Adaptive equalization circuit, adaptive equalization method and receiving device

Abstract

When the polarization separation monitor (3) detects that the correlation value between the first polarization signal and the second polarization signal exceeds a predetermined value, the control circuit (4) causes the filter tap coefficient update circuit (2) to perform the initial convergence of the tap coefficient again. The polarization separation monitor (3) determines which of the four quadrants on the IQ plane the first polarization signal and the second polarization signal belong to, calculates a correlation value on the basis of the determination result, and compares the correlation value with a predetermined value. A symbol determination circuit (7) extracts on-axis data when there is on-axis data located on the I axis or the Q axis of the IQ plane in the first polarization signal and the second polarization signal. The data on the positive side on the I axis, the data on the negative side on the I axis, the data on the positive side on the Q axis, and the data on the negative side on the Q axis among the on-axis data are each associated with one of the values representing the four quadrants on the IQ plane without overlapping with each other.

Description

Technical Field

[0001] The present disclosure relates to an adaptive equalization circuit, an adaptive equalization method, and a receiving apparatus for compensating characteristics of an optical transmission path in data communication. Background Art

[0002] In coherent optical communication, distortion of a transmission signal is compensated by digital signal processing on the receiving side, thereby enabling large-capacity transmission of several tens of Gbit / s or more. In digital signal processing, mainly wavelength dispersion compensation, frequency control / phase adjustment, polarization multiplexing separation, and polarization dispersion compensation are performed.

[0003] The processing of polarization multiplexing separation and polarization dispersion compensation is mainly performed by an adaptive equalizer. In the case where an adaptive equalizer is implemented by digital signal processing, a digital filter is generally used. By setting tap coefficients of the digital filter calculated in such a manner that distortion of the transmission signal is canceled, distortion of the transmission signal can be compensated. The tap coefficients of the digital filter correspond to the impulse response of the filter characteristics. The tap coefficients are successively updated in accordance with a condition that changes with time, and the adaptive equalizer performs compensation following changes in the polarization state.

[0004] In addition, in a receiving optical module, an X-polarized signal (hereinafter referred to as X polarization) and a Y-polarized signal (hereinafter referred to as Y polarization) synthesized on the transmitting side are separated. However, a part of the Y polarization signal remains in the separated X polarization, and a part of the X polarization signal remains in the separated Y polarization. In order to further separate data of the X polarization and data of the Y polarization, the digital filter in the adaptive equalization includes a total of four filters: two filters that output in the X polarization direction and the Y polarization direction for input of X polarization data, and two filters that output in the X polarization direction and the Y polarization direction for input of Y polarization data.

[0005] In the update of the tap coefficients of these digital filters, the following successive update algorithm is used. In the successive update algorithm, generally RLS (Recursive Least-Squares) or LMS (Least Mean Square) is used. This is an algorithm in which a known signal such as a training signal or a pilot signal is inserted into the optical signal on the transmitting side, and the tap coefficients are updated and obtained for each step size in such a manner that the error between the transmitted known signal and the true value (the value inserted on the transmitting side) of the known signal is minimized.

[0006] In addition, as a successive update algorithm, such a blind equalization method has recently been used: the tap coefficients are obtained without using a known signal. In the blind equalization method, there are the Constant Modulus Algorithm (CMA) and the Radius directed equalization (RDE) which is extended to a multi-amplitude loop for applying the CMA to Quadrature Amplitude Modulation (QAM) (for example, refer to Patent Documents 1 and 2). In these methods, the tap coefficients are updated in such a way that the error between the output of the digital filter and the value that should originally exist (in the case of a constant envelope, the "value that should exist" can be easily estimated as the expected value of the amplitude) is minimized. The tap coefficients are controlled and converged according to this algorithm.

[0007] However, in the above successive update algorithm, particularly in the blind equalization method, equivalent convergence sometimes occurs. Equivalent convergence refers to an incorrect convergence during the convergence process of the algorithm, where the filter outputs of the X-polarization data and the Y-polarization data become the same value. Mostly, both outputs tend to be the value of the X-polarization data or the value of the Y-polarization data. In this case, the polarization separation of the X-polarization data and the Y-polarization data is not correctly performed. That is, equivalent convergence becomes an index for confirming whether polarization separation has been achieved when tap coefficients are set in adaptive equalization.

[0008] The occurrence of this equivalent convergence is determined based on the correlation value of the output information of the adaptive equalization of the X-polarization data and the Y-polarization data, indicating that polarization separation cannot be performed. In particular, the probability of equivalent convergence occurring in the blind equalization method (CMA or RDE) is high. When equivalent convergence occurs, detect this occurrence and perform the successive update algorithm again. At this time, the function of detecting the occurrence of equivalent convergence is called the Miss Capture Checker (MCC).

[0009] Prior Art Documents

[0010] Patent Documents

[0011] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2012-124782

[0012] Patent Document 2: Japanese Unexamined Patent Application Publication No. 2021-190787 Summary of the Invention

[0013] Problems to be Solved by the Invention

[0014] However, in existing MCCs, according to the modulation method, even when the tap coefficients for polarization separation reach a state where they can converge correctly and perform polarization separation appropriately, equivalent convergence may sometimes be detected erroneously. That is, according to the modulation method, equivalent convergence may sometimes not be detected accurately. In such a case, there is a situation where updates are frequently performed even in a state where the tap coefficients do not need to be updated, and the adaptive equalization process becomes unstable.

[0015] The present disclosure has been made to solve the above problems, and an object thereof is to obtain an adaptive equalization circuit, an adaptive equalization method, and a receiving device that can detect equivalent convergence with high accuracy for a wide variety of modulation methods and perform adaptive equalization processing stably.

[0016] Means for Solving the Problems

[0017] The adaptive equalization circuit of the present invention includes: a digital filter that inputs the first polarized signal and the second polarized signal that have been polarization-separated and performs further polarization separation processing; a filter tap coefficient update circuit that updates the tap coefficients of the digital filter according to changes in the polarization state; a polarization separation monitor that compares the correlation value between the first polarized signal and the second polarized signal output from the digital filter with a specified value; and a control circuit that, when the polarization separation monitor detects that the correlation value has exceeded the specified value, causes the filter tap coefficient update circuit to perform the initial convergence of the tap coefficients again. The polarization separation monitor has: a sign determination circuit that determines which of the four quadrants in the IQ plane the first polarized signal and the second polarized signal belong to; a correlation operation circuit that calculates the correlation value based on the determination result of the sign determination circuit; and a comparison circuit that compares the correlation value with the specified value. In the case of a transmission modulation method in which there is on-axis data located on the I axis or the Q axis of the IQ plane in the first polarized signal and the second polarized signal, the sign determination circuit extracts the on-axis data and corresponds the positive-side data on the I axis, the negative-side data on the I axis, the positive-side data on the Q axis, and the negative-side data on the Q axis in the on-axis data to one of the values representing the four quadrants in the IQ plane without repetition.

[0018] Advantages of the Invention

[0019] According to the present disclosure, equivalent convergence can be detected with high accuracy for a wide variety of modulation methods, and adaptive equalization processing can be performed stably. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a block diagram showing an optical communication system of an embodiment.

[0021] Figure 2It is a diagram showing the adaptive equalization circuit of the embodiment.

[0022] Figure 3 It is a diagram showing the digital filter of the embodiment.

[0023] Figure 4 It is a diagram showing the polarization separation monitor of the embodiment.

[0024] Figure 5 It is a diagram showing the symbol determination circuit of the embodiment.

[0025] Figure 6 It is a flowchart of the operation of the symbol determination unit X.

[0026] Figure 7 It is a flowchart of the operation of the symbol determination unit Y.

[0027] Figure 8 It is a diagram showing the first operation example of the symbol determination circuit;

[0028] Figure 9 It is a diagram showing the second operation example of the symbol determination circuit.

[0029] Figure 10 It is a diagram showing the third operation example of the symbol determination circuit.

[0030] Figure 11 It is a diagram showing the third operation example of the symbol determination circuit.

[0031] Figure 12 It is a diagram showing a comparison example of symbol determination of 8QAM.

[0032] Figure 13 It is a diagram showing the function evaluation result in 8QAM of the adaptive equalization circuit of the embodiment.

[0033] Figure 14 It is a diagram showing the function evaluation result in 8QAM of the adaptive equalization circuit of the embodiment.

[0034] Figure 15 It is a diagram showing the function evaluation result in 8QAM of the adaptive equalization circuit of the embodiment.

[0035] Figure 16 It is a diagram showing the function evaluation result in 8QAM of the adaptive equalization circuit of the embodiment. Detailed implementation mode

[0036] Figure 1 It is a block diagram showing the optical communication system of the embodiment. The optical communication system includes a transmission device 100 and a reception device 200. The optical signal output from the transmission device 100 is transmitted to the reception device 200 through the optical fiber transmission path 300.

[0037] The transmitting device 100 includes a transmission signal processing circuit 101 and a transmission optical module 102. The receiving device 200 includes a receiving optical module 201 and a received signal processing circuit 202. The transmission signal processing circuit 101 performs prescribed processing on the input data. Specifically, the transmission signal processing circuit 101 divides the input data into horizontal polarization data X and vertical polarization data Y, and performs processing such as error correction coding, band-limiting filtering, and modulation mapping on each data. The horizontal polarization data X and the vertical polarization data Y that have undergone such processing are each represented by an in-phase component and a quadrature component and output to the transmission optical module 102. The in-phase component of the horizontal polarization data X is represented by X_I, and the quadrature component is represented by X_Q. The in-phase component of the vertical polarization data Y is represented by Y_I, and the quadrature component is represented by Y_Q. These data components are also denoted by the same symbols in the receiving device 200 described later.

[0038] In addition, in this specification, in principle, "data" represents "baseband data", and "signal" represents "optical signal" or "high-frequency signal". However, sometimes "X polarization" and "Y polarization" represent baseband data or high-frequency signals.

[0039] The transmission optical module 102 converts the horizontal polarization data X and the vertical polarization data Y into an X polarization signal and a Y polarization signal of an optical signal, respectively, synthesizes the two polarization signals, and transmits them. The transmission optical module 102 includes a signal light source 11 (signal LD), two 90° synthesizers 12, 13, and a polarization synthesizer 14. The two 90° synthesizers 12, 13 modulate the output light of the signal light source 11 using the horizontal polarization data X and the vertical polarization data Y, respectively, to convert them into optical signals. The polarization synthesizer 14 synthesizes the X polarization signal and the Y polarization signal that have been converted into optical signals. The synthesized optical signal is transmitted to the receiving device 200 through the optical fiber transmission path 300.

[0040] In the receiving device 200, the receiving optical module 201 receives the optical signal, converts the received optical signal into an electrical signal, and outputs it. The receiving optical module 201 includes a polarization separator 21, a local oscillation light source 22 (local oscillation LD), and two 90° hybrid circuits 23, 24. The polarization separator 21 separates the optical signal into two orthogonal polarization components, namely, an X polarization signal and a Y polarization signal.

[0041] 90° hybrid circuits 23 and 24 combine the output light of the local oscillation light source 22 with the respective polarization signals of the optical signal output from the polarization splitter 21, and further separate the respective polarization signals of the optical signal into an in-phase component I and a quadrature component Q. Although not shown, the 90° hybrid circuits 23 and 24 have photoelectric converters. The photoelectric converters convert the respective components (I, Q) of the X polarization signal and the Y polarization signal, which are the optical signals output from the 90° hybrid circuits 23 and 24, into electrical signals, and output the electrical signals as X polarization data (X_I, X_Q) and Y polarization data (Y_I, Y_Q). Hereinafter, the X polarization data and the Y polarization data are referred to as received signals. The "data" here represents an analog baseband signal. In addition, the above structure for obtaining the X polarization data and the Y polarization data is an example and is not limited to the above structure.

[0042] The received signal processing circuit 202 includes an AD converter 25, a wavelength dispersion compensation circuit 26, an adaptive equalization circuit 27, and a decoding circuit 28. The AD converter 25 converts the electrical signal output from the receiving optical module 201 into a digital signal. When the optical signal propagates in the optical fiber transmission path 300, the signal waveform is distorted due to wavelength dispersion. The wavelength dispersion compensation circuit 26 estimates the magnitude of the distortion of the received signal based on the digital signal output from the AD converter 25, and compensates for the distortion caused by the wavelength dispersion of the digital signal.

[0043] In addition, in the transmitting device 100, the X polarization signal and the Y polarization signal are combined and transmitted. Before the X polarization signal and the Y polarization signal are separated in the receiving device 200, polarization fluctuations occur due to the polarization mode dispersion effect of the optical fiber transmission path 300, and the signal waveform is distorted. The adaptive equalization circuit 27 performs an equalization process for compensating for the distortion caused by the polarization fluctuation of the output signal of the wavelength dispersion compensation circuit 26. In addition, the polarization separation is initially performed by the receiving optical module 201, and the adaptive equalization circuit 27 processes the polarization separation in a more complete direction. The decoding circuit 28 decodes the received signal output from the adaptive equalization circuit 27 to reproduce the original data (i.e., the input data of the transmitting signal processing circuit 101).

[0044] Before the data is decoded by the decoding circuit 28, the inputs and outputs of the respective processing circuits of the received signal processing circuit 202 are represented by four signals, namely, the in-phase component X_I and the quadrature component X_Q of the X polarization data, and the in-phase component Y_I and the quadrature component Y_Q of the Y polarization data. In each processing circuit, generally, the X polarization data and the Y polarization data are usually processed in a state where they are divided into an in-phase component and a quadrature component. The X polarization data (X_I, X_Q) and the Y polarization data (Y_I, Y_Q) are coordinate data, but in the decoding circuit 28, they are converted into logical data "1" and "0".

[0045] Figure 2FIG. is a diagram showing an adaptive equalization circuit according to an embodiment. The adaptive equalization circuit 27 includes a digital filter 1, a filter tap coefficient update circuit 2, a polarization separation monitor 3, and a control circuit 4.

[0046] The digital filter 1 receives the first polarization signal and the second polarization signal that have been polarization-separated by the wavelength dispersion compensation circuit 26, and performs further polarization separation processing to compensate for distortion and the like. The result of the compensation is provided to the filter tap coefficient update circuit 2. Signals received via the optical fiber transmission path 300 are generally affected by changes in the polarization state of the optical fiber transmission path 300. Therefore, the input signal from the wavelength dispersion compensation circuit 26 is also affected by the change in the polarization state. The filter tap coefficient update circuit 2 adaptively updates the tap coefficients of the digital filter 1 according to the change in the polarization state by a successive update algorithm. The updated tap coefficients are set for the digital filter 1. In the successive update algorithm, the tap coefficients are successively updated so that the output of the digital filter 1 becomes the original value and converges to a specified value.

[0047] At this time, the input signal input to the digital filter 1 is both X-polarization data and Y-polarization data. During the period when the filter tap coefficients of the digital filter 1 are updated in the filter tap coefficient update circuit 2, the output of the digital filter 1 is also provided to the polarization separation monitor 3. This polarization separation monitor 3 is called an equivalent convergence monitor (MC C). The polarization separation monitor 3 always calculates the correlation value between the X-polarization data and the Y-polarization data output from the digital filter 1, and compares the correlation value with a specified value to monitor whether the X-polarization data and the Y-polarization data have been properly polarization-separated. When the X-polarization data and the Y-polarization data are not properly polarization-separated, a correlation above a threshold is generated between the X-polarization data and the Y-polarization data. The polarization separation monitor 3 determines the failure of polarization separation based on this correlation. When the failure of polarization separation is determined, the control circuit 4 causes the filter tap coefficient update circuit 2 to perform the initial convergence of the tap coefficients again. Initial convergence means monitoring by the control circuit for determining whether polarization separation has been performed, regenerating the Wiener filter again by the least squares method, executing a successive update algorithm (CMA or RDE) of tap coefficients from the impulse state (a state where only the central tap is set to 1), and performing a certain process from the state of the current tap, such as only maintaining the taps on the polarization side with a larger amplitude of the tap coefficients and executing the tap coefficients on the other polarization side again from the impulse state tap coefficients.

[0048] However, in practice, a frequency error compensation circuit and a carrier phase reproduction circuit (not shown) are provided on the output side of the digital filter 1. Synchronization with the frequency or phase of the carrier is performed by these circuits, and the carrier-synchronized X-polarization data and Y-polarization data are supplied to the polarization separation monitor 3 and the decoding circuit 28.

[0049] In addition, the monitoring operation of the polarization separation monitor 3 sometimes performs incorrect detection depending on the modulation method. Generally, in the case of modulation methods such as QPSK and 16QAM, the monitor has few malfunction cases. However, for modulation methods such as 8QAM, especially those that include signals mapping the transmission-side data at points where the values of the in-phase component or the quadrature component are near 0, there are cases where the monitor malfunctions. If the monitor malfunctions, the update of unnecessary tap coefficients is repeated, and the adaptive equalization operation becomes unstable.

[0050] In contrast, the polarization separation monitor 3 of the present embodiment can reduce the malfunction of the monitor function in any modulation method. In addition, the adaptive equalization circuit 27 using the polarization separation monitor 3 can appropriately determine polarization separation, always appropriately update the filter tap coefficients, and perform a stable adaptive equalization operation. The output of the digital filter 1 with the tap coefficients appropriately updated in this way is supplied to the Figure 2 decoding circuit 28 shown.

[0051] Figure 3 is a diagram showing the digital filter of the embodiment. This digital filter 1 is an example constituted by an FIR filter. However, the digital filter 1 is not limited to this structure, as long as it is a structure in which incorrect convergence may occur in the case of a successive update algorithm that obtains filter tap coefficients through the convergence operation of a convergence algorithm.

[0052] As an example of the successive update algorithm, there are blind equalization methods such as CMA (Constant Modulus Algorithm) or RDE (Radius-directed equalization). They update the tap coefficients in such a way as to minimize the error between the output of the digital filter 1 and the value that should be (in the case of a constant envelope, the "value that should be" can be easily estimated as the expected value of the amplitude). As other examples, there are also RLS (Recursive Least-Squares) or LMS (Least Mean Square), etc. They insert a known signal such as a training signal or a pilot signal into the optical signal on the transmission side, and update the tap coefficients at each step to minimize the error between the transmitted known signal and the true value (the value set on the transmission side) of the known signal.

[0053] The digital filter 1 includes FIR (Finite Impulse Response) filters FIR_A, FIR_B, FIR_C, and FIR_D configured in a butterfly shape. Each FIR filter has N taps. However, the number of taps of the FIR filters may also be different from each other. FIR_A is a filter for X-polarized data. FIR_B is a filter for the influence from Y-polarized data to X-polarized data. FIR_C is a filter for the influence from X-polarized data to Y-polarized data. FIR_D is a filter for Y-polarized data.

[0054] The digital filter 1 uses the sum of the filtering result of FIR_A for X-polarized data and the filtering result of FIR_B for Y-polarized data as the compensation output for X-polarized data, and uses the sum of the filtering result of FIR_C for X-polarized data and the filtering result of FIR_D for Y-polarized data as the compensation output for Y-polarized data. Thereby, the polarization separation is more reliable.

[0055] The digital filter 1 uses the sum of the filtering result of FIR_A for X-polarized data and the filtering result of FIR_B for Y-polarized data as the compensation output for X-polarized data, and uses the sum of the filtering result of FIR_C for X-polarized data and the filtering result of FIR_D for Y-polarized data as the compensation output for Y-polarized data. Thereby, the polarization separation is more reliable.

[0056] In addition, the filter tap coefficients of the digital filter 1 are obtained and set by a filter tap coefficient update circuit 2. At this time, the tap coefficients of FIR_A, FIR_B, FIR_C, and FIR_D are represented by the following equations.

[0057] WXX(n + 1) = WXX(n) + μeX(n)Xout(n)·Xin*(n)

[0058] WYX(n + 1) = WYX(n) + μeX(n)Xout(n)·Yin*(n)

[0059] WXY(n + 1) = WXY(n) + μeY(n)Yout(n)·Xin*(n)

[0060] WYY(n + 1) = WYY(n) + μeY(n)Yout(n)·Yin*(n)

[0061] Here, n is a value representing the update order in the successive update algorithm. The tap coefficient WXX(n) indicates the tap coefficient group of FIR_A in the case of update order n. The tap coefficient WYX(n) indicates the tap coefficient group of FIR_B in the case of update order n. The tap coefficient WXY(n) represents the tap coefficient group of FIR_C in the case of update order n. The tap coefficient WYY(n) represents the tap coefficient group of FIR_D in the case of update order n. μ represents the step size of the update algorithm. eX(n) represents the error from the expected value in the filter output of the X-polarized data. eY(n) represents the error from the expected value in the filter output of the Y-polarized data. Xout(n) represents the filter output in the X-polarized data. Xin(n) represents the filter input in the X-polarized data. Yout(n) represents the filter output in the Y-polarized data. Yin(n) represents the filter input in the Y-polarized data. * represents conjugate or complex conjugate. Additionally, the data and tap coefficients are represented by complex numbers.

[0062] Through the above successive update algorithm, the tap coefficients are updated successively in the update order n. Eventually, the tap coefficients converge. The convergence condition is determined by the number of times of update order n, or the error between the filter output and the expected value, etc. In addition, the above formula is an example of the formula representing the successive update algorithm, and the formula representing the successive update algorithm is not limited to the above formula.

[0063] The above shows the case of updating the tap coefficients after convergence, but it is also possible to update the tap coefficients successively during the convergence process. That is, even if the convergence condition is not achieved, it is possible to adopt a method of successively updating using the tap coefficients calculated in such a way that the difference between the output of the digital filter and the value that should be present is minimized in the state where the polarization state changes.

[0064] Figure 4 is a diagram showing the polarization separation monitor of the embodiment. In Figure 4 In, Figure 2 A frequency error compensation circuit 5 and a carrier phase reproduction circuit 6 (not shown) are connected between the digital filter 1 and the decoding circuit 28. The frequency error compensation circuit 5 is a circuit that makes the frequency error between the transmitted carrier and the receiver carrier substantially zero. The carrier phase reproduction circuit 6 is a circuit that makes the phase error between these carriers substantially zero. Through the frequency error compensation circuit 5 and the carrier phase reproduction circuit 6, the X-polarized data and Y-polarized data from the digital filter 1 can be stably displayed on the IQ plane without phase rotation and phase shift. Thereby, the X-polarized data and Y-polarized data from the digital filter 1 can be accurately processed in the polarization separation monitor 3 and the decoding circuit 28. Additionally, when phase rotation and phase shift can be removed by other methods, the frequency error compensation circuit 5 and the carrier phase reproduction circuit 6 are not essential for the adaptive equalization circuit 27.

[0065] The polarization separation monitor 3 includes a sign determination circuit 7, a correlation operation circuit 8, and a comparison circuit 9. The sign determination circuit 7 determines the signs of the outputs Xout and Yout of the digital filter 1 supplied via the frequency error compensation circuit 5 and the carrier phase reproduction circuit 6. Here, consider the case where the outputs Xout and Yout of the digital filter 1 are shown on the IQ plane. Specifically, Xout has an I-axis component X_I and a Q-axis component X_Q, and Yout has an I-axis component Y_I and a Q-axis component Y_Q. The sign determination circuit 7 determines in which of the four quadrants on the IQ plane the received X-polarization data and Y-polarization data belong for each bit or each symbol. In addition, a symbol represents a unit of change in phase or amplitude in the case of multi-value modulation such as QPSK or 16QAM. In QPSK, 2 bits are 1 symbol, and in 16QAM, 4 bits are 1 symbol.

[0066] The correlation operation circuit 8 calculates the correlation value between the X-polarization data and the Y-polarization data based on the determination result of the sign determination circuit 7. The closer the X-polarization data and the Y-polarization data are to being the same, the higher the correlation value. The comparison circuit 9 compares the correlation value calculated by the correlation operation circuit 8 with a specified value and passes the comparison result to the control circuit 4.

[0067] When the correlation value exceeds the specified value, it is regarded that there is a correlation between the X-polarization data and the Y-polarization data, and it is determined that the convergence of the tap coefficients has not been properly performed. Therefore, when the polarization separation monitor 3 detects that the correlation value exceeds the specified value, the control circuit 4 causes the filter tap coefficient update circuit 2 to perform the initial convergence of the tap coefficients again. On the other hand, if the correlation value from the correlation operation circuit 8 is less than the specified value, the control circuit 4 regards that there is no correlation between the X-polarization data and the Y-polarization data and determines that the convergence of the tap coefficients has been properly performed. In this case, the control circuit 4 does not instruct the filter tap coefficient update circuit 2 to perform the initial convergence of the tap coefficients again, and the digital filter 1 continues to use the tap coefficients at this time.

[0068] Figure 5 It is a diagram showing the sign determination circuit of the embodiment. The frequency error compensation circuit 5 and the carrier phase reproduction circuit 6 are omitted. The sign determination circuit 7 includes sign determination units X, Y, and averaging circuits A, B, C, D.

[0069] The symbol determination unit X determines which quadrant of the four quadrants on the IQ plane the X-polarization data belongs to based on the I-axis component X_I and the Q-axis component X_Q of the X-polarization data from the digital filter 1. The values representing the four quadrants (the first quadrant, the second quadrant, the third quadrant, and the fourth quadrant) on the IQ plane are (+1, +1), (-1, +1), (-1, -1), and (+1, -1), respectively. When the X-polarization data belongs to the first quadrant, the symbols are A = +1, B = +1; when it belongs to the second quadrant, the symbols are A = -1, B = +1; when it belongs to the third quadrant, the symbols are A = -1, B = -1; and when it belongs to the fourth quadrant, the symbols are A = +1, B = -1.

[0070] The symbol determination unit Y determines which quadrant of the four quadrants on the IQ plane the Y-polarization data belongs to based on the I-axis component Y_I and the Q-axis component Y_Q of the Y-polarization data from the digital filter 1. The values representing the four quadrants (the first quadrant, the second quadrant, the third quadrant, and the fourth quadrant) on the IQ plane are (+1, +1), (-1, +1), (-1, -1), and (+1, -1), respectively. When the Y-polarization data belongs to the first quadrant, the symbols are C = +1, D = +1; when it belongs to the second quadrant, the symbols are C = -1, D = +1; when it belongs to the third quadrant, the symbols are C = -1, D = -1; and when it belongs to the fourth quadrant, the symbols are C = +1, D = -1.

[0071] In the case where the X-polarization data or the Y-polarization data is on-axis data located on the I-axis or the Q-axis, the quadrant to which it belongs is not clear. Therefore, the symbol determination circuit 7 extracts the on-axis data, and respectively corresponds the positive-side data on the I-axis, the negative-side data on the I-axis, the positive-side data on the Q-axis, and the negative-side data on the Q-axis in the on-axis data to one value among the values representing the four quadrants on the IQ plane without repetition, and outputs the symbols representing the corresponding quadrants. Details will be described later.

[0072] The averaging circuit A accumulates the symbol A determined by the symbol determination unit X a prescribed number of times and outputs it as averaged A. The averaging circuit B accumulates the symbol B determined by the symbol determination unit X a prescribed number of times and outputs it as averaged B. The averaging circuit C accumulates the symbol C determined by the symbol determination unit Y a prescribed number of times and outputs it as averaged C. The averaging circuit D accumulates the symbol D determined by the symbol determination unit Y a prescribed number of times and outputs it as averaged D.

[0073] Using averaged A, averaged B, averaged C, and averaged D, the averaged data XA of the X-polarization and the averaged data YA of the Y-polarization are represented as follows. In addition, j is the imaginary unit.

[0074] XA = averaged A + j * averaged B

[0075] YA = Averaged C + j × Averaged D

[0076] Then, the averaged data XA of the X polarization and the averaged data YA of the Y polarization output from the symbol determination circuit 7 are supplied to the correlation operation circuit 8. The correlation operation circuit 8 calculates the correlation value between the averaged data XA of the X polarization and the averaged data YA of the Y polarization by the following formula. "*" represents complex multiplication.

[0077]

[0078] If the X polarization data X and the Y polarization data Y have the same value (i.e., have correlation) in the output of the digital filter 1, then the averaged data XA of the X polarization and the averaged data YA of the Y polarization have the same value. Therefore, the above-mentioned correlation value is infinitely close to 1. In addition, when the signs of XA and XB are the same, the correlation value does not approach 0 but increases. In contrast, if the X polarization data X and the Y polarization data Y are substantially randomly different (i.e., have no correlation), then the values of the averaged data XA of the X polarization and the averaged data YA of the Y polarization become different signs with a probability of 1 / 2. Thus, the sign of XA * YA in the numerator of the correlation value also becomes positive or negative, and its accumulation is substantially close to 0.

[0079] In addition, the number of accumulations in the numerator and denominator of the correlation value formula is different from the specified number of times in the case of averaging by the symbol determination circuit 7. For example, the symbol determination circuit 7 can be averaged every 16 symbols, and the accumulation of the correlation value formula is set to 512 times. In this case, the correlation value is obtained every 16 × 512 = 8192 symbols.

[0080] In addition, the operation of the correlation value of the correlation operation circuit 8 is not limited to the above method. As long as the correlation value of the X polarization data X and the Y polarization data Y as the output of the digital filter can be calculated based on the averaged data XA of the X polarization and the averaged data YA of the Y polarization from the symbol determination circuit 7, any operation formula can be applied to the adaptive equalization circuit 27 of the present embodiment.

[0081] Figure 6 It is a flowchart of the operation of the symbol determination unit X. The steps executed by the symbol determination unit X vary depending on the modulation method.

[0082] Step S0: When the modulation method is 8QAM such that a part of the data is mapped to the I axis or the Q axis in the IQ plane, the process proceeds to step S1 next. On the other hand, when the modulation method is QPSK or 16QAM such that all the data is not mapped to the I axis or the Q axis in the IQ plane, steps S1 to S3 are skipped and the process proceeds to step S4.

[0083] Step S1: After inputting the I-axis component \(X_I\) and the Q-axis component \(X_Q\) of the X-polarization data from the digital filter 1, estimate the power of the X-polarization data based on the following formula.

[0084] Power of the X - polarization data = Absolute value of \(X_I\)+ Absolute value of \(X_Q\)

[0085] Power is generally represented by the square of a signal, but it can also be represented as an index of power in the above formula. The above formula can be calculated only by addition without using multiplication, so the index of power can be simply obtained. In modulation methods that mainly change phase and amplitude, the amplitudes that data can take are generally divided into several groups. In particular, the amplitudes that data can take in the modulation methods used in blind equalization methods are reduced to several types. In the case of QPSK, the amplitude is 1 type, in the case of 8QAM, the amplitude is 2 types, and in the case of 16QAM, the amplitude is 3 types. The calculation of power in this step only determines which amplitude group the data belongs to, so it is not necessary to measure the exact power value.

[0086] Step S2: Based on the magnitude of the power obtained in Step S1, determine which amplitude group the data belongs to. For example, in the case where the amplitudes that data can take are two types, namely the inner shell and the outer shell, if the power of X ≥ the set threshold, it is determined to be the outer shell, and otherwise, it is determined to be the inner shell.

[0087] Step S3: In the case where it is determined to be the inner shell in Step S2, determine the symbols A and B according to the following conditions. Here, a case is shown where the data of the inner shell is mapped to the I-axis or the Q-axis, and the data of the outer shell is mapped to the four quadrants of the IQ plane. In addition, in the case where the data of the inner shell is mapped to the quadrants of the IQ plane and the data of the outer shell is mapped to the I-axis or the Q-axis, Step S3 is executed in the case of the outer shell, and Step S4 is executed in the case of the inner shell.

[0088] When the absolute value of \(X_I\) ≥ the absolute value of \(X_Q\), if \(X_I\geq0\), it is determined as the fourth quadrant \((A, B)=( + 1, - 1)\), and if \(X_I\lt0\), it is determined as the second quadrant \((A, B)=( - 1, + 1)\). When the absolute value of \(X_I\lt\) the absolute value of \(X_Q\), if \(X_Q\geq0\), it is determined as the first quadrant \((A, B)=( + 1, + 1)\), and if \(X_Q\lt0\), it is determined as the third quadrant \((A, B)=( - 1, - 1)\).

[0089] Step S4: When it is determined to be the outer shell in Step S2, determine symbols A and B according to the following conditions. When \(X_I\geq0\), if \(X_Q\geq0\), it is determined as the first quadrant \((A, B)=(+1, +1)\); if \(X_Q\lt0\), it is determined as the fourth quadrant \((A, B)=(+1, -1)\). When \(X_I\lt0\), if \(X_Q\geq0\), it is determined as the second quadrant \((A, B)=(-1, +1)\); if \(X_Q\lt0\), it is determined as the third quadrant \((A, B)=(-1, -1)\).

[0090] Step S5: Output the symbols A and B obtained in Step S3 or Step S4. In addition, the determination in Step S3 can also be made as follows. When the absolute value of \(X_I\geq\) the absolute value of \(X_Q\), if \(X_I\geq0\), it is determined as the first quadrant \((A, B)=(+1, +1)\); if \(X_I\lt0\), it is determined as the third quadrant \((A, B)=(-1, -1)\). When the absolute value of \(X_I\lt\) the absolute value of \(X_Q\), if \(X_Q\geq0\), it is determined as the second quadrant \((A, B)=(-1, +1)\); if \(X_Q\lt0\), it is determined as the fourth quadrant \((A, B)=(+1, -1)\).

[0091] In addition, in the above example, the amplitudes that the data can take are two types, namely the inner shell and the outer shell, but it can also be applied to the case where the amplitudes that the data can take are more than three types. In this case, it is only necessary to determine which amplitude group by the same power calculation method. After the determination of the amplitude group, for the on-axis data located on the I-axis or Q-axis, allocate quadrants through Step S3. In addition, for the data originally mapped within the quadrant, determine the quadrant through the hard determination in Step S4.

[0092] In addition, the correspondence between the on-axis data located on the I-axis or Q-axis and the quadrants is not limited to the above two. As long as different data are not corresponding to the same quadrant, they can be allocated to any quadrant. For example, \((I, Q)=\{(1, 0), (-1, 0), (0, 1), (0, -1)\}\) can be corresponding like \((A, B)=\{(-1, +1), (+1, -1), (-1, -1), (+1, +1)\}\), \((A, B)=\{(+1, -1), (-1, +1), (-1, -1), (+1, +1)\}\) or \((A, B)=\{(+1, +1), (-1, +1), (-1, -1), (+1, -1)\}\), etc. As described above, if the positive-side data, negative-side data on the I-axis, positive-side data, and negative-side data on the Q-axis of the X-polarization data and Y-polarization data are respectively corresponding to the quadrants without repetition, the correlation between the X-polarization data and Y-polarization data can be detected efficiently. Such an arbitrary correspondence is also the same in the symbol determination unit Y described below.

[0093] Figure 7 It is a flowchart of the operation of the symbol determination unit Y. The operation of the symbol determination unit Y is the same as Figure 6 the operation of the symbol determination unit X shown.

[0094] Step S0: When the modulation method is such that a part of the data is mapped to the I-axis or Q-axis in the IQ plane, such as 8QAM, the process proceeds to step S1 next. On the other hand, when the modulation method is such that all the data is not mapped to the I-axis or Q-axis in the IQ plane, such as QPSK or 16QAM, steps S1 to S3 are skipped and the process proceeds to step S4.

[0095] Step S1: After the I-axis component Y_I and Q-axis component Y_Q of the Y-polarized data are input from the digital filter 1, the power of the Y-polarized data is estimated based on the following formula.

[0096] Power of Y-polarized data = absolute value of Y_I + absolute value of Y_Q

[0097] In addition, since the calculation of the power only determines which amplitude group the data belongs to, the index of the above formula can be used for comparison, and it is not necessary to measure the accurate power value.

[0098] Step S2: Based on the magnitude of the power obtained in step S1, it is determined which amplitude group the Y-polarized data belongs to. For example, when there are two types of amplitudes that the Y-polarized data can take, namely the inner shell and the outer shell, if the power of Y ≥ the set threshold, it is determined as the outer shell, and otherwise, it is determined as the inner shell.

[0099] Step S3: When it is determined as the inner shell in step S2, the symbols C and D are determined according to the following conditions. Here, a case is shown where the data of the inner shell is mapped to the I-axis or Q-axis, and the data of the outer shell is mapped to the four quadrants in the IQ plane. In addition, when the data of the inner shell is mapped to the quadrants in the IQ plane and the data of the outer shell is mapped to the I-axis or Q-axis, step S3 is executed in the case of the outer shell, and step S4 is executed in the case of the inner shell.

[0100] When the absolute value of Y_I ≥ the absolute value of Y_Q, if Y_I ≥ 0, it is determined as the fourth quadrant and (C, D) = (+1, -1), and if Y_I < 0, it is determined as the second quadrant and (C, D) = (-1, +1). When the absolute value of Y_I < the absolute value of Y_Q, if Y_Q ≥ 0, it is determined as the first quadrant and (C, D) = (+1, +1), and if Y_Q < 0, it is determined as the third quadrant and (C, D) = (-1, -1).

[0101] Step S4: When it is determined in Step S2 that it is the outer shell, determine symbols C and D according to the following conditions. When Y_I ≥ 0, if Y_Q ≥ 0, it is determined as the first quadrant (C, D) = (+1, +1); if Y_Q < 0, it is determined as the fourth quadrant (C, D) = (+1, -1). When Y_I < 0, if Y_Q ≥ 0, it is determined as the second quadrant (C, D) = (-1, +1); if Y_Q < 0, it is determined as the third quadrant (C, D) = (-1, -1).

[0102] Step S5: Output the symbols C and D obtained in Step S3 or Step S4. In addition, the determination in Step S3 can also be made as follows. When the absolute value of Y_I ≥ the absolute value of Y_Q, if Y_I ≥ 0, it is determined as the first quadrant (C, D) = (+1, +1); if Y_I < 0, it is determined as the third quadrant (C, D) = (-1, -1). When the absolute value of Y_I < the absolute value of Y_Q, if Y_Q ≥ 0, it is determined as the second quadrant (C, D) = (-1, +1); if Y_Q < 0, it is determined as the fourth quadrant (C, D) = (+1, -1).

[0103] In addition, the processing of the symbol determination unit Y is the same as that of the symbol determination unit X and can be applied to the case where the amplitude of the data that can be taken is more than three types. In addition, the correspondence between the on-axis data located on the I axis or the Q axis and the quadrant is not limited to the above two, and as long as different data do not correspond to the same quadrant, they can be assigned to any quadrant.

[0104] Figure 8 It is a diagram showing the first operation example of the symbol determination circuit. The first operation example is the case where the modulation signal is QPSK. In the case where the modulation signal is QPSK, there is no on-axis data located on the I axis or the Q axis. Therefore, after Step S0 in the Figure 6 and Figure 7 flowchart, Step S4 is executed.

[0105] In step S4, when X_I ≥ 0, if X_Q ≥ 0, the symbol determination unit X determines (A, B) = (+1, +1) as the first quadrant, and if X_Q < 0, it determines (A, B) = (+1, -1) as the fourth quadrant. When X_I < 0, if X_Q ≥ 0, the symbol determination unit X determines (A, B) = (-1, +1) as the second quadrant, and if X_Q < 0, it determines (A, B) = (-1, -1) as the third quadrant. When Y_I ≥ 0, if Y_Q ≥ 0, the symbol determination unit Y determines (C, D) = (+1, +1) as the first quadrant, and if Y_Q < 0, it determines (C, D) = (+1, -1) as the fourth quadrant. When Y_I < 0, if Y_Q ≥ 0, the symbol determination unit Y determines (C, D) = (-1, +1) as the second quadrant, and if Y_Q < 0, it determines (C, D) = (-1, -1) as the third quadrant.

[0106] The above results, (A, B) and (C, D) are equivalent to the results of hard determination of the values representing the first quadrant, the second quadrant, the third quadrant, and the fourth quadrant of QPSK at the receiving side. For example, the hard determination result of the value representing the first quadrant is A = +1, B = +1, C = +1, D = +1. The hard determination result of the value representing the second quadrant is A = -1, B = +1, C = -1, D = +1. The hard determination result of the value representing the third quadrant is A = -1, B = -1, C = -1, D = -1. The hard determination result of the value representing the fourth quadrant is A = +1, B = -1, C = +1, D = -1.

[0107] Figure 9 It is a diagram showing a second operation example of the symbol determination circuit. The second operation example is the case where the modulation signal is 16QAM. In the case where the modulation signal is 16QAM, there is no in-phase axis data or quadrature axis data on the I-axis or Q-axis. Therefore, step S4 is executed after step S0 in the Figure 6 and Figure 7 flowchart.

[0108] In step S4, when X_I ≥ 0, if X_Q ≥ 0, the symbol determination unit X determines (A, B) = (+1, +1) as the first quadrant, and if X_Q < 0, it determines (A, B) = (+1, -1) as the fourth quadrant. When X_I < 0, if X_Q ≥ 0, the symbol determination unit X determines (A, B) = (-1, +1) as the second quadrant, and if X_Q < 0, it determines (A, B) = (-1, -1) as the third quadrant. When Y_I ≥ 0, if Y_Q ≥ 0, the symbol determination unit Y determines (C, D) = (+1, +1) as the first quadrant, and if Y_Q < 0, it determines (C, D) = (+1, -1) as the fourth quadrant. When Y_I < 0, if Y_Q ≥ 0, the symbol determination unit Y determines (C, D) = (-1, +1) as the second quadrant, and if Y_Q < 0, it determines (C, D) = (-1, -1) as the third quadrant.

[0109] As a result of the above, (A, B) and (C, D) correspond to the results of determining the coordinates of all 16 data of 16QAM mapped to the IQ plane by hard determination at the receiving side. For example, the hard determination results representing the values in the first quadrant are A = +1, B = +1, C = +1, D = +1. The hard determination results representing the values in the second quadrant are A = -1, B = +1, C = -1, D = +1. The hard determination results representing the values in the third quadrant are A = -1, B = -1, C = -1, D = -1. The hard determination results representing the values in the fourth quadrant are A = +1, B = -1, C = +1, D = -1.

[0110] As described above, A, B, C, and D are obtained by hard determination based on the I-axis and Q-axis for the data in the four quadrants on the IQ plane. Therefore, in transmission modulation methods such as 64QAM or 256QAM where there is no in-axis data on the I-axis or Q-axis, A, B, C, and D are obtained by the same method as above.

[0111] Figure 10 and Figure 11 FIG. shows a third operation example of the symbol determination circuit. The third operation example is the case where the modulation signal is 8QAM. 8QAM is a transmission modulation method in which there is in-axis data on the I-axis or Q-axis in the X-polarization data and Y-polarization data. Regarding the correspondence between the in-axis data on the I-axis or Q-axis and the quadrants, two examples of Figure 10 and Figure 11 will be described. However, as described before, this correspondence is not limited to these two examples, and any quadrant can be corresponded as long as there is no repetition.

[0112] When the modulation method is 8QAM, 4 of the 8 data in the outer shell are mapped to the center of each of the 1st to 4th quadrants, and the remaining 4 data in the inner shell are mapped to the I-axis or the Q-axis. Therefore, in step S0, there are data on the axis located on the I-axis or the Q-axis, so Figure 6 and Figure 7 all steps after step S1 of the flowchart shown are executed.

[0113] In step S1, power estimation is performed. The power of the X-polarized data and the Y-polarized data are calculated by the following formulas respectively.

[0114] Power of X-polarized data = |X_I| + |X_Q|

[0115] Power of Y-polarized data = |Y_I| + |Y_Q|

[0116] Regarding the 4 data in the outer shell, when the received data is located approximately at the center of each quadrant, |X_I| and |X_Q| become approximately the same value, and in addition, |Y_I| and |Y_Q| also become approximately the same value. Therefore, the power is calculated as approximately twice the absolute value of the coordinate value on the I-axis respectively.

[0117] On the other hand, for the 4 data in the inner shell, one of |X_I| and |X_Q| is approximately near zero, and one of |Y_I| and |Y_Q| is approximately near zero. Therefore, the power is calculated as only the absolute value of the coordinate value on the I-axis or the absolute value of the coordinate value on the Q-axis respectively.

[0118] Therefore, assuming that the coordinate value of the outer shell is about 2 times that of the inner shell, the power of the outer shell is about 4 times that of the inner shell. The difference between these powers can be detected relatively large. Therefore, if a threshold is set between them, it is possible to relatively easily determine whether the received data is data of the inner shell or the outer shell.

[0119] In step S2, it is determined whether the received data is data of the inner shell or the outer shell. In the case of 8QAM, if the threshold is set to 2.5 times the coordinate value on the I-axis of the data of the inner shell as described above, when the power estimated in step S1 is higher than the set threshold, it is determined as data of the outer shell, and when it is lower than the set threshold, it is determined as data of the inner shell. In addition, the threshold is selected according to the modulation method to be a value that can more reliably determine the inner shell and the outer shell. Through this determination, in Figure 10 the example of 8QAM shown, the data located near the center of the 4 quadrants on the IQ plane are identified as data of the outer shell, and the data on the axis located on the I-axis or the Q-axis are identified as data of the inner shell.

[0120] In step S3, the following processing is performed on the data of the inner shell. When the absolute value of X_I ≥ the absolute value of X_Q, the symbol determination unit X, if X_I ≥ 0, associates the on-axis data with the value representing the fourth quadrant, and sets (A, B) = (+1, -1); if X_I < 0, associates the on-axis data with the value representing the second quadrant, and sets (A, B) = (-1, +1). When the absolute value of X_I < the absolute value of X_Q, the symbol determination unit X, if X_Q ≥ 0, associates the on-axis data with the value representing the first quadrant, and sets (A, B) = (+1, +1); if X_Q < 0, associates the on-axis data with the value representing the third quadrant, and sets (A, B) = (-1, -1). When the absolute value of Y_I ≥ the absolute value of Y_Q, the symbol determination unit Y, if Y_I ≥ 0, associates the on-axis data with the value representing the fourth quadrant, and sets (A, B) = (+1, -1); if Y_I < 0, associates the on-axis data with the value representing the second quadrant, and sets (A, B) = (-1, +1). When the absolute value of Y_I < the absolute value of Y_Q, the symbol determination unit Y, if Y_Q ≥ 0, associates the on-axis data with the value representing the first quadrant, and sets (A, B) = (+1, +1); if Y_Q < 0, associates the on-axis data with the value representing the third quadrant, and sets (A, B) = (-1, -1).

[0121] That is, regarding the data of the inner shell, the positive-side data on the I axis is associated with the value representing the fourth quadrant, the negative-side data on the I axis is associated with the value representing the second quadrant, the positive-side data on the Q axis is associated with the value representing the first quadrant, and the negative-side data on the Q axis is associated with the value representing the third quadrant. This corresponds to the case where the phase of the signal point of each data is rotated 45 degrees to the right. The values of A, B, C, and D corresponding to each quadrant are output from the symbol determination circuit 7.

[0122] In addition, regarding the data of the inner shell, it is also possible to associate the positive-side data on the I axis with the value representing the first quadrant, the negative-side data on the I axis with the value representing the third quadrant, the positive-side data on the Q axis with the value representing the second quadrant, and the negative-side data on the Q axis with the fourth quadrant. This corresponds to the case where the phase of the signal point of each data is rotated 45 degrees to the left. This case is as Figure 11 shown.

[0123] Next, in step S4, the following processing is performed on the data of the outer shell. Regarding this processing, Figure 10 the example of Figure 11is the same as the example. When X_I ≥ 0, if X_Q ≥ 0, the symbol determination unit X determines (A, B) = (+1, +1) as the first quadrant, and if X_Q < 0, it determines (A, B) = (+1, -1) as the fourth quadrant. When X_I < 0, if X_Q ≥ 0, the symbol determination unit X determines (A, B) = (-1, +1) as the second quadrant, and if X_Q < 0, it determines (A, B) = (-1, -1) as the third quadrant. When Y_I ≥ 0, if Y_Q ≥ 0, the symbol determination unit Y determines (C, D) = (+1, +1) as the first quadrant, and if Y_Q < 0, it determines (C, D) = (+1, -1) as the fourth quadrant. When Y_I < 0, if Y_Q ≥ 0, the symbol determination unit Y determines (C, D) = (-1, +1) as the second quadrant, and if Y_Q < 0, it determines (C, D) = (-1, -1) as the third quadrant.

[0124] For Figure 10 and Figure 11 the data of the outer shell of 8QAM shown, the outputs (A, B), (C, D) of the symbol determination units X and Y are the same as those of QPSK, and are the same as the results of determining the values representing each quadrant by hard determination on the receiving side.

[0125] Figure 12 FIG. is a diagram showing a comparative example of symbol determination of 8QAM. In the comparative example, for the signal points of 8QAM, like QPSK or 16QAM, hard determination is performed through the I-axis and Q-axis regardless of the power level.

[0126] In the case of 8QAM, for the 8 signal points and the 4 data of the outer shell, the hard determination results directly become the values representing the first quadrant, the second quadrant, the third quadrant, and the fourth quadrant, and the corresponding symbols (A, B), (C, D) are output. However, for the 4 data of the inner shell, the symbol determination based on the hard determination results is uncertain.

[0127] On the other hand, in the symbol determination circuit 7 using the Figure 6 and Figure 7 shown symbol determination method, in the case of 8QAM shown in Figure 10 and Figure 11 for the 4 data of the outer shell, the hard determination results directly become the symbols corresponding to each quadrant, and further for the 4 data of the inner shell, they are assigned to one of the 4 quadrants without becoming uncertain, and are determined as the symbols corresponding to that quadrant.

[0128] Therefore, even in the case of a modulation method in which a part of the data is mapped onto the I-axis or Q-axis, the symbol determination circuit 7 of the present embodiment can reliably determine the symbol without accompanying uncertain determination. In addition, their averaging can also be performed. In the correlation operations using this output, the correlation value can be stably obtained and accurately compared. As a result, the polarization separation monitor 3 including the symbol determination circuit 7 can significantly reduce the case of erroneously detecting equivalent convergence even for a modulation method such as 8QAM in which a part of the data is mapped onto the I-axis or Q-axis.

[0129] Figures 13 to 16 FIG. is a diagram showing the result of function evaluation in 8QAM of the adaptive equalization circuit according to the embodiment. These are obtained by measuring the correlation status in the correlation operation circuit 8 of the polarization separation monitor 3. However, in the adaptive equalization circuit 27, all the tap coefficients of the four FIR filters in the digital filter 1 are in a state of converging to values that correctly perform polarization separation. Since the operation of the tap coefficients converges correctly, the correlation value should preferably be a lower value in the polarization separation monitor 3. When the correlation value is high, although the operation of the tap coefficients has converged, it is erroneously detected as not converged.

[0130] Figure 13 Shows the correlation value between the I-axis component X_I of the X-polarized data and the I-axis component Y_I of the Y-polarized data. Figure 14 Shows the correlation value between the I-axis component X_I of the X-polarized data and the Q-axis component Y_Q of the Y-polarized data. Figure 15 Shows the correlation value between the Q-axis component X_Q of the X-polarized data and the I-axis component Y_I of the Y-polarized data. Figure 16 Shows the correlation value between the Q-axis component X_Q of the X-polarized data and the Q-axis component Y_Q of the Y-polarized data. The vertical axis represents the correlation value, 1 represents the maximum correlation, and 0 represents no correlation. The horizontal axis represents time. In each graph, the upper graph (points of ◆) represents the output of the correlation operation circuit of the conventional polarization separation monitor that does not apply the present embodiment. The lower graph (points of ■) represents the output of the correlation operation circuit 8 of the polarization separation monitor 3 of the present embodiment.

[0131] The output of the correlation operation circuit of the existing polarization separation monitor sometimes has a maximum value of 0.55 in Figure 14 and sometimes has a maximum value of 0.55 in Figure 16Values above 0.65 sometimes appear, and larger correlation values are detected. Assuming that the comparison value of the correlation value in the comparison circuit is set to 0.5, it can be seen that incorrect detections occur frequently. If an incorrect detection is made, the polarization separation monitor 3 determines that equivalent convergence has occurred and notifies the control circuit 4. Through this notification, the control circuit 4 instructs the filter tap coefficient update circuit 2 to perform an update again. Although the tap coefficient has converged to a value that correctly performs polarization separation, the update operation is performed again. Therefore, the tap coefficient fluctuates unnecessarily, and stable adaptive equalization processing cannot be performed.

[0132] On the other hand, the output of the correlation operation circuit 8 of the polarization separation monitor 3 according to the present embodiment is 0.2 or less in any case. If the comparison value of the correlation value in the comparison circuit 9 is set to 0.6, incorrect detections do not occur. As a result, the polarization separation monitor 3 does not notify the control circuit 4 of information on incorrect equivalent convergence. It is also possible to reduce unnecessary re-update instructions from the control circuit 4 to the filter tap coefficient update circuit 2. Thus, stable adaptive equalization processing can be continued without unnecessary fluctuations in the tap coefficient.

[0133] As described above, in actual evaluation, the effectiveness of the polarization separation monitor 3 according to the present embodiment can also be confirmed. In addition, the adaptive equalization circuit 27 having the polarization separation monitor 3 according to the present embodiment can handle any modulation method and can perform stable polarization separation operation.

[0134] Reference Numeral Explanation

[0135] 1 Digital filter; 2 Filter tap coefficient update circuit; 3 Polarization separation monitor; 4 Control circuit; 7 Symbol determination circuit; 8 Correlation operation circuit; 9 Comparison circuit; 25 AD converter; 26 Wavelength dispersion compensation circuit; 27 Adaptive equalization circuit; 201 Optical receiving module.

Claims

1. An adaptive equalization circuit, characterized in that, It has: A digital filter that inputs the first polarized signal and the second polarized signal that have been polarization-separated and performs further polarization-separation processing; A filter tap coefficient update circuit that updates the tap coefficients of the digital filter according to changes in the polarization state; A polarization separation monitor that compares the correlation value between the first polarized signal and the second polarized signal output from the digital filter with a specified value; And A control circuit that, when the polarization separation monitor detects that the correlation value has exceeded the specified value, causes the filter tap coefficient update circuit to perform the initial convergence of the tap coefficients again. The polarization separation monitor has: A sign determination circuit that determines which quadrant of the four quadrants on the IQ plane the first polarized signal and the second polarized signal belong to; A correlation operation circuit that calculates the correlation value according to the determination result of the sign determination circuit; And A comparison circuit that compares the correlation value with the specified value. In the case of a transmission modulation method in which there is on-axis data located on the I axis or the Q axis of the IQ plane in the first polarized signal and the second polarized signal, the sign determination circuit extracts the on-axis data, and the positive-side data on the I axis, the negative-side data on the I axis, the positive-side data on the Q axis, and the negative-side data on the Q axis in the on-axis data are respectively and non-repetitively corresponded to one of the values representing the four quadrants on the IQ plane.

2. The adaptive equalization circuit according to claim 1, wherein: The sign determination circuit extracts the on-axis data based on the magnitudes of the powers of the data of the input first polarized signal and second polarized signal.

3. The adaptive equalization circuit according to claim 2, wherein: The magnitude of the power is estimated based on the sum of the absolute value of the coordinate value on the I axis and the absolute value of the coordinate value on the Q axis of the data.

4. The adaptive equalization circuit according to any one of claims 1 to 3, wherein: The values representing the four quadrants on the IQ plane are (+1, +1), (-1, +1), (-1, -1), (+1, -1).

5. The adaptive equalization circuit according to any one of claims 1 to 3, wherein: In the I-axis component and Q-axis component of each of the first polarized signal and the second polarized signal, when the absolute value of the I-axis component ≥ the absolute value of the Q-axis component, if the I-axis component ≥ 0, the on-axis data is corresponded to the value representing the fourth quadrant, and if the I-axis component < 0, the on-axis data is corresponded to the value representing the second quadrant. When the absolute value of the I-axis component < the absolute value of the Q-axis component, if the Q-axis component ≥ 0, the on-axis data is corresponded to the value representing the first quadrant, and if the Q-axis component < 0, the on-axis data is corresponded to the value representing the third quadrant.

6. The adaptive equalization circuit according to any one of claims 1 to 3, wherein: Among the I-axis component and Q-axis component of the first polarization signal and the second polarization signal respectively, when the absolute value of the I-axis component ≥ the absolute value of the Q-axis component, if the I-axis component ≥ 0, the on-axis data is associated with the value representing the first quadrant, and if the I-axis component < 0, the on-axis data is associated with the value representing the third quadrant. When the absolute value of the I-axis component < the absolute value of the Q-axis component, if the Q-axis component ≥ 0, the on-axis data is associated with the value representing the second quadrant, and if the Q-axis component < 0, the on-axis data is associated with the value representing the fourth quadrant.

7. A receiving device, characterized in that, It has: A receiving optical module that converts the received optical signal into an electrical signal; An AD converter that converts the electrical signal into a digital signal; A wavelength dispersion compensation circuit that compensates for the distortion caused by the wavelength dispersion of the digital signal; And The adaptive equalization circuit according to any one of claims 1 to 3, which performs equalization processing for compensating for the distortion caused by the polarization variation of the output signal of the wavelength dispersion compensation circuit.

8. An adaptive equalization method, characterized in that, It has the following steps: The digital filter inputs the first polarization signal and the second polarization signal that have been polarization-separated, and performs further polarization separation processing; The filter tap coefficient update circuit updates the tap coefficients of the digital filter according to the change of the polarization state; The polarization separation monitor determines which quadrant among the four quadrants on the IQ plane the first polarization signal and the second polarization signal belong to, calculates the correlation value between the first polarization signal and the second polarization signal, and compares the correlation value with a specified value; And When the polarization separation monitor detects that the correlation value exceeds the specified value, the control circuit causes the filter tap coefficient update circuit to perform the initial convergence of the tap coefficients again; In the case of a transmission modulation method with on-axis data located on the I-axis or Q-axis of the IQ plane in the first polarization signal and the second polarization signal, the polarization separation monitor extracts the on-axis data, and the positive-side data on the I-axis, the negative-side data on the I-axis, the positive-side data on the Q-axis, and the negative-side data on the Q-axis in the on-axis data are respectively and non-repetitively associated with one of the values representing the four quadrants on the IQ plane.

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