The Rice rig, known as Argos, represents the largest such array yet built and will serve as a test bed for a concept known as “Massive MIMO.”
MIMO, or “multiple-input, multiple-output,” is a wireless networking technique aimed at transferring data more efficiently by having several antennas work together to exploit a natural phenomenon that occurs when signals are reflected en route to a receiver. The phenomenon, known as multipath, can cause interference, but MIMO alters the timing of data transmissions in order to increase throughput using the reflected signals.
MIMO is already used for 4G LTE and in the latest version of Wi-Fi, called 802.11ac; but it typically involves only a handful of transmitting and receiving antennas. Massive MIMO extends this approach by using scores or even hundreds of antennas. It increases capacity further by effectively focusing signals on individual users, allowing numerous signals to be sent over the same frequency at once. Indeed, an earlier version of Argos, with 64 antennas, demonstrated that network capacity could be boosted by more than a factor of 10.
“If you have more antennas, you can serve more users,” says Lin Zhong, associate professor of computer science at Rice and the project’s co-leader. And the architecture allows it to easily scale to hundreds or even thousands of antennas, he says.
Massive MIMO requires more processing power because base stations direct radio signals more narrowly to the phones intended to receive them. This, in turn, requires extra computation to pull off. The point of the Argos test bed is to see how much benefit can be obtained in the real world. Processors distributed throughout the setup allow it to test different network configurations, including how it would work alongside other emerging classes of base stations, known as small cells, serving small areas.
“Massive MIMO is an intellectually interesting project,” says Jeff Reed, director of the wireless research center at Virginia Tech. “You want to know: how scalable is MIMO? How many antennas can you benefit from? These projects are attempting to address that.”
An alternative, or perhaps complementary, approach to an eventual 5G standard would use extremely high frequencies, around 28 gigahertz. Wavelengths at this frequency are around two orders of magnitude smaller than the frequencies that carry cellular communications today, allowing more antennas to be packed into the same space, such as within a smartphone. But since 28 gigahertz signals are easily blocked by buildings, and even foliage and rain, they’ve long been seen as unusable except in special line-of-sight applications.
But Samsung and New York University have collaborated to solve this, also by using multi-antenna arrays. They send the same signal over 64 antennas, dividing it up to speed up throughput, and dynamically changing which antennas are used and the direction the signal is sent to get around environmental blockages (see “What 5G Will Be: Crazy Fast Wireless Tested in New York City”).
Meantime, some experiments have been geared toward pushing existing 4G LTE technology further. The technology can, in theory, deliver 75 megabits per second, though it is lower in real-world situations. But some research suggests it can go faster by stitching together streams of data from several wireless channels (see “LTE Advanced Is Poised to Turbocharge Smartphone Data”).
Emerging research done on Argos and in other wireless labs will help to define a new 5G phone standard. Whatever the specifics, it’s likely to include more sharing of spectrum, more small transmitters, new protocols, and new network designs. “To introduce an entirely new wireless technology is a huge task,” Marzetta says.
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