Re: [Discuss-gnuradio] UCLA ZigBee and the Capture Effect

[Matthias Wilhelm]

Hi Bjoern, 

the receiver uses FM demodulation to track phase changes, and when two signals 
collide the stronger one simply has the larger influence on the overall phase. 
This is actually a good property, because you still have the chance to receive 
one of the colliding packets.

Do you want to go on receiving the weaker packet? This may be quite tricky, 
because you cannot just separate the signals. One way I can think is successive 
interference cancelation, first receive the stronger packet, and subtract its 
signal and start receiving the weaker one. There is a ("CS-style") paper on 
this:  
Daniel Halperin, Thomas Anderson, and David Wetherall. 2008. Taking the sting 
out of carrier sense: interference cancellation for wireless LANs. In 
Proceedings of the 14th ACM international conference on Mobile computing and 
networking (MobiCom '08). ACM, New York, NY, USA, 339-350. 
DOI=10.1145/1409944.1409983 http://doi.acm.org/10.1145/1409944.1409983

Regards,
Matthias

Am 22.02.2012 um 11:42 schrieb address@hidden:

> Hi everyone,
> 
> First of all thanks a lot for any support!
> 
> I'm use the UCLA zigbee PHY (IEEE 802.15.4) on three nodes, of which one is a 
> dedicated receiver and the other two nodes are transmitting simultaneously 
> (No CSMA!).
> 
> I noticed that something like a capture effect is taking place, meaning that 
> even though the signal is synchronized to a weaker signal (signal 1), the 
> receiver appears to jump to the stronger signal (signal 2) as soon as it 
> arrives at the receiver (if the second signal is significantly stronger then 
> the first one).
> 
> I would like to influence this behavior, and hence am looking for the related 
> parameters. I just can't find them! Does anyone have a clue where this is 
> happening?
> 
> I am using the XCVR2450 Daughterboard together with the USRP1.
> 
> Any hints, suggestions or detailed help will be highly appreciated!
> 
> best regards and thanks a lot,
> 
> B