labeled ???b??? and follow the path back in time to the first transmitted bit, which is 1. In fact, for illustrative
purposes, a decision on the second bit can also be made at this time, which also equals 1. Hence
the reader can see the operations that are necessary to perform decoding.
In a typical application, the entire frame of data needs to be decoded and so we would continue to
accumulate the Hamming distances, compare the joining (or merging) paths, and then select the surviving
path. The procedure repeats until the entire data is decoded. Note, it is common to force the
very last state to be zero by inserting K zeros at the end of the frame input to the FEC encoder. Now
if tentative or final decisions are required prior to decoding the entire frame, then a good rule-of-thumb
is to wait for at least five times the constraint length before attempting to make a decision on the bits
that were first transmitted. It is generally good practice to give all the accumulated metrics enough
time to acquire their statistical distance measurements before one attempts to make any decisions on
the transmitted bits. A final point to make about this trellis-decoding operation is that we have presented
an approach where a single trellis path is chosen and its associated bit stream is passed to the
next signal processing function.
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