802.11a speeds wireless LANs

Wireless vendors face the challenge of supporting increasingly bandwidth-hungry 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.

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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.

The COFDM technique would be technically difficult and very costly without concurrent advances in the CMOS semiconductor fabrication process. Current-generation deep submicron CMOS processes have allowed for a great increase in the complexity of the baseband processors to modulate/demodulate COFDM. The fundamental advances also lower power consumption, decrease chip size and cut development costs.

Wireless vendors now have a goal to boost wireless throughput beyond 100M bit/sec. While the 802.11a standard currently tops out at 54M bit/sec in 20-MHz channels, several firms are developing and proposing high-rate extensions to the 802.11a standard. These proposals generally envision at least doubling throughput to anywhere from 108M to 155M bit/sec.

Unfortunately, developers cannot further increase the index or complexity of the modulation on each subcarrier beyond the maximum symbol rate for wireless LAN transmissions because of the amount of noise allowed. Instead, they plan to increase the bandwidth of the COFDM channel, increasing and reallocating the individual carriers, and propose different coding rate schemes.

The broad application of wireless LAN technology has arrived. Thanks to the 802. 11a standard and its advances in wireless modulation techniques, users soon will be able to access broadband video, audio and data as easily as from their wired clients. They will have more freedom to use next-generation applications for increased efficiency, productivity and mobility.

Anderson is an engineering manager at Cisco. He can be reached at freda@cisco.com.