Chapter 5. Satellite Networks.ppt

CSE475Satellite and Space Networks,MER KORAKomer.korcakmarmara.edu.trMarmara UniversityDepartment of Computer EngineeringSatellite Networks Research Laboratory(SATLAB)Department of Computer Engineering,Boazii UniversityIstanbul,Turkey,2,OUTLINE,SatellitesGEO,MEO,LEOHigh Altitude PlatformsIntegration ScenarioProblem Definition&Solution Approach,SATELLITES,3,Distance:378.000 kmPeriod:27.3 days,Basics,Satellites in circular orbitsattractive force Fg=m g(R/r)centrifugal force Fc=m r m:mass of the satelliteR:radius of the earth(R=6370 km)r:distance to the center of the earthg:acceleration of gravity(g=9.81 m/s):angular velocity(=2 f,f:rotation frequency)Stable orbitFg=Fc,4,Satellite period and orbits,10,20,30,40 x106 m,24,20,16,12,8,4,radius,satellite period h,velocity x1000 km/h,synchronous distance35,786 km,5,Basics,elliptical or circular orbitscomplete rotation time depends on distance satellite-earthinclination:angle between orbit and equatorelevation:angle between satellite and horizonLOS(Line of Sight)to the satellite necessary for connection high elevation needed,less absorption due to e.g.buildingsUplink:connection base station-satelliteDownlink:connection satellite-base stationtypically separated frequencies for uplink and downlinktransponder used for sending/receiving and shifting of frequenciestransparent transponder:only shift of frequenciesregenerative transponder:additionally signal regeneration,6,Inclination,inclination d,d,satellite orbit,perigee,plane of satellite orbit,equatorial plane,7,Elevation,Elevation:angle e between center of satellite beam and surface,e,minimal elevation:elevation needed at leastto communicate with the satellite,footprint,8,Link budget of satellites,Parameters like attenuation or received power determined by four parameters:sending powergain of sending antennadistance between sender and receivergain of receiving antennaProblemsvarying strength of received signal due to multipath propagation interruptions due to shadowing of signal(no LOS)Possible solutionsLink 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:Lossf:carrier frequencyr:distancec:speed of light,9,Atmospheric attenuation,Example:satellite systems at 4-6 GHz,elevation of the satellite,5,10,20,30,40,50,Attenuation of the signal in%,10,20,30,40,50,rain absorption,fog absorption,atmospheric absorption,e,10,Satellite Orbits,11,Satellite Orbits 2,earth,km,35768,10000,1000,LEO(Globalstar,Irdium),HEO,inner and outer VanAllen belts,MEO(ICO),GEO(Inmarsat),Van-Allen-Belts:ionized particles2000-6000 km and15000-30000 kmabove earth surface,12,GEO Satellites,No handoverAltitude:35.786 km.One-way propagation delay:250-280 ms3 to 4 satellites for global coverageMostly used in video broadcastingExample:TURKSAT satellitesAnother applications:Weather forecast,global communications,military applicationsAdvantage:well-suited for broadcast servicesDisadvantages:Long delay,high free-space attenuation,13,MEO Satellites,Altitude:10.000 15.000 kmOne-way propagation delay:100 130 ms10 to 15 satellites for global coverageInfrequent handoverOrbit period:6 hrMostly used in navigationGPS,Galileo,GlonassCommunications:Inmarsat,ICO,14,MEO Example:GPS,Global Positioning SystemDeveloped by US Dept.Of DefenceBecame fully operational in 1993Currently 31 satellites at 20.200 km.Last lunch:March 2008It 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 positionFourth satellite is needed to correct the receivers clock.Selective AvailabilityGlonass(Russian):24 satellites,19.100 kmGalileo(EU):30 satellites,23.222 km,under development(expected date:2013)Beidou(China):Currently experimental&limited.,15,LEO Satellites,Altitude:700 2.000 kmOne-way propagation delay:5 20 msMore than 32 satellites for global coverageFrequent handoverOrbit period:2 hrApplications:Earth Observation GoogleEarth image providers(DigitalGlobe,etc.)RASAT(First satellite to be produced solely in Turkey)CommunicationsGlobalstar,IridiumSearch and Rescue(SAR)COSPAS-SARSAT,16,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 kmNo intersatellite link:Ground gateways provide connectivity from satellites to PSTN and Internet.Satellite visibility time:16.4 minOperational 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.,17,Globalstar Coverage Map,18,Iridium,66 satellites(6 planes,11 sat per plane)and 10 spares.86.4 inclination:full coverage Altitude:780 kmIntersatellite links,onboard processingSatellite visibility time:11.1 minSatellites launched in 1997-98.Initial company went into bankrupcyTechnologically flawless,however:Very expensive;Awful business plan Cannot compete with GSMNow,owned by Iridium satellite LLC.280.000 subscribers(as of Aug.2008)Multi-year contract with US DoD.Satellite collision(February 10,2009).,19,COSPAS-SARSAT(Search And Rescue Satellite Aided Tracking),An international satellite-based SAR distress alert detection&information distribution system.4 GEO,5 LEO satellitesAircraft&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,20,Since 1982,30.713 persons rescued in 8.387 distress situation.,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 satellitesFrom flying mirrors to intelligent routers on sky.Major problem with LEOs:Mobility of satellites Frequent hand-overAnother important problem with satellites:Infeasible to upgrade the technology,after the satellite is launched,21,High Altitude Platforms(HAPs),Aerial unmanned platformsQuasi-stationary position(at 17-22 km)Telecommunications&surveillance Advantages:Cover larger areas than terrestrial base stationsNo mobility problems like LEOsLow propagation delaySmaller and cheaper user terminalsEasy and incremental deploymentDisadvantages:Immature airship technologyMonitoring of the platforms movement,22,HAP Coverage,23,HAP-Satellite Integration,HAPs have significant advantages.Satellites still represent the most attractive solution for broadcast and multicast servicesShould be considered as complementary technologies.,24,An Integration Scenario,Integration of HAPs and mobile satellitesEstablishment of optical links,HAPs,Satellites,25,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 min 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.,26,Optimal Assignment of Optical Links(cont.),SOLUTION APPROACHESThis optimization problem can be represented as an Integer Linear Programming(ILP)problemILP solution approaches:Exclusive search,Branch-and-bound algorithms,etc.Exponential time complexityNot feasible for large networksOptimization algorithm should be applied repeatedlyIn periodic manner:every t time unitIn event-driven manner:When a link becomes obsoleteFaster algorithm is necessary.There exists a polynomial time solution approach,27,Solution Approach,Example scenarioThree satellites&seven HAPsVisible pairs are connectedElevation angles are given on the links,28,Solution Approach(cont.),Bipartite graphFirst group:Node for each satellite transmitterSecond group:Node for each HAPEdge exists between a HAP and each transmitter of a satellite,if they are visible to each other.Weights:Elevation angles,29,Maximum weighted maximum cardinality matching,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),30,Results,31,100 HAPs(height:20 km)10 MEO Satellites:ICO(height:10350 km,inclination:45)Total time:1 day,t=1 minute,Increasing the Link Durations,Matching with maximum“average elevation angle”may result in frequent optical link switchingSwitching from one satellite to another is an expensive operation.Reduction of switching=Increasing the link durationsMethod:Favor existing links with a particular amount Weights of utilized edges are incremented by,32,Results-2,33,min=-2,Net gain function,“Efficient Integration of HAPs and Mobile Satellites via Free-space Optical Links,”Computer Networks,2011,More on Satellites,Hand-overSatellite-fixed/Earth-fixed footprintsNetwork Mobility ManagementRouting,35,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 satellitesIntra satellite handoverhandover from one spot beam to anothermobile station still in the footprint of the satellite,but in another cellInter satellite handoverhandover from one satellite to another satellitemobile station leaves the footprint of one satelliteGateway handoverHandover from one gateway to anothermobile station still in the footprint of a satellite,but gateway leaves the footprintInter system handoverHandover from the satellite network to a terrestrial cellular networkmobile station can reach a terrestrial network again which might be cheaper,has a lower latency etc.,36,Satellite-fixed vs Earth-fixed Footprints,Satellite Fixed Footprints(asynchronous handoff),Earth Fixed Footprints(synchronous handoff),37,Physical network topology is dynamic,Virtual Node topology is fixed,A satellite corresponds to a VN in a time.,Routing is carried out without considering the movement of satellites,Virtual Node,38,Routing,One solution:inter satellite links(ISL)reduced number of gateways needed forward connections or data packets within the satellite network as long as possibleonly 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 routershigher fuel consumptionthus shorter lifetimeIridium and Teledesic planned with ISLOther systems use gateways and additionally terrestrial networks,39,Features of Satellite Networks,Effects of satellite mobilityTopology is dynamic.Topology changes are predictable and periodic.Traffic is very dynamic and non-homogeneous.Handovers are necessary.Limitations and capabilities of satellitesPower 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.,40,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 handoversTo 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,2007,41,Thank You,Any questions?,