I am developing a continuous wave Doppler radar to detect human movement at a speed of 0.2-6m/s. For a radiation frequency of 24.15 GHz, the Doppler shift during longitudinal motion will be equal to ~32.2-966 Hz. In transverse motion, the Doppler shift will depend on the angle at which I cross the detection zone. I created a Doppler shift table for the velocities and intercept angles I needed. From this I concluded that the signal I needed would be in the range of 10-990Hz.
I use the InnoSent IVS-162 microwave module. I apply a constant voltage to the varactor tuning voltage input to achieve a central radiation frequency of 24.15 GHz. Next, from the signal I output, I give the signal to the amplifier and analog filters. Filters: High pass filter 4th order ~5Hz; LPF 4 order ~1500Hz. Next, I give the amplified and filtered signal to the ADC of the microcontroller. Sampling frequency 8 kHz. Next comes digital filter 8 of the order of 1100Hz. Next, I decimate the signal to a frequency of 2 kHz. Next come 5 2nd order bandpass filters for frequencies 10-300; 100-500; 300-700; 500-900; 700-990 Hz respectively. Next, I detect the signal and build the signal envelope using low-pass filter 2 of the order of 1 Hz for each frequency range (10-300; 100-500; 300-700; 500-900; 700-990).
After conducting the experiments, I saw that the lateral movements in a normal step, as expected, are in the range of up to ~100Hz and I see a signal in the frequency band 10-300Hz. With longitudinal movements at a normal pace, I see a signal already in the range of 10-300 and 100-500. But at the same time, its amplitude is approximately two times less than during transverse movement. I don't understand why this happens. After all, with longitudinal movement, the person’s area is larger.
Now I have a some questions:
- Why do I receive a signal amplitude that is approximately half as large when moving longitudinally as when moving transversely at the same distance?
- I divided the signal frequency band into 5 smaller frequency bands in order to get less noise in each frequency band and a correspondingly higher signal to noise ratio. Are there any other ways to increase the signal-to-noise ratio?
- At close distances from the microwave module (1-2m), with any type of movement, the amplitude of the signal is simply huge and, accordingly, any movement, for example, of a bird or an insect, can cause a false alarm. Is there any way to prevent this effect?
- I understand that for this microwave module you can use FMCW modulation, which will solve some of my problems, but I want to deal with CW modulation specifically.
- For processing, I use a very simple STM32G030 microcontroller and integer mathematics, since this microcontroller does not have an FPU, so any complex processing algorithms will not work for me.