Beamforming design of sum secrecy rate optimization for MU-MISO channel under sum power constraint

This paper studies the beamforming design problem of a multi-user downlink network, assuming perfect channel state information (CSI) known to the base station (BS). In this scenario, the BS is equipped with multiple antennas, and each user is wiretapped by a specific eavesdropper where each user or eavesdropper is equipped with one antenna. It is supposed that the BS employs transmit beamforming with a given requirement on sum transmitting power. The object is to maximize the sum secrecy rate (SSR) of the network, which leads to a nonconvex complicated structure, and makes he considered beamforming design problem difficult to handle. To solve this problem, a beamforming design scheme is proposed to transform the original problem into a convex approximation problem, by employing semidefinite relaxation (SDR) and first-order approximation technique based on Taylor expansion. Besides, with the advantage of low complexity, a zero-forcing (ZF) based beamforming method is presented in the case that BS is able to nullify he eavesdroppers' rate. Numerical results show that the former strategy achieves better performance than the later one, which is mainly due to the ability of optimizing beamforming direction, and both outperform the traditional signal-to-leakage-and-noise ratio (SLNR) based algorithm.