  The degrees of freedom (DoF) of the two-user Gaussian multiple-input and
multiple-output (MIMO) broadcast channel with confidential message (BCC) is
studied under the assumption that delayed channel state information (CSI) is
available at the transmitter. We characterize the optimal secrecy DoF (SDoF)
region and show that it can be achieved by a simple artificial noise alignment
(ANA) scheme. The proposed scheme sends the confidential messages superposed
with the artificial noise over several time slots. Exploiting delayed CSI, the
transmitter aligns the signal in such a way that the useful message can be
extracted at the intended receiver but is completely drowned by the artificial
noise at the unintended receiver. The proposed scheme can be interpreted as a
non-trivial extension of Maddah-Ali Tse (MAT) scheme and enables us to quantify
the resource overhead, or equivalently the DoF loss, to be paid for the secrecy
communications.
