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3-12

Viterbi Decoder Implementation

Expanding the Viterbi Algorithm

Storing the Paths

Figure 3-1

Stored B0 Path

Note that the current encoder state is part of the path! In retrospect, this must be so, as

both the state (the recreated encoder state) and its path (the potential decoder outputÑ

the recreated encoder input) represent the same data stream. To use this, store the path

byte in a memory location with address defined by the time the path bytes are stored,

offset by the current encoder state. Then, make sure the saved partial path does not

contain the current encoder state. Instead, save the current encoder state part of the path

to be stored the next time we save paths.

To do the traceback, start with the survivor path at the end, and trace the path bits back

in time to recover the decoder output. When we recover the stored path as part of our

traceback, the final bits in that path point to the offset memory location where we need

to read to continue the traceback.

Hence, if we omit the most recent 5 bits of the path, we can potentially store up to 11 bits

of path (assuming 16-bit registers). We have chosen to store 8 in this example because

reconstruction is a little easier if we have data in bytes. There is a trade-off. Storing more

path bits at a time means we donÕt need to store paths as often (fewer instructions

executed, less memory used), but the traceback reconstruction is more difficult because

we have to piece together 11-bit data into 16-bit words.

The code in our example code makes one more assumption. We assume that the data is

sent in a block, and that the transmitter ends the block by 0 filling the encoder. The effect

of 0 filling the encoder is to end everything at state 0. Hence, for traceback, start

everything at state 0.

Bit #

1 0 1 0 1 1 1 0

0 0 0 1 0 0 1 1

Encoder state

Stored path byte

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DSP56300 Datasheet(PDF) 40 Page - Freescale Semiconductor, Inc