Covert Communications enabled by Space-time-modulation IRS: Joint Phase and Frequency Optimization for 3D Beam Focusing

Intelligent reflecting surface (IRS) is widely utilized in covert communications to enhance the system performance by passive beamforming. However, transmission covertness is seriously threatened when the warder is located on the line between IRS and the legitimate receiver. To overcome this limitation, the space-time-modulation IRS (STM-IRS) is considered in this paper, which can adjust both the phase and the frequency of the reflection signal to realize three-dimensional (3D) beam focusing. To maximize the signal-to-noise ratio at the receiver, an optimization problem is formulated by jointly adjusting the transmit power, the phase and the frequency shift at each reflection element. To solve above non-convex problem with highly coupled optimization variables, a penalty-based algorithm is proposed wherein the majorization-minimization technique and block successive upper bound minimization technique are adopted. Simulation results demonstrate that 3D beam focusing can be realized by STM-IRS to distinguish the warder and the legitimate receiver in both angle and range domains. Besides, the proposed scheme outperforms the scheme without frequency shift and other existing frequency shift schemes.