Underwater wireless communication channel pulse time delay calculation simulation method under turbulence condition

Abstract

The invention discloses an underwater wireless communication channel pulse time delay calculation simulation method under a turbulence condition, and the method comprises the steps: simulating the movement condition of photons in a seawater channel through a Monte Carlo method, and obtaining the coordinates, total movement distance, weight and other information of the photons at a receiving end; and then pulse signals of the laser light source from the sending end to the receiving end under different water quality conditions and seawater turbulence conditions are obtained, and the pulse arrival time delay of the light pulse signals in the underwater channel is calculated according to the storage information of the receiving end. According to the underwater channel simulation method provided by the invention, the phase screen model is added, the turbulence condition in the ocean water body is more accurately simulated, and a pulse time delay calculation method is provided according to the photon information of the receiving end, so that the calculated time delay is more in line with the real condition of laser transmission in seawater; and a certain reference is provided for underwater wireless optical communication channel modeling and an underwater laser time service technology.

Description

Technical Field

[0001] The invention relates to the field of underwater laser timing, and in particular to a method for calculating and simulating pulse delay of an underwater wireless communication channel under turbulent conditions. Background Art

[0002] In the marine environment, underwater wireless communication technology is one of the key research and development focuses, especially underwater wireless optical communication technology, which uses the wave nature of light to transmit information. However, the propagation of optical signals underwater will be affected by various factors such as absorption and scattering as well as turbulence, resulting in signal attenuation and propagation distance limitation. Among them, the turbulence effect is one of the key factors affecting the performance of underwater wireless optical communication. Due to its irregular water flow dynamic characteristics, turbulence will cause random fluctuations and strong scattering of optical signals in water, thereby affecting the stability and reliability of optical signals.

[0003] When the laser propagates in the underwater channel, the transmission of the light beam in the underwater link is greatly affected by the seawater environment, resulting in a deviation between the pseudo-range at the receiving end of the optical pulse and the theoretical distance, resulting in an error between the actual arrival time of the pulse and the theoretical arrival time. The timing system is used to synchronize the clocks of the transmitter and the receiver to ensure accurate reception and decoding of data. In underwater laser communication, the accuracy of the timing system is crucial to the performance of the system. The timing system needs to take into account the influence of pulse delay to accurately synchronize the sending and receiving of signals. If the timing system is not accurate enough, it may cause signal decoding errors, which in turn affects the stability of communication and the efficiency of data transmission. Therefore, it is of certain application value to establish an underwater wireless communication channel under turbulent conditions and calculate its pulse delay. Summary of the invention

[0004] The present invention mainly considers the influence of underwater optical communication pulse delay on underwater laser timing technology, and constructs a channel model of seawater that is closer to real conditions through the Monte Carlo method in combination with the influence of turbulence, and proposes a method to calculate the delay of laser pulses when propagating in the underwater channel according to the transmission of photons in the simulated channel.

[0005] A method for calculating pulse delay of underwater wireless communication channel based on turbulence is characterized by comprising the following steps:

[0006] (1) A Gaussian beam is generated by given parameters such as the number of photons from a Gaussian light source, the beam waist radius, and the emission angle.

[0007] (2) Sampling the Gaussian light source generated in step (1), determining the initial position and azimuth of the photons, and generating a scattering angle.

[0008] (3) Generate a random step size based on the attenuation coefficient of seawater, which includes the absorption coefficient and the scattering coefficient.

[0009] (4) Generate a phase screen.

[0010] (5) Determine whether the photon has reached the phase screen based on the photon's coordinates, direction, and random step size. If it has reached the phase screen, proceed to step (6); if it has not reached the phase screen, proceed to step (7).

[0011] (6) After the photon passes through the phase screen, the photon coordinate direction and step length are updated, and the photon movement step length is accumulated and counted into the photon movement distance.

[0012] (7) Determine whether the photon reaches the receiving surface. If it reaches the receiving surface, proceed to step (8); if it does not reach the receiving surface, proceed to step (9).

[0013] (8) Record the coordinate information of the photons arriving at the receiving surface, the total movement distance of the photons, and the weight value of the photons, and mark that the photons are received.

[0014] (9) The photon scatters again, selects a new azimuth and scattering angle, updates the photon's coordinates and weight information, and adds the photon's moving step length to the photon's travel distance.

[0015] (10) The weight value of the rescattered photon is compared with the set survival threshold. If it is less than the threshold, the photon is determined to be dead and tracking is stopped. If it is greater than the threshold, return to step (2) and loop through steps (3) to (10) until all photons are received.

[0016] (11) After N times, stop the simulation and record the coordinate information (x, y, z) of the photon arriving at the receiving end, the total movement distance S of the photon, and the weight value W.

[0017] (12) Based on the coordinates, total movement distance, weight and other information of the photons at the receiving end, the transmission delay of the photons in the underwater channel is calculated.

[0018] Furthermore, the beam shape in step (1) is a Gaussian beam based on an LD light source.

[0019] Furthermore, the phase screen in step (4) is generated by power spectrum inversion method.

[0020] Furthermore, the random step length, scattering angle and azimuth angle of photons in step (3) and step (6) are all random numbers obtained by the Monte Carlo method.

[0021] Compared with the existing technology, the present invention has the following characteristics:

[0022] The present invention proposes a method for calculating and simulating the pulse delay of an underwater wireless communication channel under turbulent conditions. The absorption and scattering of seawater are taken into consideration, and the underwater turbulence is simulated in combination with a phase screen, so as to more accurately simulate the complex water conditions in the sea area. The motion trajectory, coordinate information, and weight information of the photons are obtained at the receiving end, and a method is proposed to obtain the pulse delay of the laser light source from the transmitting end to the receiving end under different water quality conditions and seawater turbulence, so as to provide a basis for underwater wireless laser timing calculation. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A schematic diagram of the steps of a method for calculating and simulating pulse delay of an underwater wireless communication channel under turbulent conditions provided by an embodiment of the present invention;

[0024] Figure 2 A block diagram of the underwater wireless optical communication system solution of the present invention;

[0025] Figure 3 This is a diagram of the underwater pulse signal transmission process;

[0026] Figure 4 A schematic diagram of the simulation results of pulse arrival delays at different transmission distances under weak turbulence provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0027] The implementation process of the present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0028] The simulation method proposed in the present invention is mainly applied to underwater laser communication systems and underwater laser timing technology.

[0029] like Figure 1 As shown, the present invention proposes a method for calculating and simulating the pulse delay of an underwater wireless communication channel under turbulent conditions, which includes:

[0030] The initial coordinates, azimuth and scattering angle of the photons are determined according to the type of light source, and a random step size is generated according to the attenuation coefficient of seawater.

[0031] The phase screen is generated according to the power spectrum inversion method to simulate the turbulent seawater conditions.

[0032] Set the parameters required for simulation and simulate the scattering process of photons, calculate the position of photons after each collision, and determine whether the photons reach the phase screen and the receiving surface. After reaching the receiving surface, store their coordinate position, movement distance and weight.

[0033] After the scattering process of all photons is completed, the coordinate positions, movement distances and weights of all photons arriving at the receiving surface are counted according to the stored data. The weights of all photons arriving at the receiving surface are accumulated and multiplied by the power of a single photon to obtain the received power. The arrival time of the light pulse when the received power is half is calculated, and the light pulse arrival delay is calculated by combining the coordinate position and movement distance of the photon.

[0034] Example 1

[0035] like Figure 2 As shown, a method for calculating and simulating pulse delay of underwater wireless communication channels under turbulent conditions specifically includes the following steps:

[0036] (1) A Gaussian beam is generated by given the number of photons from a Gaussian light source, the beam waist radius, the emission angle, etc.

[0037] (2) Sampling the Gaussian light source generated in step (1), determining the initial position and azimuth of the photons, and generating a scattering angle.

[0038] Assume that the initial position of the photon is in the xy plane with z=0, and take the right direction of the z axis as the positive direction. Record the angles between the photon unit transmission direction vector and the x, y, and z axes, which are denoted as θ respectively. x ,θ y and θ z The direction vector of the unit vector in the propagation direction on the x, y, and z axes is denoted by μ x , μ y and μ z

[0039]

[0040] The scattering angle θ0 = -r0 / f1 is determined according to the selected light source, where f1 = -ω0 / φ div / 2 , ω0 is the beam width, φ div / 2 is the divergence half angle, r0 is the radius sampling value Initial azimuth rnd1 and rnd2 represent two random numbers in the range (0, 1), and the initial coordinates of the photon can be obtained as:

[0041]

[0042] The initial motion direction vector is expressed as:

[0043]

[0044] Generate the scattering angle β(θ) according to the HG function:

[0045]

[0046] Where: g is the symmetry factor, θ is the scattering angle. When g = 0.924, it can more accurately describe the characteristics of seawater and obtain a more accurate fitting effect.

[0047] (3) Generate a random step size based on the attenuation coefficient of seawater, which includes the absorption coefficient and the scattering coefficient.

[0048] The random step length r of a photon is determined by the survival probability of the photon, and its calculation formula is:

[0049]

[0050] Where P l (l) Take a random number uniformly distributed between 0 and 1, c is the attenuation coefficient of seawater, c = a + b, where a is the absorption coefficient and b is the attenuation coefficient.

[0051] (4) Generate a phase screen.

[0052] A phase screen and a receiving screen are placed in sequence along the plane of z=0, with the phase screen being located between the initial position and the receiving screen, with the z-axis transmitting direction to the right being the positive direction.

[0053] The distribution function calculation formula of the random phase screen is:

[0054]

[0055] where k x and k y is the optical wave number in the x and y directions of the phase space, C is the constant factor controlling the variance of the phase screen, h(k x , k y ) is the Fourier transform of a random number with mean 0 and variance 1.

[0056] is the phase power spectrum, which is calculated as follows:

[0057]

[0058] Where k is the wave number of light in vacuum, Δz is the distance between phase screens, φ(k x , k y ) is the refractive index fluctuation spectrum of seawater, which is calculated as follows:

[0059]

[0060] The individual expressions are as follows:

[0061]

[0062] where ε is the turbulent kinetic energy dissipation rate, η is the Kolmogorov scale, ω is the ratio of temperature and salinity gradients, and χT is the mean square temperature dissipation rate, and the other parameters are:

[0063] A T =1.863×10 -2 , As = 1.9 × 10 -4 , A TS =9.41×10 -3

[0064] (5) Determine whether the photon has reached the phase screen based on the photon's coordinates, direction, and random step size. If it has reached the phase screen, proceed to step (6); if it has not reached the phase screen, proceed to step (7).

[0065] (6) After the photon passes through the phase screen, the photon coordinate direction and step size are updated.

[0066] Update the direction vector of the photon after passing through the phase screen:

[0067]

[0068] where μ x , μ y and μ z are the direction vectors of the photons before scattering.

[0069] Update the coordinates of the photon on the phase screen to:

[0070]

[0071] Among them, x m and m are the coordinates of the photon before scattering, z phase is the position of the phase screen on the Z axis.

[0072] Update the remaining step length of photons r0 = r-(z phase -z add ) / μ z , where μ z is the z-axis direction vector of the photon.

[0073] The coordinates of the photon are:

[0074]

[0075] (7) Determine whether the photon reaches the receiving surface. If it reaches the receiving surface, proceed to step (8); if it does not reach the receiving surface, proceed to step (9).

[0076] (8) Record the coordinate information of the photons arriving at the receiving surface, the total movement distance of the photons, and the weight value of the photons, and mark that the photons are received.

[0077] (9) The photon scatters again, selects a new azimuth and scattering angle, updates the photon's coordinates and weight information, and adds the photon's moving step length to the photon's travel distance.

[0078] When the photon is scattered for the next time, the direction vector of the photon can be obtained as:

[0079]

[0080] where μ xm , μ ym and μ zm is the next scattering direction vector, θ s is the angle between the next scattering step direction and the previous scattering step direction, is the azimuth of the next scattering.

[0081] When μ zm =1 simplifies to:

[0082]

[0083] Where sign is a sign function that satisfies:

[0084]

[0085] Calculate the weight value ω after scattering n =ω n-1 (b / c), where the initial weight of all photons is ω=1, n is the number of scatterings, b / c is the power loss coefficient when scattering occurs, b is the seawater scattering coefficient, c is the seawater attenuation coefficient, ω n-1 is the weight value of the last scattering;

[0086] (10) The weight value of the rescattered photon is compared with the set survival threshold. If it is less than the threshold, the photon is determined to be dead and tracking is stopped. If it is greater than the threshold, return to step (2) and loop through steps (3) to (10) until all photons are received.

[0087] (11) After N times, stop the simulation and record the coordinate information (x, y, z) of the photon arriving at the receiving end, the total movement distance S of the photon, and the weight value W.

[0088] (12) Based on the information of the photons at the receiving end, calculate the transmission delay of the photons in the underwater channel.

[0089] Combination Figure 2 Given the underwater pulse signal transmission process diagram, the pulse arrival delay Δt of the optical pulse signal in the underwater channel is calculated as follows:

[0090]

[0091] Where t1 is the emission time of the transmitter, t2 is the half-power time of the received light pulse, D is the ballistic light distance between the light transmitter and the light receiver, that is, the straight-line distance, C is the speed of light propagation underwater, D / C is the time for photons to propagate in a straight line without refraction in the underwater channel, T HW It is the period from the initial pulse from the transmitting end to the pulse half-height.

[0092] The simulation is stopped after N times, and the received power is obtained by multiplying the accumulated value of the received photon weight by the power of a single photon, where N is the number of sampled photons.

[0093] The power P of a single photon is calculated as follows:

[0094]

[0095] Where h is Planck's constant, which is 6.6260693×10 -34 , λ is the wavelength of incident light, which is 532nm.

[0096] Calculate the received power of each photon when it arrives at the receiver, and use interpolation calculation to find the photon that arrives when the received power is half. The time when this photon arrives at the receiver is the reception time of the light pulse.

[0097] The Monte Carlo random simulation method in step (12) above can be used to obtain the distance S of the photon reaching the receiving end, as well as the photon coordinates (x, y, z), where S is the accumulation of the step length between each scattering. The time t1 when the transmitter sends the pulse signal is the initial value 0, and t2 is the half-power moment of the received light pulse, which can be obtained

[0098] Δt can be further simplified as:

[0099] Calculate the straight-line distance D that the photon travels in the channel:

[0100]

[0101] Among them, x0, y0, z0 are the coordinates of the center of the transmitter. The coordinates of the transmitter default to the origin of the coordinate system. At this time, x0=y0=z0=0

[0102] The expression of D can be simplified to:

[0103]

[0104] The simplified Δt can be obtained:

[0105]

[0106] Taking the normalized power Gaussian pulse as an example, THW =30.39t, t is the pulse duration.

[0107] Among them, in steps (1) to (7), the sampling photon coordinates and the photon free step length are determined by the Monte Carlo random simulation method. As the number of simulations increases, the average value of each statistic can be taken to obtain a more accurate conclusion, avoiding the complex calculation of a large number of photons.

[0108] Among them, when the signal source 0 and 1 are transmitted with equal probability, the optimal decision threshold level is half of the maximum value of the pulse, so the half-power moment of the received optical pulse is defined as the pulse reception moment.

[0109] Example 2

[0110] In this embodiment, the positive direction of the Z axis is the light transmission direction, Z=0 is the plane where the light source is located, the transmission distance is set to 1-14m, the number of photons is 1000000, the initial photon weight ω is 1, and the propagation speed C in water is 2.248×10 8 , the wavelength λ is 532mm, the beam width ω0 of the Gaussian beam is 20mm, and the field of view of the receiving end is 90°. Phase screens are set at intervals of 0.5m on the Z axis, and the size of the phase screen is 3mX3m. The turbulence environment is set to weak turbulence, and the specific parameters are turbulence kinetic energy dissipation rate ε=10 -5 m 2 ·s -3 , the ratio of temperature and salinity gradient ω = -2.5, the mean square temperature dissipation rate x T =10 -7 K 2 ·S -1 , Kolmogorov scale η = 10 -3 , Figure 4 A schematic diagram of a pulse delay simulation result at a receiving end provided in an embodiment of the present invention.

Claims

1. A method for calculating and simulating pulse delay of underwater wireless communication channels under turbulent conditions, characterized in that: The following steps are involved: (1) A Gaussian beam is generated by given the number of photons from a Gaussian light source, the beam waist radius, the emission angle, etc. (2) Sampling the Gaussian light source generated in step (1), determining the initial position and azimuth of the photons, and generating a scattering angle. (3) Generate a random step size based on the attenuation coefficient of seawater, which includes the absorption coefficient and the scattering coefficient. (4) Generate a phase screen. (5) Determine whether the photon has reached the phase screen based on the photon's coordinates, direction, and random step size. If it has reached the phase screen, proceed to step (6); if it has not reached the phase screen, proceed to step (7). (6) After the photon passes through the phase screen, the photon coordinate direction and step length are updated, and the photon movement step length is accumulated and counted into the photon movement distance. (7) Determine whether the photon reaches the receiving surface. If it reaches the receiving surface, proceed to step (8); if it does not reach the receiving surface, proceed to step (9). (8) Record the coordinate information of the photons arriving at the receiving surface, the total movement distance of the photons, and the weight value of the photons, and mark that the photons are received. (9) The photon scatters again, selects a new azimuth and scattering angle, updates the photon's coordinates and weight information, and adds the photon's moving step length to the photon's travel distance. (10) The weight value of the rescattered photon is compared with the set survival threshold. If it is less than the threshold, the photon is determined to be dead and tracking is stopped. If it is greater than the threshold, return to step (2) and loop through steps (3) to (10) until all photons are received. (11) After N times, stop the simulation and record the coordinate information (x, y, z) of the photon arriving at the receiving end, the total movement distance S of the photon, and the weight value W. (12) Based on the coordinates, total movement distance, weight and other information of the photons at the receiving end, the transmission delay of the photons in the underwater channel is calculated.

2. The method for calculating and simulating pulse delay of an underwater wireless communication channel under turbulent conditions as claimed in claim 1 is characterized in that The phase screen in step (4) is generated by power spectrum inversion method.

3. The method for calculating and simulating pulse delay of underwater wireless communication channels under turbulent conditions as claimed in claim 1 is characterized in that In step (5), the phase screen judgment is added to the Monte Carlo simulation process to establish an underwater laser channel model that takes into account more seawater environmental influences, so that the calculated pulse delay is more realistic.

4. The method for calculating and simulating pulse delay of underwater wireless communication channels under turbulent conditions as claimed in claim 1 is characterized in that The random step length, scattering angle and azimuth angle of photons in step (3) and step (6) are all random numbers obtained using the Monte Carlo method.

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