A highly-efficient, cost-effective technique for multi-channel
QAM modulation is described. The technique employs an inverse fast-
Fourier transform (IFFT) as a multi-
channel modulator.
QAM encoding 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, the carrier frequencies are generally uniformly spaced at a channel-spacing frequency (6 MHz, for
digital cable systems in the United States). The IFFT accepts a set of complex frequency inputs, 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. Since the
channel spacing and the
symbol rate are different due to excess channel bandwidth, interpolation is used to rectify the difference. An efficient scheme for combining this interpolation with
baseband filtering and anti-imaging filtering is described.