WO2014199601A1

WO2014199601A1 - Equalizer, polarization separator using same, and equalization method

Info

Publication number: WO2014199601A1
Application number: PCT/JP2014/002991
Authority: WO
Inventors: 大作小笠原
Original assignee: 日本電気株式会社
Priority date: 2013-06-14
Filing date: 2014-06-05
Publication date: 2014-12-18
Also published as: JPWO2014199601A1; JP6314979B2

Abstract

When using a polarization separator which updates filter coefficients by CMA, since distortion of the constellation of a reproduced received optical signal increases if the optical SNR of the received optical signal is low, this polarization separator comprises a filter means which inputs a first received signal and a second received signal acquired from a polarization division multiplexed signal beam, and a filter coefficient updating means which updates the filter coefficients such that the envelope of the output signal of the filter means is constant, wherein the filter coefficient updating means calculates an output symbol distance, calculates the output symbol movement amount, which is the difference between a first output symbol distance, calculated from the output signal acquired using the first filter coefficient, and a second output symbol distance, calculated from the output signal acquired using the second filter coefficient that updates the first filter coefficient, and calculates an update amount of the filter coefficient such that the change in the output symbol movement amount relative to the first output symbol distance is point-symmetric around a prescribed target value of the first output symbol distance.

Description

Equalizer, polarization separator using the same, and equalization method

The present invention relates to an equalizer, a polarization separator using the same, and an equalization method, and more particularly to an equalizer using a constant envelope algorithm, a polarization separator using the same, and an equalization method.

Since the traffic volume of the backbone network is rapidly increasing due to the spread of the Internet, an ultrahigh-speed optical communication system of 100 Gbps is desired. As a technique for realizing such an ultrahigh-speed optical communication system, an optical digital coherent communication system using an optical phase modulation system and a polarization multiplexing / demultiplexing technique has attracted attention.

The optical phase modulation method is a method for performing data modulation on the optical phase of the transmission laser beam, rather than performing data modulation on the optical intensity of the transmission laser beam as in the conventionally used optical intensity modulation method. It is. Optical phase modulation methods include QPSK (Quadrature Phase Shift Keying) method, 8PSK (8 Phase Shift Modulation: 8-Phase Shift Keying) method, and QAM (Quadrature Amplitude Modulation Method). Etc. are known. In the optical phase modulation method, it is possible to reduce the symbol rate by assigning a plurality of bits to one symbol. As a result, the operating speed of the electric device can be reduced, and it is expected that the manufacturing cost of the apparatus can be reduced.

On the other hand, the polarization multiplexing / demultiplexing technology multiplexes two independent optical signals whose carrier waves are arranged in the same frequency band and whose polarization states are orthogonal to each other in an optical transmitter. Then, in the optical receiver, the above-described two independent optical signals are separated from the received signal. Thereby, a double transmission speed can be realized. In this case, since the symbol rate of the optical signal is halved, the operation speed of the electric device can be reduced. Therefore, according to the polarization demultiplexing technique, it is possible to reduce the manufacturing cost of the communication device.

By combining the optical phase modulation method and polarization multiplexing / demultiplexing technology described above, an ultra-high speed optical communication system capable of 100 Gbps transmission can be realized. Then, a process for compensating for optical carrier frequency deviation and optical phase deviation, and a process for separating into two independent optical signals (polarization separation process) using a digital signal processing technique, and a technique for highly accurate demodulation are proposed. Has been. Such a system is called an optical digital coherent communication system.

An example of an optical receiver used in an optical communication system using such an optical digital coherent communication system is described in Patent Document 1. FIG. 5 shows a block diagram of a related optical receiver 200 described in Patent Document 1. In FIG.

The related optical receiver 200 receives the received optical signal through the optical transmission line. Locally oscillated light having an optical frequency substantially the same as the carrier frequency of the received optical signal is input to the 90-degree hybrid 210 together with the received optical signal. The 90-degree hybrid 210 separates the received optical signal into optical signal components having a polarization state parallel to each of two orthogonal polarization axes, and a total of 4 consisting of a real part component and an imaginary part component of each optical signal component. Number of optical signals are output. These four optical signals are converted into analog electric signals by four optical detectors 221 to 224, and then converted into digital electric signals by analog-to-digital converters (ADCs) 231 to 234. The digital electric signals output from the analog-digital converters (ADCs) 231 to 234 are converted into digital electric signals sampled at the symbol rate of the received optical signal by a resampling unit (not shown), and then polarized. Input to the separation unit 240. The polarization separation unit 240 extracts two independent optical signals that are polarization multiplexed based on the four input digital electric signals. Each of the extracted optical signals is compensated for optical phase rotation by the optical carrier frequency deviation and the optical phase deviation between the received optical signal and the local oscillation light by the optical carrier frequency deviation / optical phase

deviation compensation units

251 and 252. . Finally, the

symbol identifying units

261 and 262 respectively demodulate the original transmission bit strings.

In the optical receiver used in the optical communication system based on the optical digital coherent communication method described above, the optical phase modulation method and the polarization multiplexing / demultiplexing technology are combined. In addition, the influence of the optical carrier frequency deviation and the optical phase deviation is compensated for each of the two independent optical signals subjected to polarization separation. Thereby, it is possible to realize an ultra-high speed optical communication system capable of 100 Gbps transmission.

International Publication No. 2012/105070

First, the operation of the polarization separation unit 240 used in the related optical receiver 200 will be described. FIG. 6 shows a configuration of the related polarization separation unit 240. The polarization separation unit 240 includes an equalizer including filter units 241 to 244 and filter

coefficient update units

245 and 246. The input signal 1 of the polarization separation unit 240 is a signal based on an optical signal having a polarization state parallel to one of two orthogonal polarization axes in the 90-degree hybrid 210. That is, it is a digital electric signal represented by a complex number having the digital electric signal output from the ADC 231 shown in FIG. 5 as a real part component and the digital electric signal output from the ADC 232 as an imaginary part component. Similarly, the input signal 2 of the polarization separation unit 240 is a signal based on an optical signal having a polarization state parallel to the other of the two polarization axes orthogonal to each other in the 90-degree hybrid 210. That is, the digital electric signal is represented by a complex number having the digital electric signal output from the ADC 233 shown in FIG. 5 as a real part component and the digital electric signal output from the ADC 234 as an imaginary part component.

The output signal 1 and the output signal 2 of the polarization separation unit 240 shown in FIG. 6 are digital electric signals based on two independent optical signals that are polarization multiplexed in the optical transmitter.

The filter units 241 to 244 included in the polarization separation unit 240 perform filtering processing on the input signal 1 and the input signal 2 using filter coefficients set independently for each filter unit. Thereafter, the sum of the outputs of the filter unit 241 and the filter unit 243 is set as the output signal 1, and the sum of the outputs of the filter unit 242 and the filter unit 244 is output as the output signal 2. Note that a general FIR (Finite Impulse Response) filter can be used for the filter units 241 to 244.

The filter coefficient updating unit 245 updates the filter coefficients of the filter unit 241 and the filter unit 243 according to a predetermined algorithm. Similarly, the filter coefficient update unit 246 updates the filter coefficients of the filter unit 242 and the filter unit 244. As an algorithm for updating the filter coefficients of the filter units 241 to 244 by the filter

coefficient updating units

245 and 246, a constant envelope algorithm (Constant Modulus Algorithm: CMA) is widely used. In CMA, polarization separation is performed by adaptively controlling the filter coefficients of the filter units 241 to 244 so that the envelope of the extracted optical signal is constant, that is, the light intensity is constant.

Next, a case where the filter coefficient is updated using CMA will be described. The following equation (1) shows an error function defined by CMA.

J _x (W, W ^H ) is an error function for the output signal 1, and J _y (W, W ^H ) is an error function for the output signal 2. Wherein W is a square matrix of size of 2 × 2, 1 row and the first column elements of the matrix W _(w xx), 1 row 2 column component _(w xy), 2 rows and one column component _(w yx), 2 The row 2 column component (w _yy ) represents the filter coefficients of the filter units 241 to 244, respectively. The matrix W is a matrix representing the characteristics of an optical transmission line called a Jones matrix. Note that the matrix ^WH is a Hermitian conjugate of the matrix W.

In the above description, the number of taps of the filter unit is 1 for simplicity, but the number of taps may be 2 or more. Also shown in the formula (1), the E x _'and E _y' are each output signal 1 and the output signal 2, a target value of the amplitude of the r _x and r _y are the output signals 1 and the output signal 2. E [x] represents an expected value of x.

The filter coefficient updating unit 245 sequentially updates the filter coefficients of the filter unit 241 and the filter unit 243 so that the error function J _x for the output signal 1 is minimized. The filter coefficient updating unit 246 sequentially updates the filter coefficients of the filter unit 242 and the filter unit 244 so that the error function J _y with respect to the output signal 2 is minimized.

The following equations (2) to (4) show equations for the filter

coefficient updating units

245 and 246 to update the respective filter coefficients based on the CMA error function shown in the expression (1).

Here, “μ” in the equation (2) is a step parameter for stabilizing the feedback control by adjusting the update amount of the filter coefficient. That is, “μ” is a parameter that determines the processing speed of the polarization separation process. Note that the expected value is generally replaced with an instantaneous value for calculating the update amount of the filter coefficient.

From the above equations (2) to (4), it can be seen that in the CMA, the filter coefficients of the filter units 241 to 244 are updated as follows. That is, as a symbol point on the figure constellation shown output signal of the polarization separation section 240, a direction toward the origin (or vice versa) moves on a circumference of radius r _x or r _y along, The filter coefficients of the filter units 241 to 244 are updated.

Hereinafter, a process in which the filter

coefficient update units

245 and 246 update the filter coefficients of the filter units 241 to 244 will be described in more detail. FIG. 7 is a flowchart for explaining the process of updating the filter coefficient.

First, the filter

coefficient updating units

245 and 246 calculate a cost that is a difference between the square of the radius r _x or r _y centered at the origin and the square of the distance from the origin of the output symbol (step S10). Here, the symbol means a signal point on the constellation. Then, multiplying the input symbol _{E x} or _{E y} in the cost (step S20). Further, by multiplying the product obtained by multiplying the complex conjugate E _x ^'* or E _y ^{' *} of the output symbol (step S30) by the step parameter μ (step S40), the update amount of the filter coefficient is calculated. A value obtained by adding the update amount to the filter coefficient is set as a new filter coefficient (step S50), and the new filter coefficient is set in the filter units 241 to 244 (step S60). Through the above processing, the filter

coefficient update units

245 and 246 update the filter coefficients of the filter units 241 to 244.

As described above, in the polarization separation unit 240, the filter

coefficient update units

245 and 246 update the filter coefficients of the filter units 241 to 244 using CMA, so that two independent optical signals are obtained from the received optical signal. Separation and extraction are possible.

Next, the change of the output symbol when the output symbol is recalculated using the filter coefficient updated by the CMA will be described. FIG. 8 shows the distance from the output symbol before the filter coefficient update (hereinafter referred to as “output symbol movement amount”) in this case as the distance from the origin of the output symbol (hereinafter referred to as “output symbol distance”). Is shown. Here, r _x and r _y (radius of the target circle), which are target values of the amplitude of the output signal, are set to “1”, respectively, and step parameters are not multiplied for simplicity.

In FIG. 8, the solid line indicates that the input symbol is within the target circle, the broken line indicates that the input symbol is on the target circle, and the alternate long and short dash line indicates that the input symbol is outside the target circle. In the same figure, the output symbol whose horizontal axis is the distance from the origin of the output symbol is smaller than 1, that is, the output symbol that is inside the target circle (unit circle) of radius 1 has the symbol movement amount of the vertical axis. Since it takes a positive value, it can be seen that it moves in the direction outside the unit circle. In contrast, an output symbol having an output symbol distance greater than 1 and outside the unit circle moves to the inside of the unit circle because the symbol movement amount has a negative value. Further, it can be seen that an output symbol having an output symbol distance of 1, that is, an output symbol originally on the unit circle does not move because the symbol movement amount is zero “0”.

The filter

coefficient updating units

245 and 246 update filter coefficients so that all output symbols are located on the unit circle by sequentially executing such processing.

Also, it can be seen from FIG. 8 that the output symbol movement amount outside the unit circle is much larger than the output symbol inside the unit circle. This means that a strong force to move in the direction inside the unit circle acts on the output symbols outside the unit circle. Although the output symbols are evenly distributed around the ideal symbol point due to optical noise, a force for moving the output symbol in the inner direction acts as a whole by the update process of the filter coefficient. Therefore, when the related polarization separation unit 240 described above is used, there is a problem described below.

FIGS. 9A and 9B show examples of constellation diagrams of optical signals obtained by digital signal processing in the related optical receiver 200. FIG. The QPSK method was used as the optical signal modulation method. FIG. 9A shows a case where the optical SN (Signal to Noise) ratio is 15 dB, and FIG. 9B shows a case where the optical SN ratio is 10 dB.

As described above, in the polarization separation unit 240 included in the related optical receiver 200, the filter coefficient is updated based on the above formulas (2) to (4). At this time, as shown in FIGS. 9A and 9B, when the optical S / N ratio of the received optical signal is low, the center of the symbol point greatly deviates from the ideal symbol point (symbol “+” in the figure), and the filter coefficient is updated. It can be seen that the expected value of the error function used (the above formula (1)) is not zero. As a result, the filter coefficients of the filter units 241 to 244 do not correctly represent the reverse characteristics of the transfer function of the optical transmission line or the front end, and thus there is a problem that the information of the optical transmission line cannot be correctly extracted from the filter coefficient. It was.

As described above, when the related polarization separator that updates the filter coefficient by the constant envelope algorithm (CMA) is used, when the optical S / N ratio of the received optical signal is low, the distortion of the constellation of the reproduced received optical signal becomes large. There was a problem.

An object of the present invention is to use a polarization separator that updates a filter coefficient by a constant envelope algorithm (CMA), which is the above-mentioned problem, and when a received optical signal has a low optical signal-to-noise ratio, a constellation of a reproduced received optical signal. It is an object of the present invention to provide an equalizer, a polarization separator using the equalizer, and an equalization method that can solve the problem that the distortion of the modulation increases.

The equalizer of the present invention has filter means for performing filter processing based on the filter coefficient, and filter coefficient update means for updating the filter coefficient so that the envelope of the output signal of the filter means becomes constant. The update means calculates an output symbol distance that is a distance between an output symbol that is a position on the constellation of the output signal and an origin on the constellation from the output signal, and an output signal that is obtained using the first filter coefficient An output symbol movement amount that is a difference between the first output symbol distance calculated from the above and the second output symbol distance calculated from the output signal acquired using the second filter coefficient obtained by updating the first filter coefficient The change of the output symbol movement amount with respect to the first output symbol distance is calculated with a predetermined target value of the first output symbol distance as a center. Such that referred calculates the update amount of filter coefficients.

Further, the equalizer of the present invention has filter means for performing filter processing based on the filter coefficient, and filter coefficient update means for updating the filter coefficient so that the envelope of the output signal of the filter means becomes constant, The filter coefficient updating means is a function of the output signal, and uses a point-symmetric function centered at a point at which the absolute value of the output signal is equal to a predetermined target value of the amplitude of the output signal, to update the update amount of the filter coefficient calculate.
The polarization separator of the present invention is a first that photoelectrically converts the first received light constituting the polarization multiplexed signal light and the second received light having a polarization direction orthogonal to the polarization direction of the first received light. Filter means for inputting the received signal and the second received signal and performing filter processing based on the filter coefficient, and filter coefficient updating means for updating the filter coefficient so that the envelope of the output signal of the filter means becomes constant And a filter coefficient updating unit calculates an output symbol distance which is a distance between an output symbol which is a position on the constellation of the output signal and an origin on the constellation from the output signal, and uses the first filter coefficient. Calculated from the output signal acquired using the first output symbol distance calculated from the output signal acquired in this step and the second filter coefficient updated from the first filter coefficient. An output symbol movement amount that is a difference from the output symbol distance is calculated, and a change in the output symbol movement amount with respect to the first output symbol distance is point-symmetric about a predetermined target value of the first output symbol distance. Then, the update amount of the filter coefficient is calculated.

The equalization method of the present invention outputs an output signal by performing a filtering process on an input signal based on a filter coefficient, and outputs a constellation from an output signal that is a position on the constellation of the output signal. The output symbol distance, which is the distance from the upper origin, is calculated, and the first output symbol distance calculated from the output signal acquired using the first filter coefficient and the second filter in which the first filter coefficient is updated An output symbol movement amount that is a difference from the second output symbol distance calculated from the output signal acquired using the coefficient is calculated, and a change in the output symbol movement amount with respect to the first output symbol distance is the first output symbol. The update amount of the filter coefficient is calculated so as to be point-symmetric about a predetermined target value of distance.

Also, the equalization method of the present invention outputs an output signal by performing filtering on the input signal based on the filter coefficient, and is an output signal function, where the absolute value of the output signal is the output signal. A point-symmetric function centered on a point equal to a predetermined target value of amplitude calculates the value of the function taken for the output signal, and the value obtained by normalizing the input signal with the square norm The filter coefficient update amount is calculated by multiplying the complex conjugate of the output signal by the norm normalized value, and the value obtained by adding the update amount to the filter coefficient is used as the update filter coefficient. Process.

According to the equalizer of the present invention, the polarization separator using the same, and the equalization method, even if the polarization separator that updates the filter coefficient by the constant envelope algorithm (CMA) is used, the received light Regardless of the optical signal-to-noise ratio of the signal, the constellation distortion of the reproduced received optical signal can be reduced.

It is a block diagram which shows the structure of the equalizer which concerns on embodiment of this invention. It is a flowchart for demonstrating the update process of the filter coefficient in the equalizer which concerns on embodiment of this invention. It is a figure which shows the change of the moving amount | distance of the output symbol with respect to an output symbol distance at the time of using the equalizer which concerns on embodiment of this invention. It is a constellation figure of the received optical signal at the time of using the polarization separator concerning the embodiment of the present invention, and is a case where optical SN ratio is 15 dB. It is a constellation figure of the received optical signal at the time of using the polarization separator concerning the embodiment of the present invention, and is a case where optical SN ratio is 10 dB. It is a block diagram which shows the structure of the related optical receiver used for the optical communication system by an optical digital coherent communication system. It is a block diagram which shows the structure of the related polarization separation part used for a related optical receiver. It is a flowchart for demonstrating the update process of the filter coefficient in a related polarization separation part. It is a figure which shows the change of the moving amount | distance of the output symbol with respect to an output symbol distance in the case of using a related polarization separation part. FIG. 4 is a constellation diagram of a received optical signal obtained by a related optical receiver, in which the optical SN ratio is 15 dB. FIG. 7 is a constellation diagram of a received optical signal obtained by a related optical receiver, in which the optical SN ratio is 10 dB.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing a configuration of an equalizer 100 according to an embodiment of the present invention. The equalizer 100 includes filter means 111 to 114 that perform filter processing based on the filter coefficient, and filter coefficient update means 121 that updates the filter coefficient so that the envelopes of the output signals of the filter means 111 to 114 are constant. 122. That is, the filter coefficient updating means 121 and 122 uses a constant envelope algorithm (Constant Modulus Algorithm: CMA) as an algorithm for updating the filter coefficient.

The filter coefficient updating means 121 and 122 calculate an output symbol distance which is a distance between an output symbol which is a position on the constellation of the output signal and an origin on the constellation, from the output signals of the filter means 111 to 114. Here, the first output symbol distance calculated from the output signal acquired using the first filter coefficient and the output signal acquired using the second filter coefficient after updating the first filter coefficient An output symbol movement amount that is a difference from the second output symbol distance is calculated. Then, the filter coefficient update amount is calculated so that the change of the output symbol movement amount with respect to the first output symbol distance is point-symmetric about a predetermined target value of the first output symbol distance.

With this configuration, according to the equalizer 100 of the present embodiment, the filter coefficient is updated so that the output symbol movement amount becomes equal regardless of the magnitude relationship of the output symbol distance with respect to the predetermined target value. be able to. As a result, the constellation distortion of the output signal can be reduced.

Also, the equalizer 100 can be used as a polarization separator by inputting the received signal obtained by receiving the polarization multiplexed signal light to the equalizer 100 of the present embodiment. That is, the filter means 111 to 114 photoelectrically convert the first received light constituting the polarization multiplexed signal light and the second received light having a polarization direction orthogonal to the polarization direction of the first received light, respectively. One reception signal and a second reception signal can be input. The polarization separator can be used as the polarization separator 240 of the related optical receiver 200 shown in FIG.

In this case, the same effect as described above can be obtained. That is, even when a polarization separator that updates the filter coefficient by a constant envelope algorithm (CMA) is used, the constellation distortion of the reproduced received optical signal is reduced regardless of the optical SN ratio of the received optical signal. can do.

Next, the operation of the equalizer 100 according to this embodiment will be described. The filter units 111 to 114 included in the equalizer 100 perform a filtering process on the input signal 1 and the input signal 2 using filter coefficients set independently for each filter unit. Thereafter, the sum of the outputs of the filter unit 111 and the filter unit 113 is set as an output signal 1, and the sum of the outputs of the filter unit 112 and the filter unit 114 is output as an output signal 2. As the filter means 111 to 114, a general FIR (Finite Impulse Response) filter can be used.

The filter coefficient update means 121 updates the filter coefficients of the filter means 111 and the filter means 113 according to the CMA algorithm. Similarly, the filter coefficient updating unit 122 updates the filter coefficients of the filter unit 112 and the filter unit 114 according to the CMA algorithm.

Next, processing for updating the filter coefficient using CMA will be described. The error function defined by CMA is the same as the equation (1) described above. As described above, J _x (W, W ^H ) is an error function for the output signal 1, and J _y (W, W ^H ) is an error function for the output signal 2.

The filter coefficient updating unit 121 sequentially updates the filter coefficients of the filter unit 111 and the filter unit 113 so that the error function J _x for the output signal 1 is minimized. The filter coefficient updating unit 122 sequentially updates the filter coefficients of the filter unit 112 and the filter unit 114 so that the error function J _y with respect to the output signal 2 is minimized.

The following formulas (5) and (6) show the filter coefficient update formulas in the equalizer 100 of the present embodiment.

FIG. 2 is a flowchart for explaining the process of updating the filter coefficient in the equalizer 100 of the present embodiment.

First, the filter coefficient updating means 121 and 122 calculate the value of the function that the cost function f expressed by the above equation (6) takes on the output signal (step S100). Here the cost function f _'is a function of the output signal E _m' output signal E _m that the absolute value of is equal to the target value r _m of the amplitude of the output signal _{^{(| E m '| = r}} m) around the Is a point-symmetric function. That is, the filter coefficient updating means 121, 122 is a function of the output signal, and uses a point-symmetric function centered on a point where the absolute value of the output signal is equal to the target value of the amplitude of the output signal. Calculate the update amount. In formula (6), in a case where the cost function f is a quadratic function of the output signal E _{m ',} showing a case in which the target value r _m to "1".

Next, the value of the cost function at this time is multiplied by the input signal normalized by the square norm (step S200). Further, the product obtained by multiplying the complex conjugate of the output signal normalized by the norm (step S300) is multiplied by the step parameter (step S400) to calculate the update amount of the filter coefficient. Then, a value obtained by adding the update amount to the filter coefficient is set as a new filter coefficient (update filter coefficient) (step S500), and this update filter coefficient is set in the filter means 111 to 114 (step S600). Through the above processing, the filter coefficient update means 121 and 122 update the filter coefficients of the filter means 111 to 114.

Here, by inputting a received signal obtained by photoelectrically converting the polarization multiplexed signal light to the filter means 111 to 114, it becomes possible to separate and extract two independent received signals that have been polarized and separated. In other words, the polarization separator using the equalizer of the present embodiment includes the first received light constituting the multiplexed signal light and the second received light having a polarization direction orthogonal to the polarization direction of the first received light. The first received signal and the second received signal that have been photoelectrically converted can be input to the filter means.

Next, changes in the output symbol when the equalizer 100 according to the present embodiment recalculates the output symbol using the filter coefficient updated by the CMA will be described. FIG. 3 shows the movement amount (output symbol movement amount) from the output symbol before updating the filter coefficient in this case with respect to the distance (output symbol distance) from the origin of the output symbol before updating the filter coefficient. is there. Here, r _x and r _y (radius of the target circle), which are target values of the amplitude of the output signal, are each “1”.

From FIG. 3, according to the equalizer 100 of the present embodiment using the above update equations (5) and (6), the change of the output symbol movement amount with respect to the output symbol distance is the target values r _x , r _y ( = 1) It can be seen that the point is symmetric about the center. That is, if the absolute value of the difference between the output symbol distance and the target value is equal, the absolute value of the output symbol movement amount is also equal. When the output symbol distance is larger than the target value, the output symbol distance decreases because the output symbol movement amount takes a negative value. On the other hand, when the output symbol distance is smaller than the target value, the output symbol distance increases because the output symbol movement amount takes a positive value. From the above, it can be seen that the output symbol reaches the target value with an equal amount of movement regardless of the magnitude relationship between the output symbol distance before the filter coefficient is updated and the target value.

On the other hand, when the filter coefficient updating formulas (formulas (2) to (4)) by the CMA used in the related polarization separation unit 240 are employed, the movement amount of the output symbol is shown in FIG. Thus, the target value (r _x , r _y = 1) is not point symmetric. That is, in this case, the absolute value of the movement amount of the output symbol is greater when the error between the distance from the origin of the output symbol and the target value is positive than when it is negative. Therefore, as shown in FIGS. 9A and 9B, there is a problem that distortion of the constellation increases.

However, in the equalizer 100 according to the present embodiment, the filter coefficient update formulas shown in the above formulas (5) and (6) are used. In this case, as described above, the amount of movement of the output symbol is point-symmetric with respect to the distance from the origin of the output symbol and the sign of the target value error. Therefore, the force that works so that the output symbol inside the unit circle (target circle with radius 1) moves on the unit circle by the filter coefficient update process, and the output symbol outside the unit circle moves on the unit circle The force that works is zero when the update process is stable. Therefore, the average position of output symbols is almost the same as the ideal symbol point.

FIGS. 4A and 4B show examples of constellation diagrams of received optical signals when the polarization separator according to the present embodiment is used. The QPSK method was used as the optical signal modulation method. 4A shows a case where the optical SN ratio is 15 dB, and FIG. 4B shows a case where the optical SN ratio is 10 dB.

As described above, in the polarization separator according to the present embodiment, the filter coefficient is updated based on the above-described filter coefficient update formulas (formulas (5) and (6)). 4A and 4B, it can be seen that the ideal symbol point is located substantially at the center of the symbol distribution regardless of the optical signal-to-noise ratio of the received optical signal, and a constellation with small distortion is obtained.

As described above, according to the equalizer according to the present embodiment, the polarization separator using the equalizer, and the equalization method, the polarization separator that updates the filter coefficient by the constant envelope algorithm (CMA) is used. Even so, the constellation distortion of the regenerated received optical signal can be reduced regardless of the optical signal-to-noise ratio of the received optical signal.

In the above description, the quadratic function of the output signal E _m ′ shown in Expression (6) is used as the filter coefficient update expression. But not limited to this, the cost function f used in the update equation of the filter coefficients, _'is a function of the output signal E _m' output signal E _m that the absolute value of is equal to the target value r _m of the amplitude of the output signal Any function that is point-symmetric about (| E _m ^′ | = r _m ) may be used. For example, as the cost function f (E _m ^′ , r _m ), a linear function having a slope “−a” such as f (E _m ^′ , r _m ) = − aE _m ′ + ar _m may be used. Furthermore, not only the update formula using these cost functions, but also the filter coefficient that can calculate the update amount of the filter coefficient so that the change of the output symbol movement amount with respect to the output symbol distance is point-symmetric about the target value The update formula can be used.

Further, as an update formula for the filter coefficient, an update formula having a large gradient (differential value) of the movement amount of the output symbol around the target value may be used. As a result, it becomes possible to adapt sensitively to fluctuations in the optimum filter coefficient, and it is possible to shorten the time required for convergence of the polarization separation processing in the polarization separator.

Note that the filter coefficient update formula need not use the same update formula from the start of polarization separation processing until operation. It is possible to use an update formula suitable for each situation, such as high speed required from the start time to stabilization and stability required during operation. For example, when calculating the update amount of the filter coefficient, two or more different functions may be switched and used as the cost function. Thereby, it is possible to optimize the system including the equalizer of the present embodiment and the polarization separator using the equalizer.

In the polarization separator according to the above-described embodiment, the case where the QPSK method is used as the optical signal modulation method has been described as an example. However, the present invention is not limited to this, and the polarization separator of the present embodiment can be applied to an optical signal that can be polarized and separated by CMA even when an optical signal of another modulation method is used. Is possible.

For example, it is known that polarization-demultiplexed QAM signals such as 8QAM and 16QAM can be polarized and separated using an RDE (Radius Directed Equalization) method (see, for example, Non-Patent Document 1). Since this RDE method is characterized as a CMA method having a plurality of target values, the polarization separator according to the present embodiment can be applied.

When an RDE having a plurality of target values is used as the polarization separation algorithm, one (individual target value) is selected from a plurality of target values according to the distance from the origin of the output symbol. Then, the update amount of the filter coefficient is calculated based on the error between the distance from the origin of the output symbol and the selected individual target value. At this time, the update amount of the filter coefficient can be calculated so that the movement amount of the output symbol is point-symmetric with respect to each individual target value.

As described above, even when a modulation method other than QPSK is used, by applying the polarization separator of this embodiment, the constellation distortion of the reproduced received optical signal can be reduced.

Also, if the output symbol is far away from the ideal symbol point, there is a high possibility that large optical noise is added to the output symbol. Therefore, the effect of optical noise can be further reduced by not performing the process of updating the filter coefficient for such output symbols. Specifically, for example, when the output symbol separation distance, which is the distance between the output symbol acquired using the first filter coefficient and the ideal symbol point, is smaller than a predetermined value, the calculated update amount of the filter coefficient is used. To update the first filter coefficient. On the other hand, when the output symbol separation distance is equal to or greater than the predetermined value, the process of updating the first filter coefficient using the calculated update amount of the filter coefficient may not be performed.

The present invention is not limited to the above-described embodiment, and various modifications are possible within the scope of the invention described in the claims, and it goes without saying that these are also included in the scope of the present invention. Nor.

This application claims priority based on Japanese Patent Application No. 2013-125513 filed on June 14, 2013, the entire disclosure of which is incorporated herein.

DESCRIPTION OF SYMBOLS 100 Equalizer 111-114 Filter means 121, 122 Filter coefficient update means 200 Related optical receiver 210 90 degree hybrid 221-224 Optical detector 231-234 Analog-digital converter (ADC)
240 Polarization Separation Units 241 to 244

Filter Units

245, 246 Filter

Coefficient Update Units

251, 252 Optical Carrier Frequency Deviation / Optical Phase

Deviation Compensation Units

261, 262 Symbol Identification Units

Claims

Filter means for performing filtering based on the filter coefficient;
Filter coefficient updating means for updating the filter coefficient so that the envelope of the output signal of the filter means is constant;
The filter coefficient update means includes
From the output signal, an output symbol distance that is a distance between an output symbol that is a position on the constellation of the output signal and an origin on the constellation is calculated,
The first output symbol distance calculated from the output signal acquired using the first filter coefficient and the output signal calculated from the output signal acquired using the second filter coefficient obtained by updating the first filter coefficient. 2 calculates an output symbol movement amount that is a difference from the output symbol distance of 2;
An update amount of the filter coefficient is calculated such that a change in the output symbol movement amount with respect to the first output symbol distance is point-symmetric about a predetermined target value of the first output symbol distance. .
The equalizer of claim 1,
The target value includes a plurality of different individual target values,
The filter coefficient updating means selects one individual target value based on the first output symbol distance, and a change in the output symbol movement amount with respect to the first output symbol distance is the selected individual target value. An equalizer that calculates an update amount of the filter coefficient so as to be point-symmetric with respect to.
Filter means for performing filtering based on the filter coefficient;
Filter coefficient updating means for updating the filter coefficient so that the envelope of the output signal of the filter means is constant;
The filter coefficient updating means is a function of the output signal, and uses the point symmetric function centered on a point where the absolute value of the output signal is equal to a predetermined target value of the amplitude of the output signal. An equalizer that calculates the amount of coefficient updates.
The equalizer according to claim 3, wherein
The function is a quadratic function equalizer.
The equalizer according to claim 3, wherein
The function is a linear function equalizer.
The first received light and the second received light obtained by photoelectrically converting the first received light constituting the polarization multiplexed signal light and the second received light having a polarization direction orthogonal to the polarization direction of the first received light, respectively. Filter means for inputting a received signal and performing filter processing based on a filter coefficient;
Filter coefficient updating means for updating the filter coefficient so that the envelope of the output signal of the filter means is constant;
The filter coefficient update means includes
From the output signal, an output symbol distance that is a distance between an output symbol that is a position on the constellation of the output signal and an origin on the constellation is calculated,
The first output symbol distance calculated from the output signal acquired using the first filter coefficient and the output signal calculated from the output signal acquired using the second filter coefficient obtained by updating the first filter coefficient. 2 calculates an output symbol movement amount that is a difference from the output symbol distance of 2;
The amount of update of the filter coefficient is calculated so that a change in the amount of movement of the output symbol with respect to the first output symbol distance is point-symmetric about a predetermined target value of the first output symbol distance. .
An output signal is output by performing a filtering process on the input signal based on the filter coefficient,
From the output signal, an output symbol distance that is a distance between an output symbol that is a position on the constellation of the output signal and an origin on the constellation is calculated,
The first output symbol distance calculated from the output signal acquired using the first filter coefficient and the output signal calculated from the output signal acquired using the second filter coefficient obtained by updating the first filter coefficient. 2 calculates an output symbol movement amount that is a difference from the output symbol distance of 2;
An equalization method for calculating an update amount of the filter coefficient so that a change in the output symbol movement amount with respect to the first output symbol distance is point-symmetric about a predetermined target value of the first output symbol distance .
The equalization method according to claim 7,
When the output symbol separation distance, which is the distance between the output symbol acquired using the first filter coefficient and the ideal symbol point, is smaller than a predetermined value, the first filter is used using the update amount of the filter coefficient. Update the coefficient,
If the output symbol separation distance is greater than or equal to a predetermined value, the process of updating the first filter coefficient using the update amount of the filter coefficient is not performed.
An output signal is output by performing a filtering process on the input signal based on the filter coefficient,
A function of the output signal, which is a point-symmetric function centered on a point where the absolute value of the output signal is equal to a predetermined target value of the amplitude of the output signal. Calculate the value,
The update value of the filter coefficient is calculated by multiplying the value of the function by a value obtained by normalizing the input signal with a square norm and a value obtained by normalizing the complex conjugate of the output signal with the norm,
A value obtained by adding the update amount to the filter coefficient is an update filter coefficient,
An equalization method for performing the filtering process using the updated filter coefficient.
The equalization method according to claim 9,
An equalization method that uses two or more different functions as the function when the update amount of the filter coefficient is calculated.