RX signal level calculation

Ricardo Galli gallir at uib.es
Fri Feb 21 07:53:23 EST 2003


On Friday 21 February 2003 13:01, Location Proj shaped the electrons to 
shout:
> Im also trying to determine indoor position using signal strength
> values. We are seeing a small fairly predictable variance. Our cards
...
> are 1.4.2 or 1.4.9 with the ram upload, both give a signal readings of
> between 1 and 100.  Id prefer cards that gave a higer resolution but we
> have had good results with these (1-2 meters) so far.
>
> How high is the variance you are seeing?  ours is approx 5 vaues (ie
> 40->45 or 60-65 etc..)

It depends on the antenna, and if your are using diversity ot not, and the 
txpower of the transmitters.

By fixing the txpower and enabling just one of the antenna (even if the 
card has an external antenna they have a second one, which is not 
disabled by default), I've got a variance of 1, with diversity antennas 
up to 65. 

With variable txpower, you will see a "low frequency" variation.

Check some of the plots in:

http://mnm.uib.es/~gallir/80211tracking/

The first image in http://mnm.uib.es/~gallir/80211tracking/data3.png 
(three different locations) is the signal coming from a Prism2 on a 
Windows machine. The second one if from a hostap in master mode.

In http://mnm.uib.es/~gallir/80211tracking/data4.png (four different 
locations) I put an external (omni) antenna in the receiver. The fiorst 
image is from the same AP as before. The second is from a iBook with 
Linux and the kernel airport driver (default configuration).


-- 
  ricardo galli       GPG id C8114D34



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