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I am simulating OFDM by randomly selecting members of a constellation. That is, randomly selecting the I & Q from a 64QAM constellation. This gives me 64 possible phase & amplitude values. I am using a 64-point FFT and using FFTW for the numerical FFT/IFFT computations.

My question is what to put in the zero-th element of the IFFT complex input array for FFTW. The zero-th element seems to be the DC offset. But isnt this always just zero for OFDM? Then where do I put the complex values for the 64-th frequency? Does this complex value go in the 64-th element of the IFFT input array?

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You're right, the DC carrier is usually left out (for technical reasons – you need very good receiver hardware to be able to receive that one with a good SINR). That means you simply can't use all 64 bins of your FFT!

For example, IEEE 802.11a uses, if I remember correctly, only 52 of 64 carriers; the rest is filled with nulls around DC or at the band edges.

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