For the
BER values shown, coding gains as much as approximately 6.5 dB are possible. In an effort to provide
an interim summary for the reader, we will comment on the two types of decoders presented.
First, the hard-decision decoder is presented. Here a binary decision device is placed before the VA
in order to make a decision on the received bit stream. In this case the VA can use the Hamming distance
as the path metric. The drawback of this technique is that reliable and unreliable bits are equally
weighed in the decoding process. Soft decision aims to solve this shortcoming by preserving the
amplitude of the received symbol. As we have shown in Fig. 5.37, this provides approximately 2 dB
of additional coding gain over hard decision.
Puncture Coding. The concept of puncturing the encoder comes into play when the communication
system cannot afford a powerful low-code rate coding scheme. System limitations such as transmission
bandwidth may preclude the system designer from using a lower code rate. Alternatively, this
application can be approached from another perspective. Let??™s suppose a data packet is transmitted
and it is received with an error, the transmitter can either retransmit the same packet or increase the
redundancy information in order to help the receiver decode the packet error-free.
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