Image Processing Reference
In-Depth Information
24.3 Wireless LAN Technology and Wave Propagation
In this section we discuss some basic characteristics of WLANs technology and present some of
the fundamental wave propagation effects. In Sections . and ., we discuss physical layer and
MAC/link layer approaches to overcome or at least relax some of the problems created by the
propagation effects.
24.3.1 Wireless LANs
Wireless LANs are designed for packet-switched communications over short distances (up to a few
hundred meters) and with moderate to high bitrates. As an example, the IEEE . WLAN standard
offers bitrates between and Mbps [,]. Wireless LANs usually use either infrared or radio fre-
quencies. In the latter case license-free bands like the . GHz ISM band (industrial, scientific, and
medical band) are particularly attractive, since the only restriction in using this band is a transmit
power limit. On the other hand, since anyone can use these bands, several systems have to coexist.
Radio waves below GHz propagate through walls and can be reflected on several types of surfaces,
depending on both frequency and material. Thus with radio frequencies non-line-of-sight com-
munications is possible. In contrast, systems based on infrared only allow for line-of-sight (LOS)
communications over a short distance. An example is the IrDA system [].
Wireless LANs can be roughly subdivided into ad hoc networks [] and infrastructure-based net-
works. In the latter case some centralized facilities like APs or base stations are responsible for tasks
like radio resource management, forwarding data to distant stations, mobility management, and so
on. In general, stations cannot communicate without the help of the infrastructure. In ad hoc net-
works,thereisnoprescribedinfrastructureandthestationshavetoorganizenetworkoperationby
themselves.
Infrastructure-based WLANs offer some advantages for industrial applications. Many industrial
communication systems already have an asymmetric structure which can be naturally accommo-
dated in infrastructure-based systems. The often used master-slave communication scheme serves
as an example. Furthermore, the opportunity to offload certain protocol processing tasks to the
infrastructure allows to keep mobile stations more simple and to make efficient centralized decisions.
As compared to other wireless technologies like cellular systems and cordless telephony, WLAN
technologies seem to offer the best compromise between data rate, geographical coverage, and
license-free/independent operation.
24.3.2 Wave Propagation Effects
In the wireless channel, waves propagate through the air, which is an unguided medium. he wireless
channel characteristics are significantly different from those of guided media like cables and fibers
andcreateuniquechallengesforcommunicationprotocols.
A transmitted waveform is subjected to phenomena like path loss, attenuation, reflection, diffrac-
tion, scattering, adjacent- and co-channel interference, thermal or man-made noise, and finally to
imperfections in the transmitter and receiver circuitry [,].
The path loss characterizes the loss in signal power when increasing the distance between a trans-
mitter
T
and a receiver
R
. In general, the mean received power level
E
[
P
R
x
]
can be represented as
the product of the transmit power
P
T
x
and the mean path loss
E
[
PL
]
:
E
[
P
R
x
]=
P
T
x
⋅
E
[
PL
]
