ECE 5233 Satellite Communications
Prepared by:
Dr. Ivica Kostanic
Lecture 15: Secondary atmospheric losses
effects
(Section 8.5-8.7)
Spring 2011
Outline
Tropospheric scintillation (refractive effects)
Ionospheric scintillation
Faraday rotation (polarization loss)
Rain and ice crystal depolarization
Propagation impairment counter measures
Important note: Slides present summary of the results. Detailed
derivations are given in notes.
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Tropospheric scintillation
 Losses associated with variations of the atmosphere close to the ground
 Due to weather conditions (heating and cooling), the refractive index of the atmosphere changes
 Change of refractive index changes the direction of signal propagation
 Change of direction of arrival is “modulated” by antenna pattern -> causes signal fluctuation
 Scintillation is more pronounced for higher frequencies
 Scintillation does not cause depolarization
 At low elevation angles (< 10 deg), scintillation may cause path loss behavior similar to terrestrial
multipath fading
Physical explanation of
atmospheric scintillation
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Tropospheric scintillation - modeling
 Scintillation losses depend on
o Operating frequency
o Climate
o Satellite elevation
o Antenna beam
 Modeled as additional random path loss
 Mitigation approaches
o Fade margin
o Error control coding
RSL attenuation due to scintilation
0
-0.5
Example. Scintilation losses may be
modeled as a random variable with a PDF
given by:

1
2 
f l   
2


2
 l2
exp  
2
 2
0

,


l0
,l  0
Where  is 1.2 dB.
Estimate required design margin to
guarantee reliability of 90% with respect to
the scintillation losses.
Answer: 2dB
-1
-1.5
-2
-2.5
0
20
40
60
80
100
time
120
140
160
180
Example of scintilation losses
200
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Ionospheric scintillation
 Energy from the sun causes variations to total electron content in the ionosphere
 Typical range 1018 during day, 1016 during night
 At the local sunsets/sunrises there are rapid changes of concentration that cause
changes of magnitude and phase of radio waves
 The changes are further modulated by the antenna pattern
 The net result are variations of the RSL at sunset and down
 Magnitude of the ionosphere scintillation varies with sun activity
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Faraday rotation – polarization loss
 Radio waves propagate through Earths
magnetic field
 Magnetic field changes the polarization of the
wave
Illustration of
Faraday’s
rotation
 Two negative effects:
o Increased losses due to polarization
mismatch between RX antenna and radio
wave
o Increased adjacent channel interference
 The rotation angle depends on
o Length of the path through ionosphere
o Concentration of ionosphere charges
o Operating frequency
 The effects becomes smaller with frequency
increase
Estimation of losses
XPD  20 log cot  
 – Faraday’s rotation angle
Magnetic field of the Earth
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Depolarization losses
 Rain affects two polarizations in a
different way
 Rain attenuates horizontal component
more than the vertical one
 If a linearly polarized wave has a
general orientation w.r.t. rainfall, the
wave tilts towards vertical polarization
 In a non-wind condition, raindrops
have elliptical shape with minor axis
in the vertical direction
 In wind-conditions, the orientation of
the raindrop ellipse changes – canting
angle
Definition of
canting angle
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Tilt angle
 Due to geometry – vertically polarized
transmission from the satellite is received
at a tilted angle
 Tilt depends on the earth station location
 May be estimated using
  arctan tan Le  / sin  lS  lE 
Le – latitude of earth station
le – longitude of earth station
ls – longituide of su-satellite point
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Prediction of XPD losses (ITU-R P.618-6)
Algorithm provided in the text book
Consists of eight steps
Review with students
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Propagation impairment counter measures
 Adaptive power control
 Diversity reception/transmission
 Diversity reception/transmission
 Used in high capacity FSS hubs
 Signal processing (on-board processing)
 Adaptive modulation and coding
 The signal is received/transmitted from
multiple location on the ground
 Probability of simultaneous fades is reduced
with separation between earth stations
 Adaptive power control
 Signal (on-board processing)
o TX power adjusted to compensate for
losses
 Used in VSAT systems
o Power control usually operates in closed
loop
 Measurement at the RX compared
against threshold
 If the signal falls below threshold –
feedback is sent to TX
 Uplink demodulated to the baseband and
rerouted towards different antenna beams
 Each beam examined independently where
rate, power, coding and modulation may be
varied depending in the path loss
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Propagation impairment counter measures
 Adaptive modulation and coding
o Idea: Modulation and coding changes as a function of SNR
o The lower SNR – more robust modulation and coding
o The lower SNR – lower data rate
o Link designed for availability at the worst conditions (at the lowest rate)
o If the conditions are better than worst case – higher throughput is achieved
AMC example for Florida
DVBS-2
standard
Institute of technologies
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