Browse Prior Art Database

Improvement to Spectral Shaping Technique

IP.com Disclosure Number: IPCOM000123448D
Original Publication Date: 1998-Nov-01
Included in the Prior Art Database: 2005-Apr-04
Document File: 3 page(s) / 125K

Publishing Venue

IBM

Related People

Dholakia, A: AUTHOR [+4]

Abstract

Disclosed is a method for improving spectral shaping used in a communication system. Traditionally, spectral shaping is applied to the transmit signal in order to achieve a desired spectral characteristic for the transmit signal itself. However, in certain communication systems, a signal obtained from the transmit signal by a non-linear transformation is of interest for spectral shaping. In the disclosed invention, the spectral shaping process is modified such that the non-linearly transformed signal has the desired spectral characteristic.

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Improvement to Spectral Shaping Technique

   Disclosed is a method for improving spectral shaping used
in a communication system.  Traditionally, spectral shaping is
applied to the transmit signal in order to achieve  a desired
spectral characteristic for the transmit signal itself.  However, in
certain communication systems, a signal obtained from the transmit
signal by a non-linear transformation is of interest for spectral
shaping.  In the disclosed invention, the spectral shaping process is
modified such that the non-linearly transformed signal has the
desired spectral characteristic.

   A specific embodiment of the invention can be illustrated
by considering the spectral shaping technique in the ITU-T V.90
recommendation for PCM modems.  The communication system as
envisioned in V.90 is shown in Fig. 1, comprising digital modem 10,
downstream channel 20, and analog modem 50.  The downstream channel
itself comprises digital channel 30 and analog channel 40.  The
digital channel can impair and alter the transmitted PCM codes.  Such
"digital impairments" can be viewed as applying a non-linear
transformation to the transmitted signal.  The analog channel, which
includes the subscriber loop, further impairs the signal by adding
noise and frequency-dependent distortion.  Spectral shaping is one of
the methods generally used to reduce the effects of such "analog"
impairments and applies to point 70 in Fig. 1.

Prior to data transmission, digital impairments are detected by the
analog modem using specific training signals.  The presence of
digital impairments results in the analog modem preparing two sets of
signal constellations which are sent to the digital modem: set S,
reflecting the PCM codes (and associated analog values) at point 60
in Fig. 1, and set S', reflecting the PCM codes (and associated
analog values) at point 70 in Fig. 1.  During training, the analog
modem also determines a spectral shaping filter which is suitable for
the analog channel impairments and sends these coefficients to the
digital modem.  The digital modem uses these coefficients in the
spectral shaper.

Possible non-linear transformations include robbed-bit signalling
(RBS) in which the least significant bit of a PCM code is forced to a
binary 1 or a 0 for network signalling.  Another possible non-linear
transformation is a digital attenuation pad (DPAD) used to adjust the
power level of the transmitted signal.  These non-linear
transformations can occur in tandem, as shown by one pos...