  Physical layer security (PLS) technologies are expected to play an important
role in the next-generation wireless networks, by providing secure
communication to protect critical and sensitive information from illegitimate
devices. In this paper, we propose a novel secure communication scheme where
the legitimate receiver use full-duplex (FD) technology to transmit jamming
signals with the assistance of simultaneous transmitting and reflecting
reconfigurable intelligent surface (STARRIS) which can operate under the energy
splitting (ES) model and the mode switching (MS) model, to interfere with the
undesired reception by the eavesdropper. We aim to maximize the secrecy
capacity by jointly optimizing the FD beamforming vectors, amplitudes and phase
shift coefficients for the ESRIS, and mode selection and phase shift
coefficients for the MS-RIS. With above optimization, the proposed scheme can
concentrate the jamming signals on the eavesdropper while simultaneously
eliminating the self-interference (SI) in the desired receiver. To tackle the
coupling effect of multiple variables, we propose an alternating optimization
algorithm to solve the problem iteratively. Furthermore, we handle the
non-convexity of the problem by the the successive convex approximation (SCA)
scheme for the beamforming optimizations, amplitudes and phase shifts
optimizations for the ES-RIS, as well as the phase shifts optimizations for the
MS-RIS. In addition, we adopt a semi-definite relaxation (SDR) and Gaussian
randomization process to overcome the difficulty introduced by the binary
nature of mode optimization of the MS-RIS. Simulation results validate the
performance of our proposed schemes as well as the efficacy of adapting both
two types of STAR-RISs in enhancing secure communications when compared to the
traditional selfinterference cancellation technology.
