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Using a Redundant Time Slot for Noise Reduction in Optical Pulse Position Modulation

IP.com Disclosure Number: IPCOM000106701D
Original Publication Date: 1993-Dec-01
Included in the Prior Art Database: 2005-Mar-21
Document File: 2 page(s) / 70K

Publishing Venue

IBM

Related People

Chen, KC: AUTHOR

Abstract

Disclosed is a method to reduce the observed noise in each time slot of optical Pulse Position Modulation (PPM). PPM is attractive in power-efficient free space optical communication links which suffer from strong noise, multi-path effects and intersymbol/interslot interference, and/or weak optical lumination at the photodetector(s). At the same time, no received power information is required for optimal detection. In order to improve the performance, one way is to increase the code (or signal constellation) distance as the well-known Trellis Coded Modulation (TCM) does. Another way is to reduce the observed noise so that the Euclidean distance of the code increases by a different calibration of noise scale. For a fixed energy signaling system, the distance is actually calibrated by the strength of the noise.

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Using a Redundant Time Slot for Noise Reduction in Optical Pulse Position Modulation

      Disclosed is a method to reduce the observed noise in each time
slot of optical Pulse Position Modulation (PPM).  PPM is attractive
in power-efficient free space optical communication links which
suffer from strong noise, multi-path effects and
intersymbol/interslot interference, and/or weak optical lumination at
the photodetector(s).  At the same time, no received power
information is required for optimal detection.  In order to improve
the performance, one way is to increase the code (or signal
constellation) distance as the well-known Trellis Coded Modulation
(TCM) does.  Another way is to reduce the observed noise so that the
Euclidean distance of the code increases by a different calibration
of noise scale.  For a fixed energy signaling system, the distance is
actually calibrated by the strength of the noise.  The effect of
reducing noise by one half is equivalent to doubling the code
distance while signal energy is kept the same.

      Additive White Gaussian Noise (AWGN) at the baseband was
assumed.  In the frequency domain, the information carried by PPM is
ideally decided by the separation of pulses in time domain.  For an
M-ary PPM, the information is carried primarily at M/T, M/2T,...,
M/(2M-1)T, where T is the symbol period.  The simplest form of noise
filter is a low pass filter with cutoff frequency M/T.

      A practical way to reduce observed noise is to reduce the noise
bandwidth by adding a redundant time slot at the end of each symbol
period.  This results in an (M+1)-ary PPM with the last time slot
carrying no encodi...