Secrecy Rate Maximization in RIS-Enabled OFDM Wireless Communications: The Circuit-Based Reflection Model Case

A reconfigurable intelligent surface (RIS) is a promising key enabler for improving the physical-layer security in wireless communications. This results from its reconfigurability potential to simultaneously reflect the incoming electro-magnetic waves constructively towards legitimate users (LUs) and destructively towards eavesdroppers (EDs). In this paper, we consider a wideband RIS-enabled physical-layer security system with orthogonal frequency-division multiplexing (OFDM) and present a novel design of the RIS reflection coefficients to maximize the achievable secrecy spectral efficiency. A novel Dinkelbach-based iterative algorithm is proposed which includes alternating optimization at each repetition. Albeit most research works that consider ideal modeling for the RIS elements reflection response, presented passive beamforming optimization method incorporates a phase-dependent amplitude response model, which mimics various realistic RIS implementations. Our simulation results unveil that a high-quality suboptimal solution is derived for the considered system model, which is compared to a theoretical upper bound, for the challenging case when the ED is in the vicinity of the RIS. Furthermore, practical aspects, including mutual coupling and discrete-phase elements at the RIS, that degrade the passive beamforming gain, are also examined, leading to insightful conclusions.