Reception device and frequency offset estimation method

Abstract

A reception device according to the present invention comprises: an acquisition unit that, from a transmission device, acquires a signal, the phase of which is time-interleaved; a mapping unit that maps the signal to a complex plane representing a real axis component and an imaginary axis component of the signal; and an estimation unit that estimates the amount of frequency offset of the signal on the basis of a distribution of the signal in the complex plane. The estimation unit may estimate the amount of frequency offset of the signal on the basis of the ellipticity of the distribution and the angle of the major axis of the distribution with respect to the real axis. A rotation unit may rotate the distribution so as to reduce the angle in the complex plane.

Description

Receiver and frequency offset estimation method

[0001] The present invention relates to a receiving apparatus and a frequency offset estimation method.

[0002] In the intensity modulation-direct detection method used in optical subscriber networks, the wider the bandwidth of the transmitted signal light, the lower the receiving sensitivity of the signal light can be. Furthermore, the waveform of the signal light can be degraded due to chromatic dispersion of the signal light. To address these issues, the application of digital coherent receiving technology to receiving devices (user terminals) is being considered. A receiving device that applies digital coherent receiving technology can compensate for chromatic dispersion through digital signal processing and improve receiving sensitivity.

[0003] Fig. 11 is a diagram showing an example of the configuration of a communication system in a core network. The transmitter includes a dual-polarization IQ (In-Phase, Quadrature-Phase) modulator (DP (Dual-Polarization)-IQ modulator). "TIA" shown in Fig. 11 stands for transimpedance amplifier. The transmitter uses the polarization and phase of light to superimpose information onto the signal light.

[0004] The receiving device is equipped with a polarization and phase diversity receiver. The receiving device performs photoelectric conversion using balanced receivers on the signal light separated into orthogonal polarization and orthogonal phase and the local light output from the local light source. The receiving device corrects, by digital signal processing, the chromatic dispersion, polarization rotation, and phase rotation that occur in the signal light transmitted through the transmission path. Although the optical front-end configuration in the receiving device becomes complex, wideband and highly sensitive reception and long-distance transmission of the signal light are possible.

[0005] Furthermore, in order to make the access network more economical, it is being considered to equip the receiving device with a receiver having a simpler configuration than the polarization / phase diversity receiver.

[0006] FIG. 12 is a diagram showing an example of the configuration of a communication system in an access network (see Non-Patent Document 1). The transmitting device performs Alamouti coding (polarization time block coding) on ​​the transmission signal. The transmitting device includes a digital signal processing unit for transmission (space-time multiplexing code) and a DP-IQ modulator. The receiving device includes a digital signal processing unit for reception (decoding processing unit). This allows the receiving device to receive signal light without being equipped with a polarization diversity configuration and independent of polarization.

[0007] In the homodyne system, the receiving device matches the wavelength of the signal light with the wavelength of the local light. In contrast, in Figure 12, the receiving device separates the phase of the I axis from the phase of the Q axis using the heterodyne system. In the heterodyne system, a difference is created between the wavelength of the signal light and the wavelength of the local light. In addition, the band of the signal light is down-converted to an intermediate frequency band.

[0008] The heterodyne receiver in the receiving device includes an optical splitter (e.g., a 3 dB coupler), a local oscillator, and a single balanced receiver. The digital signal processing unit of the receiving device converts the analog-to-digital (AD) converted signal superimposed on the intermediate frequency band into a baseband signal. The digital signal processing unit of the receiving device performs Alamouti decoding on the baseband signal.

[0009] 12, the number of photodetectors and the number of AD converters can be reduced to one-fourth of that required when the receiving device is equipped with a polarization and phase diversity receiver. However, if a Nyquist filter is not used, an electrical bandwidth at least twice the signal bandwidth "B" is required.

[0010] Fig. 13 is a diagram showing an example of the configuration of a communication system that performs phase diversity reception (see Non-Patent Document 2). A single-polarization IQ modulator in a transmitting device transmits a phase-modulated signal (e.g., a signal subjected to m-ary Quadrature Amplitude Modulation (mQAM)) to a receiving device. The receiving device includes an optical branching device (e.g., a 3 dB coupler), a frequency-modulated light generator, and a balanced light source. The frequency-modulated light generator includes an external modulator on the output side of the local light source for phase diversity reception.

[0011] The frequency-modulated optical generator rotates the phase of the local light by 90 degrees at twice the symbol rate. Two consecutive samples converted from analog to digital at twice the symbol rate are regarded as I-axis and Q-axis signals in the phase diversity receiver. The I-axis and Q-axis signals are mapped onto the IQ plane (complex plane).

[0012] Non-Patent Document 3 discloses a communication system that performs phase diversity reception. The single-polarization IQ modulator in the transmitting device disclosed in Non-Patent Document 3 performs time interleaving on a signal whose phase has been rotated by 90 degrees. The receiving device disclosed in Non-Patent Document 3 also performs oversampling processing on the time-interleaved signal. The receiving device decodes two consecutive samples as an I-axis signal and a Q-axis signal. This makes it possible to further reduce the receiving bandwidth.

[0013] Also, Non-Patent Document 4 discloses the configuration of a transmitting device that generates a signal whose phase is time-interleaved (hereinafter referred to as a "phase-interleaved signal").

[0014] M. S. Faruk, H. Louchet, M. S. Erkilinc, et al., “DSP algorithms for recovering single-carrier Alamouti coded signals for PON applications,” Opt. Express 24, 24083-24091 (2016).Zhengxuan Li, Fan Yin, Xingang Huang, Zhuang Ma, Yingxiong Song, and Lilin Yi, "Demonstration of a 50G-PON with a 45-dB power budget using an IQ-interleaved coherent detection scheme," Opt. Express 29, 32523-32534 (2021)S. Yan, C. Lu, A. P. T. Lau, R. Nejabati and D. Simeonidou, "Simple Intradyne Receiver with Time-switched Phase Diversity for Optical Interconnects," 2017 Asia Communications and Photonics Conference (ACP), Guangzhou, China, 2017, pp. 1-3.P. Torres-Ferrera, G. Rizzelli, H. Wang, V. Ferrero and R. Gaudino, "Experimental Demonstration of 100 Gbps / λ C-Band Direct-Detection Downstream PON Using Non-Linear and CD Compensation with 29 dB+ OPL Over 0 Km-100 Km," in Journal of Lightwave Technology, vol. 40, no. 2, pp. 547-556, 15 Jan.15, 2022, doi: 10.1109 / JLT.2021.3129446.

[0015] The communication systems disclosed in Non-Patent Documents 2 and 3 respectively achieve phase diversity reception while reducing the electrical bandwidth. However, when the frequency difference between the signal light and the local light is large, the orthogonality between the I-axis signal and the Q-axis signal is lost, resulting in degradation of signal quality. For this reason, it is desirable to realize a signal processing unit that can easily estimate the frequency difference based on signals whose orthogonality is lost.

[0016] 14 is a diagram showing an example of the configuration of a transmitting device. The transmitting device generates a phase-interleaved signal. The transmitting device transmits an original symbol (I-axis signal) and a signal (Q-axis signal) whose phase is rotated by −90 degrees relative to the phase of the original symbol at a modulation speed twice the symbol rate.

[0017] 15 is a diagram showing an example of the configuration of a receiving device. The receiving device includes a local light source, an optical splitter (e.g., a 3 dB coupler), a balanced receiver, and a transimpedance amplifier. ｒ (t)" is expressed as in equation (1).

[0018]

[0019] Here, "R" represents the efficiency of photoelectric conversion for the received signal light. ｓ " represents the intensity of the received signal light. ＬＯ " represents the intensity of the local light output from the local light source. The receiving device generates a received signal based on the beat component between the received signal light and the local light source. The modulation component is superimposed on the phase component of the received signal. In equation (1), the modulation component is expressed as the phase of the cosine "cos". In the following, the polarization state of the signal light and the polarization state of the local light are the same. "ω ｓ " represents the angular velocity of the signal light. "ω ＬＯ " represents the angular velocity of the local light. "θ ｓ " represents the phase component of the symbol in the transmitted signal light. ｎ " represents phase noise.

[0020] The reception digital signal processing unit recovers an I-axis signal and a Q-axis signal orthogonal to the I-axis signal from the reception signal. Here, the reception digital signal processing unit performs AD conversion on the reception signal at a sampling rate "2T" that is twice the symbol rate "T". The reception digital signal processing unit performs serial-parallel conversion on a signal (I-axis reception signal) corresponding to the time slot of the I-axis signal of the AD-converted signal. The reception digital signal processing unit also performs serial-parallel conversion on a signal (Q-axis reception signal) corresponding to the time slot of the Q-axis signal of the AD-converted signal. The reception digital signal processing unit inputs the serial-parallel converted signal to an adaptive equalization filter as a complex signal.

[0021] I-axis (real axis component) received signal (time signal) "S ｒＩ (t)" is expressed as in equation (2).

[0022]

[0023] Q-axis (imaginary axis component) received signal (time signal) "S ｒＱ (t)" is expressed as in equation (3).

[0024]

[0025] Since the transmitter rotates the phase by -90 degrees (=-π / 2), the received signal on the I axis, "S ｒＩ (t)" and the Q-axis received signal "S ｒＱ (t)" are considered to be quadrature signals.

[0026] FIG. 16 is a diagram showing an example of the amount of phase rotation caused by a frequency offset. The sampling timing of the received signal on the I axis is different from the sampling timing of the received signal on the Q axis. Therefore, when a frequency offset exists in the received signal, a phase rotation caused by the frequency offset occurs between the received signal on the I axis and the received signal on the Q axis. This phase rotation amount "φ = ((ω ｓ -ω ＬＯ )×T / 2)) destroys the orthogonality between the real axis component (I axis signal) and the imaginary axis component (Q axis signal).

[0027] In order to compensate for the phase rotation caused by the frequency offset, the frequency offset must be estimated. However, estimating the frequency offset requires complex signal processing, such as fast Fourier transform to convert a time-domain signal into a frequency-domain signal.

[0028] Furthermore, in a system in which the amount of phase rotation is compensated for in the subsequent stage of the adaptive equalization filter, if a phase-sensitive algorithm is used, the coefficients of the adaptive equalization filter may not converge, and therefore the signal-to-noise ratio required for the frequency offset estimation unit to estimate the amount of frequency offset may not be ensured.

[0029] As described above, there is a problem in that it is not possible to improve the accuracy of estimating the amount of frequency offset in a phase interleaved signal while suppressing the complexity of the configuration of signal processing for the phase interleaved signal.

[0030] In view of the above circumstances, an object of the present invention is to provide a receiving device and a frequency offset estimation method that can improve the accuracy of estimating the amount of frequency offset in a phase interleaved signal while suppressing the complexity of the signal processing configuration for the phase interleaved signal.

[0031] One aspect of the present invention is a receiving device comprising an acquisition unit that acquires a signal whose phase is time-interleaved from a transmitting device, a mapping unit that maps the signal onto a complex plane representing real axis components and imaginary axis components of the signal, and an estimation unit that estimates a frequency offset amount of the signal based on the distribution of the signal on the complex plane.

[0032] One aspect of the present invention is a frequency offset estimation method executed by a receiving device, the frequency offset estimation method including the steps of: acquiring a phase-time-interleaved signal from a transmitting device; mapping the signal onto a complex plane representing real axis components and imaginary axis components of the signal; and estimating a frequency offset amount of the signal based on a distribution of the signal on the complex plane.

[0033] According to the present invention, it is possible to improve the accuracy of estimating the amount of frequency offset in a phase interleaved signal while suppressing the complexity of the configuration of signal processing for the phase interleaved signal.

[0034] 1 is a diagram illustrating an example of the configuration of a communication system according to a first embodiment. ｒ 10 is a diagram illustrating an example of the distribution of the received signal "S (t)" mapped onto the IQ plane in the first embodiment. ｒ 10 is a diagram illustrating an example of parameters representing the distribution of "(t)". ​​FIG. 10 is a diagram illustrating an example of the configuration of an estimator in the first embodiment. FIG. 10 is a diagram illustrating an example of the configuration of an estimator in the second embodiment. FIG. 10 is a diagram illustrating an example of measurement processing in the second embodiment. FIG. 10 is a diagram illustrating an example of the configuration of a communication system in the third embodiment. FIG. 10 is a diagram illustrating an example of the configuration of a communication system in the fourth embodiment. FIG. 10 is a diagram illustrating an example of the configuration of a communication system in the fifth embodiment. FIG. 10 is a diagram illustrating an example of the hardware configuration of a communication device in each embodiment. FIG. 10 is a diagram illustrating an example of the configuration of a communication system in a core network. FIG. 10 is a diagram illustrating an example of the configuration of a communication system in an access network. FIG. 10 is a diagram illustrating an example of the configuration of a communication system that performs phase diversity reception. FIG. 10 is a diagram illustrating an example of the configuration of a transmitting device. FIG. 10 is a diagram illustrating an example of the configuration of a receiving device. FIG. 10 is a diagram illustrating an example of the amount of phase rotation caused by a frequency offset.

[0035] Embodiments of the present invention will be described in detail with reference to the drawings. (First Embodiment) FIG. 1 is a diagram showing an example of the configuration of a communication system 1a in the first embodiment. The communication system 1a is a system that communicates using a transmitted phase-interleaved signal. The communication system 1a achieves phase diversity reception using a simple configuration. The phase-interleaved signal may be transmitted using light or radio waves. That is, the communication system 1a may be an optical communication system or a wireless communication system. In the following, the phase-interleaved signal is transmitted using light, as an example. The modulation method of the phase-interleaved signal is not limited to a specific modulation method. In the following, the modulation method is multilevel quadrature amplitude modulation (mQAM), as an example.

[0036] The communication system 1a compensates for the phase rotation (waveform distortion) of the phase interleaved signal based on the frequency offset estimated based on the distribution of the phase interleaved signal in the IQ plane (complex plane). Here, the communication system 1a estimates the frequency offset of the phase interleaved signal by simple signal processing.

[0037] The communication system 1a includes a transmitter 2a, a transmission path 3, and a receiver 4a. The transmission path 3 includes an optical fiber.

[0038] The receiving device 4a includes, as an acquisition unit (receiving unit), a local light source 41, an optical splitter 42, two photodiodes 43, a transimpedance amplifier 44, and an AD converter 45. The receiving device 4a further includes a received digital signal processing unit 46a. The received digital signal processing unit 46a includes an adjustment unit 47, a conversion unit 48, a mapping unit 49, an estimating unit 50a, and an adaptive equalization filter 51a.

[0039] The transmitting device 2a transmits a phase-interleaved signal (signal light) via the transmission path 3 to the optical splitter 42 of the receiving device 4a.

[0040] In the receiving device 4a, a local light source 41 outputs local light to an optical splitter 42. The optical splitter 42 splits the received signal light and the local light source to each photodiode 43 "PD." Each photodiode 43 outputs a current signal corresponding to the intensity of the signal light and the local light to a transimpedance amplifier 44 "TIA." The transimpedance amplifier 44 outputs a voltage signal corresponding to the current signal to an AD converter 45. The AD converter 45 converts the voltage signal into a digital signal.

[0041] The adjustment unit 47 adjusts the sampling clock for the voltage signal. ｒＩ (t)" and the Q-axis (imaginary axis component) received signal "S ｒＱ The conversion unit 48 (serial-to-parallel conversion unit) converts the received signal "S ｒＩ (t)" and the Q-axis received signal "SｒＱ (t)" to the mapping unit 49.

[0042] The mapping unit 49 (quadrature phase restoration unit) handles each received signal as a complex number. That is, the mapping unit 49 maps the received signal "S" on the I axis to ｒＩ (t)" and the Q-axis received signal "S ｒＱ (t)) onto the IQ plane (complex plane). ｒＩ (t)" and "S ｒＱ (t)" are distributed on the IQ plane. The mapping unit 49 outputs symbol arrangement information (constellation information) of each received signal distributed on the IQ plane to the estimation unit 50a.

[0043] FIG. 2 shows the received signal “S” mapped onto the IQ plane in the first embodiment. ｒ 10 is a diagram showing an example distribution of "(t)". ​​The phase rotation amount "φ" of the received signal is the amount of phase rotation in a time period equivalent to half the symbol length of the received signal. Therefore, when the phase rotation amount "φ=π / 2" is, the frequency offset amount is signal band "B" / 2. When the phase rotation amount "φ=-π / 2" is, the frequency offset amount is signal band "B" / (-2).

[0044] When the phase rotation amount is "φ=π / 2", the shape of the distribution of the received signal is a line tilted by "θ=45 degrees" with respect to the I axis. When the phase rotation amount is "0<φ<π / 2", the shape of the distribution of the received signal is an ellipse with its major axis tilted by "θ=45 degrees" with respect to the I axis. When the phase rotation amount is "φ=0", the shape of the distribution of the received signal is close to a perfect circle. When the phase rotation amount is "-π / 2<φ<0", the shape of the distribution of the received signal is an ellipse with its major axis tilted by "θ=-45 degrees" with respect to the I axis. When the phase rotation amount is "φ=-π / 2", the shape of the distribution of the received signal is a line tilted by "θ=-45 degrees" with respect to the I axis. As such, the positive and negative sides of the angle "θ" of the major axis are reversed depending on the positive and negative sides of the phase rotation amount "φ", with the I axis as the reference axis.

[0045] More specifically, the received signal on the I axis “S ｒＩ (t)) is expressed as in the above equation (3). ｒＱ (t)" is expressed as in equation (4).

[0046]

[0047] When the phase rotation amount is "φ=π / 2", "S ｒＱ (t)" is "cos(θ ｓ -π / 2+π / 2)=cos(θ ｓ Therefore, in "φ=π / 2", "S ｒＱ (t)" is "S ｒＩ (t)”(=cos(θ ｓ On the other hand, when the phase rotation amount is "φ=-π / 2", "S ｒＱ (t)" is "cos(θ ｓ -π / 2-π / 2)=-cos(θ ｓ ) is expressed as

[0048] Furthermore, between the phase rotation amount "φ=0" and the phase rotation amount "φ=π / 2", the received signal "S ｒ Similarly, between the phase rotation amount "φ=0" and the phase rotation amount "φ=-π / 2", the distribution shape of the received signal "S ｒ The distribution shape of the received signal "S(t)" is elliptical. ｒ If the shape of the distribution of (t) is elliptical, the signal quality will be degraded.

[0049] FIG. 3 shows the received signal “S” mapped onto the IQ plane in the first embodiment. ｒ 10 is a diagram showing an example of parameters representing the distribution of the received signal "S (t)". ｒ The shape of the distribution of "(t)" is expressed, for example, by ellipticity. The ellipticity is expressed, for example, by the ratio "b / a" of the major axis length "a" to the minor axis length "b". The reference axis of the angle "θ" of the major axis of the ellipse is, for example, the I axis (real axis).

[0050] Returning to FIG. 1, the description of the exemplary configuration of the communication system 1a continues. The ellipticity and angle change depending on the phase rotation amount "φ" caused by the frequency offset. Therefore, the estimation unit 50a estimates the received signal "S ｒ The frequency offset is estimated based on the distribution of "(t)".

[0051] For example, the estimation unit 50a estimates the frequency offset amount based on the ellipticity and the positive or negative value of the angle "θ" of the major axis (the phase of the ellipse). Here, the estimation unit 50a may estimate the frequency offset amount using a data table that indicates the correspondence between the ellipticity, the angle, and the frequency offset amount, and using the ellipticity and the angle as search keys. Furthermore, the estimation unit 50a may be provided with a trained model having a neural network in advance. The estimation unit 50a may acquire the value of the frequency offset amount from the trained model by inputting the ellipticity and the angle into the trained model that has been trained in advance using a machine learning technique.

[0052] The estimation unit 50a estimates the received signal "S ｒ The adaptive equalization filter 51a performs adaptive equalization processing on the received signal whose frequency offset has been compensated for. In this way, the adaptive equalization filter 51a compensates for waveform degradation of the received signal input from the estimation unit 50a.

[0053] 4 is a diagram showing an example of the configuration of the estimator 50a (frequency offset compensator) in the first embodiment. The estimator 50a includes a replicator 501, a measurer 502a, a determiner 503, and a compensator 504.

[0054] The replicating unit 501 replicates each received signal (symbol allocation information) distributed on the IQ plane. The replicating unit 501 replicates the replicated received signal “S ｒ (t)" to the measurement unit 502a and the compensation unit 504.

[0055] The measurement unit 502a measures the received signal "S ｒThe measurement unit 502a measures the ellipticity and the angle for the distribution of "(t)." Here, the measurement unit 502a may measure the combination of the ellipticity and the angle by using the distribution of the received signal as a search key, using a data table that indicates the correspondence between the distribution of the received signal and the combination of the ellipticity and the angle. The measurement unit 502a may be provided with a trained model having a neural network in advance. The measurement unit 502a may input the distribution of the received signal to a trained model that has been trained in advance using a machine learning technique, and thereby acquire the combination of the ellipticity and the angle from the trained model.

[0056] The determination unit 503 determines the frequency offset amount based on the combination of the ellipticity and the angle. The determination unit 503 may determine the frequency offset amount using a data table indicating correspondence between combinations of ellipticity and the angle and the frequency offset amount, using the combination of the ellipticity and the angle as a search key. The determination unit 503 may be provided in advance with a trained model having a neural network. The determination unit 503 may acquire the value of the frequency offset amount from the trained model by inputting the combination of the ellipticity and the angle to the trained model that has been trained in advance using a machine learning technique.

[0057] The compensation unit 504 compensates for the received signal "S ｒ The compensation unit 504 compensates for the frequency offset of the received signal "S ｒ The compensation unit 504 outputs the signal (t) to the adaptive equalization filter 51a. The compensation unit 504 may be provided at a stage subsequent to the adaptive equalization filter 51a.

[0058] As described above, the receiving device 4a may include, for example, a local light source 41, an optical splitter 42, two photodiodes 43, a transimpedance amplifier 44, and an AD converter 45 as an acquisition unit. The acquisition unit acquires a phase-interleaved signal from the transmitting device 2a. The mapping unit 49 maps the phase-interleaved signal onto a complex plane (IQ plane) representing the real axis component (I-axis amplitude) and imaginary axis component (Q-axis amplitude) of the phase-interleaved signal. The estimating unit 50a estimates the frequency offset of the phase-interleaved signal based on the distribution (symbol arrangement) of the phase-interleaved signal on the complex plane.

[0059] This makes it possible to improve the accuracy of estimating the frequency offset amount in a phase-interleaved signal while preventing the signal processing configuration for a signal whose phase is time-interleaved (phase-interleaved signal) from becoming complicated.

[0060] Second Embodiment The second embodiment is mainly different from the first embodiment in that the distribution of the received signal on the IQ plane is rotated by a predetermined angle in a predetermined rotation direction before the frequency offset amount estimation process is performed. The second embodiment will be described focusing on the differences from the first embodiment.

[0061] 5 is a diagram showing an example of the configuration of an estimation unit 50b in the second embodiment. The estimation unit 50b includes a replication unit 501, a measurement unit 502b, a determination unit 503, and a compensation unit 504. The measurement unit 502b includes a rotation unit 505 and a detection unit 506. The measurement unit 502b may further include an averaging unit 507.

[0062] The measurement unit 502b measures the received signal "S ｒ For the distribution of "(t)", the ellipticity "b / a" and the angle "θ" of the major axis are measured. Here, the rotation unit 505 rotates the received signal distributed on the IQ plane by a predetermined angle in a predetermined rotation direction (clockwise or counterclockwise). The predetermined angle is, for example, 45 degrees. The rotation unit 505 may rotate the received signal by 45 degrees clockwise (positive direction), or may rotate the received signal by 45 degrees counterclockwise (negative direction). This reduces the angle "θ" of the major axis, making it easier to detect the ellipticity.

[0063] The detector 506 detects the ellipticity "b / a" of the received signal distributed on the IQ plane based on the amplitude of each axis of the received signal rotated on the IQ plane within a predetermined frequency offset range (signal band "B" / (±2)). That is, the detector 506 detects the I-axis amplitude (real axis component) "x" and the Q-axis amplitude (imaginary axis component) "y" for the received signal rotated on the IQ plane. Here, the detector 506 may detect the peak of the time-series amplitude for each axis. The detector 506 may also detect the angle "θ" of the major axis.

[0064] The averaging unit 507 may average the amplitude of the time series for each axis, thereby making it possible to stably detect the amplitude "x" averaged along the I axis and the amplitude "y" averaged along the Q axis even if the amplitude of each time series fluctuates due to white noise.

[0065] 6 is a diagram showing an example of measurement processing in the second embodiment. When "y / x<1" is true, the angle "θ" is a positive value. When "y / x>1" is true, the angle "θ" is a negative value. When "y / x=1" is true, the angle "θ" may be treated as a positive value or a negative value for convenience. The detection unit 506 or the averaging unit 507 easily detects the ellipticity "b / a" and the angle "θ" based on the detected "y / x" and a predetermined correspondence relationship between the phase of the ellipse before and after rotation of the distribution.

[0066] As described above, the estimation unit 50b estimates the frequency offset amount of the phase-interleaved signal based on the ellipticity "b / a (=y / x or =x / y)" of the distribution of the phase-interleaved signal and the angle "θ" of the major axis of the distribution with respect to the real axis (I axis). Here, the rotation unit 505 rotates the distribution of the phase-interleaved signal so as to reduce the angle "θ" in the IQ plane. The determination unit 503 determines the frequency offset amount based on the ellipticity "b / a" detected based on the rotated distribution and whether the angle "θ" of the major axis is positive or negative.

[0067] In this way, inter-symbol interference caused by chromatic dispersion, which reduces the accuracy of estimating the frequency offset amount, is compensated for at a stage prior to the determining unit 503. This makes it possible to further improve the accuracy of estimating the frequency offset amount in a phase interleaved signal while preventing the signal processing configuration for the phase interleaved signal from becoming complicated. Also, it is possible to determine the frequency offset amount regardless of the signal modulation method.

[0068] Third Embodiment The third embodiment differs from the second embodiment mainly in that a control signal representing the frequency offset amount is fed back to the transmitting device. The third embodiment will be described focusing on the differences from the second embodiment.

[0069] 7 is a diagram showing an example of the configuration of a communication system 1c according to the third embodiment. A received digital signal processing unit 46c includes an adjustment unit 47, a conversion unit 48, a mapping unit 49, an estimation unit 50c, an adaptive equalization filter 51c, and a DA converter 52.

[0070] To compensate for the loss of orthogonality caused by the frequency offset, the difference between the frequency of the signal light and the frequency of the local light must be less than a predetermined value. Therefore, the estimator 50c and the DA converter 52 may adjust the oscillation frequency of the local light source 41 (the frequency of the local light) based on the estimated frequency offset so that the difference between the frequency of the signal light and the frequency of the local light is less than the predetermined value. For example, the DA converter 52 may convert a digital signal representing the frequency offset into an analog signal representing the frequency offset. The DA converter 52 may output the analog signal to the local light source 41.

[0071] The estimation unit 50c may feed back a control signal representing the amount of frequency offset to the transmitting device 2c. The transmitting device 2c may adjust the frequency (wavelength) of the phase-interleaved signal to be transmitted based on the control signal representing the amount of frequency offset.

[0072] As described above, the estimation unit 50c may feed back the frequency offset amount to the transmitting device 2c, which adjusts the wavelength of the phase-interleaved signal based on the frequency offset amount. Also, the estimation unit 50c may adjust the wavelength of the local light used to acquire the phase-interleaved signal using the local light source 41 and the DA converter 52.

[0073] This makes it possible to improve the accuracy of estimating the amount of frequency offset in a phase interleaved signal while preventing the configuration of signal processing for the phase interleaved signal from becoming complicated.

[0074] (Fourth Embodiment) In the fourth embodiment, the main difference from the third embodiment is the configuration of the received digital signal processing unit. In the fourth embodiment, the difference from the third embodiment will be mainly described.

[0075] In a receiving device using Alamouti space-time block code (STBC) (see Non-Patent Document 1), the reception digital signal processing unit may perform frequency offset compensation as a frequency offset compensating unit (see Non-Patent Document 2) that modulates the frequency of local oscillator light.

[0076] In addition, when a transmitting device transmits a phase-interleaved signal (IQ-interleaved signal) to a receiving device (see Non-Patent Document 3), a receiving digital signal processing unit using Alamouti's space-time block code (see Non-Patent Document 1) may perform frequency offset compensation as a frequency offset compensating unit.

[0077] 8 is a diagram showing an example of the configuration of a communication system 1d according to the fourth embodiment. The communication system 1d includes a mapping unit 49, a compensation processing unit 53, a conversion unit 48, an adaptive equalization filter 51d, an inverse conversion unit 54, and a decoding unit 55. The communication system 1d may include the estimation unit 50c of the third embodiment instead of the compensation processing unit 53.

[0078] The mapping unit 49 maps the received signal "S ｒＩ (t)" and the Q-axis received signal "S ｒＱ (t)" is mapped onto the IQ plane.

[0079] The compensation processing unit 53 or the estimation unit 50c calculates the received signal "S ｒ Based on the distribution of the received signal "S ｒ The compensation processing unit 53 or the estimation unit 50c may feed back a control signal representing the amount of frequency offset to the transmitting device 2d. The transmitting device 2d may adjust the frequency (wavelength) of the transmission signal including the Alamouti space-time block code (polarization time block code) based on the control signal representing the amount of frequency offset. Furthermore, the compensation processing unit 53 or the estimation unit 50c may adjust the oscillation frequency of the local light source 41 (the frequency of the local light) based on the amount of frequency offset.

[0080] The converter 48 converts the frequency offset compensated received signal "S ｒ The converter 48 performs serial-to-parallel conversion on the I-axis received signal "S ｒＩ (t)" and the Q-axis received signal "S ｒＱ (t)" to the adaptive equalization filter 51d.

[0081] The adaptive equalization filter 51d includes an adaptive equalization filter circuit for Alamouti's space-time block code (for polarization time block code) (see Non-Patent Document 1) for each of the I axis and the Q axis. ｒＩ (t)" and the Q-axis received signal "S ｒＱ (t)" and the adaptive equalization process is performed on each of them.

[0082] The inverse conversion unit 54 (parallel-serial conversion unit) performs parallel-to-serial conversion on the received signals for each axis that have been subjected to adaptive equalization processing. The decoding unit 55 (mQAM decoding unit) performs multilevel quadrature amplitude modulation (mQAM) decoding processing on the received signals that have been subjected to parallel-to-serial conversion.

[0083] As described above, the compensation processing unit 53 or the estimation unit 50c calculates the received signal "S ｒ Based on the distribution of the received signal "S ｒThe adaptive equalization filter 51d compensates for the frequency offset of the received signal "S (t)" using an adaptive equalization filter circuit for space-time block coding by Alamouti (see Non-Patent Document 1). ｒＩ (t)" and the received signal "S ｒＱ (t)" and executes adaptive equalization processing.

[0084] This makes it possible to improve the accuracy of estimating the amount of frequency offset in a phase interleaved signal while preventing the configuration of signal processing for the phase interleaved signal from becoming complicated.

[0085] Fifth Embodiment In the fifth embodiment, a transmitting device compensates for chromatic dispersion occurring in a phase-interleaved signal. The transmitting device of the fifth embodiment is also applicable to each of the first to fourth embodiments. That is, the transmitting device of the fifth embodiment is also applicable to cases where dual polarization modulated signals using Alamouti space-time block codes are transmitted and received. The fifth embodiment will be described focusing on the differences from the first to fourth embodiments.

[0086] 9 is a diagram showing an example of the configuration of a communication system 1e according to the fifth embodiment. The communication system 1e includes a transmitter 2e, a transmission path 3, and a receiver 4e. The transmitter 2e includes a transmission digital signal processing unit 21, a DA converter 22, a light source 23, and an IQ modulator 24. The transmission digital signal processing unit 21 includes an mQAM encoding unit 211, an IQ interleaved signal generation unit 212, and a chromatic dispersion compensation unit 213.

[0087] In general, in space-time block coding, signal characteristics are degraded due to signal interference between blocks. For example, signals (symbols) interfere with each other between the I-axis signal and the Q-axis signal of a phase-interleaved signal due to the influence of chromatic dispersion. This causes fluctuations in the phase of the phase-interleaved signal distributed on the IQ plane, which may reduce the accuracy of ellipticity detection.

[0088] Therefore, in the communication system 1e, the transmitting device 2e provides the phase-interleaved signal with information corresponding to the inverse characteristics of chromatic dispersion (transmission path characteristics) so that the receiving device 4e can remove signal interference caused by chromatic dispersion in the phase-interleaved signal. This keeps the shape of the phase-interleaved signal distributed on the IQ plane elliptical, thereby suppressing the possibility of a decrease in the detection accuracy of the ellipticity.

[0089] The mQAM encoder 211 acquires a predetermined transmission signal sequence from a higher-level device (not shown). The mQAM encoder 211 encodes the transmission signal sequence using multi-level quadrature amplitude modulation to generate a time-series signal “S ｔ The mQAM encoder 211 generates a time-series signal "S ｔ (t)" to the IQ interleaved signal generator 212.

[0090] The IQ interleaved signal generator 212 generates a time-series signal “S ｔ That is, the IQ interleaved signal generator 212 generates a phase interleaved signal (IQ interleaved signal) based on the time series signal "S ｔ (t)" and the I-axis (real axis) transmission signal "S Ｉ (t)" and the Q-axis (imaginary axis) transmission signal "S Ｑ The chromatic dispersion compensator 213 generates the transmission signal "S Ｉ (t)" and the transmitted signal "S Ｑ (t)" and "(t)" are given information corresponding to the inverse characteristics of chromatic dispersion (transmission path characteristics).

[0091] The DA converter 22 performs digital-to-analog conversion on each phase-interleaved signal to which information corresponding to the inverse characteristics has been given. The light source 23 outputs light of a predetermined wavelength to the IQ modulator 24. The IQ modulator 24 performs modulation processing on each phase-interleaved signal converted into an analog signal using the light input from the light source 23. The IQ modulator 24 transmits signal light based on each phase-interleaved signal to the receiving device 4e via the transmission path 3.

[0092] As described above, the transmitting device 2e transmits the phase interleaved signal "S Ｉ (t)" and the transmitted phase interleaved signal "S Ｑ (t)" and "(t)", information (inverse characteristic information) corresponding to the inverse characteristic of chromatic dispersion (transmission path characteristic) is given.

[0093] This makes it possible to improve the accuracy of estimating the amount of frequency offset in a phase interleaved signal while preventing the configuration of signal processing for the phase interleaved signal from becoming complicated.

[0094] 10 is a diagram showing an example of the hardware configuration of the communication device 5 in each embodiment. The example of the hardware configuration of the communication device 5 corresponds to the example of the hardware configuration of each of the transmitting device and the receiving device in each embodiment.

[0095] The communication device 5 is realized as software by a processor 101, such as a CPU (Central Processing Unit), executing a program stored in a storage device 103 having a non-volatile recording medium (non-transitory recording medium) and a memory 102. The program may be recorded on a computer-readable recording medium. Examples of computer-readable recording media include portable media such as a flexible disk, a magneto-optical disk, a ROM (Read Only Memory), and a CD-ROM (Compact Disc Read Only Memory), and non-transitory recording media such as a hard disk or a solid state drive (SSD) built into a computer system. The communication unit 104 executes predetermined communication processing.

[0096] The communication device 5 may be realized using hardware including an electronic circuit (electronic circuit or circuitry) using, for example, an LSI (Large Scale Integrated circuit), an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array).

[0097] Although an embodiment of the present invention has been described in detail above with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention.

[0098] The present invention is applicable to communication systems such as optical communication systems and wireless communication systems.

[0099] 1a, 1b, 1c, 1d, 1e...Communication systems, 2a, 2c, 2d, 2e...Transmitting devices, 3...Transmission path, 4a, 4b, 4c, 4d, 4e...Receiving devices, 5...Communication devices, 21...Transmitting digital signal processing unit, 22...DA converter, 23...Light source, 24...IQ modulator, 41...Local light source, 42...Optical branching unit, 43...Photodiode, 44...Transimpedance amplifier, 45...AD converter, 46a, 46c, 46d...Receiving digital signal processing unit, 47...Adjusting unit, 48...Converting unit, 49...Mapping unit 50a, 50b, 50c, 50d... Estimation unit, 51a, 51c, 51d... Adaptive equalization filter, 52... DA converter, 53... Compensation processing unit, 54... Inverse conversion unit, 55... Decoding unit, 101... Processor, 102... Memory, 103... Storage device, 104... Communication unit, 211... mQAM encoding unit, 212... IQ interleaved signal generation unit, 213... Wavelength dispersion compensation unit, 501... Replication unit, 502a, 502b... Measurement unit, 503... Determination unit, 504... Compensation unit, 505... Rotation unit, 506... Detection unit, 507... Averaging unit

Claims

an acquisition unit that acquires a phase-time interleaved signal from a transmitting device; a mapping unit that maps the signal onto a complex plane that represents a real axis component and an imaginary axis component of the signal; an estimation unit that estimates a frequency offset amount of the signal based on a distribution of the signal on the complex plane; A receiving device comprising: 　 The receiving device according to claim 1 , wherein the estimating unit estimates the amount of frequency offset of the signal based on the ellipticity of the distribution and an angle of the major axis of the distribution with respect to a real axis. 　 The receiving device according to claim 2 , further comprising a rotation unit that rotates the distribution so as to reduce the angle in the complex plane. 　 The receiving device according to claim 1 , wherein the estimating unit feeds back the amount of frequency offset to the transmitting device, which adjusts the wavelength of the signal based on the amount of frequency offset. 　 The receiving device according to claim 1 , wherein the estimation unit adjusts a wavelength of a local oscillator light used to acquire the signal. 　 further comprising an adaptive equalization filter having an adaptive equalization filter circuit for space-time block coding; 6. The receiving device according to claim 4, wherein the estimator compensates for the frequency offset of the signal based on the amount of frequency offset, and inputs the signal with the frequency offset compensated for to the adaptive equalization filter. 　 a compensation unit that compensates for a frequency offset of the signal based on the estimated frequency offset amount, The receiving device according to claim 1 , wherein the compensating unit compensates for a frequency offset of the signal in which waveform deterioration caused by chromatic dispersion in a transmission path has been compensated for. 　 A frequency offset estimation method performed by a receiving device, comprising: obtaining a phase time interleaved signal from a transmitter; mapping the signal onto a complex plane representing real and imaginary axis components of the signal; estimating a frequency offset of the signal based on a distribution of the signal in the complex plane; A frequency offset estimation method comprising:

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