Performance-Complexity Tradeoffs of Concatenated FEC for Higher-Order Modulation

Multilevel coding (MLC) is compared with bit-interleaved coded modulation (BICM) from a performance-versus-complexity standpoint. In both approaches, complexity-optimized error-reducing low-density parity-check inner codes are designed for concatenation with an outer hard-decision code, for various modulation orders. The codes are designed to achieve various points on the Pareto frontier characterizing the performance-complexity tradeoff. Computer simulations of the resulting codes reveal that MLC not only provides significant advantages compared with BICM, but also outperforms several existing MLC and BICM proposals. At 25% overhead, MLC provides a net coding gain of up to 12.8 dB with 16-QAM (1.0 dB from the constrained Shannon limit), a net coding gain of up to 13.6 dB with 64-QAM (1.2 dB from the constrained Shannon limit), and a net coding gain of up to 14 dB with 256-QAM (1.65 dB from the constrained Shannon limit), all with reasonable decoding complexity.