  We investigate the physical layer security of wireless single-input
single-output orthogonal-division multiplexing (OFDM) when a transmitter, which
we refer to as Alice, sends her information to a receiver, which we refer to as
Bob, in the presence of an eavesdropping node, Eve. To prevent information
leakage, Alice sends an artificial-noise (AN) signal superimposed over her
information signal. We investigate the impact of the channel delay spread, OFDM
cyclic prefix, information/AN power allocation, and information and AN
precoders design on the achievable average secrecy rate. We consider the two
cases of known and unknown channel state information (CSI) at Alice.
Furthermore, we compare both cases of per-sub-channel processing and joint
sub-channels processing at Eve's receiver. Our numerical results show the gain
of AN injection in terms of average secrecy rate for different OFDM operating
conditions. Moreover, based on our new insights, we demonstrate that the
AN-aided scheme is effective and achieves almost the same average secrecy rate
as the full-CSI case without the need for Eve's instantaneous CSI at Alice.
