Testing of 5G radios, their physical layer, and their protocols has moved into the mainstream, with base-station, handset, and device manufacturers all getting into the act. Standards compliance is key.
Mobile network operators (MNOs) are in the midst of launching commercial 5G networks. They need base stations and devices to deliver the promise of 5G to consumers. Bringing 5G base stations and devices to market requires conformance testing to ensure consistency with the standards and interoperability among devices.
5G adds complexity to the test process because it adds new operating bands and a more complex network architecture; the standards for conformance tests are not yet complete. Here is an overview of the conformance and device acceptance test requirements for 5G that includes key considerations for network equipment and device test engineers going through this process.
5G NR conformance tests
The 3rd Generation Project Partnership (3GPP) is the de facto standards body for commercial mobile communications. The Radio Access Network (RAN) technical specifications group defines conformance testing for devices and base stations. This group is structured into several numbered working groups who set out the 5G specifications.
The documents they produce are available online. Working groups RAN4 and RAN5 focus on conformance tests. TS 38.101 – TS 38.173 (+38.307) within the TS 38 series cover radio performance and protocol aspects. TS 38.508 – TS 38.533 provide the requirements for mobile terminal conformance testing.
Conformance testing takes place once chipsets and components are assembled into systems such as mobile devices and base stations—gNBs in the context of 5G. MNOs may also require supplemental tests referred to as carrier-acceptance tests to ensure the equipment operates to the mobile operator’s specific network needs. Because the availability of spectrum bands varies by country, this makes for a complex conformance testing environment.
Conformance testing is a challenge with any cellular technology. However, 5G takes the complexity of this part of the equipment workflow several step functions higher compared to previous generations. That added complexity comes from new operating bands and the requirement for non-standalone (NSA) network operations. The 5G New Radio (NR) standard extends commercial mobile communications for frequencies from 3 GHz through 7.125 GHz, yielding significant challenges including link-budget constraints. It also introduces new bands in the millimeter-wave (mmWave) spectrum. mmWave bands represent uncharted territory for commercial cellular technologies. The new bands bring new specifications and conformance tests for both base stations and devices, as well as added complexity for carrier aggregation.
Conformance tests development also continues. Although Release 15 is “complete”, there is still work in progress in the RAN4 and RAN5 working groups to clarify how to perform the tests and finalize the performance requirements. In addition, future releases will bring supplemental conformance tests.
Conformance tests for 5G base stations
Conformance tests for base stations are organized into chapters with Chapter 6 dealing with transmitter characteristics, Chapter 7 covering receiver characteristics, and Chapter 8 focusing on performance requirements related to physical channel management.
Testing gNB transmitters
Table 1 shows Chapter 6 covers transmit power including total radiated power (TRP) and effective isotropic radiated power (EIRP), signal quality, unwanted emissions, and intermodulation. Examples of configurations for transmitter conformance tests are provided below.
Figure 1 provides an example of a time alignment error test for a base station transmitter. This setup validates the time alignment between two antenna ports with a signal analyzer.
Figure 2 shows a configuration with an interfering signal to ensure that the intermodulation distortion is lower than mandated by the specification. Many potential interfering signals in the real world can cause the transmitter of a base station to misbehave. This test validates that the base station design will tolerate such interference once deployed in the network.
Testing gNB receivers
For receiver characteristics (Chapter 7), tests cover dynamic range, adjacent channel leakage ratio (ACLR), and sensitivity, among other parameters. Figure 3 shows a configuration for intermodulation testing. This test setup validates whether the base station receiver can distinguish desired signals from unwanted ones, and reject signals that could impact transmission.
Testing gNB physical channels
Chapter 8 covers the performance tests. These tests assess how well the base station is performing as a network element by focusing on the physical communication channels. They help determine how well the system manages the physical channels – physical uplink shared channel, control channel, and random access channel – to make sure that the base station manages the nature of the physical layer as expected.
Different base stations, different testing methods
Apart from the different types of conformance tests, distinguishing base station architectures is imperative for gNB conformance testing. This impacts how to perform the conformance tests.
Base stations are becoming increasingly integrated. Type 1-O and 2-O base station architectures such as those used in small cells limit the access to antenna ports. These architectures make connected measurements impossible at both low and mmWave frequencies. They require radiated test methods. Although not as integrated, 1-H base stations also require some over-the-air measurements. Table 2 provides the test methods for the four base station configurations specified by 3GPP.
Conformance tests for 5G devices
Conformance testing is more extensive for devices compared to base stations. To start, there are more certification bodies involved in the process than just 3GPP including the Global Certification Forum (GCF) and the PCS Type Certification Review Board (PTCRB). GCF manages the certification and validation process for conformance tests for all regions but North America. PTCRB, which is part of the Cellular Telecommunications and Internet Association (CTIA), handles this process for North America. These organizations take the 3GPP specifications and refine them down to an essential and practical test suite. They also manage the validation of test cases and certification of test laboratories that perform testing services.
The device conformance process involves test houses certified by these entities as performing the tests per the standard and validated test cases. All the test equipment and test cases must be validated by these bodies for relevant geographic regions for conformance testing of 5G devices. Conformance testing is expensive and time-consuming. If a device does not pass the tests, it is likely to miss the market window. In addition to the conformance tests, a number of MNOs require supplemental tests to ensure that the devices will not disrupt their network and provide high user experience. These tests are referred to as carrier acceptance tests and vary by network operator. Several operators have already issued their 5G acceptance plan this year including AT&T and NTT DoCoMo.
RF device conformance testing
Conformance testing for devices spans tests for RF, protocol, and the interaction of the two—radio resource management (RRM). The RF tests cover the fundamental physics of the device’s RF subsystems. These tests (Table 3) include transmitter characteristics such as transmit power, signal quality, and spectrum emissions to ensure that the device generates enough power, is a good neighbor, and provides a good transmission link. Receiver tests ensure that the device rejects unwanted signals and assess the overall system sensitivity. Device conformance tests also include interworking operation and performance tests that evaluate physical channel behavior but not virtual or logical channels.
Protocol device conformance testing
Protocol conformance tests check system operation at layer 2 and 3. These tests verify messaging and timing, among other aspects (Table 4).
RRM device conformance testing
RRM tests (Table 5) relate to activities like handovers. These tests are particularly important for 5G in the context of beam management to switch the beam from one antenna to another. 5G increases RRM complexity significantly with dual connectivity between 5G and legacy radio access networks, and standalone option 2. RRM testing ensures that the radio is told what to do, and that it completes the task when managing the radio resource.
OTA considerations for conformance testing environments
5G represents a paradigm shift for commercial mobile communications with over-the-air (OTA) testing. This statement is especially true for conformance testing. Almost all device conformance tests for previous generations have been conducted using galvanic connections. Now, all mmWave conformance tests have to be managed in OTA test setups. Placing a test point in a communication system used to be easy. This is no longer the case due to heavy integration. Cabled tests are impossible for many gNBs and 5G devices. The calibration plane must be extended inside an anechoic chamber using an antenna.
On the base station front, type 1-O and 2-O base stations can only be tested using OTA methods at the radiated interface boundary (RIB). Their integration limits access to antenna ports and connected measurements. Testing over the air is more strenuous than testing with cables because it is more complex. The test takes place in an anechoic chamber. This test environment has an impact on accuracy and power levels.
For devices, there are additional considerations. A key aspect is the test type as it impacts OTA method selection. Different types of chambers are required depending on the type of conformance tests. For example:
The DFF OTA test method provides a direct link between the device under test and the probe antenna. The IFF approach provides a shorter path length using a physical transformation from a parabolic reflector between the probe antenna and the device. You can review key concepts and definitions for 5G OTA testing in 5G OTA testing: Key concepts and definitions.
Making 5G mainstream
5G technology has come a long way since the start of research efforts in 2012. The technology has reached a critical phase in its lifecycle. Consumers in select places are experiencing 5G. More than 40 networks will launch in 2020. Release 15 reached completion in mid-2019. Although many conformance tests are being finalized, and new ones will emerge with future releases of the standard, 3GPP has completed significant work on 5G – enough to get the ball rolling.
5G mainstream is imminent. The industry’s focus has shifted to conformance and acceptance tests. These tests are mandatory and essential for scaling 5G. They also present significant technical and business challenges.
Jessy Cavazos is part of Keysight’s Industry Solutions Marketing team.