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

We examine the secrecy performance of three artificial-noiseaided 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 phi 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 phi, R-B, and R-E all vary with the instantaneous channel gains.