VSAT Installation and Maintenance Training_Version_1

VSAT INSTALLATION and MAINTENANCE Organized By SKANNET Trainer: Ajuyah, Silvanus December 9th, 10th and 14th 2009

Introduction The VSAT Field Installation and Maintenance Training Course, is an Intensive Structured Program designed to ensure that Installers of bi-directional Satellite Earth Station acquire an understanding of the operational technology used in SCPC-SCPC, SCPC-DVB and SCPC-TDMA VSAT Industry. The goals are to enable the participants achieve a better understanding of the range of equipment and systems in use, and realize how they can relate to the various technical job responsibilities. The training will offer both theoretical and practical skills transfer as applicable to VSAT Field Engineers, Support Engineers and Operational Managers VSAT Installation and Maintenance Training (SKANNET)

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Training Objectives Through a series of intensive structured lecture and hands-on training the course aims to: • Provide an in-depth treatment of basic concepts relating to Satellite communication • Provide in-depth understanding of VSAT Installation and Site Survey • Provide in-depth understanding of how to use Test Equipment (Inclinometer, Compass, GPS and Spectrum Analyzer) • Provide in-depth understanding of Mechanical VSAT Assembly, Satellite Tracking and Antenna Alignment VSAT Installation and Maintenance Training (SKANNET)

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Training Objectives • Provide how to perform Peak and Poll and Commission a Full VSAT Installation • Provide an in-depth understanding of fault diagnostics • Provide specific Preventive Maintenance Procedures and Documentation • Provide Communication tips on working with Network Operation Centres (NOC) VSAT Installation and Maintenance Training (SKANNET)

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Who Should Attend • All those requiring technical understanding of VSAT System • Support Engineers • Freelance VSAT Installers • Practicing Engineers, Information Technologists, as well as managers, users and those concern about the impact of VSAT VSAT Installation and Maintenance Training (SKANNET)

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Basic Principles of Satellite Communication

What is a Satellite? • A satellite is any object that orbits or revolves around another object. For example, the Moon is a satellite of Earth, and Earth is a satellite of the Sun. • Communication satellites act as relay stations in space. People use them to transmit messages from one part of the globe to another. These messages can voice, data or video VSAT Installation and Maintenance Training (SKANNET)

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Types of Satellite Orbits • •

Before discussing satellite orbits in more general terms, it is important to understand the natural laws that control the movement of satellites. These are based on Kepler’s Laws and state that:

1. The orbital plane of any Earth satellite must bisect the Earth centrally. 2. The Earth must be at the center of any orbit. • •

There are basically three orbits: polar, equatorial, and inclined. The shape of the orbit is limited to circular and elliptical. VSAT Installation and Maintenance Training (SKANNET)

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Types of Satellite Orbits

VSAT Installation and Maintenance Training (SKANNET)

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Low Earth orbit (LEO) •

When a satellite circles close to Earth we say it's in Low Earth Orbit (LEO). Satellites in LEO are just 200 -500 miles (320 -800 kilometers) high. Because they orbit so close to Earth, they must travel very fast so gravity won't pull them back into the atmosphere. Satellites in LEO speed along at 17,000 miles per hour (27,359 kilometers per hour)! They can circle Earth in about 90 minutes.


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Polar Orbit • A Polar orbit is a particular type of Low Earth Orbit. The only difference is that a satellite in polar orbit travels a north-south direction, rather than the more common east-west direction. VSAT Installation and Maintenance Training (SKANNET)

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Why use a Polar Orbit • Polar orbits are useful for viewing the planet's surface. As a satellite orbits in a north-south direction, Earth spins beneath it in an east-west direction. As a result, a satellite in polar orbit can eventually scan the entire surface. For this reason, satellites that monitor the global environment, like remote sensing satellites and certain weather satellites, are almost always in polar orbit. No other orbit gives such thorough coverage of Earth. VSAT Installation and Maintenance Training (SKANNET)

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Geosynchronous Equatorial Orbit •

A satellite in geosynchronous equatorial orbit (GEO) is located directly above the equator, exactly 22,300 miles out in space. At that distance, it takes the satellite a full 24 hours to circle the planet. Since it takes Earth 24 hours to spin on in its axis, the satellite and Earth move together. So, a satellite in GEO always stays directly over the same spot on Earth. (A geosynchronous orbit can also be called a GeoSTATIONARYOrbit.)


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The Orbit of a Geosynchronous Satellite


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GEO Footprints •

Because they're so far away, GEO satellites have a very broad view of Earth. For instance, the footprint of one satellite covers almost all of North America.

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And, since they stay over the same spot on Earth, we always know where GEO satellites are. If our antenna points in the right direction, we'll always have direct contact with the satellite.


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Satellite Anatomy • A satellite is a complex machine. All satellites are made up of several subsystems that work together as one large system to help the satellite achieve its mission. This simplified illustration shows the key parts of a remote-sensing satellite. The main subsystems are grouped by color. VSAT Installation and Maintenance Training (SKANNET)

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Satellite Anatomy


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Satellite Anatomy


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Satellite Stabilization • Stabilization of the satellite is necessary because the Earth is not truly spherical. The Earth’s tidal motion, the Moon and the Sun have gravitational effects on the satellite, which tends to make it drift from its correct position. • An orbit that is inclined towards the equatorial plane produces a sinusoidal variation in longitude, seen from Earth as motion around an ellipse once every 24 hours. • Incorrect velocity results in incorrect altitude and a drift to the east or to the west. VSAT Installation and Maintenance Training (SKANNET)

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Satellite Position •

The satellite must be maintained in position for its required lifetime (typically 10 to 15 years). This positioning is regularly corrected to within ±0.10°.

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To extend the life of the satellites, less frequent corrections may be made. For example, keeping the satellite in its current North-South position is particularly demanding on satellite fuel reserves.

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If the North-South positioning is left unchecked, the satellite will tend to move to a natural position (Inclination) of 15°away from the geostationary orbit.

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INTELSAT allows some of its satellites to increase inclination up to about ± 3 degrees, which extends the operational life up to 3 years or more. These satellites are said to be in "inclined orbit".


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Satellite Payloads •

The payloads on communications satellites are effectively just repeaters.

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They receive the signals that are transmitted to them and then retransmit them at a different frequency back to earth.

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They receive the signals and then sometimes demodulate them to access the data, the data can then be processed before being modulated and retransmitted. The data can be stored for later retransmission or modulated using a different method, even at a different data rate. VSAT Installation and Maintenance Training (SKANNET)

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Satellite Transponders • The basic building block of any satellite communications package is the transponder. This device receives the uplink carriers, amplifies them, converts them to the correct downlink frequency band, and then transmits them, via a highpowered amplifier, back to Earth. • Today satellite can carry up to 30 transponders. VSAT Installation and Maintenance Training (SKANNET)

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VSAT Architecture


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VSAT Architecture • Very Small Aperture Terminal (VSAT) is a satellite-based telecommunications technology. • There are three components in a VSAT network: The master earth station (Teleport or Hub), The remote earth station and The satellite VSAT Installation and Maintenance Training (SKANNET)

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VSAT Network Architecture


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The Master Earth Station (Teleport) •

The first is called the Teleport. They are the ‘intermodal hubs’ of the broadband and broadcast world. It is a gateway that connect satellite circuits with terrestial fiber optics

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The Teleports are facilities located throughout the world, built for the purpose of maintaining high quality communications with orbiting satellites.

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Configuration, monitoring, and management of the VSAT network are done at this location. The master earth station has a large dish (6 m or bigger), fully redundant electronics, a self-contained backup power system, and a regulated air conditioning system.

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In addition, the master earth station is manned 24x7 throughout the year.


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The Remote Earth Station •

The Second is called the Remote Earth Station. The remote (VSAT Terminal) is comprised of the hardware installed at the customer’s premises, including the outdoor unit (ODU), the indoor unit (IDU), and the inter-facility link (IFL).

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The size depends on the data to be transmitted and its location. It can handle data, voice and video signals.

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The ODU consists of a standard VSAT dish antenna, a solid state power amplifier (SSPA) or Block Up Converter (BUC), a low noise amplifier (LNA) or Low Noise Block Converter (LNB), and a Feedhorn.

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The IDU provides this interface in the form of a modem and a router; which houses the communications electronics, including interface with the customer’s equipment such as computers

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The IFL consists of coaxial cables that connect the ODU to the IDU. VSAT Installation and Maintenance Training (SKANNET)

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The Satellite • The third component of a VSAT network is the satellite itself. All signals sent between the VSAT earth stations are beamed through the satellite. • The VSAT uses a geostationary (GEO) satellite which is orbiting at 36,000 km above the ground.


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VSAT ODU •

The outdoor unit (ODU) is mounted on the feed arm in front of the antenna and houses the Radio Frequency (RF) equipment required to transmit (TX) and receive (RX) from the antenna.

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The outdoor unit (ODU) mainly consists of these devices:

1. Low Noise Block (LNB) which is a down converter and receiver 2. Block Up Converter (BUC) this is the up converter and transmitter 3. Ortho-Mode Transducer (OMT) the Tx and Rx waveguide joint. 4. Microwave filters which protect the LNB from the Tx signals.


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VSAT IDU • The indoor unit (IDU) usually consists of a single box (normally referred to as a Modem) which should be located in a dry, cool and clean place. An office environment is ideal. • The IDU requires a stable mains supply and connection to the end user equipment. This could be further units for telephone exchanges or networks for internet or intranet connections. VSAT Installation and Maintenance Training (SKANNET)

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A Typical VSAT Terminal

Block Diagram of a Typical VSAT Terminal VSAT Installation and Maintenance Training (SKANNET)

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Basic Satellite Antenna Theory •

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A standard satellite dish antenna works by concentrating signals, that are picked up along its axis, to a single point. This point is called the focal point. The receiving amplifier is usually placed near the focal point and the concentrated signals are collected into the receiver using a small horn. This serves to further concentrate the signals to get the maximum possible signal level at the amplifier input. VSAT Installation and Maintenance Training (SKANNET)

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Basic Satellite Antenna Theory •

The focal point can be offset from the main axis so that the receiving equipment does not obstruct the beam in any way.

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This offset has the effect of raising the beam of the antenna. The exact amount that the beam is raised is equal to the amount of offset.

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Thus, if the offset is 20 degrees then the beam is raised by 20 degrees. VSAT Installation and Maintenance Training (SKANNET)

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Types of Satellite Antenna


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Basic RF Theory and Operating Frequency Range

The Electromagnetic Spectrum • The entire electromagnetic spectrum is composed of electromagnetic radiation with a wide range of energies and wavelengths: radio waves, microwaves, infrared light, visible light, ultraviolet light, x-rays, and gamma rays. • Radio frequency (RF) is a term given to the portion of the electromagnetic spectrum with long wavelengths and low frequencies. This includes radio waves and microwaves ranging from a few hertz (Hz)—a measurement of frequency in cycles per second—to about 300 billion hertz (or 300 gigahertz [GHz]). VSAT Installation and Maintenance Training (SKANNET)

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The Electromagnetic Spectrum

The Electromagnetic Spectrum VSAT Installation and Maintenance Training (SKANNET)

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Electromagnetic Fields and Waves • Electromagnetic radiation, also known as Electromagnetic wave, is a propagating wave in space with electrical and magnetic components. The electrical and the magnetic components oscillate perpendicular to each other and to the direction of the propagation.


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Characteristics of Electromagnetic Waves •

Some of the characteristics of electromagnetic waves are described below:

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E and H Fields: Electromagnetic forces act between electric charges and electric currents. For every point in space, an electromagnetic field (the force felt by a charge or current at that very point) can be defined and measured. The electric field E describes the force between charges. The magnetic field H describes the forces between currents.


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Characteristics of Electromagnetic Waves .

2 Carrier Medium: One very important quality of electromagnetic waves is that they do not need any carrier medium. There is no air or ether needed to propagate them electromagnetic waves, unlike sound, air pressure waves that propagates, that needs a carrier. Examples of electromagnetic waves are light, Xrays, microwaves and other radio waves.


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Characteristics of Electromagnetic Waves 3. Wavelength and Frequency: An electromagnetic wave, like any wave, has by its basic shape of a sinus, troughs and crests. The wavelength is the distance between two crests (or 2 troughs) and is measured in meter. The wavelength is represented by the Greek letter l(lambda). •

The frequency of a wave is its rate of oscillation and is measured in hertz (Hz). The SU unit of frequency is the 1 oscillation per second (1/s)


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Characteristics of Electromagnetic Waves

Characteristics of an Electromagnetic Wave


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Characteristics of Electromagnetic Waves • The frequency and the wavelength of a wave has the following relation: c=l*f whereas: c = the speed of light [m/s] (3x108 m/s = 300,000 km/s) l = wavelength [m] f = frequency* [1/s] (*) the frequency is also called n (Nu) VSAT Installation and Maintenance Training (SKANNET)

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Signal Polarization •

Polarization is determined by the orientation of the electric and magnetic fields radiating from the transmitting antenna.

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In satellite communications, the term Polarity refers to a physical property of the transmission of the satellite to/from earth. Since it costs a lot of money to send a satellite up to space, and since the satellite has a limited lifespan, the satellite company wishes to use as much of the frequency band as possible.


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Signal Polarization • One of the ways to do it is to use the frequency range twice, with the transmissions separated by 90 degrees from each other, so that they do not cause interference to one another. • This is called a Linear polarization. • Circular polarization uses the direction of a helix around the direction of the electric field vector. VSAT Installation and Maintenance Training (SKANNET)

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Signal Polarization • Digital signals are transmitted from the satellites on either Vertical (V) or Horizontal (H) polarity for linear feeds, or on Right (R) and Left (L) polarity for circular feeds. • Standard big dishes are most likely to have feed horn that can receive linear (H/V) polarity.


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Linear Polarization • Linear Polarization allows the to use the frequency range twice, with the transmissions separated by 90 degrees from each other, so that they do not cause interference to one another. • This can be either Cross-Pol or Co-Pol VSAT Installation and Maintenance Training (SKANNET)

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Cross-Polarization (X-Pol) •

The term Cross-Pol refers to a linear polarization or feed, in which the receive (Rx) and transmit (Tx) polarizations are in 90 degrees to each other, forming the shape of a cross, or X (this is also called X-Pol).

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This happens when the customer is working on a transponder (or a combination of transponders), that results in this setting. In order to both receive and transmit, the customer will have to have a CrossPol feed (see antenna parts).


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Co-Polarization (Co-Pol) •

The term Cross-Pol refers to a linear polarization or feed, in which the receive (Rx) and transmit (Tx) polarizations are parallel to each other.

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In order to both receive and transmit, the customer will have to have a Co-Pol feed


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Circular Polarization • A circular polarization comes in two flavors: LHCP (Left Hand Circular polarization) and RHCP (Right Hand Circular Polarization). • It is different from linear polarizations, and in order to work, requires a circular feed. The feed can be converted from LHCP to RHCP and viceversa VSAT Installation and Maintenance Training (SKANNET)

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Satellite Operating Frequency Ranges • There are 4 frequency ranges which we use in satellite communications: C-Band, Ku-Band, L-Band, and IF. • Some are used to communicate from the ground to the satellite, and some are used inside the teleport equipment


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C-Band Frequency • C-Band is used in communication from the earth station to the satellite and was the first frequency band allocated for commercial ground-tosatellite communications. • A typical C band satellite uses 3.7–4.2 GHz for downlink and 5.925–6.425 GHz for uplink. Slight variations of C band frequencies are approved for use in various parts of the world: VSAT Installation and Maintenance Training (SKANNET)

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C-Band Frequency


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Ku-Band Frequency • Ku-Band is used in communication from the earth station to the satellite. The range of frequencies is between 11 and 14 gigahertz, used increasingly by communication satellites. • Requires smaller ground antennas, but is more susceptible to rain fade (when raining, receive quality drops dramatically). VSAT Installation and Maintenance Training (SKANNET)

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C-Band Versus Ku-Band


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L-Band Frequency • L-Band is the frequency band between 950 - 2150 MHz. • It is used for communication between earth station components (modems, receivers, up and down converters).


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Intermediate Frequency (IF) • IF is the frequency band between 50 - 180 MHz. It is used for communication between earth station components (modems and transceivers).


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Uplink and Downlink Frequencies • In general, the term “Uplink” refers to the transmission from the ground to the satellite. The term “Downlink” refers to the transmission from the satellite to the ground. • The satellite, by design, receives a transmission from the ground, as long as the earth station transmitting is inside the satellite’s footprint. • The satellite bounces that transmission back down to the surface, where it is received by another earth station, assuming it also is inside the footprint (note the transmission is going out to the ENTIRE footprint, not a specific station). VSAT Installation and Maintenance Training (SKANNET)

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Uplink and Downlink Frequencies • However, the satellite cannot transmit the carrier back on the same frequency on which it was received, or it will cause interference. • The satellite therefore must convert the frequency on which is received the transmission (the UPLINK frequency) to a new frequency on which to transmit the downlink signal (the DOWNLINK frequency). VSAT Installation and Maintenance Training (SKANNET)

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Uplink and Downlink Frequencies • The conversion between the downlink and uplink frequencies is constant, so that each uplink frequency has a corresponding downlink frequency. • Thus, each frequency range is divided into an Uplink range and a downlink range that do not overlap. VSAT Installation and Maintenance Training (SKANNET)

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Uplink and Downlink Frequencies • In C-band, the conversion is done by subtracting 2225MHz from the uplink frequency so that: UF – Uplink frequency (in MHz) DF – Downlink frequency (in MHz) UF – 2225Mhz = DF • In Ku-band, the conversion is done in the same way, but the difference between the uplink and downlink frequencies is 1745MHz: UF – 1745Mhz = DF VSAT Installation and Maintenance Training (SKANNET)

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Frequency Conversions • There are two frequency bands (C-band and Kuband) which are used when communicating with the satellite from the ground, and there are two frequency bands (L-band and IF) which are used on the modems and receivers that we use in the earth stations. • The conversion between these two frequency ranges is done by specialized equipment, and is different for each frequency band. In this segment we will cover the various conversions that may take place VSAT Installation and Maintenance Training (SKANNET)

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Frequency Conversions • Local oscillators The term “Local Oscillator” (LO) refers to the amount subtracted or added to a frequency in order to convert it from one frequency band to another. The LO is a property of the equipment performing the conversion (LNB, BUC, Transceiver, etc.), and is sometimes set by the equipment type, and sometimes configurable on the device itself. VSAT Installation and Maintenance Training (SKANNET)

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Frequency Conversion • L-band Conversions to and from L-band (whether to C-band or Ku-band), are done through devices called LNB and BUC. They are separate devices, and their LO is constant and not configurable. Modems that work in the L-band frequency band require them to work. VSAT Installation and Maintenance Training (SKANNET)

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Frequency Conversion • BUC BUC stands for “Block Up Converter”. It is a device that converts from L-band to Cband/Ku-band, and therefore is responsible for the uplink (Tx) from the earth station to the satellite.


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Frequency Conversion •

BUC The BUC performs 2 functions:

1. Conversion of the signal from L-band to the appropriate frequency range 2. Amplification of the signal in order to transmit it to the satellite. VSAT Installation and Maintenance Training (SKANNET)

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Frequency Conversion • In C-band, there are two possible LOs for a BUC – 4900MHz or 7375MHz. Each performs the conversion differently: • In 4900MHz BUCs, the incoming L-band frequency from the modem is simply added to the LO, and the result (In C-band now – uplink segment), is outputted to the Antenna. MF – L-band frequency from the modem (In MHz) UF – Uplink frequency (In MHz) MF + 4900 = UF


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Frequency Conversion • In BUCs with a local Oscillator or 7375Mhz, however, the conversion is done by subtracting the incoming L-band frequency from the LO, and outputting the result: 7375 – MF = UF • If we want to calculate which frequency to put on the modem given an uplink frequency, you just have to isolate the MF variable: MF(4900) = UF – 4900 MF(7375) = 7375 – UF • In Ku-band, conversion is done in the same way, but the normal LOs are 13000MHz and 13050 MHz VSAT Installation and Maintenance Training (SKANNET)

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Frequency Conversion • Amplification – Apart from conversion, the BUC also performs amplification of the signal in order to ensure it is strong enough to be received at the satellite. The amount of amplification depends on the BUC output. Models range from 2 watt, through 5 and 10 watt, up to hundreds of watts. The minimum output needed is determined by the satellite, frequency range, size of dish, geographic location, etc.


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Frequency Conversion • BUCs usually receive their voltage from the modem (should be about 24v), but there are models with external power supplies (usually the more robust ones). • BUCs also require a 10MHz reference carrier to work, which is supplied by the modem (in very rare cases, the reference is supplied by an external device). VSAT Installation and Maintenance Training (SKANNET)

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LNB LNB stands for “Low Noise Block”. This is the device that performs the conversion from Cband/Ku-band to L-band and is therefore responsible for the downlink (Rx) part of the link.

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The LNB performs 3 functions: Conversion of the signal to L-band Amplification of the signal Cleaning the signal from noise VSAT Installation and Maintenance Training (SKANNET)

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Frequency Conversion • Conversion – All C-band LNBs use the same LO – 515Mhz. All of them also perform the same conversion. They subtract the incoming C-band frequency from 5150, and the result is the L-band frequency they output to the modem/receiver: DF – Downlink frequency (in MHz) MF – L-band frequency to the modem (in MHz) 5150 – DF = MF • In Ku-Band, there are several types of LNBs. The most common ones have LOs of 10GHz and 11.3Ghz. Be sure to verify what LO you are working with! VSAT Installation and Maintenance Training (SKANNET)

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Frequency Conversion • Amplification – The signal received from the satellite is fairly weak. The LNB amplifies the signal logarithmically, i.e. the stronger a signal is, the more amplification it receives. This is done automatically. • Cleaning – As a byproduct of the amplification process, the LNB cleans the signal from small interference and noise, which are usually smaller than carriers (hence the name “Low Noise Block”). VSAT Installation and Maintenance Training (SKANNET)

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Frequency Conversion • IF IF is the Intermediate Frequency range (50-180MHz). Conversion to and from IF to C-band/Ku-Band is done by devices called transceivers. • Transceivers The term “Transceiver” is combination of the words Transmitter and Receiver. As implied by their name, Transceivers perform frequency conversion and amplification on both the uplink and downlink.


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Frequency Conversion • Transceivers Transceivers differ from BUCs and LNBs in that their LO is not constant, but configurable. Modems working in the IF frequency band require a transceiver to communicate with the satellite. VSAT Installation and Maintenance Training (SKANNET)

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Frequency Conversion • Transceivers The configurable LO is needed to solve a fundamental problem: The IF frequency band is only 130MHz wide, while both the C-band and Ku-Band transmit bands are much wider. The variable LO is needed in order to ensure that the entire band is covered. This is how it is done: VSAT Installation and Maintenance Training (SKANNET)

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Transceivers

1. Each IF modem has a Center frequency. This frequency is part of the modem, and is not configurable. IF modems come in two flavors – one with 70Mhz as a center frequency, and a span of +/-20 MHz around it (meaning it can work in the range of 50-90MHz), and one with 140Mhz as a center frequency, with a span of +/-40Mhz, meaning it can work at 100180MHz. VSAT Installation and Maintenance Training (SKANNET)

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Frequency Conversion • 2.

Transceivers On the Transceiver, we configure a center frequency as well. This center frequency is in C-band/Ku-Band (depending on the transceiver).

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The difference between the center frequency on the transceiver and the center frequency on the modem is the LO: CF – Center frequency CF(transceiver) – CF(modem) = LO

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In effect, this causes the two center frequencies to become “matched” to each other. VSAT Installation and Maintenance Training (SKANNET)

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Modulation Scheme and Techniques

Modulation Techniques •

In telecommunications, modulation is the process of varying a periodic waveform, i.e. a tone, in order to use that signal to convey a message.

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The aim of digital modulation is to transfer a digital bit stream over an analog bandpass channel.

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A device that performs modulation is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator (sometimes detector or demod).

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A device that can do both operations is a modem (short for "Modulate-Demodulate"). In short, modulation aims to transfer information by changing one of the carrier parameters between several states (in PSK modulation, for example, the phase of the carrier is changed). VSAT Installation and Maintenance Training (SKANNET)

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Modulation Used in Modems There are the 3 basic types of modulation used in modems: 1. FSK - Frequency Shifted Keying 2. QPSK - Quadrature Phase Shifted Keying 3. QAM - Quadrature Amplitude Modulation VSAT Installation and Maintenance Training (SKANNET)

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Modulation Technique • FSK - Frequency Shift Keying Frequency Shift Keying (or FSK) is the frequency modulation of a carrier that represents digital intelligence. For Simplex or Half Duplex operation, a single carrier is used communication can only be transmitted in one direction at a time. A Mark or 1 is represented by Freq A, and a Space or 0 is represented by Freq B. FSK is not really used in satellite communications because of the inefficient use of bandwidth and frequencies. VSAT Installation and Maintenance Training (SKANNET)

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Modulation Technique • QPSK - Quadrature Phase Shift Keying Quadrature Phase Shift Keying employs shifting the phase of the carrier plus an encoding technique. QPSK is used in almost all modems. The digital information is encoded using 4 (Quad) level differential PSK. • The data is encoded as follows: DIGIT 00 01 10 11

PHASE SHIFT +90 0 180 270 VSAT Installation and Maintenance Training (SKANNET)

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Modulation Techniques • 8PSK Any number of phases may be used to construct a PSK constellation but 8-PSK is usually the highest order PSK constellation deployed. With more than 8 phases, the error-rate becomes too high.


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QAM - Quadrature Amplitude Modulation Quadrature Amplitude Modulation refers to QPSK with Amplitude Modulation. Basically, it is a mix of phase modulation and amplitude modulation. QAM phase modulates the carrier and also modulates the amplitude of the carrier.

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Phase Modulated and Amplitude Modulated Carrier: There are two types, 8-QAM and 16-QAM: 8-QAM encodes 3 bits of data (2^3=8) and 16-QAM encodes 4 bits of data (2^4=16). 16-QAM has 12 phase angles, 4 of which have 2 amplitude values! Higher data rates use much more complex QAM methods


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Modulation Technique • In summary depending on the type of modulation, 1 or more bits can be encoded per symbol. • Therefore, the better the modulation, the less bandwidth will be required for a given data-rate. The downside is that the “higher” the modulation, the more error prone it is, and we will need better error correction and receive level in order to ensure a lossless link. • There is also a direct link between the modulation and the bandwidth of the carrier. The higher the modulation, the less bandwidth will be required to deliver the same number of bits. VSAT Installation and Maintenance Training (SKANNET)

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Forward Error Correction (FEC) •

FEC stands for “Forward Error Correction”, a system of error control for data transmission whereby the sender adds redundant data to its messages, also known as an error correction code.

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This allows the receiver to detect and correct errors (within some bound) without the need to ask the sender for additional data. The advantage of forward error correction is that a back-channel is not required, or that retransmission of data can often be avoided, at the cost of higher bandwidth requirements on average.

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FEC is therefore applied in situations where retransmissions are relatively costly or impossible. This parameter indicates which errorcorrection algorithm is used in the link. The FEC type must be identical on both the transmitting and receiving sides of the carrier is order to gain a lock.


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Forward Error Correction (FEC) •

Viterbi Viterbi FEC has been around since 1967. It is an established FEC type, supported by almost all modems, and there is intercompatibility between modem types that support it.

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Turbo Turbo is a relatively new (early 1990’s) type of error-correction algorithm. It allows much better error correction than the Viterbi algorithm. This means the receive level of carriers can be lower, which in turn leads to smaller transmissions, allowing either less investment in equipment, or working in poorer conditions (rain fade, distance from center of footprint, etc.).


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Forward Error Correction (FEC) • Code rate The code rate is the measure of how much data is added to the carrier in order to provide FEC. It is measured as a fraction (1/2, ¾, 5/6, etc.). The fraction denotes how many bits go to data out of the total, with the remainder going to FEC. For example, if we have a FEC code of ¾ active, this means that out of every 4 bits, 3 are dedicated to data, and 1 goes to error correction. 5/6 means that of every 6 bits, 5 go to data and 1 to error correction and so on. VSAT Installation and Maintenance Training (SKANNET)

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Forward Error Correction (FEC) • The code rate does not come at the expanse of the data rate! • If we wish to transmit 128Kbps with a FEC rate of ¾, than the total bits transmitted on the carrier will be 128Kbps X 4/3 = 170.66Kbps. • The data rate will be delivered, but the total number of bits will be increased to accommodate the FEC code. This of course has an effect on the bandwidth of the carrier. The higher the FEC, less bits will have to be added, and the carrier will be narrower. VSAT Installation and Maintenance Training (SKANNET)

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Forward Error Correction (FEC) • In Summary, the FEC rate also determines the amount of error correction done on the carrier. As such, the lower the FEC, the more error correction will be done, and the carrier will be less error prone, allowing us to work at lower Eb/No levels while maintaining a good BER


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Symbol Rate •

In digital communications, symbol rate, also known as baud rate or modulation rate; it is the number of symbol changes (signaling events) made to the transmission medium per second using a digitally modulated signal or a line code.

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The Symbol rate is measured in baud (Bd) or symbols/second. In the case of a line code, the symbol rate is the pulse rate in pulses/second. Each symbol can represent or convey one or several bit of data. The symbol rate is related to but should not be confused with gross bitrate expressed in bit/s.

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The symbol rate is the sum of all the configurations of the carrier and is not configurable in and of itself .It is a single number.


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