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Satellite Communications/FGM/ALC mode of transparent transponder


Hi Dave...
Hope you will be alright and doing good....

I want some clarifications about FGM and ALC mode of transponders.

1- For a comm payload, in FGM mode; SFD range is say from -80 to -100 dBW/m2; and normal link uses -90 as SFD. Now if i reduce this SFD value to -98 dBW/m2; will i get a reduction in uplink earth station size? Plz explain..

2- In ALC mode of operation; Output of channel remains fixed so the d/l EIRP remains fixed. Power flux density dynamic range is available at input of channel and gain is adjusted accordingly. Now for multicarrier environment; how the gains of different carriers are adjusted by TWTA?

3- further to Q2; Is there any relationship of the gain adjusted for different carriers to (a)their power  (b) their bandwidth? any formula to calculate this?

Thanks a lot in advance..
Atif Noushad

Hi Atif,

For the effect of the SFD:

1- C/N_up = G/T + 10.log(lē/4pi) + SFD - k (where G/T  is the figure of merit of the transponder, l = wavelength, SFD = saturated flux density, k = Boltzmann's constant.

So the C/N achieved in the uplink is directly related to the SFD setting. If the SFD is made smaller, the resulting C/N that is achievable in the uplink is reduced. On the plus side, less uplink EIRP is needed to saturate the transponder so the E/S can be smaller. You will need to make a complete linkbudget to see the effect.

2- The transponder cannot set the gain for individual carriers. All carriers will be "amplified" equally. Moreover, ALC is typically used for single carrier per transponder. In multi-carrier mode, when one of the carriers reduces in power (possibly due to fading), the gain of the whole transponder will go up, also for the carriers that are not faded. This could be potentially dangerous are the other carriers could then violate their density and cause intermodulation.

3- No, see 2.

Hope this helps, if something is not clear, let me know.


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Dave Suffys


Questions related to design of satellite communication links.


Satellite communication, link budgets, DVB-S2, ACM


Telecommunications engineer, Sales Support Manager at Newtec

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