802.11n: A Survival Guide

Chapter 3. Channels, Framing, and Coding

I like big channels, I cannot lie…

—“Baby Got Bandwidth” (with apologies to Sir Mix-a-Lot)

This chapter moves from the rarefied theoretical discussion of how MIMO works into the details of how the 802.11n PHY interfaces with the physical medium and delves into the details of the techniques that are used to increase speeds.

Channel Structure and Layout

The structure of a channel in 802.11n is basically the same as 802.11a/g. Both are based on OFDM, and therefore, both divide up the radio channel into a number of subcarriers that are packed closely together and precisely enough that they are orthogonal to each other. 802.11n provides several minor improvements to the structure of the channel. Like 802.11a/g, it uses OFDM and re-uses the same modulations and numbering scheme.^[12]

Channel Structure

802.11n offers two features to increase the utilization of the radio spectrum. 802.11n retains the common 20 MHz channel width used by prior 802.11 standards. Within the 20 MHz channel, however, 802.11n improves spectral efficiency by adding subcarriers that were unused in 802.11a/g, as shown in Figure 3-1. Even though 802.11n adds four data subcarriers, increasing throughput by about 8%, it does not need to add any pilot subcarriers. Pilot subcarriers are used to provide channel measurement and calibration, and are a form of overhead. Just as MIMO increases the efficiency of a data transmission, it increases the efficiency of the pilot carrier operation. ...