Transcript Wireless Media

Chapter 4 – Transmission Media
Wireless Media
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Wireless Transmission Frequencies
1GHz to 40GHz
 Microwave frequency range
 highly directional beams are possible
 Point-to-point transmission
 Used also in satellite communications
30MHz to 1GHz
 Radio frequency range
 Omnidirectional (all directions)
 broadcast radio and TV
3 x 1011 to 2 x 1014 Hz
 Infrared portion of the spectrum
 Local point-to-point and multipoint applications
(e.g., infrared remote control, wireless LAN)
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Antennas
Electrical conductor used to radiate or collect
electromagnetic energy
Transmission antenna
 radio frequency energy from transmitter converted to
electromagnetic energy by antenna
 radiated into surrounding environment
Reception antenna
 electromagnetic energy impinging on antenna converted
to radio frequency electrical energy
 fed to receiver
Same antenna is often used for both purposes
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Antenna Radiation Pattern
Power radiated in all directions
Not same performance in all directions
as seen in a radiation pattern diagram
An isotropic antenna is a (theoretical)
point in space
reference antenna
radiates in all directions equally
with a spherical radiation pattern
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Parabolic Reflective Antenna
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Antenna Gain
 Measure of directionality of antenna
 Defined as the output power in particular direction compared to
that produced by an isotropic antenna
 The antenna gain is related to the effective area of an antenna:
G 
4  Ae

2

2
G  antenna
2
A e  effective
4  f Ae
c
 
c
gain
 carrier
area
wavelength
f
 The isotropic antenna has a gain of 1=0dB with Ae   2 4
 Example: what is the gain of a parabolic antenna with effective
area Ae  0 . 56 A  0 . 56 ( r 2 ) , radius r=1m and f=12GHz?
  c f  3  10
G 
4  Ae

2

8
12  10  0 . 025 m
9
4  0 . 56 
( 0 . 025 )
2
 35186  45 . 46 dB
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Terrestrial Microwave
Used as an alternative to coaxial and fiber cables
Requires fewer repeaters but line-of-sight
Used also for short point-to-point links between
buildings
Use a parabolic dish to focus a narrow beam onto a
receiver antenna
1-40GHz frequencies
Higher frequencies give higher data rates
Main source of loss is attenuation; the loss :
2
L dB
 4 d 
 10 log 10 
  10 log
  
 4  df 

10 
 c 
d  distance
2
dB
  wavelength
f  frequency
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Satellite Microwave
 A satellite is a microwave relay station
 Receives on one frequency band (uplink), amplifies or repeats
signal and transmits on another frequency band (downlink)
 eg. uplink 5.925-6.425 GHz & downlink 3.7-4.2 GHz
 A satellite operate on a number of frequency bands called
transponders
 The satellite requires geo-stationary orbit
 height of 35,784km
 spaced at least 3-4° apart (to minimize interference from other satellites)
 typical uses




Television distribution
long distance telephone transmission
private business networks
Global Positioning System (GPS)
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Satellite Point-to-Point Link
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Satellite Broadcast Link
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Broadcast Radio
Use broadcast radio, 30MHz - 1GHz, for:
FM radio
UHF and VHF television
Is omnidirectional
Still need line-of-sight
Suffers from multipath interference
reflections from land, water, other objects
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Infrared
modulate infrared light
are blocked by walls
no licenses required
typical uses
TV remote control
Infrared WLAN
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Wireless Propagation: Ground Wave
Ground wave is found in frequencies up to 2MHz
The best-known example of ground wave
communication is AM radio
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Wireless Propagation: Sky Wave
Sky wave propagation is found in frequencies from
2MHz to 30MHz
A sky wave signal bounces back and forth between
the ionosphere and the earth surface
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Wireless Propagation: Line-Of-Sight (LOS)
LOS propagation is found in frequencies above
30MHz
the transmitting and receiving antennas must be within
a line of sight of each other
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Refraction
 velocity of electromagnetic wave is function of material density
~3 x 108 m/s in vacuum, less in anything else
 speed changes as move between media
 Index of refraction (refractive index) is:

n  sin(  1 ) / sin(  2 )
1
2
 have gradual bending if medium density varies
 density of atmosphere decreases with height
 results in bending of radio waves towards earth
 hence optical LOS horizon and radio LOS horizon are not the same
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Optical and Radio LOS
 The optical LOS to the horizon can be expressed as:
d  3 . 57 h , where
d : the distance
between th e antenna
h: the antenna
hight in meters
and the horizon
in Kilometers
 The radio LOS to the horizon can be expressed as:
d  3 . 57
Kh
wher e K : adjustment
factor to account for the refreactio n, usually
K  4/3
 The max. distance between two antennas for LOS propagation:
d  3 . 57

Kh 1 
Kh 2

wher e h 2 and h 2 are the hight of the two antennas
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EXAMPLE
 The max. distance between two antennas for LOS transmission
if one antenna is 100m high and the other at ground level is:
d  3 . 57
Kh  3 . 57 ( 4 / 3 )100  41 Km
Now suppose that the receiving antenna is 10m high. To
achieve the same distance, how high must the transmitting
antenna be? The result is
41 Km  3 . 57

Kh 1 

Kh 2  3 . 57

( 4 / 3 ) h1 
( 4 / 3 )10

 h1  46 . 2 m
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Free Space Loss
 The free space loss for ideal isotropic antenna is:
L dB  10 log
Pt
10
2
 4 d 
   20 log
10 
  
 10 log
Pr
10
  20 log
10
d  21 . 98 dB
10
f  20 log
10
d  147 . 56 dB
2
 10 log
 4  df 
  20 log
10 
 c 
where
Pt , Pr : signal
power at the transmitt
c : speed of light
ing and receiving
antennas
( 3  10 m/s)
8
f ,  : carrier frequency
and wavelengt
h
 For other antennas, we must take into account antenna gain:
L dB  10 log
where
Pt
10
Pr
( 4 ) d
2
 10 log
10
GtG r 
2
2
G t and G r are the gains of the transmitt
ing and receiving
antennas
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Free Space Path Loss
 The free space path loss is a function of both the distance and
the carrier frequency:
L dB  20 log 10 f  20 log 10 d  147 . 56 dB
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Example
 Determine the free space loss at 4GHz for the shortest path to a
satellite from earth (35.863Km).
 
c
f

3  10
8
4  10
9
L dB   20 log
10
  20 log
 0 . 075 m
  20 log
10
d  21 . 98 dB
( 0 . 075 )  20 log
10
( 35 . 853  10 )  21 . 98 dB
10
6
 195 . 6 dB
 Now consider the antenna gain of both the satellite and earth station
antennas. Typical values are 44dB and 48dB, respectively. The free
space path loss:
L dB  195 . 6 dB  44  48  103 . 6 dB
 What is the received power at the satellite antenna if the transmitted
power from the earth station antenna is 250W?
250 W  24 dB
received
power  24  103 . 6   79 . 6 dB
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Impairments in wireless LOS transmission
Free space loss
loss of signal with distance
Atmospheric Absorption
from water vapour and oxygen absorption
Multipath
multiple interfering signals from reflections
Refraction
bending signal away from receiver
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Examples of Multipath Interference
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