Overcoming challenges threatening NTN success

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IMAGE: Keysight Technologies

By Nancy Friedrich

To heighten communications capabilities and improve situational awareness, military and government agencies increasingly leverage commercially developed technologies. Many plan to boost their connectivity capabilities with 5G non-terrestrial networks (NTNs). 

An NTN is a hybrid network, applying satellite communication (SATCOM) technology to extend existing 5G technology. 5G NTNs draw many features from 5G terrestrial networks and face many of the same challenges, adding higher reliability expectations for 5G NTN service compared to earlier SATCOM networks. 

To help assure performance of 5G NTN deployments, virtual simulation, emulation, and digital twin technology use RF system measurement science to deliver results beyond what is possible through physical testing alone.

Despite the hype over the commercial possibilities of 5G NTN, it also promises to transform capabilities for aerospace and defence. Potential 5G NTN use cases for military and government include coverage for forward battlefields or focused special operations. 

NTNs also will provide coverage to restore communications in disaster areas experiencing widespread infrastructure outages. Among transportation use cases, NTNs support logistic in-transit tracking for long-haul trucking routes, rail lines, and maritime shipping lanes.

Five challenges facing NTNs

More data, crowded spectrum

The hybrid 5G NTN provides obvious advantages as well as challenges. 

Handheld or vehicle-based user equipment (UE) tends to demand high volumes of data for video and mapping services. Additionally, sensor applications may connect user equipment with lower data rates. 

Delivering the required volumes of data means leveraging 5G signalling fundamentals for 5G NTN, including mmWave carrier frequencies and complex modulation in wide bandwidths.

5G spectrum is already tightly allocated in terrestrial networks, and an onslaught of tens of thousands of lower earth orbit (LEO) satellites and geostationary earth orbit (GEO), medium earth orbit (MEO), and high-altitude platform systems (HAPS) platforms soon operating in 5G NTNs will add to the spectrum crowding.

The space environment

Space is the foremost challenge for NTNs. Once deployed, equipment is inaccessible. 

In addition, systems must operate in an extremely harsh environment with extreme temperatures and radiation. For successful performance, systems also need to provide consistent power generation and storage. 

For all of these aspects, satellite system providers need to balance risk versus cost across the lifetime of the operation.

Size, weight, power, and cost

Another concern is the physical limits of placing high-frequency RF and computing resources in the sky. 

Size, weight, power, and cost (SWaP-C) become issues when moving away from the GEO 20 tonners into more compact LEO satellites and HAPS platforms, and payloads must transform accordingly. 

On the plus side, placing more satellites into service with smaller payloads and shorter life cycles is now feasible and cost-effective. A 5G NTN might consist of a group of satellites working together in various orbits.

Connecting in motion

5G NTNs put some things, or perhaps everything in the network, in constant motion. 

Satellite and HAPS movements factor into connection setup, signal quality, and handovers. gNodeB instances and parts of the RAN flying aloft add to the movement of any UE at the surface. 

Parameters previously fixed or confined in a small range in a 5G terrestrial network suddenly become wide-ranging variables in a 5G NTN. Tracking areas, bulk delays, Doppler shifts, signal-to-noise ratios (SNRs), and more elements take on dynamic characteristics.

The payload question

The introduction of 5G NTNs disrupts the traditional 5G terrestrial network architecture and opens up a paradigm shift in connectivity. 

Many alternatives exist for satellites and HAPS participating in gNodeB and RAN domains, some with multiple satellites in the chain scattered across miles of sky. The choice between transparent or regenerative payloads can completely change how the network organises and the resulting signal routing. 

With LEO satellites in motion, remember that all timing relationships are dynamic. At stake is the quality of service (QoS) user experience, primarily due to variable delays and complex handovers that can result in dropped connections.

Platform kinematics rapidly alter 5G NTN channel behaviour, and staging fast-moving platforms in the proper orientation long enough to gather detailed physical measurements is not an option. 

However, simulations can account for complex orbital paths and decompose real-time motion into precise detail with time-correlated analysis.

Advancing the next NTN wave

Accurate multi-domain simulation of a 5G NTN link depends on four elements: an authentic representation of complex digital modulation in a 5G waveform with real-world effects, a complete model of satellite kinematics, robust modelling of RF system signal processing, and a time-correlated view of 5G protocol decoding. 

The critical goal is validating performance in a simulation before deployment of orbital hardware.

Nancy Friedrich is industry solutions manager for aerospace and defence at Keysight Technologies

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