Massive MIMO

The big picture
NIST CTL is developing measurement methods for massive MIMO (multiple-input, multiple-output) antennas capable of supporting the gigabit data rates of 5G wireless. With today’s macrocell base station antennas, a single mobile user occupies a slice of frequency throughout the base station antenna’s 120-degree span. MIMO, in contrast, is a vehicle for spatial multiplexing. It directs narrower beams between wireless users and base stations, which lets more than one user converse with the same base station on a particular frequency. While 4G LTE employs MIMO, that standard harnesses just two to four elements per antenna. Massive MIMO will feature hundreds or even more elements, each element steering ultra-narrow, millimeter-wave “pencil-beams” toward a receiving antenna. The complexity of massive MIMO arrays calls for new approaches to antenna metrology and characterization.

What we do
Imagine a smartphone containing dozens of antennas, each communicating with one or more small base stations. As you walk around a pedestrian mall, the device in your pocket fires narrow beams of high-frequency radio waves at select base stations, which shoot the same “pencil-beams” right back. In addition, using a technique called nullforming to minimize interference, your smartphone and others nearby simultaneously cancel out their transmissions.

This is the future of MIMO: massive arrays generating ultra-narrow beam patterns in frequencies in the tens or hundreds of gigahertz, each burst steered toward its intended recipient like a radiofrequency laser beam. This focusing of radiofrequency energy will minimize interference and enable more users in a given frequency band, boost signal-to-noise ratios and enhance data rates. To no small degree, the mastery of massive MIMO will be central to achieving the leaps in data capacity that next-generation 5G wireless needs to deliver.

There’s much to be done before this vision for massive MIMO – and, by extension, the vision for 5G wireless – can be realized. CTL’s efforts to solve some of the key measurement and characterization challenges surrounding massive MIMO will be indispensable to the technology’s ultimate viability.

Among CTL’s areas of focus with massive MIMO include:

• Devising new techniques that employ statistical methods to characterizing the shape and behavior of active beams (necessary because today’s approach with limited MIMO arrays, which involves calibrating each path with laboratory-grade instrumentation, doesn’t scale).
• Extending massive MIMO antenna metrology to millimeter-wave bands and extending real-world measurement data into high-frequency MIMO channels.