Browse Prior Art Database

Forward Correction of Data Induced Noise in a Communication Channel

IP.com Disclosure Number: IPCOM000117888D
Original Publication Date: 1996-Jul-01
Included in the Prior Art Database: 2005-Mar-31
Document File: 2 page(s) / 46K

Publishing Venue

IBM

Related People

Camp Jr, WO: AUTHOR [+4]

Abstract

It is a problem with direct frequency modulation of an oscillator whose frequency is being held constant by a Phase Locked Loop (PLL), that the PLL will remove the modulation components that fall within the loop bandwidth. For the case of modulation that is a data signal, this distortion will cause an increase in Bit Error Rate (BER). The underlying effect of this combination of modulation type and an oscillator that is part of a PLL, is the addition of a signal that is simply the integration over time of the unbalance in the input data signal. The integration time constant is set by the loop bandwidth of the PLL. The unbalance in the input signal is the deviation from 1.0 of the weighted ratio of plus symbols to negative symbols. (Weighting allows for multivalued symbols.)

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Forward Correction of Data Induced Noise in a Communication Channel

      It is a problem with direct frequency modulation of an
oscillator whose frequency is being held constant by a Phase Locked
Loop (PLL), that the PLL will remove the modulation components that
fall within the loop bandwidth.  For the case of modulation that is a
data signal, this distortion will cause an increase in Bit Error Rate
(BER).  The underlying effect of this combination of modulation type
and an oscillator that is part of a PLL, is the addition of a signal
that is simply the integration over time of the unbalance in the
input data signal.  The integration time constant is set by the loop
bandwidth of the PLL.  The unbalance in the input signal is the
deviation from 1.0 of the weighted ratio of plus symbols to negative
symbols.  (Weighting allows for multivalued symbols.)

Existing methods for counteracting this problem are:
  1.  to greatly reduce the loop bandwidth,
  2.  to open the phase lock loop during modulation,
  3.  to break the message up into blocks and reverse the polarity of
       blocks as necessary to reduce the unbalance, and
  4.  to modulate both the reference oscillator for the loop as well
as
       the primary oscillator.

      If rapid channel changes are required, one cannot use (1).  If
the message length is long, one cannot use (2).  Method (4) requires
that one carefully adjust the phase and amplitude of the modulation
signal to the...