  This paper investigates artificial noise injection into the temporal and
spatial dimensions of a legitimate wireless communication system to secure its
transmissions from potential eavesdropping. We consider a multiple-input
single-output (MISO) orthogonal frequency division multiplexing (OFDM) system
in the presence of a single-antenna passive eavesdropper and derive both the
secrecy rate and average secrecy rate of the legitimate system. It is assumed
that the legitimate transmitter knows the full channel information of the
legitimate transceivers but does not know the instantaneous channel state
information of the passive eavesdropper. Closed-form expressions for the
secrecy rate and average secrecy rate are derived for the asymptotic case with
a large number of transmit antennas. We also investigate 1) the power
allocation between the data and the AN; 2) the power allocation between the
spatial and the temporal AN. Computer simulations are carried out to evaluate
the performance of our proposed artificial noise scheme.
