JP5761414B1 - Optical signal receiving apparatus and FFE optimization method provided in optical signal receiving apparatus - Google Patents

Abstract

The effect of noise added by FFE is reduced without additional components. In step S1, tap coefficients are acquired by using a tap coefficient matrix W, an output signal matrix Y, and a delay matrix U so that a training signal before transmission is the same as an output signal. In S2, the tap coefficient acquired in S1 is set, the training signal before transmission is compared with the output signal, and the sub-peak is acquired. In S3, it is determined whether or not reception is possible with the set tap coefficient. As a result of the determination in S3, when it is determined that reception is impossible, a step (S4) of inserting a row corresponding to the sub-peak into the output signal matrix Y and the delay matrix U, and the tap coefficient matrix W obtained in S4, Using the output signal matrix Y and the delay matrix U, a process (S5) of acquiring tap coefficients so that the training signal before transmission is the same as the output signal is performed. After S5, S3 is performed again. [Selection] Figure 4

Description

  The present invention relates to an optical signal receiving apparatus including an FFE (Feed Forward Equalizer), and more particularly to an optical signal receiving apparatus suitable for use in an optical access network and an FFE optimization method.

  In optical access networks that receive a wide variety of service requests from users, traffic demands are unevenly distributed in time and space, and a new network architecture for performing highly efficient network control will be required in the future.

  As an example of a network architecture that enables the provision of optimal communication capacity and integration of heterogeneous services, the wavelength of the conventional PON (Passive Optical Network) technology based on Time Division Multiplexing (TDM) technology, A study of WDM / TDM-PON in which wavelength division allocation (WDM) technology is added and wavelength allocation is performed according to traffic usage on the network is underway (see Non-Patent Document 1, for example). .

  As a result, it is possible to control the driving of a station side terminal (OLT: Optical Line Terminal) according to the traffic, and it is expected to reduce the power consumption of the center office by improving the band utilization efficiency. The purpose of this WDM / TDM-PON is to widen the cover area by increasing the capacity and increasing the number of branches, and for this purpose, OLT integration by extending the transmission distance is required.

  As a technique for extending the transmission distance, a chromatic dispersion compensation technique is used in order to cope with a problem that a waveform deteriorates due to the influence of chromatic dispersion. As a chromatic dispersion compensation technique, there is an electrical dispersion compensation (EDC) technique (see, for example, Patent Document 1). The EDC technique is a technique for correcting a distortion of a signal waveform distorted by chromatic dispersion by digital signal processing.

  One of EDC techniques is FFE (see, for example, Patent Document 2). FFE is a technique for correcting a waveform by branching a received optical signal, adding a delay, weighting, and then superimposing received signals.

JP 2006-287695 A JP 9-326728 A

Hirotaka Nakamura et al. “Wavelength Tunable WDM / TDM-PON for Flexible Service Upgrade” 2010 IEICE Society Conference B-10-40

  FIG. 1 shows the result of simulation of 80 km transmission with respect to FFE. FIG. 1 is a diagram illustrating a time waveform by simulation. In FIG. 1, the horizontal axis indicates time (unit: ps), and the vertical axis indicates signal intensity normalized by the peak of the time waveform before transmission. In FIG. 1, the time waveform before transmission is indicated by I, the time waveform when FFE is not used after transmission (without FFE) is indicated by II, and the time waveform when FFE is used after transmission (with FFE) is indicated by III. Show.

  As shown in FIG. 1, the time waveform (II) without FFE is degraded to a waveform having a wider time width than the time waveform (I) before transmission. On the other hand, the time waveform (III) with FFE has the same time width as that before transmission (I).

  Here, in the time waveform (III) with FFE after transmission, the part other than the signal should be zero level originally, but a sub-peak occurs in the adjacent part (indicated by A in the figure). Yes. For this reason, there is a problem that a noise component due to the sub-peak is added.

  2A to 2C show eye diagrams obtained by simulation. FIG. 2A is an eye diagram when FFE is not used after transmission (no FFE). In the case of no FFE, the waveform has a wide time width due to wavelength dispersion. As a result, the adjacent bit invades and the eye opening is divided into upper and lower portions, and reception is impossible.

  FIG. 2B is an eye diagram when FFE is used after transmission (with FFE). When FFE is present, an eye opening can be confirmed, but a clear waveform is not obtained due to the effect of the sub-peak. Noise due to the sub-peak can be removed by inserting an LPF (Low Pass Filter).

  FIG. 2C is an eye diagram when FFE is used after transmission and when a 10 GHz LPF is further inserted (with FFE-with LPF). By inserting the LPF, the eye opening is improved as compared with the FFE present-no LPF illustrated in FIG.

  However, an LPF that is an additional component is required, leading to an increase in cost.

  The present invention has been made in view of the above-described problems. An object of the present invention is an optical signal receiving apparatus using FFE, which reduces the influence of noise added by FFE without an additional component, and optimization of FFE included in the optical signal receiving apparatus It is to provide a method.

To achieve the above object, an optical signal receiving apparatus of the present invention branches the received optical signal into the number of taps L, the addition of delay, by adding after the weighting given by the tap coefficient w _x Output an optical signal processing unit for generating a signal, acquires the power strips coefficients, and a controller for notifying the optical signal processing unit. The control unit includes a matrix generation means for generating a tap coefficient matrix W indicating the tap coefficient w _x , an output signal matrix Y indicating the output signal, and a delay matrix U indicating a delay in the optical signal processing unit, and an output signal before transmission Approximate calculation means for acquiring tap coefficients by performing approximate calculation for the matrix equation Y = U · W in accordance with the training signal, and row insertion for inserting a row corresponding to the sub-peak into the output signal matrix Y and the delay matrix U Means, a training signal before transmission, and an output signal are compared to obtain a sub-peak, and a sub-peak obtaining means for judging whether or not the optical signal receiving apparatus can receive signals. If reception is impossible as a result of determination by the reception availability determination unit, the row insertion unit inserts a row corresponding to the sub-peak into the output signal matrix Y and the delay matrix U, and then the approximate calculation unit calculates the tap coefficient. The process of obtaining and setting the tap coefficient in the optical signal processing unit is repeated until reception becomes possible.

  Moreover, according to the optimization method of this invention, it comprises the following processes.

First, in the first process, using the tap coefficient matrix W, the output signal matrix Y, and the delay matrix U , the output signal is matched with the training signal before transmission , and an approximate calculation for the matrix equation Y = U · W is performed. to get the tap coefficients by. Next, in the second process, the tap coefficient acquired in the first process is set, the training signal before transmission is compared with the output signal, and the sub-peak is acquired. Next, in the third process, it is determined whether or not reception is possible with the set tap coefficient. As a result of the determination in the third process, if it is determined that reception is impossible, the following fourth process and fifth process are performed.

In the fourth process, rows corresponding to sub-peaks are inserted into the output signal matrix Y and the delay matrix U. In the fifth process, using the tap coefficient matrix W, the output signal matrix Y, and the delay matrix U obtained in the fourth process, the output signal is matched with the training signal before transmission , and the matrix equation Y = U · W Tap coefficients are obtained by performing approximate calculation on . After the fifth process, the third process is performed again , and the third, fourth, and fifth processes are repeated until it is determined that reception is possible as a result of the determination in the third process.

  According to the optical signal receiving apparatus and the FFE optimization method of the present invention, the rows corresponding to the sub-peaks are inserted in the matrix calculation for the influence of the sub-peaks generated when the FFE is simply used. This increases the contribution of the sub-peak in the approximate calculation, so that the influence of the sub-peak can be easily reduced without additional components.

It is a figure which shows the time waveform by simulation. A simulated eye diagram is shown. It is a schematic diagram of the optical signal receiver of this embodiment. It is a figure which shows the processing flow of the optimization method. It is a figure which shows the eye diagram obtained using PRBS.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shape, size, and arrangement relationship of each component are merely schematically shown to the extent that the present invention can be understood. In the following, a preferred configuration example of the present invention will be described. However, numerical conditions and the like are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many changes or modifications that can achieve the effects of the present invention can be made without departing from the scope of the configuration of the present invention.

  With reference to FIG. 3, an embodiment of the optical signal receiving apparatus of the present invention will be described. FIG. 3 is a schematic diagram of the optical signal receiving apparatus of this embodiment. The optical signal receiving apparatus of this embodiment includes an optical signal processing unit 100 and a control unit 200.

  The optical signal processing unit 100 branches the received optical signal, weights it by adding a delay, and then superimposes the received signals. The optical signal processing unit 100 is configured to include L weighting circuits 120-1 to L and L-1 delay circuits 110-1 to (L-1), where L is the number of taps. Here, the tap number L corresponds to the number of branches of the optical signal.

The input signal u (t) is branched into two, one being sent to the first weighting circuit 120-1 and the other being sent to the first delay circuit 110-1. The first delay signal delayed by the first delay circuit 110-1 is branched into two branches, one being sent to the second weighting circuit 120-2 and the other being sent to the second delay circuit 110-2. It is done. Similarly, the (m−1) th delay signal delayed by the (m−1) th delay circuit 110- (m−1) is branched into two, one being sent to the mth weighting circuit 120-m, and the other being It is sent to the m-th delay circuit 110-m. The (L-1) th delay signal delayed by the (L-1) th delay circuit 110- (L-1) is sent to the Lth weighting circuit 120-L. Here, in each delay circuit 110, a delay of T / K is given to the bit period T and the sampling rate K. Each weighting circuit 120 is given a weight of a tap coefficient w _x (x is an integer of 1 or more and L or less). The signals weighted by each weighting circuit 120 are added by the adding circuit 130 and output as an output signal y (t). In the following description, the output signal is also referred to as an FFE signal.

The output signal y (t) is also sent to the control unit 200. Further, the tap coefficient setting signal generated by the control unit 200 is sent to the optical signal processing unit 100. The optical signal processing unit 100 sets the tap coefficient w _x of each weighting circuit 120 to a value indicated by the tap coefficient setting signal. The optical signal processing unit 100 described above can be configured in the same way as any conventionally known suitable FFE.

  The control unit 200 includes a tap coefficient acquisition unit 210, a sub peak acquisition unit 220, a tap coefficient setting unit 230, and a reception availability determination unit 240. The tap coefficient acquisition unit 210 includes a matrix generation unit 212, a row insertion unit 214, and an approximate calculation unit 216. The control unit 200 can be configured using, for example, a programmable chip. Each function means is realized by the control unit 200 executing the program. In addition, the control unit 200 includes storage means 250 such as a RAM (Random Access Memory) and a ROM (Read Only Memory). The storage means 250 stores processing results of the respective functional means of the control unit 200, output signals from the optical signal processing unit 100, and the like so as to be read and rewritten. The storage means 250 stores a training signal having a predetermined pattern as a reference signal so that it can be read out. Details of the operation of each functional means will be described later.

Next, with reference to FIGS. 3 and 4, an FFE provided in the optical signal receiving apparatus, that is, a method for optimizing the tap coefficient w _x of the weighting circuit will be described. FIG. 4 is a diagram showing a processing flow of the optimization method.

First, in the first step (S1), a tap coefficient w _x that makes the output signal y (t) the same as a known training signal is acquired. The training signal received via the transmission path by the optical signal receiving device becomes the input signal u (t) to the optical signal processing unit 100. The input signal u (t) is deteriorated by transmission with respect to the training signal before transmission. The output signal y (t) obtained as a result of processing in the optical signal processing unit 100, that is, the FFE is expressed by the following equation (1).

The input signal u (t) is deteriorated by transmission, but the output signal y (t) is deteriorated by determining the tap coefficient so that the output signal y (t) matches the reference signal s (t) that is the training signal before transmission. Can be reduced. Here, t is considered as an integer between 1 and 2L-1. In this case, the expression (1) is expressed as Y = U using the output signal matrix Y of 2L-1 rows and 1 column, the delay matrix U of 2L-1 rows and L columns, and the tap coefficient matrix W of L rows and 1 columns. • Represented as W. Here, k of the output signal matrix Y indicating the output signal, 1 component Y _{k, 1} is y (k). The k and x components U _{k and x} of the delay matrix U indicating the degree of delay in the delay circuit 110 are u (−x + t + 1) × T / K. Further, x of the tap coefficient matrix W indicating the tap coefficient, and 1 component W _{x, 1} are w _x .

  The matrix generation means 212 generates these matrices Y, U, and W.

  As an example, a case where the number of taps L is 3 will be described. In this case, the output signal matrix Y has 5 rows and 1 column, and y (1) to y (5) are expressed by the following equations (2) to (6).

  Here, since the number of taps L is 3, only u (T / K), u (2T / K), and u (3T / K) are considered for the input signal, and u (−T / K), u (0), u (4T / K) and u (5T / K) are all considered as 0.

  As a result, the above equations (2) to (6) are expressed by the following matrix equation (7).

The tap coefficients w ₁ , w ₂ and w ₃ are the solution of this matrix equation (7). That is, the tap coefficients w ₁ , w ₂ and w ₃ are given by the following matrix equation (8).

  Therefore, the approximate calculation means 216 obtains these solutions by the least square method.

After the tap coefficient w _x is obtained by the approximate calculation means 216, the tap coefficient setting means 230 generates a tap coefficient setting signal and sends it to the optical signal processing unit 100. The optical signal processing unit 100 sets the tap coefficient w _x specified by the tap coefficient setting signal in each weighting circuit 120.

  Next, in the second step (S2), the sub-peak acquisition means 220 acquires the sub-peak by comparing the reference signal with the output signal, that is, comparing the training signal before transmission with the case where FFE is used after transmission. To do.

  Next, in the third step (S3), the reception availability determination unit 240 determines whether reception is possible. The reception permission / inhibition determination can be performed using, for example, a pseudo-random bit sequence (PRBS: Pseudo-Random Bit Sequence).

  FIGS. 5A to 5H are diagrams showing eye diagrams obtained using PRBS.

The reception availability determination unit 240 acquires a Q value indicating the eye opening ratio from the eye diagram. For example, the Q value is obtained by using the average values _SH and S _L of the signal strengths of the H level signal and the L level signal and their standard deviations σ _H and σ _L , and Q = (S _H −S _L ) / It is given by (σ _H + σ _L ).

  The reception availability determination unit 240 compares the Q value with a threshold value stored in advance in the storage unit 250. When the Q value is larger than the threshold, the reception availability determination unit 240 determines that reception is possible, and when the Q value is smaller than the threshold, determines that reception is not possible. If reception is not possible, the effect of sub-peaks can be considered. Therefore, in this case, in S40, the row insertion unit 214 acquires the tap coefficient again after inserting the row corresponding to the sub-peak into the matrix equation. For example, when a sub peak exists in y (1) with respect to y (3) which becomes a peak, a row corresponding to y (1) is added to obtain a new matrix equation (9).

This corresponds to removing the influence of the sub-peak by increasing the weighting in the least square method of the row in which the sub-peak occurs.

Thereafter, in the fifth step (S5), the approximate calculation means 216 performs the same approximate calculation as in the first step on the matrix equation to which the row has been added, and acquires the tap coefficient w _x .

  After acquiring the tap coefficient, the reception determination in S3 is performed using the new tap coefficient obtained in S5. As a result, the eye diagram shown in FIG. 5 is obtained. The process from S3 to S5 is repeated until reception is possible. 5A to 5H are eye diagrams obtained during this iteration. FIG. 5A is an eye diagram when no row is inserted. 5B to 5H insert rows 1, 3, 5, 7, 9, 11 and 15 where subpeaks are generated, respectively, and rows s where subpeaks are generated are 2, 4, 6, 8, 10, It is an eye diagram when there are 12 and 16 lines.

  If reception is possible as a result of the reception enable / disable determination in S5, a tap coefficient that enables reception is set in S6, and the process ends. In this case, for example, when the threshold value is 12.5 [dB], Q = 12.9 [dB] when s = 8, so the tap coefficient obtained when s = 8 is adopted.

  In addition, although the example which complete | finishes a process was demonstrated here, when reception became possible with Q value using PRBS, it is not limited to this.

  The tap coefficient value obtained in S1 or S5 and the Q value may be associated with each other and stored in the storage unit, and set to the tap coefficient that maximizes the Q value. In this case, since s = 10 and Q = 13.02 [dB], when s = 12 and Q = 12.98 [dB], the tap coefficient obtained when s = 10 is adopted. The Further, whether or not reception is possible may be determined by a subsequent signal processing unit or the like.

DESCRIPTION OF SYMBOLS 100 Optical signal processing part 110 Delay circuit 120 Weighting circuit 130 Adder circuit
200 Control unit
210 Tap coefficient acquisition means
212 Matrix Generation Unit 214 Row Insertion Unit 216 Approximate Calculation Unit
220 Sub-peak acquisition means 230 Tap coefficient setting means 240 Receivability determination means 250 Storage means

Claims (2)

Branches the received optical signal to the number of taps L, added to each other bit period T is the time difference divided by the sampling rate K delay, generates an output signal by adding after the weighting given by the tap coefficient w _x An optical signal processing unit;
Get the power strips coefficient, an optical signal receiving apparatus Ru and a control unit configured to notify the optical signal processing unit,
The controller is
Matrix generating means for generating a tap coefficient matrix W indicating a tap coefficient w _x , an output signal matrix Y indicating an output signal, and a delay matrix U indicating a delay amount in the optical signal processing unit;
Approximate calculation means for acquiring the tap coefficient by matching the output signal with the training signal before transmission and performing approximate calculation on the matrix equation Y = U · W ;
A row insertion means for inserting a row corresponding to a subpeak in the output signal matrix Y and the delay matrix U;
And training signal before the transmission, by comparing the output signal, and a reception determination means for determining receivability at the sub-peak acquiring means and the optical signal receiving apparatus obtains the sub-peak,
As a result of determination by the reception availability determination means, if reception is impossible, the row insertion means inserts a row corresponding to a sub-peak into the output signal matrix Y and the delay matrix U, and then the approximate calculation means includes: An optical signal receiving apparatus , wherein a tap coefficient is acquired and the process of setting the tap coefficient in the optical signal processing unit is repeated until reception is possible . Using the tap coefficient matrix W, the output signal matrix Y, and the delay matrix U , the output signal is matched with the training signal before transmission, and the tap coefficient is obtained by performing approximate calculation on the matrix equation Y = U · W. 1 process,
A second step of setting a tap coefficient acquired in the first step, comparing a training signal before transmission with an output signal, and acquiring a sub-peak;
A third step of determining whether or not reception is possible with the set tap coefficient;
As a result of the determination in the third step, it is performed when it is determined that reception is impossible.
A fourth step of inserting a row corresponding to the sub-peak into the output signal matrix Y and the delay matrix U;
The obtained in the fourth step, the tap coefficient matrix W, using the output signal matrix Y and delay matrices U, the output signal to match a training signal before the transmission, the approximate calculation for the matrix equation Y = U · W And a fifth process of obtaining the tap coefficient by performing
After the fifth step, we have row the third step again,
The optimization method , wherein the third, fourth, and fifth steps are repeated until it is determined that reception is possible as a result of the determination in the third step .

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