[OLSR-users] Fast Roaming Nodes

[Weber, Michael J. (US SSA)]

John,

I think you may need a new envelope...

I get 816.67Hz of doppler shift for 2450MHz at 100m/s closure using the
equation F' = Fo * (1/(1+(v/Co))), where v is closure speed and Co is
speed of the wave in the medium in question. For our purposes, v is 100,
Co is 3*10^8, and Fo is 2.45*10^9. If you plug common X band radar
frequencies in instead, you get the familiar ~10Hz/mph shift at X band
that we hear on the audio monitor of traffic radar. This equation is
good only for non-relativistic speeds. :-) I got it here:

http://imagine.gsfc.nasa.gov/YBA/M31-velocity/Doppler-shift-2.html


Still, I expect it will be quite a challenge to route any train-bound
traffic when the attachment point changes once per second. I might
suggest a pair of yagis on a pole near the track, one pointing up the
track, one pointing down the track, to give longer "hold times." An
antenna external to the train would also be a requirement. It will
probably be easier to get data from the train to the fixed
infrastructure via UDP, but TCP sessions will be difficult.

Good luck,
Mike

> -----Original Message-----
> From: (spam-protected) 
> [mailto:(spam-protected)] On Behalf Of John Gorkos
> Sent: Wednesday, April 05, 2006 2:26 PM
> To: (spam-protected)
> Subject: Re: [OLSR-users] Fast Roaming Nodes
> 
> 
> I'm no expert, but my back-of-the-envelope calculations for 
> 100 meters/second 
> give me a doppler shift of 31.81 MHz at 2450Mhz at 0 degrees 
> offset.  There's 
> not a chipset in the world that can find a spread spectrum 
> signal 31MHz off 
> center frequency in <1 second, especially given that it's 
> going from 31.81 
> MHz UP as it approaches to 31.81MHz DOWN as it goes past.  
> Unless you're ONLY 
> using channel 6, you're going to be out of band on your 
> transmitter.  If you 
> do only use channel 6, you're going to self interfere.
> At 900Mhz, doppler is still 11Mhz, and Canopy isn't that 
> good. Even if you stood the antennas a good 500 meters off 
> the tracks and used a 60 
> degree sector antenna, you've still got a delta of 15MHz.  
> Plus, 802.11 can't 
> associate and disassociate to the radios that fast.
> 
> Try going and talking to the NASCAR guys or the Indy car guys 
> about the 
> techniques they use to get 802.11, and full motion video off 
> the race cars. 
> Hint:  if the cars weren't going in a rough circle around the 
> antenna, it 
> wouldn't work.
> 
> John Gorkos
> 
> On Wednesday 05 April 2006 11:41, Jaime Vargas wrote:
> > Hello,
> >
> > I am considering using olsr for a commuter trail corridor. 
> Our plan is 
> > to placed routers every 500 feet along the tracks. My 
> question is can 
> > olsrd handle user moving on the mesh at 230 miles/hour (337 
> > feet/second).
> >
> > That means that user will be hopping from router to router every 
> > second.
> >
> > Has anyone tried such setup? We also expect about 40
> > users doing this roaming at the same time. We plan to
> > do some trials, but before that I will like to know any possible 
> > shortcomings.
> >
> > Thanks a lot for the help,
> >
> > Jaime
> >
> > --
> > Jaime E. Vargas
> > Why Wire, Inc.
> > 601 Birch St. SW
> > Vienna, VA 22180
> > T. 703-766-0939
> >
> >
> >
> > _______________________________________________
> > olsr-users mailing list
> > (spam-protected) https://www.olsr.org/mailman/listinfo/olsr-users
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