Wireless Energy Transfer Solutions for Sustainable Connectivity Infrastructure From Space to Ground for 6G

The sixth generation (6G) wireless networks are expected to be supported by space-to-ground infrastructures (SGIs) to enable global connectivity and intelligence. However, ensuring a sustainable and continuous energy supply for SGIs, including satellites, high-altitude platforms, and low-altitude platforms, remains a fundamental challenge. This paper presents a comprehensive survey of wireless energy transfer technologies as key enablers for sustainable 6G connectivity from space to ground. We first review fundamental WPT mechanisms, including radio-frequency and laser-based approaches, and discuss their applicability in SGI-enabled networks (SGIENs). We then examine practical energy provisioning architectures by mapping energy sources and delivery paths across ground stations, aerial relays, and space-based platforms. Furthermore, advanced technologies such as massive multiple-input multiple-output energy beamforming, cooperative energy relays, joint waveform and rectifier design, and integrated mobility-aware beam control are reviewed to enhance power delivery. Finally, we discuss network-level integration of energy, communication, sensing, and computation, and outline open challenges in end-to-end efficiency optimization and promising research directions. This survey provides a comprehensive overview and research roadmap toward self-sustainable and carbon-neutral 6G infrastructures.