Multicarrier signal waveform equalization circuit and multicarrier signal waveform equalization method

Abstract

According to the present invention, a multicarrier signal waveform equalization circuit comprises: an acquisition unit that acquires an electrical signal obtained by converting, by means of coherent detection, an optical signal which is converted into a digital modulation signal via phase modulation or quadrature amplitude modulation, split into multiple carrier waves, superimposed on a local oscillation laser, and transmitted from a transmission unit; and a crosstalk compensation unit that compensates for crosstalk between a plurality of signals superimposed on each of the plurality of carrier waves obtained from the acquired electric signal.

Description

[Technical Field]

[0001] The present invention relates to a multi-carrier signal waveform equalization circuit and a multi-carrier signal waveform equalization method. [Background technology]

[0002] In coherent optical communications, polarization / phase diversity transmission and reception has been realized, and digital signal processing utilizing the phase information of signal light obtained on the receiving side has been realized (see, for example, Non-Patent Documents 1 and 2). Crosstalk and linear distortion between polarization multiplexed signals can be equalized by controlling the adaptive filter coefficients of a digital filter, typically a finite impulse response (FIR) filter.

[0003] Furthermore, due to imperfections in analog devices used in transmitters and receivers, there is an IQ imbalance and a time delay difference (skew) between the in-phase and quadrature components of a quadrature amplitude modulation (QAM) signal, which causes crosstalk between the IQ components. Similar to crosstalk between polarization multiplexed signals, this crosstalk between the IQ components can also be equalized by adaptively controlling the filter coefficients of a digital filter (see, for example, Non-Patent Document 3). In this case, for example, it is possible to control the filter coefficients to minimize the mean square error with a reference signal.

[0004] Furthermore, a transmission method has been realized in which a time-series signal is divided into multiple carrier waves and transmitted and received as a multicarrier signal (see, for example, Non-Patent Document 4). In this case, the quadrature / amplitude error (IQ imbalance) and the time delay difference (skew) are observed as crosstalk between subcarrier signals, as shown in FIG. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Seb J. Savory, “Digital filters for coherent optical receivers,” Optics Express, Vol.16, No.2, pp.804-817, January 2008. [Non-patent document 2] K. Kikuchi, “Fundamentals of Coherent Optical Fiber Communications,” Journal of Lightwave Technology, Vol.34, No.1, pp.157-179, January 2016. [Non-patent document 3] W. Nam, H. Roh, J. Lee and I. Kang, “Blind Adaptive I / Q Imbalance Compensation Algorithms for Direct-Conversion Receivers,” IEEE Signal Processing Letters, Vol.19, No,8, pp.475-478, August 2012. [Non-patent document 4] EP d. Silva and D. Zibar, “Widely Linear Blind Adaptive Equalization for Transmitter IQ-Imbalance / Skew Compensation in Multicarrier Systems,” ECOC 2016, 42nd European Conference on Optical Communications (ECOC 2016), pp.1-3, September 2016. [Non-Patent Document 5] A. Li, Y. Zhu, W. Peng, Y. Cui and Y. Bai, “103-GBd PDM-16QAM Coherent Detection Highly Tolerant to Transmitter IQ Impairments Enabled by Real-Valued 4 x 4 MIMO Equalizer,” 45th European Conference on Optical Communication (ECOC 2019), September 2019. [Non-patent document 6] P. Skvortcov, C. Sanchez-Costa, I. Phillips and W. Forysiak, “Receiver DSP highly tolerant to transmitter IQ impairments,” Optical Fiber Communications Conference and Exhibition (OFC 2019), pp.1-3, May 2019. [Non-Patent Document 7] Y. Fan, X. Chen, W. Zhou, X. Zhou and H. Zhu, “The Comparison of CMA and LMS Equalization Algorithms in Optical Coherent Receivers,” 6th International Conference on Wireless Communications Networking and Mobile Computing (WiCOM 2010), pp.1-4, September 2010. Summary of the Invention [Problem to be solved by the invention]

[0006] Conventional equalization methods can equalize crosstalk in signals with a single carrier wave (single-carrier signal). However, in an environment where analog device imperfections and laser phase noise / frequency errors exist, conventional equalization methods cannot effectively equalize crosstalk between subcarriers in multi-carrier signals.

[0007] In view of the above circumstances, an object of the present invention is to provide a multi-carrier signal waveform equalization circuit and a multi-carrier signal waveform equalization method that can perform signal processing with higher accuracy in communications using multi-carrier signals, even in an environment where imperfections in analog devices and laser phase noise / frequency errors exist. [Means for solving the problem]

[0008] One aspect of the present invention is a multi-carrier signal waveform equalization circuit comprising: an acquisition unit that acquires an electrical signal converted from an optical signal by coherent detection, the electrical signal being an optical signal that has been converted into a digitally modulated signal by phase modulation or quadrature amplitude modulation, then divided into multiple carrier waves, and superimposed on a local oscillator laser and transmitted from a transmission unit; and a crosstalk compensation unit that compensates for crosstalk between multiple signals that were superimposed on each of the multiple carrier waves obtained from the acquired electrical signal.

[0009] Another aspect of the present invention is a multi-carrier signal waveform equalization method, which includes an acquisition step of acquiring an electrical signal obtained by converting an optical signal into a digitally modulated signal by phase modulation or quadrature amplitude modulation, dividing the optical signal into a plurality of carrier waves, superimposing the divided carrier waves on a local oscillator laser, and transmitting the optical signal from a transmitting unit, and a crosstalk compensation step of compensating for crosstalk between a plurality of signals that were superimposed on the plurality of carrier waves obtained from the acquired electrical signal. [Effects of the Invention]

[0010] The present invention enables more accurate signal processing in communications using multi-carrier signals, even in environments where imperfections in analog devices and laser phase noise / frequency errors exist. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a diagram showing the overall configuration of an optical communication system 1 according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a block diagram showing a configuration of a digital signal processing unit 41 in the first embodiment of the present invention. [Figure 3] FIG. 2 is a diagram showing the configuration of a 2×2 FIR filter included in a digital signal processing unit 41 according to the first embodiment of the present invention. [Figure 4] FIG. 3 is a flowchart of a multi-carrier signal waveform equalization process performed by a digital signal processing unit 41 in the first embodiment of the invention. [Figure 5] FIG. 10 is a block diagram showing the configuration of a digital signal processing unit 41a in a second embodiment of the present invention. [Figure 6] FIG. 10 is a diagram showing the procedure of the computer simulation performed. [Figure 7] FIG. 1 is a diagram showing representative parameters and their values ​​used in the computer simulations performed. [Figure 8] FIG. 10 is a diagram showing the results of a computer simulation performed. [Figure 9] FIG. 10 is a diagram showing the results of a computer simulation performed. [Figure 10] FIG. 2 is a diagram illustrating a frequency spectrum of a multicarrier signal. DETAILED DESCRIPTION OF THE INVENTION

[0012] This invention relates to digital signal processing technology for receiving devices in coherent optical communications. Coherent optical communications is a communications method that utilizes the wave properties of light. Note that "coherent" means having interference, and in the field of communications, it refers to the use of frequency or phase modulation. Coherent optical communications has better receiving sensitivity than intensity modulation (IM) / direct detection (DD) methods, which detect changes in the intensity of signal light using a photodiode, and is also the basic technology for wavelength division multiplexing communications, which enables the transmission of large amounts of information at terabits per second.

[0013] For example, Quadrature Phase Shift Keying (QPSK), a modulation method, uses optical phase information to transmit twice as much information as the IM-DD method. Furthermore, by taking advantage of the property that two orthogonal light waves do not intersect, different information can be transmitted on the X and Y polarizations, making it possible to transmit twice as much information. This is called Dual Polarization (DP)-QPSK. DP-QPSK can transmit four times as much information in the same signal bandwidth as the conventional IM-DD method.

[0014] In the optical signal transmitter, the digital signals of "0" and "1" are converted into in-phase (I) and quadrature (Q) components of the X and Y polarizations, respectively. The XI, XQ, YI, and YQ electrical signals are used to drive Mach-Zehnder modulators, which are then polarized and combined to generate a phase-modulated, polarization-multiplexed optical signal.

[0015] In the optical signal receiving section, the phase-modulated and polarization-multiplexed optical signal is subjected to polarization separation, and then interfered with a laser light (local light) installed in the receiving section to detect the I and Q components of the X and Y polarizations, respectively. This process is called coherent detection, as the signal is detected by interfering the signal light with the local light. The detected I and Q components of the X and Y polarizations are converted into electrical signals by a photodetector, and then converted into digital sampling data by an analog-to-digital converter (ADC) with a high sampling rate. This data is then subjected to advanced signal equalization through digital signal processing using a DSP (Digital Signal Processor), making it possible to correct signal distortions such as chromatic dispersion and polarization dispersion that are unique to optical fibers.

[0016] Hereinafter, a multi-carrier signal waveform equalization circuit and a multi-carrier signal waveform equalization method according to an embodiment will be described with reference to the drawings.

[0017] First Embodiment A first embodiment of the present invention will be described below. The multicarrier signal waveform equalization circuit in the first embodiment described below is a circuit that performs digital signal processing on a received signal that has been converted from an optical signal to an electrical signal by coherent detection. The optical signal is converted into a digitally modulated signal by phase modulation or quadrature amplitude modulation in a transmitting device, which is the communication partner, and then split into multiple carrier waves, which are superimposed on a local oscillator laser and transmitted to a receiving device. The multicarrier signal waveform equalization circuit is a circuit mounted on the receiving device.

[0018] The multicarrier signal waveform equalization circuit in the first embodiment is characterized by including a crosstalk compensation unit that compensates for crosstalk between signals superimposed on each of multiple carrier waves obtained from a received signal for each polarization (i.e., for each of the X polarization and the Y polarization). Crosstalk refers to a signal that leaks from one channel to another when signals are transmitted through multiple channels.

[0019] With this configuration, the multi-carrier signal waveform equalization circuit of the first embodiment can effectively equalize crosstalk between subcarriers in a multi-carrier signal, thereby enabling the multi-carrier signal waveform equalization circuit of the first embodiment to achieve more accurate signal processing in communications using multi-carrier signals even in an environment where imperfections in analog devices and laser phase noise / frequency errors exist.

[0020] [Configuration of optical communication system] The overall configuration of an optical communication system 1 according to the first embodiment, which is equipped with the above-described multi-carrier signal waveform equalization circuit, will now be described. Fig. 1 is a diagram showing the overall configuration of the optical communication system 1 according to the first embodiment of the present invention. As shown in Fig. 1, the optical communication system 1 includes an optical transmitter 2, an optical transmission line 3, and an optical receiver 4.

[0021] The optical transmitter 2 is configured to include an electrical signal generator 20 and an optical signal generator 21. The electrical signal generator 20 encodes information acquired from an information source (not shown) and converts it into an electrical signal waveform. The electrical signal generator 20 outputs the converted electrical signal waveform to the optical signal generator 21. The optical signal generator 21 converts the electrical signal waveform input from the electrical signal generator 20 into an optical signal. The optical signal generator 21 sends the converted optical signal to the optical transmission path 3.

[0022] The optical transmission line 3 includes at least an optical fiber 30. The optical fiber 30 is a transmission medium for an optical signal. The optical transmission line 3 may further include one or more optical amplifiers 31 that amplify the transmitted optical signal, as shown in FIG. 1, for example. The optical transmission line 3 may also include optical devices (not shown), such as an optical switch and a regenerator.

[0023] The optical receiving unit 4 includes a coherent optical receiving unit 40 and a digital signal processing unit 41. The coherent optical receiving unit 40 includes at least a 90-degree optical hybrid circuit, a local oscillator light source, a photodetector, and an optical fiber that couples these optical devices (not shown). As described above, coherent optical communication is characterized by the use of a local oscillator light source on the receiving side. The coherent optical receiving unit 40 may further include other optical devices, such as an optical attenuator.

[0024] The digital signal processing unit 41 includes the above-mentioned multi-carrier signal waveform equalization circuit. The configuration of the digital signal processing unit 41 will be explained in detail below.

[0025] [Digital signal processing section configuration] The following describes the configuration of the digital signal processing unit 41. In the first embodiment, as an example, a case will be described in which communication is performed using a polarization-multiplexed multicarrier signal with two subcarriers.

[0026] 2 is a block diagram showing the configuration of the digital signal processing unit 41 according to the first embodiment of the present invention. As shown in FIG. 2, the digital signal processing unit 41 sc1 , y sc1 , x sc2 , y sc2 For the four signal inputs, (x sc )^, (y sc1 )^, (x sc2 )^, (y sc2)^. These variables represent the following signals. For example, a variable with a hat symbol added to variable a will be represented as "(a)^".

[0027] x sc1 : Input signal (X polarization of subcarrier #1) y sc1 : Input signal (Y polarization of subcarrier #1) x sc2 : Input signal (X polarization of subcarrier #2) y sc2 : Input signal (Y polarization of subcarrier #2) (x sc1 )^: Output signal (X polarization of subcarrier #1) (y sc1 )^: Output signal (Y polarization of subcarrier #1) (x sc2 )^: Output signal (X polarization of subcarrier #2) (y sc2 )^: Output signal (Y polarization of subcarrier #2)

[0028] As shown in FIG. 2, the digital signal processing unit 41 includes crosstalk compensation units 410-1 and 410-2, waveform distortion compensation units 411-1 and 411-2, phase compensation units 412-1 to 412-4, crosstalk compensation coefficient control units 413-1 and 413-2, waveform distortion compensation coefficient control units 414-1 and 414-2, and reference signal processing units 415-1 and 415-2.

[0029] Crosstalk compensation units 410-1 and 410-2 compensate for crosstalk between signals superimposed on subcarrier #1 and subcarrier #2, respectively. Hereinafter, when there is no need to distinguish between crosstalk compensation unit 410-1 and crosstalk compensation unit 410-2, they will simply be referred to as "crosstalk compensation unit 410." Crosstalk compensation unit 410 is configured using a digital filter, typically an FIR filter.

[0030] 3 is a diagram showing the configuration of a 2×2 FIR filter included in the digital signal processing unit 41 according to the first embodiment of the present invention. As shown in FIG. 3, the crosstalk compensation unit 410 according to the first embodiment is configured using a 2×2 FIR filter including four FIR filters 4101.

[0031] The variables shown in FIG. 3 represent the following variables: a j (n): Input sample of the FIR filter at time n b ij (n): Output sample of the FIR filter at time n h ij (k): FIR filter coefficient (k=0, 1, …, N-1) N: Number of taps in the FIR filter

[0032] Furthermore, the inputs and outputs of the FIR filter are expressed as follows:

[0033]

number

[0034]

number

[0035]

number

[0036]

number

[0037] 3, only the configuration of the FIR filter 4101 of h11 is shown, but the FIR filters 4101 of h12, h21, and h22 have the same configuration as the FIR filter 4101 of h11. Hereinafter, the explanation will return to FIG.

[0038] The waveform distortion compensator 411-1 compensates for linear distortion other than the distortion that is the target of compensation by the crosstalk compensator 410-1. Similarly, the waveform distortion compensator 411-2 compensates for linear distortion other than the distortion that is the target of compensation by the crosstalk compensator 410-2. Hereinafter, when there is no need to distinguish between the waveform distortion compensator 411-1 and the waveform distortion compensator 411-2, they will be simply referred to as the "waveform distortion compensator 411." The waveform distortion compensator 411 is configured using a digital filter. Like the crosstalk compensator 410, the waveform distortion compensator 411 in the first embodiment is configured using a 2×2 FIR filter shown in FIG. 3.

[0039] Phase compensation units 412-1 to 412-4 compensate for signal phase rotation using a reference signal that is known on the receiving side. Signal phase rotation occurs due to, for example, phase noise in a transmitter / receiver. As shown in FIG. 2, in the first embodiment, phase compensation units 412-1 and 412-3 are arranged before crosstalk compensation unit 410-1, and phase compensation units 412-2 and 412-4 are arranged before crosstalk compensation unit 410-2. Hereinafter, when it is not necessary to distinguish between phase compensation units 412-1 to 412-4, they will be simply referred to as "phase compensation units 412."

[0040] The crosstalk compensation coefficient control unit 413-1 controls the FIR filter coefficients (hereinafter, sometimes simply referred to as "filter coefficients") of the crosstalk compensation unit 410-1 using a reference signal that is known on the receiving side. Similarly, the crosstalk compensation coefficient control unit 413-2 controls the filter coefficients of the crosstalk compensation unit 410-2 using a reference signal that is known on the receiving side. Hereinafter, when there is no need to distinguish between the crosstalk compensation coefficient control unit 413-1 and the crosstalk compensation coefficient control unit 413-2, they will simply be referred to as the "crosstalk compensation coefficient control unit 413." The crosstalk compensation coefficient control unit 413 can use, for example, the LMS (Least Mean Square) algorithm described in Non-Patent Document 7 to control the filter coefficients of the crosstalk compensation unit 410.

[0041] The waveform distortion compensation coefficient control unit 414-1 controls the filter coefficient of the waveform distortion compensation unit 411-1 using a reference signal that is known on the receiving side. Similarly, the waveform distortion compensation coefficient control unit 414-2 controls the filter coefficient of the waveform distortion compensation unit 411-2 using a reference signal that is known on the receiving side. Hereinafter, when there is no need to distinguish between the waveform distortion compensation coefficient control unit 414-1 and the waveform distortion compensation coefficient control unit 414-2, they will simply be referred to as the "waveform distortion compensation coefficient control unit 414." For example, the LMS algorithm described in Non-Patent Document 7 can be used to control the filter coefficient of the waveform distortion compensation unit 411 by the waveform distortion compensation coefficient control unit 414, as in the control of the filter coefficient of the crosstalk compensation unit 410.

[0042] The reference signal processing unit 415-1 transforms a reference signal known on the receiving side, which is used to update the filter coefficients of the preceding waveform distortion compensator 411-1 and the compensation amounts of the phase compensators 412-1 and 412-3, by affine transformation using the filter coefficients of the crosstalk compensator 410-1, to generate a new reference signal. Similarly, the reference signal processing unit 415-2 transforms a reference signal known on the receiving side, which is used to update the filter coefficients of the waveform distortion compensator 411-2 and the compensation amounts of the phase compensators 412-2 and 412-4, by affine transformation using the filter coefficients of the crosstalk compensator 410-2, to generate a new reference signal.

[0043] The above conversion process by the reference signal processing unit 415-1 generates a (converted) reference signal that takes into account waveform distortion that cannot be compensated for by the waveform distortion compensator 411-1 (i.e., waveform distortion that can only be compensated for by the crosstalk compensator 410-1). The converted reference signal is used by the waveform distortion compensation coefficient control unit 414-1 to control the filter coefficient of the waveform distortion compensator 411-1 using the LMS algorithm. Similarly, the above conversion process by the reference signal processing unit 415-2 generates a (converted) reference signal that takes into account waveform distortion that cannot be compensated for by the waveform distortion compensator 411-2 (i.e., waveform distortion that can only be compensated for by the crosstalk compensator 410-2). The converted reference signal is used by the waveform distortion compensation coefficient control unit 414-1 to control the filter coefficient of the waveform distortion compensator 411-2 using the LMS algorithm.

[0044] Furthermore, the converted reference signal generated by reference signal processing unit 415-1 is used to calculate the compensation amounts of phase compensation units 412-1 and 412-3, which are arranged upstream of crosstalk compensation unit 410-1. Similarly, the converted reference signal generated by reference signal processing unit 415-2 is used to calculate the compensation amounts of phase compensation units 412-2 and 412-4, which are arranged upstream of crosstalk compensation unit 410-2. Hereinafter, when there is no need to distinguish between reference signal processing unit 415-1 and reference signal processing unit 415-2, they will simply be referred to as "reference signal processing unit 415."

[0045] [Processing flow] The following describes an example of the flow of signal processing by the digital signal processing unit 41. Fig. 4 is a flowchart of multi-carrier signal waveform equalization processing by the digital signal processing unit 41 in the first embodiment of the present invention.

[0046] The waveform distortion compensator 411 acquires an input signal (here, a reference signal that is known on the receiving side). The waveform distortion compensator 411 compensates for linear distortion other than the distortion that is the target of compensation by the crosstalk compensator 410. The waveform distortion compensator 411 outputs the reference signal to the phase compensator 412.

[0047] The phase compensation unit 412 acquires the reference signal output from the waveform distortion compensation unit 411. The phase compensation unit 412 compensates for the phase rotation of the signal using the acquired reference signal. The phase compensation unit 412 outputs the reference signal to the crosstalk compensation unit 410 and the waveform distortion compensation coefficient control unit 414.

[0048] The waveform distortion compensation coefficient control unit 414 acquires the reference signal output from the phase compensation unit 412. The waveform distortion compensation coefficient control unit 414 uses the acquired reference signal to update (control) the filter coefficients of the waveform distortion compensation unit 411 using the LMS algorithm.

[0049] The crosstalk compensation unit 410 acquires the reference signal output from the phase compensation unit 412. The crosstalk compensation unit 410 compensates for crosstalk between the signals superimposed on the two subcarriers. The crosstalk compensation unit 410 outputs the reference signal to the crosstalk compensation coefficient control unit 413 and also outputs the reference signal as an output signal.

[0050] The crosstalk compensation coefficient control unit 413 acquires the reference signal output from the crosstalk compensation unit 410. The crosstalk compensation coefficient control unit 413 uses the acquired reference signal to update (control) the filter coefficients of the crosstalk compensation unit 410 using the LMS algorithm. In addition, the crosstalk compensation coefficient control unit 413 outputs information indicating the updated filter coefficients to the reference signal processing unit 415.

[0051] The reference signal processing unit 415 acquires information indicating the updated filter coefficients of the crosstalk compensation unit 410, output from the crosstalk compensation coefficient control unit 413. The reference signal processing unit 415 transforms a reference signal known on the receiving side, which is used to update the filter coefficients of the preceding waveform distortion compensation unit 411 and the compensation amount by the phase compensation unit 412, by affine transformation using the updated filter coefficients of the crosstalk compensation unit 410, to generate a new reference signal. The reference signal processing unit 415 outputs the converted reference signal to the waveform distortion compensation coefficient control unit 414.

[0052] The waveform distortion compensation coefficient control unit 414 acquires the converted reference signal output from the reference signal processing unit 415. The waveform distortion compensation coefficient control unit 414 uses the acquired converted reference signal to update (control) the filter coefficients of the waveform distortion compensation unit 411 by the LMS algorithm.

[0053] As described above, in the conversion process of the reference signal by the reference signal processing unit 415, a (converted) reference signal is generated that takes into account waveform distortion that cannot be compensated for by the waveform distortion compensator 411 (i.e., waveform distortion that can only be compensated for by the crosstalk compensator 410). Then, the converted reference signal is reflected in the control of the filter coefficient by the LMS algorithm of the waveform distortion compensation coefficient controller 414.

[0054] With such a configuration, the multi-carrier signal waveform equalization circuit provided in the digital signal processing unit 41 in the first embodiment realizes overall optimization of both the waveform distortion compensation unit 411 and the crosstalk compensation unit 410, thereby improving waveform equalization performance and signal quality.

[0055] <Second embodiment> A second embodiment of the present invention will now be described. As with the first embodiment described above, the multicarrier signal waveform equalization circuit in the second embodiment described below is a circuit that performs digital signal processing on a received signal that has been converted from an optical signal to an electrical signal by coherent detection. The optical signal is converted into a digitally modulated signal by phase modulation or quadrature amplitude modulation in a transmitting device, which is the communication partner, and then split into multiple carrier waves, which are superimposed on a local oscillator laser and transmitted to a receiving device. The multicarrier signal waveform equalization circuit is a circuit installed in the receiving device.

[0056] The multi-carrier signal waveform equalization circuit in the second embodiment is characterized by including a crosstalk compensation unit that compensates for crosstalk between signals superimposed on each of a plurality of carrier waves obtained from a received signal for each polarization (i.e., for each of the X polarization and the Y polarization).

[0057] With this configuration, the multi-carrier signal waveform equalization circuit of the second embodiment can effectively equalize crosstalk between subcarriers in a multi-carrier signal, thereby enabling the multi-carrier signal waveform equalization circuit of the second embodiment to achieve more accurate signal processing in communications using multi-carrier signals even in an environment where imperfections in analog devices and laser phase noise / frequency errors exist.

[0058] The overall configuration of the optical communication system in the second embodiment is similar to the overall configuration of the optical communication system 1 in the first embodiment shown in FIG. 1, and therefore a description thereof will be omitted.

[0059] [Digital signal processing section configuration] The configuration of the digital signal processing unit 41a in the second embodiment will be described below. In the second embodiment, as in the first embodiment described above, a case where communication is performed using a polarization-multiplexed multicarrier signal with two subcarriers will be described as an example.

[0060] 5 is a block diagram showing the configuration of a digital signal processing unit 41a according to the second embodiment of the present invention. As shown in FIG. 5, the digital signal processing unit 41a sc1 , y sc1 , x sc2 , y sc2 For the four signal inputs, (x sc )^, (y sc1 )^, (x sc2 )^, (y sc 2) ^ The signals represented by these variables are as explained in the first embodiment.

[0061] As shown in FIG. 5, the digital signal processing unit 41a is configured to include crosstalk compensation units 410-1 and 410-2, waveform distortion compensation units 411-1 and 411-2, phase compensation units 412-1 to 412-4, phase compensation units 412-5 to 412-8, crosstalk compensation coefficient control units 413-1 and 413-2, waveform distortion compensation coefficient control units 414-1 and 414-2, and reference signal processing units 415-1 and 415-2.

[0062] As shown in Figure 5, the configuration of digital signal processing unit 41a in the second embodiment differs from the configuration of digital signal processing unit 41 in the first embodiment described above (shown in Figure 2) in that it further includes phase compensation units 412-5 to 412-8.

[0063] As described above, phase compensation units 412-1 to 412-4 compensate for signal phase rotation using a reference signal known on the receiving side. In contrast, phase compensation units 412-5 to 412-8 compensate for signal phase rotation based on hard decision results of symbols input to phase compensation units 412-5 to 412-8 without using a reference signal known on the receiving side.

[0064] Phase compensation units 412-1 and 412-3, which are arranged before crosstalk compensation unit 410-1, operate in a state where they contain distortion that is to be compensated for by crosstalk compensation unit 410-1. Similarly, phase compensation units 412-2 and 412-4, which are arranged before crosstalk compensation unit 410-2, operate in a state where they contain distortion that is to be compensated for by crosstalk compensation unit 410-2. As a result, distortion will, in principle, remain in the output signals of phase compensation units 412-1 to 412-4.

[0065] In contrast, by arranging a phase compensation unit 412-5 and a phase compensation unit 412-6 after the crosstalk compensation unit 410-1 and a phase compensation unit 412-7 and a phase compensation unit 412-8 after the crosstalk compensation unit 410-2, the digital signal processing unit 41a in the second embodiment can compensate for residual distortion and further improve waveform equalization performance and signal quality.

[0066] Furthermore, similarly to the first embodiment described above, in the conversion process of the reference signal by the reference signal processing unit 415, a (converted) reference signal is generated that takes into account waveform distortion that cannot be compensated for by the waveform distortion compensation unit 411 (i.e., waveform distortion that can only be compensated for by the crosstalk compensation unit 410).The converted reference signal is then reflected in the control of the filter coefficient by the LMS algorithm of the waveform distortion compensation coefficient control unit 414.

[0067] With such a configuration, the multi-carrier signal waveform equalization circuit provided in the digital signal processing unit 41 in the second embodiment realizes overall optimization of both the waveform distortion compensation unit 411 and the crosstalk compensation unit 410, thereby improving waveform equalization performance and signal quality.

[0068] (Example) A computer simulation performed to evaluate the effects of the present invention will now be described. Fig. 6 is a diagram showing the procedure of the computer simulation performed.

[0069] As shown in Figure 6, computer simulations were performed in the following order: multicarrier signal generation, transmitting and receiving analog device imperfection loading, receiving digital signal processing with and without crosstalk compensation, and signal quality measurement.

[0070] As shown in Figure 6, multi-carrier signal generation involves the following processes in order: binary sequence generation, symbol mapping, Nyquist shaping, and multi-carrier modulation. In the binary sequence generation process, a binary sequence bit string is generated. In the symbol mapping process, the binary sequence bit string is converted into a QAM signal based on the mapping rules. In the Nyquist shaping process, a band narrowing process is performed using a Nyquist filter. In the multi-carrier modulation process, conversion is performed into a multi-carrier signal.

[0071] In the case of receiving digital signal processing with crosstalk compensation, the operation of the crosstalk compensation coefficient control unit was turned on, and in the case of receiving digital signal processing without crosstalk compensation, the operation of the crosstalk compensation coefficient control unit was turned off.In the signal quality measurement, the evaluation was performed by calculating the Q value based on the transmitted binary sequence (a sequence of "0" and "1") and the binary sequence restored from the input signal to the signal quality measurement unit (not shown).

[0072] The Q (Quality factor) value mentioned here represents the quality of the optical signal. The amplitude of the "0" and "1" of a binary signal varies due to noise, etc., and the Q value is defined by the difference between the magnitude of this spread (standard deviation) and the average amplitude. For example, if the quality deteriorates, the signal amplitude variation increases or the difference in average amplitude decreases, resulting in a smaller Q value.

[0073] Generally, the most accurate method for signal monitoring is bit error rate (BER) monitoring. However, BER monitoring has drawbacks, such as difficulty in monitoring during service operation, long measurement times when signal quality is good, and dependence on bit rate and signal format. Therefore, the issue when considering optical signal monitoring methods is how to monitor optical signal quality more accurately, quickly, and without interfering with communications, while remaining independent of bit rate and signal format (transparent). Using a method that measures the Q factor can solve many of the above problems.

[0074] Fig. 7 shows representative parameters and their values ​​used in the computer simulation. As shown in Fig. 7, 16QAM was used as the modulation method, the modulation rate per subcarrier was 69 [Gbaud], the number of multicarriers was 2, the IQ quadrature error was -10 to +10 [degrees], the skew between the transmitter IQ lanes was 0 to 2 [psec], the number of taps of the FIR filter in the waveform distortion compensation unit was 17, and the number of taps of the FIR filter in the crosstalk compensation unit was 7.

[0075] 8 and 9 are diagrams showing the results of the computer simulations that were performed. Fig. 8 shows the Q value for each IQ quadrature error, and Fig. 9 shows the Q value for each skew between the transmitter IQ lanes.

[0076] 8, for example, the Q value when the IQ quadrature error is -7.5 degrees and the Q value when the IQ quadrature error is 7.5 degrees are approximately 6.4 dB without crosstalk compensation, whereas with crosstalk compensation according to the present invention, they are approximately 6.7 dB. Therefore, the Q value when the IQ quadrature error is -7.5 degrees is increased by approximately 0.3 dB by crosstalk compensation according to the present invention.

[0077] 9, for example, the Q factor when the skew between transmitter lanes is 1.5 [psec] is approximately 4 [dB] without crosstalk compensation, whereas it is approximately 6.8 [dB] with crosstalk compensation according to the present invention. Therefore, the Q factor when the skew between transmitter lanes is 1.5 [psec] is increased by approximately 2.8 [dB] by crosstalk compensation according to the present invention.

[0078] Thus, the results of the computer simulation of the present invention shown in FIGS. 8 and 9 show that the crosstalk compensation according to the present invention improves the signal quality (Q value).

[0079] According to the above-described embodiment, the multicarrier signal waveform equalization circuit includes an acquisition unit and a crosstalk compensation unit. For example, the multicarrier signal waveform equalization circuit is a circuit constituting the digital signal processing unit 41 in the embodiment, the acquisition unit is the waveform distortion compensation unit 411 in the embodiment, and the crosstalk compensation unit is the crosstalk compensation unit 410 in the embodiment.

[0080] The acquisition unit acquires an electrical signal obtained by coherent detection of an optical signal that has been converted from a digitally modulated signal by phase modulation or quadrature amplitude modulation, then split into multiple carrier waves, and superimposed on a local oscillation laser and transmitted from a transmitter. For example, the multiple carrier waves are subcarrier #1 and subcarrier #2 in the embodiment, and the transmitter is optical transmitter 2 in the embodiment.

[0081] The crosstalk compensator compensates for crosstalk between multiple signals superimposed on multiple carrier waves obtained from the acquired electrical signal. For example, the crosstalk between the multiple signals is a crosstalk component from subcarrier #2 superimposed on subcarrier #1 and a crosstalk component from subcarrier #1 superimposed on subcarrier #2 (shown in FIG. 10) in this embodiment.

[0082] In the above multi-carrier signal waveform equalization circuit, the crosstalk compensation unit is configured by a digital filter, for example, the digital filter is a 2×2 FIR filter (shown in FIG. 3) in the embodiment.

[0083] The multi-carrier signal waveform equalization circuit further includes a phase compensation unit, for example, the phase compensation unit 412 in the embodiment.

[0084] The phase compensation unit is arranged before the crosstalk compensation unit or before and after the crosstalk compensation unit, and compensates for signal phase rotation. For example, the phase compensation unit arranged before the crosstalk compensation unit is phase compensation unit 412-1 to phase compensation unit 412-4 in the embodiment, the phase compensation unit arranged after the crosstalk compensation unit is phase compensation unit 412-5 to phase compensation unit 412-8 in the embodiment, the transmitter is optical transmitter 2 in the embodiment, and the receiver is optical receiver 4 in the embodiment.

[0085] The multi-carrier signal waveform equalization circuit further includes a waveform distortion compensator. For example, the waveform distortion compensator is the waveform distortion compensator 411 in the embodiment. The waveform distortion compensator is disposed before the crosstalk compensator, and compensates for linear distortion other than the distortion that is the target of compensation by the crosstalk compensator.

[0086] In the above multi-carrier signal waveform equalization circuit, the waveform distortion compensation unit is configured by a digital filter, for example, the 2×2 FIR filter (shown in FIG. 3) in the embodiment.

[0087] The above multi-carrier signal waveform equalization circuit further includes a reference signal processing unit, for example, the reference signal processing unit 415 in the embodiment.

[0088] The reference signal processing unit converts a reference signal, which is known on the receiving side and is used to update the filter coefficients of the digital filter constituting the waveform distortion compensating unit, into a new reference signal by affine transformation using the filter coefficients of the digital filter in the crosstalk compensating unit, and updates the filter coefficients of the digital filter constituting the waveform distortion compensating unit with the converted reference signal.

[0089] The multicarrier signal waveform equalization circuit further includes a reference signal processing unit that converts a reference signal known on the receiving side and used to update the compensation amount of a phase compensation unit arranged upstream of the crosstalk compensation unit into a new reference signal by affine transformation using filter coefficients of a digital filter in the crosstalk compensation unit, and calculates the compensation amount of the phase compensation unit using the converted reference signal.

[0090] The digital signal processing unit 41 and a part of the digital signal processing unit 41a in the above-described embodiment may be implemented by a computer. In this case, a program for implementing this function may be recorded on a computer-readable recording medium, and the program recorded on the recording medium may be loaded into a computer system and executed. Note that the term "computer system" here includes hardware such as an OS and peripheral devices. Furthermore, the term "computer-readable recording medium" refers to portable media such as a flexible disk, a magneto-optical disk, a ROM, a CD-ROM, and a storage device such as a hard disk built into a computer system.

[0091] Furthermore, the term "computer-readable recording medium" may include a medium that dynamically stores a program for a short period of time, such as a communication line when transmitting a program via a network such as the Internet or a communication line such as a telephone line, or a medium that stores a program for a fixed period of time, such as a volatile memory within a computer system that serves as a server or client in such a case. The program may also be one that realizes part of the above-mentioned functions, or one that can realize the above-mentioned functions in combination with a program already stored in the computer system, or one that can be realized using a programmable logic device such as an FPGA (Field Programmable Gate Array).

[0092] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention. [Explanation of symbols]

[0093] 1...optical communication system, 2...optical transmitter, 3...optical transmission path, 4...optical receiver, 20...electrical signal generator, 21...optical signal generator, 30...optical fiber, 31...optical amplifier, 40...coherent optical receiver, 41, 41a...digital signal processor, 410, 410-1, 410-2...crosstalk compensation unit, 411-1, 411-2...compensator, 412-1 to 412-8...phase compensation unit, 413-1, 413-2...crosstalk compensation coefficient controller, 414-1, 414-2...compensation coefficient controller, 415-1, 415-2...reference signal processor, 4101...FIR filter

Claims

1. an acquisition unit that acquires an electrical signal obtained by converting an optical signal into an electrical signal by coherent detection, the electrical signal being an optical signal that has been converted into a digital modulated signal by phase modulation or quadrature amplitude modulation, and then divided into a plurality of carrier waves and superimposed on a local oscillation laser and then transmitted from a transmission unit; a crosstalk compensation unit configured by a digital filter to which one of a plurality of signals superimposed on each of the plurality of carrier waves obtained from the acquired electrical signal is subjected to complex conjugate processing and input, and which compensates for crosstalk between subcarrier signals; A multi-carrier signal waveform equalization circuit comprising:

2. a phase compensation unit that is arranged before the crosstalk compensation unit or both before and after the crosstalk compensation unit and compensates for phase rotation of a signal; The multi-carrier signal waveform equalization circuit according to claim 1 , further comprising:

3. a waveform distortion compensator arranged before the crosstalk compensator and compensating for linear distortion other than the distortion that is the target of compensation by the crosstalk compensator; The multi-carrier signal waveform equalization circuit according to claim 1 , further comprising:

4. The waveform distortion compensation unit is configured by a digital filter.

4. The multi-carrier signal waveform equalization circuit according to claim 3.

5. a reference signal processing unit that converts a reference signal known on the receiving side, which is used to update the filter coefficients of the digital filter constituting the waveform distortion compensator, into a new reference signal by affine transformation using the filter coefficients of the digital filter of the crosstalk compensator, and updates the filter coefficients of the digital filter constituting the waveform distortion compensator with the converted reference signal; The multi-carrier signal waveform equalization circuit according to claim 4, further comprising:

6. a reference signal processing unit that converts a reference signal known on the receiving side and used to update the compensation amount of the phase compensation unit arranged in a stage preceding the crosstalk compensation unit into a new reference signal by an affine transformation using a filter coefficient of the digital filter of the crosstalk compensation unit, and calculates the compensation amount of the phase compensation unit using the converted reference signal The multi-carrier signal waveform equalization circuit according to claim 2 , further comprising:

7. an acquisition step of acquiring an electrical signal obtained by converting an optical signal into an electrical signal by coherent detection, the electrical signal being an optical signal that has been converted into a digital modulated signal by phase modulation or quadrature amplitude modulation, then divided into a plurality of carrier waves, and superimposed on a local oscillation laser and then transmitted from a transmitter; a crosstalk compensation step of compensating for crosstalk between subcarrier signals, the crosstalk compensation step being configured by a digital filter to which one of the plurality of signals superimposed on each of the plurality of carrier waves obtained from the acquired electrical signal is subjected to complex conjugate processing and input; A multi-carrier signal waveform equalization method comprising:

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