How does MATLAB do fft and ifft in matrices

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Hi, I'm trying to do the ifft and fft of matrices, but I'm getting unexpected values.
I only don't get any error when dealing with a 1x4 matrix (which is a vector).
When putting, for example, a 2x4 matrix, I obtain and 8x4 matrix, which is not what I expected. I expected a 2x4 matrix again.
When coding the fft and ifft of the matrix, I wrote:
received_signal = fft(a,n)
transmitted signal = ifft(a,n)
where a= matrix and n= dft size (i.e. total number of elements in the matrix)
In addition, I'd like the fft/ifft to calculate the ifft and fft in a row-by-row fashion, since I'm dealing with signals so I don't want the bits to get out of order.
Thanks in advance for your help.

Accepted Answer

Star Strider
Star Strider on 28 Oct 2017
The fft function computes the discrete Fourier transform on the columns (dimension 1) of a matrix, unless you tell it to compute along the rows (dimension 2).
When I do something similar, I get a Fourier-transformed matrix that is the same size as the original matrix, however complex (as expected).
If you are getting something else as a result, post your code. I suspect you are simply getting the expected complex result.
  2 Comments
Star Strider
Star Strider on 29 Oct 2017
My pleasure.
To get fft to compute along the rows, specify ‘2’ (the row dimension) for ‘dim’:
Fy = fft(y, NFFT, dim);
so:
received_signal = fft(a,n,2);
The fft function will compute the fft of all the rows (with the row dimension specified here) in matrix ‘a’. The same applies to the ifft function.

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