EXIT chart

If there are two components which exchange messages, the behaviour of the decoder can be plotted on a two-dimensional chart.

A key assumption is that the messages to and from an element of the decoder can be described by a single number, the extrinsic information.

This is true when decoding codes from a binary erasure channel but otherwise the messages are often samples from a Gaussian distribution with the correct extrinsic information.

The other key assumption is that the messages are independent (equivalent to an infinite block-size code without local structure between the components) To make an optimal code, the two transfer curves need to lie close to each other.

This observation is supported by the theoretical result that for capacity to be reached for a code over a binary-erasure channel there must be no area between the curves and also by the insight that a large number of iterations are required for information to be spread throughout all bits of a code.