802.11n: A Survival Guide

Chapter 2. MIMO and the 802.11n PHY

Row, row, row your boat,

Gently down the stream…

—Row, Row, Row Your Boat (traditional)

Increasing the speed of a network can be done in two ways. First, protocol designers can try to increase the raw speed as measured by the transmission rate, which might be termed the “go faster” approach. Increasing transmission rates has been the primary tool for the dramatic increase in wireless LAN speeds from 1 Mbps in the original 802.11 standard to 54 Mbps in 802.11a and 802.11g. Second, protocol designers can increase the speed perceived by users by increasing the efficiency of the protocol, to transmit more bits within a given period of time, an approach which might be called the “efficiency” approach. Although 802.11n uses both techniques, most of the gains come from dramatic increases in data rate. At the core of the “go faster” approach, 802.11n uses MIMO. Early 802.11n products transmitted at a data rate of 150 Mbps, and the standard laid out a clear path to data rates of up to 600 Mbps. These speeds are achievable only through the application of MIMO.

The Big Idea: MIMO and Data Streams

Before MIMO, 802.11 used a single data stream. A transmitter used one antenna, and a receiver used one antenna.^[6] The transmission link in pre-802.11n devices can be described in terms of its two components. It was called Single-Input because the receiver used one antenna, and Single-Output because the transmitter used only one antenna. Taken together, the communication ...