Effective information transmission and energy supply are essential for low-power Internet-of-things (IoT) devices, particularly in geographically constrained or disaster-affected areas. As an emerging technology, simultaneous wireless information and power transfer (SWIPT) ensures uninterrupted communication through supplying power, thereby eliminating issues related to battery depletion. Owing to the high maneuverability, unmanned aerial vehicle (UAV) has enabled SWIPT a new paradigm by providing flexible and on-demand service. However, maintaining connectivity among UAVs while they execute tasks is a critical challenge, for their powerful online decision-making relies on information sharing. Unfortunately, this issue is often overlooked in existing literature on UAV-aided wireless communication. In this paper, a novel connectivity preserved multi-agent deep reinforcement learning algorithm (CP-MARL) is designed to solve the trajectory planning problem in complex environments, where multi UAVs cooperatively provide SWIPT service for ground users (GUs) while ensuring seamless information exchange. Moreover, fairness of service received among GUs is also considered, as unequal resource allocation can lead to suboptimal system performance. Besides, reconfigurable intelligent surface (RIS) is adopted to redirect signals to achieve line-of-sight (LoS) channel. Under this framework, the fair energy efficiency is maximized by jointly optimizing the UAVs’ trajectory, the phase shift of RIS, power splitting (PS) ratio and the association relationships between UAVs and GUs. The formulated problem is inherently non-convex but the proposed CP-MARL algorithm successfully delivers online near-optimal solutions. Numerical results demonstrate that compared with benchmark solutions, the designed CP-MARL algorithm significantly enhances the energy-efficiency performance of the system, achieving 40.22% performance improvement compared to scenarios where connectivity is not maintained, along with a more stable training process. Additionally, the superiority of incorporating RIS in this system is corroborated by the 27.31% improvement in energy-efficiency.
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