Timing recovery for optical coherent receivers in the presence of polarization mode dispersion
View Patent ↗A timing recovery system generates a sampling clock to synchronize sampling of a receiver to a symbol rate of an incoming signal. The input signal is received over an optical communication channel. The receiver generates a timing matrix representing coefficients of a timing tone detected in the input signal. The timing tone representing frequency and phase of a symbol clock of the input signal and has a non-zero timing tone energy. The receiver computes a rotation control signal based on the timing matrix that represents an amount of accumulated phase shift in the input signal relative to the sampling clock. A numerically controlled oscillator is controlled to adjust at least one of the phase and frequency of the sampling clock based on the rotation control signal.
1. A method for timing recovery comprising:
receiving an input signal sampled based on a sampling clock;
generating, by a processor, a timing matrix representing coefficients of a timing tone detected in the input signal, the timing tone representing frequency and phase of a symbol clock of the input signal and the timing tone having a non-zero timing tone energy;
computing a rotation control signal based on the timing matrix, the rotation control signal representing an amount of accumulated phase shift between the input signal and the sampling clock; and
controlling an oscillator to generate the sampling clock and to adjust at least one of the phase and frequency of the sampling clock based on the rotation control signal.
2. The method of claim 1 , wherein generating the timing matrix comprises:
applying a resonator filter to the input signal to generate a band pass filtered signal;
computing an in-phase and quadrature error signal based on the band pass filtered signal, the in-phase and quadrature error signal representing an amount of phase error in each of an in-phase component and a quadrature component of the input signal; and
computing the rotation control signal based on the in-phase and quadrature error signal.
3. The method of claim 2 , wherein computing the in-phase and quadrature error signal comprises:
applying an interpolator filter to the band pass filtered signal to generate an interpolated signal, the interpolated signal having a higher sampling rate than the band pass filtered signal;
determining a correlation signal representing correlation measures between a first polarization of the interpolated signal and a second polarization of the interpolated signal; and
determining the in-phase error and quadrature error signal based on a difference between the correlation measures for symbols of an in-phase component of the correlation signal separated by a half symbol period and based on a difference between the correlation measures for symbols of a quadrature component of the correlation signal separated by a half symbol period.
4. The method of claim 2 , wherein computing the rotation control signal based on the in-phase and quadrature error signal comprises:
determining the timing matrix based on the in-phase and quadrature error signal; determining based on the timing matrix, a cycle slip number signal representing a number of full symbol period shifts of the input signal relative to the sampling clock;
determining based on the timing matrix, a phase error signal representing a difference between the phase the input signal and the phase of the sampling clock;
applying a loop filter to the phase error signal to generate a filtered phase error signal; and
generating the rotation control signal based on the cycle slip number signal and the filtered phase error signal, the rotation control signal to correct for the phase error and the number of full symbol period shifts of the input signal relative to the sampling clock.
5. The method of claim 4 , wherein determining the cycle slip number signal comprises:
computing a determinant of the timing matrix; and
generating the cycle slip number signal based on the determinant.
6. The method of claim 4 , wherein determining the phase error signal based on the timing matrix comprises:
computing a determinant of the timing matrix; and determining the phase error based on the determinant.
7. The method of claim 4 , wherein determining the phase error signal based on the timing matrix comprises:
applying a modified wave difference method to the in-phase and quadrature error signals and the timing matrix to generate the phase error signal.
8. The method of claim 2 , wherein the applying the resonator filter comprises band pass filtering the input signal around a frequency of approximately half of the symbol rate of the input signal for each polarization of the input signal.
9. The method of claim 1 , wherein the input signal comprises an impairment introduced in an optical channel including at least one: a half symbol period differential group delay, a cascaded differential group delay, a dynamic polarization mode dispersion, and a residual chromatic dispersion.
10. A non-transitory computer-readable storage medium storing instructions for timing recovery, the instructions when executed by a processor causing the processor to perform steps including:
receiving an input signal sampled based on a sampling clock;
generating a timing matrix representing coefficients of a timing tone detected in the input signal, the timing tone representing frequency and phase of a symbol clock of the input signal and the timing tone having a non-zero timing tone energy;
computing a rotation control signal based on the timing matrix, the rotation control signal representing an amount of accumulated phase shift in the input signal relative to the sampling clock; and
controlling an oscillator to adjust at least one of the phase and frequency of the sampling clock based on the rotation control signal.
11. The non-transitory computer-readable storage medium of claim 10 , wherein generating the timing matrix comprises:
applying a resonator filter to the input signal to generate a band pass filtered signal;
computing an in-phase and quadrature error signal based on the band pass filtered signal, the in-phase and quadrature error signal representing an amount of phase error in each of an in-phase component and a quadrature component of the input signal; and
computing the rotation control signal based on the in-phase and quadrature error signal.
12. The non-transitory computer-readable storage medium of claim 11 , wherein computing the in-phase and quadrature error signal comprises:
applying an interpolator filter to the band pass filtered signal to generate an interpolated signal, the interpolated signal having a higher baud rate than the band pass filtered signal;
determining a correlation signal representing correlation measures between a first polarization of the interpolated signal and a second polarization of the interpolated signal; and
determining the in-phase error and quadrature error signal based on a difference between the correlation measures for symbols of an in-phase component of the correlation signal separated by a half symbol period and based on a difference between the correlation measures for symbols of a quadrature component of the correlation signal separated by a half symbol period.
13. The non-transitory computer-readable storage medium of claim 11 , wherein computing the rotation control signal based on the in-phase and quadrature error signal comprises:
determining the timing matrix based on the in-phase and quadrature error signal;
determining based on the timing matrix, a cycle slip number signal representing a number of full symbol period shifts of the input signal relative to the sampling clock; determining based on the timing matrix, a phase error signal representing a phase error of the input signal relative to the sampling clock;
applying a loop filter to the phase error signal to generate a filtered phase error signal; and
generating the rotation control signal based on the cycle slip number signal and the filtered phase error signal, the rotation control signal to correct for the phase error and the number of full symbol period shifts of the input signal relative to the sampling clock.
14. The non-transitory computer-readable storage medium of claim 13 , wherein determining the cycle slip number signal comprises:
computing a determinant of the timing matrix; and
generating the cycle slip number signal based on the determinant.
15. The non-transitory computer-readable storage medium of claim 13 , wherein determining the phase error signal based on the timing matrix comprises:
computing a determinant of the timing matrix; and
determining the phase error based on the determinant.
16. The non-transitory computer-readable storage medium of claim 13 , wherein determining the phase error signal based on the timing matrix comprises:
applying a modified wave difference method to the in-phase and quadrature error signals and the timing matrix to generate the phase error signal.
17. The non-transitory computer-readable storage medium of claim 11 , wherein the applying the resonator filter comprises band pass filtering the input signal around a frequency of approximately half of the symbol rate of the input signal for each polarization of the input signal.
18. The non-transitory computer-readable storage medium of claim 10 , wherein the input signal comprises an impairment introduced in an optical channel including at least one: a half symbol period differential group delay, a cascaded differential group delay, a dynamic polarization mode dispersion, and a residual chromatic dispersion.
19. A receiver, comprising:
an analog front end for receiving an analog input signal and generating a digital input signal sampled based on a sampling clock;
a digital signal processor to generate a timing matrix representing coefficients of a timing tone detected in the digital input signal, the timing tone representing frequency and phase of a symbol clock of the digital input signal and the timing tone having a non-zero timing tone energy, the digital signal processor further to compute a rotation control signal based on the timing matrix, the rotation control signal representing an amount of accumulated phase shift detected in the digital input signal relative to the sampling clock; and
an oscillator control block to control an oscillator to adjust at least one of the phase and frequency of the sampling clock based on the rotation control signal.
20. The receiver of claim 19 , wherein the digital signal processor comprises:
a resonator filter to filter the digital input signal to generate a band pass filtered signal; an in-phase error and a quadrature error computation block to generate an in-phase and quadrature error signal based on the band pass filtered signal, the in-phase and quadrature error signal representing an amount of phase error in each of an in-phase component and a quadrature component of the input signal; and
a rotation computation block to compute the rotation control signal based on the in-phase and quadrature error signal.
21. The receiver of claim 20 , wherein the in-phase and quadrature error computation block comprises:
an interpolator filter to filter the band pass filtered signal to generate an interpolated signal, the interpolated signal having a higher sampling rate than the band pass filtered signal;
a correlation computation block to generate a correlation signal based on the interpolated signal, the correlation signal representing correlation measures between a first polarization of the interpolated signal and a second polarization of the interpolated signal; and
a difference computation block to generate the in-phase and quadrature error signal based on a difference between the correlation measures for symbols of an in-phase component of the correlation signal separated by a half symbol period, and based on a difference between the correlation measures for symbols of a quadrature component of the correlation signal separated by a half symbol period.
22. The receiver of claim 20 , wherein the phase error computation block comprises:
a timing matrix estimation block to generate the timing matrix based on the in-phase and quadrature error signal;
a cycle slip computation block to determine based on the timing matrix, a cycle slip number signal representing a number of full symbol period shifts of the input signal relative to the sampling clock;
a phase error computation block to generate based on the timing matrix, a phase error signal representing a phase error of the input signal relative to the sampling clock; and
a loop filter to filter the phase error signal to generate a filtered phase error signal and to generate the rotation control signal based on the cycle slip number signal and the filtered phase error signal, the rotation control signal to correct for the phase error and the number of full symbol period shifts of the input signal relative to the sampling clock.
23. The receiver of claim 22 , wherein the cycle slip computation block is further configured to:
compute a determinant of the timing matrix; and
generate the cycle slip number signal based on the determinant.
24. The receiver of claim 22 , wherein the phase error computation block is further configured to:
compute a determinant of the timing matrix; and
determine the phase error based on the determinant.
25. The receiver of claim 22 , wherein the phase error computation block is further configured to:
apply a modified wave difference method to the in-phase and quadrature error signals and the timing matrix to generate the phase error signal.
26. The receiver of claim 20 , wherein the resonator filter comprises a band pass filter having a center frequency of approximately half of the symbol rate of the digital input signal for each polarization of the digital input signal.
27. The receiver of claim 19 , wherein the input signal comprises an impairment introduced by an optical channel including at least one: a half symbol period differential group delay, a cascaded differential group delay, a dynamic polarization mode dispersion, and a residual chromatic dispersion.