Patent ID: 8879653

Claim:
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 ).