Time-varying channel transmittance estimation method and device based on frequency division multiplexing pilot frequency

Abstract

The invention discloses a method for estimating the transmittance of a time-varying channel based on a frequency division multiplexing pilot frequency, and aims to solve the problem that the existing method for estimating the transmittance by adopting average transmittance does not have the capabilities of timely feedback and dynamic adjustment. The method comprises the following steps of: measuring a regular position component and a regular momentum component of a received signal by receiving a quantum signal and a pilot signal of frequency division multiplexing in a local local oscillator continuous variable quantum key distribution system generated by a transmitting end in real time; compensating phase noise caused by frequency offset and channel jitter due to different center frequencies of two independent lasers at a receiving end and a transmitting end by using a frequency offset compensation and phase recovery algorithm, and performing band-pass filtering to obtain a recovered pilot signal; and calculating the power ratio of the obtained pilot signal to the pre-calibrated pilot signal at each moment to obtain the time-varying free space transmittance. The performance change of the system in different time periods can be accurately reflected, the communication performance of the CVQKD system can be optimized, the success rate of key transmission can be improved, and the adaptability and robustness of the system can be improved.

Description

Technical Field

[0001] The 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 local oscillator continuous variable quantum key distribution. Background Art

[0002] Nowadays, the degree of social informatization is constantly improving, and information technology is increasingly used in all walks of life, covering important fields such as military and finance. With the development of science and technology and the strengthening of economic strength, the value of information is also increasing. However, information leakage incidents occur frequently, which has brought serious harm to individuals, society and the country. Therefore, it is particularly important to ensure the secure transmission of information.

[0003] The two parties in legal communication use keys to encrypt the information they need to transmit, which increases the difficulty for third parties to eavesdrop on the information, and the security of the key determines the security of the confidential communication process. Quantum key distribution is based on quantum mechanics principles such as quantum non-cloning and quantum uncertainty relations, and can achieve theoretically unconditionally secure key distribution. It 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, according to the different physical quantities loaded with the key. After recent years of development, discrete variable quantum key distribution (DVQKD) technology is relatively mature and can achieve longer transmission distances than continuous variable quantum key distribution, but there are difficulties in preparing and detecting single-photon signals. It is not only complex and difficult to prepare, but also costly and expensive.

[0004] Continuous Variable Quantum Key Distribution (CVQKD) technology has attracted widespread attention at home and abroad due to its advantages in channel capacity, good integration with existing optical communications, and resistance to background light interference. According to the different light sources used, CVQKD can be divided into three types of protocols: coherent state, compressed 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, without the need for expensive and complex single-photon light sources and special detectors. It can make full use of the development accumulation of optical fiber communications over the years, and has significant cost advantages and high long-term reliability.

[0005] According to the different quantum state modulation formats, CVQKD technology can be divided into Gaussian modulation (GMCS) and discrete modulation scheme (DMCS). Discrete variable quantum key distribution technology is relatively mature, but there are certain difficulties and high costs in the preparation and detection of single photon signals. In contrast, continuous variable quantum key distribution technology has attracted widespread attention at home and abroad due to its advantages in channel capacity, optical communication integration and anti-background light interference. Among them, the coherent state CVQKD system has attracted much attention due to its relatively simple preparation method. It is implemented using traditional optical communication devices, avoiding the use of expensive and complex single-photon light sources and dedicated detectors, and has obvious cost advantages and long-term reliability. Gaussian modulation can ensure that the legal communication parties reach a mutual information amount close to the channel capacity, which means that it has outstanding advantages in the efficiency and quality of information transmission. In addition, with the continuous advancement of technology, researchers are also working hard to overcome the technical difficulties in the implementation of Gaussian modulation. Once these difficulties are solved, the Gaussian modulation CVQKD system will show more powerful performance and broader application prospects. Therefore, Gaussian modulation is also one of the current research hotspots of CVQKD technology.

[0006] The emergence of the local local oscillator CVQKD scheme aims to ensure the security of CVQKD implementation and improve performance. In this method, the transmitter uses the same light source to generate reference pulses and coherent quantum signals, while the receiver uses local local oscillator light to detect the reference pulses and quantum signals, and adjusts the phase of the quantum signal according to the phase information provided by the reference pulse. Since the local local oscillator light at the receiving end is not controlled by the eavesdropper, this scheme can circumvent various attacks on the local oscillator light and achieve stable control of the shot noise level, thereby achieving efficient balanced homodyne detection. The local local oscillator CVQKD scheme has the advantages of high security, easy to reach the shot noise limit, easy system integration, and support for large-scale deployment.

[0007] The development background of free-space CVQKD can be traced back to the growing demand for information security and the challenges faced by traditional encryption technologies. CVQKD has shown great potential in long-distance communications, especially in satellite communications and ground-to-ground communications. Due to its ability to effectively resist the risk of eavesdropping, CVQKD is widely recognized as a highly secure communication method in these fields. With the continuous advancement of technology, CVQKD has made significant progress in optical devices, detectors, and signal processing. These technological breakthroughs not only improve the transmission rate and security of CVQKD, but also gradually push it to become an indispensable component of future quantum communication networks. Therefore, as a cutting-edge quantum communication technology, free-space CVQKD has laid an important foundation for building a secure quantum Internet.

[0008] With respect to the above-mentioned related technologies, the inventors believe that the real-time transmittance change reflects the channel status. By monitoring the real-time transmittance change of free space CVQKD, the state changes of the communication channel, including channel noise, interference, etc., can be timely understood. This helps to identify potential security threats and problems and take corresponding measures to deal with them. At the same time, according to the real-time transmittance change, the communication parameters can be dynamically adjusted to optimize the communication performance and improve 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 it is of great significance to monitor it in real time. Timely detection of abnormal changes can help the system respond to possible security threats in a timely manner and ensure the security of communications.

[0009] However, most of the existing channel transmittance estimation is based on the average transmittance estimation of a frame signal, which does not have the ability of timely feedback and dynamic adjustment. In the face of emergencies or specific environmental conditions, the average transmittance may not accurately reflect the performance changes of the system in different time periods. Therefore, it is very meaningful to realize the real-time channel transmittance estimation of the free space local oscillator CVQKD system. Summary of the invention

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

[0011] 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 receiving and transmitting ends and the phase noise caused by channel jitter, and the recovered pilot signal is filtered out through a bandpass filter; the power ratio of the pilot signal and 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 quantum signal during modulation; the symbol rate of 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 modulates the constant complex sequence, that is, multiplying the sequence by the exponential sequence exp(j2πf p t k ), and 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 p It represents the frequency of carrier modulation, the unit is Hz, exp() represents the natural constant, j represents the imaginary unit, and π represents the circumference of a circle;

[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 the 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] A signal sending module, used for generating a frequency-division multiplexed quantum signal and a pilot signal in a local local oscillator continuous variable quantum key distribution system at the sending end, and transmitting the quantum signal and the pilot signal to the receiving end via a 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 the pilot signal transmitted through 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 receiving and transmitting ends and the phase noise caused by the channel jitter by using the frequency offset compensation and phase recovery algorithm, and to filter out the recovered pilot signal through the bandpass filter; the power ratio of the pilot signal and 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] The method for estimating the time-varying channel transmittance based on the frequency-division multiplexing pilot in one embodiment of the present invention aims at the problem that the existing average transmittance estimation does not have the ability of timely feedback and dynamic adjustment. In the face of emergencies or specific environmental conditions, the average transmittance may not accurately reflect the performance changes of the system in different time periods. The method measures the canonical position component and canonical momentum component of the received signal by receiving the frequency-division multiplexing quantum signal and pilot signal generated by the local local oscillator continuous variable quantum key distribution system generated by the transmitter in real time; and compensates the frequency deviation caused by the different center frequencies of the two independent lasers at the receiving and transmitting ends and the phase noise caused by the channel jitter by using the frequency deviation compensation and phase recovery algorithm, and obtains 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 to obtain the time-varying free space transmittance. It can accurately reflect the performance changes of the system in different time periods, help optimize the communication performance of the CVQKD system, improve the success rate of key transmission, and improve the adaptability and robustness of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 A 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 a device for estimating time-varying channel transmittance based on frequency division multiplexing pilot in one embodiment of the present invention. DETAILED DESCRIPTION

[0033] The following is a further detailed description of a method and device for estimating the transmittance of a time-varying channel based on a frequency division multiplexing pilot proposed by the present invention in conjunction with 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 mode in step S1 is baseband Gaussian modulation, and the pilot signal modulation mode 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, and 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 and the quantum signal, and j represents the imaginary unit.

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

[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 pilot and quantum signal coherent states of frequency division multiplexing are obtained |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 a circle to a circle.

[0041] Step S2, performing a pre-calibration operation, using an optical fiber cable as short as possible to directly connect the transmitting end and the receiving end, detecting and recovering the pilot signal at this time, as a 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 an optical fiber connecting line 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, using a pilot-assisted frequency offset compensation and phase recovery algorithm to compensate for the frequency offset caused by the different center frequencies of two independent lasers at the transmitting and receiving ends and the phase noise caused by channel jitter;

[0046] Step S2.4, bandpass filter out the restored pilot signal, and use the pilot signal as a pre-calibrated pilot 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 completes necessary operations such as frequency offset recovery and phase recovery;

[0049] The method for completing frequency offset recovery and phase recovery for the detected signal transmitted through the free space channel in step S4 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, using a pilot-assisted frequency offset compensation and phase recovery algorithm to compensate for the frequency offset caused by the different center frequencies of two independent lasers at the transmitting and receiving ends and the phase noise caused by channel jitter;

[0052] Step S4.3: bandpass filter out the restored pilot signal transmitted through the free space channel.

[0053] Step S5, use the pilot-based time-varying channel transmittance estimation algorithm to complete the estimation of the time-varying transmittance of the free space channel, 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, and the obtained ratio sequence is the estimated time-varying free space transmittance.

[0054] The real-time transmittance change in the local local oscillator continuous variable quantum key distribution system reflects the channel state. Monitoring the real-time transmittance change of the free space local local oscillator continuous variable quantum key distribution can timely understand the changes in the channel noise, interference and other states in the communication channel, which helps to identify potential security threats and problems and take corresponding measures to deal with them. At the same time, according to the real-time transmittance change, the communication parameters can be dynamically adjusted to optimize the communication performance and improve the success rate of key transmission. This real-time feedback mechanism can improve the adaptability and robustness of the system. The present invention combines the pilot signal scheme of frequency division multiplexing with the free space continuous variable quantum key distribution system to complete the real-time estimation of the free space channel transmittance. The proposed pilot auxiliary scheme performs data processing on the quantum signal based on pure software, so no additional physical devices are required, and there is no need to modify the optical path of the existing free space local local oscillator continuous variable quantum key distribution system; and the scheme has excellent implantability and can be directly added to the existing digital signal processing algorithm.

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

[0056] The device is used to realize that the transmitting end generates the frequency-division multiplexed quantum signal and pilot signal in the local local oscillator continuous variable quantum key distribution system, and transmits them to the receiving end through the free space channel; and realizes that the receiving end measures the canonical position component and canonical momentum component of the frequency-division multiplexed quantum signal and pilot signal transmitted through the free space channel in a heterodyne detection manner; and compensates for the frequency deviation caused by the different center frequencies of two independent lasers at the receiving and transmitting ends and the phase noise caused by channel jitter by using the frequency deviation compensation and phase recovery algorithm, and 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.

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

[0058] Module M1: The transmitter generates frequency-division multiplexed quantum signals and pilot signals 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 and 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 ), and get {xp k +j*pp k}*exp(j2πf p t k ), which is the amplitude of the pilot signal during modulation; where t k Representing time series The kth value of p =-1.25×10 9 It represents the frequency of carrier modulation, the unit is Hz, exp() represents the natural constant, j represents the imaginary unit, and π represents the circumference of a circle;

[0063] Module M1.4: Convert 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 pilot and quantum signal coherent states of frequency division multiplexing are obtained |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 a circle to a circle.

[0064] Module M2: performs a pre-calibration operation, directly connects the transmitter and the receiver with a fiber optic cable as short as possible, detects and recovers the pilot signal at this time, and uses it as a 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: Use 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 a pre-calibrated pilot signal for channel transmittance estimation.

[0070] Module M3: A free space channel is placed between the transmitter and the receiver, and 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 completes necessary operations such as frequency offset recovery and phase recovery;

[0072] Module M4 includes the following modules:

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

[0074] Module M4.2: Use 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;

[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 transmittance of the free space channel, 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, and the obtained ratio sequence is the estimated time-varying free space transmittance.

[0077] The embodiments of the present invention are described in detail above in conjunction with 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 protection scope of the present invention.

Claims

1. A method for estimating the transmittance of a time-varying channel based on a frequency-division multiplexing pilot, used in free-space local oscillator continuous variable quantum key distribution, characterized in that: include: The transmitter generates the frequency-division multiplexed quantum signal and pilot signal 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, and 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, characterized in that: 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, characterized in that: 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 quantum signal during modulation; the symbol rate of 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 modulates the constant complex sequence, that is, multiplying the sequence by the exponential sequence exp(j2πf p t k ), and 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 It represents the frequency of carrier modulation, the unit is Hz, exp() represents the natural constant, j represents the imaginary unit, and π represents the circumference of a circle; 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, characterized in that: 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 the 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 the transmittance of a time-varying channel based on a frequency-division multiplexing pilot, used in free-space local oscillator continuous variable quantum key distribution, characterized in that: include: A signal sending module, used for generating a frequency-division multiplexed quantum signal and a pilot signal in a local local oscillator continuous variable quantum key distribution system at the sending end, and transmitting the quantum signal and the pilot signal to the receiving end via a free space channel; The transmittance estimation module is used for measuring 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 by heterodyne detection at the receiving end; 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, and 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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