Constrained block codes for class-IV partial-response channels with maximum-likelihood sequence estimation

Significant improvements in magnetic storage densities have been made feasible by the application of partial-response signaling combined with maximum-likelihood sequence estimation. To enhance the performance of this technique when applied to the class-IV partial-response channel, which is recognized as being appropriate to model the magnetic recording channel, it is often required to bound the number of consecutive zeros in the recorded data sequence and its odd and even subsequences. We investigate block codes that satisfy such a constraint. In particular, we look for a set of maximal number of fixed-length sequences such that any pair of them can be concatenated without violating the constraint. In many cases, depending on the constraint and the length of the sequences, we determine such a set, and in the remaining cases, we determine at most three candidates for it. These results are used to study the best possible constrained block codes