Digital clock and data recovery scheme
View Patent ↗A method and apparatus providing clock and data recovery within an optical receiver using three CMA FIR filters with different group delays where filter coefficients are adapted to provide for a filter exhibiting a group delay of one sample period from which symbols for further processing are provided.
1. Apparatus, comprising:
a digital signal processor (DSP) adapted to receive a sequence of complex sampled values associated with a modulated optical carrier and responsively provide a corresponding sequence of symbols;
the DSP is configured to include three constant modulus algorithm (CMA) adaptive finite impulse response (FIR) filters, each filter processing the received sequence of complex sampled values according to a respective set of filter coefficients to provide thereby a respective sequence of symbols, a first filter exhibiting a first group delay, a second filter exhibiting a second group delay and a third filter exhibiting a third group delay, the first group delay initially being less than the second group delay, the second group delay initially being less than the third group delay;
the DSP is configured to include a mechanism for adapting the filter coefficients sets to constrain the exhibited group delays;
the DSP selecting for further processing the sequence of symbols provided by the second filter.
2. The apparatus of claim 1 , wherein:
the mechanism for adapting the filter coefficients selectively changes filter coefficients in response to changes in exhibited group delay.
3. The apparatus of claim 2 , wherein:
in response to the exhibited group delay of the second filter increasing by more than a threshold amount, the mechanism for adapting the filter coefficients operates to reassign filter coefficient sets as follows:
the coefficient set associated with the second filter is reassigned to the first filter;
the coefficient set associated with the second filter is shifted left by one coefficient and reassigned to the first filter; and
the coefficient set associated with the third filter is reassigned to the second filter.
4. The apparatus of claim 2 , wherein:
in response to the exhibited group delay of the second filter decreasing by more than a threshold amount, the mechanism for adapting the filter coefficients operates to reassign filter coefficient sets as follows:
the coefficient set associated with the second filter is reassigned to the third filter;
the coefficient set associated with the second filter is shifted right by one coefficient and reassigned to the first filter; and
the coefficient set associated with the first filter is reassigned to the second filter.
5. The apparatus of claim 2 , wherein:
in response to the exhibited group delay of the second filter increasing by more than a second threshold amount, the DSP operates to remove the last symbol from the provided sequence of symbols.
6. The apparatus of claim 2 , wherein:
in response to the exhibited group delay of the second filter decreasing by more than a second threshold amount, the DSP operates to add a symbol to the provided sequence of symbols.
7. The apparatus of claim 3 , wherein the threshold amount comprises approximately one sample period.
8. The apparatus of claim 5 , wherein the second threshold amount comprises approximately two sample periods.
9. The apparatus of claim 1 , further comprising:
an optical hybrid configured to mix the received optical carrier signal with a reference optical carrier signal to provide thereby two mixed optical signals, the mixed optical signal having different phase; and
a pair of light detectors, each light detector configured to generate a sequence of complex sampled values in response to a respective one of the mixed optical signals.
10. The apparatus of claim 9 , wherein the DSP is further configured to include at least one of a frequency offset correction mechanism, a phase offset correction mechanism and a symbol estimation mechanism for processing the sequence of symbols selected for further processing.
11. The apparatus of claim 9 , wherein the pair of light detectors comprise single ended light detectors.
12. The apparatus of claim 9 , further comprising a pair of optical splitters for passing respective portions of the modulated optical carrier and a local oscillator optical signal to the pair of light detectors.
13. The apparatus of claim 1 , wherein the filter group delays are calculated according to the following equation:
τ
ij
=
real
(
∑
m
mh
ij
(
m
)
∑
m
h
ij
(
m
)
)
.
14. The apparatus of claim 1 , wherein an output yi(k Ti) of each filter is adapted to the following equation:
(
y
1
,
l
(
kT
t
)
y
2
,
l
(
kT
t
)
)
=
∑
m
(
h
11
,
l
(
m
)
h
12
,
l
(
m
)
h
21
,
l
(
m
)
h
22
,
l
(
m
)
)
(
x
1
(
2
kT
t
+
m
+
c
+
l
)
x
2
(
2
kT
t
+
m
+
c
+
l
)
)
.
15. The apparatus of claim 1 , wherein the coefficients of each filter are adapted to the following equation:
h
ij
,
l
(
m
)
=
z
-
1
h
11
,
l
(
m
)
-
μ
∂
e
i
y
i
(
kT
t
)
x
j
*
(
2
kT
t
+
m
+
c
+
l
)
.
16. A method, comprising:
processing a sequence of complex sampled values associated with a modulated optical carrier according to each of three constant modulus algorithm (CMA) adaptive finite impulse response (FIR) filters, each filter processing the received sequence of complex sampled values according to a respective set of filter coefficients to provide thereby a respective sequence of symbols, a first filter exhibiting a first group delay, a second filter exhibiting a second group delay and a third filter exhibiting a third delay, the first group delay initially being less than the second group delay, the second group delay initially being less than the third group delay;
adapting the filter coefficients sets to constrain the exhibited group delays; and
selecting for further processing the sequence of symbols provided by the second filter.
17. A non-transitory computer-readable storage medium storing computer instructions which, when processed by a computer, adapt the operation of the computer to perform a method, the method comprising:
processing a sequence of complex sampled values associated with a modulated optical carrier according to each of three constant modulus algorithm (CMA) adaptive finite impulse response (FIR) filters, each filter processing the received sequence of complex sampled values according to a respective set of filter coefficients to provide thereby a respective sequence of symbols, a first filter exhibiting a first group delay, a second filter exhibiting a second group delay and a third filter exhibiting a third delay, the first group delay initially being less than the second group delay, the second group delay initially being less than the third group delay;
adapting the filter coefficients sets to constrain the exhibited group delays; and
selecting for further processing the sequence of symbols provided by the second filter.