Transmission device, control device, and transmission method

A transmission device includes a first signal processing circuit configured to average transmission quality of a first data signal based on a third data signal, a second signal processing circuit configured to average transmission quality of a second data signal based on a fourth data signal; and a processor configured to allocate a channel in a transmission path that transmits the first data signal and a channel in a transmission path that transmits the second data signal, based on a first index value indicating transmission quality of a first optical signal in the transmission path, the first optical signal being generated based on the averaged first data signal and a second index value indicating transmission quality of a second optical signal in the transmission path, the second optical signal being generated based on the averaged second data signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2019-3761, filed on Jan. 11, 2019, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a transmission device, a control device, and a transmission method.

BACKGROUND

For example, there is a technique wherein optical signals having different polarizations are multiplexed and transmitted to an optical fiber (see, for example, Patent Literature 1). In this type of optical multiplexing transmission system, not only polarization but also a wavelength and a space in an optical fiber (for example, a mode or a core) are used as an optical signal channel.

Since a transmission path is provided with not only an optical fiber but also other devices that affect the transmission characteristics of the optical signal such as an optical amplifier, the optical signal has different transmission quality such as (Signal-Noise Ratio) SNR for each channel. For example, propagation loss of the optical fiber, gain and noise figure (NF) of the optical amplifier, and nonlinear optical coefficient depend on the wavelength, mode, and core of the optical signal.

On the other hand, there is a method for improving the transmission quality of each optical signal by optimizing selection of a modulation method used for the optical signal, forward error correction (FEC) redundancy, and the like for each channel; however, the more options for the modulation method and FEC redundancy, the more complicated the configuration and control of optical signal transmitters and receivers. There is also a method for optimizing the power of the optical signal inputted to the optical fiber; however, there are influences on the transmission characteristics of other channels due to the nonlinear optical effect in the optical fiber, changes in gain of the optical amplifier, the upper limit power, and the like, and thus such a method is not effective.

For example, in an optical multiplexing transmission system that multiplexes optical signals of different modes, there is a method for averaging the transmission quality of the optical signal for each channel by Hadamard-transforming an electric field signal.

For example, as related art, Japanese Laid-open Patent Publication No. 2012-124934, K. Shibahara et al., “Space-Time Coding-Assisted Transmission for Mitigation of MDL Impact on Mode-Division Multiplexed Signals”, OFC2016, Th4C.4, and the like are disclosed.

SUMMARY

According to an aspect of the embodiments, a transmission device that transmits an optical signal, includes a first signal processing circuit configured to average transmission quality of a first data signal based on a third data signal, a second signal processing circuit configured to average transmission quality of a second data signal based on a fourth data signal, and a processor configured to allocate a channel in a transmission path that transmits the first data signal and a channel in a transmission path that transmits the second data signal, based on a first index value indicating transmission quality of a first optical signal in the transmission path through which the first optical signal is transmitted, the first optical signal being generated based on the averaged first data signal and a second index value indicating transmission quality of a second optical signal in the transmission path through which the second optical signal is transmitted, the second optical signal being generated based on the averaged second data signal.

DESCRIPTION OF EMBODIMENTS

When averaging means is implemented by, for example, a digital signal processor (DSP), it is required to fix a combination of a plurality of optical signals for averaging the transmission quality in order to avoid an increase in DSP circuit scale and a complicated configuration. Therefore, for example, when the averaging process is performed with a combination of optical signals of channels with high transmission quality, and the averaging process is performed with a combination of optical signals of channels with low transmission quality, there is a problem that variations in transmission quality are not reduced as a whole for the optical signals to be multiplexed and transmitted.

Therefore, it is an object of the present disclosure to provide a transmission device, a control device, and a transmission method capable of reducing variations in transmission quality of optical signals to be multiplexed and transmitted.

FIG. 1is a diagram illustrating a transmission quality averaging method according to a comparative example. A transmission system includes a transmitting device9aand a reception device9bcoupled via a transmission path92such as an optical fiber. As an example, the transmitting device9aand the reception device9bperform wavelength multiplexing transmission of a plurality of optical signals.

The transmitting device9aincludes signal processing units90aand90band a multiplexer91. The signal processing unit90aaverages transmission qualities of data signals S #1and S #2by unitary transforming the pair of data signals S #1and S #2as an example. In the following description, the transmission quality averaging process is referred to as “precoding”. The unitary transform includes, but not limited to, Hadamard transform, and a transform method called Space-Time codes (space-time code) or Polarization-Time codes (polarization space-time code), for example, may also be used.

The signal processing unit90balso performs precoding of another set of data signals S #3and S #4. For example, individual wavelengths λ1to λ4are allocated to the data signals S #1to S #4as channels. The data signals S #1to S #4are transformed into light of wavelengths λ1to λ4, and then multiplexed into one wavelength multiplexed optical signal by the multiplexer91such as an optical coupler and outputted to the transmission path92.

On the other hand, the reception device9bincludes a demultiplexer95and signal processing units94aand94b.The demultiplexer95demultiplexes the wavelength multiplexed optical signal inputted through the transmission path92into the data signals S #1to S #4for the wavelengths λ1to λ4. After demultiplexing, a pair of data signals S #1and S #2are transformed into electrical signals and inputted to the signal processing unit94a,while another pair of data signals S #3and S #4are transformed into electrical signals and inputted to the signal processing unit94b.

The signal processing unit94arestores the data signals S #1and S #2before the unitary transform by performing inverse transform of the unitary transform, as an example, on the pair of data signals S #1and S #2. The signal processing unit94balso restores the data signals S #3and S #4before the unitary transform by performing inverse transform of the unitary transform, as an example, on the pair of data signals S #3and S #4. In the following description, the restoration process of the data signals S #1to S #4is referred to as “decoding”.

The reception device9bmonitors an SNR as an example of the transmission quality of the data signals S #1to S #4. For example, when the transmission characteristics of the data signals S #1and S #2of the wavelengths λ1and λ2in the transmission path92are better than the transmission characteristics of the data signals S #3and S #4of the wavelengths λ3and λ4, the SNRs of the data signal S #1and S #2are larger than the SNRs of the data signals S #3and S #4(see SNR “large” and “small”). The SNR is an example of an index value indicating transmission quality.

Therefore, the difference in transmission quality between one pair of data signals S #1and S #2and the other pair of data signals S #3and S #4is increased. Accordingly, variations in transmission quality as a whole for the data signals S #1to S #4are not reduced,

Therefore, the transmitting device9aaccording to the embodiment allocates the wavelengths λ1to λ4to the data signals S #1to S #4so that the wavelengths λ1and λ2that are high SNR channels and the wavelengths λ3and λ4that are low SNR channels are combined as pairs for preceding.

FIG. 2is a diagram illustrating a transmission quality averaging method according to the embodiment. InFIG. 2, constituent's common to those inFIG. 1are denoted by the same reference numerals, and description thereof is omitted.

In this example, unlike the comparative example, the wavelength λ3is allocated to the data signal S #2, while the wavelength λ2is allocated to the data signal S #3. Therefore, the SNR of the data signal S #2is reduced due to deterioration in transmission characteristics according to the wavelength λ3, while the SNR of the data signal S #3is increased due to improvement in transmission characteristics according to the wavelength λ2.

Since the signal processing unit90aprecodes the combination of the data signals S #1and S #2of the wavelengths λ1and λ3, the SNR is averaged between the wavelength λ1that is the high SNR channel and the wavelength λ3that is the low SNR channel. Since the signal processing unit90balso precodes the combination of the data signals S #3and S #4of the wavelengths λ2and λ4, the SNR is averaged between the wavelength λ2that is the high SNR channel and the wavelength λ4that is the low SNR channel. (see SNR “Medium”).

Therefore, the difference in transmission quality between one pair of data signals S #1and S #2and the other pair of data signals S #3and S #4is reduced. Accordingly, variations in transmission quality are reduced as a whole for the data signals S #1to S #4.

A configuration for performing channel allocation as described above will be described below,

FIG. 3is a configuration diagram illustrating an example of a transmission system. The transmission system includes a transmitting device1that performs wavelength multiplexing transmission by performing channel allocation as described above, a reception device2, and a network monitoring and control device5that monitors and controls the transmitting device1and the reception device2. The network monitoring and control device5is an example of a control device that controls the transmitting device1.

The transmitting device1is an example of a transmission device, and transmits a plurality of data signals S #1to S #4after wavelength multiplexing thereof, the data signals each having a wavelength allocated as an example of a channel. A wavelength multiplexed optical signal Smux obtained by wavelength multiplexing a plurality of optical signals is inputted to the reception device2through the transmission path92. The reception device2receives the wavelength multiplexed optical signal Smux and separates the signal for each of the wavelengths λ1to λ4to restore the data signals S #1to S #4.

The transmitting device1includes a control unit30, transmitters31ato31d,and a multiplexer32. The transmitters31ato31dtransmit data signals S #1and S #2as an example of optical signals, respectively. The multiplexer32is, for example, an optical coupler, and combines the data signals S #1to S #4from the transmitters31ato31dto generate a wavelength multiplexed optical signal Smux and output the wavelength multiplexed optical signal Smux to the transmission path92.

The transmitters31aand31bprecode the pair of data signals S #1and S #2, while the transmitters31cand31dprecode the pair of data signals S #3and S #4. Among the data signals S #1and S #2and the data signals S #3and S #4, one of the pairs is an example of a first pair, and the other pair is an example of a second pair.

The reception device2includes a control unit40, a demultiplexer42, and receivers41ato41d.The demultiplexer42is, for example, an optical splitter, and demultiplexes the wavelength multiplexed optical signal Smux to each of the receivers41ato41d.As will be described later, the receivers41ato41dhave wavelength tunable optical filters, and receive the data signals S #1to S #4by filtering the wavelength multiplexed optical signal Smux for each of the wavelengths λ1to λ4.

The receivers41aand41bdecode the pair of data signals S #1and S #2, while the receivers41cand41ddecode the pair of data signals S #3and S #4. Thus, the data signals S #1to S #4before precoding are restored.

The receivers41ato41dmonitor the transmission quality of the data signals S #1to S #4and notify the control unit40of the SNR as an index value indicating the transmission quality. The control unit40generates quality information from the SNR and notifies the generated quality information to the control unit30of the transmitting device1via, for example, the network monitoring and control device5. Thus, the control unit30acquires the quality information from the reception device2. The quality information is not limited to this, and may be notified via a line, for example, that transmits a data signal from a node of he reception device2to a node of the transmitting device1.

The control unit30allocates a wavelength to each of the data signal S #1to S #4based on the quality information so that the difference in transmission quality between the data signals S #1and S #2and the data signals S #3and S #4is reduced. Examples of wavelength allocation include those illustrated inFIGS. 1 and 2, for example.

The control unit30acquires the quality information of the data signals S #1to S #4to which the wavelengths λ1to λ4are allocated, respectively, from the reception device2. Therefore, the control unit30may acquire quality information with higher accuracy, for example, compared with the case where the SNR of each of the data signals S #1to S #4is predicted from the wavelength multiplexed optical signal Smux transmitted from the transmitting device1.

As an example, the control unit30changes the wavelength λ2of the data signal S #2to the wavelength λ3so that the wavelength λ1that maximizes the SNR and the wavelength λ3that minimizes the SNR are allocated to the data signals S #1and S #2as a pair for precoding, respectively. The control unit30also changes the wavelength λ3of the data signal S #3to the wavelength λ2so that the wavelength λ2with the second largest SNR and the wavelength λ4with the second smallest SNR are allocated to the data signals S #3and S #4as a pair for precoding, respectively. The wavelengths λ1to λ4are changed by setting the central wavelength of a light source of the data signals S #1to S #4, for example.

The control unit30also notifies the control unit40in the reception device2of setting information indicating the allocation of the wavelengths λ1to λ4via the network monitoring and control device5. Based on the setting information, the control unit40sets the wavelengths λ1to λ4of the data signals S #1to S #4received by the receivers41ato41d.Therefore, the reception device2may normally receive the data signals S #1to S #4even when the transmitting device1changes the wavelength allocation for the data signals S #1to S #4. The setting information is not limited to this, and may be stored in the data signals S #1to S #4and notified to the reception device2, for example.

FIGS. 4A and 4Bare configuration diagrams illustrating an example of the transmitters31ato31d.FIGS. 4A and 4Billustrate, together with the configurations of the transmitters31aand31c,only a mapping unit801and a precoding unit802of the transmitters31band31dto be paired up with the transmitters31aand31cfor precoding; however, the overall configuration of the transmitters31band31dis the same as that of the transmitters31aand31c.

The transmitters31ato31dtransmit data signals S #1to S #4according to a digital coherent optical transmission system using a polarization multiplexing method. InFIGS. 4A and 4B, the data signals S #1to S #4that are electrical signals are expressed as Dt, while the data signals S #1to S #4that are optical signals are expressed as So.

The transmitters31ato31deach include a transmission processing circuit80, digital-to-analog converters (DACs)82ato82d,amplifiers (AMPs)84ato84d,and a modulation unit83. The modulation unit83includes Mach-Zehnder modulators (MZMs)83ato83d.The transmitters31ato31deach further include a polarization beam combiner (PBC)85, a polarization beam splitter (PBS)86, and a light source87.

The transmission processing circuit80receives an electrical data signal Dt from another device or a subsequent processing circuit, for example, and outputs digital signals Hi, Hq, Vi, and Vq generated from the data signal Dt. The data signal Dt includes, but not limited to, an Ethernet (registered trademark) signal, for example.

The digital signals Hi and Hq are an in-phase component and a quadrature phase component of an electric field signal Eh corresponding to an H polarization component (polarized light Sh) of the data signal So, respectively. The digital signals Vi and Vq are an in-phase component and a quadrature component of an electric field signal Ev corresponding to a V polarization component (polarized light Sv) of the data signal So, respectively,

The transmission processing circuit80includes a forward error correction (FEC) adding unit800, the mapping unit801, the precoding unit802, a signal insertion unit803, and a pre-equalization processing unit804. Examples of the transmission processing circuit80include, but not limited to, a digital signal processor (DSP), and a field-programmable gate array (FPGA) may be used, for example.

The FEC adding unit800inserts an FEC code, which is an example of an error correction code, into the data signal Dt. The FEC code is located at the end of the frame of the data signal Dt, for example. The FEC adding unit800outputs the data signal Dt to the mapping unit801.

The mapping unit801maps the data signal Dt to symbols according to the modulation method. Examples of the modulation method Include, but not limited to quadrature phase shift keying (QPSK) and binary phase shift keying (BPSK), and quadrature amplitude modulation (QAM) may also be used.

The mapping unit801performs serial-parallel conversion, for example, to divide the data signal Dt into two data strings to be allocated to H-axis and V-axis polarization components. The mapping unit801performs mapping processing on the data signal Dt for each data string, and outputs the data string of each polarization component to the precoding unit802as electric field signals Eh and Ev, respectively.

The electric field signals Eh and Ev are inputted to the precoding unit802from the mapping unit801in the preceding stage and the mapping unit801that is the counterpart of the precoding pair, respectively.

For example, the precoding unit802of the transmitter31areceives the electric field signals Eh and Ev from the mapping unit801in the preceding stage and the mapping unit801of the transmitter31bthat is the counterpart of the precoding pair, and the precoding unit802of the transmitter31breceives the electric field signals Eh and Ev from the mapping unit801in the preceding stage and the mapping unit801of the transmitter31a.As in the case of the transmitters31aand31b,the precoding units802of the transmitters31cand31dreceive the electric field signals Eh and Ev from the mapping units801, respectively.

Each of the precoding units802of the transmitters31aand31baverages transmission quality by performing unitary transform on the data signals S #1and S #3. Each of the precoding units802of the transmitters31cand31daverages transmission quality by performing unitary transform on the data signals S #2and S #4.

Therefore, the precoding unit802may easily perform an averaging process by general-purpose numerical calculation. For the precoding unit802, an averaging process frequency (hereinafter referred to as “precoding frequency”) is set by the control unit30. One of the precoding units802of the transmitters31aand31band the precoding units802of the transmitters31cand31dis an example of a first signal processing unit, while the other is an example of a second signal processing unit. The precoding unit802outputs the electric field signals Eh and Ev to the signal insertion unit803.

In accordance with an insertion instruction from the control unit30, the signal insertion unit803inserts a predetermined pattern reference signal for monitoring the transmission quality of the unprecoded data signals S #1to S #4into the electric field signal Eh outputted from the precoding unit802. The reference signal includes a training sequence (TS) that is a synchronization pattern of the frame of the data signal Dt and a pilot symbol (PS) that is a test pattern. The signal insertion unit803outputs the electric field signals Eh and Ev to the pre-equalization processing unit804, respectively.

The pre-equalization processing unit804performs pre-equalization processing on the electric field signals Eh and Ev. For example, the pre-equalization processing unit804suppresses the distortion of the signal waveform due to the transmission path characteristics by electrically giving the characteristics opposite to the transmission path characteristics of the transmission path92to the electric field signals Eh and Ev in advance. The pre-equalization processing unit804outputs the electric field signals Eh and Ev to the DACs82ato82das digital signals Hi, Hq, Vi, and Vq, respectively.

The DACs82ato82dconvert the digital signals Hi, Hq, Vi, and Vq into analog signals, respectively. The analog signals are amplified by the AMPs84ato84dand then inputted to the MZMs83ato83d.The DACs82ato82dmay be configured in the transmission processing circuit80.

The light source87includes, for example, a laser diode or the like, and outputs transmission light LOs having a central wavelength set by the control unit30to the PBS86. The PBS86separates the transmission light LOs into an H axis and a V axis (polarization axis). The H polarization component of the transmission light LOs is inputted to the MZMs83aand83b,respectively, and the V polarization component of the transmission light LOs is inputted to the MZMs83cand83d,respectively.

The modulation unit83optically modulates the transmission light LOs based on the electric field signals Eh and Ev to generate polarized light Sh and Sv orthogonal to each other. The MZMs83ato83doptically modulate the transmission light LOs based on the analog signals from the DACs82ato82d.For example, the MZMs83aand83boptically modulate the H-axis component of the transmission light LOs based on the analog signals from the DACs82aand82b,while the MZMs83cand83doptically modulate the V-axis component of the transmission light LOs based on the analog signals from the DACs82cand82d.

The optically modulated H-axis component and V-axis component of the transmission light LOs are inputted to the PBC85as polarized light Sh and Sv. The PBC85generates a data signal So by polarization combining the H-axis component and the V-axis component of the transmission light LOs, and outputs the data signal So to the transmission path92.

FIGS. 5A and 5Bare configuration diagrams illustrating an example of the receivers41ato41d.The receivers41ato41dreceive the data signals So from the transmitters31ato31d.FIGS. 5A and 5Billustrate, together with the configurations of the receivers41aand41c,only carrier phase recovery units703and decoding units704of the receivers41band41dto be paired up with the receivers41aand41cfor decoding; however, the overall configuration of the receivers41band41dis the same as that of the receivers41aand41c.

The receivers41ato41deach include a reception processing circuit70, analog-to-digital converters (ADCs)72ato72d,a monitoring unit78, and a front end unit79. The front end unit79includes a light source71, balanced photodiodes (PDs)73ato73d,90-degree optical hybrid circuits740and741, PBSs75and76, and an optical filter77.

The front end unit79is an example of a signal conversion unit, which receives the wavelength multiplexed optical signal Smux from the transmitters31ato31dfor conversion into electric field signals Eh and Ev, respectively, by separating the data signal So included in the wavelength multiplexed optical signal Smux into polarized light Sh and Sv. The optical filter77separates the data signal So from the wavelength multiplexed optical signal Smux and outputs the data signal to the PBS76. In this event, the central wavelength of the data signal So to be separated is set by the control unit40.

The PBS76separates the data signal So into polarized light Sh and Sv to be outputted to the 90-degree optical hybrid circuits740and741, respectively. The light source71includes a laser diode or the like, for example, and inputs local light LOr having the central wavelength set by the control unit40to the PBS75. The PBS75separates the local light LOr into H-axis and V-axis components and outputs those components to the 90-degree optical hybrid circuits740and741, respectively.

The 90-degree optical hybrid circuit740has a waveguide for causing the polarized light Sh and the H-axis component of the local light LOr to interfere with each other, and detects the polarized light Sh. The 90-degree optical hybrid circuit740outputs optical components corresponding to the amplitude and phase of the in-phase component and the quadrature phase component to the PDs73aand73b,respectively, as the detection results.

The 90-degree optical hybrid circuit741has a waveguide for causing the polarized light Sv and the V-axis component of the local light LOr to interfere with each other, and detects the polarized light Sv. The 90-degree optical hybrid circuit741outputs optical components corresponding to the amplitude and phase of the in-phase component and the quadrature component to the PDs73cand73d,respectively, as the detection results.

The PDs73ato73dconvert the optical components inputted from the 90-degree optical hybrid circuits740and741into electrical signals, and output the electrical signals to the ADCs72ato72d,respectively. The ADCs72ato72dconvert the electrical signals inputted from the PDs73ato73dinto digital signals Hi, Hq, Vi, and Vq, respectively. The digital signals Hi, Hq, Vi, and Vq are inputted to the reception processing circuit70.

The reception processing circuit70includes a dispersion compensation unit700, an adaptive equalization processing unit701, a carrier phase recovery unit703, a decoding unit704, and an error correction unit705. Examples of the reception processing circuit70include, but not limited to, a DSP, and may be an FPGA, for example. The reception processing circuit70may also include functions other than those described above.

The dispersion compensation unit700compensates for waveform distortion of the data signal So caused by the wavelength dispersion on the transmission path92, based on fixed parameters. The dispersion compensation unit700outputs the digital signals Hi, Hq, Vi, and Vq to the adaptive equalization processing unit701as electric field signals Eh and Ev.

The adaptive equalization processing unit701performs adaptive equalization processing on the electric field signals Eh and Ev. For example, the adaptive equalization processing unit701compensates for waveform distortion of the data signal So caused by wavelength dispersion, nonlinear optical effect, or the like on the transmission path92, based on dynamic parameters.

The adaptive equalization processing unit701compensates for quality degradation of the data signal So. For example, the adaptive equalization processing unit701uses a finite impulse response (FIR) filter to impart characteristics opposite to the characteristics of the transmission path92to the electric field signals Eh and Ev, thereby correcting the waveforms of the electric field signals Eh and Ev. The adaptive equalization processing unit701outputs the electric field signals Eh and Ev to the carrier phase recovery unit703.

The carrier phase recovery unit703removes phase noise components from the electric field signals Eh and Ev, estimates the correct carrier phase, and synchronizes the phases of the electric field signals Eh and Ev with the estimated carrier phase. The carrier phase recovery unit703outputs the electric field signals Eh and Ev to the decoding unit704.

The electric field signals Eh and Ev are inputted to the decoding unit704from the carrier phase recovery unit703in the preceding stage and the carrier phase recovery unit703of the decoding unit704that is the counterpart of the decoding pair.

For example, the decoding unit704of the receiver41areceives the electric field signals Eh and Ev from the carrier phase recovery unit703in the preceding stage and the carrier phase recovery unit703of the receiver41bthat is the counterpart of the decoding pair, while the decoding unit704of the receiver41breceives the electric field signals Eh and Ev from the carrier phase recovery unit703in the preceding stage and the carrier phase recovery unit703of the receiver41a.As in the case of the receivers41aand41c,the decoding units704of the receivers41band41dalso receive the electric field signals Eh and Ev from the carrier phase recovery units703, respectively. The decoding unit704decodes the electric field signals Eh and Ev and outputs the decoded signals to the error correction unit705.

The error correction unit705reproduces the data signal Dt from the electric field signals Eh and Ev, and performs error correction processing of the data signal Dt by soft decision based on the FEC code added to the data signal Dt. The error correction processing is performed for each frame of the data signal. Dt. The data signal Dt thus restored is outputted to another device.

The monitoring unit78monitors the transmission quality of the data signal Dt. The monitoring unit78calculates, for example, an SNR from the data signal Dt, and outputs the SNR to the control unit40as quality information. The monitoring unit78includes, for example, a DSP, an FPGA, or the like.

FIG. 6is a configuration diagram illustrating an example of the control unit30. The control unit30includes a central processing unit (CPU)10, a read-only memory (ROM)11, a random-access memory (RAM)12, a storage memory13, a communication port14, and a hardware interface (HW-IF) unit15. The CPU10is coupled to the ROM11, the RAM12, the storage memory13, the communication port14, and the HW-IF15through a bus19.

The ROM11stores a program for driving the CPU10. The RAM12functions as a working memory for the CPU10. The communication port14is, for example, a local area network (LAN) port and processes communications between the network monitoring and control device5and the CPU10.

When the CPU10reads the program from the ROM11, a quality information acquisition unit101and a channel allocation unit102are formed as functions. The storage memory13stores a channel setting table (TBL)130.

The quality information acquisition unit101is an example of an acquisition unit, which acquires the SNRs of the data signals S #1to S #4from the reception device2in accordance with instructions from the channel allocation unit102. The channel allocation unit102is an example of an allocation unit, which allocates any one of the wavelengths λ1to λ4as a channel to the data signals S #1to S #4based on the SNR so that a difference in transmission quality between the data signals S #1, S #2and the data signals S #3, S #4is reduced. For example, as described above, the channel allocation unit102performs wavelength allocation so that the high SNR wavelength and the low SNR wavelength among the wavelengths λ1to λ4are paired up for precoding.

The channel allocation unit102sets the central wavelength for the fight source71, for example, via the HW-IF15. The channel allocation unit102performs various settings and instructions for other hardware in the transmitting device1via the HW-IF15.

In the channel setting table130, information related to the channel allocation of the data signals S #1to S #4is registered. In the channel setting table130, for example, signal IDs #1to #4of identifiers of the data signals S #1to S #4, CH-IDs #1to #4which are channel identifiers, the wavelengths λ1to λ4corresponding to the channels, and SNRs are registered. The control unit40of the reception device2also includes a CPU circuit similar to the above, for example.

FIG. 7is a flowchart illustrating an example of channel allocation processing. The channel allocation processing is executed, for example, when the transmission quality of the data signals S #1to S #4no longer satisfies a predetermined standard, and when a new transmitter, that is, a data signal is added to the transmitting device1. Prior to this processing, the channel allocation unit102sets the precoding frequency for the precoding unit802of each of the transmitters31ato31dto one.

The channel allocation unit102instructs the signal insertion unit803to insert reference signals into the data signals S #1to S #4so that unprecoded data signals S #1to S #4are transmitted to the reception device2(Step St1). In this event, CH-IDs #1to #4, for example, are allocated to the data signals S #1to S #4as initial settings.

Next, the quality information acquisition unit101acquires the quality information including the SNR of each of the unprecoded data signals S #1to S #4from the reception device2via the communication port14(Step St2). In this event, the quality information acquisition unit101registers the SNR in the channel setting table130. The SNR when precoding is not performed is acquired by inserting reference signals into the data signals S #1to S #4; however, the present disclosure is not limited thereto, and the data signals S #1to S #4with no reference signals inserted therein may also be acquired by monitoring between the carrier phase recovery unit703and the decoding unit704of the reception device2.

Then, the channel allocation unit102selects a channel having the smallest SNR (Step St3), and selects a channel having the maximum SNR (Step St4) from channel candidates to be allocated to the data signals S #1to S #4. For example, the channel allocation unit102refers to the SNRs in the channel setting table130to select the CH-ID having the maximum SNR and the CH-ID having the minimum SNR.

Thereafter, the channel allocation unit102allocates the currently selected channel pair to the data signals S #1to S #4as the precoding pairs, respectively (Step St5). For example, the channel allocation unit102allocates the currently selected wavelength of each channel to the data signals S #1and S #2of the transmitters31aand31bas the precoding pair or the data signals S #3and S #4of the transmitters31cand31das the precoding pair.

The channel allocation unit102sets the central wavelength of the transmission light LOs in the light sources87of the transmitters31aand31bor the transmitters31cand31daccording to the wavelengths λ1to λ4of each channel being selected. Thus, the transmitters31aand31b(or the transmitters31cand31d) transmit the data signals S #1and S #2(or the data signals S #3and S #4) of the wavelength of each selected channel.

Next, the channel allocation unit102notifies the reception device2via the communication port14of setting information indicating channel allocation of the wavelength of each selected channel to the data signals S #1and S #2(or the data signals S #3and S #4) (Step St6). The control unit40of the reception device2sets the central wavelength corresponding to the wavelength allocated to the data signals S #1to S #4in the optical filter77and the light source71of each of the receivers41ato41daccording to the setting information. Accordingly, the receivers41ato41dmay normally receive the data signals S #1and S #2(or the data signals S #3and S #4).

Next, the channel allocation unit102instructs the signal insertion unit803to stop the insertion of reference signals into the data signals S #1to S #4so that the precoded data signals S #1to S #4are transmitted to the reception device2(Step St7). Then, the quality information acquisition unit101acquires the quality information including the SNR of each of the precoded data signals S #1to S #4from the reception device2via the communication port14(Step St8).

Thereafter, the channel allocation unit102determines whether or not the precoding frequency for the data signals S #1and S #2(or the data signals S #3and S #4) with each selected channel allocated thereto has reached an upper limit (Step St9). The upper limit of the precoding frequency is determined according to the hardware configuration, for example, and may be different for each of the transmitters31ato31d,or may be the same.

When the precoding frequency has not reached the upper limit (No in Step St9), the channel allocation unit102increases the precoding frequency for the precoding unit802(Step St12). Thereafter, each process after Step St3is executed again.

When the precoding frequency has reached the upper limit (Yes in Step St9), the channel allocation unit102excludes the selected channel from the channel candidates to be allocated to the remaining data signals S #3and S #4(or the data signals S #1and S #2) (Step St10). Since the data signals S #1and S #2(or the data signals S #3and S #4) to which the currently selected channel is allocated are thus determined, the channel allocation unit102updates the channel setting table130for the allocated channel.

Next, the channel allocation unit102determines whether or not there is an unselected channel among the remaining channel candidates (Step St11). When there is an unselected channel (Yes in Step St11), each process after Step St3is executed. When there is no unselected channel (No in Step St11), this process is terminated. The allocation processing is thus executed.

Next, an example of channel allocation processing will be described.

FIG. 8is a diagram illustrating an example of channel allocation. Reference numeral Ga denotes the channel setting table130before channel allocation, while reference numeral Gb denotes the channel setting table130after channel allocation.

Each channel setting table130represents a constellation on the reception side for each of the data signals S #1to S #4(signal IDs #1to #4). The modulation method for the data signals S #1to S #4is QPSK as an example, but is not limited thereto. In this example, the upper limit of the precoding frequency for each of the transmitters31ato31dis 1.

Before the allocation processing, the wavelengths λ1to λ4are set as initial settings for the data signals S #1to S #4, respectively. The SNRs of the data signals S #1to S #4indicate values when precoding is not performed. The SNRs of the data signals S #1and S #2are higher than the SNRs of the data signals S #3and S #4, the SNR varies significantly across the data signals S #1to S #4. Therefore, there is a clear difference in signal point distribution between the constellation of the data signals S #1and S #3and the constellation of the data signals S #2and S #4.

The channel allocation unit102selects a channel #1(CH-ID #1) with a wavelength λ1having a maximum SNR (14 (dB)) and a channel #4(CH-ID #4) with a wavelength λ4having a minimum SNR (5.4 (dB)). The channel allocation unit102allocates the wavelengths λ1and λ4of the channels #1and #4to the data signals S #1and S #2as the precoding pair, respectively. Since the precoding frequency has reached the upper limit, the channels #1and #4are excluded from the channel candidates.

Next, the channel allocation unit102selects a channel #2(CH-ID #2) with a wavelength λ2having the second largest SNR (11.2 (dB)) and a channel #3(CH-ID #3) with a wavelength λ3having the second smallest SNR (6.9 (dB)). The channel allocation unit102allocates the wavelengths λ2and λ3of the channels #2and #3to the data signals S #3and S #4of the precoding pair, respectively. Since the precoding frequency has reached the upper limit, the channels #3and #4are excluded from the channel candidates. Since there are no other channel candidates left, the channel allocation unit102terminates the allocation processing.

After the allocation processing, the SNRs of the precoded data signals S #1and S #2with the wavelengths λ1and λ4allocated thereto, respectively, are both 7.4 (dB), and the SNRs of the precoded data signals S #3and S #4with the wavelengths λ3and λ2allocated thereto, respectively, are both 6.4 (dB). Therefore, as compared with the SNRs before the allocation processing, the SNR variation is reduced across the data signals S #1to S #4. Accordingly, the difference in signal point distribution between the constellation of the data signals S #1and S #3and the constellation of the data signals S #2and S #4is also reduced.

As described above, the channel allocation unit102allocates the wavelengths as channels to the data signals S #1to S #4based on the SNRs so that the difference in transmission quality between the data signals S #1and S #2and the data signals S #3and S #4is reduced. Thus, variations in transmission quality across the data signals S #1to S #4dependent on the wavelength are reduced.

Although the SNR is used as the index value of transmission quality in this example, the present disclosure is not limited thereto, and an SNR margin (hereinafter simply referred to as “margin”) may be used as in the following example. The margin is an SNR difference with respect to a lower limit of the SNR determined by the FEC limit according to the encoding scheme of the FEC adding unit800.

FIGS. 9A and 9Bare flowcharts illustrating another example of channel allocation processing. InFIGS. 9A and 9B, constituents common to those inFIG. 7are denoted by the same reference numerals, and description thereof is omitted.

The quality information acquisition unit101calculates a margin from the SNR and the FEC limit (Step St2a). The margin is registered in the channel setting table130. The channel allocation unit102selects a channel with the smallest margin (Step St3a) and selects a channel with the largest margin (Step St4a) from the channel candidates. The channel allocation unit102allocates the selected channel to the data signals S #1and S #2(or the data signals S #3and S #4) (Step St5) and notifies the reception device2of the setting information (Step St6).

After acquiring the SNRs of the precoded data signals S #1and S #2(or the data signals S #3and S #4) (Step St8), the quality information acquisition unit101calculates margins from the SNR and the FEC limit. (Step St8a). The margin is registered in the channel setting table130.

When the margins of the data signals S #1and S #2(or the data signals S #3and S #4) are 0 or more (Yes in Step St21), the channel allocation unit102excludes the selected channels from the candidates (Step St22). Thus, the allocation of the selected channels is determined.

As described above, the channel allocation unit102selects channels to be allocated to the data signals S #1to S #4based on the SNRs when the precoding is not performed, thereby determining the channels to be allocated to the data signals S #1to S #4based on the SNRs when the precoding is performed. Therefore, the channel allocation unit102may accurately determine the transmission quality for each channel, and may more effectively reduce the variations in transmission quality of the data signals S #1to S #4.

Next, the channel allocation unit102determines whether or not there is any unselected channel among the remaining channel candidates (Step St23). When there is an unselected channel (Yes in Step St23), each process after Step St3ais executed. When there is no unselected channel (No in Step St23), this processing is terminated.

When any of the margins of the data signals S #1and S #2(or the data signals S #3and S #4) is less than 0 (No in Step St21), the channel allocation unit102determines whether or not the precoding frequency of the transmitters31ato31dfor the data signals with the margins less than 0 has reached the upper limit (Step St24). When the precoding frequency is less than the upper limit (No in Step St24), the channel allocation unit102performs setting for increasing the precoding frequency for the precoding unit802(Step St27). Thereafter, each process after Step St8is executed.

Thus, the channel allocation unit102increases the precoding frequency when the margin is less than 0. Therefore, the power consumption within the precoding unit802may be increase, for example; however, the margin may be improved. In this example, the transmission quality standard is that the margin is 0 or more; however, the present disclosure is not limited thereto, and the transmission quality standard may be that the margin is 1 or more, for example.

When the precoding frequency has reached the upper limit (Yes in Step St24), the channel allocation unit102determines whether or not there is any unselected channel in the remaining channel candidates (Step St25). When there is an unselected channel (Yes in Step St25), the channel allocation unit102selects, instead of the channel with the smaller SNR, the channel with the next smaller SNR among the selected channels (Step St26). For example, when the channel with the smallest SNR is selected, the channel allocation unit102selects the channel with the second smallest SNR instead of that channel. Thereafter, each process after Step St4ais executed.

As described above, when the margin is less than 0, the channel allocation unit102changes the selection of channels to be allocated to the data signals S #1to S #4when the precoding frequency has reached the upper limit. Therefore, the channel allocation unit102may perform channel allocation except for the channels for which the margin may not be improved because the precoding frequency has reached the upper limit. The channel candidates may include those left unallocated.

When there is no unselected channel (No in Step St25), this processing is terminated. The channel allocation processing is thus executed.

Next, an example of channel allocation processing will be described.

FIG. 10is a diagram illustrating another example of channel allocation. Reference numeral Gc denotes a channel setting table130before channel allocation, while reference numeral Gd denotes a channel setting table130after channel allocation.

Each channel setting table130represents a constellation on the reception side for each of the data signals S #1to S #4(signal IDs #1to #4). The modulation method for the data signals S #1to S #4is QPSK as an example, but is not limited thereto. In this example, the upper limit of the precoding frequency for each of the transmitters31ato31dis 1. In this example, unlike the example ofFIG. 8, SNR margins are added to the channel setting table130. The lower limit of the SNR based on the FEC limit is 7.0 (dB).

Before the allocation processing, the wavelengths λ1to λ4are set as initial settings for the data signals S #1to S #4, respectively. The margins of the data signals S #1to S #4indicate values when precoding is not performed. The margins of the data signals S #1and S #2are larger than 0; however the margins of the data signals S #3and S #4are smaller than 0, and the margin varies significantly across the data signals S #1to S #4. Therefore, there is a clear difference in signal point distribution between the constellation of the data signals S #1and S #3and the constellation of the data signals S #2and S #4.

The channel allocation unit102selects a channel #1(CH-ID #1) with a wavelength λ1having the maximum margin (+7.0 (dB)) and a channel #4(CH-ID #4) with a wavelength λ4with the minimum margin (−1.6 (dB)). The channel allocation unit102allocates the wavelengths λ1and λ4of the channels #1and #4to the data signals S #1and S #2as the precoding pair, respectively.

Since the margins of the precoded data signals S #1and S #2are both +0.4 (dB) (>0), the channel allocation unit102determines that the transmission quality standard is satisfied, and excludes the channels #1and #4from the channel candidates. Thus, the allocation of the channels #1and #4is determined.

Next, the channel allocation unit102selects a channel #2(CH-ID #2) with a wavelength λ2having the second largest margin (+4.2 (dB)) and a channel #3(CH-ID #3) with a wavelength λ3with the second smallest margin (−0.1 (dB)). The channel allocation unit102allocates the wavelengths λ3and λ2of the channels #3and #2to the data signals S #3and S #4as the precoding pair, respectively.

Since the margins of the precoded data signals S #3and S #4are both +1.4 (dB) (>0), the channel allocation unit102determines that the transmission quality standard is satisfied, and excludes the channels #3and #2from the channel candidates. Thus, the allocation of the channels #3and #2is determined. Since there are no other channel candidates left, the channel allocation unit102terminates the allocation processing.

After the allocation processing, the margins of the precoded data signals S #1and S #2with the wavelengths λ1and λ4allocated thereto are both +0.4 (dB), and the margins of the precoded data signals S #3and S #4with the wavelengths λ3and λ2allocated thereto are both +1.4 (dB). Therefore, compared with the margin before the allocation processing, variations in margin are reduced across the data signals S #1to S #4. Accordingly, the difference in signal point distribution between the constellation of the data signals S #1and S #3and the constellation of the data signals S #2and S #4is also reduced.

Thus, the channel allocation unit102allocates wavelengths as channels to the data signals S #1to S #4based on the margins so that the difference in transmission quality between the data signals S #1and S #2and the data signals S #3and S #4is reduced. Thus, variations in transmission quality across the data signals S #1to S #4dependent on the wavelength are reduced. As the index value for the transmission quality, a bit error rate before error correction by FEC, an error vector magnitude (EVM), or the like may also be used.

(Transmission System Using Wavelength Converters)

In the above example, the channel allocation unit102allocates wavelengths to the data signals S #1to S #4by setting the central wavelength of the transmission light LOs of the transmitters31ato31d;however, the present disclosure is not limited thereto, and the wavelengths may be allocated by setting wavelengths after wavelength conversion of the data signals S #1to S #4.

FIG. 11is a configuration diagram illustrating an example of a transmission system using wavelength converters33and43. InFIG. 11, constituents common to those inFIG. 3are denoted by the same reference numerals, and description thereof is omitted.

The transmitting device1ais another example of the transmission device, which includes transmitters31ato31f,the same number of wavelength converters (CNV)33as the transmitters31ato31f,a multiplexer32a,and a control unit30. The transmitters31eand31fhave the same configuration as that of the other transmitters31ato31d.The pair of transmitters31aand31b,the pair of transmitters31cand31d,and the pair of transmitters31eand31ftransmit data signals S #1to S #n (n: positive integer) of preceding pairs, respectively.

The transmitters31ato31ftransmit data signals S #1to S #n having a common wavelength λo, and the wavelength λo is converted into other wavelengths λ1to λn by the wavelength converter33. The wavelength converter33converts the wavelengths λo of the data signals S #1to S #n inputted from the transmitters31ato31finto wavelengths λ1to λn according to settings from the control unit30.

The control unit30allocates any one of the wavelengths λ1to λn to the data signals S #1to S #n by setting a wavelength shift amount of the wavelength converter33, for example.FIG. 11illustrates the case where the wavelengths λ1to λn are allocated to the data signals S #1to S #n, respectively, as an example. The wavelength converted data signals S #1to S #n are inputted to the multiplexer32a.

The multiplexer32ais, for example, a wavelength selection switch, which multiplexes the data signals S #1to S #n to generate a wavelength multiplexed optical signal Smux. The control unit30sets the wavelengths λ1to λn after the conversion by the wavelength converter33to the input port of the multiplexer32a.The wavelength multiplexed optical signal Smux is inputted to the reception device2avia the transmission path92.

The reception device2aincludes receivers41ato41f,the same number of wavelength converters43as the receivers41ato41f,a demultiplexer42a,and a control unit40. The wavelength multiplexed optical signal Smux is inputted to the demultiplexer42a.

The demultiplexer42ais, for example, a wavelength selection switch, which demultiplexes the data signals S #1to S #n having the wavelengths λ1to λn from the wavelength multiplexed optical signal Smux. The control unit40sets the wavelengths λ1to λn to the output port of the demultiplexer42aso that the data signals S #1to S #n are inputted to the respective receivers41a.The data signals S #1to S #n are inputted from the demultiplexer42ato the respective wavelength converters43.

The wavelength converters43each convert the wavelengths λ1to λn of the data signals S #1to S #n into a common wavelength λo according to the setting from the control unit40. The control unit40sets a wavelength shift amount of the wavelength converter43in accordance with the wavelength λo after wavelength conversion. The data signals S #1to S #n are inputted from the wavelength converters43to the receivers41ato41f.The receivers41eand41fhave the same configuration as the other receivers41ato41d.

FIG. 12is a configuration diagram illustrating an example of the wavelength converters33and43. The wavelength converters33and43include an excitation light source331such as a laser diode, a nonlinear optical element332such as a highly nonlinear fiber or a highly nonlinear semiconductor waveguide, and a band pass filter (BPF)333.

The nonlinear optical element332receives the excitation light from the excitation light source331and data signal S #i (i=1, 2, . . . , n) having wavelengths λj (=λ1to λn, λo). In the nonlinear optical element332, four-wave mixing occurs, for example, between the excitation light and the data signal S #i. Idler light generated by the four-wave mixing is extracted as a wavelength converted data signal S #i′ by the BPF333. The BPF333may be provided when it is desired to remove unwanted frequency component light, and does not have to be provided in the wavelength converters33and43.

The idler light is generated at a target position on the frequency axis across the excitation light with respect to the data signal S #i. Therefore, the wavelength of the idler light takes a target value across the central wavelength of the excitation light with respect to the wavelength λj of the data signal S #i. Therefore, the control units30and40may change the wavelength shift amount of the wavelength conversion by controlling the central wavelength of the excitation light at the excitation light source331. In this example, the case where the wavelength conversion is performed using four-wave mixing has been described; however, other nonlinear optical effects may be used for wavelength conversion.

Thus, the wavelength converters33and43are provided in the transmission system. Therefore, even when the central wavelength of the transmission light LOs that may be set to the light source87of the transmitters31ato31fand the central wavelength of the local light LOr that may be set to the light source71of the receivers41ato41fare limited, the transmitters31ato31fand the receivers41ato41fmay transmit and receive the data signals S #1to S #n having a single wavelength λo. The transmitters31ato31fand the receivers41ato41fmay transmit and receive data signals S #1to S #n having a plurality of wavelengths instead of the single wavelength λo.

The control unit30allocates wavelengths to the data signals S #1to S #n by setting the wavelength shift amount of each wavelength converter33ain the process of Step St5described above. Thus, variations in the transmission quality of the data signals S #1to S #n are reduced. The control unit40of the reception device2asets the wavelength converter43according to the setting information notified from the control unit30.

(Other Transmission Systems Using Wavelength Converters)

When the wavelength converters33and43are used as in the above example, the transmission quality of the data signals S #1to S #n after the wavelength conversion is lower than that before the wavelength conversion.

Therefore, not all the data signals S #1to S #n but only some data signals S #2, S #4, . . . , S #n may be subjected to the wavelength conversion, and precoding pairs may be formed with the other data signals S #1, S #3, . . . , S #(n-1) not to be subjected to the wavelength conversion. Thus, variations in transmission quality may be more effectively reduced.

FIG. 13is a configuration diagram illustrating another example of a transmission system using wavelength converters33aand43a.InFIG. 13, constituents common to those inFIG. 11are denoted by the same reference numerals, and description thereof is omitted.

A transmitting device1bis another example of the transmission device, which includes transmitters31ato31f,a wavelength converter (CNV)33a,multiplexers32band32c,and a control unit30. The transmitters31ato31ftransmit data signals S #1to S #n, respectively. The data signals S #1to S #n are inputted from the transmitters31ato31fto the multiplexer32b,respectively.

The multiplexer32bis a wavelength selection switch, for example, which outputs the data signals S #1to S #n from one of two output ports P1and P2to the multiplexer32cin the subsequent stage according to the setting of the control unit30. The wavelength converter33ais provided between the output port P2and the multiplexer32c.The wavelength converter33ahas the same configuration as that of the wavelength converters33and43illustrated inFIG. 12.

The multiplexer32bis, for example, a wavelength selection switch, which generates a wavelength multiplexed optical signal Smux by multiplexing the data signals S #1to S #n inputted from the output ports P1and P2of the multiplexer32bin the preceding stage. The wavelength multiplexed optical signal Smux is inputted to the reception device2bvia the transmission path92.

The control unit30allocates common wavelengths λ1to λm (m=n/2) for each of the data signals S #1to S #n to be precoding pairs, for example. For example, a common wavelength λ1is allocated to the data signals S #1and S #2, a common wavelength λ2is allocated to the data signals S #3and S #4, and a common wavelength λm is allocated to the data signals S #(n-1) and S #n.

The control unit30sets wavelengths to the output ports P1and P2so that the data signals S #1to S #n to be precoding pairs are outputted from the separate output ports P1and P2of the multiplexer32b,respectively. For example, the control unit30configures the setting such that one data signal S #1, S #3, . . . , S #(n-1) of each precoding pair is outputted from the output port P1and the other data signal S #2, S #4, . . . , S #n of each precoding pair is outputted from the output port P2(see dotted lines).

Therefore, the data signals S #1, S #3, . . . , S #(n-1) are inputted to the multiplexer32cwithout being wavelength-converted, and the data signals S #2, S #4, . . . , S #n are wavelength-converted by the wavelength converter33aand then inputted to the multiplexer32c.The control unit30sets the wavelengths λ1to λm of the data signals S #1to S #n at the input ports of the multiplexer32b.

The wavelength converter33aconverts the wavelengths λ1to λm into wavelengths λm+1 to λ2m,respectively, as denoted by reference numeral330. Therefore, the wavelength λ2of the data signal S #2is converted into the wavelength λm+1, the wavelength λ2of the data signal S #4is converted into the wavelength λm+2, and the wavelength λm of the data signal S #n is converted into the wavelength λ2m.Thus, the number of wavelengths (2m) included in the wavelength multiplexed optical signal Smux is twice the number of wavelengths (m) used by the transmitters31ato31f.

Through the wavelength conversion, the data signal S #1having the wavelength λ1and the data signal S #2having the wavelength λ2form a precoding pair, the data signal S #3having the wavelength λ2and the data signal S #4having the wavelength λ2form a precoding pair, and the data signal S #(n-1) having the wavelength λm and the data signal S #n having the wavelength λ2mform a precoding pair. For example, precoding pairs are formed between the data signals S #1, S #3, . . . , S #(n-1) not to be subjected to wavelength conversion and the data signals S#2, S #4, . . . , S #n to be subjected to wavelength conversion. Thus, variations in transmission quality may be more effectively reduced.

The control unit30allocates wavelengths by setting any of the wavelengths λ1to λm of the data signals S #1to S #n for the transmitters31ato31f(central wavelength of the transmission light LOs) in the process of Step St5. Since the relationship between the pairs having the wavelengths λ1to λm before the wavelength conversion by the wavelength converter33aand the wavelengths λm+1 to λ2mafter the wavelength conversion is fixed, any of the wavelengths λ1to λm may be allocated to the data signals S #1to S #n to form precoding pairs.

Since the wavelength λ1is converted into the wavelength λ2, for example, the control unit30may allocate the wavelengths λ1and λ2to the data signals S #1and S #2, respectively, by setting the wavelength λ1to the transmitters31aand31b,respectively. In this example, the same wavelengths λ1to λm are set in the transmitters31ato31fof the data signals S #1to S #n to form precoding pairs; however, different wavelengths λ1to λm may be set.

The reception device2bincludes demultiplexers42band42c,receivers41ato41f,a wavelength converter43a,and a control unit40. The wavelength multiplexed optical signal Smux is inputted to the demultiplexer42cthrough the transmission path92.

The demultiplexer42cis a wavelength selection switch, for example, which separates the data signals S #1to S #n having the wavelengths λ1to λ2mfrom the wavelength multiplexed optical signal Smux, and outputs the signals from one of the output ports P3and P4according to the setting from the control unit40. The data signals S #1to S #n are inputted from the demultiplexer42cto the demultiplexer42bin the subsequent stage. The wavelength converter43ais coupled between the output port P4and the demultiplexer42b.

The demultiplexer42boutputs the data signals S #1to S #n to the receivers41ato41f,respectively, according to the setting from the control unit40. The receivers41ato41freceive the data signals S #1to S#n according to the wavelengths λ1to λm set by the control unit40.

The control unit40sets the demultiplexers42band42cand the receivers41ato41fbased on the setting information notified from the control unit30of the transmitting device1b.The control unit40sets wavelengths to the output ports P3and P4so that the data signals S #1, S #3, . . . , S #(n-1) having the wavelengths λ1to λm are outputted from the output port P3and the data signals S #2, S #4, . . . , S #n having the wavelengths λm+1 to λ2mare outputted from the output port P4.

The data signals S #2, S #4, . . . , S #n are inputted to the wavelength converter43athrough the output port P4. As denoted by reference numeral430, the wavelength converter43aconverts the wavelengths λm+1 to λ2minto the wavelengths λ1to λm, respectively. The wavelength converter43ahas the same configuration as that of the wavelength converters33and43illustrated inFIG. 12.

The control unit40sets wavelengths to the input and output ports of the demultiplexer42bso that the data signals S #1, S #3, . . . , S #(n-1) having the wavelengths λ1to λm inputted to the demultiplexer42bthrough the output port P3as inputted to the receivers41a,41c,. . . ,41e.The control unit40also sets wavelengths to the input and output ports of the demultiplexer42bso that the data signals S #2, S #4, . . . , S #n having the wavelengths λ1to λm inputted to the demultiplexer42bthrough the output port P4are inputted to the receivers41b,41d,. . . ,41f.

Thus, the data signals S #1to S #n of the preceding pairs are inputted to the receivers41ato41f,respectively, to form decoding pairs. For example, the data signals S #1and S #2are inputted to the receivers41aand41b,respectively, the data signals S #3and S #4are inputted to the receivers41cand41d,respectively, and the data signal S #(n-1) and S #n are inputted to the receivers41eand41f,respectively.

According to the above configuration, since the wavelength converters33aand43aconvert the wavelengths λm+1 to λ2minto the wavelengths λ1to λm, respectively, the number of the wavelength converters33aand43ais only one as compared with the case where the wavelength converters33and43are provided for each of the data signals S #1to S #n as in the example ofFIG. 11, thus reducing the scale of hardware.

(Transmission System Using Spatial Multiplexing Transmission Method)

In the above example, the wavelength multiplexing transmission system using wavelengths as channels has been described as an example; however, variations in transmission quality may also be reduced by channel allocation as described above in a transmission system using a spatial multiplexing transmission method wherein a space inside a transmission path such as a core and a mode of an optical fiber is used as a channel.

FIG. 14is a configuration diagram illustrating an example of a transmission system using a spatial multiplexing transmission method. InFIG. 14, constituents common to those inFIG. 3are denoted by the same reference numerals, and description thereof is omitted.

A transmitting device1cand a reception device2care coupled by a transmission path93including an optical fiber with functions of at least one of a multimode and a multicore. The transmitting device1cincludes transmitters31ato31d,an optical switch (4×4 SW)34, a multiplexer39, and a control unit30. The transmitting device1cis another example of the transmission device.

The transmitters31ato31doutput data signals S #1to S #4to the optical switch34, respectively. The data signals S #1to S #4each have an arbitrary wavelength.

The optical switch34has input ports P1to P4and output ports P5to P8. The control unit30sets a coupling relationship between the input ports P1to P4and the output ports P5to P8. Therefore, the data signals S #1to S #4inputted to the optical switch34are outputted to the multiplexer39from one of the output ports P5to P8according to the setting from the control unit30. As initial settings, the data signals S #1to S #4are outputted, for example, from the output ports P5to P8.

The multiplexer39is a connector for coupling the optical switch34to the transmission path93, and has input ports Pa to Pd to which data signals S #1to S #4are inputted from the optical switch34, respectively. The input ports Pa to Pd are coupled to the output ports P5to P8of the optical switch34, respectively.

The input ports Pa to Pd are coupled to different spaces in the transmission path93. When the optical fiber of the transmission path93is multicore, the data signals S #1to S #4are inputted to different cores through the input ports Pa to Pd; on the other hand, when the optical fiber of the transmission path93is multimode, the data signal S #1to S #4are inputted to propagation paths of different modes through the input ports Pa to Pd. When the optical fiber of the transmission path93is multicore and multimode, the data signals S #1to S #4are inputted to combinations of propagation paths of different cores and modes through the input ports Pa to Pd.

Therefore, the data signals S #1to S #4are spatially multiplexed by being inputted to the transmission path93from the multiplexer39. The optical switch34is an example of a switch unit that switches a space in the transmission path93through which the data signals S #1to S #4are transmitted. The data signals S #1to S #4are inputted to the reception device2cthrough the transmission path93.

The reception device2cincludes receivers41ato41d,an optical switch (4×4 SW)44, a separator49, and a control unit40. The separator49is a connector that couples the optical switch44to the transmission path93, and outputs the data signals S #1to S #4inputted from the transmission path93to the optical switch44through the output ports Pe to Ph, respectively. The output ports Pe to Ph are coupled to the input ports Pa to Pd of the multiplexer39via the transmission path93, respectively.

The optical switch44has input ports P11to P14and output ports P15to P18. The input ports P11to P14are coupled to the output ports Pe to Ph of the separator49, respectively, while the output ports P15to P16are coupled to the receivers41ato41drespectively.

The control unit40sets a link relationship between the input ports P11to P14and the output ports P15to P18so that the data signals S #1to S #4are inputted to the receivers41ato41d,respectively. The control unit40sets the optical switch44according to the setting information indicating the setting of the optical switch34of the transmitting device1c.The control unit40acquires SNRs of the data signals S #1to S #4, for example, from the receivers41ato41d,and transmits the SNRs as quality information to the control unit30of the transmitting device1cvia the network monitoring and control device5.

The channel allocation unit102of the control unit30controls the optical switch44based on the SNR indicated by the quality information, thereby allocating the space in the transmission path93to the data signals S #1to S #4, respectively. As in the example ofFIG. 1, for example, when the SNRs of the data signals S #1and S #2of the precoding pair are higher than the SNRs of the data signals S #3and S #4of the other precoding pair, the channel allocation unit102changes the output port P6of the data signal S #2to the output port P8and changes the output port. P8of the data signal S #4to the output port P6.

For example, the channel allocation unit102configures the setting to couple the input port P2to the output port P8and couple the input port P4to the output port P6. Thus, the input port Pb of the multiplexer39to which the data signal S #2is inputted becomes the input port Pd, while the input port Pd of the multiplexer39to which the data signal S #4is inputted becomes the input port Pb.

The control unit30notifies the reception device2cof setting information indicating the settings of the output ports P5to P8of the data signals S #1to S #4to the reception device2cvia the network monitoring and control device5. The control unit40of the reception device2csets the optical switch44so that the data signals S #1to S #4are outputted from the output ports P15to P18according to the setting information.

For example, the control unit40configures the setting to couple the input port P12to the output port P18and couple the input port P14to the output port P16. Therefore, the data signal S #4outputted from the output port Pf of the separator49is inputted to the receiver41dfrom the output port P18, and the data signal S #2outputted from the output port Ph of the separator49is inputted to the receiver41bfrom the output port P16.

Thus, the space in the transmission path93through which the data signals S #2and S #4are transmitted is changed without changing the precoding and decoding pairs of the data signals S #1to S #4. Therefore, since the transmission characteristics of the data signals S #2and S #4are changed according to the space in the transmission path93, the difference in SNR between the data signals S #1and S #2and the data signals S #3and S #4may be reduced.

The control unit30allocates the space in the transmission path93as a channel to the data signals S #1to S #4by the processing illustrated inFIG. 7 or 9. An example of allocation processing will be described below.

FIG. 15is a diagram illustrating another example of channel allocation processing. Reference numeral Ge denotes a channel setting table130before channel allocation, while reference numeral Gf denotes a channel setting table130after channel allocation.

In the channel setting table130, port IDs indicating the output ports P5to P8of the optical switch34are registered instead of the wavelengths in the channel setting table130illustrated inFIG. 8. The channel allocation unit102couples the input ports P1to P4and the output ports P5to P8of the optical switch34so that the data signals S #1to S #4are outputted from the output ports P5to P8corresponding to the port IDs. In this example, the upper limit of the precoding frequency for each of the transmitters31ato31dis 1.

Prior to the allocation process, the output ports P5to P8are set to correspond to the data signals S #1to S #4as initial settings. The SNRs of the data signals S #1to S #4indicate values when precoding is not performed. The SNRs of the data signals S #1and S #2are higher than the SNRs of the data signals S #3and S #4, the SNR varies significantly across the data signals S #1to S #4.

The channel allocation unit102selects a channel #1(CH-ID #1) of the output port P5with the maximum SNR (14 (dB)) and a channel #4(CH-ID #4) of the output port P8with the minimum SNR (5.4 (dB)). The channel allocation unit102allocates the output ports P5and P8corresponding to the channels #1and #4to the data signals S #1and S #2of the precoding pair, respectively.

Thus, the space in the transmission path93to which the output ports P5and P8are coupled is allocated to the data signals S #1and S #2. Since the precoding frequency has reached the upper limit, the channels #1and #4are excluded from the channel candidates.

Next, the channel allocation unit102selects a channel #2(CH-ID #2) of the output port P7having the second largest SNR (11.2 (dB)) and a channel #3(CH-ID #3) of the output port P6having the second smallest SNR (6.9 (dB)). The channel allocation unit102allocates the output ports P6and P7corresponding to the channels #3and #2to the data signals S #3and S #4of the precoding pair, respectively.

Thus, the space in the transmission path93to which the output ports P6and P7are coupled is allocated to the data signals S #3and S #4. Since the precoding frequency has reached the upper limit, the channels #3and #4are excluded from the channel candidates. Since there are no other channel candidates left, the channel allocation unit102terminates the allocation processing.

After the allocation processing, the SNRs of the precoded data signals S #1and S #2with the output ports P5and P8allocated thereto, respectively, are both 7.4 (dB), and the SNRs of the precoded data signals S #3and S #4with the output ports P7and P6allocated thereto, respectively, are both 6.4 (dB). Therefore, as compared with the SNRs before the allocation processing, the SNR variation is reduced across the data signals S #1to S #4.

Thus, the channel allocation unit102allocates the space in the transmission path93as channels to the data signals S #1to S #4by controlling the optical switch34so that the difference in transmission quality between the data signals S #1and S #2and the data signals S #3and S #4is reduced. Thus, variations in transmission quality across the data signals S #1to S #4dependent on the space in the transmission path93are reduced.

(Transmission System Using Wavelength Multiplexing Transmission Method and Spatial Multiplexing Transmission Method)

In a transmission system using a wavelength multiplexing transmission method and a spatial multiplexing transmission method (hereinafter referred to as “wavelength/spatial multiplexing transmission method”), again, variations in transmission quality may be reduced by allocating wavelengths and a space in the transmission path93as channels to data signals.

FIG. 16is a configuration diagram illustrating an example of a transmitting device1din a transmission system using a wavelength/spatial multiplexing transmission method. The transmitting device1dis another example of a transmission device, which includes transmission units Us (#1to #m (m: positive integer)), a multiplexer35, an optical switch (m×m SW)34a,a multiplexer39a,and a control unit30.

Each transmission unit Us includes n transmitters Tx(1-x) to Tx(n-x) (x: 1, 2, . . . , m). The transmitters Tx(1-x) to Tx(n-x) transmit data signals having wavelengths λ1to λn. The data signals having the wavelengths λ1to λn are inputted to separate spaces in the transmission path93for each transmission unit Us.

The transmitters Tx(1-x) to Tx(n-x) have the same configuration as that of the transmitters31ato31dillustrated inFIGS. 4A and 4B. A pair of transmitters Tx(1-x) and Tx(2-x), a pair of transmitters Tx(3-x) and Tx(4-x), a pair of transmitters Tx(n-1-x) and Tx(n-x) each transmit data signals as a preceding pair. For each transmission unit Us, the wavelengths λ1to λn are allocated to the data signals of the transmitters Tx(1-x) to Tx(n-x). Each data signal is inputted to the multiplexer35.

The multiplexer35is a wavelength selection switch, for example, which wavelength-multiplexes the data signal by the transmission unit Us and outputs the data signal to one of the input ports #1to #m of the optical switch34a.For example, the multiplexer35wavelength-multiplexes the data signals of the transmitters Tx(1-1) to Tx(n-1) of the transmission unit Us (#1), wavelength-multiplexes the data signals of the transmitters Tx(1-2) to Tx(n-2) of the transmission unit Us (#2), and wavelength-multiplexes the data signals of the transmitters Tx(1-m) to Tx(n-m) of the transmission unit Us (#m). The multiplexer35outputs the wavelength multiplexed optical signal of each transmission unit Us to the optical switch34afrom one of the output ports #1to #m according to the setting from the control unit30.

The optical switch34ahas input ports #1to #m and output ports #1to #m. The wavelength multiplexed optical signals are inputted to the input ports #1to #m of the optical switch34afrom the output ports #1to #m of the multiplexer35.

The optical switch34acouples the input ports #1to #m to the output ports #1to #m according to the setting from the control unit30. Each wavelength multiplexed optical signal is inputted to the multiplexer39afrom one of the output ports #1to #m of the optical switch34a.The optical switch34ais an example of a switch unit.

The multiplexer39ahas the same function as that of the multiplexer39in the above example. The multiplexer39ahas input ports #1to #m coupled to the space in the transmission path93. The wavelength multiplexed optical signals are inputted to the output ports #1to #m of the multiplexer39afrom the output ports #1to #m of the optical switch34a.The wavelength multiplexed optical signal are spatially multiplexed by being inputted to the space in the transmission path93corresponding to the input ports #1to #m of the multiplexer39a.

Thus, the data signals are subjected to wavelength multiplexing and spatial multiplexing and then inputted to the transmission path93. The transmission path93includes an optical fiber having m pairs of modes and cores. When the optical fiber includes Ma modes and Mb cores, for example, m is the product (Ma×Mb) of Ma and Mb.

The channel allocation unit102of the control unit30sets the wavelengths λ1to λn (center wavelength of transmission light LOs) so that the difference in transmission quality between the data signals that form a precoding pair for the transmitters Tx(1-1) to Tx(n-1) of each transmission unit Us. Thus, one of the wavelengths λ1to λn is allocated to the data signal of each transmission unit Us. The channel allocation unit102sets the output ports #1to #m of the multiplexer35so that the wavelength multiplexed optical signals of the transmission units Us (#1to #m) are outputted from the output ports #1to #m of the multiplexer35, respectively.

The channel allocation unit102sets the link between the input ports #1to #m and the output ports #1to #m so that the difference in transmission quality between the data signals that form the precoding pair of each transmission unit Us is reduced for the optical switch34a.Thus, the space in the transmission path93is allocated to the wavelength multiplexed optical signal of each transmission unit Us.

The control unit30notifies the reception device2dof the setting of the wavelengths of the transmitters Tx(1-1) to Tx(n-1), the setting of the output ports #1to #m of the multiplexer35, and the link settings of the input ports #1to #m and the output ports #1to #m of the optical switch34aas setting information.

FIG. 17is a configuration diagram illustrating an example of a reception device2din the transmission system using the wavelength/spatial multiplexing transmission method. The reception device2dincludes reception units Ur (#1to #m), a separator49a,an optical switch (m×m SW)44a,a demultiplexer47, and a control unit40. The wavelength multiplexed optical signal from each transmission unit Us is inputted to the separator49athrough the transmission path93.

The separator49ahas the same function as that of the separator49in the above example. The separator49ahas output ports #1to #m coupled to the space in the transmission path93. The separator49aoutputs each wavelength multiplexed optical signal to the optical switch44afrom the output ports #1to #m.

The optical switch44ahas input ports #1to #m and output ports #1to #m. The wavelength multiplexed optical signals are inputted to the input ports #1to #m of the optical switch44afrom the output ports #1to #m of the separator49a.

The optical switch44acouples the input ports #1to #m to the output ports #1to #m according to the setting from the control unit40. Each wavelength multiplexed optical signal is inputted to the demultiplexer47from one of the output ports #1to #m of the optical switch44a.

The demultiplexer47is a wavelength selection switch, for example, having input ports #1to #m coupled to the output ports #1to #m of the optical switch44a.The demultiplexer47separates each wavelength multiplexed optical signal into data signals having the wavelengths λ1to λn, and outputs the data signals to each reception unit Ur according to the setting from the control unit40.

Each reception unit Ur includes n receivers Rx(1-x) to Rx(n-x). The receivers Rx(1-x) to Rx(n-x) receive the data signals having the wavelengths λ1to λn.

The receivers Rx(1-x) to Rx(n-x) have the same configuration as that of the receivers41ato41dillustrated inFIGS. 5A and 5B. A pair of receivers Rx(1-x) and Rx(2-x), a pair of receivers Rx(3-x) and Rx(4-x), a pair of receivers Rx(n-1-x) and Rx(n-x) each receive data signals as a decoding pair. For each reception unit Ur, the wavelengths λ1to λn are allocated to the data signals of the receivers Rx(1-x) to Rx(n-x).

The control unit40sets link between the input ports #1to #m and the output ports #1to #m of the optical switch44a,the wavelengths λ1to λm of the input ports #1to #m of the demultiplexer47, and the wavelengths λ1to λm of the receivers Rx(1-x) to Rx(n-x) of each reception unit Ur according to the setting information notified from the control unit30of the transmitting device1d.Thus, the receivers Rx(1-x) to Rx(n-x) of each reception unit Ur may normally receive the data signal according to the channel allocation in the transmitting device1d.

According to the above configuration, the channel allocation unit102may reduce variations in transmission loss for each channel by allocating the wavelength and the space in the transmission path93to the data signal as a channel based on the quality information of the transmission quality.

(First Modified Example of Transmitting Device1d)

In the above example, the transmitting device1dselects a space in the transmission path93for each wavelength multiplexed optical signal by one optical switch34a;however, when there are many ports (m×m), there is a risk that the optical insertion loss may be increased or the size may be increased. Therefore, a plurality of optical switches may be used as in the following example.

FIG. 18is a configuration diagram illustrating a first modified example of the transmitting device1d.InFIG. 18, constituents common to those inFIG. 16are denoted by the same reference numerals, and description thereof is omitted.

The transmitting device1dof this example includes two optical switches (m/2×m/2SW)34band34cinstead of the optical switch34a.The optical switches34band34ceach has input ports #1to #(2/m) and output ports #1to #(m/2). It is assumed that m is a multiple of 2.

The input ports #1to #(2/m) of the optical switch34bare coupled to the output ports #1to #(m/2) of the multiplexer35, respectively, while the output ports #1to #(2/m) of the optical switch34bare coupled to the input ports #1to #(m/2) of the multiplexer39a,respectively. The input ports #1to #(2/m) of the optical switch34care coupled to the output ports #1to #(m/2+1) of the multiplexer35, respectively, while the output ports #1to #(2/m) of the optical switch34bare coupled to the input ports #(j/2+1) to #m of the multiplexer39a,respectively.

Therefore, as in the case of the optical switch34ain the above example, the optical switches34band34cmay input the wavelength multiplexed optical signal of each transmission unit Us to one of the input ports #1to #m of the multiplexer39aaccording to the setting from the control unit30.

According to this configuration, since the number of the ports of each of the optical switches34band34cis smaller than that of the optical switch34ain the above example, a risk may be reduced that the optical insertion loss of each of the optical switches34band34cmay be increased or the size thereof may be increased. In the reception device2d,two optical switches with fewer ports may be provided instead of the single optical switch44aaccording to the transmitting device1dof this example.

(Second Modified Example of Transmitting Device1d)

In the above example, the transmitting device1dgenerates a wavelength multiplexed optical signal by wavelength multiplexing the data signal for each transmission unit Us by one multiplexer35; however the data signal may be wavelength multiplexed by the multiplexer for each transmission unit Us. In the above example, the optical switches34ato34care used; however, a wavelength selection switch may be used instead of the optical switch34a.

FIG. 19is a configuration diagram illustrating a second modified example of the transmitting device1d.InFIG. 19, constituents common to those inFIG. 16are denoted by the same reference numerals, and description thereof is omitted.

The transmitting device1dof this example includes m multiplexers37instead of the multiplexer35, and includes a wavelength selection switch36instead of the optical switch34a.The wavelength selection switch36has input ports #1to #m and output ports #1to #m.

The multiplexer37is an optical coupler, for example, which is provided corresponding to each transmission unit Us. The multiplexer37generates a wavelength multiplexed optical signal by multiplexing the data signals having wavelengths λ1to λn and outputs the signal to the wavelength selection switch36. Therefore, the m multiplexers37have the same function as that of the multiplexer35in the above example. The multiplexer37may be a wavelength selection switch.

The wavelength selection switch36has input ports #1to #m and output ports #1to #m. The input ports #1to #m of the wavelength selection switch36are each coupled to the multiplexer37corresponding to each transmission unit Us. The output ports #1to #m of the wavelength selection switch36are coupled to the input ports #1to #m of the multiplexer39a.Therefore, the wavelength selection switch36has the same functions as those of the optical switch34aand the two optical switches34band34cin the above example.

According to the configuration of this example, the optical switches34ato34cmay be omitted from the transmitting device1d,and the multiplexer37having fewer ports than the multiplexer35may be used. In the reception device2d,the wavelength selection switch may be provided instead of the optical switch44aand the demultiplexer may be provided for each reception unit Ur instead of the demultiplexer47according to the transmitting device1dof this example.

(Channel Allocation by Network Monitoring and Control Device5)

In the examples described above, the control units30of the transmitting devices1and1ato1dallocate channels to the data signals S #1to S #n; however, the network monitoring and control device5may allocate channels instead of the control units30. In this case, the control unit30performs setting of the central wavelength of the transmission light LOs in accordance with an instruction from the network monitoring and control device5. The control unit40of the reception device2dalso performs setting of the central wavelength of the local light LOr in accordance with an instruction of the network monitoring and control device5.

FIG. 20is a configuration diagram illustrating an example of the network monitoring and control device5. The network monitoring and control device5includes a CPU50, a ROM51, a RAM52, a hard disk drive (HDD)53, and a communication port54. The CPU50is coupled to the ROM51, the RAM52, the HDD53, and the communication port54via a bus59so as to enable mutual inputting/outputting of signals. The network monitoring and control device5may have another storage device such as a memory instead of the HDD53.

The ROM51stores a program for driving the CPU50. The RAM52functions as a working memory for the CPU50. The communication port54is a LAN port, for example, for processing communications between the control units30and40and the CPU50.

When the CPU50reads the program from the ROM51, a quality information acquisition unit501and a channel allocation unit502are formed as functions. The storage memory13stores a channel setting table (TBL)530.

The quality information acquisition unit501is an example of an acquisition unit, which acquires SNRs of the data signals S #1to S #n from the reception devices2and2ato2dthrough the communication port14in accordance with instructions from the channel allocation unit502. The channel allocation unit502is an example of an allocation unit, which allocates at least one of the wavelengths λ1to λn and the space in the transmission path93as a channel to the data signals S #1to S #n based on quality information such as the SNR or margin so that a difference in transmission quality between the respective precoding pairs including the data signals S #1to S #n is reduced.

The channel allocation processing executed by the network monitoring and control device5is the same as that illustrated inFIG. 7 or 9described above. The processing illustrated inFIGS. 7 and 9is an example of the transmission method of the embodiment.

As described above, the channel allocation units102and502allocate the channels to the data signals S #1to S #4based on the index value of transmission quality so that the difference in transmission quality between the precoding pairs including the data signals S #1to S #n is reduced. Thus, variations in transmission quality of the data signals S #1to S #n to be multiplexed and transmitted are reduced. In this event, since the data signals S #1to S #n themselves that forms precoding pairs are not changed, the possibility of an increase in the scale of hardware such as a DSP or a complicated configuration, for example, is reduced.

In the above example, the case where at least one of the wavelength and the space in the transmission path93is allocated to the data signals S #1to S #4as a channel has been described; however, the present disclosure is not limited thereto, and examples of the channel include polarization, I components, and Q components of the data signals S #1to S #4. In the above example, the precoding pair includes two data signals S #1and S #n, but is not limited thereto and may include three or more data signals S #1to S #n.

The above-described embodiment is a preferred embodiment of the present disclosure. However, the embodiment is not limited to this, and various modifications may be made without departing from the scope of the disclosure.