The UMTS Turbo Code and an Efficient Decoder Implementation Suitable for Software-Defined Radios

This paper provides a description of the turbo code used by the UMTS third-generation cellular standard, as standardized by the Third-Generation Partnership Project (3GPP), and proposes an efficient decoder suitable for insertion into software-defined radio architectures or for use in computer simulations. Because the decoder is implemented in software, rather than hardware, single-precision floating-point arithmetic is assumed and a variable number of decoder iterations is not only possible but desirable. Three twists on the well-known log-MAP decoding algorithm are 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 that 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.