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參數(shù)資料
| 型號： | DSP56600 |
| 廠商： | 飛思卡爾半導(dǎo)體(中國)有限公司 |
| 英文描述： | Implementing Viterbi Decoders Using the VSL Instruction on DSP Families |
| 中文描述： | 維特比解碼器實現(xiàn)上使用DSP的家庭教學(xué)的VSL |
| 文件頁數(shù)： | 21/108頁 |
| 文件大?。?/td> | 726K |
| 代理商： | DSP56600 |
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The Viterbi Algorithm
Viterbi Decoder
Viterbi Decoder Implementation
For More Information On This Product,
Go to: www.freescale.com
2-7
Comparing the recreated encoder output with the decoder input, we determine the
number of agreements, shown in
Figure 2-3
as the branch metric. We find a branch
metric for each transition. For each state, we track the cumulative branch metrics to form
the path metrics. These are shown as a number appearing above each state box.
To recreate the correct input sequence, we choose the recreated encoder path that best
agrees with the decoder input data. In this case, the best path is the one with the largest
final path metric. For clarity, the path metrics for the best path are distinguished with a
larger font size and bolder arrows. To recreate the input sequence (so far), we can use
two methods. The easiest is to use the state as the decoder output. Unfortunately, this
method will not work after we finish the development of the decoder. The second
method will work when we are done. To obtain the decoder output, trace the best path
(the one with the largest final path metric) back to the beginning. Now, follow the same
path forward again to obtain the input sequence by placing a 0 at the decoder output
each time we choose an upper transition, and a 1 output each time we choose the lower
transition. Using this method on the tree in
Figure 2-3
gives us 10110, which agrees with
the encoder input example.
The most troublesome aspect of this decoder is that the number of states we have to
track for each decoder input is actually the number of possible paths. For this coding
example, the number of states doubles for each input. For any reasonable number of
inputs, the amount of work and storage needed for this decoder is far too large to be
practical. To solve this problem, begin by noting that we dont really need all the data
generated by the decoder. In particular, all the work goes toward finding the path that
best agrees with the input. We only need the path that gives us the largest path metric. If
we determine that a path cannot ever have the largest path metric, we can ignore that
path for all future calculations.
To collapse the ever-growing tree in
Figure 2-3
, consider what happens if we continue
the tree for one more pair of decoder inputs. The total number of states would be 64 for
the next input pair. To keep the diagram manageable, only a pair of specially chosen
states appears in
Figure 2-4
. When we extend the tree to the next state, we get states with
six bits. Note, however, that the encoder we are attempting to trace only needs five bits
to determine its output bits. To emphasize this, the sixth (leading) bit is separated in the
state boxes. Because the extra (leading) bits do not affect the recreated encoder outputs,
we can ignore them. As a result, the 64 states collapse into 32 states again. The only
resulting complication is that each state now has two rather than one entering paths.
Figure 2-4
shows this state collapse as well as each states multiple input paths in two
example states. To correctly process the decoder input, we must next determine which of
the input paths to keep for each state.
F
Freescale Semiconductor, Inc.
n
.
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