Wireless powered sensor networks (WPSNs) have evolved as a promising paradigm for energy-efficient communications. Recently, the proliferation of reconfigurable intelligent surface (RIS) has further been envisioned as a cost-effective solution for improving wireless power transfer (WPT) efficiency. In this paper, from the practical perspective of balancing the network sustainability and reliability, we consider a RIS-enhanced partially WPSN that composed of wireless-powered energy receivers (ERs) and battery-powered information receivers (IRs). Assuming the partially WPSN operates in time division multiple access (TDMA) mode, the joint optimization of covariance matrices, downlink/uplink (DL/UL) time allocation and RIS reflecting coefficients are investigated under the minimum DL rate constraint among all IRs for maximizing the achievable UL sum rate. Specifically, the single-IR single-ER (SISE) case is first studied based on the assumption of separate DL/UL RIS reflecting coefficients, in which an alternating optimization algorithm is proposed with semi-closed-form optimal solutions. In order to reduce the hardware overhead and signal processing complexity, we also investigate the case of identical DL/UL RIS reflecting coefficients, in which an iterative optimization algorithm is developed to tackle the coupled DL/UL transmissions. Then, we extend our work to the multiple-IRs multiple-ERs (MIME) case, where both the optimization problems corresponding to separate and identical DL/UL RIS reflecting schemes become more challenging to solve. To circumvent this intractability, we propose a successive convex relaxation (SCA) based alternating optimization algorithm and a low-complexity two-step algorithm. Finally, numerical results demonstrate the superior UL sum rate performance of our proposed algorithms over the adopted benchmarks.

中文翻译：

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RIS增强型部分WPSN的最优传输策略和时间分配


无线供电传感器网络（WPSN）已发展成为节能通信的一种有前途的范例。最近，可重构智能表面（RIS）的普及被进一步设想为提高无线功率传输（WPT）效率的经济有效的解决方案。在本文中，从平衡网络可持续性和可靠性的实际角度出发，我们考虑了由无线供电能量接收器（ER）和电池供电信息接收器（IR）组成的RIS增强型部分WPSN。假设部分 WPSN 以时分多址 (TDMA) 模式运行，在所有 IR 之间的最小 DL 速率约束下研究协方差矩阵、下行链路/上行链路 (DL/UL) 时间分配和 RIS 反映系数的联合优化，以最大化可实现的 UL 总速率。具体来说，首先基于单独的DL/UL RIS反射系数的假设来研究单IR单ER（SISE）情况，其中提出了具有半封闭形式最优解的交替优化算法。为了减少硬件开销和信号处理复杂性，我们还研究了相同 DL/UL RIS 反射系数的情况，其中开发了迭代优化算法来解决耦合的 DL/UL 传输。然后，我们将工作扩展到多 IR 多 ER (MIME) 情况，其中与单独和相同的 DL/UL RIS 反射方案相对应的优化问题变得更具挑战性。为了避免这种棘手的问题，我们提出了一种基于连续凸松弛（SCA）的交替优化算法和一种低复杂度的两步算法。 最后，数值结果证明了我们提出的算法比采用的基准具有优越的 UL 总速率性能。