Artificial noise revisited: When Eve has more antennas than Alice

We consider secure communications over MIMO wiretap channels, in the presence of a passive eavesdropper with an unlimited number of antennas. In this scenario, we characterize the performance of the artificial noise scheme proposed by Goel et al., and show that non-zero secrecy capacity is available even when the eavesdropper has more antennas than the transmitter. Our results are derived based on the fraction of the transmission power used for artificial noise and the ratio of the channel noise variances of the eavesdropper and the intended receiver. Finally, we investigate the attack option for the eavesdropper to drive the secrecy rate to zero by increasing the number of antennas.