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?
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2$\begingroup$ 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. $\endgroup$– Jason ROct 30, 2013 at 17:53
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$\begingroup$ 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? $\endgroup$– ianOct 30, 2013 at 18:11
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$\begingroup$ Nevermind it would be a 10.7 Hz difference. $\endgroup$– ianOct 30, 2013 at 18:41
1 Answer
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.
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$\begingroup$ 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? $\endgroup$– ianOct 30, 2013 at 19:41
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$\begingroup$ 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. $\endgroup$– nispioOct 30, 2013 at 19:45
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$\begingroup$ 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. $\endgroup$– nispioOct 30, 2013 at 19:53
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$\begingroup$ 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? $\endgroup$– ianOct 30, 2013 at 20:05
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$\begingroup$ The bluetooth spec defines how the frequency hopping occurs, and it is not trivial. However, the receiving bluetooth device is synchronized with the transmitting bluetooth device at pairing time in such a way that it always knows what frequency the transmitter will hop to next. Otherwise, how would it know where to listen for the next packet? $\endgroup$– nispioOct 30, 2013 at 20:18