Problem with I/Q demodulation

[Kroun1]

In my simulation I use I/Q modulation and demodulation. In modulator are in-phase and quadrature part multiplied by the sine and cosine funcitons. Because this simulation runs in Matlab sine and cosine functions are discrete ( just vector ), hence spectra of output signal is multiple ( copied ). But if I use filtration of output signal demodulator does not work. In demodulator is received signal multiplied by the same cosine and sine function as in modulator and after that is filtered in low-pass filter. I can’t use so large spectra of output signal, but without it demodulator does not work. Does anybody know some solution ?

If you want, see this link, where is part of my source code with pictures of spectra:
www.home.karneval.cz/00107006/IQ_modulation

[charan langton]

You can not filter out the other lobes, they all carry information. What you probably want is a spectral shifter to do this. which would move your whole baseband spectrum which is a sin x/ x shape to a carrier frequency. The spectrum you show does not look like you have set up the modulation (or frequency upshift) correctly. It looks like a TDM signal.

Charan Langton

[Kroun1]

Maybe problem is in Quadrature an In-phase parts. They are not changing continuously but it's step function (output of OFDM modulator). What about using some raised-cosine filtering of these parts before modulation ?