System and method for precoded Faster than Nyquist signaling
Systems and methods of precoded faster than Nyquist (FTN) signalling are provided. In the transmitter, Tomlinson-Harashima Preceding (THP) is applied to produce precoded symbols. The THP is based on inter-symbol interference (ISI) due to using faster than Nyquist (FTN) signalling. An inverse modulo operation is not performed in the receiver. Instead, in the receiver, FTN processing is performed based on a matched filter output by determining log a-posteriori probability ratio LAPPR values computed for an n th bit b n of a k th received symbol and pre-computed a-priori probabilities of an extended constellation for a given pulse shape h(t) and FTN acceleration factor combination.
1. A method comprising:
in a receiver, receiving a signal containing a plurality of received symbols, each received symbol comprising a symbol of an extended constellation multiplied by a faster than Nyquist (FTN) pulse shape characterized by roll-off factor β and time acceleration factor τ after transmission over a channel;
in the receiver, performing matched filtering of the received signal based on the FTN pulse shape to produce a matched filter output;
in the receiver, for a kth received symbol v of the plurality of received symbols, without performing a modulo M operation, performing FTN processing based on the matched filter output by determining log a-posteriori probability ratio (LAPPR) values computed for each bit of the kth received symbol based on x′[k] and pre-computed a-priori probabilities of the extended constellation for a given pulse shape h(t) and τ combination, where x′[k] is the matched filter output for the kth received symbol v[k];
in the receiver, performing forward error correction (FEC) decoding based on the LAPPR values.
2. The method of claim 1 further comprising:
in the receiver, whitening filtering the matched filter output x′[k] to produce a whitening filtered output v′[k];
wherein performing FTN processing based on x′[k] comprises performing FTN processing based on the whitening filter output v′[k].
3. The method of claim 2 wherein the whitening filtering has a frequency response that is an inverse of a maximum phase representation of a spectral factorization of h(t)*h(−t), where h(t) is the faster than Nyquist pulse shape.
4. The method of claim 1 wherein determining LAPPR values comprises employing
LAPPR
(
k
)
(
b
n
)
=
log
P
(
b
n
=
1
|
v
′
[
k
]
)
P
(
b
n
=
0
|
v
′
[
k
]
)
=
log
Σ
c
i
∈
C
1
P
(
v
′
[
k
]
|
v
[
k
]
=
c
i
)
.
P
(
v
[
k
]
=
c
i
)
Σ
c
j
∈
C
0
P
(
v
′
[
k
]
|
v
[
k
]
=
c
j
)
.
P
(
v
[
k
]
=
c
j
)
or
LAPPR
(
k
)
(
b
n
)
=
log
Σ
c
i
∈
C
1
α
i
exp
(
-
d
i
2
σ
2
)
Σ
c
j
∈
C
0
β
j
exp
(
-
d
j
2
σ
2
)
or
LAPPR
(
k
)
(
b
n
)
≈
log
(
α
i
*
β
j
*
)
+
|
v
′
[
k
]
-
c
j
*
|
2
-
|
v
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[
k
]
-
c
i
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|
2
2
σ
2
.
where C t is the set of symbols in the extended constellation set with n th assigned bit b n =t, where t=0 or 1, where n is an integer between 1 and a total number of bits of the kth symbol.
5. The method of claim 1 wherein
determining the LAPPR values for each bit of the kth received symbol is based on the kth received symbol and L previous and L succeeding symbols where L≥1.
6. The method of claim 1 wherein determining LAPPR values comprises using
LAPPR
(
k
)
(
b
n
)
=
log
P
(
b
n
=
1
|
v
′
[
k
]
)
P
(
b
n
=
0
|
v
′
[
k
]
)
=
log
P
(
b
n
=
1
|
v
′
[
k
-
L
]
,
…
,
v
′
[
k
]
,
…
,
v
′
[
k
+
L
]
)
P
(
b
n
=
0
|
v
′
[
k
-
L
]
,
…
,
v
′
[
k
]
,
…
,
v
′
[
k
+
L
}
)
=
log
Σ
c
i
∈
C
1
P
(
v
[
k
]
=
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i
|
v
′
[
k
-
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,
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,
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[
k
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,
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[
k
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c
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C
0
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v
[
k
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k
-
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[
k
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Σ
c
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∈
C
1
,
c
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-
L
,
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+
L
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v
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=
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v
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Σ
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C
1
,
c
i
-
L
,
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+
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P
(
v
′
[
k
-
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′
[
k
]
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[
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|
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=
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,
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k
]
=
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v
[
k
+
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]
=
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+
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)
.
P
(
X
1
i
)
Σ
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j
∈
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j
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,
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+
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v
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[
k
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]
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+
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]
=
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P
(
X
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)
(
5
)
or
LAPPR
(
k
)
(
b
n
)
≈
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(
P
(
X
1
i
*
)
P
(
X
0
j
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)
)
+
||
v
′
-
c
j
*
||
2
-
||
v
′
-
c
i
*
||
2
2
σ
2
where C t is the set of symbols in the extended constellation set with n th assigned bit b n =t, where t=0 or 1, and where n is an integer between 1 and a total number of bits of the kth symbol.
7. A receiver comprising:
an optical-electrical front end;
a coherent optical receiver comprising an analog-to-digital converter (ADC), matched filter, polarization mode dispersion (PMD) compensator, log likelihood ratio (LLR) generator from Tomlinson-Harashima Precoding (THP) symbols;
a soft-decision FEC decoder;
wherein the LLR generator from THP symbols generates soft-decisions for use by the soft-decision forward error correction (FEC) decoder using the method of claim 1 .
8. The receiver of claim 7 wherein the LLR generator comprises a whitening filter that performs said whitening filtering and has frequency response that is an inverse of a maximum phase representation of a spectral factorization of h(t)*h(−t), where h(t) is the faster than Nyquist pulse shape.
9. The receiver of claim 7 wherein the LLR generator is configured to determine LAPPR values for each bit of the kth received symbol based on the kth received symbol and L previous and L succeeding symbols where L≥1.
10. An optical system comprising;
the receiver of claim 7 ;
a transmitter; and
the transmitter comprising:
a forward error correction (FEC) encoder;
a coherent optical transmitter comprising a quadrature amplitude modulation (QAM) mapper, a Tomlinson-Harashima Precoder (THP), a faster than Nyquist (FTN) pulse shaper, and a digital-to-analog converter;
an electrical-optical front end;
wherein the THP produces precoded symbols by applying Tomlinson-Harashima precoding based on at least one input representative of ISI that will be introduced as a result of using a faster than Nyquist pulse shape; and
wherein the pulse shaper applies pulse shaping to the precoded symbols with the faster than Nyquist (FTN) pulse shape.
11. The optical system of claim 10 wherein the THP comprises a THP filter in a feedback loop within the transmitter.
12. The optical system of claim 11 wherein the THP filter has a frequency response that is a minimum phase representation of a spectral factorization of h(t)*h(−t), where h(t) is the faster than Nyquist pulse shape.
13. The optical system of claim 11 , wherein M-ary PAM signalling is employed, the transmitter further comprises:
a modulo 2M operator in a forward path of the feedback loop.
14. The optical system of claim 11 , wherein M-ary PAM signalling is employed, the transmitter further comprising:
an adder, prior to the feedback loop, that adds an amount to PAM symbols, wherein a result of adding the amount is equivalent to a result of performing a modulo 2M operation in a forward path of the feedback loop.
15. The optical system of claim 10 , wherein the FTN pulse shape is a raised root cosine (RRC) pulse shape characterized by roll-off factor β and FTN time acceleration factor τ, with 0<τ<1.
16. The method of claim 1 further comprising:
in an optical transmitter, applying Tomlinson-Harashima Precoding (THP) to produce precoded symbols, wherein applying the THP is based on at least one input representative of inter-symbol interference (ISI) due to using faster than Nyquist (FTN) signalling;
in the optical transmitter, applying pulse shaping to the precoded symbols with a faster than Nyquist (FTN) pulse shape;
transmitting a signal based on an output of the pulse shaping;
wherein the signal received by the optical receiver is the signal transmitted by the optical transmitter after transmission over an optical link.
17. The method of claim 16 wherein applying the THP in the optical transmitter comprises using a THP filter in a feedback loop within the optical transmitter.
18. The method of claim 17 wherein the THP filter has a frequency response that is a minimum phase representation of a spectral factorization of h(t)*h(−t), where h(t) is the faster than Nyquist pulse shape.
19. The method of claim 17 , wherein M-ary PAM signalling is employed, the method further comprising:
in the optical transmitter, performing a modulo 2M operation in a forward path of the feedback loop.
20. The method of claim 17 , wherein M-ary PAM signalling is employed, the method further comprising:
in the optical transmitter, prior to the feedback loop, adding an amount to PAM symbols, wherein a result of adding the amount is equivalent to a result of performing a modulo 2M operation in the forward path of the feedback loop.
21. The method of claim 16 , wherein the FTN pulse shape is a raised root cosine (RRC) pulse shape characterized by β, and FTN time acceleration factor τ, with 0<τ<1.