Signal Processing Advances for Physical Layer Security in Communication Networks

This dissertation presents multiple novel techniques for securing wireless communications at the physical layer. Specifically, we are interested in increasing the secrecy capacity of systems employing the Alamouti space-time block code (STBC) for point to point and multiple access communications. First, we introduce a secure space-time block coding scheme for 2 and 4 transmit antennas to secure point to point transmissions in the presence of a passive eavesdropper. The bit error probability is analyzed for coherent detection as well as with differential detection. The analytical expressions are validated through simulations and demonstrate that our proposed secure STBC guarantees full diversity to legitimate users while reducing the diversity order of an eavesdropper to zero even when she has perfect channel state information. Next we investigate the physical layer security of synchronous multiple access transmissions using the Alamouti STBC in fading channels where multiple users communicate with a single intended receiver in the presence of an eavesdropper. We propose an artificialnoise-aided technique to secure the transmissions by having the Alamouti users collaborate with each other, without exchanging information, to degrade the eavesdropper’s channel. Unlike previous work, which assumes that the transmitters have complete knowledge of the legitimate as well as the eavesdropper’s channels, our proposed technique requires no communications between the users, minimal knowledge of the legitimate channel, and no channel knowledge regarding the eavesdropper.