A simple method of approximating the error floor of turbo codes with S-type permutors

An efficient method for calculating some of the first coefficients of the distance spectrum of turbo codes is presented. It is based on the evaluation of cycles in the permutation matrix that are of a special type. For S-type permutors, the calculated coefficients in the distance spectrum include the minimum distance and other low weight terms. Therefore, by applying the union bound, this method is capable of giving a very tight approximation to the error floor behavior of the corresponding turbo code - even for large permutor sizes.