Basic Principles of Digital Modulation - Digital Video and Audio Broadcasting Technology - page 234

Before the data are supplied to the error protection section, however,
they are first scrambled in order to bring movement into the data stream, to
break up any adjoining long strings of zeroes or ones into more or less
random data streams. This is done by mixing and EXOR-operations on a
pseudo random binary sequence (PRBS). At the receiving end, the data
stream now encrypted must be recovered by synchronous descrambling.
The scrambling is followed by the first FEC. The data stream is then dis-
tributed in time by means of time interleaving. This is necessary so that
during the deinterleaving at the receiving end, burst errors can be broken
up into individual errors. This can be followed by a second FEC.

EXOR

+ + out1

code rate =
data rate in /
data rate out;

in

+ out2

EXOR

Shift registers

Example: GSM, UMTS, DVB inner coder

Fig. 13.28. Convolutional coding

There is also concatenated error protection (David Forney, 1966). It is
possible to concatenate both block codes with block codes and block codes
with convolutional codes or also convolutional codes with convolutional
codes. Concatenated convolutional codes are called turbo codes. They only
made their appearance in the 90's.
It depends on the choice of modulation method and of the error protec-
tion how closely the Shannon-Limit is approached. Shannon determined
the theoretical limit of the data rate in a distorted channel of a certain band-
width. The precise formula for this is: