FMCW Radar Query!

I am working on Frequency Modulated Continuous Wave (FMCW) radar which linearly modulate frequency according to the amplitude of a sawtooth. Parameters are : Sweeptime= 1e-3, Sweepbandwidth(bw)=150 MHz and samplerate= 2*bw ; So, this FMCW signal is to be transmitted at 77 GHz (carrier frequency). Range of this radar is 200 max and range resolution comes out to be 1m as computed below Here is the code for FMCW generation. This is the same code as given in the example and I got the desired output.

fc = 77e9; c = 3e8; lambda = c/fc;

The sweep time can be computed based on the time needed for the signal to travel the unambiguous maximum range. In general, for an FMCW radar system, the sweep time should be at least 5 to 6 times the round trip time. This example uses a factor of 5.5.

range_max = 200; tm = 5.5*range2time(range_max,c);

The sweep bandwidth can be determined according to the range resolution and the sweep slope is calculated using both sweep bandwidth and sweep time.

range_res = 1; bw = range2bw(range_res,c); sweep_slope = bw/tm;

hwav = phased.FMCWWaveform('SweepTime',tm,'SweepBandwidth',bw,... 'SampleRate',fs); % generation of FMCW signal)

s = step(hwav); subplot(211); plot(0:1/fs:tm-1/fs,real(s)); xlabel('Time (s)'); ylabel('Amplitude (v)'); title('FMCW signal'); axis tight; subplot(212); spectrogram(s,32,16,32,fs,'yaxis'); title('FMCW signal spectrogram');

You can see the output in the attached imaged. Now this sweep signal needs to be transmitted at 77 GHz and according to my knowledge it needs to be modulated again.The 'frequency modulator passband' block in matlab has the following parameters : carrier frequency, initial phase and frequency deviation.

My questions are :

1) what should be the value of frequency deviation? I don't think it should be 150 MHz as my message signal has already been modulated for this bandwidth sweep

2) Enlighten me with the phenomenon of transmitting FMCW signal at carrier frequency?

3) If I generate a sawtooth waveform of same sweeptime and apply it to 'frequency modulator passband' block with parameters , fc=77 GHz, frequency deviation= 150 MHz. Is it going to modulate from 36 GHz to 36.15 GHz in order to generate a sweepbandwidth of 150 MHz?

4) Or do I have to go with the built in function of fmcw.waveform to generate my desired signal?

Awaiting reply Sir, thanking you in aniticipation

Answer 1

[Honglei Chen]

Looks like you are using Phased Array System Toolbox, in that case, have you checked out its library in Simulink? There is a FMCW block in Simulink from the toolbox. The following example may be useful to you

http://cn.mathworks.com/help/phased/examples/automotive-adaptive-cruise-control-using-fmcw-and-mfsk-technology.html

HTH

[Hassam Mahmood]

Yes I am using FMCW block but I need to transmit it over 36 Ghz. This block simply generates frequency modulated wave ranging from 0 to 500 Mhz in my case. My questions are still unanswered, I would appreciate if you can help me out

[Honglei Chen]

Most simulation would simulate the effect on the baseband signal instead of a signal at 36 GHz if that's what you have in mind. So the value 36 GHz will be used to determine the path loss, the antenna pattern, and so on but the signal is not sampled at 36 GHz. This is mainly due to (1) carrier are not removed before processing and (2) our computer is not that powerful yet. If you want to simulate more RF behavior, you can use SimRF components to replace certain blocks in the model, such as shown in the following example:

Modeling RF Front End in Radar System Simulation

But even that is modeling the RF effect on the baseband/IF signal, not at the RF carrier.

HTH

[Hassam Mahmood]

Thank you so much Mr. Honglie for your knowledgeable answer. I figured this out after reading few threads. I have another query and I will link it up and would like you to answer that one as well.