Underwater wireless optical communication physical layer security coding and decoding system and method based on Polar code
By utilizing a Polar code-based underwater wireless optical communication physical layer security encoding and decoding system, and employing channel polarization theory and dynamic adjustment mechanisms, the problems of high key management costs and security vulnerabilities in underwater wireless optical communication are solved, achieving a low-cost, high-security communication solution.
Patent Information
- Application Number
- CN202511705058.0
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-11-20
- Publication Date
- 2026-01-23
AI Technical Summary
Existing underwater wireless optical communication systems suffer from high key management costs and difficulty in achieving information-theoretic security when frequently switching communication targets. Furthermore, eavesdroppers can obtain information through scattered photons, posing a security risk.
A physical layer security encoding and decoding system for underwater wireless optical communication based on Polar codes is adopted. The information bit set is dynamically determined by the Monte Carlo construction algorithm, the channel is decomposed by channel polarization theory, and the channel condition difference between the legitimate receiver and the eavesdropping end is considered. A low-cost and miniaturized encoding and decoding system is designed, and the interruption probability is introduced as a security evaluation index to dynamically adjust the Polar code parameters.
Without increasing hardware costs, it enhances the security and reliability of underwater wireless optical communication, and can automatically adjust encoding parameters in complex underwater environments to maintain a high level of security and resist the risk of information leakage.
Smart Images
Figure CN121396346A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of underwater optical communication technology, and in particular to a physical layer security encoding and decoding system and method for underwater wireless optical communication based on Polar codes. Background Technology
[0002] Underwater communication has broad application prospects in various fields such as marine resource exploration, underwater geological monitoring, and hydrological monitoring. Among them, underwater visible light communication, due to its combined illumination and communication functions, has become a current research hotspot. Although optical communication is highly directional, its channel is still open in space, and light waves suffer energy loss and signal diffusion during underwater transmission due to absorption and scattering effects. This characteristic makes it possible for eavesdroppers to obtain information by capturing scattered photons, thus posing a security risk.
[0003] To achieve information-theoretical security, the traditional method is "one-time pad" encryption, where the sender encrypts the ciphertext using a pre-shared private key, and the receiver decrypts it using the same private key. For secure communication, the key and ciphertext must be of equal length, and the key can only be used once. However, in underwater wireless communication applications, communication partners frequently switch. Creating a separate key for each partner would drastically increase key storage and management costs, making practical deployment difficult. Physical layer secure communication (PLS) is another way to achieve information-theoretical security. It requires that the channel between legitimate communicators is superior to that of eavesdroppers, utilizing the difference in channel capacity to establish secure communication. Underwater optical wireless communication has good directionality, ensuring that legitimate receivers have better channel conditions than eavesdroppers. Therefore, PLS technology holds promise for providing a more secure communication strategy for underwater optical wireless communication.
[0004] Besides ensuring that the channel between legitimate communicating parties is superior to the eavesdropping channel, anti-eavesdropping coding is another key factor in guaranteeing secure physical layer communication. Polar codes are the first theoretically proven coding scheme to strictly reach the Shannon limit, with lower encoding and decoding complexity and more flexible code rates. However, although there has been a wealth of research on Polar codes in recent years, most of the results remain at the theoretical level, and there is very little research on applying Polar codes to underwater wireless optical communication scenarios. Summary of the Invention
[0005] The purpose of this invention is to provide a physical layer security encoding and decoding system and method for underwater wireless optical communication based on Polar codes, which ensures the reliability of the underwater wireless optical communication system and enhances its security without increasing the hardware cost of the system.
[0006] To achieve the above objectives, the present invention provides a physical layer security encoding and decoding system for underwater wireless optical communication based on Polar codes, including a transmitter, an underwater channel, a legitimate receiver, and an illegal eavesdropping terminal. The transmitter includes an encoding and modulation module, a laser emission module, and a backscatter detection module. Both the legitimate receiver and the illegal eavesdropping terminal include a single-photon detection module and a data processing module. The encoding and modulation module is used to dynamically determine the information bit set of the Polar code through the Monte Carlo construction algorithm, perform Polar code encoding and modulation on the original data, and convert the digital signal into a modulated signal. The laser emitting module is connected to the encoding and modulation module and is used to convert the modulation signal into an optical signal and transmit it to the underwater channel; The backscattering detection module is used to detect and analyze the backscattering of the optical signal after the transmitter passes through the underwater channel, and to assess the signal situation that the illegal eavesdropping terminal may detect. The underwater channel is used for transmitting and attenuating optical signals, reducing the intensity of the optical signal reaching the legitimate receiving end to the level of a single photon. The single-photon detection module is used to detect the single-photon signal after it has been attenuated by the underwater channel and convert it into an electrical signal. The data processing module, connected to the single-photon detection module, is used to demodulate the electrical signal and decode the Polar code to recover the original data.
[0007] Preferably, the Polar coding in the coding and modulation module is based on channel polarization theory, which decomposes the original channel into multiple sub-channels through polarization transformation, and performs bit allocation according to the reliability of the sub-channels; The reliability of each sub-channel is evaluated using the Monte Carlo construction algorithm. In the polarization subchannel, the th The Bhattacharyya parameter of each polarization sub-channel is defined as follows: ; in, This represents a symmetric binary input discrete memoryless channel, with the input bit sequence being... The output bit sequence is , Indicates the input of the channel ， Indicates channel The output, Indicates the transition probability; By simplifying the computational difficulty of the Monte Carlo construction algorithm through log-likelihood ratio, the first... The log-likelihood ratio of each channel is defined as: ; when , When the Bhattacharyya parameter is simplified to: ; when , When 1, the Bhattacharyya parameter simplifies to: .
[0008] Preferably, the specific steps of the Monte Carlo construction algorithm are as follows: a. Initialize the Bhattacharyya parameter of all polarization sub-channels to 0; b. Randomly generate the input bit sequence ; c. After encoding and modulation, the signal is loaded into the laser emission module. After attenuation and scattering in the underwater channel, the laser is detected by the single-photon detection module, and the output bit sequence is obtained in the data processing module. ; d. Perform Polar code decoding in the data processing module and save the log-likelihood ratio of the decoded code; f. Calculate and sort the Bhattacharyya parameters based on the decoding results, and select the top... Each of these constitutes a set of information bits, and the remaining ones... Each constitutes a frozen position.
[0009] Preferably, the encoding and modulation module includes a host computer and an FPGA. The host computer is used to output the original bit sequence to be transmitted and connects to the FPGA via a network port to transmit the original bit sequence to the FPGA. The FPGA integrates a Polar code encoder and a modulator. The Polar code encoder is used to perform Polar code encoding on the original bit sequence transmitted by the host computer, and the modulator is used to modulate the encoded codewords to convert the digital signal into a modulated signal.
[0010] Preferably, the backscattering detection module includes a high-sensitivity photomultiplier tube, an amplifier circuit, and a host computer connected in sequence. The high-sensitivity photomultiplier tube detects the intensity of the backscattered light signal and converts the signal into an electrical signal. After being amplified by the amplifier circuit, the signal is sent to the host computer for data conversion and displays the intensity of the backscattering from the underwater channel.
[0011] A physical layer secure encoding and decoding method for underwater wireless optical communication based on Polar codes includes the following steps: S1. At the transmitting end, the information bit sequence to be transmitted is encoded and modulated using a Polar code encoding and modulation module to generate a modulated signal. S2. The modulated signal is converted into an optical signal by the laser emission module and transmitted to the underwater channel; S3. Using the backscattering detection module, detect the backscattering signal at an angle of 30° to the laser emission direction, calculate the backscattering coefficient, and determine whether the backscattering coefficient is within the target range. If so, determine a new set of information bits based on the target range and use the new set of information bits to update the current set of information bits. S4. At the legitimate receiving end and the illegal eavesdropping end, detect the single-photon signal respectively, and use the decoding strategy corresponding to the current information bit set to demodulate and decode the Polar code to obtain their respective original data; S5. Calculate the bit error rate of the legitimate receiver and the illegal eavesdropping end; S6. Calculate the system interruption probability based on the bit error rate, and evaluate the communication security in combination with the backscattering coefficient; if the security does not meet the preset conditions, adjust the Polar code rate and restart the communication process.
[0012] Preferably, the specific steps of S6 include: S61. Based on the bit error rate of the legitimate receiving end and the illegal eavesdropping end, a target bit error rate is preset, and the system interruption probability is calculated. S62. If the interruption probability is 0 and the backscattering coefficient is within the target range, the system is deemed safe and communication continues. S63. If the interruption probability is not 0, pause communication, increase the number of frozen bits in the Polar code to reduce the code rate, and re-execute the Polar code encoding process.
[0013] Preferably, the S6 interruption probability The calculation formula is: ; in, This represents the minimum signal-to-noise ratio threshold that a legitimate receiver must meet to ensure reliable transmission in the system. This represents the maximum signal-to-noise ratio threshold that an unauthorized eavesdropping device must meet to ensure secure transmission within the system. This represents the instantaneous signal-to-noise ratio at the legitimate receiver. This indicates the instantaneous signal-to-noise ratio of the illegal eavesdropping device.
[0014] Therefore, the present invention employs the above-mentioned underwater wireless optical communication physical layer security encoding and decoding system and method based on Polar codes, which has the following beneficial effects: (1) By jointly analyzing the legitimate channel and the eavesdropping channel based on the Bhattacharyya parameter, this invention can accurately divide the sub-channels that are reliable only to the legitimate receiver but unreliable to the eavesdropper for transmitting information. This mechanism ensures that even if the eavesdropper obtains the signal by capturing scattered photons, the disadvantage of its channel conditions prevents it from decoding correctly, thus effectively resisting the risk of information leakage caused by underwater channel scattering.
[0015] (2) A low-cost, miniaturized prototype of an underwater blue light real-time wireless security communication system was designed and implemented. The theoretical advantages of Polar codes were transformed into a practical solution. The system can achieve both reliability and security of communication through Polar code encoding and decoding, providing information security for underwater wireless optical communication.
[0016] (3) By introducing the interruption probability as a security assessment index and designing a dynamic feedback mechanism based on the test word, the system can automatically adjust the construction parameters of the Polar code when the channel conditions change, continuously maintain a high security level, and enhance the system's adaptability in actual complex underwater environments.
[0017] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description
[0018] Figure 1 This is a structural framework diagram of a physical layer secure encoding and decoding system for underwater wireless optical communication based on Polar codes, according to the present invention. Figure 2 This is a flowchart illustrating the specific algorithm for constructing polar codes using the Monte Carlo method in a physical layer security encoding and decoding system for underwater wireless optical communication based on Polar codes, as described in this invention. Figure 3 This is a flowchart illustrating the physical layer security encoding and decoding system method for underwater wireless optical communication based on Polar codes according to the present invention. Figure 4 This is a schematic diagram illustrating the interruption probability acquisition process of a physical layer security encoding and decoding method for underwater wireless optical communication based on Polar codes according to the present invention. Detailed Implementation
[0019] The following detailed description of embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.
[0020] Example like Figure 1As shown, the present invention provides a physical layer security encoding and decoding system for underwater wireless optical communication based on Polar codes, including a transmitter, an underwater channel, a legitimate receiver, and an illegal eavesdropping end. The transmitter includes an encoding and modulation module, a laser emission module, and a backscatter detection module. Both the legitimate receiver and the illegal eavesdropping end include a single-photon detection module and a data processing module. The encoding and modulation module is used to dynamically determine the information bit set of the Polar code through the Monte Carlo construction algorithm, perform Polar code encoding and modulation on the original data, and convert the digital signal into a modulated signal; The laser emitting module, connected to the encoding and modulation module, is used to convert the modulated signal into an optical signal and transmit it to the underwater channel; The backscattering detection module is used to detect and analyze the backscattering of optical signals after the transmitter passes through the underwater channel, and to assess the signal situation that illegal eavesdropping terminals may detect. Underwater channels are used for the transmission and attenuation of optical signals, reducing the intensity of the optical signal reaching the legitimate receiver to the level of a single photon. The single-photon detection module is used to detect single-photon signals attenuated by the underwater channel and convert them into electrical signals. The data processing module, connected to the single-photon detection module, is used to demodulate the electrical signal and decode the Polar code to recover the original data.
[0021] The encoding and modulation module includes a host computer and an FPGA. The host computer outputs the original bit sequence to be transmitted and connects to the FPGA via a network port to transmit the original bit sequence to the FPGA. The FPGA integrates a Polar code encoder and a modulator. The Polar code encoder performs Polar code encoding on the original bit sequence transmitted by the host computer and outputs the corresponding Polar code encoding information. The modulator is electrically connected to the Polar code encoder and is used to modulate the encoded codewords, converting the digital signal into the corresponding modulated signal.
[0022] Polar coding is based on channel polarization theory. It decomposes the original channel into multiple sub-channels through polarization transformation and allocates bits according to the reliability of the sub-channels. The high-reliability sub-channels, i.e., good channels, are used to transmit information bits, while the low-reliability sub-channels, i.e. bad channels, are fixed as frozen bits to enhance system security.
[0023] The specific steps of Polar encoding include: Assume the Polar code encoding parameters are ,in That is, a set Number of elements, bitrate Define a set For information bits set, This involves freezing the bit vector. When configuring the channel, it's necessary to filter based on channel reliability and determine the optimal one. Each sub-channel receives information bits. The value of the information bit can be 0 or 1, but the value of the frozen bit is often 0. Assume the information sequence after bit mixing is... The sequence generated after encoding is Then we have: ; in, It is the generating matrix, which can be represented as: ; matrix , For matrix Sub-Croneck product, and has ; For Polar codes The steps for selecting each information bit include: Determine the length of the Polar code ; Calculate the Bhattacharyya parameter for each channel; After calculation Sort the Bhattacharyya parameters of each sub-channel and then select the one with the smallest Bhattacharyya parameter value. Each channel number is selected for transmitting information bits.
[0024] The Polar coding in the coding and modulation module is based on channel polarization theory. It decomposes the original channel into multiple sub-channels through polarization transformation and allocates bits according to the reliability of the sub-channels. The reliability of each sub-channel is evaluated using the Monte Carlo construction algorithm. In the polarization subchannel, the th The Bhattacharyya parameter of each polarization sub-channel is defined as follows: ; in, This represents a symmetric binary input discrete memoryless channel, with the input bit sequence being... The output bit sequence is , Indicates the input of the channel ， Indicates channel The output, Indicates the transition probability; By simplifying the computational difficulty of the Monte Carlo construction algorithm through log-likelihood ratio, the first... The log-likelihood ratio of each channel is defined as: ; when , When the Bhattacharyya parameter is simplified to: ; when , When 1, the Bhattacharyya parameter simplifies to: .
[0025] The Monte Carlo construction method has a wider range of applications compared to other construction methods, and practical channels such as underwater wireless optical communication channels are typical asymmetric channels. Therefore, this embodiment chose the Monte Carlo construction method for the construction of polar codes, such as... Figure 2 The diagram shows the specific algorithm flow for constructing polar codes based on the Monte Carlo construction method in this embodiment, including: a. Initialize the Bhattacharyya parameter of all polarization sub-channels to 0; b. Randomly generate the input bit sequence ; c. After encoding and modulation, the signal is loaded into the laser emission module. After attenuation and scattering in the underwater channel, the laser is detected by the single-photon detection module, and the output bit sequence is obtained in the data processing module. ; d. Perform Polar code decoding in the data processing module and save the log-likelihood ratio of the decoded code; f. Calculate and sort the Bhattacharyya parameters based on the decoding results, and select the top... Each of these constitutes a set of information bits, and the remaining ones... Each constitutes a frozen position.
[0026] The backscattering detection module includes a high-sensitivity photomultiplier tube, an amplifier circuit, and a host computer connected in sequence. The high-sensitivity photomultiplier tube detects the intensity of the backscattered light signal and converts the signal into an electrical signal. After being amplified by the amplifier circuit, the signal is sent to the host computer for data conversion and displays the intensity of backscattering from the underwater channel.
[0027] In this embodiment, the laser emission module, single-photon detection module, and data processing module may employ the following specific hardware.
[0028] The laser emitting module includes a DC power supply, a power amplifier, a bias converter, a laser diode, and a collimating lens, which are connected in sequence. The power amplifier is electrically connected to the FPGA and is used to amplify the modulation signal generated by the modulator. The DC power supply is electrically connected to the power supply port of the power amplifier and is used to provide a stable operating voltage to the power amplifier. The bias converter is used to DC bias the amplified modulation signal to obtain a bias signal. The laser diode is used to convert the bias signal into an optical signal. The collimating lens is used to adjust the optical path of the optical signal in the underwater channel, collimating the optical signal into a parallel beam and reflecting it into the underwater channel.
[0029] The single-photon detection module includes a microlens array, an array-type single-photon detector, and a multi-channel time-to-digital converter connected in sequence. The microlens array is used to converge the single-photon signal formed after absorption and scattering attenuation through the underwater channel to obtain a focused single-photon signal. The array-type single-photon detector is used to perform photoelectric conversion on the focused single-photon signal to generate a corresponding electrical signal. The multi-channel time-to-digital converter is used to perform time measurement and digitization processing on the electrical signal to obtain transmission data containing information related to the single-photon signal. This transmission data is the data required for subsequent signal analysis.
[0030] The data processing module includes a host computer, which connects to a multi-channel time-to-digital converter via a USB interface to perform demodulation and Polar code decoding to restore the data. The host computer integrates a demodulator and a Polar code decoder. The demodulator is used to demodulate the signal output by the single-photon detection module to obtain the corresponding baseband signal. The Polar code decoder is used to decode the baseband signal to recover the original data.
[0031] like Figure 3 As shown, a physical layer secure encoding and decoding method for underwater wireless optical communication based on Polar codes includes the following steps: S1. At the transmitting end, the information bit sequence to be transmitted is encoded and modulated using a Polar code encoding and modulation module to generate a modulated signal.
[0032] S2. The modulated signal is converted into an optical signal by the laser emission module and transmitted to the underwater channel; S3. Using the backscatter detection module, detect the backscatter signal at an angle of 30° to the laser emission direction, calculate the backscatter coefficient, and determine whether the backscatter coefficient is within the target range. If so, determine a new set of information bits based on the target range and use the new set of information bits to update the current set of information bits.
[0033] S4. At the legitimate receiving end and the illegal eavesdropping end, detect the single-photon signal respectively, and use the decoding strategy corresponding to the current information bit set to demodulate and decode the Polar code to obtain their respective original data.
[0034] In this embodiment, the specific process of detecting single-photon signals includes: S41. A single-photon signal is formed by focusing the light signal through the underwater channel using a microlens array to obtain a focused photon signal, and the focused photon signal is guided to the array single-photon detector. S42. Use an array of single-photon detectors to perform photoelectric conversion on the focused photon signal to generate a corresponding electrical signal; S43. A multi-channel time-to-digital converter is used to measure the time of arrival of the electrical signal (collect the time of arrival of the photon) and process the digital signal to obtain transmission data containing information related to the single photon signal.
[0035] S5. Calculate the bit error rate of the legitimate receiver and the illegal eavesdropping end.
[0036] S6. Calculate the system interruption probability based on the bit error rate, and evaluate the communication security in combination with the backscattering coefficient; if the security does not meet the preset conditions, adjust the Polar code rate and restart the communication process.
[0037] S61. Based on the bit error rate of the legitimate receiving end and the illegal eavesdropping end, a target bit error rate is preset, and the system interruption probability is calculated. S62. If the interruption probability is 0 and the backscattering coefficient is within the target range, the system is deemed safe and communication continues. S63. If the interruption probability is not 0, pause communication, increase the number of frozen bits in the Polar code to reduce the code rate, and re-execute the Polar code encoding process.
[0038] Interruption probability The calculation formula is: ; in, This represents the minimum signal-to-noise ratio threshold that a legitimate receiver must meet to ensure reliable transmission in the system. This represents the maximum signal-to-noise ratio threshold that an unauthorized eavesdropping device must meet to ensure secure transmission within the system. This represents the instantaneous signal-to-noise ratio at the legitimate receiver. This indicates the instantaneous signal-to-noise ratio of the illegal eavesdropping device.
[0039] The calculation process for the interruption probability is as follows: Figure 4 As shown.
[0040] The key design feature of this embodiment lies in the secondary encryption achieved through Polar code encoding and decoding of the information transmitted between the sending and receiving ends. The receiving end can then use a Polar code decoding structure for decoding, enhancing the security of information transmission. First, the channel is divided according to the Polar code channel environment, consisting of information bits, random bits, and frozen bits. The sending end uses these information bits, random bits, and frozen bits to transmit the encoded information to the receiving end, using channel coding to encrypt the information and strengthen its confidentiality. Furthermore, because the channel environment of an illegal eavesdropping channel differs from that of a legitimate receiving channel, the number of bits and their division also differ, thus preventing illegal eavesdropping channels from obtaining complete and valid information. At the receiving end, a corresponding decoder is constructed based on the Polar code channel used to transmit the information, reducing the error rate of the received information.
[0041] Therefore, the present invention adopts the above-mentioned underwater wireless optical communication physical layer security encoding and decoding system and method based on Polar code, which realizes the reliability and security of communication through Polar code encoding and decoding, and provides information security guarantee for underwater wireless optical communication.
[0042] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.
Claims
1. A physical layer secure encoding and decoding system for underwater wireless optical communication based on Polar codes, characterized in that: It includes a transmitter, an underwater channel, a legitimate receiver, and an illegal eavesdropping device. The transmitter includes a coding and modulation module, a laser emission module, and a backscattering detection module. The legitimate receiver and the illegal eavesdropping device both include a single-photon detection module and a data processing module. The encoding and modulation module is used to dynamically determine the information bit set of the Polar code through the Monte Carlo construction algorithm, perform Polar code encoding and modulation on the original data, and convert the digital signal into a modulated signal. The laser emitting module is connected to the encoding and modulation module and is used to convert the modulation signal into an optical signal and transmit it to the underwater channel; The backscattering detection module is used to detect and analyze the backscattering of the optical signal after the transmitter passes through the underwater channel, and to assess the signal situation that the illegal eavesdropping terminal may detect. The underwater channel is used for transmitting and attenuating optical signals, reducing the intensity of the optical signal reaching the legitimate receiving end to the level of a single photon. The single-photon detection module is used to detect the single-photon signal after it has been attenuated by the underwater channel and convert it into an electrical signal. The data processing module, connected to the single-photon detection module, is used to demodulate the electrical signal and decode the Polar code to recover the original data.
2. The underwater wireless optical communication physical layer security encoding and decoding system based on Polar codes according to claim 1, characterized in that: The Polar coding in the coding and modulation module is based on channel polarization theory. It decomposes the original channel into multiple sub-channels through polarization transformation and allocates bits according to the reliability of the sub-channels. The reliability of each sub-channel is evaluated using the Monte Carlo construction algorithm. In the polarization subchannel, the th The Bhattacharyya parameter of each polarization sub-channel is defined as follows: ； in, This represents a symmetric binary input discrete memoryless channel, with the input bit sequence being... The output bit sequence is , Indicates the input of the channel ， Indicates channel The output, Indicates the transition probability; By simplifying the computational difficulty of the Monte Carlo construction algorithm through log-likelihood ratio, the first... The log-likelihood ratio of each channel is defined as: ； when , When the Bhattacharyya parameter is simplified to: ； when , When 1, the Bhattacharyya parameter simplifies to: 。 3. A physical layer security encoding and decoding system for underwater wireless optical communication based on Polar codes according to claim 2, characterized in that, The specific steps of the Monte Carlo construction algorithm are as follows: a. Initialize the Bhattacharyya parameter of all polarization sub-channels to 0; b. Randomly generate the input bit sequence ; c. After encoding and modulation, the signal is loaded into the laser emission module. After attenuation and scattering in the underwater channel, the laser is detected by the single-photon detection module, and the output bit sequence is obtained in the data processing module. ; d. Perform Polar code decoding in the data processing module and save the log-likelihood ratio of the decoded code; f. Calculate and sort the Bhattacharyya parameters based on the decoding results, and select the top... Each of these constitutes a set of information bits, and the remaining ones... Each constitutes a frozen position.
4. The underwater wireless optical communication physical layer security encoding and decoding system based on Polar codes according to claim 1, characterized in that: The encoding and modulation module includes a host computer and an FPGA. The host computer is used to output the original bit sequence to be transmitted and connects to the FPGA via a network port to transmit the original bit sequence to the FPGA. The FPGA integrates a Polar code encoder and a modulator. The Polar code encoder is used to perform Polar code encoding on the original bit sequence transmitted by the host computer, and the modulator is used to modulate the encoded codewords to convert the digital signal into a modulated signal.
5. A physical layer security encoding and decoding system for underwater wireless optical communication based on Polar codes according to claim 4, characterized in that: The backscattering detection module includes a high-sensitivity photomultiplier tube, an amplifier circuit, and a host computer connected in sequence. The high-sensitivity photomultiplier tube detects the intensity of the backscattered light signal and converts the signal into an electrical signal. After being amplified by the amplifier circuit, the signal is sent to the host computer for data conversion and displays the intensity of the backscattering from the underwater channel.
6. A physical layer security encoding and decoding method for underwater wireless optical communication based on Polar codes, applied to a physical layer security encoding and decoding system for underwater wireless optical communication based on Polar codes as described in any one of claims 1-5, characterized in that, Includes the following steps: S1. At the transmitting end, the information bit sequence to be transmitted is encoded and modulated using a Polar code encoding and modulation module to generate a modulated signal. S2. The modulated signal is converted into an optical signal by the laser emission module and transmitted to the underwater channel; S3. Using the backscattering detection module, detect the backscattering signal at an angle of 30° to the laser emission direction, calculate the backscattering coefficient, and determine whether the backscattering coefficient is within the target range. If so, determine a new set of information bits based on the target range and use the new set of information bits to update the current set of information bits. S4. At the legitimate receiving end and the illegal eavesdropping end, detect the single-photon signal respectively, and use the decoding strategy corresponding to the current information bit set to demodulate and decode the Polar code to obtain their respective original data; S5. Calculate the bit error rate of the legitimate receiver and the illegal eavesdropping end; S6. Calculate the system interruption probability based on the bit error rate, and evaluate the communication security in combination with the backscattering coefficient; if the security does not meet the preset conditions, adjust the Polar code rate and restart the communication process.
7. A physical layer secure encoding and decoding method for underwater wireless optical communication based on Polar codes according to claim 6, characterized in that, The specific steps of S6 include: S61. Based on the bit error rate of the legitimate receiving end and the illegal eavesdropping end, a target bit error rate is preset, and the system interruption probability is calculated. S62. If the interruption probability is 0 and the backscattering coefficient is within the target range, the system is deemed safe and communication continues. S63. If the interruption probability is not 0, pause communication, increase the number of frozen bits in the Polar code to reduce the code rate, and re-execute the Polar code encoding process.
8. A physical layer secure encoding and decoding method for underwater wireless optical communication based on Polar codes according to claim 7, characterized in that, S6 interruption probability The calculation formula is: ； in, This represents the minimum signal-to-noise ratio threshold that a legitimate receiver must meet to ensure reliable transmission in the system. This represents the maximum signal-to-noise ratio threshold that an unauthorized eavesdropping device must meet to ensure secure transmission within the system. This represents the instantaneous signal-to-noise ratio at the legitimate receiver. This indicates the instantaneous signal-to-noise ratio of the illegal eavesdropping device.