Multi-dimensional LDPC coded modulation for high-speed optical transmission systems
View Patent ↗Arbitrarily high data transmission rates may be achieved by the use of N-dimensional, LDPC-coded modulation. N orthonormal basis functions are employed using coherent reception, resulting in a proportional increase in transmission rate with only a modest increase in bit-error ratio.
1. A transmitter for optical transmissions comprising:
at least one low density parity check (LDPC) encoder configured to encode source data;
an interleaver to interleave the encoded data;
a mapper configured to generate N coordinates, defined by a set of N orthonormal basis functions each representing a dimension, in accordance with an N-dimensional (N-D) signal constellation; and
a modulator configured to perform electrical-to-optical conversion in accordance with each of the N coordinates to provide a transmission signal, wherein the mapper determines the corresponding M-ary signal constellation point
s
i
=
1
N
∑
j
=
1
N
ϕ
i
,
j
Φ
j
(
t
)
,
and wherein φ i,j represents the possible values of the orthonormal basis functions, Φ j (t) represents the set of orthonormal basis functions at the given point in time, and M=2 N .
2. The transmitter of claim 1 , wherein N is three or greater.
3. The transmitter of claim 1 , wherein φ i,j ε{1,−1} and the M signals form the vertices of an N-dimensional hypercube.
4. The transmitter of claim 1 , wherein the at least one LDPC encoder uses a structured LDPC code.
5. A coherent receiver for optical transmissions, comprising:
a demodulator configured to receive an input signal comprising N component signals, each modulated according to one of N orthonormal basis functions, and configured to output N branches corresponding to the N signals;
a demapper configured to receive the N branches and iteratively demap the N branches based on extrinsic log likelihood ratios (LLRs);
a bit LLR calculator configured to compute LLRs for bit prediction; and
at least one LDPC decoder configured to iteratively decode bits by feeding back extrinsic LLRs to the demapper, wherein the mapper determines the corresponding M-ary signal constellation point
s
i
=
1
N
∑
j
=
1
N
ϕ
i
,
j
Φ
j
(
t
)
,
and wherein φ i,j represents the possible values of the orthonormal basis functions, Φ j (t) represents the set of orthonormal basis functions at the given point in time, and M=2 N .
6. The receiver of claim 5 , wherein N is three or greater.
7. The receiver of claim 5 , wherein each LDPC decoder decodes a separate bit stream.
8. The receiver of claim 5 , wherein the LDPC decoder iteratively decodes bits until either the LDPC decoder converges to a codeword or until a predefined number of iterations is reached.
9. A method for N-dimensional modulation, comprising:
encoding a signal with a low-density parity check (LDPC) code;
interleaving data from a plurality of encoders to a mapper;
mapping an encoded signal to N constellation coefficients, defined by a set of N orthonormal basis functions each representing a dimension, using an N-dimensional constellation; and
performing a modulation in accordance with the N constellation coefficients to provide a transmission signal;
determining corresponding M-ary signal constellation point
s
i
=
1
N
∑
j
=
1
N
ϕ
i
,
j
Φ
j
(
t
)
,
wherein φ i,j represents the possible values of the orthonormal basis functions, Φ j (t) represents the set of orthonormal basis functions at the given point in time, and M=2 N .
10. The method of claim 9 , further comprising:
coherently receiving the transmission signal;
demodulating the transmission signal into N branches corresponding to the N orthonormal basis functions;
iteratively demapping the transmission signal from the N branches using an N-dimensional signal constellation;
predicting bits using LLRs for demapped bits; and
iteratively decoding using at least one LDPC decoder configured to iteratively decode bits by feeding back extrinsic LLRs for demapping.
11. The method of claim 9 , wherein N is three or greater.
12. The method of claim 9 , wherein the LDPC code is a structured code.
13. A computer readable medium comprising a computer readable program, wherein the computer readable program when executed on a computer causes the computer to perform the method of claim 9 .