WELCOME

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Today’s Topic

Satellite Communication

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Communication Medium

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Open wire Twisted Pair UTP / STPCoaxial Thicknet / ThinnetFiber OpticMicrowave Terrestrial TransmissionMicrowave Satellite Transmission

TRANSMISSION Terrestrial & Satellite

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ECHO SYNCOM

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Frequency Bands

The up-link is a highly directional, point topoint link The down-link can have a footprint providing coverage for a substantial area "spot beam“.

Band Up link(GHz)

D link (GHz)

Issues

C 6 4 Interference With Ground Links

Ku 14 11 Attenuation Due to Rain

Ka 30 20 High Equipment Cost

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Elevation Angle:

The angle of the horizontal of the earth surface to the center line of the satellite transmission beam.

Factors in Satellite Communication

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Satellite Transponders

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ANALOG•One program per channel / transponder

•Comparatively noisy

•Lower quality with respect to VCD, DVD digital media

•Fixed reception

•Limited coverage

DIGITAL•More programs per channel / Transponder i.e. spectrum efficient•Noise-Free Reception•CD quality sound & better than DVD quality picture•Reduced transmission power•Flexibility in service planning -quality / Bandwidth trade off•Terrestrial free network

Satellite transmissionAnalog Vs Digital

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SIGNALS -Analog Vs DigitalANALOG DIGITAL

SIGNAL Continuous Discrete

REPRESENTATION Continuous Range of Values

Discontinuous Range of Values

TRANSMISSION Not Noise Immune Noise Immune

MEMORY Stored in the Form of Waves

Binary Bits

ENCRYPTION Not easy Easy

LONG DISTANCETRANSMISSION

Require amplifiers, and each amplifier adds distortion and noise

digital amplifiers regenerate an exact signal

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Block schematic of a transponder

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Introduction:

A satellite link is defined as an Earth station - satellite - Earth station connection.

The Earth station - satellite segment is called the uplink and the satellite - Earth station segment is called the downlink.

The Earth station design consists of the Transmission Link Design, or Link Budget, and the Transmission System Design.

The Link Budget establishes the resources needed for a given service to achieve the performance objectives.

EARTH STATION

Earth Station is a uplink center from which the signals are fed to Satellite for distribution in a specified area covered by the Satellite.

The signal is up-linked from the earth station and received by many down link centers in TV broad casting.

It is a very important part of satellite communication system for broadcasting of signals.

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1) PDA (Parabolic Dish Antenna)2) FEED3) LNA / LNBC4) Wave Guide / Low Loss Cable5) HPA (TWTA, SSPA, Klystrons)6) Up converter7) Modulator8) Encoder9) Multiplexer10) IRD (Integrated Receiver Decoder)

Major Components of Digital Earth Station

Digital Earth Station

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Link Bidget

• Earth Station Uplink • Earth Station Downlink

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The satellite link is composed primarily of three segments: (i) the transmitting Earth station and the uplink media; (ii) the satellite; and (iii) the downlink media and the receiving Earth station.

The carrier level received at the end of the link is a straightforward addition of the losses and gains in the path between transmitting and receiving Earth stations.

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DVB - Digital Video BroadcastingDigital Video Broadcasting (DVB) is being adopted as the standard for digital television

Main forms of DVB

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Carrier to Noise Ratio

The basic carrier-to-noise relationship in a system establishes the transmission performance of the RF portion of the system, and is defined by the receive carrier power level compared to the noise at the receiver input. For example, the downlink thermal carrier-to-noise ratio is:

C/N = C -10log(kTB)

Where:C = Received power in dBWk = Boltzman constant, 1.38*10-23 W/°K/HzB = Noise Bandwidth (or Occupied Bandwidth) in HzT = Absolute temperature of the receiving system in °K

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Transmission LossesGenerally consist of four components:

L = Lo + Latm + Lrain + Ltrack (5)

Where:Lo = free Space LossLatm = atmospheric lossesLrain = attenuation due to rain effectsLtrack = losses due to antenna tracking errors

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ENCODER

The video/audio encoding system is fully complaint to DVB/MPEG-standards.

One encoder is used for only one video/audio channel.

For more channels more encoders are used.

The output of the encoders are fed to multiplexer unit for multiple programs

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MULTIPLEXER

The outputs of all the encoders are fed to the Transport

stream multiplexer.

Multiplexed data from encodes produces a single bit

stream. Supports insertion of conditioner access.

It is configured by network management system to enable

the efficient use of transmission band width

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Up-converters

The up-conversion is required to raise the frequency of the

signal in desired band: C-band, Extended C-band or Ku-

band before transmission.

The input to up converter is 70 MHz (output of modulator)

and output of Up-converter is fed to HPA.

The up-conversion may done in stages or in one stage

directly. For example the 70 MHz signal is first converted

into L –band and then L band signal raised to desired

frequency band.

Normally L-band monitoring point is also provided in Up-

converters for monitoring purposes.

RF Stage in Communication

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High Power Amplifier

The high power amplifier is used for the final power amplification of the digital RF signal in C-band/ Ku band that is fed to the antenna.

The important parameters of HPAs are:

1. Frequency range

2. Output power at flange

3. Bandwidth

4. Gain variation (1.0 db (max.) for 40 MHz (narrow band)

5. 2.50 db for full bandwidth)

RF Stage in Communication……

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High Power Amplifier

6. Gain (at rated output)

7. RF output power

8. Gain stability: (+/- 0.25 dB per 24 hrs. (max.) at constant

drive, prime power and temperature. And +/- 1.0 dB max.

over operating temperature range)

9. Input VSWR: 1.3: 1 max, Output VSWR: 1.3: 1 max

RF Stage in Communication…….

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High Power Amplifier

The different types of HPAs are:

1.KHPA - Klystron High Power Amplifier

2.TWTA -Traveling Wave Tube Amplifier

3.SSPA- Solid state Power Amplifier

RF Stage in Communication……

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klystronAn evacuated electron-beam tube in which an initial velocity modulation imparted to electrons in the beam results subsequently in density modulation of the beam. A klystron is used either as an amplifier in the microwave region or as an oscillator.

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An electron gun to form and accelerate a beam of electrons

A focusing magnet to focus the beam of electrons through the cavities

Microwave cavities where the electron beam power is converted to

microwave power

A collector to collect the electron beam after the microwave power has been

generated

A microwave input where the microwave signal to be amplified is introduced

into the klystron

A microwave output where the amplified microwave power is taken out

Klystron……

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TWTA

The travelling wave tube is a form of thermionic valve

Used for wideband RF amplifier designs.

Used in broadcasting, radar and in satellite transponders.

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Cross section of twta

1 Electron Gun2 RF input3 Magnet4 Attenuator

5 Helix6 R F Out Put7 Vacuum Tube8 Collector

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Solid State Power Amplifier

The basic functionality of the amplifier is the same in a high power solid state amplifier.

Multiple devices are combined into power modules.

They are combined to develop the needed gain and power.

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Electromagnetic waves

Radio signals are a form of electromagnetic wave

Electromagnetic waves have both electric and magnetic components that are inseparable.

The planes of these fields are at right angles to one another and to the direction of motion of the wave.

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PolarizatioN The polarization is effectively the plane in which the electric wave vibrates.

Vertical and horizontal are the simplest forms of antenna polarization

They are known as linear polarisation .

Circular polarisation

It can be imagined that a signal propagating from an RF antenna is rotating.

Circular polarisation can be either right or left handed dependent upon the

the direction of rotation of electric vector as seen from the transmitter.

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An other form of polarisation is known as elliptical polarisation .

It occurs when there is a mix of linear and circular polarisation .

This can be visualised as the tip of the electric field vector tracing out an

elliptically shaped corkscrew.

PolarizatioN…..

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Wave guide

Waveguides are basically a device for transporting electromagnetic energy.

Typically, waveguides are hollow rectangular or circular metal tubes.

They are capable of directing power precisely to where it is needed.

Can handle large amounts of power and function as a high-pass filter.

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Circular waveguide:

This is less common than rectangular waveguide.

They have many similarities in their basic approach.

Types of RF waveguide……

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Wave guide…..

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Antenna directivity and gain

RF antennas or aerials do not radiate equally in all directions.

The actual pattern is dependent upon the type of antenna design.

Also depends on size, the environment and a variety of other factors.

The gain can be defined as a ratio of the signal transmitted in the " maximum " direction to that of a standard or reference antenna.

This may sometimes be called the " forward gain " .

The figure that is obtained is then normally expressed in decibels (dB)

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Parabolic Reflector

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Parabolic reflector antenna

Advantages :

High gain High directivity

Disadvantages

Requires reflector and drive element

Cost Size

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