Max-Min Fairness in STAR-RIS Assisted WPCNs

A simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted wireless powered communication networks (WPCNs) framework is proposed. More specifically, an STAR-RIS is deployed to assist both wireless power and information transfer between a hybrid access point (HAP) and two users. To eliminate the doubly-near-far effect in WPCNs, this paper delves into the system fairness performance based on non-orthogonal multiple access (NOMA) and time division multiple access (TDMA) schemes. As a result, two optimization problems are formulated to maximize the minimum user rate, by jointly optimizing time allocation, user transmit power, and STAR-RIS beamforming. For NOMA, the resulting intractable problem is solved via a two-layer iterative algorithm by invoking one-dimensional search and the alternating optimization (AO) method. While for TDMA, an AO based algorithm is proposed to obtain the optimal solution. Numerical results demonstrate significant performance gains can be achieved by the proposed scheme over the baseline schemes. Especially in the case of NOMA, deploying an STAR- RIS can offer a more flexible resource allocation strategy to reap better fairness between users.