Published on March 5, 2014
Satellite Link Budgets: Thinks 2 know AJAL.A.J firstname.lastname@example.org MOB: 8907305642 DEPARTMENT OF ECE UNIVERSAL ENGINEERING COLLEGE, THRISSUR- KERALA INDIA 2
UP LINK EARTH STATION A DOWN LINK EARTH STATION B 3
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 5
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 6
At the end of the day ….. • It spends its power (or) attenuates as it travels , be it wired or wireless 7
What Is a Link Budget • 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 8
LINK BUDGET A link budget is a relatively simple addition and subtraction of gains and losses within an RF link. 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.
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 10
LINK BUDGET CAN Minimize Bandwidth Power “Power limited” – too much power 1:1 Bandwidth “Bandwidth limited”
Link Budget Components • A satellite link budget should include the following parts: 1. 2. 3. 4. 5. UPLINK DOWNLINK COMBINE 1 AND 2 DEFINE PERFORMANCE LIMIT(S) COMPARE CALCULATED AND DESIRED PERFORMANCE 12
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 13
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 14
LINK 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 ANALYSIS Space Segment Parameters Carrier Parameters Earth Segment Parameters 16
Space Segment Parameters Transponder 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)
Carrier Parameters Forward Error Correction (FEC) 1/2 FEC: 1 E 2 E 3 E 4 E 5 5 6 E E 6 E 7 E 7 3/4 FEC: 1 2 3 E 4 7/8 FEC: 1 2 X 3 4 5 6 7 E E Data Bit Extra Bit
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 20
Earth Segment Parameters • Antenna Size • HPA size • Receive Antenna G/T • Location
Earth Segment Parameters Antenna Size – What? – Impact? - Diameter - Efficiency - The larger the antenna diameter, the higher the antenna gain - The larger the antenna diameter, the better the antenna G/T
"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
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