Dr. Jinjun Shan, Associate Professor of Space EngineeringDepartment of Earth and Space Science and EngineeringRoom 255, Petrie Science and Engineering BuildingTel: 416-736 2100 ext. 33854Email: jjshan@yorku.caHomepage: http://www.yorku.ca/jjshanEarth, Moon, Mars, and BeyondENG 4360 - Payload Design6.2 Communications Satellite Payload- Link BudgetDr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget2References Title: Satellite communications: system and its design technology Takashi Iida, ed.IOS Press, c2000. ISBN: 158603085X (IOS Press) Introduction to Satellite Communication, 2ndBruce R. ElbertArtech House: Space Applications Series1999Reserved at Steacie Library.Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget3Basics of Satellite Communications LinksDr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget4Configuration of Satellite Communications Links and Transmit/Receiver Power - I Exampleofasimplesatellitelink:communicationbetween two earth stations via a communications satellite. On-boardtransponder(throughrepeater orbentpipe) performs frequency conversion and amplification. Qualityoflinkisessentiallydeterminedbythesignal-to-noise ratio (S/N). Tospecifythecharacteristicsofthesatelliteportionofthe link,thecarrier-to-noise-power-densityratio(C/N0)is normally used. C: power of the propagation wave; N0: noise power density per 1 Hz. Given a signal (Ptin dBW) transmitted from a transmit earth station,determineatwhatpowerthissignalcanbe received at a receive earth station (Prin dBW)Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget5Configuration of Satellite Communications Links and Transmit/Receiver Power - II Configuration of link:Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget6Configuration of Satellite Communications Links and Transmit/Receiver Power - III The Power Balance Equation simplifies the analysis of microwave links: The Power Received = the Power Transmitted plus all gains, minus all losses.| | || | || | | || || || | | || || | | || |ra r d da sd sua u su ta t t rL G L L G GL L G L G P P + + + + + =Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget7Noise Considerations in Satellite Comm. Link Noise (1): Noise included in signal source plus thermal noise generated by modulator, frequencyconverter,andpoweramplifier.Inmostcases,thisnoiseissufficiently small compared to signal power and is negligible compared with other noise sources. Noise (2): Thermal noise from ground received by satellite antenna (often at 300K). Noise (3): Thermal noise generated by the satellite transponder and governed by the low-noise performance of the transponders first stage. Noise(4): Noisereceivedbythegroundantennainadditiontothesignalfromthe satellite; includes sky noise, atmospheric thermal noise, and terrestrial thermal noise. Noise (5): Thermal noise generated by the ground receiver and governed by the low-noise performance of the first-stage amplifier.Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget8Main Link Parameters Antenna gain EIRP Free space loss Atmospheric absorption loss Receiver noise power density Antenna noise Noise temperature Noise figure Equivalent input noise temperature G/TDr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget9Antenna Gain - I Antennagain isthemostimportantantenna characteristic in link calculations. Definition:theratioofpowerradiatedperunitsolid angle by an actual antenna in a given direction to the powerradiatedperunitsolidangleinthesame direction by a reference antenna. Absolutegain:whenthereferenceantennaisan isotropicantenna.Itisusedinsatellite-link calculations and is often denoted by dBi Relative gain: w.r.t. an idealhalf-waveantennawith no loss that is often used as a characteristic of linear antennas.Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget10Antenna Gain - II Polar coordinate system is normally used for antenna. Gain in the direction (,), G(,), can be given bywhere w(,) is power flux density in the (,) direction. If antenna bean direction is not specified, antenna gain is usually taken to mean gain in the direction of maximum radiation. The gain of parabolic antennas that are often used in satellite communications is where is aperture efficiency (50-70%), D is antenna diameter, is wavelength. In logarithmic form,t| u| u4 /) , () , (tPwG =2|.|

\|=tqDG|| ) 110 log( 102 2D f G = qDr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget11EIRP EIRP = Effective isotropic radiated power EIRPisaproductoftransmitantennagain(Gt)and transmitter output power (Pt), EIRP = PtGt EIRPvariationistypicallyduetoantennathermal distortion,satelliteattitudeinstabilities,atmospheric disturbance(i.e.rain)andunitthermalandaging effects. Example:Pt = 100 W = 20 dBW, Gt= 1000 X = 30 dBi The signal power also diminishes as it propagates to the earth and this is called the free space loss.Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget12Free Space Loss - I A basic quantity in link budget is propagation loss in free space. Inasatellitelink,itisassumedthattransmitandreceive antennasfaceeachotherbutareseparatedbyasufficient distance d [m] in free space. Gains of the transmit and receive antennas: Gtand Gr; Effective area of receive antenna Ar; Transmit power Pt; Wavelength . Power density at the reception point: Pt Gt/4d2 Received power: Since we have Ar= Gr2/4, thus24dAG P Prt t rt=22) 4 ( dG G P Pr t t rt=Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget13Free Space Loss - II Freespacelosscanbegivenastheratioofreceivepowerto transmit power Ifwetreatthetransmitandreceiveantennasasisotropic antennas, we have basic transmission loss In logarithmic form22) 4 ( dG GPPr ttrt=meters in are d,) 4 (22tdLF =| |km. inand GHz inwheredB ) 788 , 35 / log( 20 ) log( 20 5 . 183R fR f LF+ + =Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget14Example - 1 CalculatethefreespacelossoflinkbetweenAnik-F2anda ground station at Toronto. Note: Anik-F2 is the first satellite to fully commercialize the Ka-band frequency. It has 24 C-band, 32 Ku-band, and 38 Ka-band.Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget15Example - 2 CalculatethereceivedpowerbygroundstationatToronto considering free space loss using Anik-F2 data.Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget16Atmospheric Absorption Loss - I Amonggaseousmolecules,oxygenandwatervaporaretheprimary factorsunderlyingtheattenuationofradiowavesthroughresonance absorption. Attenuationcanalsooccurduetoabsorptionandscatteringprocesses caused by water drops and ice particles. Absorption is the maincauseof attenuationiftheradiusofthedropsorparticlesissufficientlysmallwrtradio wave wavelength.Pressure: 1013 hPaTemp: 15Cwater vapor content: 7.5g/m3Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget17Atmospheric Absorption Loss - II Rough Model for Sea Level Gaseous Attenuation (Curve-fit) Awater=Where is the water vapor concentration in gram per cubic meter, 7.5 g/m3 at sea level and 1 g/m3at altitude of 4 km, and f is the frequency in GHz. AO2= These two equations are for f < 57 GHz and f > 63 GHz. Between 57 GHz and 63 GHz , an average value of 14.9 dB/km is used. Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget18Atmospheric Absorption Loss - III Scale height approximationwhere hw0= 1.6 km during clear periods, 2.1 km during wet weather. Therefore, the total atmospheric absorption loss iswhere is elevation angle, usually between 5 and 90 degrees.km2 . 1 ) 60 (26 . 8310 52087 . 3 10 87185 . 1 10 32734 . 3 386 . 5km4 ) 4 . 325 (5 . 26 ) 3 . 183 (0 . 55 ) 2 . 22 (0 . 3123 5 2 3 202 2 2 0+ + + =)`+ ++ ++ + = ff f f hf f fh hw wdBsinu + =o o w wAtmh A h ALDr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget19Receiver Noise Power Density Thermal noise on the transmit side is relatively small compared to signal power. It can be ignored. However, on receive side, thermal noise has to be considered. A convenient method: select one point in the system, convert the noise in each section of the system to the value of noise atthe selected point, get the total value. Selected point is the receivers input port. Receiver noise mainly consists of antenna noise, feed-system noise, and noise from the low-noise amplifier.Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget20Antenna Noise Areceiveantennareceivesnoiseradiowaveinadditiontothe desired signal. Thermal loss of the antenna will be output as thermal noise. Noise presents a problem in the reception of weak signals as in satellite communications. Noisepowerisexpressedasabsolutetemperature,Ts.It consists of cosmic noise, noise from lighting, and thermal noisebased on atmospheric absorption. Antennathermalnoise:(1-)T0, isantennaradiation efficiency and T0is ambient temperature. AntennanoiseTa =Ts +(1-)T0.Thisiscalledtheantennas equivalent noise temperature. Note:amajorcontributiontoantennanoiseTaismadeby thermalnoisefromantennasidelobespointedtowardthe ground. Efforts are therefore made to reduce side-lobe levels so as to reduce overall noise.Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget21Noise Temperature Becausethefaintsignalsinsatellitecommunications,noise level in the receivers must be made extremely low. Low-powernoisecanbeexpressedintermsofabsolute temperature. Thermalnoisepowerperunitbandwidth(N0)=thermalnoise generatedbyresistanceatT,thenthenoiseisexpressedin termsofTandbecomesequivalenttotheaverageenergyof blackbodyradiationinthermalequilibriumatabsolute temperature T. Noisepower:N=kTB(inwatts).k- BoltzmannsConstant, 1.3810-23J/K; T - noise temperature. In logarithmic form, we haveT is in Kelvin, B is in Hz.| | W dB ) log( 10 ) log( 10 6 . 228 B T N + + =Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget22Noise Figure (NF) Noise figure (NF) is a quantity that expresses the quality of like characteristics with respect to noise and is defined aswhere Sin/Ninis the ratio of signal to noise at the links input port andSin/Nin=Sin/kTB;Sout/Nout=GSin/G(kTB+kTiB)withlinkgain G and equivalent input noise kTi. Therefore,out outin inN SN SNF//=TTNFi+ =1Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget23Equivalent Input Noise Temperature Inadditiontotheexternalnoisecoupledintothereceiver throughtheantenna,eachcomponentofareceivergenerates its own internal noise! Noise temperature cannot be measured by thermometer, can be converted to a value at the circuits input port. Amplifiercircuit:noisefigureNF,EquivalentInputNoise temperature Ti, ambient temperature T0. Then Ti = T0(NF-1) Losscircuit:linkLossLc,ambienttemperatureT0.ThenTi = T0(Lc-1),noisetemperatureattheoutputportToutcanbe expressed as Tout = T0(1-1/Lc) Seriescircuit:todetermineequivalentnoisetemperatureat input port of amplifier circuit 1. Ti = TL1+ TG1+ TL2/G1+ TG2L2/G1+ TL3L2/G1G2Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget24G/T (Gain/Tempature) G/Tistheratio ofantennagainGtoreceive-system noise temperature T. When calculating this index at the receivers input port, thevalueusedforantennagainincludesfeederloss, and noise temperature T is given by Ti. G/T variation is due to antenna thermal distortion,satelliteattitude instability, receiver thermal characteristics, etc Becauseoftheverylowsignalstrengthreceivedatthe satellite, it is essential to maximize the G/T performanceDr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget25Basics of Link DesignDr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget26C/N0 of a Satellite Link Total link C/N0can be determined by separating the link into its uplink and downlink portions, computing the C/N0 of each, and then combining the two. C/N0 for either uplink and downlink takes on the following form: The total link including inter- and intra-system interference| | || | | | || | | | | || |0 0/ N L G L L G P N Cfeed r a f t t + + =Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget27Some Important Equations in Link Budget Calculation - I Carrier to Noise Ratio (in all its incarnations) [C/N] Ratio of powers, dimensionless dB Used in combining noise and interference sources [C/N0] Ratio of power to power density, dB-Hz Removes bandwidth from the equation [C/T] Ratio of power to system noise temperature, dBW/K Results from simple equation: C/T = EIRP A + G/T [Eb/N0] (eb-no) Ratio of energy per bit to noise density, dB Measure of SNR for a digital communication system Used in evaluating error rate performance Differentmodulationforms(BPSK,QPSK,QAM,etc.)have different curves of theoretical bit error rates (BER) vs. Eb/N0.Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget28Some Important Equations in Link Budget Calculation - IIDr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget29 [C/N] = power received divided by total noise in carrier bandwidth B [C/N0]= [C/N] + 10log(B) [C/T] = [C/N0] 228.6 [Eb/N0] = [C/N0] 10log(Rb), where Rbis the information bit rateSome Important Equations in Link Budget Calculation - IIIDr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget30Example: Design of a Satellite Link Link design for a Ka-band satellite.Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget31Other Issues - I Rain Margin Attenuationduetoraincannotbeignoredatfrequencies above 10 GHz in satellite communications. Rainattenuationisnotpredictablewithgreataccuracy,but estimates can be made that allow links to be designed. Dry seasons and regions of the world with low rainfall would notsuffergreatlyfromthisphenomenon.However,linksin regions with heavy thunderstorm activity should be provided withgreaterlinkmargin,orservicemightnotbemaintained with sufficient availability to satisfy commercial requirements. What is availability? Thelinkavailabilityisexpressedasapercentageofayearwhen thelinkwillperformaspertherequiredBER(orabovethelinkthreshold).Therefore,99%availabilitystatesthatthelinkwillbe unavailable for 87.6 Hours. Typically, satellite links operate in the range of 99% to 99.5%.Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget32Other Issues - IIDr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget33Other Issues - III Interference Thesamefrequencybandsemployedinsatellite communicationsarealsoallocatedtoothertypesof businesses and applications. When configuring a satellite communications system, studies must be performed on interference not only between satellite communicationsystemsbutalsooninterferencewith terrestrial wireless communications systems using the same frequency band. (1) Interference between satellite systems. (2) Interference with terrestrial systems. (3) Intra-system interference.Dr. Jinjun Shan, Assocaite Professor of Space EngineeringCommunications Payload - Link Budget34Link Margin In Ku band networks, it is a good rule of thumb to allow 7 or 8 dB ofmarginabovethresholdatthereceivesitewithclearsky conditions.Thiswillgenerallyprovidealinkavailabilityinexcess of 99.5%. Cbandnetworksrequiremuchlessmargin,typicallyabout3dB, forthesameperformanceexpectation,sincethereisless atmospheric attenuation and rain attenuation with C band. Ka band needs more link margin for acceptable availability.