How to transmit the real+imaginary output of an IFFT over a transmission channel?
If you want to transmit a complex signal, you need two independent channels: one for the real part of the signal and another for the imaginary part. This makes sense, as the complex plane is two-dimensional. The question is, how can those channels be realized? It depends on what equipment you have available.

If you have multiple wires/DACs that can be used to convey the signal, then you can just use baseband transmission. Route the real part of your signal to one DAC and the imaginary part to the other. These would then feed separate transmission lines to carry the signals to the receiver. This method isn't often used, though. Why use multiple wires when you can just...

If you have a single wire/DAC that can be used to transmit the signal, then you can use carrier modulation. Basically, you would modulate the real and imaginary parts of the signal onto separate sinusoidal carriers (this is done by multiplication by the desired carrier), which are then observed at the receiver and reassembled into the composite complex signal that you started with.
A common technique for modulating complex signals is called I/Q modulation. This scheme uses a pair of sinusoids at the same frequency that have a phase offset of 90 degrees. This particular phase offset results in carrier signals that are orthogonal to one another, which allows the receiver to (in a perfect world) losslessly recover the two modulated signals. In practice, there will always be some &quot;crosstalk&quot; between the I and Q channels (which you could think of as carrying the real and imaginary parts of your modulating signal, respectively).


In your question, you suggested that you have a single DAC, so I would go the carrier modulation route. You'll need to select a carrier frequency appropriate for your application, taking the following things into account:

The bandwidth of your modulating signal.

The propagation medium to the receiver. If this is a wire, then you can often get by with as low of a carrier frequency as your signal bandwidth will allow (barring any limitations imposed by the path to the receiver, such as transformer coupling or other details). Wireless links often have quite high carrier frequencies (in the tens, hundreds, or thousands of MHz), depending upon characteristics such as desired range, transmit power, licensing, propagation environment, etc.

Hardware limitations in your transmitter/receiver. You need to generate the carrier-modulated signal somehow! If you don't have analog hardware to do that for you (i.e. you output a pair of baseband signals and an analog transmitter upconverts it to an I/Q carrier-modulated output), then you'll need to synthesize the carrier-modulated signal digitally. That may or may not be possible given your system's constraints on DAC sample rate, processing throughput, and so on.


FYI, the design that you described for your system sounds a lot like orthogonal frequency-division multiplexing (OFDM), albeit with a lot of unused carriers. You might find some useful information if you study up on how those systems are usually implemented.