The interference wiretap channel with an arbitrarily varying eavesdropper: Aligning interference with artificial noise

In this work, a Gaussian two-user MIMO interference channel is considered in the presence of an external eavesdropper whose channel is completely unknown to the legitimate communication parties and can be varying in an arbitrary fashion from one channel use to the next. We improve our recent result by deriving a larger achievable secrecy degrees of freedom region. In the achievable scheme, the transmitter injects artificial noise to confuse the eavesdropper, and at the same time aligns the injected noise with the interference from the other user at the intended receiver. The achieved secure degrees of freedom are shown to be closely connected to the rank of the effective channel matrix through which the eavesdropper observes the artificial noise. The precoding matrix is then designed to ensure that this rank cannot be reduced under any possible Eve´s channel state.