Coded Orthogonal Frequency Division Multiplex (COFDM) - Digital Video and Audio Broadcasting Technology - page 319

packets. Mathematically, a convolution occurs in the frequency domain,
i.e. the spectra of the rectangular window pulse and of the sinewave be-
come superimposed. In the frequency domain there is then a sin(x)/x-
shaped spectrum at the f s and -f s position instead of a discrete spectral line.
The nulls of the sin(x)/x spectrum are described by the length of the rec-
tangular window ∆t. The space between the nulls is ∆f = 1/∆t.
If then many adjacent carriers are transmitted simultaneously, the
sin(x)/x-shaped tails produced by the bursty transmission will interfere
with the adjacent carriers.
However, this interference is minimized if the carrier spacing is selected
in such a way that a carrier peak always coincides with a null of the adja-
cent carriers. This is achieved by selecting the subcarrier spacing ∆f to cor-
respond to the inverse of the length of the rectangular window, i.e. the
burst period or symbol period. Such a burst packet with many and often
thousands of modulated subcarriers is called a COFDM symbol.
COFDM
symbol
duration t
Fig. 19.7. COFDM symbol
The following holds true as COFDM orthogonality condition (Fig.
19.8.):
∆f = 1/∆t;
where ∆f is the subcarrier spacing and ∆t is the symbol period.
If, for example, the symbol period of a COFDM system is known, the
subcarrier spacing can be inferred directly, and vice versa.