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G3 FACSIMILE COMMUNICATIONS
5.5.2 Trellis coding/vitterbi decoding on V.17 MODEMs
MODEMs support various modulation systems such as differential phase
modulation and quadrature phase modulation. All of these modulation sys-
tems make the digital data correspond to analog signals and transmit infor-
mation. The signals that are sent are influenced, for example, by line noise
on the communications path and are deformed in greater or lesser degree.
When trellis coding/Vitterbi decoding is applied to the MODEM's trans-
mission system as an error correction mode, a method for performing error
correction on the line noise is conceivable.
To perform error correction, the information is appended with a redundant
bit before it is transmitted. Appending of the redundant bit causes the
transmission efficiency to drop increasingly as the redundant bit is in a cor-
relationship with faster data transmission speeds. The transmission effi-
ciency, however, can be improved by performing error correction more
times than the frequency of error occurrence according to impaired trans-
mission quality caused by line noise, for example. As described above, the
maximum likelihood decoding method calculates and compares the Ham-
ming distance for judging the most reliable-looking bit targeting binary
bits 0 and 1. Trellis coding/Vitterbi decoding on a MODEM, however,
takes line noise generated on the transmission path into consideration, and
adopts an error correction mode that uses Euclidean distance, in other
words, the distance between satellite points for the receive signals as the
method for judging the receive data.
Essentially, Euclidean distance is an analog amount, and a precision ana-
log circuit is required to achieve trellis coding/Vitterbi decoding using the
Euclidean distance. However, in practical terms it is difficult to achieve
such as error correction scheme by an analog circuit. For this reason, the
analog amount is converted to a digital amount by multi-value quantizing
the Euclidean distance, and processing is then performed by a digital cir-
cuit. Decoding where the Euclidean distance is used in this way and judg-
ment is performed according to multi-value quantized values is called
"soft-judgment decoding." "Soft-judgment Vitterbi decoding" is the result
of having applied soft-judgment decoding to Vitterbi decoding. When trel-
lis coding/Vitterbi decoding that uses Euclidean distance is applied to a
MODEM, coding by which the distance between signals in a constellation
in a MODEM's modulated signals is made the largest is performed, and at
the decoding stage, a method is used for demonstrating error correction
capabilities most effectively is adopted by judging and generating signals
based upon the points in the constellation that are the shortest distance
with the receive signal. Though actual trellis coding as recommended is
configured in accordance with the basic logic, the description relating to
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