Tap centerer method and structure for coherent optical receiver
A method and structure for tap centering in a coherent optical receiver device. The center of gravity (CG) of the filter coefficients can be used to evaluate a proper convergence of a time-domain adaptive equalizer. However, the computation of CG in a dual-polarization optical coherent receiver is difficult when a frequency domain (FD) adaptive equalizer is adopted. In this case, the implementation of several inverse fast-Fourier transform (IFFT) stages is required to back time domain impulse response. Here, examples of the present invention estimate CG directly from the FD equalizer taps and compensate for an error of convergence based off of the estimated CG. This estimation method and associated device architecture is able to achieve an excellent tradeoff between accuracy and complexity.
1. A coherent optical receiver device, the device comprising:
a compensation module configured to receive an input signal, the compensation module having a plurality of taps; and
an evaluation module coupled to the compensation module, the evaluation module being configured to synchronize the input signal and to iteratively adjust a determinant of a frequency-domain (FD) coefficient-based matrix using the plurality of taps to minimize an error of convergence;
wherein the evaluation module includes
an iterator module configured to receive a compensated input signal and a plurality of taps signals from the compensation module, the iterator module being configured to compute a determinant of a frequency-domain (FD) coefficient-based matrix using the plurality of taps signals;
a phase error module coupled to the iterator module, the phase error module being configured to adjust an error of convergence Δn d of the compensated input signal resulting in an adjusted input signal;
a loop filter coupled to the phase error module, the loop filter being configured to filter the adjusted input signal resulting in a filtered input signal; and
a feedback module coupled to the loop filter and the iterator module, the feedback module being configured to provide the filtered input signal to the iterator module to iteratively adjust the determinant of the FD coefficient-based matrix to minimize the error of convergence.
2. The device of claim 1 wherein the input signal is a dual-polarization input with an x-type polarization input and a y-type polarization input.
3. The device of claim 1 wherein the compensation module includes a non-adaptive frequency-domain (FD) equalizer, an adaptive FD equalizer, or both.
4. The device of claim 1
wherein the evaluation module iteratively adjusts the determinant according to the following equation:
ρ k+1 (Ω m ,0)=ρ k (Ω m ,0) e jαΣ i=0 k Δ{circumflex over (n)} d (i)
where ρ k (Ω m ,0)=e −j4Δn d Ω m e −jN fft Ω m (P(Ω m )P(−Ω m )) 2 ,
where
Δ
n
d
=
n
d
-
N
fft
4
,
where n d is a group delay at impedance Ω=0,
where N fft /2 is the number of the plurality of taps, and
where P(Ω m ) is a real positive number related to the magnitude of the frequency response of the input signal.
5. The device of claim 4 wherein the evaluation module is configured to estimate the group delay n d from the plurality of taps.
6. The device of claim 1 wherein the compensation module includes a chromatic dispersion (CD) equalizer module receiving the input signal and being configured to compensate for CD affecting the input signal;
wherein the compensation module includes a polarization mode dispersion (PMD) equalizer module coupled to the CD equalizer module, the PMD equalizer having a plurality of PMD taps and being configured to compensate for PMD affecting the input signal; and
wherein the plurality of taps includes the plurality of PMD taps.
7. The device of claim 1 further comprising a zero padding module coupled to the evaluation module and the compensation module, the zero padding module being configured to increase a sampling rate of the input signal; and
wherein the evaluation module includes a slicer module configured to derive a data stream from the input signal.
8. An evaluation module device, the device comprising:
an iterator module configured to receive a compensated input signal and a plurality of taps signals, the iterator module being configured to compute a determinant of a frequency-domain (FD) coefficient-based matrix using the plurality of taps signals;
a phase error module coupled to the iterator module, the phase error module being configured to adjust an error of convergence Δn d of the compensated input signal resulting in an adjusted input signal;
a loop filter coupled to the phase error module, the loop filter being configured to filter the adjusted input signal resulting in a filtered input signal; and
a feedback module coupled to the loop filter and the iterator module, the feedback module being configured to provide the filtered input signal to the iterator module to iteratively adjust the determinant of the FD coefficient-based matrix to minimize the error of convergence.
9. The device of claim 8 wherein the compensated input signal is a dual-polarization input with an x-type polarization input and a y-type polarization input.
10. The device of claim 8 wherein the iterator module computes the determinant according to the following equation:
ρ k+1 (Ω m ,0)=ρ k (Ω m ,0) e jαΣ i=0 k Δ{circumflex over (n)} d (i)
ρ k (Ω m ,0)=e −j4Δn d Ω m e −jN fft Ω m (P(Ω m )P(−Ω m )) 2 ,
where
Δ
n
d
=
n
d
-
N
fft
4
,
where n d is a group delay at impedance Ω=0,
where N fft /2 is the number of the plurality of taps, and
where P(Ω m ) is a real positive number related to the magnitude of the frequency response of the compensated input signal.
11. The device of claim 8 wherein the iterator module is configured to estimate a group delay n d from the plurality of taps signals.
12. The device of claim 8 wherein the compensated input signal comprises an input signal compensated by an adaptive FD equalizer module.
13. The device of claim 8 wherein the compensated input signal comprises an input signal compensated by a Chromatic Dispersion (CD) equalizer module and by a Polarization Mode Dispersion (PMD) equalizer module; and
wherein the plurality of taps signals comprises a plurality of PMD taps signals from the PMD equalizer module.
14. An optical communication system, the system comprising:
an optical channel having a first end and a second end;
a transmitter device coupled to the first end of the optical channel;
a receiver device coupled to the second end of the optical channel, the receiver device comprising:
a compensation module configured to receive an optical signal from the transmitter device over the optical channel, the compensation module having a plurality of taps; and
an evaluation module coupled to the compensation module, the evaluation module being configured to synchronize the optical signal and to iteratively adjust a determinant of a frequency-domain (FD) coefficient-based matrix using the plurality of taps to minimize an error of convergence;
wherein the evaluation module includes
an iterator module configured to receive a compensated input signal and a plurality of taps signals from the compensation module, the iterator module being configured to compute a determinant of a frequency-domain (FD) coefficient-based matrix using the plurality of taps signals;
a phase error module coupled to the iterator module, the phase error module being configured to adjust an error of convergence Δn d of the compensated input signal resulting in an adjusted input signal;
a loop filter coupled to the phase error module, the loop filter being configured to filter the adjusted input signal resulting in a filtered input signal; and
a feedback module coupled to the loop filter and the iterator module, the feedback module being configured to provide the filtered input signal to the iterator module to iteratively adjust the determinant of the FD coefficient-based matrix to minimize the error of convergence.
15. The system of claim 14 wherein the optical signal is a dual-polarization signal with an x-type polarization signal component and a y-type polarization signal component.
16. The system of claim 14 wherein the compensation module includes a non-adaptive frequency-domain (FD) equalizer, an adaptive FD equalizer, or both.
17. The system of claim 14 wherein the evaluation module iteratively adjusts the determinant according to the following equation:
ρ k+1 (Ω m ,0)=ρ k (Ω m ,0) e jαΣ i=0 k Δ{circumflex over (n)} d (i)
ρ k (Ω m ,0)=e −j4Δn d Ω m e −jN fft Ω m (P(Ω m )P(−Ω m )) 2 ,
where
Δ
n
d
=
n
d
-
N
fft
4
,
where n d is a group delay at impedance Ω=0,
where N fft /2 is the number of the plurality of taps, and
where P(Ω m ) is a real positive number related to the magnitude of the frequency response of the optical signal.
18. The system of claim 17 wherein the evaluation module is configured to estimate the group delay n d from the plurality of taps.
19. The system of claim 14 wherein the receiver device further comprises a zero padding module coupled to the evaluation module and the compensation module, the zero padding module being configured to increase a sampling rate of the optical signal; and
wherein the evaluation module includes a slicer module configured to derive a data stream from the optical signal.
20. The system of claim 14 wherein the compensation module includes a chromatic dispersion (CD) equalizer module receiving the optical signal and being configured to compensate for CD affecting the optical signal;
wherein the compensation module includes a polarization mode dispersion (PMD) equalizer module coupled to the CD equalizer module, the PMD equalizer having a plurality of PMD taps and being configured to compensate for PMD affecting the optical signal; and
wherein the plurality of taps includes the plurality of PMD taps.