Coded Orthogonal Frequency Division Multiplex (COFDM) - Digital Video and Audio Broadcasting Technology - page 323

main after which the Inverse Fast Fourier Transform is performed and the
result is considered at the outputs re(t) and im(t) after the IFFT, i.e. in the
time domain, and after the complex mixer.
Fig. 19.12. IFFT of a asymmetric spectrum
This begins with a spectrum which is symmetric with respect to the
band center of the COFDM channel (Fig. 19.11.). simply consisting of car-
rier No.1 and N. After the IFFT, an output signal is produced at output
re(t) which is purely cosinusoidal. At output im(t), u(t) = 0V is present. A
purely real time-domain signal is expected since the spectrum meets the
conditions of symmetry required for this. After the I/Q modulator, an am-
plitude modulated signal with suppressed carrier is produced which is only
generated by the real time-domain component (see Fig. 19.11.).
If, however, e.g. the spectral line in the upper range of the band, that is
to say the carrier at N, is suppressed and only the component at carrier
No.1 is left, a complex time-domain signal is obtained due to the asymmet-
ric spectrum (Fig, 19.12. and 19.13.). At output re(t) after the IFFT, a cosi-
nusoidal signal with half the amplitude as before is now present. In addi-
tion, the IFFT now supplies a sinusoidal output signal of the same
frequency and the same amplitude at output im(t). This produces a com-
plex signal in the time domain. If this, i.e. re(t) and im(t), is fed into the
following I/Q modulator, the modulation disappears, resulting in a single
sinusoidal oscillation converted into the carrier frequency band. A single-
sideband modulated signal is produced and the arrangement now repre-