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Sep 23rd, 2008 at 8:22am
The receive AGC will alter the receive gain so that the level of the signal+noise into the demodulator is steady. The AGC has no affect on C/N or Eb/No. The wanted signal and the noise are both amplified equally. To improve the C/N you need a bigger dish, clear sky, a lower noise LNA/LNB, better dish pointing or more transmit power from the far end.
A remote site receiving a steady outlink carrier from the hub needs AGC, mainly to deal with different LNB cable lengths. A hub burst receiver need fast AGC to adapt the gain to each incoming burst, many times per second, coming from many sites all having slightly different levels due to inevitable small misadjustments at the remote sites. It make like easier for the hub burst demod is all the remotes transmit at about the same level and on exactly the same frequency.
If you are at the hub, you can measure the C/N or Eb/No of bursts incoming from each remote site. You can then use this information to tell remote sites to change transmit power level so as to maintain a constant C/N or Eb/No at the hub, during rain for example. This is called closed loop uplink power control. It requires that remote sites have BUCs with a useful power margin. For example, BUCs rated at 2 W but normally operated in clear sky conditions at 0.5 W. This would allow up to 4 times power increase or up to +6 dB ULPC range in rain.
Best regards, Eric.
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