  This letter studies the practical design of secure transmissions without
knowing eavesdropper's channel state information (ECSI). An ECSI-irrelevant
metric is introduced to quantize the intrinsic anti-eavesdropping ability (AEA)
that the transmitter has on confronting the eavesdropper via secrecy encoding
together with artificial-noise-aided signaling. Non-adaptive and adaptive
transmission schemes are proposed to maximize the AEA with the optimal encoding
rates and power allocation presented in closed-form expressions. Analyses and
numerical results show that maximizing the AEA is equivalent to minimizing the
secrecy outage probability (SOP) for the worst case by ignoring eavesdropper's
receiver noise. Therefore, the AEA is a useful alternative to the SOP for
assessing and designing secure transmissions when the ECSI cannot be prior
known.
