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Calculating dBFs from RSSI

My RF chip doesn't have any RSSI output, just ADC I and Q. I was wondering is it possible to determine RSSI from ADC's I and Q values and adjust gain regarding to this? Someone suggested calculating RSSI as a vector magnitude.

$$ RSSI = \sqrt{I^2 + Q^2} $$

Or could I just look at the outputs from ADC and if they aren't using full digital range e.g. 12bits, after some sample period, I make my AGC decision based on this.

Any good papers available about doing digital automatic gain control based only about ADC's I and Q outputs? Because all the papers those I have found from the net, usually use RSSI value for AGC.

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  • See this answer to a related question; I think it covers your question as well. – Dave Tweed Nov 07 '12 at 12:23
  • @DaveTweed Thank you for your answer. I was just wondering because he says: "Normally, you would want to adjust your gain to stay within the maximimum vector rotatable to any phase, ie, I or Q = 2047 and the other zero, or their sum of squares = 2047, so that is what should be considered full scale." Shouldn't it be I or Q = 2047 and the other zero, or their sum of squares = 2047², not 2047? – seed Nov 08 '12 at 12:13
  • Good catch. Fixed. – Dave Tweed Nov 08 '12 at 12:27

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