The What, When, Where, and Why of 5G NR OTA Testing
The most common Q&A topics at our events are about over-the-air testing, commonly referred to as OTA. There are big changes ahead for 5G NR, and successful OTA testing will be a critical requirement for 5G NR base stations (gNBs) and user equipment (UEs). Let’s look at the four W's of 5G NR OTA.
Why OTA testing?
The short answer is that radiated tests are required to meet the 3GPP (Third Generation Partnership Project) conformance requirements in frequency range 2 (FR2: 24.25 to 52.6 GHz) and for some configurations in frequency range 1 (FR1: 450 MHz to 6 GHz).
TS 38.141-1: Part 1: Conducted conformance testing for FR1
TS 38.141-2: Part 2:
Radiated conformance testing for specific base station configurations in FR1 and FR2
TS 38.521-1: Range 1 Standalone – FR1 Conducted tests
TS 38.521-2: Range 2 Standalone – FR2 Radiated tests
TS 38.521-3: Range 1 & 2 Interworking operation with other ratios – FR1 Conducted and
FR2 Radiated
There are several reasons for these radiated test requirements. First, 5G NR designs use multi-element active antennas in both sub-6 GHz frequency bands (FR1) and mmWave frequencies (FR2). Due to the active nature of these antenna arrays, it is not possible to extract end-to-end performance from individual antenna element measurements. These antenna arrays will use narrow beams in FR2 that add a spatial 3-dimensional requirement that need to be tested over-the-air. Second, antenna arrays designed for mmWave frequencies will be highly integrated with the amplifier IC’s in the radio system. This results in no probing points for conducted measurements, making OTA the only possible approach to testing.
What is OTA Testing?
Radiated requirements in the 3GPP specifications are referred to as OTA requirements. OTA testing is not new and has been used in previous cellular generations to validate everything from radiated sensitivity, to MIMO data throughput. For 5G NR, especially for FR2, OTA will be required for the four types of radio-related conformance tests:
- RF Performance – a minimum level of signal quality
- Demodulation – data throughput performance
- RRM (radio resource management) – initial access, handover, and mobility
- Signaling – upper layer procedures
A typical OTA test solution includes a chamber enclosure, probe antennas (and link antennas for tests requiring an active call), and test equipment to generate and analyze the radiated signals in a spatial setting. The chamber provides a shielded environment so that radiated signals of known power and direction can be generated and measured in a controlled environment.
When and Where is OTA testing needed?
All UEs and base stations must pass required conformance tests before being released into the market. OTA testing is required for most FR2 conformance test cases, and for some FR1 conformance tests. Independent conformance test services are most often provided by independent and certified test labs. Test systems and the specific test cases used to perform conformance tests must be validated by independent parties to ensure that the conformance test is consistent with the standards and repeatable.
Conformance test is a major milestone in the develop of UEs and base stations, however, they are expensive and can take weeks to complete. Therefore, UE and base station manufacturers will do pre-conformance testing to ensure a “Pass” before spending time and expense on official conformance tests.
In addition, device and base station manufacturers will use OTA testing at other stages of the workflow - earlier in design to test a wider set of parameters to ensure quality and sufficient margins, and later during manufacturing to ensure a UE or base station meets its specificationand is not significantly different than the articles tested in conformance.
Once a UE passes conformance tests, operators will require a device acceptance test to ensure the device has sufficient performance to meet the goals of the operator.
Specific measurements and methodologies for measuring mmWave devices over-the-air are still being developed in RAN5 with a target completion date of May 2019 for Release-15. Subsequent releases of the standard will add to the tests that must be done.
Many Challenges Remain
5G NR introduces many exciting possibilities with mmWave operating bands. Access to more spectrum enables higher data rates and higher capacity networks. However, with the required OTA testing, there are many new challenges that will fundamentally change the way 5G NR products are tested and validated.
- Active antennas using steerable narrow beams in base stations and devices are more difficult to validate than fixed antennas
- OTA testing can introduce more measurement uncertainty than that with cabled tests
- OTA test generally takes more time than cabled tests due to the number of measurements that must be made and the requirement in many cases to change the orientation of the DUT to accommodate 3D spatial requirements
- OTA testing solutions are often more expensive than those for cabled tests
- The standards bodies are still converging on common RF channel models for both demodulation and RMM conformance tests which impacts the time to converge on standard test methods
Testing 5G NR is a paradigm shift from 4G testing. The use of narrow beams at mmWave frequencies leads to new 3D spatial requirements that can only be validated using OTA testing. There is still work to define test methods, but despite the unknows, Keysight is committed to getting our customers on the fastest path to 5G. Here are a couple of resources that will help: