1. Introduction
Low-density parity-check (LDPC) codes were discovered by Gallager in 1963 [6] and, nowadays, they are among the most promising error correcting schemes. The renewed interest in Gallager's LDPC codes can be justified by their simplicity and by their attractive performance/complexity tradeoff. Currently, LDPC codes are being considered by the standardization committees of several future communication systems as serious candidates for the error control coding.