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I'm working on a sound card based OFDM system and ran into some problems:

enter image description here

How can I perform DAC on the real and imaginary parts of the IFFT? I think they are real numbers instead of 0 and 1. Thanks.

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Imaginary part of a complex number is real. For example

$$z = 1 - 2j$$ $$\mathsf{Re}(z) = 1$$ $$\mathsf{Im}(z) = -2$$

The same will happen with the imaginary part of your FFT. In the end, you get two real signals. Then you modulate them by two sinusoids which are 90 degrees out of phase, and add the modulated signals at the output stage.

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  • $\begingroup$ Thanks for your reply! Is this modulation the same as PSK? Or is PSK a special case of this kind of modulation? $\endgroup$ – babel92 Jan 16 '14 at 0:52
  • $\begingroup$ I'm not an expert on this, but as far as I understand, you can use different modulation techniques to transmit OFDM signals. The picture you attached uses Quadrature Amplitude Modulation (everything to the right of the DAC's), but you could equivalently use PSK here. Think of OFDM as everything to the left of DAC's, and modulation as everything to the right. Those two are independent of each other and can be mixed in a variety of configurations. $\endgroup$ – Phonon Jan 16 '14 at 1:25
  • $\begingroup$ OFDM modulation cannot be obtained by a pure phase modulation. It requires both phase and amplitude modulation which is usually referred to as quadrature amplitude modulation as shown in the block diagram above. $\endgroup$ – Deve Jan 16 '14 at 8:42
  • $\begingroup$ Just for clarification because I might have misunderstood your comments: of course the OFDM subcarriers can be PSK modulated. For radio frequency up-conversion, however, an inphase/quadrature modulation as shown in the block diagramm is necessary. $\endgroup$ – Deve Jan 16 '14 at 8:54

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