Methods and apparatus for performing rate determination based on the growth of a path metric in a Viterbi decoder

摘要

A rate determination method in accordance with this invention which is of particular interest for transmission in CDMA spread-spectrum mode according to the E1A Interim standard IS-95 includes the steps of (a) receiving digital data (12) transmitted at an unknown one of n rates (R1,R1/2,R1/4,R1/8), (b) decoding in a Viterlis decoder (14), at successive ones of said n rates, said received signal and determining, for individual ones of n data rates, a value of a total cumulative metric associated with a most likely path through a decoder trellis; (c) comparing for each tentative rate the value of the total cumulative metric to a total cumulative metric threshold value (Tr-1,Tr-2,Tr-3,Tr-41,Tr-31) corresponding to an expected total cumulative metric for said tentative one of the n data rates and being one of fixed or adaptively determined as a function of the transmission channel characteristics; and (d) identifying the received data rate based on the comparison. The step of determining a total cumulative metric includes the steps of (a) processing (30) branch metrics to determine survivor metrics; (b) normalizing (32) individual ones of the survivor metrics to form normalized survivor metrics (32A); (c) accumulating (34) the normalized survivor metrics to form cumulative metrics (34A); (d) accumulating (36) normalization values used to normalize each of the normalized survivor metrics; (e) at the end of decoding, outputting a cumulative metric that corresponds to a most likely path through the trellis; and (f) combining (38) an accumulated normalization value with the outputted cumulative metric to form the total cumulative metric.