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G3 FACSIMILE COMMUNICATIONS
In other words, the area of these 16 points is four times the area of the orig-
inal 128 points. This means that this area has a tolerance of four times with
respect to data errors caused by line noise. According to the coding scheme
described above, a coding gain of about 4dB can be obtained when con-
verted to S/N ratio compared with a QAM system MODEM on which 64
points are not coded. The trellis coder in recommendation V.17 has an
elimination function for phase uncertainty. Generally, on a MODEM that
uses a phase modulation system or QAM modulation system, differential
phase modulation system is performed; in other words, the data is replaced
with the phase change of the carrier signal before it is transmitted. By this
system, when an instantaneous phase change has occurred, an error will
occur at that time. However, data from then on will not be affected. In
V.17, the differential coder that is used as a pre-stage to the trellis coder is
mounted for such kind of purpose, and has an elimination for 90°, 180°
and 270° phase uncertainty. This trellis coder is configured so that receive
data will not be in error in burst manner on an instantaneous phase hit of
90° according to the features of the repetitive structure and to the signal
mapping rules onto the signal spatial diagram shown in Fig. 5-55.
5–68
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Fig. 5-57 Mapping Rules in Trellis Coding (2)
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