I have found several sources describing how to use Doppler velocimetry/anemometry to measure the Doppler shift with analog signals, but i cannot find much about using this method for digital signals such as Bluetooth. I was wondering if it were possible to use Doppler velocimetry/anemometry with digital signals? If so i was wondering if I could use it with the Bluetooth capabilities(2400 MHz – 2483.5 MHz) of an iPhone?

  • 2
    There's nothing that would prevent you from doing the same analysis with digital signals, but you would need access to a sample of the modulated RF signal. On a cell phone, you're likely in the best case to just get a stream of demodulated bits out of the RF receiver, if not data decoded at a much higher level in the network stack. – Jason R Oct 30 '13 at 17:53
  • Could the cell phone even detect a doppler shift if it were moving at a velocity say walking speed(3 mph) away from the bluetooth signal emitter? Or is bluetooth frequency, 2.4 GHz, too large for such a small velocity? – ian Oct 30 '13 at 18:11
  • Nevermind it would be a 10.7 Hz difference. – ian Oct 30 '13 at 18:41
up vote 2 down vote accepted

If your question is whether you could ever use the Bluetooth radio in an iPhone to perform doppler velocimetry, I'm going to go out on a limb here and say no.

Most mobile devices are incredibly modular. This means that the BT/WiFi radio (they are often combined into a single chip these days) is designed to handle all of the RF signal processing necessary, and then hand over only the digital data received, but never the analog signals. By the time the signal is turned into digital data, any effects of multipath, channel noise, doppler shift, and other analog phenomena has been removed. It is highly unlikely that any mass-produced cell phone would have access to the analog signal being received by the individual radios.

  • I was told that I could not use doppler velocimetry with Bluetooth because according to bluetooth.com: Bluetooth technology operates in the unlicensed industrial, scientific and medical (ISM) band at 2.4 to 2.485 GHz, using a spread spectrum, frequency hopping, full-duplex signal at a nominal rate of 1600 hops/sec. … adaptive hopping among 79 frequencies at 1 MHz intervals gives a high degree of interference immunity and also allows for more efficient transmission within the spectrum. Aside from not be able to use the iphone to do it. is this true? – ian Oct 30 '13 at 19:41
  • All of that is true, but that is not why you can't do it on an iPhone. Since the frequency hopping is deterministic, you could still get the information you need out of the raw data, if the raw data was available to you. – nispio Oct 30 '13 at 19:45
  • ahhh i see. Thanks nispio – ian Oct 30 '13 at 19:47
  • In fact, with regards to the frequency hopping, it probably has some analogies to pulse-doppler radar. Once you have the decoded, demodulated data, you know exactly what the transmitted signal should have looked like. If you view each frequency hop as a separate pulse, you can coherently combine the information from each of the short pulses to converge on a solution. – nispio Oct 30 '13 at 19:53
  • This may sound dumb, but how could I know how the transmitted signal should have looked like? Is the transmitted signal's "expected" frequency hopping sequence contained in one of the transmitted packets? – ian Oct 30 '13 at 20:05

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