Doctoral Thesis Performance of Physical Layer Security under Correlated Fading Wire-Tap Channel

The inherent openness of wireless medium makes information security one of the most important and difficult problems in wireless networks. Physical layer security, which achieves the information-theoretic security by exploiting the differences between the physical properties of signal channels such that a degraded signal at an eavesdropper is always ensured and thus the original data can be hardly recovered regardless of how the signal is processed at the eavesdropper, has been studied as a promising approach to providing a strong form of security. By now, many research works have been devoted to understand the fundamental performance limits of physical layer security under different wire-tap channel models. It is notable that among different wire-tap channel models, the fading channel model has been an important model to efficiently capture the basic time-varying properties of wireless channels. Available works related to physical layer security study of the fading wire-tap channel are mainly based on the assumption that the channel from a transmitter to a legitimate receiver is independent of the one from the transmitter to an eavesdropper. In practice, however, the correlation among channels from a transmitter to different receivers has been frequently observed. Therefore, understanding the performance of physical layer security under the more practical correlated fading channels is of great importance for practical applications of physical layer security in wireless networks. In this thesis, we aim to provide a comprehensive study on the fundamental performance limits of physical layer security under a fading wire-tap channel, where the channel from transmitter to legitimate receiver is correlated with the one from transmitter to eavesdropper. We start the study from the scenario when the transmission power is asymptotically infinite. In particular, we first provide an informationtheoretic formulation of secure transmission over wireless fading channels at one realization of coherence interval in the high transmission power regime and show that the secrecy capacity is limited by the channel gain ratio of the main and eavesdropper channels rather than the transmission power, which is different from the Shannon’s capacity that increases with transmission power. We next characterize the asymptotic outage probability and also asymptotic outage secrecy capacity for the correlated fading wire-tap channel as the transmission power goes to infinity. We then analyze the performance of physical layer security under correlated fading wire-tap channel with a limited transmission power, a more complicate scenario compared with the one with asymptotic-infinite power. Specifically, we provide theoretical modeling for the secrecy capacity, transmission outage probability and secrecy outage probability of such systems. In particular, we first derive a simple closedform expression of secrecy capacity based on the typical Marcum Q function. This is achieved by exploring the symmetry property between the main channel and eavesdropper channel such that some complicated integration operations involved in the secrecy capacity analysis can be significantly simplified. We then derive the transmission outage probability and secrecy outage probability to depict the reliability and security performances of the concerned wire-tap channel. Finally, extensive numerical results are provided to illustrate the inherent performance tradeoffs under i fading wire-tap channel and also the potential impact of channel correlation on such tradeoffs. Our results reveal that the channel correlation between the main and eavesdropper channels has a significant impact on both secrecy capacity and outage performances. Remarkably, the impacts of correlation on the outage performances can be helpful or harmful depending on the channel conditions of both the main and eavesdropper channels and also the secrecy rate adopted in the transmission.