Secrecy for MISO broadcast channels via alternating CSIT

The two-user multiple-input single-output (MISO) broadcast channel with confidential messages (BCCM) is studied in which the nature of channel state information at the transmitter (CSIT) from each user can be of the form Ii, i = 1, 2 where I1; I2 ∈ {P;D;N}, and the forms P, D and N correspond to perfect and instantaneous, completely delayed, and no CSIT, respectively. Thus, the overall CSIT can alternate over time between 9 possible states corresponding to all possible values of I1I2, with each state occurring for λI1I2 fraction of the total duration. The main contribution of this paper is to establish the secure degrees of freedom (s.d.o.f.) region of the MISO BCCM with alternating CSIT with the symmetry assumption λI1I2 = λI2I1 . The results highlight the synergistic benefits of coding across CSIT states for secrecy and the interplay between various aspects of channel knowledge and its impact on s.d.o.f.