On Tuesday (07/02/2002 at 03:08PM -0500), Carl Wilhelm Soderstrom wrote:
> > No kidding.  Bluetooth has been seen to take out big chunks of 802.11b
> > WLANs and for a while, many large 802.11b sites were banning
> > Bluetooth gadgets from their premises.  The two definitely don't
> > coexist... at least in close proximity.
> is this just a matter of channel collision or what?

I think it's primarily an issue of front-end overload.  Whenever you have
a high power transmitter (relative to the received signal strength) operating
in the same passband, nearby, there isn't much you can do to keep it from
desensing your receiver's frontend.

So, the bluetooth transmitter basically steps on anything the 802.11b
card might receive.

Even though they are spread-spectrum, they also have a dynamic range which,
in order to keep the cost at a point we are still interested in, is
not what it could be in order to keep out the offending signal(s).

You get the same behavior often when trying to run a 2.4 GHz cordless
phone near your 802.11b stuff.  The phone just knocks the WLAN equipment
right to zero... even though they are on different "channels" and use
different spreading sequences, they're in the same passband and the
poor receiver gets nailed by the nearby transmitter.

This is the basis for the argument that XM Satellite Radio and Sirius were
making when they claimed that 802.11b stuff was screwing up their satellite
radio service--  because they basically have designed such poor receivers
(in order to keep the cost really low) that they can't reject out of band
signals 50 MHz away.  The cheaper they go, the more they let in.  The
closer the signals are in frequency, the more it costs to keep the offending
ones out.  If they're on the same frequency, as with 802.11b and Bluetooth,
you can see the challenge.

> bluetooth seems like too good a thing, to make me want to pass it up. if it
> can be made to coexist nicely with 802.11b, I'd like to know how. maybe this
> would be a good topic for a TCWUG meeting?

Hmm...  move your 802.11b to 5.7 GHz ?  :-)

Chris Elmquist   mailto:chrise at pobox.com   http://www.pobox.com/~chrise