  A novel method guaranteeing nondecreasing girth is presented for constructing
longer low-density parity-check (LDPC) codes from shorter ones. The
parity-check matrix of a shorter base code is decomposed into N (N>=2)
non-overlapping components with the same size. Then, these components are
combined together to form the parity-check matrix of a longer code, according
to a given N*N Latin square. To illustrate this method, longer quasi-cyclic
(QC) LDPC codes are obtained with girth at least eight and satisfactory
performance, via shorter QC-LDPC codes with girth eight but poor performance.
The proposed method naturally includes several well-known methods as special
cases, but is much more general compared with these existing approaches.
