Satellite Communication

1. Unit-VI
Satellite Communication
Link Design
R.S.Bhadade
MIT-WPU, Pune

2. Contents (Syllabus, 8 Hrs)
 Introduction
 Basic Transmission Theory
 System Noise Temperature and G/T Ratio
 Design of Downlinks
 Satellite Systems using small Earth Stations (ES)
 Uplink Design
 Design of Specified C/N: Combining C/N and C/I
values in Satellite Links
 System Design Examples

3.  Text Book:
Satellite Communications- T Pratt, Charles Bostian,
Jeremy Allnutt, John Wiley & Sons.
 Reference Book:
Satellite Communications- Dennis Roody, McGraw
Hill

4. Introduction
 Design of a SCS is a complex process and requiring
compromises between many factors to achieve best
performance at an acceptable cost.
 Three factors influence system design:
i. Frequency band
ii. Atmospheric propagation effects
iii. Multiple access technique

5.  Performance objectives: (in baseband channel)
Demodulator
C/N
BER (digital link)
SNR (analog link)
 C/N > 6dB
 C/N < 10 dB (digital link)-must use error correction to
improve BER calculated at the i/p of receiver ( or o/p port
of receiving antenna)
 For Noise less receiver :- It is constant at all points (RF
and IF chain)
 Overall C/N: UL and DL
IF
Amplifier
C/N

6. Design of
Satellite
system
Performance
of uplink and
downlink
Propagation
characteristics
and Rain
attenuation
Satellite
Parameters
ES
Parameters

7. Satellite Orbits: LEO, MEO, GEO

8. Antenna Look Angles
 For ES antenna alignment, Two Angles need to be
adjusted and fixed:
 Azimuth Angle
 Elevation Angle
 Jointly referred as Antenna Look Angles.
Elevation Angle (Θ:- 0 to π) Azimuth Angle(Φ:- 0 to 2π)

9. Earth Station
Uplink – ES to a Satellite
Downlink – Satellite to an ES

10. Satellite Functions
(RF to RF conversion)
1. Signal reception: Uplink frequency
2. Processing of signal:
Amplification, Filtration, and Down conversion, etc
3. Signal retransmission: Downlink frequency

11. Frequency Bands

12. Frequency Bands
 C-Band (6/4 GHz)
 Ku-Band (14/11 GHz)
 Ka-Band (30/20 GHz)
Most satellites
in GEO

13. Repeaters and Transponders
Bandwidth of Repeater = 500MHz (C-Band: 4200-3700 MHz)
Bandwidth of Each Transponder = 36MHz
No. of Transponders = 500/36 = 12 Transponders
N = (500/7MHz) = 71 TV channels
Bandwidth = 1000 MHz (Ku-Band)
N= (1000/7) = 142 TV channels
OFC:- 30 THz

14. C-Band Transponder

15. Ku-Band Transponder

16. C-Band Transponder Frequency Plan

17. Ku-Band Transponder Frequency Plan

18. DTH Antenna Position
HPBW

19. TV Signal link (FTA)
TV Rebroadcasting
Station- DD-I/II
(Sinhagad , Pune )
DD National, New Delhi
DBS-TV

20. TV Signal link (Pay)
Up-link
Down-link
DD Kendra, Worli, Mumbai
(C-Band)
DBS-TV
DBS-TV
Down-link
Ku Band (Thodapur, New Delhi)

21. Basic Transmission Theory
 Satellite and Microwave link: Line-of-Sight (LoS)
 Link analysis relates the Pt and Pr.
 The flux density and link equation can be used to
calculate the Pr.

22.  The flux density in the direction of the antenna bore sight at
a distance R meters is given by:
Pt = Output Power
Gt = Transmitting antenna gain
Pt Gt = Effective Isotropic Radiated Power or EIRP
R= Distance between source and receiving antenna.
A = Receiving antenna aperture area (m2)
Aperture efficiency:
 Accounts for losses: blockage, phase errors,
polarization, and mismatches)
 Cassegrain antennas 50 to 75%
 Horn : 90 %

24. Pr = EIRP + Gr – Lp – La – Lta – Lra (dBW)
La = attenuation in atmosphere
Lta = losses associated with transmitting antenna
Lra = losses associated with receiving antenna

25. Noise Temperature
Noise Temperature:
Pn = KTpBn
where:
k = Boltzmann’s constant (-228.6 dBW/K/Hz)
Tp = Physical temperature of source in (kelvin
degrees or dBK)
Bn = Noise bandwidth (in Hz or dBHz)

26. NOISE-EQUIVALENT BANDWIDTH
Bn:
 Ideal: Infinite (flat PSD)
 Receiver (LPF or BPF)

27. Pno=kTsBnGrx
Pn=KTsBn
C/N=PrGrx/KTsBnGrx=Pr/KTsBn
Pno= Noise power at demodulator i/p
Grx= Overall ene-to-end gain of the receiver
Ts=System noise temperature
Pn= Noise power at receiver i/p
C/N at demodulator
Pr=signal power at
receiver i/p
C= Pr for constant envelope signal (FM or PSK)

28. System Noise Temperature
Superheterodyne receiver (SHR) :
 Front end (RF amp, Mixer, and LO)
 IF amplifier and filters
 Demodulator and Baseband

29. Pn= GIFkTIFBn+GIFGmkTmBn+GIFGmGRFkBn(TRF+Tin)
Ts= [Tin+TRF+Tm/GRF+TIF/GmGRF]
Ts= [Tantenna+TLNA]

30. Noise Figure (NF)
 It is frequently used to specify the noise generated
within a device.
NF = (SNR)in/(SNR)out
NF = 1 or 0 dB (Ideal receiver)
Td = To (NF-1)
Td=Noise temperature
To= Reference temp (290 K)

31. G/T Ratio for ESs
• C/N= [PtGtGr/kTsBn][λ/4πR]2
= [PtGt/kBn][Gr/Ts] [λ/4πR]2
C/N α G/T Gr/Ts=G/T
Increasing
Increases

32. Design of Downlinks
 The design of any satellite communication is based on two
objectives:
i. Meeting a minimum C/N ratio
ii. Carrying the maximum revenue earning traffic
at minimum cost.
 All satellite links are affected by rain attenuation
 C-band:- Effect of rain is small
 Ku/Ka-band:- Effect of rain is large
 Rain attenuation is a variable phenomenon, both in time and
place
 Satellite links are designed to achieve reliabilities of 99.5 to
99.99%, averaged over a long period of time.

33. Link Budgets
 It is a tabular method for estimating the Pr and N in a radio
link. Also to calculate C/N ratio.
 It quantifies link performance
 It uses dB units for all quantities.
 The link budget must be calculated for an individual
transponder, and must be repeated for each of the individual
links.
 In a 2-way satellite link:- C/N for 4 separate links.

34. These are usually calculated for a worst case, the one
in which the link will have the lowest C/N ratio.
Factors of worst case scenario:
 Location of ES at the edge of the satellite coverage
zone
 Maximum path length (S to ES)
 Elevation angle at ES
Worst
case

35. ES antennas are assumed to be pointed directly at the
satellite, and therefore operate at their on-axis gain.
If the antenna is mispointed, a loss factor is included
for reduction in antenna gain.

37. Link margin:
 Clear air: 16.0 - 9.5 = 6.5 dB
 In Rain: 12.7 – 9.5 = 3.2 dB
Solution:
Uplink Power Control (UPC)

38. Satellite Systems Using small ESs
Small, low-cost ESs:
Satellites carry only one or two telephone or data
channels or a DBS -TV signal.
Two parameters in Pr equation:
i. Satellite transmitted power
ii. Satellite antenna beamwidth (or G)

39. Ku-Band Receiving Antennas
 DTH
 (D < 1m)
 Transponders are more
powerful
 VSAT:
 (D:1 to 2 m)
 Transponders are less
powerful

41. • A=Aca + Arain dB
• Tsky=270×(1-10-A/10) K
• TA=ɳc ×Tsky K
• Ts rain= TLNA+TA K
• ΔNrain=10 log10[Ts rain/Tsca] dB
A= Total path attenuation
Aca = Clear sky path attenuation
Arain = Rain attenuation
Tsky= Sky noise temperature
TA=Antenna noise temperature
ɳc= Coupling coefficient (90 to 95%)
ΔNrain=Increase in noise
power due increase in Ts
Tsca= System noise
temperature in clear sky
conditions

42. • Crain=Cca-Arain dB
• (C/N)dn rain = (C/N)dn ca - Arain - ΔNrain dB
 For Linear (bent pipe) transponder:
(C/N)o = (C/N)up + (C/N)dn rain
Crain= Carrier power in rain attenuation
Cca= Carrier power in clear sky conditions

43. Uplink Design
 Easier than the downlink
 Accurate specified carrier power can be presented at
the satellite transponder
 High power transmitter at ES can be used
 Transmitters are costlier than the receivers.
 Major growth in satellite communications: Point to
multipoint transmission (Cable TV and DBS-
TV/Radio)

44. • Nxp=k + Txp + Bn dBW
• Prxp= Pt + Gt + Gr – Lp - Lup dBW
• (C/N)up= 10 log10 (Pr/N) = Prxp - Nxp dB
Nxp= Noise power at transponder i/p
Txp = System noise temperature of the
transponder
Prxp = Received power at transponder i/p
Lp = path loss
Lup = all uplink losses

45. Design of Specified C/N: Combining C/N and C/I
values in Satellite Links
 Sources of Noise
 Thermal noise
 For complete satellite link
 Receiver it self
 Receiving antenna
 Sky noise
 Satellite transponder
 Adjacent satellites
 Terrestrial transmitter
 Atmospheric gases
 Rain
Demodulator
C/N
BER (digital link)
SNR (analog link)
IF
Amplifier
C/N

46.  Reciprocal C/N formula (measured in ES@ o/p of IF
amplifier)
(C/N)o =1/[1/(C/N)1 + 1/(C/N)2 + 1/(C/N) 3 + …]
(C/N)o = C/(N1+N2+N3+….) Carrier power is same
(C/N)o = C dBW- 10 log10(N1+N2+N3+…. W) dB
(C/N)o = (C/N)dn => due to transponder and ES
Demodulator
C/N
BER (digital link)
SNR (analog link)
IF
Amplifier
C/N

47. Interference:
• Intermodulation products (IM) generated by the
transponder’s nonlinear i/o characteristic.
• IM power level = (C/I) must be include in (C/N)o
• Interference from adjacent satellites when small
receiving antennas are used (e.g., VSAT and DTH)
• Interference cancellation techniques can be used to
reduce the level of interference

48. Overall (C/N)o with uplink and downlink attenuation
Effect of change in (C/N)up on (C/N)o depending on
the operating modes (or types) and gain of the
transponder
• Linear Pout = Pin+ Gxp dBW
• Nonlinear Pout = Pin+ Gxp – ΔG dBW
• Regenerative Pout = constant dBW
Pout = Power delivered by transponder HPA
to transmitting antenna
Pin = Power delivered by receiving antenna
to transponder
Gxp = gain of transponder
ΔG = loss of gain due nonlinear saturation
characteristics of transponder

49. Output backoff
 Saturated output power is the maximum output power and is nominal
transponder power rating that is usually quoted.
 ISI and IM product (FDMA) results- when the transponder operating (non-
linear characteristic) close to its saturated output power level.
 Transponders are usually operated with output backoff, to make the
characteristic more nearly linear.
 Typical values of backoff:
1 dB- single FM or PSK carrier (Input backoff: 3dB)
3 dB- FDMA with several carrier (Input backoff: 5dB)

50. Uplink and Downlink Attenuation in Rain
Rain attenuation affects both uplink and downlink
Rain attenuation is occurring on either uplink or
downlink, but not on both at the same time.
 Assumption: True for ESs are well separated
geographically
 Example: PCCOE, Ravet to Balewadi (< 20 km)-
Rain attenuation occurs on both links at the same
time.

51. Uplink Attenuation and (C/N)up
 Linear
(C/N) o uplink rain = (C/N) o clear air – Aup
 Nonlinear
(C/N) o uplink rain = (C/N) o clear air – Aup + ΔG
 Regenerative or AGC
(C/N) o uplink rain = (C/N) o clear air

52. Downlink Attenuation and (C/N)dn
 Linear
(C/N)dn rain = (C/N)dn clear air – Arain –ΔNrain
(C/N) o = 1/[1/(C/N)dn rain +1/(C/N)up]
(C/N)up is clear air and remains constant regardless
of the attenuation on the downlink

53. Design Procedure
(One-way satellite link)
1. Determine the frequency band
2. Determine the communications parameters of the satellite.
Estimate any values that are not known.
3. Determine the parameters of the transmitting and receiving
ESs.
4. Start at the transmitting ES. Establish an uplink budget and a
transponder noise power budget to find (C/N)up.
5. Find the output power of the transponder based on
transponder gain or output backoff.

54. 6. Establish a downlink power and noise budget for the
receiving ES. Calculate (C/N)dn and (C/N)o for a station at
the edge of the coverage zone (worst case)
7. Calculate SNR or BER in the baseband channel. Find the link margins.
8. Evaluate the result and compare with the specification requirements.
Change the parameters of the systems as required to obtain acceptable
(C/N)o or SNR or BER values. This may require several trial designs.
9. Determine the propagation conditions under which the link must operate.
Calculate the outage times for uplink and downlinks.
10. Redesign the system by changing some parameters if the link margin are
inadequate. Design can be implemented within the expected budget.

55. System Design Examples

57. Satellite link design using Ku-band GEO with bent
pipe transponders to distribute digital TV signals to
many receiving stations(US).
Minimum permitted overall C/N of 9.5dB

58. Ku-Band Uplink Design:
 Noise power Budget
k=Boltzmann’s constant -228.6 dBW/K/Hz
Ts= 500 K 27.0 dBK
B= 43.2MHz 76.4 dBHz
N= Transponder noise power -125.2 dBW
Pr= -125.2+30 dB -95.2 dBW
Required C/N at the i/p of transponder

59.  Gt=10 log 10[ɳe×(πD/λ)2]=55.7 dB ɳe = 68% D=5m
λ=0.0212m@14.15GHz
 Lp =10 log 10[(4πR/λ)2]=207.2 dB
 Power Budget
Pt= ES transmit power ? dBW
Gt= ES antenna gain 55.7 dB
Gr= Satellite antenna gain 31.0 dB
Lp = Free space path loss -207.2 dB
Lant= ES on 2 dB contour -2.0 dB
Lm= other losses -1.0 dB
Pr= Received power at transponder -123.5 dB
 The required power at the transponder i/p is 30 dB-123.5 dB = - 95.2 dBW
 Pt-123.5 dB= -95.2 dB= 28.3 dBW or 675 W
Required C/N at the i/p of transponder

60.  Ku-Band Downlink Design:
1/ (C/N)dn = 1/(C/N)o - 1/(C/N)up (not in dB)
(C/N)dn = 17.2 dB (C/N)o=17 dB (C/N)up= 30 dB
 Noise power budget
k=Boltzmann’s constant -228.6 dBW/K/Hz
Ts= 30+110 K 21.5 dBK
B= 43.2MHz 76.4 dBHz
N= Transponder noise power -130.7 dBW
Pr = -130.7 dBW+17.2 dB = -113.5 dBW

61.  Lp =10 log 10[(4πR/λ)2] λ=0.0262m@11.45GHz
 Lp =207.2-20 log 10[(14.15/11.45]=205.4 dB
 Pt=19 dBW-1 dB= 18 dBW (1 dB below 80 W)
 Power budget
Pt= Satellite o/p power 18.0 dBW
Gt= Satellite antenna gain 31.0 dB
Gr= ES antenna gain ? dB
Lp = Free space path loss -205.4 dB
Lant= ES on -3 dB contour of satellite antenna -3.0 dB
Lm= other losses -0.8 dB
Pr= Received power at transponder -160.2 dB
Gr-160.2 dB= -95.2 dB Gr= 46774 or 46.7 dB
D= 2.14 m Gr=ɳe×(πD/λ)2] = 46774 ɳe = 68%