Final Thesis

3.3.3 OFDM technique 
To reduce the effects owing to the radio channel is used the Orthogonal Frequency Division Multiplexing 
(OFDM). This technique consists in a multi-carrier modulation where the signal is divided in N-flows of low 
speed that modules several sub-carriers. 
The duration of the low speed symbols are selected in a way that exceed the dispersion time including the 
last echo. Every sub-carrier that is modulated has a zero in its spectrum at the frequency of the following 
sub-carrier. In such a way, they are orthogonal. To obtain this, the frequencies of every sub-carrier has to be 
separated the same value as the inverse of the low speed symbols' duration. If it chooses a large number of 
OFDM sub-carriers, the symbol's time of each modulated signal is higher than the echo delay and each one 
will be affected by flat fading. This fact, with the orthogonality of the carriers, allows the individual 
demodulation with quality. The echo problem is reduced and it turns into an interference between symbols 
(ISI), that is limited. On the other way, if the number of OFDM sub-carriers are limited, the echo delay are 
higher and the signal is affected by selective fading and is difficult to demodulate due to the high 
interferences that are in, even the signal has a great level of amplitude. 
Each sub-carrier is modulated by the data output of the serial-parallel converter. In the DVB-T standard are 
defined three possible modulations: 
 
- QPSK:
2 bits per symbol. 
- 16 QAM: 
4 bits per symbol.
 
 
 
 
 
 
 

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TAMK University of Applied Sciences 
Escola Politécnica Superior de Castelldefels 
Marcos Tejedor Alonso 
24 (44) 
- 64 QAM:
8 bits per symbol. 
The DVB-T system also allows two possible values to the number of sub-carriers: 
ü