An IEEE standard that lets a wireless LAN achieve data rates as high as 54 Mbps. Thus, the standard can support many broadband applications, letting wireless users access the most demanding applications.

To dramatically increase throughput, 802.11a proponents had to solve a major challenge of indoor radio frequency: delay spread in the current 2.4-GHz, single-carrier, delay-spread system.

The 802.11a standard cleverly solves this through a modulation technique called Coded Orthogonal Frequency Division Multiplexing (COFDM), which has found earlier application in European digital TV and audio transmission.

COFDM breaks the ceiling of the data bit rate by

1. Ssending data in a massively parallel fashion and

2. slowing the symbol rate down so each symbol transmission is much longer than the typical delay spread

A guard interval (sometimes called a cyclic prefix) is inserted at the beginning of the symbol transmission to let all delayed signals "settle" before the baseband processor demodulates the data.

COFDM slows the symbol rate while packing many bits in each symbol transmission, making the symbol rate substantially slower than the data bit rate. It maps the data signal to be transmitted into several lower-speed signals, or subcarriers, which then are modulated individually and transmitted in parallel.

COFDM also uses coding to allow for recovery of errors and to add more interference rejection by spreading information across all carriers. Interferers may jam individual carriers and the data will still get through. The COFDM physical layer allows greater scalability in delivering data over the wireless channel. The larger-spectrum allocation at 5 GHz can, therefore, be exploited for greater data rates.

From 802.11a speeds wireless LANs, Network World Tech Update, 01/29/01.

Also see 802.11b.

Additional Resources

Network World Wireless/Mobile Research Center.