An efficient software radio implementation of the UMTS turbo codec

This paper addresses some critical implementation issues involved in the development of a turbo decoder, using the UMTS specification as a concrete example. The assumption is that the decoder is to be implemented in software rather than hardware, and thus a variable number of decoder iterations is not only possible, but desirable. Three twists on the decoding algorithm am proposed: (1) a linear approximation of the correction function used by the max* operator which reduces complexity with only a negligible loss in BER performance; (2) a method for normalizing the backward recursion which yields a 12.5% savings in memory usage; and (3) a simple method for halting the decoder iterations based only on the log-likelihood ratios