Patent Publication Number: US-2023163857-A1

Title: Signal processing method and apparatus, and coherent receiver

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/CN2021/094742, filed on May 20, 2021, which claims priority to Russian Patent Application No. RU2020117812, filed on May 29, 2020. The disclosures of the aforementioned applications are hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to the optical communications field. Embodiments of the present disclosure provide a signal processing method and apparatus, and a coherent receiver. 
     BACKGROUND 
     Performance of a high-capacity optical communications system may deteriorate due to optical fiber attenuation, optical fiber chromatic dispersion, polarization mode chromatic dispersion, laser phase noise, optical fiber nonlinearity, or the like. A currently provided polarization multiplexing coherent receiver can effectively compensate for the foregoing deterioration factors in electrical domain during an electrical signal processing procedure. 
     During the electrical signal processing procedure of the existing polarization multiplexing coherent receiver, to implement the foregoing effective compensation in electrical domain, convolution of a time-domain signal and an inverse function of a channel response is implemented based on Fourier transform. Therefore, a fast Fourier transform (FFT) module and an inverse fast Fourier transform (IFFT) module need to be introduced into a structure of the polarization multiplexing coherent receiver. However, the Fourier transform has high complexity and low precision. As a result, the electrical signal processing procedure is highly complex, and accuracy of a recovered signal is low. Therefore, how to improve performance of a polarization multiplexing coherent receiver in processing an electrical signal becomes an urgent problem to be resolved. 
     SUMMARY 
     Embodiments of the present disclosure provide a signal processing method and apparatus, and a coherent receiver, to improve signal processing performance. 
     According to a first aspect, a signal processing method is provided. The signal processing method may be performed by a coherent receiver, or may be performed by a chip or a circuit disposed in a coherent receiver. This is not limited in the present disclosure. 
     The signal processing method includes: 
     first, obtaining P real-number signals, where the P real-number signals include P real-number signals obtained through analog-to-digital conversion or other P real-number signals that require digital signal processing, and this is not limited in the present disclosure; 
     then, performing at least number theoretic transform (NTT) processing on the P real-number signals to obtain P transform-domain first real-number signals; 
     then, performing at least clock recovery on the P transform-domain first real-number signals to obtain P transform-domain second real-number signals; and 
     then, performing at least polarization compensation and inverse number theoretic transform (INTT) processing on the P transform-domain second real-number signals to obtain m time-domain complex-number signals X and m time-domain complex-number signals Y, where m and P are positive integers. 
     Further, if INTT and combination processing is first performed and then polarization compensation processing is performed, in the signal processing method provided in the present disclosure, phase recovery and decoding may be performed on m time-domain complex-number signals X and m time-domain complex-number signals Y that are obtained through polarization compensation, to obtain bit signals. 
     It should be understood that if the INTT and combination processing is first performed and then the polarization compensation processing is performed, the polarization compensation processing is performed based on a time-domain signal obtained through INTT processing. Specifically, a capability of resisting a loop delay can be enhanced by performing polarization compensation processing in time domain. 
     Alternatively, if polarization compensation processing is first performed and then INTT and combination processing is performed, in the signal processing method provided in the present disclosure, phase recovery and decoding may be performed on m time-domain complex-number signals X in a first polarization direction and m time-domain complex-number signals Y in a second polarization direction that are obtained through combination, to obtain bit signals, so as to implement signal recovery. 
     It should be understood that if the polarization compensation processing is first performed and then the INTT and combination processing is performed, the polarization compensation processing is performed based on a transform-domain signal. Specifically, polarization equalization impairment compensation is performed in transform domain, and multiplication is used to replace convolution, so that power consumption can be reduced. 
     In the signal processing method provided in the present disclosure, signals in two polarization directions are obtained through analog-to-digital conversion, and the signals in the two polarization directions are input to a receiver digital signal processing Rx DSP apparatus. During a processing procedure of the Rx DSP, number theoretic transform NTT and inverse number theoretic transform INTT processing is used to replace fast Fourier transform FFT and inverse fast Fourier transform IFFT processing, to avoid high complexity and low accuracy caused by the FFT and IFFT processing, thereby improving signal processing performance. 
     With reference to the first aspect, in some implementations of the first aspect, the performing at least number theoretic transform NTT processing on the P real-number signals to obtain P transform-domain first real-number signals includes: performing NTT processing on the P real-number signals to obtain the P transform-domain first real-number signals; or 
     performing digital back propagation DBP processing on the P real-number signals to obtain P time-domain tenth real-number signals, and separately performing NTT processing on the P time-domain tenth real-number signals to obtain the P transform-domain first real-number signals. 
     If the P real-number signals undergo DBP processing, a DBP module is used to implement nonlinear-effect compensation, to increase a signal transmission distance. 
     In the signal processing method provided in the present disclosure, the P transform-domain first real-number signals may be obtained by inputting the P real-number signals to an NTT module and performing NTT processing; or the P transform-domain first real-number signals may be obtained by first inputting the P real-number signals to the DBP module and performing DBP processing, and then performing NTT processing. Different manners of obtaining the P transform-domain first real-number signals are provided, so that a structural design of the Rx DSP apparatus is more flexible. 
     With reference to the first aspect, in some implementations of the first aspect, when the P real-number signals do not undergo DBP processing, chromatic dispersion compensation needs to be performed when the P transform-domain second real-number signals are obtained based on the P transform-domain first real-number signals. To be specific, the performing at least clock recovery on the P transform-domain first real-number signals to obtain P transform-domain second real-number signals includes: performing chromatic dispersion compensation on the P transform-domain first real-number signals to obtain P transform-domain third real-number signals; and performing clock recovery on the P transform-domain third real-number signals to obtain the P transform-domain second real-number signals. 
     Alternatively, when the P real-number signals undergo DBP processing, because the DBP module has a chromatic dispersion compensation function, no additional chromatic dispersion compensation needs to be performed when the P transform-domain second real-number signals are obtained based on the P transform-domain first real-number signals. To be specific, the performing at least clock recovery on the P transform-domain first real-number signals to obtain P transform-domain second real-number signals includes: performing chromatic dispersion compensation on the P transform-domain first real-number signals to obtain the P transform-domain second real-number signals. 
     With reference to the first aspect, in some implementations of the first aspect, the P real-number signals include an in-phase real-number signal I m   x  and a quadrature real-number signal Q m   x  in the first polarization direction, and an in-phase real-number signal I m   y  and a quadrature real-number signal Q m   y  in the second polarization direction. In the foregoing case in which additional chromatic dispersion compensation is required, a chromatic dispersion compensation process includes: performing NTT processing on a chromatic dispersion impulse response I h (t) corresponding to an in-phase real-number signal and a chromatic dispersion impulse response Q h (t) corresponding to a quadrature real-number signal, to obtain a transform-domain chromatic dispersion equalization function I n (w) corresponding to the in-phase real-number signal and a transform-domain chromatic dispersion equalization function Q h (w) corresponding to the quadrature real-number signal. 
     After I h (w) and Q h (w) are obtained, 2×m transform-domain third real-number signals that are in the P transform-domain third real-number signals and that are in the first polarization direction can be determined based on I h (w), Q h (w), and an in-phase real-number signal and a quadrature real-number signal that are in the P transform-domain first real-number signals and that are in the first polarization direction, and 2×m transform-domain third real-number signals that are in the P transform-domain third real-number signals and that are in the second polarization direction can be determined based on I h (w), Q h (w), and an in-phase real-number signal and a quadrature real-number signal that are in the P transform-domain first real-number signals and that are in the second polarization direction, so as to complete chromatic dispersion compensation. 
     With reference to the first aspect, in some implementations of the first aspect, the transform-domain third real-number signals obtained through chromatic dispersion compensation and the transform-domain first real-number signals meet the following requirements: 
         I′   x ( w )= I   x ( w )· I   h ( w )− Q   x ( w )· Q   h ( w );
 
         Q′   x ( w )= Q   x ( w )· I   h ( w )+ I   x ( w )· Q   h ( w );
 
         I′   y ( w )= I   y ( w )· I   h ( w )− Q   y ( w )· Q   h ( w ); and
 
         Q′   y ( w )= Q   y ( w )· I   h ( w )+ I   y ( w )· Q   h ( w ), where
 
     I x (w) represents an in-phase real-number signal that is in the transform-domain first real-number signals and that is in the first polarization direction, Q x (w) represents a quadrature real-number signal that is in the transform-domain first real-number signals and that is in the first polarization direction, I y (w) represents an in-phase real-number signal that is in the transform-domain first real-number signals and that is in the second polarization direction, Q y (w) represents a quadrature real-number signal that is in the transform-domain first real-number signals and that is in the second polarization direction, I′ x (w) represents an in-phase real-number signal that is in the transform-domain third real-number signals and that is in the first polarization direction, Q′ x (w) represents a quadrature real-number signal that is in the transform-domain third real-number signals and that is in the first polarization direction, I′ y (w) represents an in-phase real-number signal that is in the transform-domain third real-number signals and that is in the second polarization direction, and Q′ y (w) represents a quadrature real-number signal that is in the transform-domain third real-number signals and that is in the second polarization direction. 
     Specifically, when m is equal to 1, the determining 2*m transform-domain third real-number signals that are in the P transform-domain third real-number signals and that are in the first polarization direction based on I h (w), Q h (w), and an in-phase real-number signal and a quadrature real-number signal that are in the P transform-domain first real-number signals and that are in the first polarization direction includes: 
         Ix _3= Ix _1· I   h ( w )− Qx _1· Q   h ( w );  Qx _3= Qx _1· I   h ( w )+ Ix _1· Q   h ( w ), where
 
     Ix_3 represents a transform-domain in-phase third real-number signal in the first polarization direction, Ix_1 represents an in-phase real-number signal that is in the P transform-domain first real-number signals and that is in the first polarization direction, Qx_3 represents a transform-domain quadrature third real-number signal in the first polarization direction, and Qx_1 represents a quadrature real-number signal that is in the P transform-domain first real-number signals and that is in the first polarization direction; and 
     the determining 2×m transform-domain third real-number signals that are in the P transform-domain third real-number signals and that are in the second polarization direction based on I h (w), Q h (w), and an in-phase real-number signal and a quadrature real-number signal that are in the P transform-domain first real-number signals and that are in the second polarization direction includes: 
         Iy _3= Iy _1· I   h ( w )− Qy _1· Q   h ( w );  Qy _3= Qy _1· I   h ( w )+ Iy _1· Q   h ( w ), where
 
     Iy_3 represents a transform-domain in-phase third real-number signal in the second polarization direction, Iy_1 represents an in-phase real-number signal that is in the P transform-domain first real-number signals and that is in the second polarization direction, Qy_3 represents a transform-domain in-phase third real-number signal in the second polarization direction, and Qy_1 represents an in-phase real-number signal that is in the P transform-domain first real-number signals and that is in the second polarization direction. 
     With reference to the first aspect, in some implementations of the first aspect, the DBP processing specifically includes: performing NTT processing on the P real-number signals to obtain P transform-domain fourth real-number signals; performing, in transform domain, chromatic dispersion compensation on the P transform-domain fourth real-number signals to obtain P transform-domain fifth real-number signals; performing INTT processing on the P transform-domain fifth real-number signals to obtain P time-domain ninth real-number signals; and performing nonlinear compensation on the P time-domain ninth real-number signals to obtain the P time-domain tenth real-number signals. 
     With reference to the first aspect, in some implementations of the first aspect, polarization compensation may be first performed and then INTT processing is performed, to obtain P time-domain seventh real-number signals. 
     Specifically, polarization compensation is performed on the P transform-domain second real-number signals to obtain P transform-domain sixth real-number signals; and INTT processing is performed on the P transform-domain sixth real-number signals to obtain the P time-domain seventh real-number signals. Equalization and depolarization are performed on 2×m transform-domain second real-number signals that are in the P transform-domain second real-number signals and that are in the first polarization direction to obtain 2×m transform-domain sixth real-number signals that are in the P transform-domain sixth real-number signals and that are in the first polarization direction. Equalization and depolarization are performed on 2×m transform-domain second real-number signals that are in the P transform-domain second real-number signals and that are in the second polarization direction to obtain 2×m transform-domain sixth real-number signals that are in the P transform-domain sixth real-number signals and that are in the second polarization direction. 
     Then, the P time-domain seventh real-number signals obtained through INTT processing are combined. 2×m time-domain seventh real-number signals that are in the P time-domain seventh real-number signals and that are in the first polarization direction are combined into m time-domain complex-number signals X in the first polarization direction. 2×m time-domain seventh real-number signals that are in the P time-domain seventh real-number signals and that are in the second polarization direction are combined into m time-domain complex-number signals Y in the second polarization direction. 
     With reference to the first aspect, in some implementations of the first aspect, INTT processing may be first performed and then polarization compensation is performed, to obtain a time-domain complex-number signal X and a time-domain complex-number signal Y that have undergone the polarization compensation. 
     Specifically, INTT processing is performed on the P transform-domain second real-number signals to obtain P time-domain eighth real-number signals. 2×m time-domain eighth real-number signals that are in the P time-domain eighth real-number signals and that are in the first polarization direction are combined into m time-domain complex-number signals X in the first polarization direction. 2×m time-domain eighth real-number signals that are in the P time-domain eighth real-number signals and that are in the second polarization direction are combined into m time-domain complex-number signals Y in the second polarization direction. Time-domain polarization compensation is performed on the m time-domain complex-number signals X in the first polarization direction and the m time-domain complex-number signals Y in the second polarization direction to obtain m time-domain complex-number signals X and m time-domain complex-number signals Y that have undergone the polarization compensation. 
     With reference to the first aspect, in some implementations of the first aspect, the P real-number signals include 4×m real-number signals in m transmission modes. m may be a value equal to 1 or greater than 1. P and m satisfy P=4×m. When m is equal to 1, it may be understood as single-mode transmission; or when m is greater than 1, it may be understood as multi-mode transmission. 
     According to a second aspect, a signal processing apparatus is provided, including a processor. The processor is coupled to a memory, and may be configured to execute an instruction in the memory, to implement the method in any one of the first aspect or the possible implementations of the first aspect. 
     Optionally, the apparatus further includes a communications interface, and the processor is coupled to the communications interface. 
     In an implementation, the apparatus is a digital signal processor. When the apparatus is a digital signal processor, the communications interface may be a transceiver or an input/output interface. 
     In another implementation, the apparatus is a chip configured in a digital signal processor. When the apparatus is a chip configured in a digital signal processor, the communications interface may be an input/output interface. 
     In another implementation, the apparatus is a chip or a chip system. 
     Optionally, the transceiver may be a transceiver circuit. 
     Optionally, the input/output interface may be an input/output circuit. 
     Specifically, the signal processing apparatus includes: 
     a first number theoretic transform NTT module, configured to perform NTT processing on P input signals to obtain P transform-domain first real-number signals; 
     a clock recovery module, configured to perform clock recovery on the P transform-domain first real-number signals or P transform-domain first real-number signals obtained through chromatic dispersion compensation, to obtain P transform-domain second real-number signals; 
     a polarization compensation module and a first inverse number theoretic transform INTT module, configured to process the P transform-domain second real-number signals to obtain m time-domain complex-number signals X and m time-domain complex-number signals Y; and 
     a phase recovery module and a decoding module, configured to perform phase recovery and decoding on the time-domain complex-number signal X and the time-domain complex-number signal Y to obtain bit signals, where 
     m and P are positive integers. 
     With reference to the second aspect, in some implementations of the second aspect, the P input signals include P real-number signals, and that the first number theoretic transform NTT module is configured to perform NTT processing on P input signals to obtain P transform-domain first real-number signals includes: the first number theoretic transform NTT module is configured to perform number theoretic transform on the P real-number signals to obtain the P transform-domain first real-number signals. 
     For example, the P real-number signals include 4×m real-number signals in m transmission modes. m may be a value equal to 1 or greater than 1. P and m satisfy P=4×m. When m is equal to 1, it may be understood as single-mode transmission; or when m is greater than 1, it may be understood as multi-mode transmission. 
     With reference to the second aspect, in some implementations of the second aspect, the apparatus further includes a first chromatic dispersion compensation module, configured to perform chromatic dispersion compensation on the P transform-domain first real-number signals to obtain P transform-domain third real-number signals; and the clock recovery module is configured to perform clock recovery on the P transform-domain third real-number signals to obtain the P transform-domain second real-number signals. 
     With reference to the second aspect, in some implementations of the second aspect, that the first chromatic dispersion compensation module is configured to perform chromatic dispersion compensation on the P transform-domain first real-number signals to obtain P transform-domain third real-number signals includes: 
     the first chromatic dispersion compensation module is configured to determine m transform-domain third real-number signals that are in the P transform-domain third real-number signals and that are in the first polarization direction based on a transform-domain chromatic dispersion equalization function I h (w) corresponding to an in-phase real-number signal, a transform-domain chromatic dispersion equalization function Q h (w) corresponding to a quadrature real-number signal, and an in-phase real-number signal and a quadrature real-number signal that are in the P transform-domain first real-number signals and that are in the first polarization direction; and 
     the first chromatic dispersion compensation module is configured to determine 2×m transform-domain third real-number signals that are in the P transform-domain third real-number signals and that are in the second polarization direction based on I h (w), Q h (w), and an in-phase real-number signal and a quadrature real-number signal that are in the P transform-domain first real-number signals and that are in the second polarization direction, where I h (w) and Q h (w) are obtained by performing NTT processing on a chromatic dispersion impulse response WO corresponding to the in-phase real-number signal and a chromatic dispersion impulse response Q h (t) corresponding to the quadrature real-number signal, respectively. 
     With reference to the second aspect, in some implementations of the second aspect, the transform-domain third real-number signals obtained through chromatic dispersion compensation and the transform-domain first real-number signals meet the following requirements: 
         I′   x ( w )= I   x ( w )· I   h ( w )− Q   x ( w )· Q   h ( w );
 
         Q′   x ( w )= Q   x ( w )· I   h ( w )+ I   x ( w )· Q   h ( w );
 
         I′   y ( w )= I   y ( w )· I   h ( w )− Q   y ( w )· Q   h ( w ); and
 
         Q′   y ( w )= Q   y ( w )· I   h ( w )+ I   y ( w )· Q   h ( w ), where
 
     I x (w) represents an in-phase real-number signal that is in the transform-domain first real-number signals and that is in the first polarization direction, Q x (w) represents a quadrature real-number signal that is in the transform-domain first real-number signals and that is in the first polarization direction, I y (w) represents an in-phase real-number signal that is in the transform-domain first real-number signals and that is in the second polarization direction, Q y (w) represents a quadrature real-number signal that is in the transform-domain first real-number signals and that is in the second polarization direction, I′ x (w) represents an in-phase real-number signal that is in the transform-domain third real-number signals and that is in the first polarization direction, Q′ x (w) represents a quadrature real-number signal that is in the transform-domain third real-number signals and that is in the first polarization direction, I′ y (w) represents an in-phase real-number signal that is in the transform-domain third real-number signals and that is in the second polarization direction, and Q′ y (w) represents a quadrature real-number signal that is in the transform-domain third real-number signals and that is in the second polarization direction. 
     With reference to the second aspect, in some implementations of the second aspect, the first chromatic dispersion compensation module includes a third NTT module, a combination module, and a multiplication module. The third NTT module is configured to perform NTT processing on a time-domain chromatic dispersion impulse response to obtain a chromatic dispersion equalization function, and perform NTT processing on the P transform-domain first real-number signals. The multiplication module is configured to multiply the chromatic dispersion equalization function by the P transform-domain first real-number signals obtained through NTT processing. The combination module is configured to combine signals obtained through processing by the multiplication module. 
     With reference to the second aspect, in some implementations of the second aspect, the P input signals include P time-domain tenth real-number signals; and the apparatus further includes a digital back propagation DBP module, configured to perform DBP processing on P digital signals to obtain the P time-domain tenth real-number signals. 
     When the apparatus includes the DBP module, the apparatus uses the DBP module to implement nonlinear-effect compensation on a basis of a linear impairment, to increase a signal transmission distance. 
     With reference to the second aspect, in some implementations of the second aspect, the DBP module sequentially includes: 
     a second NTT module, a second chromatic dispersion compensation module, a second INTT module, and a nonlinear compensation module, where 
     that the DBP module is configured to perform DBP processing on P digital signals to obtain the P time-domain tenth real-number signals includes: 
     the second NTT module is configured to perform NTT processing on the P digital signals to obtain P transform-domain fourth real-number signals; 
     the second chromatic dispersion compensation module is configured to perform, in transform domain, chromatic dispersion compensation on the P transform-domain fourth real-number signals to obtain P transform-domain fifth real-number signals; 
     the second INTT module is configured to perform INTT processing on the P transform-domain fifth real-number signals to obtain P time-domain ninth real-number signals; and 
     the nonlinear compensation module is configured to perform nonlinear compensation on the P time-domain ninth real-number signals to obtain the P time-domain tenth real-number signals. 
     With reference to the second aspect, in some implementations of the second aspect, the clock recovery module is configured to perform clock recovery on the P transform-domain first real-number signals to obtain the P transform-domain second real-number signals. 
     With reference to the second aspect, in some implementations of the second aspect, the apparatus further includes: 
     a combination module, where 
     the polarization compensation module is configured to perform polarization compensation on the P transform-domain second real-number signals to obtain P transform-domain sixth real-number signals; 
     the first inverse number theoretic transform INTT module is configured to perform INTT processing on the P transform-domain sixth real-number signals to obtain P time-domain seventh real-number signals; and 
     the combination module is configured to combine every two of 2×m time-domain seventh real-number signals that are in the P time-domain seventh real-number signals and that are in the first polarization direction to obtain m time-domain complex-number signals X in the first polarization direction; and 
     combine every two of 2×m time-domain seventh real-number signals that are in the P time-domain seventh real-number signals and that are in the second polarization direction to obtain m time-domain complex-number signals Y in the second polarization direction. 
     For example, the polarization compensation module includes a first butterfly filter and a second butterfly filter. 
     The first butterfly filter is configured to perform equalization and depolarization on 2×m transform-domain second real-number signals that are in the P transform-domain second real-number signals and that are in the first polarization direction to obtain 2×m transform-domain sixth real-number signals that are in the P transform-domain sixth real-number signals and that are in the first polarization direction. 
     The second butterfly filter is configured to perform equalization and depolarization on 2×m transform-domain second real-number signals that are in the P transform-domain second real-number signals and that are in the second polarization direction to obtain 2×m transform-domain sixth real-number signals that are in the P transform-domain sixth real-number signals and that are in the second polarization direction. 
     Specifically, during a signal processing procedure, when the apparatus first performs polarization compensation processing based on the polarization compensation module and then performs INTT processing based on the first INTT module, the polarization compensation processing is performed based on a transform-domain signal. Polarization equalization impairment compensation is performed in transform domain, and multiplication is used to replace convolution, so that power consumption can be reduced. 
     Alternatively, the apparatus further includes a combination module, where the first inverse number theoretic transform INTT module is configured to perform INTT processing on the P transform-domain second real-number signals to obtain P time-domain eighth real-number signals; 
     the combination module is configured to combine every two of 2×m time-domain eighth real-number signals that are in the P time-domain eighth real-number signals and that are in the first polarization direction to obtain m time-domain complex-number signals X in the first polarization direction; and 
     combine every two of 2×m time-domain eighth real-number signals that are in the P time-domain eighth real-number signals and that are in the second polarization direction to obtain m time-domain complex-number signals Y in the second polarization direction; and 
     the polarization compensation module is configured to perform time-domain polarization compensation on the m complex-number signals X and the m complex-number signals Y. 
     For example, the polarization compensation module includes a third butterfly filter. The third butterfly filter is configured to perform time-domain polarization compensation on the m time-domain complex-number signals X in the first polarization direction and the m time-domain complex-number signals Y in the second polarization direction to obtain m time-domain complex-number signals X and m time-domain complex-number signals Y that have undergone the polarization compensation. 
     Specifically, during a signal processing procedure, when the apparatus first performs INTT processing based on the first INTT module and then performs polarization compensation processing based on the polarization compensation module, the polarization compensation processing is performed based on a time-domain signal obtained through INTT processing. A capability of resisting a loop delay can be enhanced by performing polarization compensation processing in time domain. 
     According to a third aspect, a coherent receiver is provided. The coherent receiver includes the signal processing apparatus in any one of the second aspect or the possible implementations of the second aspect. 
     Further, the coherent receiver further includes a polarization beam splitter, a frequency mixer, a photoelectric detector, and an analog-to-digital converter. The polarization beam splitter is configured to obtain signals in two polarization directions. The frequency mixer is configured to perform frequency mixing processing on signals in a same polarization direction. The photoelectric detector is configured to convert strength of an optical signal into strength of an electrical signal. The analog-to-digital converter is configured to perform analog-signal-to-digital-signal conversion on a signal. 
     According to a fourth aspect, a chip is provided. The chip includes a communications interface, a memory, and a processor. The memory is configured to store a computer program. The processor is configured to read and execute the computer program stored in the memory, so that the chip implements the signal processing method in any one of the first aspect or the possible implementations of the first aspect. 
     According to a fifth aspect, a signal processing apparatus is provided, including a processor and a communications interface. The processor is coupled to a memory. The processor may be configured to execute program code in the memory, to implement the signal processing method in any one of the first aspect or the possible implementations of the first aspect. 
     According to a sixth aspect, a computer-readable storage medium is provided. The computer-readable storage medium stores a computer program. When the computer program is executed by an apparatus, the apparatus is enabled to implement the signal processing method in any one of the first aspect or the possible implementations of the first aspect. 
     According to a seventh aspect, a computer program product including an instruction is provided. When the instruction is executed by a computer, the apparatus is enabled to implement the signal processing method in any one of the first aspect or the possible implementations of the first aspect. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1  ( a )  and  FIG.  1  ( b )  are a schematic diagram of a polarization multiplexing coherent receiver; 
         FIG.  2    is a schematic diagram of an Rx DSP architecture according to an embodiment of the present disclosure; 
         FIG.  3    is a schematic diagram of another Rx DSP architecture according to an embodiment of the present disclosure; 
         FIG.  4 A  is a schematic diagram of implementing chromatic dispersion compensation based on FFT transformation;  FIG.  4 B  is a schematic structural diagram of a chromatic dispersion compensation module according to an embodiment of the present disclosure; 
         FIG.  5    is a schematic diagram of still another Rx DSP architecture according to an embodiment of the present disclosure; 
         FIG.  6    is a schematic diagram of still another Rx DSP architecture according to an embodiment of the present disclosure; 
         FIG.  7    is a schematic structural diagram of a DBP module according to an embodiment of the present disclosure; 
         FIG.  8  ( a )  is a schematic structural diagram of a first polarization compensation module according to an embodiment of the present disclosure;  FIG.  8  ( b )  is a schematic structural diagram of a second polarization compensation module according to an embodiment of the present disclosure; 
         FIG.  9  ( a )  is a schematic diagram of an Rx DSP architecture in a multi-mode transmission scenario according to an embodiment of the present disclosure;  FIG.  9  ( b )  is a schematic diagram of an Rx DSP architecture in another multi-mode transmission scenario according to an embodiment of the present disclosure;  FIG.  9  ( c )  is a schematic diagram of an Rx DSP architecture in still another multi-mode transmission scenario according to an embodiment of the present disclosure;  FIG.  9  ( d )  is a schematic diagram of an Rx DSP architecture in still another multi-mode transmission scenario according to an embodiment of the present disclosure;  FIG.  9  ( e )  is a schematic structural diagram of another DBP module according to an embodiment of the present disclosure; 
         FIG.  10  ( a )  is a schematic flowchart of a signal processing method according to an embodiment of the present disclosure;  FIG.  10  ( b )  is a schematic flowchart of DBP processing according to an embodiment of the present disclosure;  FIG.  10  ( c )  is a schematic flowchart of obtaining a time-domain complex-number signal according to an embodiment of the present disclosure;  FIG.  10  ( d )  is another schematic flowchart of obtaining a time-domain complex-number signal according to an embodiment of the present disclosure; and 
         FIG.  11    is a schematic diagram of a chip according to an embodiment of the present disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes embodiments of the present disclosure with reference to accompanying drawings. 
     The technical solutions in the embodiments of the present disclosure relate to improvement of a polarization multiplexing coherent receiver. The following first describes a conventional polarization multiplexing coherent receiver with reference to  FIG.  1  ( a )  and  FIG.  1  ( b ) .  FIG.  1  ( a )  and  FIG.  1  ( b )  are a schematic diagram of a single-mode polarization multiplexing coherent receiver. 
     It can be seen from  FIG.  1  ( a )  and  FIG.  1  ( b )  that a signal processing procedure of the single-mode polarization multiplexing coherent receiver includes the following steps. 
     An optical signal is split into signals x1 and y1 through processing by a polarization beam splitter #1 in a polarization beam splitter (Polarization beam splitter)  101 . x1 is sent to a frequency mixer #1 in a frequency mixer (Hybrid)  102 , and y1 is sent to a frequency mixer #2 in the frequency mixer  102 . 
     Local oscillator light (for example, generated by a local laser (local laser) or a transmitter) is split into signals x2 and y2 through processing by a polarization beam splitter #2 in the polarization beam splitter  101 . x2 is sent to the frequency mixer #1 in the frequency mixer  102 , and y2 is sent to the frequency mixer #2 in the frequency mixer  102 . 
     It should be noted that an optical signal or the local oscillator light has two polarization modes: transverse electric (transverse electric, TE) and transverse magnetic (transverse magnetic, TM), which may also be referred to as a polarization state X and a polarization state Y, or may also be referred to as a polarization direction X and a polarization direction Y. 
     The polarization state X and the polarization state Y are orthogonal to each other. To be specific, after undergoing polarization rotation, a light beam in a single polarization state (a polarization state Y) becomes a light beam in a polarization state X. It should be noted that X and Y herein are not an x-axis and a y-axis in a narrow sense, but are two orthogonal directions in a broad sense, for example, a horizontal +45° direction and a vertical −45° direction. 
     The frequency mixer #1 and the frequency mixer #2 output four signals. The four signals are used as input of a photoelectric detector (Photodiode)  103 , and are separately input to a photoelectric detector #1 to a photoelectric detector #4. The photoelectric detector is configured to convert strength of an optical signal into strength of an electrical signal. 
     An output signal of the photoelectric detector #1 is a signal Ix, an output signal of the photoelectric detector #2 is a signal Qx, an output signal of the photoelectric detector #3 is a signal Iy, and an output signal of the photoelectric detector #4 is a signal Qy. 
     Ix, Qx, Iy, and Qy are converted by an analog-to-digital converter #1 to an analog-to-digital converter #4 in an analog-to-digital converter (ADC)  104  respectively, and digital signals Ix, Qx, Iy, and Qy obtained through analog-to-digital conversion are output. Ix and Qx are combined by a combination module #1 in a combination module  105  to obtain a complex-number signal X. Iy and Qy are combined by a combination module #2 in the combination module  105  to obtain a complex-number signal Y. For example, the combination module in the present disclosure may also be referred to as an adder. 
     The complex-number signal X is input to an FFT module #1 in an FFT module  106 , and is processed by the FFT module #1 to obtain a frequency-domain signal X that has undergone fast Fourier transform. The complex-number signal Y is input to an FFT module #2 in the FFT module  106 , and is processed by the FFT module #2 to obtain a frequency-domain signal Y that has undergone fast Fourier transform. 
     The frequency-domain signal X is input to a chromatic dispersion compensation module #1 in a chromatic dispersion compensation (CDC) module  112 , and chromatic dispersion compensation is performed in frequency domain to obtain a frequency-domain signal X that has undergone the frequency-domain dispersion compensation. The frequency-domain signal Y is input to a chromatic dispersion compensation module #2 in the chromatic dispersion compensation module  112 , and chromatic dispersion compensation is performed in frequency domain to obtain a frequency-domain signal Y that has undergone the frequency-domain chromatic dispersion compensation. 
     The frequency-domain signal X obtained through frequency-domain chromatic dispersion compensation is input to a clock recovery module #1 in a clock recovery module  107 , and clock recovery is performed to obtain a frequency-domain signal X that has undergone the clock recovery. The frequency-domain signal Y obtained through frequency-domain chromatic dispersion compensation is input to a clock recovery module #2 in the clock recovery module  107 , and clock recovery is performed to obtain a frequency-domain signal Y that has undergone the clock recovery. 
     The frequency-domain signal X obtained through clock recovery is input to an IFFT module #1 in an IFFT module  113 , and is processed by the IFFT module #1 to obtain a time-domain signal X that has undergone inverse fast Fourier transform. The frequency domain signal Y obtained through clock recovery is input to an IFFT module #2 in the IFFT module  113 , and is processed by the IFFT module #2 to obtain a time-domain signal Y that has undergone inverse fast Fourier transform. 
     Both the time-domain signal X and the time-domain signal Y are input to a polarization compensation module  108 . Optionally, the polarization compensation module includes 2×2 butterfly filters, and performs polarization demultiplexing and impairment equalization to obtain a time-domain signal X and a time-domain signal Y that have undergone polarization compensation. Specifically, as shown in  FIG.  1  ( a )  and  FIG.  1  ( b ) , the polarization compensation module  108  further includes a coefficient update module  109 . The coefficient update module  109  is configured to update a coefficient of a filter included in the polarization compensation module  108 . 
     The complex-number signal X obtained through polarization compensation is sequentially sent to a phase recovery module #1 in a phase recovery module  110  and a decoding module #1 in a decoding (decoder) module  111  to obtain a bit signal. The complex-number signal Y obtained through polarization compensation is sequentially sent to a phase recovery module #2 in the phase recovery module  110  and a decoding module #2 in the decoding module  111  to obtain a bit signal. 
     It should be noted that, because the FFT module  106  and the IFFT module  113  are introduced into the current signal processing procedure of the coherent receiver, the following cases may occur: 
     (1) Complexity of FFT transformation increases with a quantity of transformation points at a speed of N log 2N. 
     (2) FFT transformation includes a large quantity of multiplication operations, and multiplication complexity is quite high. 
     (3) An FFT transformation matrix is a complex-number matrix based on an exponent e, and cannot accurately indicate a truncation error and a fixed-point penalty on a computer with a limited quantity of bits. 
     This embodiment of the present disclosure mainly relates to improvement of a processing procedure after the ADC. Therefore, for ease of description, the processing procedure after the ADC is collectively referred to as a receiver digital signal processing (Rx DSP) procedure, for example, an Rx DSP signal processing procedure included in a large dashed-line box in  FIG.  1  ( a )  and  FIG.  1  ( b ) . 
     For ease of understanding, several basic concepts in the present disclosure are briefly described. 
     1. Polarization Multiplexing 
     A transmission mode, that is, an HE 11  mode, for an optical signal in a single-mode optical fiber includes two sub-modes: HE x   11  and HE y   11 . The two sub-modes are independent of each other, and their polarization directions are orthogonal to each other. The two sub-modes present different forms in a transmission process: linear polarization, elliptical polarization, and circular polarization, but always remain orthogonal. X-polarization and Y-polarization in the present disclosure are two orthogonal polarization states in a multiplexing single-mode optical fiber. 
     Compared with the single-mode optical fiber, a multi-mode optical fiber can transmit signals in a plurality of modes. Currently, an LP pq  mode is commonly used. Values of p and q represent different mode field characteristics of an LP mode. 
     2. NTT Processing and INTT Processing 
     Both NTT processing and FFT processing in the present disclosure belong to transformation for implementing fast convolution. The FFT processing is complex-number transformation whose core is K=exp(jα). exp(jα) is a complex number and has a truncation error in storage. In addition, multiplication complexity of the complex number is quite high. NTT transformation is transformation that is defined in a finite field and whose core is K (K is an integer). Usually, K is 2. Therefore, a transformation matrix has no truncation error. On a binary computer, both multiplication and division of 2 can be implemented through shifting, so that multiplication can be avoided and power consumption can be reduced. 
     In addition, the INTT processing is an inverse process of the NTT processing, and IFFT is an inverse process of the FFT processing. Details are not described in the present disclosure. 
     3. Transform Domain and Time Domain 
     A time-domain signal in the embodiments of the present disclosure is a signal that is input to an Rx DSP process through analog-to-digital conversion. A signal obtained by performing FFT processing on the time-domain signal is referred to as a frequency-domain signal, and a signal obtained through NTT processing is referred to as a transform-domain signal. 
     In addition, a time-domain signal is obtained by performing IFFT processing on the frequency-domain signal, and a time-domain signal is obtained by performing INTT processing on the transform-domain signal. 
     To be specific, the time-domain signal in the present disclosure may be understood as a signal at a time granularity, and may change as time changes; the frequency-domain signal may be understood as a frequency-domain representation into which a time-domain signal is converted through transformation from time domain to frequency domain; and likewise, the transform-domain signal may be understood as a transform-domain representation into which a time-domain signal is converted through transformation from time domain to transform domain. During a process of transformation from time domain to transform domain, a time-domain signal is usually segmented, then a part is superposed at a beginning and an end, and then the signal is sent to an NTT/INTT transformation module. For example, a segment of time-domain data includes 1024 points. A time-domain signal is first divided into 512 points, then 256 points are superposed at a beginning and an end, and then the 1024 points are sent to the NTT/INTT transformation module. 512, 256, and 1024 herein are merely examples. A specific superposition proportion and a specific quantity of points in a segment are not limited in the present disclosure. A quantity of points of data transformation is an array length during the data transformation. Generally, a larger quantity of points contributes to better transformation performance. 
     4. In-Phase Real-Number Signal and Quadrature Real-Number Signal 
     Quadrature amplitude modulation is widely used in modem coherent communications. The quadrature amplitude modulation is a modulation scheme in which amplitude modulation is performed on two orthogonal carriers. The two carriers are usually sine waves with a phase difference of 90°, and therefore are referred to as orthogonal carriers. The real part of a complex-number signal is referred to as a codirectional component, and the imaginary part of the signal is referred to as a quadrature component. 
     It should be noted that both a real part and an imaginary part of a complex-number signal (for example, X=a+bj) are real numbers (for example, both a and b are real numbers). Therefore, in the present disclosure, the codirectional component is referred to as an in-phase real-number signal, and the quadrature component is referred to as a quadrature real-number signal. 
     It can be seen from the foregoing descriptions that introducing the FFT module and the IFFT module into an electrical signal processing procedure of the coherent receiver may increase complexity of the signal processing procedure. To resolve a problem in the existing electrical signal processing procedure of the coherent receiver and reduce power consumption of an algorithm, the embodiments of the present disclosure provide a signal processing method and apparatus, in which an electrical signal is processed without introducing the FFT module or the IFFT module, thereby improving performance of the polarization multiplexing coherent receiver in processing an electrical signal and reducing complexity. 
     The following describes in detail the signal processing method and apparatus provided in the embodiments of the present disclosure with reference to the accompanying drawings. The signal processing method provided in the embodiments of the present disclosure may be used by a polarization multiplexing coherent receiver to process an electrical signal in a single-mode or multi-mode transmission scenario. 
       FIG.  2    is a schematic diagram of an Rx DSP architecture according to an embodiment of the present disclosure. The Rx DSP architecture  200  includes: 
     a first NTT module  211 , a first chromatic dispersion compensation module  212 , a clock recovery module  213 , a first polarization compensation module  214 , a first INTT module  215 , a combination module  216 , a phase recovery module  217 , and a decoding module  218 . 
     The first NTT module includes an NTT module #1, an NTT module #2, an NTT module #3, and an NTT module #4. Signals I 1   x , Q 1   x , and Q 1   y  are input to the NTT module #1, the NTT module #2, the NTT module #3, and the NTT module #4 respectively. 
     Optionally, the signals I 1   x , Q 1   x , I 1   y , and Q 1   y  are real-number signals obtained through conversion by an ADC. 
     The NTT module #1, the NTT module #2, the NTT module #3, and the NTT module #4 perform NTT processing on I 1   x , Q 1   x , I 1   y , and Q 1   y  respectively, and output transform-domain first real-number signals Ix_1, Qx_1, Iy_1, and Qy_1 to the first chromatic dispersion compensation module  212 . 
     The first chromatic dispersion compensation module  212  includes a chromatic dispersion compensation module #1 and a chromatic dispersion compensation module #2. Ix_1 and Qx_1 are input to the chromatic dispersion compensation module #1, and chromatic dispersion compensation is performed to obtain transform-domain third real-number signals Ix_3 and Qx_3. Iy_1 and Qy_1 are input to the chromatic dispersion compensation module #2, and chromatic dispersion compensation is performed to obtain transform-domain third real-number signals Iy_3 and Qy_3. 
     The clock recovery module  213  includes a clock recovery module #1 and a clock recovery module #2. Ix_3 and Qx_3 are input to the clock recovery module #1, and clock recovery is performed to obtain transform-domain second real-number signals Ix_2 and Qx_2. Iy_3 and Qy_3 are input to the clock recovery module #2, and clock recovery is performed to obtain transform-domain second real-number signals Iy_2 and Qy_2. 
     The second real-number signals Ix_2, Qx_2, Iy_2, and Qy_2 are input to the first polarization compensation module  214 , and polarization demultiplexing and impairment equalization are performed to obtain transform-domain sixth real-number signals Ix_5, Qx_5, Iy_5, and Qy_5. Optionally, the first polarization compensation module  214  further includes a coefficient update module  2141 . The coefficient update module  2141  is configured to update a coefficient of a filter included in the first polarization compensation module  214 . 
     The first INTT module  215  includes an INTT module #1, an INTT module #2, an INTT module #3, and an INTT module #4. Signals Ix_5, Qx_5, Iy_5, and Qy_5 are input to the INTT module #1, the INTT module #2, the INTT module #3, and the INTT module #4 respectively. 
     The INTT module #1, the INTT module #2, the INTT module #3, and the INTT module #4 perform INTT processing on Ix_5, Qx_5, Iy_5, and Qy_5 respectively, and output time-domain seventh real-number signals Ix_6, Qx_6, Iy_6, and Qy_6 to the combination module  216 . 
     The combination module  216  includes a combination module #1 and a combination module #2. Ix_6 and Qx_6 are input to the combination module #1, and are combined to obtain a time-domain complex-number signal X. Iy_6 and Qy_6 are input to the combination module #2, and are combined to obtain a time-domain complex-number signal Y. 
     The phase recovery module  217  includes a phase recovery module #1 and a phase recovery module #2. The decoding module  111  includes a decoding module #1 and a decoding module #2. The time-domain complex-number signal X is sequentially input to the phase recovery module #1 and the decoding module #1 to obtain a recovered bit signal. The time-domain complex-number signal Y is sequentially input to the phase recovery module #2 and the decoding module #2 to obtain a recovered bit signal. 
     The Rx DSP architecture  200  shown in  FIG.  2    may be referred to as a full-transform-domain Rx DSP architecture. The full-transform-domain Rx DSP architecture not only performs chromatic dispersion compensation based on NTT/INTT, but also performs polarization compensation based on NTT/INTT, that is, performs chromatic dispersion compensation and polarization compensation in transform domain. Specifically, polarization equalization impairment compensation is performed in transform domain, and multiplication is used to replace convolution, so that power consumption can be further reduced. In addition, a quantity (size) of transformation points can be increased, so that an impairment is effectively equalized and performance is improved. 
       FIG.  3    is a schematic diagram of another Rx DSP architecture according to an embodiment of the present disclosure. The Rx DSP architecture  300  includes: 
     a first NTT module  211 , a first chromatic dispersion compensation module  212 , a clock recovery module  213 , a second polarization compensation module  310 , a coefficient update module  311 , a first INTT module  215 , a combination module  216 , a phase recovery module  320 , and a decoding module  330 . 
     Modules before clock recovery in the architecture  300  are the same as those in the architecture  200 . A difference lies in that a signal output by the clock recovery module  213  is first processed by the INTT module  215  and combined by the combination module  216 , and then input to the second polarization compensation module  310 . A process before the clock recovery is not described again. 
     The first INTT module  215  includes an INTT module #1, an INTT module #2, an INTT module #3, and an INTT module #4. Second real-number signals Ix_2, Qx_2, Iy_2, and Qy_2 are input to the INTT module #1, the INTT module #2, the INTT module #3, and the INTT module #4 respectively. 
     The INTT module #1, the INTT module #2, the INTT module #3, and the INTT module #4 perform INTT processing on Ix_2, Qx_2, Iy_2, and Qy_2 respectively, and output time-domain eighth real-number signals Ix_7, Qx_7, Iy_7, and Qy_7 to the combination module  216 . 
     The combination module  216  includes a combination module #1 and a combination module #2. Ix_7 and Qx_7 are input to the combination module #1, and are combined to obtain a time-domain complex-number signal X. Iy_7 and Qy_7 are input to the combination module #2, and are combined to obtain a time-domain complex-number signal Y. 
     The time-domain complex-number signal X and the time-domain complex-number signal Y are input to the second polarization compensation module  310 , and polarization demultiplexing and impairment equalization are performed to obtain a time-domain complex-number signal X and a time-domain complex-number signal Y that have undergone polarization compensation. The second polarization compensation module  310  further includes a coefficient update module  311 . The coefficient update module  311  is configured to update a coefficient of a filter included in the second polarization compensation module  310 . 
     The phase recovery module  320  includes a phase recovery module #1 and a phase recovery module #2. The decoding module  330  includes a decoding module #1 and a decoding module #2. The time-domain complex-number signal X obtained through polarization compensation is sequentially input to the phase recovery module #1 and the decoding module #1 to obtain a recovered bit signal. The time-domain complex-number signal Y obtained through polarization compensation is sequentially input to the phase recovery module #2 and the decoding module #2 to obtain a recovered bit signal. 
     The Rx DSP architecture  300  shown in  FIG.  3    may be referred to as an Rx DSP architecture combining time domain and transform domain. In the Rx DSP architecture combining time domain and transform domain, chromatic dispersion compensation is performed based on NTT/INTT, and polarization compensation is still performed in time domain, that is, chromatic dispersion compensation is performed in transform domain but polarization compensation is performed in time domain. Specifically, a chromatic dispersion compensation module with quite high power consumption implements compensation in transform domain, so that overall power consumption can be effectively reduced, and a polarization equalization module is left in time domain to perform compensation, so that a capability of the equalization module in resisting a loop delay is enhanced. 
       FIG.  4  ( a )  is a schematic structural diagram of a chromatic dispersion compensation module  400 . The chromatic dispersion compensation module  400  may be applied to the architecture shown in  FIG.  1  ( a )  and  FIG.  1  ( b ) . The chromatic dispersion compensation module includes an FFT module  402 , an FFT module  404 , a combination module  401 , a combination module  403 , and a multiplication module  405 . 
     Signals are transmitted in an optical fiber. Impact of chromatic dispersion of the optical fiber on the signals may be described as an impulse response. In time-domain chromatic dispersion equalization, a finite impulse response (FIR) is used for compensation. An FIR coefficient is as follows: 
     
       
         
           
             
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     β 2  indicates a chromatic dispersion coefficient of an optical fiber, z indicates a transmission distance of a signal in the optical fiber, j indicates an imaginary number, and t indicates time. Because a signal in a polarization direction x and a signal in a polarization direction y undergo a same chromatic dispersion impairment, a same function h(t) is used during equalization. 
     In the FFT-based chromatic dispersion compensation module in the signal processing procedure shown in  FIG.  1  ( a )  and  FIG.  1  ( b ) , input of the chromatic dispersion compensation module is a frequency-domain complex-number signal X (co). 
     For example, I h (t) and Q h (t) are time-domain chromatic dispersion impulse responses, I h (t) and Q h (t) are combined by the combination module  403  into a complex-number impulse response H(t), H(t) is input to the FFT module  404  to obtain a frequency-domain equalization function H(ω), and H(ω) and X(ω) are input to the multiplication module  405 , and are multiplied to obtain output of the chromatic dispersion compensation module: Xout(ω)=H(ω)·X(ω). 
       FIG.  4  ( b )  is a schematic structural diagram of a chromatic dispersion compensation module  410  according to an embodiment of the present disclosure. The chromatic dispersion compensation module  410  may be applied to the RX DSP architectures  200  and  300 . The chromatic dispersion compensation module includes an NTT module  411 , an NTT module  412 , an NTT module  419 , an NTT module  420 , a combination module  414 , a combination module  418 , a multiplication module  413 , a multiplication module  415 , a multiplication module  416 , and a multiplication module  417 . 
     In the signal processing procedures shown in  FIG.  2    and  FIG.  3   , the NTT-based chromatic dispersion compensation module separately performs chromatic dispersion compensation on a signal in a polarization direction x and a signal in a polarization direction y. A chromatic dispersion compensation process (as shown in  FIG.  4  ( b ) ) is described by using chromatic dispersion compensation in the polarization direction x as an example. 
     For example, a chromatic dispersion impulse response h(t) in the polarization direction x is split into I h (t) and Q h (t), where 
     
       
         
           
             
               
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     I h (t) and Q h (t) are input to the NTT module  419  and the NTT module  420  respectively, and are transformed to obtain chromatic dispersion equalization functions I h (w) and Q h (w) in NTT transform domain. A codirectional signal Ix(t) and a quadrature signal Qx(t) in the polarization direction x are input to the NTT module  411  and the NTT module  412  respectively, and are transformed to obtain transform-domain signals I x (w) and Q x (w). 
     It can be seen from  FIG.  4  ( b )  that a result obtained by inputting the transform-domain codirectional signal I x (w) and the codirectional component I h (w) of the equalization function to the multiplication module  413  and performing multiplication, and a result obtained by inputting the transform-domain quadrature signal Q x (w) and the quadrature component Q h (w) of the equalization function to the multiplication module  415  and performing multiplication are combined by the combination module  414  as follows: Ix_out(w)=I x (w)·I h (w)−Q x (w)·Q h (w). 
     A result obtained by inputting Q x (w) and I h (w) to the multiplication module  416  and performing multiplication, and a result obtained by inputting I x (w) and Q h (w) to the multiplication module  417  and performing multiplication are combined by the combination module  418  as follows: Q x  out(w)=Q x (w)·I h (w)—I x (w)·Q h (w). 
     Likewise, when chromatic dispersion compensation is performed in the polarization direction y, a codirectional signal I y (t) and a quadrature signal Q y (t) in the polarization direction y are input to NTT modules, and are transformed to obtain transform-domain signals I y (w) and Q y (w) respectively. A chromatic dispersion compensation process in the polarization direction y may be described by using the following formulas: 
         Iy _out( w )= I   y ( w )· I   h ( w )− Q   y ( w )· Q   h ( w ); and
 
         Qy _out( w )= Q   y ( w )· I   h ( w )+ I   y ( w )· Q   h ( w ).
 
     In an actual operation, the equalization function H(o) in Fourier transform domain in  FIG.  4  ( a )  and the chromatic dispersion equalization functions I h (w) and Q h (w) in number theoretic transform domain in  FIG.  4  ( b )  need to be calculated only once and stored in a memory, and may be repeatedly invoked for different input signals. 
     For example, one NTT transformation and one FFT transformation are used as examples to explain a difference between the two types of transformations: 
     Transformation matrices of both FFT and NTT may be represented as follows: 
     
       
         
           
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     of FFT. A transformation core α of NTT is a power of 2. Usually, α is 2 (or may be √{square root over (2)} or another value, which is not limited in the present disclosure). For example, α=2. Each element in a transformation matrix is still a power of 2, and multiplication by 2 or a power of 2 in a binary operation may be performed through bit shifting. 
     Correspondingly, for example, the core α=2. A transformation matrix H of NTT and a transformation matrix H −1  of INTT are respectively as follows: 
     
       
         
           
             
               
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     Internal working processes of the clock recovery module #1 and the clock recovery module #2 are similar to that in  FIG.  4  ( b ) , and only the chromatic dispersion equalization functions I h (w) and Q h (w) need to be replaced with functions for equalizing clock delays. A delay caused by a clock is expressed as h(t)=exp(−2πft 0 ) by using a time-domain finite impulse response. In the clock recovery module, I h (w) and Q h (w) may be replaced with NTT transformation of cos (2πft 0 ). Details are not described herein. 
       FIG.  5    is a schematic diagram of still another Rx DSP architecture according to an embodiment of the present disclosure. The Rx DSP architecture  500  includes: 
     a digital back propagation (digital back propagation, DBP) module  510 , a first NTT module  211 , a clock recovery module  213 , a first polarization compensation module  214 , a first INTT module  215 , a combination module  216 , a phase recovery module  217 , and a decoding module  218 . 
     An architecture after the first polarization compensation module  214  in the Rx DSP architecture  500  is the same as that in the Rx DSP architecture  200 . A difference lies in that a signal undergoes DBP processing before polarization compensation. Because the DBP module  510  has a chromatic dispersion compensation function, a process after the DBP module may not require the chromatic dispersion compensation module  212 . 
     Signals I 1   x , Q 1   x , I 1   y , and Q 1   y  are input to the DBP module  510 , DBP processing is performed, and time-domain tenth real-number signals Ix_4, Qx_4, Iy_4, and Qy_4 are output to the first NTT module  211 . 
     The first NTT module  211  includes an NTT module #1, an NTT module #2, an NTT module #3, and an NTT module #4. Ix_4, Qx_4, Iy_4, and Qy_4 are input to the NTT module #1, the NTT module #2, the NTT module #3, and the NTT module #4 respectively. 
     The NTT module #1, the NTT module #2, the NTT module #3, and the NTT module #4 perform NTT processing on Ix_4, Qx_4, Iy_4, and Qy_4 respectively, and output transform-domain first real-number signals Ix_1, Qx_1, Iy_1, and Qy_1 to the clock recovery module  213 . 
     The clock recovery module  213  includes a clock recovery module #1 and a clock recovery module #2. Ix_1 and Qx_1 are input to the clock recovery module #1, and clock recovery is performed to obtain transform-domain second real-number signals Ix_2 and Qx_2. Iy_1 and Q y _1 are input to the clock recovery module #2, and clock recovery is performed to obtain transform-domain second real-number signals Iy_2 and Qy_2. 
     For a process after the clock recovery module  213 , refer to the embodiment shown in  FIG.  2   . 
     The Rx DSP architecture  500  shown in  FIG.  5    may be referred to as a full-transform-domain Rx DSP architecture in which DBP is added. A difference from the full-transform-domain Rx DSP architecture shown in  FIG.  2    lies in that the DBP module is added. It should be understood that in the full-transform-domain Rx DSP architecture shown in  FIG.  2   , only a linear impairment of an optical fiber is compensated for, for example, chromatic dispersion compensation. In the full-transform-domain Rx DSP architecture in which the DBP module is added, the DBP module is used to implement nonlinear-effect compensation on a basis of a linear impairment, to increase a signal transmission distance. 
       FIG.  6    is a schematic diagram of still another Rx DSP architecture according to an embodiment of the present disclosure. The Rx DSP architecture  600  includes: 
     a DBP module  510 , a first NTT module  211 , a clock recovery module  213 , a second polarization compensation module  310 , a coefficient update module  311 , a first INTT module  215 , a combination module  216 , a phase recovery module  320 , and a decoding module  330 . 
     An architecture after the second polarization compensation module  310  in the Rx DSP architecture  600  is the same as that in the Rx DSP architecture  300 . An architecture before the second polarization compensation module  310  is the same as that in the Rx DSP architecture  500 . To be specific, a received signal obtained through conversion by an ADC first undergoes DBP processing. For a process before the DBP processing and clock recovery, refer to the embodiment shown in  FIG.  5   . For a process after the clock recovery module  213 , refer to the embodiment shown in  FIG.  3   . 
     The Rx DSP architecture  600  shown in  FIG.  6    may be referred to as an Rx DSP architecture combining time domain and transform domain in which DBP is added. A difference from the Rx DSP architecture combining time domain and transform domain shown in  FIG.  3    lies in that the DBP module is added. It should be understood that in the Rx DSP architecture combining time domain and transform domain shown in  FIG.  3   , only a linear impairment of an optical fiber is compensated for, for example, chromatic dispersion compensation. In the Rx DSP architecture combining time domain and transform domain in which the DBP module is added, the DBP module is used to implement nonlinear-effect compensation on a basis of a linear impairment, to increase a signal transmission distance. 
       FIG.  7    is a schematic structural diagram of a DBP module according to an embodiment of the present disclosure. The DBP module  700  includes: 
     a second NTT module  711 , a second chromatic dispersion compensation module  712 , a second INTT module  713 , and a nonlinear compensation module  714 , where a structure of the second chromatic dispersion compensation module  712  is shown in  FIG.  4  ( b ) , and is not described in detail herein. 
     It can be seen from the structural diagram of the DBP module shown in  FIG.  7    that the performing DBP processing on I 1   x , Q 1   x , I 1   y , and Q 1   y  to obtain the real-number signals Ix_4, Qx_4, Iy_4, and Qy_4 shown in  FIG.  5    and  FIG.  6    specifically includes the following steps. 
     The second NTT module  711  includes an NTT module #1, an NTT module #2, an NTT module #3, and an NTT module #4. I 1   x , Q 1   x , I 1   y , and Q 1   y  are input to the NTT module #1, the NTT module #2, the NTT module #3, and the NTT module #4 respectively. 
     The NTT module #1, the NTT module #2, the NTT module #3, and the NTT module #4 perform NTT processing on I 1   x , Q 1   x , I 1   y , and Q 1   y  respectively, and output fourth real-number signals Ix_4′, Qx_4′, Iy_4′, and Qy_4′ to the second chromatic dispersion compensation module  712 . 
     The second chromatic dispersion compensation module  712  includes a chromatic dispersion compensation module #1 and a chromatic dispersion compensation module #2. Ix_4′ and Qx_4′ are input to the chromatic dispersion compensation module #1, and chromatic dispersion compensation is performed in transform domain to obtain transform-domain fifth real-number signals Ix_4″ and Qx_4″. Iy_4′ and Qy_4′ are input to the chromatic dispersion compensation module #2, and chromatic dispersion compensation is performed in transform domain to obtain transform-domain fifth real-number signals Iy_4″ and Qy_4″. 
     The second INTT module  713  includes an INTT module #1, an INTT module #2, an INTT module #3, and an INTT module #4. Ix_4″, Qx_4″, Iy_4″, and Qy_4″ are input to the INTT module #1, the INTT module #2, the INTT module #3, and the INTT module #4 respectively. 
     The INTT module #1, the INTT module #2, the INTT module #3, and the INTT module #4 perform INTT processing on Ix_4″, Qx_4″, Iy_4″, and Qy_4″ respectively, and output time-domain ninth real-number signals Ix_4′″, Qx_4′″, Iy_4′″, and Qy_4′″ to the nonlinear compensation module  714 . 
     The nonlinear compensation module  714  includes a nonlinear compensation module #1 and a nonlinear compensation module #2. Ix_4′ and Qx_4′ are input to the nonlinear compensation module #1, and nonlinear compensation is performed to obtain Ix_4 and Qx_4. Iy_4′ and Qy_4′ are input to the nonlinear compensation module #2, and nonlinear compensation is performed to obtain Iy_4 and Qy_4. 
     It should be noted that the foregoing processing procedure in the DBP module needs to be repeated for Ns times. To ensure performance, when a span of a link increases, a quantity Ns of DBP iterations also needs to be increased. Different DBP solutions are not limited herein. Depending on different power consumption required by scenarios, Ns may be set to be equal to a total span of a link, or single-step DBP may be set. In the DBP module in this embodiment of the present disclosure, no FFT module or IFFT module is used; instead, the NTT module and the INTT module are used. In this way, during a signal processing procedure of the DBP module, repeated Ns transformations do not cause error accumulation. 
       FIG.  8  ( a )  is a schematic structural diagram of a first polarization compensation module  214  according to an embodiment of the present disclosure.  FIG.  8  ( b )  is a schematic structural diagram of a second polarization compensation module  310  according to an embodiment of this application. 
     It can be seen from  FIG.  8  ( a )  that the first polarization compensation module  214  in this embodiment of the present disclosure includes a butterfly filter  810  and a butterfly filter  820 . The butterfly filter  810  includes a filter  801 , a filter  802 , a filter  803 , a filter  804 , an adder  805 , and an adder  806 . The butterfly filter  820  includes a filter  807 , a filter  808 , a filter  809 , a filter  8010 , an adder  8011 , and an adder  8012 . 
     Output of the filter  801 , output of the filter  807 , output of the filter  803 , and output of the filter  809  are negative. The foregoing four signals are input to the adder  805  to obtain a signal Ix_5 that has undergone chromatic dispersion compensation. The output of the filter  801 , the output of the filter  807 , the output of the filter  803 , and the output of the filter  809  are input to the adder  805  to obtain a signal Qx_5 that has undergone chromatic dispersion compensation. Output of the filter  802 , output of the filter  808 , output of the filter  804 , and output of the filter  8010  are negative. The foregoing four signals are input to the adder  806  to obtain a signal Iy_5 that has undergone chromatic dispersion compensation. The output of the filter  802 , the output of the filter  808 , the output of the filter  804 , and the output of the filter  8010  are output to the adder  805  to obtain a signal Qy_5 that has undergone chromatic dispersion compensation. 
     A clock recovery module outputs four signals. The four signals Ix_2, Qx_2, Iy_2, and Qy_2 are input to a polarization compensation module. The polarization compensation module includes two butterfly filters. One butterfly filter represents a 2×2 two-input, two-output system. Two complex-number signals are input, and two complex-number signals are output. Because real-number signals are processed in NTT transformation, a polarization equalization part is a 4×4 four-input, four-output system. 
     The first butterfly filter  810  is for equalization and depolarization of I-path signals (including an x-polarization I-path signal Ix_2 and a y-polarization I-path signal Iy_2), and a transformation matrix is 
     
       
         
           
             
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                       hxy 
                     
                   
                 
                 
                   
                     
                       I 
                       hyx 
                     
                   
                   
                     
                       I 
                       hyy 
                     
                   
                 
               
               ] 
             
             . 
           
         
       
     
     The second butterfly filter  820  is for Q-path signals (including an x-polarization Q-path signal Qx_2 and a y-polarization Q-path signal Qy_2), and a transformation matrix is 
     
       
         
           
             
               [ 
               
                 
                   
                     
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                       hxx 
                     
                   
                   
                     
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                       hxy 
                     
                   
                 
                 
                   
                     
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             . 
           
         
       
     
     The polarization compensation module outputs four real-number signals: Ix_5, Qx_5, Iy_5, and Qy_5. A process of performing polarization compensation by the first polarization compensation module  214  is described with reference to  FIG.  8  ( a ) . 
     Input signals of the first polarization compensation module  214  are four real-number signals: Ix_2, Qx_2, Iy_2, and Qy_2, which are a codirectional signal Ix_2 in a polarization direction x, a quadrature signal Qx_2 in the polarization direction x, a codirectional signal Iy_2 in a polarization direction y, and a quadrature signal Qy_2 in the polarization direction y, respectively. 
     Specifically, a multiplication result of Ix_2 and I hxx  is input to the adder  805 , a negative value of a multiplication result of Qx_2 and Q hxx  is input to the adder  805 , a multiplication result of Iy_2 and I hxy  is input to the adder  805 , a negative value of a multiplication result of Qy_2 and is input to the adder  805 , and the adder  805  outputs a codirectional signal Ix_5 that is in the polarization direction x and that is obtained through polarization compensation. 
     A multiplication result of Ix_2 and Q hxx  is input to the adder  8011 , a negative value of a multiplication result of Qx_2 and I hxx  is input to the adder  8011 , a multiplication result of Iy_2 and Q hxy  is input to the adder  8011 , a multiplication result of Qy_2 and I hxy  is input to the adder  8011 , and the adder  8011  outputs a quadrature signal Qx_5 that is in the polarization direction x and that is obtained through polarization compensation. 
     A multiplication result of Ix_2 and I hyx  is input to the adder  806 , a negative value of a multiplication result of Qx_2 and Q hyx  is input to the adder  806 , a multiplication result of Iy_2 and I hyy  is input to the adder  806 , a negative value of a multiplication result of Qy_2 and Q hyy  is input to the adder  806 , and the adder  806  outputs a codirectional signal Iy_5 that is in the polarization direction y and that is obtained through polarization compensation. 
     A multiplication result of Ix_2 and Q hyx  is input to the adder  8012 , a multiplication result of Qx_2 and I hxy  is input to the adder  8012 , a multiplication result of Iy_2 and Q hyy  is input to the adder  8012 , a multiplication result of Qy_2 and I hyy  is input to the adder  8012 , and the adder  8012  outputs a quadrature signal Qy_5 that is in the polarization direction y and that is obtained through polarization compensation. 
     A process in which the output Ix_5, Qx_5, Iy_5, and Qy_5 of the first polarization compensation module  214  interact with an equalization matrix and the input signals Ix_2, Qx_2, Iy_2, and Qy_2 is described by using the following formula: 
     
       
         
           
             
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     For example, the first polarization compensation module  214  further includes a coefficient update module  2141  (not shown in the figure). The coefficient update module  2141  is configured to update a coefficient of a filter included in the first polarization compensation module  214 . In this embodiment, manners of updating equalization coefficient matrices 
     
       
         
           
             
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             ⁢ 
                 
             
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     of a polarization compensation module include a blind update, an update with training, an update without decision feedback, an update with decision feedback, and the like. A manner of updating a coefficient of the polarization compensation module is not limited in this embodiment. Details are not described herein. 
     It can be seen from  FIG.  8  ( b )  that the second polarization compensation module  310  in this embodiment includes 2×2 butterfly filters, and the butterfly filters include a filter  801 , a filter  802 , a filter  803 , a fourth filter  804 , an adder  805 , and an adder  806 . 
     Functions of the second polarization compensation module  310  include channel optical fiber effects such as equalization compensation, a differential group delay (DGD), and residual chromatic dispersion. An optical fiber effect may be described by using a 2×2 impairment matrix. An inverse matrix of the impairment matrix may be obtained by using different algorithms. Tap coefficients of a time-domain butterfly filter  108  are hxx, hxy, hyx, and hyy. X and Y signals are convolved with the tap coefficient matrices hxx, hxy, hyx, and hyy to obtain signals Xout and Yout that have undergone polarization impairment compensation. Common algorithms for calculating a tap coefficient matrix include a constant modulus algorithm (CMA), a least mean square (LMS), and various modified versions of the LMS or other forms such as a data-aided LMS. 
     The combination module  216  outputs a complex-number signal X in a polarization direction X, and the complex-number signal X is input to the filter  801  and the filter  803 . The combination module  216  outputs a complex-number signal Yin a polarization direction Y, and the complex-number signal Y is input to the filter  802  and the filter  804 . The filter  801  and the filter  803  provide output to the adder  805 . The filter  802  and the filter  804  provide output to the adder  806 . 
     hxx, hxy, hyx, and hyy in  FIG.  8  ( b )  represent coefficients of the filter  801 , the filter  802 , the filter  803 , and the filter  804  respectively. 
     It should be noted that although  FIG.  8  ( a )  and  FIG.  8  ( b )  are used as examples to describe the butterfly filters in the present disclosure, the butterfly filters shown in  FIG.  8  ( a )  and  FIG.  8  ( b )  are merely examples, and do not constitute any limitation on the protection scope of the present disclosure. The butterfly filters in the embodiments of the present disclosure should be understood in a broad sense, and a filter or a filter combination that can implement a deconvolution function may be referred to as a butterfly filter. 
     The Rx DSP provided in this embodiment may be further applied to a multi-mode transmission scenario. For example, the Rx DSP is applied to m transmission modes. In this case, the Rx DSP receives P real-number signals. 
       FIG.  9  ( a )  is a schematic diagram of an Rx DSP architecture in a multi-mode transmission scenario according to an embodiment of the present disclosure. The Rx DSP architecture  900  includes: 
     a first NTT module  211 , a first chromatic dispersion compensation module  212 , a clock recovery module  213 , a first polarization compensation module  214 , a first INTT module  215 , a combination module  216 , a phase recovery module  217 , and a decoding module  218 . 
     P real-number signals are input to the first NTT module  211 . The first NTT module separately performs NTT processing on the P real-number signals, and outputs P transform-domain first real-number signals to the first chromatic dispersion compensation module  212 . 
     Optionally, the P real-number signals are real-number signals obtained through conversion by an ADC. The P real-number signals include 4×m real-number signals in m transmission modes. 
     The first chromatic dispersion compensation module  212  separately performs chromatic dispersion compensation on the P transform-domain first real-number signals, and outputs P transform-domain third real-number signals to the clock recovery module  213 . 
     The clock recovery module  213  separately performs clock recovery on the P transform-domain third real-number signals, and outputs P transform-domain second real-number signals to the first polarization compensation module  214 . 
     The first polarization compensation module  214  performs polarization compensation processing on the P transform-domain second real-number signals, and outputs P transform-domain sixth real-number signals to the first INTT module  215 . 
     The first INTT module  215  separately performs INTT processing on the P transform-domain sixth real-number signals, and outputs P time-domain seventh real-number signals to the combination module  216 . 
     The combination module  216  combines the P time-domain seventh real-number signals, and outputs m complex-number signals X and m complex-number signals Y to the phase recovery module  217  and the decoding module  218 . 
     The phase recovery module  217  and the decoding module  218  perform phase recovery and decoding processing on the m complex-number signals X and the m complex-number signals Y to obtain recovered bit signals. 
       FIG.  9  ( b )  is a schematic diagram of an Rx DSP architecture in another multi-mode transmission scenario according to an embodiment of the present disclosure. The Rx DSP architecture  910  includes: 
     a first NTT module  211 , a first chromatic dispersion compensation module  212 , a clock recovery module  213 , a second polarization compensation module  310 , a first INTT module  214 , a combination module  216 , a phase recovery module  320 , and a decoding module  330 . 
     Modules before clock recovery in the architecture  910  are the same as those in the architecture  900 . A difference lies in that a signal output by the clock recovery module is first processed by the INTT module and combined by the combination module, and then input to the second polarization compensation module  108 . A process before the clock recovery is not described again. 
     The first INTT module  214  separately performs INTT processing on the P transform-domain second real-number signals, and outputs P time-domain eighth real-number signals to the combination module  216 . The combination module  216  combines the P time-domain eighth real-number signals, and outputs m complex-number signals X and m complex-number signals Y to the second polarization compensation module  310 . 
     The second polarization compensation module  310  performs polarization compensation processing on the m complex-number signals X and the m complex-number signals Y, and outputs m time-domain complex-number signals X and m time-domain complex-number signals Y obtained through polarization compensation to the phase recovery module  320  and the decoding module  330 . The second polarization compensation module  310  further includes a coefficient update module  311  (not shown in the figure). The coefficient update module  311  is configured to update a coefficient of a filter included in the second polarization compensation module  310 . 
     The phase recovery module  320  and the decoding module  330  perform phase recovery and decoding processing on the m complex-number signals X and the m complex-number signals Y to obtain recovered bit signals. 
       FIG.  9  ( c )  is a schematic diagram of an Rx DSP architecture in still another multi-mode transmission scenario according to an embodiment of the present disclosure. The Rx DSP architecture  920  includes: 
     a DBP module  510 , a first NTT module  211 , a clock recovery module  213 , a first polarization compensation module  214 , a first INTT module  215 , a combination module  216 , a phase recovery module  217 , and a decoding module  218 . 
     An architecture after the first polarization compensation module  214  in the Rx DSP architecture  920  is the same as that in the Rx DSP architecture  900 . A difference lies in that a signal undergoes DBP processing before polarization compensation. Because the DBP module  510  has a chromatic dispersion compensation function, a process after the DBP module may not require the chromatic dispersion compensation module  212 . 
     P real-number signals are input to the DBP module  510 , DBP processing is performed, and P time-domain tenth real-number signals are output to the first NTT module  211 . 
     The first NTT module  211  separately performs NTT processing on the P time-domain tenth real-number signals, and outputs P transform-domain first real-number signals to the clock recovery module  213 . 
     The clock recovery module  213  separately performs clock recovery processing on the P transform-domain first real-number signals, and outputs P transform-domain second real-number signals to the first polarization compensation module  214 . 
     For a process after the clock recovery module  213 , refer to the embodiment shown in  FIG.  9  ( a ) . 
       FIG.  9  ( d )  is a schematic diagram of an Rx DSP architecture in still another multi-mode transmission scenario according to an embodiment of the present disclosure. The Rx DSP architecture  930  includes: 
     a DBP module  510 , a first NTT module  211 , a clock recovery module  213 , a second polarization compensation module  310 , a first INTT module  215 , a combination module  216 , a phase recovery module  320 , and a decoding module  330 . 
     An architecture after the second polarization compensation module  310  in the Rx DSP architecture  930  is the same as that in the Rx DSP architecture  910 . An architecture before the second polarization compensation module  310  is the same as that in the Rx DSP architecture  920 . To be specific, a received signal obtained through conversion by an ADC first undergoes DBP processing. For a process before the DBP processing and clock recovery, refer to the embodiment shown in  FIG.  9  ( c ) . For a process after the clock recovery module  213 , refer to the embodiment shown in  FIG.  9  ( b ) . 
     For example, the DBP module  510  shown in  FIG.  9  ( c )  and  FIG.  9  ( d )  is shown in  FIG.  9  ( e ) .  FIG.  9  ( e )  is a schematic structural diagram of another DBP module according to an embodiment of the present disclosure. The DBP module  940  includes: 
     a second NTT module  711 , a second chromatic dispersion compensation module  712 , a second INTT module  713 , and a nonlinear compensation module  714 . 
     That P real-number signals are input to the DBP module  510 , and DBP processing is performed to obtain P time-domain tenth real-number signals specifically includes the following steps. 
     The P real-number signals are respectively input to P second NTT modules  711 , NTT processing is performed, and P transform-domain fourth real-number signals are output to the second chromatic dispersion compensation module  712 . 
     The second chromatic dispersion compensation module  712  performs chromatic dispersion compensation processing on the P transform-domain fourth real-number signals, and outputs P transform-domain fifth real-number signals to the second INTT module  713 . 
     The second INTT module  713  performs INTT processing on the P transform-domain fifth real-number signals, and outputs P time-domain ninth real-number signals to the nonlinear compensation module  714 . 
     The nonlinear compensation module  714  performs nonlinear compensation on the P time-domain ninth real-number signals to obtain the P time-domain tenth real-number signals. 
     Specifically, compared with performing signal processing by using the FFT module and the IFFT module, performing signal processing by using the NTT module and the INTT module has the following advantages. 
     (1) When signal processing is performed based on the NTT module and the INTT module, complexity of the Rx DSP architecture is reduced. 
     When signal processing is performed based on the FFT module and the IFFT module, during a signal transmission process, power consumption of merely chromatic dispersion compensation accounts for half of total power consumption of Rx DSP, and complexity of N-point FFT is proportional to N log 2N multiplications. Depending on different data bit widths of operations, complexity of a multiplication is equal to several times to dozens of times of that of an addition. 
     However, complexity of signal processing performed based on the NTT module and the INTT module is proportional to 2N log 2N additions. Depending on different data bit widths and different quantities N of transformation points, complexity of signal processing performed based on the NTT module and the INTT module is reduced to different degrees. For example, if a quantity of transformation points is 256, power consumption of NTT is reduced to approximately ¼ of that of FFT. 
     (2) When signal processing is performed based on the NTT module and the INTT module, a transformation matrix has no truncation error or fixed-point penalty. 
     When signal processing is performed based on the FFT module and the IFFT module, because a transformation core of FFT is an exponential function, a truncation error cannot be accurately expressed on a binary computer. In addition, a bit width of a transformation matrix is not fixed, causing a fixed-point penalty. 
     However, in signal processing performed based on the NTT module and the INTT module, because a transformation core of NTT is an integer and the transformation core is usually 2 or a power of 2, the transformation core can be accurately expressed in a memory. In addition, a bit width of a transformation matrix is fixed, without a fixed-point penalty. 
     (3) When signal processing is performed based on the NTT module and the INTT module, no error accumulates. 
     A truncation error and a fixed-point penalty in a single FFT or IFFT transformation repeatedly accumulate during DBP propagation. As a result, a final result error is larger. 
     However, there is no truncation error or fixed-point penalty in a single NTT or INTT transformation. Therefore, repeated Ns transformations do not cause error accumulation. 
     (4) When signal processing is performed based on the NTT module and the INTT module, storage can be reduced. 
     A transformation core of NTT is 2, and a transformation matrix includes 2 or a power of 2. In a binary system, a number is multiplied by a power of 2 only through shifting. Therefore, no huge transformation matrix needs to be stored in a DSP process, and only the transformation core needs to be stored. 
       FIG.  10  ( a )  is a schematic flowchart of a signal processing method according to an embodiment of the present disclosure. The following steps S 1010  to S 1050  are included. 
     S 1010 . Obtain P real-number signals. 
     For example, the P real-number signals include real-number signals that are in two polarization directions and that correspond to each of m modes. When m is equal to 1, it indicates single-mode transmission; or when m is greater than 1, it indicates multi-mode transmission. m and P are positive integers. 
     S 1020 . Obtain P transform-domain first real-number signals. 
     At least NTT processing is performed on the P real-number signals to obtain the P transform-domain first real-number signals. The following two possible implementations are included. 
     Implementation 1.1: Perform NTT processing on the P real-number signals to obtain the P transform-domain first real-number signals. 
     Implementation 1.2: Sequentially perform DBP processing and NTT processing on the P real-number signals to obtain the P transform-domain first real-number signals. 
     In the implementation 1.2, DBP processing is first performed on the P real-number signals to obtain P time-domain tenth real-number signals, and then NTT processing is performed on the P time-domain tenth real-number signals to obtain the P transform-domain first real-number signals. 
     The performing NTT processing on the P real-number signals in the implementation 1.1, and the performing NTT processing on the P time-domain tenth real-number signals that are obtained by performing DBP processing on the P real-number signals in the implementation 1.2 may be collectively referred to as performing NTT processing on P input signals. In other words, in this embodiment of the present disclosure, the P input signals include the P real-number signals, or the P time-domain tenth real-number signals that are obtained by performing DBP processing on the P real-number signals. 
     For ease of understanding, a process of performing DBP processing on the P real-number signals to obtain the P time-domain tenth real-number signals is briefly described with reference to  FIG.  10  ( b ) .  FIG.  10  ( b )  is a schematic flowchart of DBP processing according to an embodiment of the present disclosure. The following steps S 1021  to S 1024  are included. 
     S 1021 . Obtain P transform-domain fourth real-number signals. 
     NTT processing is performed on P real-number signals to obtain the P transform-domain fourth real-number signals. 
     S 1022 . Obtain P transform-domain fifth real-number signals. 
     Chromatic dispersion compensation is performed on the P transform-domain fourth real-number signals in transform domain to obtain the P transform-domain fifth real-number signals. 
     S 1023 . Obtain P time-domain ninth real-number signals. 
     INTT processing is performed on the P transform-domain fifth real-number signals to obtain the P time-domain ninth real-number signals. 
     S 1024 . Obtain P time-domain tenth real-number signals. 
     Nonlinear compensation is performed on the P time-domain ninth real-number signals to obtain the P time-domain tenth real-number signals. 
     S 1030 . Obtain P transform-domain second real-number signals. 
     At least clock recovery processing is performed on the P transform-domain first real-number signals to obtain the P transform-domain second real-number signals. Corresponding to the two possibilities in step S 1020 , the following two possible implementations are included. 
     Implementation 2.1: This corresponds to the implementation 1.1 in step S 1020 . A processing procedure includes the following steps. 
     First, perform chromatic dispersion compensation on the P transform-domain first real-number signals to obtain P transform-domain third real-number signals. 
     Then, separately perform clock recovery on the P transform-domain third real-number signals to obtain the P transform-domain second real-number signals. 
     The performing chromatic dispersion compensation on the P transform-domain first real-number signals to obtain P transform-domain third real-number signals specifically includes the following steps: 
     First, determine a chromatic dispersion impulse response I h (t) corresponding to an in-phase real-number signal and a chromatic dispersion impulse response Q h (t) corresponding to a quadrature real-number signal, and perform NTT to obtain a transform-domain chromatic dispersion equalization function I h (w) corresponding to the in-phase real-number signal and a transform-domain chromatic dispersion equalization function Q h (w) corresponding to the quadrature real-number signal. 
     Then, determine 2×m transform-domain third real-number signals that are in the P transform-domain third real-number signals and that are in a first polarization direction based on I h (w), Q h (w), and an in-phase real-number signal and a quadrature real-number signal that are in the P transform-domain first real-number signals and that are in the first polarization direction, and determine 2×m transform-domain third real-number signals that are in the P transform-domain third real-number signals and that are in a second polarization direction based on I h (w), Q h (w), and an in-phase real-number signal and a quadrature real-number signal that are in the P transform-domain first real-number signals and that are in the second polarization direction. 
     Specifically, the transform-domain third real-number signals obtained through chromatic dispersion compensation and the transform-domain first real-number signals that have not undergone chromatic dispersion compensation meet the following requirements: 
         I′   x ( w )= I   x ( w )· I   h ( w )− Q   x ( w )· Q   h ( w );
 
         Q′   x ( w )= Q   x ( w )· I   h ( w )+ I   x ( w )· Q   h ( w );
 
         I′   y ( w )= I   y ( w )· I   h ( w )− Q   y ( w )· Q   h ( w ); and
 
         Q′   y ( w )= Q   y ( w )· I   h ( w )+ I   y ( w )· Q   h ( w ), where
 
     I x (w) represents an in-phase real-number signal that is in the transform-domain first real-number signals and that is in the first polarization direction, Q x (w) represents a quadrature real-number signal that is in the transform-domain first real-number signals and that is in the first polarization direction, I y (w) represents an in-phase real-number signal that is in the transform-domain first real-number signals and that is in the second polarization direction, Q y (w) represents a quadrature real-number signal that is in the transform-domain first real-number signals and that is in the second polarization direction, I′ x (w) represents an in-phase real-number signal that is in the transform-domain third real-number signals and that is in the first polarization direction, Q′ x (w) represents a quadrature real-number signal that is in the transform-domain third real-number signals and that is in the first polarization direction, I′ y (w) represents an in-phase real-number signal that is in the transform-domain third real-number signals and that is in the second polarization direction, and Q′ y (w) represents a quadrature real-number signal that is in the transform-domain third real-number signals and that is in the second polarization direction. 
     Implementation 2.2: 
     This corresponds to the implementation 1.2 in step S 1020 . A processing procedure includes the following step: 
     Separately perform clock recovery on the P transform-domain first real-number signals to obtain the P transform-domain second real-number signals. 
     S 1040 . Obtain time-domain complex-number signals. 
     At least polarization compensation processing and INTT processing are performed on the P transform-domain second real-number signals to obtain m time-domain complex-number signals X and m time-domain complex-number signals Y. The following two possible implementations are included. 
     Implementation 3.1: Sequentially perform polarization compensation processing, INTT processing, and combination processing on the P transform-domain second real-number signals to obtain the m time-domain complex-number signals X and the m time-domain complex-number signals Y.  FIG.  10  ( c )  is a schematic flowchart of obtaining a time-domain complex-number signal according to an embodiment of the present disclosure. The following steps S 1041  to S 1043  are included. 
     S 1041 . Obtain P transform-domain sixth real-number signals. 
     Polarization compensation is performed on P transform-domain second real-number signals to obtain the P transform-domain sixth real-number signals. 
     S 1042 . Obtain P time-domain seventh real-number signals. 
     INTT processing is separately performed on the P transform-domain sixth real-number signals to obtain the P time-domain seventh real-number signals. 
     S 1043 . Perform combination to obtain m complex-number signals X and m complex-number signals Y. 
     Every two of 2×m time-domain seventh real-number signals that are in the P time-domain seventh real-number signals and that are in a first polarization direction are combined to obtain the m time-domain complex-number signals X in the first polarization direction. 
     Every two of 2×m time-domain seventh real-number signals that are in the P time-domain seventh real-number signals and that are in a second polarization direction are combined to obtain the m time-domain complex-number signals Y in the second polarization direction. 
     Implementation 3.2: Sequentially perform INTT processing, combination processing, and polarization compensation processing on the P transform-domain second real-number signals.  FIG.  10  ( d )  is another schematic flowchart of obtaining a time-domain complex-number signal according to an embodiment of the present disclosure. The following steps S 1044  to S 1046  are included. 
     S 1044 . Obtain P time-domain eighth real-number signals. 
     INTT processing is performed on P transform-domain second real-number signals to obtain the P time-domain eighth real-number signals. 
     S 1045 . Obtain m complex-number signals X and m complex-number signals Y. 
     Every two of 2×m time-domain eighth real-number signals that are in the P time-domain eighth real-number signals and that are in a first polarization direction are combined to obtain the m time-domain complex-number signals X in the first polarization direction. 
     Every two of 2×m time-domain eighth real-number signals that are in the P time-domain eighth real-number signals and that are in a second polarization direction are combined to obtain the m time-domain complex-number signals Y in the second polarization direction. 
     S 1046 . Obtain m complex-number signals X and m complex-number signals Y that have undergone polarization compensation. 
     Polarization compensation is performed on the m time-domain complex-number signals X in the first polarization direction and the m time-domain complex-number signals Y in the second polarization direction to obtain the m time-domain complex-number signals X and the m time-domain complex-number signals Y that have undergone the polarization compensation. 
     S 1050 . Obtain bit signals. 
     To obtain the recovered bit signals, phase recovery and decoding need to be performed on the time-domain complex-number signals obtained in S 1040 . 
     First, phase recovery is performed on the m time-domain complex-number signals X and the m time-domain complex-number signals Y to obtain m time-domain complex-number signals X and m time-domain complex-number signals Y that have undergone the phase recovery. 
     Then, the m time-domain complex-number signals X and the m time-domain complex-number signals Y that are obtained through phase recovery are decoded to obtain m time-domain complex-number signals X and m time-domain complex-number signals Y that have been decoded. 
     It should be noted that how to perform phase recovery and decoding on the time-domain complex-number signals is not limited in this embodiment of the present disclosure. 
     Refer to a current process of performing phase recovery and decoding on an electrical signal by a polarization multiplexing coherent receiver. 
     An embodiment of the present disclosure further provides a coherent receiver, including a polarization beam splitter, a frequency mixer, a photoelectric detector, an analog-to-digital converter, and the Rx DSP shown in  FIG.  9  ( a ) ,  FIG.  9  ( b ) ,  FIG.  9  ( c ) , or  FIG.  9  ( d ) . The polarization beam splitter, the frequency mixer, the photoelectric detector, and the analog-to-digital converter are similar to those shown in  FIG.  1  ( a )  and  FIG.  1  ( b ) . Details are not described herein again. 
     An embodiment of the present disclosure further provides a chip.  FIG.  11    is a schematic diagram of a chip  1100  according to the present disclosure. The chip  1100  includes a processor  1110 , a memory  1120 , and a communications interface  1130 . The processor  1110  is coupled to the memory  1120 . The memory  1120  is configured to store a computer program or an instruction and/or data. The processor  1110  is configured to execute the computer program or the instruction and/or the data stored in the memory  1120 , so that the method in the foregoing method embodiment is performed. 
     In a possible implementation, the chip shown in  FIG.  11    may be a signal processing apparatus including a processor  1110  and a communications interface  1130 . The processor  1110  is coupled to a memory by using the communications interface  1130 , and the processor  1110  is configured to perform the method in the foregoing method embodiment. 
     It should be understood that the foregoing embodiments are merely examples for describing the signal processing procedure provided in the present disclosure, and do not constitute any limitation on the protection scope of the present disclosure. Other simple variations of the Rx DSP architecture all fall within the protection scope of the present disclosure. A difference between the Rx DSP architecture in the present disclosure and an existing Rx DSP architecture lies in that an FFT module and an IFFT module are not used for signal processing, but an NTT module and an INTT module are used. 
     It should be further understood that, in the embodiments of the present disclosure, unless otherwise specified or in case of a logical conflict, terms and/or descriptions in different embodiments may be consistent and may be mutually referenced. Technical features in different embodiments may be combined based on an internal logical relationship of the technical features to form a new embodiment. 
     It may be clearly understood by a person skilled in the art that, for the purpose of convenient and brief description, for a detailed working process of the foregoing system, apparatus, and unit, refer to a corresponding process in the foregoing method embodiments, and details are not described herein again. 
     When the functions are implemented in the form of a software functional unit and sold or used as an independent product, the functions may be stored in a computer-readable storage medium. Based on such an understanding, the technical solutions of the present disclosure essentially, or the part contributing to the prior art, or some of the technical solutions may be implemented in a form of a software product. The software product is stored in a storage medium, and includes several instructions for instructing a computer device (which may be a personal computer, a server, or a network device) to perform all or some of the steps of the methods described in the embodiments of the present disclosure. The foregoing storage medium includes: any medium that can store program code, such as a USB flash drive, a removable hard disk, a read-only memory (ROM), a random access memory (RAM), a magnetic disk, or an optical disc.