This study investigates a robust reconfigurable intelligent surface (RIS)-assisted multiple-input multiple-output (MIMO) system for secure wireless communication, in which a multi-antenna transmitter (Alice) sends confidential messages to a multi-antenna receiver (Bob) in the presence of an eavesdropper (Eve). Unlike idealized models, the reflecting elements (REs) of the RIS are assumed to possess inherent electrical resistance, introducing a practical non-ideal effect often neglected in prior research. The aim of the study is to maximize the secrecy rate of the MIMO system under perfect knowledge of the channel state information (CSI). To achieve this, the secrecy rate maximization problem is formulated and solved using a low-complexity joint optimization framework based on an adaptive projected gradient method (PGM), which simultaneously updates both the transmit precoding matrix and the RIS phase shifts. Solving the exact problem is computationally complex. Thus, a simplified variant is further introduced that maximizes the channel power difference rather than the exact secrecy rate. The simulation results show that this approximation yields a secrecy rate close to the true optimum while significantly reducing the computational cost. In addition, the proposed PGM with an adaptive step size initialization and control mechanism substantially improves the secrecy rate and reduces the computational time compared to the conventional fixed step size PGM. Overall, the simulation results confirm the effectiveness of the proposed PGM and demonstrate that adopting a practical RIS model is essential for establishing secure RIS-assisted MIMO communication links, especially under varying RE resistance values.
W IRELESS security is a crucial aspect of fifth- generation (5G) or beyond 5G (B5G) networks. Traditionally, security in wireless transmission has relied on cryptographic techniques. However, these methods often introduce significant overheads and face challenges related to key distribution and management, which can compromise their reliability due to the inherent broadcast nature of the wireless medium [1]. Consequently, secure transmission approaches that exploit the physical characteristics of wireless channels have gained increasing attention. Unlike traditional cryptographic methods, physical layer security (PLS) methods offer notable advantages such as lower complexity and the potential for keyless secure communication [2]. The performance of PLS methods is typically evaluated using the secrecy rate, which is quantified as the highest achievable rate at which the transmitter can securely and reliably communicate with the intended user (Bob) while preventing an eavesdropper (Eve) from intercepting the transmitted information [3]. Multi-antenna techniques have been extensively employed to further improve the security of transmissions by leveraging the availability of additional spatial degrees of freedom. The secrecy rate analysis of multiple-input multipleoutput (MIMO) channels in the presence of Eve has been extensively investigated in [4]- [6], with the findings providing valuable insights into the potential of spatial processing to enhance PLS.
Reconfigurable intelligent surface (RIS) has emerged as a promising technology to enhance both the spectral efficiency and PLS in B5G wireless communication networks. An RIS consists of a large array of passive reflecting elements (REs) designed as an impedance-based metasurface embedded with reconfigurable electromagnetic components. These components allow each RE to reflect the incident waves with a precisely controllable phase shift, thereby enabling the reflected signal to be tailored as required to obtain the desired channel response [7]- [10]. For instance, by intelligently adjusting the phase shifts, the RIS can enhance the signal quality at the intended user (IU) while simultaneously reducing the signal strength at the non-intended user (NU). As a result, RIS plays a crucial role in enhancing the PLS of wireless communication systems [11]- [13]. Recent studies have therefore focused on optimizing RIS configurations to maximize the secrecy rate performance using various design and optimization strategies [14]- [17]. Collectively, these efforts highlight the central role of RIS in realizing secure and efficient wireless links through environmentadaptive signal control.
This section provides a brief overview of related studies focused on improving the achievable rate and PLS in RISassisted communication systems. A summary of the key contributions of these studies is presented in Table I. The authors in [18], [19] maximized the achievable rate in RISaided MIMO systems by obtaining the optimal values of the RIS phase shifts using either the dimension-wise sinusoidal maximization (DSM) [18] or the sum-path gain maximization (SPGM) [19] method. Rajan et al. [20] analyzed the achievable rate maximization problem in RISaided orthogonal frequency division multiplexing (OFDM) systems and proposed a gradient ascent technique with a fixed step size to determine the optimal value of the RIS phase shifts.
Chu et al. [21] addressed the secrecy rate maximization problem in RIS-assisted systems by jointly designing the precoding matrix and RIS phase shifts using the block coordinate descent (BCD) and maximization-minimization (MM) algorithms. The authors in [22] maximized the secrecy rate of RIS-aided multiple-input-single-output (MISO) systems by solving the real-valued optimization problem using a fixed step-size gradient ascent method. Li et al. [23] considered a RIS-assisted MIMO vehicular network consisting of a multi-antenna Alice, a multiantenna IU, and a multi-antenna Eve and attempted to maximize the secrecy rate of the system using semidefinite programming (SDP) and Riemannian manifold optimization (RMO) to design the active beamforming and phase shifts of the RIS elements. Shen et al. [24] proposed an alternating optimization method to jointly design the transmit covariance matrix and RIS phase shift matrix to enhance the secrecy rate in a RIS-aided multiantenna communications system. Several studies have investigated the secrecy outage probability (SOP) in RISassisted systems. For instance, Yang et al. [25] analyzed the SOP in RIS-aided SISO systems in which the NU locations were fixed, while Wang et al. [26] considered the case where the NUs were randomly located. Xiu et al. [27] investigated the secrecy rate maximization problem in RIS-assisted millimeter-wave (mmWave) communication systems under practical hardware constraints. Pala et al. [28] extended the analysis to multiple RIS, multiple IUs, and multiple NUs in a multiple user (MU)-MIMO downlink s
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