Non-terrestrial networks as a functional layer in 5G architectures
The integration of satellite segments and high-altitude platforms into fifth-generation networks responds to a need for continuous coverage that terrestrial infrastructures cannot satisfy autonomously. The non-terrestrial network model, known by its acronym NTN, defines a set of radio access nodes located outside the Earth's surface that operate as a functional extension of the mobile network core, maintaining coherence of signalling protocols and mobility management with conventional base stations. From an architectural standpoint, HAPS platforms occupy a differentiated position within this scheme. Their operational altitude, situated in the stratosphere between 17 and 22 kilometres, allows them to offer a regional-scale coverage footprint with link latencies appreciably lower than those of low-orbit systems, and with a controlled repositioning capability that satellites do not possess. This combination of characteristics makes the HAPS segment technically compatible with the latency requirements of broadband mobile access services, without requiring multi-node constellations to maintain service continuity over a given area. The operational limitations of this integration are not negligible. The management of mobility between terrestrial and non-terrestrial nodes introduces complexity into handover procedures, given that variable propagation times and changing link geometries affect synchronisation mechanisms and channel estimation. Inter-layer interference, particularly in overlapping coverage scenarios, demands coordinated spectral management policies for which fully consolidated regulatory frameworks at European level are not yet in place. The regulatory context is a determining factor for the effective deployment of these architectures. Spectrum allocation for HAPS operations in shared-use bands, as well as the definition of airspace access conditions for stratospheric platforms, remains under harmonisation within the ITU and the national bodies of the European Union Member States. This incomplete framework conditions the timescales for operational integration and the viability of service models based on roaming agreements between terrestrial network operators and non-terrestrial capacity providers. For institutional operators and government bodies with connectivity requirements in remote areas or in emergency situations, the NTN architecture offers a means of access to mobile data services without exclusive dependence on fixed infrastructure. The ability to integrate low- and medium-orbit satellite nodes together with HAPS platforms within a single management plane represents a technical evolution with direct implications for the resilience of mission-critical networks and for capacity planning in variable-demand environments.
NASSAT - Network Satellite Systems