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Shankar WL040/Bidgolio-Vol I WL040-Sample.cls June 19, 2003 16:49 Char Count= 0
132 PROPAGATIONCHARACTERISTICS OFWIRELESSCHANNELSUser # 1
(a) User #^2 (b)User # 3External networkBase stationUser has more mobilityBase stationFigure 11: (a) A line-of-sight optical wireless system. (b) An indoor optical wireless system using diffuse light.createN(N>1) multiple (diverse) independent (at least
uncorrelated) versions of the signal. If the probability
that signal goes below a threshold isp, the probability
that that all these multiple versions of the signals will
go below that thresholdsimultaneouslyispN.In other
words, if one were to choose the strongest signal from
this set, the probability that this chosen signal is above the
threshold is (1âpN), leading to fading mitigation lessen-
ing the chances of outage. The signal processing in terms
of choosing the strongest signal is known as selection di-
versity. Other forms of diversity, such as processing, max-
imal ratio combining, and equal gain combining, are also
available to combine the signals from the diverse branches
(Stein, 1987).
Diversity is implemented in the spatial domain (spatial
diversity) by having multiple transmitters or multiple re-
ceivers or in the frequency domain (frequency diversity)
by transmitting the same information over multiple fre-
quency bands, and so on. Other forms of diversity, such
as angle diversity, polarization diversity, time diversity,
and rake receiver, are also available (Shankar, 2001; Stein,
1987).
Although the techniques described here constitute ex-
amples of microscopic diversity to mitigate effects of
short-term fading, long-term fading or shadowing can
be mitigated through macroscopic diversity techniques.
Choosing the best base station that provides the strongest
signal to serve a subscriber is a form of macroscopic
diversity. It is also possible to combine the signals from
different base stations to reduce the effects of long-term
fading further.OPTICAL WIRELESS SYSTEMS
Optical wireless networks offer a simple solution to prob-
lems of electromagnetic interference in traditional in-
door wireless systems (ElBatt and Izadpanah, 2001; IEEE,1998; Heatley & Neild, 1999; Pohl, Jungnickel, & von
Helmolt, 2001). They are also not subject to any license
regulation requirements. They are simple to implement so
as to cover a large room or area without any signal leaking
through the walls or partitions. This confinement of the
signal also offers a form of security from snooping from
the adjoining rooms. Because most of the systems depend
on LOS, fading problems associated with multipath will
be less.
The optical wireless systems fall in two groups, one
using LOS systems and the other using diffuse systems.
An indoor system based on LOS is shown in Figure 11a.
The signal from the base station reaches the users via
direct paths. These LOS conditions exist in the confined
space that may also provide a limited mobility and roam-
ing. It is possible to create greater mobility using a diffuse
system in which multiple paths exist between the user and
the base station. This is shown in Figure 11b. The draw-
back of the existence of multiple paths is that it leads to
the fading condition described earlier in connection with
wireless systems in the 900- to 1,800-MHz band.
The indoor optical wireless systems use light emitting
diodes and laser diodes operating around 900 nm (900â
1,100). The receiver may be a photodiode (p-i-n) or an
avalanche photodiode (APD). These systems also have
a few major drawbacks. The interference from ambient
light is a major problem. The second is the issue of eye
safety, which may make it necessary to operate at lower
power levels. Because of these factors, the data rate is a
few hundred Mbit/s with a range of operation of a few
meters (IEEE, 1998).CONCLUDING REMARKS
This chapter presented brief overview of the topics of
interest in wireless systems. Additional reading mate-
rial may be found inIEEE Transactions on Wireless