CN114374433B - Laser line width determination method for atmospheric channel QAM communication system - Google Patents

Abstract

The invention discloses a laser linewidth tolerance determining method for an atmosphere channel QAM communication system, which aims to meet the performance requirement of the system when designing a spatial coherent laser communication system, and the linewidth of a laser is strictly controlled when the system is designed, so that linewidth tolerance parameters meeting the requirement of the error rate of the system design are obtained; the space coherent laser communication system has complicated and changeable atmosphere channels, and the current atmosphere channel information needs to be acquired to more accurately determine the linewidth tolerance of the laser meeting the system error rate. The invention can more accurately obtain the laser linewidth tolerance of the QAM communication system under the current atmosphere channel by combining the data and the model twice.

Description

Laser line width determination method for atmospheric channel QAM communication system

Technical field

The present invention relates to the technical field, and specifically relates to a method for determining the laser line width limit for an atmospheric channel QAM communication system.

Background technique

Compared with microwave communication, space laser communication has the advantages of high communication rate, high bandwidth, high security performance, free spectrum license, low power consumption, small size, etc., and has become a research hotspot for many researchers and research institutions. In addition, compared with terrestrial optical fiber communication, space laser communication not only saves costs by eliminating the need to lay optical fibers, but also has more possibilities because it is not bound by optical fiber channels. Compared with the traditional intensity modulation/direct detection space laser communication system, the spatial coherent laser communication system can not only increase the bandwidth and increase the communication rate, but also effectively suppress the background noise of the free space channel and the thermal noise of the system. This effectively improves the sensitivity of the receiver.

Although spatial coherent laser communication has improved performance due to the introduction of phase modulation, it is precisely because of the introduction of phase modulation that the spatial coherent laser communication system is very sensitive to both intensity and phase. There are two main factors that affect the performance of spatially coherent lasers, one is the linewidth of the laser, and the other is the atmospheric environment. On the one hand, the existence of the laser linewidth will cause the instability of the laser output laser phase, thereby bringing phase noise to the system and causing the performance of the spatially coherent laser communication system to deteriorate. Therefore, when designing a spatially coherent laser communication system, the linewidth of the laser must be strictly controlled.

On the other hand, spatial coherent laser communication systems generally include a long-distance or even ultra-long-distance atmospheric channel. The atmospheric channel will not only affect the distribution of optical signal intensity at the receiving end, but also cause fluctuations in the phase of the optical signal at the receiving end. The latter has a greater impact on system performance. The atmospheric channel is very complex and changeable. The atmospheric environment at different altitudes in different areas is quite different. The atmospheric environment in the morning and evening, summer and winter in the same area is also different. Therefore, in a spatially coherent laser communication system, it is necessary to obtain the atmospheric channel information at that time in order to better determine the laser line width limit that meets the system bit error rate.

To sum up, in order to meet the performance requirements of the system when designing a spatially coherent laser communication system, it is necessary to strictly control the linewidth of the laser during system design and obtain linewidth limit parameters that meet the system design bit error rate requirements; spatially coherent laser There are complex and changeable atmospheric channels in communication systems. It is necessary to obtain current atmospheric channel information in order to more accurately determine the laser line width limit that meets the system bit error rate.

The importance of laser line width tolerance and the complexity and variability of atmospheric channels are self-evident. In order to complete the QAM communication system design requirements, it is necessary to more accurately determine the line width tolerance parameters of the spatially coherent laser communication system. Since there is an atmospheric channel in the spatially coherent laser communication system, obtaining the specific model parameters of the current atmospheric channel has become a focus. The complexity and variability of the atmospheric channel have made it difficult to experimentally measure the atmospheric channel and the inaccuracies of calculating atmospheric parameters using pure models. Accuracy makes it difficult to accurately determine the line width limit parameters of the spatially coherent laser communication system.

The bit error rate performance of the existing QAM modulation system under the atmospheric channel will not only be affected by the atmospheric channel, but also be affected by the laser linewidth. Generally, when considering the laser linewidth limit of the space-related laser communication system, it cannot be very accurate. Accurately obtain line tolerance information.

Contents of the invention

In view of the deficiencies in the existing technology, the present invention provides a laser line tolerance determination method for atmospheric channel QAM communication systems, which can consider the complexity and variability of the atmospheric channel and more accurately determine the line tolerance of the spatial coherent laser communication system. Limited parameters.

In order to achieve the above objects, the present invention adopts the following technical solutions:

The embodiment of the present invention proposes a method for determining the laser line width limit for atmospheric channel QAM communication systems. The method for determining the laser line width limit includes the following steps:

S1 uses a free-space optical tracking and aiming subsystem to complete the aiming work of the transmitter and receiver, so that the signal light transmitter system emits signal light with constant power. A light intensity sensor is used at the receiver to receive the light signal and obtain the time of different light signal intensities. Distribution, calculate the average optical signal strength at the receiving end;

S2, convert the time distribution of different optical signal intensities at the receiving end obtained in step S1 into the optical signal intensity probability distribution at the receiving end, and obtain the optical signal intensity probability distribution data at the receiving end;

S3. Construct a probability density model of the optical signal intensity at the receiving end. Substitute the average optical signal intensity at the receiving end calculated in step S1 into the probability density model of the optical signal intensity at the receiving end. By changing the atmospheric turbulence scintillation variance, the reception under different atmospheric turbulence scintillation is obtained. The probability density curve of the optical signal intensity; then calculate the correlation coefficient between the probability distribution data of the receiving end optical signal intensity obtained in step S2 and the probability density curve of the received optical signal intensity under different atmospheric turbulence flickers, select the curve with the largest correlation coefficient, and obtain the receiving surface Atmospheric turbulence scintillation variance of probability density model of optical signal intensity

S4. Select a laser with determined power and linewidth at the receiving end to build the receiving end of the QAM communication system, change the intensity of the optical signal received by the receiving end, and obtain the different received light of the QAM communication system under the atmospheric channel through the digital signal processing module. Bit error rate information for signal strength;

S5, construct a bit error rate model for the QAM communication system with laser line width limit for atmospheric channels:

In the formula, is the QAM bit error rate model with laser line width limit without the influence of atmospheric turbulence, I is the received optical signal intensity,/> is the phase of the received optical signal, Δv _t is the narrow linewidth laser linewidth at the transmitting end, Δv _r is the linewidth of the local oscillator laser at the receiving end, P(I) is the probability density of different optical signal intensities I at the receiving end under the influence of atmospheric turbulence, / > is the phase of the optical signal at the receiving end under the influence of atmospheric turbulence/> Distribution probability density;

The calculation model of P(I) is as follows:

In the formula, <I> is the average optical signal intensity of the optical signal at the receiving end, is the atmospheric turbulence scintillation variance;

The calculation model is as follows:

In the formula, is the phase fluctuation variance of the received optical signal caused by atmospheric turbulence;

Change the intensity of the optical signal at the receiving end to obtain a curve of the bit error rate changing with the intensity of the received optical signal under the influence of the atmosphere on the phase fluctuation of the optical signal at the receiving end. Calculate the bit error rate data and bit error rate of different received optical signal intensities obtained in step S4. The correlation coefficient of the curve that changes with the intensity of the received optical signal is selected. The curve with the largest correlation coefficient is selected to obtain the phase fluctuation variance of the received optical signal caused by atmospheric turbulence.

S6, atmospheric turbulence scintillation variance obtained according to step S3 and the phase fluctuation variance of the received optical signal caused by atmospheric turbulence obtained in step S5/> Establish a QAM communication system bit error rate model including line width limit variables under the current atmospheric channel:

S7. According to the bit error rate model of the QAM communication system containing the line width limit variable under the current atmospheric channel obtained in step S6, and according to the bit error rate requirement index of the target QAM communication system, determine the line width limit of the laser used in QAM under the current atmospheric channel conditions. .

Further, the QAM communication system includes a transmitter and a receiver;

The transmitting end includes a narrow linewidth laser, a QAM modulator, an erbium-doped fiber amplifier, and a transmitting system;

The receiving end includes a receiving system, a light intensity sensor, an optical attenuator, a laser that determines power and line width, a coupler, a balance detector, and a digital signal processing module; the receiving system is connected to the optical attenuator and the light intensity sensor respectively, The digital signal processing module includes a demodulation module and a bit error rate calculation module.

Further, in step S1, the process of calculating the average optical signal intensity at the receiving end includes the following sub-steps:

Suppose the time-varying optical signal intensity received by the receiving end is I _re (t), and statistically obtain the time distribution T _I of different optical signal intensities at the receiving end;

The average optical signal intensity <I> at the receiving end is calculated according to the following formula:

In the formula, T is the total time for the receiving end to receive the optical signal.

Further, in step S2, the process of obtaining the probability distribution data of the optical signal intensity at the receiving end includes the following sub-steps:

According to the time distribution T _I of different optical signal strengths at the receiving end, and through the characteristic that the probability density P(I) of the optical signal intensity at the receiving end is integrated to 1, the following formula is used to calculate the probability of the optical signal intensity at the receiving end:

Further, in step S3, the atmospheric turbulence scintillation variance of the probability density model of the receiving surface optical signal intensity is obtained. The process includes the following steps:

S31. Substitute the average optical signal intensity <I> at the receiving end obtained in step S1 into the calculation model of P(I). By changing the atmospheric turbulence scintillation variance Parameters to obtain different atmospheric turbulence scintillation variances/> The P(I) curve below;

S32, calculate the receiving end optical signal intensity probability distribution data P _T (I) obtained in step S2 and the variance of different atmospheric turbulence scintillation according to the following formula The correlation coefficient of the P(I) curve below:

In the formula, Cov(P _T (I), P(I)) represents the covariance of P _T (I) and P(I), and var(P _T (I)) and var(P(I)) represent P _T respectively. The variance of (I) and P(I);

S33. According to the result of correlation coefficient r(P _T (I), P(I)), select the P(I) curve with the largest correlation coefficient to obtain the current atmospheric turbulence scintillation variance. The value of the parameter is used to obtain all the information of the P(I) calculation model under the current atmospheric turbulence.

Further, in step S4, it is assumed that the received optical signal intensity is:

I _α (t) = α I _re (t)

In the formula, α is the attenuation coefficient of the optical attenuator, and its value range is 0 to 1. The bit error rate under different I _α (t) is obtained through the digital signal processing module.

Further, in step S5, the phase fluctuation variance of the received optical signal caused by atmospheric turbulence is obtained. The process includes the following steps:

S51. Substitute the P(I) under the current atmospheric turbulence obtained in step S3 into the bit error rate model BER _QAM of the QAM communication system with laser line width limit for the atmospheric channel, by changing the received optical signal intensity and phase fluctuation variance. parameters, and obtain the BER _QAM curve with the intensity of the received optical signal under different phase fluctuation variances;

S52, calculate the values of different received light signal strengths obtained in step S4 according to the following formula: Correlation coefficient between information and BER _QAM curve with received optical signal intensity under different phase fluctuation variances:

In the formula, Express/> and covariance of BER _QAM ,/> and var(BER _QAM ) distribution representation/> and the variance of BER _QAM ;

S53, according to the correlation coefficient As a result, select the BER _QAM model with the largest correlation coefficient to obtain the phase fluctuation variance of the received optical signal caused by the current atmospheric turbulence/>

Further, in step S7, the process of determining the line width limit of the laser used by QAM under the current atmospheric channel conditions includes the following sub-steps:

According to the BERQAM model for the atmospheric channel obtained in step S6, which contains the determined scintillation variance and phase fluctuation variance of the received optical signal under current atmospheric turbulence, the maximum linewidth of the local oscillator laser of the BER of different communication systems is obtained, that is, the line width limit. According to the atmospheric QAM The bit error rate requirements of communication system design determine the line width limit under the current atmosphere.

The present invention calculates the influence parameters of atmospheric channels on spatially coherent laser communications by combining models and data. On the one hand, it reduces the risk of accuracy decline caused by too much deviation between the model and the actual environment, and avoids the need for complex processes. It requires a very difficult experiment to obtain atmospheric channel information, and the model and data combination process is carried out in two steps, which increases the accuracy of the model. Then the bit error rate model of the QAM communication system under the current atmospheric turbulence scintillation variance and the received optical signal phase fluctuation variance is obtained. Finally, according to the QAM communication system design indicators, the line width limit of the QAM communication system under the current atmospheric channel can be obtained.

The beneficial effects of the present invention are:

First, the method for determining the laser line width tolerance for the atmospheric channel QAM communication system proposed by the present invention takes the influence of the atmospheric channel into consideration when determining the line width tolerance parameters for the atmospheric QAM communication system; due to the complex and changeable atmospheric environment, It is difficult to obtain detailed information of the atmospheric channel through experiments. Most of the existing methods are analyzed through models. This method combines the model with the measured data, which avoids the accuracy of the model due to too much deviation from the actual environment. decline, and avoids obtaining atmospheric channel information through complex and difficult experiments.

Second, the method for determining the laser line width limit proposed by the present invention for the atmospheric channel QAM communication system, in the process of combining the model and data to obtain the atmospheric channel information, does not obtain all the information through one combination, but in steps. Obtained twice, this not only reduces the difficulty of combining data and models, but also improves the accuracy of the model.

Third, the method for determining the laser line width limit proposed by the present invention for the atmospheric channel QAM communication system can be completed through simple modification of the QAM communication system, and the implementation cost is relatively low.

Description of the drawings

Figure 1 is a system structure diagram of the transmitting end and receiving end according to the embodiment of the present invention.

Figure 2 is a schematic diagram of the probability density scatter diagram of the signal light intensity at the receiving end.

Figure 3 is the probability density distribution curve of the optical signal intensity at the receiving end under different atmospheric turbulence variances.

Figure 4 is a scatter diagram of the bit error rate of the QAM communication system under different received optical signal strengths.

Figure 5 is the bit error rate curve of the QAM communication system under the phase fluctuation variance of the received optical signal caused by different atmospheric turbulence and the intensity of the received optical signal.

Figure 6 is a line width limit diagram under different communication indicators BER of the QAM communication system for the atmosphere.

Figure 7 is a flow chart of a method for determining the laser line width limit for an atmospheric channel QAM communication system according to an embodiment of the present invention.

Detailed ways

The present invention will now be described in further detail with reference to the accompanying drawings.

It should be noted that terms such as "upper", "lower", "left", "right", "front", "back", etc. cited in the invention are only for convenience of description and are not used to To limit the implementable scope of the present invention, changes or adjustments in relative relationships shall also be regarded as the implementable scope of the present invention as long as the technical content is not substantially changed.

Figure 7 is a flow chart of a method for determining the laser line width limit for an atmospheric channel QAM communication system according to an embodiment of the present invention. The laser line width limit determination method includes the following steps:

S1 uses a free-space optical tracking and aiming subsystem to complete the aiming work of the transmitter and receiver, so that the signal light transmitter system emits signal light with constant power. A light intensity sensor is used at the receiver to receive the light signal and obtain the time of different light signal intensities. Distribution, calculate the average optical signal strength at the receiving end.

Illustratively, the specific process of step S1 is as follows: the signal optical power is constant. Considering the influence of atmospheric turbulence, the optical signal intensity at the receiving end will change with time. It is assumed that the time-varying optical signal intensity received by the receiving end is I _re ( t), then the time distribution T _I of different optical signal strengths at the receiving end can be statistically obtained; and the average optical signal intensity <I> at the receiving end can be obtained as:

In the formula, T is the total time for the receiving end to receive the optical signal.

S2: Convert the time distribution of different optical signal intensities at the receiving end obtained in step S1 into the optical signal intensity probability distribution at the receiving end, and obtain the optical signal intensity probability distribution data at the receiving end.

Illustratively, the specific process of step S2 is as follows:

According to the time distribution T _I of different optical signal strengths at the receiving end obtained in step 1, and through the characteristic that the probability density P(I) of the optical signal intensity at the receiving end is integrated to 1, the probability of different optical signal strengths at the receiving end can be obtained as:

From this, the probability P _T (I) of different optical signal strengths at the receiving end is obtained.

S3. Construct a probability density model of the optical signal intensity at the receiving end. Substitute the average optical signal intensity at the receiving end calculated in step S1 into the probability density model of the optical signal intensity at the receiving end. By changing the atmospheric turbulence scintillation variance, the reception under different atmospheric turbulence scintillation is obtained. The probability density curve of the optical signal intensity; then calculate the correlation coefficient between the probability distribution data of the receiving end optical signal intensity obtained in step S2 and the probability density curve of the received optical signal intensity under different atmospheric turbulence flickers, select the curve with the largest correlation coefficient, and obtain the receiving surface Atmospheric turbulence scintillation variance of probability density model of optical signal intensity

Illustratively, the specific process of step S3 is as follows: Substitute the average optical signal intensity <I> at the receiving end obtained in step S1 into the P(I) calculation model, and only one P(I) calculation model is obtained. Unknown parameter, namely atmospheric turbulence scintillation variance By changing the atmospheric turbulence scintillation variance/> Parameters to obtain different atmospheric turbulence scintillation variances/> P(I) curve below, and then calculate the correlation coefficient between the receiving end optical signal intensity probability distribution data P _T (I) obtained in step s2 and the above curve. The correlation coefficient calculation formula between P _T (I) and P (I) is as follows :

In the formula, Cov(P _T (I), P(I)) represents the covariance of P _T (I) and P(I), and var(P _T (I)) and var(P(I)) represent P _T respectively. The variance of (I) and P(I), according to the result of the correlation coefficient r(P _T (I), P(I)), select the P(I) curve with the largest correlation coefficient to obtain the current atmospheric turbulence scintillation variance The value of the parameter, thus obtaining all the information of the P(I) calculation model under the current atmospheric turbulence.

S4. Select a laser with determined power and linewidth at the receiving end to build the receiving end of the QAM communication system, change the intensity of the optical signal received by the receiving end, and obtain the different received light of the QAM communication system under the atmospheric channel through the digital signal processing module. Bit error rate information for signal strength.

Illustratively, the specific process of step S4 is as follows: change the optical attenuator at the receiving end, thereby changing the intensity of the optical signal received by the receiving end. At this time, the intensity of the received optical signal is as follows:

I _α (t) = α I _re (t) (4)

In the formula, α is the attenuation coefficient of the optical attenuator, and its value range is 0 to 1. The bit error rate under different I _α (t) is obtained through the digital signal processing module.

S5, construct a bit error rate model for the QAM communication system with laser line width limit for atmospheric channels:

In the formula, is the QAM bit error rate model with laser line width limit without the influence of atmospheric turbulence, I is the received optical signal intensity,/> is the phase of the received optical signal, Δv _t is the narrow linewidth laser linewidth at the transmitting end, Δv _r is the linewidth of the local oscillator laser at the receiving end, P(I) is the probability density of different optical signal intensities I at the receiving end under the influence of atmospheric turbulence, / > is the phase of the optical signal at the receiving end under the influence of atmospheric turbulence/> Distribution probability density.

The calculation model of P(I) is as follows:

In the formula, <I> is the average optical signal intensity of the optical signal at the receiving end, is the atmospheric turbulence scintillation variance.

The calculation model is as follows:

In the formula, is the phase fluctuation variance of the received optical signal caused by atmospheric turbulence.

Change the intensity of the optical signal at the receiving end to obtain a curve of the bit error rate changing with the intensity of the received optical signal under the influence of the atmosphere on the phase fluctuation of the optical signal at the receiving end. Calculate the bit error rate data and bit error rate of different received optical signal intensities obtained in step S4. The correlation coefficient of the curve that changes with the intensity of the received optical signal is selected. The curve with the largest correlation coefficient is selected to obtain the phase fluctuation variance of the received optical signal caused by atmospheric turbulence.

Illustratively, the specific method of step S5 is as follows: Substitute the P(I) under the current atmospheric turbulence obtained in step S3 into the bit error rate model BER _QAM of the QAM communication system for the atmospheric channel containing the laser line width limit, thus The resulting BER _QAM model leaves only one unknown parameter, which is the phase fluctuation variance of the received optical signal caused by atmospheric turbulence. By changing the received light signal intensity and phase fluctuation variance/> Parameters, obtain the BER _QAM curve with the received optical signal intensity under different phase fluctuation variances, and calculate the /> of different received optical signal intensities obtained in step 4. The correlation coefficient between the information and the above curve,/> The correlation coefficient calculation formula with BER _QAM is as follows:

In the formula, Express/> and covariance of BER _QAM ,/> and var(BER _QAM ) distribution representation/> and the variance of BER _QAM , according to the correlation coefficient/> As a result, select the BER _QAM model with the largest correlation coefficient to obtain the phase fluctuation variance of the received optical signal caused by the current atmospheric turbulence. parameter.

S6, atmospheric turbulence scintillation variance obtained according to step S3 and the phase fluctuation variance of the received optical signal caused by atmospheric turbulence obtained in step S5/> Establish a QAM communication system bit error rate model including line width limit variables under the current atmospheric channel:

S7. According to the bit error rate model of the QAM communication system containing the line width limit variable under the current atmospheric channel obtained in step S6, and according to the bit error rate requirement index of the target QAM communication system, determine the line width limit of the laser used in QAM under the current atmospheric channel conditions. .

Illustratively, the specific method of step S7 is as follows: According to the BER _QAM model for the atmospheric channel obtained in step S6 that contains the determined scintillation variance and phase fluctuation variance of the received optical signal under current atmospheric turbulence, the maximum BER of the local oscillator laser of different communication systems is obtained Linewidth, that is, the line width limit, can be determined based on the bit error rate requirements for atmospheric QAM communication system design in the current atmosphere.

Figure 1 is a system structure diagram of the transmitting end and receiving end according to the embodiment of the present invention. Referring to Figure 1, the QAM communication system of this embodiment includes a transmitter and a receiver. The transmitting end consists of a narrow linewidth laser, QAM modulator, erbium-doped fiber amplifier, transmitting system, etc. The receiving end consists of a receiving system, a light intensity sensor, an optical attenuator, a laser that determines power and linewidth, a coupler, and a balanced detector. , digital signal processing module and other components. The transmitter is used for signal modulation and transmission. The electrical signal enters the RF input end of the QAM modulator through the driving amplifier, modulates the laser generated by the narrow linewidth laser and enters the QAM modulator, and obtains an optical signal modulated by the electrical signal. The output optical signal of the QAM modulator passes through the erbium-doped fiber. After amplification by the amplifier, it enters the transmitting system for transmission, and the transmitting system transmits the modulated optical signal into the atmospheric channel. The receiving end is mainly used for receiving and demodulating optical signals. After the optical signal passing through the atmospheric channel is received by the receiving system at the receiving end, it can enter the light intensity sensor to measure the intensity of the optical signal, or it can pass through the optical attenuator and then enter the 3dB coupler and the local oscillator laser at the receiving end for coherent mixing of light. The mixed optical signal enters the balanced detector, which converts the optical signal into an electrical signal and eliminates the high-frequency part, and then enters the digital signal processing module.

Since the construction of an actual spatially coherent laser communication system involves many aspects, it is more complicated to use the test data of the actual system to illustrate. Therefore, a numerical simulation is used to illustrate the atmospheric channel QAM communication system involved in the embodiment of the present invention. The effect of the method for determining the laser line width limit is explained by using numerical simulation to illustrate the method for determining the laser line width limit for the atmospheric channel QAM communication system involved in the embodiment of the present invention. For the sake of versatility and convenience, this embodiment uses the QAM spatial coherent laser communication system of a geostationary satellite with an orbital altitude of 36,000 km. The communication system adopts the Quadrature Amplitude Modulation (QAM) method. The main parameter settings of the communication link are as follows: the communication light wavelength is 1550nm, the rate is 2.5Gbps, the transmitter laser power is 30mW, the erbium-doped fiber amplifier amplification is 30dB, and the receiver diameter is 0.1m.

After the optical tracking and aiming subsystem in free space completes the aiming work of the transmitter and the receiver, the light intensity distribution obtained by the receiver, as shown in Figure 2, is converted into the optical signal intensity distribution probability of the receiver; then this The experimental model of the embodiment simulates the P(I) curve under different atmospheric turbulence scintillation variances, and the curve shown in Figure 3 can be obtained (for intuitive display, this embodiment draws 0.02, 0.08, 0.14 and 0.20 large atmospheric turbulence here. P(I) curve under variance), calculate the correlation coefficient between the receiving end optical signal intensity probability distribution data and the above different variance curves. In this embodiment, the correlation coefficients obtained when the atmospheric turbulence variance is 0.02, 0.08, 0.14 and 0.20 are respectively 0.9969, 0.9875, 0.9840 and 0.9798, from which the current atmospheric turbulence scintillation variance is 0.02;

Next, this embodiment obtains the change in bit error rate of the QAM communication system under different received optical signal intensities by changing the received optical signal intensities. The schematic diagram is shown in Figure 4. This embodiment obtains the QAM change with the received optical signal intensity changes. Bit error rate of communication systems. Then change the intensity of the optical signal at the receiving end in the BER _QAM model, and obtain the curve of the bit error rate changing with the intensity of the received optical signal under the influence of the atmosphere on the phase fluctuation of the optical signal at the receiving end, as shown in Figure 5. Figure 5 plots the received light caused by atmospheric turbulence. The signal phase fluctuations are 1.26×10 ^-4 , 2.52×10 ^-4 , 5.02×10 ^-4 and 10.02×10 ^-4 respectively. Then calculate the correlation between the bit error rate data of different received optical signal strengths in Figure 4 and the above curve. The coefficients are 0.9992, 0.9990, 0.9988 and 0.9979 respectively. Select the curve with the largest correlation coefficient. From this, the phase fluctuation variance of the received optical signal caused by atmospheric turbulence is 1.26×10 ^-4 . At this time, this embodiment has obtained the variance of the received optical signal intensity scintillation caused by atmospheric turbulence in the current atmospheric channel and the phase fluctuation variance of the received optical signal caused by atmospheric turbulence. Then this embodiment can establish a model containing line width limit variables under the current atmospheric channel. The bit error rate model of the QAM communication system obtains the line width tolerance requirements under different communication bit error rate indicators, as shown in Figure 6. Finally, according to the bit error rate requirements of the target QAM communication system, the line width limit of the laser used in QAM under the current atmospheric channel conditions can be determined. The laser line width limit of the current atmospheric channel QAM communication system is determined through the embodiment of the present invention. Due to the combination of the model and the measured data, it not only avoids the model deviating too much from the actual environment, resulting in a decrease in accuracy, but also avoids complex and complex A very difficult experiment is required to obtain the atmospheric channel information; in addition, in this embodiment, in the process of combining the model and data to obtain the atmospheric channel information, all the information is not obtained through one combination, but is obtained twice in steps. This not only reduces the difficulty of combining data and models, but also improves the accuracy of the model.

The above are only preferred embodiments of the present invention. The protection scope of the present invention is not limited to the above-mentioned embodiments. All technical solutions that fall under the idea of the present invention belong to the protection scope of the present invention. It should be pointed out that for those of ordinary skill in the art, several improvements and modifications without departing from the principle of the present invention should be regarded as the protection scope of the present invention.

Claims (8)

1. A method for determining the laser line width limit for atmospheric channel QAM communication systems, characterized in that the laser line width limit determination method includes the following steps: S1 uses a free-space optical tracking and aiming subsystem to complete the aiming work of the transmitter and receiver, so that the signal light transmitter system emits signal light with constant power. A light intensity sensor is used at the receiver to receive the light signal and obtain the time of different light signal intensities. Distribution, calculate the average optical signal strength at the receiving end; S2, convert the time distribution of different optical signal strengths at the receiving end obtained in step S1 into the optical signal intensity probability distribution at the receiving end, and obtain the optical signal intensity probability distribution data at the receiving end; S3. Construct a probability density model of the optical signal intensity at the receiving end. Substitute the average optical signal intensity at the receiving end calculated in step S1 into the probability density model of the optical signal intensity at the receiving end. By changing the atmospheric turbulence scintillation variance, the reception under different atmospheric turbulence scintillation is obtained. The probability density curve of the optical signal intensity; then calculate the correlation coefficient between the probability distribution data of the receiving end optical signal intensity obtained in step S2 and the probability density curve of the received optical signal intensity under different atmospheric turbulence flickers, select the curve with the largest correlation coefficient, and calculate the reception Atmospheric turbulence scintillation variance of probability density model of surface light signal intensity S4. Select a laser with determined power and linewidth at the receiving end to build the receiving end of the QAM communication system, change the intensity of the optical signal received by the receiving end, and obtain the different received light of the QAM communication system under the atmospheric channel through the digital signal processing module. Bit error rate information for signal strength; S5, construct a bit error rate model for the QAM communication system with laser line width limit for atmospheric channels: In the formula, is the QAM bit error rate model with laser line width limit without the influence of atmospheric turbulence, I is the received optical signal intensity,/> is the phase of the received optical signal, Δv _t is the linewidth of the narrow linewidth laser at the transmitting end, Δv _r is the linewidth of the local oscillator laser at the receiving end, P(I) is the probability density of different optical signal intensities I at the receiving end under the influence of atmospheric turbulence, is the phase of the optical signal at the receiving end under the influence of atmospheric turbulence/> Distribution probability density; The calculation model of P(I) is as follows: In the formula, <I> is the average optical signal intensity of the optical signal at the receiving end, is the atmospheric turbulence scintillation variance; The calculation model is as follows: In the formula, is the phase fluctuation variance of the received optical signal caused by atmospheric turbulence; Change the intensity of the optical signal at the receiving end to obtain a curve of the bit error rate changing with the intensity of the received optical signal under the influence of the atmosphere on the phase fluctuation of the optical signal at the receiving end. Calculate the bit error rate data and bit error rate of different received optical signal intensities obtained in step S4. The correlation coefficient of the curve that changes with the intensity of the received light signal is selected. The curve with the largest correlation coefficient is selected to calculate the variance of the phase fluctuation of the received light signal caused by atmospheric turbulence. S6, atmospheric turbulence scintillation variance obtained according to step S3 and the phase fluctuation variance of the received optical signal caused by atmospheric turbulence obtained in step S5/> Establish a QAM communication system bit error rate model including line width limit variables under the current atmospheric channel: S7. According to the bit error rate model of the QAM communication system containing the line width limit variable under the current atmospheric channel obtained in step S6, and according to the bit error rate requirement index of the target QAM communication system, determine the line width limit of the laser used in QAM under the current atmospheric channel conditions. . 2. The laser line width determination method for atmospheric channel QAM communication system according to claim 1, characterized in that the QAM communication system includes a transmitting end and a receiving end; The transmitting end includes a narrow linewidth laser, a QAM modulator, an erbium-doped fiber amplifier, and a transmitting system; The receiving end includes a receiving system, a light intensity sensor, an optical attenuator, a laser that determines power and line width, a coupler, a balance detector, and a digital signal processing module; the receiving system is connected to the optical attenuator and the light intensity sensor respectively, The digital signal processing module includes a demodulation module and a bit error rate calculation module. 3. The laser line width limit determination method for atmospheric channel QAM communication system according to claim 1, characterized in that, in step S1, the process of calculating the average optical signal intensity of the receiving end includes the following sub-steps: Suppose the time-varying optical signal intensity received by the receiving end is I _re (t), and statistically obtain the time distribution T _I of different optical signal intensities at the receiving end; The average optical signal intensity <I> at the receiving end is calculated according to the following formula: In the formula, T is the total time for the receiving end to receive the optical signal. 4. The laser line width limit determination method for atmospheric channel QAM communication system according to claim 1, characterized in that, in step S2, the process of obtaining the receiving end optical signal intensity probability distribution data includes the following sub-steps: According to the time distribution T _I of different optical signal strengths at the receiving end, and through the characteristic that the probability density P(I) of the optical signal intensity at the receiving end is integrated to 1, the following formula is used to calculate the probability of the optical signal intensity at the receiving end: 5. The laser line width determination method for atmospheric channel QAM communication system according to claim 1, characterized in that, in step S3, the atmospheric turbulence scintillation variance of the probability density model of the receiving surface optical signal intensity is obtained. The process includes the following steps: S31. Substitute the average optical signal intensity <I> at the receiving end obtained in step S1 into the calculation model of P(I). By changing the atmospheric turbulence scintillation variance Parameters to obtain different atmospheric turbulence scintillation variances/> The P(I) curve below; S32, calculate the receiving end optical signal intensity probability distribution data P _T (I) obtained in step S2 and the variance of different atmospheric turbulence scintillation according to the following formula The correlation coefficient of the P(I) curve below: In the formula, Cov(P _T (I), P(I)) represents the covariance of P _T (I) and P(I), and var(P _T (I)) and var(P(I)) represent P _T respectively. The variance of (I) and P(I); S33. According to the result of correlation coefficient r(P _T (I), P(I)), select the P(I) curve with the largest correlation coefficient to obtain the current atmospheric turbulence scintillation variance. The value of the parameter is used to obtain all the information of the P(I) calculation model under the current atmospheric turbulence. 6. The laser line width determination method for atmospheric channel QAM communication system according to claim 5, characterized in that, in step S4, the received optical signal intensity is assumed to be: I _α (t) = α I _re (t) In the formula, α is the attenuation coefficient of the optical attenuator, and its value range is 0 to 1. The bit error rate under different I _α (t) is obtained through the digital signal processing module. 7. The laser line width limit determination method for atmospheric channel QAM communication system according to claim 6, characterized in that, in step S5, the phase fluctuation variance of the received optical signal caused by atmospheric turbulence is obtained The process includes the following steps: S51. Substitute the P(I) under the current atmospheric turbulence obtained in step S3 into the bit error rate model BER _QAM of the QAM communication system with laser line width limit for the atmospheric channel, by changing the received optical signal intensity and phase fluctuation variance. parameters, and obtain the BER _QAM curve with the intensity of the received optical signal under different phase fluctuation variances; S52, calculate the values of different received light signal strengths obtained in step S4 according to the following formula: Correlation coefficient between information and BER _QAM curve with received optical signal intensity under different phase fluctuation variances: In the formula, Express/> and covariance of BER _QAM ,/> and var(BER _QAM ) distribution representation/> and the variance of BER _QAM ; S53, according to the correlation coefficient As a result, select the BER _QAM model with the largest correlation coefficient to obtain the phase fluctuation variance of the received optical signal caused by the current atmospheric turbulence/> 8. The laser line width limit determination method for atmospheric channel QAM communication system according to claim 1, characterized in that, in step S7, the process of determining the line width limit of the laser used for QAM under the current atmospheric channel conditions includes the following sub-steps. : According to the BER _QAM model of the atmospheric channel obtained in step S6, which contains the determined scintillation variance and the phase fluctuation variance of the received optical signal under the current atmospheric turbulence, the maximum linewidth of the local oscillator laser of the BER of different communication systems, that is, the line width limit, is obtained according to the atmospheric channel. The bit error rate requirements for QAM communication system design determine the line width limit under the current atmosphere.