Average Secrecy Capacity Maximization of Rotatable Antenna-Assisted Secure Communications

TL;DR

This work tackles maximizing the average secrecy capacity in a rotatable-antenna secure link without relying on instantaneous CSI. It establishes that the optimal boresight intersects the line from the eavesdropper to the legitimate user and that the objective is quasi-concave in the adjustment parameter α, enabling a simple 1D bisection search, while also deriving a closed-form LoS-based near-optimal solution. The authors further derive a high-SNR secrecy outage probability and validate all results through simulations, showing the near-optimal LoS solution closely matches the true optimum and that the SOP bound aligns with simulations in the high-SNR regime. The findings offer practical design guidance for physical layer security using rotatable antennas in geometry-aware wireless networks.

Abstract

A rotatable antenna, which is able to dynamically adjust its deflection angle, is promising to achieve better physical layer security performance for wireless communications. In this paper, considering practical scenarios with non-real-time rotatable antenna adjustment, we investigate the average secrecy rate maximization problem of a rotatable antenna-assisted secure communication system. We theoretically prove that the objective function of the average secrecy rate maximization problem is quasi-concave with respect to an adjustment factor of the rotatable antenna. Under this condition, the optimal solution can be found by the bisection search. Furthermore, we derive the closed-form optimal deflection angle for the secrecy capacity maximization problem, considering the existence of only line-of-sight components of wireless channels. This solution serves as a near optimal solution to the average secrecy rate maximization problem. Based on the closed-form near optimal solution, we obtain the system secrecy outage probability at high signal-to-noise ratio (SNR). It is shown through simulation results that the near optimal solution achieves almost the same average secrecy capacity as the optimal solution. It is also found that at high SNR, the theoretical secrecy outage probabilities match the simulation ones.

Figures (6)

• Figure 1: Illustration of a rotatable antenna-assisted secure communication system.
• Figure 2: Average secrecy capacity $\mathbb{E}[C_s(\alpha)]$ and the secrecy capacity $C^{\text{LoS}}_s(\alpha)$ versus $\alpha$; performance comparison of our proposed optimal and near optimal solutions, where $P=16$ dBm.
• Figure 3: Average secrecy capacity $\mathbb{E}[C_s(\alpha)]$ versus $P$; performance comparison of our proposed optimal and near optimal solutions.
• Figure 4: Secrecy outage probability versus $R_s$; comparison of simulated and theoretical secrecy outage probabilities, where $P=25$ dBm.
• Figure 5: Secrecy outage probability versus $P$; comparison of simulated and theoretical secrecy outage probabilities, where $R_s=1$ bps/Hz.