the link budget · pdf file2 satcom courses university of athens er 2006-slide nr 3 the link...

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Satcom Courses University of Athens ER 2006 - Slide nr 1

SATELLITE COMMUNICATIONSSATELLITE COMMUNICATIONS

Master of Management and Economics of Master of Management and Economics of Telecommunication NetworksTelecommunication Networks

University of Athens University of Athens -- 20062006

The Link Budget

by E. by E. RammosRammosESA Senior AdvisorESA Senior Advisor


The Link BudgetThe Link Budget

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•The Link Budget Relates the Signal-to-Noise Ratio to the principal Characteristics of the Equipment on the Satellite and in the Ground Station

•The Telecommunications Equation allows the calculation of the received power as a function of the transmitted power and the antenna characteristics



Transmission side

Pe = Transmit Power

Ge = Transmit Antenna Gain

Se = Transmit Antenna Equivalent Surface

Reception side

Pr = Received Power

Gr = Receive Antenna Gain

Sr = Receive Antenna Equivalent Surface

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Isotropic antenna power flux per unit surface at distance D:

24 DPePFDπ

=



Taking into account the Transmit Antenna Gain Ge the PFD becomes:

24 DGePePFDπ

=

The product GePeGePe is called the EIRPEIRP

(Equivalent Isotropically Radiated Power)

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At the receiver side the receive antenna intercepts from the incoming wave a power:

24Pr

DSrGePeπ

=



2

4λπSrGr =

The antenna Gain is related to the effective area by:

λ is the wave wavelength

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Satellite TelecommunicationsSatellite Telecommunications

( )2

24Pr λ

πGeGrDPe

=

( )22

4Pr

DPeGeGr

πλ

=

The received power is therefore:

And the transmission attenuation is:α



Prlog10 Pedb =α

GrGeDdb −−+= )log(2022λ

α

)log(2022λ

α Ddb +=

In db the attenuation is given by:

And therefore the overall expression becomes:

For fixed antenna gain the attenuation varies as:

This is the so called Free Space AttenuationFree Space Attenuation(about 206db in Ku-Band for GEO orbit)

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Free Space Loss as a function of Frequency

180185190195200205210215220225

1 10 100

Frequency (GHz)

Atte

nuat

ion

(db)



2

4λπAnG =

The Antenna gain with respect to an isotropic antenna is given by:

2)(λπDnG =

If the antenna is circular with diameter D the gain is given by:

n is the antenna efficiency, typically 0.6 or 60%

A is the Area of the Antenna

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Gain as a function of the Diameter

0

10

20

30

40

50

60

70

10 100 1000

Diameter (cm)

Gai

n (d

b) Ku-BandC-BandL-Band



Ddbλϑ 703 =

The 3db beamwidth of the antenna is given by:

db

G3

32000ϑ

=

For a good antenna of 65 % efficiency the gain is approximately:

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3db Beamwidth

024

68

101214

161820

0 100 200 300 400 500

Diameter (cm)

3db

BW

(deg

)

Ku-BandC-BandL-Band



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( )2

24Pr λ

πAGeGr

DPe=

In real cases the various losses need to be taken into account and a loss coefficient A is added to the equation

AGrGeDdb +−−+= )log(2022λ

α

and in db



A = Atr Aprop Apol Apoint Arec

Atr = losses between transmitter output and antenna(transmission lines, duplexers, filters …)

Aprop=propagation losses in the atmosphere and ionosphere

Apol = polarisation losses (pol. mismatch )

Apoin= antenna pointing losses

Arec = losses between receive antenna and receiver(lines, duplexer, filters…)

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Respecting the order of the various phenomena the overall equation is written as:

AeGeApoinDApropApolGrArecPe +−+++++−+=λ

log2022Pr

Example:For the following assumptions:GEO satellite transmitting at a frequency of 12 GHzReceiver sensitivity -108dBWTransmit antenna 1.3m diameter (55% efficiency)Receive antenna 1m diameter (55% efficiency) Polarisation loss 1dBPointing loss 3 dBTransmission loss 1 dbReception loss 1dbFind the required Transmit power (in W)


Frequency (GHz) 12Wavelength (m) 0,025

Receive Antenna

Trans mit Antenna

Diameter (m) 1 1,3Effic iency 0,55 0,55Gain (db) 39,38 41,66

Notations Negative terms Pos itive termsRequired Power at receiver Pr -108Reception lo s s es Arec 1Receive Antenna Gain Gr -39,38Polaris ation los s es Apol 1Free Space Los s 22+20logD/λ 205,17Pointing Los s Apoin 3Trans mit antenna Gain Ge -41,66Trans mit lo s s Atr 1

Total -189,05 211,17

Trans mit Power (db) 22,12Trans mit Power (W) 162,99

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Receiver Noise Figure F

It is the ratio of the noise power Ns at the receiver output to the receiver output when only a noise source at temperature To=300K is connected at the input.

GkToBNsF =

G is the receiver gainB is the receiver frequency bandwidthK = 1.379.10-23 W/HzK is the Boltzman constant (and in dB it is equal to -228.6 dBW/HzK)



Receiver Equivalent Noise Temperature Te

It is the temperature of a noise source at the input of an ideal receiver that would generate at the ideal receiver output the same noise power generated by the real receiver.

If at the input of the real receiver is connected a noise source at temperature To then the noise power at the output is:

GkTeBGkToBNs +=

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Te (K) 7 35 75 300 900 3000F (dB) 0,1 0,5 1 3 6 10

ToTeF +=1

The Noise Figure and the equivalent noise Temperature are related by:

Typical values of Te are:

• For ground stations 10 to 100K• For satellite receivers can be much higher



The total Equivalent Noise Temperature of a series of receivers, each of gain Gi (i=1,2,3..) and of equivalent noise temperature of Tei is given by:

.....21

3

1

21 +++= GG

TG

TTT eeee

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Antenna Noise Temperature TA

All bodies radiate energy. Received by the antenna this is an external noise source.

If No is the received spectral density (W/Hz) then:

N0 = k TA

It depends on the bodies generating the noise and the antenna characteristics.

For antennas pointed to the satellites it is mainly due to:

• the sky noise• the earth radiation temperature



The sky noise is more important for frequencies above 2Ghz

• Generated by the non ionised regions of the atmosphere.• The clouds, rain etc also generate noise.• For frequencies between 1 to 15 GHz it typically below 40K.• If the sun is within the antenna beam then the noise may increase by thousands degrees!

The earth radiation noise temperature

• It has about the value of the physical temperature (around 290K)• It depends on the antenna radiation pattern and the antenna orientation

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Global Noise Temperature

For a receiver with Equivalent Noise Temperature Te, connected to an antenna with antenna Temperature TA via a line with line losses L at a physical temperature TL the global noise temperature at the receiver input is:

TeL

TLTT LA +−+= )11(

• The line loss adds about 7K noise temperature for each tenth ofdb of losses• Satellite antenna looking at the earth have TA equal to about 300K



Signal to Noise Ratio at the output of the receiver in the band B is given by:

If the signal is a carrier C=Pr and if No=N/B (W/Hz) is the spectral noise density then:

2)4

(PrDAkTB

PeGeGrN π

λ=

AkTGr

DPeGe

NC 11)()

4)(( 2

0 πλ

=

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The Signal to Noise Ratio in dB is given by:

EIRP Free Space Quality LossesLosses Factor

6.228log10log10)4log(20)log(10)(0

+−+−= ATGrDPeGe

NC

dBHz λπ



According the Shannon theorem, for a transmission without error, of a rate R (bits/sec) in a bandwidth B:

)1(log2 NCBR +≤

For digital transmission at a rate R bit/sec the energy per bit E is related to the C/N by:

BR

NoE

NC=

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The total link from a transmit station via the satellite to an other receive station includes two links:

•The UplinkUplink from the transmit station to the satellite, characterised by a signal to noise ration of (C/No)U

•The DownlinkDownlink from the satellite to the receive station, characterised by a signal to noise ratio (C/No)D

•The total link signal to noise ration is given by

111 )()()( −−− += DUT NC

NC

NC

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Propagation losses in the atmosphere

•Absorption from the gases in the atmosphere- Oxygen around 60 GHz- Water vapor around 22.5 GHZ

•Diffusion by rain etc creates interferences

•Increase of the Noise contribution by the sky

•Degradation of polarisation isolation

•Absorption due to rain: complex phenomenon depending on rain drops diameter and distribution, and increasing with thefrequency (up to about 10GHz).

•Rain statistics very important for dimensioning of link.


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Choice of Frequency Band

• Operational Constraints

- Coverage zone on the earth- Small earth station requirement (e.g. TVRO, SIT)- Volume limitation on the launcher

• Propagation Conditions

- Rain attenuation statistics (e.g. SE Asia, Africa)

• Frequency regulations

- As defined by International regulations



Frequency Bands •The radio-frequency spectrum, as defined by the International Telecommunications Union (ITU), is said to extend from 3 kHz to 3,000 GHz

• The spectrum is divided into nine bands as listed below (ITU Radio Regulations 1998, V.1, Article RRS2,“Nomenclature, Section I -Frequency and Wavelength Bands”

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Band letters •Unofficial, non-standard and consequently imprecise designations of frequency bands•Often divided into subbands, designated by suffix subscript letters. •For instance, the K band usually is divided into at least two subbands, designed Ku for frequencies in the range 10 – 14 GHz and Ka for frequencies in the range 24 – 36


Radio Spectrum Regulations

The repartition of the radio frequencies between the various services is defined in the frame of the International Telecommunications Union (ITU)

The Comite Consultatif International des Radiocommunications (CCIR) prepares the recommendations for the technical characteristics

The CCIR recommendations are made for limiting the interferences between the various systems.

Several bands are shared between space and terrestrial systems

For Space Systems the World Administrative radio Conference (WARC) allocates frequency bands to be used by various services and administrations

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•Radio Regulations -systems may operate at designated frequency bands (depending in part on regional availability and on the regulatory allocation of the country in which the system operates)Satellite Services include- Fixed Satellite Services (FSS) - Broadcast Satellite Services (BSS)- Mobile Satellite Services (MSS)

the link budget · pdf file2 satcom courses university of athens er 2006-slide nr 3 the link...

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