Covert Communications Aided by Multi-Functional IRS: Energy Harvesting, Reflecting, and Amplifying

The intelligent reflecting surface (IRS) has been widely applied in covert communications to hide the transmission behavior. However, the existing IRS relies on grid/battery power for its operation, which is unprocurable for practical covert communication applications. To address this issue, a novel multi-functional IRS (MF-IRS) is proposed for harvesting energy, signal reflecting, and amplifying simultaneously, where each element can flexibly switch between energy harvesting mode and passive/active reflection modes. To reveal the benefit of the MF-IRS for covert communications against multiple warders, the critical performance is analyzed, and effective design schemes are also provided. In particular, the detection error probabilities at warders are derived when the warders are non-collusive/collusive. In addition, the covert rate maximization problem is formulated by jointly optimizing the beamforming vector, element allocation matrices, and the reflection coefficient matrix. To solve this non-convex problem with highly coupled variables, an efficient successive convex approximation-based algorithm is proposed for the non-collusive scenario first and then extended to the collusive scenario. Simulation results demonstrate that the proposed MF-IRS always outperforms the self-sustainable passive/active IRSs, and it even outperforms the full-passive/active IRSs powered by grid/battery when the IRS is located near the signal source.