Brendan J. Frey and David J. C. MacKay 1999.
Irregular turbocodes.
Proceedings of the 37th Allerton Conference on Communication, Control and Computing 1999, Allerton House, Illinois.
Recently, several groups have increased the coding gain
of iteratively decoded Gallager codes (low density parity
check codes) by varying the number of parity check equations
in which each codeword bit participates.
In regular turbocodes, each ``systematic bit'' participates
in exactly 2 trellis sections. We construct irregular
turbocodes with systematic bits that participate in varying
numbers of trellis sections. These codes can be decoded by the
iterative application of the sum-product algorithm
(a low-complexity, more general form of the turbodecoding algorithm).
By making the original rate 1/2 turbocode of Berrou et al.
slightly irregular, we obtain a coding gain of 0.15 dB
at a block length of N = 131,072, bringing the irregular
turbocode within 0.3 dB of capacity.
Just like regular turbocodes, irregular turbocodes are linear-time encodable.
Compressed postscript,
uncompressed postscript.
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