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If you know what you are doing and have all the information at hand, you can work this out in 20 minutes or so and don't need to read this page. If you need an answer right now and have no idea, it will take a couple of weeks to learn and you would do better to seek help from your satellite network operator. Otherwise, this page is intended to be educational.

This page assumes multi-carrier operation, with a variety of carriers in the transponder, and all carriers having the same power spectral density. For the initial part of the calculation a hypothetical single carrier occupying the full transponder bandwidth must be imagined. This carrier will operate backed off, as though it were made up of very many small carriers. It will operate at less than the saturated output EIRP and need less than the PDFsat in the satellite input.

Using a link budget calculator it is possible to work out the required uplink EIRP required from the earth station.

The uplink EIRP refers to the radiated power from the dish and this comprises the addition of the transmitter (BUC or HPA) power and the gain of the antenna. You may need to allow for any long waveguide attenuation losses between the BUC/HPA and the antenna feed flange.

Transmit EIRP (dBW) = Power at the antenna flange (dBW) + transmit antenna gain (dBi).

You can get the required EIRP using either a small dish and high power BUC/HPA or a large dish and lower power amplifier. There are limits on using small antennas and very high powers as doing this causes interference to adjacent satellites. You need to meet intersystem coordination off-axis radiation limits, expressed a power spectral density versus the off axis angle.

Lets make a start. The ultimate objective is how to work out required BUC power. Gather the following satellite information, which may take a week or so:

The satellite beam coverage maps and the locations of both ends of the link on those maps.

The uplink beam map will have uplink G/T contours and associated PDFsat contours. The PFDsat contours on an uplink map will all refer to one transponder gain step setting. Verify with the satellite operator what gain setting is applied to the transponder you are considering and what effect this has on the stated PFDsat contours.

The downlink beam map will show downlink EIRP, referring to the saturated power output of one transponder.

Find the multi-carrier operating point: back-off in and back-off out, and associated carrier to intermod interference ratio. e.g. backoff in = - 4 dB, backoff out = -1.5 dB, intermodulation interference = 21 dB.

Find the transponder bandwidth and uplink and downlink frequencies.

Now invent some suitable terrestrial arrangements:

Uplink antenna diameter, uplink BUC power.

Downlink antenna diameter and downlink antenna system noise temperature.

Put all the data together into a link budget. Adjust your terrestrial ideas to try and get a reasonable C/N up and C/Ndown. Look for a positive C/Ntotal somewhere between approx 4 and 15 dB.

Adjust your transmit earth station size and power, and thus uplink EIRP, to get the correct PFD at the satellite for a transponder operating at the multi-carrier operating point. There is not much scope for altering the required PFD unless you ask the satellite operator to change the transponder gain setting. Normally you can't change the gain setting if there are multiple customers renting space in the same transponder.

Adjust your receive earth station size to create a reasonable C/Ndown, given the satellite EIRP for a transponder operating at its multi-carrier operating point. Modern LNBs are all rather similar but, for precision, read about downlink noise temperature.

Adjust your terrestrial ideas to try get a reasonable C/Nup and C/Ndown and a C/Ntotal that will actually work for you.

Note the transmit power your needed, e.g. 90 watts.

You now have a single hypothetical carrier that represents all the multiple carriers in the transponder merged together. For example your 36 MHz hypothetical carrier is a simulation of 36 separate 1 MHz carriers.

Say the resulting capacity in multi-carrier mode is 36 MHz bandwidth with a C/N of 11 dB.

Choose some modulation method and FEC coding that will work well at this C/N.

Read here about symbol rate, transmission rate and forward error correction. The information rate (your useable data rate) will be less than the transmission rate according to the FEC ratio.

This eb/no calculator may be useful.

The result you are looking for is the information bit rate of the whole transponder, as operated in multi-carrier mode. Note that careful choice of modulation method and FEC coding will optimise the bit rate.

Say the above calculations and assumptions give a bit rate of 30 Mbit/s and an uplink BUC power of 90 watts.

The 90 watts refers to a single hypothetical uplink dish and BUC transmitting a big carrier filling the transponder.

If you want to put up a carrier with 1 Mbit/s you need 3 watt BUC power in clear sky.

If you want to put up a carrier with 10 Mbit/s you need 30 watt BUC power in clear sky.

At Ku and Ka band, where there is significant rain attenuation, it is normal to use a BUC rated at several times the required clear sky power so as to provide a 6dB uplink power control range. +6 dB = 4 times the power in watts.

If you want lower cost per bit by increasing the whole transponder bit rate, to say 50 Mbit/s in the transponder, then try using a larger receive dish. Try asking for a lower gain transponder gain step and thus increasing your uplink power so as to improve the C/Nup. Obtain a higher C/Ntotal and use higher order modulation, e,g 16QAM, and less FEC.

You may optimise the combination of transmit dish size and BUC power to minimise cost, while keeping the uplink EIRP constant. A twice diameter larger dish (+6 dB gain increase) and quarter power BUC will be cheaper in terms of electricity useage.

If you want a smaller dish size you will have to accept a lower bit rate and higher operating cost per bit. A smaller dish affects both the transmit and receive performance.

► Page created 23 Feb 2015, last edited 10 April 2020 Copyright Satellite Signals Limited © 2015 all rights reserved. |