Chapter 4 Wireless Technology
159
orthogonal
frequency-
division
multiplexing
(OFDM)
a transmission tech-
nique that transmits
data over different
channels within an
assigned frequency
range. Each channel
is broadcast separately
and is referred to as
multiplexed. It can
achieve data rates as
high as 54 Mbps.
Orthogonal Frequency-Division Multiplexing
The orthogonal frequency-division multiplexing (OFDM) transmission
technique is used with wireless devices that use the 5-GHz radio band and can
achieve a data rate as high as 54 Mbps. The OFDM transmission technique divides the
allotted frequency into channels similar to frequency hopping and direct sequencing.
Orthogonal means separate side by side over a range of values. In wireless application,
the term orthogonal means there are multiple separate radio channels side by side
within an assigned radio band. Frequency division means to divide the assigned
frequency range into multiple, narrow sub-frequencies. Multiplexing is an electronics
term, which means to combine content from different sources and transmit them
collectively over a single, common carrier. By combining the three terms, OFDM
means to communicate wireless data over several different channels within an
assigned frequency range. However, in OFDM, each channel is broadcast separately
and is referred to as multiplexed.
OFDM is used in conjunction with the Unlicensed National Information
Infrastructure (U-NII) frequency ranges. The FCC divided the 5-GHz radio
frequency into three, 20-MHz channels and classifi ed them as the Unlicensed
National Information Infrastructure (U-NII). The three classifi cations are U-NII-1,
U-NII-2, and U-NII-3. See Figure 4-11. Each of the three U-NII classifi cations has
a frequency range of 100 MHz. Using the OFDM transmission technique, each
100-MHz frequency range is broken into four separate 20-MHz channels. Each
of the 20-MHz channels is further divided into fi fty-two, 300-kHz sub-channels.
Forty-eight of the fi fty-two sub-channels are used to transmit data, and the
remaining four are used for error correction. It is the large number of channels
that provide the high data rates. Additionally, communication is not affected as
adversely by interference as it is with the other techniques mentioned. If one or
two sub-channels are affected, the overall data rate is not affected.
The FCC U-NII classifi cations are based on the frequency range of the broadcast,
the allowable maximum amount of power allotted to the broadcast, and the location of
where the device may be used. There is no maximum distance measurement in feet or
meters for the different classifi cations. The maximum distances are controlled by the
maximum amount of output wattage that can be generated by the devices. The actual
range varies considerably due to infl uences such as building structures and materials,
the electromagnetic environment, and atmospheric conditions. Use the chart in
Figure 4-12 to get a relative idea of expected maximum distances.
Figure 4-10.
In direct sequencing,
a 2.4-GHz frequency
band is divided into
eleven overlapping
83-MHz channels.
Each 83-MHz
channel is further
divided into three
22-MHz channels.
Direct Sequencing Spread Spectrum (DSSS)
11 overlapping
83-MHz channels
Each 83-MHz channel
is divided into
3 22-MHz channels
2.4-GHz
frequency
band