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What to expect from 11ac's next big deal: multiuser MIMO

John Cox | July 8, 2014
The big deal about 802.11ac, or "gigabit Wi-Fi," is the promise of much higher speeds for Wi-Fi clients compared to the maximums under 802.11n today. But 11ac is actually several big deals, including something called multiuser MIMO [my-moe], which is part of the "Wave 2" 11ac features expected later this year.

Multiuser MIMO is arriving at a critical time: The number of Wi-Fi clients is exploding, thanks to smartphones and tablets, the end-user desire to avoid cellular charges, and the end-user demand for video, music, and constant social networking and updating. Not everyone near a Wi-Fi access point may have a Windows or Mac laptop, but nearly everyone is likely to have one or two or more mobile devices with Wi-Fi.

"As clients increase, access points have to cope with that," says Quantenna's Amir. "With single-user MIMO, you end up in situations where you saturate the access point, because the access point has to rotate through, say 100 clients. You run out of time, resulting in low-bandwidth service. Multiuser MIMO can improve this dramatically."

Most smartphones and even tablets with 11ac radios are likely to be single-stream clients (with just one antenna), with a maximum possible data rate of about 433Mbps. Four-stream 11ac access points with transmit beamforming are ideally suited to this kind of client population.

"The one really big question about multiuser MIMO is, how well does it work?" says Craig Mathias, Network World blogger and principal with Farpoint Group, a wireless consultancy. "And the answer is, right now, no one knows. I've only seen it in a lab. There are no production deployments. So it's unclear what benefit you will get."

Wi-Fi networking is rife with variables, he points out. While an 11ac access point with 80-MHz channels has a theoretical capacity of 1.3Gbps, "in the real world, that will not happen," he says. "Benchmark tests from the vendors show performance is all over the place. Radio performance is highly variable, and firmware, chips, board layout, drivers, antennas, prevailing radio conditions all have an effect. You cannot predict what performance you will achieve."

"The main advice I give to clients is, wait and see, and evaluate new [11ac] products as they appear," Mathias says. "It will take time to see how MU-MIMO will work and, most of all, how it will work in your specific environment."


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