On Saturday, Dec 7, 2002, at 18:14 US/Central, Kevin Lahey wrote:
Thanks.On Sat, 7 Dec 2002 07:13:46 -0800 Jim Thompson <[EMAIL PROTECTED]> wrote:Well, I've been reasonably impressed with your reliability on this list...I'm not sure I want to be an "authority". Just think of me as a geek who occasionally gets it right.
The so-called 'Doppler Effect' is caused by movement of the transmitter or receiver (or both):
Seemingly, wavelengths become shorter when they approach each other (and longer otherwise).
Maximal Doppler displacement (axial movement) in Hz:
f (Dmax) = velocity/wavelength
where velocity is in m/s, and wavelength is in meters. 40mph is just under 18 m/s, and 2.4GHz
has a wavelength of 0.125m, for a displacement of 144Hz, as a previous poster opined.
But this isn't the problem. The problem is with the coherence interval, or average time
of decorrelation, which is sort-of the inverse of the Doppler effect. (Please forgive me for
not diving into the weeds in front of the audience.)
At mobile speeds of 1 m/s (3.6 km/hr), the coherence interval is approximately 122.88 ms at
2.4 GHz. The coherence interval reduces to 24.57 ms, 12.28 ms and 6.14 ms for mobile speeds of
5 m/s (18 km/hr), 10 m/s (36 km/hr) and 20 m/s (72 km/hr).
Now, take the time it takes to send a (say) 1000 byte packet at 11Mpbs, and factor in the SIFS+ACK time.
Any off-axis movement will serve to reduce the effect, btw.
Jim
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