Browse Prior Art Database

Slicer for Phased Shift Keyed Modems

IP.com Disclosure Number: IPCOM000113285D
Original Publication Date: 1994-Aug-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 6 page(s) / 166K

Publishing Venue

IBM

Related People

Davis, GT: AUTHOR [+4]

Abstract

In a slicer implementation of digital signal processor based modems, the detection of the center of the baud period requires a high rate of sampling and processing, which becomes a problem if there is not enough processing power available. A method is disclosed which uses a threshold based slicer for determination of baud center for Phased Shift Keyed (PSK) modems to resolve this problem by a slow rate locate mechanism which reduces processing overhead, thus enabling one digital signal processor to handle multiple modem channels.

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Slicer for Phased Shift Keyed Modems

      In a slicer implementation of digital signal processor based
modems, the detection of the center of the baud period requires a
high rate of sampling and processing, which becomes a problem if
there is not enough processing power available.  A method is
disclosed which uses a threshold based slicer for determination of
baud center for Phased Shift Keyed (PSK) modems to resolve this
problem by a slow rate locate mechanism which reduces processing
overhead, thus enabling one digital signal processor to handle
multiple modem channels.

      Detection of the baud period center is one of the most crucial
steps in modem design.  It must be quick and accurate, leaving enough
time for a demodulator and decoder to recognize certain signals
necessary to complete the modem start up or handshake sequence within
a limited time.  The technique used determines the baud center by
finding a consistent signal pattern with enough power to meet the
requirements of a phase shift keyed signal.

      The threshold slicer is one of many algorithms used to complete
the phase shift keyed modem handshake sequence.  It relies on data
provided by an interpolating filter and an Automatic Gain Control
(AGC) function.  The interpolating filter is required since the baud
rate and sample rate are not even multiples of each other.  The
sample rate is fixed at 8Khz as provided by a digital
time-division-multiplexed link.  The baud rate of PSK modems is 600
symbols per second.  To facilitate ease of calculation it is
necessary to interpolate the 8Khz signal to 24Khz thus providing easy
access to signal components that are multiples of the baud rate.  The
interpolation is provided by a phase splitting filter, preferably of
the Hilbert type, containing 192 complex coefficients.  The outputs
of the filter are fed to a simple second order AGC routine.  The
results of this routine are used by the threshold slicer to determine
the baud center.  All of the above is done at twice the baud rate.

      Code execution is dependent upon sufficient energy of the
signal.  Therefore, at 101, in flow Chart 1, if carrier has not been
detected by the AGC routine, the threshold slicer routine will not
execute.  Once carrier is detected, the process of finding the baud
center is started.  The sequence of events is as follows:  1.  find a
0 (zero) threshold crossing of the real signal component, 2.  find
two more consecutive occurrences of either signal component
approaching 0, and 3.  verify demodulator power at baud center is
stable for three baud periods.

      If any of the above events does not occur, then the sequence is
restarted.  Prior statistical analysis of a PSK scrambled 1's pattern
shows that neither the real or imaginary component of the signal will
cross the 0 boundary all the time.  This is the nature of the
scrambled 1's pattern; the signal transitions to all four quadrants
of the real/imaginary...