Geostationary Satellite Communications Systems

Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada

Slide Number 1Rev -, July 2001

Vol 1: Master


Slide Number 2Rev -, July 2001 Vol 1: Master

Sec 2: Logistics

1.2.1: Course Structure

Structure• The Technical Introduction to Geostationary Satellite

Communication Systems course is 13 days in length.

• Each instruction day contains 6 hours of real instruction, allowing for lunch, prayer, or other breaks in a typical eight-hour day.

• Each day will begin at _______, and is scheduled to end at ______.

• At the discretion of the instructor, the course may be extended, either to cover specific material more closely, or to provide for the inclusion of hands-on training.

• A familiarization tour of the _____________________ facility is planned for ____________ {date}.



Sec 2: Logistics

1.2.2: Environment

Environment• You are presently in the ___________ building at _________.

• Your room designation is ______.

• In case of fire or other emergency, your nearest exit is ________.

• Fire extinguishers are located at ____________.

• The building security/emergency phone number is ______.

• The nearest first aid kit is located at _____________.

• Your nearest washroom is located ____________.

• The cafeteria or nearest recommended restaurant is located __________.



Sec 2: Logistics

1.2.2: Environment

Environment• Should you wish to receive a fax while you are a guest here,

please feel free to use ________ {phone number}.

• Your instructor’s policy on the use of cell phones while in class is _________ {state instructor’s policy}.

• Should anyone require a change of seating or other accommodation as the result of a vision or hearing impairment, please feel free to approach your instructor.

Finally . . .ARABSAT and _____________ {instructor’s name} desire that your experience here be not only educational, but enjoyable as well. Please accept our hospitality and let us know of any way we can enhance your visit.



Vol 1: Master

Section 3Section 3



Sec 3: Course Contents

1.3.1: The Course Objective

Upon completion of this course, the Upon completion of this course, the student will understand the concepts student will understand the concepts

and language of satellite and language of satellite communications and will be well communications and will be well

prepared to undertake further prepared to undertake further training in specific satellite training in specific satellite

communications systems or tasks.communications systems or tasks.

Upon completion of this course, the Upon completion of this course, the student will understand the concepts student will understand the concepts

and language of satellite and language of satellite communications and will be well communications and will be well

prepared to undertake further prepared to undertake further training in specific satellite training in specific satellite

communications systems or tasks.communications systems or tasks.

Objective Statement




1.3.2: Navigation in the Document Set

NavigationYour document set for this course is divided into 7 Volumes and is contained in 4 binders.

The Volumes are high-level topical divisions, as indicated in Part 1.3.3.

The Volumes are further subdivided into 6 paragraph levels as follows:

# Volume#.# Section#.#.# Part#.#.#.# Subject#.#.#.#.# Element#.#.#.#.#.# Point

Each paragraph level entry, with its title, is listed in the Master Table of Contents, cross referenced to its document page number.





NavigationEach page in the document is structured as shown here.

The page header simply gives the course title, the revision

number, and the revision date.





Navigation

The slide portion of the page is a reduced version of the slide

as it appears on the screen.

It is always possible to tell where you are within the document paragraph structure by looking in four places on the slide portion of the page.





Navigation

Here

Here

and Here

Here





NavigationFor example, this slide is from:

Volume 4, Earth Stations

Section 10, Power Distribution Systems

Part 4, Emergency Power Systems

Subject 4, The UPS System

Element 2, Available Configurations





Navigation

The notes portion of the page may or may not contain text. If

present, this text is always material that is in addition to, or expands upon, the material on

the slide.





Navigation

The document page number is given here. This number is

sequential across all Volumes and is the master numbering

scheme for the document set.





Navigation

The Section depth is repeated here.





NavigationA Master Index is provided to the student, located in Section 1.6. It will refer the student to the document page number on which the listed term appears.




1.3.3: Objectives for Each Volume

Volume 1 is concerned with introduction and navigation. It serves to introduce the instructor(s) to the students, the students to the instructor(s) and each other, and the Volume Set to the students.

Volume 1 also introduces the students to the environment in which the course will be taught.

In addition, Volume 1 provides reference material in the form of discussions on several key technical concepts, a glossary of terms, and a list of acronyms.

Volume 1: Master

1





Volume 2 begins in the sky. Here, satellites and the satellite concept are introduced. After a brief look at various uses for satellites, focus moves to communications. Typical satellite design features are discussed, along with testing and performance verification procedures.

Useful orbits, orbital dynamics, and the space environment are also discussed in this Volume.

The material in Volume one only introduces these topics. It is hoped that interested students will continue their study in this area.

Volume 2: Communication Satellites

2





Having introduced satellites in Volume 2, Volume 3 discusses how we use them. Various satellite system topologies are introduced, and the methods of accessing satellite resources from the ground are established.

Important technical concepts are dealt with in this Volume as well. Carrier modulation methods and error correction coding concepts are taught. As well, students will be introduced to various methods employed for dealing with the effects of satellite distance and motion.

Volume 3: Satellite Communication Principles

3





Volume 4 could well be the most important Volume for the student of this course. In it we cover all the basics of Earth Station design and maintenance. Numerous types and sizes of Earth Station are mentioned, along with interfacing techniques.

Students will profit, in particular, from the detailed discussion of a typical Earth Station block diagram with its emphasis on setup, testing, and fault finding.

Volume 4: Earth Stations

4





We’ve dealt with satellites in orbit and with Earth Stations on the ground. Now it’s time to connect the two.

Volume 5 deals with designing satellite communication links. The engineering model of a satellite link is introduced, followed by discussions of propagation, noise, and other factors that affect the link.

Detailed link calculations will be made on example links, and students will be given the opportunity to design their own link solution.

Volume 5: Link Analysis

5





Digital video is now a big part of every satellite provider’s business. Volume 6 will take the student from analog to digital video, from uncompressed to compressed digital video streams, and will introduce common standards.

Having introduced digital video itself, the course will then turn to Digital Video Broadcasting (DVB) where methods for handling digital video streams over satellite will be discussed.

Finally, new digital video standards will be mentioned, with emphasis on how the satellite industry will be affected.

Volume 6: Digital Video

6





The final Volume of this document set deals with the future. What new technologies and market forces are driving the design of communication satellites today?

Students will be introduced to regenerative processing, new frequency bands, inter-satellite links, and upcoming changes in satellite roles.

Some grand global coverage schemes, both already in the sky and still on paper, will be mentioned.

Volume 7: Emerging Trends in Satellite Communication

7



Vol 1: Master

Section 4Section 4



Vol 1: Master

1.4: Principle Technical Terms

Principle Technical TermsThe following technical terms and concepts are central to satellite communication.

• Frequency, Wavelength and Propagation• Polarization• Power (in Watts)• The Decibel (dB)• Noise• Gain to Noise Temperature Ratio (G/T)• Effective Isotropic Radiated Power (EIRP)• Power and Saturating Flux Density (PFD and SFD)• Bandwidth (BW)• Bit Error Rate (BER)

Since they are so important, and since they appear in many different areas of the satellite communication project, they will be taught first.



Sec 4: Principle Technical Terms

1.4.1: Frequency, Wavelength and Propagation

+

-

Static ElectricField E

Electromagnetic FieldsPropagating electromagnetic waves are composed of E and H fields. To see how this comes about, consider the following diagram.

Here, a battery is connected to an open circuit and static lines of force exist across the gap. These lines form an Electric, or E, field.

Note that these lines traverse space. Observing this, we are on the way to understanding propagation.

Figure 1.4.1a Static Electric Field




Electromagnetic FieldsNow suppose we have this circuit. As the switch toggles steadily between the inner and outer posts, the E field changes direction at the same rate.

a

a'sV

0 321

+V

-V

0 1 32 4

T

EE EE

The rate of change of the voltage at a/a’, and of the switch and E field as well, is known as the frequency of the pattern. Frequency is given is cycles per second, or Hertz (Hz).


Figure 1.4.1b Alternating Electric Field




Electromagnetic FieldsBecause the E field is changing in time across free space, an H field will be generated.

a

a'sV

0 321

EE EEH

H

If the switching rate of our switch is high enough, then the E and H fields will radiate into free space, but not very well.


Figure 1.4.1c Electric and Magnetic Fields




Electromagnetic FieldsTo radiate well, an antenna is required. At its simplest, an antenna is a pair of wires so arranged as to radiate efficiently.

c

c'

a

a'

E E E

H

H

H

Direction ofPropagation

t0t3 t2 t1

H

E and H energy is able to leave this basic dipole antenna and propagate away at the speed of light.


Figure 1.4.1d Propagation




Electromagnetic FieldsSince we know that the speed of propagation is that of light—about 300,000 km/s—and we know the period, or time it takes to complete one cycle of our switching pattern, we can easily calculate the distance traveled by a complete cycle.

Stated somewhat differently, we can calculate the length of one cycle, or wave, in space. This is called the wavelength, known as “lambda” (λ), and is expressed in suitable distance units: kilometers, meters, down to micro- and nanometers.

For Example:For Example: At the C-Band frequency of 6 GHz, the period of the waveform is 1 ÷ 6,000,000,000 = 0.1667 nanoseconds. In 1 second, light will travel 300,000,000 meters. Therefore, the wavelength (λ) of the transmitted energy at 6 GHz is 300,000,000 x 0.0000000001667 = 0.05 meters, or 50 mm.





1.4.2: Polarization

Y

X

H

E

Z E H

E

H

E

H

MAXIMUM ALONGZ AND X AXIS

DONUT SHAPEDPATTERN

DIRECTION OFPROPAGATION

λ

Figure 1.4.2 Nature of Radiated Fields


Slide Number 31Rev -, July 2001 Vol 1: Master, Sec 4: Principle Technical Terms

Part 2: Polarization

1.4.2.1: The Nature of Electromagnetic Waves

Electromagnetic WavesAs we have seen, energy propagates through space in the form of electric and magnetic fields.

These two fields are always paired (one cannot exist without the other) and oriented at right angles to each other.

They are also oriented at right angles to the direction of travel of the wave, which is known as the Poynting vector.

Y

X

H

E

Z E H

E

H

E

H

MAXIMUM ALONGZ AND X AXIS

DONUT SHAPEDPATTERN

DIRECTION OFPROPAGATION

λ

Figure 1.4.2.1 Nature of Radiated Fields




1.4.2.2: The Concept of Polarization

PolarizationAt any instant in time, the E and H fields originating from an antenna and travelling out of the screen toward the viewer might be spatially oriented as pictured here.

Theoretical point source antennas do not control the orientation of the fields, so that at any instant the fields could be oriented this way . . .

H

E

H

E





Polarization

or this way . . .

H

E

or this way . . .

HE





PolarizationMany kinds of antennas do control the orientation of the fields they transmit, however.

If the long axis of a dipole were physically arranged this way, for instance,

H

E

then the resulting orientation of the E and H fields would follow thus.

When energy is radiated with only one spatial orientation, the waveform is considered to be “polarized”.





Polarization

H

E

Vertical Horizontal

H

EOrbital

Plane

Figure 1.4.2.2 Satellite Horizontal and Vertical Polarization

Vol 1: Master, Sec 4: Principle Technical Terms




1.4.2.3: Linear vs Circular Polarization

Linear Polarization

Figure 1.4.2.3a Linear Polarization





PolarizationSo far, we have been discussing polarized wavefronts that are oriented either vertically or horizontally and remain that way. This arrangement is called Linear Polarization.

It is also possible to organize the wavefront so that it is a rotating vector. The energy is still considered polarized because, at any instant, only one E-Field vector orientation is being released. Over time, however, the E-Field rotates through 360º. This is called Circular Polarization.

The wavefront can be arranged to rotate either to the left or right. These are designated Left Hand Circular Polarized (LHCP) and Right Hand Circular Polarized (RHCP).

As with linear polarization, LHCP and RHCP do not interfere with each other, permitting frequency reuse.





Circular Polarization

Figure 1.4.2.3b Linear Polarization




1.4.3: Power in Watts

Power in WattsPower is the rate of doing work. The basic unit of power is the Watt (W), where 1 Watt is equivalent to 1 joule of work being done in 1 second.

The power, in Watts, of a transmission into any medium is:

P(Watts) = e2(rms) ÷ Z(medium)

The power in a slice of free space permeated by electromagnetic energy, an important concept in antenna work, may be expressed in W/m2. This is called the Power Flux Density (PFD).

Watts are linear units. That is, the difference between 1 and 2 Watts is the same real magnitude as the difference between 100 and 101 Watts.

EQ. 1.4.3 Power in Watts




1.4.3: Power in Watts

Power in WattsIn satellite communication work it is not uncommon for a piece of equipment to be driven by 0.000003 Watts, for an HPA to output 400 Watts, and for signal to arrive at the satellite at 0.00000000000001 Watts.

As can be seen, in satellite communication a wide power range must be dealt with.

Also, if we put Watts into an HPA and wish to find out how many Watts we get out, we must multiply the input by the gain of the amplifier. It is not uncommon for an HPA to boast a gain of 100,000. Even worse, if we wish to divide our transmit power down by the path loss to the satellite, we would find ourselves dividing by a number that is 20 digits long!



As we have seen, when power is expressed in the linear unit Watts, we suffer two disadvantages:• we must deal with very large and very small numbers in an

awkward fashion• when dealing with gain or loss, we must sometimes handle very

large multipliers

To get around these difficulties, power is often expressed in Decibels (dB).

The Bell--and therefore the decibel--has three advantages:• It is logarithmic, as is the response of the human ear and other

natural systems

• It can express very large and very small numbers efficiently

• It does away with large multipliers in favour of simple addition

Vol 1: Master


1.4.4: The Decibel

The Decibel



In linear calculation, this is the expression for Gain (G):

Note that Gain is unitless, acting as a multiplier.

Gain expressed in decibels has this form:

Once again, Gain is unitless. The term “dB” by itself has no units, but only represents a power ratio.

The Decibel and Gain

)()(

WattsPWattsP

Gin

out=

=

)(

)(

WattsP

WattsPLogG

in

outdB 10

Vol 1: Master


1.4.4: The Decibel

EQ. 1.4.4a Gain in Watts

EQ. 1.4.4b Gain in Decibels



The decibel is a very flexible concept, however, and can pick up units from its operands. To express a power in Watts, the same formula applies:

Notice that the power value is now expressed in dBW, where dBW is a unit. When we see “dBW” we say that “dB is referenced to 1 Watt.”

Had P been expressed in milliwatts rather than Watts, the resultant decibel unit would have been dBm, or dB referenced to 1 milliwatt.

The Decibel’s Flexibility

)()( 10 WattsdBW LogPP =

Vol 1: Master


1.4.4: The Decibel

EQ. 1.4.4c Decibels from Watts




1.4.4: The Decibel

Quantity Symbol Units dB NotationPower P Watts dBWPower P milliwatts dBmNoise Temperature T Kelvin dB-KBandwidth BW Hz dB-HzAntenna Gain (Above Isotropic) G dBiEffective Isotropic Radiated Power EIRP Watts dBWGain to Noise Temperature Ratio G/T (Kelvin)-1 dB/KBoltzmann's Constant k Watts/K/Hz dBW/K/HzNoise Power (kTB) N Watts dBWNoise Power Density (kT) N0 Watts/Hz dBW/HzCarrier to Noise Power Ratio C/N dBCarrier to Noise Power Density Ratio C/N0 Hz dB-HzFlux Density Φ Watts/m2 dBW/m2

Useful dB Expressions

Useful Decibel ExpressionsThe following table lists other dB references that will often be encountered in satellite communiction work.



A Decibel Example

Vol 1: Master


1.4.4: The Decibel

0.1 Watts 400 Watts

G = P(out) ÷ P(in) = 400 ÷ 0.1 = 4000

LinearLinear

LogarithmicLogarithmic

P(dBW in) = 10LogP(Watts in) = 10Log(0.1) = -10 dBW

P(dBW out) = 10LogP(Watts out) = 10Log(400) = 26 dBW

G(dB) = 10Log{P(Watts out) ÷ P(Watts in)} = 10Log (400 ÷ 0.1) = 36 dB

Note that the advantage of working in dB is that all we really had to do to find the gain was add P(out dBW) - P(in dBW), or 26 - (-10) = 36 dB.



Linear DecibelFunction

Gain Multiply AddLoss Divide Subtract

Sum Add (rms) ----

Vol 1: Master


1.4.4: The Decibel

Relationship and AdvantagesWe now begin to see some of the advantages of using dB, as relatively large numbers are represented by smaller ones, and operations become simple summations.

This table shows how linear and Decibel systems relate. Note that there is no direct Decibel equivalent for summation.



2 Watts = ? dBW dBWLogW 32102 == )(

2 Watts = ? dBm dBmLogW 33001

2102 =

=

.

Note that to get dBm from dB, it is only necessary to add 30.

2 Watts

G = 100

? Watts WWx 2001002 =

= ? dB dBLog 2010010100 == )(

Vol 1: Master


1.4.4: The Decibel

Some Problems for You



3 dBW

G = 20dB

? dBW dBWdBdBW 23203 =+

Notice that we added dBW and dB to get dBW, but can we add dBW and dBW?

No!No!

Some Problems for You

Vol 1: Master


1.4.4: The Decibel

Remember that there is no direct equivalent to linear addition in the logrithemic world.



∑-7.4 dBm

-12.2 dBm? dBm

Vol 1: Master


1.4.4: The Decibel

Some Problems for YouHow would we handle this real-world example? With a power meter you have measured the inputs to a 2-way combiner at -7.4 and -12.2 dBm. Loss through the combiner is 3.2 dB. What would you expect to measure at the combiner’s output?

A diagram of the problem looks like this.

Loss = 3.2 dB




1.4.4: The Decibel

Some Problems for YouBecause we cannot add dBm to dBm directly, we must first convert the dBm values back to their linear equivalent. We then add them to get a linear result.

Although we could convert the loss through the combiner to its linear equivalent and deal with it now, it will be easier to ignore it for the moment.

Our problem now looks like this:

∑0.182 milliwatts

? milliwatts0.060 milliwatts




1.4.4: The Decibel

Some Problems for YouValues in milliwatts can simply be added:

Sum(milliwatts) = 0.182 + 0.060 = 0.242 milliwatts

We now convert the result back to dBm:

Sum(dBm) = 10Log(0.242) = -6.16 dBm

∑-7.4 dBm

-12.2 dBm-6.16 dBm

No loss

Our problem now looks like this. Notice that the result is certainly not the simple addition of the dBm input values.




1.4.4: The Decibel

Some Problems for YouThe only thing we have not yet considered is the loss of the combiner. Recalling that loss is expressed as a negative value, we simply add it to the result.

Real Sum(dBm) = -6.16 + (-3.2) = -9.36 dBm

∑-7.4 dBm

-12.2 dBm-9.36 dBm

Loss = 3.2

With the power meter, we can now confirm our result.




1.4.4: The Decibel

Final Comments on dBTo convert decibels back to their linear equivalent, use:

= 1010dB

Linear

Finally, the engineer will find it useful to memorize these power relationships as an aid to quick mental calculation in the field.

•0 dBW = +30 dBm = 1 Watt•Half power is -3 dB•Twice the power is +3 dB•1/4 power is -6 dB•1/10th power is -10 dB

EQ. 1.4.4d Watts from Decibels


Slide Number 54Rev -, July 2001 Vol 1: Master, Sec 4: Principle Technical Terms Introduced

Part 5: Noise

1.4.5.1: Thermal Noise

Thermal NoiseIn satellite systems, the signal to noise ratio is impacted by three main sources:

1. thermal noise added by electrical components

2. signal losses in the chain

3. thermal noise energy received by the antenna.

Electrical Circuit NoiseThe noise in electrical circuits is caused by the random motion of electrons. This motion, in turn, relates to temperature and is therefore called thermal noise. The higher the temperature, the faster the electrons move and the greater the power of the thermal noise.



Thermal NoiseNyquist proved from thermodynamic considerations that the mean squared voltage across a resistance R measured in a bandwidth B is given by:

e n 2 = 4kTBR

Where e n = Noise Voltage

k = Boltzmann’s Constant, 1.38 x 10-23 Joules/Kelvin

T = Absolute Temperature, K

Vol 1: Master, Sec 4: Principle Technical Terms Introduced

Part 5: Noise




Thermal NoiseThe available noise power N into a matched load is then given by:

The noise power density is No = kT. This is derived by referencing the expression to a 1 Hz bandwidth, resulting in a bandwidth-independent expression.

It is common to specify the noise performance of a receiver either in terms of an equivalent noise temperature, or in terms of a noise figure.

N = en2 /4R =

4R4kTBR = kTB


Part 5: Noise


EQ. 1.4.5.1 Noise Power



Part 5: Noise

1.4.5.2: Equivalent Noise Temperature

Equivalent Noise TemperatureConsider the following noise-only model for an active device.

The equivalent noise temperature of a device is defined as the temperature of a noise generator at the device input which would produce the actual measured output noise power, assuming the device itself is noiseless.

DEVICE GAIN = G

Nout = kB (GTe) = kBTout

NoutTe

Tout = GTe



Noise Calculation in Series-Connected DevicesActive Devices in Tandem

For devices operating in tandem:

Te = Te1 + (Te2 / G1) and Tout = TeG1G2 = Te1G1G2 + Te2G2

and the output noise power is:

Nout = kTout B


Part 5: Noise

1.4.5.3: Noise Calculation in Series-Connected Devices

G1

Te1

ToutTeG2

Te2



Noise Calculation in Series-Connected DevicesThis can be modeled using ideal components as follows:

Where the noise is now inserted at the output of the noiseless circuit.

Tout is as defined on the previous slide.

In a different scenario, the circuit with ideal components can be modified again to have the noise inserted at the input of the circuit rather than the output. This is shown on the next slide.

G1

(Noiseless)

G2

Tout

Noiseless Signal(Noiseless) +


Part 5: Noise




Noise Calculation in Series-Connected Devices

Where Tin is now:

G1

(Noiseless)

G2

Tin

Noiseless Signal(Noiseless)+

Tin = Te2

Te1 +G1


Part 5: Noise


EQ. 1.4.5.3 Noise Calculation



Part 5: Noise

1.4.5.4: Carrier to Noise Ratio (C/N)

Carrier to Noise RatioThe absolute amount of noise in any system is not the whole story; we are going to insert a carrier into the system, modulated with desired information.

This carrier will also have an absolute power level, but it is the relationship between carrier and noise power that concerns us.

This relationship is called the Carrier to Noise Ratio (C/N).

A low C/N means that carrier power is not much greater than noise power. This condition will make it difficult for a demodulator to lock onto the carrier, demodulate it, and accurately reproduce the desired information it contains.

High values for C/N are necessary, but if they are too high, then power is being wasted. An optimum C/N is just sufficient to meet the QoS objectives of the link, with reasonable margins.



Part 5: Noise

1.4.5.4: Carrier to Noise Ratio (C/N)

Carrier to Noise Ratio - Measurement

C

N



Part 5: Noise

1.4.5.5: Carrier to Noise Density Ratio (C/No)

Carrier to Noise Density RatioOne problem with the C/N value is that it is bandwidth dependant: What is the bandwidth of the carrier in question? What is the equivalent noise bandwidth of the system devices that will be handling the carrier?

In link engineering it is convenient to be able to make calculations in the absence of specific knowledge about these bandwidths.

To make this possible, the industry has adopted the use of the bandwidth independent term C/No: the ratio of carrier power to the spectral density of system noise.

To put this another way: C/No is the ratio of total carrier power to the noise in a 1 Hz bandwidth in units of dB-Hz.

Using this term, calculations can be made without regard to bandwidth.



Component Noise FigureThe noise figure of a component is a measure of the device’s noise based on the deterioration in carrier to noise ratio that occurs through the component:

where f is the component noise figure, and G is the gain. Therefore we can write:

Nout = fGNin = (f - 1) GNin + GNin

= (f - 1) GkTinB + GkTinB


Part 5: Noise

1.4.5.6: Component Noise Figure

Cin

= fCout

= f GCin

NinNout Nout



Component Noise FigureThe second term, GkTinB, is just the noise at the output due to the incoming noise being amplified by the component. The first term, (f - 1)GkTinB, is the output noise contribution due to the component itself.

Component noise defined in this manner is dependent on the input noise temperature level. To avoid a noise figure definition that depends on the input temperature, the IEEE has standardized the definition so that it is always taken at an input temperature of To = 290 K, the typical ambient temperature.


Part 5: Noise




Component Noise FigureTherefore we have No = kTeBG = ( f - 1 ) kToBG

Te = ( f-1 ) To Where To = 290 K

This is an important relationship between the two common methods used to specify the noise performance of active devices—noise temperature and noise figure.

The noise figure is commonly expressed in dB terms as:

F = 10 log10 ( f ) dB


Part 5: Noise




Component Noise FigureFor two devices in tandem the overall input noise figure is:


Part 5: Noise

f1,2 = f1 + G1

f 2 - 1

G

f

G

f

1

1

2

2


G

f1,2

1,2



Component Noise FigureAttenuators operating at room temperature also generate noise power that must be included in the calculations. For a matched attenuator, the noise power flowing into and out of any section of the attenuator is

N = kToB

This section of the attenuator will reduce the input noise by the loss of the section—i.e. increase the noise by the (negative) gain—but it will also generate noise which can be specified by the equivalent noise temperature of the section. Therefore we can write the following equation:


Part 5: Noise

kTo B = kToB L

kTeB L

+ Nout = NinG + kTeBG or

where L is Loss (1/G).




Component Noise FigureTherefore, Te = To (L-1) for a device with pure loss.

The equivalent noise temperature at the output of an attenuator is therefore:


Part 5: Noise

Tout To

L-1L

=

ATTENUATOR SECTION

Nout

G =

Te

1

LNin


EQ. 1.4.5.6 Equivalent Noise Temperature of an Attenuator



Part 5: Noise

1.4.5.7: Energy per Bit to Noise Density Ratio (Eb/No)

Eb/NoIn the earlier discussion on C/N it was indicated that the noise portion could be rendered bandwidth independent by referencing it to 1 Hz, resulting in the useful C/No value.

This process could be applied to the carrier power as well, yielding Co/No. This is essentially the C/N for a 1 Hz carrier in a system with a 1 Hz noise bandwidth.

In digital communication, the Co/No idea is modified somewhat. Since the basic unit of digital communication is the “bit”, designers are more interested in the ratio of the power, or energy, in one bit to the noise power of the system.

The carrier power divided by the bit rate, then, yields the energy per bit. When this expressed as a ratio with noise density, the Energy per Bit to Noise Density Ratio (Eb/No) results.



Part 5: Noise

1.4.5.7: Energy per Bit to Noise Density Ratio (Eb/No)

Eb/NoEb/No is a “great leveler,” allowing digital system performance to be compared without respect to bandwidths or bit rates.

Thus, for instance, design engineers can compare the performance of different modems from the manufacturer’s published Eb/No specifications without concern about modem filter bandwidths or differences in coding.

It must be kept in mind, however, that Eb/No applies only to digital communication and has no relevance to analog systems.




1.4.6: Gain to Noise Temperature Ratio (G/T)

Gain to Noise TemperatureThe Gain to Noise Temperature Ratio (G/T) is known as the figure of merit of receive systems. This is an objective, accurate, one-number method of stating the ability of a receive system to process signal from system noise.

This specification is easy to use in design work, as will be demonstrated later. The reference point for G/T calculation is usually the input to the LNA, but it is important to realize that the value of G/T is independent of the reference point actually used.

G/T is also relatively easy to test in the field, and satellite operators will often require a G/T test prior to allowing an Earth Station to access a satellite.

G/T is specified in units of dB/K.





Gain to Noise Temperature

In this model:

G/T = Ga - L - 10 log (Tsys)

= 45 - 0.5 - 21.5 = 23 dB/K

LNALOSS

GL

ANTENNA

Ga = 45 dB

= 0.5 dB

TSYS= 142 K

EQ. 1.4.6 Gain to Noise Temperature





Gain to Noise TemperatureIncidentally, the noise temperature and figure of merit for the satellite receiver are calculated in the same way as those of the Earth Station.

A special consideration for the satellite is the antenna noise temperature. Unlike Earth Station antennas that are looking into "cold" space, the satellite antenna, which is designed for coverage of a portion of the Earth, is looking only at the "warm" earth.

As viewed from space, the Earth has a noise temperature of about 290 K, and this is a major factor in setting the G/T of a satellite.

The satellite G/T is specified by the manufacturer and does not have to be calculated as part of link analysis.




1.4.7: Effective Isotropic Radiated Power (EIRP)

EIRPThe concept of EIRP is based on the premise that a directional antenna can theoretically be replaced by another antenna that radiates evenly in all directions. Such an antenna would be an isotropic radiator.

If this were to happen, the EIRP would be the RF power that would have to be fed into the isotropic radiator so that it would radiate, in all directions, the same power that the directional antenna radiated along its boresight.

As a result, the EIRP is not the actual amount of energy which is emitted. It is the amount of energy that would have to be emitted by an isotropic antenna in order to give the same performance as the directional antenna.




1.4.7: Effective Isotropic Radiated Power (EIRP)

EIRPThe formula for EIRP, then, is simply:

EIRP = Antenna Gain x Carrier Power

Here, antenna gain is the gain of the directional antenna along its boresight as compared to an isotropic antenna (dBi).

EIRP values are used extensively in satellite communication link design.

EQ. 1.4.7a EIRP




1.4.8: Power and Saturating Flux Density (PFD and SFD)

PFDPower Flux Density (Φ), in Watts/m2 or dBWatts/m2, is the radiated power per unit area. It is an important concept in antenna work, since PFD is the “field strength” that receive antennas will intercept and process.

The formula relating PFD to a transmitting antenna is:

Φ = G Pin / 4π d2

where d is the slant range to the transmitting antenna.

We have seen the expression G.Pin before: it is the EIRP. Therefore, we can rewrite the formula for PFD as:

Φ = EIRP / 4π d2 EQ. 1.4.8 Power Flux Density





SFDA satellite receive antenna intercepts an electromagnetic field transmitted from an Earth Station. The PFD of the field at the satellite will be given by the formula on the previous slide, minus the path loss.

The signal is amplified by the gain of the satellite antenna, then passed through other circuitry, and delivered as drive power to an amplifier for gain and subsequent retransmission to Earth.

With any amplifier, there is a point at which further increases in drive power will not result in further increases in output power. Regardless of drive, the amplifier has reach the limit of its output power range.

This point is known as “saturation”.





SFDGenerally, most satellites are considered fixed-gain devices. To get more power on the downlink, we must hit the satellite with more power from the uplink. In other words, we are “driving” the amplifiers on the satellite from the Earth Station.

Satellite link designers are, consequently, very interested in knowing where satellite amplifier saturation occurs with respect to Earth Station transmit power.

The first step in this back-tracking process is to know the PFD directly in front of the satellite antenna that will result in amplifier saturation. This PFD value is know as the Saturating Flux Density (SFD).

SFD is, then, a simply a PFD value of particular interest to link designers.




1.4.9: Bandwidth (BW)

BandwidthThere are several important—and different—uses for the term bandwidth in satellite communication.

Carrier bandwidth is typically defined at the “3 dB down” points, i.e. the frequency range between the points on each skirt of the carrier where the level is 3 dB lower than the peak carrier level.

3dB

Pow

er

Frequency f1 f2

BW

BW = f2 - f1





BandwidthThe Information Bandwidth is simply the bit rate presented to the user. A communication link carrying the user’s data traffic at a rate of 64 kbps is considered by its users to have a bandwidth of 64 kbps.

Technically, this isn’t really a “bandwidth” at all, since it represents a rate rather than a continuous span of frequencies. This distinction is understood by communication carriers, but users often use the terms “bandwidth” and “data rate” synonymously.

It is the task of the satellite service provider to take this “information bandwidth”—the data rate—modulate it onto a carrier and send it over the satellite link. This process gives rise to three more bandwidth terms: symbol bandwidth, occupied bandwidth, and allocated bandwidth.

Not all operators use these terms in the same way.





BandwidthThe Symbol Rate Bandwidth is the actual frequency span of a carrier on the satellite, again usually defined at the 3 dB down points. The occupied bandwidth is the result of taking the baseband data stream, modulating it at a certain symbol rate (where a “symbol” can represent one or more bits), applying Forward Error Correction (FEC) coding, and transmitting it to the satellite.

Note that some satellite providers would refer to this as the Occupied Bandwidth.

The Allocated Bandwidth (Part I) is the frequency span that the satellite provider must actually reserve for the carrier. The Allocated Bandwidth is usually larger than the Symbol Rate Bandwidth by a factor of 1.2 to 1.5, thus allowing for sufficient carrier spacing.





BandwidthTo confuse matters, some providers use the term Occupied Bandwidth to refer to Allocated Bandwidth, as defined so far, rather than to the Symbol Rate Bandwidth. They would prefer to reserve the term Allocated Bandwidth to capture one further distinction.

Allocated Bandwidth (Part II): For these providers, the Allocated Bandwidth is the Occupied Bandwidth plus some additional spacing added for reasons of internal logistics.

Many providers, for instance, parcel out transponder bandwidth in specified minimum frequency step sizes. If the Occupied Bandwidth falls short of a frequency step, the bandwidth allocated to the customer will be rounded up to that frequency step.

In either case, it is always the allocated bandwidth that is actually sold to the customer.





BandwidthEvery receiver has a bandwidth over which it will accept and process information: the Receiver Bandwidth. Ideally, we would like the receiver to limit its bandwidth to that required by the signal. That way, all of the signal, but little additional noise, would make their way into the receiver.

This is usually not possible, however, and the wider the receiver’s bandwidth the more noise power it will “let in” along with the desired signal.

Noise Bandwidth is the idealized equivalent response curve of any device, such as a filter or a receiver, across which noise is to be considered.

By “idealized” we mean that the bandwidth shape is perfectly rectangular, its height normalized to the actual response of the device in question.





Bandwidth

For a filter response curve like this, for instance . . .

Noi

se P

ower

Frequency f1 f2

BW

BW = f2 - f1

. . . the equivalent noise bandwidth might look like this.

This is sometimes called a “brick wall” bandwidth.





Bandwidth

The idea here is that the area contained by the rectangle is equal to the area under the actual response curve of the device.

Consequently, for a given input noise density (No), this equivalent noise bandwidth (B) would produce at its output the same total noise power (N) as would the actual device in question.

It is this noise bandwidth that is to be used in the formula we have already seen, N = kTB, for total noise power.

In practice, for a symmetrical, flat filter, the noise bandwidth is usually about as wide as the 4 to 5 dB down points of the filter response curve.




1.4.10: Bit Error Rate (BER)

BERBit Error Rate is used as a measurement of the quality of a digital transmission system. It is the rate at which bit errors occur.

If the Bit Error Rate of a system is measured as 1 x 10–7, this means that, on average, for every 10,000,000 bits transmitted, one will be in error.

BER is often the performance parameter that a customer will specify, and engineers will design their links “around” this value.

For given coding rates, BER can be related directly to Eb/No, and therefore to C/N.

Date post:	22-Jan-2018
Category:	Engineering
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