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\section{Interleaver}

\subsection{Overview}

LDPC makes our CATS packet robust against single bit flips. However, it is not particularly robust against burst errors. This is because a burst will flip several bits in only one (or possibly two) of our LDPC chunks, while the rest remain untouched. If too many bits flip in a single chunk, it will not be possible to recover. Instead, it is better to split this burst across as many chunks as possible, to lower the likelihood that a chunk will be unrecoverable. The interleaver is responsible for reordering the CATS bits in such a way to be robust against bit flips.

In CATS, a 32-bit block interleaver is used. For a bit-string input $b_0 b_1 b_2 b_3 ... b_n$, the output from the interleaver is $b_0 b_{32} b_{64} ... b_1 b_{33} b_{65} ... b_3 b_{34} b_{66} ...$ . The receiver is responsible for de-interleaving the bit stream to get the original data.