Propagation: MobilePortable

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)
  gtbuilding penetration

3
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.

5
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
• gtCochannel interefence is a possible range
  limiting factor. ( all signals above noise level)
• gtOnly nearest cells with same frequency will
  cause problems.
• gtAll transmitters have equal power. (actually
  adaptive depending on need to reduce
  interference).
• gtAssume an S/I ratio of 18 dB is sufficient.(FM
  voice)
• gtAssume path loss to the 4th power of
  attenuation.

9
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)
• gt3 directional antennas 120 degree angles.
• gteach antenna employs a separate set of channels
• gteffect reduces the number of interfering
  channels from 6 to 2.

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.
11
Other Methods

• CELL SPLITTING

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

• gtIncrease TX power. Ex. 20 dB is multiplying TX
  pout by 100. Good for base but handheld at 700 mw
  is impractical.
• gtFrequency Diversity. 2 channels in place of 1
  in each direction. Impractical due to bandwidth
  requirement.
• gtSpread 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.
• gtCDMA. Performs well in presence of multipath.
  Several rflected data streams can be received at
  different times. Rake receiver combines power
  from various streams.
• gtSpace Diversity.Appropriate location of
  antennas to reduce multipath.