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 spectrally and energy efficient WPCNs.

中文翻译：

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由IRS协助的无线供电NOMA：真的需要动态无源波束成形吗？

智能反射面（IRS）由于具有通过智能反射重新配置信号传播环境的能力，因此是提高无线通信网络（WPCN）性能的一项有前途的技术。特别是，IRS承诺的高无源波束成形增益可以显着提高WPCN中的下行链路无线功率传输（DL WPT）和上行链路无线信息传输（UL WIT）的效率。尽管原则上可以对DL WPT和UL WIT采用不同的IRS相移，即动态IRS波束成形，但是会产生额外的信令开销和计算复杂性，但实际上是否有益仍然是一个悬而未决的问题。为了回答这个问题，我们考虑使用IRS辅助的WPCN，其中多个设备使用混合接入点（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，我们旨在通过共同优化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。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。仿真结果证明了我们提出的设计优于基准方案的有效性，并且还提供了有关IRS在实现频谱高效节能WPCN重要性方面的有用见识。