Extrinsic Information Transfer Functions: Model and Erasure Channel Properties

Extrinsic information transfer (EXIT) charts are a tool for predicting the convergence behavior of iterative decoding of concatenated codes. A general decoding model is introduced that applies to many communication situations, including the decoding of parallel concatenated (turbo) codes, serially concatenated codes, low-density parity-check codes, and repeat-accumulate codes. EXIT functions are defined using the model, and several properties of such functions are proved for erasure channels. One property expresses the area under an EXIT function in terms of a conditional entropy. A useful consequence of this result is that the design of capacity-approaching codes reduces to a curve-fitting problem for all the aforementioned codes. A second property relates the EXIT function of a code to its Helleseth-Klove-Levenshtein information functions, and thereby to the support weights of its subcodes. The relation is via a refinement of information functions called split information functions, and via a refinement of support weights called split support weights. Split information functions are used to prove a third property that relates the EXIT function of a linear code to the EXIT function of its dual.