Coded Orthogonal Frequency Division Multiplex (COFDM) - Digital Video and Audio Broadcasting Technology - page 320

In DVB-T, the following conditions apply for the so-called 2k and 8k
mode (Table 19.1.):

Table 19.1. COFDM modes in DVB-T

Mode 2k 8k

No. of subcarriers 2048 8192

Subcarrier spacing ∆f ~ 4 kHz ~ 1 kHz

~ 1 ms

Symbol duration ∆t 1/∆f = ~ 250 Μs

Orthogonality condition: f = 1/t

f

Fig. 19.8. Orthogonality condition in COFDM

19.3 Generating the COFDM Symbols

In COFDM, the information to be transmitted is first error protected, i.e. a
considerable overhead is added before this data stream consisting of pay-
load and error protection is impressed on the large number of subcarriers.
Each one of these often thousands of subcarriers must then transmit a por-
tion of this data stream. As in the single-carrier method, each subcarrier
requires mapping by which the QPSK, 16QAM or 64QAM is generated.
Each subcarrier is modulated independently of the others. In principle, a
COFDM modulator could be imagined to be composed of up to thousands
of QAM modulators, each with a mapper. Each modulator receives its
own, precisely derived carrier. All the modulation processes are synchro-