Column Control DTX

Testing 5G: Time to Throw Away the Cables Microwave Journal

Article Reprints

Back in August 2010, Microwave Journal published a series of articles called, “Masters of MIMO,” opening with an overview article from me called, “The MIMO Antenna: Unseen, Unloved, Untested!” I predicted then that MIMO over the air (OTA) testing was the biggest challenge I had seen in 20 years of standards development.

Looking back over the intervening six years, I can now see that I underestimated that challenge as the work within CTIA and 3GPP to develop MIMO OTA test methods are still in the final stages of validation. In the meantime, the sophistication of MIMO standards has grown considerably from the original LTE Release 8 through LTE-Advanced Pro in Release 14.

Yet, as I write, antenna designers still don’t have basic 2×2 MIMO performance requirements for the receivers of their Release 8 capable devices.

Looking ahead, the focus of this article is describing the test challenges of the next generation, the fifth generation. Having underestimated the difficulty in standardizing basic MIMO OTA testing for 4G (LTE), it is humbling to be making predictions this early for the next generation.

But here is my best shot: The emergence of the fifth generation of mobile communications is set to revolutionize everything we know about the design, testing and operation of cellular systems.

The primary reason for this is the assumption that to deliver on many key 5G objectives, a new air interface is required, one that will operate in the millimeter wave (mmWave) frequency bands (i.e., 28 GHz and above), with channel bandwidths around 1 GHz and higher.

To overcome the radio propagation path losses at these operating frequencies, it is assumed that both the base station and mobile devices will need to incorporate medium or large-scale antenna arrays (sometimes referred to as massive MIMO on the base station side) to maintain a usable link budget.

The result will be a 5G air interface that relies on beam-steered antennas at both ends of the link, in what will be a sparse and highly dynamic 3D narrow beam propagation environment.

For 4G, MIMO OTA testing was an obvious and useful evolution from traditional cabled testing, but MIMO OTA was never essential for 4G, since MIMO-enabled devices have been shipping for years. The untested static antennas have at least functioned, even if we don’t know how well.

However, mmWave devices with massive antenna arrays cannot be tested using cables, because there will be no possibility to add connectors for every antenna element. In addition, the dynamic (active) nature of antenna arrays means it is not 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!

Compared to the 4G MIMO OTA test challenge, with its basic 2×2 transmission mode and static 2D geometry, it seems safe to predict that what lies ahead for testing 5G will be a revolution compared to the mere evolution that we saw in the transition from 3G to 4G. To add one final opening point, there is so little time!

The industry goal to deploy 5G in the 2020 timeframe demands mmWave 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 — even without performance requirements.

MOVE TO RADIATED TESTING

Each area has its own specific needs, in terms of cost and sophistication. Design verification and production testing will be handled outside of standards bodies, while RF/baseband conformance and radiated performance test methods and requirements will be specified by 3GPP.

In addition to the UE MIMO OTA work that started in 2009, 3GPP also commenced radiated test standards in 2011 for base station active antenna array systems (AAS).1 Traditionally, as with mobile devices, all base station RF requirements such as output power, sensitivity, blocking/spurious and error vector magnitude (EVM) have been measured at the temporary antenna connectors; the actual base station antenna impact is not considered.

However, with the introduction in Release 13 of full dimension MIMO (FD-MIMO), also known as elevation beam forming, it is now accepted that the active nature of base station antennas means cabled testing without the antennas is no longer sufficient. This led 3GPP to develop the first radiated test methods for base stations.

Currently, only total radiated power (TRP) and total radiated sensitivity (TRS) tests at boresight are defined, but more will be developed in the Release 14 evolved AAS (eAAS) work item. Today’s AAS scope characterizes the antenna in a static line-of-sight channel using one of four defined test methods, all simpler than UE MIMO OTA test methods using spatial fields.

However, the AAS work does point to what will happen with basic 5G device testing when operating frequencies approach mmWave, when there will be multiple antenna elements and no temporary antenna connectors.

Tests which were straightforward using cables become much more involved when carried out OTA. Take blocking, for instance: The requirements for blocking were derived from 2D spatial system simulations, and the base station blocking levels were statistically derived from summing the power from three spatially separate mobiles.

In the conducted domain, this just looks like an omnidirectional power to be added to the wanted signal. However, if we move back to the radiated domain, how should the blocking signal be constructed?

Recreating the system level parameters implies spatially separate blocking signals interacting with the directional base station antenna, so an exhaustive test of all spatial combinations and frequencies would extend today’s already long tests beyond the pale. So there are difficult choices yet to be made about how AAS and ultimately mmWave OTA tests are to be developed.

 

×

Please have a salesperson contact me.

*Indicates required field

Preferred method of communication? *Required Field
Preferred method of communication? Change email?
Preferred method of communication?

By clicking the button, you are providing Keysight with your personal data. See the Keysight Privacy Statement for information on how we use this data.

Thank you.

A sales representative will contact you soon.

Column Control DTX