Architectures, standardisation, and procedures for 5G Satellite Communications: A survey

The 5G ecosystem is radically shifting Information and Communication Technologies (ICT) towards a system directly tailored and optimised to support vertical markets and services. In order to design and implement the ambitious 5G network, Satellite Communications (SatCom) have been recognised as a promising solution to extend and complement terrestrial networks in unserved or under-served areas, as reflected by recent commercial and standardisation endeavours. Within 3GPP, this is reflected in the novel Study Item on Non-Terrestrial Networks to introduce 5G (New Radio, as per 3GPP nomenclature) technologies in SatCom. However, typical satellite channel impairments, as large path losses, delays, and Doppler shifts, pose severe challenges to the realisation of a satellite-based 5G network. In this paper, we thoroughly review the standardisation process performed within 3GPP from Rel. 15 to the present, discussing the different technologies and the related system architectures and interfaces. Based on these discussions, we assess the impact of the satellite channel characteristics on the major physical and medium access control layers, i.e., Downlink Synchronisation, Random Access, and Hybrid Automatic Repeat reQuest. A set of potential solutions to the identified shortcomings is finally discussed.