What is NTN NB-IoT and in what situations does it offer a better alternative than other IoT technologies?
Answer: Network operators are expanding cellular services, initially aimed at consumers with smartphones, to businesses with large numbers of Internet of Things (IoT) and machine-type communication (MTC) devices. The demand for service continuity is expected to drive the evolution and expansion of networks into non-traditional areas. Non-terrestrial networks (NTNs) are becoming a major focus of research and industry as the world moves toward 5G-Advanced and, eventually, sixth-generation (6G) systems. The main advantage of NTN technology is the scalability, continuity, and ubiquity of the service, given that 7% of the world's population still lacks terrestrial cellular coverage.

Satellite communications can play a crucial role in improving communication infrastructure and bridging the digital divide. Typically, a satellite-based architecture using geosynchronous orbit (GSO), geostationary Earth orbit (GEO), medium Earth orbit (MEO), and low Earth orbit (LEO) systems provides coverage at altitudes between 400 km and 36,000 km. However, there are trade-offs among the different satellite systems in terms of performance and deployment costs.
The NTN (Network Telecommunication Network) is divided into several radio access technologies: NR-NTN, which is based on 5G New Radio (NR), and IoT-NTN, which can be based on Cat-M1 or NB-IoT. Initial deployments are primarily based on NB-IoT, offering flexibility in repurposing existing operator assets such as spectrum, core network, and access network. IoT-NTN can be used to supplement coverage where the cost of deploying a terrestrial network (TN) is prohibitive. The sectors that will benefit most from this technology are mission-critical services, public utilities, automotive, and agriculture.

How does NTN NB-IoT fit with developing standards?
Answer: The 3GPP (3rd Generation Partnership Project) consortium, responsible for developing mobile telecommunications standards, began working on enabling New Radio (NR) and IoT services via satellite in 2017. Study elements for NTNs were included in 3GPP Releases 15 and 16, and Release 17 contains the first complete set of 3GPP-compliant IoT-NTN specifications. Releases 18 and 19 include working elements (WIs) that offer enhancements for IoT-NTN and NR-NTN.
The inclusion of NTNs in the 3GPP standards is important because it provides device and chipset manufacturers with the assurance they need to incorporate satellite compatibility into their products and also benefit from economies of scale. Although some device manufacturers have long offered compatibility with GEO satellite services, they have done so on a small scale, limiting themselves to specific spectral bands and proprietary technology, which translates into high costs for customers.

What are the main challenges facing NTN devices?
Answer: There are several issues to consider when making IoT work over satellites.
Link Budget. The significant distance between the user equipment (UE) and the base station presents a challenge. The signal must travel downlink (DL) from the satellite gateway on the ground to the satellite in space using the feed link, and uplink (UL) from the satellite payload in space to the user equipment and vice versa. This results in a poor link budget, which impacts performance and causes long round-trip times (RTT). For GEO satellites, the link budget is critical, and IoT-NTN offers features such as data repeater on both UL and DL that can help maintain connectivity in areas with marginal coverage and increase demodulation and coverage performance. The received reference signal power (RSRP) could be as low as -140 dBm in IoT-NTN, which is unusual in terrestrial deployments. Test equipment with an advanced RF front-end is needed to reliably test this low-signal connectivity.
Latency. An RTT as high as 500 ms for GEO satellites will not be appropriate for latency-sensitive applications. Furthermore, there may be instances where the base station resides on the satellite payload to minimize latency and allow for greater control over mobility. The extended RTT is also problematic for certain control loops in a 3GPP network, as it can cause deadlocks because Hybrid Automatic Repeat Request (HARQ) acknowledgments are not received within the specified window. Additionally, UE channel feedback may be unusable by the time it reaches the ground base station.
Handovers. NTN cells are very large and move rapidly from a LEO perspective. Designing the network to limit signal overload and enable handover is challenging, not only depending on signal strength but also on the user's location within the cell.
Interference. Feed links from the ground gateway to the satellite and service links from the satellite to the user may use spectrum owned by mobile network operators or satellite constellation operators. It is important to carefully manage the spectrum to proactively avoid any interference between TN and NTN deployments.
Doppler shift. For satellites in non-geostationary orbit, rapid movement relative to Earth is an added complication. A LEO satellite at an altitude of 600 km, for example, travels at about 7.5 km/s and will orbit the Earth in 90 minutes. This causes Doppler frequency shifts that can reach 24 ppm.
Time drift. When a satellite moves closer to or farther from the user equipment, the reference time between the user equipment and the gNB base station shifts. This poses a challenge for synchronization and initial time advance. Furthermore, measurements of neighboring cells become more difficult, as the timing of the server cell and the neighboring cell can diverge when they are on different satellites.

What role do test platforms play in the growth of NTN IoT-NB?
Answer: Testing can be divided into three aspects: field testing, satellite testing, and UE testing.
To conduct field testing, it is important to properly design, integrate, and deploy a terrestrial network. Special care must be taken when deploying the spectrum and conducting coexistence tests between terrestrial, non-terrestrial, and traditional spectrum-using services. Anritsu's remote spectrum monitoring tools or handheld spectrum analyzers (MS2090A) are useful for this purpose. For throughput, latency, and packet loss testing, Anritsu's MT1000A network performance tester offers a simple way to test different satellite configurations.
Large-scale LEO satellites will be equipped with antennas that will need to be characterized using a combination of vector network analyzers, signal generators, and analyzers. In addition, some deployments may include regenerative architecture, i.e., a base station on the satellite. There could be different base station deployment configurations depending on the combinations of distributed components. There could be a Distributed Unit (DU)/Radio Unit (RU) on the satellite and a Centralized Unit (CU) on the ground; a gNB RU/DU/CU all in the sky; or a RU/DU/CU/Part of the Core Network all in the sky. It may also be necessary to test the capacity of the base station components, in addition to performance testing of these different combinations. A UE simulator and a SA/SG BTS are important tools for this aspect of testing.
UE testing can be divided into over-the-air (OTA) testing, radio frequency (RF) conformance testing, protocol conformance testing (PCT), RF/protocol R&D level testing, and carrier conformance testing.

What data does a conformance test collect and analyze?
Answer: Conformance tests are created to align with 3GPP requirements or operator requirements. Figure 1 shows an example of a protocol conformance test (PCT) system configured to test user equipment (UE) against protocol specifications defined in 3GPP, such as 36.521.

Figure 1: Example of a PCT system for IoT-NTN, testing user equipment against the protocol specifications defined in 3GPP.
The protocol conformance suite consists of testing different areas of the protocol stack that have been introduced from NB-IoT to NB-IoT NTN. Almost all layers of the protocol stack have been affected by the introduction of IoT-NTN. These testing procedures are standardized in 3GPP document 36.521. The testing areas include HARQ processes, new System Information Block (SIB) parameters, positioning reports, timers, and handovers.

How does this type of testing help engineers evaluate devices under real-world conditions?
Answer: It is important to thoroughly test devices with network simulators that have correctly implemented the network protocols, parameters, and conditions before they are released to the market. Often, a terrestrial or non-terrestrial network may not exist for testing devices because the necessary features/technology have not been enabled, or it may not be possible to control the live network to generate corner cases or adverse scenarios. It is critical to simulate a realistic radio environment for terrestrial and satellite base stations and test devices accordingly; troubleshooting problems that are only detected after a device has been commercially launched can be prohibitively expensive.

What is the process for adopting a conformity test?
Answer: Adopting a conformity test for test and measurement equipment involves several steps, ensuring that the equipment meets industry standards and operates reliably. Here is an outline of the typical process:
A typical process begins with understanding the relevant standards and requirements. Test specifications and procedures include 36.521-4 (TRx measurements), 36.521-3 (Performance/RRM measurements), and 36.523 (Protocol measurements).
Next, a comprehensive set of test cases must be developed, based on the conformance requirements, covering all necessary protocol/RF functionalities and scenarios. The test cases must be detailed, specifying the expected results and the criteria for passing or failing each one.
The test cases can then be implemented on test equipment, ensuring that each can be run automatically. Logging and reporting functionalities are incorporated to capture the detailed results of each run.
The accuracy and reliability of the implemented test cases are established through internal testing in collaboration with a chipset vendor. Once sufficient confidence has been built, a test case can be submitted to an accredited certification laboratory for evaluation. The laboratory will assess the equipment's conformity with the relevant standards and protocols across different bands, as required by the Global Certification Forum (GCF) or the PCS Type Certification Review Board (PTCRB), through testing across different bands and with multiple devices. The GCF and PTCRB have their own criteria for requiring original equipment manufacturers (OEMs) to include their respective tests as part of their testing qualification.
The process described here is equally applicable to operator conformity testing, with the exception that the tests are typically performed at the operator's facilities for validation and certification. Once the certified test equipment has been delivered to customers or testing laboratories, there will, of course, be a need for ongoing technical support, updates, and maintenance to address any problems that arise or changes in specifications.

What is Anritsu doing to foster innovation in this area?
Answer: IoT-NTN is an evolving technology, and as new features are introduced in future 3GPP releases, it's important to have access to the devices that enabled the first features. Partnering with diverse chipset vendors is also crucial, as not all features will be available on every chipset simultaneously.
Anritsu has worked with leading chipset and device OEM vendors, such as Sony Altair, MediaTek, Qualcomm, and Samsung, to collaborate on verifying conformance tests as soon as key features stabilize, before submitting protocol/RF conformance test results to accredited laboratories. Anritsu has also partnered with satellite network operators (SNOs) like Skylo to validate their testing requirements on Anritsu platforms.