On the Performance of BICM with Trivial Interleavers in Nonfading Channels

Recent results have shown that the performance of bit-interleaved coded modulation (BICM) using convolutional codes in nonfading channels can be greatly improved if the bit-level interleaver takes a trivial form (BICM-T), i.e., if it does not interleave the bits at all. The reported gains reach a few decibels and are obtained using a less complex BICM system. In this paper, we give a formal explanation for these results and show that BICM-T is in fact the combination of a TCM transmitter and a BICM receiver. Analytical bounds that predict the performance of BICM-T are developed and a new type of distance spectrum for the convolutional code is introduced.