A decomposition of optimal linear block codes with minimum distance \(d = 4\) and length \(4L\) into two subcodes is given such that one of the subcodes is an optimal length \(L\) code with minimum Hamming distance \(4\) and the other is a quasi-cyclic code of index \(4\). It is shown that the \(L\)-section minimal trellis diagram of the code is the product of the minimal trellis diagrams of the subcodes.
1970-2025 CP (Manitoba, Canada) unless otherwise stated.