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The integration of satellite and terrestrial networks is a promising solution for extending broadband coverage to areas not connected to a terrestrial infrastructure, as also demonstrated by recent commercial and standardisation endeavours. However, the large delays and Doppler shifts over the satellite channel pose severe technical challenges to traditional terrestrial systems, as LTE or 5G. In this paper, two architectures are proposed for a LEO mega-constellation realising a satellite-enabled LTE system, in which the on- ground LTE entity is either an eNB (Sat-eNB) or a Relay Node (Sat-RN). The impact of satellite channel impairments as large delays and Doppler shifts on LTE PHY/MAC procedures is discussed and assessed. The proposed analysis shows that, while carrier spacings, Random Access, and RN attach procedures do not pose specific issues, HARQ requires substantial modifications. Moreover, advanced handover procedures will be also required due to the satellites' movement., Comment: Submitted to IJSCN Special Issue on SatNEx IV

An unprecedented exploding demand for broadband high-speed, heterogeneous, ultra-reliable, secure, and low latency communications is motivating and leading the definition of new standards and technologies for wireless communications, known as 5G. The massive scientific and industrial interest in 5G communications is motivated by the key role that 5G systems can play in the worldwide economic and societal processes to support next generation vertical services, e.g., Internet of Things, Industry 4.0, automotive and transportation sectors, e-Health, factories of the future, etc. The integration of satellite and terrestrial networks can be a cornerstone to the realisation of the foreseen heterogeneous global system. Due to their inherent large footprint, satellites can efficiently complement and extend dense terrestrial networks, both in densely populated areas and in rural zones, as well as provide reliable Mission Critical services. While in the past terrestrial and satellite networks have evolved almost independently from each other, leading to a difficult a posteriori integration, the definition of the future 5G paradigm provides the unique chance for a fully-fledged architecture. This Special Issue aims at collecting the latest progresses in scientific, industrial, regulatory, and standardisation analyses, as well as sharing novel concepts and ideas, for a fully integrated satellite-terrestrial network. Potential topics include, but are not limited to: • System architecture: challenges and solutions • Impact of satellite channel impairments on the system physical layer • Integration with low altitude drones • Scheduling • Higher layers design • Regulatory aspects • Standards evolution • Coexistence with Advanced LTE and IoT networks • Application to 5G verticals • Cognitive radio techniques