802.11ac: A Survival Guide

Chapter 4. Beamforming in 802.11ac

You can’t depend on your network when your transmission is out of focus.

—Mark Twain, as network administrator

In wired networking, the biggest innovation of the last two decades was the introduction of Ethernet switching, which dramatically increased network capacity by moving from relatively large collision domains (a multi-port hub) to minimum-sized collision domains (a single port). Wireless LANs have offered great benefits to network users, primarily in the form of mobility, but in return have expanded the collision domain from an Ethernet switch port to the coverage area of an access point. Beamforming and its application in the form of multi-user MIMO in 802.11ac have the potential to change how networks are built and increase capacity well beyond the headline rate of the network equipment. In essence, multi-user MIMO works by taking advantage of beamforming to send frames to spatially diverse locations at the same time, building the first standardized version of an 802.11 “switch.” Beamforming is not inherently more useful in one direction of the link, but typically enterprise access points are less resource-constrained, have access to more memory, power (both computational and electrical), and have more antennas. Therefore, beamforming in the downlink direction from the AP to the client was a ripe area for innovation in the 802.11ac standard.

Beamforming Basics

Traditionally, access points have been equipped with omnidirectional antennas, ...