DSP56300 datasheet(38/108 Pages) FREESCALE

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3-10

Viterbi Decoder Implementation

Expanding the Viterbi Algorithm

Creating the Branch Metrics

The FindMetrics macro is for the most part straightforward. We begin by loading the

decoder input data to compute the branch metrics. Next, load the scaling factor, and

multiply to obtain the partial branch metric with one of the inputs. The next line

initializes address register r2 as a pointer to the table location used by state 3, where the

branch metrics are to be stored. We then finish the computation of the branch metric for

a branch with encoder output 00, followed by the metric for 01. With these two metrics,

we begin to load the branch metric table.

We can load the branch metric table in any order. To minimize the number of cycles

needed, we apply a few constraints. First, it is easier if we load the table in some

consistent manner such as using a constant address offset each time. Second, it is

convenient if we end up at the address used by state 0, because then we are initialized

for the butterfly state update. Also, we must finish generating the metric values for 11

and 10. It is most efficient if we can do this in parallel with the moves to load the branch

metric table.

The easiest way to store the branch metrics might be to start at state 15 and count down.

If we do this, it turns out that a stall is unavoidable: we cannot generate all of the

required branch metric values in time to use them with this ordering. Other loading

orders are easy to obtain by using an address register increment that is relatively prime

to the table length (and using a modulo addressing mode to wrap around). For our

table length, any odd number increment will ensure that we cover the entire table with

constant increments. Our first candidate is 3. If we start at state 3, and increment by 3Õs,

we will cover the entire table and end up at the address for state 0. These first three

branch metric values are associated with recreated encoder outputs of 00, 01 and 00.

Because the repeat of 00 gives us time to generate the extra branch metric values with

no stall, we use increments of 3.

We begin by writing the branch metric for state 3. At the same time, we negate A to

compute the metric for branches with encoder output 11, saving the metric for 00 in x1.

For the next write, we negate B to compute the metric for 10, saving the metric for 01 in

x0. We write to the branch metric table at the same time.

What follows are writes to the branch metric table. Using modulo addressing, we

increment the address by 3Õs until the entire table is filled. We write the branch metrics

determined by the encoder polynomials as they appear in Table 3-1.

Finally, note that this code has been constructed so that the address register r2 points to

the beginning of the branch metric table at the end of this routine. Because of this, the

butterfly loop is automatically initialized to load branch metrics by the end of this

routine.

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