Chapt 4 Final

8/2/2019 Chapt 4 Final

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1

Satellite Link Design


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Cost $25000/kg

Therefore heavier the satellite higher the cost -> its

recovered by selling its communication services.

During launch diameter of satellite


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Weight is driven by 2 factors

Number of transponders & Output power of

transponders.

Weight of station-keeping fuel

High power transponders require more electrical

power dimensions ofsolar cells increases.weight of

solar cells also increases.

Half of the total weight of satellite is intended to remain

in service for 15years is due to fuel.

3

Introduction-Contd..


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3 factors influence system design.

Choice of frequency band. Atmospheric propagation effects.

Multi access technique.

Major bands are 6/4, 14/11, 30/20GHz.

More GEO satellites use both 6/4 and 14/11 every 20

minimum spacing between satellites to avoid

interference from uplink earth station.

Additional satellites use 30/20GHz. But distortion dueto rain at higher frequency is more than lower

frequencyAttenuation increases as square of

frequency.

4



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LEO & MEO Covers smaller area of earths surface.

More no of satellites are required to serve a smaller

area compared to GEO.

Closer to earth

produces stronger signal

Thisadvantage is lost since earth station need low gain

Omni directional antennas because the position of

satellite is continually changing.

They use multiple beam antennaincrease gain of

satellite antenna beams and provide frequency reuse.

5



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Maritime satellite communication

Use low gain antenna at mobile unitL-band is used.

C-band is used for fixed hub station (major earth station).

Communication links are assigned to meet performance

objectives.

BER for a digital link

SNR for analog link

Both measured in baseband channel (information

signal is generated or received)

Eg: TV camera generates baseband video signalTV

receivers delivers baseband video signal to the picture

tube to form images.

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A maritime satellite communication system.



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Digital data produced by computers at baseband and BER

is measured at baseband.

Baseband channel BER(Digital)or S/N ratio(Analog)is

determined by CNR at input to demodulator in the

receiver.

C/N at demodulator input must be > 6dB

In digital linkifC/N


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C/N measured atinput ofreceiver and output terminals

ofreceiverantenna.

RF noise received along with signal and noise generated

by receiver are combined into an equivalent noise power

at input to receiver and noiseless receiver model is

used.

Noiseless receiverC/N ratio is constant at all points in

RF and IF chain.

C/N at demodulator input =C/N at receiver input

Satellite link 2 paths uplink and downlink

Overall C/N at earth station receiver depends on both

links.

Rain attenuationC/N fall below minimum permitted

valueFor high frequency(30/20GHz)Lead to link

outage. 9



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Basic Transmission Theory

Two approaches to this calculation:

1. Flux density

2. Link Equation


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Power received by an ideal antenna with area A m2.

Incident flux density is F = Pt/4R2 W/m2. Received poweris Pr= F X A = PtA/4R

2 W.

11

Basic Transmission Theory Contd..


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Isotropic Radiator

Consider an Isotropic Source (punctual radiator) radiating

Pt Watts uniformly into free space Isotropic Radiator.

At distance R, the area of the spherical shell with center at

the source is4R2

Flux density at distance R is given by

Power flux density (p.f.d.) is a measure of the power per

unit area.

2

4 R

PF t

W/m2



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13

Power Flux Density

Find the power density at the receiver.

The power radiated from a transmitter must passthrough a spherical shell on the surface of which is thereceiver.

The area of this spherical shell is4R2 .

Therefore spherical spreading loss is 1/4R2



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14

Isotropic Radiator

24 R

PF t

W/m2

Pt Watts

Distance R

Isotropic Source

Power Flux Density:Surface Area of

sphere = 4R2

encloses Pt.

Gain of isotopic

antenna=1.



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15

Antenna Gain 1

Directive antennas to get power to go in wanted direction.

Define Gain of antenna as 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



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16

Antenna Gain 2

Antenna has gain in every direction!

Usually Gain denotes the maximum gain of the antenna.

The direction ofmaximum gain is called bore sight.

is taken to be the direction in which maximum power isradiated.

Gain of the antenna is then the value ofG() at angle =0o.



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17

Received Power

Transmitter with output PtWatts driving a lossless

antenna with gain Gt, flux density in the direction of

antenna boresight at distance R is

The power available to a receive antenna of areaAe m2 we

get:

24

xR

AGPAeFP ettr

2

22W/m

44 RGP

R

EIRPFtt



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18

Effective Aperture

Real antennas have effective flux collecting areas which

are LESS than the physical aperture area.

Practical Rx antenna of area Aphywill not deliver above

Pr.Some energy is reflected and some is absorbed leads to

decrease in efficiency.

Define Effective Aperture Area Ae:

xe phyAA Where Aphy is actual (physical) aperture area.

= aperture efficiencyVery good: 75%

Typical: 55%



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19

Effective Aperture - 2

2

4

e

A

Gr

Antennas have (maximum) gain Gr related to the effective

aperture area as follows:

Where: Ae is effective aperture area.


Gain is a ratio:

It is usually expressed inDecibels (dB)

G [dB] = 10 log10 (G ratio)


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20

Back to Received Power

The power available to a receive antenna of effective area Ar =Ae m

2 is:

--(Eqn. 4.6)24x R

AGPAFP etter

Where Ar = receive antenna effective aperture area = Ae

2

4

er

AG

Inverting the equation given for gain gives:

Inverting

4

2

re

GA

--Link Equation



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21

Back to Received Power

Substituting in Eqn. 4.6 gives:

2

4

R

GGPP rttr

Friis Transmission Formula

The inverse of the term at the right referred to as Path Loss,

also known as Free Space Loss (Lp)[Path Loss]:

24

RLp

Therefore

p

rtt

rL

GGPP



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22

More complete formulation

rataap

rttr

LLLL

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



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23

Efficiency accounts for all the losses between the incident wavefront and the output port.

Illumination efficiency or aperture taper efficiency

Energy distribution produced by the feed across the aperture.

Other losses due to spillover, blockage, phase error, diffraction

effects, position and mismatch losses.

For parabolodial reflector antennas efficiency is in the range 50

to 75%.

Horn antennas efficiency is ~90%



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24

Aperture Antennas

Typical values of:

-Reflectors: 50-60%

-Horns: 65-80 %

2D

Gain

4

22 DrA

phy

2244 phye AA

Gain

Aperture antennas (horns and reflectors) have a

physical collecting area that can be easily calculated

from their dimensions:

Therefore, obtain the formula for aperture antenna

gain as:



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25

EIRP - 1

An isotropic radiator is an antenna which radiates in alldirections equally.

Antenna gain is relative to this standard

Antennas are fundamentally passive

No additional power is generated

Effective Isotropic Radiated Power (EIRP) is the amountof power the transmitter would have to produce if it wasradiating to all directions equally

Note that EIRP may vary as a function of direction becauseof changes in the antenna gain vs. angle.



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26

The output power of a transmitter HPA is: Poutwatts

Some power is lost before the antenna:

Pt=Pout/Lt watts reaches the antenna

Pt= Power into antenna

The antenna has a gain of: Gtrelative to an isotropicradiator.

This gives an effective isotropic radiated power of:

EIRP = PtGt watts relative to a 1 watt isotropic radiator

EIRP - 2

HPA

PoutLt

Pt

EIRP



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27

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


The received power Pr is commonly referred to as Carrier

Power, C.


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28

Review of Decibel


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29

What is a dB?

Decibel (dB) is the unit for 10 times the base 10logarithmic ratio of two powers

For instance: gain is defined as Pout/Pin (where Pout isusually greater than Pin)

in dB:

Similarly loss is:

dBlog10

in

out

P

PG

dBlog10

out

in

P

PL


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30

Using Decibels - 1

Rules:

Multiply A x B:

(Add dB values)

Divide A / B:

(Subtract dB values)

dB)(

dBdB

)(log10)(log10

)/(log10

1010

10

BA

BA

BA

BA

dB)(

dBdB

)(log10)(log10

)x(log10

1010

10

BA

BA

BA

BA


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31

Using Decibels - 2

Rules:

Squares:

(Multiply by 2) )dBin(x2

)(log20

)(log10x2

)(log10

10

10

2

10

A

A

A

A

Square roots:

(Divide by 2)

)dBin(x2

1

)(log210

)(log10

10

10

A

A

A


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32

References in dB

dB values can be referenced to astandard

The standard is simply appended to dBTypical examples are:

Units Reference

dBi isotropic gain antenna

dBW 1 watt

dBm 1 milliwattdBHz 1 Hertz

dBK 1 Kelvin

dBi/K isotropic gain antenna/1 Kelvin

dBW/m2

1 watt/m2

dB$ 1 dollar


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Received Power[Contd.]

In dB=>

where EIRP= 10 log(Pt

Gt

) dBW

Lp=10 log(4 R/ )=20 log(4 R/ ) dB

33



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A satellite link. LNA, low noise amplifier.

35



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36

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 & G/T Ratio


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37

System Noise Temperature

1) System noise power is proportional to system noisetemperature

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 eachcontribution

Reference these noise temperatures to the same location

Additive White Gaussian Noise (AWGN)

RXlinelossLNAantennadtransmittes TTTTTT



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38

System Noise temperature

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

Noise Power=Pn=KTpBn

Where

K=Boltzmann Constant== 1.38x10-23

J/K(-228.6 dBW/HzK)Tp=Physical temperature of source in Kelvin Degrees

Bn=Noise BW in which noise power is measured in Hz

Pn is delivered only to load that is impedance matched to the

noise source



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Noise temperature from 30K to 200k is achieved by

without physical cooling ifGaASFETamplifiers are

used.

GaASFET amplifiers operate at temperature of30K at

4GHz & 100K at 11GHz. LNA receiver for 20GHz have

noise temperature of150K.

A device with a noise temperature of Tn Kelvin (K)

produces at its output the same noise power as a black

body at a temperature Tn degrees Kelvin followed by a

noiseless amplifier with the same gain as the actual deviceThe description of a low noise component by an

equivalent noise source at the input of a noiseless

amplifier is very useful because we can add noise

temperature to determine the total noise power in a 39

Noise temperature



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Noise Temperature

Noise Temperature

If amplifier noise=0 =>Noise temperature=0K

If amplifier noiseNoise Temperature


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Noise Temperature-Performance

Performance of a receiving system is determined by

determining Ts.

Ts is the noise temperature of a noise source, located at theinput 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 antenna

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Noise Temperature-Performance[Contd.]

The noise power referred to the input of the receiver is Pn

where

Pn=KTsBn wattsCarrier to Noise Ratio is given by=

= =

43



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Noisy Model Of Receiver: Noise Analysis

Noisy devices in the receiver replaced by equivalent

noiseless blocks with same gain & noise generators at

the input to each block, hence the block produces the same

noise at its output as noisy device

Entire receiver is reduced to a single equivalent noiseless

block

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Simplified earth station receiver. BPF, band pass filter.

45

Calculation of System Noise Temperature



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Communication receiver: Has RF amplifier (LNA) &

single frequency conversion from its RF i/p to IF o/p.

RF amplifier must generate as little noise as possible-hence

called LNA.

Mixer & local oscillator form a frequency conversion stagethat down converts the RF signal to a fixed IF.

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Simplified earth station receiver. BPF, band pass filter.




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Double Conversion earth station receiver.

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Double Super heterodyne Conversion

Consists of 2 stages of frequency conversion:

The front end of the receiver is mounted behind the

antenna feedConverts incoming RF signal to a first IF in the range

900-1400 MHz[flo=2800 for C-band ,flo=10800 for

Ku-band]

Receivers accept all the signals transmitted from asatellite in a 500-MHz BW at C or Ku band

48




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Double Super heterodyne Conversion[Contd.]

900-1400 MHz signal is sent over a coaxial cable to a set-

top receiver that has another down-converter and a

tunable local oscillator & tunable channel select filter

Local oscillator is tuned to convert the incoming signal

from a selected transponder to a second IF frequency

Second IF amplifier has BW equal to spectrum of

transponder signal

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51

Receiver noise comes from several sources.

Method which reduces several sources to a singleequivalent noise source at the receiver input.

Using model in Figgives:

End)-(Front

(Mixer)(IF)

inRFRFmIF

mmIF

IFIFn

TTkBGGG

BkTGGBkTGP



i i i h C d


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GRF, Gm & GIF are the gains of the RF amplifier, mixer,

and IF amplifier.

TRF, Tm & TIF are equivalent noise temperature of the RFamplifier, mixer, and IF amplifier.

Tin is the noise temperature of the antenna, measured at

its output port.

52



B i T i i Th C d


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Noise model of receiver.All noisy units have been replaced byone

noiseless amplifier, with a single noise source Ts as its input.

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B i T i i Th C d


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55

Equate Pn :

Succeeding stages of the receiver contribute less & less noise

to the total system noise temperature.

If RF amplifier in the receiver has a high gain, noisecontributed by IF amplifier & later stages can be ignored

System noise temperature is sum of the antenna noisetemperature & the LNA noise temperature. Ts=Tantenna+TLNA.

All noise comes from antenna or is internally generated in thereceiver.

inRF

RFm

IF

RF

m

inRF TTGG

T

G

TTTT

S



B i T i i Th C d


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Model to deal with noise that reaches the receiver afterpassing through a lossy medium. Eg: Waveguides & rain

losses.

Model the noise emission as a noise source placed at the

output of the atmosphere, which is the antenna aperture.Noise model for equivalent output noise source is shown

in Fig , & it produces a noise temperature Tno given by

Tno=Tp(1-Gl) where Gl is the linear gain of the attenuating

device or medium(


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Noise model for a lossy device. The lossy device has been replaced by a

lossless device, with a single noise source Tno at its output.

57




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Noise Figure & Noise TemperatureDefine the noise generated in the device.NF=(S/N)in /(S/N)out

Noise figure is converted to noise temperature Td.

Td=To(NF-1)Noise figure is a linear ratio, not in dB, & where To is the

reference temperature used to calculate the standard noise

figure~290K.

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G/T Ratio for Earth Stations


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G/T Ratio for Earth Stations

Link equation in terms of C/N at the earth station

Thus C/N proportional to [Unit dB/K]

Describes the quality of receiving earth station or a

satellite receiving system.

Increasing increases C/N ratio.

s

r

n

tt

ns

rtt

T

G

RKB

GP

RBKT

GGPNC 22

4

4/

s

r

T

G

s

r

T

G

Chapt 4 Final

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