Demodulation of 16-qam, dcm data symbols using two hybrid-qpsk constellations

Abstract

In one embodiment, a demapper uses two hybrid-QPSK constellations to demap pairs of equalized data symbols recovered from 16-QAM, DCM OFDM symbols, wherein the equalized data symbols in a pair correspond to the same four-bit group. A first hybrid-QPSK constellation is generated by combining the real components of both 16-QAM mapping constellations onto one coordinate plane. The demapper generates a first set of two decision variables by combining the real components of each equalized data symbol in a pair to correspond to the first hybrid-QPSK coordinate plane. A log-likelihood ratio is then calculated for both decision variables in the set to determine likelihood estimates for the first and second bits of the four-bit group. This process is repeated for the imaginary components of each corresponding pair of equalized data symbols to generate likelihood estimates for the third and fourth bits of the four-bit group.

We claim: 
     
         1 . A method for demapping first and second data symbols, wherein the first data symbol corresponds to a mapping of a group of four bits using a first 16-QAM constellation and the second data symbol corresponds to a mapping of the group of four bits using a second 16-QAM constellation different from the first 16-QAM constellation, the first and second data symbols each having a real component and an imaginary component, the method comprising:
 (a) generating an estimate for a first bit in the group of four bits by applying the real components of the first and second data symbols to a first formula derived using a first hybrid-QPSK constellation generated from the first and second 16-QAM constellations;   (b) generating an estimate for a second bit in the group of four bits by applying the real components of the first and second data symbols to a second formula derived using the first hybrid-QPSK constellation;   (c) generating an estimate for a third bit in the group of four bits by applying the imaginary components of the first and second data symbols to a third formula derived using a second hybrid-QPSK constellation generated from the first and second 16-QAM constellations; and   (d) generating an estimate for a fourth bit in the group of four bits by applying the imaginary components of the first and second data symbols to a fourth formula derived using the second hybrid-QPSK constellation.   
     
     
         2 . The invention of  claim 1 , wherein the first and second data symbols are recovered by demodulating one or more OFDM symbols. 
     
     
         3 . The invention of  claim 2 , wherein the first and second data symbols are recovered from a single DCM OFDM symbol. 
     
     
         4 . The invention of  claim 1 , wherein the first formula for estimating the first bit is a first log-likelihood ratio, the second formula for estimating the second bit is a second log-likelihood ratio, the third formula for estimating the third bit is a third log-likelihood ratio, and the fourth formula for estimating the fourth bit is a fourth log-likelihood ratio. 
     
     
         5 . The invention of  claim 1 , further comprising demodulating received OFDM symbols to generate recovered data symbols corresponding to the first and second data symbols. 
     
     
         6 . The invention of  claim 5 , further comprising applying equalization to the recovered data symbols to generate the first and second data symbols. 
     
     
         7 . The invention of  claim 6 , wherein the equalization is zero-forcing equalization. 
     
     
         8 . An apparatus adapted to de-map first and second data symbols, wherein the first data symbol corresponds to a mapping of a group of four bits using a first 16-QAM constellation and the second data symbol corresponds to a mapping of the group of four bits using a second 16-QAM constellation different from the first 16-QAM constellation, the first and second data symbols each having a real component and an imaginary component, the apparatus comprising a data symbol demapper adapted to:
 (a) generate an estimate for a first bit in the group of four bits by applying the real components of the first and second data symbols to a first formula derived using a first hybrid-QPSK constellation generated from the first and second 16-QAM constellations;   (b) generate an estimate for a second bit in the group of four bits by applying the real components of the first and second data symbols to a second formula derived using the first hybrid-QPSK constellation;   (c) generate an estimate for a third bit in the group of four bits by applying the imaginary components of the first and second data symbols to a third formula derived using a second hybrid-QPSK constellation generated from the first and second 16-QAM constellations; and   (d) generate an estimate for a fourth bit in the group of four bits by applying the imaginary components of the first and second data symbols to a fourth formula derived using the second hybrid-QPSK constellation.   
     
     
         9 . The invention of  claim 8 , wherein the first and second data symbols are recovered by demodulating one or more OFDM symbols. 
     
     
         10 . The invention of  claim 9 , wherein the first and second data symbols are recovered from a single DCM OFDM symbol. 
     
     
         11 . The invention of  claim 8 , wherein the first formula for estimating the first bit is a first log-likelihood ratio, the second formula for estimating the second bit is a second log-likelihood ratio, the third formula for estimating the third bit is a third log-likelihood ratio, and the fourth formula for estimating the fourth bit is a fourth log-likelihood ratio. 
     
     
         12 . The invention of  claim 8 , further comprising an FFT transform adapted to convert received OFDM symbols into recovered data symbols corresponding to the first and second data symbols. 
     
     
         13 . The invention of  claim 12 , further comprising an equalizer adapted to apply equalization to the recovered data symbols to generate the first and second data symbols. 
     
     
         14 . The invention of  claim 13 , wherein the equalization is zero-forcing equalization. 
     
     
         15 . Apparatus for demapping first and second data symbols, wherein the first data symbol corresponds to a mapping of a group of four bits using a first 16-QAM constellation and the second data symbol corresponds to a mapping of the group of four bits using a second 16-QAM constellation different from the first 16-QAM constellation, the first and second data symbols each having a real component and an imaginary component, the apparatus comprising:
 (a) means for generating an estimate for a first bit in the group of four bits by applying the real components of the first and second data symbols to a first formula derived using a first hybrid-QPSK constellation generated from the first and second 16-QAM constellations;   (b) means for generating an estimate for a second bit in the group of four bits by applying the real components of the first and second data symbols to a second formula derived using the first hybrid-QPSK constellation;   (c) means for generating an estimate for a third bit in the group of four bits by applying the imaginary components of the first and second data symbols to a third formula derived using a second hybrid-QPSK constellation generated from the first and second 16-QAM constellations; and   (d) means for generating an estimate for a fourth bit in the group of four bits by applying the imaginary components of the first and second data symbols to a fourth formula derived using the second hybrid-QPSK constellation.