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Next, find a white wall. A small white piece of card is perhaps better for this analogy, 10cm

by 10cm placed high up in the room or on the wall... aim the torch beam at the white card.The card is our satellite.

Now comes the bit that gets hard to visualise. If you can see the light on the card, then you

can see the satellite and if your eyes are the receiver then the light is travelling from the

torch to the card and is then being reflected or bounced back and spread out over a largerarea and your eyes are receiving the light where ever you are - as long as you can see the

card.

Satellite Communications Systems

So in a satellite communications system we have a signal generator of some sort.

This is usually a modem (modulator/demodulator) which takes in digital information, suchas digital TV pictures, and modulates it (mixes) onto a signal carrier for transmission and

can also demodulate (separates) the data from a received carrier to produce the data ordigital TV picture.

Then we have a transmitter and a receiver amplifier for two way communications. If we

were only receiving we would only have the receive side of the system, this is why oursatellite TV boxes are small and affordable. It is cheaper to receive than it is to transmit.

Next we have our antenna. We point the antenna at the satellite and we can receive asignal. We can also now send our signal to the satellite if we are able to transmit.

The satellite also has an antenna, receiver, transmitter and often another antenna. The

signal it receives is at a frequency we will call 'A' but it will move this signal to frequency 'B'

before sending it back to earth. This is so that we can keep our transmit signals separatefrom our receive signals. Otherwise, our receiver may pickup our own transmit signal and

be swamped by it. The difference in these signal frequencies is known as the translationfrequency. The transmit frequency is translated into a receive frequency.

In terms of size our transmit signal is huge compared to our receive signal because it has togo a long way to the satellite.

About 36,000 Km is the average distance it has to travel for a geostationary satellite.


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This is also the reason why our receive signal is so small, because the satellite that sent it is

36,000 Km away. The receiver has to amplify this signal without amplifying the noise, in a

way our eyes do exactly that. Rain and weather generally also affects the signals. Rain, andthe atmosphere generally, can reduce the size of the signal by half, this varies depending onwhere in the world you are.

So now we can send out our signal and we can also receive someone elses.

Satellite ComponentsSatellite Payloads

A payload is the part of the satellite that performs the purpose it was put in space for. There

are many different types of satellite but communications satellites are the kind we are

interested in here. The payloads on communications satellites are effectively just repeaters.They receive the signals that are transmitted to them and then retransmit them at adifferent frequency back to earth.

Modern satellites do more than this. They receive the signals and then sometimesdemodulate them to access the data, the data can then be processed before beingmodulated and retransmitted. The data can be stored for later retransmission or modulated

using a different method, even at a different data rate.

The accompanying picture shows an anatomy of a typical satellite. You can see the uplinkreceiver chain and the downlink transmit chain. The central area shown as 'Processing', iswhere the frequency is translated or any demodulation, processing and modulation wouldtake place.

These are the basic main components of a satellite and form the payload. The reality is that

most satellites have many different antenna with separate transmit and receive chains.

These have back up and redundancy built in so that in the vent of failur all is not lost. Thegain of the on board transmitter amplifiers, configuration of which antenna they are


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connected to and even where on the earth the antenna are aimed at is all controlled from

the satellite controllers on earth. These are complex and versatile satellites and are in use

doing the job they were intended for 24 hours a day 365 days a year for many years. Theyeventually fail due to worn out parts or lack of fuel which is used to keep them stationary sowe don't have to follow them about with our dishes. They stay put because they are

manoevered by the controllers who constantly monitor and adjust the satellite.

Now that we have seen how satellites work and how the satellite terminals work we can

take a look at the applications of this technology and why it is so important.

Basic Satellite Antenna Theory

A standard satellite dish antenna works by concentrating signals, that are picked up alongits axis, to a single point. This point is called the focal point. The receiving amplifier isusually 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 themaximum possible signal level at the amplifier input.

The focal point can be offset from the main axis so that the receiving equipment does notobstruct the beam in any way. 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. Thus, if theoffset is 20 degrees then the beam is raised by 20 degrees.


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Antenna shapes and sizes vary but they all do the same thing, every antenna dish you see

including radio telescopes, which are basically receive only antennas, all perform thisfunction. they all focus the signals they receive from a specific point oin space onto a small

area where they can be received and amplified.

The principle works in exactly the same way for transmitting a signal. The transmit amplifier

produces a large signal to a horn set at the focal point. The signal is then radiated onto thedish (through the same horn used to pick up the receive signals) which in turn focuses the

transmit signal into a parallel beam of energy out into space to the satellite.

The transmit amplifier is also positioned as close to the focal point as possible to reduce

losses. The horn used to transmit and receive the signals to and from the reflector is called

a feed horn and is placed at the focal point also known as the feed point.


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Chapter 2

Satcom Systems

In any satcom system, there are some fundamental building blocks. This diagram shows

these blocks in a basic form to help youunderstand what they are.

Suppliers

Where ever the data to be transmitted

via satellite is coming from is thesupplier base. this could be anything

from an outside broadcast unit to astudio or office complex. Even banks orcommunications headquarters.

Bearers & Terrestrial Links

In order to get this data to the teleport (satellite uplink station) for onward transmission via

satellite, we use a point to point terrestrial data link sometimes known as a bearer because

is 'bears' the data or carries it. One of the most commonly used data link suppliers in theUK is BT but they are not the only supplier of such communications links. Many forms ofbearer exist and are mainly fibre optic high speed lines or lower speed kilostreams and evenpermanent leased line (telephone line) connections.

Bearers can be occassional use types such as 64k or 128k ISDN lines. Even the internet can

be used for some applications.

The end aim is to deliver error free reliable data to the teleport.

Teleports

Once the data arrives at the teleport (satellite uplink station) then it is passed to a satellite

modem or multiplexed with several other data streams before reaching the modem. The

modems then give an intermediate frequency output (I.F.) which are adjusted for level andfrequency before entering the RF system which contains the up conversion, filters and high

power amplifiers. The signal containing the suppliers data is then broadcast to the satellitethrough one or more antenna at the teleport. More than one antenna is used if the data is

required on more than one satellite to increase coverage.

Subscribers

Finally the signal is received back on earth at the subscriber terminal. This could be a large

VSAT for distribution of many data streams such as the internet or a small terminal such asa Satellite TV system or video conferencing system.


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This whole system can be two way or one way depending on the requirements.

Broadcasting is usually one way and point to point transmission is usually two way. There

are also Multicasting applications where the broadcast is sent to everyone but only thosewho are interested or requested the data can receive it.

These principles are the same for all satellite systems however big or small.

Star & Mesh Networks

A Star Network comprises of aHub satellite station which

transmits to and receives frommultiple remote subscriber sites.

This kind of network can be usedin one direction only such as

from the Hub to the subscriberand this is how the Direct-To-Home (DTH) satellite TV

systems are run.

A Mesh Network consists of

multiple Hub or mini-Hubsatellite stations all transmitting

and receiving between each

other.

This kind of network is oftenused between large

organisations such as banks and

other large corporateenterprises. This method is idealfor connecting large computer

networks at each Hub together.

Variations on these two core

network topologies are usedevery day to connect up the

world of internet, telephone,

television, banking andcountless other media andinformation systems.

Now we can revisit the

Modulation schemes touchedupon earlier and see how they work.


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Chapter 3

Anatomy of a Satcom Terminal:

A satellite communications terminal has several parts which are essential for all such

communications terminals. In other words every satcoms terminal that can transmit andreceive to and from a satellite has the following parts:

An antenna.

The antenna system comprises of a reflector, feedhorn and a mount. The size of a VSAT

antenna varies from 0.75 metres to 3.8 metres. The feedhorn is mounted on the antennaframe at its focal point by support arms. Antenna size is used to describe the ability of theantenna to amplify the signal strength.

A Feeder.

The FEEDER directs the transmitted power towards the antenna dish or collects the receivedpower from it. It consists of an array of microwave passive components.

It includes a feed horn and a circulator which is used to make sure that the transmit signals

go out through the dish and not back into the receive chain. It also makes sure that the

receive signals come from the dish into the receive chain and not into the transmit chain. Itworks much like a roundabout in principle. This is often referred to as an Orthomode

Transducer or OMT and is, these days, built into the feed assembly. It can also be apolarisation device but they all do the same job.

It also includes a Rx or receive filter which is usually a waveguide filter which tightly

controls the frequencies allowed into the receive chain. This has the effect of reducing theunwanted noise from space and prevents interference from outside of the receive band offrequencies. This is often built in to the feed assembly.

Demodulator

Modulator

Data

Interface

LNA

PA

Down

Conv.

Up

Conv.

FeederDish

To Baseband

equipment

LNB

BUC

Modem


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Similarly, there is a Tx or transmit filter which is usually a waveguide filter which tightly

controls the frequencies allowed into antenna. This has the effect of reducing the unwanted

signals from being accidentally transmitted onto the satellite and prevents interference tooutside of the transmit band of frequencies. This is often built in to the feed assembly.

LNA / LNB.

The LNA (Low Noise Amplifier) is a very good amplifier which has the job of amplifying thesmall signals picked up by the antenna without amplifying the noise. Various kinds existwhich all do the same thing, they provide enough signal level to demodulate the data from

the carrier.

The LNB is more than just an amplifier as it also handles polarisation selection and the downconversion to L-Band frequencies.

PA / BUC

The HPA (High Power Amplifier), otherwise known as a TWTA (Travelling Wave TubeAmplifier) or an SSPA (Solid State Power Amplifier), has one job. It amplifies a specific band

of frequencies by a large amount, sufficiently large to enable the antenna to beam them upto the satellite. These can range in power from a few watts upto over 1000 watts in power.The bigger the dish, usually the bigger the power amp. The largest have to be cooled usingliquid nitrogen and resemble electron microscopes. The smallest look more like a lump of

metal bolted to a small heatsink.The Block Up converter is a combination of frequency up converter and an SSPA.

Down Converters.

The down converters 'do exactly as they say on the tin', they convert signals down in

frequency. The signals arrive at the dish at anything from 10 to 40 GHz and are then filteredand amplified, they now need to be moved down the frequency spectrum so that the

equipment can be made cheaper and easier. The 1st downconverter mixes the signals withanother frequency, the result is both the sum and difference of the signals. By filtering outthe original and the sum frequencies the result is that the original frequencies are now thedifference frequencies - lower down in the frequency spectrum. An example would be the

downconversion of 10 GHz to 1 GHz which is Ku band to L Band. The 2nd downconverterthen downconverts the L Band signals to an Intermediate Frequency (IF) of around 70 MHz.this is then ready for the demodulator.

Up Converters.

The up converters 'do exactly as they say on the tin' aswell, they convert signals up infrequency. The signals are sent to the up converters at at around 70 MHz. They now need to

be moved up the frequency spectrum so that the HPA can amplify them and transmit them

through the antenna. The 1st upconverter mixes the signals with another frequency, theresult is both the sum and difference of the signals. By filtering out the original and the

difference frequencies the result is that the original frequencies are now the sum

frequencies - higher up in the frequency spectrum. An example would be the upconversionof 70 MHz to 1 GHz which is IF to L Band. The 2nd upconverter then upconverts the L Bandsignals to a Radio Frequency (RF) of around 10 GHz. this is then ready for the HPA totransmit through the antenna.


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Modulator / Demodulator. (MODEM)

Modulator / Demodulator (Modem)

These two units are often combined as one and are known as modems. Just like thecomputer modem you may have at home, these units take digital data and modulate it ontoa carrier and they demodulate the digital data from a carrier. Computer modems use audio

frequency carriers but the end result is the same.

That's it, anything else is not strictly part of the satcom system such as routers, computers,

televisions and telephones.


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Chapter 4

Antenna Pointing & Set Up:

When chosing a site for a VSAT installation, the antenna needs to be located with a

clear view to the satellite. Access to the antenna should be restricted to prevent anydamage or harm from occurring to either the antenna or people. Ideally, the antenna

should be sheltered from wind.

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

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

o Low Noise Block (LNB) which is a down converter and receivero Block Up Converter (BUC) this is the up converter and transmittero Ortho-Mode Transducer (OMT) the Tx and Rx waveguide joint.o Microwave filters which protect the LNB from the Tx signals.

Safety Is Important:

Installing a VSAT can be very dangerous especially on a roof. Follow

the instructions carefully to ensure that the antenna is correctlyassembled.

Care must be taken when working near the edge of roof tops such thatyou do not fall off or drop anything on people who may be below. Neverwork alone and always look out for each other.

Static - Permanent Mounting:

This type of installation is usually a pole mount which is bolted securely to the ground.The pole can be concreted into a hole but in both cases must be completely vertical.

Temporary Mount (Non-Penetrating):


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This is a quickly deployed mount which consists of a frameconstructed to hold the pole vertically and securely whilstproviding a large base for the addition of ballast. Ballast(heavy bricks or concrete) weigh down the mount preventingthe wind from overturning the antenna assembly. In extreme

conditions even a permanent mount can sustain damage andso a sheltered location and saftey lines attaching the mount tothe building should be considered. The antenna could

otherwise be blown of the roof of a building. A structural survey is required before anysuch mounts are installed on a roof to ensure the weight is not a risk to the building.

Tx & Rx Coax Cables:

Usually two coax cables carry the signals between the IDU and the ODU. Power for the

LNB and BUC as well as control signals are carried along these coaxes.

Ensure that the ends of the coax cables are properly terminated in F type connectors.

There should be 2mm of copper coax centre conductor protruding from the connector inorder to ensure a good connection is made.

The braided screen of the coax cable must not touch the inner copper conductor. Thiswill cause a short and damage the equipment.

Connect the coaxes to the ODU noting which are Tx and Rx then run them to the IDU.

The connectors at the ODU should be sealed against the weather with the cablesdropping down from the connectors before being taped up to the feed arm. This allowswater to harmlessly drip from the cable rather than run into the connector, in otherwords the connectors should be higher than the cables.

The indoor unit (IDU):

The indoor unit (IDU) usually consists of a single box which should be located in a dry,

cool and clean place. An office environment is ideal.

The coax cables run from the antenna ODU to the IDU and should not cause a hazardto anyone along the way.

Use cable ties to clip them safely to pipes, fencing or similar in order to make theinstallation neat, tidy and safe.


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signal to the analyser as this can cause a lot of damage to an already

expensive peice of equipment. The cable from the antenna may have DC onit and so you must first be sure that it is safe before plugging into an

analyser.

You will need a supply of DC voltage and possibly a 22kHz tone. Thereceiver can provide these up the coax cable. An RF coupler with a DC block

on one side is invaluable and is connected inbetween the receiver and thedish. The DC block side is to be connected to the analyser.

The analyser can display the signals being received in a particular band of

frequencies. Recognising these signals is key to finding your desired

satellite. Again, it helps if you already know the location of a good referencesatellite and can clearly see the signals being received from it. In my

experience satellites are found mcuh more quickly with a spectrum analysertuned to look at a known signal for a known satellite than by using a signalmeter.

You may not recognise the signals but if you are receiving signals then youcan peak up on them to get the maximum signal strength and the biggest

signals. then you can see if the receiver is getting what you expected.

By Using a Satellite TV receiver:

The TV receiver (for example a Sky DigiBox) can provide you with signal

quality and level information. This is displayed as bar graphs on the TV. Not

so useful if you are up a ladder but with the help of someone else you can betold if the signal levels are getting better or worse. If you can see the TV byputting it outside then even better. I have used an RF wireless portable TV

attached to the Rf out of a Skybox to be able to see the screen when up theladder and this was very useful.

Peaking up:

Start with the azimuth and move the dish from one side of the satellitethrough the main signal and out the other side untill you lose the signal.

Peaking th esignal simply means finding the maximum signal strength whichis known as the boresight of the satellite. Pointing directly at it. The Azimuthis only the first part however and so on to the next.

Elevation is done in the same way and there may be no actual increase if by

chance you already have the antenna on boresight.


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Antenna Polarisation Setup:

(Not necessarily required for some TV satellite dishes such as Skymini-dishes)Before alignment of the antenna with the satellite, the polarisation should besetup correctly. The polarisation is the axial rotation of the feed system(Feed horn/LNB/BUC) on the antenna.

Polarisation Setup - Step 1:

Loosen the feed assembly slightly to allow slight rotation by hand.

Set the polarisation to the nominal position.

Polarisation Setup - Step 2:

Rotate the feed assembly based on the results of the antenna anglecalculation (see below). Facing the satellite a positive rotation is clockwise

and a negative rotation is anti-clockwise.

Polarisation adjustment normally needs assistance from the NOC. The NOCcan make your antenna system transmit a continuous wave (CW) on a

frequency where it is unlikely to cause interference. The NOC uses aspectrum analyser to monitor this signal and detremine the polarisation and

may ask you to make adjustments to the feed. This should be done in verysmall amounts leaving about 20 seconds between each small turn of the

feed so that the NOC can measure any difference.


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Chapter 5

Not everyone has access to a spectrum analyser. This is an expensive and used to be a

strangely heavy piece of equipment similar to an oscilloscope but that measures signals inthe frequency domain instead of the time domain on the horizontal axis.

Spectrum Analysis Principles

The frequencies available to us range from DC (direct current) 0 Hz right upto visible lightand beyond. Within this great range of frequencies lies the radio frequency spectrum. Within

that lies our television and radio frequencies, and our satellite frequencies. It also containsradar and communications for aircraft, police, fire and ambulance, military forces and

shipping to name but a few. This is a crowded part of the spectrum. These days nearlyeverything is wireless and that means more transmissions.

Well because RF (radio frequency) waves are invisible, the only way we can see them andthen measure them accurately is to analyse the spectrum of frequencies that they are in.Just like an oscilloscope measures voltage against time.

Why do we need to analyse the frequency domain?

To do this we use what is called a spectrum analyser. This peice of equipment is two things:

expensive and complicated. They work by plotting the amplitude of a signal against thefrequency. If you look at the picture above, you can see a signal in the middle of the

analyser screen.

The noise floor (level of background noise) is visible 2 squares up from the bottom andcovers all frequencies across the screen. The signal is 5 squares tall and about half a square

wide. This all tells us a lot of information about the signal. The reason is that the screen is

divided into squares just like an oscilloscope. Instead of time being on the horizontal axis, itis frequency. Amplitude is on the vertical axis measured in dB.


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If our screen is set to 10 GHz wide (span) and our center frequency (CF) is 6 GHz, then our

signal is at 6 GHz, the far left of the screen is 1 GHz and the far right is 11 GHz. The height

of our signal is 5 squares and if our screen is set to 5 dB per square (dB/Div) then oursignal is 25 dB above the noise floor. As you can see knowing all about our screen gives usthe information we need to measure our signal.

The noise floor can also be given a value and this allows us to reference the signal to thenoise floor. This then lets us see and measure the signals to detrmine if they are the correct

size and at the correct frequency. When compared with the expected results from the linkbudget this is a very useful way of checking all is well.


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Spectrum Analysis in Practice

What is Spectrum Analysis

In order to see the signals that are being transmitted over a satellite, or indeed over anyradio frequency system, the most effective tool for visualising them is a spectrum analyser.

A spectrum analyser displays a graphical representation of small portion of radio frequency

spectrum. In a way this is similar to looking at a map of a country and zooming in to view atown, street and a house within that street.

Center Frequency and Span

The display is made up of a 10 by 10 grid of squares just like an oscilloscope. The central

verticle line represents the center frequency and either side is 5 vertical lines of freqencymaking up a span of frequencies. The horizontal lines represent amplitude in decibels. Eachsquare is given a value by the user such as 5 dB per square or division for amplitude and

say 10 MHz per division for frequency.

The user chooses these settings in order to see the signals of interest and then measure

them accurately. Other settings include Resolution Bandwidth and Video Bandwidth.

Resolution Bandwidth (RBW) is the width of a filter that is swept across the selected

Span of frequencies and this must be set to a value not less than the bandwidth of thesignal you wish to view or it will not be fully captured by the filter and this will result in thesignal being displayed inaccurately.

Video bandwidth (VBW) is used to reduce the amount of noise displayed so that signals

can be more easily seen and measured. A rule of thumb is to set the video bandwidth to a

value of 1/10th or 1/100th of the RBW. This makes measurements easier to calculate in

your head.

Reference Level

The display above shows a signal at a center frequency of 6.5 GHz within a span of

frequencies covering 20 MHz. The RBW is set to 10 KHz and the VBW is set to 1 KHz. The


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reference level is set to -40 dBm and this is the level of the base line of the display, the

bottom line of the grid. The noise floor is 2 squares up from this and the dB/Div is set to 5

dB per division. Straight away we can see that the noise floor is at -30 dBm since thebottom line is at -40 dBm and the noise is 2 squares higher at 5 dB/Div.

So now you can begin to see how these settings can be set to find and display a signal so

that it be measured. In our next Tutorial we will look at how we can measure values for

C/No and how that can be in turn used to verify a link budget.

Spectrum Analyser Measurements

C/No and C/KT Measurements

C/No is the Carrier to Noise ratio sometimes known as C/KT because noise is measured as

KT or Boltzmanns constant K multiplied by the noise temperature T.

This picture shows a signal displayed on a spectrum analyser. The signal is at a frequency of

6.5 GHz as it is in the center of the screen and the center frequency of the analyser is 6.5GHz.

Each square of the display is a division, the analyser is set to 5 dB/DIV which means thateach square is 5 dB.

The resolution bandwidth (RBW) is set to 10 KHz which is the width of the sweep filter

inside the analyser. This filter is swept across the frequency span defined by the SPAN of

the analyser, in this case 20 MHz. The RBW defines the amount of power captured by theanalyser and displayed on the screen as a signal. Having an RBW to narrow will not capture

all of the signal and will result in a less accurate display.

In order to measure the C/No we first multiply the Log of the RBW by 10.

10 x Log (10,000) = 40 dB

Next we add the size of the signal which is 5 squares or 25 dB.


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40 dB + 25 dB = 65 dB. Now because of an inherent error using this method we mustsubtract 2 dB.

65 dB - 2 dB = 63 dBc/Hz.

This gives us a fairly accurate C/No of 63 dB (carrier power) in 1 Hz bandwidth.


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Radio & Satellite Frequency Bands

If you hear Ku Band, C Band and L band being discussed and wonder what on earth people are talkingabout then fear not, it isn't some special code. Radio & Satellite frequency bands are both Historic Radarbands and ITU definitions and they are given letters to represent bands of frequencies.

Satellite Frequency Bands

L BAND 1 - 2 GHZ MOBILE SERVICES [ Mostly used for Intermediate (IF) frequencies ]( 950 - 1750 MHz is the Tx Band and 950 - 1900 MHz is the Rx Band )

S BAND 2.5 - 4 GHZ MOBILE SERVICES( Not used apart from satellite control links & HAM radio. )

C BAND 3.7 - 8 GHZ FIXED SERVICES( 5.7-6.725 GHz is the Tx Band [Linear] and 3.625-4.2 GHz is the Rx Band [circular] also 5.85-6.425 GHzis the Tx Band [cirular] )

X BAND 7.25 - 12 GHZ MILITARY

( 7.25 to 7.75 is the Rx Band and 7.9 to 8.4 is the Tx Band all circular )

Ku BAND 12 - 18 GHZ FIXED SERVICES( 13.75 - 14.5 GHz is the linear Tx Band and 10.7 - 12.75 GHz is the linear Rx Band )

Ka BAND 18 - 30.4 GHZ FIXED SERVICES( 27.5 - 31 GHz is the Tx Band [linear] and 19.7 - 20.2 GHz is the Rx Band [circular] also 29.5 - 30 GHz isthe Tx Band [circular] )

V BAND 37.5 - 50.2 GHZ FIXED SERVICES( New - not widely used yet - maybe used instead of Ka Band in the future. )

Historic Radar Frequency Bands:Band: Frequency: (GHz) Wavelength: (cm)

VHF 0.03 - 0.3 1000 - 100

UHF 0.3 - 1 100 - 30

L 1 - 2 30 - 15

S 2 - 4 15 - 7.5

C 4 - 8 7.5 - 3.75

X 8 - 12 3.75 - 2.5

Ku 12 - 18 2.5 - 1.6K 18 - 27 1.6 - 1.1

Ka 27 - 40 1.1 - 0.75

MM 40 - 100 0.75 - 0.3

ITU Definitions for Frequency Bands:


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Band: Frequency: (Hz) Wavelength: (km)

ELF 3 - 30 100,000 - 10,000

SLF 30 - 300 10,000 - 1000

ULF 300 - 3000 1000 - 100

VLF 3000 - 30k 100 - 10

LF 30k - 300k 10 - 1

MF 300k - 3M 1 - 0.1

HF 3M - 30M 0.1 - 0.01

VHF 30M - 300M 0.01 - 0.001

UHF 300M - 3G 0.001 - 0.0001

SHF 3G - 30G 0.0001 - 0.00001

EHF 30G - 300G 0.00001 - 0.000001


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Chapter 6

This picture shows a power meter, transmitter and an antenna.

EIRP & Power Measurements

In order to connect a power meter to measure the power into the antenna, a coupler is

used.

Power meters measure small amounts of power and can not be connected to transmitters

directly unless a large load or attenuator is fitted in between the transmitter and the powermeter.

This can be seen in the diagram as a coupler with 40 dB of attenuation. The coupler worksby tapping off a small part of the power which is 40 dB lower than the power passingthrough to the antenna.

The power meter will then read 40 dB lower and so by adding the 40 dB onto the power

meter reading the forward power (into the antenna) is measured as 56.7 dBm. The antennahas a gain of 20 dBi and so the EIRP of this system is simply 56.7 + 20 = 76.7 dBm.

This is the Effective Isotropic Radiated Power (EIRP) of the antenna. Now an Isotropicreference antenna radiates, or transmits equal amounts of power or signal in all directions.This is of little use and is a theoretical antenna. It has unity gain so what you put in you get

out.

So the EIRP is a comparative measurement which is used to compare every antenna to one

single reference, the Isotropic antenna. The measurement is calculated by adding theantenna gain and forward power minus any loss.

The system would normally have a transmit filter before the antenna to limit the unwantedsputious signals often generated by the transmitter. This filter gives us a loss. This loss canbe as much as 2 dB which we would subtract from the EIRP figure given above.

Each power meter is calibrated to a particular frequency by the user and the coupler is also

calibrated to give an attenuation calibrated at a particlular frequency. These frequencies arenormally listed over a range which is determined by the manufacturer.

EIRP = Forward Power + Antenna Gain - Losses


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Chapter 7

Modems - What are they?

What is a modem? How does it work?

A modem is a modulator / demodulator (modem) which is a two part system.

Transmit & Receive (Modulate & Demodulate)

In any satcom system the reason for broadcasting via satellite is that you want to get

information from one part of the world to another. So how do we get lets say our TV picturefrom our tv camera into the antenna?

The transmit path up to the satellite uses a modulator to take the digitised TV camera data

and modulate it onto a carrier signal. This is the same process as any radio transmission.Once modulated your data is hitching a ride on the carrier signal, this signal is transmittedto the satellite and back to earth again.

At the destination antenna, your signal is then received at a demodulator. The demodulatorworks just as any radio receiver, the TV data is separated from the carrier signal and this is

called demodulated. The data can then be fed into a TV once converted to the appropriateformat and watched.

This principle is the same for any modulator and demodulator. These two quite separatedevices are usually packaged together to make a Modem. They can however, be used on

their own, for example, just like your home Satellite TV receiver. This is usually a

demodulator only and separates the TV data from the carrier signal for you to watch onyour TV.

So what is QPSK?


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That's a bit of a leap but QPSK stands for Quadrature Phased Shift Keying. if this all get's abit confusing then don't worry too much as it is covered again in later Tutorials.

Keying is a method of communication and is probably a word left over from the old morse

code method. There are many modulation techniques of which QPSK is just one. It increases

the accuracy of data at higher data rates by having 4 states instead of just on and off.

This is acheived by using a sine wave which is phased shifted (delayed) by 90 degrees, 180

degrees and 270 degrees. With 0 degrees being state 1, 90 degrees being state 2 and soon. The diagram above shows 4 ellipses one in each quadrant. These vary in shape with the

data that is modulated onto each phase. Four times the amount of data can therefore be

transmitted with this technique or four times the accuracy. This is one of the most commonforms of modulation.

The most useful sounding technique is CDMA or Code Division Multiple Access. Thistechnique spreads the data across a wide area of frequency or a wide bandwidth such that

there is very little or no detectable carrier. The signal is said to be down in the noise. The

data is coded on transmission and can be recovered from the noise by decoding the signal

providing the correct range of frequencies or bandwidth is used.

CDMA also allows multiple signals to be stacked upon each other at the same frequency.This then crams in more signals in the same frequency range requiring less space. The

system is old but it is only recently being exploited to it's full potential and is being used ineverything from the latest mobile phone technology to vehicle telematics applications.

Another technique is FDMA, Frequency Division Multiple Access, and is most commonly used

with satellites. Quite simply, each access has its own frequency. The power of each carrier isbalanced across the whole satellite channel and with DAMA, Demand Assignment Multiple

Access, the frequencies of each carrier are controlled by a central control center. Each

transmitter is commanded to a frequency by the use of a common control channel. This isimplemented in our every day lives through the management of our cellular phone network.

We don't notice but our phones are continually changing frequency and power level as wespeak.

In terms of modulation, the most common is a derivitive of QPSK or is actually still QPSK:

Quadrature Phase Shift Keying - Modulation Schemes:

The 8-PSK modulation scheme utilises approx 66% of the bandwidth that QPSK does and

therefore can reduce satellite charges by up to 66%. This is an obvious benefit to satelliteusers but then there is a better scheme. 16-QAM utilises approx 50% of the bandwidth that

QPSK does and therefore can reduce satellite charges by up to 50%.

There is a catch, these are more advanced modulation schemes than QPSK but require a

higher C/No in terms of thermal noise (T) and phase noise (dBc/Hz). This results in anantenna at least twice the diameter of that required for QPSK. If the same size antenna isused then much more power from the satellite is needed and so greater charges will be

incurred which will negate the benfits of using the more advanced modulation schemes.

DVB-S2 uplink carriers provide the ability to change modulation schemes very rapidly

between QPSK, 8QAM and 16QAM and also FEC rates. Each subscriber RX site measures itsown downlink signal receive quality, (Eb/No) and reports this information back to the NOCvia its return link. If the NOC needs to send data to that RX site it operates using the best


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modulation and FEC to maximise the data throughput. Poor weather at a RX site requires a

lower modulation scheme with more FEC ensure data is received. This is a dynamic and

semi-autonomous system of control.

QPSK - Quadrature Phase Shift Keying

(From Course 3 - Modulation Techniques (Part Two) - FSK, QPSK, QAM)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:

DIBIT Phase Shift00 +9001 0

10 18011 270

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 andalso modulates the amplitude of the carrier.

Phase Modulated and Amplitude Modulated Carrier:

There are two types, 8-QAM and 16-QAM:

8-QAM encodes 3 bits of data (23=8) and 16-QAM encodes 4 bits of data (24=16).

16-QAM has 12 phase angles, 4 of which have 2 amplitude values!

Higher data rates use much more complex QAM methods.

Enough of all that, lets take a step back and look at the anatomy of a satellite

communications system otherwise known as a satcom terminal.


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Chapter 8

Modulation Techniques (Part One)AM, FM, PM TutorialNote:

Although, this section refers to telephone modems and analogue audio modulation, the

same principles apply to satellite modems. This material has been used for the tutorial as itexplains the principles well.

Modulation Techniques

Modulation techniques are methods that are used to encode digital information in an analogworld. The 3 basic modulation techniques are as follows:

o AM (amplitude modulation)o FM (frequency modulation)o PM (phase modulation)

All 3 modulation techniques employ a carrier signal. A carrier signal is a single frequency

that is used to carry the intelligence (data). For digital, the intelligence is either a 1 or 0.When we modulate the carrier, we are changing its characteristics to correspond to either a1 or 0.

AM - Amplitude Modulation

Amplitude Modulation modifies the amplitude of the carrier to represent 1s or 0s.In the

above example, a 1 is represented by the presence of the carrier for a predefined period of3 cycles of carrier. Absence--or no carrier--indicates a 0.

Advantages:

Simple to design.

Disadvantages:

Noise spikes on transmission medium interfere with the carrier signal.

Loss of connection is read as 0s.

FM - Frequency Modulation

Frequency Modulation modifies the frequency of the carrier to represent the 1s or 0s. In the

above example, a 0 is represented by the original carrier frequency, and a 1 by a muchhigher frequency (the cycles are spaced closer together).


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Advantages:

Immunity to noise on transmission medium.

Always a signal present. Loss of signal easily detected

Disadvantages:

Requires 2 frequencies

Detection circuit needs to recognize both frequencies when signal is lost.

PM - Phase Modulation

Phase Modulation modifies the phase of the carrier to represent a 1 or 0.

The carrier phase is switched at every occurrence of a 1 bit, but remains unaffected for a 0bit. The phase of the signal is measured relative to the phase of the preceding bit. The bits

are timed to coincide with a specific number of carrier cycles (3 in this example = 1 bit).

Advantage:

Only 1 frequency used

Easy to detect loss of carrier

Disadvantages:

Complex circuitry that is required to generate and detect phase changes.

Modulation Techniques (Part Two)FSK, QPSK, QAM TutorialModulation Used in Modems

Here are the 3 basic types of modulation used in modems:

o FSK - Frequency Shifted Keyingo QPSK - Quadrature Phase Shifted Keyingo QAM - Quadrature Amplitude Modulation


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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 insatellite communications because of the inefficient use of bandwidth and frequencies.

QPSK - Quadrature Phase Shift Keying

Quadrature Phase Shift Keying employs shifting the phase of the carrier plus an encodingtechnique. QPSK is used in almost all modems. The digital information is encoded using 4

(Quad) level differential PSK.

The data is encoded as follows:

DIBIT Phase Shift

00 +9001 010 180

11 270

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 andalso modulates the amplitude of the carrier.

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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Chapter 9

Base Band Digital Communications

Base band is the lowest band of communications; we may have Ku Band Radio Frequency (RF) at the top, L Band Intermediate

Frequency (IF) in the middle at the modem and then finally Base Band. The actual data that is being transported via satellitearrives at its intended destination from the modem.

Asynchronous RS-232 Communications

Asynchronous means the transmission of data without the use of an external clock signal. The timing required to recover the datafrom the communication link is encoded within the data link. Asynchronous communications allow for the transmitter and receiver

clock generators to not be exactly synchronised or aligned.

If we assume the data being sent is text based characters then the following will make more sense:

Start Bits:

Asynchronous data is framed by use of start bits, character bits, parity bits and stop bits. This allows for receiver synchronisation to

extract the data in the correct format.

[ Start Bit ] [ Data Bits ] [ Parity ] [ Stop Bits ]

1 8 0 1

The start bit is followed by 8 data bits, no parity bit and one stop bit, for a 10-bit frame.

These days there is nearly always 1 start bit and so it is often not specified.

Data Bits:

The data bits are the number of bits per character. 7 bit characters are now rarely used and more often than not you will find that 8

bit characters are used. Both 7 bit and 8 bit characters are common in the American Standard Code for Information Interchange(ASCII), format.

Parity Bits:

A parity bit is added to ensure that the number of bits in a given set of bits is always even or odd. That is the number of bits with a

value of 1. This is a simple form of error checking. 01101110 have 5 bits with a value of 1. If the parity is even the parity bit willalso be 1.

If the parity bit is not used it may be called 'mark', where the parity is always 1, or 'space' where the parity is always 0.

Stop Bits:

The minimum stop bit quantity can be more than 1. Some systems required 2 stop bits and some require 1.5 stop bits. Equipmentthat don't support fractional stop bits quantities can be set to 2 stop bits for transmit and 1 stop bit for receive.

Baud Rate or Speed:

Rather than go into the history and complexity of baud rates here, see the Wikipedia page:http://en.wikipedia.org/wiki/Baud_rate

Synchronous Data Links:

RS-232, RS-422, RS-423, X-21, RS-530, V11, V24, V35 and G703 are all examples of synchronous communications standards.

All this means is that they have both data and clock for both transmit and receive lines. Some of these standards are balanced and

others are single ended (unbalanced). These differences in the interfaces are discussed later.

---------o Tx Data

---------o Tx Clock

---------o Rx Data---------o Rx Clock

---------o Signal Ground

In its simplest form this is a synchronous un-balanced interface.
http://en.wikipedia.org/wiki/Baud_ratehttp://en.wikipedia.org/wiki/Baud_ratehttp://en.wikipedia.org/wiki/Baud_ratehttp://en.wikipedia.org/wiki/Baud_rate


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The transmit data has an associated transmit clock which is used to clock the data in or out of the sending device and the edges of

the clock are aligned or synchronised with the edges of the data.

At the receiving device the received data is clocked in or out using the receive clock which is also aligned with the edges of the

receive data.

The complexities arise with the directions of the clocks and data which are dependant on the devices in terms of DCE or DTE

interfaces. DCE stands for Data Communications Equipment and DTE stands for Data Terminal Equipment. A modem is usually a

DCE whereas a computer is usually a DTE. A DCE is normally connected straight to a DTE using pin to pin straight cabling. This

means that the Tx Data from the DTE is connected to the Tx Data on the DCE because on the DTE this is an ouput and on the DCEit is an input.

Connecting two computers via a modem link would involve a DTE (computer #1) connected to a DCE (modem #1), then over a linkto the DCE (modem #2) which is connected to the other DTE (computer #2).

Data and Clocking:

DCE data and clocks are always received on the transmit lines and transmitted on the receive lines. This is because the interfacesare always referenced to the DTE.

This is why a DTE to DCE interface cable is a straight pin to pin cable. A DCE to DCE or a DTE to DTE is a crossover cable because

the DTE interfaces are expecting to receive the transmit data and clocks on the receive data and clock inputs and visa versa. The

same goes for the DCE interfaces except they are expecting the receive data and clocks on the transmit data and clock inputs andvisa versa.

A further complexity comes from clocking schemes:

Network Timing:

In standard point to point communications, that do not involve satellite links but are effectively over copper wires end to end, the

interfaces are straight forward.

Data and clocks are sent from end A to end B and visa versa. There is little or no delay in the arrival of data and clock.

If a communications network (such as the BT network) is in-between end A and end B then the clocking scheme can become

interesting. The network sometimes supplies both the transmit and receive clocks. It is a DCE. Data is clocked into it and out of it

using the same clock. This clock is derived from a very stable atomic clock. There is no better clock.

End A and end B must use the clock to transmit the data and receive the data. For this reason most equipment has inputs for both

transmit and receive clocks. This equipment may also be a DCE and that means that it is to be connected using a crossover cabling

method as DCE to DCE.

END A END B

Tx Data o---------


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If end B wants to supply the clock to the modem or rather the network clock to clock the data out from the modem then the

modem will need to be able to use the clock from the network for both transmit and receive clocks.

Satellite Buffers:

If there is a buffer in between the modem and end B then the clock from end B is used to clock out the data from the buffer and the

modem clocks its data into the buffer using its own clock. The buffer then takes up any differences and hopefully copes with the

situation. Major differences between the clocks could result in the buffer overflowing and massive data errors reducing the

availability of the link.

Thankfully, these days the satellite buffer is built-in to the modem which is an increasingly sophisticated piece of equipment. It may

provide a host of clocking options and buffer settings to match and interface with the network.

Next we will look at the different standards in more detail.


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Data Communications - Data Interface Standards

Data Interface Standards

There are a variety of data communications standards which can leave you totally confused.Fear not, they are actually fairly simple and easy to understand.

Each standard defines the pinout (pin number and designation), connector type, voltagelevels on each pin, and the maximum data rate and maximum distance of cable run.

EIA-232 (CCITT V.24) / RS-232

This specifies a 25 pin D Type connector as a standard unbalanced (single wire signals)

interface for DCE (Data Communication Equipment - Modem) or DTE (Data TerminationEquipment - PC Comm Port or User Equipment).EIA-423 / RS-423

This specifies a 25 pin D Type connector as a standard unbalanced synchronous (data andclocks) interface for DCE (Data Communication Equipment - Modem) or DTE (Data

Termination Equipment - User Equipment). This interface is almost identical to RS-422 butfor RS-423 is unbalanced. Very similar to RS-232 but can drive longer cable lengths athigher data rates.

EIA-422 (CCITT V.11) / RS-422

This specifies a 37 pin D Type connector as a standard balanced (signal pairs) synchronous

(data and clocks) interface for DCE (Data Communication Equipment - Modem) or DTE

(Data Termination Equipment - User Equipment). This interface can also be on 9 way DTypes and 15 way D Types.

EIA-530 (CCITT ) / RS-530

This specifies a 25 pin D Type connector as a standard balanced (signal pairs) synchronous(data and clocks) interface for DCE (Data Communication Equipment - Modem) or DTE(Data Termination Equipment - User Equipment). This interface is identical to RS-422 but

uses a 25 Way connector and is also used for RS-423 which is the same interface again butunbalanced.

V.35 / EIA-449 / RS-449


(data and clocks) interface for DCE (Data Communication Equipment - Modem) or DTE(Data Termination Equipment - User Equipment).

X-21


(data and clocks) interface for DCE (Data Communication Equipment - Modem) or DTE


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(Data Termination Equipment - User Equipment). This interface has only one clock normallybut the full pinout caters for two.

Next we will have a look at the pin outs...


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Data Communications - Data Interface Pin Outs

Pinout Tables

The following Pinout tables are correct to the best of our knowledge and are reviewedregularly. Always double check pin outs against a second source.

RS-232, RS-422, RS-530, RS-423, RS-449, AUI, RJ-45, EIA Standards and X-21 Pinouts.

A pin out is the list of pins for a connector used with a particular interface such as RS-232.The pin out lists the pins and describes the function of each pin.

RS-232 Pinout ( 25 Way D Type )

Asynchronous

Pin DescriptionConnectorType

DTEDCENotes

2 Tx Data 25 Way D-Type Out In3 Rx Data In Out

4 RTS Out In RTS/CTS

5 CTS In Out RTS/CTS

6 DSR In Out DSR/DTR

7Signalground

8 DCD In Out

20 DTR Out In DSR/DTR

22 RI In Out

EIA-232 / RS-232 Pinout ( Mainly 25 Way D Type )Synchronous

PinDescription Connector Type DTE DCE Notes

2 Tx Data 25 Way D-Type Data Out Data In

3 Rx Data Data In Data Out

7 Signal ground

15 Tx Clock Clock Out Clock In

17 Rx Clock Clock Out Clock In

24 Tx Clock Clock In Clock Out

EIA-232 / RS-232 Pinout ( PC Comm Port - 9 Way D Type )Asynchronous

PinDescription Connector Type DTE(PC) DCE Notes

1 DCD Data Carrier Detect 9 Way D-Type Male Data In Data Out


3 Tx Data Data Out Data In


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4 DTR Data Terminal Ready Data Out Data In

5 Signal Ground

6 DSR Data Set Ready Data In Data Out

7 RTS Request to Send Data Out Data In

8 CTS Clear to Send Data In Data Out

9 RI Ring Indicator Data In Data Out

EIA-449 / V.35 Pinout ( Mainly 37 Way D Type or Cannon )

Synchronous

PinDescription Designation / Label DCE DTE Notes

1 Shield

2 Signal Rate Indicator S In Out

3 N/C

4

Send Data (A) SD- Data In

Data Out5 Send Timing (A) ST- Clock Out Clock In

6 Receive Data (A) RD- Data Out Data In

7 Request To Send (A) RS- In Out

8 Receive Timing (A) RT- Clock Out Clock In

9 Clear To Send (A) CS- Out In

10 Local Loopback LL In Out

11 Data Mode (A) DM- Out In

12 Terminal Ready (A) TR- In Out

13 Receiver Ready (A) RR- Out In

14 Remote Loopback RL In Out

15 Incoming Call IC Out In

16Signal Freq./Sig. RateSelect.

SF/SR+ Out/In In/Out

17 Terminal Timing (A) TT- Clock In Clock Out

18 Test Mode (A) TM- Out In

19 Signal Ground SG

20 Receive Common RC

21 N/C

22 Send Data (B) SD+ Data In Data Out

23 Send Timing (B) ST+ Clock Out Clock In

24 Receive Data (B) RD+ Data Out Data In

25 Request To Send (B) RS+ In Out

26 Receive Timing (B) RT+ Clock Out Clock In

27 Clear To Send (B) CS+ Out In

28 Terminal In Service IS In Out

29 Data Mode (B) DM+ Out In

30 Terminal Ready (B) TR+ In Out

31 Receiver Ready (B) RR+ Out In


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Chapter 9 Page 8

32 Select Standby SS Out In

33 Signal Quality SQ Out In

34 New Signal NS In Out

35 Terminal Timing (B) TT+ Clock In Clock Out

36 Standby Indicator SB Out In

37 Send Common SC

EIA-423 / RS-423 Pinout

Synchronous


2 Tx Data 25 Way D-Type Data Out Data In


7 Signal ground(Can be a variety ofConnectors)

15 Tx Clock Clock Out Clock In

17 Rx Clock Clock Out Clock In

24 Tx Clock Clock In Clock Out

X-21 Pinout

Synchronous

PinDescription Connector Type DCE DTE Notes

2 Tx Data A 15 Way D-Type Data In Data Out

9 Tx Data B Data In Data Out

4 Rx Data A Data Out Data In

11 Rx Data B Data Out Data In

8 Signal ground

6 Tx Clock A Clock Out Clock In

13 Tx Clock B Clock Out Clock In

3 Control A Handshake InHandshakeOut

Often

10 Control B Handshake InHandshakeOut

Looped

5 Indication AHandshake

OutHandshake In 3 - 5

12 Indication B HandshakeOut

Handshake In 10 -12

V.35 / EIA-442 / RS-442 Pinout (See also EIA-449 Above)Synchronous


4 Tx Data A 37 Way D Type Data Out Data In


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Chapter 9 Page 10

Note that with ethernet cables,connecting a PC or another

ethernet device to a hub, switch

or router requires a standard

cable. Connecting two devices of

the same type such as a PC to aPC or a hub to a switch requiresthe crossover cable. Some hubs

and switches have a dedicated or

switchable 'Uplink' port which can

be used with a standard cable.

stripe

2 Transmit - Green with whitestripe

Orange with white stripe

3 Receive + White with orange

stripe

White with green stripe

4 N/A Blue with white stripe Blue with white stripe

5 N/A White with blue stripe White with blue stripe

6 Receive - Orange with whitestripe

Green with white stripe

7 N/A White with brownstripe

White with brown stripe

8 N/A Brown with white

stripe

Brown with white stripe


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Chapter 10 Page 1

Chapter 10

In any satcom system, there is a need to test the path of the data from end to end to

ensure that the bit error rate (the amount of errors) is minimal or error free.

Data Communications - Test Sets

Test Sets

We use test sets to send test data across the system where it can either be received and

analysed by a compatible test set for one way testing or looped round and sent back to bereceived and analysed by the same test set.

What the test sets are looking for is errors in the received data. Since they know what wasoriginally transmitted they can accurately monitor errors in transmission.

Bit Error Rate

If you have sent 1,000,000 bits of data and receive 1 error then that is an error rate of 1 in1 million bits or 1 x E-6. Acceptable error rates start from 1 error in 10 million bits or 1 x E-

7. To measure this kind of error rate accurately the tests have to run over time to build up a

statistical picture of the errors. 1 error in 24 hours and then 2 million in the next 10 minutesis not a good link but for the first 24 hours is was a superb link.

Eb/No & Bit Error Rates

Eb/N0 is a specification of the satellite modem which states the size of the signal for a given

data rate to give a specified bit error rate. It comprises of the energy per bit in a 1 Hzbandwidth.

In order to calculate this we need the data rate.

Data Rate = 64 kBps

We simply subtract 10 Log this data rate from the measured C/N0 to give the Eb/N0.


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Eb/N0 = 54 dBc/Hz - 48 = 6

Thus, to find the required C/N0 we simply rearrange:

C/N0 = 10 Log Data Rate + Eb/No required

C/N0 = 48 + 6 = 54 dBc/Hz

The modem manufacturer states the Eb/N0 for a given data rate so you can easily know therequired C/N0 to acheive this.


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Chapter 11 Page 1

Chapter 11

Claculations

Link Budget Calculations:

What is a link budget?

A link budget is actually not as complicated as it sounds. Put simply, it is the sum of all the

losses between your transmitter and the satellite and back down again to a receiver. These

losses are reduced by any gain you have at the transmitter, satellite and receiver. So inorder to see if your signal is still going to be big enough to use after it has been sent to areceiver via satellite, the gains and losses are effectively all added together and the result

will be the net gain or loss. A loss means your signal has got smaller, and a gain means ithas got bigger.

This is a very simplified explanation, but it gives you an idea of what the link budget istrying to calculate. The following more in depth explanations talk you through each majorparameter. The maths behind all of this, and there is a lot of it, is not looked at here. Words

are much easier to understand than equations I think. Besides, maths was never my strongpoint.

These parameters may seem scary or alien to you. Fear not, learning what they are is allthat is important at this stage.

The Transmit (Uplink) Terminal:

The transmit frequency is the RF radio frequency at which this carrier wave is transmitted.Usually measured in GHz and sometimes MHz (multiplied by 1000).

The EIRP (Effective Isotropic Radiated Power) is a measure of the power which is requiredto transmit the carrier signal so that it reaches the satellite.

The G/T is a measure of the performance of the transmitter and is based on the gain of the

transmitter (the amplifier, other parts of the uplink equipment chain and the antenna) andthe noise of the equipment in the uplink chain.

o Just as in an audio system, a noisy amplifier is not as good as a quietamplifier.

o The uplink chain is the series of stages the signal goes through before

leaving the antenna on its way to the satellite.

The Lat and Long is the location of the transmit terminal on the earth. Measured in degrees

the latitude and longitude is a global position reference used by the GPS system amongstothers.

The elevation is the angle up from horizontal (0 degrees) that the antenna must point at tosee the satellite in conjunction with azimuth.


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The azimuth is the compass angle from true north that the antenna must be pointed at tosee the satellite in conjunction with elevation.

Path loss is the attenuation of the signal due to the inverse square law and the earthsatmosphere which reduces the size of your signal on its way to the satellite.

Losses are the attenuating factors within the transmitter system such as RF radio frequencycable connectors and different types of RF cable.

Lastly the margin is used to allow for extra atmospheric attenuation due to localised rain orsnow at the transmitter location.

The Satellite:

Starting with the translation frequency, this is used by the satellite to convert the

transmitted signal to a new frequency so that the satellite doesn't retransmit the signal atthe same frequency as the transmitter on earth. If it did the two signals would interfere with

each other and the result would be unusable. Instead, the signal is moved, usually down in

frequency to a 'Receive Band'.

The translation frequency is the amount by which the transmitted signal is moved in MHz.

The EIRP is again the same for the satellite as it is for the transmit terminal. Not the samevalue but the same explanation.

The G/T is also the same explanation for the satellite as it is for the transmit terminal.

The C/No Sat is the carrier signal level to noise level ratio of the transmitted signal when itreaches the satellite. This is a measure of the signal reaching the satellite after travelling

through the atmosphere into space.

The Long is the longitude of the satellite. The satellite is usually located above the equator

at a latitude of 0 degrees. Thus, only the longitude is required to identify the satelliteslocation.

Ant Gain is the gain of the receive antenna on the satellite.

Transp Gain is the gain of the transponder on the satellite, this is in effect one channel of

many that are arranged in bands of frequency on the satellite. They can be independantlycontrolled to increase or decrease the gain. They are sometimes even organised so thatthey cover different areas of the earth through the use of different antenna systems.

Transp Gain is the gain of the satellite transponder.

Req'd EIRP is the amount of power the satellite has to use to get your signal back to earth.

% EIRP is the percentage of total satellite power available for all of the signals using it, thatit must devote to your signal.

Pwr @ Sat is the actual power of your signal transmitted from the satellite.


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The Receive (Downlink) Terminal:

The Rx Freq is the receive frequency determined by the satellite translation freqency. thereceiver must be tuned to this frequency to pick up the signal from the satellite.

C/No is the carrier power to noise level ratio which is a measure of how much signal is

visible above the noise. In audio terms this would be exactly the same as music and hiss.Less hiss more music.

G/T is the same as the transmitter G/T except it is now for the receive terminal.

Lat is the same as the transmitter Latitude except it is now for the receive terminal.

Long is the same as the transmitter Longitude except it is now for the receive terminal.

Elevation is the same as the transmitter Elevation except it is now for the receive terminal.

Azimuth is the same as the transmitter Azimuth except it is now for the receive terminal.

Path Loss is the same as the transmitter Path Loss except it is now for the receive terminal.

Losses is the same as the transmitter Losses except it is now for the receive terminal.

Margin is the same as the transmitter Margin except it is now for the receive terminal takinginto account any weather at the location of the receive terminal.

The Modem:

Data Rate is described as the amount of data you wish to transmit per second. This is

measured in bits. In a link budget the calculation is usually for just one direction so this isthe data rate in one direction.

Eb/No Req'd is the energy per bit to noise level ratio that is required to provide error freedata. This is usually specified by the modem manufacturer.

Link margin is the overall amount of attenuation in any part of the satellite link that can be

tolerated by the modems before they've lost lock. Losing lock means losing the signalbecause it is too small and thus the satellite link is said to be lost, no data received.

Calculations:

With all this data and a lot of maths, the calculations can be made and the Link Marginobtained, if the Link Margin is too small, extra losses may occur due to atmosphericconditions which cause the link to fail. Rainfall when heavy can reduce received signals byaround half.

Balancing the budget is to end up with either 0 dB if no extra margin is required, or about 3dB if some protection is needed.


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Chapter 11 Page 4

Antenna Gain Calculation

The antenna gain is calculated as follows:

Ant Gain = 4 x PI x 'efficient area' = dBi

-----------------------------

wavelength (lamda)

PI = 3.1415927 approx

'efficient area' = antenna reflector area x antenna efficiency

Antenna efficiency is the ability of the antenna to radiate all of the power fed into it. An efficiency of 100% is

theoretical but in practice not achievable. The average efficiency of an antenna is in the region of 70%, but often60% is used as a worst case figure.

70% means that 30% of the input power is lost and this is due to factors such as mismatch, reflected power andimperfections. A reflector is not always a perfect reflector.

wavelength = Speed of Light = 3 x 10^8

------------------ --------------

frequency frequency


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Chapter 11 Page 5

Turnaround Frequency Calculation

The turnaround frequency is calculated by subtracting the satellite translation frequency from the uplink Transmit frequency.

This is used to calculate the Rx downlink frequency that the Transmit signal will return at, this is also known as the Return link or

Downlink frequency.

Rx Freq (T/R) = Tx Freq - Trans Freq

Example:

Tx Freq = 10.5 Ghz

Satellite Translation Freq = 650 MHz

Rx Freq = 10.5 - 0.65 = 9.85 GHz.


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Chapter 11 Page 6

Eb/No Calculation

Eb/No is the Energy per bit divided by the spectral noise density.

Eb/No is related to the C/No.

Eb/No = C/No - (10 x Log(data rate))

Example:

If the data rate is 2048 kbps or 2 Mb then Log(2048000) = approx 6.31

If the C/No is 65 dBc/Hz then

Eb/No = 65 - (10 x 6.31) = 65 - 63.1 = 1.9

Usually an Eb/No of at least 6 or more is expected so in this example the C/No is too small.


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Chapter 11 Page 7

BER Calculation

BER is the Bit Error Rate of a digital data transmission.

This is the ratio of errors to the number of bits transmitted.

BER = the number of errors in the number of bits

Example:

BER = 100 errors in 1000 bits = 100 / 1000 = 0.01

This is expressed as 1 error in 10 to the power of -2 = 1.0 E-2.


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Chapter 11 Page 8

EIRP Calculation

EIRP is the Effective Isotropic Radiated Power.

This is the comparison between the antenna under test and a perfect theoretical omni-directional antenna radiating the same

power in all directions. This is called Isotropic radiation.

The reason for EIRP is that the power out of the antenna can be determined by a simple formula based on the power into the

antenna and the gain of the antenna:

EIRP = Forward transmit power (dB) + Antenna Gain (dB).


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Chapter 11 Page 9

G/T Calculation

G/T is the figure of merit for a satellite system.

G is the Receive antenna gain.

T is the system noise temperature.

System noise temperature = antenna noise temperature + Receiver noise temperature (LNA)

Antenna noise temperature is the noise power seen at the receive output of the antenna. (To LNA)

If you are not measuring with an LNA or Receiver then the System noise temperature = antenna noisetemperature. This is not a representative value for calculating G/T since the G/T relates to the receiveperformance of both antenna and receiver.

To convert to dB, T dB = 10 log(T kelvin)

If both are measured in dB then the G/T = Ant Gain - System Noise Temp.


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Chapter 11 Page 10

C/No Calculation

C/No is the ratio of the carrier power to the noise power.

No = k x T x 1 Hz bandwidth.

No is the noise power in a 1 Hz bandwidth.

k is Boltzmann's constant = -228.6 dBW/K/Hz.

T is the system noise temperature.

C/No is sometimes referred to as the C/kT or just C/N.

C/No = Rx carrier power (dBW) = dB Hz

-----------------------------

Rx noise power (dB Hz)

That completes ourtutorials.
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Annex P a g e | 1

ANNEX

ACTSAdvanced Communications Technology Satellite. A NASA experimental satellite project to demonstrate

the use of the Ka-Band (30/20 GHz) services. A

Satellite Communications Terms & Explanations:

AmplifierA device used to boost the strength of an electronic signal.

Amplitude Modulation (AM)The baseband signal is caused to vary the amplitude or height of the carrier wave to create the desiredinformation content.

AnalogA form of transmitting information characterized by continuously variable quantities, as opposed to digitaltransmission, which is characterized by discrete bits of information in numerical steps. An analog signal isresponsive to changes in light, sound, heat and pressure.

Analog-to-Digital Conversion (ADC)Process of converting analog signals to a digital representation. DAC represents the reverse translation.

ANIKThe Canadian domestic satellite system that transmits Canadian Broadcasting Corporation's (CSC)network feeds throughout the country. This system also carries long distance voice and data servicesthroughout Canada as well as some transborder service to the U.S. and Mexico.

AntennaA device for transmitting and receiving radio waves. Depending on their use and operating frequency,antennas can take the form of a single piece of wire, a di-pole a grid such as a yagi array, a horn, a helix,

a sophisticated parabolic-shaped dish, or a phase array of active electronic elements of virtually any flator convoluted surface.

ApertureA cross sectional area of the antenna which is exposed to the satellite signal.

ApogeeThe point in an elliptical satellite orbit which is farthest from the surface of the earth. Geosynchronoussatellites which maintain circular orbits around the earth are first launched into highly elliptical orbits withapogees of 22,237 miles. When the communication satellite reaches the appropriate apogee, a rocketmotor is fired to place the satellite into its permanent circular orbit of 22,237 miles.

Apogee Kick Motor (AKM)

Rocket motor fired to circulate orbit and deploy satellite into geostationary orbit.

Apstar (Asia-Pacific Star)Name of the Chinese satellite system which carries commercial video services in the region.

ArabsatThis is the Arabsat Satellite Organization and its is headquartered in Riyadh, Saudi Arabia. It providesregional telecommunications services for the Middle East region.


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AsiaSatA satellite system covering the Asia mainland.

Asynchronous CommunicationsStream of data routed through a network as generated, rather than in organized message blocks. Mostpersonal computers send data in this format. (See ATM)

Asynchronous Transfer Mode (ATM)This is the new form of super-fast packet switching operating at speeds in the Gigabits/second.

AttenuationThe loss in power of electromagnetic signals between transmission and reception points.

Attitude ControlThe orientation of the satellite in relationship to the earth and the sun.

Audio SubcarrierThe carrier between 5 MHz and 8 MHz containing audio (or voice) information inside of a video carrier.

Automatic Frequency Control (AFC)A circuit which automatically controls the frequency of a signal.

Automatic Gain Control (AGC)A circuit which automatically controls the gain of an amplifier so that the output signal level is virtuallyconstant for varying input signal levels.

AZ/EL MountAntenna mount that requires two separate adjustments to move from one satellite to another;

AzimuthThe angle of rotation (horizontal) that a ground based parabolic antenna must be rotated through to point

to a specific satellite in a geosynchronous orbit. The azimuth angle for any particular satellite can bedetermined for any point on the surface of the earth giver the latitude and longitude of that point. It isdefined with respect to due north as a matter of easy convenience.

B-MacA method of transmitting and scrambling television signals. In such transmissions MAC (MultiplexedAnalog Component) signals are time-multiplexed with a digital burst containing digitized sound, videosynchronizing, authorization, and information.

BackhaulA terrestrial communications channel linking an earth station to a local switching network or populationcenter.

BackoffThe process of reducing the input and output power levels of a traveling wave tube to obtain more linearoperation.

Band Pass FilterAn active or passive circuit which allows signals within the desired frequency band to pass through butimpedes signals outside this pass band from getting through.


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BandwidthA measure of spectrum (frequency) use or capacity. For instance, a voice transmission by telephonerequires a bandwidth of about 3000 cycles per second (3KHz). A TV channel occupies a bandwidth of 6million cycles per second (6 MHz) in terrestrial Systems. In satellite based systems a larger bandwidth of17.5 to 72 MHz is used to spread or "dither" the television signal in order to prevent interference.

BasebandThe basic direct output signal in an intermediate frequency based obtained directly from a televisioncamera, satellite television receiver, or video tape recorder. Baseband signals can be viewed only onstudio monitors. To display the baseband signal on a conventional television set a "modulator" is requiredto convert the baseband signal to one of the VHF or UHF television channels which the television set canbe tuned to receive.

BaudThe rate of data transmission based on the number of signal elements or symbols transmitted persecond. Today most digital signals are characterized in bits per second.

BeaconLow-power carrier transmitted by a satellite which supplies the controlling engineers on the ground with a

means of monitoring telemetry data, tracking the satellite, or conducting propagation experiments. Thistracking beacon is usually a horn or omni antenna.

BeamwidthThe angle or conical shape of the beam the antenna projects. Large antennas have narrower beamwidthsand can pinpoint satellites in space or dense traffic areas on the earth more precisely. Tighterbeamwidths thus deliver higher levels of power and thus greater communications performance.

BirdSlang for a communications satellite located in geosynchronous orbit.

BitA single digital unit of information

Bit Error Rate (BER)The fraction of a sequence of message bits that are in error. A bit error rate of 10-6 means that there is anaverage of one error per million bits.

Bit RateThe speed of a digital transmission, measured in bits per second.

BlankingAn ordinary television signal consists of 30 separate still pictures or frames sent every second. Theyoccur so rapidly, the human eye blurs them together to form an illusion of moving pictures. This is thebasis for television and motion picture systems. The blanking interval is that portion of the television

signal which occurs after one picture frame is sent and before the next one is transmitted. During thisperiod of time special data signals can be sent which will not be picked up on an ordinary televisionreceiver.

Block Down ConverterA device used to convert the 3.7 to 4.2 KHz signal down to UHF or lower frequencies (1 GHz and lower).

BPSK (Binary Phase Shift Keying)A digital modulation technique in which the carrier phase can have one of two possible values, namely 0


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degrees or 180 degrees.

Broad beamA single large circular beam that covers a large geographic area

Broadcast

The sending of one transmission to multiple users in a defined group (compare to unicast).

BSS (Broadcast Satellite Service)This is the ITU designation but DBS or Direct Broadcast Service is more commonly used term in thesatellite industry.

Business TelevisionCorporate communications tool involving video transmission of information via satellite. Common uses ofbusiness television are for meetings, product introductions and training.

Buttonhook FeedA shaped piece of waveguide directing signal from the feed to the LNA behind the antenna.

BypassUse of satellite, local area network, wide area network or metropolitan area network as an alternativetransmission facility.

C BandThis is the band between 4 and 8 GHz with the 6 and 4 GHz band being used for satellitecommunications. Specifically, the 3.7 to 4.2 GHz satellite communication band is used as the down linkfrequencies in tandem with the 5.925 to 6,425 GHz band that serves as the uplink.

CarrierThe basic radio, television, or telephony center of frequency transmit signal. The carrier in an analogsignal. is modulated by manipulating its amplitude (making it louder or softer) or its frequency (shifting itup or down) in relation to the incoming signal. Satellite carriers operating in the analog mode are usually

frequency modulated.

Carrier FrequencyThe main frequency on which a voice, data, or video signal is sent. Microwave and satellitecommunications transmitters operate in the band from 1 to 14 GHz (a GHz is one billion cycles persecond).

Carrier to Noise Ratio (C/N)The ratio of the received carrier power and the noise power in a given bandwidth, expressed in dB. Thisfigure is directly related to G/T and S/N; and in a video signal the higher the C/N, the better the receivedpicture.

Cassegrain AntennaThe antenna principle that utilizes a subreflector at the focal point which reflects energy to or from a feedlocated at the apex of the main reflector.

CATVOriginally meant Community Antenna Television. Independent smaller companies in rural communitieswould build a large television receiving antenna on a nearby mountain to pick up the weak TV signalsfrom a distant metropolis. These signals were amplified, modulated onto television channels and sentalong a coaxial cable strung from house to house.


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CCITT (now TSS)Comite Consultatif Internationale de Telegraphique et Telephonique. International body, associated withthe ITU, which establishes worldwide standards for telecommunications. Reorganized to include CCIR(radio standards group) and renamed TSS (Telecommunications Standardization Sector).

CDMA

Code division multiple access. Refers to a multiple-access scheme where stations use spread-spectrummodulations and orthogonal codes to avoid interfering with one another.

ChannelA frequency band in which a specific broadcast signal is transmitted. Channel frequencies are specified inthe United States by the Federal Communications Commission. Television signals require a 6 MHzfrequency band to carry all the necessary picture detail.

CIFCommon Intermediate Format. A compromise television display format adopted by the CCITT which isrelatively easy to derive from both PAL and NTSC.

Circular Polarization

Unlike many domestic satellites which utilize vertical or horizontal polarization, the international Intelsatsatellites transmit their signals in a rotating corkscrew-like pattern as they are down-linked to earth. Onsome satellites, both right-hand rotating and left-hand rotating signals can be transmitted simultaneouslyon the same frequency; thereby doubling the capacity of the satellite to carry communications channels.

ClampA video processing circuit that removes the energy dispersal signal component from the video waveform.

Clarke OrbitThat circular orbit in space 22,237 miles from the surface of the earth at which geosynchronous satellitesare placed. This orbit was first postulated by the science fiction writer Arthur C. Clarke in Wireless Worldmagazine in 1945. Satellites placed in these orbits, although traveling around the earth at thousands ofmiles an hour, appear to be stationary when viewed from a point on the earth, since the earth is rotating

upon its axis at the same angular rate that the satellite is traveling around the earth.

C/No or C/kT or C/kTBCarrier-to-noise ratio measured either at the Radio Frequency (RF) or Intermediate Frequency (IF).

Coaxial CableA transmission line in which an inner conductor is surrounded by an outer conductor or shield andseparated by a nonconductive dielectric.

CodecCoder/decoder system for digital transmission.

Co-LocationAbility of multiple satellites to share the same approximate geostationary orbital assignment frequentlydue to the fact that different frequency bands are used.

Color SubcarrlerA subcarrier that is added to the main video signal to convey the color information. In NTSC systems, thecolor subcarrier is centered on a frequency of 3.579545 MHz, referenced to the main video carrier.

Documents

VSAT Training Book