Optical reception device, iq skew estimation method, and frame synchronization method

Abstract

A digital signal processing unit (22) of an optical reception device (2) calculates a correlation function of an I component and a Q component of a reception signal when, given that T is the time of one frame of the I component and the Q component, the time of one of the I component and the Q component is shifted with respect to the other by approximately nT (where n is an integer of not less than 1), and estimates skew between the I component and the Q component on the basis of the position of the peak of a correlation function.

Description

Optical receiving device, IQ skew estimation method, and frame synchronization method

[0001] The present invention relates to an optical receiving device, an IQ skew estimation method, and a frame synchronization method.

[0002] To cope with the increase in communication traffic, optical transceivers are required to have higher speeds and larger capacities. In order to expand the transmission capacity per channel, baud rates are increasing and modulation methods are becoming more multi-valued, so optical transceivers are required to have high transmission characteristics. The transmission characteristics of optical transceivers are expressed by a transfer function. Generally, optical transceivers have multiple lanes (an XI lane for an in-phase component of X polarization, an XQ lane for an orthogonal component of X polarization, a YI lane for an in-phase component of Y polarization, and a YQ lane for an orthogonal component of Y polarization) (see Patent Document 1).

[0003] Differences in transfer functions between lanes cause degradation of the transmission characteristic compensation performance of the entire system. Therefore, it is important to sufficiently suppress the differences between lanes. In other words, there is a need for a technology that can detect and correct IQ skew with a simple configuration. There is also a need for a technology that can achieve frame synchronization with a simple configuration.

[0004] Patent No. 6319487

[0005] The present invention has been made to solve the above problems, and aims to provide an optical receiving device and an IQ skew estimation method that can estimate IQ skew with a simple configuration, and also aims to provide an optical receiving device and a frame synchronization method that can obtain frame synchronization with a simple configuration.

[0006] The optical receiving device of the present invention is characterized by comprising: a correlation function calculation unit configured to calculate a correlation function of an I component and a Q component when one of the I component and the Q component is shifted in time by approximately nT (n is an integer equal to or greater than 1) relative to the other, when the duration of one frame of the I component and the Q component of a received signal is T; and an IQ skew estimator configured to estimate a skew between the I component and the Q component based on a position of a peak of the correlation function. The optical receiving device of the present invention is also characterized by comprising: a correlation function calculation unit configured to calculate a correlation function of an I component and a Q component when one of the I component and the Q component is shifted in time by approximately nT (n is an integer equal to or greater than 1) relative to the other, when the duration of one frame of the I component and the Q component of a received signal is T; and a frame synchronization unit configured to regard the position of the peak of the correlation function as the position of a fixed pattern included in a frame.

[0007] An IQ skew estimation method of the present invention is characterized by including a first step of calculating a correlation function between the I component and the Q component when one of the I component and the Q component is shifted in time by approximately nT (n is an integer equal to or greater than 1) relative to the other, when the frame time of the I component and the Q component of a received signal is T, and a second step of estimating the skew between the I component and the Q component based on the position of a peak of the correlation function. Also, a frame synchronization method of the present invention is characterized by including a first step of calculating a correlation function between the I component and the Q component when one of the I component and the Q component is shifted in time by approximately nT (n is an integer equal to or greater than 1) relative to the other, when the frame time of the I component and the Q component of a received signal is T, and a second step of regarding the position of the peak of the correlation function as the position of a fixed pattern included in the frame.

[0008] According to the present invention, it is possible to estimate IQ skew with a simple configuration. In the present invention, by averaging the correlation for each frame of the signal, it is possible to reduce the influence of noise and increase the intensity of the peak of the correlation function. Furthermore, the present invention is also applicable when chromatic dispersion is added to the signal received from the optical transmitter.

[0009] FIG. 1 is a block diagram showing the configuration of an optical transmission system according to an embodiment of the present invention. FIG. 2 is a block diagram showing the configuration of a digital signal processing unit of an optical receiving device according to an embodiment of the present invention. FIG. 3 is a diagram showing an example of the configuration of I and Q components of a received signal. FIG. 4 is a diagram explaining an overview of the operation of a correlation function calculation unit and an IQ skew estimator according to an embodiment of the present invention. FIG. 5 is a diagram showing the calculation result of the correlation function of the I and Q components. FIG. 6 is a flowchart explaining the operation of a correlation function calculation unit, an IQ skew estimator, and a frame synchronization unit according to an embodiment of the present invention. FIG. 7 is a diagram showing a model of chromatic dispersion for a signal. FIG. 8 is a diagram explaining the effect of an embodiment of the present invention. FIG. 9 is a block diagram showing an example of the configuration of a computer realizing an optical receiving device according to an embodiment of the present invention.

[0010] An embodiment of the present invention will now be described with reference to the drawings. Fig. 1 is a block diagram showing the configuration of an optical transmission system according to an embodiment of the present invention. The optical transmission system comprises an optical transmitter 1 and an optical receiver 2. The optical transmitter 1 and the optical receiver 2 are connected via an optical fiber 3.

[0011] The optical transmitter 1 includes a digital signal processor 10, a modulator driver 11, a light source 12, and a polarization multiplexing modulation module 13. The digital signal processor 10 symbol-maps a transmission signal obtained by performing forward error correction coding on a transmission bit string. Furthermore, the digital signal processor 10 converts each of the symbol-mapped I (in-phase) component of the X polarization of the transmission signal, the Q (quadrature) component of the X polarization of the transmission signal, the I component of the Y polarization of the transmission signal, and the Q component of the Y polarization of the transmission signal from a digital signal to an analog signal.

[0012] The modulator driver 11 amplifies the analog signal output from the digital signal processing unit 10 and drives the IQ modulators 130 and 131 of the polarization multiplexing modulation module 13 with the amplified analog signal.

[0013] The polarization multiplexing modulation module 13 includes IQ modulators 130 and 131, and a polarization combiner 132. The IQ modulator 130 outputs an X-polarized optical signal generated by modulating the light output from the light source 12 with the I component of the X-polarized wave and the Q component of the X-polarized wave output from the modulator driver 11. The IQ modulator 131 outputs a Y-polarized optical signal generated by modulating the light output from the light source 12 with the I component of the Y-polarized wave and the Q component of the Y-polarized wave output from the modulator driver 11. The polarization combiner 132 polarization-combines the X-polarized optical signal output from the IQ modulator 130 and the Y-polarized optical signal output from the IQ modulator 131, and transmits the result to the optical fiber 3.

[0014] The optical receiving device 2 includes a local oscillation light source 20, an optical front end 21, and a digital signal processing unit 22. The optical front end 21 includes a polarization separator 210, 90-degree optical hybrid circuits 211 and 212, photodetectors 213 to 216, and amplifiers 217 to 220.

[0015] The polarization separator 210 separates the optical signal from the optical fiber 3 into X-polarized and Y-polarized waves. The polarization separator 210 outputs the X-polarized optical signal to a 90-degree optical hybrid circuit 211 and outputs the Y-polarized optical signal to a 90-degree optical hybrid circuit 212.

[0016] The 90-degree optical hybrid circuit 211 causes interference between the X-polarized optical signal and the locally oscillated light output from the local oscillator light source 20, and extracts the I and Q components of the received optical field. The 90-degree optical hybrid circuit 211 outputs the extracted I and Q components of the X-polarized wave to photodetectors 213 and 214. The 90-degree optical hybrid circuit 212 causes interference between the Y-polarized optical signal and the locally oscillated light output from the local oscillator light source 20, and extracts the I and Q components of the received optical field. The 90-degree optical hybrid circuit 212 outputs the extracted I and Q components of the Y-polarized wave to photodetectors 215 and 216.

[0017] Photodetectors 213-216 are differential input type photoelectric converters. Photodetector 213 converts the I component of the received signal of X polarization into an electric signal and outputs it to amplifier 217. Photodetector 214 converts the Q component of the received signal of X polarization into an electric signal and outputs it to amplifier 218. Photodetector 215 converts the I component of the received signal of Y polarization into an electric signal and outputs it to amplifier 219. Photodetector 216 converts the Q component of the received signal of Y polarization into an electric signal and outputs it to amplifier 220. Amplifiers 217-220 amplify the electric signals output from photodetectors 213-216 and output them to digital signal processing unit 22.

[0018] 2 is a block diagram showing the configuration of the digital signal processing unit 22. The digital signal processing unit 22 includes analog-to-digital converters 221 to 224, a correlation function calculation unit 225, an IQ skew estimation unit 226, a compensation unit 227, a demapping unit 228, a decoding unit 229, and a frame synchronization unit 230.

[0019] Analog-to-digital converters 221 to 224 convert into digital signals the analog signals of the four lanes XI, XQ, YI, and YQ output from amplifiers 217 to 220. An IQ skew estimator 226 estimates the skew (delay time difference) between XI and XQ, and the skew between YI and YQ.

[0020] The compensating unit 227 compensates for the skew between XI and XQ and the skew between YI and YQ based on the skew estimation result by the IQ skew estimating unit 226. The compensating unit 227 compensates for the skew by, for example, a digital signal processing method using an FIR (Finite Impulse Response) filter or the like. The compensating unit 227 may also compensate for waveform distortion occurring in the optical fiber 3, compensate for the difference between the frequency of the light output from the light source 12 of the optical transmitting device 1 and the frequency of the locally oscillated light output from the local oscillation light source 20 of the optical receiving device 2, and compensate for phase noise. The compensating unit 227 can also synchronize each frame of the I component, synchronize each frame of the Q component, and synchronize the I component and the Q component based on the fixed pattern detection result by the frame synchronizing unit 230.

[0021] The demapping unit 228 determines the symbols of the received signal output from the compensation unit 227 and converts the determined symbols into binary data. The decoding unit 229 performs error correction decoding processing on the binary data demapped by the demapping unit 228 to obtain a received bit string. Note that the configuration of the digital signal processing unit 22 may be different from that shown in FIG. 2.

[0022] Next, the correlation function calculation unit 225 and the IQ skew estimator 226, which are characteristic components of the present invention, will be described in more detail. In this embodiment, it is assumed that the correlation function calculation unit 225 acquires a received signal as shown in FIG. 3, which is composed of an I component and a Q signal. The I component is divided into frames as shown by 100I-1, 100I-2, 100I-3, ​​and 100I-4 in FIG. 3, and each frame includes frame headers 101I-1, 101I-2, 101I-3, and 101I-4 (first fixed pattern). Similarly, the Q component is divided into frames as shown by 102I-1, 102I-2, 102I-3, and 102I-4 in FIG. 3, and each frame includes frame headers 103I-1, 103I-2, 103I-3, and 103I-4 (second fixed pattern).

[0023] The length of one frame of each I component and Q component is N samples (N is an integer equal to or greater than 2). The header length is M samples (M is an integer equal to or greater than 2, M<N). However, all of the signals in one frame may be fixed repetitive signals (N=M). In this embodiment, two or more frames are required to estimate the IQ skew. The relationship between the I component frame header and the Q component frame header is random. Meanwhile, the frame headers 101I-1, 101I-2, 101I-3, 101I-4, ... included in each I component frame are assumed to be identical. Similarly, the frame headers 103I-1, 103I-2, 103I-3, 103I-4, ... included in each Q component frame are assumed to be identical. Furthermore, this embodiment assumes that there is an offset between the frequency of the light output from the light source 12 of the optical transmitter 1 and the frequency of the local oscillator light output from the local oscillator light source 20 of the optical receiver 2.

[0024] The correlation function calculation unit 225 calculates the correlation function between the I and Q components while shifting the time of the Q component relative to the I component, as shown in Fig. 4. The IQ skew estimation unit 226 estimates the IQ skew based on the calculation result of the correlation function. When the correlation function between the I and Q components is calculated, the value of the correlation function reaches its maximum near T, 2T, 3T, and 4T, as shown in Fig. 5. The period T is a time equivalent to the frame length N.

[0025] In this embodiment, the correlation function between the I and Q components is calculated while shifting the Q component in time by (k+K) relative to the I component. The shift time K is a multiple of the period T. That is, K takes the value of nT (n is an integer equal to or greater than 1). Here, we will first explain the case where the shift time K is 0. The received signal r(t) consisting of the I and Q components can be expressed as in equation (1). θ(t) in equation (1) is expressed as in equation (2), where t is time and ω is angular frequency.

[0026]

[0027] θ(t) = ωt + φ(t) (2) If the I component is a(t) and the Q component is b(t), then from equation (1), a(t) and b(t) are expressed as follows: a(t) = Re(r(t)) = I(t) cos θ(t) - Q(t) sin θ(t) (3) b(t) = Im(r(t)) = I(t) cos θ(t) + Q(t) sin θ(t) (4)

[0028] Re() in equation (3) means the real part, and Im() in equation (4) means the imaginary part. The correlation function E between the I and Q components when the time of the Q component is shifted by k relative to the I component is ａｂ [k] is expressed as in equation (5), and Δt represents the IQ skew.

[0029] ...(5)

[0030] "I(t)Q(t+Δt+k)cos θ(t)cos θ(t+Δt+k)-Q(t)I(t+Δt+k)sin θ(t)sin θ(t+Δt+k)" in equation (5) is 0 because I(t) and Q(t) are uncorrelated with each other. "sin(2ωt+ωk+ωΔt)" in equation (5) is equivalent to the integration of one period (T), so its value is 0. When Δt is large at k=0, the value of the last line of equation (5) is 0 because the correlation within Σ disappears. Also, when Δt is small at k=0, sin() is 0, so the value of the last line of equation (5) is 0. As described above, when K=0, the I component and the Q component are uncorrelated.

[0031] Next, we will explain the cases where K=T, 2T, 3T, 4T, .... The correlation function E between the I and Q components when the time of the Q component is shifted by (k+K) relative to the I component is ａｂ [k+K, l] is expressed by equation (6), where l represents the time of the frame head position of the I component, and L represents the range of integration for calculating correlation.

[0032] ...(6)

[0033] In equation (6), when k = -Δt, the correlation function E ａｂ 6 is a flowchart explaining the operations of the correlation function calculation unit 225, the IQ skew estimation unit 226, and the frame synchronization unit 230. The correlation function calculation unit 225 calculates the correlation function E between the I and Q components when the time of the Q component is shifted by (k+K) relative to the I component. ａｂ [k+K, l] is calculated (step S100 in FIG. 6).

[0034] The IQ skew estimator 226 calculates the correlation function E ａｂ The time of the peak position of [k+K, l] is identified (step S101 in FIG. 6). Then, the IQ skew estimator 226 calculates the IQ skew based on the identified time of the peak position, the time shift value K, and the period T (step S102 in FIG. 6). For example, when K=T and the time of the Q component is shifted by (k+K) relative to the I component, peaks of the correlation function appear near T, 2T, 3T, and 4T as shown in FIG. 5. The time of the peak position that appears near T is called t１ｐ Then, the IQ skew estimator 226 calculates the IQ skew d using equation (7): １ｐ -T...(7)

[0035] Similarly, the time of the peaks appearing near 2T, 3T, and 4T is t ２ｐ , t ３ｐ , t ４ｐ Then, t ２ｐ , t ３ｐ , t ４ｐ The IQ skew d can be calculated from the equations (8) to (10) respectively. ２ｐ -2T...(8) d=t ３ｐ -3T...(9) d=t ４ｐ -4T...(10)

[0036] The IQ skew estimating section 226 may calculate the IQ skew based on one peak of the correlation function, or may use the average value of the IQ skews calculated for each of a plurality of peaks as the final IQ skew value.

[0037] Furthermore, in the above example, we have explained the case where K = T, but the IQ skew estimation unit 226 may also use the average value of the IQ skew calculated for each of multiple K (= T, 2T, 3T, 4T, ...) as the final IQ skew value.

[0038] The frame synchronization unit 230 calculates the correlation function E ａｂ The peak position of [k+K, l] is regarded as the position of the fixed pattern (frame header) (step S103 in FIG. 6). However, the correlation function E between the I and Q components is calculated by shifting the time of the Q component relative to the I component. ａｂ When [k+K, l] is calculated, the correlation function E ａｂ The peak position of [k+K, l] indicates the position of the fixed pattern of the Q component. The position of the fixed pattern of the I component is the value obtained by subtracting the IQ skew d from the time of the peak position.

[0039] Furthermore, when the correlation function between the I and Q components is calculated by shifting the time of the I component relative to the Q component, the peak position of the correlation function indicates the position of the fixed pattern of the I component. The position of the fixed pattern of the Q component is the value obtained by subtracting the IQ skew d from the time of the peak position.

[0040] Next, it will be explained that the present invention is applicable as long as the I and Q components are uncorrelated even after chromatic dispersion has been added to the transmission signal. The phenomenon of chromatic dispersion for a signal x(t) can be modeled as shown in Figure 7. FT is the Fourier transform that converts the time-domain signal x(t) into the frequency domain, H is the transfer function related to the phase of the optical fiber 3, and IFT is the inverse Fourier transform that converts the frequency-domain signal into the time domain. The signal x(t) can be expressed as in equation (11): x(t) = I(t) + jQ(t) ... (11)

[0041] The signal X(f) after the Fourier transform is expressed as in equation (12).

[0042]

[0043] The signal H(f)X(h) obtained by multiplying the signal X(f) after the Fourier transform by the transfer function H as in equation (13) is expressed as in equation (14).

[0044]

[0045] The signal x'(t) obtained by performing an inverse Fourier transform on the signal H(f)X(h) is given by equation (15).

[0046] ...(15)

[0047] Therefore, the I component I'(t) of the signal x'(t) is given by equation (16), and the Q component Q'(t) is given by equation (17).

[0048] ...(16) ...(17)

[0049] t → t ｋ , t' → t ｌ, f → f ｍ Then, when I'(t) and Q'(t) are discretized, the equations (18) and (19) are obtained.

[0050] ...(18) ...(19)

[0051] When the phases θ and θ′ are defined as in equations (20) and (21), the I component I′(t ｋ ) and Q component Q'(t ｋ ) can be expressed as in equation (22).

[0052]

[0053] ...(22)

[0054] If m in equation (22) is calculated first, equation (23) is obtained.

[0055] ...(23)

[0056] For example, Σ in equation (22) ｍ The first term in [ ] is as in equation (24), so when you take the sum of one period for m, it becomes 0 because the plus and minus cancel each other out. Σ ｍ The same applies to the second, third, and fourth terms in [ ].

[0057] ...(24)

[0058] From equations (22) to (24), equation (25) holds. ｋ ) Q'(t ｋ ) = 0 ... (25)

[0059] However, when k = l = l', equation (22) does not depend on m, so equation (25) does not hold. Therefore, we will check the correlation in this case. When k = l = l', equations (26) to (29) hold.

[0060]

[0061] When k = l = l', I'(t ｋ ) and Q'(t ｋ ) can be expressed as in equation (30).

[0062] ...(30)

[0063] In equation (30), m is in a commutative relationship. From equation (30), it was confirmed that there is no correlation between the I component and the Q component after transmission. Therefore, the present invention can be applied even after the signal has been transmitted.

[0064] 8 is a diagram illustrating the effects of this embodiment, showing the calculation results of the correlation function between the I component and the Q component. Here, the modulated signal transmitted from the optical transmitter 1 is a 100 GBd 64QAM (2os) signal, and the frequency offset between the light source 12 of the optical transmitter 1 and the local oscillator light source 20 of the optical receiver 2 is 125 MHz. The frame length of each of the I component and the Q component is 2002000 samples, and the correlation function was calculated for the case where each of the I component and the Q component has two frames. The IQ skew of the modulated signal transmitted from the optical transmitter 1 is 10 samples.

[0065] 8, the peak of the correlation function appears at a position shifted by 10 samples from K=T=2002000 samples. Therefore, the IQ skew estimator 226 can calculate the IQ skew d as 10 samples using the following equation: d=2002010-2002000=10 (31)

[0066] The calculation result of equation (31) coincides with the true value of the IQ skew. Therefore, it can be seen that the IQ skew can be accurately estimated according to this embodiment. As described above, this embodiment allows the IQ skew to be estimated with a simple configuration. In this embodiment, the correlation is averaged for each frame of the signal, thereby reducing the influence of noise and increasing the intensity of the peak of the correlation function.

[0067] In this embodiment, a frame header is used as the fixed pattern included in each frame of the signal, but it need not be a frame header; each frame need only include a fixed pattern. The optical receiving device 2 does not need to know the content of the fixed pattern. In other words, there is no need to transmit fixed pattern information to the optical receiving device 2, and the processing of the optical receiving device 2 is completed using only the received signal, so IQ skew can be estimated with a simple configuration. This embodiment is also applicable when chromatic dispersion is added to the signal received from the optical transmitting device 1. In other words, IQ skew can be estimated both before and after transmission.

[0068] In addition, when the optical transmission / reception device has multiple lanes, the correlation function calculation unit 225 and the IQ skew estimation unit 226 may estimate the IQ skew for each polarization. That is, the correlation function calculation unit 225 and the IQ skew estimation unit 226 may estimate the skew between XI and XQ using the above method, and may also estimate the skew between YI and YQ in a similar manner. Furthermore, in this embodiment, the case where the Q component is time-shifted relative to the I component has been described, but it goes without saying that the I component may also be time-shifted relative to the Q component.

[0069] The digital signal processing unit 22 of the optical receiving device 2 described in this embodiment can be realized by a computer equipped with a CPU (Central Processing Unit), a storage device, and an interface, and a program that controls these hardware resources. An example of the configuration of this computer is shown in Figure 9.

[0070] The computer includes a CPU 200, a storage device 201, and an interface device (I / F) 202. The I / F 202 is connected to the optical front end 21 and the like. In such a computer, a program for implementing the IQ skew estimation method of the present invention is stored in the storage device 201. The CPU 200 executes the processing described in this embodiment in accordance with the program stored in the storage device 201. Note that at least a part of the digital signal processing unit 22 may be implemented by an LSI (Large Scale Integration) circuit formed in an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit).

[0071] Some or all of the above embodiments can be described as, but are not limited to, the following supplementary notes.

[0072] (Supplementary Note 1) The optical receiving device of the present invention includes a correlation function calculation unit configured to calculate a correlation function of the I component and the Q component when the time of one frame of the I component and the Q component of a received signal is shifted by approximately nT (n is an integer equal to or greater than 1) with respect to the other, when the time of the I component and the Q component is T, and an IQ skew estimation unit configured to estimate the skew between the I component and the Q component based on the position of the peak of the correlation function.

[0073] (Supplementary Note 2) The optical receiving device of the present invention includes a correlation function calculation unit configured to calculate the correlation function of the I component and the Q component when the time of one frame of the I component and the Q component of a received signal is shifted by approximately nT (n is an integer equal to or greater than 1) relative to the other, when the time of the I component and the Q component is T, and a frame synchronization unit configured to regard the position of the peak of the correlation function as the position of a fixed pattern included in the frame.

[0074] (Supplementary Note 3) In the optical receiving device according to Supplementary Note 1 or 2, each frame of the I component includes the same first fixed pattern, and each frame of the Q component includes the same second fixed pattern.

[0075] (Supplementary Note 4) In the optical receiving device described in Supplementary Note 1, when the received signal is a polarization multiplexed signal, the correlation function calculation unit calculates the correlation function for each polarization, and the IQ skew estimator estimates the skew for each polarization.

[0076] (Supplementary Note 5) The IQ skew estimation method of the present invention includes a first step of calculating a correlation function between the I component and the Q component when the time of one frame of the I component and the Q component of a received signal is shifted by approximately nT (n is an integer equal to or greater than 1) relative to the time of the other, when the time of one frame of the I component and the Q component of the received signal is T, and a second step of estimating the skew between the I component and the Q component based on the position of the peak of the correlation function.

[0077] (Supplementary Note 6) The frame synchronization method of the present invention includes a first step of calculating a correlation function between the I component and the Q component when the time of one frame of the I component and the Q component of a received signal is T and one of the I component and the Q component is shifted in time by approximately nT (n is an integer equal to or greater than 1) relative to the other, and a second step of regarding the position of the peak of the correlation function as the position of a fixed pattern included in the frame.

[0078] The present invention can be applied to an optical receiving device.

[0079] 1...optical transmitting device, 2...optical receiving device, 3...optical fiber, 10...digital signal processing unit, 11...modulator driver, 12...light source, 13...polarization multiplexing modulation module, 20...local oscillation light source, 21...optical front end, 22...digital signal processing unit, 130, 131...IQ modulator, 132...polarization combiner, 210...polarization separator, 211, 212...90-degree optical hybrid circuit, 213-216...photodetector, 217-220...amplifier, 221-224...analog-to-digital converter, 225...correlation function calculation unit, 226...IQ skew estimation unit, 227...compensation unit, 228...demapping unit, 229...decoding unit, 230...frame synchronization unit.

Claims

1. When the time of one frame of the I component and the Q component of the received signal is T, a correlation function calculation unit configured to calculate the correlation function of the I component and the Q component when the time of the other is shifted by approximately nT (n is an integer of 1 or more) with respect to one of the I component and the Q component; and an IQ skew estimation unit configured to estimate the skew between the I component and the Q component based on the position of the peak of the correlation function. An optical receiver characterized by comprising.

2. When the time of one frame of the I component and the Q component of the received signal is T, a correlation function calculation unit configured to calculate the correlation function of the I component and the Q component when the time of the other is shifted by approximately nT (n is an integer of 1 or more) with respect to one of the I component and the Q component; and a frame synchronization unit configured to regard the position of the peak of the correlation function as the position of a fixed pattern included in the frame. An optical receiver characterized by comprising.

3. The optical receiver according to claim 1 or 2, wherein each frame of the I component includes the same first fixed pattern, and each frame of the Q component includes the same second fixed pattern. An optical receiver characterized by this.

4. The optical receiver according to claim 1, wherein when the received signal is a polarization multiplexed signal, the correlation function calculation unit calculates the correlation function for each polarization, and the IQ skew estimation unit estimates the skew for each polarization. An optical receiver characterized by this.

5. A first step of calculating a correlation function of the I component and the Q component when the time of the other is shifted by approximately nT (n is an integer of 1 or more) with respect to one of the I component and the Q component when the time of one frame of the I component and the Q component of the received signal is T; and a second step of estimating the skew between the I component and the Q component based on the position of the peak of the correlation function. An IQ skew estimation method characterized by including.

6. When the time of one frame of the I component and the Q component of the received signal is T, a first step of calculating a correlation function of the I component and the Q component when the time of the other is shifted by approximately nT (n is an integer of 1 or more) with respect to one of the I component and the Q component; and a second step of regarding the position of the peak of the correlation function as the position of a fixed pattern included in the frame. A frame synchronization method characterized by comprising the above steps.

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