Secure MISO Broadcast Channel: An Interplay Between CSIT and Network Topology

We study the problem of secure transmission over a Gaussian two-user multi-input single-output (MISO) broadcast channel (BC) 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 delayed 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 broadcast channel and establish inner and outer bounds on secure generalized degrees of freedom (SGDoF) region with different topology states. The encoding scheme sheds light on the usage of both resources, i.e., topology of the model and delayed channel state information at the transmitter (CSIT); and, allows digitization and multicasting 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 SGDoF. Next, we extended this model to the Gaussian MISO two receiver channel with an external eavesdropper, under the assumption that the state of the channel which is available to each receiver is conveyed either perfectly (P) or with delay (D) to the transmitter equal fractions of time, while only delayed CSI is available from the eavesdropper to the transmitter. For this case, we first establish an outer bound on the SGDoF region. Next, we develop an achievability scheme that shows that as opposed to coding independently over different states, joint encoding across the alternating CSIT and topological states enables strictly better secure rates.