ece 5233 satellite communications prepared by: dr. ivica kostanic lecture 15: secondary atmospheric...
TRANSCRIPT
ECE 5233 Satellite Communications
Prepared by:
Dr. Ivica Kostanic
Lecture 15: Secondary atmospheric losses effects
(Section 8.5-8.7)
Spring 2011
Florida Institute of technologies
Page 2
Tropospheric scintillation (refractive effects)
Ionospheric scintillation
Faraday rotation (polarization loss)
Rain and ice crystal depolarization
Propagation impairment counter measures
Outline
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
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Physical explanation of atmospheric scintillation
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Tropospheric scintillation - modeling
Scintillation losses depend ono Operating frequencyo Climateo Satellite elevation o Antenna beam
Modeled as additional random path loss Mitigation approaches
o Fade margino Error control coding
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Example. Scintilation losses may be modeled as a random variable with a PDF given by:
0,0
0,2
exp2
12
2
2
2
l
ll
lf
Where s is 1.2 dB.
Estimate required design margin to guarantee reliability of 90% with respect to the scintillation losses.
Answer: 2dB
0 20 40 60 80 100 120 140 160 180 200-2.5
-2
-1.5
-1
-0.5
0
RS
L at
tenu
atio
n du
e to
sci
ntila
tion
time
Example of scintilation losses
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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
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
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Illustration of Faraday’s rotation
Magnetic field of the Earth
cotlog20XPD
Estimation of losses
b – Faraday’s rotation angle
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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
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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
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ESe llL sin/tanarctan
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
Page 9
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Propagation impairment counter measures
Adaptive power control
Diversity reception/transmission
Signal processing (on-board processing)
Adaptive modulation and coding
Adaptive power control
o TX power adjusted to compensate for losses
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
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Diversity reception/transmission
Used in high capacity FSS hubs
The signal is received/transmitted from multiple location on the ground
Probability of simultaneous fades is reduced with separation between earth stations
Signal (on-board processing)
Used in VSAT systems
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
Page 11AMC example for DVBS-2 standard