A simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted wireless powered communication network (WPCN) is proposed, where two energy-limited devices first harvest energy from a hybrid access point (HAP) and then use that energy to transmit information back. To fully eliminate the doubly-near-far effect in WPCNs, two STAR-RIS operating protocol-driven transmission strategies, namely energy splitting non-orthogonal multiple access (ES- NOMA) and time switching time division multiple access (TS- TDMA) are proposed. For each strategy, the corresponding optimization problem is formulated to maximize the minimum throughput by jointly optimizing time allocation, user transmit power, active HAP beamforming, and passive STAR-RIS beamforming. For ES-NOMA, the resulting intractable problem is solved via a two-layer algorithm, which exploits the one-dimensional search and block coordinate descent methods in an iterative manner. For TS-TDMA, the optimal active beamforming and passive beamforming are first determined according to the maximum-ratio transmission beamformer. Then, the optimal solution of the time allocation variables is obtained by solving a standard convex problem. Numerical results show that: 1) the STAR-RIS can achieve considerable performance improvements for both strategies compared to the conventional RIS; 2) TS- TDMA is preferred for single-antenna scenarios, whereas ES- NOMA is better suited for multi-antenna scenarios; and 3) the superiority of ES-NOMA over TS-TDMA is enhanced as the number of STAR-RIS elements increases.

翻译：本文提出了一种由同时透射反射可重构智能表面（STAR-RIS）辅助的无线供能通信网络（WPCN）方案。在该网络中，两个能量受限设备首先从混合接入点（HAP）收集能量，随后利用所获能量回传信息。为彻底消除WPCN中的“双近-远”效应，提出了两种由STAR-RIS工作协议驱动的传输策略：能量分割非正交多址接入（ES-NOMA）与时间切换时分多址接入（TS-TDMA）。针对每种策略，通过联合优化时间分配、用户发射功率、HAP有源波束赋形以及STAR-RIS无源波束赋形，构建了相应的优化问题以最大化最小吞吐量。对于ES-NOMA策略，其导致的难解问题通过一种双层算法求解，该算法以迭代方式结合一维搜索与块坐标下降法。对于TS-TDMA策略，首先根据最大比传输波束赋形器确定最优有源波束赋形与无源波束赋形，随后通过求解标准凸问题获得时间分配变量的最优解。数值结果表明：1）与传统RIS相比，STAR-RIS在两种策略下均能实现显著的性能提升；2）TS-TDMA更适用于单天线场景，而ES-NOMA更适合多天线场景；3）随着STAR-RIS单元数量的增加，ES-NOMA相对于TS-TDMA的优越性进一步增强。