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4-6

Viterbi Decoder Implementation

Algorithmic Extensions

Allowing More General Branch Metrics

4.2.2

Modify Branch Metric Generation

The branch metric generation needs modification also. Example 4-2 shows the user a

different motivation for noncomplementary branch metrics. In this example, we use

convolutional code from Section 3, but take the path metric for state 0 and subtract it

from all branch metrics. This is a computationally inexpensive way to automatically

normalize the path metrics so that we need not worry about arithmetic saturation of

path metrics when decoding very long streams of data. It does, however, destroy the

symmetry of the branch metrics.

Example 4-2

Modified Branch Metric Generation Code

;*****************BRANCH METRIC MACRO*************************************

;

FUNCTION: Input data and generate branch metrics.

This function

;

subtracts the path metric for state 0 from all branch metrics

;

to provide autonormaliztion. For this decoder, the metric is a scaled

;

sum or difference of the real and imag inputs.

;

INPUTS:

;

r2 should point to the beginning of the branch metric table

;

r5 should point to the latest path metric for state 0

;

r1 should point to the next input XY data pair

;

OUTPUTS:

;

Branch metrics are stored at BRX in XY memory

;

REGISTERS USED:

;

a,b,x01,y01,r1,r2,r5, r5 unchanged,r2 unchanged (modulo req'd)

;****************************************************************

;

FindMetrics macro

move

x:(r5),a

;st. metric scaling, read last st. 0

neg

l:(r1)+,y ;negate metric|grab dec input

move

#-16,x1

;sign for real component, upper br.

mac

x1,y1,a

a,b

;comp 0x partial br. path mt. to

move

y1,x0

;mv real input to x0

mac

x1,y0,a

a,y1

;a gets 00 br. y1 gets 0x partial br

subl

a,b

b,x0

;a has 00 br, b has 11 br, save path

tfr

y1,a

a,y1

;swap 0x and 00 to compute 01 branch

mac

-x1,y0,a

b,x1

;a gets 01 br

tfr

x0,b

b,y0

;swap path metric,11 branch to y0

subl

a,b

y1,x0

;b gets 10 br, x0 has copy of 00 br.

;*****************************************************************************

;

AT this point, X is configured with br 11|00, y with 00|11, and

;

AB with 01|10, BA with 10|01 all needed for quick storage in XY

;

memory.

;*****************************************************************************

Freescale Semiconductor, Inc.

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