Artificial-Noise Alignment for Secure Multicast using Multiple Antennas

We propose an artificial-noise alignment scheme for multicasting a common-confidential message to a group of legitimate receivers. Our scheme transmits a superposition of information and noise symbols. At each legitimate receiver, the noise symbols are aligned in such a way that the information symbols can be decoded with high probability. In contrast, the noise symbols completely mask the information symbols at the eavesdroppers. Our proposed scheme does not use the knowledge of the eavesdropper's channel gains at the transmitter for alignment, yet it achieves the best-known lower bound on the secure degrees of freedom. The knowledge of the eavesdropper's channel gains is still necessary when selecting the rate of the wiretap code. Our scheme is also a natural generalization of the approach of transmitting artificial noise in the null-space of the legitimate receiver's channel, previously proposed in the literature.