Signal Mapping for Bit-Interleaved Coded Modulation Schemes to Achieve Secure Communications

In this letter, we investigate different signal mappings for bit-interleaved coded modulation (BICM) and their effect on the secrecy information rate of a wire-tap channel exhibiting Rayleigh fading. By focusing on BICM schemes using 64 QAM, 16 QAM and (1, 5, 10) signal constellations, we show that the optimal mapping depends on the range of signal-to-noise ratios (SNRs) we are interested in. We propose a new mapping algorithm, called maximum error event (MEE), to optimize the secrecy rate over a wide range of SNRs. At low SNR, MEE mapping achieves a lower secrecy rate than other well known mappings, but at medium-to-high SNRs MEE mapping achieves a significantly higher secrecy rate over a wide range of SNRs. Simulation results on 16 QAM, 64 QAM and (1, 5, 10) constellations are presented to show the achievable secrecy rates of various mappings for BICM from low to high SNRs.