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G3 FACSIMILE COMMUNICATIONS
At time n, when W2n from among three bits W1n, W2n and W3n currently
stored on the convolutional coder is 0, one of 000(=A), 010 (=B), 001(=C)
or 011(=D) is output as the coded bit string, and when W2n is 1, one of
110(=E), 101 (=F), 111(=G) or 100(=H) is output. In other words, the shift
in states has regularity where there are only four shifts in state to the next
state from the eight respective states. For this reason, we can try consider-
ing how the coded output data of this trellis coder is mapped. In mapping
of output data from the trellis coder, the Euclidean distance between output
signals that are coded as described above must be made as large as possi-
ble. With a trellis coder stipulated in V.17, the coded output data is three
bits Y0n, Y1n and Y2n, and are mapped so that the distance between sig-
nals in the eight states expressed by these three bits becomes larger. For
this reason, the seven bits that express each of the signal points of the sig-
nal spatial diagram in Fig. 5-54 are sorted into four bits of coded data Q3n,
Q4n, Q5n and Q6n, and the eight points by three bits are each expressed by
A to H, and the 16 points expressed by four bits are each expressed by 1 to
16.
5–66
State
W
W
W
n1,
n2,
n3,
A
000
C
C
D
001
B
H
010
G
E
F
E
F
011
H
G
100
C
D
D
C
A
101
H
G
110
F
F
E
111
Fig. 5-55 Shift in States in 8-state Trellis Coding
State
W
n+1 1,
B
D
A
B
A
B
E
G
H
W
W
n+1 2,
n+1 3,
000
001
010
011
100
101
110
111
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