A method and device for laser-induced underwater acoustic communication based on discrete chirp pulse spacing

Abstract

This invention discloses a laser-induced acoustic underwater communication method and apparatus based on discrete linear frequency modulated pulse intervals, belonging to the field of cross-medium communication technology. The invention first maps the binary data to be transmitted into a pulse time interval pattern, where logic "0" and "1" correspond to pulse sequences with pulse intervals that decrease or increase linearly with time, respectively, and inserts a silence protection interval. A laser is controlled to emit pulses towards the water surface at non-uniform intervals, utilizing the photoacoustic effect to excite a train of acoustic pulses with discrete linear frequency modulated characteristics in the water. After acquiring the acoustic signal, the receiving end uses a local reference signal matching the transmitting end's pattern to perform dual-channel cross-correlation demodulation, achieving data decision-making and self-synchronization based on the amplitude and position of the correlation peaks. This invention combines the cross-medium advantages of laser-induced acoustics with the high processing gain characteristics of linear frequency modulated signals, solving the problem of poor multipath resistance in traditional pulse position modulation and significantly improving communication reliability in complex underwater acoustic channels.

Description

Technical Field

[0001] This invention belongs to the field of cross-medium communication technology, and more specifically, relates to a laser-induced acoustic underwater communication method and device based on discrete linear frequency modulated pulse intervals. Background Technology

[0002] With the rapid development of marine information technology, air-to-water cross-medium communication has become a key technology for achieving integrated air-sea communication and strategic marine objectives, and is widely used in underwater robot control, marine environmental monitoring, seabed resource exploration, and military communications. Laser-induced acoustic communication enables direct communication between airborne platforms and underwater targets through laser-induced sound waves, offering advantages such as strong concealment and the elimination of the need for surface relays.

[0003] However, current underwater acoustic communication is severely affected by multipath effects. Reflections of sound waves at the sea surface, seabed, and between different water layers cause signal distortion and inter-symbol interference, significantly increasing the bit error rate. Simultaneously, laser energy fluctuations, transmission medium losses, and variations in laser incident and observation angles significantly reduce the signal-to-noise ratio, limiting communication stability and reliability. Currently, laser-induced acoustic communication mainly employs modulation methods such as on-off keying (OOK) or pulse position modulation (PPM). These methods primarily rely on the detection of single-pulse signals, exhibiting poor noise immunity and lacking effective spreading gain mechanisms. When faced with strong multipath interference, PPM signals are highly susceptible to position aliasing, leading to communication link interruptions. In contrast, while linear frequency modulation (LFM) signals possess excellent autocorrelation characteristics and multipath immunity, the working principle of lasers makes it difficult to directly generate continuous LFM acoustic signals.

[0004] Therefore, existing modulation and demodulation techniques are difficult to effectively cope with the complex channel environment of multipath effect and low signal-to-noise ratio in laser-induced acoustic communication. There is an urgent need for a communication method that can both utilize the characteristics of pulsed lasers and obtain the anti-interference gain of LFM signals to meet the requirements of highly robust cross-medium communication. Summary of the Invention

[0005] This invention addresses the technical problems of existing laser-induced acoustic communication technologies, such as poor resistance to multipath interference, low signal-to-noise ratio, and difficulty in directly generating continuous linear frequency modulated signals. It provides a laser-induced acoustic underwater communication method and device based on discrete linear frequency modulated pulse intervals.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: This invention first provides a laser-induced acoustic underwater communication method based on discrete linear frequency modulated pulse intervals, comprising the following steps: S1. Coding Mapping: Obtain the binary data sequence to be transmitted, map the binary symbols to the corresponding pulse time interval patterns, and insert a silent protection interval with a preset duration between the pulse sequences corresponding to adjacent binary symbols; among them, the first type of binary symbol corresponds to a pulse sequence with a linearly decreasing pulse interval over time, and the second type of binary symbol corresponds to a pulse sequence with a linearly increasing pulse interval over time. S2. Laser Emission Control: Generate a laser trigger signal based on the mapped pulse time interval pattern and protection interval; drive the pulsed laser to emit laser pulses towards the water surface at non-uniform moments determined by the trigger signal. S3. Acoustic Wave Excitation and Propagation: The laser pulse irradiates the water surface, and an acoustic wave pulse train with discrete linear frequency modulation characteristics is excited in the water through the photoacoustic effect. The acoustic wave pulse train propagates through the underwater channel to the receiving end. S4. Signal Acquisition: The underwater acoustic transducer at the receiving end collects the underwater acoustic signal and converts it into an electrical signal. S5. Correlation Demodulation: Construct a local reference signal corresponding to the pulse time interval pattern in S1; perform a cross-correlation operation on the collected electrical signal and the local reference signal. S6. Decision Output: Detect the correlation peak in the result of the cross-correlation operation, and determine the original binary data according to the position and amplitude of the correlation peak to complete the communication.

[0007] Preferably, in step S1, it is set that each pulse sequence corresponding to a binary symbol contains N laser pulses, the preset basic pulse interval is T base , and the step size of the pulse interval change is δ; In the pulse sequence corresponding to the first type of binary symbol, the time interval ΔT i between the i -th pulse and the i +1-th pulse satisfies the following linearly decreasing relationship: ; In the pulse sequence corresponding to the second type of binary symbol, the time interval ΔT i between the i -th pulse and the i +1-th pulse satisfies the following linearly increasing relationship:

[0008] Among them, 1 ≤ i < N and T min is the minimum pulse interval allowed by the laser.

[0009] Preferably, in step S1, a silence protection interval is inserted between the pulse sequences corresponding to two adjacent binary symbols, during which the laser remains off; the duration of the silence protection interval is set to be greater than the maximum multipath delay spread of the underwater acoustic channel in the target communication water area, so as to block the interference of the multipath echo of the previous symbol to the current symbol.

[0010] Preferably, step S2 further includes: Before emitting the laser pulse, the vertical distance between the emitting platform and the water surface is measured in real time by the laser ranging module; the focal length of the optical system is dynamically adjusted according to the vertical distance to focus the laser beam on the water-air interface and control the laser spot radius within a preset range to ensure that the laser acoustic process is in the thermal expansion mechanism region. The characteristic radius of the laser beam is R0, the absorption coefficient at the water-air interface is α, and when the laser beam is incident perpendicularly on the water surface, if the following conditions are met... α When R0 << 1, it is a weak absorption region with a radius of R0 and a height of 1 / α A cylinder, where energy can penetrate deeper; when the condition is met... α When *R0>>1, strong absorption occurs, and the energy deposition zone is located within the thin disk in contact with the water surface. The generated sound wave is a plane wave parallel to the surface. The relationship between pressure and temperature changes is as follows:

[0011] In the formula, Δ T This represents the change in water temperature. ; α The absorption coefficient is... E 0 represents the surface thermal energy density. ρ 1 represents the initial density of water. C p The specific heat capacity of water at constant pressure; c 1 represents the speed of sound in water. α V is the coefficient of volume expansion of water; Finally, the sound waves propagating from outside the energy deposition zone into the water body are obtained, represented as:

[0012] In the formula, α The absorption coefficient is... c 1 represents the speed of sound in water. a v is the coefficient of volume expansion of water. E 0 represents the surface thermal energy density. C p The specific heat capacity of water at constant pressure. z Let the coordinates be the vertical water-air interface, and the sgn function be: .

[0013] Preferably, in step S2: The laser pulse output from the laser is split into beams, and a small portion of the energy is used as a reference beam to be introduced into the energy monitoring module to monitor in real time whether the energy intensity of the emitted pulse meets the preset threshold. The remaining main beam is oriented by the optical reflection component and then incident at an angle perpendicular to the water surface to maximize the photoacoustic energy conversion efficiency and reduce the reflection loss of the beam at the water-air interface.

[0014] Preferably, in step S3: A sound wave pulse train with discrete linear frequency modulation characteristics includes several single sound wave pulses with the same duration and the same center frequency. On the time axis, the time interval between two adjacent acoustic pulses is consistent with the laser pulse interval, which makes the acoustic pulse train exhibit autocorrelation function characteristics similar to continuous linear frequency modulated signals in the matched filtering process at the receiving end, and has high distance resolution and noise resistance. When a laser beam is incident perpendicularly on the water surface and causes thermal expansion, the resulting acoustic signal's sound pressure frequency domain expression is:

[0015] In the formula, r ω is the distance from the receiving point to the sound source, and ω is the angular frequency. T The optical transmittance of the liquid. I 0 represents laser intensity. β The coefficient of thermal expansion is... a Where is the laser beam radius, C p Specific heat capacity of the liquid I (ω) represents the laser signal spectrum. E (ω) represents the electromagnetic wave energy density spectrum absorbed and converted into heat per unit time, which can be expressed as:

[0016] In the formula, k τ is the sound wave number (2π / γ, where γ is the wavelength). μ For the vertical characteristic delay time, τ a The horizontal characteristic delay time is expressed as follows, where θ is the observation angle in the vertical direction, i.e., the angle between r and the vertically downward direction;

[0017] .

[0018] Preferably, in step S4: The underwater acoustic transducer is a broadband hydrophone, and its response bandwidth covers the main energy spectrum range of laser-induced acoustic signals. Before converting the acquired analog electrical signals into digital signals, the analog electrical signals are first subjected to bandpass filtering. The passband frequency range of the bandpass filter is set to match the spectral width of a single laser acoustic pulse in order to filter out out-of-band environmental noise. Then, the filtered signal is converted into a digital discrete-time sequence by an analog-to-digital converter at a sampling rate that satisfies the Nyquist sampling theorem, and transmitted to subsequent steps for processing.

[0019] Preferably, step S5 includes: S501, Obtain standard single-pulse waveform The standard acoustic pressure waveform signal Sref(t) excited by a single laser pulse underwater is obtained in advance. This waveform signal is generated either through experimental calibration or based on the photoacoustic conversion theoretical model. S502, Constructing a local reference signal Based on the standard sound pressure waveform signal Sref(t), time delay superposition is performed according to the time interval rule to construct the first local reference signal R0(t) and the second local reference signal R1(t): Wherein, R0(t): consists of N Sref(t), and the time interval between adjacent waveforms satisfies the linear decreasing rule, corresponding to logic "0"; R1(t): consists of N Sref(t), and the time interval between adjacent waveforms satisfies the linear increasing rule, corresponding to logic "1"; S503, Related Operations The digital signal output in step S4 is cross-correlated with R0(t) and R1(t) respectively to obtain the first correlation function output representing the matching degree of logic "0" and the second correlation function output representing the matching degree of logic "1".

[0020] Preferably, step S6 includes: Within each preset symbol period search window, the maximum peak value Vpeak0 of the first correlation function output and the maximum peak value Vpeak1 of the second correlation function output are detected respectively. Compare the magnitudes of Vpeak0 and Vpeak1: If Vpeak0 > Vpeak1, then the currently demodulated binary symbol is determined to be "0"; If Vpeak1 > Vpeak0, then the currently demodulated binary symbol is determined to be "1"; Simultaneously, the time index tpeak corresponding to the winning peak is extracted and marked as the arrival time of the current symbol. Based on tpeak and combined with the preset symbol period duration, the starting position of the search window for the next symbol is calculated, thereby realizing the self-synchronization demodulation of the receiver.

[0021] Furthermore, the present invention also provides a laser-induced acoustic underwater communication device based on discrete linear frequency modulated pulse intervals, which utilizes the above-mentioned communication method, including: An aerial platform equipped with drones; The signal transmitter is located on an air platform and includes a laser, an adjustable-focus optical system, and an energy monitoring module. The adjustable-focus optical system includes an optical reflection component, which includes a beam splitter and a plane mirror. The signal receiver is equipped with a signal receiving module. The signal processing module is connected to the signal receiver. Laser ranging module; The laser beam is split into two parts by a beam distributor. One part enters the energy monitoring module, and the other part is guided to the water vapor interface through an optical reflection component, focused in a vertical incident manner, and finally received by the signal receiving module.

[0022] Compared with the prior art, the present invention has the following beneficial effects: This invention provides a laser-induced acoustic underwater communication method and apparatus based on discrete linear frequency modulated pulse intervals, suitable for achieving stable communication between airborne platforms and underwater targets in low signal-to-noise ratio and multipath channel environments. This invention innovatively employs a discrete linear frequency modulated pulse interval encoding method, enabling the laser-induced acoustic signal to possess excellent autocorrelation characteristics of a broadband signal on a macroscopic scale. Compared to traditional single-pulse modulation, significant processing gains can be obtained through cross-correlation demodulation, thereby achieving stable communication in underwater channels with low signal-to-noise ratios and strong multipath interference. Furthermore, this invention introduces a silence protection interval between adjacent symbol pulse sequences that is greater than the channel multipath delay. This design effectively blocks the interference of the multipath echo of the previous symbol on the current symbol at the physical level, significantly suppressing inter-symbol interference (ISI) and reducing the system bit error rate. Furthermore, this invention, through real-time ranging and dynamic focusing mechanisms at the transmitter, can precisely control the laser spot radius, ensuring that the interaction between the laser and water remains within the efficient "thermal expansion mechanism" region. This effectively avoids vaporization or dielectric breakdown effects caused by excessive energy density, prevents energy dissipation and waveform distortion, and guarantees the stability and efficiency of the sound source signal. At the same time, since the cross-correlation demodulation algorithm inherently possesses time synchronization characteristics, the optimal decision time can be determined using the correlation peak. This eliminates the need for additional complex hardware synchronization circuits at the receiver, significantly reducing the implementation cost and system complexity of the underwater node. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a flowchart illustrating one embodiment of the communication method of the present invention; Figure 2 This is a schematic diagram of the structure of one embodiment of the communication device of the present invention; Figure 3 This is a schematic diagram of the actual received signal waveform in an embodiment of the present invention.

[0025] Explanation of symbols in the diagram: 1. Unmanned Aerial Vehicle (UAV); 2. Signal Transmitter; 3. Laser; 4. Energy Monitoring Module; 5. Beam Splitter; 6. Plane Mirror; 7. Signal Receiving Module; 8. Signal Processing Module. Detailed Implementation

[0026] To make the technical problems, solutions, and beneficial effects of this application clearer, the following detailed description, in conjunction with the accompanying drawings and embodiments, provides a laser-induced underwater communication method and apparatus based on discrete linear frequency modulated pulse intervals. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0027] Example 1 Please see Figure 1 This invention provides a laser-induced acoustic underwater communication method based on discrete linear frequency modulated pulse intervals, comprising the following steps: S1. Encoding Mapping: Obtain the binary data sequence to be sent, map the binary symbols to the corresponding pulse time interval patterns, and insert a preset duration of silent protection interval between the pulse sequences corresponding to adjacent binary symbols; wherein, the first type of binary symbol (such as logic "0") corresponds to a pulse sequence in which the pulse interval decreases linearly with time, and the second type of binary symbol (such as logic "1") corresponds to a pulse sequence in which the pulse interval increases linearly with time. S2, Laser Emission Control: Based on the mapped pulse time interval pattern and guard interval, a laser trigger signal is generated; the pulsed laser is driven to emit laser pulses toward the water surface at non-uniform intervals determined by the trigger signal. S3. Acoustic Excitation and Propagation: When a laser pulse irradiates the water surface, it excites a series of acoustic pulses with discrete linear frequency modulation characteristics in the water through the photoacoustic effect. The acoustic pulses propagate to the receiving end through an underwater channel. S4. Signal Acquisition: The underwater acoustic transducer at the receiving end acquires underwater acoustic signals and converts them into electrical signals; S5. Correlation Demodulation: Construct a local reference signal corresponding to the pulse time interval pattern in S1; perform cross-correlation operation between the acquired electrical signal and the local reference signal; S6. Decision Output: Detect the correlation peak in the cross-correlation operation result, determine the original binary data based on the position and amplitude of the correlation peak, and complete the communication.

[0028] This invention provides a laser-induced acoustic underwater communication method based on discrete linear frequency modulated pulse intervals, suitable for achieving stable communication between airborne platforms and underwater targets in low signal-to-noise ratio and multipath channel environments. This embodiment employs a discrete linear frequency modulated pulse interval encoding method, enabling the laser-induced acoustic signal to possess excellent autocorrelation characteristics of a broadband signal on a macroscopic scale. Compared to traditional single-pulse modulation, significant processing gains can be obtained through cross-correlation demodulation, thereby achieving stable communication in underwater channels with low signal-to-noise ratios and strong multipath interference. Furthermore, this embodiment introduces a silence protection interval greater than the channel multipath delay between adjacent symbol pulse sequences. This design effectively blocks the interference of multipath echoes from the previous symbol on the current symbol at a physical level, significantly suppressing inter-symbol interference (ISI) and reducing the system bit error rate.

[0029] Furthermore, this embodiment can precisely control the laser spot radius through the real-time ranging and dynamic focusing mechanism at the transmitter, ensuring that the interaction between the laser and water is always maintained in the efficient "thermal expansion mechanism" region. This effectively avoids vaporization or dielectric breakdown effects caused by excessive energy density, prevents energy dissipation and waveform distortion, and ensures the stability and efficiency of the sound source signal. At the same time, since the cross-correlation demodulation algorithm naturally has time synchronization characteristics, the optimal decision time can be determined by using the correlation peak, so that the receiver does not need to deploy additional complex hardware synchronization circuits, which greatly reduces the implementation cost and system complexity of the underwater node.

[0030] Example 2 Furthermore, based on Example 1, this example provides a laser-induced acoustic underwater communication method based on discrete linear frequency modulated pulse intervals, specifically including the following steps: S1, Encoding Mapping and Sequence Generation Obtain the sequence of binary data to be sent.

[0031] In this embodiment, each binary symbol is set to contain N laser pulses in its corresponding pulse sequence, and the preset basic pulse interval is T. base, the step size of the pulse interval change is δ; In the pulse sequence corresponding to the first binary symbol, the i th pulse and the i +1th pulse, the time interval ΔT i satisfies the following linearly decreasing relationship: ; In the pulse sequence corresponding to the second binary symbol, the i th pulse and the i +1th pulse, the time interval ΔT i satisfies the following linearly increasing relationship:

[0032] where, 1 ≤ i <N and T min is the minimum pulse interval allowed by the laser.

[0033] Furthermore, a silent protection interval is inserted between the pulse sequences corresponding to adjacent two binary symbols. During this period, the laser is kept off; the duration of the silent protection interval is set to be greater than the maximum multipath delay spread of the underwater acoustic channel in the target communication water area to block the interference of the multipath echo of the previous symbol to the current symbol.

[0034] S2. Laser emission control In this embodiment, before emitting the laser pulse, the vertical distance between the emission platform and the water surface is measured in real time by the laser ranging module; the focal length of the optical system is dynamically adjusted according to the vertical distance to make the laser beam focus on the water-vapor interface, and the radius of the laser spot is controlled within a preset range.

[0035] Furthermore, according to the mapped pulse time interval pattern and the protection interval, a laser trigger signal is generated. Before driving the pulse laser to emit, this embodiment also executes the following control strategies: S201. Dynamic focusing and spot control: The vertical distance between the emission platform and the water surface is measured in real time by the laser ranging module; the focal length of the optical system is dynamically adjusted according to the vertical distance to make the laser beam focus on the water-vapor interface, and the radius R0 of the laser spot is controlled within a preset range to ensure that the laser-induced sound process is in the thermal expansion mechanism region and avoid the vaporization or dielectric breakdown effect caused by too high energy density.

[0036] Specifically, it is necessary to ensure that the condition α *R0 << 1 is satisfied, so as to ensure that the laser-induced sound process is in the thermal expansion mechanism region.

[0037] In this embodiment, the characteristic radius of the laser beam is R0, the absorption coefficient of the water-vapor interface is α, and when vertically incident on the water surface, when αWhen R0 << 1, it is a weak absorption region with a radius of R0 and a height of 1 / α A cylinder, where energy can penetrate deeper; when the condition is met... α When *R0>>1, strong absorption occurs, and the energy deposition zone is located within the thin disk in contact with the water surface. The generated sound wave is a plane wave parallel to the surface.

[0038] When the energy density is low, the center frequency of the excited sound wave is low, and the attenuation is small when propagating in the liquid. However, if the energy density is too low, it is difficult to effectively excite the sound signal. By adjusting the spot radius, the laser energy density is kept within a reasonable range to ensure that the energy is higher than the energy required for the thermal expansion mechanism to excite the sound signal, and the center frequency of the excited sound signal is as low as possible, so as to achieve stable and efficient sound signal excitation.

[0039] Furthermore, controlling the laser spot radius aims to ensure that the laser-induced acoustic process occurs within the thermal expansion mechanism region, avoiding vaporization or dielectric breakdown effects. When the laser energy is fixed, the spot size determines the energy density at the water surface. When the energy density is low and the water surface does not reach the boiling point, the thermoelastic pressure caused by uneven heating of the water surface excites sound waves, which is the thermal expansion mechanism. When the energy density is high and the water surface reaches the boiling point, the vaporization mechanism plays a dominant role. When the energy density is extremely high and the water surface reaches the boiling point, if energy continues to be injected before the boiling bubbles burst, the water vapor will be ionized into a plasma state, and dielectric breakdown will play a dominant role.

[0040] In this embodiment, the adjustable focusing optical system is used to control the spot radius so that the acoustic signal converted by the pulsed laser is always in the thermal expansion mechanism. The laser energy causes the water temperature to change, generating a pressure field, which in turn generates sound waves.

[0041] S202, Energy Monitoring: The laser pulse output by the laser is split into beams, and a small portion of the energy (e.g., 1%) is used as a reference beam and introduced into the energy monitoring module to monitor in real time whether the energy intensity of the emitted pulse meets the preset threshold. The remaining main beam is oriented by the optical reflection component and then incident at an angle perpendicular to the water surface to maximize the photoacoustic energy conversion efficiency and reduce the reflection loss of the beam at the water-air interface.

[0042] S3, Sound Wave Excitation and Propagation In this embodiment, the laser pulse irradiates the water surface at non-uniform times as determined in step S2, and generates sound waves in the water through the photoacoustic effect.

[0043] Under the thermal expansion mechanism, laser energy causes a change in water temperature, generating a pressure field. The relationship between the pressure p1 and the temperature change follows the following physical model:

[0044] In the formula, Δ T This represents the change in water temperature. ; α The absorption coefficient is... E 0 represents the surface thermal energy density. ρ 1 represents the initial density of water. C p The specific heat capacity of water at constant pressure; c 1 represents the speed of sound in water. α V is the coefficient of volume expansion of water.

[0045] Although the waveforms of the individual acoustic pulses are the same, the transmission time interval varies linearly (e.g., 3.0ms→4.5ms), resulting in a macroscopically varying density structure for the acoustic pulse train. This gives the acoustic pulse train discrete linear frequency modulation characteristics, and the acoustic pulse train propagates to the receiving end through an underwater channel.

[0046] Finally, the sound waves propagating from outside the energy deposition zone into the water body are obtained, represented as:

[0047] In the formula, α The absorption coefficient is... c 1 represents the speed of sound in water. a v is the coefficient of volume expansion of water. E 0 represents the surface thermal energy density. C p The specific heat capacity of water at constant pressure. z Let the coordinates be the vertical water-air interface, and the sgn function be: .

[0048] Furthermore, in this embodiment, the acoustic pulse train with discrete linear frequency modulation characteristics includes several single acoustic pulses with the same duration and the same center frequency. On the time axis, the time interval between two adjacent acoustic pulses is consistent with the laser pulse interval in S1, which makes the acoustic pulse train exhibit autocorrelation function characteristics similar to those of a continuous linear frequency modulated signal in the matched filtering process at the receiving end, and has high distance resolution and noise resistance. When a laser beam is incident perpendicularly on the water surface and causes thermal expansion, the resulting acoustic signal's sound pressure frequency domain expression is:

[0049] In the formula, r ω is the distance from the receiving point to the sound source, and ω is the angular frequency. T The optical transmittance of the liquid. I 0 represents laser intensity. βThe coefficient of thermal expansion is... a Where is the laser beam radius, C p Specific heat capacity of the liquid I (ω) represents the laser signal spectrum. E (ω) represents the electromagnetic wave energy density spectrum absorbed and converted into heat per unit time, which can be expressed as:

[0050] In the formula, k τ is the sound wave number (2π / γ, where γ is the wavelength). μ For the vertical characteristic delay time, τ a The horizontal characteristic delay time is expressed as follows, where θ is the observation angle in the vertical direction, i.e., the angle between r and the vertically downward direction;

[0051] .

[0052] By superimposing the above non-uniform intervals, the acoustic pulse train exhibits autocorrelation function characteristics similar to those of a continuous linear frequency modulated (LFM) signal during matched filtering at the receiving end, thus possessing high distance resolution and noise immunity.

[0053] S4, Sound signal reception In this embodiment, the acoustic signal induced by the laser on the water surface propagates through the underwater acoustic channel, is received by the piezoelectric hydrophone of the signal receiving module, and is converted into an electrical signal.

[0054] Specifically, the signal receiving module utilizes the piezoelectric effect to directly convert sound pressure in water into an electrical signal. When a sound wave acts on the piezoelectric element with sound pressure P, the element generates mechanical stress and a short-circuit charge on the electrode, which can be approximated as follows:

[0055] Under open-circuit conditions, this charge generates a voltage across the electrostatic capacitance of the component. Utilizing the piezoelectric effect, the sound pressure p is converted into a voltage signal V. oc The approximate relationship is:

[0056] In the formula, Q sc For short-circuit charges, V oc Open circuit voltage, d It is the piezoelectric constant. p The sound pressure acting on the surface of the component. A The effective area of ​​the electrode. t p For the thickness of the piezoelectric element, εT Let be the dielectric constant under constant stress; the negative sign in the formula indicates the polarity convention.

[0057] In this embodiment, the underwater acoustic transducer is a broadband hydrophone, whose response frequency band covers the main energy spectrum range of the laser-induced acoustic signal.

[0058] Furthermore, before converting the acquired analog electrical signals into digital signals, the acquired electrical signals are first subjected to bandpass filtering to remove out-of-band environmental noise; the passband range covers the laser acoustic frequency, such as 10kHz. (50kHz) and A / D conversion, filtering out low-frequency sea wave noise, to obtain a digital discrete sequence.

[0059] The passband frequency range of the bandpass filter is set to match the spectral width of a single laser-induced acoustic pulse to filter out out-of-band environmental noise. Then, the filtered signal is converted into a digital discrete-time sequence by an analog-to-digital converter at a sampling rate that satisfies the Nyquist sampling theorem. The acquired signal is then transmitted to the signal processing module for further processing.

[0060] S5, Related Demodulation Specifically, this embodiment includes the following steps: S501, Obtain standard single-pulse waveform The standard acoustic pressure waveform signal Sref(t) excited by a single laser pulse underwater is obtained in advance. This waveform signal is generated either through experimental calibration or based on the photoacoustic conversion theoretical model.

[0061] S502, Constructing a local reference signal Based on the standard sound pressure waveform signal Sref(t), time delay superposition is performed according to the time interval rule of S1 to construct the first local reference signal R0(t) and the second local reference signal R1(t): Wherein, R0(t): consists of N Sref(t), and the time interval between adjacent waveforms satisfies the linear decreasing rule, corresponding to logic "0"; R1(t): consists of N Sref(t), and the time interval between adjacent waveforms satisfies the linear increasing rule, corresponding to logic "1".

[0062] S503, Related Operations The digital signal output in step S4 is cross-correlated with R0(t) and R1(t) respectively to obtain the first correlation function output representing the matching degree of logic "0" and the second correlation function output representing the matching degree of logic "1".

[0063] S6, Judgment Output Within each preset symbol period search window, perform the following operations: Detect the maximum peak value Vpeak0 of the first correlation function output and the maximum peak value Vpeak1 of the second correlation function output respectively; Compare the magnitudes of Vpeak0 and Vpeak1: If Vpeak0 > Vpeak1, then the currently demodulated binary symbol is determined to be "0"; If Vpeak1 > Vpeak0, then the currently demodulated binary symbol is determined to be "1".

[0064] Simultaneously, the time index tpeak corresponding to the winning peak is extracted and marked as the arrival time of the current symbol. Based on tpeak and combined with the preset symbol period duration, the starting position of the search window for the next symbol is calculated, thereby realizing the self-synchronization demodulation of the receiver.

[0065] Example 3 Furthermore, this embodiment uses example data to verify the communication method.

[0066] like Figure 3 As shown, it is a schematic diagram of the actual received signal waveform in this embodiment.

[0067] In the encoding mapping and sequence generation, this embodiment sets each binary symbol to contain N=5 laser pulses, and the basic pulse interval is T. base =3.0ms, pulse interval change step size δ=0.5ms.

[0068] For logic "1": a trigger sequence with linearly increasing pulse intervals is generated, with the time intervals between adjacent pulses being 3.0ms, 3.5ms, 4.0ms, and 4.5ms respectively.

[0069] For logic "0": a trigger sequence with linearly decreasing pulse intervals is generated, with the time intervals between adjacent pulses being 3.0ms, 2.5ms, 2.0ms, and 1.5ms respectively.

[0070] Furthermore, a 30ms silent guard interval is inserted between the pulse sequences of adjacent symbols. Since this duration is longer than the multipath delay of common underwater acoustic channels (typically <20ms), it physically blocks the interference of the multipath aftereffects of the previous symbol on the current symbol, effectively suppressing inter-symbol interference (ISI).

[0071] This invention maps binary symbols into a pulse sequence with linearly varying pulse intervals, thereby exciting a sound wave pulse train with discrete linear frequency modulation characteristics. This indirectly achieves the anti-interference advantage of LFM signals and solves the technical problem that continuous LFM sound wave signals are difficult to generate directly.

[0072] When constructing the local reference signal in the relevant demodulation, firstly, the first local reference signal R0(t) is constructed: it consists of N Sref(t), with adjacent intervals satisfying a linear decreasing rule of 3.0ms→1.5ms (i.e. 3.0ms, 2.5ms, 2.0ms, 1.5ms), corresponding to logic "0".

[0073] Then, a second local reference signal R1(t) is constructed: it consists of N Sref(t) with adjacent intervals satisfying a linear increasing rule of 3ms→4.5ms (i.e., 3.0ms, 3.5ms, 4.0ms, 4.5ms), corresponding to logic "1".

[0074] After the local reference signal is constructed, the digital signal acquired by the receiver is cross-correlated with the first local reference signal R0(t) and the second local reference signal R1(t) to obtain the corresponding correlation function output.

[0075] Within each symbol period, peak detection is performed on the two correlated outputs to obtain the maximum correlated peak values ​​Vpeak0 and Vpeak1, respectively. When Vpeak0 is greater than Vpeak1, it is determined as logic "0", and when Vpeak1 is greater than Vpeak0, it is determined as logic "1". The corresponding peak position is used as the symbol arrival time to achieve self-synchronous demodulation.

[0076] like Figure 3 As shown, the received signal exhibits a pulse interval distribution characteristic that varies with the sign on the time axis. After correlation processing (not shown in the figure), the correlation output of the matched reference signal produces a significant correlation peak, thereby effectively distinguishing logic "0" and "1".

[0077] In addition, because a silent protection interval greater than the channel multipath delay is set, the multipath echo has little impact on the current symbol, effectively suppressing inter-symbol interference and improving the reliability of the decision.

[0078] Therefore, under the above parameter conditions, the method of the present invention can achieve stable signal demodulation and has good anti-multipath interference capability.

[0079] Example 4 Furthermore, embodiments of the present invention provide a transmedium laser-induced underwater communication device based on discrete linear frequency modulated pulse intervals, which utilizes the communication method described in the above embodiments. For example... Figure 2 As shown, the communication device mainly consists of an airborne section and an underwater section, specifically including: An aerial platform, equipped with one unmanned aerial vehicle (UAV); Signal transmitter 2 is set on an aerial platform and mounted below the UAV 1.

[0080] The signal transmitting end 2 integrates a laser 3, an energy monitoring module 4, and a tunable optical system, which includes an optical reflection component.

[0081] In this embodiment, the optical reflection component specifically includes a beam splitter 5 and a plane mirror 6.

[0082] In addition, the signal transmitter 2 is also equipped with a laser ranging module (not shown) and an adjustable focus lens group (integrated in the optical path, not shown) to help achieve dynamic adjustment of the focal length.

[0083] The signal receiver is located underwater and is equipped with a signal receiving module 7 (shown in the figure as a piezoelectric hydrophone).

[0084] The signal processing module 8 (shown as a host computer / computer terminal in the figure) is connected to the signal receiving module 7 via a cable and is used for data acquisition and processing.

[0085] In this embodiment, the beam output by the laser 3 is divided into two parts by the beam distributor. One part enters the energy monitoring module 4, and the other part is guided to the water vapor interface through the optical reflection component, focused in a vertical incident manner, and finally received by the signal receiving module 7.

[0086] The specific working principle and optical path of the communication device provided in this embodiment are as follows: Laser emission: Laser 3 (preferably an Nd:YAG laser in this embodiment) emits high-energy pulsed laser according to the discrete linear frequency modulation timing described in the above embodiment.

[0087] Beam distribution: After the laser beam is emitted, it first passes through beam splitter 5.

[0088] Beam splitter 5 reflects / transmits a small portion of the beam (reference light) to energy monitoring module 4, which monitors the stability of the emitted energy in real time using an internal optical power meter.

[0089] Most of the main beam passes through or is directed towards the plane mirror 6 after passing through the beam splitter 5.

[0090] Optical path guidance: Plane mirror 6 reflects the horizontal laser beam vertically downwards, allowing it to be incident perpendicularly onto the water-air interface. During this process, the device dynamically adjusts the optical path focal length based on the height data fed back by the laser ranging module, controlling the radius of the laser spot entering the water.

[0091] Acoustic signal conversion: The laser pulse induces ultrasonic waves on the water surface (as shown by the dashed ripples in the figure), and the acoustic waves retain the discrete linear frequency modulation characteristics of the transmitting end.

[0092] Reception and Processing: The underwater piezoelectric hydrophone 7 captures the acoustic signal and converts it into an electrical signal, which is then transmitted to the signal processing module 8. The signal processing module 8 executes a cross-correlation demodulation algorithm to reconstruct the original communication data.

[0093] This invention provides a laser-induced acoustic underwater communication method and apparatus based on discrete linear frequency modulated (LFM) pulse intervals, suitable for achieving stable communication between an airborne platform and an underwater target in low signal-to-noise ratio and multipath channel environments. The communication method first maps the binary data to be transmitted into a pulse time interval pattern, where logic "0" and "1" correspond to pulse sequences with pulse intervals that decrease or increase linearly with time, respectively, and a silence protection interval is inserted. Then, a laser is controlled to emit pulses towards the water surface at non-uniform intervals, utilizing the photoacoustic effect to excite a train of acoustic pulses with discrete LFM characteristics in the water. After acquiring the acoustic signal, the receiving end uses a local reference signal matching the transmitting end's pattern for dual-channel cross-correlation demodulation, achieving data decision-making and self-synchronization based on the amplitude and position of the correlation peaks. This invention combines the cross-medium advantages of laser-induced acoustics with the high processing gain characteristics of LFM signals, effectively solving the problem of poor multipath resistance in traditional pulse position modulation and significantly improving communication reliability in complex underwater acoustic channels.

[0094] Furthermore, this invention innovatively employs a discrete linear frequency modulated (LFM) pulse interval encoding method, enabling the laser-induced acoustic signal to possess excellent autocorrelation characteristics of a broadband signal on a macroscopic level. Compared to traditional single-pulse modulation, significant processing gains can be obtained through cross-correlation demodulation, thereby achieving stable communication in underwater channels with low signal-to-noise ratios and strong multipath interference. Based on this, this invention introduces a silence protection interval between adjacent symbol pulse sequences that is greater than the channel multipath delay. This design effectively blocks the interference of the multipath echo of the previous symbol on the current symbol at the physical level, significantly suppressing inter-symbol interference (ISI) and reducing the system bit error rate. This invention indirectly realizes the anti-interference advantage of LFM signals by mapping binary symbols to pulse sequences with linearly varying pulse intervals, thereby exciting acoustic pulse trains with discrete linear frequency modulated characteristics and solving the technical problem of the difficulty in directly generating continuous LFM acoustic signals. Furthermore, this invention, through real-time ranging and dynamic focusing mechanisms at the transmitter, can precisely control the laser spot radius, ensuring that the interaction between the laser and water remains within the efficient "thermal expansion mechanism" region. This effectively avoids vaporization or dielectric breakdown effects caused by excessive energy density, prevents energy dissipation and waveform distortion, and guarantees the stability and efficiency of the sound source signal. At the same time, since the cross-correlation demodulation algorithm inherently possesses time synchronization characteristics, the optimal decision time can be determined using the correlation peak. This eliminates the need for additional complex hardware synchronization circuits at the receiver, significantly reducing the implementation cost and system complexity of the underwater node.

[0095] In the description of this invention, it should be understood that terms such as “length”, “width”, “upper”, “lower”, “front”, “rear”, “left”, “right”, “vertical”, “horizontal”, “top”, “bottom”, “inner”, and “outer” indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0096] Furthermore, in the description of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0097] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A laser-induced acoustic underwater communication method based on discrete linear frequency modulated pulse intervals, characterized in that, Includes the following steps: S1. Encoding Mapping: Obtain the binary data sequence to be sent, map the binary symbols to the corresponding pulse time interval patterns, and insert a preset duration of silent protection interval between the pulse sequences corresponding to adjacent binary symbols; wherein, the first type of binary symbol corresponds to a pulse sequence in which the pulse interval decreases linearly with time, and the second type of binary symbol corresponds to a pulse sequence in which the pulse interval increases linearly with time. S2, Laser emission control: Based on the mapped pulse time interval pattern and guard interval, a laser trigger signal is generated; the pulsed laser is driven to emit laser pulses toward the water surface at non-uniform intervals determined by the trigger signal. S3. Acoustic Excitation and Propagation: A laser pulse irradiates the water surface, and a series of acoustic pulses with discrete linear frequency modulation characteristics are excited in the water through the photoacoustic effect. The acoustic pulses are then propagated to the receiving end through an underwater channel. S4. Signal Acquisition: The underwater acoustic transducer at the receiving end acquires underwater acoustic signals and converts them into electrical signals; S5. Correlation Demodulation: Construct a local reference signal corresponding to the pulse time interval pattern described in S1; The acquired electrical signal is cross-correlated with the local reference signal. S6. Decision Output: Detect the correlation peak in the cross-correlation operation result, determine the original binary data based on the position and amplitude of the correlation peak, and complete the communication.

2. The laser-induced acoustic underwater communication method based on discrete linear frequency modulated pulse intervals according to claim 1, characterized in that, In step S1, each binary symbol is set to have a pulse sequence containing N laser pulses, and the preset basic pulse interval is T. base The pulse interval variation step size is δ; In the pulse sequence corresponding to the first type of binary symbol, the first... i The pulse and the first i +1 pulse time interval ΔT i It satisfies the following linear decreasing relationship: ； In the pulse sequence corresponding to the second type of binary symbol, the first... i The pulse and the first i +1 pulse time interval ΔT i It satisfies the following linear increasing relationship: where 1 ≤ i <N and T min is the minimum pulse interval allowed by the laser.

3. The laser-induced acoustic underwater communication method based on discrete linear frequency modulated pulse intervals according to claim 1, characterized in that, In step S1, the silence protection interval is inserted between the pulse sequences corresponding to two adjacent binary symbols, during which the laser remains off; the duration of the silence protection interval is set to be greater than the maximum multipath delay spread of the underwater acoustic channel in the target communication water area, so as to block the interference of the multipath echo of the previous symbol to the current symbol.

4. The laser-induced acoustic underwater communication method based on discrete linear frequency modulated pulse intervals according to claim 1, characterized in that, Step S2 also includes: Before emitting the laser pulse, the vertical distance between the emitting platform and the water surface is measured in real time by the laser ranging module; the focal length of the optical system is dynamically adjusted according to the vertical distance to focus the laser beam on the water-air interface and control the laser spot radius within a preset range to ensure that the laser acoustic process is in the thermal expansion mechanism region. The characteristic radius of the laser beam is R0, the absorption coefficient of the water-air interface is α, and when the laser beam is incident perpendicularly on the water surface, if the following conditions are met... α When R0 << 1, it is a weak absorption region with a radius of R0 and a height of 1 / α A cylinder, where energy can penetrate deeper; when the condition is met... α When R0>>1, strong absorption occurs, and the energy deposition zone is located within the thin disk in contact with the water surface. The generated sound wave is a plane wave parallel to the surface. The relationship between pressure and temperature change is as follows: In the formula, Δ T This represents the change in water temperature. ; α The absorption coefficient is... E 0 represents the surface thermal energy density. ρ 1 represents the initial density of water. C p The specific heat capacity of water at constant pressure; c 1 represents the speed of sound in water. α V is the coefficient of volume expansion of water; Finally, the sound waves propagating from outside the energy deposition zone into the water body are obtained, represented as: In the formula, α The absorption coefficient is... c 1 represents the speed of sound in water. a v is the coefficient of volume expansion of water. E 0 represents the surface thermal energy density. C p The specific heat capacity of water at constant pressure. z Let the coordinates be the vertical water-air interface, and the sgn function be: 。 5. The laser-induced acoustic underwater communication method based on discrete linear frequency modulated pulse intervals according to claim 1, characterized in that, In step S2: The laser pulse output from the laser is split into beams, and a small portion of the energy is used as a reference beam to be introduced into the energy monitoring module to monitor in real time whether the energy intensity of the emitted pulse meets the preset threshold. The remaining main beam is oriented by the optical reflection component and then incident at an angle perpendicular to the water surface to maximize the photoacoustic energy conversion efficiency and reduce the reflection loss of the beam at the water-air interface.

6. A laser-induced acoustic underwater communication method based on discrete linear frequency modulated pulse intervals according to claim 1 or 2, characterized in that, In step S3: The acoustic pulse train with discrete linear frequency modulation characteristics includes several single acoustic pulses with the same duration and the same center frequency. On the time axis, the time interval between two adjacent acoustic pulses is consistent with the laser pulse interval, so that the acoustic pulse train exhibits autocorrelation function characteristics similar to a continuous linear frequency modulated signal in the matched filtering process at the receiving end, and has high distance resolution and noise resistance. When a laser beam is incident perpendicularly on the water surface and causes thermal expansion, the resulting acoustic signal's sound pressure frequency domain expression is: In the formula, r Let ω be the distance from the receiving point to the sound source, and ω be the angular frequency. T The optical transmittance of the liquid. I 0 represents laser intensity. β The coefficient of thermal expansion is... a Where is the laser beam radius, C p Specific heat capacity of the liquid I (ω) represents the laser signal spectrum. E (ω) represents the electromagnetic wave energy density spectrum absorbed and converted into heat per unit time, which can be expressed as: In the formula, k τ is the sound wave number (2π / γ, where γ is the wavelength). μ For the vertical characteristic delay time, τ a The horizontal characteristic delay time is expressed as follows, where θ is the observation angle in the vertical direction, i.e., the angle between r and the vertically downward direction; 。 7. The laser-induced acoustic underwater communication method based on discrete linear frequency modulated pulse intervals according to claim 1, characterized in that, In step S4: The underwater acoustic transducer is a broadband hydrophone, whose response frequency band covers the main energy spectrum range of the laser-induced acoustic signal. Before converting the acquired analog electrical signal into a digital signal, the analog electrical signal is first subjected to bandpass filtering. The passband frequency range of the bandpass filter is set to match the spectral width of a single laser-induced acoustic pulse in order to filter out out-of-band environmental noise. Then, the filtered signal is converted into a digital discrete-time sequence by an analog-to-digital converter at a sampling rate that satisfies the Nyquist sampling theorem, and transmitted to subsequent steps for processing.

8. The laser-induced acoustic underwater communication method based on discrete linear frequency modulated pulse intervals according to claim 2, characterized in that, Step S5 includes: S501, Obtain standard single-pulse waveform The standard acoustic pressure waveform signal Sref(t) excited by a single laser pulse underwater is obtained in advance. This waveform signal is generated either through experimental calibration or based on the photoacoustic conversion theoretical model. S502, Constructing a local reference signal Based on the standard sound pressure waveform signal Sref(t), time delay superposition is performed according to the time interval rule to construct the first local reference signal R0(t) and the second local reference signal R1(t): Wherein, R0(t): consists of N Sref(t), and the time interval between adjacent waveforms satisfies the linear decreasing rule, corresponding to logic "0"; R1(t): consists of N Sref(t), and the time interval between adjacent waveforms satisfies the linear increasing rule, corresponding to logic "1"; S503, Related Operations The digital signal output in step S4 is cross-correlated with R0(t) and R1(t) respectively to obtain the first correlation function output representing the matching degree of logic "0" and the second correlation function output representing the matching degree of logic "1".

9. A laser-induced acoustic underwater communication method based on discrete linear frequency modulated pulse intervals according to claim 8, characterized in that, Step S6 includes: Within each preset symbol period search window, the maximum peak value Vpeak0 of the first correlation function output and the maximum peak value Vpeak1 of the second correlation function output are detected respectively. Compare the magnitudes of Vpeak0 and Vpeak1: If Vpeak0 > Vpeak1, then the currently demodulated binary symbol is determined to be "0"; If Vpeak1 > Vpeak0, then the currently demodulated binary symbol is determined to be "1"; Simultaneously, the time index tpeak corresponding to the winning peak is extracted and marked as the arrival time of the current symbol. Based on tpeak and combined with the preset symbol period duration, the starting position of the search window for the next symbol is calculated, thereby realizing the self-synchronization demodulation of the receiver.

10. A laser-induced acoustic underwater communication device based on discrete linear frequency modulated pulse intervals, characterized in that, The laser-induced underwater communication method based on discrete linear frequency modulated pulse intervals as described in any one of claims 1-9 includes: An aerial platform, which is equipped with unmanned aerial vehicles (UAVs); The signal transmitting end is disposed on the air platform. The signal transmitting end includes a laser, an adjustable focusing optical system, and an energy monitoring module. The adjustable focusing optical system includes an optical reflection component, which includes a beam splitter and a plane mirror. The signal receiving end is equipped with a signal receiving module; A signal processing module, which is connected to the signal receiving end; Laser ranging module; The laser beam is split into two parts by a beam distributor. One part enters the energy monitoring module, and the other part is guided to the water vapor interface by the optical reflection component, focused in a vertical incident manner, and finally received by the signal receiving module.

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