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.