A modem includes an LDPC encoder which utilizes a deterministic H-matrix, optionally
via a generation matrix, to generate redundant parity bits for a bit block. Ones
are placed into the H-matrix in a completely diagonal manner with diagonals subdivided
into sets of diagonals. The first diagonal in each set i begins with coordinates
H(1,k), where k=(1+(i*Mj)). The remaining diagonals in the sets are
offset from the first diagonals so that the column distances between any two pairs
of diagonals is unique. In another embodiment, the H-matrix is determined by assigning
"1s" in a first column, and then assigning "1s" of subsequent columns deterministically
by causing each "1" in a previous ancestor column to generate a "1" in the next
descendant column based on the rule that a descendant is placed one position below
an ancestor except where rectangles would be generated. Interrupted descending
diagonals are generated.