revise Wireless Internet and Mobile Computing interoperability and performance from Wireless Internet And Mobile Computing book by Yu-Kwong Ricky Kwok(Colorado State University) andVincent K. N. Lau(The Hong Kong University of Science and Technology)
CHAPTER 1
THE MOBILE RADIO PROPAGATION
CHANNEL
The beaten path is the safest. (Via trita est tutissima.)
-Latin proverb
1.1 INTRODUCTION
A communication system consists of a transmitter and a receiver connected by a
channel. The channel is a black box representation of the actual physical medium
where a signal at the channel input will produce a corresponding channel output
after suffering from various distortions. Examples of a physical medium include
telephone wire, coaxial cable, radio frequency (RF), and optical fiber. Different
meda have different signal propagation characteristics and require different designs.
In this chapter we focus on understanding the signal propagation characteristics of
wireless channels.
Wireless Internet and Mobile Computing. By Yu-Kwong Ricky Kwok and Vincent K. N. Lau 1
Copyright @ 2007 John Wiley & Sons, Inc.
The simplest wireless channel is the additive white Gaussian Noise (AWGN)
channel where the output signal from the channel is given by:
where z ( t ) is the channel input and Z ( t ) is the white Gaussian channel noise. In this
channel, the received signal composes of an undistorted transmit signal contaminated
with channeI noise. In Chapter 2, we discuss the design and performance of basic
digital communication systems using AWGN channel as a simple example. Note that
AWGN channel is quite an accurate model for deep space communications between
earth station and geostationary satellites or spacecrafts.
On the other hand, for terrestrial wireless communications, the channel model is
much more complicated due to the time-varying nature as well as the multipath nature
of the wireless channel [2711. For instance, multipath refers to the situation that there
are more than one propagation path between a transmitter and a receiver. The received
signals from the multipaths are superimposed with each other. Depending on the phase
difference (or path difference) of the multipaths, the superposition may be constructive
or destructive. When the superposition is constructive (path difference around nX
for some integer n where X is the carrier wavelength), the received signal will be
strong. On the other hand, when the superposition is destructive (path difference
around nX/2 for some integer n), the received signal will be quite weak. This is
illustrated in Figure 1.1
LOS: Line-Of-Sight NLOS: Non-Line-Of-Sight
Figure 1.1 Illustration of multipath in terrestrial wireless communications
Typically, the signal envelop as a result of multipath superposition can fluctuate
over 30dB or more over a span of a few X in distance. Suppose the carrier frequency
is 3GHz, the wavelength X is around lOcm and the large fluctuation of signal strength
(35 dB) can be observed with a span of -30cm which is quite short.
On the other hand, time variation of the wireless channel results from the mobility
of the transmitter, the receiver or the environment (e.g., scatterer). Hence, the number
of multipaths, the strength of multpaths as well as the delays of the multipaths can be
time varying.
To capture different time scale of the distortions introduced by the wireless channels,
we introduce a three-level model. The first level model is called the large scale
path loss model. The path loss model focuses on the study of the long-term or large
scale variations on the average received signal strength due to the variation of distance
from the transmitter and the receiver. The second level model is called the shadowing
model which focuses on the study of the medium term variation on the medium-term
average (over hundreds of A) received signal strength due to local changes of terrain
features or man-made obstacles (such as blockage or coverage shadows). The third
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