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802.11g was the third modulation standard for Wireless LANs. It works in the same 2.4 GHz band as
802.11b, but operates at a maximum raw data rate of 54 Mbit/s, and 802.11g hardware is fully backwards
compatible with 802.11b hardware. Higher throughput is achieved by a more efficient modulation scheme,
and to reduce susceptibility to interference there are only three non- overlapping usable channels in the
United States with 25 MHz separation. Even with such separation, some interference due to side lobes
exists, though it is weaker than for “b” signals. Other emitters including Bluetooth devices, cordless
telephones, and microwave ovens—all potential sources of interference, also share this band.
The IEEE 802.11a band improves on the 802.11b/g standards and also provides for operations in an
alternative, possibly less congested band. The 802.11a band offers over three times the operating bandwidth
over the spectrum available in the 2.4 -GHz band with less susceptibility to interference. The modulation
scheme of 802.11a has fundamental propagation advantages when in a high multipath environment, such
as an indoor office, and the higher frequencies enable the building of smaller antennas with higher RF
system gain which mitigate the disadvantage of a higher band of operation.
The IEEE 802.11a band has 12 non-overlapping channels, 8 dedicated to indoor and 4 to point to point
applications. The 8 indoor carriers are spaced across 200 MHz in the lower spectrum (5.150–5.350 GHz)
and 4 point-to-point carriers are spaced across 100 MHz in the upper spectrum (5.725–5.825 GHz). The
channels are spaced 20 MHz apart, which allows for high bit rates per channel. Maximum raw data rate in
this band is 54 Mbit/s, which yields realistic net achievable throughput in the mid-20 Mbit/s. The effective
overall range of 802.11a is slightly less than that of 802.11b/g because 5 GHz signals are absorbed more
readily by walls and other solid objects in their path.
WiMAX technology is being developed under the IEEE 802.16 standard as wireless microwave back-haul
for Wi-Fi and other data services, but over time it is also expected to appear in adapter cards for laptops to
provide longer ranges than Wi-Fi at broadband speeds up to several megabits/s, depending upon the service
provider’s network. WiMAX may be offered as an alternative to cable TV or DSL broadband in areas where
such services are not available. WiMAX could also emerge as a competitor 4G cellular phone services.
For airport wireless services, WiMAX provides the flexibility of greater range and the use of licensed bands
for higher levels of security and resistance to interference, plus the potential for client-side hardware with
the economies of 802.11a/b/g devices.
The IEEE has issued standards in the 802.16 series for both fixed and mobile WiMAX. The initial standard,
802.16d (also known as 802.16-2004), is the fixed wireless version. The mobile version is called 802.16e-
2005. Most commercial interest is in the 802.16d and 16e standards, since the lower frequencies used in
these variants suffer less from inherent signal attenuation and will have better range and in-building
WiMAX is based on orthogonal frequency-division multiplexing (OFDM). Most applications are expected
to offer 5 to 20 MHz of bandwidth with data rates of 2- to 10-Mbit/s and operating ranges up to several
miles over non-line-of-sight (NLOS) paths. The fixed version supports both time-division duplex (TDD)
and frequency-division duplex (FDD) modes. WiMAX also can use most advanced antenna technologies,
like multiple-input multiple-output (MIMO), beam forming, and adaptive scanning to improve data rate,
range, and general reliability in mixed multipath conditions. The mobile version is expected to use 5- or
10-MHz channels and TDD only in a half-duplex mode.
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