Although the world's companies have created the latest wireless infrastructure, known as 4G LTE, a new device with 96 antennas is taking shape in a Rice University laboratory in Texas, which could help define the next generation of wireless technology.
The "Rice rig", known as Argos, represents the largest system built and will serve as a testbed for a concept known as "massive MIMO".
MIMO, or "multiple-input multiple-output," is a wireless networking technique designed to transfer data more efficiently by using multiple antennas to exploit a natural phenomenon that occurs when signals are reflected on their way to a receiver. This phenomenon, known as multipath, can cause interference, but MIMO alters the timing of data transmissions to increase throughput by utilizing these reflected signals.
MIMO is already used for 4G LTE and in the latest version of Wi-Fi technology, called 802.11ac, but it typically involves only a handful of transmitting and receiving antennas. Massive MIMO extends this approach by using multiple, or even hundreds, of antennas. It further increases capacity by effectively focusing signals on individual users, allowing numerous signals to be sent over the same frequency simultaneously. In fact, an earlier version of Argos, with 64 antennas, demonstrated that network capacity could be boosted by more than 10 times.
"If you have more antennas, you can serve more users," says Lin Zhong, an associate professor of computer science at Rice University and co-leader of the project. And the architecture allows you to easily scale to hundreds or even thousands of antennas, he says.
Massive MIMO requires more processing power because base stations send radio signals more directly to the phones trying to receive them. This, in turn, requires additional computing power. The goal of the Argos testbed is to see how much benefit can be gained in the real world. Processors distributed throughout the facility allow testing of different network configurations, including how to work alongside another emerging class of base stations, known as small cells, which serve 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 benefit? These projects are trying to answer that."
An alternative, or perhaps a complementary approach, to a potential 5G standard would use extremely high frequencies, around 28 gigahertz. Wavelengths at this frequency are about two orders of magnitude smaller than the frequencies used by cellular communications today, allowing more antennas to be placed in the same space, as in a smartphone. But because 28-gigahertz signals are easily blocked by buildings, and even vegetation and rain, they have always been seen as unusable except in specialized line-of-sight applications.
However, Samsung and New York University have collaborated to address this issue, also by using multi-antenna arrays. These send the same signal through 64 antennas, splitting it to boost performance, and dynamically changing which antennas are being used and the direction the signal is sent to avoid environmental interference.
Meanwhile, some experiments have focused on pushing the limits of existing 4G LTE technology. The technology can, in theory, deliver 75 megabits per second, although this is lower in real-world situations. However, some studies suggest that it can go faster by combining data streams from multiple wireless channels.
New research conducted at Argos and other wireless labs will help define a new 5G telephony standard. Whatever the details, it will likely include greater spectrum sharing, smaller transmitters, new protocols, and new network designs. "Introducing a new wireless technology is a huge undertaking," says Marzetta.
By David Talbot, MIT
