Double-RIS-Assisted Orbital Angular Momentum Near-Field Secure Communications

Due to the broadcast and open characteristics of wireless channels, near-field physical layer security has attracted much attention to facilitate wireless information security against illegitimate eavesdropping. However, highly correlated channels between the legitimate transceivers and eavesdroppers for existing near-field line-of-sight multiple-input multiple-output (MIMO) systems with low degrees of freedom make it difficult to efficiently distinguish eavesdropping channels and legitimate channels in the angular domain, thus resulting in low secrecy rates. Fortunately, orbital angular momentum (OAM) with rich phase information shows great potential to enhance the physical layer security. To significantly increase the secrecy rates of near-field wireless communications, in this paper we propose the double-reconfigurable-intelligent-surface (RIS) assisted OAM secure scheme, where RISs with few reflecting elements are easily deployed to reconstruct the direct links blocked by obstacles between the legitimate transceivers, mitigate the inter-mode interference caused by the misalignment of legitimate transceivers, and adjust the OAM beams direction to interfere with eavesdroppers. Meanwhile, due to the unique orthogonality among OAM modes, the OAM-based joint index modulation and artificial noise scheme is proposed to weaken the information acquisition by eavesdroppers while increasing the achievable rate with the low cost of legitimate communications. To maximize the secrecy rate of our proposed scheme, we develop the Riemannian manifold conjugate gradient (RMCG)-based alternative optimization (AO) algorithm to jointly optimize the transmit power allocation of OAM modes and phase shifts of double RISs. Numerical results show that our proposed double-RIS-assisted OAM near-field secure scheme outperforms the existing works in terms of the secrecy rate and the eavesdropper’s bit error rate.