A method of azimuth angle measurement based on hydroacoustic orbital angular momentum

Abstract

The present invention discloses an azimuth angle measurement method based on underwater acoustic orbital angular momentum, which is applicable to the application field of vortex waves carrying orbital angular momentum. The method is based on the wavefront distribution characteristics of vortex waves carrying special orbital angular momentum. Two coaxially transmitted superposition waves with orbital angular momentum having the same topological charge value but opposite signs and having a rotational angular velocity are generated by a uniform circular array, so that the azimuth angle of the "dark diameter" of the superposition wave in the signal receiving plane changes uniformly. A data signal received by a single target receiver located in the signal receiving plane is analyzed and processed to obtain a low-frequency signal containing envelope information of the received data signal, which is then correlated with a reference signal to obtain the peak position of the spectrum line. Finally, the target azimuth angle is calculated through the formula relationship between the peak position of the spectrum and the azimuth angle.

Description

Technical Field

[0001] The invention relates to an application field of vortex waves carrying orbital angular momentum, and in particular to an azimuth angle measurement method based on underwater acoustic orbital angular momentum. Background Art

[0002] Vortex waves carrying orbital angular momentum (OAM) have the characteristics of annular intensity distribution and spiral phase distribution with a "dark area" in the center. It has been proved that vortex waves carrying different integer multiple topological charges, that is, vortex waves carrying different OAM modes, are orthogonal. Theoretically, infinite OAM modes provide a new degree of freedom for multiplexed communication technology, with great potential to increase communication capacity exponentially, and have been widely studied in the field of modal multiplexing communication technology. At the same time, because its beam has a vortex shape, when the acting wave is a mechanical wave (such as a sound wave), it can generate a force pointing to the center of the object, so the acoustic manipulation technology based on orbital angular momentum came into being. However, the azimuth information contained in the orbital angular momentum of the vortex wave has not been well utilized. Although some studies have combined orbital angular momentum with radar detection and imaging technology, the application of orbital angular momentum in underwater acoustic positioning is still blank. Summary of the invention

[0003] The purpose of the present invention is to address the problem that the technical field of positioning methods based on orbital angular momentum is still blank in the prior art, and to propose an azimuth angle measurement method based on underwater acoustic orbital angular momentum, which expands the application potential of orbital angular momentum in positioning. By utilizing the intensity and phase distribution characteristics of the shear wave carrying orbital angular momentum, only the single-dimensional received signal of the target is analyzed and processed, so as to realize the measurement of the azimuth angle of the target in the line-of-sight scenario, and greatly reduce the complexity of digital signal processing in the positioning process.

[0004] The object of the present invention is achieved through the following technical solution: a method for measuring azimuth angle based on hydroacoustic orbital angular momentum, the method comprising the following steps:

[0005] S1: The transmitter of the positioning signal generates two coaxially transmitted vortex waves carrying orbital angular momentum with the same topological charge value but opposite signs. The two vortex waves carry exactly the same positioning signal, forming a superposition wave with a "dark path". The "dark path" is a narrow and long area at a certain azimuth angle in the propagation direction of the superposition wave, in which the signal amplitude is constantly zero;

[0006] S2: The two vortex waves carrying orbital angular momentum generated by the transmitter of the positioning signal are given a fixed rotational angular velocity ω1 and ω2 respectively, so that the phase difference Δφ between the two changes at a uniform speed, so that the superimposed wave has a rotational angular velocity ω overlay ;

[0007] S3: After the superimposed wave is transmitted in the free space, it is sampled and received by the target single receiver in the receiving plane perpendicular to the propagation axis and where the target to be located is located, and the corresponding received signal data is obtained;

[0008] S4: Extract the envelope signal characteristics of the received signal data, correlate the signal containing the envelope information with the reference signal to obtain the correlation spectrum peak, and then calculate the azimuth angle of the positioned target through the formula relationship between the spectrum peak position and the azimuth angle.

[0009] Furthermore, the S1 is specifically:

[0010] (1.1) The transmitting end is specifically any device or antenna array that can generate a coaxially transmitted vortex wave carrying a specific orbital angular momentum, such as a cascaded spiral phase plate, an acoustic hyperplane, or a uniform circular array combined with a digital phase shift network;

[0011] (1.2) The coaxially transmitted vortex waves carrying orbital angular momentum with the same topological charge but opposite signs are, specifically, the two vortex waves generated by the transmitting end have the same propagation direction, the centers of the vortex wave beams coincide, and the wavefront phase of the vortex wave carrying the orbital angular momentum is vortex-distributed, which can be expressed by the phase factor Represents, where l is the topological charge, is the spatial azimuth, i is a complex factor, the sign of the topological charge indicates the direction of the vortex phase distribution, and the phase is set to rotate counterclockwise as negative and the phase to rotate clockwise as positive;

[0012] (1.3) The two vortex waves carry exactly the same positioning signal. Specifically, when the two vortex waves generated by the transmitter are superimposed, it should be ensured that the amplitude and phase of the positioning signals carried by the two vortex waves are completely consistent from the moment of superposition, and only the signs of the topological charges of the orbital angular momentum they carry are different.

[0013] (1.4) The positioning signal is specifically a signal used for positioning composed of a synchronization header, a protection interval and a positioning symbol sequence. The positioning symbol sequence can be a constant envelope signal such as a single-frequency sinusoidal signal, a frequency shift keying (FSK) modulation signal, a phase shift keying (PSK) modulation signal, etc.;

[0014] (1.5) The azimuth angle of the “dark diameter” passing through the center of the circle can be determined by the topological charge, initial phase difference, and rotational angular velocity of the two waves carrying orbital angular momentum that constitute the superposition wave.

[0015] Furthermore, the S2 is specifically:

[0016] (2.1) The two fixed rotational angular velocities ω1 and ω2 respectively assigned to the two vortex waves are specifically to increase the phase increments ω1t and ω2t at the phase factors of the two vortex waves respectively, where t represents time, and at this time, the two vortex waves are in a state of uniform rotation around the propagation axis;

[0017] (2.2) The phase difference Δφ between the two changes at a constant speed. Specifically, the relationship between the phase difference Δφ and ω1 and ω2 in S2 can be expressed as Δφ=ω1t-ω2t=(ω1-ω2)t;

[0018] (2.3) The superimposed wave has an angular velocity ω overlay , specifically

[0019] Furthermore, the S3 is specifically:

[0020] (3.1) The free space is specifically a spatial state in which the influence of reflected waves can be ignored within the propagation range of waves carrying orbital angular momentum;

[0021] (3.2) The receiving plane is the cross section of the vortex wave carrying orbital angular momentum, containing spatial phase and amplitude information;

[0022] (3.3) The target single receiver is specifically a positioning target that can only receive one signal and has only a single antenna or a single hydrophone for positioning;

[0023] (3.4) The corresponding received signal data is specifically a sampled signal received at the target single receiver.

[0024] Furthermore, the S4 is specifically:

[0025] (4.1) Extracting the envelope signal characteristics is specifically to obtain a signal containing envelope information of the received signal data by filtering or non-coherent demodulation method;

[0026] (4.2) The reference signal is specifically a reference signal designed according to the selected orbital angular momentum topological charge, the superposition wave rotation angular velocity and the initial phase difference between the two beams. The front end of the reference signal is a synchronization head, which is used for time synchronization and determines the initial receiving time of the received signal. After that, a guard interval is set according to experience to separate the synchronization head and the positioning symbol sequence. The positioning symbol sequence after the guard interval is a standard sinusoidal signal.

[0027] (4.3) The formula relationship between the peak position and the azimuth angle is as follows: the phase φ′ of the restored single-frequency sinusoidal signal is calculated based on the peak position. If the initial azimuth angle of the “dark diameter” is The target azimuth is Then there is

[0028] The beneficial effects of the present invention are as follows: the present invention utilizes sound waves carrying orbital angular momentum to realize the measurement of target azimuth angle, broadens the application scope and application scenarios of orbital angular momentum, reduces the complexity of the positioning algorithm at the receiving end, and can also simultaneously perform communication and angle measurement positioning under the constant envelope modulation mode. This multi-transmitter and single-receiver passive positioning method can make up for the shortcomings of the commonly used attitude sensor positioning method, assist in eliminating the positioning error accumulated over time when the attitude sensor works for a long time, and increase the independent working time of the autonomous equipment underwater. In addition, this azimuth angle measurement method based on underwater acoustic orbital angular momentum can realize covert positioning without exposing the position of the target, and is a new positioning method. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a flowchart of the azimuth angle measurement method based on hydroacoustic orbital angular momentum;

[0030] Figure 2 This is a system block diagram of the azimuth angle measurement method based on hydroacoustic orbital angular momentum;

[0031] Figure 3 This is the intensity and phase distribution diagram of the superimposed OAM wave with topological charge of ±2 at different rotation phases.

[0032] Figure 4 It is the structural design of reference signal and positioning signal. DETAILED DESCRIPTION

[0033] The present invention is further described below in conjunction with the accompanying drawings and specific examples, but the implementation and protection scope of the present invention are not limited thereto.

[0034] The present invention provides an azimuth angle measurement method based on underwater acoustic orbital angular momentum, which is suitable for azimuth angle measurement of a target with only a single receiver in free space. The main contents are as follows: Figure 1 The flowchart of the azimuth angle measurement method based on hydroacoustic orbital angular momentum is shown in FIG. The specific steps are based on Figure 2 The system block diagram of the azimuth angle measurement method based on hydroacoustic orbital angular momentum is shown.

[0035] The specific implementation of the transmitter is as follows:

[0036] 1. Design positioning signal such as Figure 4As shown in (a), the front end of the signal is a synchronization header, which can be specifically composed of a linear frequency modulation symbol or an m-sequence. There is a short protection interval in the middle, which is usually not less than one tenth of the length of the synchronization header. After the protection interval is a positioning symbol sequence designed based on the orbital angular momentum angle measurement method. The positioning symbol sequence can be a single-frequency sinusoidal signal that does not carry information, a frequency shift keying (FSK) modulation signal that carries information, a phase shift keying (PSK) modulation signal that carries information, etc., which can achieve positioning while communicating;

[0037] 2. Through the digital phase shift network, the positioning signal s input to the nth array element carries the phase factor Where l and -l are a pair of topological charges of orbital angular momentum with opposite signs and the same value, is the azimuth angle of the nth array element relative to the reference line, i is a complex factor, l is the topological charge number, and the digital phase shift network specifically refers to the phase shift operation performed on the digital signal to be sent on the computer.

[0038] In order to make the two vortex waves carrying orbital angular momentum generated by the transmitter rotate uniformly around the propagation axis, a phase factor ωt that changes uniformly with the transmission time is added to the phase of each sampling point in the positioning signal to be sent by each hydroacoustic transducer at the digital phase shift network. ω is the rotational angular velocity of the corresponding waveform, t represents time, and ω1 and ω2 represent the angular velocity of the wave with a topological charge of ±l, respectively. The phase difference between the two can be expressed as

[0039] Δφ=φ1+ω1t-(ω2t+φ2)=(ω1-ω2)t+(φ1-φ2) (1)

[0040] Where φ1 and φ2 are the initial phases of the two waves respectively; the rotational angular velocity ω of the superimposed wave overlay It can be derived and calculated from its phase expression:

[0041] The signal emitted by the nth hydroacoustic transducer can be expressed as:

[0042]

[0043] therefore

[0044] The superimposed wave has a rotation phase factor of 0, The phase and intensity distribution at π is as follows Figure 3 shown.

[0045] 3. The signal carrying the orbital angular momentum with topological charge l is composed of the phase factor The positioning signal is generated by the orbital angular momentum carrying the topological charge of -l, and the signal carrying the phase factor The positioning signal is generated.

[0046] 4. Set the radius to R t , the number of array elements is N t A uniform circular array is used as the transmitting end, in which the array elements are underwater acoustic transducers. The uniform circular array is an antenna arrangement method for generating waves carrying orbital angular momentum, specifically referring to the array elements being uniformly arranged on a circle with a certain radius, and all the array elements constitute an transmitting plane; the vertical upward direction of the center of the uniform circular array is set as the reference line, and the azimuth angle of the points on the reference line on the transmitting plane is regarded as zero; the transmitting plane specifically refers to the plane where the uniform circular array of the transmitting end is located, and the reference line is also on this plane.

[0047] 5. The positioning signal carrying the phase factor described in step 3 is converted into an acoustic signal by the corresponding hydroacoustic transducer on the uniform circular array and sent. The two waves carrying orbital angular momentum with topological charge of ±1 respectively generated by the same uniform circular array are coaxial transmission. At this time, the wave emitted by the uniform circular array is the superposition wave of the "dark diameter" passing through the center of the circle.

[0048] The specific implementation of the receiving end is as follows:

[0049] 1. Use a single receiver to sample and receive the superimposed wave to obtain the corresponding received signal data, specifically:

[0050] Establish a cylindrical coordinate system, the coordinates of a point P in the field are Where ρ represents the length of the line connecting the projection of point P on the XOY coordinate plane and the origin, It represents the angle between the projection of point P on the XOY coordinate plane and the line connecting the origin and the positive direction of the X-axis, also known as the azimuth of point P. D represents the distance from point P to the XOY coordinate plane, also known as the propagation distance of the OAM wave from the transmitting array to point P. When M positioning signals s carrying orbital angular momentum are sent, the signal received at this point It can be expressed as

[0051]

[0052] where λ is the wavelength, k is the wave number, i is the complex factor, r is the position vector of point P, and r n is the position vector of the nth hydroacoustic transducer, f c is the carrier frequency, is the azimuth angle of the nth hydroacoustic transducer, Indicates l m The first-kind Bessel function of order |rr n | represents the distance between point P and the nth hydroacoustic transducer. When D>>R t When, item The denominator of |rr n | can be replaced by r, and |rr in the numerator represents the phase n |Available Instead, α is the oblique angle of point P, and there is

[0053] For the sake of convenience, we only consider two topological charges l and -l, where M = 2 and l m ∈{l,-l}, according to the properties of the Bessel function, when l is an integer, J -n (x) = (-1) n J n (x), so formula (3) can be further expressed as

[0054]

[0055] It can be found that when the topological charges in the transmitted superposition waves have the same value but opposite signs, the sign intensity is modulated by a cosine signal whose frequency is only related to the phase angle between l and the target point. Therefore, when a wave carrying orbital angular momentum with a known topological charge is sent, the amplitude of the wave at the diameter passing through the center of the circle at a specific azimuth angle is always zero, forming a "dark diameter".

[0056] 2. Process the received signal data and extract its envelope signal characteristics. Correlate the low-frequency signal containing envelope information with the reference signal to obtain the relevant spectrum peak. The reference signal is designed as Figure 4 As shown in (b), the design of the synchronization header and the guard interval is exactly the same as the reference signal, and the reference symbol sequence is a standard sinusoidal signal.

[0057] 3. The target azimuth angle is calculated through the formula relationship between the spectrum peak position and the azimuth angle.

[0058] The superimposed wave with rotational angular velocity generated in the above manner is generated and emitted by a uniform circular array, and propagates through free space to the receiving plane where the target is located. The free space specifically refers to the propagation range of the wave carrying the orbital angular momentum, which is almost not affected by the reflected wave. For example, when the reflection of the wave by the reflecting surface such as the sea surface and the seabed is negligible or the propagation path of the reflected wave does not include the location of the target, only the wave through the direct path can reach the target; the superimposed wave is sampled and received by a single receiver on the target to obtain a single set of received data carrying the location information;

[0059] The advantage of the present invention is that it utilizes the intensity and phase distribution characteristics of the vortex wave carrying orbital angular momentum, and only analyzes and processes the single-dimensional received signal of the target, so as to realize the measurement of the target azimuth in the line-of-sight scenario, which greatly reduces the complexity of digital signal processing in the positioning process, simplifies the calculation process, and can simultaneously perform communication and angle measurement, which is innovative in theory. This multi-transmitting and single-receiving passive positioning method can make up for the shortcomings of the commonly used attitude sensor positioning method, assist in eliminating the positioning error accumulated over time when the attitude sensor works for a long time, and increase the independent working time of the autonomous equipment underwater. In addition, this azimuth angle measurement method based on hydroacoustic orbital angular momentum can realize concealed positioning without exposing the position of the target, and is a new positioning method.

[0060] Those skilled in the art can understand that the above are only preferred examples of the invention and are not intended to limit the invention. Although the invention is described in detail with reference to the above examples, those skilled in the art can still modify the technical solutions recorded in the above examples or replace some of the technical features therein with equivalents. Any modification, equivalent replacement, etc. made within the spirit and principle of the invention shall be included in the protection scope of the invention.

Claims

1. A method for azimuth angle measurement based on hydroacoustic orbital angular momentum, characterized in that: The method comprises the following steps: S1: The transmitter of the positioning signal generates two coaxially transmitted vortex waves carrying orbital angular momentum with the same topological charge value but opposite signs. The two vortex waves carry exactly the same positioning signal, forming a superposition wave with a "dark path". The "dark path" is a narrow and long area at a certain azimuth angle in the propagation direction of the superposition wave, in which the signal amplitude is always zero; the details are as follows: (1.1) The transmitting end is specifically a uniform circular array that can generate coaxially transmitted vortex waves carrying a specific orbital angular momentum; (1.2) The coaxially transmitted vortex waves carrying orbital angular momentum with the same topological charge but opposite signs are, specifically, the two vortex waves generated by the transmitter have the same propagation direction, the centers of the vortex wave beams coincide, and the wavefront phase of the vortex wave carrying the orbital angular momentum is vortex-distributed, which is determined by the phase factor Represents, where l is the topological charge, is the spatial azimuth, i is a complex factor, the sign of the topological charge indicates the direction of the vortex phase distribution, and the phase is set to rotate counterclockwise as negative and the phase to rotate clockwise as positive; (1.3) The two vortex waves carry exactly the same positioning signal. Specifically, when the two vortex waves generated by the transmitter are superimposed, it should be ensured that the amplitude and phase of the positioning signals carried by the two vortex waves are completely consistent from the moment of superposition, and only the signs of the topological charges of the orbital angular momentum they carry are different. (1.4) The positioning signal is specifically a signal used for positioning composed of a synchronization header, a protection interval and a positioning symbol sequence, and the positioning symbol sequence is selected from the following constant envelope signals: a single-frequency sinusoidal signal, a frequency shift keying (FSK) modulation signal, and a phase shift keying (PSK) modulation signal; (1.5) The azimuth angle of the "dark diameter" passing through the center of the circle is determined by the topological charge, initial phase difference, and rotational angular velocity of the two waves carrying orbital angular momentum that constitute the superposition wave; S2: The two vortex waves carrying orbital angular momentum generated by the transmitter of the positioning signal are given a fixed rotational angular velocity ω1 and ω2 respectively, so that the phase difference Δφ between the two changes at a uniform speed, so that the superimposed wave has a rotational angular velocity ω overlay ; The details are as follows: (2.1) Two fixed rotational angular velocities ω1 and ω2 are assigned to the two vortex waves, respectively. Specifically, phase increments ω1t and ω2t are added to the phase factors of the two vortex waves, respectively, where t represents time. At this time, the two vortex waves are in a state of uniform rotation around the propagation axis; (2.2) The phase difference Δφ between the two changes at a constant speed. Specifically, the relationship between the phase difference Δφ and ω1 and ω2 in S2 is expressed as Δφ=ω1t-ω2t=(ω1-ω2)t; (2.3) The superimposed wave has an angular velocity ω overlay , specifically ; S3: After the superimposed wave is transmitted in free space, it is sampled and received by a single target receiver in a receiving plane perpendicular to the propagation axis where the target to be located is located, and the corresponding received signal data is obtained; the details are as follows: (3.1) The free space is specifically a spatial state in which the influence of reflected waves can be ignored within the propagation range of waves carrying orbital angular momentum; (3.2) The receiving plane is the cross section of the vortex wave carrying orbital angular momentum, containing spatial phase and amplitude information; (3.3) The target single receiver is specifically a positioning target that can only receive one signal and has only a single antenna or a single hydrophone for positioning; (3.4) the corresponding received signal data, specifically the sampled signal received at the target single receiver; S4: extracting the envelope signal characteristics of the received signal data, correlating the signal containing the envelope information with the reference signal to obtain the relevant spectrum peak, and then calculating the azimuth angle of the positioning target through the formula relationship between the spectrum peak position and the azimuth angle; the details are as follows: (4.1) Extracting the envelope signal characteristics of the received signal data specifically comprises obtaining a signal containing the envelope information of the received signal data by filtering or non-coherent demodulation method; (4.2) The reference signal is specifically a reference signal designed according to the selected orbital angular momentum topological charge, the superposition wave rotation angular velocity and the initial phase difference between the two beams. The front end of the reference signal is a synchronization head, which is used for time synchronization and determines the initial receiving time of the received signal. After that, a guard interval is set according to experience to separate the synchronization head and the positioning symbol sequence. The positioning symbol sequence after the guard interval is a standard sinusoidal signal. (4.3) The formula relationship between the peak position and the azimuth angle is as follows: the phase φ′ of the restored single-frequency sinusoidal signal is calculated based on the peak position. If the initial azimuth angle of the "dark diameter" is The target azimuth is Then there is

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