April 2, 2012

Serving More Clients with 802.11ac Multi-User MIMO

Research firm NPD In-Stat expects the 802.11ac "Very High Throughput in 5GHz" standard to trigger an explosion in Wi-Fi-enabled cellular devices, with shipments exceeding 650 million by 2015. In the same year, 802.11ac is also projected to reach one out of every four notebooks. And attach rates for 802.11ac are predicted to skyrocket in consumer electronics devices - especially those involving video delivery.

How will enterprise wireless LANs avoid being overrun by this swarm of thirsty new Wi-Fi clients? Fortunately, the answer lies in 802.11ac itself: a new technology known as multi-user multiple-input multiple-output (MU-MIMO).

What 802.11ac Brings to the Table

As described in the March 19 Webtorials TechNote, "802.11ac and 802.11ad: Faster and Fatter Wi-Fi," the new 11ac standard promises to push data over the air at rates up to 7Gbps. Multi-gigabit rates are achieved through better coding and error correction, wider channel widths (jumping from 40MHz to 160MHz) and more spatial streams (eight instead of four).

Not only does 11ac increase top wireless speeds for such demanding applications as uncompressed HD video, but it also improves rate-versus-range performance and link reliability when operating at a distance. In other words, Wi-Fi clients will be able to stray farther from WLAN access points (APs) while happily slurping up rich media.

These improvements also grow WLAN capacity. Wi-Fi devices share the air; the aggregate capacity of each channel gets distributed across connected clients that must all wait their turn to transmit. 802.11ac can therefore enable higher-density deployments. For example, if 10 clients each use 30Mbps from a 802.11n AP that delivers 300Mbps throughput using three spatial streams, 20 clients could easily share an 11ac AP supporting four spatial streams. And so on.

Reducing Competition for Shared Air

But competition for shared air is a tricky thing. 802.11n achieved major increases in capacity and throughput by tapping multiple-input multiple-output (MIMO), a technique that exploits multipath technology to simultaneously send and receive data over several spatial streams. For example, a 3x3 11n AP (one with three transmitting antennas and three receiving antennas) might divide a single user's data into three spatial streams. Each stream is transmitted through a different antenna, propagating over the air along a different path. When all streams reach the client, the data are recombined - in this case, tripling throughput.

802.11ac super-charges MIMO by introducing a multi-user (MU) mode. With MU-MIMO, APs can send different users' data along diverse spatial streams, bouncing along through the air across up to eight paths simultaneously. Where a single client with one or two antennas would not be able to fully utilize eight spatial streams offered by an 8x8 11ac AP, several Wi-Fi clients will now be able to share this larger pool of streams and antennas.

According to Anita Pandey, vice president of product marketing and vertical business development at Wi-Fi supplier Meru Networks, "Meru has seen the BYOD [bring your own device] trend explode across enterprise customers in the past year. [Using MU-MIMO] to transmit up to eight single-stream clients - smartphones are the best example - widely increases network capacity."

MU-MIMO also builds upon transmit beamforming, a technique used to focus RF energy in a given direction to improve delivery to individual clients. Matthew Gast, director of product management at Wi-Fi company Aerohive Networks, said MU-MIMO gains big advantages by using beamforming to reduce channel contention. "It's likely to work better in the downstream direction because APs will need to keep track of stations and be aware of [spatial] overlap," he said.

Enabling the Wi-Fi Explosion

In networking, the goal is to allocate available capacity in ways that maximize utilization and minimize interference. However, not every 11ac deployment should necessarily strive to use 160MHz-wide channels. Depending on the client and application mix, allocating spectrum to a larger number of narrower channels might actually accommodate more users.

Deriving the optimal channel design for each deployment is going to take some effort. But, no matter how you slice it, 802.11ac will be an important enabler for "everything Wi-Fi" and a critical tool for high-density enterprise WLANs.


If you're looking for tech detail or tests showing the benefits of MU-MIMO, I recommend this illustrated research by NTT Network Innovation Labs:


A few more resources for those wanting to learn more about 11ac --

Broadcom's 5G WiFi resource website: http://www.5gwifi.org/

IEEE article: http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=05751287

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