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Tanner Graphs

Tanner graphs are used to determine the properties of a specific LDPC code.

Consider the example decoder below...

If the decoder was implemented as an iterative decoder, we could draw a Tanner Graph as shown below...

The light blue dots represent the Estimators and the light red dots represent the Predictors.

One of the things of interest is the length of the cycles in a Tanner graph.

The following Figure shows a cycle of length 6...

Short cycles degrade performance considerably.

Cycles cannot be eliminated completely otherwise the minimum distance of the code will be < 2.