Satellite Satellite CommunicationsCommunications

Satellite-Related TermsSatellite-Related TermsEarth Stations – antenna systems on or Earth Stations – antenna systems on or near earthnear earthUplink – transmission from an earth Uplink – transmission from an earth station to a satellitestation to a satelliteDownlink – transmission from a satellite Downlink – transmission from a satellite to an earth stationto an earth stationTransponder – electronics in the Transponder – electronics in the satellite that convert uplink signals to satellite that convert uplink signals to downlink signalsdownlink signals

Ways to CategorizeWays to CategorizeCommunications Communications

SatellitesSatellitesCoverage areaCoverage area Global, regional, nationalGlobal, regional, nationalService typeService type Fixed service satellite (FSS)Fixed service satellite (FSS) Broadcast service satellite (BSS)Broadcast service satellite (BSS) Mobile service satellite (MSS)Mobile service satellite (MSS)General usageGeneral usage

Commercial, military, amateur, Commercial, military, amateur, experimentalexperimental

Difference Difference satellite terrestrial satellite terrestrial

Coverage Area – Coverage Area – moremoreLimited bandwidthLimited bandwidthGreater Precision & Greater Precision & high qualityhigh qualityCost independent Cost independent of distance as far of distance as far as within coverageas within coverage

Coverage Area – far Coverage Area – far lesslessWide BandwidthWide BandwidthLess Precision level Less Precision level & low quality& low qualityCost increases as Cost increases as the distance the distance increasesincreases

Classification of Satellite Classification of Satellite OrbitsOrbits

Circular or elliptical orbitCircular or elliptical orbit Circular with center at earth’s center Circular with center at earth’s center Elliptical with one foci at earth’s centerElliptical with one foci at earth’s centerOrbit around earth in different planesOrbit around earth in different planes

Equatorial orbit above earth’s equatorEquatorial orbit above earth’s equatorPolar orbit passes over both polesPolar orbit passes over both polesOther orbits referred to as inclined Other orbits referred to as inclined orbitsorbits

Altitude of satellitesAltitude of satellites Geostationary orbit (GEO)Geostationary orbit (GEO) Medium earth orbit (MEO)Medium earth orbit (MEO) Low earth orbit (LEO)Low earth orbit (LEO)

Geometry TermsGeometry TermsElevation angle (Elevation angle (θθ)- the angle from the )- the angle from the horizontal to the point on the center of the horizontal to the point on the center of the main beam of the antenna when the main beam of the antenna when the antenna is pointed directly at the satelliteantenna is pointed directly at the satelliteIdeally for maximum coverage would like to Ideally for maximum coverage would like to use elevation angle of 0ºuse elevation angle of 0ºMinimum elevation angle –downlinks – 5º-Minimum elevation angle –downlinks – 5º-20º20º

uplinks - 5ºuplinks - 5º

Coverage angle(Coverage angle(ββ) - the measure of the ) - the measure of the portion of the earth's surface visible to the portion of the earth's surface visible to the satellitesatellite

Minimum Elevation AngleMinimum Elevation AngleReasons affecting minimum elevation Reasons affecting minimum elevation angle of earth station’s antenna (>0angle of earth station’s antenna (>0oo))

Buildings, trees, and other terrestrial Buildings, trees, and other terrestrial objects block the line of sightobjects block the line of sightAtmospheric attenuation is greater at Atmospheric attenuation is greater at low elevation angleslow elevation anglesElectrical noise generated by the Electrical noise generated by the earth's heat near its surface adversely earth's heat near its surface adversely affects receptionaffects reception

Geostationary SatellitesGeostationary SatellitesSatellite in circular orbit of 35,863 Satellite in circular orbit of 35,863 km above the earth’s surface and km above the earth’s surface and rotates in the equatorial plane of the rotates in the equatorial plane of the earthearthRotates at exactly the same angular Rotates at exactly the same angular speed as the earth & will remain speed as the earth & will remain above the same spot on the equator above the same spot on the equator as the earth rotates.as the earth rotates.

GEO OrbitGEO OrbitAdvantages of the the GEO orbit Advantages of the the GEO orbit No problem with frequency changesNo problem with frequency changes Tracking of the satellite is simplifiedTracking of the satellite is simplified High coverage areaHigh coverage areaDisadvantages of the GEO orbitDisadvantages of the GEO orbit

Weak signal after traveling over 35,000 Weak signal after traveling over 35,000 kmkmPolar regions are poorly servedPolar regions are poorly servedSignal sending delay is substantialSignal sending delay is substantial

LEO Satellite CharacteristicsLEO Satellite CharacteristicsCircular/slightly elliptical orbit under 2000 Circular/slightly elliptical orbit under 2000 km Range of 500 to 1500 Km.km Range of 500 to 1500 Km.Orbit period ranges from 1.5 to 2 hoursOrbit period ranges from 1.5 to 2 hoursDiameter of coverage is about 8000 kmDiameter of coverage is about 8000 kmRound-trip signal propagation delay less Round-trip signal propagation delay less than 20 msthan 20 msMaximum satellite visible time from a fixed Maximum satellite visible time from a fixed point is up to 20 minpoint is up to 20 minSystem must cope with large Doppler shiftsSystem must cope with large Doppler shiftsAtmospheric drag results in orbital Atmospheric drag results in orbital deteriorationdeterioration

Advantages of LEO over Advantages of LEO over GEOGEO

Reduced propagation delayReduced propagation delaySignal is much stronger for same Signal is much stronger for same transmission power transmission power Coverage is localized so spectrum can Coverage is localized so spectrum can be better conservedbe better conservedTherefore technology is currently Therefore technology is currently proposed for communicating with proposed for communicating with mobile and personal terminals that mobile and personal terminals that need stronger signals for functionneed stronger signals for function

LEO CategoriesLEO CategoriesLittle LEOsLittle LEOs Frequencies below 1 GHz Frequencies below 1 GHz 5MHz of bandwidth 5MHz of bandwidth Data rates up to 10 kbpsData rates up to 10 kbps Aimed at paging, tracking, and low-rate Aimed at paging, tracking, and low-rate

messagingmessagingIt has over 30 satellites in the low earth It has over 30 satellites in the low earth orbit.orbit.Uses frequencies 148.00 to 150.05 MHz to Uses frequencies 148.00 to 150.05 MHz to the Satellites and 137.00 to 138.00 from the the Satellites and 137.00 to 138.00 from the satellites satellites Supports a subscriber date rates of 2.4kbps Supports a subscriber date rates of 2.4kbps to the satellite and 4.8kbps down.to the satellite and 4.8kbps down.

Big LEOsBig LEOs Frequencies above 1 GHz Frequencies above 1 GHz Support data rates up to a few megabits Support data rates up to a few megabits

per secper sec Uses CDMA Uses CDMA Uses S – band (2 GHz) for the downlink to Uses S – band (2 GHz) for the downlink to

mobile usersmobile users Earth constellations consists of 48 Earth constellations consists of 48

operating satellites and 8 sparesoperating satellites and 8 spares They are 1413-km high orbitsThey are 1413-km high orbits Offer same services as little LEOs in Offer same services as little LEOs in

addition to voice and positioning servicesaddition to voice and positioning services

LEO CategoriesLEO Categories

MEO Satellite CharacteristicsMEO Satellite CharacteristicsCircular orbit at an altitude in the range of Circular orbit at an altitude in the range of 5000 to 12,000 km5000 to 12,000 kmOrbit period of 6 hoursOrbit period of 6 hoursDiameter of coverage is 10,000 to 15,000 Diameter of coverage is 10,000 to 15,000 kmkmRound trip signal propagation delay less Round trip signal propagation delay less than 50 msthan 50 msMaximum satellite visible time is a few Maximum satellite visible time is a few hourshours

Frequency Bands Available Frequency Bands Available for Satellite for Satellite

CommunicationsCommunications

Satellite Link Performance Satellite Link Performance FactorsFactors

Distance between earth station antenna Distance between earth station antenna and satellite antennaand satellite antennaFor downlink, terrestrial distance between For downlink, terrestrial distance between earth station antenna and “aim point” of earth station antenna and “aim point” of satellitesatellite Displayed as a satellite footprint Displayed as a satellite footprint Atmospheric attenuationAtmospheric attenuation

Affected by oxygen, water, angle of Affected by oxygen, water, angle of elevation, and higher frequencieselevation, and higher frequencies

Satellite FootprintSatellite FootprintSignal from a satellite is not isotropically Signal from a satellite is not isotropically broadcast but aimed at a specific point on the broadcast but aimed at a specific point on the earth depending on the area of coverage earth depending on the area of coverage desired.desired.Center receives the highest radiated power Center receives the highest radiated power and reduces as you move away from the and reduces as you move away from the center in any direction.center in any direction.This is displayed in a pattern known as the This is displayed in a pattern known as the satellite footprintsatellite footprintIt is the effective radiated power of the It is the effective radiated power of the antenna at each point taking into account the antenna at each point taking into account the signal power fed into the antenna and the signal power fed into the antenna and the directionality of the antennadirectionality of the antennaActual power free space loss – effective powerActual power free space loss – effective power

Satellite FootprintSatellite Footprint

Atmospheric AttenuationAtmospheric AttenuationPrimary cause – Oxygen & waterPrimary cause – Oxygen & waterHumid air pronounced with fog and Humid air pronounced with fog and rainrainAngle of elevation – smaller the Angle of elevation – smaller the angle more of atmosphere that the angle more of atmosphere that the signal has to travelsignal has to travelFrequency – higher the freq. greater Frequency – higher the freq. greater the attenuationthe attenuation

Satellite Satellite Network Network ConfigurConfigurationsations

Capacity Allocation Capacity Allocation StrategiesStrategies

Frequency division multiple access Frequency division multiple access (FDMA)(FDMA)Time division multiple access (TDMA)Time division multiple access (TDMA)Code division multiple access (CDMA)Code division multiple access (CDMA)

Capacity of the communication channel is Capacity of the communication channel is divided into number of channels – top level divided into number of channels – top level FDMFDMFurther capacity allocation carried out Further capacity allocation carried out within each channelwithin each channelExample:Example:

Galaxy Satellites from PanAmSatGalaxy Satellites from PanAmSatSatellite uses C-Band frequencies & Satellite uses C-Band frequencies & provides a 500-MHz bandwidth, broken provides a 500-MHz bandwidth, broken 24, 40- MHz, squeeze 24 channels into 24, 40- MHz, squeeze 24 channels into 500 MHz by frequency reuse. Each has 500 MHz by frequency reuse. Each has a 4-MHz guard band – 36Mhz a 4-MHz guard band – 36Mhz

Frequency-Division MultiplexingFrequency-Division Multiplexing

Frequency-Division Frequency-Division MultiplexingMultiplexing

Alternative uses of channels in point-to-Alternative uses of channels in point-to-point configurationpoint configuration

1200 voice-frequency (VF) voice channels1200 voice-frequency (VF) voice channelsOne 50-Mbps data streamOne 50-Mbps data stream16 channels of 1.544 Mbps each16 channels of 1.544 Mbps each400 channels of 64 kbps each400 channels of 64 kbps each600 channels of 40 kbps each600 channels of 40 kbps eachOne analog video signalOne analog video signalSix to nine digital video signalsSix to nine digital video signals

Frequency-Division Frequency-Division Multiple AccessMultiple Access

Satellite – intermediate device to provide a Satellite – intermediate device to provide a point-to-point link between 2 earth station point-to-point link between 2 earth station INTELSAT allows a single 36-MHz to be divided INTELSAT allows a single 36-MHz to be divided using FDM into smaller channels, each using using FDM into smaller channels, each using FM, each smaller channel in turn carries no. of FM, each smaller channel in turn carries no. of VF signals using FDMVF signals using FDMAbility of multiple earth stations accessing the Ability of multiple earth stations accessing the same channel is referred to as FDMAsame channel is referred to as FDMAFactors which limit the number of subchannels Factors which limit the number of subchannels provided within a satellite channel via FDMAprovided within a satellite channel via FDMA

Thermal noiseThermal noiseIntermodulation noiseIntermodulation noiseCrosstalkCrosstalk

Forms of FDMAForms of FDMAFixed-assignment multiple access (FAMA)Fixed-assignment multiple access (FAMA) The assignment of capacity is distributed in The assignment of capacity is distributed in

a fixed manner among multiple stationsa fixed manner among multiple stations Demand may fluctuateDemand may fluctuate Results in the significant underuse of Results in the significant underuse of

capacitycapacityDemand-assignment multiple access (DAMA)Demand-assignment multiple access (DAMA)

Capacity assignment is changed as needed Capacity assignment is changed as needed to respond optimally to demand changes to respond optimally to demand changes among the multiple stationsamong the multiple stations

FAMA-FDMAFAMA-FDMAFAMA – logical links between stations FAMA – logical links between stations are preassignedare preassignedFAMA – multiple stations access the FAMA – multiple stations access the satellite by using different frequency satellite by using different frequency bandsbandsUses considerable bandwidthUses considerable bandwidth

FAMA-FDMAFAMA-FDMA

Both FDMA and FAMABoth FDMA and FAMASatellite does not perform switching functionSatellite does not perform switching functionConsiderable bandwidth is usedConsiderable bandwidth is used

DAMA-FDMADAMA-FDMAFAMA-FDMA is not efficient FAMA-FDMA is not efficient In INTELSAT FDMA scheme divides 36 Mhz In INTELSAT FDMA scheme divides 36 Mhz in 7 5Mhz blocks each caring a grp of 60 in 7 5Mhz blocks each caring a grp of 60 VF total of 420 channelsVF total of 420 channelsSingle channel per carrier (SCPC) – Single channel per carrier (SCPC) – bandwidth divided into individual VF bandwidth divided into individual VF channelschannels

A Single 36MHz channel is subdivided A Single 36MHz channel is subdivided into 800 45 kHz analog channels each into 800 45 kHz analog channels each dedicated to a VF linkdedicated to a VF linkDigital SCPC – QPSK with 64kbps service Digital SCPC – QPSK with 64kbps service

DAMA-FDMADAMA-FDMAFor fullduplex pairs of channels are For fullduplex pairs of channels are assigned to the pairs of earth stationassigned to the pairs of earth stationEach earth station is multiplexed Each earth station is multiplexed supporting a small number of user supporting a small number of user stations.stations.Attractive for remote areas with few Attractive for remote areas with few user stations near each siteuser stations near each siteSuffers from inefficiency of fixed Suffers from inefficiency of fixed assignmentassignmentTo achieve greater efficiency DAMA-To achieve greater efficiency DAMA-FDMA is usedFDMA is used

DAMA – set of subchannels in a DAMA – set of subchannels in a channel is treated as a pool of channel is treated as a pool of available links available links For full-duplex between two earth For full-duplex between two earth

stations, a pair of subchannels is stations, a pair of subchannels is dynamically assigned on demanddynamically assigned on demand

Demand assignment performed in a Demand assignment performed in a distributed fashion by earth station distributed fashion by earth station using CSCusing CSC

DAMA-FDMADAMA-FDMA

First Commercial DAMA SCPC - First Commercial DAMA SCPC - SPADESPADE((SSingle Channel per carrier, ingle Channel per carrier, PPCM,Multiple-CM,Multiple-AAccess ,ccess , D Demand-assignment emand-assignment EEquipment) quipment) Used on INTELSAT Used on INTELSAT Each subchannel carries 64-kbps QPSK Each subchannel carries 64-kbps QPSK signal – occupies 38Khz + 7Khz guardbandsignal – occupies 38Khz + 7Khz guardbandTotal of 794 channels are availableTotal of 794 channels are availableParing is such that 2 channels are Paring is such that 2 channels are 18.405Mhz apart to form a fullduplex circuit18.405Mhz apart to form a fullduplex circuitAddition – 160 kHz common signaling Addition – 160 kHz common signaling channel – 128 –kbps PSK signalchannel – 128 –kbps PSK signal

DAMA-FDMADAMA-FDMA

Reasons for Increasing Reasons for Increasing Use of TDM TechniquesUse of TDM TechniquesCost of digital components continues Cost of digital components continues to dropto dropAdvantages of digital componentsAdvantages of digital components Use of error correctionUse of error correctionIncreased efficiency of TDMIncreased efficiency of TDM

Lack of intermodulation noiseLack of intermodulation noise

FAMA-TDMA OperationFAMA-TDMA OperationTransmission in the form of repetitive Transmission in the form of repetitive sequence of framessequence of frames Each frame is divided into a number of Each frame is divided into a number of

time slotstime slots Each slot is dedicated to a particular Each slot is dedicated to a particular

transmittertransmitterEarth stations take turns using uplink channelEarth stations take turns using uplink channel Sends data in assigned time slotSends data in assigned time slotSatellite repeats incoming transmissionsSatellite repeats incoming transmissions Broadcast to all stationsBroadcast to all stationsStations must know which slot to use for Stations must know which slot to use for transmission and which to use for reception transmission and which to use for reception

FAMA-FAMA-TDMA TDMA UplinkUplink

FAMA-FAMA-TDMA TDMA

DownlinDownlinkk