Page 24 - Integrated Wireless Propagation Models

2 C h a p t e r O n e

on the direction of the mobile unit traveling toward or away from the transmitting
antenna of the base station. This is called the Doppler shift.
3. Due to the mobile unit always moving on the ground, the ground contour affects
the received signal at the mobile unit and makes the average signal strength vary
up or down. It is found that the average signal follows the lognormal distribution
and is called long-term fading. The signal along the radio path will be visualized
up and down at its mean value of the signal. Besides, the signal propagation
experiences a huge path loss due to the signal reception in the mobile environment.
It is a 40-dB-per-decade loss observed from this mobile environment instead of a
20-dB-per-decade loss observed from free space propagation.

1.3 Treatment of Mobile Signals
1 . To deal with the multipath (short-term) fading signals, we use diversity
techniques to reduce the fading: spacing diversity, polarization diversity,
angle diversity, field component diversity, and so on.3 Sometimes, the spread
spectrum, such as frequency hopping or CDMA, also reduces the fading.4
2. For dealing with the lognormal (long-term) fading, we first have to model the
signal receptions based on different natural environments.4•5 Based on the size
of the cell, there are three popular models: macrocell, microcell, and in-building
(picocell). In each of the model environments, we may encounter different
man-made structures, such as metropolitan, suburban, rural area, and open
land. There are different landscapes, such as water surface, foliage, hills, and so
on. Once we can model those particular environments, signal strength can be
easily predicted. Second, by using the output from a predicting model working
in an area of implementing a mobile system, we can engineer our system
equipment with a desired signal coverage in that area.
3. Once we understand the signal coverage in an area, we can use many techniques
to improve the reception of the signal. One very promising technique is MIM0.6
The techniques of improving signal reception or increasing the spectrum
efficiency are not covered in this book. However, in Sec. 2.18 the on-body model
is applied in the MIMO system.

1.4 History of Developing the Lee ModeF
In 1974, the first designing stage of the analog system AMTS (Advanced Mobile
Telephone Service) had already been completed.8 Bell Labs was starting to search for
ways to implement the AMTS system in the field. At that time, the cost of each cellsite 1
was over U.S. 1 million. How many cellsites are needed for an adequate signal-strength
$
coverage in a given deployed area? To answer this question, the Lee model was devel­
oped. At that time, all the propagation models were area-to-area models. Developing
a point-to-point model for predicting the signal strength along any mobile traveling
route in an area was a challenge. The Lee model was created for the point-to-point
prediction, and its accuracy needed to be verified before determining whether it
would be accepted. Bell Labs first suggested using printed DMA (Defense Map Agent)

'The term cellsite was coined by F. H. Blecher at Bell Labs. The cellsite is a base station with many system
control features.