Smearing in Range-Doppler plots

In the Matlab example "Automotive Adaptive Cruise Control Using FMCW Technology" they plot the Range-Speed Response. In the plot one can see unexpected smearing over the whole range at the main velocity. Is this smearing related to the Range-Doppler coupling or where does it come from?

Answer 1

[Hassaan]

1. Doppler Effect: The change in frequency of the radar signal due to the relative motion of the target can lead to variance over the pulse duration, causing smearing in the Doppler dimension of the radar image.
2. Range-Doppler Coupling: In FMCW radars, the Doppler shift can be influenced by the target's range changes during the chirp, leading to an elongation of the Doppler spectrum—this is range-Doppler coupling, which appears as smearing.
3. FFT Windowing/Sidelobes: Improper windowing in FFT processing can lead to sidelobes, which manifest as additional peaks or spread in the Doppler domain, resembling smearing.
4. Integration Time: A short integration time for the Doppler processing may result in insufficient Doppler resolution, causing a spread of the energy over multiple Doppler bins.
5. Target Fluctuations: Variability in the target's radar cross-section or orientation during the radar's observation time can cause the reflected signal's Doppler spectrum to spread.
6. Multi-path Interference: Reflections from different paths can combine at the receiver, interfering with each other, and causing Doppler smearing due to the phase differences.
7. Speed Resolution: Limited speed resolution of the radar system can blur distinct Doppler shifts together, especially when targets have similar but not identical velocities.

Each of these factors contributes to the Doppler domain's smearing in the radar response, and mitigating them may involve enhancements in signal processing techniques or radar system design.

To address this, you could look into:

• Improving FFT Resolution: Increasing the number of FFT points can improve the resolution but may not entirely eliminate the issue if the underlying cause is related to system limitations or environmental factors.
• Windowing: Using different window functions during the FFT process can reduce sidelobes and may mitigate some of the smearing.
• Signal Processing Techniques: Employing advanced signal processing techniques, such as clutter rejection filters or using multiple-input multiple-output (MIMO) configurations, can help to isolate the true target signal from these artifacts.

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Answer 2

[Honglei Chen]

These are the sidelobes of the FFT. I don't think this is due to the range-Doppler coupling. You can consider windowing to reduce this effect, but you pay the price of resolution.

Hope this helps.