Adaptive equalization circuit, adaptive equalization device, receiver and adaptive equalization method

Abstract

A digital filter (6) performs polarization variation compensation for data of at least two polarizations in which a plurality of known signals are inserted at predetermined positions in each frame by means of a filter in which a tap coefficient is set. Before a frame synchronization unit (4a) generates a frame signal from an output signal of a digital filter, a tap coefficient update circuit (7) updates the tap coefficient such that the difference between the amplitude value of the data and the amplitude value that the data should take is small. After a frame signal is generated, a tap coefficient updating circuit (7) receives the frame signal in a feedback manner, detects a known signal from the data on the basis of the frame signal, and updates the tap coefficient such that the difference between the detected known signal and the true value of the known signal becomes small.

Description

Technical Field

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

[0002] In coherent optical communications, digital signal processing is used on the receiving side to compensate for signal distortion, enabling high-capacity transmission exceeding tens of Gbit / s. Digital signal processing primarily involves wavelength dispersion compensation, frequency control / phase adjustment, polarization multiplexing and separation, and polarization dispersion compensation.

[0003] Polarization multiplexing and separation, as well as polarization dispersion compensation, are primarily performed by an adaptive equalizer. When implementing an adaptive equalizer through digital signal processing, a digital filter is typically used. By setting tap coefficients calculated to offset distortion in the transmitted signal, this digital filter can compensate for distortion in the transmitted signal.

[0004] Within the optical receiving module, the X-polarized signal (hereinafter referred to as X-polarization) and the Y-polarized signal (hereinafter referred to as Y-polarization) synthesized on the transmitting side are separated. A portion of the Y-polarized signal remains in the separated X-polarization signal, and a portion of the X-polarized signal remains in the separated Y-polarization signal. The adaptive equalizer's digital filter more completely separates the X-polarized and Y-polarized data. However, the aforementioned polarization dispersion and other factors are affected by fluctuations in the polarization state. Therefore, the tap coefficients of the digital filter are sequentially updated according to fluctuations in the polarization state to compensate for these fluctuations.

[0005] To update the tap coefficients of these digital filters, a successive update algorithm, such as RLS (Recursive Least-Squares) or LMS (Least Mean Square), is generally used. These algorithms insert a known signal, such as a training signal or pilot signal, into the optical signal on the transmitting side. The tap coefficients are updated at each step to minimize the error between the transmitted known signal and its true value (the value inserted on the transmitting side) (hereinafter referred to as the reference signal comparison equalization method).

[0006] As the known signals used for comparison, a relatively long training signal (TS) pattern for frame synchronization (e.g., 128, 256, or 512 symbols inserted every tens of thousands of symbols) and a relatively short pilot signal (PS) for phase synchronization (e.g., 1 to 2 symbols inserted every tens of symbols) are used. For example, the TS pattern indicating the beginning of a frame is detected and used to determine the initial tap coefficients. Subsequently, the PS pattern periodically inserted from the position of this TS pattern is detected and used to determine successive tap coefficients.

[0007] However, the TS pattern is relatively long, so the amount of calculation for detection is large and has a slight impact on the amount of data. Therefore, new communication systems are being developed in the direction of omitting or shortening the TS pattern as much as possible.

[0008] To cope with such systems, a blind equalization method is used in adaptive equalizers as a sequential update algorithm. This method determines tap coefficients without using a known signal. Examples of blind equalization methods include the Constant Modulus Algorithm (CMA) and Radius Directed Equalization (RDE), which extends CMA into a loop of multiple amplitudes to apply it to QAM (Quadrature Amplitude Modulation) (see, for example, Patent Documents 1 and 2). In these methods, the tap coefficients are updated to minimize the error between the digital filter output and the value that should have been (in the case of the constant envelope, the "should have been" value can be easily estimated as the expected value of the amplitude). The tap coefficients are controlled and converged according to this algorithm. RDE adds the function of determining an arbitrary amplitude among multiple amplitudes to CMA.

[0009] Prior art literature

[0010] Patent Literature

[0011] Patent Document 1: Japanese Patent Application Laid-Open No. 2017-225078

[0012] Patent Document 2: Japanese Patent Application Laid-Open No. 2020-17819 Summary of the Invention

[0013] Problems to be solved by the invention

[0014] In recent years, the use of adaptive equalizers that minimize the TS pattern signal and operate using blind equalization methods such as the RDE mode has increased. However, compared to adaptive equalizers that update tap coefficients by comparing with known signals such as the TS pattern and PS pattern, existing blind equalization methods suffer from issues such as poor noise tolerance, polarization variation tolerance, and tolerance to DGD (Differential Group Delay) load. DGD load is the delay difference between the X-polarization signal and the Y-polarization signal.

[0015] The present disclosure has been made to solve the above-mentioned problems, and its object is to provide an adaptive equalization circuit, an adaptive equalization device, a receiver, and an adaptive equalization method that can suppress margin degradation even when using a signal without a TS pattern or with a maximally shortened TS pattern.

[0016] Means for solving problems

[0017] The adaptive equalization circuit disclosed in the present invention is characterized in that the adaptive equalization circuit comprises: a digital filter, which compensates for polarization variations of data of at least two polarizations of a plurality of known signals inserted at specified positions in each frame through a filter with set tap coefficients; and a tap coefficient updating circuit, which calculates the tap coefficients before a frame synchronization unit generates a frame signal based on an output signal of the digital filter, so as to reduce the difference between the amplitude value of the data and the amplitude value that the data should take, and after the frame signal is generated, feeds back the frame signal, detects the known signal from the data based on the frame signal, and updates the tap coefficients so as to reduce the difference between the detected known signal and the true value of the known signal.

[0018] Effects of the Invention

[0019] According to the present disclosure, even when a signal having no TS pattern or a signal having a TS pattern shortened as much as possible is used, it is possible to suppress degradation of the margin. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a structural diagram of a receiver showing an embodiment.

[0021] Figure 2 This is a diagram showing the structure of a data frame of a transmission signal used in an optical communication system.

[0022] Figure 3 1 is a diagram showing the configuration of an adaptive equalizing device according to an embodiment.

[0023] Figure 4 is a diagram showing the relationship between a received signal and a frame signal.

[0024] Figure 5 is a diagram showing a digital filter.

[0025] Figure 6 is a diagram showing the structure of a decoding circuit.

[0026] Figure 7 It is a diagram for explaining the XY polarization switching operation.

[0027] Figure 8 This is a diagram for explaining the operation of compensating the skew between X and Y polarizations.

[0028] Figure 9 This is a diagram for explaining IQ exchange.

[0029] Figure 10 This is a diagram for explaining IQ exchange.

[0030] Figure 11 This is a flowchart showing the operation of the adaptive equalizer according to the embodiment. DETAILED DESCRIPTION

[0031] Figure 1 Receiver 100 includes an optical receiving module 10, an A / D converter 20, a chromatic dispersion compensation circuit 30, and an adaptive equalizer 40. The adaptive equalizer 40 includes an adaptive equalizer circuit 1, a frequency offset compensation circuit 2, a carrier phase recovery circuit 3, and a decoding circuit 4.

[0032] Although not shown in the figure, the transmitter divides the transmission data into X-polarized transmission data and Y-polarized transmission data, and modulates the X-polarized optical signal and the Y-polarized optical signal, respectively. The modulated X-polarized optical signal and the Y-polarized optical signal are combined and supplied as a received signal to receiver 100 via optical fiber 200.

[0033] The optical receiving module 10 separates the received signal into an X-polarized optical signal and a Y-polarized optical signal, converting them into electrical signals. The A / D converter 20 converts the output signal of the optical receiving module 10 into X-polarized data and Y-polarized data, respectively. The chromatic dispersion compensation circuit 30 performs chromatic dispersion compensation on the X-polarized data and the Y-polarized data to compensate for distortion caused by chromatic dispersion.

[0034] Adaptive equalization circuit 1 further performs polarization separation on the output signal of wavelength dispersion compensation circuit 30 and applies polarization dispersion compensation processing to compensate for polarization fluctuations. Since polarization separation and polarization dispersion conditions gradually change in response to polarization fluctuations in optical fiber 200, the tap coefficients of the digital filter within adaptive equalization circuit 1 are updated to track polarization fluctuations.

[0035] Frequency offset compensation circuit 2 compensates for the frequency error of the carrier signal between transmitter and receiver 100 for both X polarization data and Y polarization data. Carrier phase recovery circuit 3 compensates for the phase error of the carrier signal between transmitter and receiver 100 for both X polarization data and Y polarization data.

[0036] Decoding circuit 4 performs frame synchronization on the X-polarization data and Y-polarization data from carrier phase reconstruction circuit 3, and compares the X-polarization data with the Y-polarization data to check the appropriateness of the polarization separation state and the received phase relationship in the IQ plane. After performing error correction, decoding circuit 4 ultimately outputs decoded data for X-polarization and decoded data for Y-polarization as "0" and "1."

[0037] Furthermore, X-polarization data and Y-polarization data are transmitted in parallel between the various circuits of receiver 100. However, in practice, each data is processed as I data, which is a real component, and Q data, which is an imaginary component, on the IQ plane. That is, X-polarization data is processed as a set (X_I, X_Q), and Y-polarization data is processed as a set (Y_I, Y_Q). Furthermore, this specification describes the case of two polarization data, X polarization and Y polarization, as a typical example of optical communication. However, this embodiment is not necessarily limited to the case of two polarizations and can also be applied to cases where three or more polarizations are possible.

[0038] Figure 2 This diagram shows the structure of a data frame for a transmission signal used in an optical communication system. This data frame is composed of the aforementioned X-polarization transmission data and Y-polarization transmission data. Each data frame includes a synchronization pattern portion and a data portion. In this optical communication system, the synchronization pattern portion consists of approximately several digital symbols to several dozen code elements.

[0039] In typical optical communication systems such as the OTN (Optical Transport Network) transmission packets, a known training sequence (TS) pattern consisting of several hundred symbols (e.g., 128, 256, or 512 symbols) is appended once or several times to an OTU (Optical-channel Transport Unit) frame. This TS pattern is detected by the circuitry preceding the adaptive equalization circuit 1 and used for OTU frame synchronization and updating the tap coefficients of the adaptive equalization circuit 1. The tap coefficient update algorithm of the adaptive equalization circuit 1 uses a successive update algorithm using a reference signal comparison equalization method, such as RLS (Recursive Least-Squares) or LMS (Least Mean Squares), which determines the tap coefficients by comparing them with a known signal. In RLS and LMS, the tap coefficients are updated to minimize the difference between the received known signal (TS signal) and the true value of the known signal.

[0040] On the other hand, this optical communication system does not use long patterns such as the aforementioned TS pattern. Instead, it adds only a short synchronization pattern of several dozen symbols (e.g., 16 symbols). The synchronization pattern is not used for tap coefficient updates. At least initially upon receiving a received signal, there is no long reference signal such as the TS pattern for comparison. Therefore, the tap coefficient update algorithm of the adaptive equalization circuit 1 uses a blind equalization method that calculates the tap coefficients to minimize the error between the amplitude of the received signal and its expected value. Examples of blind equalization methods include the Constant Modulus Algorithm (CMA) and RDE (Radius Directed Equalization), which is a ring-like method that extends CMA to multiple amplitudes to apply it to QAM (Quadrature Amplitude Modulation).

[0041] In addition, in the data section, as data, the X-polarization transmission data is divided into multiple subframes in the frame of the X-polarization transmission data, and the Y-polarization transmission data is divided into multiple subframes in the frame of the Y-polarization transmission data. Each subframe can be set to, for example, tens of code elements to hundreds of code elements. Furthermore, at the beginning of each subframe is a known signal in which a pilot signal (PS) of 1 to 1 digital element is inserted. Therefore, the PS is inserted into the data at a fixed code element interval (subframe interval). The PS is usually set to the same amplitude. That is, in each frame of the data, multiple known signals are inserted at specified positions. In addition, in the typical optical communication system described above, the PS is also regularly inserted into the data.

[0042] In a typical optical communication system, the initial values ​​of the tap coefficients of the adaptive equalizer circuit 1 are obtained by comparing and equalizing the reference signal in the TS mode, and the tap coefficients of the adaptive equalizer circuit 1 are successively updated by periodically inserted PS according to the reference signal comparison and equalization method.

[0043] The synchronization pattern section and the PS can be different or the same between the frames of data transmitted for X polarization and the frames of data transmitted for Y polarization. However, to distinguish between X polarization and Y polarization, the synchronization pattern section generally sets different patterns. Furthermore, to prevent the tap coefficients from converging on the same polarization side (for example, the taps for the Y polarization side mistakenly converge on the X polarization side), the PS also generally sets different patterns.

[0044] Figure 3 is a block diagram illustrating an adaptive equalization device according to an embodiment. The adaptive equalization device 40 includes a pseudo-frame signal generation circuit 5, an adaptive equalization circuit 1, a frequency offset compensation circuit 2, a carrier phase recovery circuit 3, and a decoding circuit 4. The X-polarization data and Y-polarization data from the chromatic dispersion compensation circuit 30 are supplied to the pseudo-frame signal generation circuit 5. The pseudo-frame signal generation circuit 5 generates a temporary frame signal (referred to as a pseudo-frame signal) upon receiving the received signal. The adaptive equalization circuit 1 includes a digital filter 6 and a tap coefficient update circuit 7.

[0045] Figure 4 This diagram shows the relationship between the received signal and the frame signal. The pseudo-frame signal generation circuit 5 arbitrarily generates a pseudo-frame signal for the received X-polarization data and Y-polarization data. The X-polarization data and the Y-polarization data are not necessarily received simultaneously. The time difference between the X-polarization data and the Y-polarization data is called the XY polarization skew.

[0046] The X- and Y-polarization data supplied from the chromatic dispersion compensation circuit 30 are not processed by the pseudo-frame signal generation circuit 5 and are supplied along with the pseudo-frame signal to the downstream adaptive equalization circuit 1. The X- and Y-polarization data, adaptively equalized in the adaptive equalization circuit 1, are supplied along with the pseudo-frame signal via the frequency offset compensation circuit 2 and the carrier phase recovery circuit 3 to the decoding circuit 4. Upon initial reception of the received signal, the tap coefficients of the adaptive equalization circuit 1 are updated using a blind equalization method.

[0047] The pseudo frame signal initially generated by the pseudo frame signal generation circuit 5 is used as a trigger to start the operation of the adaptive equalization circuit 1 and as a reference signal for frame synchronization and various signal processing in the subsequent decoding circuit 4. The decoding circuit 4 performs various processing based on the pseudo frame signal. The decoding circuit 4 will be described later; however, it detects the true frame signal from the X-polarization data and the Y-polarization data. Typically, the frame signal detected from the X-polarization data is used as the "true frame signal." The XY polarization skew is calculated based on the difference between the frame signals detected from the X-polarization data and the Y-polarization data. This XY polarization skew is fed back to the adaptive equalization circuit 1.

[0048] The true frame signal detected based on the X-polarization data has a time difference with the pseudo frame signal. This time difference can be expressed as a symbol quantity. This difference is fed back from the decoding circuit 4 to the pseudo frame signal generation circuit 5 as frame signal information. The pseudo frame signal generation circuit 5 corrects the pseudo frame signal to a true frame signal based on this frame signal information. Furthermore, even if no correction is actually performed, substantial correction can be achieved by transmitting correction information to each circuit.

[0049] The pseudo-frame signal, corrected to a true frame signal, is supplied to the adaptive equalizer circuit 1 along with the X- and Y-polarization data. First, the adaptive equalizer circuit 1 compensates for the XY polarization skew using the tap coefficients of the digital filter 6. Specifically, the tap coefficients of the digital filter 6 are corrected so that the XY polarization skew is zero. The decoder circuit 4 then resynchronizes and confirms that there is no positional misalignment between the corrected pseudo-frame signal sent from the adaptive equalizer circuit 1 and the true frame signal detected by the decoder circuit 4, and that the XY polarization skew detection value is also zero. This allows PS to be detected as a known signal from the true frame signal. Using this PS as a known signal, the process then transitions to the tap coefficient update mode, which uses a reference signal comparison equalization method.

[0050] Figure 5 is a diagram illustrating a digital filter. Digital filter 6 includes butterfly-shaped FIR (Finite Impulse Response) filters FIR_A, FIR_B, FIR_C, and FIR_D. FIR_A is a filter for X-polarization data. FIR_B is a filter for the effects of Y-polarization data on X-polarization data. FIR_C is a filter for the effects of X-polarization data on Y-polarization data. FIR_D is a filter for Y-polarization data. Each FIR filter has N taps. However, the number of taps in each FIR filter may differ.

[0051] Digital filter 6 uses the sum of the filtering results of FIR_A for the X polarization data and FIR_B for the Y polarization data as the compensation output for the X polarization data, and uses the sum of the filtering results of FIR_C for the X polarization data and FIR_D for the Y polarization data as the compensation output for the Y polarization data. This makes polarization separation more reliable.

[0052] Furthermore, the relationship between input and output data and the tap coefficients of each filter is shown below.

[0053] Xout=Whh·Xin+Wvh·Yin

[0054] Yout=Whv·Xin+Wvv·Yin

[0055] Here, Xin is the input data of the X polarization data. Yin is the input data of the Y polarization data. Xout is the output data of the X polarization data. Yout is the output data of the Y polarization data. Whh is the series of tap coefficients of the filter FIR_A. Wvh is the series of tap coefficients of the filter FIR_B. Whv is the series of tap coefficients of the filter FIR_C. Wvv is the series of tap coefficients of the filter FIR_D.

[0056] The tap coefficients Whh, Wvh, Whv, and Wvv are obtained by a successive update algorithm of the tap coefficient update circuit 7. The successive update algorithm is generally expressed by the following equation.

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

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

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

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

[0061] Here, n is a value indicating the update order in the successive update algorithm. The tap coefficient Whh(n) indicates the tap coefficient group of FIR_A in the case of update order n. The tap coefficient Wvh(n) indicates the tap coefficient group of FIR_B in the case of update order n. The tap coefficient Whv(n) indicates the tap coefficient group of FIR_C in the case of update order n. The tap coefficient Wvv(n) indicates the tap coefficient group of FIR_D in the case of update order n. μ indicates the step size of the update algorithm. eX(n) indicates the error between the filter output of the X-polarization data and the expected value. eY(n) indicates the error between the filter output of the Y-polarization data and the expected value. In the reference signal comparison equalization method, the expected value is the reference signal, and in the blind equalization method, the expected value is the amplitude value that should be taken.

[0062] Xout(n) represents the filter output for the X polarization data at update order n. Xin(n) represents the filter input for the X polarization data at update order n. Yout(n) represents the filter output for the Y polarization data at update order n. Yin(n) represents the filter input for the Y polarization data at update order n. * represents conjugate or complex conjugate. Data and tap coefficients are represented using complex numbers.

[0063] In the successive update algorithm, the tap coefficients are repeatedly updated and converged to minimize the aforementioned error. In the reference signal comparison equalization method, not only the amplitude value but also the phase is compared as a reference signal. Therefore, compared to the blind equalization method that only compares the amplitude value, high-precision equalization compensation can be performed. Furthermore, when using a successive update algorithm such as RDE that does not use phase information to perform blind equalization for modulation schemes that can take multiple amplitudes, it is also possible to switch to a CMA method that does not require amplitude determination. As a result, it is possible to prevent erroneous determinations and perform high-precision adaptive equalization compensation. The above equation is an example of an equation representing a successive update algorithm, and the equation representing a successive update algorithm is not limited to the above.

[0064] The above-described successive update algorithm updates the tap coefficients sequentially in update order n to minimize the error between the filter output and the desired value, ultimately leading to convergence of the tap coefficients. The convergence condition is determined by factors such as the number of update orders n and the error between the filter output and the desired value. Furthermore, the tap coefficient update circuit 7 determines the tap coefficients based on the XY polarization swap information and XY polarization skew information detected by the decoding circuit 4 to compensate for the signal degradation caused by these factors.

[0065] return Figure 3 , the operation of each circuit is explained. Figure 5The butterfly-type FIR filters FIR_A, FIR_B, FIR_C, and FIR_D shown perform polarization separation and polarization dispersion compensation on the X- and Y-polarization data supplied to adaptive equalization circuit 1. The pseudo-frame signal supplied from pseudo-frame signal generation circuit 5 is not subjected to adaptive equalization; its timing is merely adjusted before it is output to subsequent circuits. The tap coefficients for each FIR filter are determined by a successive update algorithm in tap coefficient update circuit 7.

[0066] In this optical communication system, Figure 2 As shown, the TS pattern is not set at the beginning of the data frame, making frame signal detection based on the TS pattern impossible. Therefore, the beginning position of the data frame is unknown, and the PS inserted into the data portion cannot be detected. In this situation, the reference signal comparison equalization method cannot be used as a successive update algorithm. Therefore, upon initial reception of the received signal, the tap coefficient update circuit 7 uses a blind equalization method such as CMA or RDE as a successive update algorithm to update the tap coefficients of the digital filter 6. Compared to the reference signal comparison equalization method, the tap coefficient update algorithm based on the blind equalization method has slightly lower noise tolerance, polarization separation tolerance, and DGD tolerance, but it can compensate to a level that allows signal point detection on the IQ plane. The frequency offset compensation circuit 2 further compensates for the frequency offset of the X- and Y-polarization data after adaptive equalization by the adaptive equalization circuit 1. The carrier phase reconstruction circuit 3 synchronizes the phase of the data's IQ axis with that of the carrier signal. Furthermore, the adaptive equalization circuit 1 can use a pseudo-frame signal to determine the initial reception of the received signal.

[0067] Next, the decoding circuit 4 performs frame synchronization and state detection. Frame synchronization enables the detection of a true frame signal from the X- and Y-polarization data. After generating the true frame signal, the pseudo-frame signal generation circuit 5 receives feedback of the true frame signal (actually, the time difference from the pseudo-frame signal) and corrects the pseudo-frame signal to a true frame signal. The tap coefficient update circuit 7 uses the corrected pseudo-frame signal, the true frame signal, to detect the beginning of the frame and, based on this beginning position, detects the PS periodically inserted into the data portion. State detection detects IQ exchange information.

[0068] After detecting the PS, the tap coefficient update circuit 7 executes a tap coefficient update algorithm using a reference signal comparison method that uses the PS as a reference signal. The PS is periodically inserted into the data portion, so the tap coefficients are also updated sequentially due to it. At this point, the tap coefficient update algorithm switches from a blind equalization method to a reference signal comparison method. By switching to a reference signal comparison method using the PS, noise tolerance, polarization separation tolerance, and DGD tolerance are improved. Thus, in optical communication systems using signals that lack a TS mode or have a TS mode that is minimized, adaptive equalization processes such as polarization separation processing and polarization dispersion compensation processing can be performed without degrading noise tolerance, polarization separation tolerance, and DGD tolerance.

[0069] In addition, the above-described embodiment illustrates a case where a blind equalization method based on CMA (for QPSK) or RDE (for QAM) is used as the tap coefficient update algorithm before PS detection. After PS detection, the method switches to a reference signal comparison method that uses the PS signal as the reference signal. However, after PS detection, the algorithm is not limited to the reference signal comparison method; blind equalization based on CMA using PS or a hybrid of blind equalization based on CMA using PS and blind equalization based on RDE using data can also be used. In these cases, noise tolerance, polarization separation tolerance, and DGD tolerance can be improved compared to blind equalization based solely on RDE using data.

[0070] Furthermore, in the above-described embodiment, information about the true frame signal detected by the decoding circuit 4 is fed back to the pseudo frame signal generating circuit 5, and the pseudo frame signal is corrected into a true frame signal for use in the tap coefficient updating circuit 7. However, information about the true frame signal can also be fed back directly to the tap coefficient updating circuit 7 to correct and use the pseudo frame signal.

[0071] The X- and Y-polarization data, after undergoing polarization separation and polarization dispersion compensation in the adaptive equalization circuit 1, are then supplied to the frequency offset compensation circuit 2. The frequency offset compensation circuit 2 compensates for the frequency error (frequency offset) between the carrier wave of the transmitter and the carrier wave of the receiver 100, respectively, for the X- and Y-polarization data. This frequency offset can be easily compensated by applying a phase rotation on the IQ plane opposite to the phase rotation corresponding to the frequency error. This frequency offset compensation can be performed by complex multiplying the phase rotation by the data coordinate value or using the carrier phase reconstruction circuit 3.

[0072] Next, the carrier phase reconstruction circuit 3 synchronizes the phase of the received data with the phase of the carrier for both the X-polarization data and the Y-polarization data. Specifically, the I-axis and Q-axis of the received data's IQ plane are synchronized with the I-axis and Q-axis of the carrier. This allows the coordinate values ​​(signal points) of the received data to be detected in the carrier's IQ plane. This synchronization also creates four uncertainties, each 90 degrees, such as when coordinates in quadrants 2 through 4 are detected instead of the original first quadrant.

[0073] Patent Document 2 discloses a general carrier phase reconstruction method. This document discloses a method for phase synchronization based on the phase difference between a known pattern and its true value (known pattern comparison method), and a method for phase synchronization by multiplying the received signal by M times the number of modulation phases (M-th power method). The known pattern comparison method eliminates the aforementioned uncertainty, enabling accurate phase synchronization. While the M-th power method retains uncertainty, it allows phase synchronization even when no known pattern is detected.

[0074] In this system, a true frame signal is not generated when a received signal is first received. Therefore, the PS, a known pattern, cannot be detected, and carrier phase reconstruction based on the PS and known pattern comparison cannot be performed. Therefore, the aforementioned M-th power phase synchronization is initially performed. In this case, even with the aforementioned quadrant uncertainty, phase synchronization within a quadrant is possible because the four quadrants are superimposed into a single quadrant.

[0075] After the decoder circuit 4 detects a true frame signal, it can use the PS detected from the frame signal to perform known pattern comparison-type phase synchronization. However, depending on the implementation structure of the modulator on the transmitting side and the receiver 100 on the receiving side, the known pattern observable on the receiving side may cause a sign inversion or IQ coordinate inversion with respect to the known pattern inserted on the transmitting side. This presents a problem with known pattern comparison-type phase synchronization using PS.

[0076] Therefore, the state detection unit of the decoding circuit 4, described later, obtains IQ swap information. The IQ swap information indicates whether the sign, I coordinate, and Q coordinate have been swapped. For example, if PS data is transmitted with counterclockwise transitions such as (I coordinate, Q coordinate) = (+1, +1) (1st quadrant), (I coordinate, Q coordinate) = (-1, +1) (2nd quadrant), (I coordinate, Q coordinate) = (-1, -1) (3rd quadrant), and (I coordinate, Q coordinate) = (+1, -1) (4th quadrant), and if IQ swapping of the carrier occurs (when the sign of the I axis is reversed), the coordinate values ​​upon reception will be (I coordinate, Q coordinate) = (+1, +1) (1st quadrant), (I coordinate, Q coordinate) = (+1, -1) (4th quadrant), (I coordinate, Q coordinate) = (-1, -1) (3rd quadrant), and (I coordinate, Q coordinate) = (-1, +1) (2nd quadrant), indicating clockwise transitions. In the decoding circuit 4, after detecting the true frame position, the state detection unit obtains the above-mentioned IQ exchange information.

[0077] The coordinate values ​​of the reference signal on the receiving side are corrected based on this IQ exchange information, and known pattern comparison-type phase synchronization is performed. Specifically, the carrier phase reconstruction circuit 3 performs M-th-order phase synchronization upon initial reception of the received signal. After detecting a true frame signal, the carrier phase reconstruction circuit 3 obtains IQ exchange information based on state detection, converts the PS reference coordinates based on this IQ exchange information, and performs known pattern comparison-type phase synchronization.

[0078] As described above, upon initial reception of a received signal, the adaptive equalization circuit 1 operates in a blind equalization mode, while the carrier phase recovery circuit 3 operates in an M-th power phase synchronization mode. This allows the decoding circuit 4 to generate a true frame signal. Consequently, the PS position can be determined without requiring a function to detect frame synchronization in the received signal. The adaptive equalization circuit 1 can use the detected PS as a reference signal to transition to a high-precision coefficient update mode, while the carrier phase recovery circuit 3 can use the detected PS as a known signal to transition to a high-precision known pattern comparison phase synchronization mode.

[0079] The decoding circuit 4 performs frame synchronization, state detection, and data decoding on the X-polarization data and the Y-polarization data. Figure 6 2 is a diagram showing the structure of a decoding circuit. The decoding circuit 4 includes a frame synchronization unit 4a, a state detection unit 4b, and a data decoding unit 4c.

[0080] The frame synchronization unit 4a performs frame synchronization on the X polarization data and the Y polarization data based on the pseudo frame signal generated by the pseudo frame signal generation circuit 5 when the reception signal is first received. That is, a frame signal representing a frame of the X polarization data and a frame signal representing a frame of the Y polarization data are generated. In this way, a true frame signal is detected. Figure 4As shown, the true frame signal is, in principle, generated based on the X-polarization data. However, if there is a skew (delay difference) between the X-polarization data and the Y-polarization data, the state detection unit 4b detects this inter-polarization skew from the output signal of the frame synchronization unit 4a as XY inter-polarization skew information. This delay difference is fed back to the adaptive equalization circuit 1 and compensated for. After compensation, the frame signal generated based on the Y-polarization data also approaches the true frame signal.

[0081] The time difference between the true frame signal and the pseudo-frame signal is fed back as frame signal information to the pseudo-frame signal generation circuit 5. Based on the frame signal information, the pseudo-frame signal is corrected to generate the true frame signal. Essentially, the pseudo-frame signal and frame signal information are transmitted, and each circuit generates the timing of the true frame signal based on these two. Furthermore, the beginning of the data portion can be detected from the true frame signal, thereby enabling the detection of the PS (Pick 1) inserted into the data at fixed symbol intervals on the transmitting side. The PS is a known signal.

[0082] Therefore, after the frame signal information is fed back to the pseudo-frame signal generating circuit 5, as described above, in the adaptive equalization circuit 1, the tap coefficient updating algorithm is switched from the blind equalization method to the tap coefficient updating algorithm of the reference signal comparison method using PS as the reference signal, and in the carrier phase reproduction circuit 3, the phase synchronization is switched from the M-th power type to the known pattern comparison type.

[0083] The state detection unit 4b detects XY polarization swap information and XY inter-polarization skew information from the output signal of the frame synchronization unit 4a, and detects IQ swap information based on the data for each polarization. The XY polarization swap information and XY inter-polarization skew information are fed back to the tap coefficient update circuit 7 of the adaptive equalization circuit 1. The IQ swap information is fed back to the carrier phase recovery circuit 3. These pieces of information can be detected and fed back independently. It is not necessary to feed back all of them. Therefore, the state detection unit 4b can be separated according to the information to be detected. For example, the state detection unit 4b can be separated into a first part that detects the XY polarization swap information and XY inter-polarization skew information, and a second part that detects the IQ swap information.

[0084] XY polarization swap information can be determined when frame synchronization is established in the frame synchronization unit 4a. When a synchronization pattern originally inserted into the X polarization data is detected in the Y polarization data channel, or when a synchronization pattern originally inserted into the Y polarization data is detected in the X polarization data channel, it can be determined as XY polarization swap information that the XY polarization has been swapped. The adaptive equalization circuit 1 receives this XY polarization swap information as feedback and adjusts the filter tap coefficients based on this XY polarization swap information, thereby compensating for the polarization swap in the XY polarization data. Alternatively, the adaptive equalization circuit 1 can implement the XY polarization swap by swapping the adaptive equalization input data or output data on the X and Y polarization sides without swapping the tap coefficients. Alternatively, the swap can be performed via the output of the data decoding unit 4c. In the latter case, the reference signal used for the XY comparison in the carrier phase recovery circuit 3 must be swapped for phase comparison.

[0085] Figure 7 : This is a diagram for explaining the XY polarization conversion operation. The relationship between the input and output of the digital filter 6 of the adaptive equalization circuit 1 in normal operation for setting the tap coefficient is expressed by the following equation.

[0086] Xout=Whh·Xin+Wvh·Yin

[0087] Yout=Whv·Xin+Wvv·Yin

[0088] If the XY polarization swap information from the decoding circuit 4 indicates that the XY polarization data has been swapped, the tap coefficients can be swapped to easily swap the signal normally output as Xout and the signal output as Yout. The tap coefficients are swapped using the order Whh → Whv, Wvh → Wvv, Whv → Whh, and Wvv → Wvh. The input-output relationship during the swap is expressed by the following equation.

[0089] Xout=Whv · Xin+Wvv · Yin

[0090] Yout=Whh·Xin+Wvh·Yin

[0091] Here, Xout during the swap is the same as Yout during normal operation, and Yout during the swap is the same as Xout during normal operation. That is, by swapping the tap coefficients, the XY polarization data can be swapped.

[0092] Regarding XY polarization conversion, the input or output data itself can also be converted as described above. However, for ease of implementation, converting the tap coefficients significantly simplifies circuit processing. This allows the adaptive equalization circuit 1 to operate more appropriately for both X- and Y-polarization data.

[0093] XY polarization skew information can be measured when frame synchronization is established in the frame synchronization unit. The XY polarization skew can be determined by comparing the frame signal detected from the X polarization data with the frame signal detected from the Y polarization data. The adaptive equalization circuit 1 receives feedback of the XY polarization skew information and adjusts the filter tap coefficients based on the XY polarization skew information to compensate for the polarization skew.

[0094] Figure 8 This is a diagram for explaining the compensation operation of the XY polarization skew. When the adaptive equalization circuit 1 compensates for the XY polarization skew, Figure 5 The tap coefficients of the FIR filter in the Figure 8 The tap coefficients are shown. Figure 8 In FIG, only FIR_A is shown as an example. The digital filter 6 can advance or delay the output by shifting the order of the tap coefficients. The following modification of the tap coefficients means shifting the order of the tap coefficients.

[0095] Whh i (n)→Whh (i+Xskew*OverSampleRate) (n)

[0096] Wvh i (n)→Wvh (i+Xskew*OverSampleRate) (n)

[0097] Whv i (n)→Whv (i+Yskew*OverSampleRate) (n)

[0098] Wvv i (n)→Wvv (i+Yskew*OverSampleRate) (n)

[0099] The subscript i is the tap number, and in the case of an N-stage filter, i=0 to N-1. n represents the update order of the tap coefficients. Xskew is a parameter representing the shift amount of the X polarization data. Yskew is a parameter representing the shift amount of the Y polarization data. OverSampleRate is the oversampling rate. Xskew*OverSampleRate represents the shift amount in the oversampling of the X polarization data. Yskew*OverSampleRate represents the shift amount in the oversampling of the Y polarization data. For example, in the case where each symbol is 2 samples, a 1 symbol shift means shifting the tap by 2. Figure 8In the example of FIR_A, a case of shifting 2 samples is shown. In addition, the number of tap coefficients is limited, so when the tap coefficients are offset, taps with no value are generated, but in this case, zero padding is performed. Figure 8 In the example, it becomes Whh N (n)=0、Whh N+1 (n) = 0. Based on the XY polarization skew information, the tap coefficient numbers are shifted in each filter to compensate for the XY polarization skew.

[0100] By changing the tap coefficients as described above, the skew between the X polarization data and the Y polarization data is compensated during the tap coefficient update operation of the adaptive equalization circuit 1 . Therefore, the adaptive equalization circuit 1 can operate more appropriately for the X polarization data and the Y polarization data.

[0101] IQ swap information indicates uncertainty in carrier phase synchronization in carrier phase recovery circuit 3. If phase synchronization is not performed correctly, signs on the I and Q axes may be reversed, and the I and Q coordinate values ​​may be swapped. Generally, the presence of IQ swapping can be determined by comparing the values ​​with expected values.

[0102] Figure 9 and Figure 10 This diagram illustrates IQ switching. In this diagram, the I and Q axes represent carrier axes. As transmitted PS data, (I coordinate, Q coordinate) = (+1, +1) (1st quadrant), (I coordinate, Q coordinate) = (-1, +1) (2nd quadrant), (I coordinate, Q coordinate) = (-1, -1) (3rd quadrant), and (I coordinate, Q coordinate) = (+1, -1) (4th quadrant) are transmitted, shifting counterclockwise. Figure 9 The transition of the quadrants of the PS when no IQ exchange is induced is shown, and the direction is counterclockwise.

[0103] Figure 10 The figure shows the clockwise transition of the PS quadrants when IQ swapping occurs. Specifically, when IQ swapping of the carrier occurs (when the sign of the I axis is reversed), the coordinate values ​​during reception are (I coordinate, Q coordinate) = (+1, +1) (1st quadrant), (I coordinate, Q coordinate) = (+1, -1) (4th quadrant), (I coordinate, Q coordinate) = (-1, -1) (3rd quadrant), and (I coordinate, Q coordinate) = (-1, +1) (2nd quadrant), with a clockwise transition. Figure 10 For example, the case where the sign of the I axis is reversed during reception is shown. In the decoding circuit 4, after detecting the true frame position, the state detection unit 4b obtains the above-mentioned IQ exchange information.

[0104] This IQ swap information is fed back to the carrier phase reconstruction circuit 3 and used to ensure proper synchronization using the known pattern during the carrier phase reconstruction operation, using a known pattern comparison-type phase synchronization method. Specifically, the carrier phase reconstruction circuit 3 receives this IQ swap information as feedback and adjusts the coordinate data of the referenced known pattern based on the IQ swap information, thereby compensating for IQ swapping. Specifically, based on the IQ swap information, the expected value of the known pattern on the receiving side is inverted in sign on the I-axis or Q-axis, or the I-coordinate value and the Q-coordinate value are swapped.

[0105] Finally, the data decoding unit 4 c decodes the X polarization data and the Y polarization data, and outputs X polarization decoded data and Y polarization decoded data, respectively.

[0106] In addition, Figure 3 In the configuration example, the dummy frame signal generation circuit 5 is placed before the adaptive equalization circuit 1. However, if the adaptive equalization circuit 1 can know the position of the beginning of the data portion and the position of the PS by some method, the dummy frame signal generation circuit 5 can also be placed after the adaptive equalization circuit 1.

[0107] Figure 11 4 is a flowchart showing the operation of the adaptive equalizer according to the embodiment. The time-series operation of the adaptive equalizer 40 described above will be described.

[0108] Step S1: Upon receiving the received signal, the frame synchronization unit 4a has not yet generated a true frame signal based on the output signal of the digital filter 6. Therefore, the tap coefficient update circuit 7 of the adaptive equalization circuit 1 operates using a blind equalization method such as CMA or RDE mode to determine tap coefficients to minimize the difference between the data amplitude and the amplitude value the data should have. The adaptive equalization circuit 1 does not have a frame synchronization function for the input signal. The carrier phase recovery circuit 3 operates in an M-th power mode, superimposing the modulated data signal in one quadrant and compensating for the phase difference with the carrier signal in that quadrant. In other words, the carrier phase recovery circuit 3 performs phase synchronization so that the phase difference between the signal superimposed in one quadrant on the IQ plane by multiplying the output signal of the digital filter 6 by M and the phase difference between the superimposed signal and the phase that the superimposed signal should have is minimized. The dummy frame signal generation circuit 5 generates a dummy frame signal and supplies it sequentially to the adaptive equalization circuit 1, the frequency offset compensation circuit 2, the carrier phase recovery circuit 3, and the decoding circuit 4.

[0109] Step S2: Decoding circuit 4 performs frame synchronization through differential decoding. This process detects a true frame signal. The synchronization pattern is approximately 16 symbols. The difference between the detected true frame signal and the dummy frame signal generated by dummy frame signal generator circuit 5 is communicated to dummy frame generator circuit 5, which then corrects the dummy frame signal.

[0110] Step S3 : the decoding circuit 4 detects the XY polarization swap information and the XY polarization skew information, and feeds them back to the adaptive equalization circuit 1 , detects the IQ swap information, and feeds it back to the carrier phase recovery circuit 3 .

[0111] Step S4: Adaptive equalization circuit 1 detects PS from the received data based on the true frame signal and uses this to update the tap coefficients. The tap coefficient update algorithm shifts from RDE mode (blind equalization) to PS comparison mode (reference signal comparison equalization). Furthermore, the X- and Y-polarization data are optimized using XY polarization swap information and XY polarization skew information from decoding circuit 4, allowing for the calculation of more appropriate tap coefficients. Specifically, after generating a frame signal, tap coefficient update circuit 7 receives feedback from the frame signal, detects a known signal from the data based on the frame signal, and updates the tap coefficients to minimize the difference between the detected known signal and the true value of the known signal.

[0112] Step S5: Carrier phase recovery circuit 3 detects PS from the received data based on the true frame signal and uses this to perform phase compensation based on PS comparison. This shifts the phase synchronization method from the M-th power mode to the PS comparison mode (known pattern comparison). Specifically, carrier phase recovery circuit 3 receives the frame signal as feedback, detects the known signal from the data based on the frame signal, and performs phase synchronization to minimize the difference between the phase of the detected known signal and the true phase of the known signal.

[0113] As described above, the adaptive equalization circuit 1 can determine the position of the PS by obtaining the frame signal in the subsequent decoding circuit 4, without providing a frame synchronization function for the input signal. This allows the circuit to switch to a high-precision coefficient update method that uses the PS as a reference signal. Furthermore, the carrier phase recovery circuit 3 can also switch to a high-precision known pattern comparison-type phase synchronization method that uses the detected PS as a known signal.

[0114] Label Description

[0115] 1: Adaptive equalization circuit; 3: Carrier phase reproduction circuit; 4: Decoding circuit; 4a: Frame synchronization unit; 4b: Status detection unit; 5: Pseudo-frame signal generation circuit; 6: Digital filter; 7: Tap coefficient update circuit; 10: Receiving optical module; 30: Wavelength dispersion compensation circuit; 40: Adaptive equalization device; 100: Receiver.

Claims

1. An adaptive equalization circuit, characterized in that: The adaptive equalization circuit has: a digital filter for compensating for polarization variation by inserting data of at least two polarizations of a plurality of known signals at predetermined positions in each frame using a filter having set tap coefficients; and A tap coefficient updating circuit calculates the tap coefficients before a frame synchronization unit generates a frame signal based on an output signal of the digital filter, so as to reduce the difference between the amplitude value of the data and the amplitude value that the data should take. After the frame signal is generated, the frame signal is fed back and received, and the known signal is detected from the data based on the frame signal. The tap coefficients are updated so as to reduce the difference between the detected known signal and the true value of the known signal.

2. An adaptive equalization device, characterized in that: The adaptive equalization device has: The adaptive equalization circuit according to claim 1; the frame synchronization unit; and a pseudo frame signal generating circuit, which generates a pseudo frame signal, After the frame signal is generated, the pseudo-frame signal generating circuit receives feedback of the frame signal and corrects the pseudo-frame signal into the frame signal. The tap coefficient updating circuit detects the starting position of the frame based on the corrected pseudo-frame signal and detects the known signal based on the starting position.

3. An adaptive equalization device, characterized in that: The adaptive equalization device has: The adaptive equalization circuit according to claim 1; the frame synchronization unit; and a state detection unit that detects whether polarization has been switched based on the output signal of the frame synchronization unit as polarization switching information, The adaptive equalization circuit receives feedback of the polarization swap information and adjusts the tap coefficient according to the polarization swap information, thereby compensating for the polarization swap.

4. An adaptive equalization device, characterized in that: The adaptive equalization device has: The adaptive equalization circuit according to claim 1; the frame synchronization unit; and a state detection unit that detects the inter-polarization skew as inter-polarization skew information based on the output signal of the frame synchronization unit, The adaptive equalization circuit receives feedback of the inter-polarization skew information and adjusts the tap coefficient according to the inter-polarization skew information, thereby compensating for the inter-polarization skew.

5. An adaptive equalization device, characterized in that: The adaptive equalization device has: The adaptive equalization circuit according to claim 1; the frame synchronization unit; and A carrier phase recovery circuit is located between the adaptive equalization circuit and the frame synchronization unit, and performs phase synchronization between the IQ axis of the data and the IQ axis of the carrier signal. The carrier phase reproduction circuit performs phase synchronization before generating the frame signal so that the difference between the phase of the signal after the phase overlaps in one quadrant on the IQ plane by multiplying the output signal of the digital filter to the Mth power and the phase that the overlapped signal should take becomes smaller. After the frame signal is generated, the frame signal is fed back and received, and the known signal is detected from the data based on the frame signal, and phase synchronization is performed so that the difference between the phase of the detected known signal and the phase of the true value of the known signal becomes smaller.

6. The adaptive equalization device according to claim 5, characterized in that: The adaptive equalization device further includes a state detection unit, which detects IQ exchange information according to the data of each polarization based on the output signal of the frame synchronization unit. The carrier phase reconstruction circuit receives the IQ exchange information as feedback, and adjusts the coordinate data of the known pattern to be referenced according to the IQ exchange information, thereby compensating for the IQ exchange.

7. A receiver, characterized in that: The receiver has: a receiving optical module, which converts the received optical signal into the data; a wavelength dispersion compensation circuit that compensates for distortion caused by wavelength dispersion of the data; and The adaptive equalizer according to any one of claims 2 to 6, which compensates for polarization variation of an output signal of the chromatic dispersion compensation circuit.

8. An adaptive equalization method, characterized in that: The adaptive equalization method has the following steps: A digital filter having set tap coefficients performs polarization variation compensation on data of at least two polarizations of a plurality of known signals inserted at predetermined positions in each frame; Before the frame synchronization unit generates a frame signal based on the output signal of the digital filter, the tap coefficient updating circuit calculates the tap coefficient so as to reduce the difference between the amplitude value of the data and the amplitude value that the data should take; as well as After the frame signal is generated, the tap coefficient update circuit receives feedback of the frame signal, detects the known signal from the data based on the frame signal, and updates the tap coefficient to reduce the difference between the detected known signal and the true value of the known signal.

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