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Adaptive Thresholding and Parameter Estimation for PPM NASA's Jet Propulsion Laboratory, Pasadena, California
 

Summary: Adaptive Thresholding and Parameter Estimation for PPM
NASA's Jet Propulsion Laboratory, Pasadena, California
Mar 01 2005
A receiver detection threshold is repeatedly adjusted to balance competing requirements.
A method of adaptive setting of a threshold level for the detection of pulses in a pulse-position modulation
(PPM) free-space optical communication system has been developed. In simplified terms, it is desirable
to set a threshold value high enough to greatly reduce the probability (PFA as defined below) of
erroneously detecting noise as signal pulses but not so high as to greatly reduce the probability (PD as
defined below) of detecting any signal pulses that may be present along with noise. In the present
method, the threshold level is varied with time, in response to changing conditions in the optical-
communication channel, in an effort to maintain a balance between the aforesaid competing
requirements. An integral part of this adaptation scheme is a scheme for estimating key parameters of the
optical-communication channel in particular, parameters that describe the fading and total attenuation in
the channel, and parameters that characterize spreading of pulses by atmospheric and other effects. The
method can be implemented by software processing of digitized optoelectronic-detector output, and has
been tested by computational simulation.
http://www.techbriefs.com/content/view/720/34/
In the first stage of processing by this method, the digitized values of the detector output during noise-
only time slots of received PPM symbols are averaged to obtain a background level. This background
level is subtracted from the detector output in the hope of reducing or eliminating the noise component in

  

Source: Arabshahi, Payman - Applied Physics Laboratory & Department of Electrical Engineering, University of Washington at Seattle

 

Collections: Engineering