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Modulation is used to shift baseband signals into an appropriate passband position, centered around a specific carrier frequency $f_c$ that is suitable for the respective channel. E.g., for wireless mobile communication, a typical value is $f_c=1\mathrm{GHz}$, or wireless LAN, $f_c=2.5\mathrm{GHz}$,

More information, see lecture Nachrichtentechnik 2, or Communications I.