Patent ID: 8446986
Filing Date: 2013-05-21
Classification: H04L

Abstract:
1. A method of estimating log-likelihood ratios for a first stream and a second stream of samples of a received Spread Frequency Shift Keying (S-FSK) signal demodulated using a first carrier and a second carrier, respectively, samples of the first stream and of the second stream representing a same received binary value, comprising: estimating channel and noise parameters associated with a transmitted value −1 and a transmitted value +1, for the first stream and for the second stream of samples obtained from the received S-FSK modulated signal; estimating current signal-to-noise ratios for current samples of the first stream and of the second stream of samples obtained from the received S-FSK modulated signal using the channel and noise parameters; comparing the estimated current signal-to-noise ratios with values of a discrete ordered set and identifying respective pairs of consecutive values of the discrete ordered set between which the estimated current signal-to-noise ratios are comprised; estimating log-likelihood ratios for the current samples of the first stream and of the second stream using respective polynomial approximating functions defined based upon the current signal-to-noise ratios and based upon coefficients stored in a look-up table corresponding to the pairs of consecutive values of the discrete ordered set, the coefficients being related to values of a natural logarithm of a modified Bessel function of a first kind and of a zero order based upon values belonging to the discrete ordered set.