A dual-polarization dual-aperture multi-wavelength visible light wireless laser communication system

Abstract

The present invention belongs to the technical field of visible light wireless laser communication, and specifically is a dual-polarization dual-aperture multi-wavelength visible light wireless laser communication system. The system of the present invention includes a digital signal processing module at the transmitting end, two groups of five-wavelength laser transmitting modules, an in-phase or anti-phase transmitting module based on polarization multiplexing, and a dual-aperture receiving module at the receiving end, a maximum ratio combining or differential processing module, and a receiving end digital signal processing module; the present invention can realize dual-polarization in-phase transmission that complements the damage in the horizontal and vertical polarization directions, and dual-polarization differential transmission that suppresses common-mode interference. For dual-polarization in-phase transmission, two groups of lasers modulate the same signal, and the receiving end uses a maximum ratio combining algorithm to combine; for dual-polarization differential transmission, two groups of lasers modulate anti-phase signals, and the reception of the two apertures at the receiving end is differentiated. The present invention can greatly improve the reliability and robustness of communication, and help to achieve higher-speed and larger-capacity communications.

Description

Technical Field

[0001] The present invention belongs to the technical field of visible light wireless laser communication, and in particular relates to a dual-polarization dual-aperture multi-wavelength visible light wireless laser communication system. Background Art

[0002] The current new generation of mobile communications is developing towards higher speed, lower latency and larger capacity, but the spectrum resources of current wireless communications are very congested. In order to build a high-speed, large-capacity three-dimensional communication network that integrates air, land and sea, it is urgent to expand wireless communications to higher frequency bands, such as millimeter wave, terahertz, infrared, visible light, ultraviolet and other frequency bands. The wavelength of visible light is 380nm-760nm, and the frequency band is the unlicensed frequency band of 400THz-800THz. It is not affected by electromagnetic radiation and is located in the underwater transmission window. It has the potential for high-speed transmission and has become a new research hotspot in recent years [1-3]. The transmitters of visible light communication are divided into incoherent light sources such as light-emitting diodes (LEDs) and coherent light sources such as lasers (LDs). Compared with LEDs, LDs have the advantages of high coherence, small divergence angle, high power, concentrated beam and large modulation bandwidth. Visible light laser communication (VLLC) is expected to push the speed and capacity of the new generation of wireless communications to a new peak [4].

[0003] In the past decade of research, visible light laser communication systems have been mainly based on single-wavelength point-to-point communication, which can only achieve a rate of 10Gbps, and the transmission distance is concentrated within 10 meters. Abroad, in 2020, King Abdullah University of Science and Technology in Saudi Arabia (Jorge A. Holguin-Lerma, etc.) achieved 0.38-meter orthogonal amplitude modulation signal transmission based on a 480nm blue laser, with a rate of up to 10.5Gbps[5]. In 2021, the University of Oxford in the United Kingdom (William Matthews, etc.) achieved 3.45Gbps on-off keying signal transmission based on a single green laser[6]. In China, in 2016, the Industrial Technology Research Institute of Taiwan (I-Cheng Lu, etc.) achieved 11.1Gbps 1.2-meter short-distance communication based on a 682nm laser[7]. Fudan University (Qin Guojin, Lu Zhilan, etc.) achieved 100-meter long-distance high-speed communication of 6 Gbps[8] and 11.2 Gbps[9] based on blue laser and green laser respectively.

[0004] In order to further improve the capacity and spectrum utilization of communication systems, researchers have begun to consider introducing multiple transmission and multiple reception, wavelength division multiplexing, and polarization multiplexing technologies into VLLC systems in the past two years. In 2023, Guru Nanak Dev University in India (Abhishek Sharma et al.) combined polarization multiplexing and multiple input multiple output technology to achieve 60Gbps signal transmission using RGB three-color lasers

[10] . In 2024, the University of Cambridge in the United Kingdom (Cheng Chen et al.) achieved 105.36Gbps wireless indoor communication based on a 10-wavelength laser module

[11] . In China, in 2022, Yang Ming Chiao Tung University in Taiwan (Wahyu Hendra Gunawan et al.) achieved a communication rate of 21.01Gbps based on wavelength division multiplexing and non-orthogonal multiple access system, but the transmission distance could only reach 4 meters

[12] . In 2023, Fudan University (Hu Junhui et al.) independently developed an integrated RGB transmission module and achieved 46.41Gbps spatial optical communication based on bit-loaded discrete multi-tone modulation technology

[13] . In 2024, Fudan University (Zhou Yuning et al.) achieved a rate of 47 Gbps in a long-distance transmission of 100 meters based on the RGB transmission module and time-domain hybrid Huffman coding

[14] .

[0005] Based on the above literature, it can be found that most of the current visible light communication systems still use simple point-to-point communication systems, and are aimed at the transmission of one-dimensional time signals. It is difficult to fully utilize the various degrees of freedom of light waves, including wavelength, polarization state, phase, space and other dimensions. It is also difficult to observe the channel from different dimensions, which is not conducive to the improvement of transmission capacity and spectrum utilization and the expansion of application scenarios. Summary of the invention

[0006] The object of the present invention is to provide a dual-polarization dual-aperture multi-wavelength visible light wireless laser communication system with high transmission rate, large capacity and good reliability.

[0007] The dual-polarization dual-aperture multi-wavelength visible light wireless laser communication system proposed in the present invention can realize dual-polarization in-phase transmission in which the damage in the horizontal and vertical polarization directions can be complementary, and dual-polarization differential transmission with common-mode interference suppression; for dual-polarization in-phase transmission, two groups of lasers modulate the same signal, and the receiving end uses the maximum ratio combining algorithm to combine; for dual-polarization differential transmission, two groups of lasers modulate the signals with opposite phases, and the reception of the two apertures at the receiving end is differentiated. The communication system used in the present invention is a multi-wavelength visible light wireless laser communication system based on direct modulation and direct detection; the system specifically includes: a digital signal processing module (DSP) at the transmitting end, two groups of integrated five-wavelength laser transmitting modules, an in-phase (or anti-phase) transmitting module based on polarization multiplexing (referred to as a polarization multiplexing module), and a dual-aperture receiving module (APD) at the receiving end, a maximum ratio combining algorithm module (MRC) or a differential processing module (Diff), and a digital signal processing module (DSP) at the receiving end; of the two groups of laser transmitting modules, one group uses horizontal polarization and the other group uses vertical polarization for multiplexing, and is received by a dual-aperture receiving module (APD) after passing through a turbulent medium, see Figure 1 As shown; among which:

[0008] (i) the transmitting end digital signal processing module (DSP), including: a 128-level quadrature amplitude mapper (QAM), a carrier-less amplitude-phase modulator (CAP), and a software pre-equalization module;

[0009] In the DSP processing module at the transmitting end, a bit stream is randomly generated, and a 128-level quadrature amplitude mapper (QAM) maps every 7 bits into a 128QAM constellation point; the constellation point is upsampled 4 times and modulated by a carrier-free amplitude-phase modulator (CAP) to obtain a real waveform signal. Since the visible light channel responds to the signal with a high low-frequency response and a low high-frequency component response, the high-frequency component of the real waveform signal is pre-emphasized, that is, pre-equalized. At this point, the signal Tx to be transmitted is obtained at the computer end.

[0010] The signal to be transmitted is converted into a positive-phase and negative-phase analog electrical signal Tx by an arbitrary waveform generator (AWG). + With Tx - Output, where:

[0011] Tx + =Tx,Tx - =-Tx;#(1)

[0012] The output electrical signal is amplified by an electrical amplifier (EA) and then passes through an adjustable attenuator (ATT) to control the amplification factor. Next, it is driven together with a DC bias through a bias tee to emit light through a five-wavelength laser emission module; the laser emission module integrates laser diodes (LDs) with wavelengths of 685nm, 638nm, 520nm, 450nm and 405nm and a metal heat sink. The working state of the laser can be changed by adjusting the peak-to-peak voltage of the electrical signal output by the arbitrary waveform generator (AWG) and the size of the bias current.

[0013] The polarization multiplexing module uses a linear polarizer (LP) to filter the output light of two groups of five-wavelength laser emission modules into two output paths of horizontal polarization and vertical polarization Tx X With Tx Y .

[0014] For dual-polarization in-phase transmission (called Scheme 1), we have:

[0015] Tx X =Tx Y =Tx + =Tx, #(2)

[0016] For dual polarization anti-phase transmission (called Scheme 2), we have:

[0017] Tx X =Tx + =Tx,Tx Y =Tx - =-Tx, #(3)

[0018] Then, the two polarized light beams Tx X With Tx Y They are transmitted together through turbulent media such as underwater or the atmosphere.

[0019] (ii) the receiving end digital signal processing module (DSP), including waveform level post-equalization, CAP demodulation, symbol level post-equalization, QAM demapping and bit error rate (BER) testing;.

[0020] The dual aperture receiving module (APD) is a spatial diversity receiver composed of two convex lenses (Lens) and an avalanche photodiode (APD). The two APDs receive the two horizontally and vertically polarized light beams respectively and convert them into corresponding electrical signals. After passing through an electrical amplifier (EA) and an attenuator (ATT), they are input into an oscilloscope (OSC) for sampling and quantized into a digital signal Rx. X With Rx Y , input to the computer for post-processing.

[0021] The maximum ratio combining algorithm (MRC) or the differential processing (Diff) module:

[0022] For solution 1, the signals transmitted in the two polarization directions are in phase, so the maximum ratio combining (MRC) module is used to combine the Rx X With Rx Y Perform weighted summation on Rx X With Rx Y Weighted weights and Rx X With Rx Y The noise power is inversely proportional to the transmitted signal power. Since the transmitted signal power is consistent, Rx is used. X With Rx Y The signal-to-noise ratio is used as the weight w X With w Y A simple estimate of Rx X With Rx Y The method to improve the signal-to-noise ratio is to set Rx X With Rx Y Convert to the frequency domain through Fourier transform, and then calculate the area ratio of the in-band and out-band parts of the signal, that is:

[0023]

[0024] Among them, F k (Rx X ) and F k (Rx Y ) are Rx X and Rx Y The value of the Fourier transform at frequency k, f w is the signal bandwidth. The signal obtained after weighting is:

[0025] Rx M =w X ·Rx X +w Y ·Rx Y ,#(5)

[0026] For solution 2, the two polarization directions transmit signals in opposite phases, so the differential processing (Diff) module is used to combine the received signals, that is:

[0027] Rx D =Rx X -Rx Y ,#(6)

[0028] The signal Rx is obtained M (Scheme 1) or Rx D(Scheme 2) is sent to the digital signal processing module (DSP) at the receiving end; wherein, it first undergoes waveform-level post-equalization to correct the distortion of the signal waveform, and then obtains the constellation points through CAP demodulation and downsampling. The constellation points undergo symbol-level equalization and QAM demapping, and finally are restored to a bit stream, and the transmission bit error rate is calculated in combination with the transmitted bit stream.

[0029] The technical features and performance advantages of the present invention are:

[0030] The dual-polarization dual-aperture multi-wavelength visible light wireless laser communication system proposed in the present invention, namely, dual-polarization in-phase transmission with complementary damage in the horizontal and vertical polarization directions and dual-polarization differential transmission system with common-mode interference suppression, uses two sets of integrated five-wavelength transmitting modules as transmitters, one set uses horizontal polarization and the other set uses vertical polarization for multiplexing, and dual-aperture reception is adopted after passing through the turbulent medium. For dual-polarization in-phase transmission (Scheme 1), two sets of lasers modulate the same signal, and the receiving end uses the MRC algorithm for merging; for dual-polarization differential transmission (Scheme 2), two sets of lasers modulate the signals of opposite phases, and the receiving end performs differential. Experimental results show that the in-phase transmission method and differential transmission method based on dual-polarization dual-aperture can achieve transmission rates of 86.1Gbps and 88.9Gbps ​​respectively, which can achieve rate increases of 9.1Gbps and 11.9Gbps ​​respectively compared with traditional point-to-point transmission. It plays an important role in improving the reliability and robustness of visible light wireless laser communication systems, and helps to achieve higher-speed and larger-capacity communications. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a diagram of the experimental setup of the dual-polarization dual-aperture visible light wireless laser communication system based on a five-wavelength laser module of the present invention.

[0032] Figure 2 Thermal diagram of the bit error rate of the five wavelengths of (a) 685nm, (b) 638nm, (c) 520nm, (d) 450nm and (e) 405nm of this system as the bias current and the peak-to-peak value of the AWG output voltage change.

[0033] Figure 3The communication performance test of three systems: point-to-point, in-phase addition, and differential subtraction, with a wavelength of 685nm laser. Among them, (a) BER changes with Vpp; (b) The distribution of constellation points of the three systems at three working points i, ii, and iii; (c) The distribution of the constellation diagram in the first quadrant under the optimal Vpp, and the small figure is the probability density curve of the distribution of the two constellation points (1,1) and (11,7) with the smallest and largest amplitudes; (d) Under the optimal Vpp, the receiving time domain waveforms and spectra of the three systems are compared with the transmitting end; (e) The probability density curve of the time domain noise distribution and the noise spectrum comparison of the three systems.

[0034] Figure 4 The variation of communication BER with Vpp for point-to-point, in-phase addition, and differential subtraction systems with wavelengths of (a) 638nm, (b) 520nm, (c) 450nm, and (d) 405nm.

[0035] Figure 5 The achievable rates (bar graph) and BER (line graph) of the five wavelengths of this system (a) 685nm, (b) 638nm, (c) 520nm, (d) 450nm and (e) 405nm vary with the transmission bandwidth. DETAILED DESCRIPTION

[0036] The present invention is further described below in combination with specific principles, experimental systems and results.

[0037] (1) Principle and experimental platform

[0038] In order to verify the performance of the system proposed in this invention, experiments were conducted using a five-wavelength visible light wireless laser communication system based on direct modulation and direct detection. Figure 1 shown.

[0039] In the DSP processing at the transmitting end, a bit stream is randomly generated, and every 7 bits are mapped to a 128QAM constellation point. The constellation point is upsampled by 4 times and then CAP modulated to obtain a real waveform signal. The high-frequency component of the real waveform signal is pre-emphasized to obtain the signal Tx to be transmitted. It is converted into a positive and negative analog electrical signal Tx through an arbitrary waveform generator. + With Tx - Output, where

[0040] Tx + =Tx,Tx - =-Tx.#(1)

[0041] The output electrical signal is amplified by an electrical amplifier and then passes through an adjustable attenuator to control the amplification factor. It is then used together with a DC bias to drive a five-wavelength laser emission module through a bias tee to emit light. The laser module integrates laser diodes with wavelengths of 685nm, 638nm, 520nm, 450nm and 405nm and a metal heat sink. The working state of the laser can be changed by adjusting the peak-to-peak voltage of the AWG output electrical signal and the size of the bias current. In the polarization multiplexing module, a linear polarizer is used to filter the output light of the two groups of lasers into two output Txs of horizontal polarization and vertical polarization. X With Tx Y For dual polarization in-phase transmission, we have:

[0042] Tx X =Tx Y =Tx + =Tx.#(2)

[0043] For dual polarization differential transmission, we have:

[0044] Tx X =Tx + =Tx,Tx Y =Tx - =-Tx.#(3)

[0045] Next, the two polarized light beams Tx X With Tx Y Together they were transmitted through 1.2 meters of water.

[0046] At the receiving end, the dual-aperture receiving module is a spatial diversity receiver composed of two convex lenses and an avalanche photodiode. The two APDs receive the two horizontally and vertically polarized beams respectively and convert them into corresponding electrical signals. After passing through the electrical amplifier and attenuator, they are input into the oscilloscope for sampling and quantization into digital signals Rx. X With Rx Y Input to computer for post-processing.

[0047] For solution 1, the two polarization directions transmit signals of the same phase, so the maximum ratio combining algorithm is used to combine the Rx X With Rx Y Perform weighted summation on Rx X With Rx Y Weighted weights and Rx X With Rx Y The noise power is inversely proportional to the transmitted signal power. Since the transmitted signal power is consistent, Rx is used. X With Rx Y The signal-to-noise ratio is used as the weight w X With w Y A simple estimate of Rx X With RxY The method to improve the signal-to-noise ratio is to set Rx X With Rx Y Convert to the frequency domain through Fourier transform, and then calculate the area ratio of the in-band and out-band parts of the signal, that is:

[0048]

[0049] Among them, F k (Rx X ) and F k (Rx Y ) are Rx X and Rx Y The value of the Fourier transform at frequency k, f w is the signal bandwidth. The signal obtained after weighting is

[0050] Rx M =w X ·Rx X +w Y ·Rx Y .#(5)

[0051] For solution 2, the two polarization directions transmit signals in opposite phases, so differential processing is used to combine the received signals, namely:

[0052] Rx D =Rx X -Rx Y .#(6)

[0053] The signal Rx is obtained M (Scheme 1) or Rx D (Scheme 2) is sent to the receiving DSP module, and firstly undergoes waveform-level post-equalization to correct the distortion of the signal waveform. The algorithm used is Volterra filter and least mean square (LMS) equalizer. Then, constellation points are obtained through CAP demodulation and 4-fold downsampling. After symbol-level LMS equalization, the constellation points can be QAM demapped and finally restored to bit stream, and the bit error rate is calculated.

[0054] (2) Experimental results and analysis

[0055] Figure 2The following are the heat maps of the bit error rate of the five wavelengths of this system (a) 685nm, (b) 638nm, (c) 520nm, (d) 450nm and (e) 405nm as the bias current and the peak-to-peak voltage (Vpp) of the AWG output change. For these five wavelengths, the heat maps have similar change trends, that is, the BER first decreases and then increases with the increase of Vpp or bias current. The reason is that a small Vpp or bias current will make the system limited by noise, while a large Vpp or bias current will make the system work in the nonlinear region, causing waveform distortion, which will affect the transmission quality of the system. Therefore, the system has an optimal operating point. The optimal Vpp for the five wavelengths of 685nm, 638nm, 520nm, 450nm and 405nm are 600mV, 650mV, 650mV, 450mV and 700mV respectively; the optimal bias currents are 60mA, 110mA, 165mA, 85mA and 90mA respectively.

[0056] Figure 3 Taking the result of wavelength 685nm as an example, the communication performance of traditional point-to-point communication is compared with the dual polarization in-phase addition (Scheme 1) and dual polarization differential subtraction system (Scheme 2) proposed by us. The signal bandwidth is fixed at 2.3GHz, and the bias current is the optimal current 60mA. Figure 3 (a) is the variation of BER with Vpp. The gray dotted line is the BER that meets the 7% hard-decision forward error correction code bit error rate threshold of 3.8E-3. It can be seen that at this time, the point-to-point single-channel receiving system cannot meet the bit error rate threshold requirement, while the dual-polarization in-phase transmission system we proposed can meet the bit error rate threshold under the optimal Vpp, and the dual-polarization differential transmission system can work normally under Vpp of 350mV to 650mV, and under all Vpp, the BER of the point-to-point system is greater than that of the in-phase transmission system, and the result that the BER of the in-phase transmission system is greater than that of the differential transmission system holds. In the figure, we use different background colors to mark the different working states of the system. Orange indicates the noise-limited area, pink indicates that the system works in the linear area, and blue indicates the nonlinear area. A Vpp value is taken in each of these three areas, namely 400mV, 600mV and 700mV, which are denoted as i, ii, and iii respectively. Figure 3 (b) shows the distribution of constellation points of the three systems at three working points i, ii, and iii. The more dispersed the constellation points are, the easier it is to make mistakes in the division of constellations, that is, the greater the probability of misjudgment. The sizes of the constellation points at working point i are relatively uniform. As Vpp increases, the inner circle constellation points become smaller, and the outer circle constellation points first become smaller and then larger. Therefore, in the nonlinear region, the part with large signal amplitude is mainly affected. In general, the interval between the constellation points of scheme 2 is larger than that of scheme 1, and the constellation points of the point-to-point single-channel system are the most difficult to divide. Figure 3(c) shows the distribution of the constellation diagram in the first quadrant under the optimal Vpp. The small figure shows the probability density curves of the distribution of the two constellation points (1,1) and (11,7) with the smallest and largest amplitudes. It can be seen that compared with the point-to-point system, the distinction between different constellation points in the inner circle of Scheme 1 and Scheme 2 is significantly increased. Under point-to-point, Scheme 1, and Scheme 2, the peak values ​​of the probability density curves of constellation points with small amplitudes are 0.09, 0.10, and 0.12, respectively, and the half-maximum full widths are 0.84, 0.76, and 0.67, respectively, while the peak values ​​and half-maximum full widths of the probability density curves of constellation points with large amplitudes are relatively close. This shows that in turbulent media with poor signal-to-noise ratio, the use of our proposed Schemes 1 and 2 can reduce the interference of noise on the system to a certain extent. Figure 3 (d) shows the comparison of the receiving time domain waveform and spectrum of the three systems with the transmitting end under the optimal Vpp. From the perspective of the time domain waveform, the normalized mean squared error (NMSE) between the receiving signal and the transmitting signal of the point-to-point system, scheme 1, and scheme 2 are 24.04%, 23.32%, and 22.21%, respectively. The difference between the waveform and the transmitting waveform gradually decreases. From the perspective of the spectrum, scheme 1 has a slight improvement compared to the point-to-point system, while scheme 2 can suppress low-frequency and high-frequency noise to a large extent. This conclusion is Figure 3 (e) This can also be verified by comparing the probability density curves and noise spectra of the time domain noise distribution of the three systems.

[0057] Figure 4 The bit error rate of three systems with wavelengths of (a) 638nm, (b) 520nm, (c) 450nm and (d) 405nm, respectively, varies with Vpp. The results show that Figure 3 (a) Same trend.

[0058] Figure 5The achievable rate (bar graph) and BER (line graph) of the five wavelengths of (a) 685nm, (b) 638nm, (c) 520nm, (d) 450nm and (e) 405nm of this system vary with the transmission bandwidth. Since the actual system is a bandwidth-limited system and the visible light communication channel has high-frequency attenuation characteristics, as the modulation bandwidth increases, the system's inter-symbol crosstalk and other interference will become more serious, and the bit error rate will also increase. When the bit error rate is lower than the threshold value, the system supports 128QAM transmission. Since the baud rate of 128QAM is 7Baud, the achievable rate can be calculated by multiplying the modulation bandwidth by 7. When the bit error rate is higher than the threshold value, the system does not support 128QAM transmission and the modulation order must be reduced, so the achievable rate will decrease. Experimental results show that the in-phase transmission method and differential transmission method based on dual polarization and dual aperture can achieve transmission rates of 86.1 Gbps and 88.9 Gbps respectively, which are 9.1 Gbps and 11.9 Gbps higher than the traditional point-to-point transmission. The two schemes we proposed play an important role in improving the reliability and robustness of visible light wireless laser communication systems, and help to achieve higher-speed and larger-capacity communications.

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Claims

1. A dual-polarization dual-aperture multi-wavelength visible light wireless laser communication system, characterized in that: The invention realizes dual-polarization in-phase transmission in which the damage in the horizontal and vertical polarization directions can be complementary, and dual-polarization differential transmission with common-mode interference suppression; for dual-polarization in-phase transmission, two groups of lasers modulate the same signal, and the receiving end uses the maximum ratio combining algorithm to combine; for dual-polarization differential transmission, two groups of lasers modulate the signals with opposite phases, and the reception of the two apertures at the receiving end is differential; the system specifically includes: a digital signal processing module (DSP) at the transmitting end, two groups of integrated five-wavelength laser transmitting modules, an in-phase or anti-phase transmitting module based on polarization multiplexing, referred to as a polarization multiplexing module; and a dual-aperture receiving module (APD) at the receiving end, a maximum ratio combining algorithm (MRC) or a differential processing (Diff) module, and a digital signal processing module (DSP) at the receiving end; of the two groups of laser transmitting modules, one group uses horizontal polarization and the other group uses vertical polarization for multiplexing; after passing through the turbulent medium, it is received by the dual-aperture receiving module (APD); wherein: (i) the transmitting end digital signal processing module (DSP), including: a 128-level quadrature amplitude mapper (QAM), a carrier-less amplitude-phase modulator (CAP), and a software pre-equalization module; In the digital signal processing module (DSP) at the transmitting end, the randomly generated bit stream is mapped into 128QAM constellation points by a 128-level quadrature amplitude mapper (QAM); the constellation points are upsampled 4 times and modulated by a carrier-free amplitude-phase modulator (CAP) to obtain a real waveform signal; the high-frequency components of the real waveform signal are pre-emphasized, i.e., pre-equalized; thus, the signal Tx to be transmitted is obtained at the computer end; The signal to be transmitted is converted into a positive-phase and negative-phase analog electrical signal Tx by an arbitrary waveform generator (AWG). + With Tx - Output, where: Tx + =Tx,Tx - =-Tx;#(1) The output electrical signal is amplified by an electrical amplifier (EA) and then passes through an adjustable attenuator (ATT) to control the amplification factor. Then, it is driven by a bias tee together with a DC bias to emit light through a five-wavelength laser emission module. The laser emission module integrates laser diodes with wavelengths of 685nm, 638nm, 520nm, 450nm and 405nm and a metal heat sink. The working state of the laser can be changed by adjusting the peak-to-peak voltage of the electrical signal output by the arbitrary waveform generator (AWG) and the size of the bias current. The polarization multiplexing module uses a linear polarizer (LP) to filter the output light of two groups of five-wavelength laser emission modules into two output paths of horizontal polarization and vertical polarization Tx X With Tx Y ; For dual-polarization in-phase transmission, called Scheme 1, we have: Tx X =Tx Y =Tx + =Tx,#(2) For dual polarization anti-phase transmission, called Scheme 2, we have: Tx X =Tx + =Tx,Tx Y =Tx - =-Tx,#(3) Then, the two polarized light beams Tx X With Tx Y Transmitted together through underwater or atmospheric turbulent media; (ii) the receiving end digital signal processing module (DSP), including waveform level post-equalization, CAP demodulation, symbol level post-equalization, QAM demapping and bit error rate (BER) testing; The dual aperture receiving module (APD) is a spatial diversity receiver composed of two convex lenses (Lens) and an avalanche photodiode (APD); the two APDs receive the two horizontally and vertically polarized light beams respectively and convert them into corresponding electrical signals, which are input into an oscilloscope (OSC) for sampling after passing through an electrical amplifier (EA) and an attenuator (ATT), and quantized into a digital signal Rx X With Rx Y , input to the computer for post-processing; The maximum ratio combining algorithm (MRC) or the differential processing (Diff) module: For solution 1, the two polarization directions transmit the same-phase signals, and the maximum ratio combining algorithm module (MRC) is used to combine the Rx X With Rx Y Perform weighted summation on Rx X With Rx Y Weighted weights and Rx X With Rx Y The noise power is inversely proportional to the transmitted signal power. Since the transmitted signal power is consistent, Rx X With Rx Y The signal-to-noise ratio is used as the weight w X With w Y Specifically, estimate Rx X With Rx Y The method to improve the signal-to-noise ratio is to set Rx X With Rx Y Convert to the frequency domain through Fourier transform, and then calculate the area ratio of the in-band and out-band parts of the signal, that is: Among them, F k (Rx X ) and F k (Rx Y ) are Rx X and Rx Y The value of the Fourier transform at frequency k, f w is the signal bandwidth; the signal obtained after weighting is: Rx M =w X ·Rx X +w Y ·Rx Y ,#(5) For solution 2, the two polarization directions transmit signals in opposite phases, and the differential processing module (Diff) is used to combine the received signals, that is: Rx D =Rx X -Rx Y ,#(6) The signal Rx is obtained M or Rx D It is sent to the digital signal processing module (DSP) at the receiving end; it first undergoes waveform-level post-equalization to correct the distortion of the signal waveform, then obtains the constellation points through CAP demodulation and downsampling, and the constellation points undergo QAM demapping after symbol-level equalization, and finally recovers to the bit stream, and calculates the transmission bit error rate in combination with the transmitted bit stream.

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