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Referring to the satellite transmit downlink coverage pattern, the saturated EIRP on each contour will be specified. For example +36 dBW at beam centre and +32 dBW at the -4 dB contour.
If a transponder is operated with a single carrier the carrier power will be adjusted to the saturated power, or perhaps -1 dB lower less to minimise distortion and make advanced modulation methods such as 8QAM, 16QAM and 32APSK work better.
For multicarrier operation you might assuming a multi-carrier transponder operating point of -10dB input back off and -4.5dB output back off, the operating downlink EIRP, on the -4 dB contour will be +36 -4 -4.5 = +27.5 dBW. This refers to the whole transponder output power and is the aggregate of all the multiple small carriers present.
Some satellite transponders have linearisers which enable multi-carrier operation nearer to saturation.
For a 2 MHz slot, out of a transponder bandwidth of 36 MHz, the available downlink EIRP will be +27.5 -10 log(36/2) = +15 dBW . This assumes that the total power is more or less evenly divided according to bandwidth. In practice you can buy more power if you need a high power spectral density carrier, but on average other people will have to have lower power per unit bandwidth to keep the total operating point under control.
You could buy say 2 MHz bandwidth and the matching +15 dBW of power and then concentrate all the +15 dBW into a narrow carrier of say 250kHz bandwidth, while leaving 1.75 MHz of your bandwidth unused. You might do better to broaden your carrier with as much more error correction bits as possible.
So put 15 into the link budget for the downlink EIRP.
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► Page amended 25 April 2020
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