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Pulse Position Filter (PPF)

IP.com Disclosure Number: IPCOM000123712D
Original Publication Date: 1999-Mar-01
Included in the Prior Art Database: 2005-Apr-05
Document File: 5 page(s) / 191K

Publishing Venue

IBM

Related People

Feng, KD: AUTHOR

Abstract

An infrared transceiver is a high gain device designed to work in open spaces, where noise and interference poses the main limitations for system sensitivity. This paper provides a solution to filter out the output pulses from the transceivers that are caused by noise or interference for pulse position modulation such as specified in IrDa for high speed infrared communication. The method in the paper is applicable to all pulse position modulation, systems but the 4 pulse position modulation is used as an example in the following description. 1. Pulse position modulation. Pulse position modulation (PPM) encoding is achieving by defining a data symbol duration (Dt) and subsequently subdividing Dt into a set of n equal time slices called "chips".

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Pulse Position Filter (PPF)

   An infrared transceiver is a high gain device designed
to work in open spaces, where noise and interference poses the main
limitations for system sensitivity.  This paper provides a solution
to filter out the output pulses from the transceivers that are caused
by noise or interference for pulse position modulation such as
specified in IrDa for high speed infrared communication.  The method
in the paper is applicable to all pulse position modulation, systems
but the 4 pulse position modulation is used as an example in the
following description.
  1.  Pulse position modulation.  Pulse position modulation
      (PPM) encoding is achieving by defining a data symbol
      duration (Dt) and subsequently subdividing Dt into a
      set of n equal time slices called "chips".  In PPM
      schemes, each chip position within a data symbol
      represents one the possible bit combinations.  Each chip
      has a duration of Ct given by
        Ct = Dt/n The bit pair that a pulse represents
        dependents on the chip where the pulse locates.
        If n=4, it is so called 4 pulse position modulation
        where
          Dt=500 ns
          Ct=125 ns The relationship of the data bit pair and
          the positions are  shown in Fig. 1.  All combinations
          of the data bit pair and their timing are shown in
          Fig. 2.

   When the data bit pairs are 1100, the two pulses are
touched together and so called a double pulse because it has double
pulse width, all pulses for other cases are called single pulses.
The data bit pairs for a single pulse is not unique while a double
pulse corresponds to a special data bit pairs: 1100 so that a double
pulse is always taken as a synchronizing signal for decoding.
  2.  The pulse position filter operation principle
      From Fig. 2, it is obvious that the timing of rising edges
      and allying edges obey the following regulations:
      a.  when a rising edge is detected, the following
          falling edge must occur at Ct (125 ns) or 2Ct (250 ns)
          after the rising edge.
      b. When a falling edge is detected, the following
         falling edge must occur at several possible locations
         after the rising edge depending on the current data
         bit pair which is shown in Table 1.

   Any output pulses from normal signals must follow the
timing mentioned above, they can pass the pulse position filter
while any output pulses  caused by noise or interference do not have
the above timing and they are filtered out.  That is the pulse
position filter operation principle.
  3.  The structure of pulse position filter A PPF consists of
      two units: rising-falling edge position filter (RFF) and
      falling-falling edge position filter (FFF).
      3...