  This paper examines linear beamforming methods for secure communications in a
multiuser wiretap channel with a single transmitter, multiple legitimate
receivers, and a single eavesdropper, where all nodes are equipped with
multiple antennas. No information regarding the eavesdropper is presumed at the
transmitter, and we examine both the broadcast MIMO downlink with independent
information, and the multicast MIMO downlink with common information for all
legitimate receivers. In both cases the information signal is transmitted with
just enough power to guarantee a certain SINR at the desired receivers, while
the remainder of the power is used to broadcast artificial noise. The
artificial interference selectively degrades the passive eavesdropper's signal
while remaining orthogonal to the desired receivers. We analyze the
confidentiality provided by zero-forcing and optimal minimum-power beamforming
designs for the broadcast channel, and optimal minimum-MSE beamformers for the
multicast channel. Numerical simulations for the relative SINR and BER
performance of the eavesdropper demonstrate the effectiveness of the proposed
physical-layer security schemes.
