1

Satellite Communication Systems

By- Kanchan Bakade

Reference book:Satellite communication- Dennis Roddy

Satellite Communication- Pratt and Bostian

Transmission Losses Losses occur along the way, some of which are

constant. Losses for clear weather conditions = losses

which don’t vary significantly with time and losses which are calculated statistically

Free-space loss Antenna misalignment losses Fixed atmospheric and ionospheric losses Losses which are weather related which

fluctuate with time , allowed for by introducing fade margins into the transmission equation.

Transmission Losses – Free-space loss

Free-space loss = power loss that comes from the spreading of the signal in space

Most significant type of transmission loss

24

log10][][][

r

GEIRPP RR

24

log10][

r

FSL

effR FAP

4

2R

eff

GA

22

2 444

RGEIRP

G

R

EIRPP R

RR

224 m

W

R

PGF tt

Basic form of link equation:

where and

Transmission Losses – Feeder losses

Basic form of link equation only accounts for free-space loss

Other types of losses need to be accounted for Feeder losses: Losses that occur between the

receive antenna and the receiver proper Eg: Losses in the connecting waveguides, filters, and

couplers Receiver feeder losses = [RFL] dB RFL values are added to FSL Similarly losses occur at transmitting antenna too

when connected to HPA o/p – needed

Transmission losses – Antenna misalignment losses

Ideal situation: earth station and satellite antenna aligned for maximum gain

Two possible sources of off-axis loss: satellite and earth station

Off-axis loss at ES is antenna pointing loss ( only few tenths of a decibel)

Misalignment of the polarization direction - [AML] dB include both pointing and polarization losses.

The AML for uplink and downlink must be taken into account seperately.

Figure: ref. 1

Transmission Losses – Fixed atmospheric and ionospheric losses

Atmospheric gases result in losses by absorption

atmospheric absorption loss = [AA] dB Polarization loss (dB); PL= 20 log(cosθ) θ = angle of mismatch

Basic Transmission Theory

Calculation of the power received by an earth station from a satellite transmitter.

Had two approach: The use of flux density The link equation.

Transmitting source is an isotropic radiator which uniformly radiates a total power in all directions.

The flux density crossing the surface of a sphere with radius R is given by

22 W/m

4 R

PF

t

2

2 W/m

4 R

PF

t

Antenna Gain Antenna gain is G(θ) is defined as the ratio of power per unit

solid angle radiated in a direction θ to the average power radiated per unit solid angle.

We need directive antennas to get power to go in wanted direction.

Gain of antenna at θ = 0º (bore sight) get increase in power in a given direction compared to isotropic antenna.

4/

)()(

0P

PG

• P() is variation of power with angle.

• G() is gain at the direction .

• P0 is total power transmitted.

• sphere = 4solid radians

EIRP

For a transmitter with o/p power P watts driving a lossless antenna with gain G , the flux density in the direction of the antenna bore sight at a distance R meters is

PtGt=Effective Isotropic Radiated Power (EIRP) Note that EIRP may vary as a function of

direction because of changes in the antenna gain vs. angle

22 W/m

4 R

GPF tt

Received Power The power available to a receive antenna of area Ar m2 we

get:24

x R

AGPAFP ett

rr

Real antennas have effective flux collecting areas LESS THAN the physical aperture areaDefine Effective Aperture Area Ae: x e rAA Where Ar is the actual (physical) aperture area. = aperture efficiency; losses between the incident wavefront and the antenna o/p port.

Includes illumination efficiency, aperture taper efficiency of the antenna, and other losses due to spill,blockage, phase due to spill, blockage, phase errors, Diffraction effects, polarization and mismatch losses.

It is range 50 to 75% for parabolic reflector and lower for small antennas and Higher for large cassegrain antennas. Horn antenna efficiency is close to 90%.

Back to Received Power… Antennas have (maximum) gain G related to the

effective aperture area as follows:

where λ is the wavelength (m) at the frequency of operation.

The power available to a receive antenna of effective area Ar = Ae m2 is:

24 x

R

AGPAFP ett

rr

2

4

e

r

AG

Inverting…

4

2r

e

GA

2

4

eA

Gain

Back to Received Power…

2

4

RGGPP rttr

Friis Transmission Formula

• The inverse of the term at the right referred to as “Path Loss”, also known as “Free Space Loss” (Lp):

24

R

Lp

Therefore…

p

rttr L

GGPP

Signal TransmissionLink-Power Budget Formula

The decibel equation for the received power is: [PR] = [EIRP] + [GR] - [LOSSES] dBWWhere:

[PR] = received power in dBW [EIRP] = equivalent isotropic radiated power in

dBW [GR] = receiver antenna gain in dB [LOSSES] = total link loss in dB

dBW = 10 log10(P/(1 W)), where P is an arbitrary power in watts, is a unit for the measurement of the strength of a signal relative to one watt.

More complete formulation

rotherpolrataap

rttr LLLLLLL

GGPP

Demonstrated formula assumes idealized case. Free Space Loss (Lp) represents spherical spreading

only. Other effects need to be accounted for in the

transmission equation: La = Losses due to attenuation in atmosphere Lta = Losses associated with transmitting antenna Lra = Losses associates with receiving antenna Lpol = Losses due to polarization mismatch Lother = (any other known loss - as much detail as available) Lr = additional Losses at receiver (after receiving antenna)

Signal TransmissionLink-Power Budget Formula Variables

Link-Power Budget Formula for the received power [PR]: [PR] = [EIRP] + [GR] - [LOSSES]

The equivalent isotropic radiated power [EIRP] is: [EIRP] = [Pt] + [Gt] dBW, where: [Pt] is the transmit power in dBW and [Gt] is the

transmitting antenna gain in dB. [GR] is the receiver antenna gain in dB. Losses for clear sky conditions are: [LOSSES] = [FSL] + [RFL] + [AML] + [AA] + [PL],

where: [FSL] = free-space spreading loss in dB = PT/PR (in watts) [RFL] = receiver feeder loss in dB [AML] = antenna misalignment loss in dB [AA] = atmospheric absorption loss in dB [PL] = polarisation mismatch loss in dB

The major source of loss in any ground-satellite link is the free-space spreading loss.

Link Power Budget

Transmission:HPA PowerTransmission Losses (cables & connectors)Antenna Gain

EIRPTx

Antenna Pointing LossFree Space LossAtmospheric Loss (gaseous, clouds, rain)Rx Antenna Pointing Loss

Rx

Reception:Antenna gainReception Losses (cables & connectors)Noise Temperature Contribution

Pr

Translating to dBs

The transmission formula can be written in dB as:

This form of the equation is easily handled as a spreadsheet (additions and subtractions!!)

The calculation of received signal based on transmitted power and all losses and gains involved until the receiver is called “Link Power Budget”, or “Link Budget”.

The received power Pr is commonly referred to as “Carrier Power”, C.

rrotherrapolaptar LGLLLLLLEIRPP

Link Power Budget

Transmission:+ HPA Power- Transmission Losses (cables & connectors)+ Antenna Gain

EIRPTx

- Antenna Pointing Loss- Free Space Loss- Atmospheric Loss (gaseous, clouds, rain)- Rx Antenna Pointing Loss

Rx

Reception:+ Antenna gain- Reception Losses (cables & connectors)+ Noise Temperature Contribution

Pr

Now all factors are accounted for as additions and subtractions

Link Budget parameters

Transmitter power at the antenna Antenna gain compared to isotropic radiator EIRP Free space path loss System noise temperature Figure of merit for receiving system Carrier to thermal noise ratio Carrier to noise density ratio Carrier to noise ratio

Easy Steps to a Good Link Power Budget

First, draw a sketch of the link path Doesn’t have to be artistic quality Helps you find the stuff you might forget

Next, think carefully about the system of interest Include all significant effects in the link power budget Note and justify which common effects are insignificant here

Roll-up large sections of the link power budget Ie.: TXd power, TX ant. gain, Path loss, RX ant. gain, RX losses Show all components for these calculations in the detailed budget Use the rolled-up results in build a link overview

Comment the link budget Always, always, always use units on parameters (dBi, W, Hz ...) Describe any unusual elements (eg. loss caused by H20 on radome)

Simple Link Power Budget

Why calculate Link Budgets?

System performance tied to operation thresholds.

Operation thresholds Cmin tell the minimum power that should be received at the demodulator in order for communications to work properly.

Operation thresholds depend on: Modulation scheme being used. Desired communication quality. Coding gain. Additional overheads. Channel Bandwidth. Thermal Noise power.

Closing the Link We need to calculate the Link Budget in order

to verify if we are “closing the link”.Pr >= Cmin Link Closed

Pr < Cmin Link not closed

Usually, we obtain the “Link Margin”, which tells how tight we are in closing the link:

Margin = Pr – Cmin

Equivalently:Margin > 0 Link ClosedMargin < 0 Link not closed

System Noise

Random thermal motion of electron in the resistive and active devices in the receiver and also lossy components of antennas

Available noise power: Thermal noise: Flat frequency spectrum Noise power spectral density: BN ≈ 1.12 B-3dB

NNN BkTP

NN

N kTB

PN 0

System Noise – Antenna Noise

Two types of antenna noise:1. Noise originating from antenna losses 2. Sky noise = microwave radiation present

throughout the universe• Figure: Equivalent noise temperature of the sky as seen from earth station antenna

• Equivalent noise temperature of the earth as seen from the satellite antenna is about 290 K

Figure: ref. 1

System Noise – Amplifiers Single stage amplifier:

Amplifiers in cascade:

)(,0 eantout TTGkN

...21

3

1

21

GG

T

G

TTTT ee

eantS

Figures: ref. 1

)(,0,0 eant

outin TTk

G

NN

For amplifiers in cascade, it’s important to have a low noise, high gain amplifier for the first stage.

System Noise Nout= FGkTo

F = noise factor, To = Room temperature =290 K

Also Nout= Gk(Te+To)

Noise figure NF = 10logF

0)1( TFTe

1

0

1

0 )1()1(

G

TFL

G

TLTT antS

System Noise Power - 1

Performance of system is determined by C/N ratio.

Most systems require C/N > 10 dB. (Remember, in dBs: C - N > 10 dB)

Hence usually: C > N + 10 dB We need to know the noise temperature of our

receiver so that we can calculate N, the noise power (N = Pn).

Tn (noise temperature) is in Kelvins (symbol K):

2739

5320 FTKT 2730 CTKT

System Noise Power - 2 System noise is caused by thermal noise

sources External to RX system

Transmitted noise on link Scene noise observed by antenna

Internal to RX system The power available from thermal noise is:

where k = Boltzmann’s constant = 1.38x10-23 J/K(-228.6 dBW/HzK),

Ts is the effective system noise temperature, andB is the effective system bandwidth

(dBW) BkTN s

Noise Spectral Density

N = K.T.B N/B = N0 is the noise spectral density (density of noise power per hertz):

N0 = noise spectral density is constant up to 300GHz.

All bodies with Tp >0K radiate microwave energy.

(dBW/Hz) 0 ss kT

B

BkT

B

NN

System Noise Temperature

1) System noise power is proportional tosystem noise temperature

2) Noise from different sources is uncorrelated (AWGN)

Therefore, we can Add up noise powers from different contributions Work with noise temperature directly

So:

But, we must: Calculate the effective noise temperature of each

contribution Reference these noise temperatures to the same

location

Additive White Gaussian Noise (AWGN)

RXlinelossLNAantennadtransmittes TTTTTT

Typical Receiver

Noise Model

Noise is added and then multiplied by the gain of the device (which is now assumed to be noiseless since the noise was already added prior to the device)

Equivalent Noise Model of Receiver

Equivalent model: Equivalent noise Ts is added and then multiplied by the equivalent gain of the device, GRFGmGIF

(noiseless).

Calculating System Noise Temperature - 1

Receiver noise comes from several sources. We need a method which reduces several

sources to a single equivalent noise source at the receiver input.

Using model in Fig. 4.5.a gives:

End)-(Front

(Mixer)

(IF)

inRFRFmIF

mmIF

IFIFn

TTkBGGG

BkTGG

BkTGP

Calculating System Noise Temperature - 2

Divide by GIFGmGRFkB:

If we replace the model in Fig. 4.5.a by that in Fig. 4.5b

RFm

IF

RF

minRFRFmIFn GG

T

G

TTTkBGGGP

BkTGGGP sRFmIFn

Calculating System Noise Temperature - 3

Equate Eqns :

Since C is invariably small, N must be minimized.

How can we make N as small as possible?

RFm

IF

RF

minRF GG

T

G

TTTT S

For a noiseless lossy device

Tno = Tp (1 - Gl) Where Gl is linear gain of attenuating device

Tno is noise temperature at the output

Tp is the physical temperature of the device Therefore, for an attenuation of A dB,

Gl =10A/10

TinGain

Gl

Noiseless lossy device

+

Pn

Noise Source Tno

Reducing Noise Power

Make B as small as possible – just enough bandwidth to accept all of the signal power (C ).

Make TS as small as possible Lowest TRF

Lowest Tin (How?) High GRF

If we have a good low noise amplifier (LNA), i.e., low TRF, high GRF, then rest of receiver does not matter that much.

inRFRFm

IF

RF

minRF TT

GG

T

G

TTTT

S

Reducing Noise Power Discussion on Tin

Earth Stations: Antennas looking at space which appears cold and produces little thermal noise power (about 50K).

Satellites: antennas beaming towards earth (about 300 K): Making the LNA noise temperature much less

gives diminishing returns. Improvements aim reduction of size and weight.

Antenna Noise Temperature

Contributes for Tin Natural Sources (sky noise):

Cosmic noise (star and inter-stellar matter), decreases with frequency, (negligible above 1GHz). Certain parts of the sky have punctual “hot sources” (hot sky).

Sun (T 12000 f-0.75 K): point earth-station antennas away from it.

Moon (black body radiator): 200 to 300K if pointed directly to it.

Earth (satellite) Propagation medium (e.g. rain, oxygen, water vapor):

noise reduced as elevation angle increases. Man-made sources:

Vehicles, industrial machinery Other terrestrial and satellite systems operating

at the same frequency of interest.

Antenna Noise Temperature Useful approximation for Earth Station

antenna temperature on clear sky (no rain): Earth Station Antenna - Noise Temperature

15

20

25

30

35

40

45

50

0 10 20 30 40 50 60 70 80 90 100

Elevation Angle (degrees)

Ta

(K)

NOISE FIGURE AND NOISE TEMPERATURE

Noise figure : specify the noise generated within a device.

The noise temperature T=T0(NF-1)Where NF is a linear ratio, not in dB. T is the reference temperature(290K)

out

in

NS

NSNF

/

/

G/T ratio for Earth Stations When link equation rewritten in terms of

(C/N) at the earth station, we had

Thus Gr / Ts used to specify the quality of a receiving earth station.

Increasing G/T, increases the received C/N ratio.

Satellite terminals has negative G/T which is below 0 dB/K. i.e Gr < Ts

s

r

n

rt

ns

rtt

T

G

RkB

GP

RBkT

GGP

N

C22

44

System Figure of Merit

G/Ts: RX antenna gain/system temperature Also called the System Figure of Merit, G/Ts

Easily describes the sensitivity of a receive system Must be used with caution:

Some (most) vendors measure G/Ts under ideal conditions only

G/Ts degrades for most systems when rain loss increases This is caused by the increase in the sky noise component This is in addition to the loss of received power flux density

Power Budget Example - 14.1.1 Satellite at 40,000 km (range)

Transmits 2WAntenna gain Gt = 17 dB (global beam)

Calculate: a. Flux density on earth’s surface b. Power received by antenna with effective aperture of 10m2

c. Gain of receiving antenna. d. Received C/N assuming Ts =152 K, and Bw =500 MHz

a. Using Eqn. 4.3: (Gt = 17 dB = 50)

2215-

2722

dBW/m 143W/m10 x 4.97

) x(4x104

50 x 2

44

R

GP

R

EIRPF tt

(Solving in dB…)

dBW/m2 1431521120

114

dB[meter] )10x4(log x 2 R

dBW 20173)(27

102

F

dB

GtPtEIRP

Power Budget Example - 1b. Received Power

dBWW 13310 x 4.97 P

10 x )(4.97x10 A x FP14-

r

-15r

(Solving in dB…)

dBW 133

10)143(

r

r

P

AFP

c. Gain given Ae = 10 m2 and Frequency = 11GHz ( eqn. 4.7)

dBA

G er 3.52

0273.010 x 4π4

2

Power Budget Example - 1b. System Noise Temperature

dBNC

NCNC

dBWW

dBW

BTKor

KTB

dBdBdB

2.13/

)79.119(133/

13310 x 4.97 PC

97.119

99.8682.21 6.228

10 x 500 x 152x 01 x 38.1PN

14-r

623n

C/N carrier to noise ratio

rotherpolrataap

rttr LLLLLLL

GGPP

BKT

P

N

C

s

r

RFm

IF

RF

minRF GG

T

G

TTTT S

2D

G

24

R

LpFLa

Carrier to Noise Ratios

C/N: carrier/noise power in RX BW (dB) Allows simple calculation of margin if: Receiver bandwidth is known Required C/N is known for desired signal type

C/No:carrier/noise p.s.d. (dbHz) Allows simple calculation of allowable RX

bandwidth if required C/N is known for desired signal type

Critical for calculations involving carrier recovery loop performance calculations

Carrier-to-Noise Ratio

][][][][][ NBkLOSSEST

GEIRP

N

C

N

R

P

P

N

C

otherRR LR

GEIRPP1

4

2

NNN BkTP

otherNsys

R

LFSLkBT

GEIRP

N

C 111

NN

BN

C

BN

C

N

C

00

Definition of C/N ratio

Received power with all losses taken into account

Noise power

C/N ratio in product form

C/N ratio in dB form

Definition of carrier-to-noise density ratio

Power Budget Example - 2Generic DBS-TV:

Received PowerTransponder output power , 160 W 22.0 dBWAntenna beam on-axis gain 34.3 dBPath loss at 12 GHz, 38,500 km path -205.7 dBReceiving antenna gain, on axis 33.5 dBEdge of beam -3.0 dBMiscellaneous losses -0.8 dBReceived power, C -119.7 dBW

Power Budget Example - 2Noise powerBoltzmann’s constant, k -228.6

dBW/K/HzSystem noise temperature, clear air, 143 K 21.6

dBKReceiver noise bandwidth, 20MHz 73.0

dBHzNoise power, N -134.0 dBW C/N in clear air 14.3 dBLink margin over 8.6 dB threshold 5.7

dBLink availability throughout US Better than

99.7 %

55

Thank you

Carrier-to-Noise Ratio – Uplink

Uplink:

Earth station EIRP, satellite receiver feeder losses, satellite receiver G/T,

Frequency dependent calculations calculated for the uplink frequency

][][][][0

kLOSSEST

GEIRP

N

CUUU

U

subscript “U” stands for uplink

Traveling wave tube amplifiers (TWTAs)

Widely used in transponders to provide the final output power required to the transmit antenna

Provides amplification over a very wide bandwidth

Nonlinear transfer characteristic Input power of the TWTA needs to be carefully

controlled to minimize distortion Low input powers: input-output relationship is linear

Higher input powers: output power saturates

Figure: ref. 1

Uplink – Saturation Flux Density

In the uplink, the TWTA will be at the receiving end

Received signal from earth will be input to the TWTA

Saturation flux density ( Fs ): flux density required at the satellite’s receiving antenna to produce saturation of the TWTA

Using the saturation flux density, one can calculate the required EIRP at the earth station to produce saturation of the TWTA at the satellite

RFLLOSSESAFEIRP USUS 0

4

log102

0A

Effective area of an isotropic antenna subscript “S” stands for saturation

Uplink – Input Back-off

A number of simultaneous carriers present in the TWTA requires back-off of the operating point to reduce intermodulation distortion

It’s the input back-off because the received signal will be input to the TWTA

Earth station EIRP has to be reduced by this specified back-off

iUSU BOEIRPEIRP ][][][ iBO = input back-off

Uplink – Earth Station HPA Earth station high power amplifier is at the transmitting end

of the uplink Supplies the radiated power plus the transmitter feeder

losses

Earth station HPA transfer characteristic can also be nonlinear, requiring output back-off

Output back-off since HPA’s output is the transmitted signal

An HPA with a high saturation point has larger physical size and higher power consumption, but penalty for this is not as large since it’s on the earth.

][][][][ TFLGEIRPP THPA TFL = transmitter feeder losses

HPAsatHPAHPA BOPP ][][][ ,

Carrier-to-Noise Ratio – Dowlink

Downlink:

Satellite EIRP, earth station receiver feeder losses, earth station receiver G/T

Frequency dependent calculations calculated for the downlink frequency

][][][][0

kLOSSEST

GEIRP

N

CDDD

D

subscript “D” stands for downlink

Downlink – Output Back-off, Satellite TWTA Output

Output back-off:

Satellite TWTA Output: TWTA supplies radiated power and transmit feeder

losses, the saturates o/p of TWTA is given by

ODSD BOEIRPEIRP ][][][

dBBOBO Oi 5][][

DTDTWTA TFLGEIRPP ][][][][

The output of the TWTA is being transmitted, so it’s the output back-off

A rule of thumb:

OSTWTATWTA BOPP ][][][

Figure: ref. 1

Effects of Rain So far, calculations have been made for

clear-sky conditions Rainfall is a significant cause of fading in

the C band and especially in the Ku band Rainfall causes attenuation by scattering

and absorption of the radio waves Rain attenuation for horizontal polarization

is greater than for vertical polarization

Rain Attenuation Rain attenuation data is usually

available as curves or tables Tables gives percentage of time

over a year that the attenuation exceeds the dB values

Figure: ref. 1Figure: ref. 4

Radomes are truncated spherical shells composed of panels to protect the earth station antenna from the environment.

Radome transmission loss: ordinary insertion loss, scattering loss

Layer of water caused by rain introduces attenuation by absorption and reflection

Rain-fade margins – Uplink and Downlink

Uplink (satellite is receiving): Increase in noise due to rain usually not a major factor

since satellite antenna is pointed toward a “hot” earth With uplink power control, power output from the earth

station may be increased to compensate for fading Downlink (earth station is receiving): No power control since user doesn’t have control of

satellite EIRP A = rain attenuation caused by absorption Equivalent noise temperature for the rain:

ATT arain

11 rainCSsky TTT

Combined Uplink and Downlink

overall C/N ratio is less than the lower of the uplink and downlink C/N ratios

111

DUoverall N

C

N

C

N

C

Lets denote noise power per unit bandwidth by PNU and the average carrier at the same point by PRU . Therefore,

The carrier power at the end of the space link is PR (i.e received carrier power for the downlink). It is equal to ƴ (system power gain ) times the carrier power input at the satellite.

At the end-of-link, noise is ƴ PNU + PND

When not counting ƴ PNU contribution, then The combined C/No is given by

NU

RU

UO P

P

N

C

NDNU

R

DO PP

P

N

C

ND

R

DO P

P

N

C

11

1

)(

DOUO

O

D

O

U

O

R

ND

RU

NUO

RUR

R

ND

R

NU

R

NDNU

R

NO

N

C

N

CN

C

C

N

C

N

P

P

P

P

C

N

then

termiforPPsubstitute

P

P

P

P

P

PP

P

P

C

N

Intermodulation Noise Occurs whenever multiple carriers pass through a

device with nonlinear characteristics, such as TWTAs1111

IMDUoverall N

C

N

C

N

C

N

C

C/N ratio for intermodulation noise is a function of the number of carriers and their modulation characteristics, and the amplitude and phase characteristics of the high-power amplifier.

Figure: ref. 5

To reduce intermodulation noise, we can operate the traveling wave tube in a back off condition

Increasing back off decreases uplink and downlink C/N ratios

There is an optimum operating point that gives maximum overall C/N ratio as a function of back-off

System Design Example Ku-band geostationary satellite with bent pipe

transponders to distribute digital TV signals from an earth station to many receiving stations

Bent pipe transponder: transponder that amplifies the received signal and retransmits it at a different frequency

Figure: ref. 2

Need a minimum overall C/N ratio of about 9.5 dB in the TV receiver

Table of specifications

Figure: ref. 2

Ku-Band Uplink Design

Uplink Noise Power Budget

k = Boltzmann’s constant -228.6 dBW/K/Hz

T_n= 500 K 27.0 dBK

B = 43.2 MHz 76.4 dBHz

P_n= transponder noise power

-125.2 dBW

NNN BTkP ][][][][ LOSSESGEIRPP RR

Uplink Power Budget

P_t = Earth station transmitter power

P_t dBW

G_t = Earth station antenna gain

55.7 dB

G_r = Satellite antenna gain 31.0 dB

FSL = Free-space loss -207.2 dB

L_ant = Earth station on 2 dB contour

-2.0 dB

Other losses -1.0 dB

P_r = Received power at transponder

P_t - 123.5 dB

Minimum required receive power:

[P_r] = [C/N] + [P_n]

= 30 + -125.3 = -95.2 dBW

dBD

Gt 7.55*68.0log102

dBR

FSL 2.2074

log102

[P_r] = [P_t] – 123.5 dB = -95.2 dBW

=> [P_t] = 28.3 dBW => P_t = 675 W

NRN

R PPP

P

N

C

Ku-Band Downlink Design

Downlink Noise Power Budget

k = Boltzmann’s constant -228.6 dBW/K/Hz

T_n= 30 + 110 K = 140 21.5 dBK

B = 43.2 MHz 76.4 dBHz

P_n= transponder noise power

-130.7 dBW

Downlink Power Budget

P_t = Satellite station transmitter power

18.0 dBW

G_t = Satellite station antenna gain

31.0 dB

G_r = Earth station antenna gain G_r dB

FSL = Free-space loss -205.4 dB

L_ant= Earth station on 3 dB contour

-3.0 dB

Other losses -0.8 dB

P_r = Received power at earth station

G_r - 160.2 dB

dBN

C

N

C

N

C

N

C

N

C

DDUoverallD

2.176.52111

5017

overalloverall N

CdB

N

C

100030

UU N

CdB

N

C

Minimum required receive power:

[P_r] = [C/N]+[P_n] = 17.2 + -130.7 = -113.5 dBW

OSTWTATWTA BOPP ][][][ P_t, sat = 80 W => [P_t, sat] = 19 dBW

Output back-off = 1 dB

[P_t] = 19 – 1 = 18 dBW dBR

FSL 4.2054

log102

[P_r] = [G_r] – 160.2 = -113.5 dBW => [G_r] = 46.7 dB

=> G_r = 46,774

mDD

GR 14.2774,46*65.02

NRN

R PPP

P

N

C

Satellite Communication Link Design Procedure

1. Determine the frequency band in which the system must operate. Comparative designs may be required to help make the selection.

2. Determine the communications parameters of the satellite. Estimate any values that are not known.

3. Determine the parameters of the transmitting and receiving earth stations.

4. Start at the transmitting earth station. Establish an uplink budget and a transponder noise power budget to find uplink C/N in the transponder.

5. Find the output power of the transponder based on transponder gain or output back-off.

6. Establish a downlink power and noise budget for the receiving earth station. Calculate downlink C/N and overall C/N for a station at the edge of the coverage zone (worst case).

7. Calculate S/N or BER in the baseband channel. Find the link margins. 8. Evaluate the result and compare with the specification requirements.

Change parameters of the system as required to obtain acceptable overall C/N or S/N or BER values. This may require several trial designs.

9. Determine the propagation conditions under which the link must operate. Calculate outage times for the uplinks and downlinks.

10. Redesign the system by changing some parameters if the link margins are inadequate. Check that all parameters are reasonable, and that the design can be implemented within the expected budget.

The above can be found in ref. 2

Summary Transmission losses include free-space loss, feeder

losses, antenna misalignment losses and fixed atmospheric and ionospheric losses

To reduce system noise for amplifiers in cascade, have a low noise, high gain amplifier in the first stage

C/N ratio gives error probability and capacity Multiple carriers present means back-off must be

accounted for Rain attenuation can be overcome with uplink power

control, increasing the antenna diameter, or using an amplifier with higher gain and lower noise

C/N ratios add as reciprocals Space link calculations are an iterative process since

it’s hard to get it all right on the first try