the contact surface between the copper sheet 5 and the passive resonant cavity shell 1; an encapsulation layer 6 is provided on the surface of the piezoelectric sheet 3; unified electrical connection is achieved through the FR4 control substrate 2, which has a cavity structure inside to reduce the influence of the substrate on the intrinsic resonance of the piezoelectric metasurface unit; the piezoelectric sheet electrodes are connected to the pads on the FR4 control substrate 2 by wire bonding and are integrated into a unified PCB trace layout; the copper-backed piezoelectric composite structure and its electrical connection structure are encapsulated in an epoxy resin encapsulation layer; the piezoelectric metasurface unit is externally connected to two electrical loads with different inductance values, and the two electrical loads are alternately connected by switching on and off using a low-power electronic switch. By utilizing the electromechanical coupling characteristics of the piezoelectric material to change the acoustic impedance of the unit, rapid switching between 0 / π phase states is achieved, meeting the phase modulation requirements of underwater backscatter communication.

[0033] In this invention, multiple copper-backed piezoelectric composite structures are arranged in an array and uniformly installed within a passive resonant cavity housing. A low-power electronic switch and an FR4 control board are also included. The multiple copper-backed piezoelectric composite structures are arranged in an array, with each low-power electronic switch connected to a piezoelectric metasurface unit. The FR4 control board integrates a low-power MCU control board, which receives the piezoelectric metasurface codebook and generates electronic switch control signals. The FR4 control board controls the on / off state of the electronic switches through these control signals to switch the external electrical load of the piezoelectric metasurface unit, thereby changing the acoustic impedance and phase modulation state of the piezoelectric metasurface unit.

[0034] Combination Figure 4This invention discloses a method for fabricating an underwater piezo-acoustic metasurface with an array-arranged copper-backed piezoelectric composite structure. First, a copper-backed piezoelectric composite structure is prepared by bonding circular piezoelectric sheets and circular copper sheets together using epoxy adhesive. Units whose resonant frequencies and impedances match theoretical design values ​​are selected for later use. A cavity structure is fabricated within an FR4 control substrate, and a low-power MCU control board is integrated onto the FR4 control substrate. The selected copper-backed piezoelectric units are arranged in an array, and the piezoelectric electrodes are connected to the pads on the FR4 control substrate via wire bonding. Electrical interconnection between the units, electronic switches, and the MCU control board is achieved through a unified PCB trace layout. An epoxy resin material with acoustic impedance close to that of water is used to integrally encapsulate the electrically connected piezoelectric metasurface unit array and the connection points of the FR4 control board. The epoxy resin encapsulation layer thickness is controlled to be 3 mm, and curing is performed after encapsulation. The epoxy resin-encapsulated metasurface structure is assembled into an integrated metal shell made of high acoustic impedance metal. The bonding area between the metal shell and the epoxy resin encapsulation layer and the FR4 control substrate is sealed. After the external leads are led out through the waterproof connector, the connector connection is encapsulated with epoxy resin to form an integral sealed underwater piezo-acoustic metasurface assembly.

[0035] According to the piezoacoustic metasurface of the present invention, the present invention also provides an underwater piezoacoustic metasurface and a high-order backscatter modulation and demodulation method. By optimizing the unit structure and integrating a passive resonant cavity, high reflection efficiency and 180-degree phase shift are achieved within the operating frequency band, and microsecond-level phase modulation is achieved by connecting an external electrical load. Furthermore, the metasurface employs a dedicated protective and connection structure, adaptable to the underwater environment, with high unit consistency and stable beamforming effect. The fabrication and assembly method of this application achieves integrated fabrication of the piezoacoustic metasurface through standardized steps. The method greatly reduces the search space of the metasurface codebook by decoupling the beamforming and signal modulation targets. Specifically, this application first calculates a beamforming codebook for aligning reflection paths; then, by uniformly adding a phase offset to all metasurface units, the absolute phase of the superimposed reflection signal is modulated without compromising the relative phase difference required for beamforming; next, a subset of metasurface units is selected for beamforming alignment, the main lobe amplitude is adjusted by changing the subset size, and the phase of the remaining units is optimized to reduce power variance; finally, the generated codebook is error-matched with an ideal quadrature amplitude modulation (QAM) constellation diagram to select the final communication codebook. Furthermore, an adaptive multi-channel decision feedback equalizer (DFE) is used at the receiver to eliminate inter-symbol interference caused by underwater multipath propagation.

[0036] A reference codebook for beamforming is calculated based on the incident signal direction and the target reflection direction. While maintaining the relative phase difference between each metasurface unit, a uniform phase shift is applied to the reference codebook to obtain multiple sets of phase modulation codebooks with the same beam direction and gain but different total phases of the reflected signals. By selecting a subset of metasurface units participating in coherent reflection and constructing an optimized target, the states of the remaining non-beamforming subset of metasurface units are optimized to minimize interference with the target amplitude, resulting in different main lobe amplitude levels and generating an amplitude modulation codebook. Based on the standard constellation diagram of target quadrature amplitude modulation (QAM), a corresponding codebook set is selected from the candidate codebook set composed of the phase modulation codebook and the amplitude modulation codebook to achieve high-order backscatter modulation. The received backscatter signal is demodulated based on the training sequence in the data packet and multi-channel adaptive decision feedback equalization.

[0037] In one embodiment, the construction of the beamforming reference codebook and the phase modulation codebook specifically involves: calculating the incident wave at the... m Line number n Phase delay and target direction at column unit The phase delay of the reflected wave is adjusted to obtain the required compensated phase:

[0038] k represents the wave number, and d represents the spacing between adjacent cells in the metasurface array. These represent the pitch angle and azimuth angle, respectively, in the direction of target reflection. These represent the elevation angle and the azimuth angle, respectively, representing the direction of incidence of the target. The continuous compensated phase is quantized to 0 or 0 according to the following rules. Two states generate a 1-bit reference codebook. : ; In the 1-bit reference codebook Based on this, the same constant phase shift is added to all metasurface units. Construct a new translation codebook set This is to achieve a quasi-continuous change in the phase of the synthesized signal.

[0039] In one embodiment, an optimization objective is constructed to optimize the metasurface unit states of the remaining non-beamforming subsets. Specifically, this process involves: to obtain the desired main lobe amplitude... Select size as A subset of metasurface units is used for beamforming; for the remaining... Each unit is based on the actual reflected signal superposition model. Define the objective function and its tolerance threshold constraint as follows: ; ; Where Q is the desired number of modulation phases, This is the tolerance threshold. These represent the expected magnitude and the actual magnitude in the target direction, respectively. This represents the q-th 1-bit phase codebook.

[0040] In one embodiment, selecting a corresponding codebook set from the candidate codebook set to implement higher-order backscatter modulation specifically includes: Let the ideal constellation point set of the target QAM modulation scheme be... P represents the number of constellation points in the QAM modulation scheme, which is any 1-bit codebook. The corresponding complex reflection coefficient is denoted as Select an optimal codebook set from the candidate codebooks. This minimizes the error between its complex reflection coefficient in the I / Q plane and the ideal constellation point. ; In one embodiment, the received backscattered signal is demodulated based on the training sequence in the data packet and a multi-channel adaptive decision feedback equalization, specifically including: Using the training sequence in the packet preamble, a recursive least squares algorithm is applied to each symbol. Update the composite tap vector containing both feedforward and feedback taps.

[0041] ; ; ; Where b represents the current symbol time and a represents the index of the historical symbol time; This represents the multichannel regression vector at the a-th symbol time. The weighted autocorrelation matrix represents the regression vector; This represents the weighted cross-correlation vector between the regression vector and the training symbols; Indicates the forgetting factor; This represents the a-th known training symbol in the preamble training sequence; Indicates conjugate transpose; Indicates complex conjugation.

[0042] After the preamble training is completed, the converged tap vector is used. Decoding is performed on subsequent data payloads to eliminate inter-symbol interference.

[0043] This invention overcomes modulation bottlenecks and reduces complexity. Beamforming and modulation are decoupled, transforming exponential search into a low-complexity optimization process. Amplitude and phase modulation are implemented in 1-bit hardware, achieving high-order modulation such as 16-QAM, significantly improving throughput and real-time performance. Communication distance is extended and anti-interference capability is enhanced. Maintaining relative phase constancy preserves the high signal-to-noise ratio of beamforming and overcomes attenuation. Combined with multi-channel adaptive decision feedback equalization, inter-symbol interference caused by multipath effects is eliminated, ensuring decoding reliability.

[0044] Example 1: This application provides an underwater high-reflectivity piezoelectric acoustic metasurface. The metasurface includes multiple piezoelectric metasurface units, multiple low-power electronic switches, and an FR4 control substrate. The multiple piezoelectric metasurface units are arranged in a 14×14 array. Each low-power electronic switch is connected to one piezoelectric metasurface unit. The FR4 control substrate integrates a low-power MCU control board, model MSP430G2553. The MCU control board is used to receive the piezoelectric metasurface codebook and generate electronic switch control signals. The FR4 control substrate controls the on / off state of the electronic switches through the electronic switch control signals to switch the external electrical load of the piezoelectric metasurface unit, thereby changing the acoustic impedance and phase modulation state of the piezoelectric metasurface unit.

[0045] Combination Figure 1 As shown, the basic design of the piezoelectric metasurface unit is a copper-backed piezoelectric composite structure. This structure consists of a circular piezoelectric sheet and a circular copper sheet bonded together with an epoxy adhesive. See the detailed structure below. Figure 1 In (a), the circular piezoelectric element is made of PZT-5H material, with a diameter of 22mm and a thickness of 1mm, and the circular copper sheet has a diameter of 24mm and a thickness of 1mm. The piezoelectric element is on the incident side of the sound wave, and the copper sheet is on the backing side. Figure 1 As can be observed in (d) and (e), the reflection efficiency of this foundation structure is low in an underwater environment. The core reason is that the acoustic impedance difference between it and the water body is small, making it difficult to achieve efficient sound wave reflection. In an underwater environment, the reflection characteristics of the unit to incident sound waves can be characterized by the reflection coefficient Γ, whose expression is: ; in, The acoustic impedance of a superatom is represented. The acoustic impedance of water is represented by, where , From the above relationship, it can be seen that when the equivalent acoustic impedance of the unit... When the impedance is close to that of the water body, the reflection coefficient is small, resulting in low reflection efficiency.

[0046] Combination Figure 1 The equivalent acoustic impedance of the unit in the equivalent circuit model shown in (b) can be expressed as: ; in, , This is the resonant frequency of the structure. , , , , The damping coefficient is... The density of copper, The density of PZT-5H, It is the piezoelectric elastic compliance constant. This is an external electrical load.

[0047] The above expression shows that the acoustic impedance of the unit is determined by its geometry, material properties, and external electrical load. Once the device is fabricated, its geometric parameters and material properties remain fixed. Conversely, the acoustic impedance can be dynamically tuned by adjusting the external electrical load, and this tuning process directly controls the reflection coefficient and signal phase. To verify this principle, we used multiphysics finite element simulation software to analyze the phase shift characteristics under different inductance conditions and at different frequencies. Specifically, changing the external inductance... Will follow The relationship changes the electrical load impedance; such as Figure 1 As shown in (c), the simulation results confirm that adjusting the inductance value can achieve complete The phase shift verified the feasibility of the impedance-based phase control scheme.

[0048] Although the basic copper-backed piezoelectric structure can achieve phase control, its reflection efficiency in the target frequency range is low (≤50%) when operating in underwater environments. Figure 1 As shown in (d). This limitation stems from the small difference in acoustic impedance between water and elementary atoms ( Figure 1 (e) leads to a decrease in the reflection efficiency of the incident sound power.

[0049] To overcome this limitation, we introduced a passive shell to improve the effective acoustic impedance of the superatoms. By coupling a piezoelectric element in series with the high-impedance shell, the total impedance difference between the water and the superatoms is increased, thereby improving the reflection efficiency. Figure 2 As shown in (a), a passive resonant cavity shell is integrated into the copper-backed piezoelectric composite structure, and the two are connected to form a closed air cavity structure. The passive resonant cavity shell is made of aluminum alloy. By optimizing its geometric parameters and working in conjunction with the copper-backed piezoelectric composite structure, the acoustic impedance characteristics of the unit are significantly changed.

[0050] like Figure 2 As shown in (b), after introducing a passive resonant cavity, the equivalent acoustic impedance of the unit can be further expressed as: ; ; ; ; ; Wherein, air density and sound speed are denoted as... and The density of water and the speed of sound are denoted as follows: and .parameter For a Bessel function of the first kind, This is the dynamic viscosity coefficient. Parameter For the volume of the cavity, and , and For example Figure 2 The geometric parameters are shown in (a). Parameters ,in It is the resonant frequency of the new structure, which is determined by the shell and the base copper backing piezoelectric structure.

[0051] Based on the above relationships, it can be seen that the equivalent acoustic impedance of the unit depends not only on the equivalent impedance of the piezoelectric structure itself. It is also closely related to the geometric parameters of the resonant cavity structure. Therefore, in the specific implementation process, by adjusting the geometric parameters of the resonant cavity and coordinating with adjusting the external electrical load of the piezoelectric unit, a phase difference of approximately 180° can be maintained within the target operating frequency band.

[0052] During the parameter determination process, different combinations of structural parameters were analyzed and screened to simultaneously meet the requirements of high reflection efficiency and phase modulation. Implementation results show that the optimized unit can achieve a reflection efficiency of no less than 90% in the target frequency band while maintaining a stable 180° phase modulation capability.

[0053] from Figure 2 As can be clearly seen in (e), the acoustic impedance difference between the optimized unit and the water body is significantly increased, and the corresponding reflection efficiency is significantly improved within the operating frequency band. Figure 2 The (d) reflection efficiency curve further verifies this optimization effect. Two external electrical loads with different inductance values, 7.6mH and 8.8mH, are connected to the unit. A TMUX1134PWR four-channel low-power electronic switch is used, which enables alternating access of the two electrical loads through switching. The electromechanical coupling characteristics of piezoelectric materials are utilized to change the acoustic impedance of the unit, thereby achieving rapid switching between 0 / π phase states. The switching response time can reach the microsecond level, meeting the phase modulation requirements of underwater backscatter communication. Figure 2 The (c) phase shift characteristic curve verifies the stability and reliability of this phase switching.

[0054] Combination Figure 3 As shown, the piezoelectric metasurface unit achieves unified electrical connection through the FR4 control substrate. The FR4 control substrate has an internal cavity structure designed to reduce the influence of the substrate on the intrinsic resonance of the piezoelectric metasurface unit, ensuring that the unit's acoustic performance is not interfered with by the substrate structure. After the FR4 control substrate and the piezoelectric metasurface unit are electrically interconnected, the entire assembly will be encapsulated in epoxy resin and assembled with a metal shell to form a sealed structure suitable for complex underwater working environments.

[0055] This application also provides a method for preparing the above-mentioned underwater high-reflectivity piezo-acoustic metasurface, combined with... Figure 4 The process flow shown is implemented in the following steps: Step S1: Select a circular PZT-5H piezoelectric sheet with a diameter of 22mm and a thickness of 1mm and a circular copper sheet with a diameter of 24mm and a thickness of 1mm, and bond the two together with GOET-1080 epoxy adhesive to form a copper-backed piezoelectric composite structure.

[0056] Step S2: To ensure the consistency of the unit array, the resonant frequency and impedance of the prepared piezoelectric metasurface units are detected using a WK6500B impedance analyzer. Only units with resonant frequencies and impedances within ±10% of the theoretical design values ​​are retained for backup.

[0057] Step S3: First, the cavity structure is etched into the FR4 control substrate. The low-power MCU control board of model MSP430G2553 is integrated into the FR4 control substrate. Then, the selected copper-backed piezoelectric composite structure units are bonded to the FR4 PCB in a 14×14 array. Each unit is connected to two external electrical loads with different inductance values ​​of 7.6mH and 8.8mH. Electrical connections are established by soldering and wire bonding. The interconnection between the unit and the TMUX1134PWR four-channel switch and the MSP430G2553 low-power MCU control board is achieved through a unified PCB routing layout.

[0058] Step S4: Place the interconnected assembly into a soft silicone mold, inject degassed GOET-1080 epoxy mixture for overall encapsulation, and strictly control the thickness of the epoxy resin encapsulation layer to 3mm. To avoid reflection interference caused by air bubbles, the epoxy resin needs to undergo vacuum degassing treatment, and then residual air bubbles are removed by injection. A 24-hour curing process is then performed. After curing, the surface of the encapsulation layer is treated with CNC polishing to ensure that the surface flatness error is ≤±0.1mm.

[0059] Step S5: The upper and lower metal shells are made of aluminum alloy and CNC machined. The epoxy resin-encapsulated metasurface structure is bonded and fixed to the metal shell with high-strength epoxy adhesive. The external leads are led out through SH1.0 connectors. The connector connection is waterproofed and sealed with epoxy resin potting process, and finally an integral sealed underwater piezo-acoustic metasurface component is formed.

[0060] This application provides a method for high-order backscattering modulation of underwater acoustic metasurfaces. Figure 5 A schematic diagram of an underwater beamforming and high-order backscattering system according to an embodiment of this application is shown. See also: Figure 5 The system includes an underwater acoustic metasurface array and a microcontroller. The metasurface modulates the information to be transmitted by changing its reflection state. Figure 6 As shown in (a) and (b) above, the piezoelectric acoustic metasurface unit involved in this application adopts a composite structure of piezoelectric ceramic and copper backplate. The system controls... Figure 2 (b) An external electrical load in the equivalent circuit can change the acoustic reflection impedance of the metasurface unit, thereby achieving 0 and 0 at low power consumption. Phase shift modulation of two discrete states (i.e., 1-bit). This modulation method specifically includes: Step S1, refer to Figure 7 The diagram showing the spatial superposition model of acoustic metasurface reflected signals and the principle of beamforming calculates the reference codebook for beamforming, given that the direction of the main beam of the reflected signal and the direction of the incident carrier signal are both determined. Calculate the incident wave at the th Line number Phase delay and target direction at column unit The phase delay of the reflected wave is adjusted to obtain the required continuous compensated phase:

[0061] in, For wave number, The wavelength of the sound wave. This refers to the unit spacing.

[0062] The continuous compensated phase is quantized to 0 or 0 according to the following rules. Two discrete states generate a 1-bit reference codebook for the metasurface. : ; Step S2: While maintaining the relative phase difference between each metasurface unit, apply a uniform phase shift to the reference codebook to obtain a phase-modulated codebook. Add the same constant phase offset to all metasurface units Construct a new translation codebook set Since the relative phase difference of all elements remains unchanged, the beamforming main lobe gain is preserved; as After traversing the changes and quantizing them with 1-bit, multiple sets of phase modulation codebooks with the same beam direction but quasi-continuous changes in the total phase of the reflected signal are generated.

[0063] Step S3: By selecting a subset of metasurface units participating in coherent reflection and constructing an optimization objective, the states of the remaining non-beamforming subset of metasurface units are optimized to generate an amplitude modulation codebook. refer to Figure 8 The diagram shows the relationship between the main lobe amplitude of the reflected signal and the size of the metasurface unit subset participating in beamforming. To obtain the desired main lobe amplitude ratio, a size of [missing value] is selected. A subset of metasurface units is specifically designed for beamforming; For the remaining beamforming components... Each unit is based on the actual reflected signal superposition model. Define the following objective function: ; To ensure minimal power fluctuations across different phase codebooks at the same amplitude level, the following tolerance threshold constraint is introduced: ; Where Q is the desired number of modulation phases, This is the tolerance threshold. and These represent the expected magnitude and the actual magnitude in the target direction, respectively. Let q represent the q-th 1-bit phase-coded codebook; solve for this optimization objective to obtain amplitude modulation codebooks with different main lobe amplitude levels.

[0064] Step S4: Based on the standard constellation diagram of target quadrature amplitude modulation (QAM), implement high-order backscatter modulation: Figure 9 The diagram illustrates the optimal codebook mapping for different modulation orders. Let the ideal constellation point set of the target QAM modulation scheme be... ; arbitrarily generated 1-bit codebook The corresponding complex reflection coefficient is denoted as Select an optimal codebook set from the candidate codebook library generated in steps S2 and S3. This minimizes the error between its complex reflection coefficient in the I / Q plane and the ideal constellation point. ; The microcontroller queries the optimal codebook set based on the data stream to be transmitted and sends it to the metasurface, thereby realizing higher-order modulation such as 16-QAM.

[0065] On the other hand, embodiments of this application also provide a method for demodulating higher-order backscattering of underwater acoustic metasurfaces, including: Step S1: Use a multi-channel hydrophone array to capture backscattered signals and extract the training sequence from the data packet preamble.

[0066] Step S2: Construct a composite tap vector containing feedforward and feedback taps. The tap vector is updated at each symbol b using a recursive least squares algorithm: Using the training sequence in the packet preamble, a recursive least squares algorithm is applied to each symbol. Update the composite tap vector containing both feedforward and feedback taps.

[0067] ; ; ; Where b represents the current symbol time and a represents the index of the historical symbol time; This represents the multichannel regression vector at the a-th symbol time. The weighted autocorrelation matrix represents the regression vector; This represents the weighted cross-correlation vector between the regression vector and the training symbols; Indicates the forgetting factor; This represents the a-th known training symbol in the preamble training sequence; Indicates conjugate transpose; Indicates complex conjugation.

[0068] The multi-channel regression vector contains the phase-corrected sampled signals of each channel and the historical decision feedback symbols.

[0069] Step S3: After the preamble training is completed, use the converged tap vector. The method performs filtering and decoding operations on subsequent data payloads. It can adaptively and coherently merge multipath energy, effectively eliminate inter-symbol interference caused by complex underwater environments, and ensure the reliability of high-order constellation diagram decoding.

[0070] The above descriptions are merely various embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. An underwater piezo-acoustic metasurface, characterized in that, At least one piezoelectric metasurface unit is provided; The piezoelectric metasurface unit is provided with a copper-backed piezoelectric composite structure, which is composed of a piezoelectric sheet and a copper sheet bonded together. The piezoelectric sheet is the sound wave incident side, and the copper sheet is the backing side. A passive resonant cavity shell is provided, and the copper-backed piezoelectric composite structure is inserted inside the passive resonant cavity shell. The copper sheet and the passive resonant cavity shell form a closed air cavity, and the piezoelectric sheet and the passive resonant cavity shell form an open liquid cavity.

2. The underwater piezoelectric acoustic metasurface of claim 1, wherein, The copper-backed piezoelectric composite structure is circular, with the diameter of the copper sheet being larger than the diameter of the piezoelectric sheet, and the thickness of the copper sheet being equal to the thickness of the piezoelectric sheet.

3. The underwater piezoelectric acoustic metasurface according to claim 1 or 2, characterized in that, The passive resonant cavity shell is a metal shell; It consists of two covers joined together. The upper cover has an open structure, while the lower cover is a closed structure. The longitudinal cross-section of the inner cavity of both the upper and lower covers is a trapezoidal structure.

4. The underwater piezoelectric acoustic metasurface according to claim 1 or 2, characterized in that, An FR4 control substrate is stacked on the contact surface between the copper sheet and the passive resonant cavity shell; an encapsulation layer is provided on the surface of the piezoelectric sheet; The piezoelectric metasurface unit is connected to two electrical loads with different inductance values. The two electrical loads are alternately connected by switching on and off using a low-power electronic switch.

5. The underwater piezoelectric acoustic metasurface of claim 1 or 2, wherein, Multiple copper-backed piezoelectric composite structures are arranged in an array and uniformly installed inside a passive resonant cavity shell.

6. A method for high-order backscattering modulation and demodulation of an underwater piezoelectric acoustic metasurface, characterized in that, Completed using the underwater piezo-acoustic metasurface according to any one of claims 1-5, comprising: A reference codebook for beamforming is calculated based on the incident signal direction and the target reflection direction. While maintaining the relative phase difference between the various underwater piezo-acoustic metasurfaces, a uniform phase shift is applied to the reference codebook to obtain multiple sets of phase modulation codebooks. By selecting a subset of metasurface units participating in coherent reflection, an optimized target is constructed to optimize the state of the remaining non-beamforming subset of metasurface units to minimize interference with the target amplitude, obtaining different main lobe amplitude levels and generating an amplitude modulation codebook. Based on the standard constellation diagram of the target orthogonal amplitude modulation, a corresponding codebook set is selected from the candidate codebook set composed of the phase modulation codebook and the amplitude modulation codebook to achieve high-order backscatter modulation. The received backscatter signal is demodulated based on the training sequence in the data packet and multi-channel adaptive decision feedback equalization.

7. The method of claim 6, wherein, The construction of the beamforming reference codebook and the phase modulation codebook is specifically as follows: The phase delay of the incident wave at the first row and the second column cell and the phase delay of the reflected wave of the target direction are calculated to obtain the required compensation phase: ​ k represents the wave number, and d represents the spacing between adjacent cells in the metasurface array. These represent the pitch angle and azimuth angle, respectively, in the direction of target reflection. These represent the elevation angle and the azimuth angle, respectively, representing the direction of incidence of the target. Quantize continuous compensated phases to 0 or according to the following rules. Two states generate a 1-bit reference codebook. : ； In the 1-bit reference codebook Based on this, the same constant phase shift is added to all metasurface units. Construct a new translation codebook set This is to achieve a quasi-continuous change in the phase of the synthesized signal.

8. The method of claim 6, wherein, The optimization objective is used to optimize the state of the metasurface units in the remaining non-beamforming subsets. The specific process is as follows: To obtain the desired main lobe amplitude Select size as A subset of metasurface units is used for beamforming; For the remaining units, based on the actual reflection signal superposition model , the following objective function and its tolerance threshold constraint conditions are defined: ； ； wherein Q is the desired number of modulation phases, is a tolerance threshold, and are the desired amplitude and actual amplitude of the target direction, respectively. represents the qth 1-bit phase encoding codebook.

9. The high-order backscattering modulation and demodulation method of the underwater piezoelectric acoustic metasurface according to claim 6, wherein, The step of selecting a corresponding codebook set from the candidate codebook set to achieve high-order backscatter modulation specifically includes: Let the set of ideal constellation points of a target QAM modulation scheme be ; P represents the number of constellation points contained in the QAM modulation scheme, and any 1-bit codebook The corresponding complex reflection coefficient is denoted as A set of optimal codebooks is selected from the candidate codebook set , so that the error of the complex reflection coefficient thereof on the I / Q plane and the ideal constellation point is minimum. 。 10. The method for high-order backscattering modulation and demodulation of underwater piezoacoustic metasurfaces according to claim 6, characterized in that, The demodulation of the received backscattered signal based on the training sequence in the data packet and multi-channel adaptive decision feedback equalization specifically includes: Using the training sequence in the packet preamble, a recursive least squares algorithm is applied to each symbol. Update the composite tap vector containing both feedforward and feedback taps. ； ； ； Where b represents the current symbol time and a represents the index of the historical symbol time; This represents the multichannel regression vector at the a-th symbol time. The weighted autocorrelation matrix represents the regression vector; This represents the weighted cross-correlation vector between the regression vector and the training symbols; Indicates the forgetting factor; This represents the a-th known training symbol in the preamble training sequence; Indicates conjugate transpose; Indicates complex conjugation; After the preamble training is completed, the converged tap vector is used. Decoding is performed on subsequent data payloads to eliminate inter-symbol interference.

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