Satellite communication systems are a promising solution to extend and complement terrestrial networks in unserved or under-served areas, as reflected by recent commercial and standardization endeavors. In particular, 3GPP recently initiated a study item for new radio, i.e., 5G, non-terrestrial networks aimed at deploying satellite systems either as a stand-alone solution or as an integration to terrestrial networks in mobile broadband and machine-type communication scenarios. However, typical satellite channel impairments, as large path losses, delays, and Doppler shifts, pose severe challenges to the realization of a satellite-based NR network. In this paper, based on the architecture options currently being discussed in the standardization fora, we discuss and assess the impact of the satellite channel characteristics on the physical and medium access control layers, both in terms of transmitted waveforms and procedures for enhanced mobile broadband and narrowband-Internet of Things applications. The proposed analysis shows that the main technical challenges are related to the PHY/MAC procedures, in particular random access, timing advance, and hybrid automatic repeat request and depending on the considered service and architecture, different solutions are proposed.
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