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EXIT Chart

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:

  1. 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.
  2. 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 ($I_{E_{2}}$).
  3. Plotting: The two plots are plotted on the same chart, but with swapped axis for the second decoder.
  4. 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.