Accumulate repeat accumulate coded modulation

Abstract

In this paper we propose an innovative coded scheme called repeat accumulate coded modulation (ARA coded modulation). This class of codes can be viewed as serial turbo-like codes, or as a subclass of low density parity check (LDPC) codes that are combined with high level modulation. Thus at the decoder belief propagation can be used for iterative decoding of ARA coded on a graph, provided a demapper transforms the received inphase and quadrature samples to reliability of the bits. The structure of encoder for this class can be viewed as preceded repeat accumulate (RA) code or as preceded irregular repeat accumulate (IRA) code when combined with high order modulation, where simply an accumulator is chosen as a precoder to improve the performance. The ARA coded has a simple, and a very fast encoder structure when representing LDPC coded modulation. Based on density evolution through some examples for ARA code with BPSK modulation, we show that for maximum variable node degree 5 a minimum bit SNR as low as 0.2-0.3 dB from channel capacity for code rates 1/2, 2/3, and 3/4 can be achieved as the block size goes to infinity . Thus based on fixed low maximum variable node degree, its threshold outperforms not only the RA and IRA codes but also the best known unstructured irregular LDPC codes reported in the literature. Iterative decoding simulation results are provided for BPSK, QPSK, 8 PSK, and 16 QAM modulations over additive white Gaussian noise. The ARA code also has projected graph or protograph representation, that allows for high speed decoder implementation.