Part 3  Transmission Media & EM Propagations

 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

Channels Wire lines Wireless electromagnetic Fiber optics Underwater acoustic

 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, f c = 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