February 13, 2001, 3:14 PM — Wireless vendors face the challenge of supporting increasingly bandwidthhungry applications, such as voice over IP, streaming video and videoconferencing.
How will they do it? Later this year, they will release wireless devices based on the IEEE 802.11a standard. Approved in 1999, the standard lets a wireless LAN achieve data rates as high as 54M bit/sec. 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. They had to develop a way to resolve the problem of delay spread in the current 2.4-GHz, single-carrier, delay-spread system.
Delay spread is caused by the echoing of transmitted radio frequency. As these signals proceed to a certain point, such as a wireless antenna, they often bounce and echo off objects, walls, furniture and floors, and arrive at the antenna at different times due to the different path lengths. A baseband processor, or equalizer, is required to "unravel" the divergent radio frequency signals. The delay spread must be less than the symbol rate, or the rate at which data is encoded for transmission. If not, some of the delayed signal spreads into the next symbol transmission. This can put a ceiling on the maximum bit rate that can be sustained.
With current bit-rate technology, this ceiling tends to be around 10M to 20M bit/sec. The 802.11a standard cleverly solves this challenge through an innovative 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) sending 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.
