Secure Communications over Wireless Channels

In this work, we present new techniques that leverage the wireless medium in facilitating secure communications in the presence of eavesdroppers. First, we consider the secure transmission of information over an ergodic fading channel with long coherence intervals. The secrecy capacity of such a system is characterized under different assumptions on the available channel state information. We then propose a low-complexity on/off power allocation strategy which becomes asymptotically optimal as the average SNR grows. Remarkably, our results reveal the positive impact of fading on the secrecy capacity and establish the critical role of rate adaptation in enabling secure communications over slow fading channels. Moreover, we discuss the utility of user cooperation in establishing secure communication links. In particular, we construct novel cooperation strategies for the relay channel with an eavesdropper. One of the proposed strategies, i.e., noise forwarding, is used to illustrate the deaf helper phenomenon, where the relay is able to create a secure source-destination channel while being totally ignorant of the transmitted message.