Underwater wireless single-photon communication system and method based on adaptive coding and decoding

Abstract

The present invention discloses an underwater wireless single-photon communication system and method based on adaptive coding and decoding, relating to the field of underwater wireless single-photon communication systems. The system comprises: a transmitting end and a receiving end; the transmitting end comprises a data coding and modulation module and a laser emission module; the receiving end comprises a detection module and a data processing module; the data coding and modulation module is used to obtain a target LDPC coding rate and, based on the target LDPC coding rate, perform LDPC coding and modulation on the data to be transmitted to generate a corresponding modulated signal; the laser emission module is used to convert the modulated signal into an optical signal and transmit it to an underwater channel; the detection module is used to detect single-photon signals formed after the optical signal is attenuated by the underwater channel; and the data processing module is used to demodulate and LDPC decode the detected single-photon signals to obtain the original data to be transmitted. The present invention can adapt to the LDPC coding rate, thereby achieving efficient and reliable data transmission in complex underwater environments.

Description

Technical Field

[0001] The present invention relates to the field of underwater optical communications, and in particular to an underwater wireless single-photon communication system and method based on adaptive coding and decoding. Background Art

[0002] Underwater wireless optical communications, with their high speed and low latency, have shown broad application prospects in ocean exploration, deep-sea archaeology, and unmanned underwater vehicle networking. However, the underwater environment is complex and dynamic, and optical signals are subject to interference from factors such as scattering, absorption, and turbulence when propagating through water. This causes significant attenuation and fluctuations in the optical signal, thereby reducing the stability of the communication link and data transmission efficiency.

[0003] Among related technologies, most optical communication technologies rely on strong light signals to directly restore high and low voltage levels, which cannot meet the application requirements of strong attenuation channels underwater. In contrast, single-photon communication, with its extremely high detection sensitivity and extremely low power consumption, can effectively transmit signals even in low-light environments, making it an important research direction for underwater communications. However, due to the dynamic characteristics of underwater channels, single-photon communication systems still face challenges such as drastic changes in channel state and large fluctuations in bit error rates in complex waters. How to maintain high reliability under different water quality conditions remains an urgent problem to be solved.

[0004] Channel coding technology is crucial for enhancing the anti-interference capabilities of underwater single-photon communications and improving the reliability of communication systems. Low-density parity-check (LDPC) codes are widely used in wireless communication systems due to their excellent error correction performance and flexible code rate adjustment capabilities. However, most LDPC codes typically use a fixed code rate, making them difficult to adapt to the dynamic changes in underwater channels, and their communication performance fluctuates significantly under different water quality conditions. Summary of the Invention

[0005] In view of this, the present invention provides an underwater wireless single-photon communication system and method based on adaptive coding and decoding to solve the technical problems existing in the related art.

[0006] In a first aspect, the present invention provides an underwater wireless single-photon communication system based on adaptive coding and decoding, characterized in that it includes: a transmitting end and a receiving end; the transmitting end includes a data coding and modulation module and a laser emission module; the receiving end includes a detection module and a data processing module;

[0007] The data coding and modulation module is used to obtain a target LDPC coding rate, and perform LDPC coding and modulation on the data to be transmitted according to the target LDPC coding rate to generate a corresponding modulated signal; the target LDPC coding rate is one of an LDPC 1 / 2 code rate, an LDPC 2 / 3 code rate, an LDPC 3 / 4 code rate, and an LDPC 5 / 6 code rate;

[0008] The laser emission module is electrically connected to the data encoding and modulation module, and is used to convert the modulated signal into an optical signal and transmit it to the underwater channel;

[0009] The detection module is used to detect the single photon signal formed after the optical signal is attenuated by the underwater channel;

[0010] The data processing module is electrically connected to the detection module and is used to demodulate and perform LDPC decoding on the detected single-photon signal to obtain the original data to be sent.

[0011] In an optional implementation, the data coding and modulation module includes:

[0012] PC, used to obtain the target LDPC coding rate and output the target LDPC coding rate and data to be sent;

[0013] An LDPC encoder is electrically connected to the PC and is used to perform LDPC encoding on the data to be transmitted according to a target LDPC encoding rate and output a corresponding LDPC codeword;

[0014] The modulator is electrically connected to the LDPC encoder and is used to modulate the LDPC codeword to generate a corresponding modulated signal.

[0015] In an optional embodiment, the laser emission module includes:

[0016] A power amplifier, electrically connected to the data coding and modulation module, for amplifying the power of the modulation signal generated by the data coding and modulation module;

[0017] a DC power supply, electrically connected to the power amplifier and configured to provide an operating voltage to the power amplifier;

[0018] A bias device electrically connected to the power amplifier, for performing a DC bias on the modulated signal after power amplification to obtain a bias signal;

[0019] a laser diode driver, electrically connected to the bias device and configured to provide a stable current to the bias device;

[0020] The laser is electrically connected to the bias device and is used to convert the bias signal into an optical signal and transmit the optical signal to the underwater channel.

[0021] In an optional embodiment, the detection module includes:

[0022] a lens for converging single photon signals formed after the optical signal is attenuated by the underwater channel to obtain focused photon signals, and guiding the focused photon signals to an array single photon detector;

[0023] Array single-photon detector, used to perform photoelectric conversion on the focused photon signal to generate a corresponding electrical signal;

[0024] The time-to-digital converter is electrically connected to the array single-photon detector and is used to perform time measurement and digital processing on the electrical signal to obtain transmission data containing relevant information of the single-photon signal.

[0025] In an optional embodiment, the data processing module includes:

[0026] a demodulator, electrically connected to the time-to-digital converter, for demodulating the detected single-photon signal to obtain a corresponding baseband signal;

[0027] The LDPC decoder is electrically connected to the demodulator and is used to perform LDPC decoding on the baseband signal to obtain the original data to be sent.

[0028] In an optional implementation, the LDPC encoder and modulator are integrated into an FPGA, and the FPGA is connected to a PC via a network port.

[0029] In an optional implementation, the demodulator and LDPC decoder are integrated into a PC, and the PC is connected to the time-to-digital converter via a USB interface.

[0030] In a second aspect, the present invention further provides an underwater wireless single-photon communication method based on adaptive coding and decoding, comprising:

[0031] Obtaining a target LDPC coding rate, and performing LDPC coding and modulation on the data to be transmitted according to the target LDPC coding rate to generate a corresponding modulated signal; the target LDPC coding rate is one of an LDPC 1 / 2 code rate, an LDPC 2 / 3 code rate, an LDPC 3 / 4 code rate, and an LDPC 5 / 6 code rate;

[0032] Converting the modulated signal into an optical signal and transmitting the optical signal to an underwater channel;

[0033] detecting a single photon signal formed after the optical signal is attenuated through an underwater channel;

[0034] The detected single-photon signal is demodulated and LDPC decoded to obtain the original data to be sent.

[0035] In an optional implementation, obtaining a target LDPC encoding rate includes:

[0036] Measure the output optical power of the laser and the received optical power at the lens;

[0037] Calculate the attenuation coefficient of the optical signal transmitted in the underwater channel based on the output optical power of the laser and the received optical power at the lens;

[0038] According to a correspondence between a preset first rate switching threshold and an LDPC coding rate, the attenuation coefficient is compared with the first rate switching threshold; the first rate switching threshold is a plurality of attenuation coefficient intervals;

[0039] When the attenuation coefficient is within the target attenuation coefficient interval, the LDPC coding rate corresponding to the target attenuation coefficient interval is used as the target LDPC coding rate.

[0040] In an optional implementation, obtaining a target LDPC encoding rate includes:

[0041] Send test words to the LDPC encoder according to the fixed LDPC 1 / 2 code rate;

[0042] Calculate the signal-to-noise ratio and bit error rate at the output of the LDPC encoder;

[0043] The bit error rate is compared with a target bit error rate; the target bit error rate is 10 -6 ;

[0044] If the bit error rate is greater than the target bit error rate, the signal-to-noise ratio is compared with the second rate switching threshold according to a preset correspondence between the second rate switching threshold and the LDPC coding rate; the second rate switching threshold is a plurality of signal-to-noise ratio intervals; when the signal-to-noise ratio is within the target signal-to-noise ratio interval, the LDPC coding rate corresponding to the target signal-to-noise ratio interval is used as the target LDPC coding rate;

[0045] Otherwise, when the bit error rate is less than the target bit error rate and the current code rate is the LDPC 1 / 2 code rate, the sending of the test word is suspended, and after a preset time period, the test word is resent to the LDPC encoder according to the current LDPC 1 / 2 code rate.

[0046] The present invention has the following beneficial effects:

[0047] Embodiments of the present invention can address the problem in related technologies of underwater wireless optical communication systems where the receiving end experiences dramatic signal fluctuations under complex channel conditions, and where communication using a fixed bit rate cannot fully utilize the channel capacity. By monitoring the hydrological parameters of the underwater channel, measuring the attenuation coefficient, signal-to-noise ratio, and bit error rate of the optical signal in water, and feeding these back to the transmitter, the transmitter switches the bit rate using a preset program, dynamically adjusting the LDPC coding rate to optimize the transmission rate under varying water quality conditions. Embodiments of the present invention can utilize high-rate coding to increase the data transmission rate when channel conditions are favorable, and reduce the coding rate to enhance communication stability when channel attenuation is severe, thereby achieving efficient and reliable data transmission in complex underwater environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0049] Figure 1 2 is a structural framework diagram of an underwater wireless single-photon communication system based on adaptive coding and decoding according to an embodiment of the present invention;

[0050] Figure 2 1 is a flow chart of an underwater wireless single-photon communication method based on adaptive coding and decoding according to an embodiment of the present invention;

[0051] Figure 3 4 is a flow chart of a process for obtaining a target LDPC coding rate according to an embodiment of the present invention. DETAILED DESCRIPTION

[0052] To make the purpose, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative efforts shall fall within the scope of protection of the present invention.

[0053] In the description of the present invention, it should be noted that the terms "upper," "lower," "left," "right," "inner," and "outer," etc., indicating directions or positional relationships, are based on the directions or positional relationships shown in the accompanying drawings and are intended solely to simplify the description of the present invention. They do not indicate or imply that the devices or components referred to must have a specific direction, be constructed, or operate in a specific direction. Therefore, they should not be construed as limitations on the present invention. Furthermore, the terms "first," "second," etc., etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0054] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "installed," "connected," and "connected" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integral connections; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; internal connections between two components; wireless connections or wired connections. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0055] In addition, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0056] Figure 1 An underwater wireless single-photon communication system based on adaptive coding and decoding according to an embodiment of the present invention is shown, comprising: a transmitting end and a receiving end; the transmitting end comprises a data coding and modulation module and a laser emission module; the receiving end comprises a detection module and a data processing module.

[0057] Specifically, the data coding and modulation module is used to obtain a target LDPC coding rate and, based on the target LDPC coding rate, perform LDPC coding and modulation on the data to be transmitted to generate a corresponding modulated signal. The target LDPC coding rate is one of LDPC 1 / 2, LDPC 2 / 3, LDPC 3 / 4, and LDPC 5 / 6. The laser output module is electrically connected to the data coding and modulation module and is used to convert the modulated signal into an optical signal and transmit it into the underwater channel. The detection module is used to detect single-photon signals formed after the optical signal is attenuated by the underwater channel. The data processing module is electrically connected to the detection module and is used to demodulate and perform LDPC decoding on the detected single-photon signals to obtain the original data to be transmitted.

[0058] In an optional implementation, the data coding and modulation module includes: a PC (computer system), an LDPC encoder, and a modulator.

[0059] Specifically, the PC (computer system) is used to obtain a target LDPC encoding rate and output the target LDPC encoding rate and the data to be transmitted. An LDPC encoder is electrically connected to the PC and is used to perform LDPC encoding on the data to be transmitted according to the target LDPC encoding rate and output a corresponding LDPC codeword. A modulator is electrically connected to the LDPC encoder and is used to modulate the LDPC codeword to generate a corresponding modulated signal.

[0060] Preferably, the LDPC encoder and modulator can be integrated into the FPGA, the PC is connected to the FPGA via a network port, and the target LDPC coding rate and the data to be sent output by the PC are transmitted to the LDPC encoder in the FPGA via the network port.

[0061] In an optional embodiment, the laser emission module includes: a power amplifier, a DC power supply, a bias device, a laser diode driver and a laser.

[0062] Specifically, the power amplifier is electrically connected to the data coding and modulation module. Furthermore, the power amplifier is electrically connected to the modulator in the data coding and modulation module. The power amplifier is configured to amplify the modulated signal generated by the data coding and modulation module. A DC power supply is electrically connected to the power amplifier. Furthermore, the DC power supply is electrically connected to the power supply port of the power amplifier. The DC power supply is configured to provide a stable operating voltage to the power amplifier. A bias device is electrically connected to the power amplifier. The bias device is configured to apply a DC bias to the amplified modulated signal to generate a bias signal. A laser diode driver is electrically connected to the bias device. The laser diode driver is configured to provide a stable current to the bias device. A laser is electrically connected to the bias device. The laser is configured to convert the bias signal into an optical signal and transmit the optical signal into an underwater channel.

[0063] The bias switch's two input ports are connected to the power amplifier's output and the laser diode driver, respectively, while its output port is connected to the laser. The stable current provided by the laser diode driver can further regulate the laser's optical signal's intensity and switching state. Combined with the modulated and biased signal output by the bias switch, precise optical signal transmission is achieved.

[0064] In an optional embodiment, the detection module includes: a lens, an array single-photon detector and a time-to-digital converter.

[0065] Specifically, the lens is used to converge the single-photon signal formed after the optical signal is attenuated by the underwater channel to obtain a focused photon signal, and then guide the focused photon signal to the array single-photon detector. The array single-photon detector is used to perform photoelectric conversion on the focused photon signal to generate a corresponding electrical signal. The time-to-digital converter is electrically connected to the array single-photon detector and is used to measure the time and digitize the electrical signal to obtain transmission data containing information related to the single-photon signal. This transmission data is required for subsequent signal analysis.

[0066] In an optional implementation, the data processing module includes: a demodulator and an LDPC decoder.

[0067] Specifically, the demodulator is electrically connected to the time-to-digital converter and is configured to demodulate the detected single-photon signal to obtain a corresponding baseband signal. The LDPC decoder is electrically connected to the demodulator and is configured to perform LDPC decoding on the baseband signal to obtain the original data to be transmitted, i.e., to restore the original data to be transmitted.

[0068] Preferably, the demodulator and LDPC decoder can be integrated into a PC, which is connected to the time-to-digital converter via a USB interface, and the output of the time-to-digital converter is transmitted to the PC via a network port for demodulation and LDPC decoding to achieve data signal processing.

[0069] like Figure 2 As shown, the present invention also provides an underwater wireless single-photon communication method based on adaptive coding and decoding, comprising:

[0070] S1. Obtain a target LDPC coding rate, and perform LDPC coding and modulation on the data to be transmitted based on the target LDPC coding rate to generate a corresponding modulated signal; the target LDPC coding rate is one of 1 / 2, 2 / 3, 3 / 4, and 5 / 6.

[0071] S2. Convert the modulated signal into an optical signal and transmit it to the underwater channel.

[0072] S3, the single photon signal formed after the detection light signal is attenuated by the underwater channel.

[0073] Among them, the optical signal is attenuated during underwater transmission and eventually reduced to the single-photon level.

[0074] In an optional embodiment, step S3 includes:

[0075] S31, using a lens to converge a single photon signal formed after the optical signal is attenuated by the underwater channel to obtain a focused photon signal, and guiding the focused photon signal to an array single photon detector;

[0076] S32. Use an array single-photon detector to perform photoelectric conversion on the focused photon signal to generate a corresponding electrical signal.

[0077] S33. Use a time-to-digital converter to perform time measurement (collect the time when the photons arrive) and digital processing on the electrical signal to obtain transmission data containing information related to the single-photon signal.

[0078] S4. Demodulate and LDPC decode the detected single-photon signal to obtain the original data to be sent.

[0079] In an optional implementation, obtaining a target LDPC encoding rate includes:

[0080] S11a, measuring the output optical power of the laser and the received optical power at the lens.

[0081] The output optical power of the laser and the received optical power at the lens can be measured by a power meter.

[0082] S12a. Calculate the attenuation coefficient of the optical signal transmitted in the underwater channel based on the output optical power of the laser and the received optical power at the lens.

[0083] S13a. According to a preset correspondence between the first rate switching threshold and the LDPC coding rate, the attenuation coefficient is compared with the first rate switching threshold; the first rate switching threshold is a plurality of attenuation coefficient intervals.

[0084] Specifically, before communication begins, multiple LDPC coding rates can be preset for different optical attenuation coefficients, with each LDPC coding rate corresponding to an optical attenuation coefficient range. By comparing the measured optical attenuation coefficient with each optical attenuation coefficient range, the most appropriate LDPC coding rate can be obtained.

[0085] S14a. When the attenuation coefficient is within the target attenuation coefficient interval, the LDPC coding rate corresponding to the target attenuation coefficient interval is used as the target LDPC coding rate.

[0086] The target attenuation coefficient interval is any interval among multiple attenuation coefficient intervals.

[0087] In an optional embodiment, as Figure 3 As shown, the target LDPC coding rate is obtained, including:

[0088] S11b: Send a test word to the LDPC encoder according to the fixed LDPC 1 / 2 code rate to evaluate the current channel state.

[0089] It should be noted that the communication process needs to be initialized before sending the test word.

[0090] S12b. Calculate the signal-to-noise ratio and bit error rate at the output of the LDPC encoder to evaluate the channel quality.

[0091] Specifically, a test word is sent to the LDPC encoder, which passes through various modules of the system and finally outputs the original test word through demodulation and LDPC decoding. At this time, the signal-to-noise ratio and bit error rate are calculated by collecting relevant data at the output end of the LDPC encoder.

[0092] S13b, compare the bit error rate with the target bit error rate; the target bit error rate is 10 -6 .

[0093] S14b. If the bit error rate is greater than or equal to the target bit error rate (i.e., the bit error rate meets the target), then the signal-to-noise ratio is compared with the second rate switching threshold according to a preset correspondence between the second rate switching threshold and the LDPC coding rate; the second rate switching threshold is a plurality of signal-to-noise ratio intervals; when the signal-to-noise ratio is within the target signal-to-noise ratio interval, the LDPC coding rate corresponding to the target signal-to-noise ratio interval is used as the target LDPC coding rate;

[0094] Otherwise, when the bit error rate is less than the target bit error rate (i.e., the bit error rate does not meet the standard) and the current code rate is the LDPC 1 / 2 code rate, the sending of the test word is suspended. After a preset time period, the test word is sent to the LDPC encoder according to the current LDPC 1 / 2 code rate.

[0095] In an optional implementation, if the current code rate is not the LDPC 1 / 2 code rate, the signal-to-noise ratio is compared with the second code rate switching threshold based on a preset correspondence between the second code rate switching threshold and the LDPC coding code rate; the second code rate switching threshold is a plurality of signal-to-noise ratio intervals; when the signal-to-noise ratio is within a target signal-to-noise ratio interval, the LDPC coding code rate corresponding to the target signal-to-noise ratio interval is used as the target LDPC coding code rate.

[0096] The embodiment of the present invention presets a variety of LDPC coding rates for different optical attenuation coefficients, signal-to-noise ratios, and bit error rates, and corresponds to specific optical attenuation coefficient intervals and signal-to-noise ratio intervals, respectively. The system sets a target bit error rate. Before communication begins, it first measures and evaluates the underwater channel characteristics, including measuring the output optical power of the transmitting laser and the optical power at the receiving lens with a power meter to calculate the attenuation coefficient of light transmission in water, or obtaining signal-to-noise ratio and bit error rate information by sending test words. Based on the measurement results, the preset code rate switching threshold is compared and the most suitable LDPC coding code rate is selected. When the bit error rate exceeds the target bit error rate during communication, the system will switch to a lower code rate to ensure communication reliability.

[0097] In an optional implementation, the steps of LDPC encoding are:

[0098] Step 1: Design four base check matrices H for LDPC 1 / 2 code rate, LDPC 2 / 3 code rate, LDPC 3 / 4 code rate, and LDPC 5 / 6 code rate respectively. b , size is m b ×n b . Among them, for different LDPC coding rates, n b Fixed to 24, and m b The value of varies with the bit rate. The base check matrix H b It consists of two parts, namely H b =[H b1 H b2 ].

[0099] Among them, H b1 is a sparse random matrix without quadruple rings, of size m b ×k b , where the matrix elements are either -1 or non-negative integers. If the element is -1, the submatrix at the corresponding position in the check matrix H is an all-zero matrix. If the element is a non-negative integer, the submatrix at the corresponding position in the check matrix H is a right-shifted cyclic permutation matrix of the identity matrix, with the number of right shifts determined by the non-negative integer.

[0100] H b2 The size is m b ×m b , the elements h(1), h(r) and h(m b ) is a non-negative integer and satisfies h(1)=h(m b ), where r is in the range of 2≤r≤m b -1, the specific value is defined by the base check matrix of the IEEE 802.16e standard. In the first column, except for the above three positions, the remaining elements are -1. b2 Except for the first column, the rest of the structure is quasi-bidiagonal, with the elements on the two quasi-diagonals being 0 and the elements in other positions being -1.

[0101] Step 2: Use fast coding based on IEEE802.16 standard to divide the information bit vector s and the check bit vector p into segments, each segment length is z. Then the information bit vector and the check bit vector can be expressed as: s=[s1,s2,…,s kb ],p=[p1,p2,…,p mb ], at this time, the encoder outputs codeword c = [sp].

[0102] According to the check equation We can get H2·p T =H1·s T Expanding this equation yields a formula containing m bThe final check vector p=[p1,p2,…,p mb ], thus achieving fast encoding.

[0103] The embodiments of the present invention have the following beneficial effects:

[0104] (1) The use of LDPC coding and decoding technology can significantly reduce the bit error rate caused by signal attenuation and noise in underwater optical communication through efficient channel coding and decoding, thereby improving the reliability of data transmission.

[0105] (2) It can adjust the coding parameters in real time according to the changes in underwater channel conditions, improve the transmission rate when the channel conditions are good, and enhance the anti-noise ability when the channel attenuation is severe, thereby optimizing the stability and adaptability of underwater wireless single-photon communication.

[0106] (3) The system adopts a combination of PC and FPGA hardware and software, which has high integration and flexibility, can adapt to different application requirements, and is convenient for subsequent expansion and upgrading.

[0107] (4) By utilizing the high sensitivity of the single-photon detector and the error correction capability of the subsequent LDPC decoder, the impact of the signal-to-noise ratio fluctuation at the receiving end on the communication quality is effectively alleviated, and the stability of data transmission is improved.

[0108] In summary, the present invention can dynamically adjust the coding rate according to the characteristics of the underwater channel to ensure that the communication bit error rate is stable within the target range. By measuring the optical attenuation coefficient, signal-to-noise ratio and bit error rate, and combining FPGA for LDPC coding and modulation, the system achieves efficient data transmission. The receiving end uses single-photon detectors and time-to-digital conversion technology to accurately recover the signal, and completes demodulation and decoding on the PC end. The present invention can maintain stable communication performance in different water quality environments, improves the reliability and adaptability of underwater single-photon communication, and is suitable for long-distance, high-speed underwater optical communication applications.

[0109] Although the embodiments of the present invention have been described with reference to the accompanying drawings, those skilled in the art may make various modifications and variations without departing from the spirit and scope of the present invention. Such modifications and variations are all within the scope defined by the appended claims.

Claims

1. An underwater wireless single-photon communication system based on adaptive coding and decoding, characterized in that: It includes: a transmitting end and a receiving end; the transmitting end includes a data coding and modulation module and a laser emission module; The receiving end includes a detection module and a data processing module; The data coding and modulation module is used to obtain a target LDPC coding rate, and perform LDPC coding and modulation on the data to be transmitted according to the target LDPC coding rate to generate a corresponding modulated signal; the target LDPC coding rate is one of an LDPC 1 / 2 code rate, an LDPC 2 / 3 code rate, an LDPC 3 / 4 code rate, and an LDPC 5 / 6 code rate; The laser emission module is electrically connected to the data encoding and modulation module, and is used to convert the modulated signal into an optical signal and transmit it to the underwater channel; The detection module is used to detect the single photon signal formed after the optical signal is attenuated by the underwater channel; The data processing module is electrically connected to the detection module and is used to demodulate and decode the detected single photon signal to obtain the original data to be sent; The system further comprises the following method steps: Obtaining a target LDPC coding rate, and performing LDPC coding and modulation on the data to be transmitted according to the target LDPC coding rate to generate a corresponding modulated signal; the target LDPC coding rate is one of an LDPC 1 / 2 code rate, an LDPC 2 / 3 code rate, an LDPC 3 / 4 code rate, and an LDPC 5 / 6 code rate; Converting the modulated signal into an optical signal and transmitting the optical signal to an underwater channel; detecting a single photon signal formed after the optical signal is attenuated through an underwater channel; Demodulate and LDPC decode the detected single-photon signal to obtain the original data to be sent; The target LDPC coding rate may be obtained by any of the following methods: (a) Obtaining a target LDPC encoding rate includes: Measure the output optical power of the laser and the received optical power at the lens; Calculate the attenuation coefficient of the optical signal transmitted in the underwater channel based on the output optical power of the laser and the received optical power at the lens; According to a correspondence between a preset first rate switching threshold and an LDPC coding rate, the attenuation coefficient is compared with the first rate switching threshold; the first rate switching threshold is a plurality of attenuation coefficient intervals; When the attenuation coefficient is within the target attenuation coefficient interval, the LDPC coding rate corresponding to the target attenuation coefficient interval is used as the target LDPC coding rate; (b) Sending a test word to the LDPC encoder according to a fixed LDPC 1 / 2 code rate; Calculate the signal-to-noise ratio and bit error rate at the output of the LDPC encoder; The bit error rate is compared with a target bit error rate; the target bit error rate is 10 -6 ; If the bit error rate is greater than or equal to the target bit error rate, comparing the signal-to-noise ratio with the second rate switching threshold according to a preset correspondence between the second rate switching threshold and the LDPC coding rate; the second rate switching threshold is a plurality of signal-to-noise ratio intervals; when the signal-to-noise ratio is within the target signal-to-noise ratio interval, the LDPC coding rate corresponding to the target signal-to-noise ratio interval is used as the target LDPC coding rate; Otherwise, when the bit error rate is less than the target bit error rate and the current code rate is the LDPC 1 / 2 code rate, the sending of the test word is suspended, and after a preset time period, the test word is resent to the LDPC encoder according to the current LDPC 1 / 2 code rate.

2. The system according to claim 1, wherein: The data coding and modulation module includes: PC, used to obtain the target LDPC coding rate and output the target LDPC coding rate and data to be sent; An LDPC encoder is electrically connected to the PC and is used to perform LDPC encoding on the data to be transmitted according to a target LDPC encoding rate and output a corresponding LDPC codeword; The modulator is electrically connected to the LDPC encoder and is used to modulate the LDPC codeword to generate a corresponding modulated signal.

3. The system according to claim 1, wherein: The laser emission module includes: A power amplifier, electrically connected to the data coding and modulation module, for amplifying the power of the modulation signal generated by the data coding and modulation module; a DC power supply, electrically connected to the power amplifier and configured to provide an operating voltage to the power amplifier; A bias device electrically connected to the power amplifier, for performing a DC bias on the modulated signal after power amplification to obtain a bias signal; a laser diode driver, electrically connected to the bias device and configured to provide a stable current to the bias device; The laser is electrically connected to the bias device and is used to convert the bias signal into an optical signal and transmit the optical signal to the underwater channel.

4. The system according to claim 1, wherein: The detection module includes: a lens for converging single photon signals formed after the optical signal is attenuated by the underwater channel to obtain focused photon signals, and guiding the focused photon signals to an array single photon detector; Array single-photon detector, used to perform photoelectric conversion on the focused photon signal to generate a corresponding electrical signal; The time-to-digital converter is electrically connected to the array single-photon detector and is used to perform time measurement and digital processing on the electrical signal to obtain transmission data containing relevant information of the single-photon signal.

5. The system according to claim 4, characterized in that The data processing module includes: a demodulator, electrically connected to the time-to-digital converter, for demodulating the detected single-photon signal to obtain a corresponding baseband signal; The LDPC decoder is electrically connected to the demodulator and is used to perform LDPC decoding on the baseband signal to obtain the original data to be sent.

6. The system according to claim 2, wherein: The LDPC encoder and modulator are integrated into an FPGA, and the FPGA is connected to a PC via a network port.

7. The system according to claim 5, characterized in that The demodulator and LDPC decoder are integrated in a PC, and the PC is connected to a time-to-digital converter via a USB interface.

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