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VSAT technology and installation >> iDirect Forum: hubs and terminals >> TX power vs Return Carrier Bandwidth
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Message started by mmurtas on Jun 7th, 2011 at 9:49am

Title: TX power vs Return Carrier Bandwidth
Post by mmurtas on Jun 7th, 2011 at 9:49am
Hi all,

I have an iDirect Network with one forward carrier and one return carrier.
I have to increase the bandwidth of the return carrier.
Some satmodems are working with the maximum transmission power (I calculated this value under the saturation point of the BUC).
If I extend the bandwidth of the carrier (with the same modulation and FEC), satmodem will to increase the tx power in order to reach a nominal Upstream C/N.
Is there a relation between Carrier Bandwidth and TX power?

Thanks.

Title: Re: TX power vs Return Carrier Bandwidth
Post by Eric Johnston on Jun 8th, 2011 at 2:03pm
Is there a relation between Carrier Bandwidth and TX power?  Yes, a direct relationship.

If you extend the bandwidth of the carrier (with the same modulation method and FEC ratio), you will need more power in direct proportion to the bandwidth.

e.g.  
300 kbit/s needs 1 watt
330 kbit/s needs 1.1 watts  (increase is 10log330/300= 0.41 dB)
600 kbit/s needs 2 watts   (increase is 10log600/300= 3dB)

If you have run out of power increase at a remote site consider managing with less than the normal 6 dB uplink power control range for rain fades.  With a 3 dB margin you will double your traffic capacity in clear sky but will increase the outage time per year due to rain fades.

Consider re-peaking the dish pointing at the worst sites. Often the transmit pointing can be improved as the transmit beam is narrower than the receive beam. Get hold of an uplink beam coverage map (with contours marked in terms of satellite uplink G/T) and compare your site locations with the satellite uplink beam sensitivity. You can then accurately compare sites.

If you change to an FEC ratio that permits satisfactory operation at a lower Eb/No you will need less watts for the same information rate, but it will cost more in terms of satellite bandwidth (MHz) for no increase in bit rate.  Increasing bit rate will then be possible but at a further increase in satellite bandwidth costs.

When the number of dishes in the network is small the least cost route is often to have larger dishes and highly efficient modulation and FEC schemes. e.g. 8-PSK and 7/8 FEC, rather than say QPSK and 1/2 FEC.

Best regards, Eric.

Title: Re: TX power vs Return Carrier Bandwidth
Post by mmurtas on Jun 9th, 2011 at 11:33am
Ok thanks, the explanation is very clear.

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