Error performance bounds of turbo-codes employing nonuniform interleavers

Several techniques have been presented to increase the performance of turbo codes. These methods usually consider the effective free distance or critical input sequences of low weights which have to be avoided. Moreover, an analytical representation of binary turbo codes is still an open problem for employing nonuniform interleavers. Therefore, it is straightforward to separate the development of optimal turbo codes into two steps. The first step is to find a new method to describe the distance properties of turbo codes. Secondly, some criteria and algorithms are necessary to evaluate interleavers and encoders. Therefore, this paper presents a technique to derive distance spectra of turbo codes using nonuniform interleavers. From these spectra performance bounds are evaluated for different channel models and verified by simulations. The development of optimal turbo codes based on the presented analytical representation is discussed