Security Gap Improvement of BICM Systems Through Bit-Labeling Optimization for the Gaussian Wiretap Channel

In this paper, we investigate the impact of a bit-labeling scheme on security performance of bit-interleaved coded modulation (BICM) systems for the Gaussian wiretap channel, where the concept of security gap plays an important role. To exemplify this, we present optimization algorithms of bit-labeling, where a difference of the achievable information rates (AIRs) for the legitimate receiver and the eavesdropper is maximized. Our results demonstrate that the conventional Gray labeling may not be necessarily optimal in terms of security gap, and the proposed bit-labeling can significantly improve its performance at the cost of degradation in the achievable bit-error rate (BER) performance by the legitimate receiver. We also demonstrate that, for high order quadrature amplitude modulation (QAM), such as 64QAM, the dimension-wise optimization of bit-labeling may be sufficient to achieve similar security gap performance to the direct two-dimensional optimization. The proposed bit-labeling optimization approach can be combined with the state-of-the-art coding schemes to further reduce a security gap.