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SATELLITE LINKS

SATELLITE LINKS. BASIC LINK BUDGETS ALLOCATING THE AVAILABLE SATELLITE RESOURCES TO ACCOMODATE THE PARAMETERS OF THE TX & RX EARTH STATIONS. C (dBW). CARRIER POWER RECEIVED IS DEFINED BY : C = PtAe / [4pi(radius^2)] (WATTS) where, 4pi(radius^2) = SURFACE AREA OF A SPHERE

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SATELLITE LINKS

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  1. SATELLITE LINKS BASIC LINK BUDGETS ALLOCATING THE AVAILABLE SATELLITE RESOURCES TO ACCOMODATE THE PARAMETERS OF THE TX & RX EARTH STATIONS ...

  2. C(dBW) • CARRIER POWER RECEIVED IS DEFINED BY : C = PtAe / [4pi(radius^2)] (WATTS) where, 4pi(radius^2) = SURFACE AREA OF A SPHERE Pt = ISOTROPICALLY SPREAD Tx POWER Ae = EFFECTIVE AREA OF THE Rx ANTENNA • WHEN A DIRECTIONAL ANTENNA IS USED : C = PtGtAe / [4pi(radius^2)] where, Gt = Tx GAIN 2

  3. pi • A CONSTANT OF PROPORTIONALITY (USEFUL IN SOLVING FOR THE AREA OF A CIRCLE) • THE EGYPTIAN RULE FOR FINDING THE AREA : EQUALS 3.16 TIMES THE RADIUS SQUARED • WHICH WAS CLOSER TO THE TRUTH THAN THE BABYLONIAN VALUE OF 3 (BASED ON THE BIBLE) • IN ACTUALITY, THE MATHEMATICAL VALUE OF pi IS AN IRRATIONAL NUMBER 3

  4. C/T (dBW/K) • CARRIER-TO-THERMAL NOISE where, C = EIRP - LOSSES + Gr and, C/T = EIRP - LOSSES + G/T THIS IS THE HEART OF THE LINK BUDGET 4

  5. C/kT (dBHz) • CARRIER-TO-THERMAL NOISE DENSITY (WITH BOLTZMANNS CONSTANT k) C/kT = C/No = C/T + 228.6 where, kT = No = N/B = N (dBW/Hz) (IN A 1Hz BANDWIDTH) 5

  6. C/N (dB) • CARRIER-TO-NOISE IN BANDWIDTH B C/N = C/kTB where, C/kTB = C/kT - 10log(BW) and, C/kT = C/No 6

  7. Eb/No (dB) • ENERGY PER BIT - NOISE DENSITY Eb/No = C/No - 10log(R) where, R = BIT RATE (BITS/SECOND) PERFORMANCE OF DIGITAL CIRCUITS IS OFTEN MEASURED AS A SPECIFIC BER. WHICH IS RELATIVE TO THE Eb/No. 7

  8. E (dBuV/m) • ELECTRIC FIELD STRENGTH (POWER PER UNIT AREA) W = 1/2[c(PERMITTIVITY)] x E^2 (W/m^2) W = [1/2(E^2)] / Z (W/m^2) where, Z = 1 / [c(PERMITTIVITY)] W = 2E - 148.77 (dBW/m^2) E = 1/2(W + 148.77) (dBuV/m) 8

  9. EIRP(dBW) • EQUIVALENT ISOTROPICALLY RADIATED POWER EIRP = PGt (WATTS) EIRP = 10log(P) + 10log(Gt) (dBW) TYPICAL VALUES OF EIRP RANGE FROM : 0-90 dBW FOR EARTH STATIONS 20-60 dBW FOR SATELLITES 9

  10. G (dBi) • GAIN OF AN ANTENNA (AS REFERENCED TO AN ISOTROPIC RADIATOR) G = Tx PWR OF ANTENNA / ISOTROPIC Tx PWR G (PARABOLIC) = (4pi x eff x A) / WAVELENGTH^2 G = eff{[(piD x FREQ)/C]^2} G = 20logD + 20logFREQ + 10log(eff) + 20.4 TYPICAL E/S GAIN FIGURES ARE 1-60dBi SATELLITE GAIN FIGURES RANGE FROM 14-40dBi 10

  11. eff • Antenna efficiency (assumed 60-70%) • Actual values range from .2 to .75 • Conventially illuminated (large) Earth stations typically are 65-75% • Flat plate antennas are 75% efficient (Superconductive surfaces on these may further increase this value) • Satellite spacecraft antennas are usually less efficient. (40-55%, or 20-30% for multi-beam) 11

  12. BASICS OF ANTENNA GAIN • A Tx SHAPED ANTENNA FOCUSES THE Tx PWR • IF NO BEAM DIRECTIVITY IS APPLIED, THE RESULT IS AN ISOTROPIC RADIATOR. (THE SUN COULD BE USED AS AN EXAMPLE) • THEORETICAL GAIN OF A PARABOLIC IS INFINITE (THUS, THE LIMITATION IS BASED ON WAVELENGTH) • GAIN CALCULATED BY VIRTUE OF THEORETICAL IS USUALLY CONSIDERED PEAK (ON-AXIS) GAIN. • OFF-AXIS GAIN IS ALSO A SERIOUS CONSIDERATION 12

  13. ANTENNA BEAMWIDTH 13

  14. G/T (dBi/K) • FIGURE OF MERIT G/T = Gr - 10logTs where, Gr = Rx ANTENNA GAIN (dBi) Ts = Rx SYSTEM NOISE TEMP (DEGREES KELVIN) • Gr IS A FACTOR OF THE EFFICIENCY, OR SIZE OF THE ANTENNA. • Ts IS THE SUM OF ANTENNA NOISE TEMP, LNA TEMP & NOISE CONTRIBUTED BY RESISTIVE COMPONENTS BETWEEN THE ANTENNA AND LNA. 14

  15. k (dBW/Hz-K) • BOLTZMANNS CONSTANT (OF PROPORTIONALITY) k = 1.3806 x 10^-23 (W/Hz-K) k = -228.6 (dBW/Hz-K) Pn (MAX NOISE OUTPUT) = kTB where, T = ABSOLUTE TEMPERATURE B = BANDWIDTH OF INTEREST 15

  16. L (dB) • FREESPACE LOSS C = (EIRP x eff x AREA) / (4pi x S^2) G = (4pi x eff x AREA) / WAVELENGTH^2 C = EIRP x [(WAVELENGTH^2) / (4piS)^2] x Gr L = (4piS)^2 / (WAVELENGTH^2) C = EIRP - L + Gr L = 20logS(km) + 20logFREQ(GHz) + 92.45 16

  17. W (dBW/m^2) • ILLUMINATION LEVEL W = PGt / [4pi(S^2)] W = EIRP - 20logS - 71 where, THE CONSTANT 71 = 10log{4pi[(1000m/km)^2] THE MAXIMUM DISTANCE (S) = 41,679km THIS CORRESPONDS TO A SATELLITE ON THE HORIZON @ 0 DEGREES ELEVATION & MAXIMUM CENTRAL ANGLE WITH THIS VALUE USED, THE WORST-CASE LEVEL IS : W = EIRP - 163.4 17

  18. PFD (dBW/m^2) • POWER FLUX DENSITY (USUALLY DEFINED WITHIN A SPECIFIED BW) PFD = W - 10log(B/Bccir) where, W = EIRP - 163.4 (dBW/m^2) PFD = EIRP - 163.4 - 10log(B/Bccir) THE STANDARD CCIR BANDWIDTH = 4kHz (FOR C & Ku BAND SYSTEMS) 18

  19. DEFINITION OF SIGNAL QUALITY (C/T) CXR-to-THERMAL NOISE RATIO (C/No) CXR-to-NOISE DENSITY (C/N) CXR-to-NOISE POWER (S/N) SIGNAL-to-NOISE POWER 19

  20. LINK BUDGET (COMPONENTS) • TRANSMITTER POWER P (W) • ANTENNA GAIN G (dBi) • RADIATED EIRP (dBW) • ILLUMINATION LEVEL @ RCVR (dBW/m^2) • FREE SPACE LOSS (dB) • SYSTEM NOISE TEMPERATURE Ts (K) • RECEIVE FIGURE OF MERIT G/Ts (dBi/K) • CXR-to-THERMAL NOISE RATIO C/T (dBW/K) • CARRIER-to-NOISE DENSITY C/No (dBHz) • CARRIER-to-NOISE RATIO C/N (dB) 20

  21. BASIC LINK BUDGETS • COME IN VARIOUS LENGTHS & STYLES (THERE IS NO STANDARD FORMAT) • 3 KEY EQUATIONS FORM THE BASIS : • FOR MOST UPLINK BUDGETS : EIRP = 10logP + Gt C/T = EIRP - L + G/T C/kT = C/T + 228.6 • FOR MOST DOWNLINK BUDGETS : C/T = EIRP - L + G/T C/kT = C/T + 228.6 C/N = C/kT - 10logB 21

  22. THE TRANSPONDER CHARACTERISTIC PARAMETERS • THE TX/RX FREQUENCY BANDS & POLARISATIONS • THE TX/RX COVERAGE (SFD & GAIN CONTOURS) • THE TX EIRP & CORRESPONDING PFD ACHIEVED • THE RX PFD REQUIRED TO ACHIEVE THE REQ’D TX EIRP • THE G/T BASED ON THE SFD CONTOUR • NON-LINEAR CHARACTERISTICS • RELIABILITY AFTER x YEARS FOR y PERCENTAGE OR NUMBER OF CHANNELS TO REMAIN IN WORKING ORDER 22

  23. TRANSMITTER POWER (P) • USUALLY SPECIFIED IN WATTS • THE 1st NUMBER OF THE LINK BUDGET (OFTEN ADJUSTED TO OBTAIN THE DESIRED PERFORMANCE) • FOR SATELLITES, Tx POWER IS LIMITED BY THE DC POWER AVAILABLE VIA THE SOLAR ARRAY. (10-200W) • EARTH STATION TRANSMITTERS RANGE FROM 1-10KW • IF LOSSES ARE SIGNIFICANT, THE Tx POWER IS MEASURED @ THE ANTENNA INPUT FLANGE. (LOSSES BEFORE THIS POINT MAY BE DEDUCTED FROM THE Tx PWR) 23

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