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Interactive Phase Jitter Cancellor

IP.com Disclosure Number: IPCOM000113682D
Original Publication Date: 1994-Sep-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 4 page(s) / 159K

Publishing Venue

IBM

Related People

Nobakht, RA: AUTHOR

Abstract

Adaptive or interactive carrier recovery systems are essential in high-performance Quadrature Amplitude Modulated (QAM) data communications systems to correct for phase jitter and frequency offset. Present schemes adapt their structure to match the spectral properties of the impairments. However, they face the conflict between a wide bandwidth to track fast jitter and a narrow bandwidth to minimize output noise. In addition, convergence time for effective phase jitter cancellation can be too long for intermittent sources of jitter. Incorporation of powerful and cost effective adaptive carrier recovery systems is essential in achieving reliable operation of high-performance (5-8 bits/s/Hz) data communications systems.

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Interactive Phase Jitter Cancellor

      Adaptive or interactive carrier recovery systems are essential
in high-performance Quadrature Amplitude Modulated (QAM) data
communications systems to correct for phase jitter and frequency
offset.  Present schemes adapt their structure to match the spectral
properties of the impairments.  However, they face the conflict
between a wide bandwidth to track fast jitter and a narrow bandwidth
to minimize output noise.  In addition, convergence time for
effective phase jitter cancellation can be too long for intermittent
sources of jitter.  Incorporation of powerful and cost effective
adaptive carrier recovery systems is essential in achieving reliable
operation of high-performance (5-8 bits/s/Hz) data communications
systems.  These high speed data communications systems are essential
in the realization of the Remote Access Multimedia and the ability to
transmit different types of digital data (i.e., speech, image, fax,
email, etc) over the existing voice band channels.

      In this invention a minimum cost solution for effective phase
jitter cancellation has been designed.  This solution will no longer
face the bandwidth conflicts mentioned above.  In addition it will
yield a very fast response to intermittent sources of phase jitter.
In this design, the well-known data-directed second-order phase
locked loop has been used to compensate for the effects of frequency
offset.  Seven second order IIR narrowband filters have been used to
compensate for 20, 50, 60, 100, 120, 150, and 180 Hz sources of phase
jitter.  However, since these sources of phase jitter are not present
continuously at all times, a fast and cost effective interactive
algorithm has been invented in order to minimize the output noise and
improve the performance of the communication system.  The 50 Hz and
60 Hz jitter harmonics, if present, stay active during the entire
communication session.  Therefore, an algorithm has been designed to
disable the specific jitter filters whose jitter source does not
exist.  This task can be performed during the startup procedure.
Unlike the other sources of jitter, the 20 Hz phase jitter can be
intermittent.  The 20 Hz phase jitter is caused by the ring
frequency.  An algorithm has been designed, in this case, to
interactively probe for the presence of this jitter, and if present,
the algorithm will activate the 20 Hz jitter filter.  On the other
hand, once the source of this 20 Hz phase jitter disappears, then the
algorithm deactivates the 20 Hz jitter filter.  This methodology
yields the fastest and the most cost effective solution for combating
the effects of severe phase jitter without any sacrifice in
performance.
jitter cancellor.

      Fig. 1 shows the structure of the carrier recovery system which
is designed to combat the effects of frequency offset and phase
jitter.  As mentioned previously, the block representing the second
order phase lock loop used for combating...