A relay-free transmission system and a method for increasing the transmission distance of relay-free optical communication

Abstract

The present invention discloses a relay-free transmission system and a method for improving the transmission distance of relay-free optical communication, which belongs to the field of optical fiber communication. A relay-free transmission system includes: a transmitting end and a receiving end; the transmitting end includes a conjugate generation module, a sequence insertion module, a dispersion pre-compensation module and a signal modulation module; the receiving end includes a compensation and synchronization module, a channel equalization module and a coherent superposition module; an X and Y polarization conjugate dual-wave data sequence is constructed at the signal transmitting end; a training sequence and a synchronization header are inserted; dispersion pre-compensation is performed; an electrical signal is modulated onto an optical carrier and transmitted through a long-distance relay-free optical communication system; dispersion compensation and frame synchronization processing are performed at the signal receiving end; the signal is equalized using the training sequence; after carrier recovery processing, the X and Y polarization data are coherently superimposed to offset nonlinear effects. The present invention compensates for the nonlinear effects on the signal during the relay-free long-distance optical transmission process, thereby improving the relay-free transmission distance.

Description

Technical Field

[0001] The present invention belongs to the field of optical fiber communication, and more specifically, relates to a relay-free transmission system and a method for improving the transmission distance of relay-free optical communication. Background Art

[0002] Long-distance, repeater-free optical communication systems do not require any plug-in repeaters or active components in the transmission link. They are a cost-effective solution for point-to-point connections over hundreds of kilometers and are extremely advantageous in scenarios such as remote areas, power fiber communications, offshore, and cross-strait communications. To increase the distance of repeater-free transmission, amplifiers such as EDFA and ROPA have been introduced to compensate for signal attenuation. However, the introduction of new amplifiers significantly increases link costs. Furthermore, as the transmission distance increases, the signal power rises, and the fiber nonlinearity caused by the Kerr effect increases significantly, limiting the maximum signal power that can be transmitted in the fiber. This ultimately sets an upper limit on the achievable system capacity and the distance of repeater-free transmission. Compensating for nonlinear damage has become a challenge that limits the capacity and distance of repeater-free systems.

[0003] Among the existing technologies for compensating nonlinear damage, those based on digital domain nonlinear compensation algorithms, such as DBP (Digital-Back-Propagation), significantly increase the complexity of DSP. Moreover, when dealing with systems with complex amplification architectures such as relayless systems, the algorithm has high computational complexity and poor compensation effect. Optical nonlinear compensation technologies, such as OPC (Optical-Phase-Conjugation), require significant symmetry conditions, are heavily dependent on link design, significantly reduce the flexibility of optical networks, and are not suitable for relayless transmission systems.

[0004] Therefore, existing technologies for increasing the transmission distance of relayless optical communication suffer from high costs and severe nonlinear impairments. Existing technologies for compensating for nonlinear impairments are also highly complex and have low link flexibility. These shortcomings limit the potential for increasing the distance of relayless optical communication systems. Summary of the Invention

[0005] In response to the defects of related technologies, the purpose of the present invention is to provide a relayless transmission system and a method for improving the transmission distance of relayless optical communications, aiming to solve the problems of high complexity and low link flexibility of existing nonlinear damage compensation technologies, which limit the capacity and applicable distance of relayless systems.

[0006] To achieve the above object, the present invention provides a relay-free transmission system, comprising: a transmitting end and a receiving end;

[0007] The transmitting end includes a conjugate generation module, a sequence insertion module, a dispersion pre-compensation module and a signal modulation module connected in sequence;

[0008] The conjugate generation module is used to map the original data to generate a first transmission signal A on the X polarization, and perform phase conjugation on the first transmission signal A to generate a second transmission signal B on the Y polarization, where the first transmission signal A and the second transmission signal B constitute a conjugated dual-wave data sequence;

[0009] The sequence insertion module is used to sequentially insert a training sequence and a synchronization header into the front end of the conjugated dual-wave data sequence to form a transmission signal data frame;

[0010] The dispersion pre-compensation module is used to perform dispersion pre-compensation on the transmission signal data frame to form an electrical signal to be transmitted;

[0011] The signal modulation module is used to modulate the electrical signal onto an optical carrier to generate an optical signal, and transmit the optical signal to the receiving end through a relayless optical communication system;

[0012] The receiving end includes a compensation and synchronization module, a channel equalization module and a coherent superposition module connected in sequence;

[0013] The compensation and synchronization module is used to perform residual dispersion compensation and frame synchronization processing on the received optical signal to obtain the required signal data frame, and split the signal data frame into a training sequence and a conjugate dual-wave data sequence;

[0014] The channel equalization module is used to calculate a transmission channel matrix H based on the training sequence equalization processing, and use the transmission channel matrix H to equalize the conjugate dual-wave data sequence in which the X and Y polarizations are mutually overlapped to obtain a first transmission signal A and a second transmission signal B that are aliased;

[0015] The coherent superposition module is used to coherently superpose the polarization data of the first transmission signal A and the second transmission signal B, compensate for the optical fiber nonlinear damage suffered by the signal during transmission in the relay-free transmission system, and obtain the restored original signal.

[0016] Optionally, the relay-free transmission system further includes: a transmitting end laser, a coherent transmitter, a transmitting end high-power EDFA, a remote pump amplifier ROPA, a remote pump gain unit RGU, a receiving end EDFA, an optical filter OBPF, a receiving end laser and a coherent receiver;

[0017] The transmitting laser is used to provide an optical carrier for the coherent transmitter;

[0018] The coherent transmitter is used to modulate the electrical signal to be transmitted onto an optical carrier to generate an optical signal;

[0019] The transmitting end high power EDFA is used to perform a first gain amplification on the optical signal;

[0020] The remote pump amplifier ROPA is used to remotely provide pump light to the remote pump gain unit RGU;

[0021] The remote pump gain unit RGU is used to perform a second gain amplification on the optical signal after the first gain amplification after it has passed through a preset first transmission distance;

[0022] The receiving end EDFA is used to perform a third gain amplification on the optical signal after the second gain amplification after passing through the preset second transmission distance;

[0023] The optical filter OBPF is used to filter the optical signal after the third gain amplification;

[0024] The receiving end laser is used to provide local oscillator light for the coherent receiver;

[0025] The coherent receiver is used to receive the filtered optical signal.

[0026] In a second aspect, the present invention further provides a method for improving the transmission distance of unrelayed optical communication, which is applied to the system described in any one of the first aspects, comprising:

[0027] S1. The signal transmitting end maps the original data to generate a first transmit signal A on the X polarization, and performs phase conjugation on the first transmit signal A to generate a second transmit signal B on the Y polarization. The first transmit signal A and the second transmit signal B constitute a conjugated dual-wave data sequence.

[0028] S2, inserting a training sequence and a synchronization header into the front end of the conjugated dual-wave data sequence to form a transmission signal data frame;

[0029] S3. Perform dispersion pre-compensation on the transmission signal data frame to form an electrical signal to be transmitted;

[0030] S4, modulating the electrical signal to be transmitted onto an optical carrier to generate an optical signal, and transmitting the optical signal to the receiving end through a relayless optical communication system;

[0031] S5. The signal receiving end performs residual dispersion compensation and frame synchronization processing on the received optical signal to obtain the required signal data frame, and splits the signal data frame into a training sequence and a conjugate dual-wave data sequence;

[0032] S6. Calculate a transmission channel matrix H based on the training sequence equalization processing, and use the transmission channel matrix H to equalize the conjugate dual-wave data sequence in which the X and Y polarizations are mutually aliased to obtain a first transmission signal A and a second transmission signal B that are aliased.

[0033] S7. Coherently superimpose the polarization data of the first transmission signal A and the second transmission signal B to compensate for the optical fiber nonlinear damage suffered by the signal during transmission in the relayless communication system, and obtain a restored original signal.

[0034] Optionally, the relationship between the first signal to be sent X and the second signal to be sent Y is:

[0035] E y (0, t) = E x (0, t) * , E y (ω)=E x (-ω) *

[0036] Among them, E x (0, t), E y (0, t) represent the light field vectors at the initial positions of X and Y polarization at time t, respectively. x (ω), E y (ω) is the frequency domain expression of the light field vector.

[0037] Optionally, step S3 includes:

[0038] Based on the communication channel conditions, electrical domain dispersion pre-compensation is performed to form the electrical signal to be transmitted. The weights of the frequency domain transfer function in the electrical domain dispersion compensation algorithm are set as follows:

[0039]

[0040] Where c is the speed of light, L1=L / 2 is half of the transmission distance, D is the fiber dispersion coefficient, f is the baseband signal frequency, and f c is the center frequency of the optical signal, j is an imaginary unit, and the channel conditions include the fiber dispersion coefficient, the fiber link length and the baseband signal frequency.

[0041] Optionally, step S4 includes:

[0042] S41: modulating the electrical signal to be transmitted onto an optical carrier through a coherent transmitter to generate an optical signal, and performing a first gain amplification on the optical signal through a high-power EDFA at the transmitting end;

[0043] S42: After the first gain amplification, the optical signal is transmitted into the optical fiber, passes through a preset first transmission distance, and reaches the remote pump gain unit RGU;

[0044] S43: The RGU performs a second gain amplification on the optical signal, and transmits the optical signal after the second gain amplification through a preset second transmission distance to the receiving end EDFA;

[0045] S44: After the EDFA at the receiving end performs a third gain amplification on the optical signal, the optical signal passes through the optical filter OBPF and reaches the coherent receiver at the receiving end.

[0046] Optionally, step S5 specifically includes:

[0047] S51. The signal receiving end compensates for the residual dispersion. The weight of the frequency domain conversion function in the dispersion compensation algorithm of the signal receiving end is set to:

[0048]

[0049] Where L2 = L / 2, which is half the transmission distance without pre-compensation, D is the fiber dispersion coefficient, f is the baseband signal frequency, and f c is the center frequency of the optical signal, j is the imaginary unit;

[0050] S52. Based on the synchronization header sequence added by the signal transmitting end, a start-stop synchronization method is used to search and identify the synchronization header from the received optical signal, find the starting position of each frame of data, and obtain a transmission signal data frame;

[0051] S53. Split the signal data frame into two segments, a training sequence and a conjugate dual-wave data sequence, according to the data lengths of the original sequence and the training sequence.

[0052] Optionally, the calculating and obtaining a transmission channel matrix H according to the training sequence equalization processing includes:

[0053] Perform equalization on the training sequence in the signal data frame. The matrix of the equalization algorithm is:

[0054]

[0055] Among them, E X1 、E Y1 Indicates the X and Y polarization data after equalization according to the training sequence, E x1 、E y1 This represents the aliased X and Y polarization training sequence data before equalization. Substituting the above data into the channel correlation matrix H, the matrix H can be expanded as follows:

[0056]

[0057] Among them, h xx 、h xy 、h yx 、h yy Indicates the tap coefficients corresponding to the four filters in the equalization algorithm.

[0058] Optionally, the transmission channel matrix H calculated based on the training sequence is used to perform equalization processing on the conjugate dual-wave data with overlapping X and Y polarizations. The expression is:

[0059]

[0060] Among them, E x1 、E y1 They represent the X polarization and Y polarization of the conjugate double-wave data sequence respectively. At this time, the X and Y polarization data are overlapped with each other. x2 、E Y2 It represents the first transmission signal A and the second transmission signal B obtained by equalization based on the matrix H to eliminate aliasing.

[0061] Optionally, step S7 specifically includes:

[0062] At the receiving end, the polarization data of the first transmission signal A and the second transmission signal B are coherently superimposed to obtain the restored original signal:

[0063] E x (L, t) + E y (L, t) * =2E(0, t)

[0064] Among them, E x (L, t), E y (L, t) represents the light field vector at position L at time t for X and Y polarization respectively, and E(0, t) represents the original signal sent.

[0065] Compared with the prior art, the above technical solutions conceived by the present invention can achieve the following beneficial effects:

[0066] 1. The present invention provides a relay-free transmission system, which is based on a phase-conjugated structure of a first transmitted signal A and a second transmitted signal B. It utilizes the coherent superposition elimination of opposite nonlinear damages on A and B to perform nonlinear compensation without changing the original system architecture. It ensures network flexibility to a great extent, compensates for the nonlinear effects on the signal during long-distance optical transmission without relays, improves the signal-to-noise ratio of the received signal, and thus extends the transmission distance of the relay-free system. At the same time, there is no need to add additional amplifying devices, and only simple operations in the digital domain are required to extend the transmission distance, saving the cost of link laying.

[0067] 2. The present invention provides a method for increasing the transmission distance of relayless optical communications. This method, algorithmically, requires only the addition of X and Y phase conjugation data at the transmitter and coherent superposition at the receiver to achieve nonlinear compensation, thus reducing DSP pressure. This simple and flexible method offsets the nonlinear effects of X and Y polarization nonlinearities, compensating for the nonlinear effects of long-distance relayless optical transmission. This improves the signal-to-noise ratio (SNR) of the received signal, effectively extending the transmission distance of relayless systems and addressing the nonlinear limitations of power increases in relayless systems. The method is suitable for relayless transmission applications in deserts and submarine optical cables. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Figure 1 A schematic diagram of the structure and transmission of a relay-free transmission system provided by an embodiment of the present invention;

[0069] Figure 2 A flow chart of a method for improving the transmission distance of unrelayed optical communication provided by an embodiment of the present invention;

[0070] Figure 3 A schematic diagram of the power distribution of signal light along a link in a relay-free system provided by an embodiment of the present invention;

[0071] Figure 4 The method provided by the embodiment of the present invention is Q under different fiber input powers 2 Factor comparison chart, where PC-QPSK is the effect of this application. DETAILED DESCRIPTION

[0072] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0073] The contents involved in the above embodiment are described below in conjunction with a preferred embodiment.

[0074] In response to the problems existing in the prior art, this application proposes a relayless transmission system and a method for improving the transmission distance of relayless optical communication, which effectively compensates for nonlinear damage in the relayless optical communication system, and extends the transmission distance of the relayless optical communication with a simple and flexible architecture, providing an effective method for making the relayless optical communication system have a longer distance and larger capacity.

[0075] like Figure 1 As shown, a relay-free transmission system includes: a transmitting end and a receiving end;

[0076] The transmitting end includes a conjugate generation module, a sequence insertion module, a dispersion pre-compensation module and a signal modulation module connected in sequence;

[0077] The conjugate generation module is used to map the original data to generate a first transmission signal A on the X polarization, and perform phase conjugation on the first transmission signal A to generate a second transmission signal B on the Y polarization, where the first transmission signal A and the second transmission signal B constitute a conjugated dual-wave data sequence;

[0078] The sequence insertion module is used to sequentially insert a training sequence and a synchronization header into the front end of the conjugated dual-wave data sequence to form a transmission signal data frame;

[0079] The dispersion pre-compensation module is used to perform dispersion pre-compensation on the transmission signal data frame to form an electrical signal to be transmitted;

[0080] The signal modulation module is used to modulate the electrical signal onto an optical carrier to generate an optical signal, and transmit the optical signal to the receiving end through a relayless optical communication system;

[0081] The receiving end includes a compensation and synchronization module, a channel equalization module and a coherent superposition module connected in sequence;

[0082] The compensation and synchronization module is used to perform residual dispersion compensation and frame synchronization processing on the received optical signal to obtain the required signal data frame, and split the signal data frame into a training sequence and a conjugate dual-wave data sequence;

[0083] The channel equalization module is used to calculate a transmission channel matrix H based on the training sequence equalization processing, and use the transmission channel matrix H to equalize the conjugate dual-wave data sequence in which the X and Y polarizations are mutually overlapped to obtain a first transmission signal A and a second transmission signal B that are aliased;

[0084] The coherent superposition module is used to coherently superpose the polarization data of the first transmission signal A and the second transmission signal B, compensate for the nonlinear interference of light on the signal during transmission in the relayless transmission system, and obtain the restored original signal.

[0085] Optionally, the relay-free transmission system further includes: a transmitting end laser, a coherent transmitter, a transmitting end high-power EDFA, a remote pump amplifier ROPA, a remote pump gain unit RGU, a receiving end EDFA, an optical filter OBPF, a receiving end laser and a coherent receiver;

[0086] The transmitting laser is used to provide an optical carrier for the coherent transmitter;

[0087] The coherent transmitter is used to modulate the electrical signal to be transmitted onto an optical carrier to generate an optical signal;

[0088] The transmitting end high power EDFA is used to perform a first gain amplification on the optical signal;

[0089] The remote pump amplifier ROPA is used to remotely provide pump light to the remote pump gain unit RGU;

[0090] The remote pump gain unit RGU is used to perform a second gain amplification on the optical signal after the first gain amplification after it has passed through a preset first transmission distance;

[0091] The receiving end EDFA is used to perform a third gain amplification on the optical signal after the second gain amplification after passing through the preset second transmission distance;

[0092] The optical filter OBPF is used to filter the optical signal after the third gain amplification;

[0093] The receiving end laser is used to provide local oscillator light for the coherent receiver;

[0094] The coherent receiver is used to receive the filtered optical signal.

[0095] The present invention provides a relayless transmission system that is simple, flexible, and low-cost, making it suitable for relayless transmission applications in deserts and submarine optical cables. The system utilizes a coherent transceiver architecture and consists of a transmitter, a relayless transmission link, and a receiver. The transmitter includes a coherent transmitter, a transmitting laser, and a transmitter signal preprocessing unit; the receiver includes a coherent receiver, a receiving laser, and a receiving signal processing unit. Among them, as mentioned above, the signal preprocessing unit at the transmitting end includes a conjugate generation module, a sequence insertion module, a dispersion pre-compensation module and a signal modulation module, and the signal processing unit at the receiving end includes a compensation and synchronization module, a channel equalization module and a coherent superposition module; further, the coherent transmitter is composed of a DAC, 4 EA amplifiers and a dual-bias IQ modulator, which coherently modulates the electrical signal onto the optical carrier emitted by the transmitting laser to form an output optical signal; the relay-free transmission link is realized by the transmitting end EDFA, the receiving end EDFA and the ROPA-pumped RGU to achieve three-time signal amplification; the coherent receiver coheres the filtered optical signal with the local receiving laser, and then processes it through the receiving end signal processing unit.

[0096] Based on the above embodiment, the present invention further provides a method for improving the transmission distance of unrelayed optical communication, which is applied to the system of the above embodiment, comprising:

[0097] S1. The signal transmitting end maps the original data to generate a first transmit signal A on the X polarization, and performs phase conjugation on the first transmit signal A to generate a second transmit signal B on the Y polarization. The first transmit signal A and the second transmit signal B constitute a conjugated dual-wave data sequence.

[0098] S2, inserting a training sequence and a synchronization header into the front end of the conjugated dual-wave data sequence to form a transmission signal data frame;

[0099] S3. Perform dispersion pre-compensation on the transmission signal data frame to form an electrical signal to be transmitted;

[0100] S4, modulating the electrical signal to be transmitted onto an optical carrier to generate an optical signal, and transmitting the optical signal to the receiving end through a relayless optical communication system;

[0101] S5. The signal receiving end performs residual dispersion compensation and frame synchronization processing on the received optical signal to obtain the required signal data frame, and splits the signal data frame into a training sequence and a conjugate dual-wave data sequence;

[0102] S6. Calculate a transmission channel matrix H based on the training sequence equalization processing, and use the transmission channel matrix H to equalize the conjugate dual-wave data sequence in which the X and Y polarizations are mutually aliased to obtain a first transmission signal A and a second transmission signal B that are aliased.

[0103] S7. Coherently superimpose the polarization data of the first transmission signal A and the second transmission signal B to compensate for the nonlinear interference of light on the signal during transmission in the relay-free transmission system, and obtain a restored original signal.

[0104] refer to Figure 1-4 The system includes 6 nodes P1 to P6 from the signal transmitting end to the signal receiving end. This embodiment shows the optical signal power ranging from -5dBm to 14dBm, and the received signal Q under different fiber input powers. 2Factor change. The gain of the amplifier unit during the optical signal transmission process is illustrated using a transmitted optical power of -6dBm. The signal transmitter uses a high-power EDFA at the transmitter to amplify the -6dBm power optical signal to 10dBm. From node P1 to P2, the transmitting EDFA unit provides 16dB gain. After the first gain amplification, the optical signal is transmitted into the optical fiber, and after a preset first transmission distance, it reaches the remote pump gain unit RGU. Combined with the EDFA amplified signal power and the optimal gain range of the back-end ROPA, the preset first signal transmission distance in this embodiment is 200km. After the optical signal is transmitted through the preset first distance, the power attenuates to -24dBm. The ROPA remote pumping RGU module is used to perform a second gain amplification on the optical signal to -4dBm. From node P3 to P4, the RGU unit provides 20dB gain. The optical signal after the second gain amplification passes through the preset second transmission distance and reaches the receiving EDFA. Combined with the signal power after ROPA pumping and the optimal receiving power at the receiving end, the preset second signal transmission distance in this embodiment is 200km. After the optical signal continues to transmit over the second preset distance, its power attenuates to -38dBm. A low-noise EDFA is then used at the receiving end to amplify the optical signal three times to -21dBm. From node P5 to P6, the receiving EDFA unit provides 17dB of gain. This amplified signal is filtered through an optical filter to obtain the wavelength signal to be processed. After coherent reception, signal processing is performed. The local oscillator laser injected into the coherent receiver has a linewidth of 10kHz.

[0105] The preprocessing process of the signal at the signal transmitting end compensates for the dispersion of L / 2=200 km, and the coherent receiver at the signal receiving end compensates for the remaining dispersion of L / 2=200 km of the obtained signal.

[0106] Optionally, the relationship between the first signal to be sent X and the second signal to be sent Y is:

[0107] E y (0, t) = E x (0, t) * , E y (ω)=E x (-ω) * (1)

[0108] Among them, E x (0, t), E y (0, t) represent the light field vectors at the initial positions of X and Y polarization at time t, respectively. x (ω), E y (ω) is the frequency domain expression of the light field vector.

[0109] Optionally, step S3 includes:

[0110] Based on the communication channel conditions, electrical domain dispersion pre-compensation is performed to form the electrical signal to be transmitted. The weights of the frequency domain transfer function in the electrical domain dispersion compensation algorithm are set as follows:

[0111]

[0112] Where c is the speed of light, L1=L / 2 is half of the transmission distance, D is the fiber dispersion coefficient, f is the baseband signal frequency, and f c is the center frequency of the optical signal, j is an imaginary unit, and the channel conditions include the fiber dispersion coefficient and the baseband signal frequency.

[0113] Based on the above, the electrical signal obtained by pre-compensating for electrical domain dispersion exhibits a symmetrical dispersion state when transmitted in the relayless transmission system provided by this application. At this time, the dynamic optical network environment of the transmission system has a symmetrical dispersion graph characteristic. The symmetrical dispersion graph of the dynamic optical network environment is constructed as follows:

[0114] C(z)=-C(Lz)(3)

[0115] Where L is the length of the entire transmission link, It represents the accumulated dispersion damage of the optical signal when it is transmitted along the link, z represents the transmission distance, and β2 represents the group velocity dispersion coefficient.

[0116] Optionally, step S4 includes:

[0117] S41: modulating the electrical signal to be transmitted onto an optical carrier through a coherent transmitter to generate an optical signal, and performing a first gain amplification on the optical signal through a high-power EDFA at the transmitting end;

[0118] S42: After the first gain amplification, the optical signal is transmitted into the optical fiber, passes through a preset first transmission distance, and reaches the remote pump gain unit RGU;

[0119] S43: The RGU performs a second gain amplification on the optical signal, and transmits the optical signal after the second gain amplification through a preset second transmission distance to the receiving end EDFA;

[0120] S44: After the EDFA at the receiving end performs a third gain amplification on the optical signal, the optical signal passes through the optical filter OBPF and reaches the coherent receiver at the receiving end.

[0121] Optionally, step S5 specifically includes:

[0122] S51. The signal receiving end performs residual dispersion compensation on the L / 2 dispersion that has not been pre-compensated. The weight of the frequency domain transfer function in the dispersion compensation algorithm at the signal receiving end is set to:

[0123]

[0124] Where L2 = L / 2, which is half the transmission distance without pre-compensation, D is the fiber dispersion coefficient, f is the baseband signal frequency, and f c is the center frequency of the optical signal, j is the imaginary unit;

[0125] S52. Based on the synchronization header sequence added by the signal transmitting end, a start-stop synchronization method is used to search and identify the synchronization header from the received optical signal, find the starting position of each frame of data, and obtain the required signal data frame;

[0126] S53. Split the signal data frame into two segments, a training sequence and a conjugate dual-wave data sequence, according to the data lengths of the original sequence and the training sequence.

[0127] Optionally, the calculating and obtaining a transmission channel matrix H according to the training sequence equalization processing includes:

[0128] Perform equalization on the training sequence in the signal data frame. The matrix of the equalization algorithm is:

[0129]

[0130] Among them, E X1 、E Y1 Indicates the X and Y polarization data after equalization according to the training sequence, E x1 、E y1 This represents the aliased X and Y polarization training sequence data before equalization. Substituting the above data into the channel correlation matrix H, the matrix H can be expanded as follows:

[0131]

[0132] Among them, h xx 、h xy 、h yx 、h yy Indicates the tap coefficients corresponding to the four filters in the equalization algorithm.

[0133] Furthermore, taking the constant modulus signal as an example, the CMA algorithm is used to depolarize the signal. The iterative update of the H matrix elements can be expressed as:

[0134] h xx (n+1)=h xx (n)+μ(1-|E X1 (n)| 2 )E X1 (n)E x1 (n) * (7)

[0135] h xy (n+1)=h xy(n)+μ(1-|E X1 (n)| 2 )E X1 (n)E y1 (n) * (8)

[0136] h yx (n+1)=h yx (n)+μ(1-|E Y1 (n)| 2 )E Y1 (n)E x1 (n) * (9)

[0137] h yy (n+1)=h yy (n)+μ(1-|E Y1 (n)1 2 )E Y1 (n)E y1 (n) * (10)

[0138] Where n represents the nth code element and μ is the algorithm step size. The channel correlation matrix H can be calculated based on the training sequence data and the above formula.

[0139] Optionally, the transmission channel matrix H calculated based on the training sequence is used to perform equalization processing on the conjugate dual-wave data with overlapping X and Y polarizations. The expression is:

[0140]

[0141] Among them, E x1 、E y1 They represent the X polarization and Y polarization of the conjugate double-wave data sequence respectively. At this time, the X and Y polarization data are overlapped with each other. X2 、E Y2 It represents the first transmission signal A and the second transmission signal B obtained by equalization processing to eliminate aliasing.

[0142] On the basis of the above method, it also includes: performing carrier recovery on the first transmission signal A and the second transmission signal B obtained after equalization, specifically performing frequency offset processing and phase recovery processing on the first transmission signal A and the second transmission signal B.

[0143] Optionally, step S7 specifically includes:

[0144] The polarization data of the first transmission signal A and the second transmission signal B are coherently superimposed to cancel the nonlinearity. The specific elimination process is as follows:

[0145] The nonlinear distortion term δE of the transmitted signal x，yThe expression of (L, ω) is:

[0146]

[0147] The above dimensionless nonlinear transfer function is defined as:

[0148]

[0149] Where L is the length of the transmission link, G(z) is the signal power evolution, C(z) is the accumulated dispersion along the transmission link, and L eff is the effective length.

[0150] Substituting formulas (3) and (13) into formula (12), we can obtain that the nonlinear distortion effect on the polarization data of the phase-conjugated first transmission signal A and the second transmission signal B is:

[0151] δE y (L,ω)=-[δE x (L, -ω)] * , δE y (L, t) = -[δE x (L, t)] * (14)

[0152] According to formula (14), the nonlinear distortions experienced by the two polarization state signals of the first transmitted signal A and the second transmitted signal B are anti-correlated. By coherently superposing the two phase conjugate signals at the receiving end, the first-order nonlinear effect can be offset to obtain the restored original signal:

[0153] E x (L, t) + E y (L, t) * =2E(0, t) (15)

[0154] In a specific embodiment, the present invention provides a relay-free transmission system using a dual-polarization QPSK modulation format. The transmission medium is a single-mode optical fiber with an attenuation coefficient of 0.17dB / km. The link architecture consists of a high-power EDFA, a ROPA, a low-noise EDFA, and two 200km optical fiber transmission links. The receiving end receives the data coherently after optical filtering by the OBPF and then processes it using the aforementioned algorithm of the present invention. The compensation effect is as follows: Figure 4As shown, as the transmitted optical power increases, the more serious the nonlinear damage suffered by the QPSK signal, the more significant the compensation effect of the PC-QPSK proposed in this application on nonlinear damage. When the signal is injected into the fiber at a high power of 14dBm, the signal is severely affected by nonlinearity. The method proposed in this invention can bring nearly 10dB performance improvement, of which 3dB is due to the power increase brought about by the superposition of X and Y polarization signals. This performance improvement result shows that the nonlinear compensation method for relayless optical communication proposed in this application can effectively compensate for the nonlinear damage caused by distance increase and power increase, can effectively extend the transmission distance of the relayless system, and solve the problem of nonlinear limitation on distance increase caused by power increase in the relayless system.

[0155] The present invention provides a relayless optical communication system and a relayless transmission system. Based on the phase-conjugated structure of a first transmitted signal A and a second transmitted signal B, the system performs nonlinear compensation by utilizing the coherent superposition elimination of opposite nonlinear damages to A and B without changing the original system architecture. This ensures network flexibility to a great extent, compensates for the nonlinear effects on the signal during the relayless long-distance optical transmission process, improves the signal-to-noise ratio of the received signal, and thus extends the transmission distance of the relayless system. At the same time, no additional amplifier devices are required, and the transmission distance can be extended by simple operations in the digital domain, saving the cost of link laying.

[0156] This invention provides a method for increasing the transmission distance of relayless optical communications. Algorithmically, this method requires only the addition of X and Y phase conjugation data at the transmitter and coherent superposition at the receiver to achieve nonlinear compensation, thus reducing DSP pressure. This simple and flexible method offsets the nonlinear effects of X and Y polarization nonlinearities, compensating for the nonlinear effects of long-distance relayless optical transmission, improving the signal-to-noise ratio of the received signal. This method can effectively extend the transmission distance of relayless systems and address the nonlinear limitations on distance caused by increased power in relayless systems. The method is suitable for relayless transmission applications in deserts and submarine optical cables.

[0157] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A relay-free transmission system, characterized in that: include: Transmitter and receiver; The transmitting end includes a conjugate generation module, a sequence insertion module, a dispersion pre-compensation module and a signal modulation module connected in sequence; The conjugate generation module is used to map the original data to generate a first transmission signal A on the X polarization, and perform phase conjugation on the first transmission signal A to generate a second transmission signal B on the Y polarization, where the first transmission signal A and the second transmission signal B constitute a conjugated dual-wave data sequence; The sequence insertion module is used to sequentially insert a training sequence and a synchronization header into the front end of the conjugated dual-wave data sequence to form a transmission signal data frame; The dispersion pre-compensation module is used to perform dispersion pre-compensation on the transmission signal data frame to form an electrical signal to be transmitted; The signal modulation module is used to modulate the electrical signal onto an optical carrier to generate an optical signal, and transmit the optical signal to the receiving end through a relayless optical communication system; The receiving end includes a compensation and synchronization module, a channel equalization module and a coherent superposition module connected in sequence; The compensation and synchronization module is used to perform residual dispersion compensation and frame synchronization processing on the received optical signal to obtain the required signal data frame, and split the signal data frame into a training sequence and a conjugate dual-wave data sequence; The channel equalization module is used to calculate a transmission channel matrix H based on the training sequence equalization processing, and use the transmission channel matrix H to equalize the conjugate dual-wave data sequence in which the X and Y polarizations are mutually overlapped to obtain a first transmission signal A and a second transmission signal B that are aliased; The coherent superposition module is used to coherently superpose the polarization data of the first transmission signal A and the second transmission signal B, compensate for the optical fiber nonlinear damage suffered by the signal during transmission in the relay-free transmission system, and obtain the restored original signal.

2. The relay-free transmission system according to claim 1, wherein: Also includes: Transmitting laser, coherent transmitter, transmitting high-power EDFA, remote pump amplifier ROPA, remote pump gain unit RGU, receiving EDFA, optical filter OBPF, receiving laser and coherent receiver; The transmitting laser is used to provide an optical carrier for the coherent transmitter; The coherent transmitter is used to modulate the electrical signal to be transmitted onto an optical carrier to generate an optical signal; The transmitting end high power EDFA is used to perform a first gain amplification on the optical signal; The remote pump amplifier ROPA is used to remotely provide pump light to the remote pump gain unit RGU; The remote pump gain unit RGU is used to perform a second gain amplification on the optical signal after the first gain amplification after it has passed through a preset first transmission distance; The receiving end EDFA is used to perform a third gain amplification on the optical signal after the second gain amplification after passing through the preset second transmission distance; The optical filter OBPF is used to filter the optical signal after the third gain amplification; The receiving end laser is used to provide local oscillator light for the coherent receiver; The coherent receiver is used to receive the filtered optical signal.

3. A method for improving the transmission distance of unrelayed optical communication, applied to the system according to any one of claims 1-2, characterized in that: include: S1. The signal transmitting end maps the original data to generate a first transmit signal A on the X polarization, and performs phase conjugation on the first transmit signal A to generate a second transmit signal B on the Y polarization. The first transmit signal A and the second transmit signal B constitute a conjugated dual-wave data sequence. S2, inserting a training sequence and a synchronization header into the front end of the conjugated dual-wave data sequence to form a transmission signal data frame; S3. Perform dispersion pre-compensation on the transmission signal data frame to form an electrical signal to be transmitted; S4, modulating the electrical signal to be transmitted onto an optical carrier to generate an optical signal, and transmitting the optical signal to the receiving end through a relayless optical communication system; S5. The signal receiving end performs residual dispersion compensation and frame synchronization processing on the received optical signal to obtain the required signal data frame, and splits the signal data frame into a training sequence and a conjugate dual-wave data sequence; S6. Calculate a transmission channel matrix H based on the training sequence equalization processing, and use the transmission channel matrix H to equalize the conjugate dual-wave data sequence in which the X and Y polarizations are mutually aliased to obtain a first transmission signal A and a second transmission signal B that are aliased. S7. Coherently superimpose the polarization data of the first transmission signal A and the second transmission signal B to compensate for the optical fiber nonlinear damage suffered by the signal during transmission in the relayless communication system, and obtain a restored original signal.

4. The method according to claim 3, wherein The relationship between the first signal X to be sent and the second signal Y to be sent is: E y (0,t)=E x (0,t) * ,E y (ω)=E x (-ω) * Among them, E x (0,t),E y (0, t) represent the light field vectors at the initial positions of X and Y polarization at time t, respectively. x (ω), E y (ω) is the frequency domain expression of the light field vector.

5. The method according to claim 3, wherein The step S3 comprises: Based on the communication channel conditions, electrical domain dispersion pre-compensation is performed to form the electrical signal to be transmitted. The weights of the frequency domain transfer function in the electrical domain dispersion compensation algorithm are set as follows: Where c is the speed of light, L1=L / 2 is half of the transmission distance, D is the fiber dispersion coefficient, f is the baseband signal frequency, and f c is the center frequency of the optical signal, j is an imaginary unit, and the channel conditions include the fiber dispersion coefficient, the fiber link length and the baseband signal frequency.

6. The method according to claim 3, wherein The step S4 comprises: S41: modulating the electrical signal to be transmitted onto an optical carrier through a coherent transmitter to generate an optical signal, and performing a first gain amplification on the optical signal through a high-power EDFA at the transmitting end; S42: The optical signal after the first gain amplification is transmitted into the optical fiber, passes through a preset first transmission distance, and reaches the remote pump gain unit RGU; S43: The RGU performs a second gain amplification on the optical signal, and transmits the optical signal after the second gain amplification through a preset second transmission distance to the receiving end EDFA; S44: After the EDFA at the receiving end performs a third gain amplification on the optical signal, the optical signal passes through the optical filter OBPF and reaches the coherent receiver at the receiving end.

7. The method according to claim 3, wherein The step S5 specifically includes: S51. The signal receiving end compensates for the residual dispersion. The weight of the frequency domain conversion function in the dispersion compensation algorithm of the signal receiving end is set to: Where L2 = L / 2, which is half the transmission distance without pre-compensation, D is the fiber dispersion coefficient, f is the baseband signal frequency, and f c is the center frequency of the optical signal, j is the imaginary unit; S52. Based on the synchronization header sequence added by the signal transmitting end, a start-stop synchronization method is used to search and identify the synchronization header from the received optical signal, find the starting position of each frame of data, and obtain a transmission signal data frame; S53. Split the signal data frame into two segments, a training sequence and a conjugate dual-wave data sequence, according to the data lengths of the original sequence and the training sequence.

8. The method according to claim 3, wherein The step of calculating and obtaining a transmission channel matrix H according to the training sequence equalization processing includes: Perform equalization on the training sequence in the signal data frame. The matrix of the equalization algorithm is: Among them, E X1 、E Y1 Indicates the X and Y polarization data after equalization according to the training sequence, E x1 、E y1 This represents the aliased X and Y polarization training sequence data before equalization. Substituting the above data into the channel correlation matrix H, the matrix H can be expanded as follows: Among them, h xx 、h xy 、h yx 、h yy Indicates the tap coefficients corresponding to the four filters in the equalization algorithm.

9. The method according to claim 8, wherein The transmission channel matrix H calculated based on the training sequence is used to equalize the conjugate dual-wave data with overlapping X and Y polarizations. Its expression is: Among them, E x1 、E y1 They represent the X polarization and Y polarization of the conjugate double-wave data sequence respectively. At this time, the X and Y polarization data are overlapped with each other. X2 、E Y2 It represents the first transmission signal A and the second transmission signal B obtained by equalization based on the matrix H to eliminate aliasing.

10. The method according to claim 3, wherein Step S7 specifically includes: At the receiving end, the polarization data of the first transmission signal A and the second transmission signal B are coherently superimposed to obtain the restored original signal: E x (L,t)+E y (L,t) * =2E(0,t) Among them, E x (L,t),E y (L, t) represents the light field vector at position L at time t for X and Y polarization respectively, and E(0, t) represents the original signal sent.

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