| 摘要 |
A highly-efficient, cost-effective technique for multi-channel QAM modulation is described. The technique employs an inverse fast-Fourier transform (340) as a multi-channel modulator. QAM encoding (304A-304n) expresses QAM symbols as constellation points in the complex plane such that each QAM symbol represents a specific phase and amplitude of a carrier frequency to which it is applied. In multi-channel systems (300), the carrier frequencies are generally uniformly spaced at a channel-spacing frequency (6MHz, for digital cable systems in the United States). The IFFT accepts a set of complex frequency inputs (308A), each representing the complex frequency specification (i.e., phase and amplitude) of a particular frequency. The inputs are all uniformly spaced, so assuming that the IFFT is sampled at a rate to provide the appropriate frequency spacing between its frequency-domain inputs, the IFFT will produce a time domain representation of QAM symbols applied to its various inputs modulated onto carriers with the desired channel separation.
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