Two-Hop IRS-Assisted Wireless Communications Exploiting Statistical CSI

This letter investigates the ergodic sum rate maximization problem for statistical channel state information-based two-hop intelligent reflecting surface (IRS)-assisted wireless systems. Our target is to maximize the ergodic sum rate by jointly optimizing the phase shift of the reflecting elements at the two-hop IRSs and the active beamforming vectors at the base station. To achieve this target, we first derive an approximation of the ergodic sum rate by considering the Rician channel model. Based on this approximation, we then formulate an optimization problem, which is non-convex and difficult to be solved. To solve this problem, we divide the original problem into two subproblems by using the alternating optimization (AO) method. Meanwhile, we solve all subproblems by using the Lagrangian dual transform and quadratic transform methods. By iteratively optimizing all the subproblems until converges, we obtain an AO-based scheme. The simulation results show that the proposed scheme outperforms the conventional scheme.