Intelligent reflecting surface (IRS) can be densely deployed in complex environments to create cascaded line-of-sight (LoS) links between base stations (BSs) and users, which significantly enhance the signal coverage. In this paper, we consider the wireless power transfer (WPT) from a multi-antenna BS to multiple energy users (EUs) by exploiting the signal beam routing via multi-IRS reflections. First, we present a baseline beam routing scheme with each IRS serving at most one EU, where the BS transmits wireless power to all EUs simultaneously while the signals to different EUs undergo disjoint sets of multi-IRS reflection paths. Under this setup, we aim to tackle the joint beam routing and resource allocation optimization problem by jointly optimizing the reflection paths for all EUs, the active/passive beamforming at the BS/each involved IRS, as well as the BS's power allocation for different EUs to maximize the minimum received signal power among all EUs. Next, to further improve the WPT performance, we propose two new beam routing schemes, namely dynamic beam routing and subsurface-based beam routing, where each IRS can serve multiple EUs via different time slots and different subsurfaces, respectively. In particular, we prove that dynamic beam routing outperforms subsurface-based beam routing in terms of minimum harvested power among all EUs. In addition, we show that the optimal performance of dynamic beam routing is achieved by assigning all EUs with orthogonal time slots for WPT. A clique-based optimization approach is also proposed to solve the joint beam routing and resource allocation problems for the baseline beam routing and proposed dynamic beam routing schemes. Numerical results are finally presented, which demonstrate the superior performance of the proposed dynamic beam routing scheme to the baseline scheme.

翻译：智能反射面（IRS）可密集部署于复杂环境中，在基站（BS）与用户之间创建级联视距（LoS）链路，从而显著增强信号覆盖。本文考虑利用多IRS反射的信号波束路由，实现从多天线基站到多个能量用户（EU）的无线能量传输（WPT）。首先，我们提出一种基线波束路由方案，其中每个IRS最多服务一个EU，基站同时向所有EU传输无线功率，而不同EU的信号经由不相交的多IRS反射路径传输。在此设定下，我们旨在通过联合优化所有EU的反射路径、基站/每个参与IRS的有源/无源波束成形，以及基站对不同EU的功率分配来最大化所有EU中最小接收信号功率，从而解决联合波束路由与资源分配优化问题。其次，为进一步提升WPT性能，我们提出两种新型波束路由方案，即动态波束路由和子表面波束路由，其中每个IRS可通过不同时隙或不同子表面分别服务多个EU。特别地，我们证明动态波束路由在所有EU的最小收集功率方面优于子表面波束路由。此外，我们表明动态波束路由的最优性能通过为所有EU分配正交时隙进行WPT实现。针对基线波束路由及所提动态波束路由方案，我们还提出一种基于团结构的优化方法以解决联合波束路由与资源分配问题。最后，数值结果展示了所提动态波束路由方案相较于基线方案的优越性能。