  In this paper, we employ Cooperative Rate-Splitting (CRS) technique to
enhance the Secrecy Sum Rate (SSR) for the Multiple Input Single Output (MISO)
Broadcast Channel (BC), consisting of two legitimate users and one
eavesdropper, with perfect Channel State Information (CSI) available at all
nodes. For CRS based on the three-node relay channel, the transmitter splits
and encodes the messages of legitimate users into common and private streams
based on Rate-Splitting (RS). With the goal of maximizing SSR, the proposed CRS
strategy opportunistically asks the relaying legitimate user to forward its
decoded common message. During the transmission, the eavesdropper keeps
wiretapping silently. To ensure secure transmission, the common message is used
for the dual purpose, serving both as a desired message and Artificial Noise
(AN) without consuming extra transmit power comparing to the conventional AN
design. Taking into account the total power constraint and the Physical Layer
(PHY) security, the precoders and time-slot allocation are jointly optimized by
solving the non-convex SSR maximization problem based on Sequential Convex
Approximation (SCA) algorithm. Numerical results show that the proposed CRS
secure transmission scheme outperforms existing Multi-User Linear Precoding
(MU-LP) and Cooperative Non-Orthogonal Multiple Access (C-NOMA) strategies.
Therefore, CRS is a promising strategy to enhance the PHY security in
multi-antenna BC systems.
