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Topic ClosedLink Budgets A Beginners Guide

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Direct Link To This Post Topic: Link Budgets A Beginners Guide
    Posted: 14/August/2008 at 16:14

Beginners Guide To Link Budgets

Link Budget Calculations:

Before we begin, let's just point out a few things to those who have arrived here looking for Link Budget calculations... They are just a click away.
 
If you are looking for calculations such as G/T, Eb/No, Antenna Gain, EIRP or C/No then try the 'Useful Calculations' Pages in our 'Tutorials' section.
 
 
For free online tools and software, including link budget software, and our Azimuth and Elevation calculator visit our Tools page.

Click here for a System Noise Temperature Calculator.

To obtain values for satellite G/T and EIRP refer to the satellite footprint specifications. Here is a list of links to some of the available SES-ASTRA satellite footprints - use the interactive fleet map to access the beam contours and EIRP information - and SES-SIRIUS satellite footprints. See also the LyngSat website for more satellite beam contour maps. The contours will show you how much to deduct from the boresight (maximum) EIRP depending on the antenna locations. The footprint diagrams may sometimes also state the satellite G/T.

Right now that's out of the way let's continue...
 
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 in order to see if your signal is still going to be big enough to use after it has been sent to a receiver 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 it has got bigger.

This is a very simplified explanation, but it gives you an idea of what the link budget is trying to calculate. The following more in depth explanations talk you through each major parameter. 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 strong point.

These parameters may seem scary or alien to you. Fear not, learning what they are is all that 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 required to transmit the carrier signal so that it reaches 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 amongst others.

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

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

Path loss is the attenuation of the signal due to the inverse square law and the earths atmosphere which reduces the size of your signal on its way to the satellite. Inverse square law is explained here.

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

Lastly the margin is used to allow for extra atmospheric attenuation due to localised rain or snow at the transmitter location. For high frequencies this can be as much as 10 dB but an average margin is around 3 dB.

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 at the 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 same value but the same explanation.

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

The C/No Sat is the carrier signal level to noise level ratio of the transmitted signal when it reaches 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 satellites location.

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 independantly controlled to increase or decrease the gain. They are sometimes even organised so that they 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, that it must devote to your signal.

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

The Receive (Downlink) Terminal:

The Rx Freq is the receive frequency determined by the satellite translation freqency. the receiver 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.

The G/T is a measure of the performance of the receiver and is based on the gain of the receive antenna and the noise of the equipment in the receive or downlink chain. G/T is often measured at the input to the Low Noise Amplifier (LNA) and is this case consists of the antenna gain and antenna noise temperature. This includes the feed and any receive filters.

    • Just as in an audio system, a noisy amplifier is not as good as a quiet amplifier.
    • The receive chain is the series of stages the signal goes through from being received at the antenna to being available for demodulation.
    • The Low Noise Amplifier is key to a good antenna system as well as the size of the Antenna.

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 taking into 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 is the data rate in one direction.

Eb/No Req'd is the energy per bit to noise level ratio that is required to provide error free data. 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 signal because 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 Margin obtained, if the Link Margin is too small, extra losses may occur due to atmospheric conditions which cause the link to fail. Rainfall when heavy can reduce received signals by around half.

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

Now we can move on to the real world practical tools for engineers. Spectrum Analysers and how they help to visualise and measure the received signals.
 


Edited by Satcoms UK - 17/June/2015 at 21:05
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