Data-aided multi-symbol phase estimation for optical differential multilevel phase-shift keying signals
View Patent ↗A data-aided, multi-symbol phase estimation (MSPE) scheme is described for improving receiver sensitivity in the direct-detection of optical differential multi-level phase-shift keying (ODmPSK) signals including optical differential quadrature phase-shift keying (ODQPSK) signals, ODQPSK signals with amplitude shift keying (ODQPSK+ASK), optical differential 8-level phase-shift keying (OD8PSK) signals with eight phase levels, and optical differential phase-shift keying signals with more than eight phase levels. The use of data-aided MSPE substantially reduces the “differential detection penalty,” with receiver sensitivity approaching that of coherent detection schemes.
1. A method for determining a data content of an optical differential multi-level phase-shift keying (ODmPSK) signal, the method comprising:
demodulating the ODmPSK signal with optical delay interferometers (ODIs) to generate two quadrature optical components;
directly detecting the two quadrature optical components to generate an in-phase electrical signal, u I , and a quadrature electrical signal, u Q ;
performing a data-aided multi-symbol phase estimation (MSPE) on the in-phase electrical signal and the quadrature electrical signal to generate at least two improved decision variables; and
recovering a plurality of data tributaries representing the data content based on the improved decision variables.
2. The method of claim 1 , wherein the ODmPSK signal is an optical differential quadrature phase-shift keying (ODQPSK) signal.
3. The method of claim 1 , wherein the ODmPSK signal is an optical differential 8-level phase-shift keying signal (OD8PSK).
4. The method of claim 1 , wherein the ODmPSK signal is modulated with amplitude-shift keying (ASK), thereby being an ODmPSK+ASK signal.
5. The method of claim 4 , comprising:
detecting an intensity of the ODmPSK+ASK signal;
generating a normalizing signal that is inversely related to the signal intensity of a previous symbol; and
using the normalizing signal in performing the data-aided MSPE.
6. The method of claim 5 , wherein performing the data-aided MSPE includes performing a nonlinear phase shift compensation.
7. The method of claim 5 , wherein the normalizing signal is obtained using recovered ASK data.
8. The method of claim 1 , wherein an optical delay between the optical paths of each ODI is approximately one symbol period, I.
9. The method of claim 1 , wherein an optical phase offset between the paths of the first ODI is π/m and an optical phase offset between the paths of the second ODI is π/m−π/2, where m is the number of phase levels of the ODmPSK signal.
10. The method of claim 1 , wherein the data-aided MSPE uses the detected electrical signals and previously recovered data symbols to obtain two improved decision variables, x I and x Q , in accordance with the following expressions:
x I ( n )= Re[x ( n )], x Q ( n )= Im[x ( n )], and
x ( n )≈[ u I ( n )+ ju Q ( n )]+ w·[u I ( n )+ ju Q ( n )]· x ( n− 1)·exp(− j Δφ( n− 1))·exp(− jπ/m ),
where w is a forgetting factor, Δφ(n−1) is an optical phase difference between the (n−1)th symbol and the (n−2)th symbol, and exp(−jΔφ(n−1)) is estimated by using previously recovered data symbols.
11. The method of claim 10 , wherein the forgetting factor w is in a range of approximately 0.3 to 0.8.
12. The method of claim 1 , wherein two of the at least two improved decision variables (x I , x Q ) are used to generate the other decision variables (dv) in accordance with the following expressions:
dv
(
π
p
/
m
)
=
cos
(
p
-
1
m
π
)
x
I
-
sin
(
p
-
1
m
π
)
x
Q
,
dv
(
π
p
/
m
-
π
/
2
)
=
sin
(
p
-
1
m
π
)
x
I
+
cos
(
p
-
1
m
π
)
x
Q
,
where p=3, 5, . . . , m/2−1.
13. An apparatus for determining a data content of an optical differential multi-level phase-shift keying (ODmPSK) signal, comprising:
means for demodulating the ODmPSK signal to generate two quadrature optical components through optical delay interferometers (ODIs);
means for directly detecting the two quadrature optical components to generate an in-phase electrical signal, u I , and a quadrature electrical signal, u Q ;
means for performing a data-aided multi-symbol phase estimation (MSPE) on the in-phase electrical signal and the quadrature electrical signal to generate at least two improved decision variables; and
means for recovering a plurality of data tributaries representing the data content based on the improved decision variables.
14. The apparatus of claim 13 , wherein the ODmPSK signal is an optical differential quadrature phase-shift keying (ODQPSK) signal.
15. The apparatus of claim 13 , wherein the ODmPSK signal is an optical differential 8-level phase-shift keying signal (OD8PSK).
16. The apparatus of claim 13 , wherein the ODmPSK signal is modulated with amplitude-shift keying (ASK), thereby being an ODmPSK+ASK signal.
17. The apparatus of claim 13 , comprising:
means for detecting an intensity of the ODmPSK+ASK signal; and
means for generating a normalizing signal that is inversely related to the signal intensity of a previous symbol,
wherein the means for performing a data-aided MSPE includes means for using the normalizing signal in the MSPE.
18. The apparatus of claim 17 , wherein the means for performing a data-aided MSPE includes nonlinear phase shift compensation means.
19. The apparatus of claim 17 , wherein the normalizing signal is obtained using recovered ASK data.
20. The apparatus of claim 13 , wherein an optical delay between the optical paths of each ODI is approximately one symbol period, T.
21. The apparatus of claim 20 , wherein an optical phase offset between the paths of the first ODI is π/m, and an optical phase offset between the paths of the second ODI is π/m−π/2, where m is the number of phase levels of the ODmPSK signal.
22. The apparatus of claim 13 , wherein the means for directly detecting each quadrature optical component includes a balanced detector.
23. The apparatus of claim 13 , wherein the means for performing a data-aided MSPE uses the detected electrical signals and previously recovered data symbols to obtain two improved decision variables, x I and x Q , in accordance with the following expressions:
x I ( n )= Re[x ( n )], x Q ( n )= Im[x ( n )], and
x ( n )≈[ u I ( n )+ ju Q ( n )]+ w·[u I ( n )+ ju Q ( n )]· x ( n− 1)·exp(− j Δφ( n− 1))·exp(− jπ/m ),
where w is a forgetting factor, Δφ(n−1) is an optical phase difference between the (n−1)th symbol and the (n−2)th symbol, and exp(−jΔφ(n−1)) is estimated by using previously recovered data symbols.
24. The apparatus of claim 23 comprising logic means for obtaining the term exp(−jΔφ(n−1)) from previously recovered data symbols.
25. The apparatus of claim 23 comprising a digital-to-analog converter (DAC) for outputting the real and imaginary parts of the term exp(−jΔφ).
26. The apparatus of claim 23 , wherein the forgetting factor w is in a range of approximately 0.3 to 0.8.
27. The apparatus of claim 13 comprising means for using two of the at least two improved decision variables (x I , x Q ) to generate the other decision variables (dv) in accordance with the following expressions:
dv
(
π
p
/
m
)
=
cos
(
p
-
1
m
π
)
x
I
-
sin
(
p
-
1
m
π
)
x
Q
,
dv
(
π
p
/
m
-
π
/
2
)
=
sin
(
p
-
1
m
π
)
x
I
+
cos
(
p
-
1
m
π
)
x
Q
,
where p=3, 5, . . . , m/2−1.
28. The apparatus of claim 13 , comprising:
a first analog-to-digital converter (ADC) for digitizing in-phase electrical signal, u I ; and
a second analog-to-digital converter (ADC) for digitizing the quadrature electrical signal, u Q ;
wherein the means for performing a data-aided MSPE includes a digital signal processor (DSP) which processes the digitized in-phase and quadrature electrical signals.
29. The apparatus of claim 28 , comprising:
a first automatic gain control (AGC) means for performing AGC on the in-phase electrical signal before it is digitized by the first ADC; and
a second automatic gain control (AGC) means for performing AGC on the quadrature electrical signal before it is digitized by the second ADC.
30. The apparatus of claim 28 , wherein the means for performing a data-aided MSPE includes an application specific integrated circuit (ASIC).
31. The apparatus of claim 28 , wherein the means for performing a data-aided MSPE includes a field-programmable gate array (FPGA).
32. The apparatus of claim 28 , wherein the means for performing a data-aided MSPE uses a finite number of iterations in a range of approximately 4 to 20.