IEEE 802.11 for Outdoor Environment

As was mentioned at the beginning of this chapter, a growing number of
cable operators are considering using IEEE 802.11 equipment to provide
data services in specific rural areas, where there is a disperse population
around an urban nucleus or where the estimated penetration factor is low,
making unprofitable the laying of optical fiber or the use of LMDS type systems.
Under these conditions, the existence of APs and PCMCIA-type cards,
and therefore, low-cost user equipment, along with freedom from operational
license applications, make the use of the IEEE 802.11 infrastructure
very attractive. To this end, manufacturers such as Avaya offer complete
solutions permitting the widespread deployment of wireless IP in outdoor
environments.
This deployment makes use of the same equipment as that for indoors,
with some additions at the software level. In order to provide better coverage,
the manufacturers offer antennas with higher gain than the indoor ones have,
especially on the user side. Figure 4.32 shows two typical outdoor antennas,
an omnidirectional one and a directional one. The gain for the omnidirectional antenna is 7 dBi, and the gain for the
directional one is 14 dBi. The first is associated with the AP, and the second
with the user terminal.
To calculate the maximum distances for placing the user terminals
from the APs, the following expression for propagation losses can be used:
Attenuation (dB) for GHz band dB ( ) km 2.4 =100 +20log d (4.1)
Note that an open space propagation model has been assumed. It is also
necessary to ensure line of sight between transmitter and receiver along the
whole propagation path, particularly for the Fresnel zone, whose radius in
the 2.4-GHz band can be obtained from
Fresnel Radius (meters) ( ) km km =34⋅ + 812 2 . d d /. (4.2)
Table 4.8 shows typical distances separating APs, with omnidirectional
antennas, and user equipment for differing regulations and user antenna
gains.
Finally, it should be commented that the manufactures provide,
through the software configuration of the outdoor equipment, the possibility
of limiting the bandwidth assigned to each user, fixing a parameter known as
throttle. Typical values of throttle are 64, 128, 256, 384, and 512 Kbps.