Adding a Rate-1 Third Dimension to Turbo Codes

Thanks to the message passing principle, turbo decoding is able to provide strong error correction near the theoretical (Shannon) limit. However, the minimum Hamming distance (MHD) of a turbo code may not be sufficient to prevent a detrimental change in the error rate vs. signal to noise ratio curve, the so-called flattening. Increasing the MHD of a turbo code may involve using component encoders with a large number of states, devising more sophisticated internal permutations, or increasing the dimension of the turbo code, i.e. the number of component encoders. This paper addresses the latter option and proposes a modified turbo code, in which some of the parity bits stemming from the classical component encoders are encoded by a rate-1, third encoder. The result is a significantly increased MHD, which improves turbo decoder performance at low error rates, at the expense of a very small increase in complexity. In this paper, we compare the performance of the proposed turbo code with that of the DVB-RCS turbo code and the DVB-S2 LDPC code. Comparisons with more complex 16-state TCs are also reported.