Wireless Networks
Lecture 3
Introduction to Wireless Communication
Dr. Ghalib A. Shah
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Outlines
Review of previous lecture #2
Multiplexing
Transmission Mediums
Propagation modes
Multi-path propagation
Fading
Summary of today’s lecture
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Last Lecture Review
Wireless Transmission
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Digital data analog signal
Baseband/bandpass signal
Encoding techniques/Modulation
Receiver synchronization / Demodulation
Noises
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Thermal noise
Intermodulation noise
Crosstalk
Impulse Noise
Manmade noise / Natural noise
Losses / Gain
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Multiplexing
Capacity of transmission medium usually exceeds
capacity required for transmission of a single signal
Multiplexing carrying multiple signals on a single
medium
► More efficient use of transmission medium
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Multiplexing
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Reasons for Widespread Use of Multiplexing
Cost per kbps of transmission facility declines with
an increase in the data rate
Cost of transmission and receiving equipment
declines with increased data rate
Most individual data communicating devices require
relatively modest data rate support
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Multiplexing Techniques
Frequencydivision multiplexing (FDM)
► Takes advantage of the fact that the useful bandwidth of
the medium exceeds the required bandwidth of a given
signal
Timedivision multiplexing (TDM)
► Takes advantage of the fact that the achievable bit rate of
the medium exceeds the required data rate of a digital
signal
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Frequencydivision Multiplexing
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Timedivision Multiplexing
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Classifications of Transmission Media
Transmission Medium
► Physical path between transmitter and receiver
Guided Media
► Waves are guided along a solid medium
► E.g., copper twisted pair, copper coaxial cable, optical fiber
Unguided Media
► Provides means of transmission but does not guide electromagnetic
signals
► Usually referred to as wireless transmission
► E.g., atmosphere, outer space
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Unguided Media
Transmission and reception are achieved by means
of an antenna
Configurations for wireless transmission
► Directional
► Omnidirectional
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General Frequency Ranges
Microwave frequency range
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1 GHz to 40 GHz
Directional beams possible
Suitable for pointtopoint transmission
Used for satellite communications
Radio frequency range
► 30 MHz to 1 GHz
► Suitable for omnidirectional applications
Infrared frequency range
► Roughly, 3x1011 to 2x1014 Hz
► Useful in local pointtopoint multipoint applications within
confined areas
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Terrestrial Microwave
Description of common microwave antenna
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Parabolic "dish", 3 m in diameter
Fixed rigidly and focuses a narrow beam
Achieves lineofsight transmission to receiving antenna
Located at substantial heights above ground level
Due to attenuation particularly rainfall, requires repeaters/amplifiers
placed farther apart 10100 km.
Applications
► Long haul telecommunications service
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4 – 6 GHz band is common
But due to increased congestion 11 GHz is coming into use now
Microwave links provide TV signals to local CATV and then distributed
to subscribers via coaxial cable.
► Short pointtopoint links between buildings
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Enterprise offices, university campuses
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Satellite Microwave
Description of communication satellite
► Communication satellite is Microwave relay station
► Used to link two or more groundbased microwave
transmitter/receivers
► Receives transmissions on one frequency band (uplink), amplifies or
repeats the signal, and transmits it on another frequency (downlink)
► Broadcast in nature
Applications
► Television distribution
► Longdistance telephone transmission
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Used for pointtopoint trunks between telephone exchange offices.
► Private business networks
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Satellite microwave
Transmission characteristics
► Optimum range is 1-10 GHz
► Below 1 GHz, significant natural noise (solar, galactic,
atmospheric) and manmade
► Above 10 GHz, higher attenuation due to atmospheric
absorption
► Mostly use 5.925-6.425 for uplink and 3.7-4.2 GHz for downlink
referred as 4/6 GHz band
► Due to saturation, 12/14 GHz band has been developed.
Uplink: 14-14.5, downlink: 11.7-12.2 GHz
► In Future, 20/30GHz. Uplink: 27.5-30.0, downlink: 17.7-20.2
GHz
► Long propagation delay of about 250 ms, which is noticeabe in
telephone conversation.
► Broadcast in nature and suitable for TV broadcast service.
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Broadcast Radio
Description of broadcast radio antennas
► Omnidirectional
► Antennas not required to be dishshaped
► Antennas need not be rigidly mounted to a precise alignment
Applications
► Broadcast radio
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VHF and part of the UHF band; 30 MHZ to 1GHz
Covers FM radio and UHF and VHF television
Characteristics
► Because of longer wavelength, radio waves relatively suffer less
attenuation.
► Prime source of impairments is multipath interference. Reflection
from land water and human made objects can create multiple paths.
► Less sensitive to rainfall
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Propagation Modes
Groundwave propagation
Skywave propagation
Lineofsight propagation
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Ground Wave Propagation
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Ground Wave Propagation
Follows contour of the earth
Can Propagate considerable distances
Frequencies up to 2 MHz, which are low
frequencies and have tendency to tilt downwards
EM waves of low frequency are scattered by the
atmosphere such that they do not penetrate the upper
atmosphere.
Example
► AM radio
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Sky Wave Propagation
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Sky Wave Propagation
Signal reflected from ionized layer of atmosphere back down
to earth
Signal can travel a number of hops, back and forth between
ionosphere and earth’s surface
Reflection effect caused by refraction
Examples
► Amateur radio
► CB radio
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LineofSight Propagation
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LineofSight Propagation
Transmitting and receiving antennas must be within line of
sight
► Satellite communication – signal above 30 MHz not reflected by
ionosphere
► Ground communication – antennas within effective line of site due to
refraction
Refraction – bending of microwaves by the atmosphere
► Velocity of electromagnetic wave is a function of the density of the
medium
► When wave changes medium, speed changes
► Wave bends at the boundary between mediums
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LineofSight Equations
Optical line of sight
d
3.57 h
Effective, or radio, line of sight
d
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3.57
h
d = distance between antenna and horizon (km)
h = antenna height (m)
K = adjustment factor to account for refraction, rule of
thumb K = 4/3
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LineofSight Equations
Maximum distance between two antennas for LOS
propagation:
3.57
h1
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h1 = height of antenna one
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h2 = height of antenna two
h2
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