Beamforming Optimization for RIS-Aided SWIPT in Cell-Free MIMO Networks

Reconfigurable intelligent surface(RIS) is a promising technology that utilizes massive low-cost elements to reflect incident signals to desired direction. In this paper, we consider a RIS-aided cell free multiple-input multiple-output(MIMO) network that composed of multiple access points(APs), multiple RISs, multiple information receivers(IRs), and multiple energy receivers(ERs). Under the maximum transmit power constraints of the APs and the minimum total energy harvesting power constraints of the ERs,we aim to maximize the weighted sum-rate(WSR) by jointly optimizing the beamforming matrices and the passive RIS reflection matrices. However, the formulated problem is nonconvex and very challenging to solve. Therefore, we propose an alternating optimization algorithm to obtain a suboptimal solution. Explicitly, the proposed algorithm decomposes the original optimization problem into three subproblems that can be efficiently solved. To overcome the nonconvexity of the subproblems, a successive convex approximation(SCA) based algorithm is proposed to tackle the energy harvesting constraints. Moreover, an alternating direction method of multipliers(ADMM) based algorithm is proposed to optimize the constant modulus constrained RIS reflecting elements. Finally, simulations are conducted to demonstrate the performance advantages of the proposed algorithms.