Robust QoS-based transmit beamforming design in MISO wiretap channels

In this paper, a robust quality-of-service (QoS) based secret transmit beamforming design problem is discussed for a multiple-input single-output (MISO) system which works under cooperative jamming (CJ) scheme with a helper transmitting jamming signals to confuse the eavesdropper. For the channel state information (CSI), we assume that the CSI for the legitimate link is available and the CSI for the wiretap channel is imperfect. Our system design goal is to optimize the worst-case performance of the network under the situation: minimizing the signal-to-interference-and-noise ratio (SINR) at the eavesdropper and meanwhile guaranteeing the SINR at legitimate receiver. Thus the spatial distribution of jamming signals and the beamforming vector of source are jointly optimized under a total power constraint. As this optimization problem is difficult to tackle, a method is proposed with proofs to obtain exact solver of it. Our analysis reveals that optimizing the beamforming vector of source is a better strategy compared to optimizing the spatial covariance matrix of information signals. Numerical results affirm this and show the advantage of robust transmit designs compared to a non-robust approach in lowering the SINR at the eavesdropper in the presence of channel uncertainties.