On bit-interleaved turbo-coded modulation with low error floors

In this work we introduce an algorithm to optimize the performance in the error floor region of bit-interleaved turbo-coded modulation (BITCM) on the additive white Gaussian noise (AWGN) channel. The key ingredient is an exact turbo code weight distribution algorithm producing a list of all codewords in the underlying turbo code of weight less than a given threshold. Using the union bounding technique, we show that a well-designed bit-interleaver is crucial to have a low error floor. Furthermore, the error rate performance in the waterfall region depends on the bit-interleaver, since the level of protection from channel noise on the bit-level depends on the bit-position and the neighboring bit values within the same symbol in the transmitted sequence. We observe a trade-off between error rate performance in the waterfall and error floor regions as illustrated by an extensive case study of a high-rate BITCM scheme.