Digital Signal Processing Reference
In-Depth Information
13.8 Channel Coding/Forward Error Correction
In addition to the most suitable modulation method, the most appropriate
error protection, i.e. channel coding, is selected from among the character-
istics of the respective transmission channel. The current aim is to ap-
proach the Shannon-Limit as closely as possible. This section discusses
commonly used error protection mechanisms and creates the foundations
for the transmission methods in digital television.
Channel coding
Block
codes
Concatenated
codes
(1966: David Forney)
Convolutional
codes
(1955: Elias,
1967: Andrew Viterbi)
Data
Code
Cyclical
group code
using
group/field
theory
of linear
algebra
Block code+
interleaver+
block code
+
+
out1
in
Block code+
interleaver+
Convol.
code
+
out2
BCH
(1960)
General
block code
Hamming
(1950)
Turbo
codes
(1993:
concat.
convol.
codes)
ReedSolomon
(1963)
LDPC
(1963: Gallager)
Fig. 13.25.
Channel coding
Before information is transmitted, source encoding is used for changing
it into a form in which it can be transmitted in as little space as possible.
This simply means that it is compressed as well as is possible and toler-
able. After that, error protection is added before the data are sent on their
journey. This corresponds to channel coding. The error-protected data are
then digitally modulated onto a sinusoidal carrier after which the informa-
tion is sent on its way, subjected to interference such as noise, linear and
nonlinear distortion, discrete and wide-band interferers, intermodulation,
multipath propagation etc. Due to the varying degree of signal quality at
the receiving end, this causes bit errors after its demodulation back to a
data stream. Using the error protection added in the transmitter (FEC -
Forward Error Correction), errors can then be corrected to a certain extent
in the channel decoder. The bit error rate is reduced back to a tolerable
amount, or to zero. The information is then processed in such a way that it
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