Interplay Between Delayed CSIT and Network Topology for Secure MISO BC

Abstract: We study the problem of secure transmission over a Gaussian two-user multi-input single-output (MISO) broadcast channel under the assumption that links connecting the transmitter to the two receivers may have unequal strength statistically. In addition to this, the state of the channel to each receiver is conveyed in a strictly causal manner to the transmitter. We focus on a two state topological setting of strong v.s. weak links. Under these assumptions, we first consider the MISO wiretap channel and establish bounds on generalized secure degrees of freedom (GSDoF). Next, we extend this model to the two-user MISO broadcast channel and establish inner and outer bounds on GSDoF region with different topology states. The encoding scheme sheds light on the usage of both resources, i.e., topology of the model and strictly causal channel state information at the transmitter (CSIT); and, allows digitization and multi-casting of overheard side information, while transmitting confidential message over the stronger link. Furthermore, for a special class of channels, we show that the established bounds agree and so we characterize the sum GSDoF.