Physical layer information hiding method based on time-frequency domain joint modulation

Abstract

The invention belongs to the technical field of optical transmission, and particularly relates to a physical layer information hiding method based on time-frequency domain joint modulation. According to the invention, a ciphertext is modulated in a discrete multi-tone modulation mode and then is loaded on a pulse amplitude modulation signal, so that the signal shaped like pulse amplitude modulation is used for optical transmission; the method comprises the specific steps of signal modulation, signal transmission and signal interpretation. According to the method, amplitude information and frequency information are added during optical signal modulation, so that time-frequency domain joint modulation is realized; because the distribution of discrete multi-tone signals is Gaussian distribution, the observation receiving end constellation can only identify pulse amplitude modulation signals, and information hiding is realized. Meanwhile, the plaintext and the ciphertext are transmitted in a fusion manner, the signal-to-noise ratios between the two channels can be selected from each other, the application range is expanded, the method can be integrated into various existing communication systems and networks, and an optional scheme is provided for future network security and special application.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical transmission, and in particular relates to a physical layer information hiding method. Background Art

[0002] As optical fiber transmission is widely used in physical layer communications, information confidentiality and secure transmission have become a major requirement for special applications and national defense security. Confidential transmission at the physical layer is mainly based on information encryption, that is, using keys to convert plaintext information into ciphertext to prevent unauthorized access and understanding of information content. Chaotic encryption algorithms have been proposed for physical layer signal encryption. Their unique chaotic encryption constellation diagrams can prevent receivers that do not have key authorization from recovering signal constellations. However, existing encryption technologies have the problem of poor concealment, and there is a clear difference between encrypted and unencrypted constellations. This allows attackers to easily identify encrypted and unencrypted signals at the physical layer and focus on research. On the other hand, in the fields of image and sound wave transmission, there are currently more concealed information hiding technologies that can integrate ciphertext into normal information, making it difficult for attackers to identify, but the physical layer lacks similar technologies. Summary of the invention

[0003] The purpose of the present invention is to provide a physical layer information hiding method based on time-frequency domain joint modulation with excellent concealment effect.

[0004] The physical layer information hiding method based on time-frequency domain joint modulation provided by the present invention modulates the ciphertext in a discrete multi-tone modulation manner and then loads it onto a pulse amplitude modulation signal, thereby realizing that a signal in the form of pulse amplitude modulation is used for optical transmission. Compared with the traditional physical layer encryption technology, the present invention can hide the ciphertext information, and the signal constellation is no different from the pulse amplitude modulation signal. The specific steps include:

[0005] (1) Signal modulation

[0006] The ciphertext bit information is modulated into a discrete multi-tone modulation format through a discrete multi-tone modulation module; after a single frame of ciphertext information is modulated by discrete multi-tone, the time domain signal after inverse fast Fourier transform is recorded as: DMT (t);

[0007] The pulse amplitude modulation module is used to modulate the plaintext bit information into a pulse amplitude modulation format; the signal of the plaintext information after pulse amplitude modulation is recorded as s PAM (t);

[0008] The peak-to-average ratio and normalized amplitude of the modulated ciphertext signal are controlled through the ciphertext loading module; the signal of hard limiting and maximum amplitude normalization of the ciphertext discrete multi-tone signal is recorded as: s′ DMT(t). The ciphertext signal is further loaded onto the pulse amplitude signal modulated by the plaintext according to the preset loading intensity factor α, and finally a time-frequency domain joint modulation signal is modulated by the ciphertext loading, which includes the plaintext and the hidden ciphertext, and is expressed as: s PAM (t)+α·s′ DMT (t);

[0009] (2) Signal transmission

[0010] Providing an optical carrier signal through a laser;

[0011] A signal generator is used to generate a time-frequency domain jointly modulated digital baseband signal carrying ciphertext and plaintext;

[0012] The digital signal is converted into an analog signal through a digital-to-analog converter (DAC) to drive the optical modulator;

[0013] Amplify tiny electrical signals through an electrical amplifier (EA);

[0014] The intensity of the driving electrical signal is modulated onto the input optical carrier through an intensity modulator;

[0015] amplifying the optical signal output by the intensity modulator through a fiber amplifier;

[0016] Through a section of optical fiber, used to transmit the channel;

[0017] The received optical signal is converted into photoelectricity by a photoelectric detector;

[0018] The received analog signal is converted into a digital signal through an analog-to-digital converter (ADC) for further digital signal processing;

[0019] (3) Information interpretation, including:

[0020] Through filtering algorithms (such as constant modulus algorithm, minimum mean square error algorithm, etc.), the pulse modulation information with inter-symbol interference after transmission is restored;

[0021] Through the plaintext decryption module, the filtered signal y(t) is subjected to PAM judgment and the plaintext information is decrypted;

[0022] Through the ciphertext decryption module, the judged plaintext symbol is remodulated into a PAM signal and subtracted from y(t). The discrete multi-tone modulated signal obtained by the subtraction can be decrypted to obtain the ciphertext information.

[0023] Compared with the prior art, the present invention uses the addition of amplitude information and frequency information during optical signal modulation to achieve joint modulation in the time-frequency domain. Since the discrete multi-tone signal itself is distributed as a Gaussian distribution, the observation of the receiving end constellation can only identify the pulse amplitude modulated signal, thus achieving information hiding. At the same time, the present invention combines plaintext and ciphertext for transmission, and the signal-to-noise ratio between the two channels can be traded off, which expands the scope of application and can be integrated into various existing communication systems and networks, providing a selection solution for future network security and special applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of a communication system using the information hiding method based on time-frequency domain joint modulation proposed by the present invention.

[0025] Figure 2 The OSNR is 15dB, the transmission baud rate is 15GBd, the transmission is 25km single-mode optical fiber, and the signal diagram at the receiving end is shown.

[0026] Figure 3 It is the signal y(t) after digital signal processing (resampling, clock recovery, filtering and equalization).

[0027] Figure 4 The plaintext symbol is remodulated into a PAM signal and subtracted from y(t) to obtain the ciphertext DMT signal diagram.

[0028] Figure 5 The plaintext symbol is remodulated into a PAM signal and subtracted from y(t) to obtain the ciphertext DMT signal spectrum.

[0029] Figure 6 The QPSK signal diagram is obtained by demodulating the DMT signal.

[0030] Numbers in the figure: 1 is the plaintext bit sequence, 2 is the pulse amplitude modulation module, 3 is the ciphertext bit sequence, 4 is the discrete multi-tone modulation module, 5 is the ciphertext loading module, 6 is the signal generator, 7 is the electric amplifier, 8 is the laser source, 9 is the intensity modulator, 10 is the optical fiber amplifier, 11 is the transmission optical fiber, 12 is the receiver, 13 is the receiving end digital signal processing module, 14 is the plaintext decryption module, 15 is the ciphertext signal extraction module, 16 is the ciphertext decryption module, 17 is the time-frequency domain joint modulation signal constellation diagram of the plaintext and ciphertext loaded at the transmitting end, 18 is the signal constellation diagram after filtering and equalization at the receiving end, and 19 is the signal constellation after the ciphertext signal is extracted. DETAILED DESCRIPTION

[0031] The present invention will be further described below in conjunction with the accompanying drawings.

[0032] The present invention proposes a method for hiding information based on time-frequency domain joint modulation for a communication system, and its block diagram is as follows: Figure 1Specifically including:

[0033] In the signal modulation stage, the plaintext bit sequence (1) is modulated using the pulse amplitude modulation module (2), that is, modulated into the PAM4 format, that is, each symbol is represented by one of four levels and carries 2 bits; the ciphertext bit sequence (3) is modulated using the discrete multi-tone modulation module (4), that is, modulated into the QPSK format built into the DMT signal, with an IFFT point number of 128, a cyclic prefix length of 16, and 46 built-in data subcarriers; and the modulated signals of the two are input into the ciphertext loading module (5). The ciphertext loading module loads the ciphertext signal into the plaintext signal. The specific process includes hard limiting and maximum amplitude normalization of the discrete multi-tone modulation (DMT) signal, and loading the ciphertext signal into the pulse amplitude signal modulated by the plaintext according to the preset loading intensity factor α. Finally, after the ciphertext loading, the time-frequency domain joint modulation signal (including the plaintext and the hidden ciphertext) is modulated as follows: s PAM (t)+α·s′ DMT (t); obtain the signal constellation after the ciphertext loading is completed (17).

[0034] In the signal transmission stage, the baseband digital signal is fed into the signal generator module (6), and the baseband signal is processed by the digital signal of the transmitting end (including digital signal processing: resampling, pre-equalization, encoding, etc.), and then converted into an electrical signal. After being amplified by the electrical amplifier (7), it is used to drive the intensity modulator (9) to modulate the signal onto the optical carrier provided by the laser source (8). After being amplified by the optical fiber amplifier (10), the optical signal is transmitted through the optical fiber (11). The receiving end receives the optical signal through the receiver module (12), and the specific process includes photoelectric detection, analog-to-digital conversion, etc. The digital signal is processed by the digital signal processing module (13) of the receiving end (specifically including resampling, clock recovery, filtering and equalization, etc.) and then used for signal interpretation.

[0035] In the signal interpretation stage, the processed signal is first fed into the plaintext interpretation module (14) for interpretation, which specifically includes judgment and decoding. Users who need ciphertext interpretation can additionally access the ciphertext signal extraction module (15) for extracting the ciphertext signal. The specific process is: based on the PAM symbol obtained by plaintext interpretation, the PAM signal is re-modulated, and then the difference is made with the original signal to extract the DMT signal. The DMT signal recovery process (fast Fourier transform, etc.) is further performed to obtain the ciphertext constellation (19); after the ciphertext interpretation module (16) performs judgment and decoding, the ciphertext information can be interpreted.

[0036] Those skilled in the art will appreciate that the above embodiments are specific examples for implementing the present invention, and in actual applications, various changes may be made thereto in form and detail without departing from the spirit and scope of the present invention.

[0037] The present invention has been simulated, specifically as follows: OSNR: 15dB, transmission baud rate 15GBd, transmission 25km single-mode fiber, the receiving end signal is Figure 2 shown.

[0038] The signal y(t) after digital signal processing (resampling, clock recovery, filtering and equalization), Figure 3 shown.

[0039] At this time, the signal at each level is a natural Gaussian distribution, and it is difficult to recognize the discrete multi-tone modulation ciphertext signal modulated in the frequency domain hidden therein.

[0040] By performing PAM4 judgment and decoding on y(t), the plaintext information can be obtained. The bit error rate before forward error correction coding is: 8.42×10 -4 , which is lower than the hard decision threshold of 0.0038 with 7% overhead, reliable transmission can be achieved.

[0041] The plaintext symbol is remodulated into a PAM signal and subtracted from y(t) to obtain the ciphertext DMT signal. Figure 4 As shown, the spectrum is Figure 5 shown.

[0042] Demodulate the DMT signal to obtain the QPSK signal, see Figure 6 shown.

[0043] After alignment, judgment and decoding are performed to obtain ciphertext information, the bit error rate is 0.0379, which is lower than the soft decision threshold of 0.04 with 25% overhead, and reliable transmission can be achieved.

Claims

1. A physical layer information hiding method based on time-frequency domain joint modulation, characterized in that: By modulating the ciphertext in a discrete multi-tone modulation manner and then loading it onto a pulse amplitude modulation signal, a signal in the form of pulse amplitude modulation is used for optical transmission; the specific steps are as follows: (1) Signal modulation The ciphertext bit information is modulated into a discrete multi-tone modulation format through a discrete multi-tone modulation module; after a single frame of ciphertext information is modulated by discrete multi-tone, the time domain signal after inverse fast Fourier transform is recorded as: DMT (t); A pulse amplitude modulation module is used to modulate the plaintext bit information into a pulse amplitude modulation format; The signal of the plaintext information after pulse amplitude modulation is denoted as s PAM (t); The peak-to-average ratio and normalized amplitude of the modulated ciphertext signal are controlled through the ciphertext loading module; the signal of hard limiting and maximum amplitude normalization of the ciphertext discrete multi-tone signal is recorded as: s′ DMT (t); further, the ciphertext signal is loaded into the pulse amplitude signal modulated by the plaintext according to the preset loading intensity factor α, and finally the time-frequency domain joint modulation signal is modulated by the ciphertext loading, which contains the plaintext and the hidden ciphertext, and is expressed as: s PAM (t)+α·s′ DMT (t); (2) Signal transmission Providing an optical carrier signal through a laser; A signal generator is used to generate a time-frequency domain jointly modulated digital baseband signal carrying ciphertext and plaintext; The digital signal is converted into an analog signal through a digital-to-analog converter to drive the optical modulator; Amplify tiny electrical signals through electrical amplifiers; The intensity of the driving electrical signal is modulated onto the input optical carrier through an intensity modulator; amplifying the optical signal output by the intensity modulator through a fiber amplifier; Through a section of optical fiber, used to transmit the channel; The received optical signal is converted into photoelectricity by a photoelectric detector; The received analog signal is converted into a digital signal through an analog-to-digital converter for further digital signal processing; (3) Information interpretation, including: Through filtering algorithms, pulse modulation information with inter-symbol interference after transmission is restored; Through the plaintext decryption module, the filtered signal y(t) is subjected to PAM judgment and the plaintext information is decrypted; Through the ciphertext decryption module, the judged plaintext symbol is remodulated into a PAM signal and subtracted from y(t). The discrete multi-tone modulated signal obtained by the subtraction can be decrypted to obtain the ciphertext information.

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