Satellite Communications - Lecture 26

Satellite CommunicationsLecture 26OverviewRequirement of Satellite CommunicationSatellite UpLink and DownLinkTypes of SatellitesSatellite Foot Print (Coverage Area)Satellite Transmission BandsUpLink and DownLink FrequenciesSignal Propagation DelayTransponderEffect of Rain on Satellite CommunicationMicrowave Communication (Why)Satellite System ElementsLossesCapacity Allocation Strategies2Why Satellite Communication?The Earth is a sphere & the microwave frequencies travel in straight line but to connect two regions very far away on the two side of the sphere, the link requires lot of repeaters because of Earth’s curvature. A single satellite can do the magic linking the continents with one repeater.3Motivation to use Satellites4Communication SatelliteA Communication Satellite can be looked upon as a large microwave repeaterIt contains several transponders which listens to some portion of spectrum, amplifies the incoming signal and broadcasts it in another frequency to avoid interference with incoming signals.56SatelliteIt is a repeater which receives signal from Earth at one frequency, amplify it & transmit it back to Earth at other frequency.7EARTH STATIONThere are two earth station in a simple Satellite communication link. One transmits the signal to satellite called transmitting Earth station.The other receives the signal from satellite called receiving Earth Station.8UPLINK & DOWN LINKThe communication link from Transmitting earth station to satellite is called Up-link.The communication link from satellite To receiving earth station is called Down-link.9Altitudes of Orbits Above the EarthThere are 3 common types of satellite based on altitude, i.e. GEO, MEO & LEOOrbit Altitude Missions possibles Low-Earth orbit LEO 250 to 1,500 km Earth observation, meteorology, telecommunications (constellations) Medium-Earth orbit MEO 10,000 to 30,000 km Telecommunications (constellations), positioning, science Geostationary Earth orbit GEO 35,786 km Telecommunications, positioning, science Elliptical orbit Between 800 and 27,000 km Telecommunications Hyperbolic orbit Up to several million km Interplanetary missions 1011Types of Satellite OrbitsBased on the inclination, i, over the equatorial plane:Equatorial Orbits above Earth’s equator (i=0°)Polar Orbits pass over both poles (i=90°)Other orbits called inclined orbits (0° needs handoffEarth stations must track satellite or have Omni directional antennasLarge constellation of satellites is needed for continuous communication (66 satellites needed to cover earth)Requires complex architectureRequires tracking at ground18HEO - Highly Elliptical OrbitsHEOs (i = 63.4°) are suitable to provide coverage at high latitudes (including North Pole in the northern hemisphere)Depending on selected orbit (e.g. Molniya, Tundra, etc.) two or three satellites are sufficient for continuous time coverage of the service area.All traffic must be periodically transferred from the “setting” satellite to the “rising” satellite (Satellite Handover)19Satellite Orbits20Why Satellites Remain in Orbits2122Orbital PeriodThe time taken by a satellite to complete one rotation in its orbit is called its period. The GEO satellite takes 23 hrs & 56 minutes & 4.1 Seconds to complete its rotation which is approximately equal to the period of rotation of earth around its axis. This is why it appears to be stationary by the observer on Earth moving with the same speed as that of satellite. So one GEO stationary satellite can serve a ground user round the clock.23Orbital PeriodSatellite SystemOrbital Height (Km)Orbital Velocity (Km/Sec)Orbital Period(H M S)Intelsat (GEO)35,7863.074723 56 4.1New ICO (MEO)10,2554.89545 55 48.4Iridium (LEO)1,4697.12721 55 17.8Notice as altitude decreases, the velocity must be increased to minimize the gravitational effect.24Coverage Area of SatelliteThe Earth surface covered by satellite radiations is called FOOT PRINT. The coverage area is inversely proportional to frequency. The foot print will be large if the frequency of down link is low.25GEO satellite CoverageOne GEO can cover 1/3 of earth surface so the earth is divided in 3 regions.AOR (Atlantic Ocean Region)POR (Pacific Ocean region)IOR (Indian Ocean region)26FREQUENCIES for Satellite CommunicationLetter DesignationFrequency rangeUSEL band1 to 2 GHzSatellite phone, GPSS band2 to 4 GHzSatellite phoneC band4 to 8 GHzTV transmissionX band8 to 12 GHzKu band12 to 18 GHzTV transmission, CommunicationK band18 to 26.5 GHzKa band26.5 to 40 GHzSatellite InternetQ band30 to 50 GHzExperimentalU band40 to 60 GHzExperimental27Satellite Transmission BandsFrequency BandDownlinkUplinkC3,700-4,200 MHz5,925-6,425 MHzKu11.7-12.2 GHz14.0-14.5 GHzKa17.7-21.2 GHz27.5-31.0 GHzThe C band is the most frequently used. The Ka and Ku bands are reserved exclusively for satellite communication but are subject to rain attenuation28FREQUENCIES For Uplink & Down linkUplink uses higher frequency than the down link.Frequency of satellite is always specified as UPLINK frequency/ Down link Frequency e.g. C band 6/4 GHz Ku band14/11 GHz Ka band30/20 GHz29Signal Propagation DELAY Using c= 3*10 ^ 8 m/s & time= distance(altitude)/ speedUplink delay from earth station to Satellite.Round trip delay 4* uplink delay.All other delays in signal coding, compression, & processing on Satellite & earth Station are neglected.orbitAverage altitude of OrbitUplink DelayRound trip delayLEO800 Km2.7 ms10.8 msMEO10,355 Km34.5 ms138 msGEO35,786 Km119.3 ms480 ms = ½ Second 30Round trip delay of GEO signal31TransponderThe BW of satellite is divided into channels allowing many earth stations to use this BW. The Electronics to support each channel is called Transponder.Each transponder consist of band pass filter to select particular channel, down converter & output amplifier.Each satellite can have a large number of active & spare transponders typically 12 to 44 active transponder in each satellite.The total BW of transponder is usually 36,54 or 72 MHz.32Transponder33Effect of rain on signalRain heavily effects the wireless communication above 10 GHz. So Ku band & Ka band will be effected by rain & specially above 20 GHz the Ka Band link can fail during heavy rain fall. 34Why fup is always Higher than fdown?The beam of higher frequency is narrow & that of lower is broad. As the earth station has to target the signal to a small point (satellite) in space so it does it by using narrow beam produced by higher frequency. While the Satellite has to cover a large area on earth to provide services to many Earth station so it does it by using broad beam produced by lower frequency.As the rain effects higher frequencies more than lower one so they need to be boosted up more to overcome the propagation losses. The Energy can be given to signal much more easily on earth than on satellite because the satellite has limited power resources like solar cells & batteries so we use higher frequencies on Earth & amplify them with enough power supply resources we have on Earth35Up & down frequencies of C band36Satellite SignalsBase band signals can be analog or digital. But the final signal transmitted towards the satellite or from the satellite is always analog as it is an RF signal.37Increasing bandwidth of SatelliteMultiplexingUse of Audio & video compression in telephone & TV signals.Frequency ReuseSame frequency but orthogonal polarization.Increasing angular spacing of two satellites in same orbit using similar frequencies.38Life of SatelliteA satellite has two life.Design life: it is the predicted life of the electronic systems working in satellite.Maneuver life: It is the life during which full maneuver capabilities exist in satellite to change its position. It depends on fuel tank storage capacity & it is usually less than design life.At the end of useful GEO satellite life , it is raised to graveyard orbit.39Uses of SatelliteMilitary communicationTelecommunicationSatellite phone.VSAT (very small aperture terminal)Cable TVDBS (Direct Broadcast Satellite) TV (with Dish Antenna)GPS (Global Positioning System)Satellite InternetWeather forecastingPhotographyGIS (geographical Information System)X-Ray & infrared view of universeNavigationDeep Space exploration& many more4041Satellite Missions42Satellite Microwave TransmissionSatellites can relay signals over a long distanceGeostationary SatellitesRemain above the equator at a height of about 22300 miles (geosynchronous orbits)Travel around the earth in exactly the same time, the earth takes to rotate43Why Do Microwaves are Used for Satellite Communications?High enough frequency to carry the information, long enough wavelength to penetrate the atmosphere. They do not rapidly disperse in the atmosphere so the power does not need to be very high to reach a distant point. They can be focused by a suitable dish to increase the reception of low power signals. They can be modulated to carry the signal and are resistant to interference. They travel in straight enough lines to be able to aim them. They are easy to produce and easy to detect. 44Satellite System Elements45Space SegmentSatellite Launching PhaseTransfer Orbit PhaseDeploymentOperationTT&C - Tracking Telemetry and Command StationSSC - Satellite Control Center, a.k.a.:OCC - Operations Control CenterSCF - Satellite Control FacilityRetirement Phase46Ground SegmentCollection of facilities, Users and ApplicationsEarth Station = Satellite Communication Station (Fixed or Mobile)47Satellite Uplink and DownlinkDownlinkThe link from a satellite down to one or more ground stations or receiversUplinkThe link from a ground station up to a satellite.Some companies sell uplink and downlink services to television stations, corporations, and to other telecommunication carriers. A company can specialize in providing uplinks, downlinks, or both. 48Why Two Frequencies for Uplink and Downlink in Satellite ComThe reason the uplink and downlink frequencies are different in satellites is because otherwise the satellite's transmitter and receiver would interfere with one another. The signals have to operate on different frequencies.ButIf you could send a signal, then wait, the receiver could be protected from the transmitted signal on the same frequency, but with high speed, continuous transmission, the receiver cannot be turned off while the transmitter is transmitting. (an example of something that transmits and receives on the same frequency is pulsed RADAR, where the transmitter sends out a pulse, and then the echo is picked up by the receiver) 49Why Uplink Frequency is Higher than Downlink FrequencyThe signals have to cross the atmosphere which presents a great deal of attenuation. The higher the frequency, the more is the signal loss and more power is needed for reliable transmission.A satellite is a light-weight device which cannot support high-power transmitters on it. So, it transmits at a lower frequency (higher the frequency, higher is the transmitter power to accommodate losses) as compared to the stationary earth station which can afford to use very high-power transmitters. This is compensated by using highly sensitive receiver circuits on the earth station which is in the line-of-sight (LOS) of the satellite.50Uplink/Downlink Mobile/SatelliteA mobile is a portable device which cannot afford high-power transmission as it has a small battery with limited power. The 'free space path loss' comes to play. The higher the transmitting frequency, the higher is the loss. Since a mobile station (cellphone) cannot afford to transmit at high power to compensate for this loss, it must transmit on a lower frequency as a lower frequency presents lesser free space path loss. Therefore, mobile-to-base station (uplink) frequencies are lower than base station-to-mobile(downlink) frequencies.51Satellite Uplink and Downlink52Satellite CommunicationWhen using a satellite for long distance communications, the satellite acts as a repeater.An earth station transmits the signal up to the satellite (uplink), which in turn retransmits it to the receiving earth station (downlink).Different frequencies are used for uplink/downlink.53Satellite Transmission LinksEarth stations Communicate by sending signals to the satellite on an uplinkThe satellite then repeats those signals on a downlinkThe broadcast nature of downlink makes it attractive for services such as the distribution of TV programs54Direct to User ServicesOne way Service (Broadcasting)Two way Service (Communication)55Satellite SignalsUsed to transmit signals and data over long distancesWeather forecastingTelevision broadcastingInternet communicationGlobal Positioning Systems56Advantages of Satellite CommunicationCan reach over large geographical areaFlexible (if transparent transponders) Easy to install new circuits Circuit costs independent of distance Broadcast possibilities Temporary applications (restoration) Niche applications Mobile applications (especially "fill-in") Terrestrial network "by-pass" Provision of service to remote or underdeveloped areas User has control over own network 1-for-N multipoint standby possibilities 57Disadvantages of Satellite CommunicationLarge up front capital costs (space segment and launch) Terrestrial break even distance expanding (now approx. size of Europe) Interference and propagation delay Congestion of frequencies and orbits 58When to use SatellitesWhen the unique features of satellite communications make it attractive When the costs are lower than terrestrial routing When it is the only solution Examples:Communications to ships and aircraft (especially safety communications) TV services - contribution links, direct to cable head, direct to homeData services - private networks Overload traffic Delaying terrestrial investments 1 for N diversity Special events 59When to use TerrestrialPSTN - satellite is becoming increasingly uneconomic for most trunk telephony routes but, there are still good reasons to use satellites for telephony such as: thin routes, diversity, very long distance traffic and remote locations. Land mobile/personal communications - in urban areas of developed countries new terrestrial infrastructure is likely to dominate (e.g. GSM, etc.) but, satellite can provide fill-in as terrestrial networks are implemented, also provide similar services in rural areas and underdeveloped countries 60Frequency Bands Allocated to the FSS(Fixed service satellite)Frequency bands are allocated to different services at World Radio-communication Conferences (WRCs). Allocations are set out in Article S5 of the ITU Radio Regulations. It is important to note that (with a few exceptions) bands are generally allocated to more than one radio services.CONSTRAINTS Bands have traditionally been divided into “commercial" and "government/military" bands, although this is not reflected in the Radio Regulations and is becoming less clear-cut as "commercial" operators move to utilize "government" bands.61Earth’s atmosphereSource: All about GPS [www.kowoma.de]62Atmospheric LossesDifferent types of atmospheric losses can disturb radio wave transmission in satellite systems:Atmospheric absorptionAtmospheric attenuationTraveling ionospheric disturbances63Atmospheric AbsorptionEnergy absorption by atmospheric gases, which varies with the frequency of the radio waves.Two absorption peaks are observed (for 90º elevation angle):22.3 GHz from resonance absorption in water vapour (H2O)60 GHz from resonance absorption in oxygen (O2)For other elevation angles:[AA] = [AA]90 cosec 64Atmospheric AttenuationRain is the main cause of atmospheric attenuation (hail, ice and snow have little effect on attenuation because of their low water content).Total attenuation from rain can be determined by:A = L [dB]where  [dB/km] is called the specific attenuation, and can be calculated from specific attenuation coefficients in tabular form that can be found in a number of publicationswhere L [km] is the effective path length of the signal through the rain; note that this differs from the geometric path length due to fluctuations in the rain density. 65Traveling Ionospheric DisturbancesTraveling ionospheric disturbances are clouds of electrons in the ionosphere that provoke radio signal fluctuations which can only be determined on a statistical basis.The disturbances of major concern are:Scintillation;Polarisation rotation.Scintillations are variations in the amplitude, phase, polarisation, or angle of arrival of radio waves, caused by irregularities in the ionosphere which change over time. The main effect of scintillations is fading of the signal.66What is Polarisation?Polarisation is the property of electromagnetic waves that describes the direction of the transverse electric field. Since electromagnetic waves consist of an electric and a magnetic field vibrating at right angles to each other.it is necessary to adopt a convention to determine the polarisation of the signal.Conventionally, the magnetic field is ignored and the plane of the electric field is used.67Types of PolarisationLinear Polarisation (horizontal or vertical):the two orthogonal components of the electric field are in phase;The direction of the line in the plane depends on the relative amplitudes of the two components.Circular Polarisation:The two components are exactly 90º out of phase and have exactly the same amplitude.Elliptical Polarisation:All other cases.Linear PolarisationCircular PolarisationElliptical Polarisation68Satellite CommunicationsAlternating vertical and horizontal polarisation is widely used on satellite communicationsThis reduces interference between programs on the same frequency band transmitted from adjacent satellites (One uses vertical, the next horizontal, and so on)Allows for reduced angular separation between the satellites.6970Ways to CategorizeCommunications SatellitesCoverage areaGlobal, regional, nationalService typeFixed service satellite (FSS)Broadcast service satellite (BSS)Mobile service satellite (MSS)General usageCommercial, military, amateur, experimental71Classification of Satellite OrbitsCircular or elliptical orbitCircular with center at earth’s center Elliptical with one foci at earth’s centerOrbit around earth in different planesEquatorial orbit above earth’s equatorPolar orbit passes over both polesOther orbits referred to as inclined orbitsAltitude of satellitesGeostationary orbit (GEO)Medium earth orbit (MEO)Low earth orbit (LEO)72Minimum Elevation AngleReasons affecting minimum elevation angle of earth station’s 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 reception73GEO 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 substantial74LEO 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 deterioration75LEO 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 services76MEO 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 hours77Frequency Bands Available for Satellite Communications78Satellite 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 frequencies79Satellite Network Configurations80Capacity Allocation StrategiesFrequency division multiple access (FDMA)Time division multiple access (TDMA)Code division multiple access (CDMA)81Frequency-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 signals82Frequency-Division Multiple AccessFactors which limit the number of subchannels provided within a satellite channel via FDMAThermal noiseIntermodulation noiseCrosstalk83Forms 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 stations84FAMA-FDMAFAMA – logical links between stations are preassignedFAMA – multiple stations access the satellite by using different frequency bandsUses considerable bandwidth85DAMA-FDMASingle channel per carrier (SCPC) – bandwidth divided into individual VF channelsAttractive for remote areas with few user stations near each siteSuffers from inefficiency of fixed assignmentDAMA – 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 CSC86Reasons for Increasing Use of TDM TechniquesCost of digital components continues to dropAdvantages of digital componentsUse of error correctionIncreased efficiency of TDMLack of intermodulation noise87FAMA-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 88FAMA-TDMA Uplink89FAMA-TDMA Downlink90Summary91Requirement of Satellite CommunicationSatellite UpLink and DownLinkTypes of SatellitesSatellite Foot Print (Coverage Area)Satellite Transmission BandsUpLink and DownLink FrequenciesSignal Propagation DelayTransponderEffect of Rain on Satellite CommunicationMicrowave Communication (Why)Satellite System ElementsLossesCapacity Allocation Strategies