BUC Power vs Bandwidth

If you keep the modulation method (e.g. BPSK, QPSK, 8PSK etc) and the FEC ratio (e.g 1/2, 7/8. 0.91 etc) unchanged then the BUC power required is directly proportional to the bit rate, so you need twice the power for 2 Mbit/s compared with 1 Mbit/s.

The above information is only useful if you already know the specific BUC power required for a specific bit rate.

The bandwidth (measured in Hz, kHz or MHz etc, and to be paid for as a fractional transponder lease) depends on the information bit rate, FEC ratio and modulation method. See https://www.satsig.net/symbol01.htm for example calculations.

If you do not know the BUC power required for any bit rate you must start with a link budget.

Talk to the satellite operator and get their help, explaining what you want to do such including your uplink bit rate and preferred dish size and frequency band. They will be able to do full link budget calculations for you taking into account your location and the relevant teleport hub location and receive dish G/T. These calculations will result in the EIRP required from your site. You can achieve this required EIRP using a variety of BUC powers and transmit antenna size (and gain) as EIRP = BUC power + antenna TX gain. There will be a lower limit on your dish size to minimise interference from you into adjacent satellite networks.

Try playing with a link budget like this example:
https://www.satsig.net/link-budgets/paksat-vsat-return-link.htm

Note this provides for a hypothetical composite carrier of about 30 MHz bandwidth with a C/N of 11 dB. Choose a modulation type and FEC that fits this (allowing for rain margin/availability and whatever BER you want.). Determine the composite information bit rate (all carriers active).

If you want 1 Mbit/s divide the BUC power needed by ratio to the total bit rate. Reduce the BUC size further by using a larger dish.

Note that in an iDirect network you need a margin for uplink power control. A BUC rated at 4 times the clear sky power is suggested (giving +6dB UPPC range).

Best regards, Eric.

Hi,

It means if your uplink is 1Mbps and working fine then you will need 3dB more BUC power to uplink 2 Mbps, 6dB more BUC power for 4Mbps etc. 3dB more means double the BUC size. (2Watts -> 4Watts , 4Watts->8Watts, 8Watts->16 Watts etc).

Hope this is clear.

Dear Balkan Telecom and Eric, thanks alot for your explanation! I still have a question, how do we come to the conclusion that 1 MHz needs 1.5 Watt which is needed for 1 Mbps throughput?

Thanks in Advance!

I am using 1.8 m Antenna C band for remote &
5 watt BUC.
My customer want to use 4M internet access.
Is it possible..?
if not how much BUC power should to need to gain
internet 2-4M for customer..?
thanks

Just a recap on how to get started with link budgets:

First you need to get a good understanding of the G/T, PFD and EIRP data about the satellite transponder.

Get hold of the coverage beam maps.

The uplink coverage map will show uplink G/T contours. Note down the G/T for your uplink location. This is the only reliable figure which does not need adjustment.

The uplink coverage map contours are also needed to find the uplink PFDsat for your site. PFDsat figures are shown against each contour lines. Unfortunately you need to know  what gain step setting of the transponder they refer to. Ask the satellite operator.  They can tell you what is the gain setting of each transponder. Make sure you understand which contour the PFDsat refers to. It might be the beam centre. It might be some beam edge (e.g -4.5 dB down), or other contour.

Also, while you are at it, ask what input and output back off they require for single carrier and multicarrier operation and what carrier/intermod is expected. For single carrier in the transponder 0 dB or 0.5 dB output back off might be applicable, for multicarrier operation maybe -2 to -3 dB. 

The downlink EIRP coverage map will show the EIRP levels on the downlink beam when the transponder is saturated with a single carrier. When the transponder is operated with multiple carriers the transponder operating point needs to be backed off to reduce intermodulation interference to about -21 dB. The output back off might be something like -2 or -3 dB reduction from maximum EIRP, depending on how linear is the satellite downlink high power amplifier.

You will often find that actual operating conditions do not match your calculated link budget. The reason may be that the contours are "the specification" and the actual satellite performance is better, e.g. higher G/T and higher EIRP. Over the satellite life the performance may degrade to "the specification" values. Your own allowances for uplink, intermodulation and downlink interference will not match reality, varying over the years.

Now you can start a link budget calculation.

I hope that helps.