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Transcript
Chapter 4 – Transmission Media
Wireless Media
1/23
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)
2/23
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
3/23
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
4/23
Parabolic Reflective Antenna
5/23
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
4f 2 Ae

c2
G  antenna gain
Ae  effective area

c
 carrier 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 108 12 109  0.025m
4Ae 4  0.56
G 2 
 35186  45.46dB
2

(0.025)
6/23
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 :
 4d 
 4df 
 10 log 10 
  10 log 10 
 dB
  
 c 
2
LdB
2
d  distance
  wavelength
f  frequency
7/23
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)
8/23
Satellite Point-to-Point Link
9/23
Satellite Broadcast Link
10/23
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
11/23
Infrared
modulate infrared light
are blocked by walls
no licenses required
typical uses
TV remote control
Infrared WLAN
12/23
Wireless Propagation: Ground Wave
Ground wave is found in frequencies up to 2MHz
The best-known example of ground wave
communication is AM radio
13/23
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
14/23
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
15/23
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:
1
n  sin( 1 ) / sin( 2 )
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
16/23
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 and the horizon in Kilometers
h: the antenna hight in meters
 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 Kh1  Kh2

wher e h2 and h2 are the hight of the two antennas
17/23
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  41Km
Now suppose that the receiving antenna is 10m high. To
achieve the same distance, how high must the transmitting
antenna be? The result is
41Km  3.57



Kh1  Kh2  3.57 (4 / 3)h1  (4 / 3)10

 h1  46.2m
18/23
Free Space Loss
 The free space loss for ideal isotropic antenna is:
2
P
 4d 
LdB  10 log 10 t  10 log 10 
  20 log 10   20 log 10 d  21.98dB
Pr
  
 4df 
 10 log 10 
  20 log 10 f  20 log 10 d  147.56dB
 c 
2
where
Pt , Pr : signal power at the transmitt ing and receiving antennas
c : speed of light (3 108 m/s)
f ,  : carrier frequency and wavelengt h
 For other antennas, we must take into account antenna gain:
Pt
(4 ) 2 d 2
LdB  10 log 10  10 log 10
Pr
Gt Gr 2
where Gt and Gr are the gains of the transmitt ing and receiving antennas
19/23
Free Space Path Loss
 The free space path loss is a function of both the distance and
the carrier frequency:
LdB  20 log 10 f  20 log 10 d  147.56dB
20/23
Example
 Determine the free space loss at 4GHz for the shortest path to a
satellite from earth (35.863Km).
c 3 108
 
 0.075m
9
f 4 10
LdB  20 log 10   20 log 10 d  21.98dB
 20 log 10 (0.075)  20 log 10 (35.853 106 )  21.98dB
 195.6dB
 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:
LdB  195.6dB  44  48  103.6dB
 What is the received power at the satellite antenna if the transmitted
power from the earth station antenna is 250W?
250W  24dB
received power  24  103.6  79.6dB
21/23
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
22/23
Examples of Multipath Interference
23/23