Flexible Intelligent Metasurfaces for Enhancing Secure MISO Communications

Abstract

This letter investigates flexible intelligent metasurface (FIM)-aided secure multiple-input single-output (MISO) communications in the presence of a multi-antenna eavesdropper. By exploiting the morphing capability of the FIM, the transmit array geometry can be dynamically reconfigured to reshape the legitimate and eavesdropping channels. We formulate a secrecy rate maximization (SRM) problem by jointly optimizing the transmit covariance matrix and the FIM surface shape. For a given FIM surface shape, the optimal transmit covariance matrix is shown to be rank one and is obtained in closed form from the dominant generalized eigenvector. For a fixed transmit covariance matrix, a projected gradient ascent (PGA) method is developed to optimize the FIM shape. Based on these two subproblems, a block coordinate ascent (BCA) algorithm is proposed. Moreover, for the special line-of-sight (LoS) scenario, we derive a globally optimal closed-form joint transmit covariance matrix and FIM surface shape design under a sufficient morphing-range condition. Numerical results demonstrate that FIM effectively improves the secrecy rate over a conventional rigid antenna array (RAA). Moreover, FIM achieves a transmit-power saving of about 2.5 dB compared with the RAA benchmark at the same secrecy rate.