The emergence of 5G mobile communications is set to revolutionize everything we know about the design, testing, and operation of cellular systems. The industry goal to deploy 5G in the 2020 timeframe demands mmWave over-the-air (OTA) test solutions and requirements in little more than half the time taken to develop the basic 4G MIMO OTA test methods we have today.
If you highlight anything from this blog post, know this:
We are going to have to move all of our testing to radiated, not just some of it like we do today, and that's a big deal.
First, a bit of background on the move from cabled to radiated testing, and then I’ll discuss the three main areas of testing that we're going to have to deal with: RF test, demodulation test, and radio resource management.
Millimeter-wave devices with massive antenna arrays cannot be tested using cables because there will be no possibility to add connectors for every antenna element. The dynamic (active) nature of antenna arrays means it isn’t possible to extrapolate end-to-end performance from measurements of individual antenna elements. So yes, for testing 5G, it really is time to throw away the cables…whether we want to or not!
A new radio design starts with the reality of the deployment environment, in this case a mmWave one. How this behaves isn’t a committee decision, it’s just the laws of physics. Next, we model the radio channel and once we have a model, we can design a new radio specification to fit the model. Then, we design products to meet the new radio specifications, and finally we test those products against our starting assumptions in the model.
If we have got it right—in other words, if the model is sufficiently overlapped with reality—then products that pass the tests should work when they are deployed in the real environment. That's the theory.
This process works well at low frequencies. For mmWave, however, there is a big step up as the difference in the propagation conditions is enormous.
Now let’s look at the categories of radio requirements that we're going to have to measure—that is, what we measure and the environments we measure them in.
For RF, it’s about what is already familiar—power signal quality, sensitivity—and those are all measured in an ideal line-of-sight channel.
With regards to demodulation, throughput tests will be done in non-ideal (faded) conditions as was the case for LTE MIMO OTA. There we had 2D spatial channels, but for mmWave, the requirement will be 3D spatial channels because the 2D assumptions at low frequencies are no longer accurate enough.
Radio resource management (RRM) requirements are about signal acquisition and channel-state information (CSI) reporting, signal tracking, handover, etc. That environment is even more complicated because now we’ll have a dynamic multi-signal 3D environment unlike the static geometry we have for the demodulation tests.
Opportunities and Challenges
The benefits of 5G and mmWave have been well publicized. There's a lot of spectrum that will allow higher network capacity and data rates, and we can exploit the spatial domain and get better efficiencies. However, testing all of this has to be done over the air and that presents a number of challenges that we have to solve if we're going to have satisfied 5G customers.
We know that we're going to have to use active antennas on the devices in base stations, and those are hard to deal with.
We know that spatial tests are slower than cabled, so you can expect long test times.
We've got the whole issue of head, hand, body blocking on devices—it's something that isn’t being considered for release-15 within 3GPP but will still impact customer experience.
We know that OTA testing requires large chambers and is expensive.
We know OTA accuracy is not as good as cabled testing—we're going to have to get used to that.
Channel models for demodulation and RRM tests haven’t been agreed upon yet, which is impacting agreement on baseline test methods for demodulation and RRM.
There's a paradigm shift going on because of mmWave. We used to work below 6 GHz and the question we asked at < 6 GHz frequencies was, "How good is my signal?" That question led to the development of non-spatial conducted requirements. The question now for mmWave is, "Where is my signal?" That's going to lead to the development of 3D spatial requirements and OTA testing. This is a fundamental shift in the industry.
It’s going to be a tall order…testing 5G mmWave devices.
Keysight is committed to getting our customers on the fastest path to 5G. Stay tuned as Keysight continues to roll out 5G testing methodologies and system solutions. Meanwhile, explore the 5G resources currently available.