A method for sensing atmospheric channel state in laser communication

Abstract

The present invention discloses an atmospheric channel state sensing method for laser communication, which belongs to the technical field of atmospheric laser communication. The method of the present invention places two photodetectors at positions d1 and d2 away from the receiving optical axis on the plane of the receiving end, respectively, and detects the refractive index structure constant and the extinction coefficient by statistically analyzing the mean value of the optical power fluctuations and combining the long-period average light intensity of the dynamic optical link; according to the theoretical derivation results of the power fluctuation probability density function, the power fluctuation probability density function on the receiving optical axis can be detected by the optical power fluctuations on the two photodetectors; combined with the detected refractive index structure constant, extinction coefficient, and power fluctuation probability density function, the average bit error rate of laser communication is evaluated and predicted, overcoming the technical problem that the bit error rate evaluation method in the prior art has a data transmission and reception time of up to several hours and is not suitable for the atmospheric channel of laser communication that changes for tens of minutes.

Description

Technical Field

[0001] The present invention belongs to the technical field of atmospheric laser communication, and in particular relates to an atmospheric channel state perception method for laser communication. Background Art

[0002] Compared with radio frequency communication, laser communication has the advantages of high bandwidth, low power consumption, small size and easy installation, strong anti-interference ability, and no need for frequency license. It is an important development direction for the next generation of mobile communications.

[0003] The atmospheric channel is the primary factor that restricts the performance of laser communication. The turbulence effect in the atmospheric channel will cause the intensity and phase fluctuations of the laser, which will greatly deteriorate the communication bit error rate; the attenuation effect in the atmospheric channel will cause the energy loss of the laser, which will seriously affect the working distance. Moreover, the atmospheric attenuation and turbulence effects also vary with time and space.

[0004] In order to study the complex and changeable atmospheric channels, the refractive index structure constant C that characterizes the turbulence intensity is used to calculate the n 2 and the extinction coefficient α, which characterizes the attenuation intensity E , and developed a variety of detection methods. For example: detection of refractive index structure constant C n 2 Temperature pulsator, scintillation meter, etc.; detect extinction coefficient α E The above typical methods are to detect C n 2 and the extinction coefficient α E , the refractive index structure constant C cannot be detected simultaneously in the laser link n 2 and the extinction coefficient α E Therefore, the prior art discloses the refractive index structure constant C n 2 and the extinction coefficient α E Synchronous detection technology. One method is to measure the atmospheric refractive index structure constant through the beam drift effect under the condition of satisfying the aperture constraint, and estimate the atmospheric extinction coefficient through the speckle power, so as to achieve synchronous measurement; another method is to use the characteristic that the atmospheric refractive index structure constant and extinction coefficient are implicit in the received power of a certain aperture on the long-period speckle accumulation image, and solve the nonlinear measurement equation through the power measurement results at two different apertures to achieve synchronous measurement.

[0005] However, only the refractive index structure constant C is obtained n 2 and the extinction coefficient α E It is still not possible to fully evaluate the atmospheric channel state. This is because the average bit error rate of laser communication is related to the average signal-to-noise ratio and the power fluctuation probability density function PDF.n 2 and the extinction coefficient α E Only the average signal-to-noise ratio and the power fluctuation probability density function under ideal conditions can be accurately estimated. However, under actual atmospheric conditions, the relevant parameters in the power fluctuation probability density function under the multiplicative superposition of atmospheric turbulence and tracking errors will deviate from the ideal conditions, directly leading to inaccurate estimation of the average bit error rate.

[0006] In summary, in order to meet the needs of quickly and accurately evaluating laser communication performance and optimizing means of suppressing atmospheric effects, it is urgent to carry out targeted research on comprehensive perception technology of atmospheric channel states. Summary of the invention

[0007] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide an atmospheric channel state perception method for laser communication.

[0008] The technical problem proposed by the present invention is solved in this way:

[0009] A method for sensing atmospheric channel state for laser communication, the specific process is as follows:

[0010] The receiving end is located at a distance d from the receiving optical axis. 1 and d 2 The photodetectors are placed at the positions of the two photodetectors; the average photovoltage collected at the rear ends of the two photodetectors <V(d 1 , L)> and <V(d 2 , L)> are respectively expressed as:

[0011]

[0012] Where L is the transmission distance between the transmitter and the receiver, P 0 is the transmitting power of the transmitter, S 0 is the detection area of ​​the photodetector, K is the conversion coefficient of the photovoltage and optical power of the photodetector; W LT is the long-period beam radius, W is the radius of the laser beam on the receiving plane without atmosphere, W = 0.5 × (D t +θ div L), D t is the transmit aperture, θ div is the beam divergence angle of the emitted laser; is the Rytov variance, k is the wave number, Λ=2L / (kW 2 ), is the refractive index structure constant; σ P is the standard deviation of the position deviation caused by the tracking error, σ P =σ θ L, σθ is the standard deviation of the tracking error angle; α E is the extinction coefficient, exp means seeking the exponential power;

[0013] Let x 1 =exp(-α E L), Dividing formula (3) by formula (4) yields:

[0014]

[0015] Substituting formula (5) into formula (3), we get:

[0016]

[0017] According to formula (5) and formula (6), α E =-lnx 1 / L,

[0018] Furthermore, the long-period average light intensity of the dynamic optical link under the combined effects of the atmospheric channel and tracking error is<I(r,L)> It is expressed as:

[0019]

[0020] Where r is the off-axis distance, i.e., the radial distance between any point on the receiving end plane and the receiving optical axis, I T is the central light intensity of the emitted laser on the emission optical axis, W 0 is the plane beam radius of the emitted laser;

[0021] Average received power at the photodetector<P(r,L)> It is expressed as:

[0022]

[0023] Among them, P 0 is the transmission power,

[0024] The conversion coefficient between the photoelectric voltage and optical power of the photodetector is K, and the average photovoltage collected by the two photodetectors is <V(d 1 , L)> and <V(d 2 , L)> are expressed as formula (3) and formula (4) respectively.

[0025] Furthermore, under the influence of atmospheric turbulence, the normalized light intensity fluctuation probability density function f(I a ) is expressed as:

[0026]

[0027] Among them, Ia is the central light intensity of the receiving end on the receiving optical axis under the influence of atmospheric turbulence;

[0028] The probability density function f of the normalized light intensity fluctuation at the receiving end at the off-axis distance r under the influence of tracking error e (I e )for:

[0029]

[0030] Among them, I e is the light intensity at the receiving end at a distance r from the axis under the influence of tracking error; W st is the short-period beam radius, I 0 (·) is the first kind of zero-order modified Bessel function;

[0031] The probability density function f of the normalized light intensity fluctuation at the receiver under the influence of atmospheric turbulence and tracking error is r The expression of (I) is:

[0032]

[0033] in, I is the light intensity at the receiving end at an off-axis distance r under the influence of atmospheric turbulence and tracking errors.

[0034] Furthermore, let the off-axis distance r in equation (9) be 0, and the normalized light intensity fluctuation probability density function f on the receiving optical axis under the influence of atmospheric turbulence and tracking error is 0 (I) is expressed as:

[0035]

[0036] Rytov variance Corrected Rytov variance It is expressed as:

[0037]

[0038] Among them, A fix is the correction factor;

[0039] use By correcting equation (10), under the influence of atmospheric turbulence and tracking error, the probability density function of the normalized light intensity fluctuation on the receiving optical axis after correction is expressed as:

[0040]

[0041] The probability density function f of the light intensity fluctuation on the receiving optical axis under the influence of atmospheric turbulence, tracking error and aperture effect0 (I, D) is:

[0042]

[0043] in, is the variance of light intensity fluctuation after aperture averaging, A(D) is the aperture smoothing factor, A(D) = [1 + 1.07 (kD 2 / 4L) 7 / 6 ] -1 , D is the receiving aperture diameter.

[0044] Furthermore, the correction factor A fix Take 1.2~1.6.

[0045] Furthermore, the power fluctuation probability density function f on the receiving optical axis is 0 (P) is:

[0046]

[0047] in, S is the receiving aperture area of ​​the receiving optical axis, and P represents the receiving power at the receiving optical axis;

[0048] Average bit error rate <ber>It is expressed as:

[0049]

[0050] in, erfc(·) is the error residual function; σ sn is the shot noise; R is the detector responsivity, r = qη / hv, q = 1.6×10 -19 C is the electron charge, η is the detector quantum efficiency, and Planck constant h = 6.625×10 -34 J·S, light wave frequency v = c / λ, speed of light c = 3×10 8 m / s,B W is the noise bandwidth and λ is the laser wavelength.

[0051] The beneficial effects of the present invention are:

[0052] The method of the present invention only requires two off-axis photodetectors to simultaneously detect the refractive index structure constant C n 2 , extinction coefficient α E , power fluctuation probability density function, is a simple and easy method for sensing the state of the atmospheric channel, which can be used to evaluate and predict the communication bit error rate online and adjust the working mode of the laser communication system in time. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 It is a schematic diagram of the overall scheme of the method of the present invention;

[0054] Figure 2 This is a speckle intensity diagram obtained by using phase screen simulation in the embodiment;

[0055] Figure 3 Schematic diagram of an equivalent power detection scheme in an embodiment;

[0056] Figure 4 : is a diagram of the regional receiving power sequence in the embodiment, where (a) is region 1, (b) is region 2;

[0057] Figure 5 (a) is a probability density curve of off-axis light intensity before and after correction when the transmission distance is 700m and the off-axis distance is 0.008m in the embodiment, Figure 5 (b) Figure 5 (a) The corresponding probability density curve of the main axis light intensity before and after Rytov variance correction, Figure 5 (c) is the off-axis light intensity probability density curve before and after correction when the transmission distance is 700m and the off-axis distance is 0.01m. Figure 5 (d) Figure 5 (c) The corresponding probability density curve of the main axis light intensity before and after the Rytov variance correction, Figure 5 (e) is the off-axis light intensity probability density curve before and after correction when the transmission distance is 700m and the off-axis distance is 0.015m. Figure 5 (f) Figure 5 (e) Corresponding probability density curves of the main axis light intensity before and after correction using the corresponding Rytov variance;

[0058] Figure 6 (a) is a probability density curve of off-axis light intensity before and after correction when the transmission distance is 1000m and the off-axis distance is 0.01m in the embodiment, Figure 6 (b) Figure 6 (a) The corresponding probability density curve of the main axis light intensity before and after Rytov variance correction, Figure 6 (c) is the off-axis light intensity probability density curve before and after correction when the transmission distance is 1000m and the off-axis distance is 0.015m. Figure 6 (d) Figure 6 (c) The corresponding probability density curve of the main axis light intensity before and after Rytov variance correction;

[0059] Figure 7 (a) is the embodiment Figure 5 (a) The corresponding comparison of the probability density function curve of the receiving power fluctuation of the receiving main axis aperture, Figure 7 (b) Figure 5 (c) The corresponding comparison of the probability density function curve of the receiving power fluctuation of the receiving main axis aperture, Figure 7 (c) Figure 5 (e) The corresponding comparison of the probability density function curve of the receiving power fluctuation of the receiving main axis aperture, Figure 7 (d) Figure 6 (a) The corresponding comparison of the probability density function curve of the receiving power fluctuation of the receiving main axis aperture, Figure 7 (e) Figure 6 (c) Comparison of the corresponding probability density function curves of the receiving power fluctuation of the receiving main axis aperture. DETAILED DESCRIPTION

[0060] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0061] This embodiment provides an atmospheric channel state sensing method for laser communication. The overall scheme schematic diagram is as follows: Figure 1 As shown in the figure, the transmitting and receiving ends are mobile platform laser communication terminals, and the two ends achieve optical link stability under relative motion conditions through capture, tracking, and alignment mechanisms. At this time, after the emitted laser is transmitted through the atmosphere, speckles are formed on the receiving plane, which are randomly modulated by atmospheric turbulence, attenuation, and tracking errors.

[0062] Place two photodetectors (PIN or APD detectors) on the receiving end plane at a distance d from the receiving optical axis. 1 and d 2 The data acquisition circuit collects the optical power fluctuation data on the two photodetectors. By statistically analyzing the average value of the optical power fluctuation and combining it with the long-period average light intensity of the dynamic optical link, the refractive index structure constant C can be detected. n 2 And extinction coefficient. According to the theoretical derivation of power fluctuation probability density function, the power fluctuation probability density function on the receiving optical axis can be detected through the optical power fluctuation on the two photodetectors. Combined with the detected refractive index structure constant C n 2 , extinction coefficient α E , power fluctuation probability density function, and evaluate and predict the average bit error rate of laser communication, overcoming the technical problem that the bit error rate evaluation method in the existing technology has the problem of data transmission and reception time being as long as several hours and is not suitable for the atmospheric channel of laser communication that changes for tens of minutes.

[0063] The specific process of the method described in this embodiment is as follows:

[0064] Long-period average light intensity of dynamic optical links under the combined effects of atmospheric channel and tracking error<I(r,L)> It can be expressed as:

[0065]

[0066] Where r is the radial distance between any point on the receiving end plane and the receiving optical axis, and L is the transmission distance between the transmitting and receiving ends; I T W is the central light intensity of the emitted laser on the emission optical axis; 0 W is the plane beam radius of the emitted laser; LT is the long-period beam radius, W is the beam radius of the receiving plane without atmosphere, D t is the transmit aperture, θ div is the beam divergence angle of the emitted laser; is the Rytov variance, and k is the wave number, Λ=2L / (kW 2 );σ P is the standard deviation of the position deviation caused by the tracking error, σ P =σ θ L, σ θ is the standard deviation of the tracking error angle.

[0067] When the transmission distance is much larger than the photodetector area S 0 When the average received power on the photodetector is<P(r,L)> (i.e., the statistical average of multiple instantaneous received powers within a few seconds) can be expressed as:

[0068]

[0069] Among them, P 0 is the transmission power,

[0070] The light intensity fluctuation on the receiving plane is composed of two parts of multiplicative superposition, one part is the light intensity fluctuation caused by atmospheric turbulence, and the other part is the light intensity fluctuation caused by tracking error. The light power fluctuation caused by atmospheric turbulence is related to the shear wind speed, which is generally less than kHz; the disturbance frequency of tracking error is generally less than kHz. Therefore, the bandwidth of the photovoltage data acquisition circuit at the back end of the photodetector can be selected as 100kHz.

[0071] Assuming that the conversion coefficient between the photovoltage and the optical power on the photodetector is K, the two distances d from the receiving optical axis are 1 ,d 2 The average photovoltage collected by the photodetector back end <V(d 1 , L)> and <V(d 2 , L)> can be expressed as:

[0072]

[0073] In d 1 ,d 2 and tracking error σ P If x is known, 1 =exp(-α E L), Dividing equation (3) by equation (4) to simplify it, we get:

[0074]

[0075] Substituting formula (5) into formula (3), we get:

[0076]

[0077] According to formula (5) and formula (6), we can finally get α E =-lnx 1 / L,

[0078] It is generally believed that the light intensity fluctuation caused by weak atmospheric turbulence obeys a log-normal distribution. In the case of point reception, the normalized light intensity fluctuation probability density function f(I a ) can be expressed as:

[0079]

[0080] Among them, I a is the central light intensity of the receiving end on the receiving optical axis under the influence of atmospheric turbulence;

[0081] Since the speckle intensity profile is unevenly distributed, the intensity fluctuation will also occur under the influence of tracking error. At a certain off-axis distance r, in the case of point reception, the normalized intensity fluctuation probability density function f caused by tracking error is e (I e )for:

[0082]

[0083] Among them, I e is the light intensity at the receiving end at a distance r from the axis under the influence of tracking error; W st is the short-period beam radius, I 0 (·) is the first kind of zero-order modified Bessel function.

[0084] Normalized light intensity fluctuation probability density function f under tracking error and atmospheric turbulence multiplicative modulation r The expression of (I) is:

[0085]

[0086] in, I is the light intensity at the receiving end at an off-axis distance r under the influence of atmospheric turbulence and tracking errors.

[0087] Let the off-axis distance r = 0 in equation (9), corresponding to the normalized intensity fluctuation probability density function f on the receiving axis: 0 (I) can be expressed as:

[0088]

[0089] Affected by factors such as the transmission distance, the light spot received on the receiving plane may not be an ideal plane wave, causing the actual Rytov variance to deviate from the theoretical value, which in turn causes the theoretical function obtained by equation (9) to deviate from the actual measured off-axis light intensity fluctuation probability density function. Therefore, it is necessary to add a correction coefficient before the Rytov variance and perform optimal fitting on this coefficient. The corrected Rytov variance It can be expressed as:

[0090]

[0091] Among them, A fix is the correction coefficient, ranging from 1.2 to 1.6.

[0092] Therefore, combined with the corrected Rytov variance After correcting formula (10), it can be expressed as:

[0093]

[0094] Formula (12) is the normalized light intensity fluctuation probability density function when the receiving axis is point reception. When the main axis is aperture reception, affected by the aperture averaging effect, the light intensity fluctuation probability density function considering the influence of atmospheric turbulence, tracking error and aperture effect will change to:

[0095]

[0096] in, is the variance of light intensity fluctuation after aperture averaging, A(D) is the aperture smoothing factor, A(D) = [1 + 1.07 (kD 2 / 4L) 7 / 6 ] -1 , D is the receiving aperture diameter.

[0097] The refractive index structure constant implied in equations (9), (12), and (13) is All of them can be accurately measured, and then, without adding other hardware, the Rytov variance corrected in the off-axis case can be used to accurately predict the probability density function of the light intensity fluctuation received by the main axis aperture.

[0098] The probability density function f of the normalized light intensity fluctuation received by the main axis aperture predicted by equation (13) is 0 (I, D) can further obtain the power fluctuation probability density function f received by the main axis aperture 0 (P) is:

[0099]

[0100] in, S is the receiving aperture area of ​​the main axis, and P represents the receiving power at the receiving main axis;

[0101] Furthermore, the average bit error rate can be obtained:

[0102]

[0103] in, erfc(·) is the error residual function. Only the shot noise σ is considered sn hour, R is the detector responsivity, R = qη / hv. q = 1.6 × 10 -19 C is the electron charge, η is the detector quantum efficiency, h = 6.625 × 10 -34 J·S is Planck's constant, v = c / λ is the frequency of light waves, c = 3×10 8 m / s is the speed of light. W is the noise bandwidth and λ is the laser wavelength.

[0104] This embodiment verifies that the long-period average light intensity and light intensity fluctuation probability density function of the phase screen output speckle are consistent with the theory described in the Chinese patent application "A single-layer phase screen simulation method for wireless laser communication" by statistically analyzing the speckle output by the classic single-layer phase screen simulation method. The speckle obtained by phase screen simulation is used to simulate the actual experimental situation, and then the above algorithm is verified. In the phase screen simulation experiment, the wavelength is λ = 850nm and the emission radius W 0 = 0.007m, propagation distance L = 2400m, grid points are 1024×1024, phase screen size is 0.4m, and the refractive index structure constant is set to The phase screen simulation program can be used to obtain Figure 2 There are 1000 speckle images in total. In each speckle image, two small squares with the same position and size and equal side length are selected, such as Figure 3 As shown. Through the light intensity data sequence of the speckle image, the received power sequence in the two regions can be obtained as Figure 4 As shown, the average received power can be further calculated and the specific equations corresponding to equations (3) and (4) can be obtained to obtain C n 2 and α E In the simulation experiment, d 1 =27.34mm, d 2 =12.11mm, the side length of the square is 4.83mm, and the received power in area D1 is 1.8309×10 -5 W, the received power in area D2 is 1.9829×10 -5 W, and finally solve C n 2 =10 -13.66 m -2 / 3 , α E =1.6dB / km.

[0105] In solving the corresponding C n 2 and α E After that, the power fluctuation probability density curve received by the main axis aperture is predicted, and then the average bit error rate can be evaluated. The off-axis light intensity probability density curves before and after correction when the off-axis distances are d = 0.008m, d = 0.01m, and d = 0.015m are as follows: Figure 5 (a) Figure 5 (c) Figure 5 As shown in (e), the corresponding Rytov variance can be used to obtain the probability density curve of the main axis light intensity before and after correction. Figure 5 (b) Figure 5 (d) Figure 5 (f) is shown, where the histogram represents the simulation results of the phase screen, the green curve is the theoretical curve before Rytov variance correction, and the red curve is the theoretical curve after Rytov variance correction. Under the condition of the same transmission distance and different off-axis distance, the optimal correction coefficient can be obtained through fitting. fix =1.3, and the corrected theoretical curve is closer to the phase screen simulation result. When the off-axis distances are 0.008m, 0.010m, and 0.015m, respectively, the correlation coefficients between the theoretical curve before correction and the off-axis probability density curve obtained by phase screen simulation are 0.969, 0.969, and 0.969, respectively, according to formula (9). Combined with the Rytov variance corrected by formula (11), the correlation coefficients between the corrected theoretical curve and the phase screen result are 0.987, 0.987, and 0.983, respectively; according to formula (10), the correlation coefficients between the corresponding spindle theoretical curve before correction and the phase screen result are 0.979, 0.973, and 0.973, and further according to formula (12), the correlation coefficients between the corrected spindle theoretical curve and the phase screen result are 0.991, 0.991, and 0.991. The above data show that under the conditions of the same transmission distance and different off-axis distances, after the Rytov variance is corrected by the same correction coefficient, the degree of fit between the off-axis and main-axis light intensity fluctuation probability density theoretical curves and the phase screen simulation results is improved compared with before correction.

[0106] When the transmission distance is changed to L = 1000m, the off-axis light intensity probability density curves before and after correction when the off-axis distances are 0.010m and 0.015m respectively are as follows: Figure 6 (a) and Figure 6 As shown in (c), the probability density curve of the main axis light intensity before and after correction can be further obtained in the corresponding case. Figure 6 (b) and Figure 6 As shown in (d), the histogram represents the simulation results of the phase screen. The green curve is the theoretical curve before Rytov variance correction, and the red curve is the theoretical curve after Rytov variance correction. After fitting, the optimal correction coefficient after changing the transmission distance is obtained. fix =1.5, and the corrected theoretical curve is closer to the phase screen simulation result. When the off-axis distances are 0.010m and 0.015m respectively, the correlation coefficients between the theoretical curve before correction and the off-axis probability density curve obtained by the phase screen simulation are 0.857 and 0.905 respectively according to formula (9). Combined with the Rytov variance corrected by formula (11), the correlation coefficients between the corrected theoretical curve and the phase screen result are 0.937 and 0.957 respectively; according to formula (10), the correlation coefficients between the corresponding spindle theoretical curve before correction and the phase screen result are 0.926 and 0.919, and further according to formula (12), the correlation coefficients between the corrected spindle theoretical curve and the phase screen result are 0.982 and 0.978. From the above data, it can be seen that when the transmission distance changes, the correction coefficient A fix will change accordingly, but in the case of transmission distance and different off-axis distance, the correction factor A fix The results are kept consistent, and the fitting degree between the revised theoretical curve and the phase screen simulation results is greatly improved.

[0107] When the spindle receiving aperture diameter D = 0.02m, Figure 7 As shown, corresponding to Figure 5 and Figure 6 The probability density curve of the spindle aperture received power fluctuation under different conditions, Figure 7 (a) is the embodiment Figure 5 (a) The corresponding comparison of the probability density function curve of the receiving power fluctuation of the receiving main axis aperture, Figure 7 (b) Figure 5 (c) The corresponding comparison of the probability density function curve of the receiving power fluctuation of the receiving main axis aperture, Figure 7 (c) Figure 5 (e) The corresponding comparison of the probability density function curve of the receiving power fluctuation of the receiving main axis aperture, Figure 7 (d) Figure 6 (a) The corresponding comparison of the probability density function curve of the receiving power fluctuation of the receiving main axis aperture, Figure 7 (e) Figure 6 (c) The corresponding comparison of the probability density function curves of the receiving power fluctuation of the receiving main axis aperture. The histogram is the power fluctuation statistically calculated based on the phase screen simulation results. The green curve is the theoretical curve before Rytov variance correction, and the red curve is the theoretical curve after correction. Figure 7 In (a)-(e), the correlation coefficients between the theoretical curve before correction and the probability density curve of spindle aperture received power fluctuation obtained by phase screen simulation statistics are 0.941, 0.931, 0.931, 0.897, and 0.894, respectively, and the correlation coefficients between the theoretical curve after correction and the phase screen result are 0.964, 0.958, 0.958, 0.958, and 0.955, respectively. The fitting degree of the probability density curve of spindle aperture received power fluctuation after correction and the phase screen simulation result is slightly lower, which may be caused by the inaccuracy of the aperture smoothing factor A(D). The derivation of aperture average theory is very complicated and needs further correction in subsequent research.

[0108] Although the fitting degree of the corrected probability density curve of the spindle aperture received power fluctuation is slightly lower than that of the phase screen simulation result, the estimation accuracy of the average bit error rate is still high. Since the smaller received power can be regarded as noise, and this noise has a greater impact on the estimation of the average bit error rate, combined with the minimum detection threshold of the photodetector, only the received power greater than 2.5×10 -7 Power fluctuation probability density curve of part W. Figure 7 In (a)-(e), the average bit error rates obtained from the phase screen simulation results are 6.53×10 -9 ,8.83×10 -9 ,8.83×10 -9 ,7.18×10 -9 , 3.12×10 -8 , the average bit error rates calculated based on the probability density curve before correction are 1.79×10 -9 , 2.42×10 -9 , 2.42×10 -9 4.60×10 -9 , 1.94×10 -8 The average bit error rates calculated based on the corrected probability density curve are 3.02×10 -9 4.08×10 -9 4.08×10 -9 , 1.19×10 -8 ,4.98×10 -8 .

[0109] Comprehensive phase screen simulation verification results, the C n 2 The synchronous measurement method of extinction coefficient and power fluctuation probability density function has a good effect, can accurately evaluate the average bit error rate, and realizes the rapid and accurate evaluation of the atmospheric channel state of laser communication.< / ber>

Claims

1. A method for sensing atmospheric channel state for laser communication, characterized in that: The specific process is: Photodetectors are placed at positions d1 and d2 away from the receiving optical axis on the plane where the receiving end is located; the average photovoltage collected by the rear ends of the two photodetectors<V(d1,L)> and<V(d2,L)> Respectively expressed as: Where L is the transmission distance between the transmitter and the receiver, P0 is the transmission power of the transmitter, S0 is the detection area of ​​the photodetector, K is the conversion coefficient of the photovoltage and optical power of the photodetector; W LT is the long-period beam radius, W is the radius of the laser beam on the receiving plane without atmosphere, W = 0.5 × (D t +θ div L), D t is the transmit aperture, θ div is the beam divergence angle of the emitted laser; is the Rytov variance, k is the wave number, Λ=2L / (kW 2 ), is the refractive index structure constant; σ P is the standard deviation of the position deviation caused by the tracking error, σ P =σ θ L, σ θ is the standard deviation of the tracking error angle; α E is the extinction coefficient, exp means seeking the exponential power; Let x1 = exp(-α E L), Dividing formula (3) by formula (4) yields: Substituting formula (5) into formula (3), we get: According to formula (5) and formula (6), α E =-lnx1 / L, 2. The atmospheric channel state perception method for laser communication according to claim 1, characterized in that: Long-period average light intensity of dynamic optical links under the combined effects of atmospheric channel and tracking error<I(r,L)> It is expressed as: Where r is the off-axis distance, i.e., the radial distance between any point on the receiving end plane and the receiving optical axis, I T is the central light intensity of the emitted laser on the emission optical axis, W0 is the plane beam radius of the emitted laser; Average received power at the photodetector<P(r,L)> It is expressed as: Where P0 is the transmit power, The conversion coefficient between the photoelectric voltage and optical power of the photodetector is K, and the average photovoltage collected by the two photodetectors is<V(d1,L)> and<V(d2,L)> They are expressed as formula (3) and formula (4) respectively.

3. The atmospheric channel state perception method for laser communication according to claim 1, characterized in that: Under the influence of atmospheric turbulence, the probability density function f(I a ) is expressed as: Among them, I a is the central light intensity of the receiving end on the receiving optical axis under the influence of atmospheric turbulence; The probability density function f of the normalized light intensity fluctuation at the receiving end at the off-axis distance r under the influence of tracking error e (I e )for: Among them, I e is the light intensity at the receiving end at a distance r from the axis under the influence of tracking error; W st is the short-period beam radius, I0(·) is the first kind of zero-order modified Bessel function; The probability density function f of the normalized light intensity fluctuation at the receiver under the influence of atmospheric turbulence and tracking error is r The expression of (I) is: in, I is the light intensity at the receiving end at an off-axis distance r under the influence of atmospheric turbulence and tracking errors.

4. The atmospheric channel state perception method for laser communication according to claim 3 is characterized in that: Let the off-axis distance r = 0 in equation (9), and the normalized light intensity fluctuation probability density function f0(I) on the receiving optical axis under the influence of atmospheric turbulence and tracking error is expressed as: Rytov variance Corrected Rytov variance It is expressed as: Among them, A fix is the correction factor; use By correcting equation (10), under the influence of atmospheric turbulence and tracking error, the probability density function of the normalized light intensity fluctuation on the receiving optical axis after correction is expressed as: Under the influence of atmospheric turbulence, tracking error and aperture effect, the probability density function f0(I, D) of the light intensity fluctuation on the receiving optical axis is: in, is the variance of light intensity fluctuation after aperture averaging, A(D) is the aperture smoothing factor, A(D) = [1 + 1.07 (kD 2 / 4L) 7 / 6 ] -1 , D is the receiving aperture diameter.

5. The atmospheric channel state sensing method for laser communication according to claim 4, characterized in that: Correction factor A fix Take 1.2~1.

6.

6. The atmospheric channel state perception method for laser communication according to claim 4, characterized in that: The power fluctuation probability density function f0(P) on the receiving optical axis is: in, S is the receiving aperture area of ​​the receiving optical axis, and P represents the receiving power at the receiving optical axis; Average bit error rate <ber> It is expressed as:< / ber> in, erfc(·) is the error residual function; σ sn is the shot noise; R is the detector responsivity, R = qη / hv, q = 1.6×10 -19 C is the electron charge, η is the detector quantum efficiency, and Planck constant h = 6.625×10 -34 J·S, light wave frequency v = c / λ, speed of light c = 3×10 8 m / s,B W is the noise bandwidth and λ is the laser wavelength.

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