FREQUENCY OFFSET ESTIMATION DEVICE, RECEIVER, FREQUENCY OFFSET ESTIMATION METHOD, AND PROGRAM

Abstract

This frequency offset estimation device comprises: a correlation processing unit that obtains a correlation pattern, which is a waveform pattern indicating the correlation between a reception signal spectrum and a detection pattern, on the basis of a detection pattern, which is a prescribed waveform pattern, and a reception signal spectrum indicating the power spectrum of the reception signal or an amplitude spectrum based on the power spectrum; and an estimation unit that estimates a frequency offset amount on the basis of the peak position of the correlation pattern obtained by the correlation processing unit.

Description

[Technical field]

[0001] The present invention relates to a frequency offset estimation device, a receiving device, a frequency offset estimation method, and a program. [Background technology]

[0002] Coherent optical communication systems can transmit information by utilizing the freedom of polarization, amplitude, and phase of the light used. This allows coherent optical communication systems to realize even larger capacity optical communication than intensity modulation optical communication systems that transmit information using only changes in the intensity of light. In addition, coherent optical communication systems can relatively easily compensate for the effects of signal distortion, such as chromatic dispersion and polarization mode dispersion, which are unavoidable in optical signal transmission using optical fibers, by using digital signal processing. Therefore, coherent optical communication systems are advantageous in long-distance transmission as well, and are widely used mainly as trunk line optical communication systems.

[0003] In a coherent optical communication system, high wavelength stability is required for the lasers used in the transmitter and receiver. In a coherent optical communication system, a transmitter is equipped with a transmission laser and a receiver is equipped with a local oscillator laser. Since the transmission laser and the local oscillator laser are used for modulation and coherent detection, respectively, it is desirable that the wavelength of the transmission laser and the wavelength of the local oscillator laser match. However, in reality, it is difficult to accurately match the wavelength of the transmission laser and the wavelength of the local oscillator laser due to, for example, manufacturing tolerances and temperature changes in the lasers. In addition, each laser is allowed to have a frequency error of about 1.8 [GHz] to 2.5 [GHz]. Therefore, if the direction of the frequency error of the transmission laser and the direction of the frequency error of the local oscillator laser are opposite to each other, a frequency offset of up to 3.6 [GHz] to 5 [GHz] occurs.

[0004] Such a frequency offset adversely affects signal processing in a receiver. Therefore, it is necessary to estimate the frequency offset amount and then compensate for the frequency offset based on the estimation result. Since the frequency offset cannot be estimated on the transmitting side, it must be estimated on the receiving side. Conventionally, there is a technique for estimating the frequency offset, for example, the technique described in Patent Document 1. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2011 / 129389 Summary of the Invention [Problem to be solved by the invention]

[0006] Although the estimation method described in Patent Document 1 can estimate the frequency offset amount with high accuracy, it is premised on completing signal processing such as chromatic dispersion compensation, polarization mode dispersion compensation, and MIMO (Multiple-Input and Multiple-Output) processing for polarization channel separation prior to estimating the frequency offset amount. These signal processing processes may be adversely affected by frequency offset. In particular, in a modulation method using multi-level modulation of 16QAM (Quadrature Amplitude Modulation) or more, which has been increasingly used in recent years, it may be impossible to perform these signal processing processes when there is a frequency offset. In addition, in order to perform chromatic dispersion compensation, it is necessary to first estimate the amount of chromatic dispersion, but depending on the estimation method, it is necessary to perform the estimation in a state in which the frequency offset is compensated. Therefore, in this case, it is necessary to perform compensation for the frequency offset prior to chromatic dispersion compensation. For the above reasons, there is a demand for a technology that can estimate and compensate for the frequency offset prior to signal processing for compensating for signal distortion such as chromatic dispersion and polarization mode dispersion caused by optical fiber transmission.

[0007] In view of the above circumstances, an object of the present invention is to provide a frequency offset estimation device, a receiving device, a frequency offset estimation method, and a program capable of compensating for a frequency offset using a received signal in which signal distortion has not been compensated for. [Means for solving the problem]

[0008] One aspect of the present invention is a frequency offset estimation device including: a correlation processing unit that obtains a correlation pattern, which is a waveform pattern that indicates a correlation between the received signal spectrum and the detection pattern, based on a received signal spectrum that indicates a power spectrum of the received signal or an amplitude spectrum based on the power spectrum, and a detection pattern that is a predetermined waveform pattern; and an estimation unit that estimates a frequency offset amount based on a peak position of the correlation pattern obtained by the correlation processing unit.

[0009] One aspect of the present invention is a receiving device for a coherent optical communication system, comprising: a correlation processing unit that obtains a correlation pattern indicating a correlation between a received signal spectrum and a detection pattern, the detection pattern being a predetermined waveform pattern, based on a received signal spectrum indicating a power spectrum of a received signal or an amplitude spectrum based on the power spectrum; an estimation unit that estimates a frequency offset amount based on a peak position of the correlation pattern obtained by the correlation processing unit; and a compensation unit that compensates for a frequency offset based on the frequency offset amount estimated by the estimation unit.

[0010] One aspect of the present invention is a frequency offset estimation method including: a correlation processing step in which a computer obtains a correlation pattern indicating a correlation between the received signal spectrum and the detection pattern, the correlation pattern being a predetermined waveform pattern, based on a received signal spectrum indicating a power spectrum of the received signal or an amplitude spectrum based on the power spectrum, and an estimation step in which a computer estimates a frequency offset amount based on a peak position of the correlation pattern obtained by the correlation processing step.

[0011] One aspect of the present invention is a program for causing a computer to function as the frequency offset estimation device described above. Effect of the Invention

[0012] According to the present invention, the frequency offset can be compensated for using a received signal in which the signal distortion has not been compensated for. [Brief description of the drawings]

[0013] [Figure 1] FIG. 2 is a diagram illustrating an example of a power spectrum of a received signal with no frequency offset. [Diagram 2] FIG. 2 is a diagram illustrating an example of a power spectrum of a received signal having a frequency offset. [Diagram 3] FIG. 2 is a diagram illustrating an example of the spectrum of a Nyquist-shaped single carrier signal. [Figure 4] FIG. 2 is a diagram illustrating an example of the spectrum of a Nyquist-shaped multicarrier signal. [Diagram 5] 13 is a diagram showing an example of correlation calculation between a received signal spectrum and a detection pattern in the case of a single carrier signal. FIG. [Figure 6] 13 is a diagram showing an example of correlation calculation between a received signal spectrum and a detection pattern in the case of a multicarrier signal. FIG. [Figure 7] This is a plot of a first-order Hermite wavelet. [Figure 8] This is a plot of a second-order Hermite wavelet. [Figure 9] FIG. 13 is a diagram showing an example of a detection pattern configured in a rectangular shape. [Figure 10] 1 is a block diagram showing a functional configuration of a receiving device 1 according to an embodiment of the present invention. [Figure 11] FIG. 13 is a block diagram showing a functional configuration of a receiving device 1a according to a modified embodiment of the present invention. [Figure 12] 4 is a flowchart showing an operation of the receiving device 1 in the embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

[0015] The receiving device in this embodiment compensates for the frequency offset using a received signal that is not compensated for signal distortions such as chromatic dispersion and polarization mode dispersion caused by optical fiber transmission, because in order to estimate and compensate for these signal distortions, it may be necessary to first compensate for the frequency offset.

[0016] Therefore, the receiving device in this embodiment estimates and compensates for the frequency offset using a feature that is not affected by signal distortion due to optical fiber transmission. Specifically, for example, the power spectrum of a signal does not change due to optical fiber transmission. The receiving device in this embodiment estimates and compensates for the frequency offset using the power spectrum of the received signal or the amplitude spectrum, which is the square root of the power spectrum.

[0017] The receiving device in this embodiment generates a power spectrum by transforming the main signal into the frequency domain using FFT (Fast Fourier Transform) and then performing absolute value processing. Alternatively, the receiving device generates an amplitude spectrum by transforming the main signal into the frequency domain using FFT and then performing absolute value squaring processing. The receiving device may further perform averaging processing. The receiving device calculates the correlation between the power spectrum or amplitude spectrum of the received signal and the detection pattern, and estimates the frequency offset amount by finding the peak position of the correlation.

[0018] [Method of estimating frequency offset] One method for estimating the frequency offset is to estimate it based on the center of gravity of the power spectrum of the received signal. If the power spectrum is symmetrical, the shift in the center of gravity of the power spectrum corresponds to the frequency offset.

[0019] 1 and 2 are diagrams for explaining the estimation of the frequency offset amount based on the center of gravity of the power spectrum, and show the power spectrum of a received signal that has been converted from an optical signal to a baseband signal by coherent detection in a receiving device.

[0020] FIG. 1 shows the power spectrum of a received signal with no frequency offset. As shown in the figure, the center of gravity of the power spectrum of a received signal with no frequency offset coincides with the DC (Direct Current) position. On the other hand, FIG. 2 shows the power spectrum of a received signal with a frequency offset. The position of the received signal with a frequency offset is shifted by the amount of the frequency offset. Accordingly, as shown in FIG. 2, the center of gravity of the power spectrum of the received signal also shifts from the DC position by the amount of the frequency offset. This may make it possible to estimate the amount of the frequency offset.

[0021] However, the above-mentioned method of estimating the frequency offset amount based on the shift in the center of gravity of the power spectrum will cause an error if the transmission line loss spectrum is asymmetric. For example, in optical fiber transmission, an optical bandpass filter may be used in the transmission line, and if the central wavelength of the optical bandpass filter and the central wavelength of the signal are relatively shifted, the optical bandpass filter will make the signal spectrum asymmetric. In this case, the shift in the center of gravity of the power spectrum will not match the frequency offset amount. Therefore, in order to estimate the frequency offset amount without being affected by the transmission line conditions, it is necessary to use a feature other than the center of gravity of the power spectrum.

[0022] Another method for estimating the frequency offset amount is to estimate it based on pattern matching. For example, it is possible to calculate the convolution of the expected value of the power spectrum and the power spectrum of the received signal, and use the maximum value as the frequency offset amount. However, this estimation method is no different from the above method in that errors occur when the transmission line loss spectrum is asymmetric.

[0023] Therefore, as another method for estimating the frequency offset, a method of estimating the frequency offset based on the position of the spectrum edge of the received signal can be considered. In recent coherent optical transmission technology, Nyquist shaped signals with a small roll-off coefficient are often used. The spectrum of a Nyquist shaped signal has a waveform in which both ends of the signal band are sharply cut off. The position of this steep edge does not change due to the loss spectrum in the transmission path. Therefore, by detecting the spectrum edge of the received signal, it is possible to estimate the frequency offset with higher accuracy.

[0024] In recent years, subcarrier modulation, which divides the signal band into multiple subcarriers, has been used to reduce power consumption and suppress waveform distortion caused by nonlinear optical effects in optical fibers. Subcarrier modulation differs from other multicarrier modulation methods, such as OFDM (Orthogonal Frequency Division Multiplexing), in that spectral gaps occur between the subcarriers. Detecting the location of these spectral gaps makes it possible to estimate the frequency offset with higher accuracy.

[0025] Fig. 3 is a diagram showing an example of the spectrum of a Nyquist-shaped single-carrier signal. In the case of a single carrier, the amount of frequency offset can be estimated based on the positions of roll-off (spectral edges) at both ends of the received signal. Fig. 4 is a diagram showing an example of the spectrum of a Nyquist-shaped multi-carrier signal. In the case of a multi-carrier, the amount of frequency offset can be estimated based on at least one of the dip (spectral gap) between subcarriers and the positions of roll-off (spectral edges) at both ends of the received signal.

[0026] [Correlation calculation between received signal spectrum and detection pattern] In order to detect a spectrum edge or a spectrum gap, the receiving device in this embodiment performs a correlation calculation between the received signal spectrum and a detection pattern. As the detection pattern, a pattern that reacts specifically only to a spectrum edge or a spectrum gap must be prepared in advance.

[0027] As a detection pattern for detecting a spectrum edge, for example, a pattern based on the second derivative of the expected value of the received signal spectrum can be used. sig (f) and the detection pattern is R(f). The received signal spectrum I sig When a detection pattern is formed by the second derivative of (f), the detection pattern R(f) can be expressed as the following equation (1).

[0028]

number

[0029] The receiving device receives the signal spectrum I sig By calculating the correlation between (f) and detection pattern R(f), a correlation pattern K(δ) represented by the following equation (2) can be obtained.

[0030]

number

[0031] Since the correlation pattern K(δ) has a sharp peak at the position of the frequency offset, the receiving device can estimate that δ, which gives the maximum value of the correlation pattern K(δ), is the value of the frequency offset amount.

[0032] The receiving device may use the expected value of the received signal spectrum as the detection pattern R(f). However, using the second derivative rather than the expected value of the received signal spectrum can provide a steeper spectrum edge, thereby further reducing the occurrence of errors.

[0033] FIG. 5 is a diagram showing an example of correlation calculation between a received signal spectrum and a detection pattern in the case of a single carrier signal. In FIG. 5, (a) shows the received signal spectrum I sig 6(f) and 6(b) show the detection pattern R(f), and 6(c) shows the correlation pattern K(δ) that is the result of correlation calculation between the received signal spectrum and the detection pattern.

[0034] As shown in FIG. 5(c), the correlation pattern K(δ) has a steep peak at a position corresponding to the center of the received signal, so the receiving device can estimate the amount of frequency offset by detecting the position of the peak.

[0035] 6 is a diagram showing an example of correlation calculation between a received signal spectrum and a detection pattern in the case of a multi-carrier signal. In FIG. 6, (a) shows the received signal spectrum I sig 6(f) and 6(b) show the detection pattern R(f), and 6(c) shows the correlation pattern K(δ) that is the result of correlation calculation between the received signal spectrum and the detection pattern.

[0036] As shown in Fig. 6(c), the correlation pattern K(δ) has a steep peak at the position corresponding to the center of the received signal, so the receiving device can estimate the frequency offset by detecting the peak position. Note that only the correlation pattern K(δ) near the origin is drawn in Fig. 6(c).

[0037] The receiving device may use a Hermite wavelet as the detection pattern R(f) instead of using the derivative of the expected value of the received signal spectrum. A Hermite wavelet is obtained by normalizing the Nth derivative of a Gaussian function by the L2 norm and then inverting the sign. A wavelet based on the first derivative of a Gaussian function is called a first-order Hermite wavelet, denoted as ψ1 here. A wavelet based on the second derivative of a Gaussian function is called a second-order Hermite wavelet, denoted as ψ2 here.

[0038] The first-order Hermite wavelet ψ1 can be used as a pattern R(f) for detecting a spectrum edge. The first-order Hermite wavelet ψ1 is expressed by the following equation (3).

[0039]

number

[0040] FIG. 7 is a plot of the Hermite wavelet ψ1(x) expressed by equation (3).

[0041] The second-order Hermite wavelet ψ2 can be used as a pattern R(f) for detecting a spectrum gap by inverting the sign. The second-order Hermite wavelet ψ2 is expressed by the following equation (4).

[0042]

number

[0043] FIG. 8 is a plot of the Hermite wavelet ψ2(x) expressed by equation (4).

[0044] Furthermore, the receiving device may configure the detection pattern R(f) by using a combination of multiple wavelets in accordance with the shape of the received signal spectrum.

[0045] [Reduction of calculation amount] In addition, the receiving device may replace the detection pattern R(f) with a simpler pattern to perform correlation calculations in order to reduce the amount of calculations. In digital circuit calculations, the calculation of multiplying by a power of 2 and the calculation of 1 / power of 2 can be performed by only bit shifting, so the amount of calculations is reduced compared to multiplication by an arbitrary coefficient. Instead of using the Hermite wavelet, the receiving device may replace the detection pattern R(f) with a simple pattern configured in a rectangular shape as shown in FIG. 9. FIG. 9 is a diagram showing an example of a detection pattern for a 4-subcarrier multicarrier signal configured in a rectangular shape in order to reduce the amount of calculations.

[0046] [Functional configuration of receiving device] The following describes the functional configuration of the receiving device 1. Fig. 10 is a block diagram showing an example of the functional configuration of the receiving device 1 in an embodiment of the present invention. As shown in Fig. 10, the receiving device 1 includes an LO laser 10, a coherent OE (Optical to Electrical) conversion unit 20, ADCs 30-1 to 30-4, and a frequency offset estimation unit 50.

[0047] The LO laser 10 is a local oscillator laser that outputs local oscillator light whose frequency and phase match the received optical signal. The coherent OE converter 20 performs coherent detection on the received optical signal using the local oscillator light output from the LO laser 10, and converts the received optical signal into a 4-lane baseband electrical signal.

[0048] Each of the four ADCs (Analog to Digital Converters) 30-1 to 30-4 takes in the four-lane electrical signals output from the coherent OE conversion unit 20 and converts them into digital signals. The four-lane digital signals are in-phase and quadrature components of the horizontally polarized waves of the received optical signal, and in-phase and quadrature components of the vertically polarized waves. An imaginary unit multiplier j1 and an imaginary unit multiplier j2 are connected to the ADCs 30-2 and 30-4 that output the quadrature components, respectively.

[0049] The imaginary unit multiplier j1 advances the phase of the quadrature component output by ADC30-2 by 90 degrees on the complex plane and outputs it. The output of ADC30-1 and the output of imaginary unit multiplier j1 are combined to generate a horizontally polarized received signal in which the in-phase component output by ADC30-1 is the real component and the quadrature component output by imaginary unit multiplier j1 is the imaginary component. The imaginary unit multiplier j2 advances the phase of the quadrature component output by ADC30-4 by 90 degrees on the complex plane and outputs it. The output of ADC30-3 and the output of imaginary unit multiplier j2 are combined to generate a vertically polarized received signal in which the in-phase component output by ADC30-3 is the real component and the quadrature component output by imaginary unit multiplier j2 is the imaginary component.

[0050] The horizontally polarized received signal and the vertically polarized received signal, which are expressed as complex numbers and converted into the digital signals, are output to a main signal demodulation processing block (not shown) at the subsequent stage in the receiver 1. Thereafter, the digitized received signal is subjected to demodulation processing, whereby information bits are extracted from the received signal.

[0051] 10, the horizontally polarized received signal and the vertically polarized received signal, which are converted into digital signals and expressed as complex numbers, are also branched and input to frequency offset estimation unit 50. As shown in FIG. 10, frequency offset estimation unit 50 includes FFT calculation units 51-1 to 51-2, absolute value calculation units 52-1 to 52-2, a frame accumulator 53, a correlation processing unit 54, a detection pattern storage unit 55, and a peak detection unit 56.

[0052] The FFT calculation units 51-1 to 51-2 acquire the branched and input received signals for each polarization. The FFT calculation units 51-1 to 51-2 convert the received signals for each polarization into the frequency domain by FFT. The FFT calculation unit 51-1 outputs the received signal converted into the frequency domain to the absolute value calculation unit 52-1, and the FFT calculation unit 51-2 outputs the received signal converted into the frequency domain to the absolute value calculation unit 52-2.

[0053] The absolute value calculation unit 52-1 acquires the received signal converted into the frequency domain output from the FFT calculation unit 51-1. The absolute value calculation unit 52-2 acquires the received signal converted into the frequency domain output from the FFT calculation unit 51-2. The absolute value calculation units 52-1 to 52-2 take the square of the absolute value of the received signal converted into the frequency domain to generate a power spectrum.

[0054] The absolute value calculation units 52-1 to 52-2 may generate the amplitude spectrum by simply taking the absolute value of the received signal converted into the frequency domain. The absolute value calculation units 52-1 to 52-2 output the generated power spectrum (or amplitude spectrum) to the frame accumulator 53.

[0055] The frame accumulator 53 acquires the power spectrum (or amplitude spectrum) output from each of the absolute value calculators 52-1 to 52-2. The frame accumulator 53 averages the power spectrum (or amplitude spectrum) by accumulating it over a plurality of FFT frames. This is to prevent instantaneous fluctuations in the power spectrum (or amplitude spectrum) of the received signal from affecting the accuracy of estimating the frequency offset amount. The frame accumulator 53 outputs the accumulated power spectrum (or absolute value spectrum) to the correlation processor 54.

[0056] The correlation processing unit 54 acquires the accumulated power spectrum (or absolute value spectrum) output from the frame accumulating unit 53. The correlation processing unit 54 also acquires the detection pattern stored in the detection pattern storage unit 55. The correlation processing unit 54 performs correlation calculation between the accumulated power spectrum (or absolute value spectrum) and the detection pattern to acquire a correlation pattern. The correlation processing unit 54 outputs the correlation pattern to the peak detecting unit 56.

[0057] The detection pattern storage unit 55 stores detection patterns in advance. As described above, a detection pattern is a predetermined waveform pattern for detecting a spectrum edge or a spectrum gap in a received signal spectrum.

[0058] The detection pattern storage unit 55 is configured by, for example, a storage medium such as a random access memory (RAM), a read only memory (ROM), a hard disk drive (HDD), or a solid state drive (SSD), or a combination of these storage media. Note that, for example, the detection pattern storage unit 55 may be configured not to be provided in the receiving device 1 but in an external device, and the receiving device 1 may acquire the detection pattern from the external device.

[0059] The peak detection unit 56 acquires the correlation pattern output from the correlation processing unit 54. The peak detection unit 56 detects the peak position of the correlation pattern. The peak detection unit 56 assumes that the detected peak position is the center position of the power spectrum, and estimates the deviation between the peak position and the DC position as the frequency offset amount. The peak detection unit 56 outputs information indicating the estimated frequency offset amount to a compensation unit (not shown) in a subsequent stage in the receiving device 1 that compensates for the frequency offset. The compensation unit (not shown) compensates for the frequency offset based on the acquired information.

[0060] (Modification) The functional configuration of the receiving device in this embodiment may be, for example, a functional configuration of a receiving device 1a shown in Fig. 11. The receiving device 1a includes an LO laser 10, a coherent OE conversion unit 20, ADCs 30-1 to 30-4, FFT calculation units 40-1 to 40-2, and a frequency offset estimation unit 50a. The frequency offset estimation unit 50a includes absolute value calculation units 52-1 to 52-2, a frame accumulator 53, a correlation processing unit 54, a detection pattern storage unit 55, and a peak detection unit 56.

[0061] As shown in the figure, the functional configuration of the receiving device 1a in this modified example differs from the functional configuration of the receiving device 1 described above in that the frequency offset estimation unit 50a does not include an FFT calculation unit, and instead uses FFT calculation units 40-1 to 40-2 that are included in the received signal processing function of the receiving device 1a.

[0062] In general, a receiving device in a coherent optical communication system is often equipped with a chromatic dispersion compensator. In addition, a chromatic dispersion compensator generally performs signal processing in the frequency domain. For this reason, a receiving device in a coherent optical communication system is often equipped with an FFT calculation unit in advance. The receiving device 1a of this modified example does not include an FFT calculation unit in the frequency offset estimation unit 50a, but is configured to share the FFT calculation units 40-1 to 40-2 that are equipped in advance in the receiving device 1a for use in signal processing of the main signal also in the frequency offset estimation process.

[0063] (Operation of receiving device) An example of the operation of the receiving device 1 in this embodiment will be described below. Fig. 12 is a flowchart showing the operation of the receiving device 1 in the embodiment of the present invention.

[0064] The LO laser 10 outputs a local oscillator light having a frequency and phase that coincides with the received optical signal (step S001). The coherent OE converter 20 performs coherent detection on the received optical signal using the local oscillator light output from the LO laser 10, and converts the received optical signal into a 4-lane baseband electrical signal (step S002).

[0065] Each of the four ADCs 30-1 to 30-4 receives the four-lane electrical signals output from the coherent OE converter 20 and converts them into digital signals (step S003). The imaginary unit multiplier j1 advances the phase of the quadrature component output by the ADC 30-2 by 90 degrees on the complex plane and outputs it. The imaginary unit multiplier j2 advances the phase of the quadrature component output by the ADC 30-4 by 90 degrees on the complex plane and outputs it.

[0066] The FFT calculation units 51-1 to 51-2 convert the input reception signals for each polarization into the frequency domain by FFT (step S004). The absolute value calculation units 52-1 to 52-2 take the square of the absolute value of the reception signals converted into the frequency domain to generate a power spectrum (step S006).

[0067] The frame accumulator 53 averages the power spectrum by accumulating it over a plurality of FFT frames (step S005). The correlation processor 54 calculates the correlation between the accumulated power spectrum and the detection pattern to obtain a correlation pattern (step S007). The peak detector 56 regards the peak position of the correlation pattern as the center position of the power spectrum, and estimates the deviation between the peak position and the DC position as the frequency offset amount (step S008).

[0068] This is the end of the operation of the receiving device 1 shown in the flowchart of Fig. 12. The operation of the receiving device 1a in this modified example is basically the same.

[0069] As described above, the receiving device 1 in the embodiment of the present invention and the receiving device 1a in the modified embodiment of the present invention estimate the frequency offset amount by detecting the peak position of the correlation pattern obtained by calculating the correlation between the power spectrum or amplitude spectrum of the received signal and the detection pattern. In the case of single carrier modulation, the receiving device 1 and the receiving device 1a use, as the detection pattern, a waveform pattern intended to detect the roll-off (spectral edge) of the power spectrum or the amplitude spectrum. In the case of multicarrier modulation, the receiving device 1 and the receiving device 1a use, as the detection pattern, a waveform pattern intended to detect at least one of a dip (spectral gap) between subcarriers and roll-off (spectral edge) at both ends of the received signal. For example, the receiving device 1 and the receiving device 1a use, as the detection pattern, a waveform obtained by second-order differentiation and sign-inversion of the envelope waveform of the power spectrum or the amplitude spectrum, or a function approximating the waveform.

[0070] With such a configuration, the receiving device 1 in the embodiment of the present invention and the receiving device 1a in the modified embodiment of the present invention estimate the frequency offset amount by using the shape of the power spectrum that does not change due to optical fiber transmission. This allows the receiving device 1 and the receiving device 1a to estimate and compensate for the frequency offset even if a received signal is used that is not compensated for signal distortion such as chromatic dispersion and polarization mode dispersion caused by optical fiber transmission. Therefore, the receiving device 1 and the receiving device 1a can estimate and compensate for the frequency offset prior to signal processing to compensate for the signal distortion.

[0071] According to the above-described embodiment, the frequency offset estimation device includes a correlation processing unit and an estimation unit. For example, the frequency offset estimation device is the frequency offset estimation unit 50 and the frequency offset estimation unit 50a in the embodiment, the correlation processing unit is the correlation processing unit 54 in the embodiment, and the estimation unit is the peak detection unit 56 in the embodiment. The correlation processing unit obtains a correlation pattern, which is a waveform pattern indicating the correlation between the received signal spectrum and the detection pattern, based on a received signal spectrum indicating a power spectrum of the received signal or an amplitude spectrum based on the power spectrum, and a detection pattern, which is a predetermined waveform pattern. The estimation unit estimates the frequency offset amount based on the peak position of the correlation pattern obtained by the correlation processing unit.

[0072] The detection pattern is a waveform pattern for detecting a spectral edge of a received signal when the received signal is a single-carrier signal, and is a waveform pattern for detecting at least one of a spectral edge of the received signal and a spectral gap between subcarriers of the received signal when the received signal is a multi-carrier signal.

[0073] The detection pattern may be a waveform pattern generated using the second derivative of the expected value of the received signal spectrum, or a rectangular pattern that approximates the waveform pattern.

[0074] The detection pattern for detecting the above-mentioned spectral edge may be a waveform pattern generated based on a Hermite wavelet based on a first derivative of a Gaussian function, and the detection pattern for detecting the above-mentioned spectral gap may be a waveform pattern generated based on a Hermite wavelet based on a second derivative of a Gaussian function.

[0075] The detection pattern may be a rectangular waveform pattern that is similar to a waveform pattern generated based on a Hermite wavelet.

[0076] The receiving device 1 and the receiving device 1a in each of the above-mentioned embodiments may be partly or entirely realized by a computer. In that case, a program for realizing the function may be recorded in a computer-readable recording medium, and the program recorded in the recording medium may be read into a computer system and executed to realize the function. The term "computer system" as used herein includes hardware such as an OS and peripheral devices. The term "computer-readable recording medium" refers to portable media such as flexible disks, optical magnetic disks, ROMs, and CD-ROMs, and storage devices such as hard disks built into a computer system. The term "computer-readable recording medium" may also include a medium that dynamically holds a program for a short period of time, such as a communication line when a program is transmitted via a network such as the Internet or a communication line such as a telephone line, and a medium that holds a program for a certain period of time, such as a volatile memory inside a computer system that is a server or client in that case. The above-mentioned program may be a program for realizing part of the above-mentioned function, or may be a program that can realize the above-mentioned function in combination with a program already recorded in the computer system, or may be a program that is realized using a programmable logic device such as an FPGA (Field Programmable Gate Array).

[0077] 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 designs that do not deviate from the gist of the present invention are also included. [Explanation of symbols]

[0078] 1, 1a... receiving device, 10... LO laser, 20... coherent OE conversion unit, 40-1, 40-2... FFT calculation unit, 50, 50a... frequency offset estimation unit, 51-1, 51-2... FFT calculation unit, 52-1, 52-2... absolute value calculation unit, 53... frame accumulation unit, 54... correlation processing unit, 55... detection pattern storage unit, 56... peak detection unit

Claims

1. a correlation processing unit that obtains a correlation pattern, which is a waveform pattern that indicates a correlation between the received signal spectrum and the detection pattern, based on a received signal spectrum that indicates a power spectrum of the received signal or an amplitude spectrum based on the power spectrum, and the detection pattern that is a waveform pattern that reacts specifically to only spectrum edges and spectrum gaps; an estimation unit that estimates a frequency offset amount based on a peak position of the correlation pattern obtained by the correlation processing unit; 2. A frequency offset estimation apparatus comprising:

2. A correlation processing unit that obtains a correlation pattern, which is a waveform pattern indicating a correlation between the received signal spectrum and the detection pattern, based on a received signal spectrum indicating a power spectrum or an amplitude spectrum based on the power spectrum and a detection pattern which is a predetermined waveform pattern; an estimation unit that estimates a frequency offset amount based on a peak position of the correlation pattern obtained by the correlation processing unit; Equipped with The detection pattern is a waveform pattern for detecting a spectrum edge of the received signal when the received signal is a single carrier signal, When the received signal is a multicarrier signal, the waveform pattern is for detecting at least one of a spectral edge of the received signal and a spectral gap between subcarriers of the received signal. Frequency offset estimator.

3. The detection pattern is a waveform pattern generated using a second-order differential of an expected value of the received signal spectrum, or a rectangular pattern that approximates the waveform pattern.

3. The frequency offset estimation device according to claim 1 or 2.

4. the detection pattern for detecting the spectrum edge is a waveform pattern generated based on a Hermite wavelet based on a first derivative of a Gaussian function, The detection pattern for detecting the spectrum gap is a waveform pattern generated based on a Hermite wavelet based on a second derivative of a Gaussian function.

3. The frequency offset estimating apparatus according to claim 2.

5. The detection pattern is a rectangular waveform pattern that is approximate to a waveform pattern generated based on the Hermite wavelet.

5. The frequency offset estimating apparatus according to claim 4.

6. A receiving device for a coherent optical communication system, comprising: a correlation processing unit that obtains a correlation pattern indicating a correlation between a received signal spectrum and a detection pattern, the detection pattern being a waveform pattern that reacts specifically only to a spectrum edge and a spectrum gap, based on a received signal spectrum indicating a power spectrum of the received signal or an amplitude spectrum based on the power spectrum; an estimation unit that estimates a frequency offset amount based on a peak position of the correlation pattern obtained by the correlation processing unit; a compensation unit that compensates for a frequency offset based on the frequency offset amount estimated by the estimation unit; A receiving device comprising:

7. a correlation processing step in which a computer obtains a correlation pattern indicating a correlation between the received signal spectrum and the detection pattern, based on a received signal spectrum indicating a power spectrum of the received signal or an amplitude spectrum based on the power spectrum, and a detection pattern which is a waveform pattern that specifically reacts only to spectral edges and spectral gaps; an estimation step of estimating a frequency offset amount based on a peak position of the correlation pattern obtained by the correlation processing step; The frequency offset estimation method includes:

8. A program for causing a computer to function as the frequency offset estimation device according to any one of claims 1 to 5.

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