OFDM QAM Pulse Shape

[mikohama]

Hi, i'm having a problem in the understanding of a QAM modulated OFDM signal and I couldnt find a satisfying answer in any tutorial whatsoever. Hope you can help me.

As far as I understood, if i have a OFDM system without QAM, I can just take the bits and multiply them on the sinosoidal carriers so that the rectangular bitform "creates" the typical sinc-spectrum. As done in Multitone, if I'm correct.

Now, if I map those bits with QAM, i will mathematically have sth like e^jPhi to be multiplied with the carriers functions, right? Here's my problem. Does it mean that, as e^jPhi=cos(Phi)+jsin(Phi), my OFDM symbol spectrum is just some dirac deltas? nothing of a sinc spectrum? Or must I do pulse shaping after the QAM-mapping?

[mikohama]

OR.. is it the D/A converter after the IFFT that causes the sinc spectrum?

Please help me on that, i'm really confused, and i have to do a presentation on ofdm soon.

Maybe there's a block diagram in which the time and freq. signal is depicted after each block?!?

[charan langton]

Take a sinusoid, modulate it using QPSK. If you do the constellation of this signal, you will see four points. But in OFDM, we have many QPSK signals all with four constellation points but at different frequencies. When you add a bunch of these together, you will not be able to extract a constellation from the resulting signal.

If instead each signal is shaped by a QAM modulation, i.e. has a four level signal and has sixteen constrllation points, then the same holds. Each harmonic will have 16 constellation points but when you add a bunch of these together, all sense of individual constellation points is lost. You can't make them out. (BTW) we generally do not pulse shape the individual carriers in OFDM because a OFDM signal is bandwidth efficient.

Charan Langton