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SATELLITE COMMUNICATIONS BISHOY MOFIED OMAR SHAABAN VARTAN SHOUSHANIAN 1
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SATELLITE COMMUNICATIONS. BISHOY MOFIED OMAR SHAABAN VARTAN SHOUSHANIAN. 1. Outline. Introduction How satellite works History of satellites Satellite Frequency Bands Digital modulation schemes Satellite components Orbital altitudes GPS satellites Antenna Gain efficiency - PowerPoint PPT Presentation

Transcript of SATELLITE COMMUNICATIONS

Satellite Communications

SATELLITE COMMUNICATIONSBISHOY MOFIEDOMAR SHAABANVARTAN SHOUSHANIAN1

11OutlineIntroductionHow satellite worksHistory of satellitesSatellite Frequency BandsDigital modulation schemesSatellite componentsOrbital altitudesGPS satellitesAntenna Gain efficiencyAntenna Examples

2Satellites Theory3

History of Satellites(Read Only)

Early in October 1957 communications stations started picking up a regular beeping noise coming from space.

The signals were coming from Sputnik 1, the world's first man-made satellite by the soviet union.

At 1958, NASA successfully launched Explorer 1, the first American satellite.

4History of Satellites(Read Only)The first satellite picture of Earth came from NASA's Explorer 6 in 1959In 1963 , the world's first geosynchronous communications satellite NASA Syncom 2 was launched as Its earlier sister, Syncom 1, had been blown up on launch earlier that year, but the second version was a huge success.

Now a days 3,600 satellites are orbiting the earth out of 6,600 satellite launched the rest are part of the space debris

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Advantages of SatellitesHigh channel capacity (>100 Mb/s)Low error rates (Pe ~ 10-6)Bit Rate (Bits/Sec) = BW * log2(1 + SNR)Stable cost environment (no long-distance cables or national boundaries)Wide area coverage (whole North America, for instance)

6Disadvantages of SatellitesExpensive to launchExpensive ground stations requiredCannot be maintainedLimited orbital space (geosynchronous)

7How satellite worksA Earth Station sends message modulated in GHz range. (Uplink) Satellite Receive signal and retransmit it back to the earth. (Downlink)8Microwave frequency range 300MHz to 300GHzSatellite frequency range 1GHz to 170GHz divided into bands with letters Wide bandwidthLine of sight required

Satellite communication bands(read only)99

Satellite communication bands1010Satellite communication bandsL band 0.950 - 1.450 GHzGPS at 1.57542 GHzC band3.7 - 4.2 GHz (Downlink)5.925 - 6.425 GHz (Uplink)Ku band 11.7 - 12.2 GHz (Downlink) 14 - 14.5 GHz (Uplink)Ka band 18.3 - 18.8, 19.7 - 20.2 GHz (Downlink)30 GHz (Uplink)V band40 - 75 GHz60 GHz allocated for unlicensed (WiFi) use70, 80, and 90 GHz for other wireless1111Uplink and downlink

Satellite communication bands(read only)

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Digital Modulation schemes

14Phase Shift Key ModulationBinary phase-shift keying:Sinusoidal wave with 0 and 180 represent binary zero and one .

Offset quadrature phase-shift keying:uses phase shift of 45,135,225,315 deg to represent 11,01,00,10

This doubles the channel bit rate

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Offset quadrature phase-shift keying(read only)The I/Q modulator mixes the I signal with the carrier cosine wave, and it mixes the Q signal with the same carrier sin waveThere are many ways to build a satellite, but the key parts are always the same and these include:

Communication and processing system Antenna, Transceivers and On-board processor.Propulsion system: Rocket motors and Thrusters.Power System : Fuel tanks,Solar panels,Batteries.

Satellite system components161616Communication and processing-Transponder can either operate on bent pipe principle or regenerative principle Bent pipe:sending back to earth of what goes in with only amplification and a shift from uplink to downlink frequency.Regenerative:use on-board processing, to demodulate, decode, re-encode and modulated aboard the satellite.

Satellite system components171717

Bent Pipe PrincipleRegenerative Principle18Bent Pipe AdvantagesReduced power and weight (simpler)DisadvantagesInter modulation between carriers to be avoidedNo error correction Regenerative AdvantagesReduce error rate (error correction) Noise doesn't carry overIncreased system flexibilityDisadvantagesIncreased power consumptionHeavier payloadDifficult to change signal format(change modulation scheme)

Satellite system components191919

-Transponder example (Bent pipe)Satellite system components202020Example Base station to user flowSatellite system components21

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Travelling wave tube amplifier

Low noise block downconverter (LNB) diagram

23Read only:Dual-band LNBs

These will typically have two alternative local oscillator frequencies, for example 9.75 GHz and 10.6 GHz with the higher frequency option selected using a 22 kHz tone injected into the cable. Such an LNB may be used to receive 10.7 - 11.7 GHz using the lower 9.75 GHz LO frequency or the higher band 11.7 - 12.75 GHz using the higher 10.6 GHz LO frequency.LNB Frequency stability

All LNBs used for satellite TV reception use dielectric resonator stabilised local oscillators. The DRO is just a pellet of material which resonates at the required frequency. Compared with quartz crystal a DRO is relatively unstable with temperature and frequency accuracies may be +/- 250 kHz to as much as +/- 2 MHz at Ku band. This variation includes both the initial value plus variations of temperature over the full extremes of the operating range. Fortunately most TV carriers are quite wide bandwidth (like 27 MHz) so even with 2 MHz error the indoor receiver will successfully tune the carrier and capture it .LNB supply voltages

The DC voltage power supply is fed up the cable to the LNB. Often by altering this voltage it is possible to change the polarisation or, less commonly, the frequency band. Voltages are normally 13 volts or 19 volts.24The propulsion system- large rocket motor and a smaller thruster rockets that keep the satellite at that location. -forces like the pressure of the solar wind, the effects of the Earth's and moon's gravity,..-Attitude control through spinning gyroscopes.

Satellite system components252525

26Power System-Solar panel with batteryPower degrades over long time-Radioactive isotopes Low power over long time-Fuel CellsHigh power but needs resupply

Satellite system components272727TYPES OF SATELLITES (BASED ON ORBITS)Geostationary or geosynchronous earth orbit (GEO) Low Earth Orbit (LEO)Medium Earth Orbit (MEO)

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~35000 Km ~8000-16000 Km ~1600 Km 292929GEOSYNCHRONOUS EARTH ORBIT (GEO)GEO satellites are synchronous with respect to earth. Looking from a fixed point from Earth, these satellites appear to be stationary.Covers around 1/3 (120 degrees latitude) of the earths surface.Three conditions which lead to a Geosynchronous satellites :The satellite should be placed 37,786 kms (approximated to 36,000 kms) above the surface of the earth. These satellites must travel in the rotational speed of earth, and in the direction of motion of earth, that is eastward. The inclination of satellite with respect to earth must be 0.

3030GEOSYNCHRONOUS EARTH ORBIT (GEO)Disadvantages of GEO: Northern or southern regions of the Earth (poles) have more problems receiving these satellites due to the low elevation above a latitude of 60, i.e., larger antennas are needed in this case. The transmit power needed is relatively high which causes problems for battery powered devices. These satellites cannot be used for small mobile phones. The biggest problem for voice and also data communication is the high latency as without having any handovers, the signal has to at least travel 72,000 kms. Transferring a GEO into orbit is very expensive3131LOW EARTH ORBIT (LEO) SATELLITESThese satellites are placed 500-1500 kms above the surface of the earth.As LEOs circulate on a lower orbit, hence they exhibit a much shorter period that is 95 to 120 minutes.Each LEO satellite will only be visible from the earth for around ten minutes.

3232LOW EARTH ORBIT (LEO) SATELLITESDisadvantage: The biggest problem of the LEO concept is the need for many satellites if global coverage is to be reached. (50200 satellites)One general problem of LEOs is the short lifetime of about five to eight years due to atmospheric drag and radiation from the inner Van Allen belt1.The short time of visibility with a high elevation requires additional mechanisms for connection handover between different satellites.the need for routing of data packets from satellite to if a user wants to communicate around the world.

3333 MEDIUM EARTH ORBIT (MEO) SATELLITESMEOs can be positioned somewhere between LEOs and GEOs,both in terms of their orbit and due to their advantages and disadvantages. Using orbits around 10,000 kmThe system only requires a dozen satellites which is more than a GEO system, but much less than a LEO system.These satellites move more slowly relative to the earth's rotation allowing a simpler system design (satellite periods are about six hours). 3434MEDIUM EARTH ORBIT (MEO) SATELLITESDisadvantage : Again, due to the larger distance to the earth, delay increases to about 7080 ms. the satellites need higher transmit power and special antennas for smaller footprints.3535THE GLOBAL POSITIONING SYSTEM (GPS) The Global Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are traveling at speeds of roughly ~11000 Km an hour.

3636The whole idea behind GPS is to use satellites in space as reference points for locations here on earth. GPS satellites use a "triangulate," system where the GPS receiver measures distance using the travel time of radio signals. By using triangulation, we can accurately measure our distance and find out position from three satellites position anywhere on earthTHE GLOBAL POSITIONING SYSTEM (GPS) 3737THE GLOBAL POSITIONING SYSTEM (GPS) Even though the satellites positions are constantly monitored, they can't be watched every second.

The atomic clocks they use are very, very precise but they're not perfect. Minute discrepancies can occur, and these translate into travel time measurement errors.

The signal may not actually get to the ground station receivers first. It may bounce off various objects before it gets to the receivers.

3838Antenna Design

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