11a and 802.11j: 5-GHz OFDM PHY

a and 802 11j 5 GHz OFDM PHY

Go West, young man, and grow up with the country!

John B.L. Soule[*]

[*] This is often attributed to Horace Greeley, who did much to popularize it.

The 2.4 GHz ISM bands are crowded, and often choked with non-802.11 traffic. In an attempt to develop higher data rates, the 802.11 working group standardized a physical layer using unlicensed spectrum around 5 GHz. Large tracts of undeveloped spectrum were available for unlicensed use, and far fewer devices have used the spectrum.

802.11a was standardized in 1999, but it took a significant amount of time to bring products to market. 802.11a hardware finally hit the market in late 2001. Today, the best known manufacturer of 802.11a chipsets is Atheros Communications. In form and function, they look similar to any other wireless LAN card. Most are CardBus cards, although some additional form factors are used more widely now. The range of very high data rates is quite short, but generally, 802.11a has a comparable data rate to 802.11b at comparable distance.

Initially, 802.11a was designed for the 5-GHz Unlicensed National Information Infrastructure (U-NII) bands in the United States. With the success of 802.11a in the American market, other regulators developed rules to allow 802.11a. The 802.11 working group supported these efforts through 802.11h for Europe, which was discussed in Chapter 8, and 802.11j for Japan.

This chapter begins with a qualitative introduction to the basis of OFDM. When all the mathematical formalism is stripped away, OFDM is a method for chopping a large frequency channel into a number of subchannels. The subchannels are then used in parallel for higher throughput. I anticipate that many readers will skip the first section, either because they are already familiar with OFDM or are interested only in how the frequency bands are used and how the PCLP wraps up frames for transmission.