1 meter is *probably* a safe assumption for coexistence of 802.11 and
802.15.4 in most use cases.
It is definitely *not* a good assumption for 802.11 and BT. These already
are collocated in the same deice (phone, PDA, laptop) and require
simultaneous operation. One reason it works is both are operating at fairly
low duty cycle today and so there is the opportunity for good luck. Some
cleverness, some luck ;-).
-Ben
----- Original Message -----
From: "Christian Herzog" <[EMAIL PROTECTED]>
To: "'Mustafa Hasan'" <[EMAIL PROTECTED]>; "'Adams Jon'"
<[EMAIL PROTECTED]>; <[EMAIL PROTECTED]>
Cc: <[email protected]>
Sent: Tuesday, July 15, 2008 8:44 AM
Subject: Re: [6lowpan] Regarding coexistence with 802.11n
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Mustafa Hasan
Sent: Tuesday, July 15, 2008 8:08 AM
To: Adams Jon; [EMAIL PROTECTED]
Cc: [email protected]
Subject: Re: [6lowpan] Regarding coexistence with 802.11n
Dear Jon & Geoff,
Thank you very much for such a nice and detail mail.... It helped me
a
lot. I want to add few comments regarding this issues, after my
little
research in this field.
1. Even thr are some free channels available for different region,
but
still creating problem for IEEE802.15.4 traffic. Actually, high
power
of wifi is killing ieee802.15.4 traffic (testbed results).
2. The current research on this issue is interestingly biased, some
are saying its not a big problem (e.g ZigBee Alliance) and others
(e.g. Z-Wave Alliance) are saying its a big issue.
The 802 family of standards has some modicum of coexistence "built-in"
as Jon suggested - our experience is that a serious interference
source are non-802 devices including various FHSS devices that pay
little to no heed to whatever else is in the band. The often have no
coexistence considerations and blatantly use the band as if they were
the only device in town.
Some of these devices are getting smarter though (but for their own
selfish reasons). Various first generation 2.4GHz cordless phones
used to horrible spectrum users by using the entire 2.4GHz band with
little regard for anything else. They seem to be taking some steps
now to segment the band and avoid Wi-Fi sized areas now; probably not
to help Wi-Fi but to make themselves work better by avoiding sections
of the band that don't work well for *them*.
3. Distance between ieee802.15.4 and ieee802.11x is a big problem in
this case. If I summarize all the research efforts (ieee802.19
coexistence assurance documents, white paper(ZigBee, Z-Wave) and
many
research papers), its not possible that 15.4 & 11x stay together in
a
distance of <2 meters (even tried with offset 20MHz).
Our practical experience is 2 meters is plenty in most situations,
even 1 meter is enough but getting riskier.
A lot of the vendor/organization white papers seem incredibly biased
to me and upon reading them, I basically get the idea that the network
running right on my desk couldn't possibly work (and might even kill
me ;) when clearly it is working.
4. thr are many potential application require this distance to be in
few centimeters. May be some technique in upper layers can give
little
bit reliability... but is it enough???
I am looking for some help, comments and suggestion, how coexistence
problem can be solved for very short distance scenario...
For a few centimeters, this problem may very well be unsolvable (at
reasonable cost/effort). The only way the upper layers can
accommodate something as brutal as this is through coordination and
knowing that the radio will effectively be "blinded" at specific
times. For uncoordinated networks, this isn't really practical.
At centimeter distances, *everything* can act like an antenna and it
is very easy to saturate a radio receiver which effectively blinds it
from receiving anything - as you pointed out, even significant
frequency separation isn't enough when things are closely located;
there is just so far to run in a relatively small band.
Even closely located *receivers* can interfere with each other even if
they never transmit; we have had to use shielding between closely
colocated receivers in some multi-receiver designs to prevent them
from interfering with each other. In simple terms, if it oscillates,
it radiates and something either sensitive enough or close enough will
get effected by such radiation possibly to the point of incorrect
operation.
Few centimeter separation of transmitters is a difficult problem to
solve and would likely be out of scope of this effort.
--
Christian Herzog Software Technologies Group, Inc.
mailto:[EMAIL PROTECTED] http://www.stg.com
+1 (708) 547-0110 x225 FAX +1 (708) 547-0783
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