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BUC Power vs Bandwidth

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Dec 23rd, 2014 at 7:19am  
Hi All,

Can anyone provide me a formula or a calculator about the relation of the BUC power to the Satellite BW? I wanted to know how the BUC capacity (BUC Power) needed for any Upstream Carrier (1 mbps, 1.5mbps, 2 mbps).

Thanking you in advance.

SAT.
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Reply #1 - Dec 23rd, 2014 at 9:48pm  
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 http://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:
http://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.
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« Last Edit: Dec 24th, 2014 at 2:46pm by Admin1 »  
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Reply #2 - Feb 19th, 2015 at 7:19am  
Eric, what do you mean about this "If you want 1 Mbit/s divide the BUC power needed by ratio to the total bit rate."?

thanks in advance
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Reply #3 - Feb 19th, 2015 at 8:33am  
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 3Mbps etc. 3dB more means double the BUC size. (2Watts -> 4Watts , 4Watts->8Watts, 8Watts->16 Watts etc).

Hope this is clear.
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Reply #4 - Feb 19th, 2015 at 10:24am  
Thank you Balkan Telecom for that contribution. Quite right.

Assume a link budget simulation:  A single 30 MHz wide carrier fills the transponder, with the transponder backed off so as to represent multicarrier operation. Input back off -4 dB, output back-off -1.5 dB. You get 11dB C/N overall, so it works and you may then choose any modulation/coding you wish that is fine with 11 dB C/N.

Say it needs 45 watts BUC power with a 1.2m uplink dish.

Having completed the design you could then fill the transponder with:
30 individual carriers, each 1 MHz wide, from 30 1.2m dishes, each with 1.5 watts BUC power.
or
15 individual carriers, each 1 MHz wide, from 15 1.2m dishes, each with 1.5 watts BUC power.
and 1 big carrier 15 MHz wide, from a 1.2m dish and 22.5watts.
or
15 individual carriers,  each 1 MHz wide, from 15 1.2m dishes, each with 1.5 watts BUC power.
and 1 big carrier 15 MHz wide, from a larger 2.4m dish with 5.625watts.
Note that the bigger 2.4m dish has 4 times (+6dB) the gain of a 1.2m antenna, so to get the same EIRP you divide HPA power by 4, or -6 dB.

Do ask if you or others have more queries.

Best regards, Eric
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Reply #5 - Feb 22nd, 2015 at 5:33am  
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!
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Reply #6 - Feb 23rd, 2015 at 5:18pm  
I've tried to answer the question here:
How to work out BUC power

Best regards, Eric.
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