Secure multiple-input single-output communication - Part II: δ-secrecy symbol error probability and secrecy diversity

The authors consider secure beamforming with artificial noise in a multiple-input single-output multiple-eavesdropper (MISOME) wiretap channel, where a transmitter has access to full channel state information (CSI) of a legitimate channel but only partial CSI of eavesdropper channels. In the second part of this study, the authors first put forth a new notion of symbol error probability (SEP) for confidential information - called the `δ-secrecy SEP´ - to connect the reliability and confidentiality of the legitimate communication in MISOME wiretap channels. For single-antenna colluding and non-colluding eavesdroppers, the authors then quantify the diversity impact of secure beamforming with artificial noise on the δ-secrecy SEP and show that the artificial -noise strategy with n_t transmit antennas preserves the secrecy diversity of order n_t - n_e for n_e colluding eavesdroppers and n_t - 1 for n_e non-colluding eavesdroppers, respectively. In addition, the authors determine the optimal power allocation between the information-bearing signal and artificial noise to minimise the δ-secrecy SEP in the presence of weak or strong eavesdroppers, and further develop the switched power allocation for general eavesdropping attacks.