  In this paper, we take all users' secrecy rate requirements into
consideration and propose a rate-splitting multiple access (RSMA)-based secure
beamforming approach to maximize the weighted sum-rate (WSR), where multi-user
interference (MUI) is partially decoded and partially treated as noise. User
messages are split and encoded into common and private streams. Each user not
only decodes the common stream and the intended private stream, but also
eavesdrops other users' private streams. To solve the introduced non-convex
security constraints in the formulated problem, a successive convex
approximation (SCA)-based approach is first proposed to maximize the
instantaneous WSR under perfect channel state information at the transmitter
(CSIT). When only imperfect CSIT is available, to ensure robustness, a weighted
ergodic sum-rate maximization problem is formulated. Then, a joint weighted
minimum mean square error and SCA-based alternating optimization algorithm is
proposed to optimize the precoder. Numerical results demonstrate that, compared
with baseline schemes, the performance advantage of the proposed RSMA-based
secure beamforming design has been shown in terms of WSR and robustness to
channel errors while ensuring all users' security requirements thanks to its
powerful interference management capability.
