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ECE 5233 Satellite Communications
Prepared by:
Dr. Ivica Kostanic
Lecture 13: Propagation effect and link margin calculation
(Section 8.1-8.3)
Spring 2014
Florida Institute of technologies
Page 2
Design for reliability
Components of the atmospheric losses
Abortion losses
Cloud losses
Losses associated with rain
Outline
Important note: Slides present summary of the results. Detailed derivations are given in notes.
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Link performance and availability
Page 3
Key link budget equation
arr LFSPLGP EiRP
Pr – received power
EiRP – effective radiated power
FSPL – free space path loss
La – atmospheric losses
Note: La is a random variable that changes due to condition of the atmosphere between TX and RX
Two thresholds are defined
1. Performance threshold – link’s performance above target
2. Availability threshold – link is not available due to bad performance
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Components of atmospherics losses
Many components of loss (green – attenuation, blue – depolarization and refraction)
o Atmospheric absorption (gaseous effects)
o Cloud attenuation (aerosol and ice particles)
o Rain attenuation
o Tropospheric scintillation (refractive effects)
o Ionospheric scintillation
o Faraday rotation (polarization loss)
o Rain and ice crystal depolarization
Losses are frequency dependent – affects different bands in different manner
All losses are random variables with spatial and temporal distribution
Many years of careful measurements have established spatial and temporal distribution of the loss contributing components
The most significant attenuation comes from rain effects (C, Ku and Ka bands)
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Note: Link design is performed with a margin that ensures that the system performance and availability targets are met for desired time. The margin is referred to as the “fade margin”
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Atmospheric absorption
Atmospheric attenuation due to oxygen and water
Oxygen absorption peaks: 60GHz and 120GHs
Water vapor absorption peaks: 22GHz and 185GHz
Attenuation is relatively small for all frequencies below 50GHz
ITU graph provides zenith attenuation. For other elevation angles
Page 5Range of interest for commercial satellites
9010
sinlog10dBdB ,
el
elzatmatm aa
Note: typically margin of about 1dB is used for atmospheric losses
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Cloud attenuation
Become important for frequencies above 10GHz
Difficult to predict due to wide variety of cloud types
On the order of 0.1 to 0.2 dB/km
Increases with frequency and temperature
Increases with lower elevation angles
Typical margin 1-2dB at frequencies around 30GHz (smaller for frequencies below)
Page 6
Reference: G. W. Stimson, Airborne Radar, SciTech Publishing, 1998
Note: the length of the satellite link path through the clouds is usually quite small except for very low elevation angles
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Rain attenuation
Most significant source of attenuation in C, Ku and Ka satellite bands
Random attenuation – needs to be dealt with using probability tools
Three basic steps in calculating rain attenuationo Step 1: determine the rain rate threshold exceeded at a
given link reliability threshold
o Step 2: determine specific attenuation in dB/km corresponding to the rain rate
o Step 3: estimate the effective length of the path and calculate the overall rain attenuation
There are two broad categorieso Stratiform rain
o Convective rain
Stratiform rain o Over large geographic areas
o Relatively low rain rate
Convective raino Over small geographical areas
o High intensity thunderstorm
o Irregular profile of the rainfall
Satellite links reliability depends mostly on convective rains. There is usually enough margin in the link design for the low rates associated with the stratiform rains
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Note 1: only small portion of the satellite path goes through rain
Note 2: rain intensity along the path may vary
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Rain characterization
Principle tool – cumulative distribution function (CDF) of rain rate
Curves accumulated over many years and are accurate on average
Significant variability from year to year – especially at low time percentages of interest to satellite design
Different sources provide different averaging time (time resolution)
o ITU recommends using 1min resolution
Different sources provide different spatial averaging
o ITU recommends interpolation methods
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Example CCDF curve for rain rate
0 20 40 60 80 100 120 140 160 180 20010
-4
10-3
10-2
10-1
100
101
rain rate [mm/h]
frac
tion
of t
ime
rain
rat
e is
exc
eede
d Example: Rain rate of80mm/h is exceeded in 0.2%of time
Note: CCDF curves are frequently given in tabular format
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Rain climate maps
Developed by ITU
World divided into 15 different regions (A-Q)
Rain rate CDF tabulated for each climate region
The CDF based on long term average and are within +/- 10%
Not very accurate, but simple and widely used for basic calculations
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ITU climate map for the US
Percentage of time (%)
A B C D E F G H J K L M N P Q
10 0.1 0.5 0.7 2.1 0.6 1.7 3 2 8 1.5 2 4 5 12 24
0.3 0.8 2 2.8 4.5 2.4 4.5 7 4 13 4.2 7 11 15 34 49
0.1 2 3 5 8 6 8 12 10 20 12 15 22 35 65 72
0.03 5 6 9 13 12 15 20 18 28 33 33 40 65 105 96
0.01 8 12 15 19 22 28 30 32 35 60 60 63 95 145 115
0.003 14 21 26 29 41 54 45 55 45 105 105 95 140 200 142
0.001 22 32 42 42 70 78 65 83 55 150 150 120 180 250 170
Climate map rain rate CCDF in mm/h
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Rainfall exceedance contour maps
Developed by ITU
Provide 1 min temporal resolution
ITU REC P.837-x provides method for calculating CCDF of rain rates
Recommendation provides 0.01 % maps
Page 10Example of ITU rainfall exceedance map (0.01%)
Note: ITU REC P.837-5 is uploaded to the website
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Example
Determine rain rate exceeded in 0.1% of time for Melbourne, FL
1. Using ITU climate maps
2. Using ITU exceedance curves
Answer:
3. 95 mm/hour
4. 80 mm/hour
Note: Melbourne is close to boundary between regions M and N. Using average between the two: (65+95)/2=80mm/hour
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