An independent dual-single-sideband direct detection system based on geometric shaping and digital chaos encryption

Abstract

The present invention discloses an independent dual-single-sideband direct detection system based on geometric shaping and digital chaos encryption, which belongs to the field of terahertz communication. In the detection system, the external cavity laser ECL at the transmitting end generates a continuous optical carrier, which is input to the I / Q modulator. The TX-DSP module uses geometric shaping and three-dimensional multi-vortex chaotic system technology to generate two independent vector radio frequency signals of GS-QPSK and GS-NQAM, drives the I / Q modulator to modulate the input optical carrier, and after amplification, inputs it to a single-row carrier photodetector for beat frequency to obtain an independent double-sideband terahertz signal, which is input to the mixer at the receiving end for mixing with the generated local oscillator signal to achieve signal down-conversion and obtain a baseband signal; then it is input to the RX-DSP module for normalization, down-sampling, signal separation, demapping, symbol-level and bit-level decryption, and finally obtains two channels of original data to calculate the bit error rate. The present invention improves the communication capacity of the system and improves the security of independent double-sideband signal transmission.

Description

Technical Field

[0001] The invention relates to an independent dual-single-sideband direct detection system based on geometric shaping and digital chaos encryption, and belongs to the field of terahertz communication. Background Art

[0002] With the rise of high-bandwidth application scenarios such as cloud computing, virtual / augmented reality, and high-definition video conferencing, higher requirements are placed on the capacity of communication systems.

[0003] To meet the needs of large-capacity data transmission, researchers have successively proposed optical single-sideband (OSSB), optical independent sideband (O-ISB), and double-sideband (DSB) schemes. However, traditional SSB (single-sideband) modulation can only transmit one sideband, while the other sideband is empty. Although typical DSB modulation transmits an additional sideband compared to SSB modulation, its spectral efficiency is reduced by half because both sidebands of the DSB signal carry the same data. In addition, optical filter-based O-ISB modulation requires two optical filters and two photodetectors, making the system design complex and costly.

[0004] Recently, a system for direct detection of independent double-sideband (DSB) signals using an I / Q modulator has attracted the attention of researchers. This system combines the advantages of typical DSB and SSB modulation, improving spectral efficiency and avoiding spectrum waste. It also uses digital signal processing to separate the independent DSB signals, offering advantages over traditional O-ISB modulation, such as a simpler structure and lower cost.

[0005] However, as the communication capacity and number of users increase, this independent double-sideband scheme brings huge challenges to data security. Digital chaotic encryption is applied to physical layer secure communications due to its nonlinearity, randomness, complex and unpredictable key sequences.

[0006] While many different independent double-sideband modulation schemes have been proposed, digital chaotic encryption technology has yet to be incorporated into independent double-sideband signal generation and detection schemes based on an I / Q modulator and a single photodetector. Furthermore, the highest modulation format currently used for independent double-sideband signal generation based on I / Q modulators is 16QAM, which has the limitation of being unable to extend to higher-order modulation formats on either side of the frequency band. Summary of the Invention

[0007] To address the data security challenges associated with high-order modulation and high-capacity communication in existing independent double-sideband (DSB) systems, this paper proposes an independent dual-SSB direct detection system based on geometric shaping and digital chaotic encryption. This system utilizes geometric shaping on both the left and right independent sidebands to achieve high-order modulation of the independent DSBs, thereby increasing the system's communication capacity. Furthermore, a three-dimensional multi-vortex chaotic system is employed to encrypt the transmitted signal at both the bit and symbol levels, enhancing the security of independent DSB signal transmission.

[0008] The independent dual-single-sideband direct detection system includes a transmitter and a receiver. The transmitter introduces geometric shaping and digital chaos encryption technology, uses the photon beat frequency method to generate a terahertz signal, which is then transmitted to the receiver through a single-mode optical fiber, and digital signal processing is used to restore two independent digital vector signals.

[0009] The transmitter includes: external cavity laser ECL, TX-DSP module, I / Q modulator, single-mode fiber, erbium-doped fiber amplifier and single-line carrier photodetector;

[0010] An external cavity laser (ECL) generates a continuous optical carrier and feeds it into an I / Q modulator. The I / Q modulator is also connected to a TX-DSP module, which utilizes geometric shaping and three-dimensional multi-vortex chaotic system techniques to generate two independent vector RF signals, GS-QPSK and GS-NQAM, to drive the I / Q modulator to modulate the input optical carrier. The modulated optical signal is then transmitted via a single-mode fiber to an erbium-doped fiber amplifier for amplification. Finally, it is fed into a single-carrier photodetector for beat frequency analysis, generating an independent double-sideband terahertz signal.

[0011] The receiving end includes: mixer, local oscillator and RX-DSP module;

[0012] The independent double-sideband terahertz signal is input into the mixer and mixed with the local oscillator signal generated by the local oscillator to achieve signal down-conversion and obtain the baseband signal; then it is input into the RX-DSP module, and normalization, down-sampling, cascaded multi-mode equalization, signal separation, blind phase search, symbol-level decryption, demapping and bit-level decryption are performed in sequence to finally obtain two channels of original data and calculate the bit error rate.

[0013] The independent dual-single-sideband direct detection system based on geometric shaping and digital chaos encryption is specifically implemented as follows:

[0014] Step 1: In the TX-DSP module, define the initial key and use the three-dimensional multi-vortex chaotic system based on the Jerk model to solve the three-dimensional multi-vortex chaotic equation to obtain a set of chaotic sequences {x, y, z} corresponding to the initial key.

[0015] The chaotic system is described as a set of ordinary differential equations:

[0016]

[0017] Where x, y, z are three state variables, t is the time step, and a is a constant system parameter.

[0018] Step 2: Perform PRNG operation on the chaotic sequence {x, y, z} to obtain the binary sequence {x', y', z'}.

[0019] The PRNG operation process is: {x', y', z'} = mod {Extract((x, y, z), m), 2} ，

[0020] Where mod{.} represents the modulo operation, and Extract((x,y,z),m) represents the integer obtained by extracting the mth digit of the decimal part of {x,y,z}.

[0021] Step 3: Use two pseudo-random binary code generators to generate two binary sequences PRBS1 and PRBS2, and perform XOR operation with the binary z' sequence to achieve the first layer of bit-level chaotic encryption; then use geometric shaping technology to perform GS-QPSK mapping and GS-NQAM mapping on the encrypted PRBS1 and PRBS2 respectively, to generate two independent digital vector baseband signals S l and S r ;

[0022] Among them S l and S r Represent the left-band and right-band digital vector baseband signals respectively;

[0023] Step 4: Generate the constellation conjugate rotation vector K using the binary x' and y' sequences Conjugate and K Rotation , for the digital vector baseband signal S l and S r Perform symbol-level constellation rotation encryption to obtain a digital vector baseband signal and

[0024] The specific process of constellation rotation encryption is as follows:

[0025] First, check whether the constellation points after symbol mapping have four-fold symmetry. If so, directly perform constellation rotation encryption. Otherwise, it is necessary to first find the four-fold symmetry center, then use translation to make it a four-fold symmetric constellation, and then perform constellation rotation encryption.

[0026] The translation operation of the constellation point is: S l '=S l -S0;Sr '=S r -S0

[0027] S0 represents the coordinates of the center of the four-fold symmetry, S l ' and S r 'represent the quadruple symmetric signals of the left and right bands after translation, respectively.

[0028] Constellation conjugate rotation vector K Conjugate and K Rotation The calculation formula is:

[0029] K Rotation =mod(x'-y',4);

[0030]

[0031] The constellation rotation encryption formula with the signal on the left is as follows:

[0032]

[0033] The constellation rotation encryption formula with the signal on the right is as follows:

[0034]

[0035] Where sng(.) is the symbolic function, S l " indicates S l 'Signal after constellation rotation encryption; S r " indicates S r 'Signal after constellation rotation encryption;

[0036] The inverse translation signal after constellation rotation encryption is:

[0037]

[0038] Step 5: Signal and After upsampling and root raised cosine filtering, the frequencies are f s and -f s The digital local oscillator source is digitally mixed to achieve digital up-conversion processing and the carrier frequencies are f s and -f s Two independent vector RF signals.

[0039] Step 6: Separate the real and imaginary parts of the two independent vector RF signals and feed them into the in-phase I and quadrature-phase Q inputs of the same dual-channel digital-to-analog converter to generate the driving signals for the I / Q modulator. and

[0040] Step 7: Use a single-mode laser to generate a center frequency of f c Continuous optical carrier and input into I / Q modulator; at the same time, the driving signal and drive the in-phase and quadrature components of the I / Q modulator respectively. The I / Q modulator output contains f c -f s and f c +f s A carrier signal with two frequency components.

[0041] Step 8: After the two carrier signals are amplified by the erbium-doped fiber amplifier through the single-mode fiber, they are transmitted to the same single-line carrier photodetector to beat the frequency, generating a frequency of 2f s of terahertz signals.

[0042] Step 9: The terahertz signal is down-converted and filtered to obtain an independent double-sideband baseband vector signal. The received signal is restored to its original binary information sequence through normalization, downsampling, CMMA, BPS, signal separation, symbol-level decryption, demapping, and bit-level decryption. The signal is then compared with the generated PRBS1 and PRBS2 to calculate the bit error rate.

[0043] The principle of the signal separation is as follows:

[0044] The expression for synthesizing the two carrier signals modulated by the I / Q modulator is:

[0045]

[0046] Among them E CW (t) represents the emission power of the external cavity laser, J -1 (.) and J1(.) are the first kind of Bessel functions, β is the modulation depth of the modulator, A l ,A r ω l ,ω r , Represent the amplitude, angular frequency and phase of the left and right band signals respectively.

[0047] After being modulated by the I / Q modulator, the left and right band carrier signals beat against each other at the photodetector and beat against the other side bands. The generated photocurrent can be used to separate the left and right band signals using the phase mapping relationship.

[0048] The photocurrent is expressed as:

[0049]

[0050] Where R represents the photoelectric conversion efficiency, and A0 represents the DC voltage introduced by the non-ideality of the modulator bias voltage.

[0051] The first term in the above equation is the target received signal, and the remaining terms are the noise generated by the signal beat frequency;

[0052] The amplitude of the received signal in the first term is related to J -1 (βA l )J1(βA r ) is related to the frequency (ω l +ω r ) is the sum of the frequencies of the left and right signals, and the phase It is the phase sum of the left and right band signals.

[0053] The advantages of the present invention are:

[0054] 1. An independent dual-SSB direct detection system based on geometric shaping and digital chaotic encryption generates chaotic sequences through a three-dimensional multi-vortex chaotic system. Utilizing XOR encryption using PRNG operations and constellation encryption using conjugate rotation vector operations, this system achieves both bit-level and symbol-level dual encryption of independent double-sideband signals, thus improving the security of the independent double-sideband direct detection system.

[0055] 2. An independent dual-SSB direct detection system based on geometric shaping and digital chaos encryption. This system uses geometric shaping technology to adjust the coordinate mapping relationship between the constellation points on the left and right sidebands. This overcomes the limitations of typical independent double-sideband systems, which suffer from a single mapping scheme and an inability to perform high-order modulation. This effectively increases channel capacity and provides a high-order modulation scheme for independent double-sideband signals.

[0056] 3. An independent dual-single-sideband direct detection system based on geometric shaping and digital chaos encryption, which can achieve two independent left and right sidebands with a resolution of about 10 46 Encrypted space and 10 -15 The key sensitivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 This is a structural diagram of an independent dual-single-sideband direct detection system based on geometric shaping and digital chaos encryption of the present invention;

[0058] Figure 2 This is an optional flow chart of an independent dual-single-sideband system based on geometric shaping and digital chaos encryption according to the present invention;

[0059] Figure 3 is an optional phase diagram of a three-dimensional multi-vortex chaotic system according to an embodiment of the present invention;

[0060] Figure 4 This is a schematic diagram of an optional ideal constellation diagram and mapping relationship according to an embodiment of the present invention;

[0061] Figure 5 is a schematic diagram of an optional constellation rotation encryption according to an embodiment of the present invention;

[0062] Figure 6 is a spectrum diagram of an optional I / Q modulator output signal according to an embodiment of the present invention;

[0063] Figure 7 is a constellation diagram restored by a receiving end according to an embodiment of the present invention;

[0064] Figure 8 is a graph showing the relationship between the bit error rate and the received optical power at the receiving end under BTB and 20 km single-mode optical fiber transmission according to an embodiment of the present invention; DETAILED DESCRIPTION

[0065] To facilitate understanding and implementation of the present invention by those skilled in the art, the present invention is further described below in detail with reference to the accompanying drawings and embodiments. It is apparent that the embodiments described are merely partial embodiments of the present invention, not all embodiments. All other embodiments derived by those skilled in the art based on the embodiments of the present invention without creative effort shall fall within the scope of protection of the present invention.

[0066] The present invention proposes an independent dual-single-sideband direct detection system based on geometric shaping and digital chaotic encryption. Geometric shaping technology is introduced at the transmitting end to increase the system transmission capacity. At the same time, a three-dimensional multi-vortex chaotic system is used to perform digital chaotic encryption on the transmitting end signal, thereby improving the security of the independent dual-single-sideband direct detection system.

[0067] The independent dual single sideband direct detection system is as follows Figure 1 As shown in the figure, it includes a transmitter and a receiver. The transmitter introduces geometric shaping and digital chaos encryption technology, uses the photon beat frequency method to generate terahertz signals, and then transmits them through a 20km single-mode optical fiber. The receiver uses digital signal processing to recover two independent digital vector signals through normalization, downsampling, CMMA, BPS, signal separation, chaos decryption and demapping operations.

[0068] The sending end includes:

[0069] An external cavity laser (ECL), the output of which serves as the input optical carrier of the I / Q modulator.

[0070] The TX-DSP module uses geometric shaping technology and a three-dimensional multi-vortex chaotic system to generate two independent vector RF signals: encrypted GS-QPSK and GS-NQAM (N=16 in this example), which are used to drive the I / Q modulator to modulate the optical carrier.

[0071] I / Q modulator;

[0072] 20 km single-mode optical fiber (SSMF) is used to transmit the modulated optical signal.

[0073] Erbium-doped fiber amplifier (EDFA) is used to amplify optical signals after being transmitted through single-mode optical fiber.

[0074] A single-column charge carrier photodetector (UTC-PD) is used to beat the optical signal to obtain an independent double-sideband terahertz signal.

[0075] An external cavity laser (ECL) is connected to an I / Q modulator, which generates a continuous optical carrier for input. The I / Q modulator is also connected to a TX-DSP module, which uses geometric shaping and three-dimensional multi-vortex chaotic system technology to generate two independent vector RF signals: GS-QPSK and GS-NQAM (N represents the modulation order). These signals drive the I / Q modulator to modulate the input optical carrier. The modulated optical signal is then transmitted over a 20km single-mode fiber and amplified by an erbium-doped fiber amplifier. Finally, it is input to a single-carrier photodetector (UTC-PD) for beat frequency analysis, generating an independent double-sideband terahertz signal.

[0076] The receiving end includes:

[0077] The mixer is used to down-convert the received signal to obtain a baseband signal.

[0078] Local oscillator (LO), which serves as the input to the mixer.

[0079] The RX-DSP module performs digital signal processing, including normalization, downsampling, CMMA, BPS, signal separation, symbol-level decryption, demapping, bit-level decryption, and bit error rate calculation.

[0080] The independent double-sideband terahertz signal is input into a mixer and mixed with the local oscillator signal generated by the local oscillator to achieve signal down-conversion and obtain the baseband signal. This is then input into the RX-DSP module for normalization, downsampling, cascaded multimode equalization, signal separation, blind phase search, symbol-level decryption, demapping, and bit-level decryption. Ultimately, the two channels of raw data are obtained and the bit error rate is calculated.

[0081] The independent dual-single-sideband direct detection system based on geometric shaping and digital chaos encryption is as follows: Figure 2 As shown, the implementation process is as follows:

[0082] Step 1: In the TX-DSP module, define the initial key and use the three-dimensional multi-vortex chaotic system based on the Jerk model to solve the three-dimensional multi-vortex chaotic equation to obtain a set of chaotic sequences {x, y, z}.

[0083] The initial key is {x0=-0.1, y0=0.05, z0=0.1}

[0084] The phase diagram of the chaotic system is shown in Figure 3 As shown, it can be described as a set of ordinary differential equations:

[0085]

[0086] Among them, x, y, and z are three state variables, t is the time step, and its value is 0.01; a is a constant system parameter. When a = 0.6, the multi-vortex chaotic system exhibits hyperchaotic behavior.

[0087] Step 2: Perform PRNG operation on the three state variables of the chaotic sequence {x, y, z} to obtain the binary sequence {x', y', z'};

[0088] The PRNG operation process is {x', y', z'} = mod {Extract((x, y, z), m), 2};

[0089] Where mod{.} represents a modulo operation, and Extract((x,y,z),m) represents an integer obtained by extracting the mth digit of the decimal part of {x,y,z}. In this example, m=15.

[0090] Step 3: Use two pseudo-random binary code generators to generate two binary sequences (Pseudo-Random Binary Sequences), expressed as PRBS1 and PRBS2, and perform bit-by-bit XOR operations on the generated PRBS1 and PRBS2 and the binary z' sequence to achieve the first layer of bit-level chaotic encryption. Then, use geometric shaping technology to perform GS-QPSK mapping and GS-NQAM mapping on the encrypted PRBS1 and PRBS2 respectively, generating two independent digital vector baseband signals S l and S r ;

[0091] Among them S l and S r Represent the left and right digital vector baseband signals respectively; the constellation diagram is as follows Figure 4 (a) and Figure 4 (b) shown.

[0092] Step 4: Generate the constellation conjugate rotation vector K using the x' and y' sequences generated in step 2 Conjugate and K Rotation , the digital vector baseband signal S generated in step 3 l and S r Perform symbol-level constellation rotation encryption, and the signals after constellation rotation encryption are expressed as and

[0093] The constellation points of GS-QPSK signal have four-fold symmetry, which can be directly encrypted by constellation rotation. Taking the constellation point with coordinates (0, 1) as an example, the encryption process is shown as follows: Figure 5 (ac) shown. Figure 5 (a), (b), and (c) represent the constellation diagrams when not encrypted, and The encrypted constellation diagram in this case.

[0094] The constellation points of the GS-16QAM signal do not have four-fold symmetry. First, we need to find the coordinates of the four-fold symmetry center (4, 4), then use translation to make it a four-fold symmetric constellation and then perform constellation rotation encryption. Finally, after the encryption is completed, perform reverse translation to restore it to the original constellation. Taking the constellation point with coordinates (5, 7) as an example, the encryption process is shown as follows Figure 5 (dh) shown. Figure 5 (d) represents the original constellation diagram, Figure 5 (e) shows the constellation diagram after four-fold symmetric translation, Figure 5 (f) and (g) represent and The encrypted constellation diagram in the case of Figure 5 (h) shows the encrypted constellation diagram after inverse translation.

[0095] The specific process of constellation rotation encryption is as follows:

[0096] First, check whether the constellation points after symbol mapping have four-fold symmetry. If so, directly perform constellation rotation encryption. Otherwise, it is necessary to first find the four-fold symmetry center, then use translation to make it a four-fold symmetric constellation, and then perform constellation rotation encryption.

[0097] The translation operation of the constellation point is: S l '=S l -S0;S r '=S r -S0

[0098] The translation operation ensures the specific four-fold symmetry of the constellation points, where S0 represents the coordinates of the four-fold symmetry center, S l ' and S r 'represent the quadruple symmetric signals of the left and right bands after translation, respectively.

[0099] Constellation conjugate rotation vector K Conjugate and K Rotation The calculation formula is:

[0100] K Rotation =mod(x'-y',4);

[0101]

[0102] The constellation rotation encryption formula with the signal on the left is as follows:

[0103]

[0104] The constellation rotation encryption formula with the signal on the right is as follows:

[0105]

[0106] Where sng(.) is the sign function, S l " indicates S l 'Signal after constellation rotation encryption; S r " indicates S r 'Signal after constellation rotation encryption;

[0107] The inverse translation signal after constellation rotation encryption is:

[0108]

[0109] Step 5: Encrypt the digital vector baseband signal ( and ) after upsampling and root raised cosine filtering, and then using the frequencies f s and -f s The digital local oscillator source is digitally mixed to achieve digital up-conversion processing and the carrier frequencies are f s and -f s Two independent vector RF signals.

[0110] This example uses a digital vector baseband signal ( and ) After 8 times upsampling and passing through a root raised cosine filter with a roll-off factor of 0.01, digital mixing is performed using digital local oscillator sources with frequencies of 60 GHz and -60 GHz respectively to achieve digital up-conversion processing, resulting in two independent vector RF signals with carrier frequencies of 60 GHz and -60 GHz respectively.

[0111] Step 6: Separate the real and imaginary parts of the two independent vector RF signals and feed them into the in-phase I and quadrature-phase Q inputs of the same dual-channel digital-to-analog converter to generate the driving signals for the I / Q modulator. and

[0112] Step 7: Use a single-mode laser to generate a center frequency of f c The continuous optical carrier is input into the I / Q modulator. At the same time, the and Drive the in-phase and quadrature components of the I / Q modulator respectively, and the output contains f c -f s and f c +f s A carrier signal with two frequency components.

[0113] f c The value is 193.1THz; the I / Q modulator outputs a carrier signal containing two frequency components, 193.04THz and 193.16THz, and its spectrum is as follows Figure 6 shown.

[0114] Step 8: After the two carrier signals are transmitted through a 20km single-mode fiber and amplified by an erbium-doped fiber amplifier, they are transmitted to the same single-line carrier photodetector to beat the frequency, generating a frequency of 2f s Terahertz signal.

[0115] The frequency of the terahertz signal generated in this example is 120 GHz.

[0116] Step 9: The terahertz signal is down-converted and filtered to obtain an independent double-sideband baseband vector signal. The signal is normalized, down-sampling, CMMA, BPS, signal separation, symbol-level decryption, demapping, and bit-level decryption to restore the original binary information sequence. The received signal is then compared with the PRBS1 and PRBS2 generated in step 1 to calculate the bit error rate.

[0117] In this example, the terahertz signal is down-converted and filtered to obtain an independent double-sideband baseband vector signal. After normalization, 8-fold downsampling, CMMA with a step size of 0.01, and BPS equalization, the received 64QAM signal constellation diagram is as follows: Figure 7 shown.

[0118] The independent double-sideband 64QAM signals are then separated. The signal separation principle is as follows:

[0119] The optical signal modulated by the I / Q modulator can be expressed as:

[0120]

[0121] Among them E CW (t) represents the external cavity laser emission power, J -1 (.) and J1(.) are the first kind of Bessel functions, β is the modulation depth of the modulator, A l ,A r ω l ,ω r , Represent the amplitude, angular frequency and phase of the left and right band signals respectively.

[0122] The modulated left and right side signals beat against each other and the other side bands at the photodetector, generating photocurrents that can be separated from the left and right side signals using phase mapping.

[0123] The photocurrent is expressed as:

[0124]

[0125] Where R represents the photoelectric conversion efficiency, and A0 represents the DC voltage introduced by the non-ideality of the modulator bias voltage.

[0126] The first term in the above equation is the target received signal, and the second to sixth terms are the noise generated in the signal beat frequency, which can be removed by an electrical filter in digital signal processing.

[0127] From the first term of the above formula, we can know that the amplitude of the received signal is related to J -1 (βA l )J1(βA r ) is related to the frequency (ω l +ω r ) is the sum of the frequencies of the left and right signals, and the phase It is the phase sum of the left and right band signals.

[0128] The mapping relationship is as follows Figure 4 As shown in (a), (b), and (c).

[0129] Then use the constellation conjugate rotation vector and Perform symbol-level decryption and demapping, perform bit-level decryption on the z' sequence generated in step 2, and restore the original binary information sequence of the received signal. Then compare it with PRBS1 and PRBS2 generated in step 1 to calculate the bit error rate.

[0130] Security concerns focus on the bit error rate. Ideally, the bit error rate for legally decrypting an encrypted signal is 0, but when encountering illegal decryption, the bit error rate is around 0.5 (i.e., the encrypted signal cannot be decrypted). Therefore, the bit error rate can reflect the security of the encryption process. Figure 8 As shown in the figure, the system's BER performance gradually improves with increasing received optical power. At the same time, when the communication system encounters brute force attacks, the system's receiving-end BER remains around 0.5, indicating that the system effectively enhances data security and prevents unauthorized users from stealing information.

[0131] The communication capacity problem is solved by increasing the modulation order. The higher the modulation order, the greater the communication capacity.

[0132] The highest modulation order achieved by previous independent double-sideband systems was 16QAM, and the mapping method was fixed, making high-order expansion impossible.

[0133] The present invention uses geometric shaping to change the coordinates of the left and right sideband constellation points, thereby changing the mapping relationship. The left sideband maintains GS-QPSK mapping, while the right sideband performs GS-NQAM mapping, where N can be 4, 16, 64, etc., and the corresponding modulation order can be 16QAM (4*4), 64QAM (4*16), 256QAM (4*64), etc., increasing the modulation order and achieving an increase in communication capacity.

Claims

1. An independent dual-single-sideband direct detection system based on geometric shaping and digital chaos encryption, characterized in that: The system includes a transmitter and a receiver. The transmitter introduces geometric shaping and digital chaos encryption technology, uses photon beat frequency to generate terahertz signals, and then transmits them to the receiver through a single-mode optical fiber. Digital signal processing is used to recover two independent digital vector signals. The transmitter includes: external cavity laser ECL, TX-DSP module, I / Q modulator, single-mode fiber, erbium-doped fiber amplifier and single-line carrier photodetector; The receiving end includes: mixer, local oscillator and RX-DSP module; The specific implementation process of the independent dual single-sideband direct detection system is as follows: Step 1: In the TX-DSP module, define the initial key and use the three-dimensional multi-vortex chaotic system based on the Jerk model to solve the three-dimensional multi-vortex chaotic equation to obtain a set of chaotic sequences {x, y, z} corresponding to the initial key; Step 2: Perform PRNG operation on the chaotic sequence {x, y, z} to obtain the binary sequence {x', y', z'}; The PRNG operation process is: {x', y', z'} = mod{Extract((x, y, z), m), 2}; Where mod{.} represents the modulo operation, and Extract((x,y,z),m) represents the integer obtained by extracting the mth digit of the decimal part of {x,y,z}; Step 3: Use two pseudo-random binary code generators to generate two binary sequences PRBS1 and PRBS2, and perform XOR operation with the binary z' sequence to achieve the first layer of bit-level chaotic encryption; use geometric shaping technology to perform GS-QPSK mapping and GS-NQAM mapping on the encrypted PRBS1 and PRBS2 respectively, to generate two independent digital vector baseband signals S l and S r ; Among them S l and S r Represent the left-band and right-band digital vector baseband signals respectively; Step 4: Generate the constellation conjugate rotation vector K using the binary x' and y' sequences Conjugate and K Rotation , for the digital vector baseband signal S l and S r Perform symbol-level constellation rotation encryption to obtain a digital vector baseband signal and Step 5: Signal and After upsampling and root raised cosine filtering, the frequencies are f s and -f s The digital local oscillator source is digitally mixed to achieve digital up-conversion processing and the carrier frequencies are f s and -f s Two independent vector RF signals; Step 6: Separate the real and imaginary parts of the two independent vector RF signals and feed them into the in-phase I and quadrature-phase Q inputs of the same dual-channel digital-to-analog converter to generate the driving signals for the I / Q modulator. and Step 7: Use the external cavity laser ECL to generate a center frequency of f c Continuous optical carrier and input into I / Q modulator; at the same time, the driving signal and drive the in-phase and quadrature components of the I / Q modulator respectively. The I / Q modulator output contains f c -f s and f c +f s A carrier signal with two frequency components; Step 8: After the carrier signal is transmitted through a single-mode fiber and amplified by an erbium-doped fiber amplifier, it is transmitted to the same single-line carrier photodetector to generate a beat frequency of 2f s The terahertz signal; Step 9: The independent double-sideband terahertz signal is input into the mixer and mixed with the local oscillator signal generated by the local oscillator to achieve signal down-conversion and filtering processing to obtain an independent double-sideband baseband vector signal; then it is input into the RX-DSP module and normalized, down-sampled, cascaded multi-mode equalization, signal separation, blind phase search, symbol-level decryption, demapping and bit-level decryption are performed in sequence to restore the original binary information sequence of the received signal and compare it with the generated PRBS1 and PRBS2 to calculate the bit error rate.

2. The independent dual-single-sideband direct detection system based on geometric shaping and digital chaos encryption according to claim 1, characterized in that: In step 4, the specific process of constellation rotation encryption is as follows: First, check whether the constellation points after symbol mapping have four-fold symmetry. If so, directly perform constellation rotation encryption; otherwise, it is necessary to first find the four-fold symmetry center, then use translation to make it a four-fold symmetric constellation, and then perform constellation rotation encryption; The translation operation of the constellation point is: S l '=S l -S0;S r '=S r -S0 S0 represents the coordinates of the center of the four-fold symmetry, S l ' and S r 'represent the quadruple symmetric signals of the left and right bands after translation; Constellation conjugate rotation vector K Conjugate and K Rotation The calculation formula is: K Rotation =mod(x'-y',4); The constellation rotation encryption formula with the signal on the left is as follows: The constellation rotation encryption formula with the signal on the right is as follows: Where sng(.) is the symbolic function, S l " indicates S l 'Signal after constellation rotation encryption; S r " indicates S r 'Signal after constellation rotation encryption; The inverse translation signal after constellation rotation encryption is:

3. The independent dual-single-sideband direct detection system based on geometric shaping and digital chaos encryption according to claim 1, characterized in that: In step 1, the chaotic system is described as a set of ordinary differential equations: Where x, y, z are three state variables, t is the time step, and a is a constant system parameter.

4. The independent dual-single-sideband direct detection system based on geometric shaping and digital chaos encryption according to claim 1, characterized in that: In step nine, the principle of signal separation is as follows: The expression for synthesizing the two carrier signals modulated by the I / Q modulator is: Among them E CW (t) represents the emission power of the external cavity laser, J -1 (.) and J1(.) are the first kind of Bessel functions, β is the modulation depth of the modulator, A l ,A r ω l ,ω r , Represent the amplitude, angular frequency and phase of the left and right band signals respectively; After being modulated by the I / Q modulator, the left and right band carrier signals beat against each other at the photodetector and beat against the other side bands. The generated photocurrent can be used to separate the left and right band signals using the phase mapping relationship. The photocurrent is expressed as: Where R represents the photoelectric conversion efficiency, A0 represents the DC voltage introduced by the non-ideality of the modulator bias voltage; The first term in the above equation is the target received signal, and the remaining terms are the noise generated by the signal beat frequency; The amplitude of the received signal in the first term is related to J -1 (βA l )J1(βA r ) is related to the frequency (ω l +ω r ) is the sum of the frequencies of the left and right signals, and the phase It is the phase sum of the left and right band signals.

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