Patent ID: 6834088
Filing Date: 2004-12-21
Classification: H04L

Abstract:
A method for calculating a log-likelihood ratio for each bit of a symbol modulated and transmitted in a communication system, the symbol comprising one or more bits according to a predetermined mapping scheme having corresponding characteristics, the method comprising the steps of:receiving the modulated symbol and demodulating the modulated symbol with the at least one receiver; and calculating a corresponding log-likelihood ratio for each of the one or more bits of the demodulated symbol by eliminating calculation steps used to determine the log-likelihood ratio using predetermined logic conditions based on the corresponding characteristics of the predetermined mapping scheme, wherein the predetermined mapping scheme includes a square Karnaugh mapping, wherein the corresponding characteristics of the predetermined mapping scheme include mirror symmetry for a constellation of symbol values about horizontal and vertical axes for all of the one or more bits in each of the symbols in the constellation except for two predetermined bits of each symbol in those symbols having more than two bits, wherein each of the two predetermined bits has opposing values for symbols on respective opposing sides of the horizontal or vertical axes, and wherein the demodulated symbol is formatted as a two's complement having a sign bit, one or more integer bits and one or more fractional bits, and wherein the predetermined logic conditions include: a) performing intermediate difference calculations in the directions of the horizontal and vertical axes using the two's complement sign bit, the one or more integer bit and the one or more fractional bits, the difference calculations based on the asymmetric and symmetric characteristics of the symbol bits in the Karnaugh mapped symbol constellation to achieve a plurality of difference values; b) determining two minimum distances of each bit in the received demodulated symbol to corresponding two closest constellation points of the Karnaugh mapped symbol constellation having corresponding bit values of one and zero, respectively, by selecting one or more of the plurality of difference values based on selection criterion predetermined for each particular bit of the demodulated symbol, the selection criterion based on characteristics of the Karnaugh mapping; and c) squaring the determined two minimum distances of each bit and taking the difference of the squares to obtain the log-likelihood ratio of each bit of the demodulated symbol.