Part 3 Part 3 Transmission Media & EM Transmission Media & EM PropagationsPropagations

Provides the connection between the transmitter and receiver.1. Pair of wires – carry electric signal.2. Optical fiber – carries the information

on a modulated light beam.3. Free space – information-bearing signal

is radiated by antenna

Signal transmission problem

additive noise – generated internally by

components used to implement the

communication system.

Interference from other users of the channel.

Minimizing noise effects

◦ Increasing the power of transmitted signal.

Constraint

◦ Limited power level

◦ Channel bandwidth availability

Underwater acoustic

Fiber optics

Wireless electromagnetic

Wire lines

Channels

Wire Lines Channel

◦ Signals transmitted are distorted in both

amplitude and phase.

– corrupted by noise.

◦ Carry a large percentage of daily

communication around the world.

Twisted pair Coaxial cable

Fiber Optics Channel◦ Low signal attenuation◦ Highly reliable photonic devices◦ Large bandwidth available◦ Services : voice, data facsimile and video◦ Tx – light source

(e.g. LED, laser)◦ Rx – photodiode◦ Noise source : photodiodes & amplifiers

Wireless Electromagnetic Channels◦ Electromagnetic energy is coupled to the

propagation medium by antenna (radiator)◦ Antenna size & configuration – Frequency of

operation◦ Efficient radiation – antenna longer than 1/10

λ

Example A radio station transmitting in AM

frequency band, fc = 1MHz, λ = 300 m,

requires antenna at least 30 m.

Various frequency bands of the electromagnetic spectrum

◦ Ground-wave propagation

◦ Sky-wave propagation

◦ Line-of-sight (LOS)

Surface-wave propagation Dominant mode of propagation Frequency band: 0.3 – 3 MHz Applications: AM broadcasting, maritime

radio broadcasting Disturbances for signal transmission: atmospheric noise, man-made noise,

thermal noise.

Transmitted signals being reflected from ionosphere

Frequency : above 30 MHz Little loss Problem : Signal Multipath Application : Satellite communications

VHF band and higher Limited by curvature of earth Problem : Thermal noise (Rx front end) Cosmic noise (pick-up by antenna) Application: A TV antenna mounted on a

tower of 300 m height to provide a broad coverage area (67km)

Underwater acoustic channels◦ EM waves do not propagate over long distances

under water except at extremely low frequencies◦ Expensive – because of the large and powerful

transmitters required◦ Problem : Attenuation – skin depth

Multipath channel – signals reflections from the surface and the bottom of the sea.

Noise : ambient ocean acoustic noise, man-made acoustic noise