Page 26 - Integrated Wireless Propagation Models

4 C h a p t e r O n e

When a mobile unit travels along a street, the mobile signal received can be pre­
dicted. This book discusses the tools for predicting the mean signal level within differ­
ent areas, such as in rural, suburban, and urban environments, and with many different
characters, including the details of the terrain, building clutter and the extent of foliage,
and over the water along the radio path. Some factors are more important than others.
The prediction is only a statistical estimate, as the measured data can vary about the
predicted mean as the mobile moves around within a small area.
The mobile signal is a fading signal. We can separate the fading signal into two parts.
The first part is that the mean signal level is varied following lognormal statistics with a
standard deviation, depending on the nature of the local environment. The variation of the
local mean signal level is called long-term (lognormal) fading. The second part is that, on
top of the slow variation of the local mean signal, there is a rapid and deep variation
known as a short-term (or Rayleigh) fading, caused by multipath propagation in the imme­
diate vicinity of the mobile. This follows Rayleigh statistics over fairly short distances.
There are other important characteristics of the signal channel, such as noise and interfer­
ence. When a wideband channel is used, issues of frequency-selective fading and lSI arise.
When a mobile communication system was first deployed in the 1980s, a powerful
transmitter was used on a high site to cover a large cell with few signal channels. Since
the natural radio spectrum is limited, we need to balance the requirements of area cov­
erage and system capacity during system planning. The AMPS system lowered the
transmitter power and applied the frequency reuse scheme to effectively increase both
the area coverage and the system capacity at the same time. The prediction tools can
help engineers optimize system performance.
The provision of a wide area coverage will always involve the development of an
infrastructure of radios and/ or line links to connect a number of base stations via one
or more control points so that the nearest base station to any mobile can be used to send
messages to and receive from that mobile. Structuring a national network entirely using
radio links is clearly complicated and costly. An alternative is to use the public tele­
phone network (PTN), which is already available. If the PTN is used as the backbone
infrastructure to connect the base stations together, then there are many connection
points between the base stations and the fixed network. Each cellsite (base station) has
to cover only a small area. This in itself is a major step toward achieving much greater
frequency reuse in a cellular system with many cellsites. Moreover, in principle, a
mobile within the coverage area of any cellsite needs to connect itself to the facilities of
the national and international telephone networks.
The potential of this deployment strategy was realized from the start, but before
any systems could be implemented some major issues in the field had to be solved from
the signal strength prediction tools, and the tools have to be updated continuously as
new issues arise.

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1.6 M o b i l e Ra i o Signal: Fa i n g Signal
1.6.1 Conditions of Mobile Signal Reception

1.6 1 . 1 In a Standstill Condition
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The received mobile signal strength at one location can be strong, but just moving a
wavelength away from its location, the signal can be very weak. This is because the