Secure Degrees of Freedom of Two-User Two-Hop X-Channel

In this paper, Secure Degrees of Freedom (SDoF) of two-user two-hop X-channel is investigated. We consider a situation in this channel in which the transmitters and one of the relays benefit from alternating Channel State Information (CSI) including perfect, delayed and no CSI. In another regime from CSI viewpoint, we consider blind transmitters alongside a relay having access to alternating CSI, and there exists feedback from one of the receivers to the relay. For this situation, we also study the case in which the receivers are permitted to cooperatively play a part in ensuring secrecy. Corresponding to each case, we drive an upper bound on SDoF and devise a scheme which successfully obtains the upper bound. So, we present the optimal SDoF and achievable schemes for all considered cases.