Maximum Secrecy Throughput Analysis of Practical RIS-Aided mmWave Systems Over Unified FTR/IFTR Fading Channels

In this paper, we consider a millimeter-wave (mmWave) wiretap scenario where a multiple-antenna base station (Alice) communicates with a legitimate user (Bob) via a reconfigurable intelligent surface (RIS) amid multiple colluding eavesdroppers (Eves) using maximal-ratio combining. We formulate the maximum effective secrecy throughput (EST) that satisfies reliability and secrecy constraints. Our key contributions are threefold. Firstly, we propose an analytical framework that unifies fluctuating two-ray (FTR) and independent FTR (IFTR) fading distributions into a single model. While FTR/IFTR distributions are recognized as the statistical models suitable for the mmWave fading channel, they have traditionally been treated as separate models. Secondly, we propose a novel approximation method to statistically characterize end-to-end channels, accounting for the combined impact of the non-uniform RIS amplitude response and phase estimation errors at Bob and Eves over the unified FTR/IFTR channels. Finally, a new approximate closed-form expression of the EST is derived, extensively validated through Monte Carlo simulations. Our numerical findings highlight the superior benefits of deploying a larger RIS over increasing the number of antennas at Alice and the improved EST performance in a mmWave scenario compared to the microwave counterpart.