I have a 24GHz motion detector (SMR-333 website, sparse datasheet) that gives I and Q, without signal processing. How can I use these values to detect motion? I only need to detect if a person moves past the sensor (from 2" to 48"). The direction and other aspects of the movement are not important to me.

I'm primarily a programmer. In the past I've done audio processing, for example using FFTs and CFAR for frequency detection. This is my first radar experience. Radar is a very deep topic but I'm hoping my relatively simple requirements can be met without too much trouble. I have read some basics here.

  • $\begingroup$ Unfortunately given what you described I think the question and response is much too broad than what can be provided here. There are many kinds of radar chips and the IQ data and the details of that are also quite broad. I recommend directly contacting the vendor and see how they can help you (reference design etc) or hire a consultant $\endgroup$ Feb 24 at 23:17
  • $\begingroup$ Thanks @DanBoschen. Given how sparse the datasheet is, I don't have high hopes the manufacturer will provide assistance. It basically says only that the IQ data is 120-360 µVrms. I can easily collect the data to build I and Q waveforms. My question is how do I interpret those waveforms over time to detect motion. This seems pretty specific and doesn't depend on the radar IC details. $\endgroup$
    – NateS
    Feb 24 at 23:31
  • $\begingroup$ It depends a lot on the radar IC details—- is this an FMCW or pulse radar IC or… talk to the manufacturer, if reputable they have applications engineers that can help you. They are there to sell chips. Good luck with your project! $\endgroup$ Feb 24 at 23:50

1 Answer 1


According to your links, this seems to be a CW radar that sends out pulses of a carrier frequency and demixes the received signal in a simple homodyne architecture, although as mentioned it would definitely be much easier and probably necessary to get more information from the manufacturer.

First, you have to possibly amplify(?), filter and sample the I and Q-path signals correctly and "isolate", i.e., cut out the samples from a transmit pulse based on the timing provided. Then you combine the paths as in $s[n] = I[n] + \mathrm{j}Q[n]$. I denote $\mathrm{j} = \sqrt{-1}$ as the imaginary unit. You combine I and Q-paths this way to be able to receive a complex-valued baseband signal.

CW radar operates directly based on the Doppler shift of moving objects so that if there is movement from a single source the signal of a single sampled pulse would be something like \begin{equation}\label{eq:sig}\tag{1} s[n] = \exp(\mathrm{j}2\pi f_D \mathrm{T_s} n) + \nu[n] \,, \end{equation} where $\nu[n]$ is assumed AWGN, $\mathrm{T_s}$ is the sampling time and the Doppler shift frequency $f_D$ is \begin{equation} f_D = \dfrac{\Delta v}{c} f_0 \end{equation} with $c$ being propagation velocity (basically the speed of light), $f_0$ the carrier frequency of the transmit signal, and $\Delta v$ the relative radial velocity of the object.

Ideally all you need to do at this point is to detect or estimate this frequency, e.g. with FFT and CFAR-based processing as you mentioned. If there is some strong enough frequency peak(s) present, there is (radial) movement. You might want to process several pulses sequentially to be more reliable in your detection of movement.

Note that you could technically just sample and process the real-valued signal from a single path and could still detect movement with much reduced complexity. The disadvantage would be a lack of being able to distinguish direction of movement (i.e., sign of frequency) and a 3dB loss in useful signal power as compared to noise. For the full IQ processing you need to have two ideally identical paths of (amplification-)filtering-sampling and then combine the sampled data as described. If you only take the I-path and sample that single output, you just get the real value of the complex baseband signal, i.e., $\cos(\cdot)$ instead of $\exp(\mathrm{j}(\cdot))$ in \eqref{eq:sig}. You can of course still detect frequencies in this signal the same way, as $\cos(\cdot) = \dfrac{\exp(\mathrm{j}\cdot)+\exp(-\mathrm{j}\cdot)}{2}$. You mentioned you have audio experience; you almost certainly worked with real-valued data there. The frequency detection/estimation task then is in principle the same here, so this solution might be easier if you do not want to think about IQ-receivers and complex signals too much.

  • $\begingroup$ This is great info and gives me some clues for further reading, thanks! For s[n]=I[n]+jQ[n], what is j? Your last paragraph is intriguing, as I don't need direction. For that do I process I or Q or both waveforms? I'm waiting for parts to arrive, then I'll build a test device and visualize the IQ data. $\endgroup$
    – NateS
    Feb 28 at 17:47
  • 1
    $\begingroup$ I edited my answer to address your comment. $\endgroup$
    – mateC
    Mar 1 at 10:57
  • 1
    $\begingroup$ Here are further details along with an evaluation demo kit from the manufacturer: media.digikey.com/pdf/Data%20Sheets/InnoSenT/… , From this it does appear to be an FMCW radar system. These further details on FMCW may also be helpful to you: dsp.stackexchange.com/questions/85817/… $\endgroup$ Mar 1 at 13:24

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.