Extrinsic Information Transfer Chart (EXIT) Chart is a method invented by S.ten Brink [2] used to evaluate performance
of any iterative decoder and give a feedback on the required number of iterations to reach successful
decoding, if successful decoding is possible.
The EXIT Chart method can be briefly described as follows:
- General description : The chart is plotted as the a priori mutual information $I_A$ before message is decoded
versus the extrinsic mutual information $I_E$ after decoding.
- Mechanism : The extrinsic output ($I_{E_{1}}$) of the first decoder is passed to the second decoder as a priori
input ($I_{A_{2}}$) then the same is repeated for the extrinsic output of the second decoder ($I_{E_{2}}$).
- Plotting: The two plots are plotted on the same chart, but with swapped axis for the second decoder.
- Evaluation: The decoding iterative behavior is given by a staircase function in the tunnel
between the two curves. The staircase functions starts at point (0,0) (no mutual information) and a successful
decoding is achieved when the point (1,1) is reached.