On error floor and free distance of turbo codes

Turbo codes have excellent performance at low and medium signal-to-noise ratios (SNR) very close to the Shannon limit, and are at the basis of their success. However, a turbo code performance curve can change its slope at high SNR if the code free distance is small. This "error floor" phenomenon is not acceptable for applications requiring very low values of bit error rates. A knowledge of the free distance and its multiplicity allows one to analytically estimate the error floor. An algorithm for computing the turbo code free distance, based on the notion of constrained subcodes, is described. Some considerations on the free distance distribution of turbo codes with growing interleaver length are also provided.