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Doppler shift in a radio wave at 10 mph? Wouldn't the Doppler shift be around 36 Hz in that case [1]? That's about 0.015 PPM. Seems temperature would have a big

Doppler shift in a radio wave at 10 mph? Wouldn't the Doppler shift be around 36 Hz in that case [1]? That's about 0.015 PPM. Seems temperature would have a bigger impact on frequency stability than that. [1] ∆f = f0 * (∆v/v0) = 2.4 GHz * (4.5 / 3e8) = 36 Hz (using 10 mph = 4.5 m/s) Edit: I'm certainly not an expert in RF. I checked my intuition against the datasheet of a radio module I've been working with (RFM69HCW), and found in a footnote of an equation (on page 31): > crystal tolerance is in the range of 50 to 100 ppm Here's a link to that datasheet: https://cdn.sparkfun.com/datasheets/Wireless/General/RFM69HCW-V1.1.pdf https://cdn.sparkfun.com/datasheets/Wireless/General/RFM69HC... It seems that the Doppler shift (measured in PPM) would be well within 0.1% of the crystal's tolerance. Edit #2: Doppler shift is 0.015 PPM; not 0.150 PPM as I originally stated.

swiley 10y ago
Keep in mind, modern WiFi uses OFDM where the signal is transmitted via a number of simultaneous carriers offset by very small frequencies. So while the center of the 20Mhz band might be shifted just a little the small carries might be shifted far enough onto each other that the signal is unintelligible.
- davidcuddeback 10y ago
  If the individual carriers aren't overlapping in the transmitter, they won't be overlapping in the receiver, either. Each one will be shifted proportionally (∆f/f0=∆v/v0) in the same direction. The signal is still intelligible. It's just shifted. A radio is a physical device subject to manufacturing variance, temperature fluctuations, and aging effects. It has to be manufactured to tolerate frequency deviations caused by these effects. What I was trying to show in my analysis is that the effect due to Doppler shift at 10 mph is orders of magnitude less than at least one of these effects. If the Doppler shift at 10 mph makes the signal unintelligible, then the other effects I mentioned should as well. For example, a transmitter with 50 PPM accuracy could cause a 2.4 GHz signal to drift by up to 120 KHz (0.00012 GHz). That's about 3000 times more than the 36 Hz that I calculated for Doppler shift at 10 mph.

swiley 10y ago

Keep in mind, modern WiFi uses OFDM where the signal is transmitted via a number of simultaneous carriers offset by very small frequencies. So while the center of the 20Mhz band might be shifted just a little the small carries might be shifted far enough onto each other that the signal is unintelligible.

davidcuddeback 10y ago
If the individual carriers aren't overlapping in the transmitter, they won't be overlapping in the receiver, either. Each one will be shifted proportionally (∆f/f0=∆v/v0) in the same direction. The signal is still intelligible. It's just shifted. A radio is a physical device subject to manufacturing variance, temperature fluctuations, and aging effects. It has to be manufactured to tolerate frequency deviations caused by these effects. What I was trying to show in my analysis is that the effect due to Doppler shift at 10 mph is orders of magnitude less than at least one of these effects. If the Doppler shift at 10 mph makes the signal unintelligible, then the other effects I mentioned should as well. For example, a transmitter with 50 PPM accuracy could cause a 2.4 GHz signal to drift by up to 120 KHz (0.00012 GHz). That's about 3000 times more than the 36 Hz that I calculated for Doppler shift at 10 mph.