Transcript Document
CSE475
Satellite and Space Networks
ÖMER KORÇAK
[email protected]
Marmara University
Department of Computer Engineering
Satellite Networks Research Laboratory (SATLAB)
Department of Computer Engineering,
Boğaziçi University
Istanbul, Turkey
OUTLINE
• Satellites
– GEO, MEO, LEO
• High Altitude Platforms
• Integration Scenario
• Problem Definition & Solution Approach
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SATELLITES
Distance: 378.000 km
Period: 27.3 days
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Basics
• Satellites in circular orbits
– attractive force Fg = m g (R/r)²
– centrifugal force Fc = m r ²
– m: mass of the satellite
– R: radius of the earth (R = 6370 km)
– r: distance to the center of the earth
– g: acceleration of gravity (g = 9.81 m/s²)
– : angular velocity ( = 2 f, f: rotation frequency)
• Stable orbit
– Fg = Fc
2
r3
gR
2
( 2 f )
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Satellite period and orbits
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satellite
period [h]
velocity [ x1000 km/h]
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16
12
8
4
synchronous distance
35,786 km
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20
30
radius
40 x106 m
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Basics
elliptical or circular orbits
complete rotation time depends on distance satellite-earth
inclination: angle between orbit and equator
elevation: angle between satellite and horizon
LOS (Line of Sight) to the satellite necessary for connection
high elevation needed, less absorption due to e.g. buildings
Uplink: connection base station - satellite
Downlink: connection satellite - base station
typically separated frequencies for uplink and downlink
– transponder used for sending/receiving and shifting of
frequencies
– transparent transponder: only shift of frequencies
– regenerative transponder: additionally signal regeneration
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Inclination
plane of satellite orbit
satellite orbit
perigee
d
inclination d
equatorial plane
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Elevation
Elevation:
angle e between center of satellite beam
and surface
minimal elevation:
elevation needed at least
to communicate with the satellite
e
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Link budget of satellites
• Parameters like attenuation or received power determined by four
parameters:
sending power
gain of sending antenna
distance between sender
and receiver
gain of receiving antenna
• Problems
L: Loss
f: carrier frequency
r: distance
c: speed of light
4 r f
L
c
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varying strength of received signal due to multipath propagation
interruptions due to shadowing of signal (no LOS)
• Possible solutions
Link Margin to eliminate variations in signal strength
satellite diversity (usage of several visible satellites at the same time)
helps to use less sending power
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Atmospheric attenuation
Attenuation of
the signal in %
Example: satellite systems at 4-6 GHz
50
40
e
rain absorption
30
fog absorption
20
10
atmospheric
absorption
5° 10°
20°
30°
elevation of the satellite
40°
50°
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Satellite Orbits
Distance (km)
Period
Low Earth Orbit (LEO)
700 - 2000
~2 hr
Medium Earth Orbit (MEO)
10.000 – 15.000
~6 hr
Geosynchronous Earth Orbit (GEO)
36.000
24 hr
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Satellite Orbits 2
GEO (Inmarsat)
HEO
MEO (ICO)
LEO
(Globalstar,
Irdium)
inner and outer Van
Allen belts
earth
1000
Van-Allen-Belts:
ionized particles
2000 - 6000 km and
15000 - 30000 km
above earth surface
10000
35768
km
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GEO Satellites
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•
•
•
•
No handover
Altitude: ~35.786 km.
One-way propagation delay: 250-280 ms
3 to 4 satellites for global coverage
Mostly used in video broadcasting
– Example: TURKSAT satellites
• Another applications: Weather forecast, global
communications, military applications
• Advantage: well-suited for broadcast services
• Disadvantages: Long delay, high free-space attenuation
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MEO Satellites
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•
•
•
•
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Altitude: 10.000 – 15.000 km
One-way propagation delay: 100 – 130 ms
10 to 15 satellites for global coverage
Infrequent handover
Orbit period: ~6 hr
Mostly used in navigation
– GPS, Galileo, Glonass
• Communications: Inmarsat, ICO
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MEO Example: GPS
• Global Positioning System
– Developed by US Dept. Of Defence
– Became fully operational in 1993
– Currently 31 satellites at 20.200 km.
• Last lunch: March 2008
• It works based on a geometric principle
– “Position of a point can be calculated if the distances between this point
and three objects with known positions can be measured”
• Four satellites are needed to calculate the position
– Fourth satellite is needed to correct the receiver’s clock.
• Selective Availability
• Glonass (Russian): 24 satellites, 19.100 km
• Galileo (EU): 30 satellites, 23.222 km, under development
(expected date: 2013)
• Beidou (China): Currently experimental & limited.
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LEO Satellites
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•
•
•
•
•
Altitude: 700 – 2.000 km
One-way propagation delay: 5 – 20 ms
More than 32 satellites for global coverage
Frequent handover
Orbit period: ~2 hr
Applications:
– Earth Observation
• GoogleEarth image providers (DigitalGlobe, etc.)
• RASAT (First satellite to be produced solely in Turkey)
– Communications
• Globalstar, Iridium
– Search and Rescue (SAR)
• COSPAS-SARSAT
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Globalstar
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Satellite phone & low speed data comm.
48 satellites (8 planes, 6 sat per plane)
and 4 spares.
52˚ inclination: not covers the polar
regions
Altitude: 1.410 km
No intersatellite link: Ground gateways
provide connectivity from satellites to
PSTN and Internet.
Satellite visibility time: 16.4 min
Operational since February 2000.
315.000 subscribers (as of June 2008)
Currently second-generation satellites
are being produced (by Thales Alenia
Space) and 18 satellites launched in
2010 and 2011.
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Globalstar – Coverage Map
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Iridium
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66 satellites (6 planes, 11 sat per plane)
and 10 spares.
86.4˚ inclination: full coverage
Altitude: 780 km
Intersatellite links, onboard processing
Satellite visibility time: 11.1 min
Satellites launched in 1997-98.
Initial company went into bankrupcy
– Technologically flawless, however:
– Very expensive; Awful business plan
– Cannot compete with GSM
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Now, owned by Iridium satellite LLC.
280.000 subscribers (as of Aug. 2008)
Multi-year contract with US DoD.
Satellite collision (February 10, 2009).
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COSPAS-SARSAT
(Search And Rescue Satellite Aided Tracking)
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An international satellite-based
SAR distress alert detection &
information distribution system.
4 GEO, 5 LEO satellites
Aircraft & maritime radiobeacons
are automatically activated in case
of distress.
Newest beacons incorporate GPS
receivers (position of distress is
transmitted)
Supporters are working to add a
new capability called MEOSAR.
– The system will put SAR processors
aboard GPS and Galileo satellites
Since 1982, 30.713 persons rescued
in 8.387 distress situation.
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Satellites - Overview
• GEOs have good broadcasting capability, but long
propagation delay.
• LEOs offer low latency, low terminal power
requirements.
• Inter-satellite links and on-board processing for
increased performance and better utilization of satellites
– From flying mirrors to intelligent routers on sky.
• Major problem with LEOs: Mobility of satellites
– Frequent hand-over
• Another important problem with satellites:
– Infeasible to upgrade the technology, after the satellite is
launched
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High Altitude Platforms
(HAPs)
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•
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Aerial unmanned platforms
Quasi-stationary position (at 17-22 km)
Telecommunications & surveillance
Advantages:
– Cover larger areas than terrestrial base
stations
– No mobility problems like LEOs
– Low propagation delay
– Smaller and cheaper user terminals
– Easy and incremental deployment
• Disadvantages:
– Immature airship technology
– Monitoring of the platform’s movement
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HAP Coverage
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HAP-Satellite Integration
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HAPs have significant advantages.
Satellites still represent the most attractive solution for broadcast and
multicast services
Should be considered as complementary technologies.
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An Integration Scenario
HAPs
Satellites
• Integration of HAPs and mobile satellites
• Establishment of optical links
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Optimal Assignment of
Optical Links
CONSTRAINTS:
• A satellite and a HAP should have line of sight in order to
communicate with each other.
– Elevation angle between HAP and satellite should be larger than a
certain εmin value.
• Number of optical transmitters in satellites is limited.
– A satellite can serve maximum of Hmax HAPs.
• One to many relation between HAPs and satellites.
AIMS:
• As much HAP as possible should be served (Maximum utilization)
• Average of elevation angles between HAPs and satellites should be
maximized.
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Optimal Assignment of
Optical Links (cont.)
SOLUTION APPROACHES
• This optimization problem can be represented as an Integer Linear
Programming (ILP) problem
• ILP solution approaches: Exclusive search, Branch-and-bound
algorithms, etc.
– Exponential time complexity
– Not feasible for large networks
• Optimization algorithm should be applied repeatedly
– In periodic manner: every ∆t time unit
– In event-driven manner: When a link becomes obsolete
• Faster algorithm is necessary.
• There exists a polynomial time solution approach
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Solution Approach
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Example scenario
Three satellites & seven
HAPs
Visible pairs are
connected
Elevation angles are
given on the links
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Solution Approach (cont.)
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Bipartite graph
First group: Node for each
satellite transmitter
Second group: Node for each
HAP
Edge exists between a HAP
and each transmitter of a
satellite, if they are visible to
each other.
Weights: Elevation angles
Maximum weighted maximum cardinality matching
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Max Weighted Max Cardinality Matching
• Matching: A subset of edges, such that no two edges
share a common node.
• Maximum cardinality matching: Matching with
maximum number of edges.
• Maximum weighted maximum cardinality matching:
Maximum cardinality matching where the sum of the
weights of the edges is maximum.
• Hungarian Algorithm: O(n3)
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Results
100 HAPs (height: 20 km)
10 MEO Satellites: ICO (height: 10350 km, inclination: 45˚)
Total time: 1 day, Δt=1 minute
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Increasing the Link Durations
• Matching with maximum “average elevation angle” may
result in frequent optical link switching
– Switching from one satellite to another is an expensive
operation.
• Reduction of switching = Increasing the link durations
• Method: Favor existing links with a particular amount γ
– Weights of utilized edges are incremented by γ
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Results - 2
εmin=-2
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Net gain function
G( )
L( )
LD( ) LD(0)
LD(0)
Aavg (0) Aavg ( )
Aavg (0)
NGF ( ) G( ) (1 ) L( )
“Efficient Integration of HAPs and Mobile Satellites
via Free-space Optical Links,” Computer Networks, 2011
More on Satellites
•
•
•
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Hand-over
Satellite-fixed / Earth-fixed footprints
Network Mobility Management
Routing
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Handover in satellite systems
•
Several additional situations for handover in satellite systems compared to
cellular terrestrial mobile phone networks caused by the movement of the
satellites
– Intra satellite handover
• handover from one spot beam to another
• mobile station still in the footprint of the satellite, but in another cell
– Inter satellite handover
• handover from one satellite to another satellite
• mobile station leaves the footprint of one satellite
– Gateway handover
• Handover from one gateway to another
• mobile station still in the footprint of a satellite, but gateway leaves the
footprint
– Inter system handover
• Handover from the satellite network to a terrestrial cellular network
• mobile station can reach a terrestrial network again which might be
cheaper, has a lower latency etc.
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Satellite-fixed vs Earth-fixed
Footprints
Satellite Fixed Footprints
(asynchronous handoff)
Earth Fixed Footprints
(synchronous handoff)
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Virtual Node
Routing
is corresponds
carried out without
considering
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A satellite
in a time.
Physical network
topology
is dynamicto aVN
Virtual
Node topology is fixed
the movement of satellites
Routing
• One solution: inter satellite links (ISL)
reduced number of gateways needed
forward connections or data packets within the satellite network as
long as possible
only one uplink and one downlink per direction needed for the
connection of two mobile phones
• Problems:
more complex focusing of antennas between satellites
high system complexity due to moving routers
higher fuel consumption
thus shorter lifetime
• Iridium and Teledesic planned with ISL
• Other systems use gateways and additionally terrestrial networks
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Features of Satellite Networks
• Effects of satellite mobility
–
–
–
–
Topology is dynamic.
Topology changes are predictable and periodic.
Traffic is very dynamic and non-homogeneous.
Handovers are necessary.
• Limitations and capabilities of satellites
– Power and onboard processing capability are limited.
– Implementing the state-of-the-art technology is difficult.
– Satellites have a broadcast nature.
• Nature of satellite constellations
– Higher propagation delays.
– Fixed number of nodes.
– Highly symmetric and uniform structure.
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Routing & Network MM
• Considering these issues various routing & MM
techniques are proposed. Main ideas are:
– To handle dynamic topology changes with minimum overhead.
– To prevent an outgoing call from dropping due to link handovers
– To minimize length of the paths in terms of propagation delay
and/or number of satellite hops.
– To prevent congestion of some ISLs, while others are idle (Load
balancing).
– To perform traffic-based routing.
– To provide better integration of satellite networks and terrestrial
networks.
– To perform efficient multicasting over satellites.
“Exploring the Routing Strategies in Next-Generation Satellite Networks”
IEEE Wireless Communications, 2007
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Thank You
Any questions?