A physical layer security enhanced wireless optical communication method based on dynamic threshold modulation

Abstract

The application provides a physical layer security enhanced wireless optical communication method based on dynamic threshold modulation, belongs to the technical field of wireless optical communication, is realized based on a wireless optical communication system, and comprises a sending end, a legal receiving end and a eavesdropping end analysis model.The method comprises the following steps: firstly, generating and synchronizing a dynamic random threshold sequence;secondly, PWM modulation and transmission of the sending end based on the dynamic random threshold sequence;thirdly, synchronous demodulation of the receiving end based on the dynamic random threshold sequence;and finally, fixed threshold decision analysis of the eavesdropping end.The application introduces a dynamically changing decision threshold at the physical layer, and has the following advantages in improving the security of wireless optical communication, reducing the implementation complexity and enhancing the environmental adaptability: the physical layer protection strength is high, the implementation mechanism is simple and efficient, the parameter is adjustable, the eavesdropping protection effect can be maximized under the premise of guaranteeing the quality of legal communication, and the method is suitable for different communication distances and application scenarios.

Description

Technical Field

[0001] This invention belongs to the field of wireless optical communication technology and relates to a physical layer security enhancement method for wireless optical communication based on dynamic threshold modulation. Specifically, it relates to a laser communication system, and particularly to a method for modulation, synchronous demodulation, and anti-eavesdropping enhancement based on dynamic random threshold sequences. It is applicable to various wireless optical communication systems that use on-off keying (OOK) and threshold decision mechanisms, and is especially suitable for short-range wireless optical communication scenarios with high security requirements. Background Technology

[0002] Wireless optical communication technology has attracted widespread attention for its advantages such as high bandwidth, resistance to electromagnetic interference, and abundant spectrum resources, particularly in indoor short-range communication, high-speed transmission between devices, and communication in electromagnetically sensitive environments. Existing wireless optical communication systems typically employ intensity modulation methods such as on / off keying (OOK) and pulse width modulation (PWM), and complete data recovery through photodetectors, sampling circuits, and fixed threshold decision circuits. For example, Chinese invention patent CN202510566963.6 discloses a "wireless optical frequency communication antenna and communication method based on OOK modulation technology," which receives optical signals through photodiodes and uses fixed threshold decision to achieve data transmission. Another example is Jorge Baranda et al.'s paper "An SDR Implementation of a Visible Light Communication System Based on the IEEE 802.15.7 Standard," which discloses a software-defined radio implementation scheme for visible light communication based on the IEEE 802.15.7 standard. This scheme primarily focuses on visible light communication link construction, low-cost hardware implementation, and real-time transmission verification.

[0003] While the existing solutions mentioned above have advantages such as simple structure and low hardware implementation cost, their focus is on completing the modulation, transmission, reception, and fixed threshold decision of the wireless optical communication link. They do not offer a specific solution to the problem that fixed threshold decision can easily expose physical layer signal characteristics. Under traditional OOK or fixed-threshold PWM decision methods, eavesdroppers only need to use a photodetector to capture changes in light intensity and compare them using a global mean threshold or an empirical threshold to recover the original binary information to some extent. Therefore, in short-range wireless optical communication scenarios, traditional solutions relying solely on fixed decision thresholds are insufficient to effectively suppress eavesdropping risks. Summary of the Invention

[0004] To address the vulnerability of traditional OOK modulation to eavesdropping due to its fixed threshold, this invention proposes a physical layer security enhancement method for wireless optical communication based on a dynamic random threshold sequence. This method generates a dynamic random threshold sequence locally and synchronously between the communicating parties, replacing the traditional fixed decision threshold with a decision benchmark that dynamically changes with the symbol. The PWM transmission amplitude at the transmitting end varies around the dynamic random threshold sequence. The legitimate receiver can accurately demodulate based on the synchronized dynamic random threshold sequence, while eavesdroppers, unable to discern the changing pattern of the dynamic random threshold sequence, can only estimate the decision using a fixed threshold, leading to a significantly higher demodulation error rate. This achieves enhanced security for wireless optical communication at the physical layer.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A physical layer security-enhanced wireless optical communication method based on dynamic random threshold modulation is disclosed, which is implemented based on a wireless optical communication system. The wireless optical communication system includes a transmitter, a legitimate receiver, and an eavesdropping analysis model. The eavesdropping analysis model simulates the process by which an unauthorized receiver, without access to the dynamic random threshold sequence, makes a fixed-threshold decision on intercepted optical signals. The transmitter includes a raw data generation module, a dynamic random threshold sequence generation module, a PWM modulation module, a driving circuit, and a laser diode. The legitimate receiver includes a silicon photodiode, an edge synchronization module, a sampling processing module, a local dynamic threshold sequence generation module, and a decision demodulation module. The physical layer security-enhanced wireless optical communication method includes the following steps: The first step is the generation and synchronization of dynamic random threshold sequences; The sending end and the legitimate receiving end pre-set communication parameters and random sequence generation rules, and synchronously generate dynamic random threshold sequences locally at both ends. The dynamic random threshold sequence Used for subsequent modulation at the transmitting end and determination of legitimate reception at the receiving end, specifically: Step 1.1: The raw data generation module generates the raw binary data sequence to be sent. Both communicating parties pre-set the length of the raw binary data sequence to be sent. Threshold dynamic range and the judgment offset Δ; where, Indicates the total number of symbols to be sent. Indicates the lower limit of the dynamic threshold. Δ represents the upper limit of the dynamic threshold, and Δ represents the emission amplitude offset on both sides of the dynamic threshold.

[0006] Step 1.2: The dynamic random threshold sequence generation module at the sending end and the local dynamic random threshold sequence generation module at the legitimate receiving end agree on a unified random sequence generation rule, enabling the sending end and the legitimate receiving end to generate completely synchronized dynamic random threshold sequences locally. The random sequence generation rules include an initial key, a pseudo-random algorithm, a sequence indexing method, and a threshold value range.

[0007] Step 1.3: The dynamic random threshold sequence generation module at the sending end and the local dynamic random threshold sequence generation module at the legitimate receiving end generate a dynamic random threshold sequence of the same length as the data to be sent, based on the threshold dynamic range set in Step 1.1 and the random sequence generation rules agreed upon in Step 1.2, respectively. ={ },in for The dynamic threshold uniformly distributed within the range represents the first... The demodulation threshold of 1 bit; the dynamic random threshold sequence The dynamic random threshold sequence can be generated through a pseudo-random algorithm or obtained from random sources such as channel environmental noise after consistent quantization and synchronization by both communicating parties, to ensure the dynamic random threshold sequence. The unpredictability.

[0008] The second step involves PWM modulation and transmission at the transmitting end based on a dynamic random threshold sequence; The PWM modulation module at the transmitting end is based on the dynamic random threshold sequence obtained in the first step. and the original binary data sequence Calculate the transmission amplitude corresponding to each bit. and will launch amplitude The signal is mapped to a physical PWM signal and then driven by a driver circuit to drive the laser diode to emit light signals, specifically as follows: Step 2.1, the PWM modulation module targets the first signal to be transmitted. bits Combined with the corresponding dynamic threshold Offset from judgment Calculate the target launch amplitude ;in, ={ } represents the original binary data sequence. Indicates the first bits, Indicates the first The normalized transmit amplitude corresponding to each bit is also used as the PWM duty cycle parameter for that bit. The mapping relationship satisfies: if ,but ;like ,but Among them, the threshold dynamic range and the decision offset satisfy... and To ensure It is within the effective PWM duty cycle range.

[0009] Step 2.2, the PWM modulation module modulates the target transmission amplitude obtained in step 2.1. Mapped to a physical PWM signal, generating a duty cycle of within one bit period. The pulse width modulation waveform.

[0010] In step 2.3, the transmitting end controls the laser diode through the driving circuit to output a PWM signal whose light intensity fluctuates in real time with the duty cycle. Utilizing the fixed start edge and pre-defined bit period of the PWM signal, the receiving end can directly perform bit-synchronous sampling without the need for additional auxiliary signals.

[0011] The third step is synchronous demodulation at the receiving end based on a dynamic random threshold sequence; The legitimate receiver sequentially performs photoelectric conversion, edge synchronization, sampling extraction, and dynamic threshold determination on the PWM signal transmitted in the second step through a silicon photodiode, an edge synchronization module, a sampling processing module, and a decision demodulation module, and outputs the demodulated binary bit sequence, specifically: Step 3.1: The silicon photodiode converts the captured PWM signal back into an electrical signal; the edge synchronization module locks the bit period boundary based on the fixed start edge of the PWM signal and the pre-agreed bit period, so that the first... Each sampling interval and the local dynamic random threshold sequence In Correspondingly, the wireless optical communication system uses this to realize the sampling interval and the local dynamic random threshold sequence. Precise alignment.

[0012] Step 3.2: Within the bit period locked in step 3.1, the sampling processing module samples the electrical signal. Let the total number of sampling points within a single bit period be... The number of high-level sampling points is Then the proportion of high level (duty cycle) is After normalization, the average received amplitude Duty cycle Satisfying linear mapping relationship = ,in Indicates the first The signal amplitude extracted within one bit period.

[0013] Step 3.3: The decision demodulation module obtains the local dynamic random threshold sequence of the legitimate receiver. Dynamic threshold at the corresponding position The signal amplitude extracted in step 3.2 With dynamic threshold Make a real-time judgment; if Then determine the first If each bit is 1; Then determine the first Each bit is 0.

[0014] Step 3.4: Repeat steps 3.1 to 3.3 for boundary alignment, sampling extraction, and dynamic threshold decision-making until all processes are complete. It outputs a demodulated binary bit sequence and a set of symbols.

[0015] Step 4: Fixed threshold judgment analysis of the eavesdropping terminal; The eavesdropping analysis model extracts the amplitude of the intercepted PWM optical signal without obtaining a dynamic random threshold sequence, and makes a decision using a global mean threshold or a preset fixed threshold. The demodulation result of the eavesdropping terminal is compared with the original binary data sequence to obtain the bit error rate of the eavesdropping terminal, which is used to evaluate the security enhancement effect of the physical layer security enhancement wireless optical communication method.

[0016] The beneficial effects of this invention are as follows: Compared with existing technologies, this invention has significant advantages in improving the security of wireless optical communication, reducing implementation complexity, and enhancing environmental adaptability by introducing a dynamically changing decision threshold at the physical layer. These advantages are specifically reflected in the following: (1) High physical layer protection: This invention replaces the traditional fixed decision threshold with a dynamically evolving random sequence by generating and synchronizing a dynamic random threshold sequence (steps 1.1-1.3). This causes eavesdroppers to have an extremely high error rate because they cannot know the evolution of the threshold, thus blocking the illegal recovery of information from the underlying architecture.

[0017] (2) Simple and efficient implementation mechanism: The present invention adopts a PWM mapping and transmission scheme based on dynamic threshold (steps 2.1-2.3), which directly completes security enhancement in the modulation stage without running complex encryption and decryption algorithms at the protocol layer, significantly reducing the computational resource consumption and processing latency of the embedded terminal.

[0018] (3) Parameter adjustability: The present invention can adjust the system parameters according to the actual communication scenario by setting the threshold range (step 1.3) and the decision offset Δ (step 2.1). Among them, adjusting the decision offset Δ can optimize the anti-noise performance of both parties in legitimate communication, and adjusting the threshold fluctuation range can further reduce the demodulation accuracy of eavesdroppers. Under the premise of ensuring the quality of legitimate communication, the eavesdropping protection effect is maximized and adapted to different communication distances and application scenarios. Attached Figure Description

[0019] Figure 1 For the dynamic random threshold sequence of the sending end With launch amplitude Mapping diagram; Figure 2 A schematic diagram of a PWM waveform with a fixed start edge; Figure 3 For legitimate receivers, a dynamic random threshold sequence is used. Real-time judgment graph; Figure 4 A fixed threshold decision diagram for the eavesdropping terminal; Figure 5 This is a flowchart of the method of the present invention. Detailed Implementation

[0020] The present invention will be further illustrated below with reference to specific implementation examples.

[0021] This implementation case uses the MATLAB simulation platform to simulate the signal-to-noise ratio. In an additive white Gaussian noise channel environment, a physical layer security enhancement wireless optical communication method based on dynamic threshold modulation is verified. The wireless optical communication system includes a transmitter, a legitimate receiver, and an eavesdropping analysis model. The transmitter includes a raw data generation module, a dynamic random threshold sequence generation module, a PWM modulation module, a driving circuit, and a laser diode. The legitimate receiver includes a silicon photodiode, an edge synchronization module, a sampling processing module, a local dynamic random threshold sequence generation module, and a decision demodulation module. The eavesdropping analysis model simulates the process by which an unauthorized receiver, without access to the dynamic random threshold sequence, makes a fixed-threshold decision on intercepted optical signals. Specific implementation steps are as follows: Figure 5 As shown below: The first step is the generation and synchronization of dynamic random threshold sequences; The sending end and the legitimate receiving end pre-set communication parameters and random sequence generation rules, and synchronously generate dynamic random threshold sequences locally at both ends. The dynamic random threshold sequence Used for subsequent modulation at the transmitting end and determination of legitimate reception at the receiving end, specifically: Step 1.1: The raw data generation module generates the raw binary data sequence to be sent; both communicating parties pre-set the length of the raw binary data sequence to be sent. Threshold dynamic range And the decision offset Δ, where, Indicates the total number of symbols to be sent. Indicates the lower limit of the dynamic threshold. represents the upper limit of the dynamic threshold, and Δ represents the emission amplitude offset on both sides of the dynamic threshold; in this simulation verification, we take . , , , The above parameters satisfy and This ensures the subsequent launch amplitude. It is within the effective range of normalized transmit amplitude and PWM duty cycle.

[0022] Step 1.2: The dynamic random threshold sequence generation module at the sending end and the local dynamic random threshold sequence generation module at the legitimate receiving end agree on a unified random sequence generation rule, enabling the sending end and the legitimate receiving end to generate completely synchronized dynamic random threshold sequences locally. In this random sequence generation rule, the sender and the legitimate receiver share the initial key. They also use the same pseudo-random number generation algorithm, sequence indexing method, and threshold range.

[0023] Step 1.3: The dynamic random threshold sequence generation module at the sending end and the local dynamic random threshold sequence generation module at the legitimate receiving end generate a dynamic random threshold sequence of the same length as the data to be sent, based on the threshold dynamic range set in Step 1.1 and the random sequence generation rules agreed upon in Step 1.2, respectively. ={ },in for The dynamic threshold, uniformly distributed within the range, represents the first... The demodulation threshold for each bit; using the key within the range of [0.3, 0.7]. Generate the dynamic random threshold sequence Subsequently, simulations yielded the dynamic thresholds corresponding to the first three code elements as follows: , , The dynamic random threshold sequence It can also be obtained from random sources such as channel environmental noise after consistent quantization and synchronization processing by both communicating parties, in order to improve the dynamic random threshold sequence. The unpredictability.

[0024] The second step involves PWM modulation and transmission at the transmitting end based on a dynamic random threshold sequence; The PWM modulation module at the transmitting end is based on the dynamic random threshold sequence obtained in the first step. and the original binary data sequence Calculate the transmission amplitude corresponding to each bit. , will launch amplitude The signal is mapped to a physical PWM signal and then driven by a driver circuit to drive the laser diode to emit light signals, specifically as follows: Step 2.1, the PWM modulation module targets the first signal to be transmitted. bits Combined with the corresponding dynamic threshold Calculate the target launch amplitude with the decision offset Δ ;in, ={ } represents the original binary data sequence. Indicates the first The _i_th bit, ai represents the _i_th bit. The normalized transmit amplitude corresponding to each bit is also used as the PWM duty cycle parameter for that bit. The mapping relationship satisfies: if ,but ;like ,but Taking the first three digits of the original data {0, 1, 0} as an example, combined with the first three dynamic thresholds obtained in step 1.3 and... ,available , , .

[0025] Step 2.2, the PWM modulation module modulates the target transmission amplitude obtained in step 2.1. Mapped to a physical PWM signal, generating a duty cycle of [value] within one bit period. The pulse width modulation waveform. When the total number of sampling points in each bit period is set to N=100, This serves as a control parameter for the duration of the high-level signal within that cycle; that is, This does not represent the number of sampling points themselves, but rather the normalized transmit amplitude and the corresponding PWM duty cycle. The number of high-level sampling points is determined by the duty cycle parameter. The mapping relationship with the number of sampling points N in a single cycle is determined.

[0026] Step 2.3: The transmitting end controls the laser diode through the driving circuit, outputting a PWM signal whose light intensity fluctuates in real time with the duty cycle. Utilizing the fixed start edge and pre-defined bit period of the PWM signal, the receiving end can directly perform bit-synchronous sampling without the need for additional auxiliary signals. For example... Figure 1 As shown, the transmitting amplitude at the transmitting end Around the dynamic random threshold sequence Changes, among which hour Located in the corresponding Above, hour Located in the corresponding Below; as Figure 2 As shown, the starting edge of each bit period remains fixed, while the high-level width within the period varies. It changes with the changes.

[0027] The third step is synchronous demodulation at the receiving end based on a dynamic random threshold sequence; The legitimate receiver sequentially performs photoelectric conversion, edge synchronization, sampling extraction, and dynamic threshold determination on the PWM signal transmitted in the second step through a silicon photodiode, an edge synchronization module, a sampling processing module, and a decision demodulation module, and outputs the demodulated binary bit sequence, specifically: Step 3.1: The silicon photodiode converts the captured PWM signal back into an electrical signal; the edge synchronization module locks the bit period boundary based on the fixed start edge of the PWM signal and the pre-agreed bit period, so that the first... Each sampling interval and the local dynamic random threshold sequence In Correspondingly, the wireless optical communication system uses this to realize the sampling interval and the local dynamic random threshold sequence. Precise alignment.

[0028] Step 3.2: Within the bit period locked in step 3.1, the sampling processing module samples the electrical signal. Let the total number of sampling points within a single bit period be... The number of high-level sampling points is The proportion of high level, i.e., the duty cycle, is: After normalization, the average received amplitude Duty cycle Satisfying linear mapping relationship = ,in Indicates the first The signal amplitude extracted within each bit period. In an additive white Gaussian noise channel environment, the receiver extracts the average received amplitude within each bit period. The Transmission amplitude at the transmitting end Based on this, it is subject to channel noise disturbance.

[0029] Step 3.3: The decision demodulation module obtains the local dynamic random threshold sequence of the legitimate receiver. Dynamic threshold at the corresponding position The signal amplitude extracted in step 3.2 With dynamic threshold Make a real-time judgment; if Then determine the first If each bit is 1; Then determine the first Each bit is 0. For example... Figure 3 As shown, the legitimate receiver possesses a dynamic random threshold sequence synchronized with the sender. It can be based on the dynamic threshold that changes bit by bit. For the received amplitude A judgment is made to recover the original binary data.

[0030] Step 3.4: Repeat steps 3.1 to 3.3 for boundary alignment, sampling extraction, and dynamic threshold decision-making until all processes are complete. Each symbol is output and the demodulated binary bit sequence is generated; the error rate of the valid receiver is statistically analyzed. This indicates that the method can maintain a low bit error rate in the noisy environment.

[0031] Step 4: Fixed threshold judgment analysis of the eavesdropping terminal; The eavesdropping analysis model, without obtaining the dynamically generated random threshold sequence simultaneously from both communicating parties, extracts the amplitude of the intercepted PWM optical signal and uses the global average value of the intercepted signal as a fixed decision threshold for forced demodulation. For example... Figure 4 As shown, a fixed decision threshold cannot adapt to a dynamically random threshold sequence that changes randomly bit by bit. Therefore, it is easy to misclassify items that are below or above a fixed threshold but should actually be determined by the corresponding dynamic threshold. Error determination of the code element in the judgment; statistical analysis shows that the demodulation error rate of the eavesdropping terminal is... Security gain .

[0032] Results analysis: As can be seen, this invention transforms the traditional fixed decision threshold into a dynamic random threshold sequence generated locally and synchronously by both communicating parties, enabling the legitimate receiver to demodulate accurately. Meanwhile, eavesdroppers, unable to discern the changing patterns of the dynamic random threshold sequence, experience a significantly higher demodulation error rate compared to the legitimate receiver, thereby enhancing the physical layer security of wireless optical communication.

[0033] The above embodiments are merely illustrative of the implementation methods of the present invention, but should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the protection scope of the present invention.

Claims

1. A method for enhancing physical layer security in wireless optical communication based on dynamic threshold modulation, characterized in that, The physical layer security enhancement wireless optical communication method is implemented based on a wireless optical communication system. The wireless optical communication system includes a transmitter, a legitimate receiver, and an eavesdropping analysis model. The transmitter includes a raw data generation module, a dynamic random threshold sequence generation module, a PWM modulation module, a driving circuit, and a laser diode. The legitimate receiver includes a silicon photodiode, an edge synchronization module, a sampling processing module, a local dynamic threshold sequence generation module, and a decision demodulation module. The eavesdropping analysis model is used to simulate the process of an unauthorized receiver that has not obtained the dynamic random threshold sequence making a fixed threshold decision on the intercepted optical signal. The physical layer security-enhanced wireless optical communication method includes the following steps: The first step is the generation and synchronization of dynamic random threshold sequences; The sending end generates the original binary data sequence to be sent. The sending end and the legitimate receiving end pre-set communication parameters and random sequence generation rules, and synchronously generate dynamic random threshold sequences locally at both ends. Dynamic random threshold sequence Used for subsequent modulation at the transmitting end and determination of legitimate receiver status; The second step involves PWM modulation and transmission at the transmitting end based on a dynamic random threshold sequence; The PWM modulation module at the transmitting end is based on the dynamic random threshold sequence obtained in the first step. and the original binary data sequence Calculate the emission amplitude corresponding to each bit. and will launch amplitude It is mapped to a physical PWM signal and driven by a driving circuit to drive the laser diode to emit light signals; The third step is synchronous demodulation at the receiving end based on a dynamic random threshold sequence; The legitimate receiver sequentially performs photoelectric conversion, edge synchronization, sampling extraction, and dynamic threshold determination on the PWM signal transmitted in the second step through a silicon photodiode, an edge synchronization module, a sampling processing module, and a decision demodulation module, and outputs the demodulated binary bit sequence. Step 4: Fixed threshold judgment analysis of the eavesdropping terminal; The eavesdropping analysis model extracts the amplitude of the intercepted PWM optical signal without obtaining a dynamic random threshold sequence, and makes a decision using a global mean threshold or a preset fixed threshold.

2. The physical layer security enhancement wireless optical communication method based on dynamic threshold modulation according to claim 1, characterized in that, The first step is specifically as follows: Step 1.1: The raw data generation module generates the raw binary data sequence to be sent. Both communicating parties pre-set the length of the raw binary data sequence to be sent. Threshold dynamic range and the judgment offset Δ; where, Indicates the total number of symbols to be sent. Indicates the lower limit of the dynamic threshold. This represents the upper limit of the dynamic threshold, and Δ represents the emission amplitude offset on both sides of the dynamic threshold. Step 1.2: The dynamic random threshold sequence generation module at the sending end and the local dynamic random threshold sequence generation module at the legitimate receiving end agree on a unified random sequence generation rule, enabling the sending end and the legitimate receiving end to generate completely synchronized dynamic random threshold sequences locally. ; Step 1.3: The dynamic random threshold sequence generation module at the sending end and the local dynamic random threshold sequence generation module at the legitimate receiving end generate a dynamic random threshold sequence of the same length as the data to be sent, based on the threshold dynamic range set in Step 1.1 and the random sequence generation rules agreed upon in Step 1.2, respectively. ={ },in for The dynamic threshold uniformly distributed within the range represents the first... The demodulation threshold for each bit.

3. The physical layer security enhancement wireless optical communication method based on dynamic threshold modulation according to claim 2, characterized in that, In step 1.2, the random sequence generation rules include the initial key, pseudo-random algorithm, sequence indexing method, and threshold value range.

4. The physical layer security enhancement wireless optical communication method based on dynamic threshold modulation according to claim 2, characterized in that, In step 1.3, the dynamic random threshold sequence It is generated through a pseudo-random algorithm, or obtained from random sources such as channel environmental noise after being consistently quantized and synchronized by both communicating parties.

5. The physical layer security enhancement wireless optical communication method based on dynamic threshold modulation according to claim 4, characterized in that, The second step is as follows: Step 2.1, the PWM modulation module targets the first signal to be transmitted. bits Combined with the corresponding dynamic threshold Offset from judgment Calculate the target launch amplitude ;in, ={ } represents the original binary data sequence. Indicates the first bits, Indicates the first The normalized transmit amplitude corresponding to each bit is also used as the PWM duty cycle parameter for that bit; the mapping relationship satisfies: if ,but ;like ,but Among them, the dynamic range of the threshold and the decision offset satisfy... and To ensure Within the effective PWM duty cycle range; Step 2.2, the PWM modulation module modulates the target transmission amplitude obtained in step 2.

1. Mapped to a physical PWM signal, generating a duty cycle of within one bit period. Pulse width modulation waveform; Step 2.3: The transmitting end controls the laser diode through the driving circuit to output a PWM signal whose light intensity fluctuates in real time with the duty cycle; taking advantage of the fixed start edge and pre-defined bit period of the PWM signal, the receiving end can directly perform bit synchronization sampling without the need for additional auxiliary signals.

6. The physical layer security enhancement wireless optical communication method based on dynamic threshold modulation according to claim 5, characterized in that, The third step specifically involves: Step 3.1: The silicon photodiode converts the captured PWM signal back into an electrical signal; the edge synchronization module locks the bit period boundary based on the fixed start edge of the PWM signal and the pre-agreed bit period, so that the first... Each sampling interval and the local dynamic random threshold sequence In Correspondingly, the wireless optical communication system realizes the sampling interval and the local dynamic random threshold sequence. Precise alignment; Step 3.2: Within the bit period locked in step 3.1, the sampling processing module samples the electrical signal. Let the total number of sampling points within a single bit period be... The number of high-level sampling points is The proportion of high level, i.e., the duty cycle, is: ; After normalization, the average received amplitude Duty cycle Satisfying linear mapping relationship = ,in Indicates the first The signal amplitude extracted within each bit period; Step 3.3: The decision demodulation module obtains the local dynamic random threshold sequence of the legitimate receiver. Dynamic threshold at the corresponding position The signal amplitude extracted in step 3.2 With dynamic threshold Make a real-time judgment; if Then determine the first If each bit is 1; Then determine the first Each bit is 0; Step 3.4: Repeat steps 3.1 to 3.3 for boundary alignment, sampling extraction, and dynamic threshold decision-making until all processes are complete. It outputs a demodulated binary bit sequence and a set of symbols.

7. The physical layer security enhancement wireless optical communication method based on dynamic threshold modulation according to claim 6, characterized in that, In the fourth step, the demodulation result of the eavesdropping end is compared with the original binary data sequence to obtain the bit error rate of the eavesdropping end, which is used to evaluate the security enhancement effect of the physical layer security enhancement wireless optical communication method.

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