SATELLITE LINKS

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(radius2) (WATTS)
• where,
• 4pi(radius2) 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(radius2)
• where,
• Gt Tx GAIN

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

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

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)

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

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.

8
E (dBuV/m)

• ELECTRIC FIELD STRENGTH
• (POWER PER UNIT AREA)
• W 1/2c(PERMITTIVITY) x E2 (W/m2)
• W 1/2(E2) / Z (W/m2)
• where,
• Z 1 / c(PERMITTIVITY)
• W 2E - 148.77 (dBW/m2)
• E 1/2(W 148.77) (dBuV/m)

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

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) / WAVELENGTH2
• G eff(piD x FREQ)/C2
• G 20logD 20logFREQ 10log(eff) 20.4
• TYPICAL E/S GAIN FIGURES ARE 1-60dBi
• SATELLITE GAIN FIGURES RANGE FROM 14-40dBi

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)

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

13
ANTENNA BEAMWIDTH
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.

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

16
L (dB)

• FREESPACE LOSS
• C (EIRP x eff x AREA) / (4pi x S2)
• G (4pi x eff x AREA) / WAVELENGTH2
• C EIRP x (WAVELENGTH2) / (4piS)2 x Gr
• L (4piS)2 / (WAVELENGTH2)
• C EIRP - L Gr
• L 20logS(km) 20logFREQ(GHz) 92.45

17
W (dBW/m2)

• ILLUMINATION LEVEL
• W PGt / 4pi(S2)
• W EIRP - 20logS - 71
• where,
• THE CONSTANT 71 10log4pi(1000m/km)2
• THE MAXIMUM DISTANCE (S) 41,679km
• THIS CORRESPONDS TO A SATELLITE ON THE HORIZON _at_
  0 DEGREES ELEVATION MAXIMUM CENTRAL ANGLE
• WITH THIS VALUE USED, THE WORST-CASE LEVEL IS
• W EIRP - 163.4

18
PFD (dBW/m2)

• POWER FLUX DENSITY
• (USUALLY DEFINED WITHIN A SPECIFIED BW)
• PFD W - 10log(B/Bccir)
• where,
• W EIRP - 163.4 (dBW/m2)
• PFD EIRP - 163.4 - 10log(B/Bccir)
• THE STANDARD CCIR BANDWIDTH 4kHz
• (FOR C Ku BAND SYSTEMS)

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

20
LINK BUDGET (COMPONENTS)

• TRANSMITTER POWER P (W)
• ANTENNA GAIN G (dBi)
• RADIATED EIRP (dBW)
• ILLUMINATION LEVEL _at_ RCVR (dBW/m2)
• 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)

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

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 REQD 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

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 _at_ THE ANTENNA INPUT FLANGE.
• (LOSSES BEFORE THIS POINT MAY BE DEDUCTED FROM
  THE Tx PWR)