Page 269 - Antennas for Base Stations in Wireless Communications

242 Chapter Seven

the WLAN is used to achieve the connections among different networks.
For instance, a wireless Ethernet bridge connects the devices in a wired
Ethernet network to a wireless LAN. The APs in a WLAN can be set up
as repeaters when connecting all of the access points within a network
through wires is difficult. Therefore, the WLAN can serve as a wireless
distribution system if required.
The WLANs operate in the unlicensed Industrial, Scientific, and
Medical (ISM) bands, namely, the 2.4-GHz band with a frequency
range of 2.4–2.485 GHz and the 5-GHz bands with frequency ranges of
5.150–5.350 GHz, 5.470–5.725 GHz, and 5.725–5.850 GHz. The channel
bandwidth within each band varies from 5 MHz to 20 MHz. Generally,
the data rate of a wireless communication link increases with a larger
bandwidth, but decreases with mobility. Table 7.1 shows the various
IEEE 802.11 standards. In general, the actual data rate in a typical
office environment is about half of its maximum. Furthermore, the data
of a conventional wireless communication link is much less than that
of the multiple-input-multiple-output (MIMO) setup based on IEEE
802.11n. The data rate of 11-Mbps for IEEE 802.11b remains relatively
constant for a distance of up to 50 meters, whereas the data rate of 54-
Mbps for IEEE 802.11a declines linearly to about 11 Mbps at a distance
of 50 meters. One of the main challenges in deploying and operating
WLAN stems from the fact that other wireless systems, such as Wireless
Personal Area Networks (WPANs), Bluetooth (IEEE 802.15.1), ZigBee
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(IEEE 802.15.4), and microwave ovens share the same unlicensed fre-
quency bands, which can lead to interference among electrical devices
and systems.
In addition, the standardization bodies in individual countries, for
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instance, the Federal Communications Commission (FCC) in the United
States and the European Telecommunications Standards Institute
4
(ETSI) in the European Union, regulate electromagnetic radiation. The
effective isotropic radiated power (EIRP) defined by FCC Rules part
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15.247 for an omnidirectional antenna of less than 6-dBi gain is 1 W.
The IEEE Standard C95.1-1991 recommends that the power density for
human exposure to radio frequency (RF) and microwave electromagnetic
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field emissions be 1–10 mW/cm from 1–10 GHz. As the lower frequency
is more penetrative in nature, a lower radiation restriction is enforced.

TABLE 7.1 IEEE 802.11 Standard Family
Frequency Maximum data Radius Radius
IEEE 802.11 bands, GHz rate, Mbps Modulation indoor, m outdoor, m
a 5 54 OFDM < 35 < 120
b 2.4 11 DSSS < 38 < 140
g 2.4 54 OFDM < 38 < 140
n 2.4, 5 600 OFDM < 70 < 250