16 QAM and OFDM

thavamaran

thavamaran

@thavamaran-uIkf9a • Oct 21, 2024

hi guys, currently im building a passband system of 5 GHz, so I have simulated 16 QAM and its perfectly working fine at 5 GHz.

But I am very new to OFDM, I have been studying on the background of OFDM and all of them only talking about baseband OFDM. Im not interested with using the MATLAB function to generate baseband 16-QAM, cause I have to analyse the changes of BER and EVM with changes of frequency.

So my question is, the generated 16 QAM signal are not in complex value, cause I have summed the I and Q channel at the end, so from the summation of this I and Q channel, I actually got the 16 QAM signal which is very trivial.

Then I have to shift from serial to parallel before performing IFFT in OFDM, but my question is, my signal is not complex, how could I IFFT my signal? should I use (FFT) after the serial to parallel conversion and IFFT back again, but it doesnt make sense.

Please advice me on this. Thank you! Cheers!

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  • durga ch

    durga ch

    @durga-TpX3gO • Sep 6, 2010

    is your question; why do we have to IFFT and then do FFT?
    the reason lies in cyclic prefx and circular convolutions.
    see when you do a IFFT, you are converting the signal from frequency to time domain which makes it easier to add cyclic prefix, now as to why do we add cyclic prefix- its to eleimate the ISI and the same signal is retried from receiver by doing an FFT to the signal.

    what else you want to know?

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  • thavamaran

    thavamaran

    @thavamaran-uIkf9a • Sep 6, 2010

    Hi Durga, thanks for the reply. Now I get the idea of IFFT.

    Anyway I am designing a 16-QAM OFDM passband system, all the examples are only showing baseband system.

    So when I modulate data with 16-QAM passband signal, I don't know the next step of feeding into OFDM. which is really complicated.

    Thanks very much, cheers!

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  • durga ch

    durga ch

    @durga-TpX3gO • Sep 6, 2010

    I think you will have to consider your signal in baseband and then do the rest of OFDM process and then convert the signal back into passband.The reason being - coding, modulation etc are all done on baseband signals , once you convert you can again follow the same steps for baseband

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