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GSAT-18 C-band link Budget : Outlink from teleport to a remote site

Satellite: GSAT-18 at 55E.

Transponder Bandwidth (MHz) 36 MHz
Uplink / Downlink Frequencies (GHz) 5950 / 3725 MHz.
Cross-Polarization Separation (dBW)  assumed 27 dB up and down.
EIRP (saturated) (dBW) 38.0 dBW
Uplink SFD (dBW/m) -85 dBW/m^2
Uplink G/T  (dB/K)> -2 dB/K
Carrier to Intermod = 22 dB

Although we only want a small carrier, say 2 or 8 Mbits, we will start by preparing a link budget for a whole transponder set at the specified input and output backoff operating point, suitable for multi-carrier operation. The resulting earth station uplink power will be far too high for a single small carrier, but it is easy to then divide the total uplink EIRP to get the required earth station EIRP for whatever uplink carrier size we want.

This initial simulation is of the aggregation of all the many small carriers. This is NOT a single carrier per transponder, big 30 MHz TV carrier simulation.

Input and output back off = -6.5 and -3.5 dB. The purpose of this is to operate near the top of the linear region where the intermodulation level will be acceptable (typically 21 dB). Higher up the intermodulation degrades the overall result, lower down the lower power degrades the overall result. Modern satellite often have output amplifiers with linearisers which means they can operate nearer saturation and still get 21 dB C/IM. Quite obviously operating with 3 dB output back off means wasting half the satellite output power. So try to combine all your traffic into one large 30 MHz carrier using the full transponder power, with perhaps 0.5 dB output back off to help advanced modulation methods like 8PSK and 16QAM to work well.

So downlink EIRP is 38 - 3.5 = 34.5 dBW

Uplink PFD = -85 - 6.6 = -92 dBW/m^2

Assumed range to satellite = 37600 km (using your correct figure is suggested)

Fill in the details below and see what is the uplink PFD at the satellite.  Adjust the BUC power and possibly the uplink earth station diameter to get get the required uplink PFD

Obtaining the PFD of -90 dBW/m^2 is important as this will reflect back and tell you there required earth station EIRP per 36 MHz. It is then very easy to determine the earth station EIRP required for any smaller carrier occupied bandwidth = X - 10 log( wanted carrier bandwidth / 36)

I have adjusted the uplink earth station size and power to get a PFD of -90 dbW/m^2. The result was 6.1m dia dish plus 190 watts. You can adjust these as you wish, while keeping the PFD at -90 dbW/m^2. If you already have an uplink dish use the size for that and adjust the power only.

Now adjust the figure marked Transponder 'gain' till you get +34.5 dBW as the EIRP down. In this case 124.5 is needed. This number links the input to the satellite to the output from the satellite. Don't adjust this figure from now on. The satellite is now operating in backed-off, linear, mode suitable for multi-carrier operation.

Uplink frequency GHz
Uplink antenna diameter m
Uplink antenna transmit gain dBi
Uplink antenna, power at the feed W
Uplink EIRP dBW
Range (35778 - 41679) km
Uplink path loss dB
Uplink pfd at satellite dBW/m^2
Bandwidth Hz
Satellite uplink G/T dB/K
Uplink C/N dB
Transponder "gain" input PFD to output EIRP
Downlink frequency GHz
Downlink receive antenna diameter m
Downlink system noise temperature(antenna+LNA) K
Downlink receive antenna gain dBi
Downlink receive antenna G/T dB/K
Downlink satellite EIRP dBW
Downlink path loss dB
Downlink C/N dB
Uplink C/interference dB
Uplink C/N dB
Satellite C/intermod dB
Downlink C/N dB
Downlink C/interference dB
Total link C/N dB

You can now adjust the the receive earth station size to obtain a total clear sky C/N that matches your proposed carrier type, nodulation method and FEC type and ratio. I have chosen a 3.7m receive antenna and obtain 12.34 dB clear sky C/N. If this is acceptable now proceed to determine the uplink power needed for any carrier bandwidth. If not acceptable then change your requirements for modulation, coding or receive earth station size.  It is up to you how you use the power and bandwidth of your lease. Try to use both up equally, which may mean careful choice of modulation and coding to bet the highest information bit rate versus weather margin etc.

For example try a leased occupied bandwidth of 3.6 MHz.   This is a 10 times reduction on the 36 MHz transponder bandwidth. So reduce your uplink power from 190 watts to 19 watts.  Simultaneously reduce your noise bandwidth from 30 MHz to 3 MHz (input 3000000 Hz). You should see that the overall C/N remains unchanged at 12.25 dB.  Use the formula 10 log (B/36MHz) to get the ratio in dB

When trying your own carrier sizes start with the wanted information bit rate. Add modulation, FEC, roll off, and performance data from manufacturers specifictions. You need to know what is the noise bandwidth (approx same as symbol rate) and the occupied bandwidth. The occupied bandwidth is what you lease - and pay for, so that you don't interfere with the adjacent users of the transponder.


Any problems or comments please e-mail eric@satsig.net

Page started: 11 Dec 2018, amended 13 Jan 2019