December 12, 2000, 2:57 PM —
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 HREF="http://www.itworld.com/Net/1748/ITW000829Geir/">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,
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 HREF=http://www.ofdm-forum.com/>OFDM Forum is in the process of
establishing a common, global OFDM standard for wireless transmission.