Receiver algorithms for coherent detection of polarization-multiplexed optical signals
View Patent ↗A digital signal processor (DSP) operating within, for example, an optical receiver wherein the DSP processes complex sample streams derived from a received digitally modulated optical signal, the DSP configured to perform a method comprising: using a filter adaptation algorithm (FAA), processing digitized complex sample streams for each of a sequence of unitary matrix different starting conditions associated with the FAA to establish a converged FAA.
1. A digital signal processor (DSP), comprising a memory configured to process complex symbol streams derived from a received digitally modulated optical signal, the DSP configured to perform a method, comprising:
using a filter adaptation algorithm (FAA), processing the complex symbol streams for each of a sequence of predefined unitary matrix different starting conditions associated with the FAA to establish a converged FAA.
2. The DSP of claim 1 , wherein the digitally modulated signal comprises a polarization division multiplexed (PDM) signal.
3. The DSP of claim 1 , wherein the digitally modulated signal comprises one of a quadrature amplitude modulated (QAM) signal and an ASK/PSK modulated signal.
4. The DSP of claim 1 , wherein the received digitally modulated optical signal comprises a PDM optical signal including a 16-QAM optical signals supplying data at approximately 112 Gb/s via a 25-GHz WDM grid.
5. The DSP of claim 1 , wherein the FAA comprises a constant modulus algorithm (CMA).
6. The DSP of claim 1 , wherein the FAA comprises a decision directed least-mean-squares algorithm.
7. The DSP of claim 1 , wherein the starting conditions for the FAA are of the following form:
(
h
xx
k
h
xy
k
h
yx
k
h
yy
k
)
=
1
2
(
ⅇ
j
a
cos
ϑ
ⅇ
j
b
sin
ϑ
ⅇ
j
(
π
+
a
-
b
)
sin
ϑ
cos
ϑ
)
.
8. The DSP of claim 1 , wherein the starting conditions for the FAA are of the following form:
(
h
xx
k
h
xy
k
h
yx
k
h
yy
k
)
=
1
2
(
ⅇ
jφ
cos
ϑ
ⅇ
-
jφ
sin
ϑ
-
ⅇ
j2φ
sin
ϑ
cos
ϑ
)
for
k
=
{
N
/
2
,
N
/
2
+
1
}
and
with
φ
∈
[
0
;
π
4
[
and
ϑ
∈
[
0
;
π
[
.
9. The DSP of claim 1 , wherein the starting conditions for the FAA are of the real-valued form:
(
h
xx
k
h
xy
k
h
yx
k
h
yy
k
)
=
1
2
(
cos
ϑ
sin
ϑ
-
sin
ϑ
cos
ϑ
)
.
10. The DSP of claim 1 , wherein the sequence of different starting conditions associated with the FAA comprise a sequence of polarization rotation matrices.
11. The DSP of claim 1 , wherein the received digitally modulated optical signal is part of a wavelength-division multiplexed (WDM) optical signal.
12. The DSP of claim 1 , wherein the converged FAA comprises an FAA having starting conditions providing a first converged outcome.
13. The DSP of claim 1 , wherein the DSP is further configured to process an initial plurality of complex symbol streams to provide retimed complex symbol streams, the DSP processing the initial plurality of digital symbol streams according to the steps of:
recovering a clock signal within the initial plurality of complex symbol streams by detecting a tone; and
down sampling the initial plurality of complex symbol streams using the recovered clock signal to provide the retimed complex symbol streams, wherein the recovered clock signal frequency is in an integer relationship with the tone frequency.
14. The DSP of claim 13 , wherein the clock signal frequency is 28-GHz and the tone frequency is 14-GHz.
15. A computer non-transitory readable medium having stored thereon software instructions which, when processed by a computer, perform a method, comprising:
processing complex symbol streams derived from a received digitally modulated optical signal using a filter adaptation algorithm (FAA) having starting conditions selected from each of a sequence of predefined unitary matrix starting conditions to establish a converged FAA.
16. Apparatus, comprising:
a digital signal processor (DSP) configured to process complex symbol streams derived from a received digitally modulated optical signal, the DSP configured to perform a method comprising:
using a filter adaptation algorithm (FAA), processing digitized complex symbol streams for each of a sequence of predefined unitary matrix different starting conditions associated with the FAA to establish a converged FAA.
17. The apparatus of claim 16 , wherein the DSP is further configured to process the complex symbol streams using the converged FAA according to at least one of a source separation function, an equalization function and a timing phase function to provide decomposed PDM signals.
18. The apparatus of claim 16 , wherein the converged FAA comprises starting conditions providing a fastest convergence from among the sequence of predefined unitary matrix different starting conditions.
19. The apparatus of claim 16 , wherein the converged FAA comprises the FAA having starting conditions providing a first outcome that has converged.
20. The apparatus of claim 16 , further comprising:
using a decision-directed algorithm for final convergence and tracking of the digitized complex symbol streams to provide a digitized symbol stream representing symbols received according to predefined constellation.
21. The apparatus of claim 20 , wherein the FAA is implemented using any of a constant modulus algorithm (CMA), a Richardson-Lucy deconvolution, a Bussgang method, a polyspectra techniques and a decision-directed algorithms.