Introduction
While recent years have witnessed revolutionary progress in deploying the fifth generation (5G) wireless network, both academia and industry have been enthusiastically exploring the roadmap to design the future sixth generation (6G) wireless network. Driven by the emergence of promising Internet of Everything (loE) applications, such as extended reality, industrial automation, and tactile Internet, 6G is envisioned to target more ambitious network performance than 5G, including truly global coverage and ubiquitous connectivity over 3D space, ultra-high data rate, and extremely high reliability and low latency, which may not be fully achieved by existing technologies for 5G.