Stochastic modeling of the convergence behavior of concatenated codes

Turbo codes are decoded by a concatenated scheme of recursively applying extrinsic information from previous steps. EXIT charts, recently introduced by Ten Brink (IEEE Transact. on Comms., vol.49(10), p.1727-1737, 2001) are an interesting approach to explain the surprising efficiency of turbo codes. In this paper, we analyze the convergence behavior on the basis of a purely stochastic model, avoiding the emulation of constituent decoders in inner steps. Mutual information trajectories are used to depict the convergence performance. It is shown how the convergence speed depends on the appropriate selection of system and model parameters.