The receive block diagram showing the depuncturing operations is shown
in Fig. 5.40.
The advantages in using this punctured coding technique are given below
??? Allows for use of a general rate 1/2 Viterbi decoder to detect a wide range of punctured codes. For
example, R 3/4,7/8, and so on.
??? This punctured BER performance is very close to the nonpunctured BER for this R 3/4 system
example.
??? We can use the less complex Viterbi decoder for rate 1/2 versus the more complex R 3/4 Viterbi
decoder.
Rate
input
ouptut
3 bits
(6 2) bits
3
4
One disadvantage in using this punctured coding technique is that the truncation path length
increases as the punctured code rate decreases. For example, start with R 1/2 and K 7 code, then
the following rules-of-thumb hold true.
Code rate Truncation path length (T)
R 1/2 >5K bits
R 3/4 >10K bits
R 7/8 >15K bits
Another disadvantage is that soft decision decoding must be used to retrieve the lost information
caused by puncturing at the transmitter. The 2-dB gain is needed to compensate for the intentionally
induced errors to meet the code rate requirements.
In Fig. 5.41 we present the BER performance results for the punctured R 3/4 and R 7/8 codes.
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