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Link Budget

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Posts: 9
Dec 21st, 2020 at 3:03pm  
Thank you so much Eric,

Kindly tell me how to calculate the frequency for Both ends of VSAT Terminal If I have 2 M available BW, both terminal are 1.5m KU band, SCPC,
more over I need to know some basic parameters for VSAT link budget, if you can guide me for formulae and the small definition of LB terms , I would be grateful to you.
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Reply #1 - Jan 5th, 2021 at 4:04pm  
You are trying to make two SCPC links between 1.5m Ku band antennas. You have 2 MHz bandwidth available.
Allocate 1 MHz to each carrier.

When you buy 2 MHz you will be told the centre frequencies for the uplink and downlink. For example 14.227 GHz for the uplink and 10.927 GHz for the downlink.

If both sites are in the same beam you could have 2 MHz contiguous.  You will be able to see your own signals.  If the two sites are in different beams, e.g. one in USA, the other in Africa, then you need two 1 MHz slots, in different transponders. 

Invent a link budget for one carrier, like this: SCPC link budget

Adjust all the input numbers yourself aiming for a workable uplink and downlink. e.g C/Nup = 20 dB, C/Ndown = 13 dB.

If you know the satellite transponder uplink G/T towards your sites then use that data.  You need uplink and downlink beam coverage maps for your satellite (G/T, PFDsat and EIRP). Also the transponder gain setting.

Set the carrier symbol rate and bandwidth to say 800 ksps and 800 kHz to minimise interfering with or being interfered by carriers either side of your two 1 MHz slots.

Set the transmit power to something reasonable, say 0.1 to 100 watts. My guess is that about 2 - 3 watts might by about right.  A 10 watt BUC might be a good idea if you want 6 dB of uplink power control for rain.

Is the resulting uplink C/N any good?. If too high (e.g greater than 25 dB) or too low (lessthan 10 dB) then adjust tx power and carrier bandwidth. Use a larger uplink dish if necessary.

Having made a start with the uplink path you need to find out how much downlink EIRP it will produce. You need the transponder gain setting and the network operations centre's decision on multi-carrier backed off operating point.

To do this yourself play with a full transponder link budget (e.g. set bandwidth to 72 MHz) plus making a reasonable assumption about PFDsat. You are operating multicattier so about 5dB input back off and 2 dB output back off.  The transponder output EIRP will be 2 dB low. Assume about 21 dB carrier to intermod ratio.

A particular input power flux density will produce so much downlink EIRP.  Calculate the downlink EIRP yourself and input that to the link budget. Use a suitable size receive dish.

Check overall C/N is good, considering interference and intermodulation and rain allowance.

Having got a reasonavle overall total C/N (say 12 dB in 800 kHz) then try different modulation methods and FEC coding to increase your customer information rate while keeping the bit error rate and rain fade acceptable.

If you really want the maximum capacity use bigger dishes and Comtech CinC modems and transmit two 1.8 MHz carriers on top of each other.

Read all the transponder operating rules and regulations particularly about maximum off-axis and spurious emissions and power spectral density.

More information about satellite link budgets: about satellite link budgets

Explanation of EIRP:  Explanation of EIRP

Explanation of PFD:  Explanation of PFD
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