OFDM using DSP Builder

Before I continued with my design, I wanted to make sure I understood how to use the FFT(IFFT) megacore block properly.

To test, I wrote a .m file where I performed the IFFT using the Matlab's built in ifft() function and Altera's IFFT megacore block using the .m file interface created when the block is generated.

I got the result from Matlab's ifft function and Altera's IFFT block to come out to be exactly the same (note about scaling later).

Now, I implemented the FFT block inside of DSP Builder and created a dummy setup with the same inputs as the data in the .m file. I compared the output of the Altera FFT block (from dsp builder) to the output of the Altera FFT (from matlab .m file) and they are totally different.

Here's a screenshot giving you an idea of what I'm talking about:

http://img353.imageshack.us/img353/5056/ifftcomparisonmh7.th.png (http://img353.imageshack.us/img353/5056/ifftcomparisonmh7.png)

I'm attaching both files.

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Now about the scaling issue (this might be related to my problem).

I have read many different things about "scaling" related the ifft block.

You have to scale your FFT/IFFT block output by

2^(-exponent)

I have also read, that if your input is very small, then it's recommended to scale it on the input side and the same on the output side (aside from the 2^(-exponent)).

In my .m file, to match the Matlab ifft block output with the Altera IFFT, I tried many scaling factor and somehow came upon this magical scaling property which works for all cases (different N points, different data or twiddle precision):

Scale input by:2^(Nbits)
Scale output by:2^(-exponent) / (2^(Nbits+Nbits-1))
WHEREo Nbits = log2(N) + 1
o N     = transform length (points)

The thing is, I don't know why it works, but it does (at least in the .m file). For different N (transform length/points) and different data/twiddle precision, it just works!

Maybe I'm completely off here. I need some more help!

Thanks