A structured parity-check matrix H is proposed, wherein H is an expansion of a base matrix H.sub.b and wherein H.sub.b comprises a section H.sub.b1 and a section H.sub.b2, and wherein H.sub.b2 comprises a first part comprising a column h.sub.b having an odd weight greater than 2, and a second part comprising matrix elements for row i, column j equal to 1 for i=j, 1 for i=j+1, and 0 elsewhere. The expansion of the base matrix H.sub.b uses identical submatrices for 1s in each column of the second part H'.sub.b2, and the expansion uses paired submatrices for an even number of 1s in h.sub.b.