Active Intelligent Reflecting Surface Assisted Covert Communications

In this paper, the covert communication system with the assistance of an active intelligent reflecting surface (IRS) is investigated. Unlike the conventional passive IRS, which reflects signals without amplification, the active IRS can jointly amplify the amplitude and adjust the phase of the reflected signals to overcome the multiplicative fading effect. However, the additional noise introduced by the active IRS is not conducive to the signal demodulation at the destination and increases the detection probability by the warder. To overcome this problem and maximize the covert rate of the system, we jointly design the transmission power at the transmitter and the reflection coefficient of each reflection element at the active IRS. Since the optimization variables are coupled in the objective function and the constraints, an efficient semi-definite relaxation combined alternating optimization algorithm is proposed to solve the non-convex problem, where each subproblem is solved optimally. Numerical results demonstrate that the active IRS assisted system outperforms the passive one in term of the covert rate even when the number of reflection elements is large (such as ten thousand of elements), since the active IRS can amplify the incident signal to overcome the multiplicative fading effect.