Wireless Communication: Overview of basic concepts

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Wireless Communication Overview of basic
concepts

• Narayan Mandayam

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Wired Vs. Wireless Communication
Wired Wireless



Each cable is a different channel
One media (cable) shared by all
High signal attenuation
Signal attenuation is low
High interference noise co-channel interference
adjacent channel interference
No interference
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Why go wireless ?

• Advantages
• Sometimes it is impractical to lay cables
• User mobility
• Cost

• Limitations
• Bandwidth
• Fidelity
• Power
• (In)security

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EM Spectrum
X rays
Gamma rays
visible
UV
infrared
?
1 MHz
1 kHz
1 GHz
1 THz
1 PHz
1 EHz
Propagation characteristics are different in each
frequency band
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Unlicensed Radio Spectrum
?
12cm
5cm
33cm
26 Mhz
83.5 Mhz
125 Mhz
902 Mhz
2.4 Ghz
5.725 Ghz
2.4835 Ghz
5.850 Ghz
928 Mhz
802.11b Bluetooth Microwave oven
802.11a
cordless phones baby monitors WaveLan
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Understanding wireless communication
Tx
Rx

• How does signal propagate ?
• How much attenuation take place ?
• How does signal look like at the receiver ?

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Radio Propagation
Three basic propagation mechanisms
Scattering ? gtgt D
Diffraction ? ? D
Reflection ? ltlt D

• Propagation effects depend on not only on the
  specific portion of spectrum used for
  transmission, but also on the bandwidth (or
  spectral occupancy) of the signal being
  transmitted
• Spatial separation of Tx-Rx

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Propagation in the Real World
a wave can
be absorbed
penetrate
reflect
bend
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Propagation
And, the higher frequencies will usually
encounter more loss in real world situations
(again, smaller cells? more base stations?)
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The Cluttered World of Radio Waves
hills
girders
rain
hallways
windows
vehicles
trees
walls
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Evaluating Frequencies

• 50 MHz- Good for range outdoors (antenna size,
  bending and penetrating), no foliage problems.
  Sees metallic building structures, doesnt pass
  through windows or down corridors, needs large
  antenna (2 meter). TV?
• 450 MHz to 2 GHz - Good compromise for
  cellular-type systems. Antenna small, but big
  enough for outdoor range. Minor foliage effects.
  OK for windows walls and corridors. (450 might
  be best, but ...) (Range issue for 2 GHz systems-
  more bases)
• 5-20 GHz- Antenna too small for range. Foliage
  and rain effects. Indoor microcells?
  Point-to-point? Satellites to ground stations?

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Path loss in dB
Path loss from source to d2 70dB
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dBm ( absolute measure of power)
40 dBm
10,000 times
0 dBm
- 1,000 times
-30 dBm