Physical layer:
Wireless Transmission
Sharif University of Technology
The Electromagnetic Spectrum
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When electrons move, they create electromagnetic
waves that can propagate through space (even in a
vacuum).
The fundamental relation between f, , and c (in
vacuum) is:
when l is in meters and f is in MHz, lf 300. For example,
100-MHz waves are about 3 meters long, 1000-MHz
waves are 0.3-meters long, and 0.1-meter waves have
a frequency of 3000 MHz.
The Electromagnetic Spectrum
Wide band
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Most transmissions use a narrow frequency band to
get the best reception
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Wide band
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Frequency hopping spread spectrum
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change frequencies hundreds of times per second
security
avoids multipath fading
Example: 802.11,Bluetooth
Wide band
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Direct sequence spread spectrum
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spread the signal over a wide frequency band
used in cell phones: second and third generation mobile
phones
Radio Waves
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Radio waves are easy to generate, can travel long distances,
and can penetrate buildings easily, so they are widely used for
communication, both indoors and outdoors. The properties of
radio waves are frequency dependent.
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Low frequency
 penetrate buildings
 follow curvature of earth
 low bandwidth
 omnidirectional
Higher frequencies
 straight line
 bounce off of obstacles
 absorbed by rain
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Radio Transmission
Microwave Transmission
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microwave communication is widely used for
long-distance telephone communication,
mobile phones, television distribution.
microwaves travel in nearly straight lines
can be narrowly focused
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much higher signal-to-noise ratio
microwaves do not pass through buildings
multipath fading
absorption by water
The Politics of the Electromagnetic
Spectrum
The ISM (Industrial, Scientific, Medical) bands in the United States
Infrared and Millimeter Waves
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widely used for short-range
they are relatively directional, cheap, and
easy to build
they do not pass through solid objects
Lightwave Transmission
Communication Satellites
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communication satellite can be thought of as
a big microwave repeater in the sky.
transponders
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each of which listens to some portion of the spectrum,
amplifies the incoming signal, and then rebroadcasts it at
another frequency to avoid interference with the incoming
signal
Communication Satellites
Geostationary Satellites