Propagation Degrades
RF Signals
Attenuation in free space: signal gets weaker as it travels over long distances.
Radio signal spreads out – free space loss.
Refraction and absorption in the atmosphere.
Frequency dependent!
Obstacles can weaken the signal through absorption or reflection.
Part of the signal is redirected.
Multi-path effects: multiple copies of the signal interfere with each other.
Mobility: moving the receiver causes another form of self interference.
Big change in signal strength.
Refraction
Speed of EM signals depends on the density of the material.
Vacuum: 3 x 108m/sec.
Denser: slower.
Density is captured by refractive index.
Explains “bending” of signals in some environments.
E.g. sky wave propagation.
But also local, small scale differences in the air.
Noise Sources
Thermal noise
Caused by agitation of the electrons.
Function of temperature.
Affects electronic devices and transmission media.
Intermodulation noise
Result of mixing signals.
Cross talk
Picking up other signals.
E.g. from other source-destination pairs.
Impulse noise
Irregular pulses of high amplitude and short duration.
Harder to deal with.
Other LOS Factors
Absorption of energy in the atmosphere
Very serious at specific frequencies, e.g. water vapor (22 GHz) and oxygen (60 GHz).
Obviously objects also absorb energy.
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