3. 3
Last Lecture Review
Wireless Transmission
► Digital data analog signal
► Baseband/bandpass signal
► Encoding techniques/Modulation
► Receiver synchronization / Demodulation
Noises
► Thermal noise
► Intermodulation noise
► Crosstalk
► Impulse Noise
► Manmade noise / Natural noise
Losses / Gain
4. 4
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
6. 6
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
7. 7
Multiplexing Techniques
Frequency-division multiplexing (FDM)
► Takes advantage of the fact that the useful bandwidth of
the medium exceeds the required bandwidth of a given
signal
Time-division multiplexing (TDM)
► Takes advantage of the fact that the achievable bit rate of
the medium exceeds the required data rate of a digital
signal
10. 10
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
11. 11
Unguided Media
Transmission and reception are achieved by means
of an antenna
Configurations for wireless transmission
► Directional
► Omnidirectional
12. 12
General Frequency Ranges
Microwave frequency range
► 1 GHz to 40 GHz
► Directional beams possible
► Suitable for point-to-point 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 point-to-point multipoint applications within
confined areas
13. 13
Terrestrial Microwave
Description of common microwave antenna
► Parabolic "dish", 3 m in diameter
► Fixed rigidly and focuses a narrow beam
► Achieves line-of-sight transmission to receiving antenna
► Located at substantial heights above ground level
► Due to attenuation particularly rainfall, requires repeaters/amplifiers
placed farther apart 10-100 km.
Applications
► Long haul telecommunications service
• 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 point-to-point links between buildings
• Enterprise offices, university campuses
14. 14
Satellite Microwave
Description of communication satellite
► Communication satellite is Microwave relay station
► Used to link two or more ground-based 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
► Long-distance telephone transmission
• Used for point-to-point trunks between telephone exchange offices.
► Private business networks
15. 15
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.
16. 16
Broadcast Radio
Description of broadcast radio antennas
► Omnidirectional
► Antennas not required to be dish-shaped
► Antennas need not be rigidly mounted to a precise alignment
Applications
► Broadcast radio
• 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 multi-path interference. Reflection
from land water and human made objects can create multiple paths.
► Less sensitive to rainfall
19. 19
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
21. 21
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
23. 23
Line-of-Sight 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
24. 24
Line-of-Sight Equations
Optical line of sight
Effective, or radio, line of sight
• d = distance between antenna and horizon (km)
• h = antenna height (m)
• K = adjustment factor to account for refraction, rule of
thumb K = 4/3
h
d 57
.
3
h
d
57
.
3
25. 25
Line-of-Sight Equations
Maximum distance between two antennas for LOS
propagation:
• h1 = height of antenna one
• h2 = height of antenna two
2
1
57
.
3 h
h
26. 26
Example
Let h1 = 100 m, h2 = 0 or the second antenna is at
ground level.
D = 3.57 ( 4/3x100)^1/2 + 0 = 41 km.
Now suppose that h2 = 10m. To achieve same
distance, what must be h1?
41 = 3.57(Kh1)^1/2+(13.3)^1/2
h1 = 46.2m
27. 27
Propagation Factors
The transmitter’s power output
The frequency being transmitted
The effect of the Earth’s shape in between the
points
The conductivity of the Earth along the
transmission path
The microclimate through which the signal
passes
27
28. 28
Multipath Propagation
Reflection - occurs when signal encounters a surface that is
large relative to the wavelength of the signal
Diffraction - occurs at the edge of an impenetrable body that
is large compared to wavelength of radio wave
Scattering – occurs when incoming signal hits an object
whose size in the order of the wavelength of the signal or less
30. 30
The Effects of Multipath Propagation
Multiple copies of a signal may arrive at different
phases
► If phases add destructively, the signal level relative to
noise declines, making detection more difficult
Intersymbol interference (ISI)
► One or more delayed copies of a pulse may arrive at the
same time as the primary pulse for a subsequent bit
31. 31
Types of Fading
Fast fading
► Rapid variation in signal strength occurs over distance
about one-half of wavelength.
► At 900 Mhz cellular badn, lambda is 0.33 m.
Slow fading
► Users cover distance well in excess of a wavelength as it
passes buildings of different heights, vacant lots and so on
► A slow variation in signal strength.
Flat fading
Selective fading
32. 32
Fading channel
Additive white Gaussian noise (AWGN) channel
► Signal is degraded only by thermal noise
► Accurate for space communication and some wire
communication such as coaxial cable.
Rayleigh fading
► Fading occurs when there are multiple indirect paths but no direct
LOS path
► Suitable for Outdoor environment
Rician fading
► When there exist a direct LOS path in addition to multiple paths.
► Suitable for smaller cells and indoor environment
33. 33
Summary of today’s lecture
Multiplexing
► FDM, TDM
Transmission Mediums
► Guided media
► Unguided media
• Microwave
• Radio waves
• Infra red
Propagation modes
► Ground wave propagation
► Sky-wave propagation
► LOS propagation
Multi-path propagation
Fading
Next lecture
► Error detecting and correcting techniques
