Cryptography Reference
In-Depth Information
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−
1
Figure 7.16 - Implementing turbo decoding does not require explicit knowledge of
In addition, the two extrinsic information memories can be merged into a
single one, observing that extrinsic information that has just been read and
exploited by a decoder no longer has to be retained. It can thus be replaced
immediately afterwards by another datum, which can be the extrinsic informa-
tion output from the same decoder. Figure 7.17 illustrates this process, which
imposes a slight hypothesis: working indices
i
and
j
havethesameparityand
permutation
i
=Π(
j
)
inverses the parity. For example, with the permutation
defined by (7.4), this hypothesis is satisfied if the departure index
i
0
is odd and
the length of the message
k
is even.
Figure 7.17 - In practice, the storage of the extrinsic information uses only a single
memory.
The extrinsic information memory is divided in two pages corresponding to
the sub-sets of the even and odd addresses. Access to these two pages, in a dual-
port memory, is alternated regularly. In Figure 7.17(a), in the even cycles, DEC1
reads at an even address and writes the extrinsic information produced during
the previous cycle, via a buffer memory with unit delay, to an odd address.
Meanwhile, DEC2 reads at an even address and writes to an odd address. In
