5G from space: implications for NTN test and measurement

NTN base station test and measurement

We are undergoing a paradigm shift - the term “base station” no longer truly applies to non-terrestrial networks (NTN). Instead, network nodes are integrated into satellites and move relative to the Earth’s surface. In the long term - for 6G, that is -– multi-orbit networks will be a reality, with 3-dimensional network nodes at all LEO, MEO and GEO altitudes.

There are various architectural approaches that are currently in the standardization process:

• Initially, as defined in Release 17, transparent mode will be used. In other words, the satellite will act as a kind of repeater, with the 5G NR radio signal generated and received in a terrestrial node (gNB). The communication between the terrestrial gNB and the satellite will take place via the feeder link between the satellite and a terrestrial gateway. The direct connection between the satellite and device is called the serving link.
• The future regenerative mode, currently discussed as a work item in Release 19, will incorporate either the entire or disaggregated gNB functions into the satellite access node (SAN). The objective is to have faster scheduling decisions as well as more processing and computing power in the satellite node. However, this comes at the price of higher complexity.

Currently, there are two documents suggesting standardization requirements that will be important for future SAN tests:

• TS 38.108 describes requirements for NTN receivers and transmitters.
• TS 38.181 describes the actual test requirements.
• TS 38.101-5 describes the specifications for NTN user equipment (UE) testing

Figure 1 below shows a brief outline of test scenarios and a symbolic setup for a SAN operating in NTN transparent payload mode. The device under test (DUT) consists of three functional blocks: the satellite (described as NTN RF payload), the gateway and the non-NTN network functions (gNB).

The RF interface tests can be roughly divided into:

• Transmitter tests (TX)
• Receiver sensitivity (RX)
• Receiver performance (RX performance)

Tests at the transmitter have an approach similar to that for the terrestrial case, with metrics such as transmit power (TX power, TX power control), modulation quality (EVM) and spectral transmit characteristics (ACLR, spurious emissions, SEM). A signal analyzer is the ideal test instrument here. Depending on the satellite node category, connection to the test instrument can be established via a cable-based connection or over-the-air (OTA). OTA tests enable verification of directional antennas, which are used for beamforming. This type of test requires full anechoic chambers (FAC) as well as positioning systems.

There are two different approaches to receiver tests:

• For metrics such as receiver sensitivity, a reference test signal is sent to the DUT using a signal generator. The result of this test is the block error rate (BLER) in the receiver or the data throughput. 3GPP specifications require throughput to reach a 95% threshold of a defined reference channel at a minimum input level to pass the sensitivity test. Due to component disaggregation, the injection point of the RF signal is at the satellite input, but the BLER can only be determined in the gNB protocol stack.
• The second approach is based on RX performance, which is similar to sensitivity, with respect to the metric of 95% in throughput. However, performance tests simulate a stress situation for the receiver by, for example, applying a fading profile to the test signal or adding interfering signals.

Test equipment for NTN user equipment

In principle, terminal equipment for 5G satellite communications has the same transmitter and receiver requirements as those for terrestrial networks. However, the devil is in the details: there will be several different test setups and methodologies depending on the NTN UE capability and use case. By way of example, the NTN-IoT device category will use a low complexity architecture.

In addition, use cases such as messaging or small data sets typically do not request a certain QoS profile and are very delay tolerant. Future NTN UE, such as very-small-aperture terminal (VSAT) types, will incorporate more sophisticated methods like beamforming, higher frequencies and wider bandwidth. This will require extended testing. The frequency spectrum is crucial for NTNs because there are numerous possible arrangements: NTN bands can overlap with terrestrial bands, be adjacent to each other or have a sufficient safety margin. As such, the test campaign should also consider some coexistence scenarios.

3GPP is working on extending the UE requirements in satellite communications with the TS 38.101-5 specification. This document extends the UE requirements specification series TS 38.101-x to include NTN aspects and covers relevant metrics:

• Transmit power
• Spectral bandwidth
• Modulation quality
• Receiver sensitivity
• Spectrum emissions (SEM, ACLR, spurious emissions)

Adequate UE testing requires a system simulator that can handle a connection that includes the entire protocol stack and allows RF testing as well as protocol testing. Figure 2 below provides an overview of this setup type. The UE is the DUT connected to the system simulator either via cable or in an OTA chamber. This system simulator performs both RF and protocol tests, where protocol tests are especially important for checking connection and mobility scenarios.

One requirement of NTN terminals is terrestrial position determination. Therefore, positioning based on GNSS signals is a mandatory NTN UE capability. The satellite station transmits its own orbital data via system information and supports the UE in correcting the time offset and Doppler shift.

In an NTN test system for conformance testing, a signal generator can simulate the GNSS signal to enable UE position determination. In addition, type approval and regulatory testing require extended spectral measurements, such as spurious emissions and RX performance tests. The 5G system simulator can include additional T&M instruments, such as signal generators and analyzers, to support these needs for additional interferer scenarios or extended spectrum analysis.

The R&S®CMX500 mobile radio tester supports fully independent LTE/FR1 and FR2 RF signaling and measurement options as well as all current and future 3GPP band combinations, with data throughput of up to 20 Gbps at the IP level. It follows the one-platform strategy of Rohde & Schwarz, offering total frequency bandwidths of up to 10 GHz and preparing users for current and future test challenges. With its intuitive web-based R&S®CMsquares graphical user interface, this one-box tester sets the new standard for testing 5G from space.