JP2019036908A - Skew compensator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To compensate for a skew between a plurality of polarized lights. A receiving device receives a plurality of polarized lights having different planes of polarization from an optical fiber 10. The receiving device includes an error rate measuring unit (20IX, 20QX, 20IY, 20QY) for measuring the code error rate for each of the in-phase component signal and the orthogonal component signal obtained from each polarized light, and the in-phase component obtained from each polarized light. The delay or advance of each of the in-phase component signal obtained from each polarized light and the other signal of the orthogonal component signal is adjusted with reference to the timing of the reference signal which is one of the signal and the orthogonal component signal. It is provided with a skew compensation unit (18IX, 18QX, 18IY, 18QY). The skew compensator adjusts the delay or advance of each of the other signals based on the code error rate measured for each of the other signals. [Selection diagram] Fig. 1

Description

  The present invention relates to a skew compensator, and more particularly to a device used in a polarization multiplexing modulation receiver.

  Optical communication systems that transmit information by light are widely used. The optical communication system includes an optical transmitter and an optical receiver connected by an optical fiber. The optical transmission device modulates light with a digital signal to generate an optical signal, and transmits the optical signal to an optical fiber. The optical receiving apparatus performs a demodulation process on the optical signal received from the optical fiber and extracts a digital signal.

  Some optical communication systems use a polarization multiplexing modulation system. In an optical signal transmitted by this method, a plurality of polarized lights having different polarization planes are superimposed. Each polarized light is modulated by a digital signal. As a modulation method, QPSK (Quadrature Phase Shift Keying) in which a phase is associated with a symbol code in a predetermined symbol period, or QAM (Quadrature Amplitude Modulation) in which an amplitude and a phase are associated with a symbol code in a predetermined symbol period. Etc. are used. Here, the symbol code refers to a predetermined digit digital code for modulating each polarized light.

  Patent Document 1 below describes a digital optical coherent transmission apparatus using a polarization multiplexing modulation system. The receiver in this apparatus acquires a constellation indicating a vector locus for each polarized light, and obtains an error of the received signal based on the constellation. In the receiver, the skew between the in-phase component signal (I signal) and the quadrature component signal (Q signal) included in the received signal is compensated based on the error of the received signal. Here, the skew between the in-phase component signal and the quadrature component signal refers to an error indicating a timing shift between the in-phase component signal and the quadrature component signal. Skew may occur due to inadequate performance of a coherent receiver that performs polarization separation and quadrature detection at the receiver.

JP 2013-207603 A

  In the conventional polarization multiplexing modulation system, the skew between the in-phase component signal and the quadrature component signal is obtained for each of the two polarized lights included in the optical signal, and the skew is individually obtained for each of the two polarized lights. Compensated. Therefore, the timing shift between the in-phase component signal and the quadrature component signal obtained from one polarized light and the in-phase component signal and the quadrature component signal obtained from the other polarized light is not compensated. For this reason, when a group of digital signals are transmitted separately in different polarized lights, it is difficult to decode the digital signals from the two polarized lights with high accuracy.

  An object of the present invention is to compensate for a skew between a plurality of polarized lights.

  The present invention relates to an error rate for measuring a code error rate for each of an in-phase component signal and a quadrature component signal obtained from each polarized light in a skew compensator used in a receiver that receives a plurality of polarized lights having different polarization planes. The in-phase component signal and the quadrature component signal obtained from each polarized light with reference to the timing of a reference signal that is one of the in-phase component signal and the quadrature component signal obtained from each of the polarized light and the measurement unit A skew compensator for adjusting a delay or a lead of each of the other signals, the skew compensator based on a code error rate measured for each of the other signals. The delay or advance of each of the signals is adjusted.

  Preferably, the reference signal is a signal having the smallest code error rate among the in-phase component signal and the quadrature component signal obtained from each of the polarized lights.

  Preferably, the skew compensation unit adjusts each delay or advance of the other signal based on a code error rate while changing each delay or advance of the other signal by a predetermined step size. Are sequentially performed for each of the other signals.

  Preferably, the skew compensation unit repeatedly executes the adjustment process on the other signals, and the step size in the adjustment process executed later is smaller than the step size in the adjustment process executed first. .

  According to the present invention, it is possible to compensate for skew between a plurality of polarized lights.

It is a figure which shows the optical receiver which concerns on embodiment of this invention. It is a flowchart of a skew setting process.

  FIG. 1 shows an optical receiver according to an embodiment of the present invention. This optical receiving apparatus receives a polarization multiplexed modulation optical signal from the optical fiber 10, and the decoding unit 26 decodes the digital signal. The received signal received by the optical receiving device includes two optical signals having orthogonal polarization planes. When an xyz orthogonal coordinate system is defined on the optical fiber 10 and the propagation direction is the z axis, the received signal has an X polarization signal having a polarization plane parallel to the xz plane, and Y having a polarization plane parallel to the yz plane. It includes the polarization signal.

  For each polarization signal of the received signal, QPSK (Quadrature Phase Shift Keying) in which a phase is associated with a symbol code in a predetermined symbol period, or QAM in which an amplitude and a phase are associated with a symbol code in a predetermined symbol period Modulation such as (Quadrature Amplitude Modulation) is applied. Of the polarization multiplexing modulation schemes, those in which the modulation scheme is QPSK are particularly called polarization multiplexed quadrature quadrature phase shift keying (DP-QPSK), and those in which the modulation scheme is 16QAM. In particular, it is called a polarization multiplexed 16QAM modulated signal (DP-16QAM: Dual Polarization 16 Quadrature Amplitude Modulation).

  The received signal may include a plurality of polarization multiplexed modulation signals having different wavelengths. The optical receiver separates the received signal into an X polarization signal and a Y polarization signal, performs orthogonal detection processing on each polarization signal using a local optical signal of a desired wavelength, and further performs digital demodulation processing. The digital signal is decoded from the polarization multiplexed modulation signal of the desired wavelength.

  Next, a specific configuration and processing of the optical receiving device will be described. The optical receiver includes a receiving terminal 12, a coherent receiver 14, A / D converters 16IX, 16QX, 16IY, 16QY, a skew compensation unit 18IX, 18QX, 18IY, 18QY, an error rate measuring unit 20IX, 20QX, 20IY, 20QY, a wavelength. A variable light source 22, a control unit 24, and a decoding unit 26 are provided.

  The skew compensation units 18IX, 18QX, 18IY, 18QY, the error rate measurement units 20IX, 20QX, 20IY, 20QY, and the control unit 24 compensate the respective skews of the in-phase component signal and the quadrature component signal obtained from the X polarization signal, Furthermore, a skew compensator that compensates for the skews of the in-phase component signal and the quadrature component signal obtained from the Y polarization signal is configured.

  The skew compensation units 18IX, 18QX, 18IY, and 18QY, the error rate measurement units 20IX, 20QX, 20IY, and 20QY, the control unit 24, and the decoding unit 26 are configured by a processor that realizes the function of each component by executing a program. May be. Further, the skew compensation units 18IX, 18QX, 18IY, and 18QY, the error rate measurement units 20IX, 20QX, 20IY, and 20QY, the control unit 24, and the decoding unit 26 may be individually configured by digital circuits.

  An optical fiber 10 is connected to the receiving terminal 12 of the optical receiver. The optical signal input from the optical fiber 10 to the receiving terminal 12 is input to the coherent receiver 14.

  The variable wavelength light source 22 outputs a local optical signal to the coherent receiver 14. The wavelength of the local optical signal output from the wavelength tunable light source 22 is variable, and the wavelength tunable light source 22 outputs a local optical signal having a wavelength according to the control of the control unit 24. That is, the control unit 24 controls the wavelength tunable light source 22 to set the wavelength of the local optical signal to the wavelength of the signal from which the digital signal is extracted from the plurality of polarization multiplexed modulation signals included in the received signal.

  The coherent receiver 14 separates the received signal into an X polarization signal and a Y polarization signal. The coherent receiver 14 performs quadrature detection processing on the X polarization signal having the same wavelength as the local optical signal, and outputs an in-phase component signal IX and a quadrature component signal QX of the X polarization signal. The in-phase component signal IX and the quadrature component signal QX of the X polarization signal are input to the A / D converter 16IX and the A / D converter 16QX, respectively. Further, the coherent receiver 14 performs a quadrature detection process on the Y polarization signal having the same wavelength as the local optical signal, and outputs an in-phase component signal IY and a quadrature component signal QY of the Y polarization signal. The in-phase component signal IY and the quadrature component signal QY of the Y polarization signal are input to the A / D converter 16IY and the A / D converter 16QY, respectively. Depending on the performance of the coherent receiver 14, skew may occur in the in-phase component signal IX and the quadrature component signal QX of the X-polarized signal, and the in-phase component signal IY and the quadrature component signal QY of the Y-polarized signal. In addition to the coherent receiver 14, there may be a case where a skew has already occurred at the optical signal transmission source.

  The A / D converters 16IX and 16QX convert the in-phase component signal IX and the quadrature component signal QX of the X polarization signal into digital sampling signals, respectively, and output them to the skew compensators 18IX and 18QX, respectively. The A / D converters 16IY and 16QY convert the in-phase component signal IY and the quadrature component signal QY of the Y polarization signal into digital sampling signals, respectively, and output them to the skew compensation units 18IY and 18QY, respectively. The digital sampling signal is obtained by sampling an analog signal at a sampling time interval and expressing each sample value as a digital value.

  Each skew compensation unit is set with a delay time by a skew setting process described later. Each skew compensator delays and outputs the signal by the delay time set by the skew setting process. That is, the skew compensation units 18IX and 18QX delay the in-phase component signal IX and the quadrature component signal QX of the X polarization signal by a preset delay time, respectively, and output them to the error rate measurement units 20IX and 20QX. Skew compensation units 18IY and 18QY delay in-phase component signal IY and quadrature component signal QY of the Y-polarized signal by a preset delay time, and output them to error rate measurement units 20IY and 20QY, respectively.

  The skew compensation units 18IX and 18QX also delay the in-phase component signal IX and the quadrature component signal QX of the X polarization signal by a preset delay time, and output the delayed signals to the decoding unit 26, respectively. Similarly, the skew compensators 18IY and 18QY delay the in-phase component signal IY and the quadrature component signal QY of the Y-polarized signal by a preset delay time and output the delayed signals to the decoder 26, respectively.

  The decoding unit 26 performs digital demodulation processing on the in-phase component signal IX and the quadrature component signal QX of the X polarization signal, and the in-phase component signal IY and the quadrature component signal QY of the Y polarization signal, and decodes the digital signal. . In the digital demodulation process, the decoding unit 26 generates a symbol synchronization signal representing the timing of symbol synchronization based on the in-phase component signal and the quadrature component signal of each of the X polarization signal and the Y polarization signal. Decoding section 26 outputs the symbol synchronization signal to error rate measuring sections 20IX, 20QX, 20IY and 20QY.

  Each error rate measuring unit measures the code error rate of the signal output from each skew setting unit according to the timing indicated by the symbol synchronization signal, and outputs it to the control unit 24. The code error rate represents the number of erroneous codes within a certain time. Examples of the code error rate include BER (Bit Error Rate) representing the number of bit errors per second and FER (Frame Error Rate) representing the number of bit errors per frame. That is, the error rate measuring units 20IX and 20QX measure the code error rates EIX and EQX of the in-phase component signal IX and the quadrature component signal QX of the X polarization signal, respectively, and output them to the control unit 24. The error rate measuring units 20IY and 20QY measure the code error rates EIY and EQY for the in-phase component signal IX and the quadrature component signal QX of the Y polarization signal, respectively, and output them to the control unit 24.

  Next, the skew setting process will be described. FIG. 2 shows a flowchart of processing executed by the control unit 24 in the skew setting processing. This process may be performed when the optical receiver is activated, or may be performed together with a process of decoding a digital signal from the optical signal received from the optical fiber 10. In the following description, the unit of the signal path from the A / D converter to the error rate measurement unit is called a lane. In the example shown in FIG. 1, the optical receiving apparatus has a signal path of 2 lanes for the X polarization, a signal path of 2 lanes for the Y polarization, and a signal path of 4 lanes in total. Yes.

  In the skew setting process, the optical signal input to the receiving terminal 12 may be modulated by a digital signal suitable for this process. The control unit 24 identifies the lane with the smallest code error rate by identifying the smallest one of the code error rates EIX, EQX, EIY, and EQY (S101). When there are two or more lanes with the lowest code error rate, for example, the control unit 24 selects one randomly selected from these lanes as the lane with the lowest code error rate. The control unit 24 sets the reference delay time d for the reference skew compensation unit that is the skew compensation unit in the lane with the smallest code error rate (S102). As a result, the reference skew compensation unit outputs a reference signal obtained by delaying the signal input to itself by a reference delay time.

  The control unit 24 executes the minimum point search process on the skew compensation unit in one of the other three lanes excluding the lane of the reference skew compensation unit (S103).

  Here, the minimum point search processing is performed by changing the delay time of the adjustment target skew compensation unit (skew compensation unit that is not the reference skew compensation unit) with a predetermined step size, and the code error rate of the lane including the adjustment target skew compensation unit. Is a process of obtaining a delay time that minimizes the code error rate and setting the delay time in the adjustment target skew compensator. That is, the minimum point search process is an adjustment process that adjusts the delay or advance of the signal output from the adjustment target skew compensator based on the code error rate with reference to the timing of the reference signal output from the reference skew compensator. It is. The delay time in the adjustment target skew compensator is, for example, a change range from a delay time d−δ obtained by subtracting the symbol period δ from the reference delay time d to a delay time d + δ obtained by adding the symbol period δ to the reference delay time d. It is changed with a step size of 0.1δ which is 1/10 of the symbol period δ. When there are two delay times that minimize the code error rate in the change range from the delay time d−δ to the delay time d + δ, the average value of the two delay times is set in the adjustment target skew compensator. May be.

  The control unit 24 sequentially executes the above-described minimum point search processing also for the other two adjustment target skew compensation units (S103).

  The control unit 24 determines whether the largest maximum error rate among the code error rates EIX, EQX, EIY, and EQY is less than a predetermined threshold Te (S104). The control unit 24 ends the process when the maximum error rate is less than the threshold Te. On the other hand, when the maximum error rate is equal to or greater than the threshold value Te, the control unit 24 executes the minute / minimum point search process described below in order for each of the three adjustment target skew compensation units (S105).

  The micro / minimum point search process reads the code error rate of the lane that includes the adjustment target skew compensation unit while changing the delay time of the adjustment target skew compensation unit around the delay time set at the start. Thus, the delay time that minimizes the code error rate is obtained, and the delay time is set in the adjustment target skew compensator. That is, the micro / minimum point search process is an adjustment process for adjusting the delay or advance of the signal output from the adjustment target skew compensator based on the code error rate.

  The minute / minimum point search process is repeatedly executed until the maximum error rate becomes less than a predetermined threshold Te. The change range of the delay time in the minute / minimum point search process executed later is smaller than the change range of the delay time in the minute / minimum point search process executed immediately before. For example, the change range of the delay time in the minute / minimum point search process executed later is set to 1/10 of the change range of the delay time in the minute / minimum point search process executed immediately before.

  Further, the step size of the delay time in the minute / minimum point search process executed later is smaller than the step size of the delay time in the minute / minimum point search process executed immediately before. For example, the increment of the delay time in the minute / minimum point search process executed later is set to 1/10 of the increment of the delay time in the minute / minimum point search process executed immediately before.

  In the initial minute / minimum point search process, the delay time set in the minimum point search process in step S103 is set in each adjustment target skew compensator at the start thereof.

  In the first minute / minimum point search process, the delay time in the adjustment target skew compensator is, for example, the delay time d0 from the delay time d−0.1δ obtained by subtracting 0.1δ from the delay time d0 set in step S103. Is changed in increments of 0.01 δ within a change range up to a delay time d 0 +0.1 δ obtained by adding 0.1 δ. When there are two delay times that minimize the code error rate in the change range from the delay time d0-0.1δ to the delay time d0 + 0.1δ, the average value of the two delay times is used as the adjustment target skew compensation. It may be set to a part.

  The control unit 24 determines whether or not the largest maximum error rate among the code error rates EIX, EQX, EIY, and EQY is less than a predetermined threshold Te (S106). The control unit 24 ends the process when the maximum error rate is less than the threshold Te. On the other hand, when the maximum error rate is equal to or greater than the threshold Te, the control unit 24 makes the change range and step size in the minute / minimum point search process smaller than the values in the minute / minimum point search process executed earlier ( S107), the minute / minimum point search process (S105) is executed again for each adjustment target skew setting unit.

  According to the skew setting process, the delay time in each adjustment target skew compensator is set so that the maximum error rate is less than the predetermined threshold Te. If the delay time in the adjustment target skew compensation unit is shorter than the delay time in the reference skew compensation unit, the signal of the lane including the adjustment target skew compensation unit proceeds with respect to the lane signal including the reference skew compensation unit. On the other hand, if the delay time in the adjustment target skew compensation unit is longer than the delay time in the reference skew compensation unit, the signal of the lane including the adjustment target skew compensation unit is delayed with respect to the lane signal including the reference skew compensation unit. In other words, according to the skew setting unit, the delay or advance of each signal output from another adjustment target skew compensation unit is adjusted based on the timing of the reference signal output from the reference skew compensation unit.

  If the possibility that the maximum error rate is less than the threshold Te is sufficiently high even if step 105 is not executed a plurality of times, the control unit 24 does not have to execute steps S106 and S107.

  In the conventional polarization multiplexing modulation system, the skew between the in-phase component signal and the quadrature component signal is obtained for each of the two polarized lights included in the optical signal, and the skew is individually obtained for each of the two polarized lights. Compensated. Therefore, the timing shift between the in-phase component signal and the quadrature component signal obtained from one polarized light and the in-phase component signal and the quadrature component signal obtained from the other polarized light is not compensated. For this reason, when a group of digital signals are transmitted separately in different polarized lights, it is difficult to decode the digital signals from the two polarized lights with high accuracy.

  According to the light receiving device according to the present embodiment, the delay times in the skew compensation unit are sequentially set for the three lanes excluding the reference lane so that the maximum error rate is less than the predetermined threshold Te. Therefore, the influence among the in-phase component signal IX of the X polarization signal, the quadrature component signal QX of the X polarization signal, the in-phase component signal IY of the Y polarization signal, and the quadrature component signal QY of the Y polarization signal is considered. In addition, the skew in each lane is compensated, and the digital signal is decoded with high accuracy from the two polarized lights.

  10 optical fiber, 12 receiving terminal, 14 coherent receiver, 16IX, 16QX, 16IY, 16QY A / D converter, 18IX, 18QX, 18IY, 18QY skew compensation unit, 20IX, 20QX, 20IY, 20QY error rate measuring unit, 22 wavelengths Variable light source, 24 control unit, 26 decoding unit.

Claims (4)

In a skew compensator used in a receiving device that receives a plurality of polarized lights having different polarization planes,
An error rate measuring unit that measures a code error rate for each of the in-phase component signal and the quadrature component signal obtained from each of the polarized lights;
The other of the in-phase component signal and the quadrature component signal obtained from each polarized light with reference to the timing of the reference signal that is one of the in-phase component signal and the quadrature component signal obtained from each of the polarized light. A skew compensator for adjusting the delay or advance of each of the signals of
The skew compensator adjusts the delay or advance of each of the other signals based on a code error rate measured for each of the other signals. The skew compensator according to claim 1,
The reference signal is
A skew compensator characterized by being a signal having the smallest code error rate among in-phase component signals and quadrature component signals obtained from the respective polarized lights. The skew compensator according to claim 1 or 2,
The skew compensation unit includes:
An adjustment process for adjusting each delay or advance of the other signal based on a code error rate while changing the delay or advance of the other signal by a predetermined step size, for each of the other signals A skew compensator, which is sequentially performed one by one. The skew compensator according to any one of claims 1 to 3,
The skew compensation unit includes:
Repeatedly performing the adjustment process on the other signals;
The skew compensator, wherein the step size in the adjustment process executed later is smaller than the step size in the adjustment process executed first.

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