12 DVB-H physical layer transmission block diagram.
TABLE 9.1 Transmission Mode Definitions
Transmission Total no. of Useful data Continual pilot TPS
mode subcarriers subcarriers subcarriers subcarriers
2K 1,705 1,512 45 17
4K 3,409 3,024 89 34
8K 6,817 6,048 177 68
The OFDM signal consists of both pilot and data information. The pilots are extremely important
in producing the required QoS performance. Namely, they should be used for time synchronization,
frequency synchronization, channel estimation, as well as support other signal processing functions
in the receiver. The pilot information is scattered across the subcarrier and across the OFDM frame.
Note this pilot information is transmitted at a higher power level in order to aid in the aforementioned
functionality.
It is important to now discuss the BWs that the DVB standard can operate in, namely 5 MHz,
6 MHz, 7 MHz, and 8 MHz. Hence for a given transmit BW, increasing the transmission mode
will increase the number of subcarriers or decrease the intercarrier frequency spacing. As discussed
earlier, this is detrimental in the presence of a large Doppler spread. Also as the BW
decreases so does the maximum possible useful bit rate in the DVB system.
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