MIMO and beamforming: Papers tell the story

Article By : Martin Rowe

A white paper that came across my desk provided another look at MIMO testing for 5G. Here's a list of other papers you can download.

If you’re not currently designing multiple input multiple output (MIMO) antennas or beamforming systems but would like to get your feet wet, you can find many papers on the topic. An email from IEEE pointed me to “Antenna Array Testing – Conducted and Over the Air: The Way to 5G,” a white paper from Rohde & Schwarz. Always looking to learn more or reinforce my knowledge of 5G, I registered and downloaded the paper.

The paper provides a good overview of MIMO testing, which is a complex topic. What surprised me, however, was that IEEE would send an e-mail about a paper that also mentions specific products. I would expect that from other sources including our own TechOnline, but not from IEEE. I get daily emails from IEEE, some of which provide links to technical papers that you might expect to find at a conference or in IEEE Transactions.

Like many papers, live seminars, and webinars, “Antenna Array Testing – Conducted and Over the Air: The Way to 5G” does provide some solid technical information and I don’t mind the product pitch so long as I also get an education. After all, it’s free. While you can get the paper through IEEE, you can also download from TechOnline, I later discovered.

The 30-page paper opens with “What is 5G,” but immediately focuses on the MIMO and beamforming aspects of 5G. It doesn’t cover the 5G new radio (5GNR) specification contained in 3GPP release 15, but it doesn’t need to because MIMO and beamforming are part of earlier 3GPP releases. The introduction does explain why MIMO and beamforming are needed in 5G. It’s really a case of economics. By focusing beams directly at users, 5G will be more energy efficient. Less energy is wasted transmitting signals to unneeded places. Think of all those cell towers along the highway in sparsely populated areas. Today, much of the signal power goes into the trees or the ground. 5G antennas will direct the signals toward the highway where there are users.

Not being an antenna engineer, I had barely any idea of how phased-array antennas—the basis for beamforming—worked. This paper explained the concept in one paragraph, comparing a single dipole antenna to a line of smaller antennas. It then carried that concept to two-dimensional antenna arrays. Next, it explained the concept of analog and digital beamforming, another topic I’ve heard at conferences but needed the basics.

Phased-array antenna concept
Several short antennas all for beam steering as where a single antenna of the same length will not. Image courtesy of Rohde & Schwarz

The bulk of the paper (section 4) covers over-the-air (OTA) testing and why it’s needed. Having the brief explanation of analog and digital beamforming helped to clarify why OTA testing is required and why cabled testing won’t work for 5G.

OTA testing, based on 3GPP releases 13 and 14, describes near-field and far-field testing. Base stations may be tested in the near field, with far-field properties calculated from near-field measurements. For user equipment (UE) such as handsets, far-field measurements can be made in a small chamber because of the UE’s small size. The paper discusses test techniques for measuring field strength of individual beams.

This paper is one of several you can find that cover test and measurement for MIMO and beamforming. Others include:

Martin Rowe covers test and measurement for EDN and EE Times. Contact him at martin.rowe@AspenCore.com

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