Satellite Communications/Link budget
I have some doubt about link budget on the uplink.
All analysis start with finding the Eirp from the ground station: given the required IPDF for the specific transponder and taking into account the G/T countur value for the ground station location, spreeding loss, pointing loss and atmospheric losses are added to find the required EIRP to saturate the transponder. Here is my doubt: once we find the EIRP necessary to have a good working uplink as specified by satellite operator (with that EIRP we get the required IPDF) why the link budget goes on to find C/N on the uplink. Besides, on the C/N calculation FSA is considered instead of spreading loss which is a highest "loss" with respect to spreading loss meaning that I am not going to reach the required IPDF anymore. I am really confused.
First let me correct some terminology from your question. I am not sure what is IPDF, but I understand that it may mean Power Flux Density (PDF) and IPDF may mean Input Power Flux Density (at transponder from Up link, if so it should be IPFD, after considering IBO at transponder Input). Normally it is termed SPFD (Saturated Power Flux Density) in Satellite Technology.
EIRP is the power level calculated for SPFD but the transponder INPUT is receiving NOISE also along with this POWER in Up link, so effective C/N also is to be calculated for that EIRP. Not just saturation levels but noise performance calculations are required for ANY RECEIVE System for capability to detection and processing: EIRP may be very high but if C/No is also LOW, the transponder cannot process, reason being it cannot detect carrier from noise.
So the transponder Uplink budget provides C/N for transponder processing which is considered with Transponder gain (combined Power and Noise) and provides what may be the Output C/N of the transponder.
So calculation of C/N is as much important and is more necessary to enable Link performance optimisation, both in Up and Down links.
For calculation of Noise performance Band width and Modulation methods are considered to set the margins. Noise band width is calculated on certain empirical guidelines that give optimised performance, and practically realisable Margins.
Hope this clears your doubts!