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

From: Matthias Wilhelm
Subject: Re: [Discuss-gnuradio] UCLA ZigBee and the Capture Effect
Date: Thu, 23 Feb 2012 12:14:31 +0100


you should check out ucla_ieee802_15_4_packet_sink.cc, the receiver checks if a 
valid chipping sequence for a payload symbol was found (after synch), and 
starts searching for a new SFD in case there is no possibly matching symbol. 

unsigned char c = decode_chips(d_shift_reg);
if(c == 0xff){ // something is wrong. restart the search for a sync
} ...

What might happen is the following:

preamble, SFD (jammer) -> sync ... payload (jammer) ... first 0 of preamble 
(sender) completely destroys a symbol such that the receiver gives up ... 
receiver syncs with the rest of the preamble (sender) and happily receives 
packet (sender).
To prevent such "re-capture", you can modify the receiver to just continue 
receiving anyway, and picking some symbol at random when no decision can be 
made. Or you can fiddle with the threshold of how many chips must match, it may 
be too conservative (decode_chips should always return a symbol in this case). 
I think this is also what most COTS receivers do, they honor the packet length 
in the PHY header and try to find symbols in any case.


Am 23.02.2012 um 11:49 schrieb address@hidden:

> Hi Matthias,
> Thanks a lot for your suggestions.
> The thing is, one transmitter (jammer) is sending packets nonstop at a 
> constant transmit power, while the second transmitter (sender) sends a packet 
> every millisecond or so.
> In this scenario, the receiver is always synchronized with the jammer and 
> even if the signal from the sender is stronger, the receiver only decodes a 
> fraction of the packets from the sender (around 1%). This sounds reasonable 
> as the receiver locks itself to the signal from the jammer when nothing else 
> is around and stays locked (remains decoding the incoming signal) even if a 
> stronger signal from the sender arives. (Only between the jammers packets, 
> when the receiver looks for new preambles, there's a chance for the receiver 
> to sync with the sender instead of the jammer.)
> But at some point, when the signal from the sender is significantly stronger 
> (around 7-10db), the behavior changes and all the packets from sender 2 get 
> decoded.
> Now it surprised me that this happend at all, that the packet delivery rate 
> from sender to receiver increased at some point instead of only relying on 
> the timings of preamble and sfd of sender and jammer. That's why I'm looking 
> for any phase lock loops and control mechanisms related to the capture effect.
> Any suggestions?
> best regards and thanks again for your time
> B
> Zitat von "Matthias Wilhelm" <address@hidden>:
>> 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

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