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Unified Trellis-Coded Modulation Coding Scheme for a 16.8 and 19.2 Kbps Full Duplex Modem

IP.com Disclosure Number: IPCOM000113672D
Original Publication Date: 1994-Sep-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 6 page(s) / 215K

Publishing Venue

IBM

Related People

Nobakht, RA: AUTHOR

Abstract

In this article a 512 point, shell mapped, 90 degree rotationally invariant Trellis-Coded Modulation (TCM) coding scheme for an IBM proprietary 16.8 and 19.2 Kbps full duplex modem has been designed which will enable a modem to send more bits for the same bandwidth. This design is based on the CCITTs rate 2/3, 8 state trellis structure.

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Unified Trellis-Coded Modulation Coding Scheme for a 16.8 and 19.2
Kbps Full Duplex Modem

      In this article a 512 point, shell mapped, 90 degree
rotationally invariant Trellis-Coded Modulation (TCM) coding scheme
for an IBM proprietary 16.8 and 19.2 Kbps full duplex modem has been
designed which will enable a modem to send more bits for the same
bandwidth.  This design is based on the CCITTs rate 2/3, 8 state
trellis structure.

      TCM is used for data communication with the purpose of gaining
noise immunity over uncoded transmission without altering the data
rate.  TCM is a combined coding and modulation scheme for improving
reliability of a digital transmission system without increasing the
transmitted power or the required bandwidth.  In a power-limited
environment, the desired system performance should be achieved with
the smallest possible power.  One solution is the use of
error-correcting codes, which increase the power efficiency by adding
extra bits to the transmitted symbol sequence.  This procedure
requires the modulator to operate at a higher data rate and hence
requires a larger bandwidth.  In a bandwidth-limited environment,
increased efficiency in frequency utilization can be obtained by
choosing higher order modulation schemes (e.g., 8-PSK instead of
4-PSK), but a larger signal power would be needed to maintain the
same signal separation and hence the same error probability.  The
trellis code solution combines the choice of a higher-order
modulation scheme with that of a convolutional code, while the
receiver, instead of performing demodulation and decoding in two
separate steps, combines the two operations into one.  In this
invention a 512 point constellation scheme has been designed which is
capable of transmitting either 7 or 8 information bits per symbol
duration (one bit is used for convolutional encoding).

      The methodology is based on a simple shell mapping scheme which
allows the 16.8 Kbps constellation to be included as a subset of the
19.2 Kbps constellation, therefore eliminating the necessity for
having to design a separate constellation for the 16.8 Kbps modem.
This will also reduce the average signal energy and save a
significant amount of data store memory required for the storage of
the 16.8 constellation symbols.  With a standard V32bis symbol rate
of 2400 Hz this yields an effective bit rate of 16800 or 19200 bits
per second.  This design is based on the CCITT's rate 2/3, 8 state
trellis structure.  At the present, the world's fastest standard
modem is the V32bis with the highest data rate of 14.4 Kbps.  These
new rates will enable the modem to send more bits for the same
bandwidth.  Also, with their proprietary constellation scheme, this
new rate can be included as an addition to the V32bis protocols.

      Assume a discrete-time, continuous-amplitude model for the
transmission of data on the additive white Gaussian noise channel.
In this communication model, intro...