  Sixth-Generation (6G) technologies will revolutionize the wireless ecosystem
by enabling the delivery of futuristic services through terrestrial and
non-terrestrial transmissions. In this context, the Non-Terrestrial Network
(NTN) is growing in importance owing to its capability to deliver services
anywhere and anytime and also provide coverage in areas that are unreachable by
any conventional Terrestrial Network (TN). The exploitation of the same radio
technology could greatly facilitate the integration of NTNs and TNs into a
unified wireless system. Since New Radio (NR) is the de facto standard to
deliver manifold heterogeneous services in terrestrial wireless systems, 3GPP
is investigating new solutions to extend NR to NTNs. In this paper, the
constraints that NTN features place on NR procedures are investigated by going
thoroughly into 3GPP specifications; strengths and weaknesses of the NR
technology in enabling typical 6G services on NR-enabled NTNs are identified;
finally, open issues and insights are provided as guidelines to steer future
research towards 6G NTNs.
