Digital Signal Processing Reference
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sents an SSB modulator. Changing the frequency of the stimulating quan-
tity at the frequency level only changes the frequency of the cosinusoidal
and sinusoidal output signals at re(t) and im(t). re(t) and im(t) have exactly
the same amplitude and frequency and a phase difference of 90 degrees as
before. The decisive factor in understanding this type of COFDM imple-
mentation is that, in principle, this mutual relationship applies to all sub-
carriers. For every subcarrier, im(t) is always at 90 degrees to re(t) and has
the same amplitude.
Fig. 19.13.
IFFT with altered frequency
Including more and more carriers produces a signal with ever more ran-
dom appearance for re(t) and im(t), the real and imaginary part-signals
having a 90° phase relation to one another in the time domain.
im(t) is said to be the Hilbert Transform of re(t). This transform can be
imagined to be a 90° phase shifter for all spectral components. If both time
domain signals are fed into the I/Q modulator following, the actual
COFDM symbol is produced. In each case, the corresponding upper or
lower COFDM subband is suppressed by this type of modulation, provid-
ing a thousandfold phase-shift-type single-sideband modulator. Many ref-
erences, some of which date back more than 20 years, contain notes re-
garding single-sideband modulators of this phase-shifting type. It is only
due to the fact that each subcarrier at re(t) and im(t) has the same ampli-
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