In this work, we have constructed RLL codes that can be used effectively with an outer LDPC code in the standard concatenation configuration. The main idea in the construction is the use of a two-state trellis representation for the sequences satisfying a (d, k) constraint. The constructed RLL codes have a free distance of 2 on this two-state trellis. This enables the use of a low-complexity, 2-state BCJR algorithm for soft-decoding the RLL codes in an iterative decoder. The method can be generalized to construct codes for other constraints as well.
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