Method and device for estimating time-varying channel transmittance based on frequency division multiplexing pilot

Abstract

The present invention discloses a method for estimating time-varying channel transmittance based on frequency-division multiplexing pilot signals. This method addresses the problem that existing methods using average transmittance estimation lack timely feedback and dynamic adjustment capabilities. By receiving the frequency-division multiplexed quantum signal and pilot signal generated by the transmitter in real time in a local oscillator continuous variable quantum key distribution system, the canonical position component and canonical momentum component of the received signal are measured. Frequency offset compensation and phase recovery algorithms are used to compensate for the frequency offset caused by the different center frequencies of the two independent lasers at the transmitter and receiver, as well as the phase noise caused by channel jitter. The recovered pilot signal is then bandpass filtered to obtain the recovered pilot signal. The power ratio of the pilot signal to the pre-calibrated pilot signal at each moment is calculated to obtain the time-varying free-space transmittance. This method accurately reflects the performance changes of the system over different time periods, helps optimize the communication performance of the CVQKD system, improves the success rate of key transmission, and enhances the adaptability and robustness of the system.

Description

Technical Field

[0001] The present invention belongs to the technical field of time-varying channel transmittance calculation, and in particular relates to a method for estimating the time-varying channel transmittance based on a frequency division multiplexing pilot in free-space local oscillator continuous variable quantum key distribution. Background Art

[0002] Today, as society continues to advance in informatization, information technology is increasingly being applied across all industries, including key sectors such as military and finance. With the advancement of science and technology and the strengthening of economic strength, the value of information is also increasing. However, information leaks are a frequent occurrence, posing serious risks to individuals, society, and the nation. Therefore, ensuring the secure transmission of information is of paramount importance.

[0003] When two parties in legitimate communication encrypt the information they need to transmit using a key, it increases the difficulty for third parties to eavesdrop. The security of the key determines the security of the confidential communication process. Quantum key distribution, based on quantum mechanics principles such as quantum non-cloning and the quantum uncertainty relation, can achieve theoretically unconditionally secure key distribution and has been a research hotspot in recent years. Quantum key distribution can be divided into two main technical approaches: discrete variable and continuous variable protocols, depending on the physical quantity loaded with the key. After recent developments, discrete-variable quantum key distribution (DVQKD) technology has become relatively mature and can achieve longer transmission distances than continuous-variable quantum key distribution. However, it faces difficulties in preparing and detecting single-photon signals, making it not only complex and difficult to prepare, but also costly and expensive.

[0004] Continuous Variable Quantum Key Distribution (CVQKD) technology has attracted widespread attention both at home and abroad due to its advantages in channel capacity, good integration with existing optical communications, and resistance to background light interference. CVQKD can be divided into three types of protocols according to the light source used: coherent state, squeezed state, and entangled state. The coherent state CVQKD system has attracted widespread attention due to the simplicity of coherent state preparation. Traditional optical communication devices can be used to prepare and detect coherent states, eliminating the need for expensive and complex single-photon light sources and dedicated detectors. This can fully utilize the years of development and accumulation of optical fiber communications, and has significant cost advantages and high long-term reliability.

[0005] Based on the quantum state modulation format, CVQKD technology can be categorized as Gaussian modulation schemes (GMCS) and discrete modulation schemes (DMCS). Discrete variable quantum key distribution (DQKD) technology is relatively mature, but it faces certain difficulties and high costs in generating and detecting single-photon signals. In contrast, continuous variable QKD technology has attracted widespread attention both domestically and internationally due to its advantages in channel capacity, optical communication integration, and immunity to background light interference. Among them, coherent state CVQKD systems have attracted significant attention due to their relatively simple construction. They utilize traditional optical communication components, avoiding the need for expensive and complex single-photon light sources and specialized detectors, offering significant cost advantages and long-term reliability. Gaussian modulation can ensure that legitimate communicating parties achieve mutual information close to the channel capacity, which means it offers significant advantages in information transmission efficiency and quality. Furthermore, with continuous technological advancements, researchers are working to overcome the technical challenges of Gaussian modulation implementation. Once these challenges are overcome, Gaussian modulation CVQKD systems will demonstrate even stronger performance and broader application prospects. Therefore, Gaussian modulation remains a current research hotspot in CVQKD technology.

[0006] The local local oscillator (LO) CVQKD scheme was developed to ensure the security of CVQKD implementations and improve performance. In this approach, the transmitter generates a reference pulse and a coherent quantum signal using the same light source. The receiver uses local LO light to detect both the reference pulse and the quantum signal, adjusting the phase of the quantum signal based on the phase information provided by the reference pulse. Because the local LO light at the receiver is not controlled by an eavesdropper, this scheme can circumvent various attacks on the LO light and achieve stable control of the shot noise level, thereby enabling efficient balanced homodyne detection. The local oscillator (LO) CVQKD scheme offers advantages such as high security, ease of achieving the shot noise limit, ease of system integration, and support for large-scale deployment.

[0007] The development of free-space CVQKD can be traced back to the growing demand for information security and the challenges faced by traditional encryption technologies. CVQKD demonstrates tremendous potential in long-distance communications, particularly in satellite and ground-to-ground communications. Due to its effective protection against eavesdropping, CVQKD is widely recognized as a highly secure communication method in these areas. With continuous technological advancements, significant progress has been made in CVQKD optical devices, detectors, and signal processing. These breakthroughs not only increase the transmission speed and security of CVQKD, but also gradually establish it as an indispensable component of future quantum communication networks. Therefore, as a cutting-edge quantum communication technology, free-space CVQKD lays an important foundation for building a secure quantum internet.

[0008] With respect to the related technologies mentioned above, the inventors believe that real-time transmittance changes reflect the channel status. By monitoring the real-time transmittance changes of free-space CVQKD, it is possible to promptly understand the state changes of the communication channel, including channel noise, interference, and other conditions. This helps to identify potential security threats and problems and take appropriate measures to address them. At the same time, based on the real-time transmittance changes, communication parameters can be dynamically adjusted to optimize communication performance and increase the success rate of key transmission. This real-time feedback mechanism can improve the adaptability and robustness of the system. In addition, changes in transmittance may indicate potential attacks or interference, so real-time monitoring of transmittance is of great significance. Promptly detecting abnormal changes can help the system respond to possible security threats in a timely manner and ensure the security of communications.

[0009] However, existing channel transmittance estimation methods are mostly based on the average transmittance of a single frame of signal, lacking the ability to provide timely feedback and dynamic adjustment. In the face of emergencies or specific environmental conditions, the average transmittance may not accurately reflect changes in system performance over time. Therefore, it is very important to implement real-time channel transmittance estimation for free-space local oscillator CVQKD systems. Summary of the Invention

[0010] The purpose of the present invention is to provide a method for estimating time-varying channel transmittance based on frequency division multiplexing pilot, so as to improve the real-time performance of real-time channel transmittance estimation in a free space local oscillator CVQKD system.

[0011] In order to solve the above problems, the technical solution of the present invention is:

[0012] A method for estimating time-varying channel transmittance based on frequency division multiplexing pilot, used in free-space local oscillator continuous variable quantum key distribution, the method comprising:

[0013] The transmitter generates the quantum signal and pilot signal for frequency division multiplexing in the local oscillator continuous variable quantum key distribution system and transmits them to the receiver via the free space channel.

[0014] The receiving end uses heterodyne detection to measure the canonical position component and canonical momentum component of the frequency-division-multiplexed quantum signal and pilot signal transmitted through the free-space channel; and uses frequency offset compensation and phase recovery algorithms to compensate for the frequency offset caused by the different center frequencies of the two independent lasers at the transmitting and receiving ends and the phase noise caused by channel jitter, and the recovered pilot signal is obtained through bandpass filtering; the power ratio of the pilot signal to the pre-calibrated pilot signal at each moment is calculated, and the obtained ratio sequence is the estimated time-varying free-space transmittance.

[0015] According to an embodiment of the present invention, the transmitting end uses a coherent state with a block length of N to perform baseband Gaussian modulation on the coherent state of the quantum signal and to perform carrier modulation on the coherent state of the pilot signal.

[0016] According to an embodiment of the present invention, the transmitting end generates a frequency-division multiplexed quantum signal and a pilot signal in a local local oscillator continuous variable quantum key distribution system, further comprising:

[0017] The quantum random number generator first generates a random complex number sequence of length N. After mapping, the complex number sequence {xq k +j*pq k} obeys Gaussian distribution, which is the amplitude of the quantum signal during modulation; the symbol rate of the quantum signal is f q , the unit is samples / s; where xq k +j*pq k represents the kth complex number in the complex number sequence, and j represents the imaginary unit;

[0018] Regenerate a complex constant sequence of length N {xp k +j*pp k}, where xp k ≡A p ,pp k ≡A p ; Among them, xp k +j*pp k represents the kth complex constant in the complex constant sequence, A p represents the amplitude ratio of the pilot signal and the quantum signal, j represents the imaginary unit;

[0019] Carrier modulation is performed on the constant complex sequence, that is, the sequence is multiplied by the exponential sequence exp(j2πf p t k ), get {xp k +j*pp k}*exp(j2πf p t k ), as the amplitude of the pilot signal during modulation; where t k Representing time series The kth value of f p Indicates the frequency of carrier modulation in Hz, exp() represents a natural constant, j represents an imaginary unit, and π represents pi;

[0020] The amplitude of the quantum signal and the amplitude of the pilot signal are directly added together, and after modulation, the frequency-division multiplexed pilot and quantum signal coherent states are obtained.

[0021] According to an embodiment of the present invention, the pre-calibrated pilot signal is obtained by the following steps:

[0022] Use a fiber optic cable to directly connect the transmitter and receiver;

[0023] The receiving end uses heterodyne detection to measure the canonical position component and canonical momentum component of the frequency-division multiplexed quantum signal and pilot signal;

[0024] Use frequency offset compensation and phase recovery algorithms to compensate for the frequency offset caused by the different center frequencies of the two independent lasers at the receiving and transmitting ends, and the phase noise caused by channel jitter;

[0025] The recovered pilot signal is band-pass filtered and used as a pre-calibrated pilot signal for channel transmittance estimation.

[0026] A device for estimating time-varying channel transmittance based on frequency division multiplexing pilot, used in free-space local oscillator continuous variable quantum key distribution, comprising:

[0027] The signal transmission module is used to generate the frequency-division multiplexed quantum signal and pilot signal in the local oscillator continuous variable quantum key distribution system at the transmitting end, and transmit them to the receiving end via the free space channel;

[0028] The transmittance estimation module is used at the receiving end to measure the canonical position component and canonical momentum component of the frequency-division-multiplexed quantum signal and pilot signal transmitted through the free-space channel by heterodyne detection. It also uses frequency offset compensation and phase recovery algorithms to compensate for the frequency offset caused by the different center frequencies of the two independent lasers at the transmitting and receiving ends and the phase noise caused by channel jitter, and then extracts the recovered pilot signal through bandpass filtering. The power ratio of the pilot signal to the pre-calibrated pilot signal at each moment is calculated, and the obtained ratio sequence is the estimated time-varying free-space transmittance.

[0029] Due to the adoption of the above technical solution, the present invention has the following advantages and positive effects compared with the prior art:

[0030] A method for estimating time-varying channel transmittance based on frequency-division multiplexing pilots, according to one embodiment of the present invention, addresses the problem that existing methods using average transmittance estimation lack timely feedback and dynamic adjustment capabilities. This method, which may not accurately reflect system performance changes over different time periods in the face of emergencies or specific environmental conditions, receives frequency-division multiplexed quantum signals and pilot signals generated by the transmitter in real time. The canonical position and momentum components of the received signals are measured. Frequency offset compensation and phase recovery algorithms are used to compensate for the frequency offset caused by the different center frequencies of the two independent lasers at the transmitter and receiver, as well as phase noise caused by channel jitter. The recovered pilot signal is then bandpass filtered to obtain the recovered pilot signal. The time-varying free-space transmittance is then calculated by calculating the power ratio of the pilot signal to the pre-calibrated pilot signal at each moment. This method accurately reflects system performance changes over different time periods, helping to optimize the communication performance of the CVQKD system, improve the success rate of key transmission, and enhance the system's adaptability and robustness. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Flow chart of a method for estimating time-varying channel transmittance based on frequency division multiplexing pilot in one embodiment of the present invention;

[0032] Figure 2 FIG. 4 is a block diagram of an apparatus for estimating time-varying channel transmittance based on frequency-division multiplexing pilots according to an embodiment of the present invention. DETAILED DESCRIPTION

[0033] The following is a detailed description of a method and device for estimating time-varying channel transmittance based on frequency division multiplexing pilots proposed by the present invention, with reference to the accompanying drawings and specific embodiments. The advantages and features of the present invention will become more apparent from the following description and claims.

[0034] This embodiment discloses a method for estimating the transmittance of a time-varying channel based on a frequency division multiplexing pilot, which is used in a free-space local oscillator continuous variable quantum key distribution system. Figure 1 , the method comprises the following steps:

[0035] Step S1: The transmitting end generates a frequency-division multiplexed quantum signal and a pilot signal in a local local oscillator continuous variable quantum key distribution system;

[0036] The quantum signal modulation method in step S1 is baseband Gaussian modulation, and the pilot signal modulation method is carrier modulation. The two are combined together by frequency division multiplexing. The preparation method of the coherent state at the transmitting end is as follows, that is, the baseband Gaussian modulated quantum signal and the carrier modulated pilot signal are frequency division multiplexed. The coherent state modulation method is:

[0037] Step S1.1, the quantum random number generator generates a length of N=10 6A random complex sequence, after mapping, the complex sequence {xq k +j*pq k} obeys Gaussian distribution, the sequence is the amplitude of the quantum signal during modulation, and the symbol rate of the quantum signal is f g =10 9 , the unit is samples / s; where xq k +j*pq k represents the kth complex number in the complex number sequence, and j represents the imaginary unit.

[0038] Step S1.2, generate a length of N = 10 6 The complex constant sequence {xp k +j*pp k}, where xp k ≡A p ,pp k ≡A p , A p =20; where xp k +j*pp k represents the kth complex constant in the complex constant sequence, A p represents the amplitude ratio of the pilot signal to the quantum signal, and j represents the imaginary unit.

[0039] Step S1.3, perform carrier modulation on the constant complex sequence in step S1.2, that is, multiply the sequence by the exponential sequence exp(jj2πf p t k ), get {xp k +j*pp k}*exp(jj2πf p t k ), the sequence is the amplitude of the pilot signal during modulation; where t k Representing time series The kth value of f p =-1.25×10 9 Indicates the frequency of carrier modulation in Hz, exp() represents a natural constant, j represents an imaginary unit, and π represents pi;

[0040] Step S1.4, the amplitude of the quantum signal {xq k +j*pq k} and the amplitude of the pilot signal {xp k +j*pp k}*exp(j2πf p t k ) are directly added, and after modulation, the frequency division multiplexing pilot and quantum signal coherent state|xq k +j*pq k +(xpk +j*pp k )*exp(jj2πf p t k )>; where exp() represents a natural constant, j represents an imaginary unit, and π represents the ratio of circumference to circumference.

[0041] Step S2: Perform a pre-calibration operation by directly connecting the transmitter and receiver with a fiber optic cable as short as possible, detect and recover the pilot signal at this time, and use it as the pre-calibration pilot signal for channel transmittance estimation.

[0042] The method of using the pre-calibration operation in step S2 to directly detect the signal output by the transmitting end is:

[0043] Step S2.1, directly connect the transmitting end and the receiving end with a fiber optic cable as short as possible;

[0044] Step S2.2: The receiving end uses heterodyne detection to measure the canonical position component and canonical momentum component of the frequency-division multiplexed quantum signal and the pilot signal;

[0045] Step S2.3: Use a pilot-assisted frequency offset compensation and phase recovery algorithm to compensate for the frequency offset caused by the different center frequencies of the two independent lasers at the transmitting and receiving ends and the phase noise caused by channel jitter;

[0046] Step S2.4: Band-pass filter the recovered pilot signal and use the pilot signal as a pre-calibrated pilot signal for channel transmittance estimation.

[0047] Step S3, inserting a free space channel between the transmitting end and the receiving end, and transmitting the signal sent by the transmitting end to the receiving end through the free space channel;

[0048] Step S4: The receiving end receives the frequency-division-multiplexed quantum signal and pilot signal transmitted from the free-space channel and performs necessary operations such as frequency offset recovery and phase recovery.

[0049] In step S4, the method for performing frequency offset recovery and phase recovery on the detected signal transmitted through the free space channel is:

[0050] Step S4.1: The receiving end uses heterodyne detection to measure the canonical position component and canonical momentum component of the frequency-division-multiplexed quantum signal and the pilot signal transmitted through the free-space channel;

[0051] Step S4.2: Use a pilot-assisted frequency offset compensation and phase recovery algorithm to compensate for the frequency offset caused by the different center frequencies of the two independent lasers at the transmitting and receiving ends and the phase noise caused by channel jitter;

[0052] Step S4.3: Band-pass filter the recovered pilot signal transmitted via the free space channel.

[0053] In step S5, the time-varying channel transmittance estimation algorithm based on the pilot is used to complete the estimation of the time-varying transmittance of the free space channel. That is, the power ratio of the pilot signal obtained in step S4.3 and the pre-calibrated pilot signal in step S2.4 at each moment is calculated. The obtained ratio sequence is the estimated time-varying free space transmittance.

[0054] Real-time transmittance changes in a local local oscillator continuous variable quantum key distribution system reflect the channel state. Monitoring the real-time transmittance changes of free-space local local oscillator continuous variable quantum key distribution can promptly understand changes in channel noise, interference, and other states in the communication channel, which helps identify potential security threats and issues and take appropriate measures to address them. At the same time, based on real-time transmittance changes, communication parameters can be dynamically adjusted to optimize communication performance and increase the success rate of key transmission. This real-time feedback mechanism can improve the adaptability and robustness of the system. The present invention combines a frequency-division multiplexing pilot signal scheme with a free-space continuous variable quantum key distribution system to achieve real-time estimation of free-space channel transmittance. The proposed pilot-assisted scheme uses pure software to process quantum signals, thus requiring no additional physical devices and no modification to the optical path of the existing free-space local local oscillator continuous variable quantum key distribution system. Furthermore, the scheme is highly implantable and can be directly incorporated into existing digital signal processing algorithms.

[0055] Based on the same concept, this embodiment also provides a device for estimating time-varying channel transmittance based on a frequency-division multiplexing pilot, for use in a free-space local oscillator continuous-variable quantum key distribution system. This device can be implemented by executing the process steps of the method for estimating time-varying channel transmittance based on a frequency-division multiplexing pilot in free-space local oscillator continuous-variable quantum key distribution. In other words, those skilled in the art can understand the method for estimating time-varying channel transmittance based on a frequency-division multiplexing pilot in free-space local oscillator continuous-variable quantum key distribution as a preferred embodiment of a real-time transmittance estimation device for a free-space local oscillator continuous-variable quantum key distribution system.

[0056] The device is used to enable the transmitter to generate frequency-division multiplexed quantum signals and pilot signals in a local oscillator continuous variable quantum key distribution system, and transmit them to the receiver via a free-space channel; and to enable the receiver to measure the canonical position component and canonical momentum component of the frequency-division multiplexed quantum signals and pilot signals transmitted via the free-space channel by heterodyne detection; and to compensate for the frequency offset caused by the different center frequencies of the two independent lasers at the transmitter and receiver and the phase noise caused by channel jitter using frequency offset compensation and phase recovery algorithms, and to obtain the recovered pilot signal through bandpass filtering; and to calculate the power ratio of the pilot signal to the pre-calibrated pilot signal at each moment, and the obtained ratio sequence is the estimated time-varying free-space transmittance.

[0057] For details, please see Figure 2 , the device includes the following modules:

[0058] Module M1: The transmitter generates the frequency-division multiplexed quantum signal and pilot signal in the local oscillator continuous variable quantum key distribution system.

[0059] Module M1 includes the following modules:

[0060] Module M1.1: Quantum random number generator generates a length of N = 10 6 A random complex sequence, after mapping, the complex sequence {xq k +j*pq k} obeys Gaussian distribution, the sequence is the amplitude of the quantum signal during modulation, and the symbol rate of the quantum signal is f g =10 9 , the unit is samples / s; where xq k +j*pq k represents the kth complex number in the complex number sequence, and j represents the imaginary unit.

[0061] Module M1.2: Generate length N = 10 6 The complex constant sequence {xp k +j*pp k}, where xp k ≡A p ,pp k ≡A p , A p =20; where xp k +j*pp k represents the kth complex constant in the complex constant sequence, A p represents the amplitude ratio of the pilot signal to the quantum signal, and j represents the imaginary unit.

[0062] Module M1.3: Carrier modulate the constant complex sequence in step S1.2, that is, multiply the sequence by the exponential sequence exp(j2πf p t k ), get {xp k +j*pp k}*exp(j2πf p t k ), the sequence is the amplitude of the pilot signal during modulation; where t k Representing time series The kth value of f p =-1.25×10 9 Indicates the frequency of carrier modulation in Hz, exp() represents a natural constant, j represents an imaginary unit, and π represents pi;

[0063] Module M1.4: Transform the amplitude of a quantum signal {xq k +j*pq k} and the amplitude of the pilot signal {xp k +j*pp k}*exp(j2πf p t k ) are directly added, and after modulation, the frequency division multiplexing pilot and quantum signal coherent state|xq k +j*pq k +(xp k +j*pp k )*exp(j2πf p t k )>; where exp() represents a natural constant, j represents an imaginary unit, and π represents the ratio of circumference to circumference.

[0064] Module M2: Performs pre-calibration operations by directly connecting the transmitter and receiver with a fiber optic cable as short as possible, detecting and recovering the pilot signal at this time, which is used as the pre-calibration pilot signal for channel transmittance estimation.

[0065] Module M2 includes the following modules:

[0066] Module M2.1: Use a fiber optic cable as short as possible to directly connect the transmitter and receiver;

[0067] Module M2.2: The receiving end uses heterodyne detection to measure the canonical position component and canonical momentum component of the frequency-division multiplexed quantum signal and pilot signal;

[0068] Module M2.3: Uses pilot-assisted frequency offset compensation and phase recovery algorithms to compensate for the frequency offset caused by the different center frequencies of the two independent lasers at the transmitting and receiving ends, and the phase noise caused by channel jitter;

[0069] Module M2.4: Bandpass filter the recovered pilot signal and use the pilot signal as the pre-calibrated pilot signal for channel transmittance estimation.

[0070] Module M3: A free space channel is placed between the transmitter and the receiver. The signal sent by the transmitter is transmitted to the receiver through the free space channel.

[0071] Module M4: The receiving end receives the frequency-division-multiplexed quantum signal and pilot signal transmitted from the free-space channel and performs necessary operations such as frequency offset recovery and phase recovery;

[0072] Module M4 includes the following modules:

[0073] Module M4.1: The receiver uses heterodyne detection to measure the canonical position and momentum components of the frequency-division-multiplexed quantum signal and pilot signal transmitted through a free-space channel.

[0074] Module M4.2: Uses pilot-assisted frequency offset compensation and phase recovery algorithms to compensate for frequency offset and phase noise caused by channel jitter caused by the different center frequencies of the two independent lasers at the transmitting and receiving ends.

[0075] Module M4.3: Bandpass filter the recovered pilot signal transmitted through the free space channel.

[0076] Module M5: Use the pilot-based time-varying channel transmittance estimation algorithm to complete the estimation of the time-varying free-space channel transmittance, that is, calculate the power ratio of the pilot signal obtained in step S4.3 and the pre-calibrated pilot signal in step S2.4 at each moment. The obtained ratio sequence is the estimated time-varying free-space transmittance.

[0077] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to the above embodiments. Even if various changes are made to the present invention, if these changes fall within the scope of the claims of the present invention and their equivalents, they still fall within the scope of protection of the present invention.

Claims

1. A method for estimating the transmittance of a time-varying channel based on a frequency-division multiplexing pilot, for use in free-space local oscillator continuous variable quantum key distribution, characterized in that: include: The transmitter generates the quantum signal and pilot signal for frequency division multiplexing in the local oscillator continuous variable quantum key distribution system and transmits them to the receiver via the free space channel. The receiving end uses heterodyne detection to measure the canonical position component and canonical momentum component of the frequency-division-multiplexed quantum signal and pilot signal transmitted through the free-space channel; Frequency offset compensation and phase recovery algorithms are used to compensate for the frequency offset caused by the different center frequencies of the two independent lasers at the transmitting and receiving ends and the phase noise caused by channel jitter. The recovered pilot signal is filtered out through a bandpass filter. The power ratio of the pilot signal to the pre-calibrated pilot signal at each moment is calculated, and the obtained ratio sequence is the estimated time-varying free-space transmittance.

2. The method for estimating time-varying channel transmittance based on frequency division multiplexing pilot according to claim 1, wherein: The transmitting end adopts a coherent state with a block length of N, performs baseband Gaussian modulation on the coherent state of the quantum signal, and performs carrier modulation on the coherent state of the pilot signal.

3. The method for estimating time-varying channel transmittance based on frequency division multiplexing pilot according to claim 1, wherein: The transmitting end generates a frequency-division multiplexed quantum signal and a pilot signal in a local local oscillator continuous variable quantum key distribution system, further comprising: The quantum random number generator first generates a random complex number sequence of length N. After mapping, the complex number sequence {xq k +j*pq k } obeys Gaussian distribution, which is the amplitude of the quantum signal during modulation; the symbol rate of the quantum signal is f q , the unit is samples / s; where xq k +j*pq k represents the kth complex number in the complex number sequence, and j represents the imaginary unit; Regenerate a complex constant sequence of length N {xp k +j*pp k }, where xp k ≡A p ,pp k ≡A p ; Among them, xp k +j*pp k represents the kth complex constant in the complex constant sequence, A p represents the amplitude ratio of the pilot signal and the quantum signal, j represents the imaginary unit; Carrier modulation is performed on the constant complex sequence, that is, the sequence is multiplied by the exponential sequence exp(j2πf p t k ), get {xp k +j*pp k }*exp(j2πf p t k ), as the amplitude of the pilot signal during modulation; where t k Representing time series The kth value of g p Indicates the frequency of carrier modulation in Hz, exp() represents a natural constant, j represents an imaginary unit, and π represents pi; The amplitude of the quantum signal and the amplitude of the pilot signal are directly added together, and after modulation, the frequency-division multiplexed pilot and quantum signal coherent states are obtained.

4. The method for estimating time-varying channel transmittance based on frequency division multiplexing pilot according to claim 1, wherein: The pre-calibrated pilot signal is obtained by the following steps: Use a fiber optic cable to directly connect the transmitter and receiver; The receiving end uses heterodyne detection to measure the canonical position component and canonical momentum component of the frequency-division multiplexed quantum signal and pilot signal; Use frequency offset compensation and phase recovery algorithms to compensate for the frequency offset caused by the different center frequencies of the two independent lasers at the receiving and transmitting ends, and the phase noise caused by channel jitter; The recovered pilot signal is band-pass filtered and used as a pre-calibrated pilot signal for channel transmittance estimation.

5. A device for estimating time-varying channel transmittance based on frequency-division multiplexing pilot, used in free-space local oscillator continuous variable quantum key distribution, characterized in that: include: The signal transmission module is used to generate the frequency-division multiplexed quantum signal and pilot signal in the local oscillator continuous variable quantum key distribution system at the transmitting end, and transmit them to the receiving end via the free space channel; The transmittance estimation module is used at the receiving end to measure the canonical position component and canonical momentum component of the frequency-division multiplexed quantum signal and pilot signal transmitted through the free space channel by heterodyne detection; Frequency offset compensation and phase recovery algorithms are used to compensate for the frequency offset caused by the different center frequencies of the two independent lasers at the transmitting and receiving ends and the phase noise caused by channel jitter. The recovered pilot signal is filtered out through a bandpass filter. The power ratio of the pilot signal to the pre-calibrated pilot signal at each moment is calculated, and the obtained ratio sequence is the estimated time-varying free-space transmittance.

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