Throughout its history, cellular communication has been confined to ground-based networks of towers and other cellular communications equipment. However, that is changing with 5G. The Third Generation Partnership Project’s (3GPP’s) Release 17, finalized last year, included the first enhancements to support 5G non-terrestrial networks (NTNs), which hold the potential to meld cellular terrestrial networks with networks incorporating satellites and other high-altitude platforms, such as balloons.
NTNs have many potential use cases, including boosting the reach of 5G to remote corners of the world that lack cellular infrastructure and regions where the infrastructure is disrupted, such as in the wake of a large-scale natural disaster. They can also enable communications service on moving aircraft, ships and trains in remote areas where it is not possible to build terrestrial network infrastructure, provide redundancy to amplify service continuity for machine-to-machine communications and IoT devices and bolster the reliability of critical communications.
While some initial implementation of NTN-capable service and equipment have already been deployed (notably iPhone 14’s satellite-enabled emergency capabilities), many more robust offerings are in the works. However, successfully deploying and operating these innovations will require the cellular and satellite industries to develop successful business models and innovation in implementation and testing.
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A new breed of service
From a technical perspective, there are some key differences between satellite and cellular phone service that need to be considered for 5G NTN implementation and testing (see Figure 1). Traditional satellite phone service relies on specialized phones with large antennas. Implementing satellite services is challenging for conventional smartphones because the distance between the satellite and the subscriber results in low signal levels, which requires cellphones to have larger, higher-gain antennas.
However, conventional cellphones see the satellite the same way as any other network cell. Unmodified cellphones can connect to satellites within a limited cell radius, where the satellite cell conditions are almost identical throughout the cell for multiple users. This requires splitting the network into relatively small cells, with the users within a few miles from each other.
Figure 1: Graphical representation of NTN communications transmission from satellites (Source: Keysight Technologies)
For cellular networks, adding satellite users generates new requirements for handling customer connections and interactions between the satellite and cellular networks. Base stations play a key role in 5G NTNs, which differs significantly from satellite phone service in the form of network-assisted operations. In 5G NTNs, base stations help to establish and maintain the connection between the cellphone and satellite.
Because of their distance from Earth and continual movement, satellite signals generate lower-power signals and introduce Doppler error in the frequency domain. The movement also creates an increased delay in changing the satellite link, which the phone and network must tolerate to provide seamless service. The network modifies the timing of the radio frames to match the user location, then pre-compensates to offset the link frequency error caused by the movement of the satellite, making it easier for the cellphone to tolerate the satellite link characteristics. The cellphone relies on network broadcast system information blocks (SIBs) containing information about satellite locations and velocities to estimate link conditions, delay and Doppler error in transmission frequency.
5G NTN communication relies on several key techniques. Minimizing the frequency error requires estimating the link conditions and making counteractions in the radio link between the network and the cellphone. Making the link from the gateway to the satellite (the feeder link) invisible to the cellphone in the downlink requires adjusting the timing. Finally, the NTN needs to handle the link from the satellite to the service subscriber (service link) from the NTN-capable cellphone, including minimizing the frequency error in the phone-to-satellite uplink.
3GPP has introduced several modifications to the 5G protocol stack for satellite link use. One of these modifications is new timing and retransmission procedures that double the number of hybrid automatic repeat request (HARQ) retransmission processes from 16 to 32 to increase tolerance for delays in receiving satellite signals.
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Testing challenges of non-terrestrial networks
At the beginning of NTN development, it is critical to narrow down the possible causes of problems that relate to new functionality of the system under test (SUT). Increased delay between the network and cellphones is the main new characteristic that a channel model uses to mimic and test at the system level when adding satellite links in a cellular network.
Fixed-delay testing is a good place to start to check the estimated satellite delay between the base station—known as gNodeB (gNB) in 5G networks—and the cellphone and the tolerance for the delay without other satellite link characteristics. Fixed-delay testing reveals the retransmission performance of the satellite links, as well as the timing and timing adjustments with the addition of NTN support.
With geostationary orbits almost equal to static-delay lines, static-delay testing is helpful to understand the behavior of these satellites.
5G NTN test cases (Source: Keysight Technologies)
In addition to delay, frequency error caused by satellite movements is the key difference between satellite connections and cellular networks. Low-Earth–orbit (LEO) satellites with orbital altitudes between 500 and 2,000 km and velocities between 7 and 8 km/s (which typically have a lot of Doppler error in the link) are especially prone to frequency errors.
Doppler error can vary from some tens of kilohertz up to the megahertz range, depending on the elevation angle toward the ground receiver and the frequency. Large frequency errors result in challenges to testing (see Figure 2).
Figure 2: Satellite orbit models and test scenarios for NTN testing generated by Keysight’s Channel Studio GCM (Source: Keysight Technologies)
Testing with real and simulated base stations
5G NTN test planning starts with defining the content of the SUT and interfaces of the device under test (DUT). A proper test environment must simulate the network connection and link to the satellite and look at the service from the DUT point of view. It is possible to conduct this testing using a real network access point or a simulated access point.
Connectivity is often the main challenge for the NTN test environment, particularly when the SUT contains a real cellphone and a real base station. The typical test setup uses either a cable between the phone and the base station or antennas for over-the-air testing along with adjustable or programmable attenuators to control link power. Accurately testing the satellite link delay, frequency errors and dynamic Doppler in NTNs is a challenge that is best addressed through channel emulation.
Channel emulators enable delay and Doppler variations over time to represent link conditions that are approximate to those in the field. The resulting channel model contains the rules that define link behavior during testing. The channel model parameters contain dynamic signal attenuation, delay and Doppler, which, when taken together, recreate the radio channel identical to the real world.
It is also possible to conduct testing using a simulated base station by using network emulation technology. A network emulator can simulate the functionality of a full cellular base station. The network emulator represents the 5G service to the DUT. As with a real base station, the network emulator requires the satellite trajectory information and the ephemeris data—information about a given satellite’s position and velocity—encoded into the SIB broadcasted to the cellphone.
The position and velocity of all satellites orbiting Earth are monitored, and their position and velocity are publicly available for commercial satellites. This data can be used for recreating orbits for testing purposes.
NTNs will have a profound effect on wireless communications in the years to come. The technology will enable valuable new use cases and solidify service to all corners of the globe. As with any wireless technology, the success of NTN products and services will depend to a large degree on the implementation of robust and accurate test, measurement and emulation capabilities and strategies.
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