adaptive impairment mitigation techniques for satellite
TRANSCRIPT
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Communications ResearchCentre Canada
David V. Rogers
Communications Research Centre Canada
Ottawa, Canada
Adaptive Impairment MitigationAdaptive Impairment MitigationTechniques forTechniques for
Satellite CommunicationsSatellite Communications
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Overview of PresentationOverview of Presentation
• Summary of Propagation Impairments AffectingSatellite Communication Systems
• Summary of Impairment Mitigation Techniques
• Example Techniques and Propagation Issues– Site Diversity
– Uplink Power Control (ULPC)
– Adaptive Depolarization Compensation
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PROPAGATION IMPAIRMENTS AFFECTING SATELLITECOMMUNICATION SYSTEMS
Impairment Physical Cause Prime Importance
Signal attenuation, sky noise increase Gaseous absorption; scatter/emission Systems at frequencies above about
Signal depolarization Frequency-reuse systems (cross-polar
Signal scintillations Small-margin systems; paths at low(tropospheric and ionospheric) elevation angles; antenna tracking
Refraction, atmospheric multipath Systems operating at low elevation
Reflection multipath, shadowing, Mobile-satellite servicesblockage
Propagation delays and delay TDMA and position-locationvariations systems; adaptive control
Intersystem interference Frequency sharing (terminal siting,
from precipitation hydrometers
Differential phase shift anddifferential attenuation caused bynonspherical scatterers (e.g.,raindrops, ice crystals)
Forward scatter from refractive-indexvariations in atmosphere
Atmospheric density variations; ducts,elevated layers
Interaction with earth’s surface,vegetation, objects on earth’s surface
Free-space propagation time;variation in composition oftroposphere and ionosphere
Ducting, diffraction, troposcatter,terrain scatter, precipitation scatter
10 Ghz
interference)
systems
angles; antenna tracking; antennaisolation
path planning, etc.)
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Jokoinen, FinlandFrequency 30 GHzPath elevation angle 20.5°
Annual time percentage abscissa is exceeded
Pat
h A
ttenu
atio
n (d
B)
Composite sum of propagation contributions to annual cumulative statistics of30-GHz attenuation, derived from radiosonde data (Salonen et al.)
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0.001
0.01
0.1
1
0 4 8 12 16
% T
ime
Abs
ciss
a E
xcee
ded
Attenuation (dB)
1994
1995
1996
1997
1998
Annual Average ('94-'98)
Statistics of 20-GHz ACTSattenuation with respect toclear sky (Vancouver data)
Interannual (and seasonal) variations in propagation impairments can be large,and average impairment statistics can be strongly influenced by severe years.
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IMPAIRMENT MITIGATION TECHNIQUES
Techniques Main Impairment(s) Main System Application
Site diversity (multiple Path attenuation and fading, FSS earth stations and MSS feeder links forearth stations) depolarization f > 10 GHz; BSS systems delivering time-
sensitive information (e.g., TV programming)
Path/orbital diversity Path attenuation and fading; Standard FSS systems; other satellite systems(multiple satellites) shadowing and blockage using nongeostationary constellations
Transmit power control Path attenuation/fading FSS and MSS feeder-link uplinks; CDMAsystems
Depolarization Path depolarization (cross- FSS earth stations (C-band or low-path anglecompensation polarization interference) systems at Ku-band)
Frequency diversity Any frequency-sensitive [Usually spectrally inefficient; difficult systemimpairment configuration issues]
Time diversity Any time-varying impairment Transmission of relatively time-insensitiveinformation (e.g., packets)
Digital methods (variable Path attenuation and fading Adaptively improve E /N (requires spareb o
symbol rate, FEC, etc.) system resources in many cases)
Service Designations:FSS = Fix-Satellite Service; MSS = Mobile-Satellite Service; BSS = Broadcast-Satellite Service
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Site DiversitySite Diversity
(two or more earth terminals to provide path diversity)(two or more earth terminals to provide path diversity)
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Dual-site diversity configuration. Site separation D and path elevation angle θ.(H and V are horizontal/vertical path separations; tracks are path projections.)
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Three years of rain rate datafrom KDD Labs, Japan
Normalized conditional probability to exceed rain rate R jointly at twopoints decreases with increasing separation between those points,and is the prime technical basis for earth-terminal site diversity
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Dual-site diversity gain vs. site separation, parameterized in terms ofsingle-path attenuation, compared to measurements (Hodge)
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Ku-band site diversity configuration and interconnect link designed by COMSAT.Mainly for downlink protection; uplink switch is installed for redundancy.
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Transmit Power ControlTransmit Power Control
(transmitter power adjusted in response to variations in(transmitter power adjusted in response to variations in
path fading; more common for path fading; more common for uplinkuplink applications) applications)
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Instantaneous fade ratio for beacon time series showing scatter in ratioInstantaneous fade ratio for beacon time series showing scatter in ratio
0 . 5
1
1 . 5
2
2 . 5
3
3 . 5
0 2 4 6 8 1 0 1 2
0
5
1 0
1 5
2 0
2 5
27.5
-GH
z/20
.2-G
Hz
Att
enu
atio
n R
atio
27.5
-GH
z A
tten
uat
ion
(dB
)
20.2-GHz Attenuation (dB)
OTTAWA, Event of Aug. 22, 1997
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0.001
0.01
0.1
1
0 5 10 15Attenuation (dB)
20 GHz
30 GHz%
Tim
e A
bsci
ssa
Exc
eede
d
Measured (‘94-’98)
Scaled from 30 GHz
Scaled from 20 GHz
Measured (‘94-’98)
Long-term frequency-scalingbehavior with ITU-R relation
(Vancouver ACTS data)
In addition to short-term fluctuations in scaling ratio between uplink and downlinkattenuation (previous figure), there is also uncertainty in the long-term
frequency-scaling relation required for uplink power control
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High correlation of signal scintillations for a common transmit/receive apertureindicates frequency-scaling power control method can also work for this effect
(Relative power levels on plot are arbitrary and intentionally offset for illustration)
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One year of data(Cox & Arnold)
Concurrently-measured 28/19-GHz attenuation showing equiprobableand median instantaneous attenuation ratios, and 10% and 90% spread
in attenuation ratio, indicating variability in frequency-scaling ratio
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Data of previous figure plotted as attenuation ratio, with uplink power controlerrors εu of ±1 and ±2 dB derived by assuming average fade ratio of 2.2,
indicating difficulty in maintaining ±2 dB control errors at Ka-band
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– ACTS measurement, Aug.10, 1998
ACTS 20-GHz beacon data. Baseline modeled and deleted by 4th-order sinsusoid.Detection flag shows when fade detected; derived fade depth shown at bottom.
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Adaptive Polarization CompensationAdaptive Polarization Compensation
(introduce differential phase and/or attenuation to counteract(introduce differential phase and/or attenuation to counteract
propagation-path contributions that cause propagation-path contributions that cause crosspolarizationcrosspolarization;;
of interest for dual-polarization frequency-reuse systems)of interest for dual-polarization frequency-reuse systems)
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General four-port representation of adaptive depolarization compensationnetwork. Cross-coupling arms allow insertion of differential phase andattenuation to counteract the differential phase and attenuation induced
by asymmetrical raindrops and ice crystals on the propagation path.
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Path depolarization is more severe for a given path attenuation at lowerfrequencies. Depolarization generally most important at C-band.
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Differential attenuation and phase contributions to path cross-polarizationdiscrimination (XPD) at 11-GHz path and path 30° elevation angle, andcombined effect. Differential phase generally more important to XPD.
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Joint 19-GHz attenuation/depolarization statistics, illustrating relative systemimportance of attenuation vs. depolarization at Ka band (Cox & Arnold).
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Wideband 4/6-GHz phase-only depolarization compensator using cascaded,individually-driven 180° and 90° polarizers, developed for INTELSAT
(low-loss system can be placed between antenna and LNA/HPA assembly)