Adaptive equalization in coherent receivers using a Stokes space update algorithm

Patent metadata

Field	Value
Publication number	US-9515745-B2
Application number	US-201414198858-A
Country	US
Kind code	B2
Filing date	Mar 6, 2014
Priority date	Mar 6, 2014
Publication date	Dec 6, 2016
Grant date	Dec 6, 2016

Abstract

A coherent optical receiver including an optical transducer, an adaptive filter, and a processor updates the adaptive filter according to a metric derived in Stokes space. The optical transducer receives an optical signal corresponding to a modulated signal of symbols. The optical transducer also determines a first signal corresponding to a first polarization of the optical signal and a second signal corresponding to a second polarization of the optical signal. The adaptive filter recovers a first equalized signal and a second equalized signal from the first signal and the second signal. The first equalized signal and the second equalized signal form an equalized modulated signal of symbols. The processor calculates a set of Stokes parameters from the equalized modulated signal and updates the adaptive filter based on a metric derived from the set of Stokes parameters.

First claim

What is claimed is:

1. A method comprising:
receiving an optical signal corresponding to a modulated signal comprising a plurality of modulated symbols;
determining a first signal corresponding to a first polarization of the optical signal;
determining a second signal corresponding to a second polarization of the optical signal;
filtering the first signal and the second signal with an adaptive filter to obtain a first equalized signal and a second equalized signal, the first equalized signal and the second equalized signal forming an equalized modulated signal comprising a plurality of equalized modulated symbols;
computing a set of Stokes parameters from the equalized modulation signal; and
updating the adaptive filter by minimizing or maximizing a metric derived from the set of Stokes parameters, wherein metric m is computed as:

m
=

-

S
0

+

2
⁢

S
0

⁢

r

⁢

cos
⁡

(

t
2

)

,

where m is the metric to be minimized or maximized, S 0 is a magnitude of a constellation vector in Stokes space, r is a magnitude of a vector comprising the set of Stokes parameters calculated from the equalized modulation signal, and t is an angle between the constellation vector and the vector comprising the set of Stokes parameters calculated from the equalized modulation signal.

2. The method of claim 1 , wherein the equalized modulation signal comprises a pair of polarization multiplexed modulation symbols corresponding to the first polarization and the second polarization.

3. The method of claim 1 , wherein filtering the first signal and the second signal with an adaptive filter comprises filtering the first and second signals with a butterfly equalizer, the butterfly equalizer comprising a plurality of complex tap weights, and wherein updating comprises updating the plurality of complex tap weights.

4. The method of claim 1 , wherein the first polarization comprises a linear polarization in an x-direction, the first signal corresponds to an electric field E x in the x-direction, and the first equalized signal corresponds to an electric field E xe , in the x-direction.

5. The method of claim 4 , wherein the second polarization comprises a linear polarization in a y-direction orthogonal to the x-direction, the second signal corresponds to an electric field E y in the y-direction, and the second equalized signal corresponds to an electric field E ye in the y-direction.

6. The method of claim 5 , wherein filtering the first signal and the second signal with an adaptive filter comprises filtering the first and second signal with a butterfly equalizer, the butterfly equalizer comprising a plurality of complex tap weights corresponding to a plurality of signals equalized at a previous update cycle, and wherein an n th equalized x-polarization signal E xe (n) corresponding to the first equalized signal is computed by E xe (n)=E x T h xx +E y T h xy , an n th equalized y-polarization signal E ye (n) corresponding to the second equalized signal is computed by E ye (n)=E x T h yx +E y T h yy , where E x T and E y T are vectors comprising the electric field E x and the electric field E y , respectively, for a plurality of previously received samples in the optical signal, and h xx , h xy , and h yx , are vectors comprising complex tap weights corresponding to the plurality of previously received samples.

7. The method of claim 6 , wherein the Stokes parameters comprise S 1e , S 2e , and S 3e defined by S 1e (n)=|E xe (n)| 2 −|E ye (n)| 2 , S 2e (n)=2 {E xe (n)E ye *(n)}, and S 3e (n)=2ℑ{E xe (n)E ye *(n)}, where {A} denotes the real portion of a complex number A, ℑ{A} denotes the imaginary part of the complex number A, and A* denotes the complex conjugate of the complex number A.

8. The method of claim 1 , wherein the equalized modulation signal comprises a Polarization Multiplexed Quadrature Amplitude Modulation (PM-QAM) signal or a Polarization Multiplexed Phase Shift Keying (PM-PSK) signal.

9. An apparatus comprising:
an optical transducer configured to:
receive an optical signal corresponding to a modulated signal comprising a plurality of modulated symbols;
obtain a first signal corresponding to a first polarization of the optical signal; and
obtain a second signal corresponding to a second polarization of the optical signal;

an adaptive filter configured to recover a first equalized signal and a second equalized signal, the first equalized signal and the second equalized signal forming an equalized modulated signal comprising a plurality of equalized modulated symbols; and
a processor configured to:
calculate a set of Stokes parameters from the equalized modulated signal; and
update the adaptive filter by minimizing or maximizing a metric derived from the set of Stokes parameters, wherein metric m is computed as:

m
=

-

S
0

+

2
⁢

S
0

⁢

r

⁢

cos
⁡

(

t
2

)

,

where m is the metric to be minimized or maximized, S 0 is a magnitude of a constellation vector in Stokes space, r is a magnitude of a vector comprising the set of Stokes parameters calculated from the equalized modulation signal, and t is an angle between the constellation vector and the vector comprising the set of Stokes parameters calculated from the equalized modulation signal.

10. The apparatus of claim 9 , wherein the equalized modulation signal comprises a pair of polarization multiplexed modulation symbols corresponding to the first polarization and the second polarization.

11. The apparatus of claim 9 , wherein the adaptive filter comprises a butterfly equalizer having a plurality of complex tap weights, and wherein the processor is configured to update the plurality of complex tap weights.

12. The appar

Assignees

• Cisco Tech Inc

Inventors

• Bosco Gabriella
• Visintin Monica
• Poggiolini Pierluigi
• Forghieri Fabrizio
• Carena Andrea
• Curri Vittorio

Classifications

• H04B10/6162 Primary

  Compensation of polarization related effects, e.g., PMD, PDL · CPC title