dc

An interleaver installed between the two decoders is used here to scatter error bursts
coming from output. DI block is a demultiplexing and insertion module. It
works as a switch, redirecting input bits to at one moment and to at
another. In OFF state, it feeds both and inputs with padding bits (zeros).
Consider a memoryless AWGN channel, and assume that at k-th iteration, the
decoder receives a pair of random variables:
,
where and are independent noise components having the same
variance . is a k-th bit from encoder output.
Redundant information is demultiplexed and sent
through DI to (when ) and to (when ).
yields a soft decision, i.e.:
and delivers it to . is called the logarithm of the likelihood
ratio (LLR). is the a posteriori probability (APP) of the data bit
which shows the probability of interpreting a received bit as . Taking the LLR into
account, yields a hard decision, i.e. a decoded bit.
It is known that the Viterbi algorithm is unable to calculate APP, thus it cannot be
used in . Instead of that, a modified BCJR algorithm is used. For , the Viterbi
algorithm is an appropriate one.
However, the depicted structure is not an optimal one, ecause
only a proper fraction of the available redundant information. In order to improve the
structure, a feedback loop is used (see the dotted line on the figure).
uses