Link Budget for Xbee

Hey Folks,
I was curious about the Xbee range in free space.  I put together a spreadsheet with a derivation of the Friis Equation (somebody check my work if they get a chance).  I know very little about 802.15.4, so I cannot speak in an educated manner about target link budgets, interference avoidance, packet detection, guardbands etc.   Some things you should know:
  • Digi claims that the sensitivity of the xbee is -100dBm, I suspect that it is not quite that sensitive
  • Digi does not provide an SNR requirement, so I would recommend 3dbm for the lowest datarate (this is similar to 802.11b)
  • Noise in the 2.4 spectrum outdoors is very high especially in places with tall buildings.  XM Radio puts 40kW terrestrial repeaters in cities with tall buildings in an adjacent spectrum.  I have not taken noise into account, but if you want to be conservative change the sensitivity to -92 dBm
  • I used 0db gain for the antennas because I assume that the antenna gain of a simple 3dBi omni is negated by polarity mismatch - however I do have some ideas about how to reliably put a 3dbi Omni on the plane with a fixed 6 dbi "uptilt" omni (or 6dbi patch) on the ground.  This would have the effect of more than doubling the range.
Anyway, enjoy the calcs.

http://spreadsheets.google.com/ccc?key=0AqpOpoOZx7jhdFdsR1FlcGJ1ZEh2N0NrNWdNNGpsdkE&hl=en


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Replies

  • I figured out the xbee SNR. To get an Bit Error Rate in the 10^-5 range (certainly good enough) you need an SNR of 5dB. That makes sense I guess.
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