Secure Communications via Active IRS Assisted SWIPT NOMA Networks

Active intelligent reflecting surface (IRS) has attracted great attention due to the characteristic of changing the propagation channels by amplifying the amplitude and adjusting the phase shift of signals. In this article, we apply the active IRS into a simultaneous wireless information and power transfer multigroup nonorthogonal multiple access system and focus on improving the secure transmission performance. Specifically, from the overall system performance perspective, we first formulate a power minimization problem by jointly optimizing the beamforming at the base station, power allocation coefficient in each group, power splitting ratio at each user, and the IRS matrix. Then, we consider the fairness among users and aim to maximize the minimum secrecy rate (SR). In order to solve these two nonconvex problems, two efficient algorithms based on alternating optimization and block coordinate descent are proposed by applying the semidefinite relaxation, successive convex approximation, Schur complement, arithmetic-geometric mean inequality and convex upper bound substitution methods. Through simulations, we verify the convergence of the algorithms. The results show that both the power consumption and minimum SR performance can be enhanced by deploying the active IRS compared to the baseline schemes under passive IRS and random phase shift active IRS aided cases.