Enhancing the Secrecy Rate With Direction-Range Focusing With FDA and RIS

One of the great potentials to improve the confidentiality in mmWave/THz at the physical layer of technical communication, measured by the secrecy rate, lies in the use of reconfigurable intelligent surfaces (RISs). However, an important open problem arises when the eavesdropper is aligned with the legitimate user or near the RIS or legitimate user. The limitation stems from the high directional gain caused by the dominant line-of-sight (LOS) path in high-frequency transmission and the high energy leakage in the proximity of the RIS and the legitimate user. To address these issues, we employ frequency diverse arrays (FDA) at the base station (BS) associated with random inverted transmit beamforming and reflective element subset selection (RIBES). More specifically, we consider a passive eavesdropper with unknown location and design the transmit beamforming and RIS configuration based on the channel information of the legitimate user only. In this context, the secrecy rate with the proposed transmission technique is evaluated in the case of deterministic eavesdropper channel, demonstrating that we can ensure a secure transmission regarding both direction and range. Furthermore, assuming no prior information about the eavesdropper, we describe the wiretap region and derive theworst-case secrecy ratein closed form. The latter is further optimized by determining the optimal subset sizes of the transmit antennas and reflective elements. Simulations validate the closed-form expressions and demonstrate that we can effectively improve the secrecy rate, especially when the eavesdropper is near the RIS or the legitimate user.