Density evolution and LDPC code optimization for interleaver division multiple access

We consider multi-stage decoding in the interleaver division multiple access (IDMA) scheme with identical low-density parity-check (LDPC) component codes. We use density evolution to show that, with a proper choice of codes, the LDPC-IDMA scheme nearly achieves the capacities of Gaussian non-fading and fading channels. We discuss several power allocation methods suitable for this scheme and propose a new method based on the density evolution algorithm. When this power allocation is employed in systems in which the overall code rate is given and fixed, we show that it is beneficial to use lowrate component codes.