This document discusses several key considerations for path loss and signal propagation in mobile environments: 1) Path loss increases with the fourth power of distance rather than the square law in fixed wireless, and antenna gain improvements are lower due to extensive multipath effects from vehicles and buildings. 2) Cellular systems operate at higher frequencies than earlier mobile systems, resulting in smaller antennas and greater path loss. Fades occur more frequently as the mobile unit moves in and out of constructive and destructive interference areas. 3) Techniques like sectorization, cell splitting, increased transmit power, frequency/space diversity, and CDMA help to mitigate fading and maximize coverage and capacity in mobile wireless networks.

1. Propagation: Mobile/Portable
• Previous discussion of path loss and multipath
considerations assumed fixed stations.
• A greater degree of clutter exists in a mobile
environment (vehicles, buildings etc.) leading to
multiple reflections.
• The general path loss equation no longer applies
(square law attenuation) due to extensive
multipath and shadowing.

2. Path loss in an urban environment
ADDITIONAL CONSIDERATIONS:
a.) 40 log d (not 20 log d) therefore 4th power of distance.
b.) figure of improved gain by doubling antenna height is 3 dB (not
6 dB).
c.) path loss increase with frequency due to a reduction in antennas
effective area. ( ex.: cell systems operate at higher frequencies
than earlier systems discussed therefore smaller antennas).
d.) At low frequencies (VHF) only large objects introduce
multipath while at UHF and higher smaller objects become
factors. ( offset by the fact that signal can now propagate
through windows)
e.) Signal loss of up to 20 dB (800 MHz) =>building penetration

3.   d
h
h
f
Lp
log
log
55
.
6
9
.
44
log
82
.
13
log
16
.
26
75
.
68





The Hata model for path loss in an urban
environment.

4. Fast Fade
• As a mobile user travels they are moving in and
out of constructive and deconstructive areas of
interference.
• If they are at a point where signals are in phase
they will add. When moving a distance of ¼
wavelength that increases the direct path there will
be an equivalent reduction of the reflected path
resulting in a 180 degree phase shift and
cancellation.

T

5. fv
c
v
v
T
2
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the
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the
of
frequncy
f
where
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:
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Note: fades occur at distances of ½ wavelength.

6. Repeater Systems
• FACTORS:
Radio horizon
Base station location and elevation of antenna
Half duplex or full duplex
Full duplex requires a TX and RX offset therefore
a duplexer is required.

7. Cell Systems
Factors:
Radio horizon not a factor in achieving range limiting.
Frequency reuse through placement of antenna and
appropriate low transmitter power to achieve operation
within a designated cell structure.
12 or 7 cell systems with available bandwidth divided
among cells therefore reuse.
Co-channel interference is the range limiting factor
Repeaters located in the center of a theoretical hexagon
shaped cell.

8. Determining the number of cells.
ASSUMPTIONS:
=>Cochannel interefence is a possible range limiting factor. (
all signals above noise level)
=>Only nearest cells with same frequency will cause
problems.
=>All transmitters have equal power. (actually adaptive
depending on need to reduce interference).
=>Assume an S/I ratio of 18 dB is sufficient.(FM voice)
=>Assume path loss to the 4th power of attenuation.

9. 6
.
4


r
d
q
dB
q
r
d
I
S
7
.
18
6
.
74
6
6
.
4
6
6
4
4
4
4





Where: q is the geometric ratio of d and r
r is distance the edge of the cell is from
center.
d is the distance from our cell center
to the center of an interfering cell.
The number of interfering cells in a 7
cell system.
Note: 18.7 dB is at the maximum acceptable threshold. Moving to a
12 cell system will have 11 interfering cells yielding a S/I of
20.7dB. There is a 2 dB improvement but less frequency reuse.

10. Other Methods
SECTORIZE: (assume 7 cells)
=>3 directional antennas 120 degree angles.
=>each antenna employs a separate set of channels
=>effect reduces the number of interfering channels from 6 to
2.
dB
q
r
d
I
S
5
.
23
8
.
222
2
6
.
4
2
2
4
4
4
4





Drawback: each cell acts like 3 cells with 1/3 the number of
channels. A 21 cell repeating pattern therefore less efficient
for frequency reuse than 12 cell system.
dB
8
.
4
2
6
log
10
:
by
Improves 







11. Other Methods
CELL SPLITTING:
2
2
464
.
3
:
therefore
3.464r
a
cell
one
of
area
a
covered
be
to
area
total
A
:
where
r
A
N
a
A
N





The number of cells required is inversley proportional to the
square of the cell radius.Reducing the cell r by 2 increases
the number of cell sites required by a factor of 4. As cell
sizes are reduced available spectrum use becomes more
efficient. Drawback is equipment cost.

12. Fading Remedies
=>Increase TX power. Ex. 20 dB is multiplying TX pout by 100. Good
for base but handheld at 700 mw is impractical.
=>Frequency Diversity. 2 channels in place of 1 in each direction.
Impractical due to bandwidth requirement.
=>Spread Spectrum. Distribution of signal information over a range of
frequencies. Fading of a NARROW channel causes a small loss of data
that can be corrected by error correction.
=>CDMA. Performs well in presence of multipath. Several rflected data
streams can be received at different times. Rake receiver combines
power from various streams.
=>Space Diversity.Appropriate location of antennas to reduce
multipath.