JP6411289B2 - Optical receiver and optical receiving method - Google Patents

Description

  The present invention relates to an optical receiver configuration and an optical reception method for minimizing an offset frequency of an OFDM signal in an optical coherent system.

  High-level OFDM (Orthogonal Frequency Division Multiplexing) and optical coherent detection technology are attracting attention as a means to improve system performance and economy and flexibly support new services in next-generation optical access systems .

  High multi-level OFDM allows an inexpensive transceiver having a narrow frequency band due to its high band utilization efficiency and enables flexible use of multiple carriers. Through the power setting in units of subcarriers, it is possible to cope with signal spectrum loss at a specific frequency caused by chirp dispersion from the light source and the optical fiber (for example, see Non-Patent Document 1).

  In optical coherent detection, a receiver sensitivity close to the shot noise limit can be obtained by placing an LO (Local Oscillator) light source in the receiver, so that a PON (Passive Optical Network) system can be extended and multi-branched. Due to the simplicity of required components and digital signal processing, it is expected to use a combination of a baseband OFDM signal obtained by optical intensity modulation and optical coherent detection.

  In a time division multiple (TDM) PON system, a plurality of optical network units (ONUs) perform point-to-multipoint communication with one optical line terminator (OLT) via an optical splitter. The OLT and ONU are composed of an optical transmitter and an optical receiver, and perform uplink / downlink communication in a predetermined time slot.

  Uplink frame signals from a plurality of ONUs are multiplexed by time division multiple access (TDMA) and sent to the OLT. Based on a unique identification number assigned to each ONU, dynamic band allocation (DBA) is performed so that the OLT arrival times of the upstream frame signals from each ONU do not overlap. (For example, refer nonpatent literatures 2 and 3.).

L. Anet Neto, et al. “On the Interest of Chirped Lasers for AMOOFDM Transmissions in Long Distance PON Networks”, Proc. Conf. Optical Fiber Communication, OWK4, Mar. 2011. NTT Technical Journal 2005.9. Technology Basic Course "GE-PON Technology" 2nd NTT Technical Journal 2005.10. Technology Basic Course "GE-PON Technology" 3rd

  In an optical coherent system, there is basically a frequency difference that varies with time between the LO light source and the transmission light source, so that the optical receiver outputs a bandpass signal having the difference as an offset frequency. When the frequency difference is large, the band pass signal exceeds the frequency band of the analog front end of the optical receiver on the spectrum, which causes spectrum loss of the signal.

  When power setting is performed in units of subcarriers, power mismatching occurs. Then, signal orthogonality damage and symmetry damage between the real part and the imaginary part of the baseband OFDM signal are caused by OFDM signal demodulation FFT (Fast Fourier Transform).

  Since these problems cannot be solved by post-digital signal processing, signal damage resulting from the offset frequency causes the communication performance of the optical coherent system to be significantly reduced. Therefore, a light source whose output optical frequency is very stable is required. However, it is difficult to realize OPLL (Optical Phase Locked Loop) which performs a carrier tracking loop at the optical end.

  An object of the present invention is to provide an optical coherent receiver in which an offset frequency is minimized with a simple configuration by using a beating component and a basic OFDM demodulation processing unit in order to solve the above problems. In addition, by detecting and operating only the beating component without referring to the information of the OFDM signal, it has characteristics of having a signal degradation tolerance in the estimation of the offset frequency and having no dependency on the format of the OFDM signal.

  When two light sources having different optical frequencies are input, the photodetector (photodiode) outputs a beating component having the frequency difference as a frequency. In order to minimize the offset frequency, the offset frequency is estimated using the beating component, and the optical frequency of the light source is controlled based on the estimated information.

  In order to achieve the above object, in the present invention, the peak of the beating component detected together with the received signal is detected and estimated as an offset frequency until the absolute value of the estimated offset frequency falls below a set target value. The optical frequency generated by the LO light source is controlled through the wavelength control circuit.

Specifically, the optical receiver according to the present invention is:
An optical coherent receiver that receives the received optical signal and local light output from the LO light source, and coherently detects the received optical signal;
Performs a fast Fourier transform by the FFT unit to the received data in the time domain obtained from the output of the optical coherent receiver, the OFDM demodulation processing unit for performing a demodulation process,
An offset frequency estimator that estimates an offset frequency of the OFDM signal using a beating component having a frequency that is an optical frequency difference between the received optical signal and the local light ;
To below the target value which the absolute value is set in the offset frequency estimated by the offset frequency estimation unit, and a wavelength control unit for controlling the generated optical frequency of the LO light source,
Equipped with a,
The offset frequency estimator is
A first peak detector for detecting a peak of the beating component from an output of the FFT unit;
A narrowband FFT unit that converts only the beating component from the received data in the time domain into the frequency domain according to the output of the first peak detector;
A second peak detector for detecting the peak of the beating component from the output of the narrowband FFT unit;
With
The offset frequency is estimated from the output of the second peak detector .

In the optical receiver according to the present invention,
The offset frequency estimator is
A delay device for delaying the time domain received data;
A downsampler that downsamples the received data output from the delay unit with a downsampling coefficient corresponding to the output of the first peak detector, and inputs the downsampler to the narrowband FFT unit;
May be further provided .

Specifically, the optical receiver according to the present invention is :
An optical coherent receiver that receives the received optical signal and local light output from the LO light source, and coherently detects the received optical signal;
An OFDM demodulation processing unit that performs fast Fourier transform on the received data in the time domain obtained from the output of the optical coherent receiving unit by an FFT unit, and performs demodulation processing;
An offset frequency estimator that estimates an offset frequency of the OFDM signal using a beating component having a frequency that is an optical frequency difference between the received optical signal and the local light;
A wavelength control unit that controls an optical frequency generated by the LO light source until an absolute value of the offset frequency estimated by the offset frequency estimation unit falls below a set target value;
With
The wavelength controller is
Wavelength dithering is performed at the initial estimation of the offset frequency, and the sign of the offset frequency is a positive value when the optical frequency output by the LO light source is higher than the optical frequency of the transmission light source, and a negative value when low. When the sign is positive with respect to the LO light source, control is performed to lower the output optical frequency, and when the sign is negative, control is performed to increase the output optical frequency. After the control, when the offset frequency estimated by the offset frequency estimation unit increases, the sign is inverted.
It is characterized by that .

Specifically, the optical receiver according to the present invention is :
An optical coherent receiver that receives the received optical signal and local light output from the LO light source, and coherently detects the received optical signal;
An OFDM demodulation processing unit that performs fast Fourier transform on the received data in the time domain obtained from the output of the optical coherent receiving unit by an FFT unit, and performs demodulation processing;
An offset frequency estimator that estimates an offset frequency of the OFDM signal using a beating component having a frequency that is an optical frequency difference between the received optical signal and the local light;
A wavelength control unit that controls an optical frequency generated by the LO light source until an absolute value of the offset frequency estimated by the offset frequency estimation unit falls below a set target value;
With
The OFDM demodulation processing unit includes:
A One-tap frequency domain equalizer that compensates for signal degradation caused by fluctuations in offset frequency within subcarrier spacing;
A tap coefficient calculation unit that calculates a tap coefficient of the One-tap frequency domain equalizer based on the offset frequency estimated by the offset frequency estimation unit is further provided.
It is characterized by that .

In the optical receiver according to the present invention,
After minimizing a large offset frequency using the wavelength control unit and the LO light source, by using a variation in small offset frequency within a subcarrier interval using the tap coefficient calculation unit and the One-tap frequency domain equalizer Signal degradation may be compensated .

Specifically, the optical receiver method of the optical receiver according to the present invention is:
An optical coherent reception procedure for inputting a received optical signal and local light output from the LO light source and coherently detecting the received optical signal;
OFDM demodulation processing procedure for performing fast Fourier transform by FFT section on time domain reception data obtained from output in the optical coherent reception procedure, and performing demodulation processing;
An offset frequency estimation procedure for estimating an offset frequency of an OFDM signal using a beating component having a frequency that is an optical frequency difference between the received optical signal and the local light;
A wavelength control procedure for controlling the optical frequency generated by the LO light source until the absolute value of the offset frequency estimated in the offset frequency estimation procedure falls below a set target value;
And
The offset frequency estimation procedure includes:
A first peak detection procedure for detecting the peak of the beating component from the output of the FFT unit;
A narrowband FFT procedure for transforming only the beating component from the received data in the time domain into the frequency domain according to the output in the first peak detection procedure;
A second peak detection procedure for detecting the peak of the beating component from the output in the narrowband FFT procedure;
Including
Estimating the offset frequency from the output in the second peak detection procedure
It is characterized by that .

Specifically, the optical receiver method of the optical receiver according to the present invention is:
An optical coherent reception procedure for inputting a received optical signal and local light output from the LO light source and coherently detecting the received optical signal;
OFDM demodulation processing procedure for performing fast Fourier transform by FFT section on time domain reception data obtained from output in the optical coherent reception procedure, and performing demodulation processing;
An offset frequency estimation procedure for estimating an offset frequency of an OFDM signal using a beating component having a frequency that is an optical frequency difference between the received optical signal and the local light;
A wavelength control procedure for controlling the optical frequency generated by the LO light source until the absolute value of the offset frequency estimated in the offset frequency estimation procedure falls below a set target value;
And
In the wavelength control procedure,
Wavelength dithering is performed at the initial estimation of the offset frequency, and the sign of the offset frequency is a positive value when the optical frequency output by the LO light source is higher than the optical frequency of the transmission light source, and a negative value when low. When the sign is positive with respect to the LO light source, control is performed to lower the output optical frequency, and when the sign is negative, control is performed to increase the output optical frequency. After the control, when the offset frequency estimated in the offset frequency estimation procedure increases, the sign is inverted.
It is characterized by that .

Specifically, the optical receiver method of the optical receiver according to the present invention is:
An optical coherent reception procedure for inputting a received optical signal and local light output from the LO light source and coherently detecting the received optical signal;
And OFDM demodulation processing procedure performs a fast Fourier transform by the FFT unit performs demodulation processing on the received time domain data obtained from the output of the above light coherent reception procedure,
An offset frequency estimation procedure for estimating an offset frequency of an OFDM signal using a beating component having a frequency that is an optical frequency difference between the received optical signal and the local light ;
To below the target value which the absolute value is set in the offset frequency estimated by the offset frequency estimation procedure, a wavelength control procedure for controlling the generated optical frequency of the LO light source,
And
The OFDM demodulation processing procedure includes:
A One-tap frequency domain equalization procedure that compensates for signal degradation caused by variations in offset frequency within subcarrier spacing;
A tap coefficient calculation procedure for calculating a tap coefficient in the One-tap frequency domain equalization procedure is further performed based on the offset frequency estimated in the offset frequency estimation procedure .

  According to the present invention, by using a beating component and a basic OFDM demodulation processing unit, it is possible to provide an optical coherent receiver with a simple configuration and with a minimum offset frequency. Further, by detecting and operating only the beating component without referring to the information of the OFDM signal, it is possible to estimate the offset frequency having signal degradation tolerance and being independent of the format of the OFDM signal.

2 shows an exemplary configuration of an optical coherent baseband OFDM receiver according to the first embodiment. The relationship of the frequency and beating component of the OFDM signal used with the optical coherent baseband OFDM receiver which concerns on this embodiment is shown. 6 shows an exemplary configuration of an optical coherent baseband OFDM receiver according to a second embodiment. 10 illustrates an exemplary configuration of an optical coherent baseband OFDM receiver according to a third embodiment. An example of a structure of the optical coherent baseband OFDM receiver which concerns on Embodiment 3 'is shown. 6 shows an exemplary configuration of a communication apparatus according to a fourth embodiment. 10 shows an exemplary configuration of a communication apparatus according to a fifth embodiment. 10 is a timing chart of uplink frame signals in a plurality of ONUs according to the fifth embodiment. 10 illustrates an example of a configuration of a communication apparatus according to a sixth embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, this invention is not limited to embodiment shown below. These embodiments are merely examples, and the present invention can be implemented in various modifications and improvements based on the knowledge of those skilled in the art. In the present specification and drawings, the same reference numerals denote the same components.

(Embodiment 1)
In the present embodiment, when two light sources having different optical frequencies are input to the optical coherent baseband OFDM receiver, a beating component output from the photodetector is used. The offset frequency is estimated using the beating component, and the offset frequency is minimized by controlling the optical frequency of the light source based on the estimated information.

  Specifically, the offset frequency estimation method is shown in the first and second embodiments. Embodiments 3 and 6 show a method of updating the equalizer and pre-emphasis tap coefficients using the estimated offset frequency. Further, Embodiments 5 and 6 show application examples of the above features to the PON system through cooperation with DBA. In the sixth embodiment, an OLT optical receiver includes an offset frequency estimation unit, and an ONU optical transmitter includes a wavelength control circuit and pre-emphasis.

  FIG. 1 shows a configuration applied to an optical coherent baseband OFDM receiver. The optical coherent OFDM receiver 10 that functions as an optical coherent baseband OFDM receiver includes an optical coherent receiving unit 11, an OFDM demodulation processing unit 15, and an offset frequency estimation unit 22. The optical coherent receiver 11 and the OFDM demodulation processor 15 are connected by an ADC 14 (Analog-to-Digital Converter), and the offset frequency estimator 22 and the optical coherent receiver 11 function as a wavelength controller. 24 and LO light source 25 functioning as a local light source are connected in this order.

  The optical coherent receiving unit 11 includes an optical carrier processing unit 12 and a balanced photodetector 13. The OFDM demodulation processing unit 15 includes an amplitude conversion unit 16, a frame synchronization unit 17, an N-FFT unit 18, a One-tap frequency domain equalizer 19, and a QAM signal determination unit 21. The offset frequency estimation unit 22 includes a peak detector 23.

  In the present embodiment, in the estimation of the offset frequency, the N-FFT unit 18 in the OFDM demodulation processing unit 15 is used to perform frequency conversion. The balanced photodetector 13 of the optical coherent receiver 11 outputs a baseband OFDM signal and a beating component. FIG. 2 shows the relationship between the frequency and the beating component of the OFDM signal used in the optical coherent OFDM receiver 10 according to the present embodiment. The frequency of the beating component is the frequency difference between the optical carriers of the transmitter light source and the receiver LO light source 25.

  The signal demodulation N-FFT unit 18 of the OFDM demodulation processing unit 15 converts the beating component together with the OFDM signal into the frequency domain. The peak detector 23 branched from the main signal system detects the peak value of the beating component from the OFDM signal, and sets the frequency value at the peak as an offset frequency. Accordingly, the offset frequency is estimated in units of OFDM symbols.

  In the estimation of the offset frequency, when the sampling frequency of the FFT input data is Fs and the size of the FFT is N, the frequency estimation range is 0 to Fs / 2 (Hz) and the frequency resolution is Fs / N (Hz). Defined.

  The wavelength control circuit 24 located at the subsequent stage of the offset frequency estimation unit 22 changes the optical frequency of the wavelength locker built-in type or the general LO light source 25 based on the information on the estimated offset frequency to minimize the offset frequency. Thereafter, the one-tap frequency domain equalizer 19 included in the OFDM demodulation processing unit 15 is activated.

(Embodiment 2)
In the optical receiver according to the present embodiment, in the estimation of the offset frequency, frequency conversion is performed by using a combination with an FFT prepared separately. The down sampler 29 in FIG. 3 receives as input the offset frequency (Δf) obtained in the configuration of the first embodiment and the received data in the time domain branched from the N-FFT unit 18 input stage. The received data is time-delayed so that frequency estimation is performed within the same received frame as the signal demodulation N-FFT unit 18.

  The down sampler 29 down-samples the sampling frequency of the received data from Fs to Fs ′, and reduces the frequency estimation range of the M-size FFT to 1.1 · Δf with Δf taking into account a 10% margin. Therefore, the downsampling coefficient (P = Fs / Fs ′) is set to 2.2 · Δf.

  The narrowband M-FFT unit 31 converts the downsampled received data into the frequency domain, and the peak detector 23 detects the peak of the beating component and estimates the offset frequency (Δf ′). The frequency resolution becomes (Fs / P) / M, which is improved by the downsampling factor with respect to Fs.

  According to the first and second embodiments described above, since only the beating component is detected and operated without referring to the OFDM signal information, it has signal degradation tolerance in the estimation of the offset frequency and has no dependency on the format of the OFDM signal. .

  In the configuration according to the present embodiment, adaptive estimation is performed according to the value of the offset frequency, so that both a wide frequency estimation range and high frequency resolution can be achieved. In addition, since the required FFT size can be reduced as compared with a frequency estimator configured by only a single FFT, digital signal processing can be simplified and time domain resolution can be improved.

(Embodiment 3)
FIG. 4 shows a configuration for updating the tap coefficient of the one-tap frequency domain equalizer 19 from the offset frequency information estimated in the first and second embodiments. The reference tap coefficient of the tap coefficient calculation unit 33 is equalization information in a state where no offset frequency exists. Therefore, this information is defined by N subcarrier frequencies designated by FFT for signal demodulation.

  The tap coefficient calculation unit 33 outputs an optimum tap coefficient that matches the subcarrier frequency that is changed by the offset frequency. For this purpose, the interpolator 35 interpolates the reference tap coefficient using the new frequency sequence obtained by adding the estimated offset frequency to the subcarrier frequency of the reference tap coefficient held by the reference tap coefficient holding unit 34, and obtains a new tap coefficient. Ask. Each time the offset frequency is estimated, the tap coefficient is continuously updated.

  The one-tap frequency domain equalizer 19 uses the updated tap coefficient to compensate for signal degradation caused by a change in offset frequency within the subcarrier interval. In this configuration, since the tap coefficient is updated based on the offset frequency information acquired from the beating component, it is possible to improve transmission efficiency without acquiring a separate pilot pattern as well as acquiring the optimum tap coefficient.

(Embodiment 3 ')
As shown in FIG. 5, after subtracting a larger offset frequency in the first or second embodiment having the wavelength control circuit 24 and the LO light source 25 in the subsequent stage in the subsequent stage of the offset frequency estimation unit 22, the subcarrier can be obtained by using the third embodiment. Compensates for signal degradation due to small offset frequency variations within an interval.

(Embodiment 4)
FIG. 6 shows a configuration applied to an optical communication system in which communication apparatuses A and B are connected one-to-one. Each communication device 20 includes an OFDM transmitter 26, an optical coherent OFDM receiver 10, and an optical multiplexer / demultiplexer 36. The OFDM transmitter 26 includes a transmission light source 27. The optical coherent OFDM receiver 10 includes an offset frequency estimation unit 22, a wavelength control circuit 24, an LO light source 25, a tap coefficient calculation unit 33, and a One-tap frequency domain equalization. And a container 19.

  A communication device 20 connected via an optical fiber includes an OFDM transmitter 26 which is a baseband OFDM transmitter and the optical coherent OFDM receiver 10 described above, and performs bidirectional communication.

  In the initial process when the communication device is initially connected, the OFDM transmitter 26 transmits a pilot OFDM pattern, and the optical coherent OFDM receiver 10 turns off the LO light source 25 and puts the balanced photodetector 13 into single-ended mode. Switch to perform direct detection. From the received pilot pattern, the OFDM demodulation processing unit 15 obtains the reference tap coefficient of the one-tap frequency domain equalizer 19.

  Thereafter, the optical coherent reception mode is restored, and the initial offset frequency is estimated from the offset frequency estimation unit 22 of the first and second embodiments. A wavelength dithering method or the like is used only for the initial offset frequency, and the wavelength control circuit 24 determines a code indicating the frequency order of the transmission light source 27 and the LO light source 25 as the offset frequency.

  When the optical frequency of the output of the LO light source 25 is higher than that of the transmission light source 27, it is determined as a positive value. Based on the information on the estimated offset frequency, the wavelength control circuit 24 changes the optical frequency of the output of the LO light source 25. When the sign of the offset frequency is positive, the optical frequency is lowered, and when it is negative, the optical frequency is raised.

  The estimation of the offset frequency and the control of the optical frequency are repeated until the absolute value of the estimated offset frequency falls below the set target value. If the estimated offset frequency increases after controlling the optical frequency, the sign of the offset frequency is inverted.

  When the second embodiment is applied, in order to obtain a wide frequency estimation range for the initial offset frequency and to obtain a high frequency resolution for the offset frequency close to the target value, adaptive estimation is performed according to the value of the offset frequency.

  When the estimated offset frequency satisfies the target value, the optical coherent OFDM receiver 10 ends the initial process and starts data communication. Even during data communication, the estimation of the offset frequency and the control of the optical frequency are repeated to minimize the offset frequency that varies with time.

  The fine fluctuations that occur even when the offset frequency is minimized by activating the tap coefficient calculation unit 33 and the one-tap frequency domain equalizer 19 described above in the third embodiment and using continuously updated tap coefficients. Is compensated for signal degradation (corresponding to Embodiment 3 ′).

(Embodiment 5)
FIG. 7 shows a configuration applied to the PON system. The PON system according to the present embodiment includes a plurality of ONUs 40 and OLTs 30 connected by an optical fiber 32 via an optical splitter 37. Each ONU 40 includes an OFDM transmitter 26, an optical coherent OFDM receiver 10, and an optical multiplexer / demultiplexer 36. The OLT 30 includes an OFDM transmitter 26, a dynamic band allocation (DBA) function unit 38, and information storage. An optical coherent OFDM receiver 39-1 having a unit 41.

  In the TDM-based downlink PON, each optical coherent OFDM receiver 10 executes the fourth embodiment for a plurality of ONUs 40. Offset frequency estimation, optical frequency control and equalization are continuously performed on all downlink TDM frame signals regardless of the destination.

  FIG. 8 shows a timing chart of upstream frame signals in the plurality of ONUs 40 according to this embodiment. Since the TDMA-based uplink PON has an optical coherent OFDM receiver 39-1 that functions as one OLT receiver, Embodiment 4 is executed in units of ONU frames in cooperation with DBA. Since the DBA function calculates the time of the upstream frame signal assigned to each ONU 40, the ONU recognition number and the time information of the head and tail of the frame can be acquired. At the time (t1) at the end of the frame, the control voltage value (Ck) of the wavelength control circuit 24 obtained at t1 and the tap coefficient (Wk ′) of the equalizer are attached with an ONU identification number (ONU # k) and stored. .

  The Ck and Wk ′ values corresponding to the ONU identification number are retrieved from the storage unit in accordance with the time (t2) at the beginning of the next frame, and the wavelength control circuit 24 and the tap coefficient calculation unit 33 set the LO light source 25 to this new value. And drive the equalizer. In consideration of the stabilization time of the light source and the equalizer, the above driving operation can be performed at a guard time before the ONU frame arrives at the OLT receiver.

  After the driving operation is completed, offset frequency estimation and optical frequency control are performed. Also, equalization is performed from the updated tap coefficient. The initial process of obtaining the reference tap coefficient and the offset frequency within the target value is executed after the identification number is assigned to the ONU 40 in the discovery (ranging) process of the PON system.

(Embodiment 6)
FIG. 9 shows a configuration in which a wavelength control circuit 24 functioning as a light source control circuit and an OFDM modulation processing unit 48 having a pre-emphasis function are placed in an OFDM transmitter 46 on the ONU side for a TDMA-based upstream PON system. . The optical coherent OFDM receiver 39-2 on the OLT side obtains an offset frequency (Δfk) and a tap coefficient from the upstream frame through cooperation with the DBA described in the fifth embodiment. Thereafter, an ONU recognition number (ONU # k) is added to the inverse function (1 / Wk ′) calculated from the tap coefficient and the offset frequency, and stored.

  The OFDM transmitter 26 on the OLT side inserts information of Δfk and 1 / Wk ′ corresponding to the ONU identification number into the downstream frame and transmits it to the ONU receiver. Based on the information (Δfk) received from the ONU receiver, the wavelength control circuit 24 of the ONU transmitter controls the optical frequency of the transmission light source. At this time, contrary to the fourth embodiment relating to the LO light source control, the optical frequency is increased when the sign of the offset frequency is positive, and is decreased when the sign is negative.

  The information (1 / Wk ′) received from the ONU receiver is a frequency domain pre-emphasis tap coefficient in the OFDM modulation processing unit 48, and the pre-emphasis is driven based on this coefficient. In this embodiment, as described in the third embodiment, the tap coefficient is continuously updated every time the offset frequency is estimated.

  In the present embodiment, control and pre-emphasis of the light source in the ONU transmission unit are performed through cooperation with the DBA and the downstream PON system.

  The present invention can be applied to the information communication industry.

10: Optical coherent OFDM receiver 11: Optical coherent receiving unit 12: Optical carrier processing unit 13: Balanced photodetector 14: ADC
15: OFDM demodulation processing unit 16: amplitude conversion unit 17: frame synchronization unit 18: N-FFT unit 19: One-tap frequency domain equalizer 20: communication device 21: QAM signal determination unit 22: offset frequency estimation unit 23: Peak detector 24: wavelength control circuit 25: LO light source 26, 46: OFDM transmitter 27: transmission light source 28: delay device 29: downsampler 30: OLT
31: Narrowband M-FFT unit 32: Optical fiber 33: Tap coefficient calculation unit 34: Reference tap coefficient holding unit 35: Interpolator 36: Optical multiplexer / demultiplexer 37: Optical splitter 38: Dynamic band allocation (DBA) function 39 -1, 39-2: Optical coherent OFDM receiver 40: ONU
41: Information storage unit 48: OFDM modulation processing unit

Claims (8)

An optical coherent receiving unit that receives a received optical signal and local light output from a LO (Local Oscillator) light source, and that coherently detects the received optical signal;
Performs a fast Fourier transform by the FFT unit to the received data in the time domain obtained from the output of the optical coherent receiver, the OFDM (Orthogonal Frequency Division Multiplexing) demodulation processing unit for performing a demodulation process,
An offset frequency estimator that estimates an offset frequency of the OFDM signal using a beating component having a frequency that is an optical frequency difference between the received optical signal and the local light ;
To below the target value which the absolute value is set in the offset frequency estimated by the offset frequency estimation unit, and a wavelength control unit for controlling the generated optical frequency of the LO light source,
Equipped with a,
The offset frequency estimator is
A first peak detector for detecting a peak of the beating component from an output of the FFT unit;
A narrowband FFT unit that converts only the beating component from the received data in the time domain into the frequency domain according to the output of the first peak detector;
A second peak detector for detecting the peak of the beating component from the output of the narrowband FFT unit;
With
An optical receiver characterized in that the offset frequency is estimated from an output of the second peak detector . The offset frequency estimator is
A delay unit Ru delays the received data of the time domain time,
A downsampler that downsamples the received data output from the delay unit with a downsampling coefficient corresponding to the output of the first peak detector, and inputs the downsampler to the narrowband FFT unit ;
The optical receiver according to claim 1 , further comprising: An optical coherent receiver that receives the received optical signal and local light output from the LO light source, and coherently detects the received optical signal;
An OFDM demodulation processing unit that performs fast Fourier transform on the received data in the time domain obtained from the output of the optical coherent receiving unit by an FFT unit, and performs demodulation processing;
An offset frequency estimator that estimates an offset frequency of the OFDM signal using a beating component having a frequency that is an optical frequency difference between the received optical signal and the local light;
A wavelength control unit that controls an optical frequency generated by the LO light source until an absolute value of the offset frequency estimated by the offset frequency estimation unit falls below a set target value;
With
The wavelength controller is
Performs wavelength dithering during the initial estimate of the offset frequency, the sign of the offset frequency, the LO when the light source is output to the optical frequency higher than the optical frequency of the transmission source is a positive value, lower case negative respectively determined, the LO if the sign is positive with respect to the light source to lower the output light frequency, if the sign is negative have line control to the effect that increase the output optical frequency, the LO source after the control, the optical receiver characterized by inverting the sign when said offset frequency estimated by the offset frequency estimation unit increases. An optical coherent receiver that receives the received optical signal and local light output from the LO light source, and coherently detects the received optical signal;
An OFDM demodulation processing unit that performs fast Fourier transform on the received data in the time domain obtained from the output of the optical coherent receiving unit by an FFT unit, and performs demodulation processing;
An offset frequency estimator that estimates an offset frequency of the OFDM signal using a beating component having a frequency that is an optical frequency difference between the received optical signal and the local light;
A wavelength control unit that controls an optical frequency generated by the LO light source until an absolute value of the offset frequency estimated by the offset frequency estimation unit falls below a set target value;
With
The OFDM demodulation processing unit includes:
Bei give a One-tap frequency domain equalizer for compensating for signal degradation caused by variations in the offset frequency within the sub-carrier interval,
Said offset said based on the offset frequency estimated by the frequency estimation unit, an optical receiver you further comprising a tap coefficient calculation unit for calculating the tap coefficients of the One-tap frequency domain equalizer. After minimizing a large offset frequency using the wavelength control unit and the LO light source, by using a variation in small offset frequency within a subcarrier interval using the tap coefficient calculation unit and the One-tap frequency domain equalizer The optical receiver according to claim 4 , wherein signal degradation is compensated. An optical coherent reception procedure for inputting a received optical signal and local light output from the LO light source and coherently detecting the received optical signal;
And OFDM demodulation processing procedure performs a fast Fourier transform by the FFT unit performs demodulation processing on the received time domain data obtained from the output of the above light coherent reception procedure,
An offset frequency estimation procedure for estimating an offset frequency of an OFDM signal using a beating component having a frequency that is an optical frequency difference between the received optical signal and the local light ;
To below the target value which the absolute value is set in the offset frequency estimated by the offset frequency estimation procedure, a wavelength control procedure for controlling the generated optical frequency of the LO light source,
And
The offset frequency estimation procedure includes:
A first peak detection procedure for detecting the peak of the beating component from the output of the FFT unit;
A narrowband FFT procedure for transforming only the beating component from the received data in the time domain into the frequency domain according to the output in the first peak detection procedure;
A second peak detection procedure for detecting the peak of the beating component from the output in the narrowband FFT procedure;
Including
An optical reception method for an optical receiver, wherein the offset frequency is estimated from an output in the second peak detection procedure . An optical coherent reception procedure for inputting a received optical signal and local light output from the LO light source and coherently detecting the received optical signal;
And OFDM demodulation processing procedure performs a fast Fourier transform by the FFT unit performs demodulation processing on the received time domain data obtained from the output of the above light coherent reception procedure,
An offset frequency estimation procedure for estimating an offset frequency of an OFDM signal using a beating component having a frequency that is an optical frequency difference between the received optical signal and the local light ;
To below the target value which the absolute value is set in the offset frequency estimated by the offset frequency estimation procedure, a wavelength control procedure for controlling the generated optical frequency of the LO light source,
And
In the wavelength control procedure,
Wavelength dithering is performed at the initial estimation of the offset frequency, and the sign of the offset frequency is a positive value when the optical frequency output by the LO light source is higher than the optical frequency of the transmission light source, and a negative value when low. When the sign is positive with respect to the LO light source, control is performed to lower the output optical frequency, and when the sign is negative, control is performed to increase the output optical frequency. An optical receiver method for an optical receiver, comprising: inverting the sign when the offset frequency estimated in the offset frequency estimation procedure increases after control . An optical coherent reception procedure for inputting a received optical signal and local light output from the LO light source and coherently detecting the received optical signal;
And OFDM demodulation processing procedure performs a fast Fourier transform by the FFT unit performs demodulation processing on the received time domain data obtained from the output of the above light coherent reception procedure,
An offset frequency estimation procedure for estimating an offset frequency of an OFDM signal using a beating component having a frequency that is an optical frequency difference between the received optical signal and the local light ;
To below the target value which the absolute value is set in the offset frequency estimated by the offset frequency estimation procedure, a wavelength control procedure for controlling the generated optical frequency of the LO light source,
And
The OFDM demodulation processing procedure includes:
A One-tap frequency domain equalization procedure that compensates for signal degradation caused by variations in offset frequency within subcarrier spacing;
The optical reception of the optical receiver further comprising a tap coefficient calculation procedure for calculating a tap coefficient in the One-tap frequency domain equalization procedure based on the offset frequency estimated in the offset frequency estimation procedure. Method.

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