CS C/N after ASI

Assuming the adjacent carriers are of similar power spectral density.

I would follow the modem manufacturers recommendations regarding carrier spacing and ignore Adjacent carrier interference in the link budget. If someone insists on seeing a figure I suggest something like 28dB.

There are many interferers you can include in the link budget, terrestrial, adjacent beams, adjacent satellites, cross-pol, intermodulation, up and down. Typical values 21 - 30dB. You choose..

You can spend ages in the lab measuring BER versus carrier spacing and writing up learned reports, but I doubt it is worth it. Contact the modem manufacturer. They will have done studies and be able to claim wonderful filter roll-offs and carrier spacings.

What does CS mean ?

Best regards, Eric

Choose your MODCOD based on overall link budget C/N and the bandwidth you have leased. Theoretically you should choose the one that gives maximum data throughput, but if your system is marginal and subject to rain, has limited or no uplink power margin etc then a more conservative and less efficient MODCOD is advisable, particularly if your customers will complain at the slightest outage.

Eric, CS means Clear Sky

Thanks alot Eric for the clarification.

If your application is VSAT uplinking of outside broadcast TV then a high uplink bit rate is appropriate to the TV quality required.  A lower downlink speed will be needed for feedback and control from a hub studio.

Two way video needs the same high bit rate in both directions.

Two way voice needs the same bit rate each way.

Continuous services, like the above are very demanding on satellite capacity, and expensive. Dedicated capacity is required to avoid congestion causing breaks.

Internet browsing via VSAT needs a high download speed and low uplink speed, ratio approx. 5:1 to 10:1.  Internet browsing is highly intermittent and may average only 5 kbit/s up and 30kbit/s down, so cost is low as the available capacity of the satellite is time shared by many users. Congestion and packet delays may occur.

Simple answer:  Ask the satellite operator to tell you what you need.

Complex answer: So you get to learn and understand about link budgets :

The first step is to obtain information about the satellite:

For the uplink and downlink beams and transponder of interest:

Downlink beam coverage map marked with EIRPdown (dBW) contours.

Uplink beam coverage map marked with G/Tup(dB/K)  and PFDsat (dBW/m^2) contours.  Ask and clarify the PFDsat for your particular transponder and your location.  The PFDsat contour values can probably  be changed on a per transponder basis by altering the transponder gain setting on the satellite.

If you rent just a portion of a transponder, say 2 MHz, you additionally need the Transponder Operating Point. This is a pair of "back-off" figures, e.g. - 4dB input back-off and -1.4 dB output back-off. Ask what is the carrier to intermodulation noise ratio at this operating point.

Find out the location and diameter of the far end hub earth station that will receive your outside broadcast signal.

Your objective will be to determine what uplink dish and power you need, for your choice of bit rate.

Having obtained all the above you can then start playing with link budgets.
Start here:  https://www.satsig.net/link-budgets/paksat-vsat-return-link.htm

Note that outside broadcast TV tends to involve a small antenna and a moderately high power transmitter.  Such arrangements (antenna size and power spectral density) must be approved by the satellite operator as there is risk of unacceptable interference into nearby satellites.  Always transmit at low power first under verbal supervision from the NOC.   Beware the RF radiation hazard from high power microwave transmitters.

If someone says you can't transmit your wanted bit rate then ask for an explanation.

Inmarsat provide world wide beam coverages, suitable for maritime use.
Read: https://www.inmarsat.com/wp-content/uploads/2013/10/Inmarsat_FleetBroadband_Best...
Offering 256 kbit/s streaming data (up or down) with 55cm radome antenna and 3 axis dynamic tracking.
Higher quality video may be recorded on the ship and then later played back and transmitted at lower speed. 

If you intend to operate in a limited sea area where there is good beam coverage from a national or regional satellite operator then you may find a better deal.

Hello,
Please explain me  what is off axis, I found this in some document but I am not sure what is off axis . for example off axis affect to EIRP spectral density ....

so that means power density of sidelobe? if diameter large will have low sidelobe will better then smaller dish?

Thanks,
Is highier frequency will smaller beam width (boresight)  ?

thanks,
Could you tell me how to choose an antenna?

thank you so much,
i see many modem use spectral spread, what is benefit of technology