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Satellite and Space Networks

Satellite and Space Networks. OUTLINE. Satellites GEO, MEO, LEO High Altitude Platforms Integration Scenario Problem Definition & Solution Approach. SATELLITES. Distance: 378.000 km Period: 27.3 days. Basics. Satellites in circular orbits attractive force F g = m g (R/r)²

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Satellite and Space Networks

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  1. Satellite and Space Networks

  2. OUTLINE • Satellites • GEO, MEO, LEO • High Altitude Platforms • Integration Scenario • Problem Definition & Solution Approach

  3. SATELLITES Distance: 378.000 km Period: 27.3 days

  4. 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

  5. Satellite period and orbits 24 satellite period [h] velocity [ x1000 km/h] 20 16 12 8 4 synchronous distance 35,786 km 10 20 30 40 x106 m radius

  6. 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

  7. Inclination plane of satellite orbit satellite orbit perigee d inclination d equatorial plane

  8. Elevation Elevation: angle e between center of satellite beam and surface minimal elevation: elevation needed at least to communicate with the satellite e footprint

  9. 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 • 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 L: Loss f: carrier frequency r: distance c: speed of light

  10. Atmospheric attenuation Attenuation of the signal in % Example: satellite systems at 4-6 GHz 50 40 rain absorption e 30 fog absorption 20 10 atmospheric absorption 5° 10° 20° 30° 40° 50° elevation of the satellite

  11. Satellite Orbits

  12. Satellite Orbits 2 GEO (Inmarsat) HEO MEO (ICO) LEO (Globalstar,Irdium) inner and outer Van Allen belts earth 1000 10000 Van-Allen-Belts: ionized particles 2000 - 6000 km and 15000 - 30000 km above earth surface 35768 km

  13. GEO Satellites • 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

  14. MEO Satellites • 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

  15. 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.

  16. LEO Satellites • 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

  17. Globalstar • 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.

  18. Globalstar – Coverage Map

  19. Iridium • 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 • Now, owned by Iridium satellite LLC. • 280.000 subscribers (as of Aug. 2008) • Multi-year contract with US DoD. • Satellite collision (February 10, 2009).

  20. COSPAS-SARSAT(Search And Rescue Satellite Aided Tracking) • 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.

  21. 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

  22. High Altitude Platforms (HAPs) • 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

  23. HAP Coverage

  24. HAP-Satellite Integration • HAPs have significant advantages. • Satellites still represent the most attractive solution for broadcast and multicast services • Should be considered as complementary technologies.

  25. An Integration Scenario • Integration of HAPs and mobile satellites • Establishment of optical links HAPs Satellites

  26. 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.

  27. 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

  28. Solution Approach • Example scenario • Three satellites & seven HAPs • Visible pairs are connected • Elevation angles are given on the links

  29. Solution Approach (cont.) • 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

  30. 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)

  31. Results 100 HAPs (height: 20 km) 10 MEO Satellites: ICO (height: 10350 km, inclination: 45˚) Total time: 1 day, Δt=1 minute

  32. 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

  33. Results - 2 εmin=-2

  34. Net gain function “Efficient Integration of HAPs and Mobile Satellites via Free-space Optical Links,” Computer Networks, 2011

  35. More on Satellites • Hand-over • Satellite-fixed / Earth-fixed footprints • Network Mobility Management • Routing

  36. 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.

  37. Satellite-fixed vs Earth-fixed Footprints Satellite Fixed Footprints (asynchronous handoff) Earth Fixed Footprints (synchronous handoff)

  38. Virtual Node Routing is carried out without considering the movement of satellites A satellite corresponds to a VN in a time. • Physical network topology is dynamic • Virtual Node topology is fixed

  39. 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

  40. 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.

  41. 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

  42. Thank You Any questions?

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