Concatenated codes: serial and parallel

An analogy is examined between serially concatenated codes and parallel concatenations whose interleavers are described by bipartite graphs with good expanding properties. In particular, a modified expander code construction is shown to behave very much like Forney´s classical concatenated codes, though with improved decoding complexity. It is proved that these new codes achieve the Zyablov bound δ_Z on the minimum distance. For these codes, a soft-decision, reliability-based, linear-time decoding algorithm is introduced, that corrects any fraction of errors up to almost δ_Z/2. For the binary-symmetric channel, this algorithm´s error exponent attains the Forney bound previously known only for classical (serial) concatenations.