Digital Signal Processing Reference
In-Depth Information
clusive-OR gate. This breaks up long sequences of ones and zeroes which
maybe present in the data stream. This is called energy dispersal. In single-
carrier methods, energy dispersal is required for preventing the carrier vec-
tor from staying at constant positions. This would lead to discrete spectral
lines. But error protection, too, only operates correctly if there is move-
ment in the data signal. This is the reason why this scrambling is carried
out at the beginning of the FEC also in the COFDM method. Every 24 ms,
the shift register arrangement is loaded with all ones and thus reset.
Reset after 24 ms frame
P(x)=x
9
+x
5
+1;
1
11
1
1
1
1
1
1
12345
6789
+
EXOR
Data in
+
Scrambled data out
EXOR
Fig. 26.18.
Data scrambling
O
ut
Puncturing
In
+
+
+
+
Coderate
+
+
+
+
+
+
+
Coderate = in / out;
8/9, 8/10, 8/11, …8/32
Fig. 26.19.
DAB convolutional coding with puncturing
Such an arrangement is also found in the receiver and must be synchro-
nised with the transmitter. Mixing again with the same PRBS in the re-
ceiver restores the original data stream.
This is followed by the convolutional coding. The convolutional coder
used in DAB (followed by puncturing) is shown in Fig. 26.19. The data
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