Patent Publication Number: US-9853765-B2

Title: Signal processing device and signal processing method for optical polarization multiplexed signal

Description:
This application is a National Stage Entry of PCT/JP2014/053853 filed on Feb. 19, 2014, which claims priority from Japanese Patent Application 2013-081395 filed on Apr. 9, 2013, the contents of all of which are incorporated herein by reference, in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to a signal processing device and a signal processing method. 
     BACKGROUND ART 
     With the spread of the Internet, an amount of data that is communicated is increased. In order to deal with this, there is a need to increase a capacity of a transmission path. As one technology for realizing a greater increase in capacity, there is a multilevel modulation (quadrature amplitude modulation: QAM) scheme. An optical signal on which modulation in compliance with the QAM scheme is performed in a transmitter is demodulated in a digital-coherent type light receiver. 
     In optical communication in compliance with the QAM scheme, there is a major problem of a nonlinear effect to which the optical signal is subjected upon propagating along the transmission path. When the optical signal is subjected to the nonlinear effect while in the transmission path, a phase of the optical signal is rotated. Because complicated phase information is handled in the QAM scheme, if the optical signal is subjected to phase rotation due to the nonlinear effect, correct phase information cannot be demodulated at the time of receiving. 
     In contrast, a nonlinear compensation scheme called backpropagation is disclosed in Non-Patent Document 1. This compensation scheme is a scheme in which dispersion compensation is performed little by little and nonlinear compensation is performed immediately after each dispersion compensation operation is performed, and thus waveform distortion is compensated for while a propagation waveform is traced backward from the receiving side to the transmitting side. 
     However, in the backpropagation, when a dispersion compensation function and a nonlinear compensation function are combined as one nonlinear compensation stage, there is a need to increase the number of stages for the nonlinear compensation. The dispersion compensation function is realized by a linear distortion compensation circuit, and the nonlinear compensation function is realized by a nonlinear distortion compensation circuit. Because the linear distortion compensation circuit performs the dispersion compensation in a frequency domain, the linear distortion circuit includes an FFT/IFFT circuit. Because the FFT/IFFT circuit is large in circuit scale, when a mounting area for an LSI and power consumption are considered, only several FFT/IFFT circuits can be mounted in one signal processing device. 
     In contrast, a compensation scheme called filtered backpropagation is also disclosed in Non-Patent Document 1. In the filtered backpropagation, an amount of a time average of amounts of phase rotation, which is calculated from a signal strength, is used for the nonlinear compensation, and thus the number of stages for the nonlinear compensation is reduced. Furthermore, in Non-Patent Document 1, a low pass filter is used for the time average of the amounts of phase rotation. 
     Additionally, a technique for setting a coefficient of the low pass filter described above is disclosed in Non-Patent Document 2. In Non-Patent Document 2, demodulation is performed without the nonlinear compensation being performed on the received optical signal. A difference between a symbol position that is demodulated and an ideal symbol position for the signal is monitored, and thus the coefficient of the low pass filter is determined. 
     RELATED DOCUMENT 
     Non-Patent Document 
     
         
         [Non-Patent Document 1] Liang B. Du and Arthur J. Lowery, “Improved single channel backpropagation for intra-channel fiber nonlinearity compensation in long-haul optical communication systems”, OPTICS EXPRESS, Vol. 18, No. 16, pp 17075-17088, 2010 
         [Non-Patent Document 2] Lei Li et al., “Implementation Efficient Nonlinear Equalizer Based on Correlated Digital Backpropagation”, Proc. Conf. OFC OWW3, 2011 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     In a method that is disclosed in Non-Patent Literature 2, it is assumed that a signal that is received can be demodulated without the nonlinear compensation. For this reason, the method cannot apply to the received signal of low quality which requires the nonlinear compensation for demodulation. 
     An object of the present invention is to make it possible to set a coefficient of nonlinear compensation even if demodulation is not performed in a case where optical signal that is polarization multiplexed and multilevel modulated is received and is demodulated. 
     Solution to Problem 
     According to an aspect of the present invention, there is provided a signal processing device including: an electric signal generation unit that generates an electric signal based on optical signal which is polarization multiplexed and multilevel modulated and which is transmitted over a transmission path; a linear compensation unit that performs processing which compensates for dispersion that occurs on the optical signal in the transmission path to the electric signal, using a first filter coefficient; a second coefficient setting unit that determines a second coefficient that determines a width on a time axis, which has to be considered when compensating for a nonlinear effect that occurs on the optical signal in the transmission path, using an amount of the dispersion; and a nonlinear compensation unit that compensates the electric signal for the nonlinear effect, using the second filter coefficient. 
     According to another aspect of the present invention, there is provided a signal processing method including: generating an electric signal based on optical signal which is polarization multiplexed and multilevel modulated and which is transmitted over a transmission path; performing processing which compensates for dispersion that occurs on the optical signal in the transmission path to the electric signal, using a first filter coefficient; determining a second filter coefficient that determines a width on a time axis, which has to be considered when compensating for a nonlinear effect that occurs on the optical signal in the transmission path, using an amount of the dispersion; and compensating the electric signal for the nonlinear effect, using the second filter coefficient. 
     Advantageous Effects of Invention 
     According to the present invention, a coefficient of nonlinear compensation can be set even if demodulation is not performed in a case where optical signal that is polarization multiplexed and multilevel modulated is received and is demodulated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects described above, and other objects, features and advantages are further made apparent by suitable exemplary embodiments described below and the following accompanying drawings. 
         FIG. 1  is a diagram illustrating a configuration of an optical communication system relating to a first exemplary embodiment. 
         FIG. 2  is a diagram illustrating one example of a functional configuration of an optical reception device. 
         FIG. 3  is a diagram illustrating a functional configuration of an optical reception device according to a second exemplary embodiment. 
         FIG. 4  is a diagram for describing a functional configuration of a distortion compensation unit. 
         FIG. 5  is a diagram illustrating one example of a functional configuration of a nonlinear compensation unit. 
         FIG. 6  is a diagram for describing processing by a number-of-taps calculation unit and by a second coefficient calculation unit in detail. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Exemplary embodiments of the present invention are described referring to the drawings. Moreover, in all the drawings, like constituent elements are given like reference numerals, respectively, and descriptions thereof will not be repeated. 
     First Exemplary Embodiment 
       FIG. 1  is a diagram illustrating a configuration of an optical communication system according to a first exemplary embodiment. The optical communication system according to the present exemplary embodiment includes an optical transmission device  10  and an optical reception device  20 . The optical transmission device  10  and the optical reception device  20  are connected to each other through a transmission path  30 . The transmission path  30  is configured using an optical fiber or the like. The optical communication system is a system that communicates using a quadrature amplitude modulation (QAM) scheme. 
     An optical transmission device  10  (a signal processing device) generates an optical signal that is polarization multiplexed and multilevel modulated, by modulating light using multiple signals that are to be transmitted. The generated optical signal is transmitted to the optical reception device  20  over the transmission path  30 . The optical reception device  20  demodulates the received optical signal. When propagating along the transmission path  30 , the optical signal is subjected to a linear effect (a dispersion effect) and a nonlinear effect. The optical reception device  20  performs processing for compensating for these effects. 
       FIG. 2  is a diagram illustrating one example of a functional configuration of the optical reception device  20 . The optical reception device  20  includes an electric signal generation unit  200 , a linear compensation unit  301 , a nonlinear compensation unit  300 , and a second coefficient setting unit  400 . The electric signal generation unit  200  generates an electric signal, based on the optical signal that is received over the transmission path  30 . The linear compensation unit  301  performs processing for compensating for dispersion that occurs on optical signal in the transmission path  30 , on the electric signal, using a first filter coefficient. The second coefficient setting unit  400  determines a second filter coefficient for compensating for the nonlinear effect that occurs on the optical signal along the transmission path, using an amount of dispersion that occurs in the transmission path  30 . The nonlinear compensation unit  300  performs processing for compensating for the nonlinear effect, on the electric signal, using the second filter coefficient that is determined by the second coefficient setting unit  400 . 
     The optical signal is a pulse signal. Then, the nonlinear effect to which the optical signal is subjected while being transmitted over the transmission path  30  occurs because a certain pulse on a time axis is subjected to an influence of the certain pulse itself and a pulse that is positioned adjacent to the certain pulse. For this reason, the nonlinear effect to which the pulse is subjected is determined by an expansion of the width of the pulse. On the other hand, the expansion of the width of the pulse is determined by an amount of dispersion of the optical signal. Therefore, when the second filter coefficient is determined using the amount of dispersion that occurs in the transmission path  30 , the nonlinear effect can be compensated for with high accuracy. For this reason, even if modulation is not performed, a coefficient for nonlinear compensation can be set. 
     If configurations of the optical communication system and of the transmission path  30  are determined, the amount of dispersion that occurs in the transmission path is determined almost uniquely. Therefore, according to the present exemplary embodiment, after the optical transmission device  10  and the optical reception device  20  are installed, when the amount of dispersion that occurs in the transmission path  30  is measured, the second filter coefficient can be determined. 
     Second Exemplary Embodiment 
     An optical communication system according to the present exemplary embodiment has the same configuration as the optical communication system according to the first exemplary embodiment except for a configuration of the optical reception device  20 . 
       FIG. 3  is a diagram illustrating a functional configuration of the optical reception device  20 . The optical reception device  20  includes a local optical source (LO)  210 , an optical 90-degree hybrid  220  (an interference unit), an optical-to-electric (O/E) converter  230 , an analog-to-digital (AD) converter (ADC)  240 , and a signal processing unit  100 . The signal processing unit  100  is configured as one semiconductor device. 
     An optical signal and a local light from the local optical source  210  are input into the optical 90-degree hybrid  220 . The optical 90-degree hybrid  220  causes the optical signal and the local light to interfere with each other with a phase difference of 0 and thus generates a first optical signal (I x ) and causes the optical signal and the local light to interfere with each other with a phase difference of π/2 and thus generates a second optical signal (Q x ). Furthermore, the optical 90-degree hybrid  220  causes the optical signal and the local light to interfere with each other with a phase difference of 0 and thus generates a third optical signal (I y ) and causes the optical signal and the local light to interfere with each other with a phase difference of π/2 and thus generates a fourth optical signal (Q y ). The first optical signal and the second optical signal form a set of signals and the third optical signal and the fourth optical signal form a set of signals. 
     The optical-to-electric converter  230  performs optical-to-electric conversion on four optical signals (output lights) that are generated by the optical 90-degree hybrid  220 , and thus generates four analog signals. 
     The AD converter  240  converts each of the four analog signals that are generated by the optical-to-electric converter  230 , into a digital signal (quantization). 
     The signal processing unit  100  processes the four digital signals that are generated by the AD converter  240  and thus generates a demodulation signal that results from demodulating the optical signal. Specifically, the signal processing unit  100  includes a polarization signal generation unit  110 , a distortion compensation unit  102 , a polarization demultiplexing unit  104 , and a demodulator  106 . 
     The polarization signal generation unit  110  includes addition units  112  and  114 . The addition unit  112  performs addition processing on the digital signal that is generated from the first optical signal (I x ) and the digital signal that is generated from the second optical signal (Q x ) and thus generates a first polarization signal (E x ). The addition unit  114  performs addition processing on the digital signal that is generated from the third optical signal (I y ) and the digital signal that is generated from the fourth optical signal (Q y ) and thus generates a second polarization signal (E y ). Specifically, E x  and E y  are obtained according to Expressions (1) and (2) as follows.
 
[Math. 1]
 
 E   x   =I   x   +jQ   x   (1)
 
[Math. 2]
 
 E   y   =I   y   +jQ   y   (2)
 
     The distortion compensation unit  102  performs the processing for compensating for the linear effect and the nonlinear effect to which the optical signal is subjected while propagating along the transmission path  30 . The distortion compensation unit  102  will be described in detail below. 
     The polarization demultiplexing unit  104  performs filter calculation for each polarization. The demodulator  106  compensates for a frequency difference and a phase difference between the optical signal and the local light, and demodulates the signal that is transmitted. 
       FIG. 4  is a diagram for describing a functional configuration of the distortion compensation unit  102 . The distortion compensation unit  102  has at least one processing stage including a linear compensation unit  301  and a nonlinear compensation unit  300 . Moreover, in a case where the number of processing stages is small (for example, equal to or smaller than five stages), it is preferable that the final stage in the distortion compensation unit  102  is the linear compensation unit  301  (a second dispersion compensation unit). However, in a case where the number of processing stages is equal to or greater than ten stages, the final stage in the distortion compensation unit  102  may not be the linear compensation unit  301 . 
     Moreover, even in any cases where the final stage in the distortion compensation unit  102  is the linear compensation unit  301  and where the final stage therein is the nonlinear compensation unit  300 , a sum of amounts of dispersion compensation that is performed by the linear compensation unit  301  included in the distortion compensation unit  102  is equal to an amount of dispersion that the optical signal receives while in the transmission path  30 . 
     The linear compensation unit  301  compensates for the linear effect to which the optical signal is subjected while in the transmission path  30 . The linear compensation unit  301  includes, for example, a fast Fourier transform (FFT) unit, a filter unit, and an inverse fast Fourier transform (IFFT) unit. The FFT performs FFT calculation on a signal that is input. The filter unit performs filter calculation on the signal, using the first filter coefficient for compensating for the dispersion effect to which the optical signal is subjected in the transmission path. The IFFT unit performs IFFT calculation on the signal on which the filter processing is performed. 
     The nonlinear compensation unit  300  compensates for the nonlinear effect to which the optical signal is subjected in the transmission path  30 , using the second filter coefficient. 
     Furthermore, the distortion compensation unit  102  includes a first coefficient setting unit  420  and the second coefficient setting unit  400 . The first coefficient setting unit  420  sets the first filter coefficient in the linear compensation unit  301 . The first filter coefficient may be calculated by the first coefficient setting unit  420  using the dispersion to which the optical signal is subjected in the transmission path  30 , and may be directly input into the first coefficient setting unit  420  from the outside. 
     The second coefficient setting unit  400  includes a number-of-taps calculation unit  402  and a second coefficient calculation unit  404 . The number-of-taps calculation unit  402  determines the number of taps using the dispersion to which the optical signal is subjected in the transmission path  30 . The second coefficient calculation unit  404  separates a predetermined function at equal intervals into sections of which the number is the same as the number of taps, and sets a value of a function in each of the multiple sections as the second filter coefficient. Then, the second coefficient calculation unit  404  sets the calculated second filter coefficient in the nonlinear compensation unit  300 . Setting processing of the second coefficient by the second coefficient setting unit  400  will be described in detail below. 
     Moreover, the dispersion to which the optical signal is subjected while in the transmission path  30  is input from a dispersion setting unit  500 . 
       FIG. 5  is a diagram illustrating one example of a functional configuration of the nonlinear compensation unit  300 . In the example in  FIG. 5 , the nonlinear compensation unit  300  performs compensation processing in compliance with filtered back propagation. However, the nonlinear compensation unit  300  may perform processing in compliance with a different scheme. 
     The nonlinear compensation unit  300  includes strength calculation units  302  and  304 , an addition unit  305 , a filter unit  306 , a phase modulator  308 , delay units  310  and  314 , and multiplication units  312  and  316 . The strength calculation unit  302  calculates a strength of a polarization signal E x  and calculates an amount of phase rotation that is based on the strength. The strength calculation unit  304  calculates a strength of a polarization signal E y  and calculates an amount of phase rotation that is based on the strength. The addition unit  305  adds the amount of phase rotation that is calculated by the strength calculation unit  302 , and the amount of phase rotation that is calculated by the strength calculation unit  304 . The filter unit  306  multiplies the amount of phase rotation that is output by the addition unit  305  by a coefficient (the second filter coefficient: h(n), which is described above) for temporal averaging. The phase modulator  308  uses the amount of phase rotation after being processed by the filter unit  306 , and thus calculates a coefficient for compensating for phase rotation. Then, this coefficient is multiplied by the polarization signal E x  after being delayed by the delay unit  310  by the multiplication unit  312 , and is multiplied by the polarization signal E y  after being delayed by the delay unit  314  by a multiplication unit  316 . Moreover, the delay units  310  and  314  are provided to synchronize the polarization signals E x  and E y  to a coefficient calculation timing. 
     Moreover, the nonlinear compensation unit  300  that is illustrated in  FIG. 5  performs processing in compliance with Expressions (3) and (4) as follows. 
     
       
         
           
             
               
                 
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       FIG. 6  is a diagram for describing processing by the number-of-taps calculation unit  402  and by the second coefficient calculation unit  404  in detail. 
     As described according to the first exemplary embodiment, the nonlinear effect to which the optical signal is subjected upon being transmitted over the transmission path  30  occurs because a certain pulse on a time axis is subjected to an influence of the certain pulse itself and a pulse that is positioned adjacent to the certain pulse. For this reason, the nonlinear effect to which the pulse is subjected is determined by an expansion of the width of the pulse. Therefore, the larger the dispersion, the greater the width of the pulse, and a time width becomes greater that has to be considered upon calculating the amount of phase rotation that results from the nonlinear effect. 
     On the other hand, the second coefficient calculation unit  404  separates a predetermined function at equal intervals into sections of which the number is the same as the number of taps, and sets a value of a function in each of the sections as the second filter coefficient. 
     Then, the number-of-taps calculation unit  402  multiplies the amount of dispersion by a proportional coefficient, and thus calculates the number of taps. Thereby, as the dispersion is larger, the number of taps is increased. As a result, the time width becomes greater that is set in the second coefficient calculation unit  404 . 
     Moreover, the proportional coefficient, for example, is set by a manager of the optical communication system based on the function and the dispersion that are used by the second coefficient calculation unit  404 . 
     Furthermore, in an example in  FIG. 6 , a function used by the second coefficient calculation unit  404  is determined in such a manner that the function has a maximum value in a tap that is positioned in the center and has minimum values in taps that are positioned in both of the ends. In the example in  FIG. 6 , the tap that is positioned in the center and the taps that are positioned at both of the ends are connected by a straight line, but may be connected by a curved line. 
     Furthermore, in the function that is used by the second coefficient calculation unit  404 , the maximum value, the minimum value, and the shape of a line that connects the maximum value and the minimum value are fixed, but the number of taps from the maximum value to the minimum value is not fixed. Then, a line (that is, a function) connecting the maximum value and the minimum value is divided into as many sections as the number that is half the number of taps, and the second filter coefficient corresponding to each tap is determined as a value of the function in each of the multiple sections. For this reason, in a case where the number of taps that is calculated by the number-of-taps calculation unit  402  is increased, a difference of two second filter coefficients that correspond to taps adjacent to each other becomes smaller. 
     According to the present exemplary embodiment, even if the demodulation is not performed, the second filter coefficient used in the nonlinear compensation can be set. 
     The exemplary embodiments of the present invention are described above referring the drawings, but these are only examples of the present invention, and various configurations other than the configurations described above may be employed. 
     This application claims priority from Japanese Patent Application No. 2013-081395, filed on Apr. 9, 2013, the contents of which are incorporated by reference herein in its entirety.