Syed Khurram IqbalSystem Architect (Pakistan and Central Asia)O3B Networks

04/10/23

2

Link Budget Overview

• What is link budget? A computation to verify/simulate the performance of a satellite link

• Why link budget is needed? To optimize between these factors:-1.Limited transponder power and bandwidth2.Desired link performance3.Also take into account of external factors (e.g adjacent satellite)

•Why is link budget important? By understanding link budget, one can:1.Estimate required capacity for equipment and bandwidth2.Come up with options to improve/optimize the link quality3.Understand the pros and cons of satellite links4.Easily understand various problems and effects (e.g interferences)

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Learning Steps for Link Budget

Step 1: Know the Definitions

- Understand the parameters involved in link budget

Step 2: Know the Relationships

- Understand how each parameter effects other parameters and link performance- Equations

Step 3: Play with the parameters

- Identify the requirements- Identify the limitations- Optimize

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Link Budget Parameters

Satellite Parameters

• Transponder Bandwidth• Satellite G/T• Transponder EIRPdn

Uplink Parameters

• Transmit Location• Transmit Antenna Size• HPA Size Carrier Parameters

• Symbol Rate• Modulation Type• FEC

Downlink Parameters

• Receive Location(s)• Antenna Size (s)• Eb/No Threshold

External Parameters

• Adjacent Satellite • Interferences

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Transponder Bandwidth

THAICOM 5 Extended C-band Global Beam

THAICOM 5 Standard C-band Regional Beam THAICOM 5 Extended C-band Regional Beam

THAICOM 5 Extended C-band Regional Beam THAICOM 5 Standard C-band Regional Beam

THAICOM 5 Extended C-band Global Beam

5945

3440

3425

3480 3520 3560 3600 3640 3680

36253585354535053465 3720 3760 3800 3840 3880 3920 3960 4000 4040 4080 4120 4160

67056665662565856545

6650661065706530

6465 6505

6450 64905985 6025 6065 6105 6145 6185 6225 6265 6305 6345 6385

TM1 TM21G 2G 3G 4G 5G 6G 7G

1E 2E 3E 4E 5E 6E 1 2 3 4 5 6 7 8 9 10 11 12

1G 2G 3G 4G 5G 6G 7G

1 2 3 4 5 6 7 8 9 10 11 12 1E 2E 3E 4E 5E E6

TM1 TM2

Vertical

Horizontal

Horizontal

Vertical

TM1 : 4197.875 MHzTM2 : 4198.300 MHz

SPOT (V)SPOT (V)

Uplink (MHz) : 5.925 - 6.425 and 6.425 - 6.725 GHz

Downlink (MHz) : (3.405 - 3.700 and 3.700 - 4.200 GHz)

TM1: 4199. 2 MHz SPOT (V)TM2: 4199. 8 MHz SPOT (V)

Thaicom 1A , 2 and 5 Standard C-Band Transponders: 12 ( each 36 MHz)Thaicom 5Extended C-Band Regional : 6 (each 36 MHz)Extended C-Band Global: 7 (each 36MHz)

Satellite Parameters

• Transponder Bandwidth• Satellite G/T• Transponder EIRPdn

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Satellite G/T

Thaicom 5 Standard C-Band G/T

1.60

0.50

0.50

0

0

0

-1

-1

-1-2

-2

-2

-3

-3

-3

-4

-4

-4

-4-5

-5-5

-5

SA

TS

OF

T

-4.00 -2.00 0.00 2.00 4.00 6.00 8.00Theta*cos(phi) in Degrees

-2.00

0.00

2.00

4.00

6.00

8.00

Theta*sin(phi) in Degrees

Uplink Parameters

• Transmit Location• Transmit Antenna Size• HPA Size

G/T Sat Satellite

Satellite Parameters

• Transponder Bandwidth• Satellite G/T• Transponder EIRPdn

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Required Uplink Power : EIRPup

Satellite G/T is the gain of the satellite’s receive antenna

Locations with high satellite G/T contour require less ‘uplink power (EIRPup)’

Uplink location G/T Sat EIRPup HPA size, Tx Antenna Size

HPA

TP

EIRPup

G/T Sat

EIRPup (dBw) = 10 log P T + G Ant - LF

PT : Input Power to Tx Antenna (Watts)G Ant: Transmit Antenna Gain (dBi)LF : Feeder Loss (dB)

P T

G Ant

L F

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HPA Sizing

Case Study: Determine the HPA size required to uplink one 128kbsp and one 64kbps carriers. Tx antenna size is 2.4m.

EIRPup1 (for 128k) = 45.6dBw

EIRPup2 (for 64k) = 42.6dBw

EIRPupTotal = 10log[10(EIRPup1/10) + 10(EIRPup2/10)]

= 47.4 dBw

GAnt = 41.6 dBi (from antenna spec)

LF = 1dB (actual loss may be higher need more uplink power)

Pout = EIRPupTotal –GAnt + LF = 6.8 dBw

More than one carrier from HPA, needs to back off to avoid intermods. (see. HPA Characteristic)

OBOhpa = 3dB

Saturated output power , PS = Pout + OBOhpa = 9.8 dBw

Required HPA Size = 10^ (PS/10) = 9.55 Watts

HPA

G AntL F

Pout EIRPupPT

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Transponder EIRPdn

Downlink Parameters

• Receive Location(s)• Antenna Size (s)• Eb/No Threshold

EIRPdn40.53

40

40

40

39

39

39

38

38

38 37

37

37

36

36

36

35

35

35

35

34

34

34

34

33

33

33

33

SA

TS

OF

T

-4.00 -2.00 0.00 2.00 4.00 6.00 8.00Theta*cos(phi) in Degrees

-2.00

0.00

2.00

4.00

6.00

8.00

Th

eta

*sin(p

hi) in

De

gre

es

Thaicom 5 Standard C-Band EIRPdn Contour[ All standard c-band transponders on T5 have same contour pattern]

Satellite

Satellite Parameters

• Transponder Bandwidth• Satellite G/T• Transponder EIRPdn

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Transponder EIRPdn (continue)

Power Flux Density (PFD) :: Total input power to transponder PFD Total = ∑ PFD Carrier

PFD Carrier = EIRPup – Loss

Saturated Flux Density (SFD) :: Total input power to transponder at saturation pointSFD = -(80+G/Tsat ) – (Atten Max – Atten)

IBO = SFD – PFD ; OBO is determined from IBO from transponder characteristic curveEIRPdn = EIRPdn Max – OBO

TP

SFDEIRPdn TP

PFDTotal

EIRPup

Loss

Atten

Input-Output Characteristic is similar to HPA

EIRPup :: Total uplink power (each carrier)

Atten :: Transponder input attenuation setting

Loss :: Spreading loss between earth to satellite

Input

Output

Bi

Bo

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Carrier EIRPdn

Transponder power is shared by all carriers using on the transponder Each carrier must operate within allowable limit of power allocated for its bandwidth

Transponder Operation Modes

• Single Carrier Mode OBO = 0 dB Max Transponder EIRPdn : 40 dBw

Allowable EIRPdn per carrier = 40dBW – 0 dB = 40 dBw

• Two Carriers ModeOBO = 2 dB

Max Transponder EIRPdn : 40 dBw – 2dB = 38 dBwAllowable EIRPdn per carrier = 38 dBw – 3dB = 35 dBw

• Multiple Carriers ModeOBO = 4 dB

Max Transponder EIRPdn : 40 dBw – 4dB = 36dBwAllowable EIRPdn per carrier : 10log[(x/36)*{10^((36)/10)}]x : bandwidth (in MHz) of the carrier

36MHz

18MHz 18MHz

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Simplified Link Model

1

C/N Total

1

C/I IntermodC/NUplink

1

C/N Downlink

1

C/I adjacent

1= + + + +

1

C/I x-pol

TPEIRPdn

EIRPup

C/I Intermod

C/I x-pol

C/I adjacentC/N DownlinkC/NUplink

uplink downlinktransponder external

Received signal quality :: Eb/No =(C/N total* Bandwidth)/(Information rate)

C/N Total

Received Signal (C/N Total )

C/I Intermod

C/I x-pol

C/I x-pol

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Symbol Rate and Bandwidth

Carrier Parameters

• Symbol Rate• Modulation Type• FEC

- Directly relates with bandwidth requirement

Bandwidth = Information Rate x ( 1/ FEC) x (1/ Mod Type) x (1 / RS) x BT Product

BT (Bandwidth Time) Product = 1 + Roll-off = { 1.2 , 1.25, 1.35}

Affects the ‘shape’ of the carrierNot much important in link budgetLimited options in modems/ encoders

Symbol Rate

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Carrier Parameters

• Symbol Rate• Modulation Type• FEC

Carrier Modulation Type

Modulation Types

BPSK : 1 bit per symbol

QPSK : 2 bits per symbol

8PSK : 3 bits per symbol

16 QAM : 4 bits per symbolHigher modulation types needs less bandwidth but need more uplink power

Bandwidth Requirement

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Carrier Modulation Type (continue)

Case Study: Customer A has lease capacity X MHz which is fully occupied with 6 QPSK carriers. Customer A wants to put one more link ( 2 carriers) without leasing more bandwidth.

Solution: Change the modulation from QPSK to 8-PSK for all carriers

Advantage : solution for limited bandwidth option

Disadvantage: Need higher uplink power -> HPA/ODU size need to recheck if enough AND check power utilization on transponder is within limit

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Carrier Parameters

• Symbol Rate• Modulation Type• FEC

Carrier FEC

1/2 FEC:

Data Bit

Extra Bit

1 E 2 E 3 E 4 E 5 E 6 E 7 E

3/4 FEC:

1 2 3 E 4 5 6 E 7

7/8 FEC:

1 2 3 4 5 6 7 E

E

X

Forward Error Correction (FEC)

Coding Types :{ Turbo, Viterbi , Reed Solomon (RS) }

Purpose is to enhance the link quality

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Carrier Parameters

• Information Rate• Modulation Type• FEC

Carrier FEC ( continue )

DVB : Viterbi + RSVSAT : Viterbi or Turbo

Viterbi : { 1/2, 2/3, 3/4, 5/6, 7/8 }Turbo : { 5/6, 3/4, 7/8 }RS : {188/204 , 112/126 , …}

Bandwidth

Power

Viterbi Coding requires higher power same bandwidth requirement as turbo coding

Reed Solomon Coding requires same power higher bandwidth requirement as turbo coding

Turbo Coding requires less power than Viterbi coding same bandwidth requirement as Viterbi coding some modems may not support

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Carrier Parameters

• Information Rate• Modulation Type• FEC

Carrier FEC ( continue )

Relates to Service Quality

BER (Bit Error Rate)

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

3

3.3

3.6

3.9

4.2

4.5

4.8

5.1

5.4

5.7 6

6.3

6.6

6.9

7.2

7.5

7.8

8.1

8.4

8.7 9

Viterbi Rate 1/2 Viterbi Rate 3/4 Viterbi Rate 7/8

BE

R

Eb/No

“Higher FEC rate requires higher Eb/No”

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Carrier FEC (continue)

Eb/No =(C/N total* Bandwidth)/(Information rate)

Where:

Eb = Energy per bit (W/bit)

No = Noise Power Density (W/Hz)

Carrier Parameters

• Information Rate• Modulation Type• FEC

FEC Relates to Service Quality

Service Quality is measured by BER

BER : shows amount of error occurring in transmission

BER Relates to Eb/No Probability of Error = 0.5 e –Eb/No

Eb/No Relates to other factors of satellite link

Downlink Parameters

• Receive Location(s)• Antenna Size (s)• EbNo Margin

Eb/No Margin : 2dB

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Carrier FEC (continue)

Case Study: A broadcast carrier of 4.5MHz bandwidth (QPSK- 3/4) is operating with maximum allowable EIRPdn level. A group of viewers from location X cannot receive well due to low EIRPdn at their location and thus face low link margin. CND does not allow the customer to increase uplink power because it will overuse power on transponder.

Solution 1: Using bigger receive antenna size ( >=3m) will increase link margin. This solution may be hard to implement if many receive sites (home users) involved.

Solution 2: Reducing FEC from 3/4 to 1/2 will improve link margin

Advantage: Link margin improves without overusing transponder power.

Disadvantage: Needs to reduce information rate to keep same symbol rate ( and bandwidth).

Bandwidth = Symbol Rate x BT Product = Information Rate x ( 1/ FEC) x (1/ Mod Type) x (1 / RS) x BT Product

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Summary

Up Link EIRPUp Link Pattern AdvantageTransponder Gain StepDown Link Pattern AdvantageReceive Antenna Gain

Free Space LossesWaveguide LossesAtmospheric LossesRain Attenuation

E/S and satellite Intermodulation

Up Link Thermal NoiseDown Link Thermal NoiseAdjacent SatelliteCross-pol Interference

+

-

--

Service Quality : BER C/NTotal

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HPA Characteristics

Input

Output Linear Region

Single Carrier Response

Multiple Carrier Response

Saturation Points

Operating Points

Bo

Bi

Bo : Output Back-offBi : Input Back-off

Maximum HPA Power:: Total output power at saturation point of single carrier response Input Back Off (IBO) :: Ratio of input power at saturation point to desired operating point Output Back Off (OBO) :: Ratio of maximum (saturation) output power to actual operating point

Different response curve for single carrier and multiple carrier modes

Multiple carrier saturate at lower input level than single carrier

Higher output back off is needed for multiple carrier mode to keep the operating point within linear region

Non-linear operating point produce intermods

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