Transcript Wireless Communications and Networks

EC 723
Satellite Communication Systems
Mohamed Khedr
http://webmail.aast.edu/~khedr
Mz
Bz
Magn etic Field
Ro ll:
Tx = - MzBy
By
By
cr oss torq ue:
Tz = MxBy
pitch:
Ty = - Mx Bz
Mx
Bz
Bz
By
ro ll:
Tx = MyBz
My
Grades
Load
Percentage
Date
Midterm Exam
30%
Week of 3
December 2007
Final Exam
30%
Participation
10%
Report and
presentation
30%
Starting week
11th
Textbook and website

Textbook: non specific

Website: http://webmail.aast.edu/~khedr
Syllabus
Week 1
Overview
Week 2
Orbits and constellations: GEO, MEO and LEO
Week 3
Satellite space segment, Propagation and
satellite links , channel modelling

Tentatively
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
Presentations
Week 13
Presentations
Week 14
Presentations
Week 15
Presentations
Exploded view of a spinner satellite based on the
Boeing (Hughes) HS 376 design. INTELSAT IVA
(courtesy of Intelsat).
a) A spinner satellite,
INTELSAT IV A
(courtesy of Intelsat).
(b) A three-axis stabilized satellite, INTELSAT V (courtesy
of Intelsat).
SPACECRAFT SUBSYSTEMS





Attitude and Orbital Control System (AOCS)
Telemetry Tracking and Command (TT&C)
Power System
Communications System
More usually TTC&M Antennas
Telemetry, Tracking,
Command, and Monitoring
Telemetry:Automatic transmission and
measurement of data from remote sources
by wire or radio or other means
We will look at each in turn
Typical tracking, telemetry,
command and monitoring
system.
Bathtub curve for probability of failure.
AOCS

AOCS is needed to get the satellite into
the correct orbit and keep it there




Orbit insertion
Orbit maintenance
Fine pointing
Major parts


Attitude Control System
Orbit Control System
Look at these next
ORBIT MAINTENANCE - 1



MUST CONTROL LOCATION IN GEO &
POSITION WITHIN CONSTELLATION
SATELLITES NEED IN-PLANE (E-W) & OUTOF-PLANE (N-S) MANEUVERS TO MAINTAIN
THE CORRECT ORBIT
LEO SYSTEMS LESS AFFECTED BY SUN AND
MOON BUT MAY NEED MORE ORBITPHASING CONTROL
FINE POINTING


SATELLITE MUST BE STABILIZED TO
PREVENT NUTATION (WOBBLE) Move
unsteadily
THERE ARE TWO PRINCIPAL FORMS OF
ATTITUDE STABILIZATION


BODY STABILIZED (SPINNERS, SUCH AS
INTELSAT VI)
THREE-AXIS STABILIZED (SUCH AS THE ACTS,
GPS, ETC.)
DEFINITION OF AXES - 1

ROLL AXIS


PITCH AXIS


Rotates around the axis tangent to the orbital plane (N-S
on the earth)
Moves around the axis perpendicular to the orbital plane
(E-W on the earth)
YAW AXIS

Moves around the axis of the subsatellite point
DEFINITION OF AXES - 2
Earth
o
Equator
s
Yaw
Axis
Roll
Axis
Pitch
Axis
TTC&M

MAJOR FUNCTIONS

Reporting spacecraft health
Monitoring command actions

Determining orbital elements




TTC&M is often
a battle between
Operations (who
want every little
thing monitored
and Engineering
who want to hold
data channels to a
minimum
Launch sequence deployment
Control of thrusters
Control of payload (communications, etc.)
TELEMETRY - 1

MONITOR ALL IMPORTANT






TEMPERATURE
VOLTAGES
CURRENTS
SENSORS
NOTE: Data are usually
multiplexed with a priority
rating. There are usually
two telemetry modes.
TRANSMIT DATA TO EARTH
RECORD DATA AT TTC&M STATIONS
TELEMETRY - 2

TWO TELEMETRY PHASES OR MODES

Non-earth pointing



NOTE: for critical
During the launch phase telemetry channels
During “Safe Mode” operations when the
spacecraft loses tracking data
Earth-pointing


During parts of the launch phase
During routine operations
TRACKING





MEASURE RANGE REPEATEDLY
CAN MEASURE BEACON DOPPLER OR
THE COMMUNICATION CHANNEL
COMPUTE ORBITAL ELEMENTS
PLAN STATION-KEEPING MANEUVERS
COMMUNICATE WITH MAIN CONTROL
STATION AND USERS
COMMAND

DURING LAUNCH SEQUENCE




SWITCH ON POWER
DEPLOY ANTENNAS AND SOLAR PANELS
POINT ANTENNAS TO DESIRED LOCATION
IN ORBIT


MAINTAIN SPACECRAFT THERMAL BALANCE
CONTROL PAYLOAD, THRUSTERS, ETC.
COMMUNICATIONS SUB-SYSTEMS






Primary function of a communications satellite (all
other subsystems are to support this one).
Only source of revenue
Design to maximize traffic capacity
Downlink usually most critical (limited output power,
limited antenna sizes).
Early satellites were power limited
Most satellites are now bandwidth limited.
Typical satellite antenna patterns and coverage zones. The antenna for
the global beam is usually a waveguide horn. Scanning beams and
shaped beams require phased array antennas or reflector antennas with
phased array feeds.
Typical coverage patterns for Intelsat satellites over the
Atlantic Ocean.
Contour plot of the
spot beam of ESA’s
OTS satellite
projected onto the
earth. The contours
are in 1 dB steps,
normalized to 0 dB at
the center of the
beam.
Radio Propagation: Atmospheric Losses

Different types of atmospheric losses can perturb radio wave
transmission in satellite systems:
 Atmospheric absorption;
 Atmospheric attenuation;
 Traveling ionospheric disturbances.
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
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.
Radio Propagation:
Traveling Ionospheric Disturbances


Traveling ionospheric disturbances are clouds of
electrons in the ionosphere that provoke radio signal
fluctuations which can only be determined on a
statistical basis.
The disturbances of major concern are:



Scintillation;
Polarisation rotation.
Scintillations are variations in the amplitude, phase,
polarisation, or angle of arrival of radio waves,
caused by irregularities in the ionosphere which
change over time. The main effect of scintillations is
fading of the signal.
Signal Polarisation:
What is Polarisation?

Polarisation is the property of
electromagnetic waves that
describes the direction of the
transverse electric field. Since
electromagnetic waves consist
of an electric and a magnetic
field vibrating at right angles to
each other it is necessary to
adopt a convention to
determine the polarisation of
the signal. Conventionally, the
magnetic field is ignored and
the plane of the electric field is
used.
Signal Polarisation:
Types of Polarisation

Linear Polarisation (horizontal
or vertical):



Circular Polarisation:

Linear Polarisation Circular Polarisation
Elliptical Polarisation 
the two orthogonal
components of the electric
field are in phase;
The direction of the line in
the plane depends on the
relative amplitudes of the two
components.
The two components are
exactly 90º out of phase and
have exactly the same
amplitude.
Elliptical Polarisation:

All other cases.
Signal Polarisation:
Satellite Communications

Alternating vertical
and horizontal
polarisation is widely
used on satellite
communications to
reduce interference
between programs on
the same frequency
band transmitted from
adjacent satellites
(one uses vertical, the
next horizontal, and
so on), allowing for
reduced angular
separation between
the satellites.
Information Resources for Telecommunication Professionals
[www.mlesat.com]
Signal Polarisation:
Depolarisation

Rain depolarisation:


Since raindrops are not perfectly spherical, as a polarised wave crosses a
raindrop, one component of the wave will encounter less water than the other
component.
There will be a difference in the attenuation and phase shift experienced by
each of the electric field components, resulting in the depolarisation of the
wave.
Polarisation vector relative to the major and minor axes of a raindrop.
Source: Satellite Communications, Dennis Roddy, McGraw-Hill
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)


Source: Satellite Communications,
Dennis Roddy, McGraw-Hill
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.
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
Refractive effects
(tropospheric
scintillation) cause
signal loss.
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 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
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°
elevation of the satellite
40°
50°
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.
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
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
Antenna Gain


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( ) 
P0 / 4
• P() is variation of power with angle.
• G() is gain at the direction .
• P0 is total power transmitted.
• sphere = 4 solid radians
Signal Transmission
Link-Power Budget Formula Variables

Link-Power Budget Formula for the received power [PR]:


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:






[PR] = [EIRP] + [GR] - [LOSSES]
[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.
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)
Pt Gt Gr
Pr 
L p La Lta Lra L pol Lother Lr
Transmission Formula

Some intermediate variables were also defined
before:
Pt =Pout /Lt
EIRP = Pt Gt
Where:

Pt = Power into antenna

Lt = Loss between power source and antenna

EIRP = effective isotropic radiated power
•Therefore, there are
many ways the formula
could be rewritten. The
user has to pick the one
most suitable to each
need.
Pt Gt Gr
Pr 
L p La Lta Lra L pol Lother Lr
EIRP x Gr

L p La Lta Lra L pol Lother Lr
Pout Gt Gr

Lt L p La Lta Lra L pol Lother Lr
Link Power Budget
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
Translating to dBs

The transmission formula can be written in dB as:
Pr  EIRP Lta  Lp  La  Lpol  Lra  Lother  Gr  Lr



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.
Link Power Budget
Tx
EIRP
Transmission:
+ HPA Power
- Transmission Losses
(cables & connectors)
+ Antenna Gain
Now all factors are accounted for
as additions and subtractions
- 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
Easy Steps to a Good Link Power Budget

First, draw a sketch of the link path



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




Doesn’t have to be artistic quality
Helps you find the stuff you might forget
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)
Simple Link Power Budget
Parameter
Frequency
Transmitter
Transmitter Power
Modulation Loss
Transmission Line
Loss
Transmitted Power
Value
Totals
11.75
40.00
3.00
0.75
Units
GHz
dBm
dB
dB
36.25 dBm
Transmit Antenna
Diameter
Aperture Efficiency
Transmit Antenna
Gain
0.5
0.55
m
none
33.18 dBi
Slant Path
Satellite Altitude
Elevation Angle
35,786
14.5
km
degrees
Slant Range
Free-space Path Loss
Gaseous Loss
41,602
206.22
0.65
km
dB
dB
Rain Loss (allocated)
Path Loss
3.50
dB
210.37 dB
Parameter
Receive Antenna
Random Loss
Diameter
Aperture Efficiency
Gain
Polarization Loss
Effective RX Ant.
Gain
Value
0.50
1.5
0.6
43.10
0.20
Received Power
Summary
Transmitted Power
Transmit Anntenna
Gain
EIRP
Path Loss
Effective RX
Antenna Gain
Received Power
Totals
Units
dB
m
none
dBi
dB
42.40 dB
-98.54 dBm
36.25
33.18
dBm
dBi
69.43 dBmi
210.37 dB
42.4 dBi
-98.54 dBm
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.
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
Margin < 0
 Link Closed
 Link not closed
Carrier to Noise Ratios

C/N:




Allows simple calculation of margin if:
Receiver bandwidth is known
Required C/N is known for desired signal type
C/No:


carrier/noise power in RX BW (dB)
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
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
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 + 10 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):
5
 
T K   T C  273
0
 
T K   T 0 F  32
 9  273
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:
N  kTs B (dBW)
where k
= Boltzmann’s constant
= 1.38x10-23 J/K(-228.6 dBW/HzK),
Ts is the effective system noise temperature, and
B is the effective system bandwidth
Thank you