More Wireless Fundamentals

1
More Wireless Fundamentals

• Wireless Propagation
• Link Budgets

2
Wireless Propagation

• As your wave travels, it may be
• Attenuated (weakened in strength) by distance and
  obstructions
• Reflected by obstructions (sometimes resulting in
  multipath problems)
• Diffracted (partially blocked or bent around
  obstructions)
• Refracted by the atmosphere (variable, usually
  bending slightly toward Earth)

3
Fresnel Zones

• Line of Sight (LOS) transmission needs 60 of
  the Fresnel Zone clear of obstructions
• At the midpoint of a 2-mile link
• Green visible lights Fresnel Zone diameter is
  0.8 inches
• 2.4 GHzs Fresnel Zone diameter is 32.9 feet

4
A Typical Complete Link Budget

• Transmitter power output
• Antenna cable loss
• Antenna gain
• Free-space path loss
• Antenna gain
• Antenna cable loss
• Receiver sensitivity threshold

5
Free Space Path Loss

• PL 96.6 10 log(d2) 10 log(f2) dB
• PL path loss in dB
• d distance in miles
• f frequency in GHz
• For 2.4 GHz at 1 mile
• PL 96.6 10 log(12) 10 log(2.42) dB
• 96.6 0 7.6
• 104.2 dB

6
Example Putting It All Together
Trans 30 dBm
Coax -8 dBm
Ant 12 dBm
Path Loss -104 dBm
Ant 24 dBm
Coax -7 dBm
Rcvr -80 dBm
30 dBm
22 dBm
34 dBm
-70 dBm
-46 dBm
-53 dBm
27 dB
System Operating Margin or Fade Margin 27
dB (general rule should be at least
10dB) Tricky question Doubling the distance
would reduce the SOM by about 6 dB. Why?