EC 723 Satellite Communication Systems

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EC 723 Satellite Communication Systems

• Mohamed Khedr
• http//webmail.aast.edu/khedr

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Syllabus
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 Presentations
Week 13 Presentations
Week 14 Presentations
Week 15 Presentations

• Tentatively

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Radio Propagation Atmospheric Losses

• Different types of atmospheric losses can perturb
  radio wave transmission in satellite systems
• Atmospheric absorption
• Atmospheric attenuation
• Traveling ionospheric disturbances.

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Radio PropagationAtmospheric 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 AA90 cosec ?

Source Satellite Communications, Dennis Roddy,
McGraw-Hill
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Radio PropagationAtmospheric 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.

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Signal PolarisationCross-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
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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
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Atmospheric attenuation
Attenuation of the signal in
Example satellite systems at 4-6 GHz
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rain absorption
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fog absorption
e
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10
atmospheric absorption
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10
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elevation of the satellite
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(No Transcript)
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Signal TransmissionLink-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.

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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

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Signal TransmissionEquivalent 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 (lt PT).

(a) Transmitting antenna Source Satellite
Communications, Dennis Roddy, McGraw-Hill
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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(?) is variation of power with angle.
• G(?) is gain at the direction ?.
• P0 is total power transmitted.
• sphere 4p solid radians

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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

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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

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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

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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

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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
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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
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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)

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Signal TransmissionLink-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.

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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
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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.

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Link Power Budget
Now all factors are accounted for 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
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Easy Steps to a Good Link Power Budget

• First, draw a sketch of the link path
• Doesnt 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)

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Simple Link Power Budget
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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.
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Closing the Link

• We need to calculate the Link Budget in order to
  verify if we are closing the link.
• Pr gt Cmin ? Link Closed
• Pr lt 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 gt 0 ? Link Closed
• Margin lt 0 ? Link not closed

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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

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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

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System Noise Power - 1

• Performance of system is determined by C/N ratio.
• Most systems require C/N gt 10 dB.
• (Remember, in dBs C - N gt 10 dB)
• Hence usually C gt 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)

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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 Boltzmanns constant
• 1.38x10-23 J/K(-228.6 dBW/HzK),
• Ts is the effective system noise temperature,
  andB is the effective system bandwidth

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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 gt0K radiate microwave energy.

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

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Calculating System Noise Temperature - 2

• Divide by GIFGmGRFkB
• If we replace the model in Fig. 4.5.a by that in
  Fig. 4.5b

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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?

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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.

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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.

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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 ? 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.

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Antenna Noise Temperature

• Useful approximation for Earth Station antenna
  temperature on clear sky (no rain)

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So many trade-offs !!!
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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 earths 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)
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Power Budget Example - 1

• b. Received Power

(Solving in dB)

• c. Gain given Ae 10 m2 and Frequency 11GHz (
  eqn. 4.7)

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Power Budget Example - 1

• b. System Noise Temperature

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Power Budget Example - 2

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

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Power Budget Example - 2

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

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Thank you