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UNIT 4SATELLITE LINK DESIGN

IntroductionThe design of satellite communication system is a complex process requiring compromises between many factors to achieve the best performance at an acceptable cost

The cost to build and launch a GEO satellite is about $25000 per kgIntroductionWeight is the critical factorHeavier the satellite higher the cost capital cost of the satellite must be recovered over its lifetime by selling communication servicesIntroductionWeight of the satellite is driven by 2 factorsThe number and output power of transponders on the satelliteWeight of the station keeping fuelHalf of the total weight of satellites intended to remain in service for 15 years may be fuelIncrease in total output power of the transponders raises the demand for electrical power increases the dimensions of the solar cellsadding more weight to the satelliteIntroductionThree other factors influence system designThe choice of frequency bandAtmospheric propagation effectsMultiple access technique

ITU RADIO REGULATIONSTable 4.1 pg:97Major frequency allocations for fixed satellite service and broadcasting satelliteTable 4.2 pg:98Major frequency allocations for mobile satellite servicesRegion I Europe, Africa and Northern AsiaRegion II North and South AmericaRegion III remainder of asiaIntroductionThe major bands are6/4 GHz14/11 GHz30/20GHz

For every 2o on the geostationary orbit there is a satellite using both 6/4 GHz and 14/11 GHzMinimum spacing for GEO to avoid interferenceIntroductionRAIN in the atmosphere attenuates radio signalsEffect is more severe as the frequency increasesLittle attenuation at 4 and 6 GHzSignificant attenuation above 10GHzAttenuation through rain (in dB) increases roughly as the square of frequencyIntroductionLEO, MEO compared to GEOCloser to earthAdvantage of strong signalsPosition change rapidlyLow gain antennas at earth terminalsLEO, MEO satellites use multiple beam antennas to increase the gain of the satellite antenna beams and also to provide frequency reuseIntroductionMobile satellite terminals must operate with low gain antennas at the mobile unitLink between the satellite and the major earth station (hub station)is usually in a different frequency band as it is a fixed linkMaritimeSatellite Communication system

Link Performance criteriaDesign of all communication links should meet certain performance objectivesUsually specified Bit Error Rate (BER) or probability of error in a digital link and S/N ratio in an analog linkMeasured in a baseband channelBaseband channel is where an information carrying signal is generated or receivedLink Performance criteriaA baseband channel BER or S/N is determined by C/N ratio at the input to the demodulator in the receiverC/N>6dB for most satellite comm. applicationsDigital links C/N below 10dB must use Error correction codes to improve BER Analog links using FM require wideband FM to achieve improvement in S/N relative to C/NS/N = 40dB in TelevisionS/N = 30dB in Speech signalsLink Performance criteriaS/N or BER is specifiedMeasured at demodulator outputAt basebandLink Performance criteriaOverall C/N at the earth station receiver depends on both uplink and downlink

Both links must achieve required performance

Heavy rain C/N ratio fall below a permitted value especially 30/20GHz band link outageLink Performance criteriaDesigning a satellite system requires knowledge of the required performance of the uplink and downlink, the propagation characteristicsrain attenuation for the frequency band being used at the earth station locations and the parameters of the satellite and the earth stationsLink Performance criteriaSometimes all this information in not availableThe designer must estimate values and produce tables of system performance with assumed scenariosDesigning is not complete at first attemptTrial design Refined until workable compromise is achievedBasic Transmission Theory: Isotropic Radiator

Basic Transmission Theory: Isotropic Radiator

Basic Transmission TheoryFor a transmitter with output Pt watts driving a lossless antenna with gain Gt

Basic Transmission TheoryEIRP effective isotropically radiated powerproduct of transmitter power and antenna gain in terms of an equivalent isotropic source with power PtGt watts, radiating uniformly in all directionsBasic Transmission Theory: Received power

Basic Transmission Theory: Received power

Basic Transmission Theory: Received power

Basic Transmission Theory: Link Budget Equation

Basic Transmission Theory: Path Loss

Basic Transmission TheoryTherefore

In decibel terms

Basic Transmission Theory: Link Budget Equation with Losses in Link

Link Budget Equation with Losses in Link

A Satellite link, LNA

System Noise temperatureNoise temperature way of determining how much THERMAL NOISE is generated by active and passive devices in the receiving system

At microwave frequencies, a black body with a physical temperature Tp degrees kelvin, generates electrical noise over a wide bandwidth

System Noise temperatureThe noise power is given byPn=kTPBn

k=Bolzmanns constant = 1.39x10-23 J/K=-28.6 dBW/K/HzTp=physical temperature of source in kelvin degBn=Noise bandwidth in which the noise power is measured, HzSystem Noise temperatureThe noise power is given byPn=kTPBnPn is the available noise power (W)Delivered only to the load that is impedance matched to the noise sourcekTp is a noise power spectral density (W/Hz)Density is constant for all radio frequencies up to 300GHzSystem noise temperature (Ts)Noise temperature of a noise source, located at the input of a noiseless receiver, which gives the same noise power as the original receiver, measured at the output of the receiver and usually includes noise from the antennaNoise powerThe noise power at the demodulator input isPno=kTsBnGrxGrx=overall end-to-end gain of the receiverThis is a ratio not in decibelsThis can also be said as the gain of the receiver from RF input to demodulator input

Noise powerNoise power referred to the input of the receiver isPn=kTsBn wattsC/N ratio at the demodulator input (C/N)=(PrGrx)/PnoPno=kTsBnGrx

Therefore C/N=Pr/kTsBnCalculation of system noise temperature

Calculation of system noise temperature

LNB-Low noise block converter900-1400MHzSet top receiverDBS TV 20MHzCalculation of system noise temperature

Noise Model of receiver

Noise Model of receiverCalculation of System Noise Temperature

Noise model of receiver

Alternate method for calculation of system noise temperature

Alternate method for calculation of system noise temperature

Note:When the RF amplifier in the receiver front end has a high gain, the noise contributed by the IF amplifier and later stages can be ignored system noise temperature is simply the sum of the antenna noise temperature and the LNA noise temperatureTs=Tantenna+TLNANoise Model for a Lossy deviceIn some circumstances, we need to use a different model to deal with the noise that reaches the receiver after passing through lossy medium e.g:waveguide transmission line and rain losses

Noise Figure and Noise TemperatureNoise figure of a receiver was described as a measure of the noise produced by a practical receiver as compared with the noise of an ideal receiver

Noise Figure and Noise TemperatureEffective Noise Temperature of the network is defined as that temperature Tnat the input of the network which would account for the noise NN=kTnBnG

Noise Figure and Noise Temperature

G/T ratio for Earth StationsTransmitters characterized by EIRPReceivers characterized by Gr/TsThe Gain of the antenna divided by the noise temperature of the receiver

Received power Pr, is commonly referred to as carrier power, C This is the input to the receiver LNA First step in determining C/N

G/T ratio for Earth Stations

Design of DownlinkDesign of satellite communication is based on two objectives:1) Meeting a minimum C/N for a specified percentage of time and 2) Carrying the maximum revenue earning traffic at minimum costDesign of DownlinkAll satellite communications links are affected by rain attenuationC band links can be designed to achieve 99.99% reliability because the rain attenuation rarely exceeds 1 or 2 dB0.01% corresponds to 52.56 min an yearThe link is said to suffer an outage Design of DownlinkKa-band links cannot be designed to achieve 99.99% reliability because rain attenuation generally exceeds 10dB and often 20dBOutage times of 8 to 40 hours are usually tolerated in Ka-band linksWhat is a Link Budget? A link budget is the accounting of all of the gains and losses from the transmitter, through the medium (free space, cable, waveguide, fiber, etc.) to the receiver in a telecommunication system. It accounts for the attenuation of the transmitted signal due to propagation, as well as the antenna gains, feedline and miscellaneous losses.It is first step an engineer will take in order to determine feasibility of any given systemA link budget calculation is also an excellent means to understand various factors which may be traded off to realize a given cost and reliability of a communication system

The Link Budget

Link BudgetsC/N ratio calculation is simplifiedIt is a tabular method for evaluating the received power and noise power in a radio linkLink budgets invariably use decibel units for all quantities so that signal and noise powers can be calculated by addition and subtractionIt is usually impossible to design a satellite link at the first attemptLink budgets make the task much easierLink BudgetsThe link budget must be calculated for an individual transponder, and must be repeated for each of the individual linksWhen a bent pipe transponder is used the uplink and downlink C/N ratios must be combined to give an overall C/NLink budgets are usually calculated for a worst caseLink Budget Example: C-Band Downlink for Earth Coverage BeamLink budgets are usually calculated for a WORST CASELink will have the lowest C/N ratioFactors - WORST CASE scenarioEarth station located at the edge of the satellite coverage zone where the received signal is typically 3dB lower than in the center of the zone because of the satellite antenna pattern

Link Budget Example: C-Band Downlink for Earth Coverage BeamFactors - WORST CASE scenario2. Maximum path length from the satellite to earth station3. A low elevation angle at the earth station giving the highest atmospheric path attenuation in clear air

Link Budget Example: C-Band Downlink for Earth Coverage BeamFactors - WORST CASE scenario4. Max rain attenuation on the link causing loss of received signal power and an increase in receiving system noise temperatureFew more points to be consideredEdge of the coverage pattern of the satellite antenna and the longest path usually go togetherEarth station antennas are assumed to be pointed directly at the satellite and therefore operate at their on-axis gainIf the antenna is mispointed, a loss factor is included in the link budget to account for the reduction in antenna gainDesign of Satellite Communication link

Link Budget Example: C-Band Downlink for Earth Coverage BeamSatellite used is in GEOSTATIONARY EARTH ORBITCarries 24 C-band transponders, each with a bandwidth of 36MHz.Downlink band is 3.7-4.2GHzUses orthogonal circular polarizationProviding an effective bandwidth of 864MHzLink Budget Example: C-Band Downlink for Earth Coverage BeamSatellite provides coverage of visible earthSubtending an angle of 17o Using a Global beam antenna

Relation between Antenna Beamwidth and Gain

Link Budget Example: C-Band Downlink for Earth Coverage BeamThe on-axis Gain of the global beam antenna is approximately 20 dB

Link Budget Example: C-Band Downlink for Earth Coverage BeamThe C/N ratio for the downlink is calculated in clear air conditions and also in heavy rain

The Saturated output power of the transponder = 20W = 13dBWAssume an output backoff of 2dBTherefore on-axis EIRP of the transponder and the antenna = PtGt =(13-2)+20 =31dbWIn this example the transmitted signal is a single 30MHz bandwidth analog FM-TV channel .

Recall..When more than one signal shares a transponder using FDMA, The power amplifier must be run below its maximum output power to MAINTAIN LINEARITY and REDUCE INTERMODULATION PRODUCTS

The degree to which the transmitter output power is reduced below its peak output is known as output backoff

Link Budget Example: C-Band Downlink for Earth Coverage BeamFor analog TV transmission, Receiver noise bandwidth is set to 27MHzThe receiving earth station has an antenna with an aperture diameter of 9ma gain of 49.7dB at 4GHz assuming an aperture efficiency of 65%Receiving system noise temperature of 75K in clear airG/T ratio for this earth station is G/T=49.7 10log(75) = 30.9 dBK-1Max. path length for a GEO satellite link is 40,000kmPath loss of 196.5dB at 4GHzLink Budget Example: C-Band Downlink for Earth Coverage BeamAt C-band, propagation losses are small but the slant path through the atmosphere will suffer a typical attenuation of 0.2dB in clear airAllowing an additional 0.5dB margin in the link design to account for miscellaneous losses to ensure that the link budget is realisticAntenna mispointingPolarization mismatchAntenna degradationLink Budget Example: C-Band Downlink for Earth Coverage BeamThe earth station receiver C/N ratio is first calculated for clear air conditions with no rain in the slant pathThe C/N ratio is then recalculated taking account of the effects of rainThe minimum permitted OVERALL C/N ratio for this link is 9.5dB corresponding to the FM threshold of an analog satellite TV receiverC-Band GEO Satellite Link Budget in Clear Air

C-Band GEO Satellite Link Budget in Clear AirWe have a downlink C/N of 16 dB in clear air, giving a link margin of 6.5dB

C-Band GEO Satellite Link Budget in RainThis link margin is reduced when there is rain in the slant pathHeavy rain in the slant path can cause up to 1dB of attenuationReduces the received power by 1dB and increases the noise temperature of the receiving systemC-Band GEO Satellite Link Budget in RainUsing the output noise model with a medium temperature of 273K andThe total atmospheric attenuation due to (clear air + rain) is (0.2dB+1dB) = 1.2dBWhen rain drops cause attenuation, they radiate additional noise whose level depends on the attenuation

Recalling equivalent output noise source modelFrom the noise model

Tno = Tp ( 1 GI)

Tno = noise temperatureTp = physical temperature of the device or mediumGI = linear gain (less than unity, not in dB) of the attenuating device or medium

Recalling the relation between gain and attenuationFor an attenuation of A dB, the value of GI is given by GI=10-A/10C-Band GEO Satellite Link Budget in RainSky noise temperature in (clear air + rain) isTno = Tp ( 1 GI) GI=10-A/10A=1.2dBTsky=273(1 - 0.7585) = 66KC-Band GEO Satellite Link Budget in RainIn clear air the sky noise temperature is 13KThis is the result of 0.2dB clear air attenuationTno = Tp ( 1 GI) GI=10-A/10A=0.2dBTsky=273(1 0.9549) = 13K

C-Band GEO Satellite Link Budget in RainClear air sky noise temperature = 13K(Clear air + rain) sky noise temperature = 66KReceiving system noise temperarure = 75KNoise temperature of the atmosphere only due to rain is (66K 13K) = 53KTherefore the noise temperature of the receiving system is increased to (75K + 53K) = 128KC-Band GEO Satellite Link Budget in RainNoise temperature of the atmosphere only due to rain is (66K 13K) = 53K This result in 1dB of rain attenuation in the slant pathTherefore there is an increase in system noise temperature of 2.3dBResulting a C/N ratio in rain = 12.7dBC-Band Downlink Budget in Rain

Link Budget Example: C-Band Downlink for Earth Coverage BeamTherefore the link margin of (12.7 9.5) = 3.2 dB is obtained in C/N ratio in rain Leaving a small margin of 2dB for unexpected losses to guarantee a particular level of reliability in the linkRemaining 1.2 dB of link margin can be traded against other parameters in the system. For example reduction in receiving antenna gain by 1.2 dBLink Budget Example: C-Band Downlink for Earth Coverage BeamFor example reduction in receiving antenna gain by 1.2 dBAntenna size can be reduced from 9m to 7.8m

Satellite systems using small earth stationsThe C/N ratio in the home receiver (DBS-TV receiver) will fall when rain is in the path between the satellite and the receiving antennaMuch reduction in C/N ratio is caused by an increase in the sky noise temperatureSatellite systems using small earth stationsThe following calculations show how the system noise temperature and (C/N)dn are determined when rain attenuation is present

Satellite systems using small earth stationsNot all of the incident noise energy from the sky is output by the antenna and a coupling coefficient is used c of 90 to 95%Therefore antenna noise temperature in rain

Satellite systems using small earth stationsLNA is assumed to be placed right at the feed horn so that there is no waveguide or coaxial run between the feed horn of the antenna and the LNAAssuming no feed losses

Satellite systems using small earth stations

UPLINK DesignAnalysis of uplink requires calculation of the power level at the input to the transponder so that the uplink C/N ratio can be foundLink equation is used to make this calculation, using either a specified transponder C/N ratio or a required transponder output power level

UPLINK DesignLet (C/N)up be the specified C/N ratio in the transponder, measured in a noise bandwidth Bn HzBandwidth Bn Hz is the bandwidth of the band-pass filter in the IF stage of the earth station receiver for which the uplink signal is intended

UPLINK DesignWhere Txp = system noise temperature of the transponder in dBKBn=dBHz , Nxp = dBW

Prxp=Pt+Gt+Gr-Lp-Lup dBW

C/N = 10 log[Prxp/kTxpBn] = Prxp Nxp dB

UPLINK DesignPrxp = C/N + Nxp

Note:To achieve a similar satellite EIRP with a small-diameter earth stations, A high power earth station transmitter is required This has the disadvantage that the interference level at the adjacent satellites risesSince the small earth station antenna has a wider beam

Uplink Power Control (UPC)This can be used to combat uplink rain attenuationThe transmitting earth station monitors a Beacon signal from the satelliteWatches for reductions in power indicating rain fading on the downlink

Automatic Monitoring and Control of transmitted uplink power is used in 14 GHz uplink earth stations to maintain C/N ratio in the satellite transponder during periods of rain attenuationNote:Since the downlink is always at a different frequency from the uplink, a downlink attenuation of A dB must be scaled to estimate uplink attenuationThe uplink attenuation is given by Aup = Adown x [fup/fdown]a dB

Where [fup/fdown]a is the scaling factora is typically between 2.0 and 2.4Design of Uplink - problemA transponder of a Ku-band satellite has a linear gain of 127 dB and a nominal output power at saturation of 5 W. The satellites 14 GHz receiving antenna has a gain of 26 dB on axis, and the beam covers western Europe.Calculate the power output of an uplink transmitter that gives an output power of 1W from the satellite transponder at a frequency of 14.45GHz when the earth station antenna has a gain of 50 dB and there is a 1.5dB loss in the waveguide run between the transmitter and the antenna. Assume that the atmosphere introduces a loss of 0.5 dB under clear sky conditions and that the earth station is located on -2dB contour of the satellites receiving antennaSolution:

Design of Uplink - problemIf rain in the path causes attenuation of 7dB for 0.01% of the year, what output power is required for the transmitter to guarantee that a 1W output can be obtained from the satellite transponder for 99.99% of the tear if uplink power control is used? Solution: If we provide extra 7dB of output power to compensate for fading on the path due to rain, the transmitter output power will be: Pt=7.2 + 7=14.2dBW or 26.3 W

Design for specified C/N: Combining C/N and C/I values in satellite linksLearning Objectives Solving calculations of Link Budget for various satellite systems We have learnt the calculation of C/N ratio for a single link (uplink or downlink) We shall now learn the calculation of C/N ratio for a complete satellite communication link

Combining C/N and C/I values in Satellite LinksTo measure BER or S/N ratio in the baseband channel of an E/S receiver we find C/N in the IF amplifier at the input of the demodulatorThe noise present in the IF amplifier comes from many sourcesTill now in our analysis of uplinks and downlinks, we have considered only the receiver thermal noise and noise radiated by atmospheric gases and rainWhen a complete satellite link is engineered, the noise in the earth station IF amplifier will have contributions from:Receiver itselfReceiving AntennaSky noiseSatellite Transponder from which it receives the signalAdjacent satellite and terrestrial transmitters which share the same frequency bandCombining C/N and C/I values in Satellite Links

Combining C/N and C/I values in Satellite LinksSince the Noise power in the individual C/N ratios is referenced to the carrier power at that point all the C values are the sameExpanding the formula by cross multiplying gives the overall (C/N)o as a power ratio, not in decibels

Combining C/N and C/I values in Satellite LinksInterference from the adjacent satellites is likely whenever small receiving antennas are used, as with VSATs(Very Small Aperture Terminals) and DBS TV receiversFDMA mode Intermodulation products

Combining C/N and C/I values in Satellite Links

Combining C/N and C/I values in Satellite Links Numerical Example

Overall (C/N)o with Uplink and Downlink attenuation

Overall (C/N)o with Uplink and Downlink attenuationUplink and Downlink Attenuation in RainRain attenuation affects uplink and downlink differentlyUsually it is assumed that rain either occurs on uplink or downlink for geographically separated E/SsGeographic separation of >20km is consideredHeavy rains occur with random geographic distribution, so probability of significant attenuation on both uplink and downlink simultaneously is smallIn our analysis of uplink and downlink attenuation effects, it will be assumed that one link is attenuated and the other is operating in clear airOverall (C/N)o with Uplink and Downlink attenuationUplink Rain Attenuation and (C/N)upNo increase in uplink noise power because satellite antenna beam sees the top of cumulonimbus clouds above the rain, which are always colder than 270 KRain attenuation on uplink path reduces the carrier power at the satellite receiver inputReduction in (C/N)up is in direct proportion to the attenuation

Overall (C/N)o with Uplink and Downlink attenuationUplink Rain Attenuation and (C/N)upWith transponder is linear mode:Output power will be reduced equal to attenuation ,AupdB

This will cause (C/N)dn to fall by Aup dB

Thus,(C/N)o will also be reduced by Aup dBOverall (C/N)o with Uplink and Downlink attenuationUplink Rain Attenuation and (C/N)upFor linear transponder:(C/N)o uplink rain=(C/N)o clearair AupdBFor Nonlinear transponder:(C/N)o uplink rain=(C/N)o clearair Aup + G dBFor Regenerative transponder (AGC):(C/N)o uplink rain=(C/N)o clearair dB

Overall (C/N)o with Uplink and Downlink attenuationDownlink Rain Attenuation and (C/N)dnE/S receiver noise temperature changes significantly with rain in downlinkSky noise temp can increase to close to physical temp of individual rain dropsThe received power level,C, is reducedThe noise power, N,in the receiver increases

Overall (C/N)o with Uplink and Downlink attenuationDownlink Rain Attenuation and (C/N)dn

For linear transponder:(C/N)dn rain=(C/N)dn clearair -Aup-Nrain dB

The overall C/N is given by(C/N)o= 1/[1/(C/N)dn rain +1/(C/N)up] dB

Satellite Communication Link Design ProcedureThe design for a one way communication link can be summarized by following 10 steps. The return link design follows same procedure.

1.Determine the frequency band in which the system must operate. Comparative designs may be required to help make the selection2.Determine the communications parameters of the satellite. Estimate any values that are not known3.Determine the parameters of the transmitting and receiving earth stations4.Start at the transmitting earth station. Establish an uplink budget and a transponder noise power budget to find (C/N)up in the transponder5.Find the output power of the transponder based on transponder gain of output backoff6.Establish a downlink power and noise budget for the receiving earth station. Calculate (C/N)dn and (C/N)o for a station at edge of coverage zone(worst case)7.Find link margins. Calculate S/N or BER

Satellite Communication Link Design Procedure8. Evaluate the result and compare with specification requirements. Change parameters of the system as required to obtain acceptable (C/N)o values. This may require several trial designs9. Determine the propagation conditions under which the link must operate. Calculate outage time for both uplinks and downlinks10. Redesign system by changing some parameters if link margins are inadequate. Check all parameters are reasonable and that the design can be implemented within the expected budget.System Design Example

System Design Example

Ku-Band Uplink Design

Ku-band satellite television distribution system

Ku-Band Uplink Design

Ku-Band Downlink Design

Ku-Band Downlink Design

Ku-Band Downlink Design

Ku-Band Downlink Design

Rain Effects at Ku-BandUplinkKu band path to the satellite station heavy rain attenuation 6dBUplink C/N in clear air = 30dBTherefore C/N in the transponder falls to 24dB (30-6)Linear transponder transponder output power falls to (18 6) = 12dBW Downlink C/N falls by 6dB (17.2 6) =11.2dBOverall (C/N)o falls by 6dB (17 6) =11dBMinimum Overall C/N = 9.5dB (given)Link margin available (in rain) = 11 9.5 = 1.5dB(C/N)up=30dB; (C/N)down = 17.2dB ; (C/N)o=17dBLink margin available on uplink = 17 9.5 = 7.5dB

Downlink Attenuation and Sky Noise Increase11.45GHz path between satellite and receiving station rain attenuation = 5dBAssuming 100% coupling of sky noise into antenna noiseClear air gaseous attenuation = 0.5dBCorresponding sky noise temperature = 270(1 10-A/10) = 270(1 10-0.05) = 29.36KApproximated to 30KTotal path attenuation from atmosphere and rain = 5.5dBCorresponding sky noise temperature = 270(1 10-A/10) = 270(1 10-0.55) =194K

Downlink Attenuation and Sky Noise Increase

Downlink Attenuation and Sky Noise Increase