Unmanned aerial vehicle to-ground space optical communication adaptive coding modulation method

Abstract

The invention discloses an unmanned aerial vehicle to-ground space optical communication adaptive coding modulation method. The method comprises the following steps: constructing an unmanned aerial vehicle to-ground space optical communication system model; constructing a fifteen-order pseudo-random binary code as an electric signal input of the space optical communication system model, and receiving the coded and modulated electric signal after the electric signal passes through the space optical communication system model in the form of a laser signal; constructing a preliminary adaptive coding modulation scheme under medium turbulence; constructing a threshold value of FER upper and lower limit switching in a formulated coding modulation scheme based on medium turbulence, and constructing a hybrid switching strategy based on channel SNR and communication FER; and the performance is verified in an unmanned aerial vehicle space optical communication system with variable turbulence. According to the unmanned aerial vehicle-to-ground space optical communication adaptive coding modulation method provided by the invention, a proper coding modulation format is selected on the premise of satisfying a feedforward error correction threshold error rate in an atmospheric channel with turbulence change, and the accuracy and throughput of a space optical communication system are improved.

Description

Technical Field

[0001] The present invention relates to the field of space optical communication, and in particular to an adaptive coding and modulation method for space optical communication between unmanned aerial vehicles and the ground. Background Art

[0002] Space optical communication is an advanced communication technology that uses lasers to transmit data in atmospheric channels. In some areas where optical fiber cannot be easily installed, traditional wireless communication technology is often supplemented by radio frequency communication, and space optical communication is considered to be a superior choice in this scenario due to its high efficiency and rapid deployment. Drones themselves have the characteristics of convenient carrying, flexible deployment and wide coverage, which complement the advantages of space optical communication. Especially in scenarios such as aerial photography, high-definition images and videos, and three-dimensional reconstruction that require high-bandwidth communication, they show great development potential. Therefore, space optical communication based on drones has received widespread attention.

[0003] However, UAV space optical communication faces many challenges. When transmitting in the atmospheric channel, laser beams are easily affected by factors such as humidity, temperature and wind speed, which causes random fluctuations in the refractive index of light along the propagation path, triggering adverse effects such as light spot flickering, beam expansion and light spot drift, namely atmospheric turbulence. These phenomena seriously affect the phase stability and intensity consistency of the spatial optical signal received by the ground receiver, thereby significantly reducing the accuracy and quality of communication.

[0004] Since space optical communication is easily affected by atmospheric turbulence and produces a high bit error rate, in order to overcome the signal fluctuations caused by changes in channel states, adaptive transmission technology is used to select a more suitable transmission scheme according to the channel state to obtain better communication quality. Adaptive Coding and Modulation (ACM) is an adaptive scheme that is relatively easy to implement in hardware and is suitable for the hardware environment on drones. However, in actual applications, changes in the intensity of atmospheric turbulence in the atmospheric channel will cause the adaptive coding and modulation scheme formulated under fixed turbulence to be unable to adapt to changes in the turbulence intensity of the atmospheric channel, affecting communication performance. Summary of the invention

[0005] In view of the above-mentioned defects of the prior art, the technical problem to be solved by the present invention is that the change of atmospheric turbulence intensity in the atmospheric channel will cause the adaptive coding and modulation scheme formulated under fixed turbulence to be unable to adapt to the change of turbulence intensity in the atmospheric channel, thus affecting the communication performance. The present invention provides an adaptive coding and modulation method for UAV-to-ground space optical communication, which is an adaptive coding and modulation method for UAV-to-ground space optical communication based on a hybrid switching strategy. It selects a suitable coding and modulation format under the premise of satisfying the Forward Error Correction (FEC) bit error rate in the atmospheric channel with turbulent changes, thereby improving the accuracy and throughput of the space optical communication system.

[0006] To achieve the above object, the present invention provides an adaptive coding and modulation method for UAV-to-ground space optical communication, which specifically includes the following steps:

[0007] Construct a model of UAV-to-ground space optical communication system;

[0008] A fifteen-order pseudo-random binary code is constructed as an electrical signal input of the space optical communication system model, and the coded modulated electrical signal is received after passing through the space optical communication system model as a laser signal;

[0009] Construct a preliminary adaptive coding modulation scheme under medium turbulence to verify the impact of turbulence intensity changes on this adaptive coding modulation scheme;

[0010] In the established coding modulation scheme based on medium turbulence, the thresholds for switching between upper and lower limits of FER are constructed, the FER switching scheme is obtained, and a hybrid switching strategy based on channel SNR and communication FER is constructed.

[0011] The performance of the adaptive coding modulation scheme based on the hybrid switching strategy is verified in the turbulent and changeable UAV space optical communication system.

[0012] Furthermore, a UAV-to-ground space optical communication system model is constructed, wherein the UAV-to-ground space optical communication system model includes a UAV transmitter, an atmospheric channel, and a UAV receiver. The UAV transmitter uses laser as a carrier, first performs channel coding on the data stream, modulates it to the light source generated by the laser by an optical modulator, then transmits the laser to the atmospheric channel through a transmitting antenna, and the UAV receiver receives the laser. The photodetector first converts the received optical signal into an electrical signal, and then demodulates and decodes it.

[0013] Furthermore, in the UAV-to-ground space optical communication system, the communication data electrical signal is coded and modulated at the UAV transmitting end and then transmitted into the space optical atmospheric channel as a laser signal, and the ground receiving station demodulates and decodes the laser signal to obtain the communication data.

[0014] Furthermore, a fifteen-order pseudo-random binary code is constructed as the electrical signal input of the space optical communication system model, and the coded modulated electrical signal is received after passing through the space optical communication system model as a laser signal. Specifically, the fifteen-order pseudo-random binary code electrical signal is constructed, coded and modulated by the UAV transmitter, and then modulated to a 1550nm laser carrier through an MZM modulator and sent to the UAV-to-ground space optical communication atmospheric channel. After the ground receiver receives the laser signal from the atmospheric channel, it is demodulated and decoded into an electrical signal after passing through an APD photodiode and a filter for reception.

[0015] Furthermore, a preliminary adaptive coding and modulation scheme under medium turbulence is constructed, and the coding and modulation format with the highest communication bit efficiency under the current SNR is obtained while meeting the FEC bit error rate threshold, and the turbulence intensity is selected as medium turbulence.

[0016] Furthermore, a preliminary adaptive coding modulation scheme under medium turbulence is constructed, and the formula followed is as follows:

[0017] R p =max(M i ,Rc j ,P e (h)≤P)

[0018] Among them, R p represents the communication bit efficiency of the adaptive coding modulation scheme based on fixed turbulence intensity, and P represents the forward error correction bit rate threshold of BER.

[0019] Furthermore, the thresholds for switching between the upper and lower limits of FER are constructed in the coded modulation scheme based on fixed turbulence, and the FER switching scheme is obtained. A hybrid switching strategy based on channel SNR and communication FER is constructed. The formula of the combined switching strategy is as follows:

[0020]

[0021] M i =2 i (i=1,2),Rc j ∈(1 / 2,2 / 3,4 / 5)

[0022] in, represents the communication bit efficiency of the adaptive coding modulation scheme based on the hybrid switching strategy, P represents the forward error correction bit rate threshold of BER. On the basis of step 3, the upper and lower limit thresholds of FER need to be added, which are f0 and f1, respectively. They are determined according to the adaptive coding modulation scheme with certain redundancy under medium turbulence.

[0023] Furthermore, in the IM / DD communication system, the modulation formats are OOK and PAM4, and the code rates of the LDPC codes are {1 / 2, 2 / 3, 4 / 5}.

[0024] Furthermore, the performance of the adaptive coding modulation scheme based on the hybrid switching strategy is verified in the UAV space optical communication system with changeable turbulence. Specifically, one of the two variables, turbulence intensity and SNR, is fixed in the UAV space optical communication system, while the other variable is changed, and the communication BER obtained by the hybrid switching strategy is used to verify the performance of the scheme.

[0025] Technical Effects

[0026] The invention discloses an adaptive coding and modulation method for UAV-to-ground space optical communication, which is an adaptive coding and modulation method for UAV-to-ground space optical communication based on a hybrid switching strategy. The curve of the ACM scheme using hybrid threshold switching is more stable when the turbulence intensity changes, and has good adaptability to the change of turbulence intensity. A coding and modulation format that is relatively suitable for the channel state can be selected, which effectively improves the communication capacity of the channel and reduces the communication bit error rate. Under the premise of satisfying the Forward Error Correction (FEC) bit error rate in the atmospheric channel with turbulent changes, a suitable coding and modulation format is selected to improve the accuracy and throughput of the space optical communication system. In practical applications, the change of atmospheric turbulence intensity in the atmospheric channel will cause the adaptive coding and modulation scheme formulated under fixed turbulence to be unable to adapt to the change of turbulence intensity in the atmospheric channel, affecting the communication performance.

[0027] The concept, specific structure and technical effects of the present invention will be further described below in conjunction with the accompanying drawings to fully understand the purpose, characteristics and effects of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a flow chart of an adaptive coding and modulation method for UAV-to-ground space optical communication according to a preferred embodiment of the present invention;

[0029] Figure 2 It is a preferred embodiment of the present invention, a method for adaptive coding and modulation of UAV-to-ground space optical communication, based on a hybrid switching strategy;

[0030] Figure 3 It is a preferred embodiment of the present invention, a UAV to ground space optical communication adaptive coding modulation scheme with different threshold switching mechanisms and an ideal adaptive coding modulation scheme SNR difference and turbulence relationship;

[0031] Figure 4 It is a preferred embodiment of the present invention, a method for adaptive coding and modulation of UAV-to-ground space optical communication under weak turbulence BER performance;

[0032] Figure 5 The present invention is a preferred embodiment of an adaptive coding and modulation method for UAV-to-ground space optical communication under strong turbulence. DETAILED DESCRIPTION

[0033] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0034] In the following description, specific details such as specific internal procedures and techniques are provided for the purpose of illustration rather than limitation, so as to provide a thorough understanding of the embodiments of the present invention. However, it should be clear to those skilled in the art that the present invention may be implemented in other embodiments without these specific details. In other cases, detailed descriptions of well-known systems, devices, circuits, and methods are omitted to prevent unnecessary details from obstructing the description of the present invention.

[0035] like Figure 1 As shown, the present invention provides an adaptive coding and modulation method for UAV-to-ground space optical communication. After the fifteenth-order pseudo-random binary code passes through the UAV transmitter, it is modulated to a 1550nm laser carrier through an MZM modulator and sent to the space optical communication channel. After the receiving end passes through an APD photodiode and a filter, the processed electrical signal is subjected to channel estimation and FER calculation, and the result is transmitted to the transmitting end through a feedback channel. The transmitting end selects the coding and modulation format according to the feedback result.

[0036] The following steps are involved:

[0037] Step 1: Construct a UAV-to-ground space optical communication system model;

[0038] Step 2: construct a fifteen-order pseudo-random binary code as an electrical signal input of the space optical communication system model, and receive the coded and modulated electrical signal after passing through the space optical communication system model;

[0039] Step 3: construct a preliminary adaptive coding modulation scheme under medium turbulence to verify the impact of turbulence intensity changes on this adaptive coding modulation scheme;

[0040] Step 4: construct the thresholds for switching between upper and lower limits of FER in the formulated coding modulation scheme based on medium turbulence, obtain the FER switching scheme, and construct a hybrid switching strategy based on channel SNR and communication FER;

[0041] Step 5: Verify the performance of the adaptive coding modulation scheme based on the hybrid switching strategy in the turbulent and changeable UAV space optical communication system.

[0042] Specifically, step 1, construct a UAV-to-ground space optical communication system model, which includes a UAV transmitter, an atmospheric channel, and a UAV receiver. The UAV transmitter uses laser as a carrier, first performs channel encoding on the data stream, modulates it to the light source generated by the laser by an optical modulator, then transmits the laser to the atmospheric channel through a transmitting antenna, and the UAV receiver receives the laser. The photodetector first converts the received optical signal into an electrical signal, and then demodulates and decodes it.

[0043] In the UAV-to-ground space optical communication system, the communication data electrical signal is coded and modulated at the UAV transmitting end and then transmitted into the space optical atmospheric channel as a laser signal. The ground receiving station demodulates and decodes the laser signal to obtain the communication data.

[0044] In the UAV-to-ground space optical communication channel, the signal x sent by the transmitter and the signal y received by the receiver can be expressed as:

[0045] y=ηIx+noise

[0046] Among them, η is the APD photoelectric conversion sensitivity, h is the link gain, and noise is the additive Gaussian white noise with an average power of zero. The atmospheric turbulence attenuation effect obeys the Gamma-Gamma distribution, and the Retov variance is used to represent the turbulence size.

[0047] The channel link gain h is mainly determined by atmospheric attenuation and atmospheric turbulence:

[0048] I=I l I a

[0049] Among them, I l and I a They represent the effects of atmospheric attenuation and atmospheric turbulence respectively.

[0050] The atmospheric turbulence probability model follows the Gamma-Gamma distribution. The model uses small-scale eddy currents I x Scattering effects and large-scale eddies I y The refraction effect is used to explain the light intensity flicker, and the formula is as follows:

[0051] I a =I x I y

[0052] The probability density functions are as follows:

[0053]

[0054] Therefore, the probability density function of light intensity flicker caused by atmospheric turbulence is:

[0055]

[0056] Where K n (.) is the second kind of n-order modified Bessel function, Γ(.) is the gamma function, α and β are the effective numbers of large-scale vortices and small-scale vortices, respectively, and their formulas are:

[0057]

[0058] in, is the refractive index structure constant, k=2π / λ is the light wave number, and λ is the wavelength.

[0059] The atmospheric channel model that obeys the Gamma-Gamma atmospheric turbulence distribution model can reflect the impact of the drone-to-ground space light model on the signal in actual communication, and simulate the intensity fluctuations and phase fluctuations generated in signal communication, so as to facilitate the subsequent establishment of a data set simulating real atmospheric channel communication.

[0060] Step 2: Construct a fifteen-order pseudo-random binary code as the electrical signal input of the space optical communication system model, modulate and encode the input electrical signal, and then receive it after passing through the space optical communication system; construct a fifteen-order pseudo-random binary code electrical signal, encode and modulate it through the UAV transmitter, modulate it to a 1550nm laser carrier through an MZM modulator and send it to the UAV-to-ground space optical communication atmospheric channel. After the ground receiver receives the laser signal from the atmospheric channel, it demodulates and decodes it into an electrical signal after passing it through an APD photodiode and filter for reception.

[0061] Step 3: Construct a preliminary adaptive coding and modulation scheme under fixed turbulence. The adaptive coding and modulation scheme uses the highest possible coding and modulation order to improve the system throughput while ensuring the BER, that is, to obtain the coding and modulation format with the highest communication bit efficiency under the current SNR while meeting the FEC bit error rate threshold. The turbulence intensity is selected as medium turbulence to provide a preliminary adaptive coding and modulation scheme for step 4. The formula followed is as follows:

[0062] R p =max(M i ,Rc j ,P e (h)≤P)

[0063] Among them, R p represents the communication bit efficiency of the adaptive coding modulation scheme based on fixed turbulence intensity, and P represents the forward error correction bit rate threshold of BER.

[0064] In the adaptive coding and modulation scheme based on fixed turbulence in this embodiment, the factor measuring the system performance is the bit efficiency R, which is defined as the number of effective bits transmitted per symbol on average, excluding coding redundancy. The purpose of the traditional adaptive coding and modulation scheme is to select the coding and modulation format with the maximum bit efficiency R according to the channel state while ensuring that the communication bit error rate is based on the FEC threshold of BER (3.9×10 -3 ).

[0065] The adaptive coding and modulation scheme based on fixed turbulence can flexibly select a suitable coding and modulation format according to the change of the signal-to-noise ratio of the channel when the turbulence intensity remains unchanged, ensuring the throughput of communication.

[0066] Step 4: Construct a threshold for switching the upper and lower limits of FER in the coding and modulation scheme based on fixed turbulence to obtain an FER switching scheme, and construct a hybrid switching strategy based on the channel SNR and communication FER. The formula of the hybrid switching strategy is as follows:[[]]

[0067]

[0068] M i = 2 i (i = 1, 2), Rc j ∈(1 / 2, 2 / 3, 4 / 5)

[0069] where, represents the communication bit efficiency of the adaptive coding and modulation scheme based on the hybrid switching strategy, P represents the forward error correction code rate threshold of BER. On the basis of step 3, the upper and lower limits of FER need to be increased to f0 and f1 respectively, which are determined according to the adaptive coding and modulation scheme with a certain redundancy under medium turbulence. In the IM / DD communication system of the present invention, the modulation formats are OOK and PAM4, and the code rates of LDPC coding are {1 / 2, 2 / 3, 4 / 5}

[0070] In terms of the FER switching strategy, the upper and lower limits of FER are denoted as f0 and f1 respectively. At the receiving end, if the FER of the LDPC code exceeds f0, it indicates that the current channel turbulence intensity has increased, and the coding and modulation format should be reduced. Similarly, if the FER is lower than f1, it means that the current channel turbulence intensity has decreased, and the complexity of the coding and modulation format should be increased, so as to maximize the bit efficiency R under the condition of f1 < f < f0.

[0071] Although the adaptive coding modulation scheme formulated in step 3 under a fixed turbulence intensity can adapt to the channel state for a long time without the need for FER-based correction, when the turbulence intensity changes, the adaptive coding modulation scheme will no longer adapt to the current channel. The hybrid switching strategy proposed in step 4 can improve this situation, which allows the adaptive system to switch flexibly according to changes in channel SNR and turbulence intensity, which has a significant improvement in the quality of space optical communications in atmospheric channels.

[0072] Step 5: Verify the performance of the adaptive coding modulation scheme based on the hybrid switching strategy developed in step 4 in the turbulent and changeable UAV space optical communication system.

[0073] In the UAV space optical communication system, one of the two variables, turbulence intensity and SNR, is fixed, and the other variable is changed. The communication BER obtained by the hybrid switching strategy is used to verify the performance of the scheme. The results are shown in Figure 3 , 4 , 5. Figure 3 In the present invention, the performance of the adaptive coding modulation scheme based on the hybrid switching strategy is verified, and the performance of the embodiment of the present invention is compared with that of the adaptive coding modulation scheme based on fixed turbulence. By changing the turbulence intensity and comparing the average SNR difference between the two and the ideal adaptive coding modulation scheme, it can be seen that the embodiment of the present invention has significantly more stable performance when the turbulence changes. This is analyzed in detail below.

[0074] Figure 3It is the relationship between the SNR difference and turbulence intensity (Turbulence Index) of the adaptive coding modulation scheme with different threshold switching mechanisms and the ideal adaptive coding modulation scheme. It can be seen that when the ACM scheme that only adopts the SNR switching threshold is placed at a turbulence lower than the original setting, its SNR difference is greater than 0, which indicates that the switching threshold of the ACM scheme at this time lags behind the ideal ACM scheme switching threshold, that is, when the channel conditions become good enough to afford a higher level of coding and modulation format, the ACM scheme does not switch in time, resulting in a waste of bit efficiency and communication capacity. Similarly, when the turbulence intensity is greater than the original turbulence, the average switching threshold difference of the traditional ACM scheme is less than 0, which indicates that the switching threshold of the ACM scheme at this time is ahead of the ideal ACM scheme under the current turbulence, that is, it switches to a higher level of coding and modulation format when the channel conditions are not good enough, which will cause an increase in the communication bit error rate and may seriously cause communication interruption. The curve of the ACM scheme with mixed threshold switching is more stable when the turbulence intensity changes, and its average value is about 0.35dB. Compared with the traditional ACM scheme, the absolute value of the SNR difference can reach nearly 4dB when the turbulence changes greatly, which has obvious robustness. Since a certain amount of redundancy is reserved when formulating the FER upper and lower limit switching thresholds of the LDPC code, its SNR difference curve is slightly greater than 0 at different turbulences, indicating that its coding modulation switching lags slightly behind the ideal ACM scheme, but the gap is not obvious, indicating that it has good adaptability to changes in turbulence intensity and can select a coding modulation format that is relatively suitable for the channel state, effectively improving the communication capacity of the channel and reducing the communication bit error rate.

[0075] Figure 4 , 5 It is the BER performance of the adaptive coding modulation scheme based on the hybrid switching strategy under weak and strong turbulence. Figure 4 It can be seen that after the adaptive coding and modulation scheme proposed in this paper adopts FEC threshold switching, the coding and modulation format is switched promptly after the channel conditions are met. When the channel conditions are good, the coding and modulation format is switched upward in time, and when the channel conditions are poor, the coding and modulation are switched downward in time. The SNR sensitivity of the overall adaptive coding and modulation scheme is improved by an average of 1.8dB. Figure 5 It can be seen that the ACM scheme proposed in this paper can downgrade the coding modulation format that does not meet the bit error rate requirements, and obtain a coding modulation scheme with a BER lower than FEC. The FER threshold proposed in this paper increases the SNR threshold of the ACM scheme by an average of 4dB, ensuring smooth communication and achieving relatively ideal communication quality.

[0076] The preferred specific embodiments of the present invention are described in detail above. It should be understood that a person skilled in the art can make many modifications and changes based on the concept of the present invention without creative work. Therefore, any technical solution that can be obtained by a person skilled in the art through logical analysis, reasoning or limited experiments based on the concept of the present invention on the basis of the prior art should be within the scope of protection determined by the claims.

Claims

1. An adaptive coding and modulation method for UAV-to-ground space optical communication, characterized in that: The following steps are involved: Construct a model of UAV-to-ground space optical communication system; A fifteen-order pseudo-random binary code is constructed as an electrical signal input of the space optical communication system model, and the coded modulated electrical signal is received after passing through the space optical communication system model as a laser signal; Construct a preliminary adaptive coding modulation scheme under medium turbulence to verify the impact of turbulence intensity changes on this adaptive coding modulation scheme; In the established coding modulation scheme based on medium turbulence, the thresholds for switching between upper and lower limits of FER are constructed, the FER switching scheme is obtained, and a hybrid switching strategy based on channel SNR and communication FER is constructed. The performance of the adaptive coding modulation scheme based on the hybrid switching strategy is verified in the turbulent and changeable UAV space optical communication system.

2. The method for adaptive coding and modulation of UAV-to-ground space optical communication according to claim 1, characterized in that: Construct a UAV-to-ground space optical communication system model, where the UAV-to-ground space optical communication system model includes a UAV transmitter, an atmospheric channel, and a UAV receiver. The UAV transmitter uses laser as a carrier, first performs channel coding on the data stream, modulates it to the light source generated by the laser by an optical modulator, and then transmits the laser to the atmospheric channel through a transmitting antenna. The UAV receiver receives the laser, and the photodetector first converts the received optical signal into an electrical signal, and then demodulates and decodes it.

3. The method for adaptive coding and modulation of UAV-to-ground space optical communication according to claim 1, characterized in that: In the UAV-to-ground space optical communication system, the communication data electrical signal is coded and modulated at the UAV transmitting end and then transmitted into the space optical atmospheric channel as a laser signal. The ground receiving station demodulates and decodes the laser signal to obtain the communication data.

4. The method for adaptive coding and modulation of UAV-to-ground space optical communication according to claim 1, characterized in that: A fifteen-order pseudo-random binary code is constructed as the electrical signal input of the space optical communication system model, and the coded modulated electrical signal is received after passing through the space optical communication system model as a laser signal. Specifically, the fifteen-order pseudo-random binary code electrical signal is coded and modulated by the UAV transmitter, and then modulated to a 1550nm laser carrier through an MZM modulator and sent to the UAV-to-ground space optical communication atmospheric channel. After the ground receiver receives the laser signal from the atmospheric channel, it is demodulated and decoded into an electrical signal after passing through an APD photodiode and a filter for reception.

5. The method for adaptive coding and modulation of UAV-to-ground space optical communication according to claim 1, characterized in that: A preliminary adaptive coding and modulation scheme under medium turbulence is constructed, and the coding and modulation format with the highest communication bit efficiency under the current SNR is obtained while meeting the FEC bit error rate threshold. The turbulence intensity is selected to be medium turbulence.

6. The method for adaptive coding and modulation of UAV-to-ground space optical communication according to claim 5, characterized in that: Construct a preliminary adaptive coding modulation scheme under medium turbulence, and follow the formula as follows: R p =max(M i ,Rc j ,P e (h)≤P) Among them, R p represents the communication bit efficiency of the adaptive coding modulation scheme based on fixed turbulence intensity, and P represents the forward error correction bit rate threshold of BER.

7. The method for adaptive coding and modulation of UAV-to-ground space optical communication according to claim 6, characterized in that: In the coding modulation scheme based on fixed turbulence, the thresholds for switching between the upper and lower limits of FER are constructed, and the FER switching scheme is obtained. A hybrid switching strategy based on channel SNR and communication FER is constructed. The formula of the combined switching strategy is as follows: M i =2 i (i=1,2),Rc j ∈(1 / 2,2 / 3,4 / 5) in, represents the communication bit efficiency of the adaptive coding modulation scheme based on the hybrid switching strategy, P represents the forward error correction bit rate threshold of BER. On the basis of step 3, the upper and lower limit thresholds of FER need to be added, which are f0 and f1, respectively. They are determined according to the adaptive coding modulation scheme with certain redundancy under medium turbulence.

8. The method for adaptive coding and modulation of UAV-to-ground space optical communication according to claim 7, characterized in that: In the IM / DD communication system, the modulation formats are OOK and PAM4, and the code rates of LDPC coding are {1 / 2, 2 / 3, 4 / 5}.

9. The method for adaptive coding and modulation of UAV-to-ground space optical communication according to claim 1, characterized in that: The performance of the adaptive coding modulation scheme based on the hybrid switching strategy is verified in the UAV space optical communication system with changeable turbulence. Specifically, one of the two variables, turbulence intensity and SNR, is fixed in the UAV space optical communication system, while the other variable is changed, and the communication BER obtained by the hybrid switching strategy is used to verify the performance of the scheme.

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