An underwater single photon communication scattering characteristic measurement system and method

Abstract

The application discloses an underwater single-photon communication scattering characteristic measurement system and method, and belongs to the technical field of underwater wireless optical communication. In view of the problem that the scattering characteristics of an underwater optical channel cannot be accurately and reproducibly measured and analyzed in the prior art, a backscattering receiving end and a multipath effect receiving end are equivalent to a band-pass filter model, through the physical model, the scattering characteristics of the underwater channel are quantified as a measurable band-pass frequency response curve, so that the upper and lower cutoff frequencies under a given water quality condition and the optimal modulation bandwidth with the least scattering effect are determined. Based on the quantitative analysis of low-frequency backscattering and high-frequency multipath effect, the application can realize efficient and stable data transmission in a complex underwater environment, and significantly improve the reliability and spectrum utilization efficiency of the underwater optical communication system. The technology provides a key test means and design basis for underwater high-speed communication, and has important scientific value and broad industrialization prospect.

Description

Technical Field

[0001] This invention belongs to the field of underwater wireless optical communication technology, specifically relating to an underwater single-photon communication scattering characteristic measurement system and method. Background Technology

[0002] Underwater communication has significant application value in marine exploration, unmanned underwater vehicle communication, environmental monitoring, marine scientific research, and national defense. With the deepening of the marine economy and the national strategy of building a maritime power, the demand for high-speed, reliable underwater data transmission is becoming increasingly urgent. Traditional underwater communication methods mainly include radio communication and underwater acoustic communication. However, radio signal propagation underwater is limited by strong absorption in the water, generally only suitable for shallow water environments and unable to meet the needs of deep-water communication. While underwater acoustic communication can achieve long-distance transmission, it suffers from low bandwidth, high transmission delay, and severe susceptibility to environmental noise and multipath interference, making it difficult to meet the requirements of high bandwidth, high reliability, and low latency data transmission. This communication bottleneck severely restricts the development of cutting-edge applications such as underwater unmanned system collaboration and deep-sea real-time monitoring networks.

[0003] Underwater optical communication, as an emerging technology, utilizes the high frequency, high bandwidth, fast propagation speed, and strong directionality of light waves to achieve high-speed, low-latency underwater data transmission, becoming an effective means to overcome the limitations of traditional communication methods. Especially in single-photon communication mode, highly sensitive single-photon detectors can significantly extend the communication distance. However, underwater optical communication still faces many challenges in practical applications, mainly due to scattering and absorption effects in water.

[0004] During underwater optical propagation, optical signals are affected by scatterers such as water molecules, suspended particles, microorganisms, and bubbles, causing photon path deviations and resulting in backscattering and multipath effects. Backscattering refers to photons being reflected back towards the receiver after encountering scatterers such as suspended particles, thus creating noise signals and reducing the signal-to-noise ratio. This effect is particularly significant in turbid water or under conditions of high particle concentration, potentially leading to significant fluctuations in single-photon reception counts. Multipath effects refer to photons undergoing reflection and refraction during propagation, generating multiple propagation paths to the receiver, creating time delay distributions, and resulting in inter-symbol interference, affecting the bit error rate and transmission efficiency of the communication system. How to accurately measure and quantitatively characterize these two scattering effects is a key scientific issue in the design and optimization of underwater optical communication systems.

[0005] Currently, research on the scattering characteristics of underwater single-photon communication mainly relies on theoretical modeling and experimental simulation. Theoretical methods include radiative transfer equations and Monte Carlo simulations, used to predict the path, attenuation, and scattering characteristics of light propagating underwater. Experimental methods, by controlling water conditions, particle concentration, and light source parameters, combined with single-photon detectors and time-correlated single-photon counting systems, measure data such as photon counts, pulse broadening, and delay distribution to quantify backscattering and multipath effects. However, existing experimental methods often suffer from insufficient controllability and repeatability, limited data volume, and difficulty in simulating real-world aquatic environments, making it difficult to provide comprehensive and reliable data support for single-photon communication system design. More critically, existing methods lack a systematic measurement method that directly correlates scattering physics with channel frequency domain characteristics, failing to provide a clear basis for bandwidth selection in communication systems. This has become a key bottleneck restricting the transition of underwater optical communication technology from the laboratory to engineering applications.

[0006] Therefore, there is an urgent need for a systematic, controllable, and repeatable experimental platform for accurately measuring and analyzing the scattering characteristics of underwater single-photon communication channels. Summary of the Invention

[0007] To address the problem of existing technologies being unable to accurately and reproducibly measure and analyze the scattering characteristics of underwater optical channels, this invention provides a system and method for measuring the scattering characteristics of underwater single-photon communication. This invention systematically models backscattering and multipath effects as a bandpass filter model for channel transmission. Specifically, backscattering noise is concentrated in the low-frequency band, determining the lower cutoff frequency of the channel's bandpass response; while the time delay broadening and inter-symbol interference of multipath effects on high-frequency signals determine the upper cutoff frequency of the channel's bandpass response. Through this physical model, the scattering characteristics of the underwater channel are quantified into a measurable bandpass frequency response curve.

[0008] For duplex underwater optical communication systems, backscattering is a key factor limiting communication performance. When a laser transmitter emits a modulated optical signal into the water, photons undergo elastic scattering along their propagation path by scatterers such as suspended particles, bubbles, and organic matter in the water. Some photons return along a path in the reverse direction, close to the incident direction, to the detection system near the transmitter. Because these scattered photons highly coincide with the signal light emission time, they exhibit near-instantaneous superposition noise in the time domain and wideband low-frequency enhancement characteristics in the frequency domain. Therefore, the interference of backscattering at the receiver can be considered equivalent to an additive noise source, whose noise power decreases with increasing frequency, causing significant noise crosstalk to the transmission of low-frequency modulated signals, thus determining the lower cutoff frequency of the channel bandpass response. The power spectral density of backscattered noise can be modeled as:

[0009] (1)

[0010] in, For signal modulation frequency, The noise constant related to the incident light power. The noise cutoff characteristic frequency is related to the size distribution of the scatterer and the turbidity of the water. The model shows that the lower the modulation frequency, the stronger the backscattering noise and the more severe the deterioration of the signal-to-noise ratio.

[0011] The multipath effect arises from the multiple scattering of light signals in water, resulting in multiple propagation paths. Different scattering paths correspond to different photon propagation times, causing a random delay spread in the photon arrival time at the receiver. Let the multipath impulse response of the water body under unit pulse excitation be... Then the received optical signal can be expressed as the convolution of the transmitted signal and h(t). Under multiple scattering conditions, It can be modeled as an integral superposition of multiple scattering paths:

[0012] (2)

[0013] in, The unit impulse function represents an ideal instantaneous pulse; The path delay time for the photon; These are the gain coefficients corresponding to different delay paths; and Minimum and maximum measurable path delays, respectively; time delay spread due to multipath effects. This determines the coherence bandwidth of the channel. For high-frequency modulated signals, its signal period is... Comparable to, or even smaller than, inter-symbol interference generated between different paths will lead to frequency-selective fading, equivalent to the channel's suppression of high-frequency components. Therefore, multipath effects limit high-frequency transmission and determine the upper cutoff frequency of the channel's bandpass response. :

[0014] (3)

[0015] By combining low-frequency noise suppression from backscattering with high-frequency fading limitation from multipath effects, underwater optical communication channels exhibit typical bandpass characteristics in the frequency domain. The channel's equivalent frequency response... It can be uniformly modeled as follows:

[0016] (4)

[0017] in, The lower cutoff frequency is dominated by backscattering. The upper cutoff frequency is determined by the multipath effect. This represents the maximum transmission coefficient in the mid-frequency band of the channel. This bandpass filter model quantifies the scattering characteristics of the underwater channel into a measurable frequency response curve, and experimental measurements are performed under different water quality conditions. and This allows for the precise determination of the optimal modulation bandwidth that minimizes scattering effects, providing a clear basis for parameter selection in communication system design.

[0018] Based on the above theoretical model, the underwater single-photon communication scattering characteristic measurement method and system provided by the present invention achieves high-precision measurement and verification of the parameters of the bandpass filter model through experimental means of frequency sweep excitation and single-photon time correlation counting.

[0019] To achieve the above objectives, the present invention employs the following technical solutions:

[0020] An underwater single-photon communication scattering characteristic measurement system, the system comprising: a transmitter, an underwater channel, a backscatter receiver, and a multipath effect receiver;

[0021] The backscatter receiver and the multipath effect receiver are modeled as bandpass filter models for channel transmission;

[0022] The transmitting end includes a data modulation module and a laser emission module;

[0023] The data modulation module is used to generate a modulation signal that can be frequency-switched within a preset bandwidth.

[0024] The laser emission module is electrically connected to the data modulation module and is used to convert the modulation signal into an optical signal and transmit it to the underwater channel;

[0025] Both the backscattering receiver and the multipath effect receiver include a photon detection module and a data processing module.

[0026] The photon detection module is used to receive the single-photon signal formed after attenuation by the underwater channel, and to perform photoelectric conversion and time measurement on the single-photon signal, and output photon arrival time data.

[0027] The data processing module is electrically connected to the photon detection module and is used to demodulate and statistically analyze the photon arrival time data to obtain backscattering characteristics and multipath effect characteristics.

[0028] The data modulation module includes an FPGA unit, a clock generation unit, a digital-to-analog converter unit, and a PC unit;

[0029] The FPGA unit is electrically connected to the PC unit and is used to generate a modulation signal according to the modulation frequency.

[0030] The clock generation unit is electrically connected to the FPGA unit and the digital-to-analog converter unit respectively, and is used to provide clock signals;

[0031] The digital-to-analog converter is electrically connected to the FPGA unit and is used to convert the electrical signal output by the FPGA unit into an analog signal.

[0032] The PC unit is used to configure the modulation frequency of the modulation signal.

[0033] The laser emission module includes a DC power supply, a power amplifier, a laser diode driver, a biaser, and a laser.

[0034] The DC power supply is electrically connected to the power supply port of the power amplifier to provide the operating voltage;

[0035] The input terminal of the power amplifier is connected to the output terminal of the digital-to-analog converter unit, and is used to receive and amplify the modulation signal;

[0036] The laser diode driver is electrically connected to the biaser and is used to provide DC bias current to the biaser;

[0037] The bias unit is electrically connected to the power amplifier and is used to superimpose the modulated signal after power amplification with the DC bias current.

[0038] The laser is connected to the output of the biaser and is used to convert the modulated signal after superimposed DC bias into an optical signal and transmit it to the underwater channel.

[0039] The underwater channel consists of a cuboid water tank and water mixed with scattering particles; by replacing different types of scattering particles, the multipath effect and backscattering characteristics under different water quality conditions can be measured.

[0040] The photon detection module includes a receiving lens, a single-photon detector, and a time-to-digital converter.

[0041] The receiving lens is used to collect the single-photon signal formed after attenuation by the underwater channel and focus it on the photosensitive surface of the single-photon detector;

[0042] The single-photon detector is used to perform photoelectric conversion on the focused single-photon signal to generate an electrical signal;

[0043] The time-to-digital converter is electrically connected to the single-photon detector and is used to measure and digitize the electrical signal to obtain data containing photon arrival time information.

[0044] The data processing module includes a demodulator electrically connected to a time-to-digital converter (TD-to-DC converter) for demodulating the data output by the TD-to-DC converter to obtain the statistical characteristics of the modulated signal. The data processing module is also used to plot a frequency response curve based on the statistical characteristics of the modulated signal.

[0045] The demodulator is integrated into a PC, which is connected to a time-to-digital converter via a USB interface.

[0046] A method for measuring the scattering characteristics of underwater single-photon communication, the method being implemented based on the system, and the method comprising the following steps:

[0047] Step S1: Add different scattering particles to the water and measure the parameters under the water quality conditions; configure the modulation frequency information on the PC unit and send it to the FPGA unit, which then generates the corresponding modulation signal.

[0048] Step S2: After the modulation signal is amplified by the power amplifier, a DC bias is superimposed through the biaser to drive the laser to convert it into an optical signal and transmit it to the underwater channel;

[0049] Step S3: At the backscatter receiver and the multipath effect receiver, the optical signal scattered by the underwater channel is detected respectively. The optical signal is attenuated by the water body and finally reduced to the level of a single photon. The receiving lens is used to collect the single photon signal attenuated by the underwater channel and focus it on the photosensitive surface of the single photon detector.

[0050] Step S4: The backscattering receiver and the multipath effect receiver use a single-photon detector to perform photoelectric conversion on the focused photon signal to generate a corresponding electrical signal; the arrival time of the electrical signal is measured by a time-to-digital converter and digitally processed to obtain photon arrival time data at different frequencies; the photon arrival time data is demodulated and processed to recover the modulation signal, and the scattering characteristics are statistically obtained.

[0051] Step S5: Extraction of backscattered noise background curve and lower cutoff frequency The determination

[0052] Based on the photon arrival time data at different frequencies acquired by the backscatter receiver, the backscatter photon count rate corresponding to each modulation frequency is obtained after demodulation. The backscatter noise power spectral density exhibits an exponential decay characteristic with increasing frequency. By fitting the measured data with an exponential function, the backscatter noise background curve is obtained. The formula is:

[0053] ,

[0054] in, For signal modulation frequency, The noise constant related to the optical power before entering the underwater channel. The noise cutoff frequency is related to the underwater channel size distribution and water turbidity.

[0055] Define the lower cutoff frequency The frequency at which the backscattered noise power drops to a preset multiple (e.g., the 3dB bandwidth point) of the mid-frequency reference noise power; below the lower cutoff frequency. In this frequency band, backscattered noise significantly raises the noise floor at the receiver, leading to a deterioration in the signal-to-noise ratio, making it unsuitable for reliable communication.

[0056] Step S6: Multipath Delay Spread Extraction and upper cutoff frequency The determination

[0057] Based on photon arrival time data at different frequencies acquired by the multipath effect receiver, the impulse response of the reconstructed channel is determined under pulse excitation or equivalent frequency sweep measurement conditions. The formula is:

[0058] ,

[0059] in, The unit impulse function represents an ideal instantaneous pulse; The path delay time for the photon; These are the gain coefficients corresponding to different delay paths; and Minimum and maximum measurable path delays, respectively; time delay spread due to multipath effects. This determines the coherence bandwidth of the channel;

[0060] Through the impulse response Waveform analysis was performed to extract the root mean square delay spread of photon arrival time. Based on the inverse relationship between channel coherence bandwidth and delay spread, the upper cutoff frequency determined by the multipath effect is determined. The formula is: ;

[0061] Higher than the upper cutoff frequency In certain frequency bands, inter-symbol interference (ISI) causes a sharp increase in the bit error rate and a significant decrease in channel transmission performance.

[0062] Step S7: Integration of channel passband characteristics and selection of optimal communication frequency band

[0063] The lower cutoff frequency determined by the combined backscatter receiver The upper cutoff frequency determined by the receiver and the multipath effect To obtain the available passband range of the underwater optical communication channel. Within this passband range, based on the measured channel frequency response curve... By fitting the data, the sub-band with the flattest gain and smallest frequency response fluctuation within the passband is determined. This sub-band is considered the optimal communication band with the least impact from scattering characteristics, and it represents the optimal operating bandwidth for the underwater single-photon communication system under given water quality conditions. (Channel frequency response curve) The formula is:

[0064] ,

[0065] in, This represents the maximum transmission coefficient in the frequency band of the channel.

[0066] Step S8: Construction of a bandwidth parameter library under different water quality conditions

[0067] By changing the type and concentration of scattering particles in the underwater channel and repeating the above steps, the water quality parameters (such as attenuation coefficient, particle concentration, turbidity, etc.) and channel bandwidth parameters ( , This database establishes a correspondence between the optimal communication frequency band and the corresponding frequency band. This database can directly provide prior knowledge and design basis for adaptive modulation, dynamic bandwidth switching, and power control strategies in practical underwater optical communication systems.

[0068] Compared with the prior art, the present invention has the following advantages:

[0069] 1. This invention systematically maps the physical mechanism of underwater light-matter interaction—backscattering and multipath effect—to a bandpass filter model for channel transmission. Starting from the physical essence, it establishes a direct correlation between scattering characteristics and channel filtering parameters, overcoming the shortcomings of traditional theoretical modeling and random experiments in analyzing physical mechanisms.

[0070] 2. By revealing the physical processes by which backscattering mainly causes low-frequency noise enhancement and attenuation, determining the lower cutoff frequency, and multipath effect leads to high-frequency time delay broadening and inter-symbol interference, determining the upper cutoff frequency, this invention can quantitatively separate the differential effects of the two on different frequency bands, providing a clear physical basis for bandwidth selection and modulation optimization.

[0071] 3. The system provided by this invention supports controllable microparticle injection and water parameter adjustment, and can simulate environments with different turbidity and particle concentrations, ensuring high repeatability and controllability of the experiment. It helps to obtain high-precision and reliable experimental data and overcomes the problems of single conditions and insufficient controllability of existing experimental methods.

[0072] 4. By combining single-photon detection and time-correlated single-photon counting techniques, this invention can accurately measure statistical characteristics such as photon arrival time, pulse broadening, and signal delay distribution, enabling quantifiable and reproducible system characterization of underwater single-photon communication channels. This provides direct reference and engineering guidance for anti-scattering signal processing in complex underwater environments. Attached Figure Description

[0073] Figure 1 A structural framework diagram of an underwater single-photon communication scattering characteristic measurement system provided in an embodiment of the present invention;

[0074] Figure 2 This is a flowchart illustrating a method for measuring the scattering characteristics of underwater single-photon communication, provided in an embodiment of the present invention. Detailed Implementation

[0075] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0076] In the description of this invention, it should be noted that the terms "upper", "lower", "left", "right", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0077] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can also refer to the internal connection of two components; they can refer to a wireless connection or a wired connection. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0078] Furthermore, 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.

[0079] An underwater single-photon communication scattering characteristic measurement system, such as Figure 1 As shown, the system includes: a transmitter, an underwater channel, a backscatter receiver, and a multipath effect receiver;

[0080] The backscatter receiver and the multipath effect receiver are modeled as bandpass filter models for channel transmission;

[0081] The transmitting end includes a data modulation module and a laser emission module;

[0082] The data modulation module is used to generate a modulation signal that can be frequency-switched within a preset bandwidth.

[0083] The laser emission module is electrically connected to the data modulation module and is used to convert the modulation signal into an optical signal and transmit it to the underwater channel;

[0084] Both the backscattering receiver and the multipath effect receiver include a photon detection module and a data processing module.

[0085] The photon detection module is used to receive the single-photon signal formed after attenuation by the underwater channel, and to perform photoelectric conversion and time measurement on the single-photon signal, and output photon arrival time data.

[0086] The data processing module is electrically connected to the photon detection module and is used to demodulate and statistically analyze the photon arrival time data to obtain backscattering characteristics and multipath effect characteristics.

[0087] The data modulation module includes an FPGA unit, a clock generation unit, a digital-to-analog converter unit, and a PC unit;

[0088] The FPGA unit is electrically connected to the PC unit and is used to generate a modulation signal according to the modulation frequency.

[0089] The clock generation unit is electrically connected to the FPGA unit and the digital-to-analog converter unit respectively, and is used to provide clock signals;

[0090] The digital-to-analog converter is electrically connected to the FPGA unit and is used to convert the electrical signal output by the FPGA unit into an analog signal.

[0091] The PC unit is used to configure the modulation frequency of the modulation signal.

[0092] The laser emission module includes a DC power supply, a power amplifier, a laser diode driver, a biaser, and a laser.

[0093] The DC power supply is electrically connected to the power supply port of the power amplifier to provide the operating voltage;

[0094] The input terminal of the power amplifier is connected to the output terminal of the digital-to-analog converter unit, and is used to receive and amplify the modulation signal;

[0095] The laser diode driver is electrically connected to the biaser and is used to provide DC bias current to the biaser;

[0096] The bias unit is electrically connected to the power amplifier and is used to superimpose the modulated signal after power amplification with the DC bias current.

[0097] The laser is connected to the output of the biaser and is used to convert the modulated signal after superimposed DC bias into an optical signal and transmit it to the underwater channel.

[0098] The underwater channel consists of a cuboid water tank and water mixed with scattering particles; by replacing different types of scattering particles, the multipath effect and backscattering characteristics under different water quality conditions can be measured.

[0099] The photon detection module includes a receiving lens, a single-photon detector, and a time-to-digital converter.

[0100] The receiving lens is used to collect the single-photon signal formed after attenuation by the underwater channel and focus it on the photosensitive surface of the single-photon detector;

[0101] The single-photon detector is used to perform photoelectric conversion on the focused single-photon signal to generate an electrical signal;

[0102] The time-to-digital converter is electrically connected to the single-photon detector and is used to measure and digitize the electrical signal to obtain data containing photon arrival time information.

[0103] The data processing module includes a demodulator electrically connected to a time-to-digital converter (TD-to-DC converter) for demodulating the data output by the TD-to-DC converter to obtain the statistical characteristics of the modulated signal. The data processing module is also used to plot a frequency response curve based on the statistical characteristics of the modulated signal.

[0104] The demodulator is integrated into a PC, which is connected to a time-to-digital converter via a USB interface.

[0105] A method for measuring the scattering characteristics of underwater single-photon communication, the method being implemented based on the system described above, such as... Figure 2 As shown, the method includes the following steps:

[0106] Step S1: Add different scattering particles to the water and measure the parameters under the water quality conditions; configure the modulation frequency information on the PC unit and send it to the FPGA unit, which then generates the corresponding modulation signal.

[0107] Step S2: After the modulation signal is amplified by the power amplifier, a DC bias is superimposed through the biaser to drive the laser to convert it into an optical signal and transmit it to the underwater channel;

[0108] Step S3: At the backscatter receiver and the multipath effect receiver, the optical signal scattered by the underwater channel is detected respectively. The optical signal is attenuated by the water body and finally reduced to the level of a single photon. The receiving lens is used to collect the single photon signal attenuated by the underwater channel and focus it on the photosensitive surface of the single photon detector.

[0109] Step S4: The backscattering receiver and the multipath effect receiver use a single-photon detector to perform photoelectric conversion on the focused photon signal to generate a corresponding electrical signal; the arrival time of the electrical signal is measured by a time-to-digital converter and digitally processed to obtain photon arrival time data at different frequencies; the photon arrival time data is demodulated and processed to recover the modulation signal, and the scattering characteristics are statistically obtained.

[0110] Step S5: Extraction of backscattered noise background curve and lower cutoff frequency The determination

[0111] Based on the photon arrival time data at different frequencies acquired by the backscatter receiver, the backscatter photon count rate corresponding to each modulation frequency is obtained after demodulation. The backscatter noise power spectral density exhibits an exponential decay characteristic with increasing frequency. By fitting the measured data with an exponential function, the backscatter noise background curve is obtained. The formula is:

[0112] ,

[0113] in, For signal modulation frequency, The noise constant related to the optical power before entering the underwater channel. The noise cutoff frequency is related to the underwater channel size distribution and water turbidity.

[0114] Define the lower cutoff frequency The frequency at which the backscattered noise power drops to a preset multiple (e.g., the 3dB bandwidth point) of the mid-frequency reference noise power; below the lower cutoff frequency. In this frequency band, backscattered noise significantly raises the noise floor at the receiver, leading to a deterioration in the signal-to-noise ratio, making it unsuitable for reliable communication.

[0115] Step S6: Multipath Delay Spread Extraction and upper cutoff frequency The determination

[0116] Based on photon arrival time data at different frequencies acquired by the multipath effect receiver, the impulse response of the reconstructed channel is determined under pulse excitation or equivalent frequency sweep measurement conditions. The formula is:

[0117] ,

[0118] in, The unit impulse function represents an ideal instantaneous pulse; The path delay time for the photon; These are the gain coefficients corresponding to different delay paths; and Minimum and maximum measurable path delays, respectively; time delay spread due to multipath effects. This determines the coherence bandwidth of the channel;

[0119] Through the impulse response Waveform analysis was performed to extract the root mean square delay spread of photon arrival time. Based on the inverse relationship between channel coherence bandwidth and delay spread, the upper cutoff frequency determined by the multipath effect is determined. The formula is: ;

[0120] Higher than the upper cutoff frequency In certain frequency bands, inter-symbol interference (ISI) causes a sharp increase in the bit error rate and a significant decrease in channel transmission performance.

[0121] Step S7: Integration of channel passband characteristics and selection of optimal communication frequency band

[0122] The lower cutoff frequency determined by the combined backscatter receiver The upper cutoff frequency determined by the receiver and the multipath effect To obtain the available passband range of the underwater optical communication channel. Within this passband range, based on the measured channel frequency response curve... By fitting the data, the sub-band with the flattest gain and smallest frequency response fluctuation within the passband is determined. This sub-band is considered the optimal communication band with the least impact from scattering characteristics, and it represents the optimal operating bandwidth for the underwater single-photon communication system under given water quality conditions. (Channel frequency response curve) The formula is:

[0123] ,

[0124] in, This represents the maximum transmission coefficient in the frequency band of the channel.

[0125] Step S8: Construction of a bandwidth parameter library under different water quality conditions

[0126] By changing the type and concentration of scattering particles in the underwater channel and repeating the above steps, the water quality parameters (such as attenuation coefficient, particle concentration, turbidity, etc.) and channel bandwidth parameters ( , This database establishes a correspondence between the optimal communication frequency band and the corresponding frequency band. This database can directly provide prior knowledge and design basis for adaptive modulation, dynamic bandwidth switching, and power control strategies in practical underwater optical communication systems.

[0127] Contents not described in detail in this specification are prior art known to those skilled in the art. Although illustrative specific embodiments of the invention have been described above to facilitate understanding by those skilled in the art, it should be understood that the invention is not limited to the scope of the specific embodiments. Various modifications are readily apparent to those skilled in the art as long as they fall within the spirit and scope of the invention as defined and determined by the appended claims, and all inventions utilizing the concept of this invention are protected.

Claims

1. An underwater single-photon communication scattering property measurement system, characterized by, The system includes: a transmitter, an underwater channel, a backscatter receiver, and a multipath effect receiver; The backscatter receiver and the multipath effect receiver are modeled as bandpass filter models for channel transmission; The transmitting end includes a data modulation module and a laser emission module; The data modulation module is used to generate a modulation signal that can be frequency-switched within a preset bandwidth. The laser emission module is electrically connected to the data modulation module and is used to convert the modulation signal into an optical signal and transmit it to the underwater channel; Both the backscattering receiver and the multipath effect receiver include a photon detection module and a data processing module. The photon detection module is used to receive the single-photon signal formed after attenuation by the underwater channel, and to perform photoelectric conversion and time measurement on the single-photon signal, and output photon arrival time data. The data processing module is electrically connected to the photon detection module and is used to demodulate and statistically analyze the photon arrival time data to obtain backscattering characteristics and multipath effect characteristics.

2. The underwater single photon communication scattering property measurement system according to claim 1, characterized in that, The data modulation module includes an FPGA unit, a clock generation unit, a digital-to-analog converter unit, and a PC unit; The FPGA unit is electrically connected to the PC unit and is used to generate a modulation signal according to the modulation frequency. The clock generation unit is electrically connected to the FPGA unit and the digital-to-analog converter unit respectively, and is used to provide clock signals; The digital-to-analog converter is electrically connected to the FPGA unit and is used to convert the electrical signal output by the FPGA unit into an analog signal. The PC unit is used to configure the modulation frequency of the modulation signal.

3. The underwater single photon communication scattering property measurement system according to claim 2, characterized in that, The laser emission module includes a DC power supply, a power amplifier, a laser diode driver, a biaser, and a laser. The DC power supply is electrically connected to the power supply port of the power amplifier to provide the operating voltage; The input terminal of the power amplifier is connected to the output terminal of the digital-to-analog converter unit, and is used to receive and amplify the modulation signal; The laser diode driver is electrically connected to the biaser and is used to provide DC bias current to the biaser; The bias unit is electrically connected to the power amplifier and is used to superimpose the modulated signal after power amplification with the DC bias current. The laser is connected to the output of the biaser and is used to convert the modulated signal after superimposed DC bias into an optical signal and transmit it to the underwater channel.

4. The underwater single photon communication scattering property measurement system according to claim 3, characterized in that, The underwater channel consists of a cuboid water tank and water mixed with scattering particles; by changing different types of scattering particles, the multipath effect and backscattering characteristics under different water quality conditions can be measured.

5. The underwater single-photon communication scattering characteristic measurement system according to claim 4, characterized in that, The photon detection module includes a receiving lens, a single-photon detector, and a time-to-digital converter. The receiving lens is used to collect the single-photon signal formed after attenuation by the underwater channel and focus it on the photosensitive surface of the single-photon detector; The single-photon detector is used to perform photoelectric conversion on the focused single-photon signal to generate an electrical signal; The time-to-digital converter is electrically connected to the single-photon detector and is used to measure and digitize the electrical signal to obtain data containing photon arrival time information.

6. The underwater single-photon communication scattering characteristic measurement system according to claim 5, characterized in that, The data processing module includes a demodulator electrically connected to a time-to-digital converter (TD-to-DC converter) for demodulating the data output by the TD-to-DC converter to obtain the statistical characteristics of the modulated signal. The data processing module is also used to plot a frequency response curve based on the statistical characteristics of the modulated signal.

7. The underwater single-photon communication scattering characteristic measurement system according to claim 6, characterized in that, The demodulator is integrated into a PC, which is connected to a time-to-digital converter via a USB interface.

8. A method for measuring the scattering characteristics of underwater single-photon communication, the method being implemented based on the system described in claims 1-7, the method comprising the following steps: Step S1: Add different scattering particles to the water and measure the parameters of the water quality under different scattering particle conditions; configure the modulation frequency information on the PC unit and send it to the FPGA unit, and the FPGA unit generates the corresponding modulation signal. Step S2: After the modulation signal is amplified by the power amplifier, a DC bias is superimposed through the biaser to drive the laser to convert it into an optical signal and transmit it to the underwater channel; Step S3: At the backscatter receiver and the multipath effect receiver, the optical signal scattered by the underwater channel is detected respectively. The optical signal is attenuated by the water body and finally reduced to the level of a single photon. The receiving lens is used to collect the single photon signal attenuated by the underwater channel and focus it on the photosensitive surface of the single photon detector. Step S4: The backscatter receiver and the multipath effect receiver use a single-photon detector to perform photoelectric conversion on the focused photon signal to generate a corresponding electrical signal; the arrival time of the electrical signal is measured by a time-to-digital converter and digitally processed to obtain photon arrival time data at different frequencies; Demodulate and process the photon arrival time data to recover the modulation signal and statistically determine the scattering characteristics. Step S5: Extraction of backscattered noise background curve and lower cutoff frequency The determination Based on the photon arrival time data at different frequencies obtained by the backscatter receiver, the backscatter photon count rate corresponding to each modulation frequency is obtained after demodulation. The power spectral density of backscattered noise exhibits an exponential decay characteristic with increasing frequency. An exponential function fit is performed on the measured data to obtain the backscattered noise background curve. The formula is: ， in, For signal modulation frequency, The noise constant related to the optical power before entering the underwater channel. The noise cutoff frequency is related to the underwater channel size distribution and water turbidity. Define the lower cutoff frequency The frequency at which the backscatter noise power drops to a preset multiple of the mid-frequency reference noise power; Step S6: Multipath Delay Spread Extraction and upper cutoff frequency The determination Based on photon arrival time data at different frequencies acquired by the multipath effect receiver, the impulse response of the reconstructed channel is determined under pulse excitation or equivalent frequency sweep measurement conditions. The formula is: ， in, The unit impulse function represents an ideal instantaneous pulse; The path delay time for the photon; These are the gain coefficients corresponding to different delay paths; and Minimum and maximum measurable path delays, respectively; time delay spread due to multipath effects. This determines the coherence bandwidth of the channel; Through the impulse response Waveform analysis was performed to extract the root mean square delay spread of photon arrival time. Based on the inverse relationship between channel coherence bandwidth and delay spread, the upper cutoff frequency determined by the multipath effect is determined. The formula is: ; Step S7: Integration of channel passband characteristics and selection of optimal communication frequency band The lower cutoff frequency determined by the combined backscatter receiver The upper cutoff frequency determined by the receiver and the multipath effect To obtain the available passband range of the underwater optical communication channel. Within this passband range, based on the measured channel frequency response curve... By performing a fitting process, the sub-band within the passband is determined as the optimal communication band, which represents the optimal operating bandwidth of the underwater single-photon communication system under given water quality conditions; channel frequency response curve. The formula is: ， in, This represents the maximum transmission coefficient in the frequency band of the channel. Step S8: Construction of a bandwidth parameter library under different water quality conditions By changing the type and concentration of scattering particles in the underwater channel and repeating the above steps, a database of the correspondence between water quality parameters and channel bandwidth parameters can be established.

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