Digital coherent optical receiver, adaptive equalizer, and digital coherent optical communication method

A digital coherent optical receiver includes a processor that is operative to separate electric signals obtained by converting an optical signal into a horizontal signal component and a vertical signal component; to generate a histogram of the horizontal signal component and the vertical signal component as outputs of the equalizing filter; and to determine a presence/absence of local convergence based on distribution of the histogram of the horizontal signal component and the histogram of the vertical signal component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2010-274933, filed on Dec. 9, 2010, the entire contents of which are incorporated herein by reference.

FIELD

The present invention relates to a digital coherent optical receiver, an adaptive equalizer, and a digital coherent optical communication method.

BACKGROUND

In the ultrahigh-speed communication at 40 Gbps and 100 Gbps, there is the problems of an insufficient optical signal-to-noise ratio (OSNR) and a linearity distortion of wavelength dispersion etc. The digital coherent reception system using an analog-to-digital converter has attracted considerable attention as a solution for the problems.

Used in the transmission at 100 Gbps or higher is polarization orthogonal modulation for modulating data independent of the polarization having vertical polarization orthogonal to horizontal polarization. Since the polarization state in optical fiber may cause a high-speed polarization fluctuation at 10 KHz or higher, it is necessary for a reception unit to perform high-speed polarization control for separating a vertical polarization signal and a horizontal polarization signal while following the high-speed polarization fluctuation. The adaptive equalizer is used for high-speed polarization control.

FIG. 1illustrates an example of an adaptive equalizer11. The adaptive equalizer11has four FIR (finite impulse response) filters12-1through12-4and a filter coefficient application control circuit13.

InFIG. 1, a horizontal signal component Eh=(Ih, Qh) and a vertical signal component Ev=(Iv, Qv) on the input side of the adaptive equalizer11are signals including polarization fluctuation components. A horizontal signal component Eh′=(Ih′, Qh′) and a vertical signal component Ev′=(Iv′, Qv′) on the output side are polarization-separated signals.

Each of the FIR filters12-1and12-2receives the horizontal signal component Eh=(Ih, Qh) and the vertical signal component Ev=(Iv, Qv), and a signal obtained by combining the components is output as a vertical signal component E′v=(I′v, Q′v).

The filter coefficient application control circuit13controls the tap coefficient and the combination ratio of the four FIR filters12-1through12-4.

The above-mentioned adaptive equalizer11can adaptively update the tap coefficient and the combination ratio of the FIR filters in real time in an update time sufficiently higher than the polarization fluctuation of optical fiber, thereby realizing a stable reception state against the fluctuation of the polarization state and the PMD. The PMD refers to polarization mode dispersion.

Proposed as adaptive equalization methods for the adaptive equalizer are a method using a training symbol, decision directed-least mean squares (DD-LMS), a constant modulus algorithm (CMA method) method, etc. The CMA method is a type of blind equalization not requiring a training symbol, and a method of controlling the tap coefficient of a filter so that the peak power of a signal after the adaptive equalization can be constant. The CMA method has a merit of having a simpler circuit than the method of using a training symbol and the DD-LMS, and being capable of converging independent of a tap coefficient initial value.

FIG. 2is a configuration of an adaptive equalizer using the CMA method. An adaptive equalizer21has four FIR filters22-1through22-4and a filter coefficient application control circuit23. The tap coefficient of the FIR filter22-1is expressed as hhh, the tap coefficient of the FIR filter22-2is expressed as hvh, the tap coefficient of the FIR filter22-3is expressed as hhv, and the tap coefficient of the FIR filter22-4is expressed as hvv.

The update equation of the tap coefficient of the filter in the CMA method is expressed as follows.
H(n+1)=H(n)−μ·rn*(|yn|2−γ)yn rn*=(Eh*,Ev*)=((Ih,−Qh),(Iv,−Qv)): FIR input signalyn=(Eh′,Ev′): FIR output signal
H (n): tap coefficient, γ: target amplitude constant

The filter coefficient application control circuit23controls the tap coefficient of the filter so that the peak power of the signal can be constant by the equation above.

When the digital signal processing exceeding 100 Gbps is realized using the CMOS circuit, a number (for example, more than 500 lanes) of circuits are provided in parallel, some of which are selected and fed back. In this case, depending on the initial value of the tap coefficient of a FIR filter, the FIR filter may converge to a local minimum solution, and a waveform may not be correctly formed.

FIG. 3illustrates the constellations on the horizontal polarization side and the vertical polarization side. As illustrated inFIG. 3, the constellation on the horizontal (H) polarization side converges around the center, and the horizontally polarization side converges to a local minimum solution.

To detect the convergence to a local minimum solution using a forward error correction (FEC) counter etc., it is necessary to complete the extraction of a frame by terminating the processes of a local emission frequency offset estimation unit, a carrier wave phase estimation unit, a determination unit, a frame synchronization unit, etc. Therefore, there has been the problem that it takes a long time to detect the convergence to a local minimum solution (hereafter referred to as a local convergence).

In addition, there has been the problem with the CMA method that the uniquness of a convergent solution is not guaranteed. That is, with the adaptive equalizer using the CMA method, there are cases in which a horizontal polarization signal and a vertical polarization signal can be separated into two different signals, and in which the same polarization signal is output separated into a horizontal polarization signal and a vertical polarization signal.

To solve the above-mentioned problem in the CMA method, the logarithm partial differentiation value of the target probability density function of a separation output signal of a polarization separator for polarization-separating an input signal is calculated, and based on the logarithm polarization differentiation value, the inclination of the target optimum function for optimizing the distribution of a multiple output signal is calculated. Furthermore, the technology of avoiding the equivalence convergence by updating the coefficient of a filter based on the inclination obtained by the calculation is known (for example, patent document 1).

In addition, a technology of calculating one tap coefficient of a FIR filter, which outputs a horizontal polarization signal or a vertical polarization signal, from another tap coefficient is known. Even when equivalence convergence occurs, the equivalence convergence can be avoided by regenerating a tap coefficient (for example, non-patent document 1).

DOCUMENT OF PRIOR ART

Patent Document

SUMMARY

The digital coherent optical receiver includes a processor that is operative to separate electric signals obtained by converting an optical signal into a horizontal signal component and a vertical signal component; to generate a histogram of the horizontal signal component and the vertical signal component as outputs of the equalizing filter; and to determine a presence/absence of local convergence based on distribution of the histogram of the horizontal signal component and the histogram of the vertical signal component.

The digital coherent optical receiver disclosed by the present invention shortens the time required in determining local convergence.

DESCRIPTION OF EMBODIMENTS

FIG. 4is an example of the configuration of the digital coherent optical receiver. A digital coherent optical receiver31has a 90° hybrid circuit32, analog-to-digital converters (ADC)33-1through33-4, a Demux unit34, a wavelength dispersion compensation unit35, and an adaptive equalizer36. The digital coherent optical receiver31includes a frequency offset control unit37, a carrier wave phase estimation control unit38, a determination unit39, and a frame synchronization unit40. The analog-to-digital converters33-1through33-4, the Demux unit34, the wavelength dispersion compensation unit35, the adaptive equalizer36, etc. can be realized by a hardware circuit such as a CMOSIC etc. The wavelength dispersion compensation unit35, the adaptive equalizer36, the frequency offset control unit37, the carrier wave phase estimation control unit38etc. can be realized by a digital signal processor.

The 90° hybrid circuit32has an opto-electric conversion function, and a current-voltage conversion function (transimpedance amplifier (TIA)). The 90° hybrid circuit32extracts from signal light using local emission an optical signal having a 90° phase difference with respect to the optical signal which is in phase with the local emission, converts the extracted optical signal into an electric signal, and separates the signal into an I component and a Q component.

Analog-to-digital converters13-1through13-4sample an analog signal output from the 90° hybrid circuit32with the timing synchronous with a clock signal and converts it into a digital signal.

The Demux unit34spreads in parallel the I component and the Q component of a signal so that the signal can be processed in parallel in a plurality of circuits in the subsequent stage. The Demux unit34and the subsequent circuits are provided with a circuit having a plurality of lanes to parallel-spread the I component and the Q component of a signal, or to process a parallel-spread signal.

The wavelength dispersion compensation unit35compensates for the wavelength dispersion of the horizontal signal component Eh=(Ih, Qh) and the vertical signal component Ev=(Iv, Qv) including the polarization component output from the Demux unit34.

The adaptive equalizer36adaptively updates the tap coefficient of a filter, and outputs the horizontal signal component Eh′=(Ih′, Qh′) and the vertical signal component Eh′=(Iv′, Qh′) by separating the polarization component. The adaptive equalizer36has a plurality of circuits for processing the I components and the Q components of the parallel-spread signal.

The frequency offset control unit37compensates for the frequency offset of the local emission on the transmission and reception sides. The carrier wave phase estimation control unit38estimates and compensates for the phase difference of the carrier wave.

The determination unit39demodulates the data by determining the signal point on the I an Q planes. The frame synchronization unit40constructs the demodulated data into a frame in a determined format.

The digital coherent optical receiver31can use a circuit of another well known configuration not limited to the circuit in the above-mentioned configuration.

FIG. 5illustrates the configuration of the adaptive equalizer36according to the first embodiment. The adaptive equalizer36includes four FIR filters42-1through42-4, a filter coefficient application control circuit43, two histogram generation units44hand44v, a local convergence determination and tap coefficient correction circuit45, and a selection circuit (SEL)46.

The FIR filters42-1through42-3receive the horizontal signal component Eh=(Ih, Qh) including the polarization component. The FIR filters42-2through42-4receive the vertical signal component Ev=(Iv, Qv) including the polarization component.

The tap coefficient of the FIR filters42-1through42-4is corrected by the filter coefficient application control circuit43or the local convergence determination and tap coefficient correction circuit45. As a result, a polarization-separated horizontal signal component Eh′=(Ih′, Qh′) is obtained as a signal obtained by combining the outputs of the FIR filters42-1and42-2. Similarly, the polarization-separated vertical signal component Ev′=(Iv′, Qv′) is obtained as a signal obtained by combining the outputs of the FIR filter42-3and42-4.

The filter coefficient application control circuit43outputs an updated tap coefficient to the selection circuit46based on the horizontal signal component Eh and vertical signal component Ev including the polarization components and the polarization-separated horizontal signal component Eh′ and vertical signal component Ev′. The filter coefficient application control circuit43is an example of a filter calculation circuit.

The histogram generation unit44hdetermines where the amplitude value (for example, a sum of squares of the I and Q components of a signal) of the horizontal signal component Eh′ is located in the range which is delimited at equal intervals between 0 and a certain value (maximum value), and generates a histogram having the amplitude value of the horizontal signal component Eh′.

Hereafter in the present embodiment, the sum of squares ((P=I2+Q2) of the I and Q components of each signal of the horizontal and vertical signal components is referred to as an amplitude value of the signal.

The histogram generation unit44vdetermines where the amplitude value of the vertical signal component Ev′ is located in the range which is delimited at equal intervals between 0 and a certain value (maximum value), and generates a histogram having the amplitude value of the vertical signal component Ev′. The data of the generated histogram of the horizontal signal component Eh′ and the vertical signal component Ev′ is output to the local convergence determination and tap coefficient correction circuit45.

FIG. 6is an example of a histogram generated by the histogram generation units44hand44v. In this example, the maximum value of the signal amplitude is equally divided into 30 steps from INDEX_0to INDEX_29to generate a histogram. For example, INDEX_0corresponds to the amplitude of “0”, and INDEX_29corresponds to the maximum value of the amplitude through the value of 29/39 of the maximum value.

The histogram generation units44hand44vinFIG. 5determine to which value in the range of INDEX_0through INDEX_29the amplitude value of the horizontal signal component and the vertical signal component corresponds, accumulate the number of pieces of data of the corresponding amplitude value, and generate a histogram.

The local convergence determination and tap coefficient correction circuit45determines the presence/absence of local convergence based on the distribution of the histogram generated by the histogram generation units44hand44vor the position of the peak value. Then, when it is determined that the local convergence exists in the horizontal signal component or the vertical signal component, a switch signal directing a selection switch is output to the selection circuit46, and the tap coefficient on the normal convergence side is output as an initial value of the tap coefficient.

The selection circuit46normally selects the tap coefficient output from the filter coefficient application control circuit43, and outputs it to the FIR filters42-1through42-4. Upon receipt of the switch signal from the local convergence determination and tap coefficient correction circuit45, the circuit selects the tap coefficient output from the local convergence determination and tap coefficient correction circuit45, and outputs it to the FIR filters42-1through42-4. Thus, when the local convergence is detected, the tap coefficient on the side on which no local convergence has occurred is output from the selection circuit46as an initial value of the tap coefficient on the side on which the local convergence has occurred.

FIG. 7is a flowchart of the operation of the adaptive equalizer according to the first embodiment.

When a wavelength dispersion compensation setting process by the wavelength dispersion compensation unit35is completed (S11), an adaptive equalization process is started by the adaptive equalizer36(S12). In the adaptive equalization process in step S12, the tap coefficient of the FIR filters42-1through42-4is updated using the tap coefficient output from the filter coefficient application control circuit43or the local convergence determination and tap coefficient correction circuit45.

Next, it is determined whether or not a CMA convergence time has passed (S13). The CMA convergence time is determined in advance to determine the local convergence.

If the CMA convergence time has passed (YES in S13), a monitor start notification is output, and a histogram accumulating process in step S14is performed. In the process in step S14, for example, histogram generation units44-1and44-2start accumulating a histogram.

When a monitor completion notification is output, it is determined using the histogram whether or not the local convergence exists (S15). In the process in step S15, for example, the local convergence determination and tap coefficient correction circuit45determines the presence/absence of the local convergence based on the histogram generated by the histogram generation units44hand44v.

If it is determined that there is local convergence (YES in S15), control is passed to step S16, and the initial value of the tap coefficient is recalculated. In the process in step S16, for example, the local convergence determination and tap coefficient correction circuit45calculates the initial value of the tap coefficient on the side on which the local convergence has occurred using the tap coefficient on the side on which no local convergence has occurred (horizontal signal component or vertical signal component).

Next, the CMA using the tap coefficient used up to the point is temporarily stopped, and the tap coefficient on the side on which the local convergence has occurred is changed to the initial value recalculated in step S16(S17). In the process in step S17, for example, the local convergence determination and tap coefficient correction circuit45directs the selection circuit46to switch the tap coefficient on the side on which the local convergence has occurred. Thus, the selection circuit46outputs the initial value of the tap coefficient output from the local convergence determination and tap coefficient correction circuit45to the FIR filter on the side on which the local convergence has been detected.

When no local convergence is detected (NO in S15), control is passed to step S18, and the process in the next stage is started. The process in the next stage refers to, for example, a frequency offset compensating process etc. by the frequency offset control unit37.

FIG. 8illustrates the configuration of the histogram generation units44hand44v. It is not necessary to generate a histogram in all adaptive equalizers36in a plurality of lanes, but a histogram may be generated in an arbitrary number of lanes. Then, using the generated histograms, the tap coefficient of another lane may be corrected. The example inFIG. 8refers to a case in which the histogram generation units44hand44vare provided for 3 lanes when the adaptive equalizers36are provided for 64 lanes.

InFIG. 8, each of the histogram generation units44hand44vincludes power value calculation units51hand51v, threshold determination units52hand52v, histogram counters53hand53v, and monitor output counters54hand54v. The histogram generation units44hand44vhave a counter monitor unit55and a read monitor unit56.

The character h of the reference numeral of each circuit refers to a circuit for a horizontal signal component, and the character v refers to a circuit for a vertical signal component.

The power value calculation unit51hfor a horizontal signal component calculates a sum of the value HI2obtained by squaring the I component of the horizontal signal component Eh and the value HQ2obtained by squaring the Q component as a power value (amplitude value) of the horizontal signal component Eh.

The power value calculation unit51vfor a vertical signal component calculates a sum of the value VI2obtained by squaring the I component of the vertical signal component Ev and the value VQ2obtained by squaring the Q component as a power value (amplitude value) of the vertical signal component.

The threshold determination unit52hfor a horizontal signal component compares the amplitude value calculated by the power value calculation unit51hwith the threshold of each section of the histogram, and determines to which section the amplitude value corresponds. Then, the unit outputs to the histogram counter53hfor the corresponding section a signal for increment of the count value.

The histogram counter53hfor a horizontal signal component includes 30 histogram counters53h-0through53h-29corresponding to the number of sections (for example, 30) of the histogram. The histogram counter53vfor a vertical signal component also includes 30 histogram counters53v-0through53v-29corresponding to the number of histograms.

The monitor output register54hfor a horizontal signal component includes a plurality of (30 in this case) monitor output registers54h-0through54h-29corresponding to the histogram counters53h-0through53h-29. Each of the monitor output registers54h-0through54h-29holds the count values of the corresponding histogram counters53h-0through53h-29, and outputs them as the accumulated values INDEX_0through INDEX_29of the horizontal signal components.

Similarly, the monitor output register54vfor a vertical signal component includes a plurality of monitor output registers54h-0through54h-29corresponding to the histogram counters53v-0through53v-29. Each of the monitor output registers54h-0through54h-29holds a count of a corresponding histogram counters53v-0through53v-29, and outputs them as accumulated values of INDEX_0through INDEX_29for vertical signal components.

Upon receipt of the monitor start notification, the counter monitor unit55starts monitoring the histogram counters53hand53v. When any count value of the histogram counters53h-0through53h-29reaches the upper limit or when the total count value of all counters reaches the upper limit of the number of monitor accumulation, the counter monitor unit55outputs the monitor completion notification.

Upon receipt of the monitor completion notification from the counter monitor unit55, the local convergence determination and tap coefficient correction circuit45reads the accumulated value of the monitor output registers54hand54v, and generates a histogram of the horizontal signal component and the vertical signal component.

The read monitor unit56monitors whether or not a read of the accumulated value of the monitor output registers54hand54vhas been performed, and if the accumulated value has been read by the local convergence determination and tap coefficient correction circuit45, the unit clears the histogram counters53hand53v.

Described next is the local convergence determining operation by the local convergence determination and tap coefficient correction circuit45.

Described first is the method (hereafter referred to as a first local convergence determining method) for determining the local convergence by comparing the number of pieces of data of amplitude values larger than a determination point, which is an amplitude value, with the number of pieces of data of amplitude values equal to or smaller than the determination point. It is an example of a case in which the presence/absence of the local convergence is determined based on the distribution of a histogram.

FIG. 9illustrates the configuration of a local convergence determination circuit61by the above-mentioned first local convergence determining method.

The local convergence determination circuit61includes, for example, two circuits, that is, a circuit for determining the local convergence of a horizontal signal component and a circuit for determining the local convergence of a vertical signal component. The local convergence determination circuit61is a part of the local convergence determination and tap coefficient correction circuit45inFIG. 5.

InFIG. 9, the local convergence determination circuit61includes 30 selectors62-0through62-29for switching the destination of the accumulated values INDEX_0through INDEX_29based on the value (for example, a reference value) of the threshold Index, and a threshold index holding unit63for holding the threshold Index. The local convergence determination circuit61also includes a lower total calculation unit64for calculating a lower total value by adding the accumulated values (number of pieces of data) of the amplitude value equal to or smaller than the threshold Index, and an upper total calculation unit65for calculating an upper total value by adding the accumulated values of the amplitude value larger than the threshold Index. Furthermore, the local convergence determination circuit61includes a local convergence determination unit66for comparing the lower total value with the upper total value to determine the presence/absence of the local convergence.

The threshold index holding unit63holds the INDEX value as the boundary between the upper side and the lower side of INDEX_0through INDEX_29.

The selectors62-0through62-29compares the threshold Index held in the threshold index holding unit63with any INDEX number in INDEX_0through INDEX_29. When the INDEX number is equal to or smaller than the threshold Index, the accumulated value is output to the lower total calculation unit64. When the INDEX number A is larger than the threshold Index, the accumulated value is output to the upper total calculation unit65.

The lower total calculation unit64calculates the lower total value by adding the accumulated values output from the selectors62-0through62-29. The upper total calculation unit65calculates the upper total value by adding the accumulated values output from the selectors62-0through62-29.

The local convergence determination unit66compares the lower total value with the upper total value, and determines the local convergence when the lower total value is larger than the upper total value. When the lower total value is equal to or smaller than the upper total value, it is determined as normal convergence.

FIGS. 10A and 10Bare explanatory views of the first local convergence determining method. The vertical axes inFIGS. 10A and 10Bindicate the INDEX number corresponding to the amplitude, and the horizontal axes indicate the number of pieces of data (data frequency). The amplitude value increases in the direction indicated by the arrow inFIGS. 10A and 10B.

FIGS. 10A and 10Billustrate the case in which, for example, INDEX_15is set as a threshold (threshold Index) as the determination criterion of the local convergence.

As illustrated inFIG. 10A, when the total of the number of pieces of data having the INDEX number equal to or smaller than the threshold (for example, INDEX_15) is equal to or smaller than the total of the number of pieces of data of INDEX equal to or exceeding the threshold, it is determined as normal convergence.

As illustrated inFIG. 10B, when the total of the number of pieces of data of INDEX smaller than the threshold is larger than the total of the number of pieces of data of INDEX equal to or exceeding the threshold, it is determined as local convergence.

Described next is the method (hereafter referred to as a second local convergence determining method) for determining the presence/absence of the local convergence depending on whether the peak value of the histogram is located on the upper side or the lower side using an amplitude value as a determination point. It is an example of a case in which the presence/absence of the local convergence is determined based on the distribution (for example, the position of the peak value) of the histogram.

FIG. 11illustrates the configuration of a local convergence determination circuit71in the second local convergence determining method.

The local convergence determination circuit71includes a maximum data number selection unit72, a threshold Index holding unit73, and a threshold determination unit74.

The maximum data number selection unit72specifies the INDEX number of the maximum number of pieces of data (maximum frequency Index) in the number of pieces of data of INDEX_0through INDEX_29output from the histogram generation units44hand44v, and outputs the INDEX number to the threshold determination unit74.

The threshold Index holding unit73holds the threshold Index as a determination standard.

The threshold determination unit74compares the INDEX number of the maximum number of pieces of data output from the maximum data number selection unit72with the threshold Index held in the threshold Index holding unit73, and determines whether or not the INDEX number of the maximum number of pieces of data is larger than the threshold Index. That is, it determines whether or not the amplitude value of the maximum number of pieces of data is larger than the specified amplitude value (threshold or reference value).

If the INDEX number of the maximum number of pieces of data is larger than the threshold Index, it is determined as normal convergence. That is, if the amplitude value of the maximum number of pieces of data is (equal to or) larger than a specified amplitude threshold in the histogram, it is determined as normal convergence.

On the other hand, if the INDEX number of the maximum number of pieces of data is smaller than the threshold Index, it is determined as local convergence. That is, if the amplitude value of the maximum number of pieces of data is smaller than the amplitude threshold in the histogram, it is determined as local convergence.

FIGS. 12A and 12Bare explanatory views of the second local convergence determining method. The vertical axes inFIGS. 12A and 12Bindicate the INDEX number corresponding to the amplitude, and the horizontal axes indicate the number of pieces of data.

FIGS. 12A and 12Bare examples of the case in which, for example, INDEX_14is set as a threshold Index.

As illustrated inFIG. 12A, when the peak value of the amplitude of the histogram is (equal to or) larger than the amplitude value of the threshold Index, it is determined as normal convergence.

On the other hand, as illustrated inFIG. 12B, when the peak value of the amplitude of the histogram is smaller than the amplitude value of the threshold Index, it is determined as local convergence.

Described next is the method (hereafter referred to as the third local convergence determining method) of determining the presence/absence of the local convergence depending on whether or not the number of pieces of data of the amplitude value 0 is larger than the threshold. It is an example of a case in which the presence/absence of the local convergence is determined based on the distribution of the histogram.

FIG. 13illustrates the configuration of the local convergence determination circuit in the third local convergence determining method.

A local convergence determination circuit81includes a threshold determination unit82and a threshold holding unit83. The threshold determination unit82compares the number of pieces of data of INDEX_0with the threshold held in the threshold holding unit83, and determines the presence/absence of the local convergence. The threshold determination unit82determines normal convergence when the number of pieces of data of INDEX_0is equal to or smaller than the threshold. The threshold determination unit82determines the local convergence when the number of pieces of data of INDEX_0is larger than the threshold.

FIGS. 14A and 14Bare explanatory views of the third local convergence determining method. The vertical axes inFIGS. 14A and 14Bindicate the INDEX number corresponding to the amplitude, and the horizontal axes indicate the number of pieces of data (data frequency).

As illustrated inFIG. 14A, when the number of pieces of data of INDEX_0is equal to or smaller than the threshold, it is determined as normal convergence.

On the other hand, as illustrated inFIG. 14B, when the number of pieces of data of INDEX_0is larger than the threshold, it is determined as local convergence.

Described next is the method for calculating the tap coefficient initial value when the local convergence is determined.FIGS. 15 and 16are explanatory views of the method for calculating a tap coefficient initial value.

When the local convergence is determined, a tap coefficient initial value is set using a tap coefficient of the side on which the local convergence has not occurred.

FIG. 15is an explanatory view (1) of the method for calculating a tap coefficient initial value, andFIG. 16is an explanatory view (2) of the method for calculating a tap coefficient initial value.

In the description below, it is assumed that the FIR filter42-1uses a tap coefficient hhh, the FIR filter42-2uses a tap coefficient hvh, the FIR filter42-3uses a tap coefficient hhv, and the FIR filter42-4uses a tap coefficient hvv. The relationship between the FIR filters42-1through42-4and the tap coefficients is the same as that illustrated inFIG. 2.

FIG. 15is an example of calculating the initial value of the tap coefficient hhvof the FIR filter32-3for separating the vertical signal component from the tap coefficient hvhof the FIR filter42-2for separating the horizontal signal component when the local convergence is detected in the vertical signal component.

The tap coefficient hvhof the FIR filter42-2on the horizontal side indicating the normal convergence can be expressed by (tk)=(avh(tk), bvh(tk)).

The tap coefficient hhvof the FIR filter42-3on the vertical side indicating the local convergence can be calculated by the following equation from the tap coefficient hvhof the FIR filter42-2on the horizontal side indicating the normal convergence.
hhv(tk)=(avh(−tk),bvh(−tk))

That is, the tap coefficient hhv(tk) of the FIR filter42-3on the vertical side indicating the local convergence inverts the value of tkbased on the tap center as a reference using the tap coefficient hvh(tk) on the horizontal side, and obtains a value as a result of inverting the sign of the I component avhof the data. The value tkspecifies the position of the tap for the tap center.

FIG. 16is an example of a case in which the initial value of the tap coefficient hvvof the FIR filter42-4for separating the vertical signal component from the tap coefficient hhhof the FIR filter42-1on the horizontal side is calculated.

The tap coefficient hhhof the FIR filter42-1can be expressed by hhh(tk)=(ahh(tk), bhh(tk)).

The tap coefficient hvvof the FIR filter42-4on the vertical side indicating the local convergence can be calculated by the following equation using the tap coefficient hhhof the FIR filter42-1on the horizontal side indicating the normal convergence.
hvv(tk)=(ahh(−tk),−bhh(−tk))

That is, the tap coefficient hvv(tk) of the FIR filter42-4on the vertical side can be obtained by inverting the sign of the Q component bhhof the data after inverting the value of tkbased on the tap center as a reference using the tap coefficient hhh(tk) on the horizontal side.

According to the first embodiment above, the adaptive equalizer36determines the presence/absence of the local convergence. Therefore, the time required to make determination can be shorter than in the method of determining the local convergence after a frame is constructed by determining a signal point.

FIG. 17illustrates the configuration of an adaptive equalizer91according to the second embodiment. InFIG. 17, the circuit block also illustrated inFIG. 5is assigned the same reference numeral, and the detailed description is omitted here.

The adaptive equalizer91includes four FIR filters42-1through42-4, the filter coefficient application control circuit43, the histogram generation units44hand44v, and a local convergence/peak selection/tap coefficient correction circuit92.

The local convergence/peak selection/tap coefficient correction circuit92determines the presence/absence of the local convergence using a histogram. If the local convergence has occurred, it calculates a tap coefficient on the side on which the local convergence has occurred from the tap coefficient on the side on which no local convergence has occurred. In addition, the local convergence/peak selection/tap coefficient correction circuit92compares the peak values of the histograms between the horizontal and vertical sides, and calculates a tap coefficient initial value on one side using a tap coefficient having a larger peak value on the other side.

Thus, the tap coefficient initial value for protection against the equivalence convergence can be set using a tap coefficient on the side having a larger peak value of signal amplitude, that is, using a tap coefficient on the side on which it is estimated that signal quality is higher.

FIG. 18illustrates a histogram on the horizontal side and the histogram on the vertical side. InFIG. 18, the vertical axis indicates an INDEX number corresponding to the amplitude value of a signal, and the horizontal axis indicates the number of pieces of data of each INDEX.

In the example inFIG. 18, the peak value of the histogram on the horizontal side is higher than the peak value of the histogram on the vertical histogram. Therefore, the tap coefficient initial value on the vertical side is calculated using the tap coefficient on the horizontal side.

FIG. 19is a flowchart of the operation of the adaptive equalizer91according to the second embodiment.

When the wavelength dispersing process is completed in the wavelength dispersion compensation unit35(S21), the adaptive equalization process is started (S22). Next, it is determined whether or not the convergence time of the CMA (constant modulus algorithm) has passed (S23).

If the convergence time has passed (YES in S23), control is passed to step S24, the accumulation of the histogram is started, and a histogram of a specified number of samples is generated. In the process in step S24, for example, the histogram generation units44hand44vgenerate a histogram of the horizontal signal component and the vertical signal component.

Next, it is determined whether or not the local convergence has occurred (S25). In the process in step S25, for example, the local convergence/peak selection/tap coefficient correction circuit92determines the presence/absence of the local convergence in the first, second, or third local convergence determining method.

When the local convergence has not occurred, control is passed to step S26, and it is determined whether or not the comparison of the peak values of the histograms has been performed between the horizontal signal component and the vertical signal component. If the local convergence is determined (YES in S25), control is passed to step S28.

If the comparison of the peak values of the histograms has not been performed (NO in S26), control is passed to step S27, and the peak values of the histograms are compared between the horizontal signal component and the vertical signal component.

Next, the tap coefficient initial value is recalculated in step S28. The recalculation of the tap coefficient initial value in step S28is performed when the local convergence is determined in step S25(YES in S25) or after the peak value comparing process is performed in step S27. That is, if the local convergence is determined in step S25, the tap coefficient initial value on the side on which the local convergence has occurred is calculated using the tap coefficient on the normal convergence side. If the comparison of the peak values of the histograms is performed in step S27, the tap coefficient initial value having a smaller peak value is recalculated using the tap coefficient having a larger peak value (horizontal signal component or vertical signal component).

FIG. 20is an explanatory view of the peak value of a histogram. The vertical axis inFIG. 20indicates the number of pieces of data, and the horizontal axis indicates an amplitude value.

The triangular point inFIG. 20indicates the histogram of the amplitude value of the horizontal signal component, and the square point indicates the histogram of the amplitude value of the vertical signal component. In the example inFIG. 20, since the peak value of the histogram of the horizontal signal component is larger than the peak value of the vertical signal component, the tap coefficient initial value on the vertical side is calculated using the tap coefficient on the horizontal side.

Back inFIG. 19, the CMA equalizing process is temporarily stopped in step S29, and the tap coefficient on the local convergence side is corrected using the tap coefficient initial value calculated in step S28. Then, in step S22, the adaptive equalization process is started using the corrected tap coefficient.

If it is determined in step S26that the peak comparison has already been performed (YES in S26), control is passed to step S30, and the process in the next stage, for example, the frequency offset compensating process etc. is performed.

In the flowchart illustrated inFIG. 19, the comparison of the peak values of the histogram is performed only once, but the comparison of the peak values can be performed plural times.

FIG. 21illustrates an example of a peak selection circuit101in the local convergence/peak selection/tap coefficient correction circuit92.

The peak selection circuit101includes a highest frequency selection unit102for selecting the maximum value of the histogram of the horizontal signal component, a highest frequency selection unit103for selecting the maximum value of the histogram of the vertical signal component, and a value comparison unit104.

The highest frequency selection unit102selects the peak value in INDEX_H_0through INDEX_H_29indicating the number of pieces of data of the amplitude value of the horizontal signal component, and outputs the selected peak value as the maximum count value H to the value comparison unit104. INDEX_H_0through INDEX_H_29are output from, for example, the monitor output register54hof the horizontal signal component inFIG. 8.

The highest frequency selection unit102selects the peak value in the INDEX value of INDEX_V_0through INDEX_V_29indicating the number of pieces of data of the amplitude value of the vertical signal component, and outputs the selected peak value as the vertical maximum count value V to the value comparison unit104. INDEX_V_0through INDEX_V_29are output from the monitor output register54vof the vertical signal component inFIG. 8.

The value comparison unit104compares the maximum count value H on the horizontal side with the maximum count value V on the vertical side, and specifies the smaller maximum count value as a tap coefficient to be corrected.

When the peak selection circuit101selects a signal to be selected, the local convergence/peak selection/tap coefficient correction circuit92corrects the tap coefficient of the FIR filter on the horizontal or vertical side to be corrected using the tap coefficient of the FIR filter having a larger maximum count value.

In the above-mentioned second embodiment, using a tap coefficient having a larger peak value of the histogram of the amplitude of a signal, the initial value of the other tap coefficient is calculated. Thus, using a tap coefficient of a filter which applies equalizatioin to a signal of higher signal quality, another tap coefficient can be corrected, thereby preventing the degradation of signal quality, and avoiding the equivalence convergence. Furthermore, using the above-mentioned first, second, or third local convergence determining method, the presence/absence of the local convergence can be determined in a shorter time.