​How to Test Nonterrestrial Networks (NTNs) on Earth​

Nonterrestrial Networks (NTNs) are wireless communication systems beyond the Earth's surface, using satellites in low, medium, and geostationary orbits, alongside high-altitude platforms for transmission. These systems deliver wireless connectivity to remote areas through spaceborne vehicles. NTNs are now in the early stages of supporting enhanced mobile broadband, including fixed and mobile cell connections, multi-connectivity, and wide-to-local area IoT and public-safety applications. The hope for NTNs is that they will help to close the digital divide, boost network resilience, and facilitate edge and mobile cell hybrid connectivity.

Testing nonterrestrial networks on Earth presents several challenges. The complexity introduced by NTNs is significant as signals must reach farther into the atmosphere and deal with weather and atmospheric conditions as the main source of interference. Other technical challenges include doppler (relative velocity of earth and satellites), staying connected to a moving object, and connecting to cell phones whose antennas are not ideal for long range wireless communication. Additionally, today's devices often lack the hardware to connect with distant satellites, necessitating changing existing satellites to connect more easily to unmodified 5G smartphones. Other NTN test challenges influenced by the large distances include delay, large Doppler shifts from rapidly moving satellites, signal attenuation, and handovers between terrestrial and satellite links. 

6 Tests for NTN

Rigorous testing and simulation play pivotal roles in ensuring NTN systems meet the demands of real-world applications. From satellite simulators that offer low risk testing environments to network simulations that prototype complex RF systems, each method addresses a unique aspect of NTN development and readiness. 

Verification of the Ground Segment

Satellite simulators are digital replicas of the satellites that run software or perform actions exactly as the satellite would, but on a computer on Earth. This simulation removes the risk associated with testing on the physical satellite and provides a realistic simulation of the spacecraft, the ground segment, and the space environment. This is used for testing and verification of the ground segment which ensures the correct installation and effectiveness of ground systems that protect personnel and circuits from hazardous currents and damaging fault conditions. The test is done prior to the launch of the spacecraft, development of flight operations procedures, and training of the flight control team.

Satellite simulators are digital replicas of the satellites that run software or perform actions exactly as the satellite would, but on a computer on Earth. This simulation removes the risk associated with testing on the physical satellite and provides a realistic simulation of the spacecraft, the ground segment, and the space environment. This is used for testing and verification of the ground segment which ensures the correct installation and effectiveness of ground systems that protect personnel and circuits from hazardous currents and damaging fault conditions. The test is done prior to the launch of the spacecraft, development of flight operations procedures, and training of the flight control team.

Network Simulation

Network simulation is used to prototype and design platforms for complex RF systems in NTNs. It involves creating a virtual model of the network and using it to analyze its performance and behavior under different conditions. This is crucial for understanding the capabilities and expected performance of each proposed radio access technology.

Stress and Performance Tests

Engineers use RF channel emulation serves to develop and verify direct-to-device NTN features and to conduct stress tests on device performance across various use scenarios. It involves creating a simulated RF environment and testing how the device performs under various signal conditions.

Addressing the Multipath Effect, Fading, and Propagation

This test is necessary to ensure that the signals transmitted by the NTN can effectively reach the intended receivers despite the challenges posed by the multipath effect (where signals take multiple paths and arrive at different times), fading (where signal strength decreases because of various factors), and propagation (how signals travel through the medium). The test involves transmitting signals and measuring their strength and quality at the receiver end.

Scale and High-Capacity Network Testing

Verifying that the NTN can handle many connections and high data rates is this test's crucial purpose. The process involves the creation of a simulated environment where the network is subjected to high traffic loads, and its performance is observed.

Field Condition Tests

Important to ensure that the NTN can perform well under real-world conditions, this test involves testing the network under various environmental conditions and scenarios that it might encounter in actual operation.

Testing Limitations

Testing NTNs on Earth is subject to a variety of constraints, including technical, logistical, and financial limitations.

1. Latency Issues—Satellite-based systems suffer from delays caused by the vast distances between satellites and the earth. This impacts real-time applications. These latency issues can be challenging to accurately replicate and test on Earth.
2. Low Throughput and High Latency— The low throughput and high latency of an NTN connection may be tolerable for low-bandwidth connections that otherwise would have no connectivity at all, but it can introduce difficulties in a network supporting hundreds of simultaneous connections, with constant uplink and downlink traffic communications that require low latency.
3. Integration with Terrestrial Networks—5G nonterrestrial networks introduce new implementation and testing challenges, including integration with terrestrial networks. This integration is complex and can be difficult to test thoroughly.
4. Satellite Link Delays and Doppler Errors— The large distances involved in NTN communication introduce significant delays and Doppler shifts. These factors can be challenging to accurately simulate and test on Earth.

How NI Supports NTN Test

NI provides efficient, cost-effective methods for NTN design and test that enhance satellite communication systems. We offer a variety of test equipment and software solutions that are instrumental in the design, testing, and implementation of NTNs. Our approach focuses on signal integrity, robust connectivity, and adapting to evolving satellite communication standards.

One piece of equipment, the PXI Vector Signal Transceiver (VST), combines an RF and baseband vector signal analyzer and generator with a user-programmable FPGA. It also includes high-speed serial and parallel digital interfaces for real-time signal processing and control from baseband to mmWave. The VST capabilities makes it ideal for measuring NTN network performance with its wider bandwidth, increased frequency, and improved RF metrics.