chap9

Satellite Communications

Satellite-Related TermsEarth Stations antenna systems on or near earthUplink transmission from an earth station to a satelliteDownlink transmission from a satellite to an earth stationTransponder electronics in the satellite that convert uplink signals to downlink signals

Ways to CategorizeCommunications SatellitesCoverage areaGlobal, regional, nationalService typeFixed service satellite (FSS)Broadcast service satellite (BSS)Mobile service satellite (MSS)General usageCommercial, military, amateur, experimental

Classification of Satellite OrbitsCircular or elliptical orbitCircular with center at earths center Elliptical with one foci at earths centerOrbit around earth in different planesEquatorial orbit above earths equatorPolar orbit passes over both polesOther orbits referred to as inclined orbitsAltitude of satellitesGeostationary orbit (GEO)Medium earth orbit (MEO)Low earth orbit (LEO)

Geometry TermsElevation angle - the angle from the horizontal to the point on the center of the main beam of the antenna when the antenna is pointed directly at the satelliteMinimum elevation angleCoverage angle - the measure of the portion of the earth's surface visible to the satellite

Minimum Elevation AngleReasons affecting minimum elevation angle of earth stations antenna (>0o)Buildings, trees, and other terrestrial objects block the line of sightAtmospheric attenuation is greater at low elevation anglesElectrical noise generated by the earth's heat near its surface adversely affects reception

GEO OrbitAdvantages of the the GEO orbit No problem with frequency changesTracking of the satellite is simplifiedHigh coverage areaDisadvantages of the GEO orbitWeak signal after traveling over 35,000 kmPolar regions are poorly servedSignal sending delay is substantial

LEO Satellite CharacteristicsCircular/slightly elliptical orbit under 2000 kmOrbit period ranges from 1.5 to 2 hoursDiameter of coverage is about 8000 kmRound-trip signal propagation delay less than 20 msMaximum satellite visible time up to 20 minSystem must cope with large Doppler shiftsAtmospheric drag results in orbital deterioration

LEO CategoriesLittle LEOsFrequencies below 1 GHz 5MHz of bandwidth Data rates up to 10 kbpsAimed at paging, tracking, and low-rate messagingBig LEOsFrequencies above 1 GHz Support data rates up to a few megabits per secOffer same services as little LEOs in addition to voice and positioning services

MEO Satellite CharacteristicsCircular orbit at an altitude in the range of 5000 to 12,000 kmOrbit period of 6 hoursDiameter of coverage is 10,000 to 15,000 kmRound trip signal propagation delay less than 50 msMaximum satellite visible time is a few hours

Frequency Bands Available for Satellite Communications

Satellite Link Performance FactorsDistance between earth station antenna and satellite antennaFor downlink, terrestrial distance between earth station antenna and aim point of satelliteDisplayed as a satellite footprint (Figure 9.6)Atmospheric attenuationAffected by oxygen, water, angle of elevation, and higher frequencies

Satellite Footprint

Satellite Network Configurations

Capacity Allocation StrategiesFrequency division multiple access (FDMA)Time division multiple access (TDMA)Code division multiple access (CDMA)

Frequency-Division MultiplexingAlternative uses of channels in point-to-point configuration1200 voice-frequency (VF) voice channelsOne 50-Mbps data stream16 channels of 1.544 Mbps each400 channels of 64 kbps each600 channels of 40 kbps eachOne analog video signalSix to nine digital video signals

Frequency-Division Multiple AccessFactors which limit the number of subchannels provided within a satellite channel via FDMAThermal noiseIntermodulation noiseCrosstalk

Forms of FDMAFixed-assignment multiple access (FAMA)The assignment of capacity is distributed in a fixed manner among multiple stationsDemand may fluctuateResults in the significant underuse of capacityDemand-assignment multiple access (DAMA)Capacity assignment is changed as needed to respond optimally to demand changes among the multiple stations

FAMA-FDMAFAMA logical links between stations are preassignedFAMA multiple stations access the satellite by using different frequency bandsUses considerable bandwidth

DAMA-FDMASingle channel per carrier (SCPC) bandwidth divided into individual VF channelsAttractive for remote areas with few user stations near each siteDAMA set of subchannels in a channel is treated as a pool of available links For full-duplex between two earth stations, a pair of subchannels is dynamically assigned on demandDemand assignment performed in a distributed fashion by earth station using CSC

Reasons for Increasing Use of TDM TechniquesCost of digital components continues to dropAdvantages of digital componentsUse of error correctionIncreased efficiency of TDMLack of intermodulation noise

FAMA-TDMA OperationTransmission in the form of repetitive sequence of framesEach frame is divided into a number of time slotsEach slot is dedicated to a particular transmitterEarth stations take turns using uplink channelSends data in assigned time slotSatellite repeats incoming transmissionsBroadcast to all stationsStations must know which slot to use for transmission and which to use for reception

FAMA-TDMA Uplink

FAMA-TDMA Downlink