# Interpretation of FFT FMCW Doppler radar results

After a lot of research and help from the users here, I managed to do some FFT and signal processing in my first time in my life :)

I have got the following figure, which shows one peak frequency, but the strange thing is the range is 0, however I was putting the radar into a wall... so I expect at least some distance.. How would I interpret the results ? Is it a problem of coding ? or how would I verify that the signal processing is correct?

x = complex(ResultI, ResultQ);

D = 64;

N=512; %FFT size
m1=reshape(x,length(x)/D,D); %generating matrix ---> each row showing range info for one period AND each column showing number of periods
[My,Ny]=size(m1');
win=hamming(Ny);
m2=(win*ones(1,My));applying window for sidelobe reduction (in time domain)
M2=(fft(m2,2*N)); %First FFT for range information
M3=fftshift(fft(M2',2*D)); %Second FFT for doppler information
[My,Ny]=size(M3);


If anyone interested here is the I/Q data from 4 channels

and the full matlab code that reads the I/Q and do FFT on them http://www.mediafire.com/file/gcj13keakb8mkf7/MATLAB.zip

• I don't understand why you think that a FFT of the spectrum will give velocity information in a FMCW radar. The first FFT(which you do on time domain data) gives you both velocity and distance, it's just hard to separate them. Doing a FFT on that nets you the ...periodicity of the distance to targets, i guess?
– jms
May 26 '18 at 9:47
• @jms I have read many papers that says doing a second FFT will yeild the velocity... so how would I seperate the first FFT to get the velocity from it ? May 26 '18 at 9:51
• @jms look at that excerpt of the paper imgur.com/a/RUUm66v May 26 '18 at 10:15
• cstools.asme.org/csconnect/FileUpload.cfm?View=yes&ID=39498 May 26 '18 at 11:56
• @jms Fs = 125khz, Rate of change is 5800MhZ/12.uS May 26 '18 at 17:59