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# Link Budget for very low bit rate return links from VSAT remote sites in North Africa

## Assumptions:

Return links from VSATs to the hub are expected to comprise very small TDMA or SCPC carriers with QPSK 3/4 rate FEC with 9600 bit/s customer information rate.

e.g. 9600 bit/s,   12800 bit/s transmission rate.  6400 sps symbol rate,  noise bandwidth 7680Hz    10,000 Hz spacing

All remote sites are on the uplink contour of +6 dBK and transponder gain step is set to PFDsat=-91 dBW/m2

The transponder multi-carrier operating point is -6 dB in and -2 dB out.  C/Intermod = 21 dB at this point.

Assume the whole transponder is filled with 3600 carriers, each with 10 kHz allocated bandwidth.

Total power spectral density produced at the satellite of all carriers combined is -91 -6 = -97 dBW/m2

Saturated downlink EIRP towards the hub site at beam edge = 52.5 dBW.

Operating PFD produces a total downlink EIRP = 52.5 - 2 = 50.5 dBW

This conversion (-97 in and + 50.5 out = 147.5 ) has been put into the above link budget as Eric's fiddle factor. In this case applies to multi carrier operation. I have joined the uplink to the downlink in the maths. The factor needs to be changed if you adjust the satellite gain step attenuator or operating point - for example, for a single big carrier operation at saturation.

The remote site transmit BUC is 0.5 watt rated, operated at 0.02 W in clear sky conditions.  If you give them higher rated power BUCs beware the way the remote people will keep turning the level up till your whole transponder is over-saturated and no one is getting any service!

The input figures are all guesses and estimates.  You need to overwrite the figures with your own assumptions.

Amend any white boxes and then click the calculate button to obtain results in the green boxes.

Using the default values gives an overall C/N of 14 so it will work well.

The link budget above is not yet optimised for maximum bit rate for least cost.  Try getting the 9600 into 5 kHz.

Modulation options include BPSK. QPSK, 8-PSK, 16-QAM, 32-APSK. The higher order versions need a higher C/N to work but occupy less bandwidth.

FEC options range from about 1/4 to 9/10.  The higher versions, like 5/6 or 7/8, need a higher C/N to work but generate less transmission bit rate so occupy less bandwidth.

You need details of your equipment (both transmit and receive) capabilities. This document https://www.satelliteguys.us/xen/attachments/dvbs-2_guide-pdf.33411/ gives details of high bit rate Tandberg DVB-S2 equipment, which has a wide range of modulation and FEC options.

Note that in networks where the hub can tell the remotes what to do it is possible to implement adaptable modulation and coding, which is responsive to the weather and operational degradations.

You won't get any satellite owner  to lease you 5 kHz but you might lease 1 MHz and then put up 200 of your own carriers. Work out how you are going to get your receivers to tune to correct carrier. Concern yourself with phase noise also.