NTNs: Filling Coverage Gaps to Enhance Public Safety Communication Systems

March 28, 2024 Rachel Edwards

First responders racing to save lives and property in the face of emergencies must contend with various obstacles. Communication shouldn’t be one of them, yet coverage gaps in terrestrial networks caused by fire, earthquakes, heavy storms, and other natural disasters can cause dropped transmissions. For this reason, non-terrestrial networks (NTNs) are becoming a key element of public safety networks.

NTNs deliver reliable and continuous communication to first responders, even in challenging environments and hazardous natural phenomena that hinder terrestrial networks. The result is that NTNs aid in servicing mission-critical scenarios more efficiently than terrestrial networks because they don’t operate on the earth’s surface. To optimize NTN performance in mission-critical applications, extensive testing must be performed during deployment, installation, and maintenance.

How NTNs Operate

Three types of satellites are typically used in NTNs. Low-Earth Orbit (LEO) and Medium-Earth Orbit (MEO) satellites circulate around the earth between 300 km – 15000 km and 700 km - 25000 km, respectively. Both have typical beam footprints of 100 km – 1000 km. Geostationary Orbit (GEO) satellites transmit signals from a notional station at 35786 km, keeping its position fixed with a typical beam footprint of 200 km - 3500 km. GEO satellites provide longer, more inclusive coverage than LEOs and MEOs due to their higher altitudes.

Some NTNs can also utilize drones and high-altitude platform systems (HAPS). Examples include balloons, airships, or any self-powered winged aircraft that fly or float up to about 20 km.

Satellite Architectures

3GPP aims to solve 5G coverage gaps by developing standards that certify high-functioning NTNs to help ensure service continuity in areas where 5G cannot operate on terrestrial networks alone. This includes public safety networks for first responders. 3GPP recommends two types of architectures that create and transmit 5G services through satellite-based NTNs:

Transparent Payload – In this architecture, the satellite serves as an analog RF repeater for the feeder and service link. RF filtering, frequency conversion, and amplification are done by the satellite. The result is the waveform signal repeated by the payload remains unchanged (figure 1).

Figure 1: Transparent payload architecture

Regenerative Payload – The satellite(s) act(s) as a 5G base station in the sky, as RF filtering, frequency conversion, amplification, as well as coding/decoding modulations, switching, and/or routing are done at the satellite. The signal from Earth is regenerated, with the NR-Uu interface operating on the service link, while the N2 and N3 operate over a satellite radio interface on the feeder link (figure 2).

Figure 2: Regenerative payload architecture.

Both architectures emit two potential satellite beams to provide coverage, depending on the specific location and application. A moving beam is active when a satellite with fixed beams casts a moving footprint on the earth’s surface. A fixed beam is consistently adjusted as the satellites orbit the earth to maintain coverage of the same geographical area. The moving beam moves relative to a fixed position on earth, while fixed beams can be adjusted to cover an area predicated on the satellite orbiting above the horizon of that geographical area.

Ensuring Coverage During Emergencies

Specific challenges must be addressed for NTNs to extend 5G coverage for emergency use. Recognizing and addressing potential issues during early production will improve NTN operation. One key concern is signal strength due to satellites operating at high altitudes and transmitting numerous elliptically-shaped coverage beams. Another issue is increased handoffs and dropped signals due to NTN sites consistently orbiting the earth’s atmosphere.

While those are two main considerations, many others exist. For example, some NTN satellites can travel up to 15,000 MPH, creating a Doppler shift in frequency. Broadcast delays occur when the satellite speed and the pull of the earth’s orbit consistently change. The varying frequency shift needs to be counteracted by the UE, and the distance for the signal to reach the UE is further in an NTN, which can increase path loss. Measuring accurate frequency and timing between the many beams and cells’ connected coverage and transmissions with terrestrial networks will aid in the troubleshooting process.

NTN Testing Considerations

According to 3GPP Release 17, proper testing procedures can maintain and improve the quality of NTNs while also troubleshooting technical challenges in cell mobility and round-trip delays. Therefore, comprehensive testing is necessary to ensure the best possible Quality of Service (QoS).

NTN integrity is necessary to provide QoS. The varying distances, speed, and mobility of the satellite and UE must be maintained during testing. This is achieved by emulating different UE mobility and fading reports, including fast-moving satellites. A high radio link facility and analytical management of significant Doppler shifts can help avoid numerous connectivity issues with NTNs and terrestrial networks while managing multi-path mesh networks between satellites, satellites-to-ground, and satellites-to-aircraft.

Anritsu offers an extensive portfolio of test solutions to mitigate issues related to NTNs. For example, long-term spectrum monitoring tools address issues generated by equipment upgrades and interference from RF services operating in spectrum adjacent to satellite frequencies.

Learn More About NTNs

NTNs are a vital component of public safety and emergency response strategies with their ability to provide quality 5G coverage to first responders. Significant differences in safeguarding lives and minimizing the impact of disasters and emergencies are achieved by integrating NTNs into public safety communications infrastructure.

To learn more about NTNs, how they are used in public safety networks, and testing approaches, download our new white paper.

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