FSO communication method for adaptive switching of SISO / MIMO mode

Abstract

The invention discloses an FSO communication method for adaptive switching of SISO / MIMO modes. The FSO communication method comprises the following steps: establishing an FSO composite atmospheric channel model; a laser module, a modulation and demodulation module and a real-time bit error rate monitoring and feedback module are written based on a GNURRadio platform, an oscilloscope module is set to observe signal waveforms, and a self-adaptive SISO / MIMO-FSO link model is built; and error rate data of SISO and MIMO modes under different influences are obtained, a threshold value is determined based on a switching criterion of an FEC error rate upper bound, and a suitable optimal signal transmission mode is selected. The transmitting end realizes dynamic switching of the transmission modes according to the feedback information, the SISO link is adopted to reduce energy consumption when the channel quality is relatively good, the MIMO link is switched to enhance signal strength and reliability when the channel quality is relatively poor, and a theoretical basis is provided for keeping robustness and stability of an FSO system in a variable environment.

Description

Technical Field

[0001] The present invention belongs to the technical field of free space optical communication (FSO), and in particular relates to a FSO communication method for adaptively switching between SISO / MIMO modes. Background Art

[0002] One of the most important and fundamental issues in wireless communications is the scarcity of radio frequency spectrum. Due to the shortage of radio frequency spectrum, the allocation of additional radio frequency bandwidth can no longer meet the requirements of high data transmission rates. Compared with traditional radio frequency communications, FSO has become a research hotspot in the future communication field due to its advantages of high bandwidth, no spectrum restrictions and high security. In urban communications, FSO can be used as a supplement or substitute for fiber optic communications, especially in areas where optical fiber is difficult to lay, to achieve high-bandwidth data transmission between buildings, such as high-speed Internet access and local area network interconnection. In military communications, FSO is suitable for tactical data transmission on the battlefield due to its high confidentiality and anti-interference capabilities, supporting the interconnection of command and control, reconnaissance and surveillance, and combat systems. In addition, in scenarios where communication systems need to be deployed quickly, such as disaster relief, emergency response, and large-scale events, FSO can quickly establish temporary communication links and provide reliable data transmission services.

[0003] In general, the performance of FSO communication links depends mainly on the climate and physical characteristics of the installation site. The main factors affecting performance include atmospheric attenuation, scintillation caused by turbulence, pointing errors caused by aperture alignment or building movement, solar interference and line of sight obstructions, which often lead to fluctuations in signal strength and reduced signal quality, thus affecting the stability and performance of the communication system. Turbulence is one of the main challenges in free-space optical communications. Turbulence causes strong intensity fluctuations of optical signals during transmission, which affects the signal quality at the receiving end. In addition, pointing error is also an important issue, that is, the inaccurate pointing of the light beam between the transmitter and the receiver due to thermal expansion and slight vibrations of the building leads to signal loss and reduced signal strength received at the receiving end. The energy attenuation of optical signals during transmission is called link loss. As the transmission distance increases, the link loss gradually increases, thereby limiting the transmission distance and reliability of the FSO system, especially in severe weather conditions such as heavy fog. Water molecules and suspended particles in the atmosphere absorb and scatter optical signals, further causing signal attenuation and significantly reducing the transmission distance throughput.

[0004] In the existing technology, MIMO is a reliable technology to achieve spatial diversity and alleviate the fading caused by atmospheric turbulence and fog in long-distance data transmission. Adaptive transmission technology (such as adaptive modulation, adaptive rate, and adaptive power) provides a feasible solution to suppress the influence of atmospheric turbulence on laser signal transmission and improve the communication performance of the system under various channel conditions. Different MIMO transmission modes can be selected according to the channel conditions: diversity, multiplexing or hybrid mode, to obtain the maximum average channel capacity while meeting a reliable average bit error rate (BER).

[0005] In FSO, the beam can be modulated using analog or digital signals. SDR (Software-DefinedRadio) provides a reusable and future-proof platform by combining the RF to baseband transceiver physical layer and the digital intelligent processor. This provides many advantages. First, as an open source software and hardware collaborative development platform, GNURadio provides a rich set of modules and tools to support rapid prototyping. Through GNURadio's graphical interface and Python programming interface, researchers can flexibly perform rapid iteration and optimization design of FSO systems, making the process of designing and testing FSO systems more efficient. Secondly, compared with traditional hardware implementation solutions, researchers can use GNURadio to perform preliminary design and performance evaluation of the system without the need for expensive hardware equipment, significantly reducing development costs. Finally, GNURadio allows researchers to perform system simulation and actual hardware testing on the same platform. Through integration with hardware devices such as USRP, system design can be verified in a simulation environment and seamlessly transitioned to an actual hardware environment for further testing and verification. Summary of the invention

[0006] The purpose of the present invention is to provide a SISO / MIMO mode adaptive switching FSO communication method, which solves the problem of low communication reliability of the existing free space optical communication system in the composite atmospheric channel.

[0007] The technical solution adopted by the present invention is: a FSO communication method for adaptive switching of SISO / MIMO modes, comprising the following steps: Step 1: Establish an FSO composite atmospheric channel model that considers the combined effects of atmospheric turbulence, pointing error, and fog loss; Step 2: Based on the FSO composite atmospheric channel model obtained in step 1, an adaptive SISO / MIMO-FSO communication system model is constructed and a signal is generated; Step 3: Under different turbulence intensities, pointing errors, and visibility ranges, the bit error rate data of the signals under the SISO and MIMO modes are obtained based on the communication system model obtained in step 2, and a threshold lookup table for mode switching is established according to the bit error rate data; Step 4: The transmitter transmits the signal into the atmospheric composite channel. After receiving the signal, the receiver compares the channel state information with the available thresholds in the threshold lookup table and selects the best applicable signal transmission mode in the SISO and MIMO modes.

[0008] The present invention is also characterized in that: The FSO composite atmospheric channel model established in step 1 is expressed as: (1) In the formula, is the optical power at the receiving end; is the optical power at the transmitting end; Indicates the responsivity of the receiver; is additive Gaussian white noise; is the normalized channel fading coefficient, expressed as , represents the random component of atmospheric turbulence, represents the random component of the pointing error, Represents the random component of fog loss.

[0009] Random component of fog loss It's about wavelength Function: (2) In the formula, is the transmission distance, is the attenuation coefficient, expressed as: (3) In the formula, and are the molecular absorption coefficient and the aerosol absorption coefficient, respectively. is the molecular scattering coefficient, is the aerosol scattering coefficient after fog attenuation, given by the Kim model: (4) In the formula, is the visibility level, is the wavelength of the light source, is the size distribution of the scattering fog particles of the Kim model.

[0010] Random Components of Atmospheric Turbulence Using the Gamma-Gamma turbulence model to characterize, the probability density function is: (11) In the formula, is the Gamma function, It means the second type The modified Bessel function of order, and is a parameter related to small-scale turbulent eddies and large-scale turbulent eddies, expressed as: (12) (13) In the formula, is the Rytov variance value, expressed as: (8) In the formula, is the wave number, is the transmission distance, is the refractive index structure constant, expressed as: (9) In the formula, is the atmospheric pressure, is the temperature, The distance between temperature difference.

[0011] Random component of pointing error It is expressed as: (14) In the formula, is the radial displacement of the light spot; express = 0 received power; is the equivalent beam waist radius and , , represents the error function, is the beam waist radius, is the receiver radius.

[0012] The adaptive SISO / MIMO-FSO communication system model built in step 2 includes a laser module, a modulation module, a channel module encapsulating the FSO composite atmospheric channel model, a demodulation module, and a real-time bit error rate monitoring feedback module. When generating a signal, the laser power and wavelength of the laser module are set, the DBPSK modulation format is used, clock recovery is performed, and a receiver matching filter is provided to eliminate inter-code interference. At the same time, the signal is downsampled to restore the phase and frequency synchronization of the digital modulation signal.

[0013] The process of establishing the threshold lookup table in step 3 is as follows: keep the fog loss, pointing error and geometric loss parameters unchanged, change the atmospheric turbulence, and establish different mode transmission switching lookup tables based on different atmospheric turbulences at multiple link distances based on the upper limit of the FEC bit error rate; keep the pointing error, atmospheric turbulence and geometric loss parameters unchanged, change the fog loss, and establish different mode transmission switching lookup tables based on different visibility at multiple link distances based on the upper limit of the FEC bit error rate; keep the fog loss, atmospheric turbulence and geometric loss parameters unchanged, change the pointing error, and establish different mode transmission switching lookup tables based on different pointing errors at multiple link distances based on the upper limit of the FEC bit error rate.

[0014] Step 4 is specifically as follows: the transmitting antennas TA and TB transmit the optical signal generated by the laser module into the atmospheric composite channel, which is then received by the receiving antennas RA and RB. The transmitting antenna TB is opened or closed according to the channel state information CSI. When the atmospheric visibility of the channel is greater than the visibility threshold, or the refractive index structure constant is less than the refractive index structure constant threshold, or the jitter standard deviation of the pointing error is less than the jitter standard deviation threshold, that is, when the bit error rate is lower than the FEC bit error rate upper limit, the transmitting antenna TB is closed and the SISO link is used for transmission to save transmission power; when the atmospheric visibility of the channel is lower than the visibility threshold, or the refractive index structure constant is greater than the refractive index structure constant threshold, or the jitter standard deviation of the pointing error is greater than the jitter standard deviation threshold, that is, when the bit error rate is higher than the FEC bit error rate upper limit, the transmitting antenna TB is opened and the MIMO link is used for transmission to obtain diversity gain.

[0015] The beneficial effects of the present invention are as follows: the FSO communication method of the present invention for adaptively switching the SISO / MIMO modes realizes dynamic switching of the transmission mode at the transmitting end according to feedback information, adopts the SISO link to reduce energy consumption when the channel quality is good, and switches to the MIMO link to enhance signal strength and reliability when the channel quality is poor, thus providing a theoretical basis for the FSO system to maintain robustness and stability in a changing environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a schematic diagram of the principle of the FSO communication method of the SISO / MIMO mode adaptive switching of the present invention; FIG2 (a) is a curve showing the system bit error rate changing with turbulence when L=300m, visibility V=1000m, and pointing error jit=0.02m in the present invention; FIG2( b ) is a curve showing the system bit error rate changing with turbulence when L=500 m, visibility V=1000 m, and pointing error jit=0.02 m in the present invention; FIG2 (c) is a curve showing the system bit error rate changing with turbulence when L=1500m, visibility V=1000m, and pointing error jit=0.02m in the present invention; Figure 3 (a) shows the link distance L = 300m and turbulence intensity in the present invention. =10 -15 , the curve of system bit error rate changing with visibility when pointing error jit=0.02m; Figure 3 (b) shows the link distance L = 500m and turbulence intensity in the present invention. =10 -15 , the curve of system bit error rate changing with visibility when pointing error jit=0.02m; Figure 3 (c) shows the link distance L = 1500m and turbulence intensity in the present invention. =10 -15 , the curve of system bit error rate changing with visibility when pointing error jit=0.02m; Figure 4 (a) shows the link distance L = 300m and turbulence intensity in the present invention. =-15, the system bit error rate changes with pointing error when visibility V=1000m; Figure 4 (b) shows the link distance L = 500m and turbulence intensity in the present invention. =-15, the system bit error rate changes with pointing error when visibility V=1000m; Figure 4 (c) shows the link distance L = 1500m and turbulence intensity in the present invention. =-15, the system bit error rate curve changes with pointing error when visibility V=1000m. DETAILED DESCRIPTION

[0017] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0018] Example 1 The present invention provides a FSO communication method for adaptive switching of SISO / MIMO modes, which is implemented according to the following steps: Step 1: Establish an FSO composite atmospheric channel model that considers the combined effects of atmospheric turbulence, pointing error, and fog loss; Step 2: Based on the FSO composite atmospheric channel model obtained in step 1, an adaptive SISO / MIMO-FSO communication system model is constructed and a signal is generated; Step 3: Under different turbulence intensities, pointing errors, and visibility ranges, the bit error rate data of the signals under the SISO and MIMO modes are obtained based on the communication system model obtained in step 2, and a threshold lookup table for mode switching is established according to the bit error rate data; Step 4: The transmitter transmits the signal into the atmospheric composite channel. After receiving the signal, the receiver compares the channel state information with the available thresholds in the threshold lookup table and selects the best applicable signal transmission mode in the SISO and MIMO modes.

[0019] Example 2 The present invention provides a FSO communication method for adaptively switching between SISO / MIMO modes, such as Figure 1 As shown, the specific steps are as follows: Step 1: Write system modules based on the GNURadio platform. The modules are connected in series. The signal passes through the established laser module, modulation module, channel module (atmospheric turbulence module, pointing error module and fog loss module to simulate the impact of the atmospheric environment on the signal), demodulation module and real-time bit error rate monitoring feedback module in turn. Set up an oscilloscope module to observe the signal waveform. Among them, the channel module encapsulates the FSO composite atmospheric channel model that considers the combined effects of atmospheric turbulence, pointing error and fog loss: In the FSO communication system, when the light beam is transmitted along the horizontal path in the atmospheric channel, it will be affected by the atmospheric channel fading. It is expressed as: (1) In the formula, is the transmitting optical power, represents the responsivity of the receiver, is additive white Gaussian noise, is the normalized channel fading coefficient. Due to the combined effect of the three influencing factors, the channel model consists of three parts and can be expressed as , represents the random component of atmospheric turbulence, represents the random component of the pointing error, Represents the random component of fog loss.

[0020] 1) For FSO links, the random component of fog loss It's about wavelength Function: (2) In the formula, is the transmission distance, is the attenuation coefficient, expressed as: (3) In the formula, and are the molecular absorption coefficient and the aerosol absorption coefficient, respectively. is the molecular scattering coefficient, is the aerosol scattering coefficient after fog attenuation, given by the Kim model: (4) In the formula, is the visibility level, is the wavelength of the light source, The size distribution of the scattered fog particles of the Kim model is shown in Table 1. The visibility levels on foggy days are shown in Table 1. A horizontal visibility distance of less than 1 km is called fog. A horizontal visibility distance of 200 to 500 meters is called heavy fog. A horizontal visibility distance of 50 to 200 meters is called dense fog. A horizontal visibility of less than 50 meters is called strong dense fog.

[0021] Table 1 Fog visibility levels

[0022] (5) 2) Atmospheric turbulence is caused by the uneven distribution of air density due to the temperature and pressure differences in different areas of the air. Atmospheric turbulence can cause rapid fluctuations in light intensity, a phenomenon called scintillation effect, which causes the light intensity on the receiving plane of the FSO system to vary in time and space. For different thermal expansions in the channel, the scintillation index used to estimate the turbulence effect is: (6) In the formula, represents the average value, is the light intensity, assuming plane wave propagation, It is expressed as: (7) In the formula, is the circular aperture after scaling of the Fresnel zone, is the wave number, is the receiving aperture diameter, is the transmission distance.

[0023] In laser transmission, the transmission distance is In the Kolmogorov turbulence spectrum, the plane wave usually uses the Rytov variance value to describe strong and weak fluctuations: (8) In the formula, is the refractive index structure constant, expressed as: (9) In the formula, is the atmospheric pressure, is the temperature, The distance between temperature difference.

[0024] The turbulence intensity levels corresponding to different values ​​of the refractive index structure constant are: (10) For the turbulence module, the module uses Gamma-Gamma distribution to generate random numbers, and its probability density function is: (11) In the formula, is the Gamma function, It means the second type The modified Bessel function of order, and is a parameter related to small-scale turbulent eddies and large-scale turbulent eddies, expressed as: (12) (13) The input signal is attenuated by the Gamma-Gamma random number, and the output signal is the signal attenuated by turbulence. The Gamma-Gamma turbulence model is used to characterize the random component of atmospheric turbulence. .

[0025] 3) Pointing error is also one of the key factors affecting FSO systems. FSO transceivers transmit highly directional narrow beams that must be precisely transmitted to the aperture at the receiving end, but in fact, buildings are always in a state of slight movement. This movement is the result of many factors, including thermal expansion, wind sway and vibration. Due to the narrowness of the transmit beam and the limited field of view of the receiver, the sway of the building will affect the alignment of the transceiver, resulting in communication interruption. The random component of pointing error The expression is: (14) In the formula, is the radial displacement of the light spot; express = 0 received power; is the equivalent beam waist radius and , , represents the error function, is the beam waist radius, is the receiver radius.

[0026] Considering zero boresight pointing error, the elevation angle and horizontal displacement obey independent identical Gaussian distributions, and the radial displacement of the receiver is Obeying Rayleigh distribution: (15) In the formula, represents the jitter variance of the receiver. From equations (14) and (15), we can know that The probability density function of can be expressed as: (16) In the formula, Indicates the receiver equivalent beam radius Pointing error jitter standard deviation ratio.

[0027] Step 2: Build an adaptive SISO / MIMO-FSO link model based on the FSO composite atmospheric channel model established in step 1. The generated signal passes through the laser module, modulation module, channel module, demodulation module and real-time bit error rate monitoring feedback module. Set the oscilloscope module to observe the signal waveform.

[0028] Among them, the laser power of the laser module is set to 300mw, and the wavelength is 1500nm. After the signal passes through the laser module, it passes through the modulation module, and the modulation format is DBPSK. After the modulated DBPSK signal passes through the turbulence, fog loss, geometric loss and pointing error modules connected in series, it is input to the demodulation module for signal demodulation. The demodulation module uses the PolyphaseClock Sync multi-phase clock synchronization module for signal optimization. This module performs clock recovery and provides a receiver matching filter to eliminate ISI, and downsamples the signal at the same time. The signal after the PolyphaseClock Sync module still has phase and frequency offset problems. The output signal is then passed through the Costas Loop module to restore the phase and frequency synchronization of the digital modulation signal. The Costas loop first multiplies the received signal with its own copy with different phases, estimates the phase error of the signal based on the result of the complex multiplier output, and then adjusts the phase of the signal through a feedback loop to minimize the error. In addition, the Costas Loop can also estimate the frequency error caused by the carrier drift of the signal during transmission, and adjust the frequency of the signal based on the frequency error to ensure that the received signal is synchronized with the local reference signal. In this way, the Costas loop can synchronize the phase and frequency of the digital modulated signal at the receiving end, thereby effectively demodulating the signal and restoring the original digital data. The signal after passing through the Costas Loop module finally enters the differential demodulation module and successfully restores the original signal.

[0029] Step 3: After executing step 2, the bit error rate data of the SISO system and the MIMO system under different visibility ranges, different turbulence intensities and different pointing errors are obtained, and a threshold lookup table for mode switching is established based on the data.

[0030] Step 4: The transmitting antennas TA and TB transmit the optical signal generated by the laser into the atmospheric composite channel, which is then received by the receiving antennas RA and RB. The transmitting antenna TB is opened or closed according to the threshold obtained in step 2 to realize the switching between the system adaptive SISO and 2×2 MIMO modes and complete the adaptive FSO communication.

[0031] The atmospheric visibility V in the channel is greater than the visibility threshold V (Threshold), or the refractive index structure constant Less than the refractive index structure constant threshold (Threshold), or the jitter standard deviation Jit of the pointing error is less than the jitter standard deviation threshold Jit (Threshold), that is, the system bit error rate is lower than the upper limit of the FEC bit error rate (3.8×10 -3 ), the transmitting antenna TB is turned off, and the system uses SISO link for transmission to save transmission power. When the atmospheric visibility V of the channel is lower than the visibility threshold V (Threshold), or the refractive index structure constant Greater than the refractive index structure constant threshold (Threshold), or the jitter standard deviation Jit of the pointing error is greater than the jitter standard deviation threshold Jit (Threshold), that is, the system bit error rate is higher than the upper limit of the FEC bit error rate (3.8×10 -3 ), the transmitting antenna TB is turned on, and the system uses MIMO links for transmission to obtain diversity gain, thereby alleviating the signal loss caused by atmospheric composite channels. Under different visibility ranges, different turbulence intensities, and different pointing errors, the base feedback mode continues to work, allowing the system to continuously switch adaptive modes.

[0032] Example 3 Figures 2(a) to (c) show the bit error rates of SISO and MIMO links using DBPSK modulation format as the refractive index structure constant increases when the visibility is fixed at 1000m and the pointing error jitter is 0.03m at link distances of 300m, 500m, and 1500m. The upper limit of mode switching is the maximum bit error rate that can be corrected by FEC forward error correction code, which is 3.8×10 -3 That is, before the bit error rate of the SISO link reaches the upper limit of FEC, the system needs to switch to a MIMO link with a bit error rate lower than the upper limit of FEC to ensure normal communication of the system, until the MIMO link can no longer meet the communication requirements.

[0033] When the link distance is 300m, the channel conditions are good and the link loss is low. The bit error rate of the MIMO link is slightly better than that of the SISO link. The SISO system saves power and does not need to be switched. When the link distance is increased to 500m, the MIMO link is-13 < <10 -11 The bit error rate is better than that of SISO link in the range of 10 -12 < <10 -11 When the link length continues to increase to 1500m, the MIMO link has a turbulence intensity of 10 -15 < <10 -12 The bit error rate is better than that of SISO link in the range of 10 -14 < <10 -13 As the transmission distance increases, the loss increases. The larger the range of the refractive index structure constant where the MIMO link bit error rate is lower than that of the SISO link, the more obvious the MIMO system advantage is. The switching visibility threshold lookup table is shown in Table 2: Table 2 Turbulence switching threshold lookup table

[0034] Example 4 Figure 3 (a) to (c) show the fixed visibility and turbulence intensity at 300m, 500m and 1500m link distances, respectively. =10 -15 , pointing error jitter is 0.03m, and the bit error rate of SISO and MIMO links using DBPSK signal format varies with visibility. The switching strategy is: the switching upper limit uses the maximum bit error rate that can be corrected by the FEC forward error correction code 3.8×10 -3 That is, before the bit error rate of the SISO link reaches the upper limit of FEC, the system needs to switch to a MIMO link with a bit error rate lower than the upper limit of FEC to resist channel fading to ensure normal communication of the system, until the MIMO link can no longer meet the communication requirements. The four grayscale intervals represent different levels of fog (see Table 1).

[0035] For a 300m link, when the visibility V > 300m, the performance of the SISO link is nearly the same as that of the MIMO link, and the SISO link can be adopted by the system. The bit error rate of the MIMO link is better than that of the SISO link within the range of visibility 140m < V < 200m, and the switching point of the system is V = 190m. However, under the conditions of strong thick fog and partial thick fog (V < 170m), due to the extremely poor channel, the MIMO system cannot meet the communication requirements either. For a 500m link, when the visibility V > 500m, the performance of the SISO link is close to that of the MIMO link, and the SISO link can be adopted by the system. The bit error rate of the MIMO link is better than that of the SISO link within the range of visibility 200m < V < 500m, and the switching point of the system is V = 360m. However, under the conditions of "strong thick fog", "thick fog" and partial "fog" (V < 330m), due to the extremely poor channel, the MIMO system cannot meet the communication requirements either. For a 1500m link, when the visibility V > 800m, the performance of the SISO link is similar to that of the MIMO link, and the SISO link can be adopted by the system. The bit error rate of the MIMO link is better than that of the SISO link within the range of visibility 500m < V < 800m, and the switching point of the system is V = 580m. However, under the conditions of "strong thick fog", "thick fog", "fog" and partial "mist" (V < 530m), due to the extremely poor channel and the influence of detector noise, the MIMO system cannot meet the communication requirements either. The switching visibility threshold lookup table is shown in Table 3: Table 3 Switching Visibility Threshold Lookup Table

[0036] Example 5 Figures 4(a) to (c) show the curves of the bit error rates of the SISO and MIMO links with the DBPSK signal format varying with the pointing error jitter Jit at a fixed visibility of 1000m, a pointing error jitter of 0.03m, and a fixed weak turbulence intensity =10 -15 , for SISO and MIMO links with a DBPSK signal format. The switching strategy is: the upper switching limit adopts the maximum bit error rate 3.8×10 -3 that can be corrected by the FEC forward error correction code. That is, before the bit error rate of the SISO link reaches the FEC upper bound, the system needs to switch to the MIMO link with a bit error rate lower than the FEC upper bound to ensure the normal communication of the system until the MIMO link can no longer meet the communication requirements.

[0037] For a 300m link, when the pointing error jitter Jit>0.06m, the SISO link cannot meet the communication requirements, and the MIMO link can always meet the system communication requirements within the range of the pointing error jitter Jit<0.08m, and the system switching point is Jit=0.06m. For a 500m link, when the pointing error jitter Jit>0.04m, the SISO link cannot meet the communication requirements, and the MIMO link cannot meet the system communication requirements when the pointing error jitter Jit>0.07m, and the system switching point is Jit=0.04m; for a 1500m link, due to the poor link conditions, the SISO and MIMO links cannot always meet the system communication requirements. The switching threshold lookup table is shown in Table 4: Table 4 Pointing error switching threshold lookup table

[0038] Example 6 The FSO communication method for adaptive switching of SISO / MIMO modes of the present invention adopts DBPSK coding mode, and the receiving end feeds back the atmospheric channel instantaneous state estimation information to the transmitting end, and the transmitting end compares the information with the switching threshold to realize dynamic switching of the transmission mode. When the channel quality is good, the SISO link is adopted to simplify the system and reduce energy consumption, and when the channel quality is poor, it is switched to the MIMO link to enhance the signal strength and reliability. Considering the influence of atmospheric joint fading caused by fog loss, pointing error, and atmospheric turbulence, the influence of different visibility, different turbulence intensity and different pointing error jitter degree on the performance of the adaptive switching system under various link distances is simulated and analyzed based on the upper bound of the FEC bit error rate, and a lookup table for transmission switching of different modes is established, which provides a theoretical basis for the FSO system to maintain robustness and stability in a changing environment.

Claims

1. An FSO communication method for adaptive switching of SISO / MIMO modes, characterized in that: The following steps are involved: Step 1: Establish an FSO composite atmospheric channel model that considers the combined effects of atmospheric turbulence, pointing error, and fog loss; Step 2: Based on the FSO composite atmospheric channel model obtained in step 1, an adaptive SISO / MIMO-FSO communication system model is constructed and a signal is generated; Step 3: Under different turbulence intensities, pointing errors, and visibility ranges, the bit error rate data of the signals under the SISO and MIMO modes are obtained based on the communication system model obtained in step 2, and a threshold lookup table for mode switching is established according to the bit error rate data; Step 4: The transmitter transmits the signal into the atmospheric composite channel. After receiving the signal, the receiver compares the channel state information with the available thresholds in the threshold lookup table and selects the best applicable signal transmission mode in the SISO and MIMO modes.

2. The FSO communication method of SISO / MIMO mode adaptive switching according to claim 1, characterized in that: The FSO composite atmospheric channel model established in step 1 is expressed as: （1） In the formula, is the optical power at the receiving end; is the optical power at the transmitting end; Indicates the responsivity of the receiver; is additive Gaussian white noise; is the normalized channel fading coefficient, expressed as , represents the random component of atmospheric turbulence, represents the random component of the pointing error, Represents the random component of fog loss.

3. The FSO communication method of SISO / MIMO mode adaptive switching as claimed in claim 2, characterized in that: The random component of the fog loss It's about wavelength Function: （2） In the formula, is the transmission distance, is the attenuation coefficient, expressed as: （3） In the formula, and are the molecular absorption coefficient and the aerosol absorption coefficient, respectively. is the molecular scattering coefficient, is the aerosol scattering coefficient after fog attenuation, given by the Kim model: （4） In the formula, is the visibility level, is the wavelength of the light source, is the size distribution of the scattering fog particles of the Kim model.

4. The FSO communication method of adaptively switching between SISO and MIMO modes as claimed in claim 2, characterized in that: The random component of atmospheric turbulence Using the Gamma-Gamma turbulence model to characterize, the probability density function is: （11） In the formula, is the Gamma function, It means the second type The modified Bessel function of order, and is a parameter related to small-scale turbulent eddies and large-scale turbulent eddies, expressed as: （12） （13） In the formula, is the Rytov variance value, expressed as: （8） In the formula, is the wave number, is the transmission distance, is the refractive index structure constant, expressed as: （9） In the formula, is the atmospheric pressure, is the temperature, The distance between temperature difference.

5. The FSO communication method of SISO / MIMO mode adaptive switching as claimed in claim 2, characterized in that: The random component of the pointing error It is expressed as: （14） In the formula, is the radial displacement of the light spot; express = 0 received power; is the equivalent beam waist radius and , , represents the error function, is the beam waist radius, is the receiver radius.

6. The FSO communication method of SISO / MIMO mode adaptive switching according to claim 1, characterized in that: The adaptive SISO / MIMO-FSO communication system model constructed in the step 2 includes a laser module, a modulation module, a channel module encapsulating an FSO composite atmospheric channel model, a demodulation module, and a real-time bit error rate monitoring feedback module. When generating a signal, the laser power and wavelength of the laser module are set, the DBPSK modulation format is adopted, the clock is recovered, and a receiver matching filter is provided to eliminate inter-code interference. At the same time, the signal is downsampled to restore the phase and frequency synchronization of the digital modulation signal.

7. The FSO communication method of adaptively switching between SISO and MIMO modes according to claim 1, characterized in that: The process of establishing the threshold lookup table in step 3 is as follows: keeping the fog loss, pointing error and geometric loss parameters unchanged, changing the atmospheric turbulence, and establishing different mode transmission switching lookup tables based on different atmospheric turbulences at multiple link distances based on the upper limit of the FEC bit error rate; keeping the pointing error, atmospheric turbulence and geometric loss parameters unchanged, changing the fog loss, and establishing different mode transmission switching lookup tables based on different visibility at multiple link distances based on the upper limit of the FEC bit error rate; keeping the fog loss, atmospheric turbulence and geometric loss parameters unchanged, changing the pointing error, and establishing different mode transmission switching lookup tables based on different pointing errors at multiple link distances based on the upper limit of the FEC bit error rate.

8. The FSO communication method of SISO / MIMO mode adaptive switching according to claim 1, characterized in that: The step 4 is specifically as follows: the transmitting antennas TA and TB transmit the optical signal generated by the laser module into the atmospheric composite channel, which is then received by the receiving antennas RA and RB, wherein the transmitting antenna TB is selected to be opened or closed according to the channel state information CSI. When the atmospheric visibility of the channel is greater than the visibility threshold, or the refractive index structure constant is less than the refractive index structure constant threshold, or the jitter standard deviation of the pointing error is less than the jitter standard deviation threshold, that is, when the bit error rate is lower than the FEC bit error rate upper limit, the transmitting antenna TB is closed and the SISO link is used for transmission to save transmission power; when the atmospheric visibility of the channel is lower than the visibility threshold, or the refractive index structure constant is greater than the refractive index structure constant threshold, or the jitter standard deviation of the pointing error is greater than the jitter standard deviation threshold, that is, when the bit error rate is higher than the FEC bit error rate upper limit, the transmitting antenna TB is opened and the MIMO link is used for transmission to obtain diversity gain.

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