Optical receiver and program

Abstract

To enable the number of taps of each FIR filter of an optical receiver to be approximate.SOLUTION: An optical receiver comprises: m-th or the like means of generating an m-th transmission signal (m is an integer number from 1 to N) on the basis of a signal from a first reception signal to an N-th reception signal. The m-th or the like means comprises: an n-th pass for generating an m-th transmission signal component by processing the n-th reception signal (n is an integer number from 1 to N); and synthesis means of generating the m-th transmission signal by synthesizing the m-th transmission signal component generated on the basis of a signal from the first pass to the N-th pass. The n-th pass contains filter means and delay means. A width of a time window of the filter means of a pass that is different from the m-th pass from the first pass to the N-th pass is set on the basis of a delay difference between a first transportation delay of an n-th transmission light from an optical transmitter to the optical receiver and a second transportation delay of the m-th transmission light.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present disclosure relates to optical receivers in optical communication systems. [Background technology]

[0002] Patent Document 1 discloses a spatial multiplexing optical communication system using a coupled multi-core optical fiber. According to Patent Document 1, in order to reduce the signal processing load in the optical receiver, the number of taps of each FIR (finite impulse response) filter in the MIMO (multiple-input multiple-output) equalizer of the optical receiver is set within a predetermined range. The number of taps of the FIR filter corresponds to the width of the time window for filtering. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2017-152811 Summary of the Invention [Problem to be solved by the invention]

[0004] Since the propagation delay difference between each signal light transmitted in a spatial multiplexing optical communication system is different, the optimal number of taps for each FIR filter is different. In the configuration described in Patent Document 1, the number of taps for each FIR filter is set within a predetermined range without considering the propagation delay difference between each signal light, so the number of taps is not optimized.

[0005] The present disclosure provides a technique for optimizing the number of taps in each FIR filter of an optical receiver. [Means for solving the problem]

[0006] According to one aspect of the present disclosure, an optical receiver receives, as first to N-th received light, first to N-th transmitted light that an optical transmitter generates based on first to N-th transmitted signals (N is an integer of 2 or more) and transmits to an optical transmission line, the first to N-th transmitted light, the first to N-th transmitted light including receiving means for outputting first to N-th received signals based on the first to N-th received light, and m-th equalization means for generating an m-th transmitted signal (m is an integer from 1 to N) based on the first to N-th received signals, the m-th equalization means processing the n-th received signal (n is an integer from 1 to N) to generate the m-th transmitted signal component. and combining means for generating the m-th transmission signal by combining the m-th transmission signal components generated in each of the first to N-th paths, wherein the n-th path includes filter means for filtering the n-th reception signal and delay means provided upstream or downstream of the filter means, and a width of a time window of the filter means of a path other than the m-th path among the first to N-th paths is set based on a delay difference between a first propagation delay of the n-th transmission light from the optical transmitter to the optical receiver and a second propagation delay of the m-th transmission light from the optical transmitter to the optical receiver. The delay amount in the delay means of the nth path is set based on a delay difference between the first propagation delay and a maximum value of the propagation delay from the first transmission light to the Nth transmission light from the optical transmitter to the optical receiver. are. [Effects of the Invention]

[0007] According to the present disclosure, the number of taps in each FIR filter of an optical receiver can be made appropriate. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram illustrating the configuration of a spatial multiplexing optical communication system. [Figure 2] FIG. 1 is a diagram illustrating the configuration of an optical receiver. [Figure 3] Configuration diagram of a MIMO equalizer. [Figure 4] FIG. 10 is a diagram illustrating the configuration of the mth equalization unit. [Figure 5] A diagram showing the delay difference of three transmitted beams. [Figure 6] 4A and 4B are diagrams showing examples of the width of the time window of each FIR filter in each equalization unit. [Figure 7] 5A and 5B are diagrams showing examples of the width of a time window of each delay circuit in each equalization unit; DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention as claimed, and not all combinations of features described in the embodiments are necessarily essential to the invention. Two or more of the features described in the embodiments may be arbitrarily combined. Furthermore, the same reference numerals are used for the same or similar components, and redundant explanations will be omitted.

[0010] FIG. 1 shows a spatial multiplexing optical communication system used to explain this embodiment. An optical transmitter 1 transmits N signals (N is an integer equal to or greater than 2) each carrying information to an optical receiver 2 via an optical transmission line 3. Hereinafter, the N signals (electrical domain) in the optical transmitter 1 will be referred to as the first transmission signal to the Nth transmission signal. The optical transmitter 1 converts the nth transmission signal (n is an integer from 1 to N) into an nth transmission light, spatially multiplexes the first transmission light to the Nth transmission light, and transmits the multiplexed light to the optical receiver 2. Any spatial multiplexing method may be used. For example, if the optical transmission line 3 has a multimode optical fiber, the optical transmitter 1 converts each of the first transmission light to the Nth transmission light into a different propagation mode and outputs the multiplexed light to a core of the multimode optical fiber. Furthermore, for example, if the optical transmission line 3 has a coupled multi-core optical fiber, the optical transmitter 1 outputs the nth transmission light to one core of the coupled multi-core optical fiber.

[0011] 2 is a configuration diagram of the optical receiver 2. The optical receiver 2 receives spatially multiplexed first to Nth received lights from the optical transmission line 3. The nth received light corresponds to the nth transmitted light. For example, if the optical transmission line 3 has a multimode optical fiber, the nth received light is received light in the same propagation mode as the nth transmitted light. Furthermore, if the optical transmission line 3 has a coupled multi-core optical fiber, the nth received light is received light output from the core to which the nth transmitted light is input. A receiving unit 21 of the optical receiver 2 demodulates or photoelectrically converts the nth received light to output an nth received signal.

[0012] Due to crosstalk in spatial multiplexing, the nth received light contains a component of the nth transmitted light as well as a component of the transmitted light that is different from the nth transmitted light. In other words, the nth received light contains components of each of the first to Nth transmitted lights. Therefore, the nth received signal also contains a component of each of the first to Nth transmitted signals. In other words, the first to Nth received signals each contain a component of the mth transmitted signal (m is an integer from 1 to N) (hereinafter referred to as the mth transmitted signal component). For this reason, the MIMO equalizer 22 performs MIMO processing to extract and combine the mth transmitted signal components contained in each of the first to Nth received signals. The MIMO equalizer 22 outputs a signal obtained by combining the mth transmitted signal components as the mth transmitted signal.

[0013] 3 is a configuration diagram of the MIMO equalizer 22. The MIMO equalizer 22 has N equalization sections, namely, first to Nth equalization sections. The mth equalization section generates and outputs the mth transmission signal based on the first to Nth received signals. The first to Nth equalization sections have basically the same configuration.

[0014] FIG. 4 shows the configuration of the mth equalization unit. The mth equalization unit has first to Nth paths for processing the first to Nth received signals, respectively. The nth path has a delay circuit 5 and an FIR filter 6. The nth received signal is delayed by the delay circuit 5 of the nth path and filtered by the FIR filter 6. The FIR filter 6 of the nth path extracts the mth transmission signal component included in the nth received signal. The combiner 7 generates the mth transmission signal by combining (adding) the N mth transmission signal components received from the first to Nth paths. The delay circuit 5 is provided to adjust the time difference between the mth transmission signal components from the first to Nth paths. Note that, although the delay circuit 5 is arranged upstream of the FIR filter 6 in FIG. 4, the delay circuit 5 may also be arranged between the FIR filter 6 and the combiner 7. In this case, the delay circuit 5 applies a delay not to the nth received signal, but to the mth transmission signal component extracted from the nth received signal by the FIR filter 6.

[0015] In the following, assuming N=3, the width of the time window (the number of taps) of the FIR filter 6 provided in the MIMO equalizer 22 for outputting the first to third transmission signals and the amount of delay provided by the delay circuit 5 will be described. Since N=3, the optical transmitter 1 transmits first to third transmission lights based on the first to third transmission signals to the optical transmission line 3. The propagation delays of the first to third transmission lights from the optical transmitter 1 to the optical receiver 2 are different. For example, if the optical transmission line 3 has a multimode optical fiber, the propagation modes of the first to third transmission lights will be different, and the propagation delays of the first to third transmission lights in the optical transmission line 3 will be different due to the difference in propagation modes. Therefore, the propagation delays of the first to third transmission signals from the optical transmitter 1 to the optical receiver 2 will also differ depending on the propagation delays of the first to third transmission lights. In the following explanation, it is assumed that the propagation delay from the optical transmitter 1 to the optical receiver 2 increases in the order of the first transmitted light, the second transmitted light, and the third transmitted light. More specifically, as shown in Fig. 5, it is assumed that the second transmitted light (second transmitted signal) has a delay A greater than that of the first transmitted light (first transmitted signal), and the third transmitted light (third transmitted signal) has a delay B greater than that of the second transmitted light (second transmitted signal).

[0016] FIG. 6 shows the width of the time window of the FIR filter 6 provided in each path of the first to third equalization units of the MIMO equalizer 22. The kth equalization unit (k is an integer from 1 to 3) outputs the kth transmission signal. The kth path processes the kth reception signal. According to FIG. 6, the second path of the first equalization unit uses an FIR filter 6 with a time window width of A. The reason for using the FIR filter 6 with a time window width of A in the second path of the first equalization unit, in other words, the reason for using the FIR filter 6 with a time window width of A to extract the first transmission signal component from the second reception signal, will be explained below.

[0017] Due to crosstalk from the first transmitted light to the second transmitted light, a component of the first transmitted light transmitted by the optical transmitter 1 at a certain timing is included in the second transmitted light over a period A corresponding to the delay difference between the first transmitted light and the second transmitted light. Therefore, the first transmitted signal component corresponding to the first transmitted light transmitted by the optical transmitter 1 at a certain timing is included in the second received signal over the period A. In order to extract the first transmitted signal component included in the second received signal over this period A, an FIR filter 6 with a time window width of A is used for the second received signal. For the same reason, an FIR filter 6 with a time window width of A+B corresponding to the delay difference between the first transmitted light and the third transmitted light is used for the third path of the first equalization unit.

[0018] Furthermore, an FIR filter 6 with a time window width of X1 is used for the first path of the first equalization unit. X1 is a predetermined value, and is determined, for example, to be equal to or larger than the delay difference due to the polarization of the first transmitted light in the optical transmission line 3. Specifically, the value of X1 is determined so that it is equal to or larger than the maximum value of the delay difference between the first transmitted light of the first polarization and the first transmitted light of the second polarization. Note that the delay difference due to differences in polarization is much smaller than the delay difference (A, B, etc.) of different signal lights in spatial multiplexing, so the delay difference due to differences in polarization can be ignored in the FIR filters 6 used for the second and third paths of the first equalization unit. In the first equalization unit, the time window width of the FIR filter 6 for the first path is the smallest.

[0019] The same applies to the time window width of the FIR filter 6 in each path of the second equalization unit and the third equalization unit. That is, the first path of the second equalization unit uses an FIR filter 6 with a time window width of A, the second path uses an FIR filter 6 with a time window width of X2, and the third path uses an FIR filter 6 with a time window width of B. Furthermore, the first path of the third equalization unit uses an FIR filter 6 with a time window width of A+B, the second path uses an FIR filter 6 with a time window width of B, and the third path uses an FIR filter 6 with a time window width of X3.

[0020] To state the above more generally, the width of the time window of the FIR filter 6 of the n-th path, which is different from the m-th path among the 1st to Nth paths of the m-th equalization unit, is set based on the delay difference between the first propagation delay of the n-th transmitted light and the second propagation delay of the m-th transmitted light. Furthermore, the width of the time window of the FIR filter 6 of the m-th path of the m-th equalization unit is set to a predetermined value. Alternatively, the width of the time window of the FIR filter 6 of the m-th path of the m-th equalization unit is set based on the delay difference due to the difference in polarization of the m-th transmitted light. The width of the time window of the FIR filter 6 of the m-th path of the m-th equalization unit is the smallest among the widths of the time windows of the FIR filters 6 of the 1st to Nth paths.

[0021] FIG. 7 shows the delay amount provided by the delay circuit 5 provided in each path of the first to third equalization units of the MIMO equalizer 22. As shown in FIG. 7, the delay amount provided by the delay circuit 5 of the n-th path is the same for each of the first to third equalization units. First, a delay circuit 5 that provides a delay amount of (A+B) / 2 is used for the first path, a delay circuit 5 that provides a delay amount of B / 2 is used for the second path, and a delay circuit 5 that provides a delay amount of 0 is used for the third path. The delay circuits 5 are provided to compensate for differences in delay between the transmission signal components extracted from each received signal. In this embodiment, as shown in FIG. 7, a delay circuit 5 that provides a delay amount corresponding to half the delay difference between the third transmitted light, which has the largest delay, and the n-th transmitted light is provided for the n-th path. Note that, since there is no need to provide a delay to the third received signal, the delay circuit 5 for the third received signal may be omitted. The delay amounts provided by the delay circuits 5 of the first to Nth paths of the same equalization unit may be the values ​​shown in Fig. 6 plus a predetermined value D. In this case, a delay circuit 5 that provides a delay amount of D+((A+B) / 2) is used for the first path, a delay circuit 5 that provides a delay amount of D+(B / 2) is used for the second path, and a delay circuit 5 that provides a delay amount of D is used for the third path.

[0022] To state the above more generally, the delay amount in the delay circuit 5 of the nth path is set based on the delay difference between the first propagation delay of the nth transmitted light and the maximum propagation delay from the first transmitted light to the Nth transmitted light. More specifically, the delay amount in the delay circuit 5 of the nth path is set based on half the delay difference between the first propagation delay and the maximum propagation delay from the first transmitted light to the Nth transmitted light.

[0023] In this embodiment, for example, the propagation delay from the optical transmitter 1 to the optical receiver 2 for each of N transmitted lights (transmitted signals) is measured in advance. Then, based on the measured propagation delay, the width of the time window of each FIR filter 6 and the amount of delay to be applied by each delay circuit 5 are determined.

[0024] For example, the control unit 23 (see FIG. 2) of the optical receiver 2 can be configured to dynamically control the width of the time window of the FIR filter 6. For example, a training period is periodically set for each transmission signal. The optical transmitter 1 transmits a training pattern during the training period of the m-th transmission signal. Information indicating the training pattern of each transmission signal is stored in advance in the control unit 23 of the optical receiver 2. The control unit 23 determines the signal-to-noise ratio (SNR) of the m-th transmission signal by comparing the training pattern with a pattern included in the training period of the m-th transmission signal generated by the m-th equalizer unit of the MIMO equalizer 22. The control unit 23 measures the SNR while changing the width of the time window of the FIR filter 6 of each path of the m-th equalizer, thereby determining the width of the time window of each FIR filter 6 that maximizes the SNR. In addition to the width of the time window of the FIR filter 6, the delay amount of each delay circuit 5 can also be controlled in the same way.

[0025] Note that the present invention is not limited to determining the SNR using a training pattern. For example, the SNR of the mth transmission signal output by the mth equalization unit may be determined by any method, and the width of the time window of each FIR filter 6 or the amount of delay of each delay circuit 5 may be controlled so as to increase the SNR.

[0026] The MIMO equalizer 22 can be realized by a computer program that, when executed on a device having one or more processors, causes the device to function as the above-described MIMO equalizer 22. These computer programs can be stored in a computer-readable storage medium or distributed via a network.

[0027] The invention is not limited to the above-described embodiment, and various modifications and variations are possible within the scope of the gist of the invention.

[0028] The above configuration allows the number of taps in each FIR filter in the optical receiver to be optimized, thereby contributing to Goal 9 of the United Nations' Sustainable Development Goals (SDGs), which is to "Build resilient infrastructure, promote sustainable industrialization, and foster innovation." [Explanation of symbols]

[0029] 21: Receiver, 22: MIMO equalizer, 5: Delay circuit, 6: FIR filter, 7: Synthesizer

Claims

1. An optical receiver that receives, as first to N-th received lights, first to N-th transmitted lights that an optical transmitter generates based on first to N-th transmitted signals (N is an integer of 2 or more) and transmits to an optical transmission line, the first to N-th transmitted lights, a receiving means for outputting first to Nth received signals based on the first to Nth received lights; an m-th equalization means for generating an m-th transmission signal (m is an integer from 1 to N) based on the first to N-th reception signals; Equipped with The mth equalization means an nth path that processes an nth received signal (n is an integer between 1 and N) to generate an mth transmitted signal component; a combining means for combining the mth transmission signal components generated in the first path to the Nth path, respectively, to generate the mth transmission signal; Equipped with the nth path includes a filter means for filtering the nth received signal, and a delay means provided upstream or downstream of the filter means; a width of a time window of the filter means for a path other than the m-th path among the first path to the N-th path is set based on a delay difference between a first propagation delay of the n-th transmitted light from the optical transmitter to the optical receiver and a second propagation delay of the m-th transmitted light from the optical transmitter to the optical receiver; An optical receiver, wherein the delay amount in the delay means of the nth path is set based on the delay difference between the first propagation delay and the maximum value of the propagation delay from the first transmitted light to the Nth transmitted light from the optical transmitter to the optical receiver.

2. 2. The optical receiver according to claim 1, wherein the width of the time window of said filter means of said mth path is set to a predetermined value.

3. 2. The optical receiver according to claim 1, wherein a width of the time window of said filter means of said mth path is set based on a delay difference due to a difference in polarization of said mth transmitted light from said optical transmitter to said optical receiver.

4. 2. The optical receiver according to claim 1, wherein the width of the time window of said filter means of said mth path is smallest among the widths of the time windows of said filter means of said first path to said Nth path.

5. 5. The optical receiver according to claim 1, wherein the delay amount in said delay means of said nth path is set based on a value that is half the delay difference between said first propagation delay and said maximum value.

6. 5. The optical receiver according to claim 1, further comprising: a control unit that controls a width of a time window of the filter unit of the nth path of the mth equalization unit based on a quality of the mth transmission signal generated by the combining unit of the mth equalization unit.

7. A computer program executed by one or more processors of an optical receiver that receives, as first to N-th received lights, first to N-th transmitted lights that an optical transmitter generates based on first to N-th transmitted signals (N is an integer of 2 or more) and transmits to an optical transmission line, and outputs first to N-th received signals based on the first to N-th received lights, When the computer program is executed by the one or more processors, the one or more processors function as an m-th equalization means for generating an m-th transmission signal (m is an integer from 1 to N) based on the first to N-th reception signals; The mth equalization means an nth path that processes an nth received signal (n is an integer between 1 and N) to generate an mth transmitted signal component; a combining means for combining the mth transmission signal components generated in the first path to the Nth path, respectively, to generate the mth transmission signal; Equipped with the nth path includes a filter means for filtering the nth received signal, and a delay means provided upstream or downstream of the filter means; a width of a time window of the filter means for a path other than the m-th path among the first path to the N-th path is set based on a delay difference between a first propagation delay of the n-th transmitted light from the optical transmitter to the optical receiver and a second propagation delay of the m-th transmitted light from the optical transmitter to the optical receiver; a computer program, wherein the delay amount in the delay means of the nth path is set based on the delay difference between the first propagation delay and the maximum value of the propagation delay from the first transmitted light to the Nth transmitted light from the optical transmitter to the optical receiver.

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