An LDPC encoding system produces an encoded block by encoding the data as i data sub-blocks using an LDPC code H.sub.1, XOR'ing the H.sub.1 sub-block code words to generate a block code word and further encoding the block code word using an LDPC code H.sub.2 to produce p parity bits. The encoded block consists of a concatenation of the H.sub.1 code words and the p parity bits. To decode the encoded block, the system decodes each H.sub.1 sub-block code word and, if all of the H.sub.1 sub-block code words converge, the block has been successfully decoded. Otherwise, if one of the H.sub.1 code words fails to converge, the system performs an "extended" decoding operation in which the system produces a vector C by XOR'ing the i-1 properly decoded H.sub.1 sub-block code words. The system then combines the vector C with the sign values of the failed H.sub.1 sub-block code word, appends the parity bits and decodes the result as an H.sub.2 code word over the H.sub.2 LDPC code. For a next iteration, the system removes the vector C from the non-parity sign bits of the decoded H.sub.2 code word and provides the results along with the associated reliability information to an ISI detector for further decoding to produce an updated failed H.sub.1 code word. The updated H.sub.1 sub-block code word is then iteratively decoded as part of an H.sub.2 code word, and so forth, by combing and removing the vector C.