I. Introduction
Turbo codes introduced by Berrou et al. in [1], and Gallager's low-density parity-check (LDPC) codes [2], rediscovered by MacKay in [3], are considered among the most powerful error-correction schemes of today due to their low decoding complexity and near-capacity performance on a wide variety of channels. Recently, serially concatenated codes, such as very simple repeat accumulate (RA) codes, and hybrid concatenated codes (HCCs), i.e., mixed parallel and serial structures combining the features of the two concatenations [4]–[13], have also attracted some attention, since they yield better minimum distances than turbo codes.