Intelligent reflecting surface (IRS) is a promising technology to improve the performance of wireless powered communication networks (WPCNs) due to its capability to reconfigure signal propagation environments via smart reflection. In particular, the high passive beamforming gain promised by IRS can significantly enhance the efficiency of both downlink wireless power transfer (DL WPT) and uplink wireless information transmission (UL WIT) in WPCNs. Although adopting different IRS phase shifts for DL WPT and UL WIT, i.e., dynamic IRS beamforming , is in principle possible but incurs additional signaling overhead and computational complexity, it is an open problem whether it is actually beneficial. To answer this question, we consider an IRS-assisted WPCN where multiple devices employ a hybrid access point (HAP) to first harvest energy and then transmit information using non-orthogonal multiple access (NOMA). Specifically, we aim to maximize the sum throughput of all devices by jointly optimizing the IRS phase shifts and the resource allocation. To this end, we first prove that dynamic IRS beamforming is not needed for the considered system, which helps reduce the number of IRS phase shifts to be optimized. Then, we propose both joint and alternating optimization based algorithms to solve the resulting problem. Simulation results demonstrate the effectiveness of our proposed designs over benchmark schemes and also provide useful insights into the importance of IRS for realizing spectrum and energy efficient WPCNs.

中文翻译：

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IRS 辅助无线供电 NOMA：我们真的需要 DL 和 UL 中的不同相移吗？

智能反射面 (IRS) 是一种很有前途的技术，可提高无线供电通信网络 (WPCN) 的性能，因为它能够通过智能反射重新配置信号传播环境。特别是，IRS 承诺的高无源波束成形增益可以显着提高 WPCN 中下行无线功率传输 (DL WPT) 和上行无线信息传输 (UL WIT) 的效率。尽管对 DL WPT 和 UL WIT 采用不同的 IRS 相移，即，动态 IRS 波束成形 ，原则上是可能的，但会导致额外的信令开销和计算复杂性，它是否真的有益是一个悬而未决的问题。为了回答这个问题，我们考虑一个 IRS 辅助的 WPCN，其中多个设备采用混合接入点 (HAP) 首先收集能量，然后使用非正交多址 (NOMA) 传输信息。具体来说，我们的目标是通过联合优化 IRS 相移和资源分配来最大化所有设备的总吞吐量。为此，我们首先证明所考虑的系统不需要动态 IRS 波束成形，这有助于减少要优化的 IRS 相移数量。然后，我们提出了基于联合和交替优化的算法来解决由此产生的问题。