Digital Signal Processing Reference
In-Depth Information
The meaning of the words 'payload carrier' is clear: these are simply the
carriers used for the actual data transmission. The edge carriers at the up-
per and lower channel edge are set to zero, i.e. they are inactive and carry
no modulation at all, i.e. their amplitudes are zero. The continual pilots are
located on the real axis, i.e. the I (in-phase) axis, either at 0 degrees or at
180 degrees and have a defined amplitude. The continual pilots are
boosted by 3 dB compared with the average signal power and are used in
the receiver as phase reference and for automatic frequency control (AFC),
i.e. for locking the receive frequency to the transmit frequency. The scat-
tered pilots are scattered over the entire spectrum of the DVB-T channel
from symbol to symbol and virtually constitute a sweep signal for the
channel estimation. Within each symbol, there is a scattered pilot every
12th carrier. Each scattered pilot jumps forward by three carrier positions
in the next symbol, i.e. in each case two intermediate payload carriers will
never become a scattered pilot whereas those at every 3rd position in the
spectrum are sometimes payload carriers and sometimes scattered pilots.
The scattered pilots are also on the I axis at 0 degrees and 180 degrees and
have the same amplitude as the continual pilots.
0 1 2 3 4 5 6 7 8 9 10 11 12
= scattered pilot
= payload carrier
Fig. 20.4.
Change of position of the Scattered Pilots
The TPS carriers are located at fixed frequency positions. For example,
carrier No. 50 is a TPS carrier. TPS stands for Transmission Parameter
Signalling. These carriers represent virtually a fast information channel via
which the transmitter informs the receiver about the current transmission
parameters. They are DBPSK (differential bi-phase shift keying) modu-
lated and are located on the I axis either at 0 degrees or at 180 degrees.
They are differentially coded, i.e. the information is contained in the dif-
ference between one and the next symbol. All the TPS carriers in one
symbol carry the same information, i.e. they are all either at 0 degrees or
all at 180 degrees on the I axis. At the receiving end, the correct TPS car-
rier position of 0 degrees or 180 degrees is then determined by majority
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