Hence you would
expect going to a larger interleaver size can drive us closer to our low SNR goal. A disadvantage is
that larger interleaver sizes require more turbo iterations and larger memory in the turbo decoder.
Lastly, we should also mention the delays caused by the large interleaving and de-interleaving blocks.
These are both interesting and challenging design trade-offs.
Turbo Decoder. Let us first start by mentioning there are a few choices available to the system
designer concerning the turbo decoder method to use. It is not our intention to compare all of them;
we will concentrate on the maximum a posteriori probability (MAP) decoder. As discussed above, the
interleaver is placed between the input of the two encoders in order to decorrelate their respective
inputs. The decoders make use of this property when iterating. The decoders operate in a concatenated
fashion where the output of one decoder is the input to the second decoder. After each iteration errors
are corrected.
The soft decisions will be passed from one decoder to another decoder in the form of loglikelihood
ratios defined as (with yj??™s representing the received signals).
(m 1, 2) (5.81)
Which can be rewritten as
(5.82)
(5.
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