  We consider the problem of secure communication with multi-antenna
transmission in fading channels. The transmitter simultaneously transmits an
information bearing signal to the intended receiver and artificial noise to the
eavesdroppers. We obtain an analytical closed-form expression of an achievable
secrecy rate, and use it as the objective function to optimize the transmit
power allocation between the information signal and the artificial noise. Our
analytical and numerical results show that equal power allocation is a simple
yet near optimal strategy for the case of non-colluding eavesdroppers. When the
number of colluding eavesdroppers increases, more power should be used to
generate the artificial noise. We also provide an upper bound on the
signal-to-noise ratio (SNR) above which the achievable secrecy rate is positive
and show that the bound is tight at low SNR. Furthermore, we consider the
impact of imperfect channel state information (CSI) at both the transmitter and
the receiver and find that it is wise to create more artificial noise to
confuse the eavesdroppers than to increase the signal strength for the intended
receiver if the CSI is not accurately obtained.
