Reliability-based coded modulation with low-density parity-check codes

In this letter, we consider the interleaver design in bit-interleaved coded modulation (BICM) with low-density parity-check (LDPC) codes. The design paradigm is to provide more coding protection through iterative decoding to bits that are less protected by modulation (and are thus less reliable at the output of the demodulator). The design is carried out by an ad hoc search algorithm over the column permutations of the parity-check matrix. Our simulations show that the proposed reliability-based coded modulation scheme can improve the error-rate performance of conventional BICM schemes based on regular LDPC codes by a few tenths of a decibel, with no added complexity.

I. Introduction and Preliminary Discussions

Bit-Interleaved coded modulation (BICM) is a simple, well-known method for achieving both power and spectral efficiency [1], [4], [18]. While it was originally proposed for fading channels [18], BICM with Gray mapping has proved to perform very close to the capacity of an additive white Gaussian noise (AWGN) channel as well [1], [4]. Interleaving is required to break down any correlation between the bits which may exist due to the memory of the channel, or for memoryless channels, due to the nonbinary modulation. The existence of an interleaver also allows for the pragmatic, but quite effective, approach of mixing and matching powerful coding schemes with different signal sets, avoiding the joint design of coding and modulation [1].