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2-14

Viterbi Decoder Implementation

The Viterbi Algorithm

Algorithmic Enhancements

Figure 2-9

Applying an Offset to Obtain Odd Symmetry in the Branch Metrics

A second modification to the algorithm is the ability to use soft decisions. By this we

mean that the input data need not be just a 0/1 decision on a receiver statistic. We can

assign intermediate values to indicate the reliability of the decision. In this way, a 0.001

can be a very reliable 0, a .75 can be a moderately reliable one, and a .49 is just barely a

zero. The Viterbi algorithm generalizes to handle soft decisions with no trouble; the

additions for the metric updates simply have more bits. By using soft decisions, the

ability of the decoder to correct errors is greatly improved.

Note that from an implementation on our DSP view, this modification is almost

invisible, as 16-bit or 24-bit arithmetic will be done regardless of the number of bits in the

branch metrics. Of course, provisions to protect path metric values from overflow may

have to change for some systems.

00100

10100

01000

01001

00/-1

11/1

Decoder

input

00100

10100

01000

01001

11/1

00/-1

After subtracting

one from each

branch metric, the

updates can be

be done by adding

and subtracting 1.

Similarly, the lower

state update can be

done by subtracting 1

first, then adding 1

for the lower branch.

Same metric, but the

add/sub order is

reversed.

Freescale Semiconductor, Inc.

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