1. EC 723 Satellite Communication Systems
Mohamed Khedr

2. Syllabus Tentatively Week 1 Overview Week 2
Orbits and constellations: GEO, MEO and LEO
Week 3
Satellite space segment, Propagation and satellite links , channel modelling
Week 4
Satellite Communications Techniques
Week 5
Satellite error correction Techniques
Week 6
Multiple Access I
Week 7
Multiple access II
Week 8
Satellite in networks I
Week 9
INTELSAT systems , VSAT networks, GPS
Week 10
GEO, MEO and LEO mobile communications
INMARSAT systems, Iridium , Globalstar, Odyssey
Week 11
Presentations
Week 12
Week 13
Week 14
Week 15
Tentatively

3. Radio Propagation: Atmospheric Losses
Different types of atmospheric losses can perturb radio wave transmission in satellite systems:
Atmospheric absorption;
Atmospheric attenuation;
Traveling ionospheric disturbances.

4. Radio Propagation: Atmospheric Absorption
Energy absorption by atmospheric gases, which varies with the frequency of the radio waves.
Two absorption peaks are observed (for 90º elevation angle):
22.3 GHz from resonance absorption in water vapour (H2O)
60 GHz from resonance absorption in oxygen (O2)
For other elevation angles:
[AA] = [AA]90 cosec 
Source: Satellite Communications, Dennis Roddy, McGraw-Hill

5. Radio Propagation: Atmospheric Attenuation
Rain is the main cause of atmospheric attenuation (hail, ice and snow have little effect on attenuation because of their low water content).
Total attenuation from rain can be determined by:
A = L [dB]
where  [dB/km] is called the specific attenuation, and can be calculated from specific attenuation coefficients in tabular form that can be found in a number of publications;
where L [km] is the effective path length of the signal through the rain; note that this differs from the geometric path length due to fluctuations in the rain density.

6. Signal Polarisation: Cross-Polarisation Discrimination
Depolarisation can cause interference where orthogonal polarisation is used to provide isolation between signals, as in the case of frequency reuse.
The most widely used measure to quantify the effects of polarisation interference is called Cross-Polarisation Discrimination (XPD):
XPD = 20 log (E11/E12)
To counter depolarising effects circular polarising is sometimes used.
Alternatively, if linear polarisation is to be used, polarisation tracking equipment may be installed at the antenna.
Source: Satellite Communications,
Dennis Roddy, McGraw-Hill

7. Illustration of the various propagation loss mechanisms on a typical earth-space path
The ionosphere can cause the electric vector of signals passing through it to rotate away from their original polarization direction, hence causing signal depolarization.
the sun (a very “hot” microwave and millimeter wave source of incoherent energy), an increased noise contribution results which may cause the C/N to drop below the demodulator threshold.
The absorptive effects of the atmospheric constituents cause an increase in sky noise to be observed by the receiver
Refractive effects (tropospheric scintillation) cause signal loss.
The ionosphere has its principal impact on signals at frequencies well below 10 GHz while the other effects noted in the figure above become increasingly strong as the frequency of the signal goes above 10 GHz

8. Atmospheric attenuation
Attenuation of
the signal in %
Example: satellite systems at 4-6 GHz 50 40
rain absorption 30
fog absorption e 20 10
atmospheric absorption 5°
10°
20°
30°
40°
50°
elevation of the satellite

10. Signal Transmission Link-Power Budget Formula
Link-power budget calculations take into account all the gains and losses from the transmitter, through the medium to the receiver in a telecommunication system. Also taken into the account are the attenuation of the transmitted signal due to propagation and the loss or gain due to the antenna.
The decibel equation for the received power is:
[PR] = [EIRP] + [GR] - [LOSSES]
Where:
[PR] = received power in dBW
[EIRP] = equivalent isotropic radiated power in dBW
[GR] = receiver antenna gain in dB
[LOSSES] = total link loss in dB
dBW = 10 log10(P/(1 W)), where P is an arbitrary power in watts, is a unit for the measurement of the strength of a signal relative to one watt.

11. Link Budget parameters
Transmitter power at the antenna
Antenna gain compared to isotropic radiator
EIRP
Free space path loss
System noise temperature
Figure of merit for receiving system
Carrier to thermal noise ratio
Carrier to noise density ratio
Carrier to noise ratio

12. Signal Transmission Equivalent Isotropic Radiated Power
An isotropic radiator is one that radiates equally in all directions.
The power amplifier in the transmitter is shown as generating PT watts.
A feeder connects this to the antenna, and the net power reaching the antenna will be PT minus the losses in the feeder cable, i.e. PS.
The power will be further reduced by losses in the antenna such that the power radiated will be PRAD (< PT).
(a) Transmitting antenna
Source: Satellite Communications, Dennis Roddy, McGraw-Hill

13. Antenna Gain sphere = 4p solid radians
We need directive antennas to get power to go in wanted direction.
Define Gain of antenna as increase in power in a given direction compared to isotropic antenna.
P() is variation of power with angle.
G() is gain at the direction .
P0 is total power transmitted.
sphere = 4p solid radians

14. EIRP - 1
An isotropic radiator is an antenna which radiates in all directions equally
Antenna gain is relative to this standard
Antennas are fundamentally passive
No additional power is generated
Gain is realized by focusing power
Effective Isotropic Radiated Power (EIRP) is the amount of power the transmitter would have to produce if it was radiating to all directions equally
Note that EIRP may vary as a function of direction because of changes in the antenna gain vs. angle

15. EIRP - 2 The output power of a transmitter HPA is: Pout watts
Some power is lost before the antenna:
Pt =Pout /Lt watts reaches the antenna
Pt = Power into antenna
The antenna has a gain of:
Gt relative to an isotropic radiator
This gives an effective isotropic radiated power of:
EIRP = Pt Gt watts relative to a 1 watt isotropic radiator
HPA
Pout Lt Pt
EIRP

16. Received Power
We can rewrite the power flux density now considering the transmit antenna gain:
The power available to a receive antenna of area Ar m2 we get:

17. Effective Aperture
Real antennas have effective flux collecting areas which are LESS than the physical aperture area.
Define Effective Aperture Area Ae:
Where Aphy is actual (physical) aperture area.
Very good: 75%
Typical: 55%
 = aperture efficiency
Antennas have (maximum) gain G related to the effective aperture area as follows:

18. Back to Received Power…
The power available to a receive antenna of effective area Ar = Ae m2 is:
Where Ar = receive antenna effective aperture area = Ae
Inverting…

19. Back to Received Power…
Friis Transmission Formula
The inverse of the term at the right referred to as “Path Loss”, also known as “Free Space Loss” (Lp):
Therefore…

20. More complete formulation
Demonstrated formula assumes idealized case.
Free Space Loss (Lp) represents spherical spreading only.
Other effects need to be accounted for in the transmission equation:
La = Losses due to attenuation in atmosphere
Lta = Losses associated with transmitting antenna
Lra = Losses associates with receiving antenna
Lpol = Losses due to polarization mismatch
Lother = (any other known loss - as much detail as available)
Lr = additional Losses at receiver (after receiving antenna)

21. Signal Transmission Link-Power Budget Formula Variables
Link-Power Budget Formula for the received power [PR]:
[PR] = [EIRP] + [GR] - [LOSSES]
The equivalent isotropic radiated power [EIRP] is:
[EIRP] = [PS] + [G] dBW, where:
[PS] is the transmit power in dBW and [G] is the transmitting antenna gain in dB.
[GR] is the receiver antenna gain in dB.
[LOSSES] = [FSL] + [RFL] + [AML] + [AA] + [PL], where:
[FSL] = free-space spreading loss in dB = PT/PR (in watts)
[RFL] = receiver feeder loss in dB
[AML] = antenna misalignment loss in dB
[AA] = atmospheric absorption loss in dB
[PL] = polarisation mismatch loss in dB
The major source of loss in any ground-satellite link is the free-space spreading loss.

22. Link Power Budget Tx Rx EIRP Transmission: HPA Power
Transmission Losses
(cables & connectors)
Antenna Gain
Antenna Pointing Loss
Free Space Loss
Atmospheric Loss (gaseous, clouds, rain)
Rx Antenna Pointing Loss
Reception:
Antenna gain
Reception Losses
(cables & connectors)
Noise Temperature Contribution Rx Pr

23. Translating to dBs The transmission formula can be written in dB as:
This form of the equation is easily handled as a spreadsheet (additions and subtractions!!)
The calculation of received signal based on transmitted power and all losses and gains involved until the receiver is called “Link Power Budget”, or “Link Budget”.
The received power Pr is commonly referred to as “Carrier Power”, C.

24. Link Power Budget Now all factors are accounted for Tx
as additions and subtractions Tx
EIRP
Transmission:
+ HPA Power
Transmission Losses
(cables & connectors)
+ Antenna Gain
Antenna Pointing Loss
Free Space Loss
Atmospheric Loss (gaseous, clouds, rain)
- Rx Antenna Pointing Loss
Reception:
+ Antenna gain
Reception Losses
(cables & connectors)
+ Noise Temperature Contribution Rx Pr

25. Easy Steps to a Good Link Power Budget
First, draw a sketch of the link path
Doesn’t have to be artistic quality
Helps you find the stuff you might forget
Next, think carefully about the system of interest
Include all significant effects in the link power budget
Note and justify which common effects are insignificant here
Roll-up large sections of the link power budget
Ie.: TXd power, TX ant. gain, Path loss, RX ant. gain, RX losses
Show all components for these calculations in the detailed budget
Use the rolled-up results in build a link overview
Comment the link budget
Always, always, always use units on parameters (dBi, W, Hz ...)
Describe any unusual elements (eg. loss caused by H20 on radome)

26. Simple Link Power Budget

27. Why calculate Link Budgets?
System performance tied to operation thresholds.
Operation thresholds Cmin tell the minimum power that should be received at the demodulator in order for communications to work properly.
Operation thresholds depend on:
Modulation scheme being used.
Desired communication quality.
Coding gain.
Additional overheads.
Channel Bandwidth.
Thermal Noise power.
We will see more on
these items in the
next classes.

28. Closing the Link
We need to calculate the Link Budget in order to verify if we are “closing the link”.
Pr >= Cmin  Link Closed
Pr < Cmin  Link not closed
Usually, we obtain the “Link Margin”, which tells how tight we are in closing the link:
Margin = Pr – Cmin
Equivalently:
Margin > 0  Link Closed
Margin < 0  Link not closed

29. Carrier to Noise Ratios
C/N: carrier/noise power in RX BW (dB)
Allows simple calculation of margin if:
Receiver bandwidth is known
Required C/N is known for desired signal type
C/No: carrier/noise p.s.d. (dbHz)
Allows simple calculation of allowable RX bandwidth if required C/N is known for desired signal type
Critical for calculations involving carrier recovery loop performance calculations

30. System Figure of Merit G/Ts: RX antenna gain/system temperature
Also called the System Figure of Merit, G/Ts
Easily describes the sensitivity of a receive system
Must be used with caution:
Some (most) vendors measure G/Ts under ideal conditions only
G/Ts degrades for most systems when rain loss increases
This is caused by the increase in the sky noise component
This is in addition to the loss of received power flux density

31. System Noise Power - 1
Performance of system is determined by C/N ratio.
Most systems require C/N > 10 dB.
(Remember, in dBs: C - N > 10 dB)
Hence usually: C > N dB
We need to know the noise temperature of our receiver so that we can calculate N, the noise power (N = Pn).
Tn (noise temperature) is in Kelvins (symbol K):

32. System Noise Power - 2 System noise is caused by thermal noise sources
External to RX system
Transmitted noise on link
Scene noise observed by antenna
Internal to RX system
The power available from thermal noise is:
where k = Boltzmann’s constant
= 1.38x10-23 J/K( dBW/HzK),
Ts is the effective system noise temperature, and B is the effective system bandwidth

33. Noise Spectral Density
N = K.T.B  N/B = N0 is the noise spectral density (density of noise power per hertz):
N0 = noise spectral density is constant up to 300GHz.
All bodies with Tp >0K radiate microwave energy.

34. System Noise Temperature
1) System noise power is proportional to system noise temperature
2) Noise from different sources is uncorrelated (AWGN)
Therefore, we can
Add up noise powers from different contributions
Work with noise temperature directly
So:
But, we must:
Calculate the effective noise temperature of each contribution
Reference these noise temperatures to the same location
Additive White Gaussian Noise (AWGN)

35. Typical Receiver
(Source: Pratt & Bostian Chapter 4, p115)

36. Noise Model
(Source: Pratt & Bostian Chapter 4, p115)
Noise is added and then multiplied by the gain of the device (which is now assumed to be noiseless since the noise was already added prior to the device)

37. Equivalent Noise Model of Receiver
(Source: Pratt & Bostian Chapter 4, p115)
Equivalent model: Equivalent noise Ts is added and then multiplied by the equivalent gain of the device, GRFGmGIF (noiseless).

38. Calculating System Noise Temperature - 1
Receiver noise comes from several sources.
We need a method which reduces several sources to a single equivalent noise source at the receiver input.
Using model in Fig. 4.5.a gives:

39. Calculating System Noise Temperature - 2
Divide by GIFGmGRFkB:
If we replace the model in Fig. 4.5.a by that in Fig. 4.5b

40. Calculating System Noise Temperature - 3
Equate Eqns :
Since C is invariably small, N must be minimized.
How can we make N as small as possible?

41. Reducing Noise Power
Make B as small as possible – just enough bandwidth to accept all of the signal power (C ).
Make TS as small as possible
Lowest TRF
Lowest Tin (How?)
High GRF
If we have a good low noise amplifier (LNA), i.e., low TRF, high GRF, then rest of receiver does not matter that much.

42. Reducing Noise Power Discussion on Tin
Earth Stations: Antennas looking at space which appears cold and produces little thermal noise power (about 50K).
Satellites: antennas beaming towards earth (about 300 K):
Making the LNA noise temperature much less gives diminishing returns.
Improvements aim reduction of size and weight.

43. Antenna Noise Temperature
Contributes for Tin
Natural Sources (sky noise):
Cosmic noise (star and inter-stellar matter), decreases with frequency, (negligible above 1GHz). Certain parts of the sky have punctual “hot sources” (hot sky).
Sun (T  f-0.75 K): point earth-station antennas away from it.
Moon (black body radiator): 200 to 300K if pointed directly to it.
Earth (satellite)
Propagation medium (e.g. rain, oxygen, water vapor): noise reduced as elevation angle increases.
Man-made sources:
Vehicles, industrial machinery
Other terrestrial and satellite systems operating at the same frequency of interest.

44. Antenna Noise Temperature
Useful approximation for Earth Station antenna temperature on clear sky (no rain):

45. So many trade-offs !!!

46. Power Budget Example - 1 4.1.1 Satellite at 40,000 km (range)
Transmits 2W
Antenna gain Gt = 17 dB (global beam)
Calculate: a. Flux density on earth’s surface
b. Power received by antenna with effective aperture of 10m2
c. Gain of receiving antenna.
d. Received C/N assuming Ts =152 K, and Bw =500 MHz
a. Using Eqn. 4.3: (Gt = 17 dB = 50)
(Solving in dB…)

47. Power Budget Example - 1 b. Received Power (Solving in dB…)
c. Gain given Ae = 10 m2 and Frequency = 11GHz ( eqn. 4.7)

48. Power Budget Example - 1
b. System Noise Temperature

49. Power Budget Example - 2 Generic DBS-TV: Received Power
Transponder output power , 160 W dBW
Antenna beam on-axis gain dB
Path loss at 12 GHz, 38,500 km path dB
Receiving antenna gain, on axis dB
Edge of beam dB
Miscellaneous losses dB
Received power, C dBW

50. Power Budget Example - 2 Noise power
Boltzmann’s constant, k dBW/K/Hz
System noise temperature, clear air, 143 K dBK
Receiver noise bandwidth, 20MHz dBHz
Noise power, N dBW
C/N in clear air dB
Link margin over 8.6 dB threshold dB
Link availability throughout US Better than 99.7 %

51. Thank you