I was recently reminded of my over-the-air (OTA) experience with 5G channel sounding. It was like black magic at the time, and now, as it turns out, is vitally important for the success of 5G.
At Keysight, we realized early on that making measurements at millimeter-wave (mmWave) frequencies would be difficult. What we didn’t realize was that there would be so little information in the standards regarding how to test this far along in the development of 5G. The first 5G New Radio (NR) draft specification was released in December 2017. It documents the 3GPP physical layer, but absent are the specifications for the mmWave test environment. That means the chances of getting lost in your 5G OTA measurements goes up dramatically. And that’s where I want to help.
Let’s review some of the things you should know about for 5G OTA measurements.
Are OTA tests really required?
Here’s a question you are probably asking yourselves right about now: is OTA testing really needed? And here’s my answer: ABSOLUTELY!
Current sub 6 GHz RF performance tests are mostly done using cables. That changes when you move to massive MIMO in sub 6 GHz or mmWave frequencies. At mmWave frequencies, beamforming antenna technologies are used to overcome higher path loss and signal propagation issues and to take advantage of spatial selectivity by using narrow signal beams. Phased-array antennas, such as those shown in figure 1, are typically highly integrated devices, with antenna elements bonded directly to ICs, making it difficult, if not impossible, to connect and probe. OTA enables test, but it introduces a more challenging ‘air interface’ between the component or device and the base station wherethe imperfections in the air interface need to be accounted for during test.
Figure 1. Example mmWave antenna arrays
You Have To Test OTA, But Now What?
Okay, so you need to test OTA, but what kind of tests? The types of measurements for 5G products vary throughout the development lifecycle and are different for an UE versus a base station. During design and development, RF parametric tests such as transmitted power, transmit signal quality, and spurious emissions are done for radiated transmitter tests. Base stations add tests such as occupied bandwidth and adjacent channel leakage ratio (ACLR), to name a few. Beam-pattern measurements in 2D and 3D and beamsteering or null-steering performance tests are also done during R&D. Conformance testing is also done to ensure the device meets 3GPP minimum requirements. These can be grouped into RF, demodulation, radio resource management (RRM), and signaling tests. This white paper provides an explanation of these tests: OTA Test for Millimeter-Wave 5G NR Devices and Systems.
OTA tests are typically conducted in the radiated near-field or radiated far-field region of the antenna system under test. Measurements in the far-field are conceptually the simplest type of OTA measurement and an approved method identified by 3GPP. A typical far-field anechoic chamber is shown in figure 2. With the appropriate probing and test equipment, 2D and 3D beam patterns and RF parametric tests can be performed. The challenge is selecting a reasonable chamber that won’t take up your entire lab space. The length of a far-field chamber is roughly determined by 2D2/λ, where D is the diameter of the device being tested. With this in mind, a 15-cm device at 28 GHz would therefore require a 4.2-m chamber as shown in figure 3. These chambers will be large and quite expensive.
Near/far boundary (m)
Figure 3. How far is far-field?
An alternative for 5G RF tests that is being used by market leaders and is now being considered by 3GPP is the compact antenna test range (CATR). In Figure 4, a CATR uses a reflector so that it looks like the waveform is coming from a long way away. This seems to be a very promising direction for 5G OTA testing, and 5G market leaders are seeing this as a comprehensive and accurate test method. However, that’s not the case for RRM test where there is no clear solution because of the many open issues due to the dynamic, multi-signal 3D environment with signal tracking and handovers.
Figure 4. Compact antenna test range (CATR)
If We Could Only Tell the Future
Yes, I know life would be a whole lot easier if we knew what to expect, but unfortunately the jury is still out on this one. What I can tell you about OTA is that progress is being made.
There are many really smart PhDs working on solutions, but it’s going to take time to get these testing methods into the standards. In the meantime, I solutions coming from market leaders working directly with test vendors to enable OTA tests of the first 5G devices and basestations. These OTA test solutions are the ones to watch, as they will pave the way for the standards.
If you are looking for more information on 5G OTA testing, then I highly recommend watching Malcolm Robertson’s video: Testing 5G: OTA and the Connectorless World or reading the : OTA Test for millimeter-Wave 5G NR Devices and Systems white paper. Looking forward, I’ll keep you posted on important 5G topics and developments in future blogs.