  Cognitive satellite-terrestrial networks (STNs) have been recognized as the
promising architecture for addressing spectrum scarcity in the next generation
communication networks. In this letter, we investigate the secure transmission
schemes in the cognitive STNs where the interference from terrestrial base
station (BS) is introduced to enhance security of the satellite link. Our
objectives are to minimize the transmit power by jointly optimizing the
cooperative beamforming and artificial noise (AN) while guaranteeing the
secrecy rate constraint for satellite link, the information rate constraint for
terrestrial link and the total transmit power constraint. Both scenarios of the
perfect and imperfect channel cases are respectively considered. These
constraints make the optimization problems formulated non-convex and
challenging, which are efficiently solved via certain transformations to be
formulated more tractable versions, respectively. Numerical results are
provided to corroborate the proposed schemes.
