Apparatus and method for estimating polarization skew

1. An apparatus for estimating polarization skew, comprising:

a processor configured to acquire a plurality of coefficients of an adaptive equalizer of a receiving device, the plurality of coefficients of the adaptive equalizer comprise a polarization skew of a transmitting device and a polarization skew of the receiving device; and

a calculator configured to calculate the polarization skew of the transmitting device and the polarization skew of the receiving device according to the plurality of coefficients of the adaptive equalizer,

wherein the calculator is configured to:

calculate a part of the plurality of coefficients of the adaptive equalizer to obtain primary diagonal element parameters φ + (ƒ), and calculate another part of the plurality of coefficients of the adaptive equalizer to obtain secondary diagonal element parameters φ − (ƒ);

calculate a sum of and a difference between the polarization skew τ xy,T of the transmitting device and the polarization skew τ xy,R of the receiving device according to the primary diagonal element parameters φ + (ƒ) and the secondary diagonal element parameters φ − (ƒ); and

calculate τ xy,T and τ xy,R according to the sum of and the difference between τ xy,T and τ xy,R .

2. The apparatus according to claim 1 , wherein the processor is further configured to determine a structure of the adaptive equalizer according to the plurality of coefficients of the adaptive equalizer.

3. The apparatus according to claim 1 , wherein the plurality of coefficients of the adaptive equalizer further comprise: in-phase quadrature skew of an x polarization state of the receiving device, and/or in-phase quadrature skew of a y polarization state of the receiving device; and

the calculator calculates the plurality of coefficients of the adaptive equalizer to eliminate components of the in-phase quadrature skew of the x polarization state of the receiving device and the in-phase quadrature skew of the y polarization state of the receiving device, and reserve components of the polarization skew of the transmitting device and the polarization skew of the receiving device.

4. The apparatus according to claim 1 , wherein following formulae are used in calculating the sum of and the difference between τ xy,T and τ xy,R :

in a case where two polarization states outputted by the adaptive equalizer are not interchanged,

τ

xy

,

T

+

τ

xy

,

R

=

1

2

⁢

π

*

slope

⁡

(

φ

+

⁡

(

f

)

)

,



f



≤

f

0

,

⁢

τ

xy

,

T

-

τ

xy

,

R

=

1

2

⁢

π

⁢

slope

⁡

(

φ

-

⁡

(

f

)

)

,



f



≤

f

0

;

in a case where the two polarization states outputted by the adaptive equalizer are interchanged,

-

(

τ

xy

,

T

-

τ

xy

,

R

)

=

1

2

⁢

π

*

slope

⁡

(

φ

+

⁡

(

f

)

)

,



f



≤

f

0

,

⁢

-

(

τ

xy

,

T

+

τ

xy

,

R

)

=

1

2

⁢

π

⁢

slope

⁡

(

φ

-

⁡

(

f

)

)

,



f



≤

f

0

;

where, slope(⋅) denotes a function used for calculating a slope, ƒ 0 >0, and ƒ=n/N*ƒs, N being a positive integer and denoting the number of taps of the adaptive equalizer, n being an integer greater than or equal to −N+ceil(N/2) and less than or equal to ceil(N/2)−1, cell(⋅) denoting a rounded up function, and fs being a sampling rate of input signals of the adaptive equalizer.

5. The apparatus according to claim 4 , wherein the adaptive equalizer is of a 4×4 structure, and the primary diagonal element parameters and the secondary diagonal element parameters are expressed as follows:

φ +,4×4 (ƒ)=0.5*unwrap(arg(( H yi−y1 +j*H yi−yq )*( H yq−yi +j*H yq−yq )*( H xi−xi +j*H xi−xq )**( H xq−xi +j*H xq−xq )*)),

φ −,4×4 (ƒ)=0.5*unwrap(arg(( H xi−y1 +j*H xi−yq )*( H xq−yi +j*H xq−yq )*( H yi−xi +j*H yi−xq )**( H yq−xi +j*H yq−xq )*));

where, arg(⋅) denotes an argument calculation operation, unwrap(⋅) denotes operation correcting radian phase angles, (⋅)* denotes a conjugate operation, and H is a frequency response of the adaptive equalizer, that is, Fourier transform of the plurality of coefficients of the adaptive equalizer.

6. The apparatus according to claim 4 , wherein the adaptive equalizer is of a 4×2 structure, and the primary diagonal element parameters and the secondary diagonal element parameters are expressed as follows:

φ

+

,

4

×

2

⁡

(

f

)

=

1

2

⁢

arg

⁡

[

H

y

⁢

i

-

y

*

H

y

⁢

q

-

y

*

H

xi

-

x

*

*

H

xq

-

x

*

]

,

⁢

φ

-

,

4

×

2

⁡

(

f

)

=

1

2

⁢

arg

⁡

[

H

x

⁢

i

-

y

*

H

x

⁢

q

-

y

*

H

yi

-

x

*

*

H

yq

-

x

*

]

;

where, arg(⋅) denotes an argument calculation operation, (⋅)* denotes a conjugate operation, and H is a frequency response of the adaptive equalizer, that is, Fourier transform of the plurality of coefficients of the adaptive equalizer.

7. The apparatus according to claim 4 , wherein the adaptive equalizer is of a 2×2 structure, and the primary diagonal element parameters and the secondary diagonal element parameters are expressed as follows:

φ

+

,

2

×

2

⁡

(

f

)

=

1

2

⁢

arg

⁡

[

H

y

-

y

*

H

x

-

x

*

]

,

⁢

φ

-

,

2

×

2

⁡

(

f

)

=

1

2

⁢

arg

⁡

[

H

x

-

y

*

H

y

-

x

*

]

;

where, arg(⋅) denotes an argument calculation operation, (⋅)* denotes a conjugate operation, and H is a frequency response of the adaptive equalizer, that is, Fourier transform of the plurality of coefficients of the adaptive equalizer.

8. The apparatus according to claim 1 , wherein the processor is further configured to acquire the plurality of coefficients of the adaptive equalizer of the receiving device multiple times after polarization states of received signals are changed, and process the plurality of coefficients of the adaptive equalizer.

9. A method for estimating polarization skew, comprising:

acquiring a plurality of coefficients of an adaptive equalizer of a receiving device; the plurality of coefficients of the adaptive equalizer comprise a polarization skew of a transmitting device and a polarization skew of the receiving device; and

calculating the polarization skew of the transmitting device and the polarization skew of the receiving device according to the plurality of coefficients of the adaptive equalizer,

wherein the calculating the polarization skew of the transmitting device and the polarization skew of the receiving device according to the plurality of coefficients of the adaptive equalizer comprises:

calculating a part of the coefficients of the adaptive equalizer to obtain primary diagonal element parameters φ + (ƒ), and calculate another part of the coefficients of the adaptive equalizer to obtain secondary diagonal element parameters φ − (ƒ),

calculating a sum of and a difference between the polarization skew τ xy,T of the transmitting device and the polarization skew τ xy,R of the receiving device according to the primary diagonal element parameters φ + (ƒ) and the secondary diagonal element parameters φ − (ƒ), and

calculating τ xy,T and τ xy,R according to the sum of and the difference between τ xy,T and τ xy,R .