LAUSR

This work considers a time-varying Rayleigh block fading broadcast channel formed by two users and an eavesdropper, external to the legitimate network. One of the users is the legitimate/trusted user and the other an untrusted user. The transmitter uses superposition coding to broadcast information to both users. The aim is to ensure the confidentiality of the legitimate user information against the untrusted user and eavesdropper. To satisfy these secrecy constraints, this work proposes an innovative secure channel-training method applied to a cooperative jamming scenario. The jammer uses interference alignment to achieve secrecy against the untrusted user; meanwhile, the developed channel-training method is designed exploiting the time-varying nature of the channel to extend the system secrecy also to the external eavesdropper. An information theoretical evaluation of the proposed scheme demonstrates that (1/4, 1/4) positive secure degrees of freedom are achievable in the presence of both untrusted users and external eavesdroppers.