[Tinyos-help] RSSI to dbm convertion, puzzled me for a month!

[陶利强]

Hello everyone:
  I am implementing a power control algorithm in mica2 node, I modify s-mac to add the power control part.
  I use the formula  Pt-min=Sensitivity*Pt/Pr to compute the minimum’s transmitting power.  where Pr is  RSSI, Pt is sender node current transmitting power, Sensitivity is -105dbm 
  According to  http://www.xbow.com/Support/Support_pdf_files/MPR-MIB_Series_Users_Manual.pdf
  RSSI(dBm)=-51.3*Vrssi-49.2 for 433 and 315 MHz Motes
  Vrssi=Vbatt*ADC_Counts/1024 ( here ADC_Counts is the raw_rssi, is it correct?)
    Vbatt=Vref*ADC_FS/ADC_Count
  Where:
  Vbatt=Battery voltage
  ADC_FS=1024
  Vref=External voltage reference=1.223V
 
 So Rssi (dBm) =-51.3*(Vbatt*raw_rssi/1024)-49.2 
My program is as follows:
  // convert rssi to dbm 
   rssi=(-51*voltage*raw_rssi/1024 )-49;      // convert rssi to dbm
     here, voltage I use mv,not v , is it correct?
 
I do experiments, the result show raw rssi is [15,29] [31,48] [65,85], when distance is 1meter,7meter,and 14meter.
But when it converted to dbm using the above formula, the result is  [-1783 ,-887 ] ,[ -2956,-1936 ], [-1987 , -712], I know the result is wrong. But, I can’t find where the problem is?
 

   

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