Secrecy Degrees of Freedom of the 2 $\times$ 2 $\times$ 2 Interference Channel With Delayed CSIT

We consider the two-hop interference channel with confidential messages (ICC), where each transmitter sends one confidential message to its intended destination through two trusted relays. We study the secrecy degrees of freedom (SDoF) of the considered 2 × 2 × 2 ICC with delayed channel state information at transmitters (CSIT). We settle the optimal SDoF of the considered network to be (1/2, 1/2). Thus, the considered network has a strictly larger sum SDoF than a two-user one-hop ICC with even full CSIT. Moreover, we study the SDoF of the network with limited Shannon feedback, that only one relay can obtain the delayed CSIT and output feedback from the destinations. The optimal SDoF is also shown to be (1/2, 1/2), revealing that only limited feedback can also bring SDoF gain. Our proposed achievable schemes in both cases are integrated forms of the retrospective interference alignment and artificial noise transmission.