VP305 datasheet(13/85 Pages) MITEL

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DRAFT - PRELIMINARY DATA

VP305/6

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on the title page of this document.

1.3.

The Viterbi Decoder block.

The Viterbi decoder input data format uses three bit, two's complement coding representation of

the data in the following form, (binary numbers):

I or Q value

Interpretation

011

most likely one

010

001

000

least likely one

111

least likely zero

110

101

100

most likely zero

Table 2. Viterbi decoder input format.

The first task for the Viterbi decoder is to 'de-puncture' the data. This is a process of restoring data

bits which have been removed (punctured), prior to transmission, to improve the transmission

efficiency. The de-puncture code rate must be programmed into the VIT MODE register. The

following patterns are used:

VITCR[2:0]

Code rate

Input bit stream

Output bit stream

1/2

I = X1

Q = Y1

X = 1

Y = 1

2/3

I = X1Y2Y3

Q = Y1X3Y4

X = 1010

Y = 1111

3/4

I = X1Y2

Q = Y1X3

X = 101

Y = 110

5/6

I = X1Y2Y4

Q = Y1X3X5

X = 10101

Y = 11010

7/8

I = X1Y2Y4Y6

Q = Y1Y3X5X7

X = 1000101

Y = 1111010

Table 3. Viterbi decoder code rate.

The zeros in the above table represent unknown data bit values, effectively error bits. The decoder

uses a trace back trellis technique to remove the uncertainty and recover the correct data. The

trace back depth is 128.

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