Frequency offset using FFT in Matlab

Question

I am working on implementing BFSK modulation and demodulation using frequency hopping, and I need to check the results with Matlab. The result that I get from my BFSK program is an array of sine values ​​corresponding to the carrier signal for different bits. These signals are bandwidth signals centered at 2.51 kHz with a bandwidth of 4 kHz around it. But to check the demodulation step, I need to convert the signal to baseband. Therefore, I need to focus my output signal at a frequency of 0 Hz.

I know that creating a complex 2.51 kHz signal and multiplying it in the time domain will do the job. But is there any method that I can use with the fft() function in matlab to shift the center frequency of the signal, and then take the ifft() signal to generate the main band signal.

Thanks Anshu

Answer 1

It should be completely possible. Just rotate the complex vector that appears from the fft process before calculating the ifft.

Pseudo code will be:

 shiftIndex = 20; %// This value would depend on the frequency shift requited Y = fft(x); %// Where x is the input vector z = ifft ( [Y((shiftIndex+1):end); Y(1:shiftIndex)] ); 

For help on the number of boxes you need to move, you may find the following question helpful.,

https://dsp.stackexchange.com/questions/2970/how-to-make-frequency-axis-for-even-and-odd-fft-length/2971#2971

This reminds me, you will probably get faster answers to questions like DSP.stackexchange.com in the future.