802.11ac: A Survival Guide

Chapter 2. The PHY

There’s fifty-seven channels and no free airtime...

—Bruce Springsteen, singing “57 Channels (And Nothin’ On)” as a wireless administrator

Large improvements in 802.11 speeds typically have resulted from the introduction of a big new idea. Introducing multi-carrier transmission with OFDM increased speeds in the transition from 802.11b to 802.11a/g. Creating MIMO systems did the same in the move from 802.11a/g to 802.11n. 802.11ac, however, does not introduce a new way of transmitting data over the air. Though they are both extended beyond what was introduced in 802.11n, the techniques used to put bits on the air in 802.11ac will be familiar to anybody familiar with 802.11n: MIMO and wide channels. Raw speed increases in the PHY come from three sources: a higher number of MIMO streams, wider channels, and a finer modulation that can pack more bits into each unit of airtime.

Extended MIMO Operations

One of the major techniques used by 802.11ac to increase throughput is the extension of MIMO from a system that supports four spatial streams to one that supports eight. As with all other MIMO systems, each spatial stream requires its own transmission system; building an 802.11ac AP that supports eight spatial streams would therefore require an antenna array with eight independent radio chains and antennas. Taken as a single protocol feature, extending to eight spatial streams alone doubles throughput over an equivalent 802.11n system by doing something equivalent to doubling ...