Independent double-single-side-band direct detection system based on geometric shaping and digital chaotic encryption

Abstract

The invention discloses an independent double-single-side-band direct detection system based on geometric shaping and digital chaotic encryption, and belongs to the field of terahertz communication. According to the detection system, an external cavity laser ECL of a sending end generates continuous optical carriers, the continuous optical carriers are input into an I / Q modulator, a TX-DSP module generates two independent vector radio frequency signals of GS-QPSK and GS-NQAM through geometric shaping and a three-dimensional multi-vortex chaos system technology, the I / Q modulator is driven to modulate the input optical carriers, and the optical carriers are transmitted to a receiver. After being amplified, the terahertz signals are input into a one-way carrier photoelectric detector for frequency beating to obtain independent double-sideband terahertz signals, the independent double-sideband terahertz signals are input into a frequency mixer of a receiving end for frequency mixing with generated local oscillation signals, down-conversion of the signals is achieved, and fundamental frequency signals are obtained; and then the data are input to an RX-DSP module, normalization, down-sampling, signal separation, de-mapping, symbol-level and bit-level decryption and other operations are carried out, and finally two paths of original data are obtained to solve the bit error rate. According to the invention, the system communication capacity is improved, and the security of independent double-sideband signal transmission is improved.

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

[0004] Recently, an independent double-sideband direct detection system based on I / Q modulator has attracted the attention of researchers. This system combines the advantages of typical double-sideband and single-sideband modulation, improves spectrum efficiency, and avoids spectrum waste. At the same time, digital signal processing technology is used to separate independent double-sideband signals. Compared with traditional O-ISB modulation, it has the advantages of simple structure and low cost.

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

[0006] At present, although many different schemes have been proposed for independent double-sideband modulation, the independent double-sideband signal generation and detection scheme based on I / Q modulator and single photodetector has not yet introduced digital chaos encryption technology. At the same time, the highest modulation format of independent double-sideband signal generation based on I / Q modulator in recent years is 16QAM, and it has the limitation that it cannot be extended to high-order modulation formats on both side bands. Summary of the invention

[0007] In view of the data security issues caused by the difficulty of high-order modulation and large-capacity communication in existing independent double-sideband systems, the present invention proposes an independent dual-single sideband direct detection system based on geometric shaping and digital chaos encryption, which uses geometric shaping for the left and right independent sidebands to achieve high-order modulation of independent double-sidebands, thereby improving the communication capacity of the system. At the same time, the transmission signal is encrypted at the bit level and symbol level using a three-dimensional multi-vortex chaotic system, thereby improving the security of independent double-sideband signal transmission.

[0008] The independent dual single-sideband direct detection system includes a transmitting end and a receiving end. The transmitting end 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 receiving end through a single-mode optical fiber, and uses digital signal processing 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] The external cavity laser ECL generates a continuous optical carrier and inputs it to the I / Q modulator; the I / Q modulator is also connected to the 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, to drive the I / Q modulator to modulate the input optical carrier. The modulated optical signal is transmitted to the erbium-doped fiber amplifier through a single-mode optical fiber for amplification. Finally, it is input to a single-line carrier photodetector for beat frequency to obtain 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 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 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] Where 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, and then use translation to make it a four-fold symmetric constellation before performing 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.

[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 sign 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 send them to the in-phase I and quadrature-phase Q inputs of the same dual-channel digital-to-analog converter to generate the driving signal of the I / Q modulator. and

[0040] 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 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 generate a beat frequency of 2f s of terahertz signals.

[0042] Step 9: The terahertz signal is processed by down-conversion and filtering to obtain an independent double-sideband baseband vector signal. The received signal is restored to the original binary information sequence through normalization, down-sampling, CMMA, BPS, signal separation, symbol-level decryption, demapping and bit-level decryption. It 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 of synthesizing two carrier signals modulated by the I / Q modulator is:

[0045]

[0046] Where 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] The left and right side carrier signals modulated by the I / Q modulator beat each other at the photodetector and beat at the other side bands. The generated photocurrent can be used to separate the left and right side 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 formula is the target received signal, and the remaining terms are the noise generated in 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 single-sideband direct detection system based on geometric shaping and digital chaos encryption generates a chaotic sequence through a three-dimensional multi-vortex chaotic system, and uses the XOR encryption of PRNG operation and the constellation encryption of conjugate rotation vector operation to achieve bit-level and symbol-level dual encryption of independent double-sideband signals, thereby improving the security of the independent double-sideband direct detection system.

[0055] 2. An independent dual-single sideband direct detection system based on geometric shaping and digital chaos encryption adjusts the coordinate mapping relationship of the constellation points on the left and right sidebands through geometric shaping technology, overcoming the problem of a single mapping method and inability to perform high-order modulation in a typical independent double-sideband system. It can effectively improve channel capacity and provide 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. Compared with the typical independent double sideband system, the left and right independent sidebands are 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 It is an optional independent dual single sideband system flow chart based on geometric shaping and digital chaos encryption of 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 is an optional ideal constellation diagram and a schematic diagram of a 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 20km single-mode optical fiber transmission according to an embodiment of the present invention; DETAILED DESCRIPTION

[0065] In order to facilitate those skilled in the art to understand and implement the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. Obviously, the described embodiments are only partial embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work should 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. The geometric shaping technology is introduced at the transmitting end to achieve the effect of increasing the transmission capacity of the system. 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, it includes a transmitting end and a receiving end. The transmitting end introduces geometric shaping and digital chaos encryption technology, uses the photon beat frequency method to generate a terahertz signal, and then transmits it through a 20km single-mode optical fiber. The receiving end 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 the ECL is used 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 of encrypted GS-QPSK and GS-NQAM (N=16 in this example) to drive the I / Q modulator to modulate the optical carrier.

[0071] I / Q modulator;

[0072] 20km 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] The single-column charge carrier photodetector (UTC-PD) is used to beat the optical signal to obtain an independent double-sideband terahertz signal.

[0075] The external cavity laser ECL is connected to the I / Q modulator, which generates a continuous optical carrier and inputs it to the I / Q modulator; the I / Q modulator is also connected to the 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), to drive the I / Q modulator to modulate the input optical carrier. The modulated optical signal is transmitted through a 20km single-mode optical fiber, and the transmitted optical signal is amplified by an erbium-doped fiber amplifier. Finally, it is input to a single-line carrier photodetector (UTC-PD) for frequency beating to obtain 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, bit error rate calculation and other operations.

[0080] The independent double-sideband terahertz signal is input into the mixer to mix 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 for normalization, down-sampling, cascaded multi-mode equalization, signal separation, blind phase search, symbol-level decryption, demapping and bit-level decryption, and finally two channels of original data are obtained to calculate the bit error rate.

[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, use the three-dimensional multi-vortex chaotic system based on the Jerk model, 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] Wherein 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, and generate two independent digital vector baseband signals S l and S r ;

[0091] Where S l and S r Represent the left and right digital vector baseband signals respectively; its constellation diagram is as follows Figure 4 (a) and Figure 4 (b) as 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 The symbol-level constellation rotation encryption is performed, and the signals after constellation rotation encryption are expressed as and

[0093] The constellation points of the GS-QPSK signal have four-fold symmetry, and 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 diagram 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 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, and then use translation to make it a four-fold symmetric constellation before performing 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.

[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, and two independent vector RF signals with carrier frequencies of 60 GHz and -60 GHz are obtained.

[0111] Step 6: Separate the real and imaginary parts of the two independent vector RF signals and send them to the in-phase I and quadrature-phase Q inputs of the same dual-channel digital-to-analog converter to generate the driving signal of 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. 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 shown in the figure Figure 6 shown.

[0114] Step 8: After the two carrier signals are transmitted through a 20km single-mode optical fiber and amplified by an erbium-doped optical fiber amplifier, they are transmitted to the same single-line carrier photodetector to generate a beat 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 processed by down-conversion and filtering to obtain an independent double-sideband baseband vector signal. The signal is normalized, down-sampled, CMMA, BPS, signal separation, symbol-level decryption, demapping and bit-level decryption to restore the received signal to the original binary information sequence, and 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] Then the independent double-sideband 64QAM signal is separated. The signal separation principle is as follows:

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

[0120]

[0121] Where 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 band signals beat each other at the photodetector and beat the other side bands, and the generated photocurrent can be used to separate the left and right side band signals by using the phase mapping relationship;

[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 formula 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, demapping, and bit-level decryption by performing bit-by-bit XOR operations on the z' sequence generated in step 2. Restore the received signal to the original binary information sequence, and 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 encrypted signals during legal decryption 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 size of the bit error rate can reflect the security of the encryption process. Figure 8 As shown in the figure, it can be seen that with the increase of received optical power, the BER performance of the system gradually improves. At the same time, when the communication system encounters brute force cracking, the BER of the system receiving end remains at around 0.5, indicating that the system effectively enhances data security and prevents illegal 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] In the present invention, geometric shaping is used to change the coordinates of the left and right sideband constellation points, thereby changing the mapping relationship. The left sideband maintains GS-QPSK mapping, and the right sideband performs GS-NQAM mapping, where N can be 4, 16, 64, ..., and the corresponding modulation order can be 16QAM (4*4), 64QAM (4*16), 256QAM (4*64), ..., etc., which increases the modulation order and realizes the increase of communication capacity.

Claims

1. An independent dual single-sideband direct detection system based on geometric shaping and digital chaos encryption, characterized in that: 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 to the receiver through a single-mode optical fiber. Digital signal processing is used to restore 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 external cavity laser ECL generates a continuous optical carrier and inputs it to the I / Q modulator; the I / Q modulator is also connected to the 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, to drive the I / Q modulator to modulate the input optical carrier. The modulated optical signal is transmitted to the erbium-doped fiber amplifier through a single-mode optical fiber for amplification; finally, it is input to a single-line carrier photodetector for beat frequency, and an independent double-sideband terahertz signal is obtained; The receiving end includes: mixer, local oscillator and RX-DSP module; 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 normalized, down-sampled, cascaded multi-mode equalized, signal separated, 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.

2. The independent dual single sideband direct detection system based on geometric shaping and digital chaos encryption as claimed in claim 1, characterized in that: The specific implementation process 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, and generate two independent digital vector baseband signals S l and S r ; Where 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 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, and then use translation to make it a four-fold symmetric constellation before performing 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 sign 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: 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 send them to the in-phase I and quadrature-phase Q inputs of the same dual-channel digital-to-analog converter to generate the driving signal of the I / Q modulator. and 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 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 the single-mode fiber and amplified by the 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 terahertz signal is processed by down-conversion and filtering to obtain an independent double-sideband baseband vector signal. The received signal is restored to the original binary information sequence through normalization, down-sampling, CMMA, BPS, signal separation, symbol-level decryption, demapping and bit-level decryption, and then compared with the generated PRBS1 and PRBS2 to calculate the bit error rate.

3. The independent dual single sideband direct detection system based on geometric shaping and digital chaos encryption as claimed in claim 2, 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 as claimed in claim 2, characterized in that: In the step nine, The principle of the signal separation is as follows: The expression of synthesizing two carrier signals modulated by the I / Q modulator is: Where 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. The left and right side carrier signals modulated by the I / Q modulator beat each other at the photodetector and beat at other side bands, and the generated photocurrent can realize the separation of the left and right side signals by 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 formula is the target received signal, and the remaining terms are the noise generated in 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.

Images