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Satellite Link Budgets: Thinks 2 know

AJAL.A.Jprofessorajal@gmail.comMOB: 8907305642

DEPARTMENT OF ECEUNIVERSAL ENGINEERING COLLEGE, THRISSUR- KERALAINDIA

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EARTH STATION

A

EARTH STATION

B

DOWN

LINK

UP

LINK

Satellite Link

The ultimate goal of satellite is to provide satisfactory transmission relayed between earth stations.

Satellite always use band pass channels.

Estimation

• You are planning for a tour.

• The estimate prepared = Rs 5,000 /-

• Back home after tour

Amount in wallet = Rs 5,00 /-

You pat yourself on the back for a job well done because you still have Rs 5,00 /- left in your wallet

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The only difference …

• We do something similar with communication links , called creating a link budget.

• The only difference is that

Traveler signal

Instead of Rs, it starts out with power

Rs power

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At the end of the day …..

• It spends its power (or) attenuates as it travels , be it wired or wireless

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• It is a theoretical calculation of end-to-end performance for a communications path under a specific set of conditions.

• Sometimes the conditions are stated; most often at least some of them are implied or assumed.

• Every link budget implies everything not included is irrelevant.– Sometimes this is true

What Is a Link Budget

A link budget is a relatively simple addition and subtraction of gains and losses within an RF link.

LINK BUDGET

When these gains and losses of various components are determined and summed, the result is an estimation of end-to-end system performance in the real world.

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Why is a Link Budget Important

• A link budget is used to predict performance before the link is established.– Show in advance if it will be acceptable– Show if one option is better than another– Provide a criterion to evaluate actual

performance

LINK BUDGET CAN Minimize BandwidthP

ow

er

Bandwidth

1:1

“Power limited” – too much power

“Bandwidth limited”

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Link Budget Components

• A satellite link budget should include the following parts:

1. UPLINK

2. DOWNLINK

3. COMBINE 1 AND 2

4. DEFINE PERFORMANCE LIMIT(S)

5. COMPARE CALCULATED AND DESIRED PERFORMANCE

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LINK BUDGET EVALUATION

• The basic question is: Is the operating margin large enough?– It must be positive to account for the items

listed in the budget– It also must cover the variations caused by

everything that was not explicitly included

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EVALUATION (continued)

• Some factors we consider important are:– Satellite non-linearity– Satellite transmit power for your signal– Interference (including CDMA if implemented)– Allowance for future (worse) conditions– Lifetime of the system under evaluation– How closely can it be maintained at the

parameters used in the budget

• Performance objectives for digital links consist of:

• BER for normal operating conditions

• Link Availability, or percentage of time that the link has a BER better than a specified threshold level

LINK BUDGET

LINK BUDGET ANALYSIS

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Space Segment Parameters

Carrier Parameters

Earth Segment Parameters

Transponder Parameters

Space Segment Parameters

• G/T Receive [dB/K]• Saturation Flux Density (SFD) [dBW/m2]• EIRP (Saturation) [dBW] • Transponder Bandwidth [MHz]• Transponder IBO (Input Back-off) [dB]• Transponder OBO (Output back-off) [dB]• Transponder Intermodulation Interference (C/IM) [dB]

Carrier Parameters

• Modulation Technique [BPSK/QPSK]

• Data Rate [bps]

• Forward Error Correction Rate (FEC)

• Reed Solomon Coding Rate (R-S)

• Turbo Coding (TPC)

Forward Error Correction (FEC)

Carrier Parameters

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

Why Optimal FEC Rates ?

– To achieve optimum use of power and bandwidth

– To provide best possible availability within system limitations

– Transmission Rate = Data Rate * 1/FEC

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Earth Segment Parameters

• Antenna Size

• HPA size

• Receive Antenna G/T

• Location

Antenna Size

Earth Segment Parameters

– What? - Diameter- Efficiency

– Impact? - The larger the antenna diameter, the higher the antenna gain

- The larger the antenna diameter, the better the antenna G/T

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"goodness"

• The G /TdBK is known as the "goodness" measurement of a receive system.

Satellite Orbits

Distance (km) Period

Low Earth Orbit (LEO) 700 - 2000 ~2 hr

Medium Earth Orbit (MEO) 10,000 – 15,000 ~6 hr

Geosynchronous Earth Orbit (GEO) 36,000 24 hr

The End!

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