Transmission Media / Channels

Introduction 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

Cont’d... Signal transmission problem  additive noise – generated internally by components used to implement the communication system.  Interference from other users of the channel.

Cont’d... Minimizing noise effects –Increasing the power of transmitted signal. Constraint –Limited power level –Channel bandwidth availability

Cont’d... Channels Wire lines Wireless electromagnetic Fiber optics Underwater acoustic

Cont’d... 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.

Cont’d... Twisted pair Coaxial cable

Cont’d... 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

Cont’d...

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

Cont’d... Example A radio station transmitting in AM frequency band, f c = 1MHz, λ = 300 m, requires antenna at least 30 m.

Cont’d... Various frequency bands of the electromagnetic spectrum

Cont’d... Mode of propagation of EM waves i.Ground-wave propagation ii.Sky-wave propagation iii.Line-of-sight (LOS)

GROUND-WAVE PROPAGATION 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.

Cont’d...

SKY-WAVE PROPAGATION Transmitted signals being reflected from ionosphere Frequency : above 30 MHz Little loss Problem : Signal Multipath Application : Satellite communications

Cont’d... Antenna at different angles > f c

LINE-OF-SIGHT (LOS) PROPAGATION 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)

Cont’d...

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

Cont’d... Multipath channel – signals reflections from the surface and the bottom of the sea. Noise : ambient ocean acoustic noise, man-made acoustic noise

END OF PART 2