Three Artificial-Noise-Aided Secure Transmission Schemes in Wiretap Channels

We examine the secrecy performance of three artificial-noise-aided secure transmission schemes, namely, the partially adaptive, fully adaptive, and on-off schemes. To this end, we provide new analysis to facilitate the optimization of the fraction φ of the transmit power allocated to the useful signal and redundancy rate R_E. Surprisingly, our examination indicates that the partially adaptive scheme, in which only the codeword rate R_B varies with the instantaneous channel gains, significantly outperforms the on-off scheme, in which both R_B and R_E vary. This performance gain can be characterized in terms of a higher average secrecy rate, subject to an upper bound on the secrecy outage probability. Furthermore, our results also demonstrate that the partially adaptive scheme can achieve almost the same secrecy performance as the fully adaptive scheme, which is of a much higher complexity, where φ, R_B, and R_E all vary with the instantaneous channel gains.