This work considers the artificial noise (AN) assisted physical layer security. At first, we consider the artificial noise scheme for securing wireless communications over multiple-input multiple-output multi-antenna eavesdropper (MIMOME) channels. In this work, we develop a lower bound on the ergodic secrecy rate of the artificial noise scheme and derive an analytic closed-form expression of the lower bound without resorting any numerical integrations nor special functions. Moreover, the lower bound does not assume any asymptotes for system parameters, thus the derived lower bound explicitly elucidates how the system parameters exert influence on the secrecy rate in various cases to some of which the previous work cannot be applied. It will also be shown that the bound is tight over wide combinations of system parameters, and a normalized difference between the bound and secrecy rate vanishes with a growing number of antennas at the transmitter. The derived bound is analytically so amenable that it enables one to not only extend the results of the previous work but also explore untouched aspects of the AN scheme. Specifically, by investigating the optimal power allocation between the information-bearing and artificial noise signals, we can finally answer an intriguing question: when is the AN signal beneficial? Besides, it makes possible to study a new important system parameter called the minimum required transmit power for a certain target secrecy rate, which has never been touched before.
Secondly, we consider the AN scheme for correlated wiretap channels which consists mutually correlated main and wiretap channels. Since the channel correlation between the main and wiretap channels both increases the amount of information leakage to the eavesdropper and decreases the effective interference at the eavesdropper caused by the AN signal, the optimal power allocation should be reconsidered. Thus, we extend the lower bound developed for uncorrelated MIMO wiretap channel to mutually correlated multiple-input single-output (MISO) wiretap channel. From the extended lower bound, we identify the effects of channel correlation on the ergodic secrecy rate and the optimal power allocation. It is noteworthy that the channel correlation does not monotonically increase or decrease the optimal fraction of total power allocated to the AN signal, thus a careful power allocation should be adopted for enhancing the secrecy performance of correlated wiretap channels.
Finally, we consider the AN scheme with the legitimate receiver who has a full-duplex capability which enables itself to capture the secret message from the transmitter and simultaneously generate a jamming signal to strengthen security. While there have been studies on similar setups, most of them focus on the design of the jamming signal with the assumption of full channel-state information (CSI) and/or system parameter optimization based on numerical evaluations. On the contrary, this work introduces a tight lower bound on an achievable ergodic secrecy rate as a versatile analytic tool and derive a closed-form expression for the bound. To confirm the analytic results, we carry out numerical evaluations of the ergodic secrecy rate which are compared with the proposed lower bound.