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Summary

Viterbi Decoder Implementation

5-3

5.1

SUMMARY

This application report began with an explanation of the Viterbi algorithm in Section 2,

introduced a basic Viterbi decoder in Section 3, and offered some enhancements to the

decoding assembly code in Section 4. The explanations in Section 2 and Section 3 are

detailed enough for the ideas behind the code to be understood. The goal is to make it

easy to take and modify this code to produce efficient code for any desired application of

the Viterbi algorithm.

We chose a nontrivial industry standard code as our example, and there are some

statistics that are worth noting. Table 5-1 presents some notable statistics for code

presented in this note. In the table, we compare the basic Viterbi decoder code of

Section 3

with the basic code in Section 4. The basic code requires about 215 clock cycles

per decoder input to decoder the data. As an example, this means that at a received data

rate of 13,350 per second (the IS-136 rate if all six slots in a frame are voice data to be

decoded) 2.87 MIPS are required.

If we use generalized branch metrics, the numbers increase, because the branch metric

calculation is more complicated. Note, however, that they lower again as the Pre ACS

and ACSFlush routines are included. Once all the enhancements of Section 4 are

included, the computational load drops to 2.81 MIPS, more than 3% below what the rate

would be if we just used the generalized branch metrics.

5.2

CONCLUSIONS

In conclusion, note that the relative improvement is dependent on the length of the data

packet. Shorter packets show greater improvement. For example, an encoded voice data

slot in IS-136 uses a packet size of only 89 decoder inputs instead of our example 168. For

packets of this size, the percent change is 4.8%Ñlarge enough to make the extra code

worth considering.

Freescale Semiconductor, Inc.

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