Velocity in FMCW Radar

 

1	If a target moves with a certain velocity, its distance changes over successive chirps.
2	As a result, the propagation path of the reflected signal differs from chirp to chirp.
3	This causes a frequency and phase difference in the beat signals from the same target across chirps.

 

 

λ  wavelength of RF center frequency f_d  doppler frequency T_c  chirp duration k_d  doppler bin index

 

 

 

 

 

 

 

 

 

 

1. Signal Flow and Processing

Since phase differences are much easier to detect than minor beat frequency variations,

the Doppler FFT is applied along the chirp index axis of the Range FFT output at a fixed range bin.

This enables the estimation of Doppler frequency and target velocity from the phase change rate.

 

 

 

 

 

 

example by target’s velocity

 

 

 

 

 

one target

 

multi-target

 

 

 

 

 

 

2. example

Velocity is typically calculated by collecting and processing data over one frame.

In the 128 point raw FFT output, bins 0 to 63 represent increasing approaching velocity, while bins 64 to 127 represent increasing receding velocity.

 

Parameter	Symbol	Value	Unit
Chirp Start Freq.	f_chirp_start	24.05	GHz
Chirp Cutoff Freq.	f_chirp_stop	24.25	GHz
Chirp Duration	T_c	1,000	us
Doppler FFT Size	N (N_d)	128
BIN @peak	k (k_d)	45