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Link Budget Calculations
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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
04/10/23 10
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
04/10/23 12
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
04/10/23 23
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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