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

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




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 (H
2
O)
60 GHz from resonance absorption in oxygen (O
2
)
For other elevation angles:

[AA] = [AA]
90 cosec 
Source: Satellite Communications , Dennis Roddy, McGraw-Hill



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.

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 (E
11
/E
12
)


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

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 absorptive effects of the atmospheric constituents cause an increase in sky noise to be observed by the receiver 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.
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

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




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:

[P
R
Where:
] = [EIRP] + [G
R
] - [LOSSES]


[P
R
] = received power in dBW
[EIRP] = equivalent isotropic radiated power in dBW


[G
R
] = receiver antenna gain in dB
[LOSSES] = total link loss in dB dBW = 10 log
10
(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.










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





An isotropic radiator is one that radiates equally in all directions.
The power amplifier in the transmitter is shown as generating P
T watts.
A feeder connects this to the antenna, and the net power reaching the antenna will be P
T minus the losses in the feeder cable, i.e. P
S
.
The power will be further reduced by losses in the antenna such that the power radiated will be P
RAD
(< P
T
).
(a) Transmitting antenna
Source: Satellite Communications , Dennis Roddy, McGraw-Hill



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 (

)
G (

)

P
0
/ 4

• P( 
) is variation of power with angle.
• G( 
) is gain at the direction

.
• P
0 is total power transmitted.
• sphere = 4
 solid radians






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





The output power of a transmitter HPA is:
P out watts
Some power is lost before the antenna:
P t
P
=P t out
/ L t watts reaches the antenna
= Power into antenna
The antenna has a gain of:
P out
HPA
L t
G t relative to an isotropic radiator
This gives an effective isotropic radiated power of:
EIRP = P t
G t watts relative to a 1 watt isotropic radiator
EIRP
P t

• We can rewrite the power flux density now considering the transmit antenna gain:

F

EIRP
4

R
2

P t
G t
4

R
2
W/m
2
The power available to a receive antenna of area A r m 2 we get:
P r

F x A r

P t
G t
4

R
A r
2


Real antennas have effective flux collecting areas which are LESS than the physical aperture area.

Define Effective Aperture Area Ae:
A e

A phy
x

Where A phy is actual (physical) aperture area.

= aperture efficiency
Very good: 75%
Typical: 55%
• Antennas have (maximum) gain G related to the effective aperture area as follows:
Gain

4


2
A e


The power available to a receive antenna of effective area A r
= A e m 2 is:
P r

F x A r

P t
G t
4

R
A e
2
Where A r
= receive antenna effective aperture area = A e
G
 r
4


2
A e
Inverting…
A
 e
G r
4


2

P r

P t
G t
G r

4

R
2
Friis Transmission Formula
• The inverse of the term at the right referred to as “Path
Loss”, also known as “Free Space Loss” (Lp):
L p

4

R

2
Therefore…
P
 r
P t
G t
G r
L p




Demonstrated formula assumes idealized case.
Free Space Loss (L p
) represents spherical spreading only.
Other effects need to be accounted for in the transmission equation:






L
L
L
L
L a ta ra pol
= Losses due to attenuation in atmosphere
= Losses associated with transmitting antenna
= Losses associates with receiving antenna other
= Losses due to polarization mismatch
= (any other known loss - as much detail as available)
Lr = additional Losses at receiver (after receiving antenna)
P
 r
P t
G t
G r
L p
L a
L ta
L ra
L pol
L other
L r






Link-Power Budget Formula for the received power [P
R

[P
R
] = [EIRP] + [G
R
] - [LOSSES]
The equivalent isotropic radiated power [EIRP] is:
]:



[EIRP] = [P
S
] + [G] dBW, where:
[P
S
] is the transmit power in dBW and [G] is the transmitting antenna gain in dB.
[G
R
] is the receiver antenna gain in dB.
[LOSSES] = [FSL] + [RFL] + [AML] + [AA] + [PL], where:
(in watts)




[FSL] = free-space spreading loss in dB = P
T
[RFL] = receiver feeder loss in dB
/P
R
[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.

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
P r


The transmission formula can be written in dB as:
P r

EIRP

L ta

L p

L a

L pol

L ra

L other

G r

L r

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 .

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
Now all factors are accounted for as additions and subtractions
Reception:
+ Antenna gain
- Reception Losses
(cables & connectors)
+ Noise Temperature
Contribution
Rx
P r





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 H
2
0 on radome)

Parameter
Frequency
Transmitter
Transmitter Power
Modulation Loss
Transmission Line
Loss
Transmitted Power
Value Totals Units Parameter
11.75 GHz
40.00
3.00
0.75
Receive Antenna
dBm Random Loss
dB
dB
Diameter
Aperture Efficiency
Transmit Antenna
Diameter
Aperture Efficiency
Transmit Antenna
Gain
Slant Path
Satellite Altitude
Elevation Angle
Slant Range
Free-space Path Loss 206.22
Gaseous Loss
Rain Loss (allocated)
Path Loss
0.5
36.25 dBm Gain
Polarization Loss
m
Effective RX Ant.
Gain
0.55
35,786
14.5
41,602
0.65
3.50
none Received Power
33.18 dBi
Summary
km Transmitted Power
degrees Transmit Anntenna
Gain
km
dB
dB
dB
210.37 dB
EIRP
Path Loss
Effective RX
Antenna Gain
Received Power
Value Totals Units
0.50 dB
1.5
0.6
m
none
43.10
0.20
36.25
33.18
dBi
dB
42.40 dB
-98.54 dBm
dBm
dBi
69.43 dBmi
210.37 dB
42.4 dBi
-98.54 dBm




System performance tied to operation thresholds.
Operation thresholds C min 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.

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



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/N
 o
: 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


G/T

 s
: RX antenna gain/system temperature
Also called the System Figure of Merit, G/T s
Easily describes the sensitivity of a receive system

Must be used with caution:


Some (most) vendors measure G/T s under ideal conditions only
G/T
 s 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






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 + 10 dB
We need to know the noise temperature of our receiver so that we can calculate N, the noise power (N = P n
T n
K):
(noise temperature) is in Kelvins (symbol
T
 

T
0
 

).
273
T
 


T
0
 

32

5
9

273



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:
N
 kT s
B (dBW) where
T s k = Boltzmann’s constant
= 1.38x10
-23 J/K(-228.6 dBW/HzK), is the effective system noise temperature, and
B is the effective system bandwidth


N = K.T.B per hertz):
 N/B = N
0 is the noise spectral density (density of noise power
N
0

N
B
 kT s
B
B
 kT s
(dBW/Hz)


N
0
= noise spectral density is constant up to 300GHz.
All bodies with Tp >0K radiate microwave energy.


1) System noise power is proportional to system noise temperature
2) Noise from different sources is uncorrelated (AWGN)
Additive White Gaussian Noise (AWGN)

Therefore, we can



So:
Add up noise powers from different contributions
Work with noise temperature directly
T s

T

T antenna

T
LNA

T lineloss

T
RX
But, we must:


Calculate the effective noise temperature of each contribution
Reference these noise temperatures to the same location

(Source: Pratt & Bostian Chapter 4, p115)

(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)

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, G
RF
G m
G
IF
(noiseless).




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:
P n

G


IF kT
IF
G
IF
G
IF
B (IF)
G m kT m
G m
G
RF
B (Mixer) kB

T
RF

T in

(Front End)

Calculating System Noise
Temperature - 2

Divide by G
IF
G m
G
RF kB:
P n

G
IF
G m
G
RF kB

 T
RF

T in

T m
G
RF

T
IF
G m
G
RF



If we replace the model in Fig. 4.5.a by that in
Fig. 4.5b
P n

G
IF
G m
G
RF kT s
B

Calculating System Noise
Temperature - 3

Equate Eqns :
T

S



T
RF

T in

T m
G
RF

T
IF
G m
G
RF





Since C is invariably small, N must be minimized.
How can we make N as small as possible?

Reducing Noise Power



Make B as small as possible – just enough bandwidth to accept all of the signal power (C ).
Make T
S
 as small as possible
Lowest T
RF
T


(How?)
If we have a good low noise amplifier (LNA), i.e., low
RF
Lowest T
High G
RF
, high G much.
RF in
, then rest of receiver does not matter that
T
S



 T
RF

T in

T
G m
RF

G
T m
IF
G
RF




T
RF

T in

Reducing Noise Power
Discussion on T in


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.

Antenna Noise Temperature



Contributes for T in
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  12000 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.

Antenna Noise Temperature

Useful approximation for Earth Station antenna temperature on clear sky (no rain):
Earth Station Antenna - Noise Temperature
50
45
30
25
40
35
20
15
0 10 20 30 40 50 60
Elevation Angle (degrees)
70 80 90 100

G


D

2
 
C
N

P r
KT s
B
P
 r
P t
G t
G r
L p
L a
L ta
L ra
L pol
L other
L r
L p

4

R

2
L a

F
T

S



T
RF

T in

T m
G
RF

T
IF
G m
G
RF




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 10m 2 c. Gain of receiving antenna.
d. Received C/N assuming Ts =152 K, and Bw =500 MHz

EIRP
4

R
2

P t
4

G t
R
2

2 x 50
4

x(4x10
7
)
2
(Solving in dB…)

4.97
x 10
15
W/m
2  
143 dBW/m
2
EIRP

( Pt

Gt )

3

17

20 dBW
R
4

2 
2 x log
10
( 4 x 10
7

11 dB
) dB[meter]
2
F

20

11

152
 
143 dBW/m2

b. Received Power
P r

F x A

(4.97x10
-15
) x 10
P r

4.97
x 10
14
W
 
133 dBW
(Solving in dB…)
P r

F

A

(

143 )

10
P r
 
133 dBW c. Gain given Ae = 10 m 2 and Frequency = 11GHz ( eqn. 4.7)
G r

4


2
A e
 4π x 10
0 .
0273

52 .
3 dB

b. System Noise Temperature
N

P n

KTB

1 .
38 x 1 0

23 x 152 x 500 x 10
6 or

K dB

T dB

B dB
 
228 .
6

21 .
82

86 .
99
 
119 .
7 9 dBW
C
C /

N
P r

4.97
x 10
14
W

C

N
 
133

 
133 dBW
(

119 .
79 )
C / N
 
13 .
2 dB

Generic DBS-TV:
Received Power
Transponder output power , 160 W 22.0 dBW
Antenna beam on-axis gain
Path loss at 12 GHz, 38,500 km path
Receiving antenna gain, on axis
Edge of beam
34.3 dB
-205.7 dB
33.5 dB
-3.0 dB
Miscellaneous losses
Received power, C
-0.8 dB
-119.7 dBW

Noise power
Boltzmann’s constant, k -228.6 dBW/K/Hz
System noise temperature, clear air, 143 K
Receiver noise bandwidth, 20MHz dBHz
Noise power, N
21.6 dBK
73.0
-134.0 dBW
C/N in clear air
Link margin over 8.6 dB threshold
Link availability throughout US
14.3 dB
5.7 dB
Better than 99.7 %