  Spectrum is the most valuable resource in communication system, but
unfortunately, so far, a half of the spectrum has been wasted. In this paper,
we will see that the negative frequency not only has a physical meaning but
also can be used in communication. In fact, the complete description of a
frequency signal is a rotating complex-frequency signal, in a complete
description, positive and negative frequency signals are two distinguishable
and independent frequency signals, they can carry different information. But
the current carrier modulation and demodulation do not distinguish positive and
negative frequencies, so half of the spectrum resources and signal energy are
wasted. The complex-carrier modulation and demodulation, proposed by this
paper, use the complex-frequency signal as a carrier signal, the negative and
positive frequency can carry different information, so the spectrum resources
are fully used, the signal energy carried by complex-carrier modulation is
focused on a certain band, so the signal energy will not be lost by the
complex-carrier demodulation.
