Soft-demapping of QAM signals
View Patent ↗This invention concerns soft-decision demapping of Quadrature Amplitude Modulation (QAM) signals to enable soft-decision channel decoding in a communications system. In a first aspect the invention is a method for performing the soft-decision demapping of Quadrature Amplitude Modulation (QAM) signals to enable soft-decision channel decoding in a communications system. The method comprises the steps of Extracting baseband signals from both I-and-Q channels. Sampling the baseband signals to extract a stream of complex numbers. Converting the stream of complex numbers to frequency domain vectors with components for each subcarrier frequency. Approximating bit log-likelihood ratios for each symbol directly from the real and imaginary parts of the corresponding frequency vector, without equalization by the estimated channel. And, soft-decoding of the channel codes using the approximated log-likelihood ratios. In other aspects the invention concerns a device for performing the method and software for performing the method.
1. A method for soft-decision demapping of Quaternary Phase Shift Keying (QPSK) signals to enable soft-decision channel decoding in a communications system, the method comprising the steps of:
extracting baseband signals from both in-phase (I) and quadrature (Q) phase channels;
sampling the baseband signals to extract a stream of complex numbers;
converting the stream of complex numbers to frequency domain vectors with components for each subcarrier frequency;
approximating bit log-likelihood ratios for each symbol directly from the real and imaginary parts of the corresponding frequency vector; and
soft-decoding of the channel codes using the approximated bit log-likelihood ratios;
wherein the approximates of the log-likelihood ratios (LLRs) are derived by decomposing the optimum LLR as a superposition of a linear function and a nonlinear function of the frequency domain vectors, and the linear part is then directly mapped to the approximate LLR;
wherein for QPSK constellation the log likelihood ratios (LLR's) of bits b 0 and b 1 which are denoted by {tilde over (b)} 0 and {tilde over (b)} 1 are defined as follows:
{tilde over (b)} 0 depends only on a n , while
{tilde over (b)} 1 depends only on c n
where a n and c n is calculated from the corresponding frequency domain vector z n and the estimated channel {tilde over (h)} n as follows:
a n =Re ( {tilde over (h)} n ) Re ( z n )+ Im ( {tilde over (h)} n ) Im ( z n )
c n =Re ( {tilde over (h)} n ) Im ( z n )− Im ( {tilde over (h)} n ) Re ( z n ).
2. A method according to claim 1 , wherein the approximated LLR is given by:
b
~
0
=
a
n
σ
n
2
b
~
1
=
(
d
2
/
d
1
)
c
n
σ
n
2
where:
σ n 2 is the noise variance in z n ; and
d 1 and d 2 are the constellation amplitude in b 0 and b 1 respectively.
3. A method according to claim 2 , wherein the noise variance σ n 2 is constant across all OFDM subcarriers and symbols; and there is equal error protection (EEP), then the optimum LLR is given by:
{circumflex over (b)} 0 =a n
{circumflex over (b)} 1 =c n .
4. A method for soft-decision demapping of 16Quadrature Amplitude Modulation (QAM) signals to enable soft-decision channel decoding in a communications system, the method comprising the steps of:
extracting baseband signals from both in-phase (I) and quadrature (Q) phase channels;
sampling the baseband signals to extract a stream of complex numbers;
converting the stream of complex numbers to frequency domain vectors with components for each subcarrier frequency;
approximating bit log-likelihood ratios for each symbol directly from the real and imaginary parts of the corresponding frequency vector; and
soft-decoding of the channel codes using the approximated bit log-likelihood ratios;
wherein the approximates of the log-likelihood ratios (LLRs) are derived by decomposing the optimum LLR as a superposition of a linear function and a nonlinear function of the frequency domain vectors, and the linear part is then directly mapped to the approximate LLR;
wherein for 16QAM constellation the approximate bit LLR's {tilde over (b)} 0 , {tilde over (b)} 1 , {tilde over (b)} 2 and {tilde over (b)} 3 are given by:
b
~
0
≈
1
σ
n
2
a
n
b
~
1
≈
2
d
1
h
~
n
2
-
a
n
σ
n
2
b
~
2
≈
d
2
/
d
1
σ
n
2
c
n
b
~
3
≈
2
(
d
2
2
/
d
1
)
h
~
n
2
-
(
d
2
/
d
1
)
c
n
σ
n
2
where a n and c n are calculated from the corresponding frequency domain vector z n and the estimated channel {tilde over (h)} n as follows:
a n =Re ( {tilde over (h)} n ) Re ( z n )+ Im ( {tilde over (h)} n ) Im ( z n )
C n =Re ( {tilde over (h)} n ) Im ( z n )− Im ( {tilde over (h)} n ) Re ( z n );
σ n 2 is the noise variance in z n ; and
d 1 and d 2 are the constellation amplitude in I-channel and Q-channel respectively.
5. A method according to claim 4 , wherein the noise variance σ n 2 , is constant across all OFDM subcarriers and symbols; and there is equal error protection (EEP), then the optimum LLR is given by:
{circumflex over (b)} 0 =a n
{circumflex over (b)} 1 =|{tilde over (h)} n | 2 −|a n |
{circumflex over (b)} 2 =c n
{circumflex over (b)} 3 ≈|{tilde over (h)} n | 2 −|c n |.