QoS-constrained robust beamforming in MISO wiretap channels with a helper

In this paper, we develop robust beamforming strategies for multiple-input single-output (MISO) wiretap channels with a helper. The channel state information (CSI) for the legitimate link is assumed to be available while the CSI for the eavesdropper´s channel is imperfect, and is assumed to be norm-bounded by some known constant. We optimize the worst-case performance for the following problems: 1) minimizing the signal to interference-plus-noise ratio (SINR) at the eavesdropper while maintaining an SINR constraint at the intended receiver, 2) maximizing the SINR at the intended receiver while forcing an SINR constraint at the eavesdropper, and 3) minimizing the global transmit power while satisfying SINR constraints at both the legitimate user and the eavesdropper. Simulation examples show the advantages of the robust designs over the non-robust counterparts.