eIRA Codes for Coded Modulation Systems

In this paper we discuss the design of linear time encodable extended irregular repeat-accumulate (eIRA) codes for bit-interleaved coded modulation (BICM) systems. We interpret eIRA codes as a particular instance of the recently introduced multi-edge type (MET) low-density parity-check (LDPC) codes. This allows us to take into account during the design step both the systematic nature of these codes and the different level of protection given to each bit by the choice of input alphabet and mapping function. Furthermore, we extend to the eIRA-BICM case the method used by Roumy et al., IEEE Trans. Inf. Theory, 2004, to optimize LDPC codes for the binary input AWGN (bi-AWGN) channel. This method is based on EXIT functions and leads to a linear program. Using this method we design capacity-approaching eIRA codes for 4PAM and 8PSK. For both short and long block lengths these codes have performance approximately 0.1 dB worse than LDPC codes optimized with density evolution and with similar decoding complexity.