  In magnetic-recording systems, consecutive sections experience different
signal to noise ratios (SNRs). To perform error correction over these systems,
one approach is to use an individual block code for each section. However, the
performance over a section affected by a lower SNR is weaker compared to the
performance over a section affected by a higher SNR. Spatially-coupled (SC)
codes are a family of graph-based codes with capacity approaching performance
and low latency decoding. An SC code is constructed by partitioning an
underlying block code to several component matrices, and coupling copies of the
component matrices together. The contribution of this paper is threefold.
First, we present a new partitioning technique to efficiently construct SC
codes with column weights 4 and 6. Second, we present an SC code construction
for channels with SNR variation. Our SC code construction provides local error
correction for each section by means of the underlying codes that cover one
section each, and simultaneously, an added level of error correction by means
of coupling among the underlying codes. Third, we introduce a low-complexity
interleaving scheme specific to SC codes that further improves their
performance over channels with SNR variation. Our simulation results show that
our SC codes outperform individual block codes by more than 1 and 2 orders of
magnitudes in the error floor region compared to the block codes with and
without regular interleaving, respectively. This improvement is more pronounced
by increasing the memory and column weight.
