Runlength-Limited Sequences and Shift-Correcting Codes: Asymptotic Analysis

Abstract: This work is motivated by the problem of error correction in bit-shift channels with the so-called $ (d,k) $ input constraints (where successive $ 1 $'s are required to be separated by at least $ d $ and at most $ k $ zeros, $ 0 \leq d < k \leq \infty $). Bounds on the size of optimal $ (d,k) $-constrained codes correcting a fixed number of bit-shifts are derived, with a focus on their asymptotic behavior in the large block-length limit. The upper bound is obtained by a packing argument, while the lower bound follows from a construction based on a family of integer lattices. Several properties of $ (d, k) $-constrained sequences that may be of independent interest are established as well; in particular, the exponential growth-rate of the number of $ (d, k) $-constrained constant-weight sequences is characterized. The results are relevant for magnetic and optical information storage systems, reader-to-tag RFID channels, and other communication models where bit-shift errors are dominant and where $ (d, k) $-constrained sequences are used for modulation.