3.4 Code-block
segmentation :
The LTE Turbo-coder
internal interleaver is only defined for a limited number of code-block sizes
with a maximum block size of 6144 bits. In case the transport block, including
the transport-block CRC, exceeds this maximum code-block size, code-block
segmentation as illustrated in Figure (3.8) is applied before Turbo coding.
Code-block segmentation implies that the transport block is segmented into
smaller code blocks that match the set of code-block sizes defined for the
Turbo coder.
Figure 3.8
In order to ensure that the size of
each code block is matched to the set of available code-block sizes, filler
bits may have to be inserted at the head of the first code block. Note that
filler bits may be needed also if there is no actual code block segmentation
that is if the transport-block size does not exceed the maximum code-block size.
As can be seen in Figure 54
code-block segmentation also implies that an additional (24 bits) CRC is
calculated for and appended to each code block.
Having a CRC per code block allows for early detection of correctly
decoded code blocks and corresponding early termination of the iterative
decoding of that code block.
This can be used to reduce the
terminal processing effort and power consumption. It should be noted that, in
case of no code-block segmentation that is in case of a single code block no
additional code-block CRC is applied.
One could argue that, in case of
code-block segmentation, the transport-block CRC is redundant as the set of
code-block CRCs should indirectly provide information about the correctness of
the complete transport block. However, code-block segmentation is only applied
to large transport blocks, in which case the extra overhead of the additional,
and partly redundant, transport-block CRC is insignificant. The transport-block
CRC also adds additional error-detection capabilities and thus reduces the risk
for undetected errors in the decoded transport block.
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