Optical transmission device and system

Abstract

The purpose of the present disclosure is to provide: an optical transmission device that can mitigate noise generated by IQ mixing in a sub-carrier multiplexing scheme; a system; a method; and a non-transitory computer-readable medium. An optical transmission device (11) according to the present disclosure comprises: a pilot addition means (111) that generates a first digital signal by adding a first pilot signal to a first data signal and generates a second digital signal by adding a second pilot signal to a second data signal; and an optical modulation means (112) that generates an optical modulation signal by optically modulating the first digital signal by a first sub-carrier (SC1) included in a frequency band which is negative relative to the center frequency of a used frequency band and optically modulating the second digital signal by a second sub-carrier (SC2) included in a frequency band which is positive relative to the center frequency of the used frequency band, and that transmits the optical modulation signal, wherein the pilot addition means (111) does not transmit the second pilot signal during transmission of the first pilot signal.

Description

[Technical field]

[0001] The present disclosure relates to an optical transmission device, a system, a method, and a non-transitory computer-readable medium, and in particular to an optical transmission device, a system, a method, and a non-transitory computer-readable medium capable of reducing noise generated by IQ mixing in a subcarrier multiplexing method. [Background technology]

[0002] In a digital coherent optical transmission system that realizes communication speeds of 1 Tbps or more, a digital subcarrier (SC: Sub Carrier) multiplexing method is essential. When an orthogonal modulation method is used in the digital SC multiplexing method, there is a problem that noise due to IQ mixing occurs in paired subcarriers due to delay differences, amplitude errors, and differences in frequency characteristics between in-phase (I: Inphase) components and quadrature (Q: Quadrature) components, and the reception characteristics are deteriorated due to the noise. Specifically, a MIMO (Multi Input Multi Output) equalizer provided on the receiving side to compensate for the frequency characteristic difference between the in-phase component and the quadrature component derives filter coefficients of a FIR (Finite Impulse Response) filter in a state in which noise due to IQ mixing is included, and thus the reception characteristics are deteriorated. Note that the in-phase component is sometimes called the I lane, and the quadrature component is sometimes called the Q lane.

[0003] Non-Patent Document 1 discloses that in wireless communication, the influence of IQ mixing is avoided by periodically and alternately setting the pilot signal of one side subcarrier of OFDM (Orthogonal Frequency Division Multiplexing) to zero, and optimal filter coefficients for transmission and reception are derived. Non-Patent Document 1 does not disclose the reduction of noise generated by IQ mixing in a digital coherent optical transmission system using a multiple subcarrier multiplexing method.

[0004] Non-Patent Document 2 discloses that in single-carrier digital coherent optical transmission, the frequency characteristics of transmitting and receiving devices are compensated for using 8x2 MIMO provided on the receiving side. Non-Patent Document 2 does not disclose the reduction of noise generated by IQ mixing in a digital coherent optical transmission system using multiple subcarrier multiplexing.

[0005] Non-Patent Document 3 discloses that in single-carrier digital coherent optical transmission, the frequency characteristics of a receiving device are compensated for using 4x2 MIMO provided on the receiving side. Non-Patent Document 3 does not disclose the reduction of noise generated by IQ mixing in a digital coherent optical transmission system using multiple subcarrier multiplexing. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] M. Sandell et al., “Novel IQ imbalance estimation for wideband MIMO OFDM systems with CFO”, 2019 IEEE Global Communications Conference (GLOBECOM) [Non-Patent Document 2] T. Kobayashi et al., “35-Tb / s C-Band Transmission Over 800 km Employing 1-Tb / s PS-64QAM Signals Enhanced by Complex 8 × 2 MIMO Equalizer “, 2019 Optical Fiber Communications Conference and Exhibition (OFC) [Non-Patent Document 3] Edson Porto da Silva et al., “Widely Linear Equalization for IQ Imbalance and Skew Compensation in Optical Coherent Receivers“, Journal of Lightwave Technology (Volume: 34, Issue: 15, Aug.1, 1 2016) Summary of the Invention [Problem to be solved by the invention]

[0007] As mentioned above, the SC multiplexing method used in high-speed digital coherent optical transmission systems has the problem that reception characteristics are degraded by noise caused by IQ mixing that occurs in paired subcarriers.

[0008] An object of the present disclosure is to provide an optical transmission device, a system, a method, and a non-transitory computer-readable medium that solves the above-mentioned problems. [Means for solving the problem]

[0009] An optical transmission device according to the present disclosure includes: a pilot adding means for adding a first pilot signal to a first data signal to generate a first digital signal, and adding a second pilot signal to a second data signal to generate a second digital signal; an optical modulation means for optically modulating the first digital signal with a first subcarrier included in a negative frequency band with respect to a center frequency of a frequency band used, and for optically modulating the second digital signal with a second subcarrier included in a positive frequency band with respect to the center frequency of the frequency band used to generate an optically modulated signal, and for transmitting the optically modulated signal; Equipped with The pilot adding means The second pilot signal is not transmitted during transmission of the first pilot signal; While the second pilot signal is being transmitted, the first pilot signal is not transmitted.

[0010] An optical transmission device according to the present disclosure includes: pilot adding means for adding a first pilot signal to a first data signal to generate a first digital signal, adding a second pilot signal to a second data signal to generate a second digital signal, adding a third pilot signal to a third data signal to generate a third digital signal, and adding a fourth pilot signal to a fourth data signal to generate a fourth digital signal; an optical modulation means for optically modulating the first digital signal with X polarization using a first subcarrier included in a negative frequency band with respect to a center frequency of a used frequency band, optically modulating the second digital signal with X polarization using a second subcarrier included in a positive frequency band with respect to the center frequency of the used frequency band to generate an X optical modulated signal, optically modulating the third digital signal with Y polarization using the first subcarrier, and optically modulating the fourth digital signal with Y polarization using the second subcarrier to generate a Y optical modulated signal, and transmitting the X optical modulated signal and the Y optical modulated signal; a polarization combining means for combining the X optical modulated signal and the Y optical modulated signal; Equipped with The pilot adding means During transmission of the first pilot signal and the third pilot signal, the second pilot signal and the fourth pilot signal are not transmitted; The first pilot signal and the third pilot signal are not transmitted while the second pilot signal and the fourth pilot signal are being transmitted.

[0011] An optical transmission device according to the present disclosure includes: pilot adding means for adding a first pilot signal to a first data signal to generate a first digital signal, adding a second pilot signal to a second data signal to generate a second digital signal, adding a third pilot signal to a third data signal to generate a third digital signal, and adding a fourth pilot signal to a fourth data signal to generate a fourth digital signal; an optical modulation means for optically modulating the first digital signal with a first subcarrier included in a negative frequency band with respect to a center frequency of a used frequency band, optically modulating the second digital signal with a second subcarrier included in a negative frequency band with respect to the center frequency of the used frequency band, optically modulating the third digital signal with a third subcarrier included in a positive frequency band with respect to the center frequency of the used frequency band, and optically modulating the fourth digital signal with a fourth subcarrier included in a positive frequency band with respect to the center frequency of the used frequency band to generate an optical modulated signal, and transmitting the optically modulated signal; Equipped with The pilot adding means During transmission of the first pilot signal and the second pilot signal, the third pilot signal and the fourth pilot signal are not transmitted; The first pilot signal and the second pilot signal are not transmitted while the third pilot signal and the fourth pilot signal are being transmitted.

[0012] The system according to the present disclosure comprises: An optical transmission device; another optical transmission device that receives an optically modulated signal from the optical transmission device via an optical transmission line; Equipped with The optical transmission device includes: a pilot adding means for adding a first pilot signal to a first data signal to generate a first digital signal, and adding a second pilot signal to a second data signal to generate a second digital signal; an optical modulation means for optically modulating the first digital signal with a first subcarrier included in a negative frequency band with respect to a center frequency of a frequency band used, and for optically modulating the second digital signal with a second subcarrier included in a positive frequency band with respect to the center frequency of the frequency band used to generate the optically modulated signal, and for transmitting the optically modulated signal; having The pilot adding means The second pilot signal is not transmitted during transmission of the first pilot signal; The first pilot signal is not transmitted while the second pilot signal is being transmitted; Another optical transmission device, an optical signal receiving means for receiving the first digital signal obtained by coherently detecting the optical modulated signal and the second digital signal obtained by coherently detecting the optical modulated signal; a position detection means for detecting a position of the first pilot signal within the first digital signal and a position of the second pilot signal within the second digital signal; a frequency characteristic difference compensating means for compensating for a frequency characteristic difference between an in-phase component and a quadrature component of the first data signal and a frequency characteristic difference between an in-phase component and a quadrature component of the second data signal, using the first pilot signal and the second pilot signal; has.

[0013] The method according to the present disclosure comprises: adding a first pilot signal to the first data signal to generate a first digital signal, and adding a second pilot signal to the second data signal to generate a second digital signal; optically modulating the first digital signal with a first subcarrier included in a negative frequency band with respect to a center frequency of a frequency band used, and optically modulating the second digital signal with a second subcarrier included in a positive frequency band with respect to the center frequency of the frequency band used to generate an optically modulated signal, and transmitting the optically modulated signal; not transmitting the second pilot signal during transmission of the first pilot signal; not transmitting the first pilot signal while transmitting the second pilot signal; Equipped with.

[0014] The present disclosure relates to a non-transitory computer readable medium, comprising: adding a first pilot signal to the first data signal to generate a first digital signal, and adding a second pilot signal to the second data signal to generate a second digital signal; optically modulating the first digital signal with a first subcarrier included in a negative frequency band with respect to a center frequency of a frequency band used, and optically modulating the second digital signal with a second subcarrier included in a positive frequency band with respect to the center frequency of the frequency band used to generate an optically modulated signal, and transmitting the optically modulated signal; not transmitting the second pilot signal during transmission of the first pilot signal; not transmitting the first pilot signal while transmitting the second pilot signal; A program that causes a computer to execute the above is stored. Effect of the Invention

[0015] According to the present disclosure, it is possible to provide an optical transmission device, a system, a method, and a non-transitory computer-readable medium capable of reducing noise generated by IQ mixing in a subcarrier multiplexing system. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a block diagram illustrating a digital coherent optical transmission system. [Diagram 2] FIG. 13 is a schematic diagram illustrating a waveform after subcarrier synthesis. [Diagram 3] FIG. 13 is a schematic diagram illustrating a waveform after subcarrier synthesis. [Figure 4] 1 is a block diagram illustrating a MIMO equalizer of an optical transmission device and its input / output. [Diagram 5] 1 is a block diagram illustrating an optical transmission device according to a first embodiment. [Figure 6] 1 is a block diagram illustrating an optical transmission device according to a first embodiment. [Figure 7] 1 is a block diagram illustrating an optical transmission device according to a first embodiment. [Figure 8] 1 is a block diagram illustrating a system according to a first embodiment. [Figure 9] 1 is a block diagram illustrating a MIMO equalizer according to a first embodiment. [Figure 10] 3 is a schematic diagram illustrating a state of transmission of a pilot signal according to the first embodiment. FIG. [Figure 11] 2 is a schematic diagram illustrating a MIMO equalizer of the optical transmission device according to the first embodiment and its input / output. [Figure 12] 2 is a schematic diagram illustrating a MIMO equalizer of the optical transmission device according to the first embodiment and its input / output. [Figure 13] 1 is a block diagram illustrating a MIMO equalizer according to a first embodiment. [Figure 14] 1 is a block diagram illustrating a MIMO equalizer according to a first embodiment. [Figure 15] 11 is a block diagram illustrating a 2×2 MIMO equalizer according to a second embodiment. FIG. [Figure 16] FIG. 11 is a block diagram illustrating a 4x2 MIMO equalizer according to a third embodiment. [Figure 17] FIG. 13 is a block diagram illustrating a 4×2 MIMO equalizer according to a fourth embodiment. [Figure 18] 13 is a schematic diagram illustrating a part of a receiving unit of an optical transmission device according to a fifth embodiment. FIG. [Figure 19] 13 is a schematic diagram illustrating a part of a transmitting unit of an optical transmission device according to a fifth embodiment. FIG. [Figure 20] 13 is a schematic diagram illustrating a part of a receiving unit of an optical transmission device according to a sixth embodiment. FIG. [Figure 21] 13 is a schematic diagram illustrating a part of a transmitting unit of an optical transmission device according to a sixth embodiment. FIG. [Figure 22] FIG. 23 is a schematic diagram illustrating a state of transmission of a pilot signal according to a seventh embodiment. [Diagram 23] FIG. 23 is a schematic diagram illustrating a state of transmission of a pilot signal according to a seventh embodiment. [Figure 24] FIG. 23 is a schematic diagram illustrating a state of transmission of a pilot signal according to a seventh embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In each drawing, the same or corresponding elements are given the same reference numerals, and for clarity of explanation, duplicate explanations will be omitted as necessary.

[0018] <Challenges> First, the details of the problem will be described. FIG. 1 is a block diagram illustrating a digital coherent optical transmission system.

[0019] As shown in FIG. 1, the transmitting means of the optical transmission device of the optical transmission system generates a data signal by performing processing such as encoding on information to be transmitted. The transmitting means of the optical transmission device generates an I component SC1-XI of the X polarization of the first subcarrier, a Q component SC1-XQ of the X polarization of the first subcarrier, an I component SC1-YI of the Y polarization of the first subcarrier, and a Q component SC1-YQ of the Y polarization of the first subcarrier from the data signal. The transmitting means of the optical transmission device optically modulates each of these components with a Mach-Zehnder (MZ) type modulator to generate an optically modulated signal. The transmitting means of the optical transmission device polarization-combines the generated optically modulated signal and transmits it to another optical transmission device via an optical transmission path. The I component is sometimes called an in-phase component, and the Q component is sometimes called an orthogonal component. The optical transmission path is sometimes called an optical fiber.

[0020] The receiving means of the other optical transmission device performs polarization separation of the optically modulated signal received from the optical transmission device, and then converts the signal into an I component of the X polarization of the first subcarrier and a Q component of the X polarization of the first subcarrier using a 90-degree hybrid. The receiving means of the other optical transmission device converts the signal into an I component of the Y polarization of the first subcarrier and a Q component of the Y polarization of the first subcarrier using another 90-degree hybrid. The receiving means of the other optical transmission device coherently detects these converted components (signals) and converts them into electrical signals, and samples these electrical signals using a high-speed ADC (Analog Digital Converter). The receiving means of the optical transmission device performs digital signal processing on the sampled signals to correct waveform distortion. In the digital signal processing, the waveform distortion is corrected using a MIMO equalizer, which will be described later. The receiving means of the optical transmission device performs error correction on the corrected data to reproduce the information.

[0021] Here, the influence of IQ mixing occurring in the transmitting means of an optical transmission device will be described.

[0022] FIG. 2 is a schematic diagram illustrating a waveform after subcarrier synthesis. FIG. 2 shows the waveform after the first and second subcarriers are combined. FIG. 2 shows the case where the frequency characteristics of the I lane are equal to the frequency characteristics of the Q lane. FIG. 3 is a schematic diagram illustrating a waveform after subcarrier synthesis. FIG. 3 shows the waveform after the first and second subcarriers are combined. FIG. 3 shows a case where the frequency characteristics of the I lane are not equal to the frequency characteristics of the Q lane. 2 and 3, the horizontal axis indicates frequency, and the vertical axis indicates power. FIG. 4 is a block diagram illustrating a MIMO equalizer of an optical transmission device and its inputs and outputs.

[0023] As shown in FIG. 2, when the frequency characteristics of the I lane are equal to the frequency characteristics of the Q lane, that is, when there is no difference between the frequency characteristics of the I lane and the frequency characteristics of the Q lane, there is no effect due to IQ mixing.

[0024] On the other hand, as shown in FIG. 3, when the frequency characteristics of the I lane are not equal to the frequency characteristics of the Q lane, that is, when there is a difference between the frequency characteristics of the I lane and the Q lane, the conjugate component SC2 of the second subcarrier is mixed with the first subcarrier SC1 by IQ mixing. * In addition, the conjugate component SC1 of the first subcarrier is added to the second subcarrier SC2 by IQ mixing. * In other words, IQ mixing occurs due to the conjugate components of each subcarrier SC. Note that IQ mixing occurs between paired subcarriers SC (first subcarrier SC1 and second subcarrier SC2 in Fig. 3) due to differences in delay, amplitude, frequency characteristics, etc. between the I lane and Q lane.

[0025] The conjugate component of the second subcarrier, SC2, is generated by IQ mixing. * and the conjugate component SC1 of the first subcarrier * As a result of this, a signal with a waveform as shown in FIG. 4 is input to the input of the MIMO equalizer. Here, for the MIMO equalizer, the conjugate component SC2 of the second subcarrier * and the conjugate component of the first subcarrier SC1 * The difference between the frequency characteristics of the I and Q receivers in the optical transmission equipment is interference or noise. The MIMO equalizer is a device for compensating for the difference in frequency characteristics between the I and Q receivers in the optical transmission equipment. Therefore, in an environment where noise is input to the MIMO equalizer, it is difficult to compensate for the difference in frequency characteristics between the I and Q receivers using the MIMO equalizer.

[0026] In an environment where noise is included due to IQ mixing, it is difficult to derive the optimal filter coefficients for the MIMO equalizer, and the reception characteristics deteriorate (degrade). In other words, the filter coefficients do not converge to the optimal values, so the reception characteristics deteriorate.

[0027] Therefore, in the first embodiment, the conjugate component SC2 of the second subcarrier is mixed by IQ mixing. * and the conjugate component SC1 of the first subcarrier *Even when the above-mentioned two types of mixing occur, the MIMO equalizer is prevented from being affected by them. A specific configuration for preventing the MIMO equalizer from being affected by IQ mixing will be described in the first embodiment.

[0028] [Embodiment 1] <Device configuration: 2SC configuration (minimum configuration)> FIG. 5 is a block diagram illustrating an optical transmission device according to the first embodiment. FIG. 5 illustrates a minimum configuration of the optical transmission device according to the first embodiment. For simplicity, FIG. 5 shows only the configuration of the transmitting means of the optical transmission device.

[0029] As shown in FIG. 5, the optical transmission device 11 according to the first embodiment includes a pilot adding unit 111 and an optical modulating unit 112.

[0030] The pilot adding means 111 adds (inserts) a first pilot signal to a first data signal to generate a first digital signal, and adds (inserts) a second pilot signal to a second data signal to generate a second digital signal.

[0031] The optical transmission device 11 may include a data signal generating means for performing processing such as encoding on information to be transmitted to generate a data signal. The data signal includes a first data signal and a second data signal. The pilot adding means may also be referred to as a pilot inserting means. The first pilot signal and the second pilot signal may also be collectively referred to as a pilot signal. The pilot signal is a known signal required to reproduce the data signal.

[0032] The optical modulation means 112 optically modulates the digital subcarrier multiplexed signal as follows. The optical modulation means 112 optically modulates the first digital signal with a first subcarrier SC1 included in a negative frequency band with respect to the center frequency of the frequency band used, and optically modulates the second digital signal with a second subcarrier SC2 included in a positive frequency band with respect to the center frequency of the frequency band used to generate an optically modulated signal. Specifically, the optical modulation means 112 optically modulates the in-phase component SC1-I and the quadrature component SC1-Q of the first digital signal with the first subcarrier SC1, and optically modulates the in-phase component SC2-I and the quadrature component SC2-Q of the second digital signal with the second subcarrier SC2 to generate an optically modulated signal. Thereafter, the optical modulation means 112 transmits the generated optically modulated signal. Note that a frequency band that is negative with respect to the center frequency refers to a frequency band that is lower than the center frequency. Also, a frequency band that is positive with respect to the center frequency refers to a frequency band that is equal to or higher than the center frequency.

[0033] The optical modulation means 112 may optically modulate each of the first digital signal and the second digital signal by phase modulation or quadrature modulation.

[0034] The optical modulation means 112 may perform optical modulation using a Mach-Zehnder (MZ) type modulator.

[0035] The pilot adding means 111 does not transmit the second pilot signal while the first pilot signal is being transmitted, and also does not transmit the first pilot signal while the second pilot signal is being transmitted.

[0036] As a result, during the transmission of the first pilot signal of the first subcarrier SC1, the conjugate component SC2 of the second subcarrier caused by IQ mixing * Similarly, during the transmission of the second pilot signal of the second subcarrier SC2, the conjugate component SC1 of the first subcarrier due to IQ mixing does not occur. * As a result, no interference or noise (conjugate component SC2 of the second subcarrier) occurs. * and the conjugate component of the first subcarrier SC1 *In an environment where no interference occurs, the MIMO equalizer can derive optimal filter coefficients, and reception characteristics do not deteriorate.

[0037] As a result, according to the first embodiment, it is possible to provide an optical transmission device, a system, a method, and a non-transitory computer-readable medium capable of reducing noise generated by IQ mixing in a subcarrier multiplexing system.

[0038] The optical transmission device 11 may include a first IQ conversion means for converting the first digital signal into an in-phase component and a quadrature component, and a second IQ conversion means for converting the second digital signal into an in-phase component and a quadrature component. In this case, the optical modulation means 112 optically modulates each of the in-phase component and the quadrature component of the first digital signal with the first subcarrier SC1, and optically modulates each of the in-phase component and the quadrature component of the second digital signal with the second subcarrier SC2 to generate an optically modulated signal.

[0039] The optical transmission device 11 may further include a subcarrier generating unit that generates a plurality of subcarriers. In this case, the first subcarrier SC1 and the second subcarrier SC2 may be selected from the plurality of subcarriers.

[0040] Moreover, the optical modulation means 112 of the optical transmission device 11 may further include an optical signal combining means for combining an in-phase component and a quadrature component of the optically modulated signal after optical modulation.

[0041] Furthermore, the optical transmission device 11 may include a pilot signal generating means for generating a first pilot signal and a second pilot signal.

[0042] Furthermore, the pilot adding means 111 may transmit the first pilot signal and the second pilot signal alternately.

[0043] Furthermore, the pilot adding means 111 may determine the length of the transmission time for transmitting the first data signal and the length of the transmission time for transmitting the first pilot signal based on the data amount of the first data signal. For example, when the data amount of the first data signal is greater than the data amount of the first pilot signal, the pilot adding means 111 sets the length of the transmission time for transmitting the first data signal to be longer than the length of the transmission time for transmitting the first pilot signal.

[0044] <Equipment configuration: 2SC XY polarization configuration> A case of a two-subcarrier XY polarization configuration will be described. FIG. 6 is a block diagram illustrating an optical transmission device according to the first embodiment. FIG. 6 illustrates an XY polarization configuration of the 2SC of the optical transmission device according to the first embodiment. For simplicity, FIG. 6 shows only the configuration of a transmitting section of an optical transmission device.

[0045] As shown in FIG. 6, an optical transmission device 11 having a 2SC XY polarization configuration includes a pilot adding unit 111, an optical modulating unit 112, and a polarization combining unit 113.

[0046] The pilot adding means 111 adds a first pilot signal to the first data signal to generate a first digital signal, adds a second pilot signal to the second data signal to generate a second digital signal, adds a third pilot signal to the third data signal to generate a third digital signal, and adds a fourth pilot signal to the fourth data signal to generate a fourth digital signal.

[0047] The optical modulation means 112 optically modulates the first digital signal with X polarization using a first subcarrier SC1 included in a negative frequency band with respect to the center frequency of the frequency band used, and optically modulates the second digital signal with X polarization using a second subcarrier SC2 included in a positive frequency band with respect to the center frequency of the frequency band used to generate an X optical modulation signal. Specifically, the optical modulation means 112 optically modulates the in-phase component SC1-XI and the quadrature component SC1-XQ of the first digital signal with X polarization using the first subcarrier SC1, and optically modulates the in-phase component SC2-XI and the quadrature component SC2-XQ of the second digital signal with X polarization using the second subcarrier SC2 to generate an optical modulation signal.

[0048] The optical modulation means 112 optically modulates the third digital signal with Y polarization using a first subcarrier SC1 included in a negative frequency band with respect to the center frequency of the frequency band used, and optically modulates the fourth digital signal with Y polarization using a second subcarrier SC2 included in a positive frequency band with respect to the center frequency of the frequency band used to generate a Y optical modulation signal. Specifically, the optical modulation means 112 optically modulates the in-phase component SC1-YI and the quadrature component SC1-YQ of the third digital signal with Y polarization using the first subcarrier SC1, and optically modulates the in-phase component SC2-YI and the quadrature component SC2-YQ of the fourth digital signal with Y polarization using the second subcarrier SC2 to generate an optical modulation signal.

[0049] The optical modulation means 112 transmits the generated X optical modulated signal and Y optical modulated signal. The polarization combining means 113 combines the X optical modulated signal and the Y optical modulated signal.

[0050] The pilot adding means 111 does not transmit the second pilot signal and the fourth pilot signal while transmitting the first pilot signal and the third pilot signal. Also, the pilot adding means 111 does not transmit the first pilot signal and the third pilot signal while transmitting the second pilot signal and the fourth pilot signal.

[0051] <Device configuration: 4SC configuration> A four-subcarrier configuration will be described. FIG. 7 is a block diagram illustrating an optical transmission device according to the first embodiment. FIG. 7 shows a 4SC configuration of the optical transmission device according to the first embodiment. For simplicity, FIG. 7 shows only the configuration of a transmitting section of an optical transmission device.

[0052] As shown in FIG. 7, the 4SC optical transmission device 11 includes a pilot adding unit 111 and an optical modulating unit 112.

[0053] The pilot adding means 111 adds a first pilot signal to the first data signal to generate a first digital signal, adds a second pilot signal to the second data signal to generate a second digital signal, adds a third pilot signal to the third data signal to generate a third digital signal, and adds a fourth pilot signal to the fourth data signal to generate a fourth digital signal.

[0054] The optical modulation means 112 optically modulates a first digital signal with a first subcarrier SC1 included in a negative frequency band with respect to the center frequency of the frequency band used, optically modulates a second digital signal with a second subcarrier SC2 included in a negative frequency band with respect to the center frequency of the frequency band used, optically modulates a third digital signal with a third subcarrier SC3 included in a positive frequency band with respect to the center frequency of the frequency band used, and optically modulates a fourth digital signal with a fourth subcarrier SC4 included in a positive frequency band with respect to the center frequency of the frequency band used to generate an optically modulated signal.

[0055] Specifically, the optical modulation means 112 optically modulates the in-phase component SC1-I and the orthogonal component SC1-Q of the first digital signal on the first subcarrier SC1, optically modulates the in-phase component SC2-I and the orthogonal component SC2-Q of the second digital signal on the second subcarrier SC2, optically modulates the in-phase component SC3-I and the orthogonal component SC3-Q of the third digital signal on the third subcarrier SC3, and optically modulates the in-phase component SC4-I and the orthogonal component SC4-Q of the fourth digital signal on the fourth subcarrier SC4 to generate an optically modulated signal.

[0056] The optical modulation means 112 transmits the generated optical modulated signal.

[0057] The pilot adding means 111 does not transmit the third pilot signal and the fourth pilot signal while transmitting the first pilot signal and the second pilot signal. Also, the pilot adding means 111 does not transmit the first pilot signal and the second pilot signal while transmitting the third pilot signal and the fourth pilot signal.

[0058] <System configuration> The system configuration will be described. FIG. 8 is a block diagram illustrating a system according to the first embodiment.

[0059] As shown in Fig. 8, a system 10 includes an optical transmission device 11 and another optical transmission device 12 that receives an optically modulated signal from the optical transmission device 11 via an optical transmission line. The optical transmission device 11 has already been described, so a description thereof will be omitted. The optical transmission device and the other optical transmission device have the same function. The system may be referred to as an optical transmission system.

[0060] In explaining the system, an example will be given in which an optically modulated signal is transmitted from an optical transmission device 11 to another optical transmission device 12. For simplicity, only the transmitting means of the optical transmission device 11 will be explained, and only the receiving means of the other optical transmission device 12 will be explained.

[0061] The other optical transmission device 12 includes an optical signal receiving unit 126 , a position detecting unit 127 , and a frequency characteristic difference compensating unit 128 .

[0062] The optical signal receiving means 126 receives a first digital signal obtained by coherently detecting the optically modulated signal, and a second digital signal obtained by coherently detecting the optically modulated signal.

[0063] The position detection means 127 detects the position of the first pilot signal in the first digital signal and the position of the second pilot signal in the second digital signal.

[0064] The frequency characteristic difference compensating means 128 compensates for the frequency characteristic difference between the in-phase component and the quadrature component of the first data signal using the first pilot signal.

[0065] The frequency characteristic difference compensating means 128 compensates for the frequency characteristic difference between the in-phase component and the quadrature component of the second data signal using the second pilot signal.

[0066] The frequency characteristic difference compensation means 128 has a MIMO (Multi Input Multi Output) equalizer for determining the frequency characteristic difference and performing compensation based on the frequency characteristic difference. The MIMO equalizer compensates for the frequency characteristic difference between the in-phase component and the quadrature component.

[0067] FIG. 9 is a block diagram illustrating a MIMO equalizer according to the first embodiment. FIG. 9 shows a 2x2 MIMO equalizer. FIG. 9 shows the case where the first subcarrier SC1 is on and the second subcarrier SC2 is off.

[0068] As shown in FIG. 9, the frequency characteristic difference compensation means 128 has a MIMO (Multi Input Multi Output) equalizer. The MIMO equalizer has a plurality of FIR (Finite Impulse Response) filters for compensating for the frequency characteristic difference. Each of the plurality of FIR filters has a filter coefficient. 11 , h 12 , h 21 , h 22 indicates the filter coefficients of the FIR filter.

[0069] The frequency characteristic difference compensation means 128 detects a frequency characteristic difference of the pilot signal using a MIMO equalizer. The frequency characteristic difference compensation means 128 compensates for a frequency characteristic difference of the data signal based on the detected frequency characteristic difference using the MIMO equalizer.

[0070] The difference in frequency characteristics between the in-phase and quadrature components (IQ) is mainly due to the devices (components) implemented in the optical transmission equipment. Therefore, the fluctuation period of the frequency characteristic difference is on the order of minutes or hours. Therefore, adaptive equalization such as a MIMO equalizer is not necessary, and an equalizer with fixed filter coefficients (fixed equalizer) may be used.

[0071] Therefore, the frequency characteristic difference compensation means 128 may obtain and calculate the predetermined filter coefficients using a MIMO equalizer before the system is put into operation. Also, the frequency characteristic difference compensation means 128 may obtain the predetermined filter coefficients during the operation of the system. Such a MIMO equalizer using the predetermined filter coefficients obtained in advance is called a fixed equalizer.

[0072] The frequency characteristic difference compensation means 128 may detect the frequency characteristic difference by operating a MIMO equalizer using a predetermined filter coefficient obtained in advance. Moreover, the frequency characteristic difference compensation means 128 compensates for the frequency characteristic difference of the data signal based on the detected frequency characteristic difference.

[0073] <Operation> The operation of the optical transmission device according to the first embodiment will be described. FIG. 10 is a schematic diagram illustrating a state of transmission of a pilot signal according to the first embodiment. The horizontal axis of FIG. 10 indicates time, and the vertical axis indicates frequency. FIG. 11 is a schematic diagram illustrating a MIMO equalizer and its input / output in the optical transmission device according to the first embodiment. FIG. 11 shows a state when the first pilot signal is transmitted. FIG. 12 is a schematic diagram illustrating a MIMO equalizer of the optical transmission device according to the first embodiment and its input / output. FIG. 12 shows a state when the second pilot signal is transmitted.

[0074] IQ mixing occurs between subcarriers SC that are paired around DC, for example, the first subcarrier SC1 and the second subcarrier SC2 (see FIG. 3).

[0075] In order to reduce the influence of IQ mixing, the optical transmission device 11 according to the first embodiment does not transmit the second pilot signal of the second subcarrier SC2 while transmitting the first pilot signal of the first subcarrier SC1, as shown in Fig. 10. Also, the optical transmission device 11 does not transmit the first pilot signal of the first subcarrier SC1 while transmitting the second pilot signal of the second subcarrier SC2. In other words, the optical transmission device 11 generates and transmits a subcarrier multiplexed signal in which a pair of subcarriers SC (first subcarrier SC1 and second subcarrier SC2) that are symmetrical to DC on the frequency axis are alternately set to zero on the time axis.

[0076] It should be noted that the number of subcarriers SC is not limited to 2. The number of subcarriers SC may be a number other than 2 as long as it is a multiple of 2 (an even number).

[0077] The optical transmission device 11 periodically alternates between setting the pilot signal of one of the subcarriers SC to zero, so that the received first subcarrier SC1r contains the conjugate of the second subcarrier SC2r, as shown in FIG. * Also, as shown in FIG. 12, the received second subcarrier SC2r does not include the conjugate of the first subcarrier SC1r. * Does not include.

[0078] As a result, when deriving the filter coefficients of a MIMO equalizer that compensates for the difference in frequency characteristics between the in-phase component and the quadrature component (between the I lane and the Q lane) in the receiving section of an optical transmission device, it is possible to avoid the effects of IQ mixing and derive optimal filter coefficients.

[0079] As a result, according to the first embodiment, it is possible to provide an optical transmission device, a system, a method, and a non-transitory computer-readable medium capable of reducing noise generated by IQ mixing in a subcarrier multiplexing system.

[0080] Note that the pilot signal may be inserted periodically with respect to the data signal. Further, the optical transmission device 11 may sequentially update the filter coefficients of the MIMO equalizer using the pilot signal. Also, since the transmission time of the pilot signal with respect to the entire transmission time is determined, even if the transmission of the pilot signal of one subcarrier SC is turned off, the transmission capacity of the data signal does not change.

[0081] <Specific Example of MIMO Equalizer> <2x2 MIMO> Here, a specific example of the MIMO equalizer provided in another optical transmission device 12 (which may be the optical transmission device 11) will be described. 2x2 MIMO will be described.

[0082] As shown in FIG. 9 described above, in the MIMO equalizer of the frequency characteristic compensation means 128, the first subcarrier SC1r received by another optical transmission device 12 and the conjugate SC1r of the first subcarrier * are input. In Embodiment 1, while the first subcarrier SC1 is being transmitted, the second subcarrier SC2 is not transmitted (see FIG. 10). Therefore, the second subcarrier SC2r and the conjugate SC2r of the second subcarrier * are not input.

[0083] The MIMO equalizer compensates for the frequency characteristic difference between the I and Q of the receiver (reception unit) of another optical transmission device 12. The MIMO equalizer compensates for the frequency characteristic difference between the I and Q based on the pilot signal included in the received first subcarrier SC1r.

[0084] The relationship between the first subcarrier SC1 and the received subcarrier SC1r is shown in Equation 1. Also, the relationship between the second subcarrier SC2 and the received subcarrier SC1r is shown in Equation 2. However, h 11 h 21 h 12 h 22 represents the filter coefficients of the FIR filter included in the MIMO equalizer. Also, since the first subcarrier SC1 is on and the second subcarrier SC2 is off, SC2 = 0.

[0085] SC1=h 11 SC1r+h 21 SC1r ** (Formula 1) SC2=h 12 SC1r * +h 22 SC1r * =0 (formula 2)

[0086] FIG. 13 is a block diagram illustrating the MIMO equalizer according to the first embodiment. FIG. 13 shows a 2x2 MIMO equalizer. FIG. 13 shows the case where the first subcarrier SC1 is off and the second subcarrier SC2 is on.

[0087] As shown in FIG. 13, the MIMO equalizer includes a second subcarrier SC2r and a conjugate of the second subcarrier SC2r received by another optical transmission device 12. * In the first embodiment, the first subcarrier SC1 is not transmitted while the second subcarrier SC2 is being transmitted (see FIG. 10). Therefore, the MIMO equalizer receives the first subcarrier SC1r and the conjugate of the first subcarrier SC1r. * Do not enter:

[0088] The MIMO equalizer compensates for the difference in frequency characteristics between I and Q of a receiver (receiving section) of another optical transmission device 12. The MIMO equalizer compensates for the difference in frequency characteristics between I and Q based on a pilot signal included in the received second subcarrier SC2r.

[0089] The relationship between the first subcarrier SC1 and the received subcarrier SC1r is shown in Equation 3. Moreover, the relationship between the second subcarrier SC2 and the received subcarrier SC2r is shown in Equation 4. Since the first subcarrier SC1 is off and the second subcarrier SC2 is on, SC1=0.

[0090] SC1=h 11 SC2r * +h 21 SC2r * =0 (Formula 3) SC2=h 12SC2r ** +h 22 SC2r (Eq. 4)

[0091] The MIMO equalizer calculates the filter coefficients of the FIR filter based on Equations 1, 2, 3, and 4. The MIMO equalizer compensates for the difference in frequency characteristics between the I and Q signals of the receiving means by using the calculated filter coefficients of the FIR filter.

[0092] <4x4 MIMO> Explaining 4x4 MIMO. FIG. 14 is a block diagram illustrating the MIMO equalizer according to the first embodiment. FIG. 14 shows a 4x4 MIMO equalizer in the case of polarization multiplexing of XY polarization.

[0093] As shown in Fig. 14, the MIMO equalizer has the following components during a given period: the first subcarrier X-polarized wave SC1xr, the first subcarrier Y-polarized wave SC1yr, and the conjugate of the second subcarrier X-polarized wave SC2xr. * and the conjugate of the Y polarization of the second subcarrier SC2yr * In addition, the MIMO equalizer receives the conjugate SC1xr * and the conjugate of the Y polarization of the first subcarrier SC1yr * The second subcarrier X-polarized wave SC2xr and the second subcarrier Y-polarized wave SC2yr are input.

[0094] The first subcarrier X-polarized SC1x and the received subcarriers (SC1xr, SC1yr, SC2xr * , SC2yr * ) is shown in Equation 5. The relationship between the Y polarized wave SC1y of the first subcarrier and the received subcarrier is shown in Equation 6. The relationship between the X polarized wave SC2x of the second subcarrier and the received subcarrier is shown in Equation 7. The relationship between the Y polarized wave SC2y of the second subcarrier and the received subcarrier is shown in Equation 8. However, h 11 From 44 denotes the filter coefficient of the FIR filter in the MIMO equalizer.

[0095] SC1x =h 11 SC1xr + h 21 SC1yr + h 31 SC2xr * +h 41 SC2yr * (Equation 5) SC1y =h 12 SC1xr + h 22 SC1yr + h 32 SC2xr * +h 42 SC2yr * (Equation 6) SC2x =h 13 SC1xr * +h 23 SC1yr * +h 33 SC2xr + h 43 SC2yr (Equation 7) SC2y =h 14 SC1xr * +h 24 SC1yr * +h 34 SC2xr + h 44 SC2yr (Equation 8)

[0096] In the first embodiment, the subcarriers of the X-polarized wave and the Y-polarized wave are paired. The pilot signals of the subcarriers SC of the X-polarized wave and the Y-polarized wave are controlled to be turned on and off at the same timing. Specifically, the X-polarized wave SC1x of the first subcarrier and the Y-polarized wave SC1y of the first subcarrier are turned on, and the X-polarized wave SC2x of the second subcarrier and the Y-polarized wave SC2y of the second subcarrier are turned off. Also, the X-polarized wave SC1x of the first subcarrier and the Y-polarized wave SC1y of the first subcarrier are turned off, and the X-polarized wave SC2x of the second subcarrier and the Y-polarized wave SC2y of the second subcarrier are turned on. Note that IQ mixing occurs between the I and Q components of the X-polarized wave and between the I and Q components of the Y-polarized wave, so the X-polarized wave and the Y-polarized wave may be independently controlled to be turned on and off. Specifically, the X-polarized wave SC1x of the first subcarrier is turned on, and the X-polarized wave SC2x of the second subcarrier is turned off. Also, the first subcarrier Y polarized wave SC1y is on and the second subcarrier Y polarized wave SC2y is off. Also, the on / off conditions of the subcarriers are not limited to the above conditions. For example, any one subcarrier may be on and all other subcarriers may be off.

[0097] The MIMO equalizer calculates the filter coefficients of the FIR filter based on Equation 5, Equation 6, Equation 7, Equation 8, and the on / off conditions of the subcarriers. The MIMO equalizer uses the calculated filter coefficients of the FIR filter to compensate for the difference in frequency characteristics between the I and Q signals in the receiving section.

[0098] As described above, by using a 4x4 MIMO equalizer, it is possible to support XY polarization multiplexing. The number of subcarriers SC can be an integer multiple of 2 (2, 4, 6, ...). In this case, 2x2 MIMO is used for each set of subcarriers SC. For example, in the case of two subcarriers, one set of 2x2 MIMO is used, and in the case of four subcarriers, two sets of 2x2 MIMO are used.

[0099] [Embodiment 2] The system 20 according to the second embodiment is different from the system 10 according to the first embodiment in that the separate optical transmission device 12 has a light source error compensation means. The light source error compensation means compensates for a frequency error between a modulation light source that the optical transmission device 11 has for optical modulation and a detection light source that the separate optical transmission device 12 has for coherent detection.

[0100] FIG. 15 is a block diagram illustrating a 2×2 MIMO equalizer according to the second embodiment. The 2x2 MIMO equalizer according to the second embodiment includes a frequency error detection means, a compensation means, and a light source error compensation means.

[0101] The first subcarrier SC1 and the received subcarriers (SC1r, SC2r * ) is shown in Equation 9. In addition, the relationship between the second subcarrier SC2 and the received subcarrier (SC1r * , SC2r) and is shown in Equation 9.

[0102] SC1 =(Δfh 11 -Δfh 13 )SC1r+(Δfh 21 -Δfh 23 )SC2r * (Formula 9) SC2 =(Δfh 12 -Δfh 14 )SC1r * +(Δfh 22 -Δfh 24 )SC2r (Equation 10) Here, Δf indicates a frequency error between the light source of the transmitting means of the optical transmission device 11 and the light source of the receiving means of another optical transmission device 12. The MIMO equalizer according to the second embodiment compensates for the difference in frequency characteristics between the IQ of the transmitting means (transmitting section) and the receiving means (receiving section) by using the calculated filter coefficients of the FIR filter.

[0103] [Embodiment 3] The system 30 according to the third embodiment differs from the system 10 according to the first embodiment in that another optical transmission device 12 has a chromatic dispersion compensating means. The chromatic dispersion compensating means compensates for the in-phase and quadrature components of chromatic dispersion that occurs when an optically modulated signal transmits through an optical transmission line.

[0104] FIG. 16 is a block diagram illustrating a 4×2 MIMO equalizer according to the third embodiment. In FIG. 16, the first subcarrier SC1 is the in-phase component I SC1 and the quadrature component Q SC1 The second subcarrier SC2 is divided into the in-phase component I SC2 and the quadrature component Q SC2 The component after passing through the chromatic dispersion compensator (CDC) (chromatic dispersion compensated component) was divided into I SC1c , Q SC1c , I SC2c , Q SC2 It was decided. The 4x2 MIMO equalizer according to the third embodiment includes a frequency error detection means and a compensation means (CDC: Chromatic Dispersion Compensation).

[0105] As shown in FIG. 16, the chromatic dispersion compensation means uses a chromatic dispersion compensator (CDC) to compensate for the in-phase component I SC1 and the quadrature component Q SC1 and the in-phase component I of the chromatic dispersion of the second subcarrier SC2 SC2 and the quadrature component Q SC2 The chromatic dispersion compensator is disposed, for example, in front of the 4x2 MIMO equalizer. The chromatic dispersion compensating means may compensate for chromatic dispersion using, for example, a chromatic dispersion compensating filter.

[0106] When an IQ signal, which is a mixture of an in-phase component and a quadrature component, is compensated for by a chromatic dispersion compensator, IQ mixing occurs. Therefore, in the third embodiment, the in-phase component (I signal) and the quadrature component (Q signal) are dispersion compensated separately. This makes it possible to reduce the occurrence of IQ mixing.

[0107] The first subcarrier SC1 and the in-phase component of the received first subcarrier SC1 after CDC I SC1c and the component Q after CDC of the orthogonal components SC1c and the component I after CDC of the in-phase component of the received second subcarrier SC2 SC2c and the component Q after CDC of the orthogonal components SC2c The relationship between and is shown in Equation 11. In addition, the component I SC1c and the component Q after CDC of the orthogonal components SC1c and the component I after CDC of the in-phase component of the received second subcarrier SC2 SC2c and the component Q after CDC of the orthogonal components SC2c The relationship between is shown in Equation 12.

[0108] SC1 =(h 11 I SC1c +jh 21 Q SC1c )+(h 31 I * SC2c +jh 41 Q * SC2c ) (Formula 11) SC2 =(h 12 I * SC1c +jh 22 Q * SC1c )+(h 32 I SC2c +jh 42 Q SC2c ) (Formula 12)

[0109] [Embodiment 4] The system 40 according to the fourth embodiment is a system in which the system 20 according to the second embodiment and the system according to the third embodiment are simultaneously applied.

[0110] FIG. 17 is a block diagram illustrating a 4×2 MIMO equalizer according to the fourth embodiment. The 4x2 MIMO equalizer according to the fourth embodiment has a chromatic dispersion compensator disposed in the front stage to compensate for the difference in frequency characteristics between IQ on the transmitter side and the receiver side.

[0111] The first subcarrier SC1 and the in-phase component I of the received first subcarrier SC1 SC1 and the quadrature component Q SC1 and the in-phase component I of the received second subcarrier SC2 SC2 and the quadrature component Q SC2 The relationship between and is shown in Equation 13. In addition, the in-phase component I SC1 and the quadrature component Q SC1 and the in-phase component I of the received second subcarrier SC2 SC2 and the quadrature component Q SC2 The relationship between is shown in Equation 14.

[0112] SC1 =((Δfh 11 -Δfh 13 )I SC1c +j(Δfh 21 -Δfh 23 )Q SC1c ) +((Δfh 31 -Δfh 33 )I * SC2c +j(Δfh 41 -Δfh 43 )Q * SC2c ) (Formula 13) SC2 =((Δfh 12 -Δfh 14 )I * SC1c +j(Δfh 22 -Δfh 24 )Q * SC1c ) +((Δfh 32 -Δfh 34 )I SC2c +j(Δfh 42 -Δfh 44 )Q SC2c ) (Formula 14)

[0113] [Embodiment 5] A system 50 according to the fifth embodiment detects and compensates for frequency characteristic differences using an equalizer while the multiple subcarriers SC are independent of each other. For example, when an equalizer is provided in a receiving means of an optical transmission device, the equalizer is provided after separating the multiple subcarriers SC. Also, for example, when an equalizer is provided in a transmitting means of an optical transmission device, the equalizer is provided before combining the multiple subcarriers SC. Note that combining is sometimes referred to as multiplexing.

[0114] <When installed after separation of subcarriers> A case will be described in which the equalizer is provided after separation of the multiple subcarriers SC. FIG. 18 is a schematic diagram illustrating a part of a receiving unit of an optical transmission device according to the fifth embodiment. As shown in FIG. In FIG. 18, the received first subcarrier is designated as SC1r, and the received second subcarrier is designated as SC2r. In the fifth embodiment, the second pilot signal on the second subcarrier SC2r is not received while the first pilot signal on the first subcarrier SC1r is being received, and the first pilot signal on the first subcarrier SC1r is not received while the second pilot signal on the second subcarrier SC2r is being received. For simplicity, in Fig. 18, the waveform during reception of the first subcarrier SC1r and the waveform during reception of the second subcarrier SC2r are shown overlapping each other. Note that both the first data signal on the first subcarrier SC1r and the second data signal on the second subcarrier SC1r are received.

[0115] As shown in FIG. 18, the equalizer compensates for the frequency characteristic difference after separating the first subcarrier SC1r and the second subcarrier SC2r. The first subcarrier SC1 includes the first digital signal, and the second subcarrier SC2 includes the second digital signal. IQ mixing is caused by the device, and adaptive equalization such as that of the MIMO equalizer is not necessary. Therefore, the equalizer according to the fifth embodiment is a fixed equalizer. In the fixed equalizer, a predetermined filter coefficient is used as the filter coefficient of the equalizer. The filter coefficient of the fixed equalizer uses a value calculated by an external computer based on the output data of the ADC (see FIG. 1). The filter coefficient of the fixed equalizer uses a value calculated in advance by an external computer by performing processing of any of the MIMO equalizers according to the first to fourth embodiments described above, for example.

[0116] <When installed before combining (multiplexing) subcarriers> A case will be described in which an equalizer is provided before combining a plurality of subcarriers SC. FIG. 19 is a schematic diagram illustrating a part of a transmitting unit of an optical transmission device according to the fifth embodiment. As shown in FIG.

[0117] As shown in Fig. 19, in order to compensate for the difference in frequency characteristics between the in-phase component and the quadrature component (between I and Q) of the subcarrier SC on the transmitting side of the optical transmission device, a transmitting-side MIMO equalizer may be provided in the transmitting means of the optical transmission device. The optical transmission device uses the equalizer to compensate for the difference in frequency characteristics in the transmitting means.

[0118] The frequency characteristic difference is, for example, a difference in frequency characteristics of delay and amplitude between the in-phase component and the quadrature component of the first pilot signal, and is also, for example, a difference in frequency characteristics of delay and amplitude between the in-phase component and the quadrature component of the second pilot signal.

[0119] Specifically, the optical transmission device has a transmission-side frequency characteristic difference compensation means on the transmission side, similar to the reception side, which compensates for the transmission-side frequency characteristic difference between the in-phase component and the quadrature component of the first data signal by using a first pilot signal, and compensates for the transmission-side frequency characteristic difference between the in-phase component and the quadrature component of the second data signal by using a second pilot signal.

[0120] The transmitting-side frequency characteristic difference compensation means includes a transmitting-side MIMO equalizer. The transmitting-side MIMO equalizer includes a plurality of transmitting-side FIR (Finite Impulse Response) filters for compensating for the transmitting-side frequency characteristic difference. Each of the plurality of transmitting-side FIR filters has a transmitting-side filter coefficient.

[0121] The transmitting-side frequency characteristic difference compensation means compensates for the transmitting-side frequency characteristic difference by using a transmitting-side MIMO equalizer instead of the frequency characteristic difference compensation means of the optical transmission device receiving the subcarrier SC.

[0122] The difference in frequency characteristics between the in-phase component and the quadrature component (between I and Q) is mainly caused by the devices (components) mounted in the optical transmission equipment. Therefore, the fluctuation period of the frequency characteristic difference is on the order of minutes or hours. Therefore, adaptive equalization such as that of a MIMO equalizer is not necessary. Therefore, the equalizer according to the fifth embodiment is an equalizer with fixed filter coefficients (fixed equalizer).

[0123] The transmitting-side frequency characteristic difference compensation means obtains predetermined transmitting-side filter coefficients in advance using a transmitting-side MIMO equalizer. Specifically, the predetermined transmitting-side filter coefficients use values ​​calculated by an external computer based on output data of the ADC (see FIG. 1). The predetermined transmitting-side filter coefficients use values ​​calculated in advance by an external computer performing processing of the MIMO equalizer of the above-mentioned second or fourth embodiment. Thereafter, the transmitting-side frequency characteristic difference compensation means operates the transmitting-side MIMO equalizer using the obtained predetermined transmitting-side filter coefficients to compensate for the transmitting-side frequency characteristic difference.

[0124] That is, a transmitting-side MIMO equalizer (fixed equalizer) having predetermined transmitting-side filter coefficients set thereto compensates for the transmitting-side frequency characteristic difference before combining the first digital signal on the first subcarrier SC1 and the second digital signal on the second subcarrier SC2.

[0125] [Embodiment 6] A system 60 according to the sixth embodiment uses an equalizer to compensate for differences in frequency characteristics when multiple subcarriers SC are multiplexed (combined). For example, when an equalizer is provided in a receiving means of an optical transmission device, the equalizer is provided before separating the multiple subcarriers SC. Also, for example, when an equalizer is provided in a transmitting means of an optical transmission device, the equalizer is provided after combining the multiple subcarriers SC. Note that multiplexing may also be referred to as combining.

[0126] <When installed before separating multiple subcarriers> A case will be described in which the equalizer is provided before separation of the multiple subcarriers SC. FIG. 20 is a schematic diagram illustrating a part of a receiving unit of an optical transmission device according to the sixth embodiment. As shown in FIG. In FIG. 20, the received first subcarrier is designated as SC1r, and the received second subcarrier is designated as SC2r. In the sixth embodiment, the second pilot signal on the second subcarrier SC2r is not received while the first pilot signal on the first subcarrier SC1r is being received, and the first pilot signal on the first subcarrier SC1r is not received while the second pilot signal on the second subcarrier SC2r is being received. For simplicity, in Fig. 20, the waveform during reception of the first subcarrier SC1r and the waveform during reception of the second subcarrier SC2r are shown overlapping each other. Note that both the first data signal on the first subcarrier SC1r and the second data signal on the second subcarrier SC1r are received.

[0127] As shown in FIG. 20, the SC collective equalizer compensates for the difference in frequency characteristics before separating the first digital signal of the first subcarrier SC1r and the second digital signal of the second subcarrier SC2r. IQ mixing is caused by the device, and adaptive equalization like that of the MIMO equalizer is not necessary. Therefore, the SC collective equalizer according to the sixth embodiment is a fixed equalizer. In the fixed equalizer, a predetermined filter coefficient is used as the filter coefficient of the equalizer. The filter coefficient of the fixed equalizer uses a value calculated by an external computer based on the output data of the ADC (see FIG. 1). The filter coefficient of the fixed equalizer uses a value calculated in advance by an external computer performing processing of any of the MIMO equalizers according to the first to fourth embodiments. The filter coefficient of the SC collective equalizer uses a predetermined filter coefficient, and the SC collective equalizer is operated as a fixed equalizer to perform SC collective compensation for the frequency characteristic difference between IQ. The filter coefficient of the SC collective equalizer (fixed equalizer) may be periodically updated.

[0128] <When provided after combining multiple subcarriers> A case will be described in which the equalizer is provided after combining a plurality of subcarriers SC.

[0129] FIG. 21 is a schematic diagram illustrating a part of a transmitting unit of an optical transmission device according to the sixth embodiment. As shown in FIG. In the sixth embodiment, the second pilot signal on the second subcarrier SC2 is not transmitted while the first pilot signal on the first subcarrier SC1 is being transmitted, and the first pilot signal on the first subcarrier SC1 is not transmitted while the second pilot signal on the second subcarrier SC2 is being transmitted. Note that both the first data signal on the first subcarrier SC1 and the second data signal on the second subcarrier SC1 are received.

[0130] As shown in FIG. 21, the SC lumped equalizer on the transmitting side compensates for the frequency characteristic difference on the transmitting side after mixing the first digital signal of the first subcarrier SC1 and the second digital signal of the second subcarrier SC2. IQ mixing is caused by the device, and adaptive equalization such as that of the MIMO equalizer is not necessary. Therefore, the SC lumped equalizer on the transmitting side according to the sixth embodiment is a fixed equalizer. In the fixed equalizer, a predetermined filter coefficient is used as the filter coefficient of the equalizer. The filter coefficient of the fixed equalizer uses a value calculated by an external computer based on the output data of the ADC (see FIG. 1). The filter coefficient of the fixed equalizer uses a value calculated in advance by an external computer performing processing of the MIMO equalizer of the second or fourth embodiment described above. The filter coefficient of the SC lumped equalizer uses a predetermined filter coefficient, and the SC lumped equalizer is operated as a fixed equalizer to perform SC lumped compensation for the frequency characteristic difference between IQ. The filter coefficient of the SC lumped equalizer (fixed equalizer) may be periodically updated.

[0131] [Embodiment 7] The optical transmission device 71 according to the seventh embodiment is characterized by the transmission pattern of the subcarriers SC. FIG. 22 is a schematic diagram illustrating a state of transmission of a pilot signal according to the seventh embodiment. FIG. 23 is a schematic diagram illustrating a state of transmission of a pilot signal according to the seventh embodiment. FIG. 24 is a schematic diagram illustrating a state of transmission of a pilot signal according to the seventh embodiment. 22 to 24, the horizontal axis indicates time, and the vertical axis indicates frequency.

[0132] As shown in FIG. 22, an optical transmission device 71 transmits a data signal and a pilot signal in a pattern in which the pilot signal is inserted between a data signal and another data signal.

[0133] 23, the optical transmission device 71 may transmit a pilot signal at all times for the purpose of calibrating a device that generates a frequency characteristic difference between I and Q. This transmission pattern is referred to as a first transmission pattern.

[0134] 24, the optical transmission device 71 may transmit the pilot signal at all times. This transmission pattern is called a second transmission pattern. The switching period from the first subcarrier SC1 to the second subcarrier SC2 in the second transmission pattern is longer than that in the first transmission pattern.

[0135] In the above embodiment, the present invention has been described as being configured as hardware, but the present invention is not limited to this. The present invention can also be realized by having a CPU (Central Processing Unit) execute a computer program to process each component.

[0136] In the above embodiment, the program can be stored and supplied to the computer using various types of non-transitory computer readable media. The non-transitory computer readable media includes various types of tangible storage media. Examples of the non-transitory computer readable media include magnetic recording media (specifically, flexible disks, magnetic tapes, and hard disk drives), magneto-optical recording media (specifically, magneto-optical disks), CD-ROMs (Read Only Memory), CD-Rs, CD-R / Ws, semiconductor memories (specifically, mask ROMs, PROMs (Programmable ROMs), and EPROMs (Erasable PROMs)), flash ROMs, and RAMs (Random Access Memory). The program may also be supplied to the computer by various types of transitory computer readable media. Examples of the transitory computer readable media include electrical signals, optical signals, and electromagnetic waves. The transitory computer readable media can supply the program to the computer via wired communication paths such as electric wires and optical fibers, or wireless communication paths.

[0137] Although the present invention has been described above with reference to the embodiment, the present invention is not limited to the above. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the invention.

[0138] The present invention is not limited to the above-described embodiment, and can be modified as appropriate without departing from the spirit and scope of the present invention.

[0139] A part or all of the above-described embodiments can be described as, but is not limited to, the following supplementary notes. (Appendix 1) a pilot adding means for adding a first pilot signal to a first data signal to generate a first digital signal, and adding a second pilot signal to a second data signal to generate a second digital signal; an optical modulation means for optically modulating the first digital signal with a first subcarrier included in a negative frequency band with respect to a center frequency of a frequency band used, and for optically modulating the second digital signal with a second subcarrier included in a positive frequency band with respect to the center frequency of the frequency band used to generate an optically modulated signal, and for transmitting the optically modulated signal; Equipped with The pilot adding means The second pilot signal is not transmitted during transmission of the first pilot signal; The first pilot signal is not transmitted while the second pilot signal is being transmitted. Optical transmission equipment. (Appendix 2) A data signal generating means is further provided for encoding information to be transmitted to generate a data signal, the data signals include the first data signal and the second data signal; 2. The optical transmission device according to claim 1. (Appendix 3) a first IQ conversion means for converting the first digital signal into an in-phase component and a quadrature component; a second IQ conversion means for converting the second digital signal into an in-phase component and a quadrature component; Further equipped with The optical modulation means includes: optically modulating each of an in-phase component and a quadrature component of the first digital signal with the first subcarrier, and optically modulating each of an in-phase component and a quadrature component of the second digital signal with the second subcarrier to generate the optical modulated signal; 3. The optical transmission device according to claim 1 or 2. (Appendix 4) the optical modulation means has an optical signal combining means for combining an in-phase component and a quadrature component of the optically modulated signal after optical modulation; 4. The optical transmission device according to claim 1 , (Appendix 5) Further comprising subcarrier generating means for generating a plurality of subcarriers; The first subcarrier and the second subcarrier are selected from the plurality of subcarriers. 5. The optical transmission device according to claim 1 . (Appendix 6) the optical modulation means optically modulates each of the first digital signal and the second digital signal by a phase modulation method or a quadrature modulation method; 6. The optical transmission device according to claim 1 . (Appendix 7) The optical modulation means performs optical modulation using a Mach-Zehnder (MZ) type modulator. 7. The optical transmission device according to claim 1 . (Appendix 8) Further comprising a pilot signal generating means for generating the first pilot signal and the second pilot signal. 8. The optical transmission device according to claim 1 . (Appendix 9) The pilot adding means alternately transmits the first pilot signal and the second pilot signal. 9. The optical transmission device according to claim 1 . (Appendix 10) The pilot adding means determines a length of a transmission time for transmitting the first data signal and a length of a transmission time for transmitting the first pilot signal based on a data amount of the first data signal. 10. The optical transmission device according to claim 1 . (Appendix 11) pilot adding means for adding a first pilot signal to a first data signal to generate a first digital signal, adding a second pilot signal to a second data signal to generate a second digital signal, adding a third pilot signal to a third data signal to generate a third digital signal, and adding a fourth pilot signal to a fourth data signal to generate a fourth digital signal; an optical modulation means for optically modulating the first digital signal with X polarization using a first subcarrier included in a negative frequency band with respect to a center frequency of a used frequency band, optically modulating the second digital signal with X polarization using a second subcarrier included in a positive frequency band with respect to the center frequency of the used frequency band to generate an X optical modulated signal, optically modulating the third digital signal with Y polarization using the first subcarrier, and optically modulating the fourth digital signal with Y polarization using the second subcarrier to generate a Y optical modulated signal, and transmitting the X optical modulated signal and the Y optical modulated signal; a polarization combining means for combining the X optical modulated signal and the Y optical modulated signal; Equipped with The pilot adding means During transmission of the first pilot signal and the third pilot signal, the second pilot signal and the fourth pilot signal are not transmitted; The first pilot signal and the third pilot signal are not transmitted during transmission of the second pilot signal and the fourth pilot signal. Optical transmission equipment. (Appendix 12) pilot adding means for adding a first pilot signal to a first data signal to generate a first digital signal, adding a second pilot signal to a second data signal to generate a second digital signal, adding a third pilot signal to a third data signal to generate a third digital signal, and adding a fourth pilot signal to a fourth data signal to generate a fourth digital signal; an optical modulation means for optically modulating the first digital signal with a first subcarrier included in a negative frequency band with respect to a center frequency of a used frequency band, optically modulating the second digital signal with a second subcarrier included in a negative frequency band with respect to the center frequency of the used frequency band, optically modulating the third digital signal with a third subcarrier included in a positive frequency band with respect to the center frequency of the used frequency band, and optically modulating the fourth digital signal with a fourth subcarrier included in a positive frequency band with respect to the center frequency of the used frequency band to generate an optical modulated signal, and transmitting the optically modulated signal; Equipped with The pilot adding means During transmission of the first pilot signal and the second pilot signal, the third pilot signal and the fourth pilot signal are not transmitted; The first pilot signal and the second pilot signal are not transmitted during transmission of the third pilot signal and the fourth pilot signal. Optical transmission equipment. (Appendix 13) An optical transmission device; another optical transmission device that receives an optically modulated signal from the optical transmission device via an optical transmission line; Equipped with The optical transmission device includes: a pilot adding means for adding a first pilot signal to a first data signal to generate a first digital signal, and adding a second pilot signal to a second data signal to generate a second digital signal; an optical modulation means for optically modulating the first digital signal with a first subcarrier included in a negative frequency band with respect to a center frequency of a frequency band used, and for optically modulating the second digital signal with a second subcarrier included in a positive frequency band with respect to the center frequency of the frequency band used to generate the optically modulated signal, and for transmitting the optically modulated signal; having The pilot adding means The second pilot signal is not transmitted during transmission of the first pilot signal; The first pilot signal is not transmitted while the second pilot signal is being transmitted; Another optical transmission device, an optical signal receiving means for receiving the first digital signal obtained by coherently detecting the optical modulated signal and the second digital signal obtained by coherently detecting the optical modulated signal; a position detection means for detecting a position of the first pilot signal within the first digital signal and a position of the second pilot signal within the second digital signal; a frequency characteristic difference compensating means for compensating for a frequency characteristic difference between an in-phase component and a quadrature component of the first data signal using the first pilot signal, and for compensating for a frequency characteristic difference between an in-phase component and a quadrature component of the second data signal using the second pilot signal; having system. (Appendix 14) Another optical transmission device has a light source error compensation means for compensating for a frequency error between a modulation light source that the optical transmission device has for optical modulation and a detection light source for the coherent detection. 14. The system of claim 13. (Appendix 15) the optical signal receiving means has a chromatic dispersion compensating means for compensating for an in-phase component and a quadrature component of chromatic dispersion generated by the optical modulated signal transmitting through the optical transmission line; 15. The system of claim 13 or 14. (Appendix 16) The chromatic dispersion compensation means compensates for the in-phase and quadrature components of the chromatic dispersion using a chromatic dispersion compensator (CDC). 16. The system of claim 15. (Appendix 17) The frequency characteristic difference compensation means has a MIMO (Multi Input Multi Output) equalizer, the MIMO equalizer has a plurality of FIR (Finite Impulse Response) filters for compensating for the frequency characteristic difference; Each of the plurality of FIR filters has filter coefficients; The frequency characteristic difference compensation means compensates for the frequency characteristic difference by using the MIMO equalizer. 14. The system of claim 13. (Appendix 18) The frequency characteristic difference compensation means comprises: determining predetermined filter coefficients using the MIMO equalizer; operating the MIMO equalizer using predetermined filter coefficients to compensate for the frequency characteristic difference; 18. The system of claim 17. (Appendix 19) The MIMO equalizer compensates for the frequency characteristic difference after separating the first digital signal and the second digital signal. 19. The system of claim 17 or 18. (Appendix 20) The MIMO equalizer compensates for the frequency characteristic difference before separating the first digital signal and the second digital signal. 19. The system of claim 17 or 18. (Appendix 21) the optical transmission device has a transmission-side frequency characteristic difference compensation means, at a transmission side, for compensating for a transmission-side frequency characteristic difference between an in-phase component and a quadrature component of the first data signal by using the first pilot signal, and compensating for a transmission-side frequency characteristic difference between the in-phase component and the quadrature component of the second data signal by using the second pilot signal; the transmitting-side frequency characteristic difference compensation means has a transmitting-side MIMO equalizer, the transmitting MIMO equalizer has a plurality of transmitting finite impulse response (FIR) filters for compensating for the transmitting frequency characteristic difference, Each of the plurality of transmitting FIR filters has a transmitting filter coefficient; the transmitting-side frequency characteristic difference compensation means compensates for the transmitting-side frequency characteristic difference by using the transmitting-side MIMO equalizer instead of the frequency characteristic difference compensation means of another of the optical transmission devices. 14. The system of claim 13. (Appendix 22) The transmission side frequency characteristic difference compensation means comprises: determining predetermined transmitting filter coefficients using the transmitting MIMO equalizer; operating the transmitting-side MIMO equalizer using predetermined transmitting-side filter coefficients to compensate for the transmitting-side frequency characteristic difference; 22. The system of claim 21. (Appendix 23) The transmitting-side MIMO equalizer compensates for the transmitting-side frequency characteristic difference before combining the first digital signal and the second digital signal. 23. The system of claim 22. (Appendix 24) the transmitting-side MIMO equalizer combines the first digital signal and the second digital signal, and then compensates for the transmitting-side frequency characteristic difference; 23. The system of claim 21 or 22. (Appendix 25) adding a first pilot signal to the first data signal to generate a first digital signal, and adding a second pilot signal to the second data signal to generate a second digital signal; optically modulating the first digital signal with a first subcarrier included in a negative frequency band with respect to a center frequency of a frequency band used, and optically modulating the second digital signal with a second subcarrier included in a positive frequency band with respect to the center frequency of the frequency band used to generate an optically modulated signal, and transmitting the optically modulated signal; not transmitting the second pilot signal during transmission of the first pilot signal; not transmitting the first pilot signal while transmitting the second pilot signal; A method for providing the above. (Appendix 26) adding a first pilot signal to the first data signal to generate a first digital signal, and adding a second pilot signal to the second data signal to generate a second digital signal; optically modulating the first digital signal with a first subcarrier included in a negative frequency band with respect to a center frequency of a frequency band used, and optically modulating the second digital signal with a second subcarrier included in a positive frequency band with respect to the center frequency of the frequency band used to generate an optically modulated signal, and transmitting the optically modulated signal; not transmitting the second pilot signal during transmission of the first pilot signal; not transmitting the first pilot signal while transmitting the second pilot signal; A non-transitory computer-readable medium on which a program for causing a computer to execute the program is stored. [Explanation of symbols]

[0140] 10, 20, 30, 40, 50, 60: System 11, 71: Optical transmission equipment 111: Pilot addition means 112: Light modulation means 113: Polarization combining means 12: Another optical transmission device 126: Optical signal receiving means 127: Position detection means 128: Frequency characteristic difference compensation means SC: Subcarrier SC1: First subcarrier SC2: Second subcarrier SC3: Third subcarrier SC4: 4th subcarrier

Claims

1. a pilot adding means for adding a first pilot signal to a first data signal to generate a first digital signal, and adding a second pilot signal to a second data signal to generate a second digital signal; an optical modulation means for optically modulating the first digital signal with a first subcarrier included in a negative frequency band with respect to a center frequency of a frequency band used, and for optically modulating the second digital signal with a second subcarrier included in a positive frequency band with respect to the center frequency of the frequency band used to generate an optically modulated signal, and for transmitting the optically modulated signal; Equipped with The pilot adding means The second pilot signal is not transmitted during transmission of the first pilot signal; The first pilot signal is not transmitted while the second pilot signal is being transmitted; determining a length of a transmission time for transmitting the first data signal and a length of a transmission time for transmitting the first pilot signal based on a data amount of the first data signal; determining a length of a transmission time for transmitting the second data signal and a length of a transmission time for transmitting the second pilot signal based on a data amount of the second data signal; Optical transmission equipment.

2. A data signal generating means is further provided for encoding information to be transmitted to generate a data signal, the data signals include the first data signal and the second data signal; 2. The optical transmission device according to claim 1.

3. a first IQ conversion means for converting the first digital signal into an in-phase component and a quadrature component; a second IQ conversion means for converting the second digital signal into an in-phase component and a quadrature component; Further equipped with The optical modulation means includes: optically modulating each of an in-phase component and a quadrature component of the first digital signal by the first subcarrier, and optically modulating each of an in-phase component and a quadrature component of the second digital signal by the second subcarrier to generate the optical modulated signal; 3. The optical transmission device according to claim 1 or 2.

4. the optical modulation means has an optical signal combining means for combining an in-phase component and a quadrature component of the optically modulated signal after optical modulation; 4. The optical transmission device according to claim 1.

5. Further comprising subcarrier generating means for generating a plurality of subcarriers; the first subcarrier and the second subcarrier are selected from the plurality of subcarriers; 5. The optical transmission device according to claim 1.

6. the optical modulation means optically modulates each of the first digital signal and the second digital signal by a phase modulation method or a quadrature modulation method; 6. The optical transmission device according to claim 1.

7. The optical modulation means performs optical modulation using a Mach-Zehnder (MZ) type modulator.

7. The optical transmission device according to claim 1.

8. further comprising a pilot signal generating means for generating the first pilot signal and the second pilot signal; 8. The optical transmission device according to claim 1.

9. pilot adding means for adding a first pilot signal to a first data signal to generate a first digital signal, adding a second pilot signal to a second data signal to generate a second digital signal, adding a third pilot signal to a third data signal to generate a third digital signal, and adding a fourth pilot signal to a fourth data signal to generate a fourth digital signal; an optical modulation means for optically modulating the first digital signal with X polarization using a first subcarrier included in a negative frequency band with respect to a center frequency of a used frequency band, optically modulating the second digital signal with X polarization using a second subcarrier included in a positive frequency band with respect to the center frequency of the used frequency band to generate an X optical modulated signal, optically modulating the third digital signal with Y polarization using the first subcarrier, and optically modulating the fourth digital signal with Y polarization using the second subcarrier to generate a Y optical modulated signal, and transmitting the X optical modulated signal and the Y optical modulated signal; a polarization combining means for combining the X-modulated light signal and the Y-modulated light signal; Equipped with The pilot adding means During transmission of the first pilot signal and the third pilot signal, the second pilot signal and the fourth pilot signal are not transmitted; During transmission of the second pilot signal and the fourth pilot signal, the first pilot signal and the third pilot signal are not transmitted; determining a length of a transmission time for transmitting the first data signal and a length of a transmission time for transmitting the first pilot signal based on a data amount of the first data signal; determining a length of a transmission time for transmitting the second data signal and a length of a transmission time for transmitting the second pilot signal based on a data amount of the second data signal; determining a length of a transmission time for transmitting the third data signal and a length of a transmission time for transmitting the third pilot signal based on a data amount of the third data signal; determining a length of a transmission time for transmitting the fourth data signal and a length of a transmission time for transmitting the fourth pilot signal based on a data amount of the fourth data signal; Optical transmission equipment.

10. An optical transmission device; another optical transmission device that receives an optically modulated signal from the optical transmission device via an optical transmission line; Equipped with The optical transmission device includes: a pilot adding means for adding a first pilot signal to a first data signal to generate a first digital signal, and adding a second pilot signal to a second data signal to generate a second digital signal; an optical modulation means for optically modulating the first digital signal with a first subcarrier included in a negative frequency band with respect to a center frequency of a frequency band used, and for optically modulating the second digital signal with a second subcarrier included in a positive frequency band with respect to the center frequency of the frequency band used to generate the optically modulated signal, and for transmitting the optically modulated signal; having The pilot adding means The second pilot signal is not transmitted during transmission of the first pilot signal; The first pilot signal is not transmitted while the second pilot signal is being transmitted; Another optical transmission device, an optical signal receiving means for receiving the first digital signal obtained by coherently detecting the optically modulated signal and the second digital signal obtained by coherently detecting the optically modulated signal; a position detection means for detecting a position of the first pilot signal within the first digital signal and a position of the second pilot signal within the second digital signal; a frequency characteristic difference compensating means for compensating for a frequency characteristic difference between an in-phase component and a quadrature component of the first data signal by using the first pilot signal, and for compensating for a frequency characteristic difference between an in-phase component and a quadrature component of the second data signal by using the second pilot signal; having system.

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