Use of a $d$-Constraint During LDPC Decoding in a Bliss Scheme (0707.3925v1)

Abstract: Bliss schemes of a run length limited (RLL) codec in combination with an LDPC codec, generate LDPC parity bits over a systematic sequence of RLL channel bits that are inherently redundant as they satisfy e.g. a $d=1$ minimum run length constraint. That is the subsequences consisting of runs of length $d=1$, viz. $...010...$ and $...101...$, cannot occur. We propose to use this redundancy during LDPC decoding in a Bliss scheme by introducing additional $d$-constraint nodes in the factor graph used by the LDPC decoder. The messages sent from these new nodes to the variable or codeword bit nodes exert a ``force'' on the resulting soft-bit vector coming out of the LDPC decoding that give it a tendency to comply with the $d$-constraints. This way, we can significantly reduce the probability of decoding error.