Page 36 - Integrated Wireless Propagation Models

14 C h a p t e r 0 n e

1.6. 7 Short-Term and Long-Term Fa i ng Signal
d
Short-term fading, or simply fading, is used to describe the rapid fluctuation of the
amplitude of a radio signal over a short period of time or travel distance. Long-term
fading is obtained by averaging a long piece of a short-term fading signal. Short-term
fading is usually observed from the measurement data caused by multipath waves in
the field, while long-term fading smoothes out the real-time signal variation in the
short-term fading and preserves the slow variation of the fading caused by the contours
of terrain, hills, forests, and buildings. Therefore, the long-term fading data are created
1
from the short-term fading data by averaging them to achieve a local mean. 8 The statis­
tics of long-term fading provides a way to compute an estimate of the path loss as a
function of distance and other factors.

1. 7 Co-Channel I n terference Created from the Frequency Reuse Scheme
The frequency reuse concept was first proposed by Doug Ring at Bell Labs in 1957.8 Lee
has shown the implementation of frequency reuse/4 which serves as a guideline for
designing cellular systems. Lee has also concluded in general that the frequency reuse
factor K has to be 7 for a requirement of C I = 18 dB when the path loss exponent y = 4.
I
In this section, a simulator is introduced for further studying the relationship between
frequency reuse factor K and the path loss exponent y. Theoretically, y = 40 dB per
decade. However, this path loss exponent value y varies due to different human-made
environments. The frequency reuse can generate many tiers of co-channel cells. For K = 3,
the first tier has six co-channel interfering cells, and the second tier has 12 co-channel
interfering cells. For K = 7, the first tier has six co-channel interfering cells, and the
second tier also has 12 co-channel interfering cells. Sectorization is way to reduce co­
channel interference. The system may need to first calculate the tolerable co-channel
interfence that the system can allow then decide what frequency reuse factor number
should be used. From what we can obtain from the prediction tool, the optimized
frequency reuse factor varies with path loss exponent as well as with sectorization.
The tool also can calculate the relative distribution of the co-channel interference
from the first and higher tiers. The effect of power control on C I I can also be obtained
from the prediction tool.
1. . 1 Basic Concepts24
7
Frequency reuse is the core concept of the cellular mobile radio system. In a frequency
reuse system, users in different cells may simultaneously use the same frequency
channel. Thus, the system increases capacity and improves spectrum efficiency.
)
Frequency reuse factor K defines the frequency reuse pattern (may not be unique .
o
I t i s the number f cells n a frequency reuse cluster. The frequency reuse patterns for
i
1
K = 4, 7 are shown in Fig. . 7.1. 1 .
i
The frequency reuse distance D s the minimum distance between two cells that use
the same frequency, that is, the co-channel cells. It can be determined from
D = .J3K · R (1.7.1.1)
where R is the radius of the cell. Reuse of an identical frequency channel in different
cells will create co-channel interference that has to be eliminated with various tech­
niques. Otherwise, it can become a major problem. The co-channel interference can be