Page 23 - Integrated Wireless Propagation Models

CHAPTER 1

Introduction to Modeling

Mobile Signals in

Wireless Communications

1.1 Why Write This Book?
Almost 50 years ago, mobile communications research was born at Bell Laboratories; 1
the major task at that time was to focus on understanding the natural phenomenon of a
mobile signal's propagation along a transmission path. This is similar to diagnosing a
disease in medicine. Then the treatment process follows. If the diagnosis is accurate,
then the treatment for the disease will usually be more effective. Therefore, in this book,
we try to put together diagnostic methods and model the natural phenomenon to give
the reader an overall view of all the tools that have been used to diagnose signal propa­
gation. Afterward, signal strength can be predicted from many different models, which
are based on the natural phenomenon.

1.2 Differences Between Free Space Communications a n d Mobile
o
i
Comm u n i c ations n Propagati n
There are three major phenomena created y mobile signals: 2
b
1 . Because the mobile unit is always surrounded by human-made structures and
buildings or is inside the buildings, many multi path waves may be created by
the reflections of the transmitting signal, causing the resultant strength of the
signal to be different at different locations. Also, the reflected signals do not
arrive at the mobile unit simultaneously. Thus, the time delay between the first
wave and the last wave causes the distortion among the received digital
symbols. This is called intersymbol interference (lSI) in the data transmission.
2. Because the mobile unit moves from one location to another location along the
travel path, the signal strength received at the mobile unit from a radio path at
each location is different. These variations in signal strength vary along the
radio paths and are called short-term signal fading. The fading rate can be fast
or slow, depending on the mobile speed. Also due to the mobile speed, the
frequency of the signal carrier will be shifted. It can be higher or lower, depending

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