OFDM opens the door to wireless broadband networks


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Spread spectrum modulation has been the basis for many IEEE 802.11- and 802.11b- compliant wireless LANs. Through the use of frequency hopping and direct sequence, these wireless LANs provide data rates from 1 to 11 Mbps. The demand for wireless broadband LANs and metropolitan-area networks, though, is pushing the envelope on these relatively low data rates. Also, there's not much hope that spread spectrum will ever deliver much higher data rates because of inefficient use of bandwidth.

For higher speed applications, you should consider the newer and more efficient wireless network products based on OFDM (orthogonal frequency division multiplexing). I feel that OFDM will revolutionize the wireless network industry.

How does OFDM improve the situation?

An OFDM-based system divides a high-speed information signal into multiple lower-speed "sub-signals" that the system transmits simultaneously at different frequencies. For example, the IEEE 802.11a standard specifies an OFDM Physical Layer that splits the information signal across 52 separate sub-carriers. Four of these are pilot sub-carriers that the system uses as a reference to disregard frequency or phase shifts of the signal during transmission. The remaining 48 sub-carriers provide separate wireless "pathways" for sending the information in a parallel fashion.

The division of the information flow and use of the pilot sub-carriers make OFDM signals much more resilient to interfering RF signals and multi-path propagation. In addition, the parallel-form of transmission over multiple sub-carriers enables IEEE 802.11a-based wireless LANs to operate at data rates up to 54 Mbps. These higher data rates and robust communications enable the implementation of wireless LANs supporting higher-speed applications operating over wider areas, where the environment is somewhat hostile toward radio transmissions.

Okay, what's the catch?

An issue with OFDM is that there's no single standard. In addition to IEEE 802.11a, HiperLAN/2 implements a similar, but different version of OFDM. To make matters worse, there are several proprietary implementations of OFDM as companies attempt to outshine each other with better efficiency and more features. To counter this problem, the OFDM Forum is in the process of establishing a common, global OFDM standard for wireless transmission.

The OFDM Forum is currently exploring specific wireless industry issues dealing with interoperability between market segments, harmonized standards, and spectrum allocation. The eventual standard will solve interoperability issues and provide economies of scale when manufacturing applicable products. Of course this results in lower costs to the end users.

Very few companies today have OFDM-based wireless LAN products on the market. Wi-LAN offers the I.Will product line, an OFDM-based system that operates at 30 Mbps.

You can expect many more products based on OFDM to begin appearing on the market in early 2001. If you're planning the implementation of a wireless LAN needing to support higher speed applications or where interference may be a significant problem, consider using I.Will or waiting until there are more systems to choose from.

Stay tuned; next time we'll take a closer look at options for enabling wireless Internet access within metropolitan areas.

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