The secrecy degrees of freedom (SDoF) of the Gaussian multiple-input andsingle-output (MISO) wiretap channel is studied under the assumption thatdelayed channel state information (CSI) is available at the transmitter andeach receiver knows its own instantaneous channel. We first show that astrictly positive SDoF can be guaranteed whenever the transmitter has delayedCSI (either on the legitimate channel or/and the eavesdropper channel). Inparticular, in the case with delayed CSI on both channels, it is shown that theoptimal SDoF is 2/3. We then generalize the result to the two-user GaussianMISO broadcast channel with confidential messages and characterize the SDoFregion when the transmitter has delayed CSI of both receivers. Interestingly,the artificial noise schemes exploiting several time instances are shown toprovide the optimal SDoF region by masking the confidential message to theunintended receiver while aligning the interference at each receiver.
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