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7/30/2019 6. Long Distance Links
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Long-Distance Links
Telecommunication Engineering
www.ee.ui.ac.id/wasp
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Propagation of Wave
The propagation of electromagnetic waves is dependenton the frequency
The propagation characteristics are the result of changesin the radio-wave velocity as a function ofaltitude andboundary conditions
The wave velocity is dependent on air temperature, airdensity, and levels of air ionization
Ionization (free electrons) of the rarified air at highaltitudes has a dominant effect on wave propagation in
the MF and HF bands The ionization is caused by ultraviolet radiation from the
sun, as well as cosmic rays
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Propagation of Wave
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Propagation of Wave
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Propagation of Wave
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Propagation of Wave
The dominant ionized regions are D, E, F1, and F2 The D layer is located closest to the Earths surface at an
altitude of about 45 or 55 miles
For f > 300 kHz, it acts as a RF sponge to absorb(attenuate) the waves
The attenuation is inversely proportional to frequencyand becomes small for frequencies above 4 MHz
For f < 300 kHz, it provides refraction (bending) of RFwaves
It is most pronounced during the daylight hours, withmaximum ionization when the sun is overhead, andalmost dissapears at night
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Propagation of Wave
The E layer has a height of 65 to 75 miles, has maximumionization around noon and disappears after sunset
The F layer has a height of 90 to 250 miles
It ionizes rapidly at sunrise, with its peak ionization in
early afternoon and decay slowly after sunset The F region splits into two: F1 and F2 during the day and
combines into one layer at night
The F region is the most predominant medium in
providing reflection of HF waves
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Propagation of Wave
Three dominant propagation characteristics: Ground wave
Sky wave
LOS
Ground wave propagation is the dominant mode ofpropagation for frequencies < 2 MHz
Here, the wave tends to follow the contour of the Earth
because the diffraction of wave causes it to propagate
along the surface of the Earth What is the lowest radio frequency that can be used?
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Propagation of Wave
For efficient radiation, the antenna needs to be longer
than 1/10 of a wavelength For example, for signaling with a carrier frequency of 10
kHz, the antenna length is minimum 3000 m
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Propagation of Wave
Sky wave propagation is the dominant mode ofpropagation in the 2 to 30 MHz frequency range
Here, long-distance coverage is obtained by reflecting the
wave at the ionosphere and at the Earths boundaries
In the ionosphere the waves are refracted gradually in aninverted U shape because the index of refraction varies
with altitude as the ionization density changes
The refraction index of the ionosphere is given by
2
811
Nn
f refractive indexn
3free electron density (electrons/m )N frequency (Hz)f
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Propagation of Wave
Typical N values range between 10^10 and 10^12
depending on the time of day, season, sunspots In an ionized region because and outside the
ionized region because
1n 0N
1n 0N
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Propagation of Wave
In the ionized region the waves will be bent according toSnells law
The layer D is present during the day and absorbsfrequencies below 4 MHz
Thats why in AM broadcast, the distant stations cannotbe heard during the day, but at night the layer disappearsand distant AM stations can be heard via sky wavepropagation
Sky wave propagation is caused by reflection from the Flayer
sin r in
angle of incidencei angle of refractionr
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Propagation of Wave
LOS propagation is the dominant mode for frequenciesabove 30 MHz
Here the electromagnetic wave propagates in a straight
line
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LOS Systems
The LOS mode has the disadvantage that forcommunication between two terrestrial stations, the
signal path has to be above the horizon
Otherwise, the Earth will blockthe LOS path
The antennas need to be placed on tall towers so thatthe receiver antenna can see the transmitting antenna
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LOS Systems
To the optical horizon, k (bending characteristic) = 1
To the radio horizon, k =4/3
The design of an LOS microwave linkinvolves 5 basic steps: Setting performance requirements
Site selection and preparation of a path profile to determine antennatower heights
Carrying out a path analysis (link budget) Physically running a path/site survey
Installation of equipment and test of the system prior to cutting itover to carry traffic
3 [ ][ ] 2.9
2o
h md km
[ ] 2.9 2 [ ]rd km h m
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Site Selection
Select operational site where we will install and operate radioequipment
Path profile of each link to determine the heights of radiotower to achieve LOS
Steps to obtain path profile:
Obtain good toplogical maps of the region Draw a straight line with a long straight edge connecting the two
sites identified
Follow along down the line identifying obstacles and their heights
Calculate earth curvature (EC)
Calculate the Fresnel zone clearance for each obstacle
Add a value of additional height for vegetation and a growth factor
Draw a straight line from left to right connecting the two highestobstacle locations on the profile
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Site Selection
To calculate the EC we must account for the radio raypath bending by using K-factor
When K-factor is greater than 1, the ray beam bendstowards the earth
Whne K-factor is less than 1, the ray beam bends awayfrom the earth
The following formula applies
where is the distance from the transmit site to theobstacle in question and is the distance from thatobstacle to the receive site
1 20.078d d
h mK
1d
2d
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Site Selection
How to find the K-factor?
Fresnel zone clearance
1 2
[ ] 17.3d km d km
R mF GHz D km
1 2D d d
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Link Budget
A path analysis is carried out to dimension the link Establishing operating parameters such as transmitter
power output, parabolic antenna aperture (diameter),
receiver noise figure
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Link Budget
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Link Budget
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Link Budget
For operating frequencies up to about 10 GHz, path lossis synonymous with free-space path loss
Free-space path loss is given by
EIRP is calculated by adding decibel units: transmittedpower (in dBm or dBW), the transmission line losses in
dB, and antenna gain in dBi
Example: If a microwave transmitter has 1 W of power
output, the waveguide loss is 3 dB and the antenna gain is
34 dBi, the EIRP is
[ ] 92.4 20log [ ] 20log [ ]PL dB F GHz D km
.output trans lineEIRP dBW P dBW Loss dB G dBi
0 3 34 31EIRP dBW dB dBi dBW
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Link Budget
Isotropic Receive Level (IRL) is the RF power levelimpinging on the receive antenna
Receive signal level (RSL) is the power level at the input
port if the first active stage in the receiver
IRL dBW EIRP dBW PL dB
. .rec ant trans lineRSL dBW IRL dBW G dB Loss dB
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Link Budget
Example: Suppose the IRL was -121 dBW, the receiveantenna gain was 31 dB, and the line losses were 5.6 dB.
The RSL would be
The thermal noise level of a receiver is a function of thereceiver noise figure and its bandwidth
The thermal noise power level in a 1-Hz bandwidth of a
perfect receiver operating at absolute zero is
The thermal noise level of a perfect receiver operating at
room temperature is
121 31 5.6 95.6RSL dBW dB dB dBW
228.6nP dBW Hz
228.6 10log 290 204nP dBW Hz K dBW Hz
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Link Budget
We can convert noise figure to noise temperature inkelvins with the following formula
The thermal noise power level of a device operating atroom temperature is
Example: A microwave receiver has a noise figure of 8 dB
and its bandwidth is 10 MHz. The thermal noise level is
10log 1 290eNF dB T
the effective noise temperature of a deviceeT
204 10lognP dBW Hz NF dB BW Hz
6204 8 10log 10 10 126nP dBW Hz dB dBW
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Link Budget
S/N is widely used in analog transmission systems as onemeasure of signal quality
In digital systems the basic measure of transmission
quality is BER
In digital radio links, the ratio Eb/N0 is used as themeasure of signal quality
Eb/N0 means energy per bit per noise spectral density
ratio
N0 is simply the thermal noise in 1 Hz of bandwidth or
0 204N dBW Hz NF dB
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Link Budget
Example: Suppose a receiver has a noise figure of 2.1 dB,what is the thermal noise level in 1 Hz of bandwidth. The
N0 is
Eb is the signal energy per bit and defined as
Example: The RSL into a certain receiver was -89 dBW
and bit rate was 2.048 Mbps. The Eb value is
Then, the formula for Eb/N0 is
0 204 2.1 201.9N dBW Hz dB dBW Hz
or 10logbE RSL dBm dBW bit rate
689 10log 2.048 10 152.11bE dBW dBW
0 10log 204bE N RSL dBW bit rate dBW NF dB