Digital Signal Processing Reference
In-Depth Information
(
a
)
(
b
)
(
c
)
FIGURE 10.49.
CCS plots using case 4: (
a
) input to convolutional encoder; (
b
) output from
convolutional encoder (between 0 and 1); (
c
) output from a Viterbi decoder.
simply counting how many bits are different between the received channel symbol
triad and the possible channel symbol triad. The results can only be zero, one, two,
or three. The Hamming distance (or other metric) values computed at each time
instant, for the paths between the states at the previous time instant and the states
at the current time instant, are called
branch metrics
. For the first time instant, these
results are saved as
accumulated error metric
values associated with states. From the
second time instant on, the accumulated error metrics are computed by adding the
previous accumulated error metrics to the current branch metrics.
Consider that at
t
1, 000 is received at the input of the decoder.
.
The only pos-
sible channel symbol triads that could have been received are 000 and 111. The
Hamming distance between 000 and 000 is zero. The Hamming distance between
=
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