Coded Orthogonal Frequency Division Multiplex (COFDM) - Digital Video and Audio Broadcasting Technology - page 326

Since nowadays analog, i.e. non-ideal, I/Q modulators are very often
used because of the direct modulation method, the effects arising can only
be explained in this way.
The more carriers (Fig. 19.14.), the more random the appearance of the
corresponding COFDM symbol. Even just 12 single carriers placed in rela-
tively random order with respect to one another result in a COFDM sym-
bol with stochastic appearance. The symbols are calculated and generated
section by section in pipeline fashion. The same number of data bits are
always combined and modulated onto a large number of up to thousands of
COFDM subcarriers. Firstly, real- and imaginary-part tables are produced
in the frequency domain and then, after the IFFT, tables for re(t) and im(t)
which are stored in downstream memories. Period by period, a COFDM
symbol of the exact constant length of ∆t = 1/∆f is then generated. Be-
tween these symbols, a guard interval of defined but often adjustable
length is maintained.
Fig. 19.16. COFDM symbols with guard interval
Inside this guard interval, transient events due to echoes can decay
which prevents inter-symbol interference. The guard interval must be
longer than the longest echo delay time of the transmission system. At the
end of the guard interval, all transient events should have decayed. If this
is not the case, additional noise is produced due to the inter-symbol inter-
ference which, in turn, is a simple function of the intensity of the echo.
However, the guard intervals are not simply set to zero. Usually, the end
of the next symbol is keyed precisely into this time interval (Fig. 19.17.)