  In this paper, we present the vision of our project 6G for Connected Sky
(6G-SKY) to integrate terrestrial networks (TNs) and non-terrestrial networks
(NTNs) and outline the current research activities in 6G research projects in
comparison with our project. From the perspectives of industry and academia, we
identify key use case segments connecting both aerial and ground users with our
6G-SKY multi-layer network architecture. We explain functional views of our
holistic 6G-SKY architecture addressing the heterogeneity of aerial and space
platforms. Architecture elements and communication links are identified. We
discuss 6G-SKY network design and management functionalities by considering a
set of inherent challenges posed by the multi-layer 3-dimensional networks,
which we termed as combined airspace and NTN (combined ASN). Finally, we
investigate additional research challenges for 6G-SKY project targets.
