Comparison of Constructions of Irregular Gallager Codes

David J C MacKay, Simon T Wilson and Matthew C Davey

(1) The low density parity check codes whose performance is closest to the Shannon limit are based on irregular graphs. We compare alternative methods for constructing these graphs and find that `super-Poisson' constructions give significantly better empirical performance.

(2) Low density parity check codes normally take N^2 time to encode, because they are defined in terms of non-systematic parity check matrices. We investigate constructions which allow the encoding to be made faster to see whether any performance loss results, looking both at regular and irregular Gallager codes.

postscript (Cambridge UK).

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